Base for container for heat storage

Abstract

A heat storage reservoir and a method of construction of a heat storage reservoir. The heat storage reservoir (103) comprises an outer container (102) and an inner container (106) for containing heat storage material. The inner container comprises a panel (222, 224, 226) that does not change in size in response to expansion or contraction of the base of the outer container on heating or cooling. The panel is slidably mounted on the base of the outer container, and the side walls of the inner container are mounted on said panel.

Description

[0001] TITLE OF THE INVENTION

[0002] Base for Container for Heat Storage

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a heat storage reservoir and a method of construction of a heat storage reservoir, and in particular to the construction of the base of such a reservoir.

[0005] BACKGROUND OF THE INVENTION

[0006] Many energy generation technologies, especially renewable sources such as wind and solar power, deliver energy in a pattern not coincident with the local energy consumption. Therefore, storage of energy for later use is an important aspect of the energy infrastructure. Today, many such technologies do exist, such as chemical batteries and thermal storage solutions. However, most solutions are expensive compared to the amount of energy stored, or have a limited number of operational cycles (chargedischarge), substantially increasing the cost of stored energy compared to energy used directly. Therefore, a solution which is scalable to store large amounts of energy at a low cost with a high number of operational cycles would be advantageous.

[0007] FR2981736 describes a heat storage system for storing solar thermal energy, especially for heating buildings, to allow use in winter of the solar thermal energy collected in summer. The heat storage system comprises: a sealed enclosure with an inlet and a heat transfer fluid outlet; an energy storage medium, which is preferably spherical particles of a glass, ceramic, or vitrified asbestos, preferably of a diameter between 1 mm and 1 cm, in the sealed enclosure; and a vacuum pump in fluid communication with the enclosure. It is proposed that the heat losses of the sealed enclosure, once heat has been transferred to the energy storage medium, can be reduced by pumping out the heat transfer fluid from the sealed enclosure and maintaining the pressure in the sealed enclosure below atmospheric pressure for the duration of the storage period. This is said to reduce thermal losses by convective and conductive exchanges between the energy storage medium and the heat transfer fluid.

[0008] GB2485836 describes the use of a pebble bed to cool gas in the event that a hot gas needs to be diverted from a turbine, for example in the event of an emergency shut down of a thermal power plant. The pebble bed intervenes between the hot gas stream and the cooler conventionally provided for use in such situations and is said to reduce thermal stress on the cooler by absorbing some of the heat of the gas and thus reducing the temperature to which the cooler is initially exposed during hot gas diversion away from the turbine. The pebbles in the pebble bed are made of a solid super-refractory material such as silicon carbide, mullite or alumina, and may have a diameter in the range of 12 mm to 30 mm. The pebble bed may be cooled by passing a fluid coolant therethrough, such as water, liquid nitrogen, steam or nitrogen gas, and a two-stage cooling process is envisaged in which gas coolant, such as steam, is delivered to the bed, followed by a liquid coolant such as water. The pebble bed may be provided with a condensate drain to remove any condensate produced during the heating or cooling of the bed. The cooling of the bed may be carried out by recirculating the fluid coolant through the pebble bed and a heat exchanger for cooling the coolant after it has passed through the pebble bed. The container for the pebbles of the pebble bed may be a low alloy steel shell lined with refractory material such as firebrick and with an inner lining of super refractory material such as silicon carbide, mullite or alumina. The pebbles may be supported on a perforated layer comprising a high alloy steel platform, which may alternatively or additionally be upwardly domed. The container may be formed as a cylindrical pressure vessel with domed upper and lower ends.

[0009] GB2509894 describes a thermal energy storage system and a method of storing and discharging thermal energy, in which a heated heat transfer liquid is flowed through a bed of particulate material, preferably rock or mineral material, to transfer heat from the heat transfer liquid to the particulate material for storage, and in which, to discharge the stored heat from the particulate material, gas is flowed through the bed of particulate material. The container for the particulate material is described as thermally insulated and lined with an impervious (liquid / gas tight) liner. The base of the packed bed is described as a grill that allows oil to rain out of the bottom into a sump.

[0010] CH703413 describes a heat storage system for use with an air source heat pump, in which the storage system is in the form of an insulated box filled with rock grains or gravel through which air is circulated to warm or to cool the rock grains or gravel as needed. A grating is provided near the base of the container to support the rock grains or gravel. The heat storage space can be a tank space previously occupied by an oil tank and which has been insulated by means of aerated concrete on the inner side of the walls, a basement room, or a thermally insulated large capacity container constructed specially to function as the heat storage space.

[0011] US2012 / 241122 relates to a heat storage system in which heat transfer fluids are phase change fluids and heat transfer to the storage medium is predominantly by phase change of the heat transfer fluid. The storage medium may also be a phase transfer material. The phase change materials may be contained in canisters, for example of metal tubing.

[0012] EP2942591 describes a stratified heat storage using regions of different storage materials, including both sensible heat storage in for example sand or cement, plus encapsulated phase change materials in other regions of the storage, the different materials being separated for example by meshes. The phase change materials need to be provided with expansion space within their capsules if those are rigid, and refers to US2012005561 which describes providing helical grooves along a pipe containing a phase change material to facilitate deformation of the capsule without risk of damage as a result of change of volume of the phase change material during melting.

[0013] LIS4192144 describes a pebble bed heat storage, “pebble” being used here to describe the form rather than the substance of the heat storage granules. The container for these “pebbles” may be a pressure container of steel or prestressed concrete or of prestressed iron. The depicted container in Fig 1 is formed of a plurality of cast iron segments bound by axial and circumferential prestressed cables. It is insulated on the inner side by insulation plates bounded by inner and outer metallic liners. The insulation at the bottom of the container can include ceramic bricks or layered steel plates, or a support place above the insulation, to support the contents of the container.

[0014] US4286141 describes a pebble bed contained in a container with perforated retaining walls of stainless steel screen sheathed in insulation such as ceramic bricks or fibre material. A sheet steel outer housing surrounds the insulation.

[0015] W02020 / 043854 describes a heat storage system comprising a substantially gas tight heat storage reservoir made out of materials suitable to withstand the operating conditions in terms of temperature or pressure, for example concrete or metal.

[0016] WO2018 / 157895 describes a heat reservoir comprising an air tight metal container insulated with ceramic stone wool on the outside.

[0017] The majority of the prior art heat storage reservoirs use a steel tank to contain the heat and the heat storage medium. It has been observed by the present inventors that the use of steel involves a significant degree of expansion and contraction on heating and cooling the reservoir. Where the reservoir has a steel base that supports the side walls of the reservoir, this expansion and contraction places significant stresses on the overall structure of the reservoir, and, where the reservoir uses a solid granular material as the heat storage medium, this can settle on expansion of the solid granular material to occupy the expanded volume of the reservoir, and will not revert to its previous configuration when the reservoir cools. The settling of the solid granular material thus prevents the reservoir from adopting its original configuration, and may distort or cause stresses in the material of the reservoir. Over time this expansion and contraction will make the reservoir brittle and cause it to rupture.

[0018] SUMMARY OF THE INVENTION

[0019] It has been recognized by the present inventors that it is important that the heat storage reservoir that contains solid granular heat storage material is able to withstand repeated heating and cooling cycles without cracking or rupture, in order that the heat storage reservoir has a long useful life.

[0020] It is an object of the invention to provide an improved construction of a heat storage reservoir suitable for use with a heat storage material in the form of solid granules.

[0021] It is an object of the invention to provide a heat storage reservoir capable of withstanding numerous cycles of heating and cooling without cracking or rupture.

[0022] It is a further object of the invention to reduce the cost of thermal energy storage.

[0023] It is a further object of the invention to provide a thermal energy solution using a larger fraction of natural materials with a low carbon footprint.

[0024] It is a further object of the invention to enhance durability, simplify maintenance and reduce barriers towards replacement of the thermal energy storage.

[0025] It is a further object of the invention to provide an alternative to the prior art.

[0026] Described herein is a base for a heat storage reservoir, a heat storage reservoir comprising said base, and a heat storage system comprising said heat storage reservoir, each aiming to fulfil at least one of the desirable aspects mentioned above.

[0027] In some cases in the prior art, the heat storage reservoir comprises an inner container and an outer container. The inner container holds within it solid granular material as the heat storage medium. The outer container is larger than the inner container, such that there is a void between the outer and inner container when the heat storage reservoir is charged with heat. In this arrangement, the outer container may expand and contract on heating without causing problems of undue stress as it is the inner container that contains the solid granular material that is able to settle on expansion of the container, thus the inner container receives the stress from the settling process. However, if the side walls of the inner container are supported on the base of the outer container, the expansion and contraction of the base of the outer container places stress on the side walls of the inner container, as the base of the outer container expands to a greater degree than does the inner container and its contents on heating. Further, where the inner container is filled with granular heat storage material, such as rocks or stones, and this material does not expand to the same extent as the inner container itself, for example where the inner container is made of steel, the granular heat storage material can settle into a new position on expansion of the inner container and will then not permit contraction of the inner container to its original dimensions, but places further stress on the inner container side walls. In these cases, the present inventors have found that it is advantageous to decouple the side walls of the inner container from the base of the outer container by mounting the side walls of the inner container on one or more slidably mounted panels, such that the side walls of the inner container are not forced to move as a result of the expansion or contraction of the base of the outer container.

[0028] In prior art containers, the base may for instance be formed from a single sheet of material having different thermal expansion properties from the side walls, e.g. the base may be a single metal plate on which the rest of the structure is built. The side wall may be pretensioned in the hoop direction and / or comprise materials so as to have minimal expansion in a radial direction or at least matched expansion to the granular heat storage material it contains. This prevents the granular heat storage material settling in the additional space created in the expanded container, which could prevent the container from contracting when cooled and give rise to stresses in the wall material that may damage the container. However, if the base experiences more thermal expansion than the side walls and is coupled to the side walls, the forces arising from the expansion of the base are coupled to the side walls and cause it to similarly expand, leading to the same problems. Thus, the invention decouples these forces by supporting the side walls on a plurality of plates that “float” on a bed of particular matter, e.g. roll on smooth pebbles (e.g. sea pebbles) or rollers or bearings or the like. Thus, the plates can move during thermal expansion relative to each other, but do not move significantly from the centre point of the container, thus minimizing the forces that are transmitted to the side walls and preventing shifting of the heat transfer material during thermal cycling and the potential resulting damage. Accordingly, in a first aspect, the present invention provides a heat storage reservoir container comprising an outer container and an inner container for containing heat storage material, the inner container comprising a panel that does not change in size in response to expansion or contraction of the base of the outer container on heating or cooling, the panel being slidably mounted on the base of the outer container, and the side walls of the inner container being mounted on said panel.

[0029] Suitably, the panel may be a single plate of a material that does not undergo significant expansion or contraction over the temperature range of intended operation of the heat storage reservoir. For example, a ceramic or concrete panel may be used. The single plate may be constructed of a plurality of connected subunits that together function as a single plate. Such a single plate may comprise a through hole or perforations in order to provide an outlet for heat transfer fluid at the base of the outer container. This is not needed where an outlet for heat transfer fluid is provided at the side of the inner container above its base.

[0030] Preferably, however, the panel comprises a plurality of metal plates that are each slidably mounted on the base of the outer container and are arranged to be able to move with respect to one another. Again, a through hole or perforations may be present in order to provide an outlet for heat transfer fluid at the base of the outer container. This is not needed where an outlet for heat transfer fluid is provided at the side of the inner container above its base.

[0031] The metal plates are each able to expand and contract on heating and cooling of the reservoir. However, in contrast to the situation where the side walls of the inner container are mounted on a single metal plate, the use of a plurality of separate panels that are slidably mounted allows the expansion of the base of the outer container not to be translated into movement of the walls of the reservoir mounted thereon, and thus decouples the expansion and contraction forces imposed by expansion or contraction of the base of the outer container from the inner container side walls and permits the side walls to maintain their original position despite the expansion or contraction of the base of the outer container of the heat storage reservoir.

[0032] This may be achieved by use of spaced panels with a suitable expansion gap between them. However, preferably, the panels are arranged to overlap one another at adjacent edges and mounted such that the panels are able to slide in relation to one another, so that the movement caused by expansion or contraction of each individual panel is independent of the others, and decouples the movement from the inner container side walls. Further, in either case, the panels are able to move with respect to the base of the outer container, such that expansion or contraction of the base of the outer container does not translate into forces tending to move the inner container side walls, as the panels on which these are mounted are able to move with respect to the base of the outer container. These two factors both allow the inner container side walls to maintain their original position during heating and cooling of the reservoir, or to allow the inner container side walls to expand and contract only to the degree inherent to the material and construction of the inner container side walls themselves rather than to the degree dictated by expansion or contraction of the outer container base, reducing stresses on the inner container.

[0033] The side walls of the inner container are connected to the one or more panels such that they do not move with respect to the one or more panels when the panels move. This may be achieved solely by the frictional forces between the side walls and the one or more panels; however, preferably the side walls are connected to the one or more panels by suitable connectors. For example, where the side walls and the panels are metal the side walls and panel may be welded together. Alternatively, the side walls and the panels may be bolted together.

[0034] Suitably, the panels are slidably mounted on a bed of particulate material that allows the panels to move in relation to one another and the base of the outer container. For example, the panels can be placed on a bed of gravel, stones, pebbles or ball bearings. The particulate material should be selected to have a size, shape and surface characteristics allowing the motion of the panels resting thereon. The choice of suitable particulate material depends on the nature of the heat transfer fluid used in the heat storage reservoir, as the base of the outer container in which the particulate material is provided is filled with this fluid. Where the heat transfer fluid is an oil, for example a mineral oil, natural oil or silicone oil, this will lubricate the particulate material. In this case, the particulate material may be irregular in shape and size, as the lubrication will still allow the panels to slide on the particulate material. For example, in this case the use of gravel, flat stones or cylindrical particles is practical, in particular beach sand, beach or sea or river bed pebbles or sea or river bed rocks, as these will have been smoothed and rounded by the action of water upon them. However, where the heat transfer fluid is not suitable to lubricate the particulate material, the particulate material is preferably selected to have smooth spherical particles with a narrow size distribution. Where perforated panels are used, it is important that the perforations be smaller in size than the particulate material on which the panels are supported, to prevent the particulate material from passing through the panels into the inner container to any significant extent.

[0035] The metal plates may be solid or perforated. The metal should be selected to remain rigid under the conditions of operation of the heat storage reservoir, and to be able to support the weight of the inner container walls, plus the heat storage medium contained in the inner container. It should also be selected to withstand the environmental conditions provided by the overall heat storage system, for example the reactivity of the heat transfer fluid or its breakdown products. Preferably, the metal is steel. Where the heat transfer fluid does not produce corrosive conditions, the steel is preferably carbon steel or black steel, as these are cost effective and have appropriate strength. However, if the heat transfer fluid is capable of corroding carbon or black steel, then stainless steel may be chosen instead. For example, repeated heating of natural oil may cause it to degrade to form acids that would attack carbon or black steel.

[0036] The dimensions of the metal plates should be selected such that they can withstand the weight of the inner container side walls while being able to slide on the particulate material. Preferably, the plates are between 10 and 20 mm thick. Preferably, the plates are between 0.5m and 1 ,5m in length and / or width. For a rectangular plate of 1m x 2m, it is expected that a thickness of 10mm or 20 mm would be sufficient.

[0037] Suitably, the panels may be connected by one or more wires or bands to limit the extent to which the radius of the base formed by the panels is able to increase. This may be in the form of a single wire or band threaded through eyes or loops mounted on the outer edges of the panels with respect to the inner container, with the ends of the wire or band joined by a tensionable fastener. Alternatively, separate wires or bands may be provided to join the outer edges of pairs or groups of panels, together forming a series of wires or bands that restrains the whole circumference of the base.

[0038] In a second aspect, the present invention provides a heat storage reservoir comprising a container having at least a base and a peripheral side wall for containing a granular heat storage material, the base comprising: a plurality of base plates that support the peripheral side wall, the base plates being supported by a bed of particulate matter such that the base plates are able to move relative to each other to accommodate thermal expansion or contraction of the base such that stresses arising from a difference in the thermal expansion or contraction of the base relative to the peripheral side wall are not coupled to the side wall.

[0039] As described above, this avoids using a single sheet of material for the base which may have different thermal expansion properties from the side walls, and which may cause unwanted expansion of at least the lower part of the container where the base transmits expansion forces to the side walls. Instead, this aspect of the invention decouples these forces by supporting the side walls on a plurality of plates that “float” on a bed of particulate matter, e.g. roll on smooth pebbles (e.g. sea pebbles) or rollers or bearings or the like. Thus, the plates can move during thermal expansion relative to each other, but do not move significantly from the centre point of the container, thus minimizing the forces that are transmitted to the side walls and preventing shifting of the heat transfer material during thermal cycling and the potential resulting damage.

[0040] In an embodiment, the side wall of the inner container comprises: a plurality of wall elements arranged such that each wall element abuts or partially overlaps adjacent wall elements to form a continuous peripheral side wall for containing the granular material; at least one hoop tension element binding together the wall elements by which radially outward movement of the wall elements due to expansion of the granular material is resiliently resisted in use.

[0041] In an embodiment, the wall elements are gabions comprising a metal cage for containing rocks. The hoop tension element may incorporate elements of the metal cage.

[0042] In another embodiment, the wall elements are panels, each panel partially overlapping the adjacent panels at longitudinal edges such as together to form the periphery of the container, slidably mounted with respect to one another such that the panels are displaceable radially while maintaining a continuous periphery.

[0043] The hoop tension element may be a wire or cable extending around the outside of the wall elements and / or extending through eyelets or open loops formed on the inside faces of the wall elements.

[0044] Preferably, the heat storage reservoir of any aspect described above further comprises insulation surrounding or lining the outer container, in order to reduce heat losses from the heat storage reservoir. Suitably, the insulation may be selected from mineral wool, glass wool, rock wool, ceramic wool, natural wool, and mixtures thereof.

[0045] Preferably, the solid granular material has a granule diameter of from 10 mm to 500 mm, such as from 10 mm to 300 mm. Preferably, the solid granular material has a granule diameter of greater than 10 mm. Preferably, the size range of the solid granular material is a difference of a factor of 10, preferably 20, and more preferably 30, in diameter of the solid granules. Preferably, the volume of solid granular material has a void fraction of from 10% to 40%, more preferably 15% to 35%, and most preferably 20% to 25%. Preferably, the average width of the voids between the granules is from 10 mm to 30 mm.

[0046] Preferably, the solid granular material is selected from rock granules, mineral granules and mixtures thereof. Preferably, the solid granular material is selected from granite, basalt, marble, diamond, quartz, flint, pebbles obtained from a beach, sea bed or river bed, or mixtures thereof. Suitably, the rock granules or mineral granules may be manufactured granules. Preferably, the rock granules or mineral granules are obtained from naturally occurring rock or mineral, and more preferably are naturally occurring rock or minerals that have been divided into suitably sized granules.

[0047] Alternatively, the solid granular material may be an encapsulated phase change material. Suitably, the phase change material may be selected from eutectic salts or organic materials with a melting point in a suitable range. Suitable eutectic salts include potassium nitrate and sodium nitrate. Suitable organic materials include wax. Suitable encapsulating materials include metal, polymer or concrete. Preferably, the granules of encapsulated phase change material are spherical.

[0048] Alternatively, the solid granular material may be made from a slag or waste material from processing or purifying metals, such as copper slags or iron slags.

[0049] Preferably, the heat storage reservoir is designed to be pressurized to at most 1 bar overpressure. That is, the heat storage reservoir is preferably designed to withstand a maximum internal pressure of 1 bar gauge, or 2 bar absolute. More preferably, the maximum internal pressure is 0.02 bar gauge.

[0050] In a third aspect, the present invention provides a heat storage system, comprising the heat storage reservoir of the first of second aspect of the invention.

[0051] Preferably, the heat storage system further comprises an input system for the introduction of heat to the heat storage reservoir. Suitably, the input system comprises an input heat source for heating, superheating or evaporating, the liquid heat transfer fluid. Suitably, the input heat source may comprise direct electrical heating, a heat exchanger with a hot fluid on the opposite side of the heat transfer fluid, illumination by concentrated sunlight, or a source of solar heat, for example a solar concentrator that is used to heat a fluid.

[0052] Preferably, the heat storage system further comprises an output system for the extraction of heat from the heat storage reservoir. Suitably, the output system comprises an output heat sink for cooling the heated or superheated heat transfer fluid and condensing the evaporated heat transfer fluid. The output heat sink may suitably comprise any apparatus that requires a source of heat, such as a turbine.

[0053] All of the features described may be used in combination in so far as they are not incompatible therewith.

[0054] BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 shows an embodiment of the container base of the invention in a heat storage reservoir

[0056] Figure 2 shows the embodiment of Fig 1 in plan view

[0057] Figure 3 shows the heat storage system and heat storage reservoir to which the invention may be applied.

[0058] Figure 4 shows an embodiment of the construction of the heat storage reservoir of the invention.

[0059] Figure 5 shows an alternative embodiment of the construction of the heat storage reservoir of the invention

[0060] Figure 6 shows a preferred arrangement of panels for a modular construction of the heat storage reservoir.

[0061] Figure 7 shows a plan view of the construction of the inner container of the heat storage reservoir

[0062] Figure 8 shows the construction of a gabion

[0063] Figure 9 shows the arrangement of two adjacent gabions as part of the inner container side wall

[0064] Figure 10 shows an embodiment of the tensionable connector of the wire 150. DESCRIPTION OF THE INVENTION

[0065] This invention makes use of a heat storage system comprising a heat storage reservoir containing a volume of solid granular material, and particularly concerns the construction of the heat storage reservoir, such as to allow for repeated charging and discharging of heat to and from the heat storage system without compromising the integrity of the heat storage reservoir.

[0066] The below description is made in the context of a heat storage system in which the heat transfer fluids used to charge heat to the granular material and / or to discharge heat from the granular material undergo a phase change during transfer of heat to or from the granular material. However, it will be understood that the construction of the heat storage reservoir may equally be applied to systems in which the heat transfer fluids do not undergo phase change during transfer of heat to or from the granular material. The below description also includes the possibility of pressure reduction of the partial pressure in the heat storage reservoir due to non-condensable species, which, while preferred in some cases, is optional.

[0067] The heat storage system according to the invention comprises an input system, a heat storage reservoir, and an output system. Furthermore, the invention may include a system for recovering different fractions of the used heat transfer fluids, and a system for removing all heat transfer fluids from the heat storage reservoir, which is preferable for maintenance or end-of-life deconstruction.

[0068] The input system comprises a system for introducing to the heat storage reservoir a heated heat transfer fluid.

[0069] Preferably, the input system comprises a system for generating in the heat storage reservoir a saturated vapour of heat transfer fluid(s) at a pressure close to or below ambient pressure. A typical implementation would be to have a primary fluid circuit (the heat source) and the heat transfer fluid to be evaporated to pass through a heat exchanger transferring heat from the heat source to the heat transfer fluid, thereby evaporating the heat transfer fluid. The evaporated heat transfer fluid is then passed into the heat storage reservoir as vapour. Alternatively, the heat transfer fluid can be superheated by the input heat source, in order that it remains liquid until it is introduced into the heat storage reservoir, and at least partially evaporates on or shortly after its introduction into the heat storage reservoir. This latter arrangement can allow for a simpler apparatus than including an evaporation device in the input system. As a further alternative, the heat transfer fluid can be heated by the input heat source to be introduced into the heat storage reservoir as a heated liquid.

[0070] The heat storage reservoir comprises a volume of a granular material, where the granules of said material are preferably non-porous. The granular nature of the material will ensure that voids will be formed between the granules in such a way that the voids will form an interconnected grid through which the heat transfer fluid from the input system can flow. Contact between the heated heat transfer fluid and the solid granular material allows for heat transfer from the heat transfer fluid to the solid granular material, thus charging the heat storage reservoir with heat.

[0071] In the embodiment in which the heat transfer fluid is introduced into the volume of solid granular material as an evaporated heat transfer fluid, provided that the granules are not porous and that the granules have a temperature below the boiling point of the heat transfer fluid, the evaporated heat transfer fluid will condense on the surface of the granules, thereby releasing the heat of evaporation of the fluid, which is absorbed by the granules, thus storing the heat. After condensation, the now liquid (and thereby denser) heat transfer fluid will be collected in the bottom (by the means of gravity) of the reservoir and be removed by mechanical means, e.g. by a pump. The higher the fraction of the heat transfer fluid that is removed in the liquid phase, rather than the vapour phase, the higher thermodynamic efficiency the system will have.

[0072] Again in the context of the above embodiment, when the granules by this heat absorption reaches a temperature close to the boiling point of said heat transfer fluid, this process will no longer be able to move energy from the evaporated heat transfer medium to the heat reservoir. However, by employing a multitude of heat transfer liquids with different boiling points used in series, heat can be transferred to the storage until the storage reaches the boiling temperature of the heat transfer fluid having the highest boiling point. The reason for not using a single fluid with a high boiling point in the input system is that a typical heat source (e.g. a concentrated solar power plant) will be more effective at transferring heat to the heat transfer fluid the colder the heat transfer fluid is. This temperature will be set by the boiling point of the used heat transfer liquid as the heat source liquid will not cool below the boiling point of the heat transfer fluid in the heat exchanger. The control and selection of which heat transfer fluid is to be introduced will typically be done through temperature monitoring of the heat reservoir.

[0073] By using condensation of a vapor phase fluid to transfer the heat to the reservoir, three major advantages are obtained over using a tubed system. First of all, no tubes are required in the heat reservoir, thereby significantly reducing the cost of the reservoir. Secondly, the granularity of the storage can be tuned to give different input / output power of the system (by controlling the surface area to volume ratio of the system). The last major advantage is that such a system is self-leveling in regard to the temperature distribution of the thermal storage. This effect is due to the volume change when the evaporated heat transfer fluid condenses. Given a colder volume of the heat reservoir, the rate of condensation will be higher in this volume, and hence the mass flow to this volume will increase, thereby increasing the heating rate of this particular colder volume until the temperature is the same as the rest of the volume. This feature is especially important given the interchange of different heat transfer fluids as function of the temperature of the storage. If a high ratio of the supplied evaporated heat transfer liquid is not condensed (or is re-evaporated by a higher evaporation point fluid), the heat transfer efficiency of the system will be lowered. Therefore, good volumetric control of the temperature is an important feature of the system, which here is realized by using a heat transfer process (evaporation / condensation) which also gives rise to a volume and density change.

[0074] However, even where the heat is not transferred by means of a phase change, the first two listed advantages above are achieved.

[0075] Again in the context of the above embodiment, it has been discovered that reducing the residual air pressure, ie the gas pressure contribution from non-condensable species, in the heat storage reservoir during heat transfer greatly increases the internal heat transfer rate. This is due to the increased rate of both evaporation and condensation of the heat transfer fluid, as well as increased rate of diffusion of evaporated liquid between colder and hotter zones in the storage.

[0076] Again in the context of the above embodiment, a system to maintain low residual air pressure in the system has been developed. Most systems that are operated at less than atmospheric pressure will have small leaks causing air to enter the system, thus slowly increasing the residual air pressure, which will reduce the internal heat transfer over time. By using a standard vacuum pump connected directly to the system, both residual air and the heat transfer fluid (to some extent) will be evacuated, which is not a preferable situation. However, by inserting a long and narrow tube which is maintained at ambient temperature between the storage and the vacuum pump, with the vacuum pump being placed higher than the storage, the active fluid will condense in the tube and return to the storage due to gravity as liquid that can be recovered using the normal recovery system of the thermal storage, whereas the residual air will not condense at ambient temperature and hence be evacuated. In order to further minimize the evacuated active fluid, the tube or a section thereof can be actively cooled to a temperature where the vapor pressure of the active fluid is even lower.

[0077] Again in the context of the above embodiment, a further feature of the system is that the heat reservoir granules should preferably not be porous, as condensation would then happen in the pores of the material, which to a large extent would prevent the condensed liquid from running down to the mechanical liquid collection system. If run down is prevented, the liquid will re-evaporate once the next heat transfer fluid is employed (at a higher temperature), with poorer thermodynamic efficiency as a result. Furthermore, it would also require higher volumes of (typically expensive) heat transfer fluids to be used in the system, resulting in a more expensive system. A way to further reduce the need for heat transfer fluids and a way to improve the charging / discharging characteristic of the system is to surface treat the granules such that the liquid heat transfer fluid will form drops on the surface and thereby run off faster.

[0078] However, where a phase change of the heat transfer fluid is not the primary means of heat transfer between the heat transfer fluid and the volume of solid granular material, the porosity and / or surface properties of the solid granular material are of lesser importance, as the cost of the heat transfer fluid in such cases is lower.

[0079] The output system works in the opposite way to the input system; a shower of cool heat transfer fluid is supplied at the top of the reservoir to contact the heated solid granular material such that heat is transferred from the solid granular material to the heat transfer fluid, which is then removed from the reservoir, to discharge heat from the heat storage reservoir.

[0080] In the context of the embodiment in which a phase change of the heat transfer fluid takes place during heat transfer, the output system supplies a shower of liquid heat transfer fluid at the top of the reservoir. Once the liquid heat transfer liquid reaches contact with the hot granules of the heat reservoir, the liquid heat transfer medium will evaporate, thus absorbing energy and increasing in volume. The volume increase will make the evaporated heat transfer liquid escape the heat reservoir (which is not designed to withstand high overpressure, such as pressures of greater than 1 bar gauge / 2 bar absolute, but which is fluid-tight) to an output heat exchanger system where the hot and evaporated heat transfer fluid will condense and thereby transfer the heat of evaporation to be used in another process, e.g. the water / steam in a steam turbine or the pressure fluid in an organic rankine cycle (ORC) system, or to water / steam in a steam generator. After condensation in the heat exchanger, the liquid fluid may be passed into the reservoir again in a cyclical process. Once the temperature of the heat reservoir reaches the boiling point of the fluid, a lower boiling point fluid must be employed. The reason for not starting to use the lowest boiling point liquid is that the temperature at which the heat energy is extracted (which equals the boiling point of the used fluid) at should normally be as high as possible, e.g. to ensure a higher efficiency of electricity generation in a Carnot process (e.g. steam turbine I ORC generator).

[0081] Again in the context of the above embodiment, as the system may make use of multiple heat transfer fluids in both the input and output system, it will be advantageous to include a mechanism to separate and separately store the different heat transfer liquids, so they can be employed numerous times in both systems, in an optimal thermodynamic way.

[0082] Again in the context of the above embodiment, a further feature of the system is that moving the heat transfer liquid from the input system to the reservoir, and the reservoir to the output system, respectively, does not require the use of mechanical pumps. The phase change from liquid to gas in the output system, and the expansion that causes, drives the heat transfer fluid through the gas outlet. The phase change from gas to liquid in the input system causes a volume reduction, which draws further gaseous heat transfer liquid into the reservoir. Furthermore, by arranging the inlets and outlets of the reservoir accordingly, gravity can be used to collect the condensed liquids from the reservoir or the output system, respectively.

[0083] However, where a phase change of the heat transfer fluid is not the primary means of transfer of heat between the heat transfer fluid and the solid granular material, the use of mechanical pumps may be required.

[0084] A typical realization of the heat storage reservoir is to use stone or rocks having a size range chosen to produce the required void fraction in the reservoir such that the heat transfer fluid can circulate freely around the solid granular material. Typical dimensions (depending on how fast energy needs to be extracted and how large the volume of the reservoir is) will be in the range 10-500 mm, such as 10 mm to 400 mm. A typical size range will be a difference of a factor of 10, preferably 20, and more preferably 30, in diameter of the solid granules in order to form the required network of voids around the granules, as having a very broad size distribution will typically result in densely packed structures. A void fraction of at least 10% is required to allow for circulation of the heat transfer fluid. However, a large void fraction does not make efficient use of the space in the reservoir, as the greater the mass of solid granular material contained in the reservoir the more heat can be stored in it. Suitably a void fraction of from 10% to 40% may be used, more preferably 15% to 35%, and most preferably 20% to 25%. Furthermore, it will also be dependent on the local source of materials. Another realization could be to use metal containers with a phase change material within. This would add cost, but allow for the storage of more energy at the phase change temperature of said phase change material. This may be a preferable solution if the volume of the reservoir is constricted. The same void fraction considerations apply when the solid granular material is in the form of containers of phase change material.

[0085] The inventors have recognized that the use of solid granular material within the heat storage reservoir can cause stress on the material used to construct the heat storage reservoir. In particular, where the heat storage reservoir is made of steel, the steel will expand when the reservoir is charged with heat, and contract when heat is discharged from the reservoir. Where the material used to construct the heat storage reservoir has a higher expansion coefficient, ie expands more than, the material(s) used for the solid granular material, on charging the heat storage reservoir with heat, the solid granular material can shift position and settle into a new arrangement occupying the expanded volume of the heat storage reservoir. However, when the heat is discharged from the reservoir, the material used to construct the heat storage reservoir contracts to a greater degree than the solid granular material, and in so doing meets resistance from the solid granular material, which will not shift back into its original position, but instead acts to distort and strain the material used to construct the heat storage reservoir. Repeated cycles of heating and cooling can lead to cracking and rupture of the heat storage reservoir.

[0086] In order to address this problem, the present inventors have proposed specific constructions for the heat storage reservoir.

[0087] In one such construction, the present inventors have recognized that the use of closed restraining loops, such as wires or bands, to bind together panels of material to form a container for the solid granular material when constructing the heat storage reservoir can prevent there being problematic expansion and contraction that strains the material of the heat storage reservoir such as to cause cracking or rupture. The closed restraining loops provide the tension necessary to maintain the continuous periphery of the container against the static force exerted by the solid granular material inside the container, and also against the additional force exerted by the expansion of the solid granular material and the panels on charging the heat storage reservoir with heat. The panels should be sufficiently rigid to maintain their general form when forces are applied to them, for example by the solid granular material or by the wires or bands used to bind them together. The wires or bands can be tensioned during construction of the heat storage reservoir. This has the advantage that the wires or bands will not be able to expand to any significant extent on charging the heat storage reservoir with heat, and thus settling or shifting of the solid granular material will be prevented. Alternatively, the wires or bands need not be tensioned during construction of the heat storage reservoir any further than is required to bind the panels together into the form of the inner container. In this case, initial charging of the reservoir with heat will permit expansion of the inner container and shifting of the solid granular material, thus creating tension in the bands and preventing repeated movement of the solid granular material and resulting strain on the material of the heat storage reservoir. This latter arrangement has safety advantages compared with actively tensioning the wires or bands during construction, as it reduces the possibility of accident to construction workers if a wire or band under tension is released or snaps.

[0088] Suitably, the bands or wires may be made of any material that can withstand the temperatures to which the heat storage reservoir will be exposed, such as up to 400 C. The bands or wires should also be sufficiently flexible that they can be formed to surround the panels of material forming the walls of the inner container of the heat storage reservoir. While it may be possible to use carbon fibre or polymer plastics materials for the bands or wires, it is expected that the preferred material will be metal, as this is cheap, available and processable into the required form. Preferably the bands or wires are of steel. It is expected that most types of steel will be suitable. To minimize the thermal expansion and elastic modulus of the bands, and thus minimize the degree of possible expansion of the inner container and shifting of the solid granular material on charging the reservoir with heat, carbon steel may be preferred.

[0089] The panels of material bound together with the bands or wires to form the inner container of the heat storage reservoir may suitably be formed of metal, such as steel, and preferably carbon steel. The panels may be perforated or formed of a mesh; in this case the maximum size of the perforations or the holes in the mesh must be smaller than the smallest diameter of the solid granular material to be contained by the panels. Alternatively, the bands or wires may be used in conjunction with the first construction in which the inner container is formed of gabions. In another such construction, the inner container of the heat storage reservoir comprises a plurality of wall elements which are gabions comprising a metal cage for containing rocks. In this case, the hoop tension elements can be provided as an integral aspect of the plurality of wall elements forming the periphery of the container. As the gabions comprise a metal cage, which has faces of metal mesh, connecting together the inner faces of adjacent gabions into a loop which forms the periphery of the container creates a hoop tension element from the metal mesh of the inner faces of the gabions together with the connectors that join the adjacent inner faces. Initial charging of the reservoir with heat will permit expansion of the inner container and shifting of the solid granular material, thus creating tension in the hoop tension element formed of the inner faces of the gabions and their connectors, and preventing repeated movement of the solid granular material and resulting strain on the material of the heat storage reservoir, as described above for the second embodiment. Further, the use of rock or stone filled gabions as the wall elements is cost effective and allows for straightforward construction. If desired, additional hoop tension elements in the form of bands or wires may be provided as described for the second embodiment of the first aspect of the invention.

[0090] In both constructions the wires or bands may extend around the outside of the wall elements, or extend through eyelets or open loops formed on the inside faces of the wall elements. The former arrangement has the advantage of spreading the force exerted by the wires or bands across the widest possible area. The latter arrangement is expected to be more geometrically stable.

[0091] In both constructions, an outer, preferably fluid-tight, container is provided surrounding the inner container; suitably the outer container may be of metal such as steel, or of concrete. As the inner container protects the outer container from the effect of shifting and settling of the solid granular material, there is no strain from the solid granular material placed on the outer container. The outer container may be dimensioned to be larger than the inner container by a few cm, or to allow for a gap of up to 1 m or more between the inner container and the outer container, for example if it is desired to provide space to access the gap between the containers, perhaps for maintenance purposes. The gap may also provide space for suitable insulation to be provided to line the outer container or to surround the inner container. Alternatively or additionally, insulation may be provided on the outside of the outer container. If the outer container is not constructed to be substantially fluid tight, it is possible to blanket it with an inert gas such as nitrogen or argon by maintaining a positive pressure of inert gas in the space between the inner and outer containers. The heat storage reservoir is fluid tight or blanketed with inert gas to exclude oxygen, which may cause decomposition of heat transfer fluid at elevated temperatures. If the heat storage reservoir is fluid tight this also prevents the escape of gaseous heat transfer fluid from the reservoir. Where the heat storage reservoir is not fluid-tight but is instead blanketed with inert gas, a condenser may be provided maintained at a temperature below the boiling point of the heat transfer fluid in order to condense and to recover any escaping gaseous heat transfer fluid.

[0092] In the second construction, the inner container is comprised of gabions, and the gabions are connected at their vertices at the inner diameter of the ring by connecting wires. This construction is cost effective, as it uses a lower mass of metal in the gabions than in the panels of the first construction. In addition, the use of the gabions effectively provides the hoop tension element inherently by connecting the mesh inner faces of the abutting gabions. In the first construction, the panels from which the inner container is constructed are a plurality of overlapping slidable panels bound together into the required form of container by bands or wires. The use of overlapping slidable panels allows the panels to move in relation to one another in response to expansion and contraction of the panels on charging and discharging heat from the heat storage reservoir, respectively. The bands or wires need not be tensioned during construction of the container, as movement of the panels resulting from expansion on heating will apply tension to the wires or bands such that they become tensioned over time. However, if preferred, the wires or bands may be tensioned during construction.

[0093] Preferably, the heat storage reservoir is in the form of a cylinder, in order to avoid sharp bends in the wires or bands around corners that create additional stress. However, it will of course be understood that other geometries are possible, such as a cuboid with rounded edges to avoid such stresses.

[0094] The advantages of the heat storage reservoir of the invention are particularly important for larger reservoirs, since the degree of expansion and contraction, and shifting and settling of the solid granular material, increases with reservoir size. Where the reservoir is cylindrical, it is expected that benefit will be found for reservoirs of greater than 1.5 m diameter, and particular benefit greater than 5 m in diameter, such as greater than 10 m diameter. Where other geometries of reservoir are used, it is expected that the benefit of the invention will apply to reservoirs having basal cross-sectional areas corresponding to the areas of the abovementioned diameters of cylindrical reservoirs.

[0095] The invention furthermore relates to a heat storage reservoir in which the reservoir is preferably designed to be maximally pressurized at less than 1 bar overpressure, more preferably by less than 0.5 bar overpressure, even more preferably by less than 0.25 bar overpressure, more preferably by less than 0.1 bar overpressure, even more preferably by less than 0.02 bar overpressure, and most preferably not being designed to be pressurized at above ambient pressure. To operate at pressures below those requiring the use of a pressure vessel and compliance with the safety regulations for operating at elevated pressure leads to significant cost savings in terms of safety factors built in for construction of pressure vessels and associated documentation costs. Here “overpressure” refers to pressure above ambient pressure, thus bar gauge.

[0096] The invention furthermore preferably relates to a heat storage reservoir where the operating temperature ranges from ambient temperature to 500 °C. Suitably, the maximum temperature to which the volume of solid granular material can be heated in the invention is 250 °C, or 300 °C, or 350 °C, or 400 °C, or 450 °C, such as 475 °C, or 500 °C.

[0097] The invention furthermore relates to a heat storage system without any gas-phase mechanical pumps for pumping heat transfer fluid between the said input system, the said thermal reservoir and the said output system.

[0098] By evaporation heat is meant the enthalpy of evaporation.

[0099] By active fluid is meant the heat transfer fluid that is being used to transfer the heat from the heat source to the storage by evaporation in the input system and condensation in the storage, or the fluid used to transfer the heat from the storage to the output system by evaporation in the storage and condensation in the output system.

[0100] By gas pressure contribution from non-condensable species is meant the pressure of inactive gases in the system, e.g. the pressure of atmospheric air, nitrogen or other gases that are not evaporating or condensing in the temperature window of operation of the thermal storage. This is also referred to as “residual air pressure”.

[0101] By convex granule is meant a shape of a granule where no significant amount of liquid can assemble in concave regions on the surface of the granule, and hence the liquid will run off due to gravitational drag in the liquid. For the purpose of this application, a granule is defined as convex if liquid volume equaling less than 1% of the volume of the granule can be assembled in concave surface regions of the granule.

[0102] By granular is meant a material comprised of individual cohesive parts capable of forming a mechanically stable aggregate with voids (or air) in between the individual granules. By diameter of the granular material is meant the size of the aperture of a mesh through which the granular material will pass. Thus, a granule with a diameter of between 10 and 20 mm can pass through a 20 mm aperture mesh sieve, but not through a 10 mm aperture mesh sieve. Hence, the requirements for the size range of the granular material defined by the diameter does not imply the need for the granular material to consist of spherical objects.

[0103] By size distribution is meant the relative spread of the size of the object. The distribution may follow a normal distribution or other distributions, and the spread is defined to be two standard deviations, equal to have 95% of the objects within the spread.

[0104] By pressurized is meant a construct designed to be able to be mechanically stable at significant internal overpressure. In this context, significant is defined as more than 1 bar overpressure.

[0105] By phase change material is meant a material which changes between solid and liquid phase at a specific temperature.

[0106] By thermodynamic efficiency is meant the energy quality loss (or entropy gain) from the input to the output system. For example, where the temperature gradient between a heat transfer fluid and a solid material granule is very large, the temperature loss on heat transfer between the fluid and the granule is large, and the thermodynamic efficiency of the transfer is thus low. On the other hand, where the temperature gradient is small, but sufficient to allow the phase change of the heat transfer fluid on contact with the granule to a sufficient degree to give an acceptable transfer rate, the thermodynamic efficiency of the transfer is much higher and the temperature loss accompanying the transfer is much less.

[0107] By boiling point is meant the boiling point at atmospheric pressure.

[0108] By “X is in fluid connection with Y” is meant that a fluid flow path is provided between element X and element Y.

[0109] The apparatus according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0110] Figure 1 is a cross section view which shows an embodiment of the container base of the invention in the context of a heat storage reservoir 103 comprising an inner container 106 and an outer container 102. The outer container 102 is a cylindrical tank of steel, comprising an inlet 128 for heat transfer fluid 104 at the upper end of the tank, here depicted as a showerhead inlet, and an outlet 116 for heat transfer fluid at the lower end of the tank. The inner container 106 is of cylindrical form, its diameter being smaller than that of the outer container 102. The upper end of the inner container 106 is open, to permit ingress of heat transfer fluid 104 from inlet 128. In the interior of inner container 106 is a volume of solid granular material 108, suitably rock or stone, that functions as a heat storage medium. The base of the inner container 106 is comprised of a plurality of steel plates 222, 224, 226. The outermost plates 226 support the side walls of inner container 106. Plates 222 are arranged to underlap the edges of plates 224 and 226 at their adjacent edges. All of plates 222, 224, 226 are supported on a bed of gravel 214 which fills the base of the outer container 102. The plates 222, 224, 226 are able to move with respect to one another and with respect to the base of the outer container 102 as they are able to roll on the bed of gravel in a plane that extends over the plane of the base of the outer container. The bed of gravel 214 is bathed in the heat transfer fluid 104, here an oil that lubricates the gravel particles and facilitates the plates 222, 224, 226 sliding over them. Outlet 116 is provided in the side of outer tank 102 immediately above the plates 226.

[0111] In this Figure, the side walls of the inner container 106 are constructed from a plurality of gabions 510, which are steel mesh cubes containing stone or rock of a size larger than the apertures of the steel mesh. Outer restraining wires 150 surround each layer of gabions 510. Further description of this side wall construction is given in relation to Figure 5 below. It is to be understood that other side wall constructions are possible, as described herein.

[0112] Figure 2 shows the construction in plan view of an embodiment of the base of the container in the context of a heat storage reservoir 103 comprising an inner container 106 and an outer container 102. The inner container 106 is of cylindrical form, its diameter being smaller than that of the outer container 102. Outer container 102 is insulated by layer of insulation 104 on its inner face. Inner container 106 is supported on a plurality of steel panels 226. In the Figure, dashed lines are used to indicate the outline of panels that are underneath an adjacent panel. Each panel 226 is generally rectangular in shape, though it may have its outer edge rounded off to reflect the cylindrical shape of inner container 106. For example, each panel 226 may be a 1m x 2m panel of 10 mm or 20 mm thickness. The panels 226 are arranged in a ring with their outer edges spaced apart and their inner edges overlapping one another. Suitably the panels 226 may be arranged so that alternate panels underlap and overlap the neighbouring panel, as depicted in Fig 2, or each panel may underlap the neighbouring panel on one side and overlap the neighbouring on the other side, or a combination of these arrangements. The panels 226 are supported on a bed of particulate material 214, such as gravel bathed in the oil the serves as the heat transfer fluid of the system, such that the panels are able to move with respect to the base of the outer container that supports the bed of particulate material. Further, expansion of each individual panel does not affect the positioning of neighboring panels due to the overlapping arrangement. The inner container 106, which may be constructed in any suitable manner, is connected to each of the supporting panels 226, for example by welding or bolting. The outer periphery of the ring of panels 226 is surrounded by a wire 228 whose ends are joined by tensionable fastener 229. Within the outer ring of panels 226 is provided an intermediate ring of panels 224 and a central panel 222, of a similar construction to the panels 226 except that rounding of the outer edges is omitted. The panels 224 are arranged in a ring with their outer edges spaced apart and their inner edges overlapping one another. In the Figure, the intermediate ring of panels is shown as underlapping both the outer ring of panels 226 and the central panel 222, but it will be appreciated that it may overlap both, or underlap some and overlap others. In addition, alternate panels 224 may underlap and overlap the neighbouring panel, as depicted in Fig 2, or each panel may underlap the neighbouring panel on one side and overlap the neighbouring on the other side, or a combination of these arrangements. Again, as for outer ring of panels 226, the panels 224 and 222 are supported on a bed of particulate material 214, such as gravel bathed in the oil the serves as the heat transfer fluid of the system, such that the panels are able to move with respect to the base of the outer container that supports the bed of particulate material, and expansion of each individual panel does not affect the positioning of neighboring panels due to the overlapping arrangement.

[0113] In use, the heat storage reservoir 103 of Fig 1 and Fig 2 is charged with heat by addition of hot heat transfer fluid to the inner container 106 via inlet 128 to bathe the solid granular material 108 and transfer its heat thereto, before being withdrawn through the outlet 116. The heating of the heat storage reservoir causes the base of the outer container 102 to expand, the panels 226, 224 and 222 to expand, and the material of the inner container 106 to expand. The solid granular material 108 does not expand so much as the before listed aspects of the system. The expansion of the base of the outer container 102 is decoupled from the side walls of the inner container 106 due to the side walls being supported on the panels 226 which are able to move on their supporting bed of gravel 214 with respect to the base of the outer container 102; thus, the outer container base can expand without causing outward movement of the side walls of the inner container 106. Expansion of inner container 106 will cause outward movement of the panels 226, to the extent permitted by the wire 228 surrounding the periphery of the panels 226. The tension of the wire 228 may be selected to permit a small degree of expansion that is not sufficient to allow significant settling of the solid granular material. Each panel 226, 224, 222 may expand independently without affecting the overall size of the container base due to their spaced and overlapping arrangement. On discharging the heat from the heat storage reservoir, cool heat transfer fluid is introduced through inlet 228 to bathe the heated solid granular material 108 and to transfer heat to the fluid before it is withdrawn through outlet 116. The cooling of the heat storage reservoir causes the base of the outer container 102 to contract, the panels 226, 224 and 222 to contract, and the material of the inner container 106 to contract. The solid granular material 108 does not contract so much as the before listed aspects of the system. The contraction of the base of the outer container 102 is decoupled from the side walls of the inner container 106 due to the side walls being supported on the panels 226 which are able to move on their supporting bed of gravel 214 with respect to the base of the outer container 102; thus, the outer container base can contract without causing inward movement of the side walls of the inner container 106. Contraction of inner container 106 will cause inward movement of the panels 226, to their original position prior to heating, as the settling of the solid granular material 108 was not permitted by the expansion on heating. Each panel 226, 224, 222 may contract independently without affecting the overall size of the container base due to their spaced and overlapping arrangement.

[0114] Figure 3 depicts an embodiment of a heat storage system 100 comprising a heat storage reservoir 103. The heat storage reservoir 103 is substantially fluid tight when the various inlets and outlets provided are closed.

[0115] The heat storage reservoir 103 comprises an outer container 102 and an inner container 106, the construction of which inner container will be described later with reference to Figs 4 and 5.

[0116] The heat storage reservoir may be made of any material suitable to withstand the conditions of use, both in terms of temperature and pressure, and suitable materials include concrete or metal for the outer container 102. For use in the context of the present invention, the heat storage reservoir need not be designed to withstand a large positive overpressure, as pressurising the reservoir to pressures significantly above atmospheric pressure, such as in excess of one bar gauge, is not envisaged. The heat storage reservoir also comprises insulation 104 to prevent heat loss from the reservoir, with suitable insulation being such materials as mineral wool, glass wool or rock wool and other ceramic or stone wools, or other suitable materials as well known to the skilled person. While this is shown between the outer container 102 and inner container 106 in Figs 2-5, it is also possible to place this outside the outer container 102. When the insulation 104 is placed outside the outer container 102, the choice of insulation material is not limited, and may be a low cost material such as mineral wool, but cladding is required to protect it from the ambient environment. When the insulation 104 is placed between the outer container 102 and the inner container 106, the choice of materials for the insulation 104 must take into account the possibility that it may be exposed to heat transfer fluid and must not break down in contact with hot heat transfer fluid or absorb the heat transfer fluid, so non absorbent, non porous materials such as expanded glass or ceramic bricks would be suitable.

[0117] Within the heat storage reservoir 103 is provided an inner container 106 containing a volume of solid granular material 108, which functions to store heat by receiving heat from an input heat transfer fluid, and, when the stored heat is required, by transferring heat to an output heat transfer fluid. In certain cases, the heat transfer may take place at least partially on input by the contact of the input heat transfer fluid with the volume of solid granular material 108 causing a phase change in the heat transfer fluid from the gaseous state to the liquid state, and on output by the contact of the output heat transfer fluid with the volume of solid granular material 108 causing a phase change in the heat transfer fluid from the liquid state to the gaseous state.

[0118] In order that the solid granular material may carry out this function it is preferably a non-porous material, at least at its surface, and preferably throughout each granule, in order that heat transfer fluids passing over the surface of the solid granular material are not trapped within the pores of the material to any significant degree. Further, it is preferred that the solid granular material is of a generally convex shape, again to prevent heat transfer fluid collecting in concave areas, or recesses, of the surface of the material granules. For these reasons, it is preferred that the solid granular material is capable of containing a volume of heat transfer fluid of less than 1% by volume of the granules within the pores of the material granules that are accessible to the heat transfer fluid. Similarly, the solid granular material is preferably of a shape which permits a volume of heat transfer fluid of less than 1% by volume of the granules to occupy convex or recessed areas of the surface of the granules. For the purposes of these definitions, a pore is considered to have a diameter of less than 10 mm, and a concave area or recessed area on the surface of a granule is considered to have a diameter of 10 mm or more, as the former size is considered to allow liquid to be held to the granule surface by the capillary forces exerted by the pore, whereas this does not apply for larger concave areas or recessed areas. These features are preferred in the event that the heat transfer fluid used is a high cost material such that the trapping of significant quantities in the volume of solid granular material represents a significant increase in expense in operation of the heat storage reservoir, or where a multitude of heat transfer fluids are used such that they are not mixed as a result of trapped heat transfer fluid in the volume of solid granular material. However, where a single heat transfer fluid is used, and / or where the heat transfer fluid is of low cost, these properties of the volume of solid granular material are not required.

[0119] It is also preferred that the solid granular material is non-porous in order to improve the capacity for heat storage for each granule.

[0120] It is thus envisaged that heat transfer will take place by contact between the heat transfer fluid and the surface of the solid granular material, and that the heat transfer fluid is able to pass freely over the surface of the solid material granules within the heat storage reservoir 103. Thus, the size and shape of the solid granular material must be chosen to permit free flow of the heat transfer fluid around and between the solid granules of material. It has been found by the present inventors that a diameter for the material granules of between 10 and 500 mm is suitable, such as greater than 10 mm to 500 mm, such as 10 mm to 400 mm, such as 10 to 300 mm, or greater than 10 mm to 300 mm. For example, an average diameter of 150 mm may be selected. Further, it is preferred that the diameters of the granules of solid material are selected to arrive at a void fraction for the granules of solid material within the inner container 106 of at least 10% to allow for the circulation of heat transfer fluid within the volume of solid granular material. The void fraction should not be too great, as this reduces the mass of granular material present in the heat storage reservoir to store heat; a suitable maximum void fraction may be 40%. Preferably, the void fraction is in the range of from 15% to 35%, more preferably from 20% to 30%, and most preferably from 20% to 25%. In order to achieve this, a range of different sizes for the granules of solid material is required. Suitably, the sizes of the granules may vary by a factor of 10, preferably a factor of 20, and more preferably a factor of 30, in terms of the diameter of the granules. The void fraction ranges given previously may also be defined in terms of filling ratio, ie the amount of volume of the inner container 106 occupied by the granules of solid material, which must not exceed 90%, and may suitable range from 60% to 90%, preferably from 65% to 85%, more preferably from 70% to 80%, and most preferably from 70% to 75%. The average width of a void between granules might suitably be in the range of 10 to 30 mm.

[0121] The present inventors have also found that the use of a range of different sizes for the solid granules provides for a better thermal response. Smaller granules will heat or cool more rapidly than larger granules, but have a lower overall capacity for heat; larger granules will take longer to heat or cool but have a higher overall capacity for heat. Use of a mixture of sizes allows for gradual update and release of heat in a controlled manner.

[0122] Suitably, the solid granular material may be selected from rock granules or mineral granules. The materials should be chosen to be suitable to undergo numerous cycles of heating and cooling to the desired maximum storage temperature without breaking into significantly smaller pieces or decomposing. It is expected that using temperatures of up to 500 °C will not cause such problems for most rock granules or mineral granules. Suitable natural rock or natural mineral materials for the solid granular material may be granite, basalt, marble, diamond, quartz, flint or other similar rocks or minerals, or mixtures thereof. Further, it is envisaged that it would be possible to make use of rock that has been crushed, formed into granules and then sintered in order to reduce its porosity compared with the natural state, if desired, or other manufactured or non-natural rock or mineral products, such as synthetic diamond or engineered granite. However, the use of natural materials is preferred, more preferably locally obtainable natural materials, especially in the context of providing thermal storage for a renewable energy generation system. Taking into account the size and shape considerations for the granular material set out above, a particularly preferable material may be pebbles obtained from the sea, beach or riverbed, as these have already been worn into a smooth and more evenly convex shape than is usually obtained by the quarrying of rock on land. However, of course, quarried rock may be suitably smoothed, polished, size graded and / or shaped in order to arrive at a suitable granule size and shape for use in the present invention.

[0123] Alternatively, and particularly where space for the heat storage reservoir is restricted, an encapsulated phase change material may be used as the solid granular material to which heat is transferred for storage. The phase change material is solid when heat has been discharged from the heat storage reservoir, and becomes liquid when heat is transferred to it from a heat transfer fluid. In general, a phase change material permits a heat storage reservoir of around half the volume of the one containing rock or mineral granules for the same heat storage capacity, as the phase change allows a much larger heat capacity for the phase change material at the temperature at which the phase change takes place, compared with a material for which a phase change does not take place at that temperature. In addition, this property can allow a much higher proportion of the stored heat to be obtained at a high temperature, if the phase change occurs at a high temperature, compared with a solid granular material for which a phase change does not occur, as a large proportion of the stored heat is released on change of phase from liquid to solid, rather than simply as a result of reducing temperature. However, the cost is significantly higher. Suitable phase change materials may include eutectic salts such as potassium nitrate or sodium nitrate, or organic materials such as wax with a melting point within a suitable range for the heat transfer to be performed. It is envisaged that the phase change material may be encapsulated in a suitable material, such as metal, polymer or concrete, to form the solid material granules. The choice of the encapsulating material will depend on the intended use of the heat storage reservoir, in particular the rate at which the heat transfer must occur. For example, if seasonal heat storage is desired, the heat transfer can occur slowly, and the material used to encapsulate the phase change material need not be a highly conducting material, and so concrete might be an appropriate choice. Where the heat transfer must occur more quickly, for example over the timescale of minutes or hours, however, a more rapidly conducting material must be selected to encapsulate the phase change material, with polymer or metal being more appropriate as these can form a thin capsule and / or conduct heat more rapidly than concrete. The properties of the granules of encapsulated phase change material are preferably as given above for solid granular materials generally. As the granules of encapsulated phase change material are manufactured, there is complete control over the size, size distribution, shape and surface properties of the granules. Preferably, the granules of encapsulated phase change material are spherical.

[0124] The heat storage system 100 further comprises an input system 126 for the introduction of heat to the heat storage reservoir 103, ie charging of the heat storage reservoir 103. The input system 126 comprises input inlet 128 for introduction of heated liquid, superheated liquid, or gaseous, heat transfer fluid into the heat storage reservoir 103, liquid recovery system 116 for collection of cooled liquid heat transfer liquid from the heat storage reservoir 103, and input heat source 130 for heating, superheating or evaporating, the liquid heat transfer fluid. Liquid recovery system 116, input heat source 130 and input inlet 128 are in fluid connection with one another in order to recirculate heat transfer fluid through the input system 126.

[0125] Input inlet 128 may be of any suitable form to introduce heated liquid, superheated liquid or gaseous heat transfer fluid into the heat storage reservoir 103 in order that the heated liquid, superheated liquid or gaseous heat transfer fluid contacts the solid granular material 108; for example, while Figure 1 depicts the input inlet 128 placed towards the upper part of the volume of solid granular material 108, and extending across part of the width of the volume of solid granular material, these positions are not to be construed as limiting. Further, separate inlets may be provided where different heat transfer fluids are to be used.

[0126] Below the lower portion 114 of the inner container 106 is provided a liquid recovery system 116 for collection of liquid draining down through the volume of solid granular material. As depicted in Figure 3, the lower portion 114 of the inner container 106 may be provided in such a form as to encourage the draining down of liquid under gravity, for example a sloped surface at the lower portion of the inner container 106. Suitably, liquid collected at the liquid recovery system 116 may be pumped away, for example using a mechanical centrifugal pump (not shown), and / or further draining of fluid from the surface of the solid granular material 108 may be encouraged by means such as application of vibrations or soundwaves to the granular material 108. However, as will be appreciated, other arrangements allowing collection of liquid at the lower portion of the inner container 106 may be used. Further, separate liquid recovery systems may be provided where different heat transfer fluids are to be used.

[0127] Input heat source 130 may comprise any device or method to add heat to the heat transfer fluid, such as direct electrical heating, a heat exchanger with a hot fluid on the opposite side of the heat transfer fluid, illumination by concentrated sunlight, a source of solar heat, for example a solar concentrator that is used to heat a fluid, or any other means to heat the heat transfer fluid in the input system. The input heat source 130 may comprise, or further comprise, a heat exchanger for transfer of heat to the heat transfer fluid to cause its heating, superheating or evaporation. Where different heat transfer fluids are to be used, it may be preferred to provide a separate heat exchanger for each fluid, or separate flow paths through a single exchanger for each fluid.

[0128] The heat storage system 100 further comprises an output system 132 for the extraction of heat from the heat storage reservoir 103, ie discharging of the heat storage reservoir 103. The output system 132 comprises output inlet 110 for introduction of liquid heat transfer fluid into the heat storage reservoir 103, output outlet 118 through which heated, superheated or evaporated heat transfer fluid exits the heat storage reservoir 102, and output heat sink 134 for cooling the heated or superheated heat transfer fluid and condensing the evaporated heat transfer fluid. Output outlet 118, output heat sink 134, and output inlet 110 are in fluid connection with one another in order to recirculate heat transfer fluid through the output system 132.

[0129] Output inlet 110 may be of any suitable form for the introduction of liquid heat transfer fluid to the volume of solid granular material. Suitably, the output inlet 110 may be in the form of one or more distributors in the upper portion of the heat storage reservoir 103, such as one or more spray nozzles or one or more rotating distributors. Figure 3 depicts a single rotating distributor arm 112, but it will be understood by the skilled person that alternative distributors may be provided, and that a suitable number and type of such distributors may be provided in order to ensure even distribution of the heat transfer fluid across the top of the volume of solid granular material with which heat is to be exchanged with the heat transfer fluid. Further, separate inlets may be provided where different heat transfer fluids are to be used. Preferably, output inlet 110 is placed above the volume of solid granular material in order that the liquid heat transfer fluid can be distributed through the volume of solid granular material by operation of gravity alone.

[0130] Output outlet 118 may be of any suitable form to allow heated, superheated or gaseous heat transfer fluid to be withdrawn from the heat storage reservoir 103 and to pass to the output heat sink 134; for example, while Figure 3 depicts the output outlet 118 placed towards the upper part of the volume of solid granular material 108, and extending across part of the width of the volume of solid granular material, these positions are not to be construed as limiting. Further, separate output outlets may be provided where different heat transfer fluids are to be used. Where a liquid heat transfer fluid is to be withdrawn from the heat storage reservoir, suitably a mechanical pump may be provided in the output system.

[0131] Output heat sink 134 may suitably comprise any apparatus that requires a source of heat, such as a turbine. The output heat sink 134 may further comprise a heat exchanger for transfer of heat from the heated, superheated or gaseous heat transfer fluid to cause its cooling and, where relevant, condensation. Where different heat transfer fluids are to be used, it may be preferred to provide a separate heat exchanger for each fluid, or separate flow paths through a single exchanger for each fluid.

[0132] While the figure depicts a single input system 126 and single output system 132, it will be understood that more than one of each such system may be provided, for example in order to allow the separate circulation of different heat transfer fluids to be used at different stages of the heat transfer process, during charging and / or discharging of heat from the heat storage system, as necessary. The heat storage reservoir suitably further comprises a temperature monitor placed to monitor the temperature of the volume of solid granular material.

[0133] The heat storage reservoir 103 further comprises a residual pressure reduction system 120 in fluid connection with the heat storage reservoir 103 to maintain low residual air (or gas) pressure within the volume of solid granular material, where residual air (or gas) pressure refers to the pressure resulting from air or gas that is not, and does not comprise, heated, superheated or evaporated heat transfer fluid, or other gases that undergo a phase change within the temperature range of operation of the heat storage reservoir. The residual pressure reduction system may suitably comprise an outlet leading to a pressure reduction unit 124 such as a vacuum pump. Preferably, the pressure reduction system further comprises a condenser 122 maintained at a temperature sufficiently lower than the boiling point of the heat transfer fluid that contact between the condenser and any gaseous heat transfer fluid causes the gaseous heat transfer fluid to be condensed, thus reducing the evacuation of the heat transfer fluid from the heat storage reservoir by the residual pressure reduction system. Suitably, the condenser 122 may take the form of a tube, which is comprised in the fluid connection between the pressure reduction unit 124 and the volume of solid granular material, and which slopes downward from the pressure reduction unit 124 towards the volume of solid granular material to assist in returning condensed heat transfer liquid to the heat storage reservoir 103. A suitable form for the condenser 122 is a spiral tube as shown in Figure 3. Suitably, the condenser 122 is maintained at a suitable temperature to condense evaporated heat transfer fluid simply by exposure to the ambient environment outside the heat storage reservoir 103; however, it will be appreciated that other forms of temperature control are possible, such as the inclusion of a flowing water jacket surrounding the flow path connecting the pressure reduction unit 124 and the volume of solid granular material. Alternatively, the condenser 122 may take the form of a cold finger or cold trap placed in the fluid connection between the pressure reduction unit 124 and the volume of solid granular material. A pressure monitor may be provided to monitor the gas pressure contribution of non-condensable species, for example placed between the pressure reduction unit and the condenser.

[0134] The heat storage system may suitably further comprise a system for removal of all heat transfer fluids from the heat storage reservoir and / or the input system and / or the output system. This is useful in order to decommission the heat storage system for cleaning or maintenance, or at the end of its useful life. Suitably, the removal of heat transfer fluids may comprise heating the heat transfer fluids present in the heat storage system to evaporate them, such that they pass through the output outlet 118, are condensed by the output heat sink 134, and are then drained off through a suitable valve provided in output system 132, for example to a storage vessel. Alternatively, the removal of the heat transfer fluids can be achieved by heating the heat transfer fluids present in the heat storage system to evaporate them, and use of the pressure reduction system, where provided, to pump the evaporated heat transfer fluids out of the heat storage reservoir. The condenser 122 would in that case need to be maintained at a suitable temperature to prevent condensation and return of the heat transfer fluid to the heat storage reservoir, or provided with a suitable outlet to allow draining of the condensed heat transfer fluid, for example to a storage vessel, without it returning to the heat storage reservoir. Further alternative means for removal of heat transfer fluids from the heat storage vessel that are apparent to the skilled reader may also be provided.

[0135] In use, a liquid heat transfer fluid in input system 126 is heated, directly or indirectly, by input heat source 130 such that it is heated, superheated, or evaporates, and is introduced in its heated, superheated or gaseous state through the input inlet 128 such that it contacts the solid granular material 108 within heat storage reservoir 103. The granules of solid material 108 are at this stage at a temperature below the temperature to which the heat transfer fluid has been heated by the input heat source 130. Accordingly, on contact of the heated, superheated or evaporated heat transfer fluid with the surface of the solid granular material 108, the heat transfer fluid transfers heat to the solid granular material 108 and, where the heat transfer fluid was evaporated by the input heat source 130, condenses onto the surface of the granules. Due to the void space provided between the granules of the granular material, the cooled and / or condensed liquid heat transfer fluid flows freely downwards in the heat storage reservoir to be collected in the lower part of the reservoir 114 and drained through the liquid recovery system 116. This drained cooled and / or condensed liquid heat transfer fluid is recirculated via input heat source 130 to be heated, superheated or evaporated once again and reintroduced through the input inlet 128 into the heat storage reservoir to transfer further heat to the solid granular material 108.

[0136] When the solid granular material 108 reaches a temperature very close to the temperature to which the heat transfer fluid is heated by input heat source 130, or the boiling point of the heat transfer fluid, this process will no longer be able to transfer energy from the heated, superheated or evaporated heat transfer medium to the heat reservoir efficiently, as a sufficient degree of heat transfer (which is an equilibrium process) of the heat transfer fluid will not reliably take place on contact with the solid granular material 108. If it is desired to transfer further heat into the heat storage reservoir, a second heat transfer liquid having a higher boiling point may be used in place of the first heat transfer liquid. The use of two or more heat transfer liquids having progressively increasing boiling points allows the transfer of heat into the solid granular material until the temperature of the solid granular material reaches, or approaches, the boiling temperature of the heat transfer fluid having the highest boiling point.

[0137] Where the use of a superheated heat transfer fluid is envisaged, input inlet 128 may be of any suitable form to introduce superheated heat transfer fluid into the heat storage reservoir 103 in order that superheated or gaseous heat transfer fluid contacts the solid granular material 108. For example, input inlet 128 may comprise a pressure reduction valve at any chosen position between the input heat source 130 and the heat storage reservoir 103, but preferably placed close to the heat storage reservoir 103. The pressure reduction valve allows the pressure upstream of the valve to be maintained at a level such that the superheated heat transfer fluid does not evaporate to any significant extent, but is primarily in the liquid state. However, downstream of the pressure reduction valve, the pressure is maintained at a level such that the heat transfer fluid can evaporate at least partially, and enters the heat storage vessel 103 in a gaseous state. Alternatively, a pressure reduction valve need not be provided: where the pressure in the heat storage vessel 103 is reduced by the residual pressure reduction system 120, the pressure in the heat storage reservoir 103 may be maintained sufficiently lower than the pressure in the input inlet 128 that the superheated heat transfer fluid remains liquid in the input inlet 128 but evaporates on entering the heat storage reservoir 103. Preferably, in this circumstance, the input inlet 128 enters the heat storage reservoir 103 at the upper end of the reservoir, and the input heat source 130 is positioned lower than the point at which the input inlet 128 enters the heat storage reservoir 103, in order that the superheated liquid does not evaporate in the input heat source but only at the highest point of the input system as this will be the point of lowest pressure.

[0138] In use, the liquid heat transfer fluid in input system 126 is heated, directly or indirectly, by input heat source 130 such that it becomes superheated to above its normal boiling point at atmospheric pressure, but does not evaporate, and is introduced through the input inlet 128 in a volatile liquid state and evaporates on, or subsequent to, entry to the volume of solid granular material. The gaseous heat transfer fluid then contacts the solid granular material 108 within heat storage reservoir 103. The granules of solid material 108 are at this stage at a temperature below the boiling point of the heat transfer fluid. Accordingly, on contact of the evaporated heat transfer fluid with the surface of the solid granular material 108, the heat transfer fluid transfers heat to the solid granular material 108 and condenses onto the surface of the granules. Where the heat transfer fluid does not completely evaporate on entry to the volume of solid granular material, or immediately subsequent thereto, some heat transfer may also take place by contact between heated liquid heat transfer fluid and the solid granular material 108. While the temperature of the solid granular material 108 is lower than the boiling point of the heat transfer fluid at the pressure maintained in the volume of solid granular material, but the superheated liquid transfer fluid has a temperature higher than the boiling point of the heat transfer fluid at the pressure maintained in the volume of solid granular material, liquid heat transfer fluid introduced into the volume of solid granular material will evaporate, and any liquid heat transfer fluid that does not evaporate will flow to the bottom of the chamber and heat the condensed heat transfer fluid collected at the bottom of the chamber causing, if sufficient heat transfer fluid at a temperature above its boiling point is admixed, partial evaporation thereof. The evaporated heat transfer fluid will then contact the solid granular material in gaseous form to transfer heat thereto by condensation onto the surface of the solid granular material. Due to the void space provided between the granules of the granular material, the condensed heat transfer fluid flows freely downwards in the heat storage reservoir to be collected in the lower part of the reservoir 114 and drained through the liquid recovery system 116. This drained condensed heat transfer fluid is recirculated via input heat source 130 to be superheated and optionally evaporated once again and reintroduced through the input inlet 128 into the heat storage reservoir to transfer further heat to the solid granular material 108.

[0139] The choice of number and type of heat transfer fluids depends on the temperature of both the heat source and the intended use. The choice will influence the thermodynamic efficiency as the boiling points of each heat transfer liquid will define the possible input and output temperatures. By having few (immiscible or azeotropic) fluids, a relatively larger difference in boiling point will be realized, and by having more azeotropic fluids, the better thermodynamic performance the system will have, but at an increased cost and complexity level. Typical differences in boiling point for different liquids will be in the range of 10 °C-80 °C. Having smaller boiling point differences by using more azeotropic fluids will improve the thermodynamic performance to the maximum level, but would also require a more advanced system to control the mixture and collect and store the fluids.

[0140] It has been determined by the present inventors that it is more efficient to use a succession of heat transfer fluids having progressively increasing boiling points rather than a single high boiling point heat transfer fluid, because a typical input heat source 130 will transfer heat more effectively to the heat transfer liquid if the heat transfer liquid is colder, due to the higher temperature gradient between the source and the heat transfer liquid. Use of a high boiling point heat transfer fluid initially will result in returned heat transfer fluid exiting through liquid recovery system 116 being at a temperature only slightly below its boiling point. Accordingly, the temperature gradient between the input heat source 130 and the heat transfer liquid to be evaporated is less than it would be if a lower boiling point fluid had been used.

[0141] However, the inventors have realized that, where a pressure reduction system is provided, it is advantageous to select a small number of heat transfer fluids for use, such as one fluid, two fluids, three fluids or four fluids having different boiling points, as the pressure reduction system used in the present invention can allow pressure adjustment during the charging and / or discharging of the heat storage reservoir which in turn can adjust the boiling temperature of a given heat transfer fluid to a desired value, thus giving the same effect as the use of a succession of heat transfer fluids with different boiling points. Thus, a given heat transfer fluid may be used at a range of pressures from the lowest pressure selected during the charging or discharging of the heat storage reservoir up to ambient pressure, and thus be used at a range of effective boiling points as dictated by the pressure in the volume of solid granular material or phase change of material.

[0142] The heat transfer fluid may be one that is solid at normal ambient temperature and pressure, so long as it is in the correct state - liquid or gas - to undergo the required contact with the granular material for heat transfer under the conditions of use. The heat transfer fluid should be one that is stable to repeated heating and cooling over the temperature range of operation of the heat storage system, and which is not flammable at any of the temperatures of operation of the heat storage system. Preferably, a heat transfer fluid for use in the present invention has a low melting point and a high normal boiling point. The first heat transfer fluid used for charging a heat storage reservoir is preferably liquid at ambient or slightly above ambient temperature. Preferably, a heat transfer fluid for use in the present invention reaches 1 bar vapor pressure at as high a temperature as possible. Suitable heat transfer fluids are oils (biological or mineral), silicones or other substances where the molecular weight can be tailored to get the feasible properties, or, for higher temperature use, sulphur.

[0143] In many cases, the upper temperature limit for operation of a heat transfer fluid is determined by the breakdown temperature of the fluid, or of specific molecular bonds of the fluid, in particular for organic components or silicones that break down in the range of 350-400 °C in the presence of oxygen. It is anticipated by the present inventors that such heat transfer fluids may be used at temperatures exceeding this range where a pressure reduction system is provided due to the low residual air pressure used, and thus the very low oxygen content of the atmosphere in the heat storage reservoir.

[0144] From an economic and environmental point of view, the heat transfer fluid should preferably be cheap, non-toxic and non-corrosive.

[0145] It will be appreciated that heat transfer between the heat transfer fluid and the volume of solid granular material will be due partially to the heat of the phase change taking place when the heat transfer fluid condenses (on charging) or evaporates (during discharging) on contact with the solid granular material, where the heat transfer fluid undergoes a phase change, and partially due to the temperature change of the heat transfer fluid on contact with the solid granular material, which, where a phase change of the heat transfer fluid takes place, brings the heat transfer fluid to the temperature at which its phase change takes place. It is preferred that the fraction of heat transfer to and from the heat storage reservoir that takes place through phase change of the said heat transfer fluid is at least 50%, more preferably 60%, more preferably 70%, even more preferably 80%, even more preferably 90% and most preferably more than 95%. The skilled reader will appreciate that the proportion of the heat transferred due to the phase change is able to be calculated or modelled by known means. The higher the proportion of the heat transfer that is due to the phase change, the less the process is dependent on temperature gradient between the heat transfer fluid and the solid granular material or the phase change material; thus, the heat transfer can take place under almost isothermal conditions, allowing a significant degree of heat transfer to take place without the need for provision of a large temperature gradient. The size of the temperature gradient needed will depend on such factors as the identity of the heat transfer fluid, its surface tension, the pressure in the volume of solid granular material, the volume of the heat transfer material, and the purpose of the storage (for example, seasonal heat storage uses a large volume of solid granular material, and is expected to be charged and discharged slowly, such as over a period of weeks or months, and so a small temperature gradient is feasible). Suitably, there may be a maximum temperature difference between the heat transfer fluid and the volume of solid granular material or volume of phase change material at the time of contact of 50 °C, such as 45 °C, or 40 °C, such as 35 °C, or 30 °C, such as 20 °C, or 15 °C. In order for the phase change in the heat transfer fluid to take place on contact with the solid granular material or phase change material, there may be a minimum temperature difference between the heat transfer fluid and the volume of solid granular material or volume of phase change material at the time of contact of 2 °C, such as 5 °C, or 10 °C, such as 15 °C, or 20 °C, such as 25 °C or 30 °C.

[0146] It has further been found by the present inventors that there are significant advantages to using solid granular material to store heat, compared with using a system in which tubes are provided within a solid material such as a concrete block. First, no tubes are required in the heat reservoir thereby reducing the cost and complexity of manufacturing the reservoir. Second, the size of the solid granular material 108 used in the heat storage reservoir 103 can be adjusted in order to adjust the input / output power of the system by means of controlling the surface to volume ratio of the solid granular material 108. Further, where the heat transfer takes place at least partially by means of a phase change of the heat transfer fluid on contact with the solid granular material, the heat storage system 100 of the present invention inherently permits equalisation of the temperature distribution within the heat storage reservoir due to the volume change occurring on condensing the evaporated heat transfer fluid on contact with the solid granular material. Any colder area of the volume of solid granular material will cause a higher rate of condensation of the evaporated heat transfer fluid, thus causing mass flow to the colder area to increase and increasing the heating rate of this colder area until the temperature is the same as the remainder of the volume. That this self-equalisation of the temperature can be attained is particularly important when one wishes to use a succession of heat transfer fluids having increasing boiling point to charge the heat storage reservoir 103 with heat, as, if a high ratio of the supplied evaporated heat transfer liquid is not condensed or is re-evaporated by the introduction of a heat transfer fluid having a higher boiling point, the heat transfer efficiency of the system will be lowered. Fourth, the condensation of the heat transfer fluid on contact with the solid granular material 108 allows the transfer of significant heat from the heat transfer fluid to the solid granular material 108 almost independently of the temperature difference between the fluid and the granular material 108. As long as the solid granular material 108 is sufficiently cooler than the boiling point of the heat transfer fluid to cause its condensation, the heat transferred will equal, where the temperature difference between the fluid and the granular material is at the minimum required for condensation of the heat transfer fluid, or exceed, where the temperature difference between the fluid and the granular material is greater than the minimum required for condensation of the heat transfer fluid, the enthalpy of the gas-liquid phase change of the heat transfer fluid.

[0147] It has further been realised by the present inventors that reducing the residual air pressure within the heat storage reservoir 103 significantly increases the internal heat transfer rate where heat transfer takes place at least partially by phase change of the heat transfer fluid. Without wishing to be bound by theory, the present inventors believe that this is due to the increased rate of evaporation (during discharge) and / or condensation (during charging) of the heat transfer fluid, as well as an increased rate of diffusion of evaporated heat transfer liquid between colder zones and hotter zones within the heat storage reservoir.

[0148] It has been described in FR2981736 that evacuation of a heat storage chamber following its charging with heat is advantageous in terms of reducing the heat loss from the heat storage chamber during storage period. However, the discovery of the present inventors relates specifically to residual pressure reduction in the heat storage reservoir during charging and / or discharging of heat from the reservoir, and not during the storage period. While it is an option to reduce the residual pressure in the heat storage reservoir during charging and to maintain that lower pressure during the storage period in order to minimise the work needed to reduce the residual pressure during discharge of heat from the storage reservoir, the purpose of the residual pressure reduction of the heat storage reservoir is to improve the internal heat transfer rate between the heat transfer fluid and the solid granular material in the heat storage reservoir where the heat transfer takes place at least partially as a result of a phase change of the heat transfer fluid, and to tailor the boiling point of the heat transfer fluid to maximise the efficiency of the heat transfer. The use of residual pressure reduction during charging and / or discharging of the heat storage system is not suggested in FR2981736, and indeed would not result in any improvement in that system, which is designed to use air as the heat transfer fluid, and thus does not make use of the phase change of the heat transfer fluid as is required in the present embodiment. Further, it is only the residual air pressure that is to be reduced during charging and discharging of the heat storage reservoir. The overall pressure of the system will comprise contributions both from residual air (or other non-condensing gas) pressure and the vapour pressure of the heat transfer fluid. Removal only of noncondensing gases from the heat storage reservoir allows the improvement of the heat transfer between the heat transfer fluid and the solid granular material where the heat transfer is due at least partially to a phase change of the heat transfer fluid.

[0149] It has been found that reduction of the residual air pressure to 200 mbar or less allows the benefits of improved heat transfer which takes place at least partially as a result of phase transfer to be achieved, with lower residual air pressure being preferred. For example a maximum residual air pressure of 175 mbar, or 150 mbar, such as 125 mbar, or 100 mbar, such as 75 mbar, or 50 mbar, such as 25 mbar, or 10 mbar, such as 5 mbar, may be used. Depending on the type of residual pressure reduction system used, a minimum residual air pressure of 25 mbar, 20 mbar, 15 mbar, 10 mbar, 5 mbar, 2 mbar, 1 mbar or less than 1 mbar may be achievable.

[0150] Accordingly, before and / or during the introduction of heated, evaporated or superheated heat transfer fluid through the input inlet 128, the residual air pressure of the heat storage reservoir is reduced by the residual pressure reduction system 120, suitably by operation of a vacuum pump attached to an outlet in the heat storage reservoir. The residual pressure reduction system is preferably operated during the entire cycle of introducing heat to the heat storage reservoir in order to prevent any small leaks in the heat storage reservoir causing air or gases to enter the system and thus increasing the residual air pressure and reducing the internal heat transfer rate over time. However, it is contemplated as an alternative that the residual pressure is reduced by operation of the residual pressure reduction system prior to introduction of evaporated or superheated heat transfer fluid to the solid granular material, and that the residual pressure reduction system ceases to operate before evaporated or superheated heat transfer fluid is introduced into the heat storage reservoir.

[0151] It has been noted by the present inventors that, where the pressure reduction system is to be operated during introduction of the heated, evaporated or superheated heat transfer fluid to the heat storage reservoir, simple use of a vacuum pump or similar device attached to an outlet to the heat storage reservoir would result in the removal both of the residual air within the heat storage reservoir and the heat transfer fluid, in particular gaseous heat transfer fluid, which is not a preferable situation when seeking to increase the internal heat transfer rate and / or to recirculate the heat transfer fluids for repeated use. Accordingly, the present inventors have sought to overcome this difficulty by use of condenser 122 maintained at a temperature cooler than the boiling point of the heat transfer liquid, but above a temperature at which constituents of air, especially oxygen, can be condensed, and suitably at ambient temperature, between the solid granular material and the vacuum pump in order to condense any evaporated heat transfer fluid that comes in contact with condenser 122 and return it to the heat storage reservoir and prevent its removal by the vacuum pump. Suitably, condenser 122 may take the form of a long narrow tube maintained at a temperature lower than the boiling temperature of the heat transfer liquid and placed between the storage and the vacuum pump 124, with the vacuum pump being placed higher than the storage in order that any evaporated heat transfer fluid condensing on the cooled tube will flow down the tube and thus be returned into the heat storage reservoir. Suitably, the condenser is maintained at ambient temperature by placing condenser 122 outside the heat storage reservoir 103. Residual air does not condense at the temperature of the device 122 and hence will be evacuated through outlet 120. A further option might be to incorporate a cold trap or cold finger as known in the art, in which active cooling is applied to a section of tube to prevent the evacuation of heat transfer fluid.

[0152] Once heat transfer to the heat storage reservoir is complete, the outlets and inlet(s) may be closed in any suitable manner to seal the heat storage reservoir for the period of time for which heat storage is required. During this time, the residual air pressure may be maintained at its reduced pressure, or may be allowed to return to an ambient pressure. The former is preferred to reduce the energy expenditure in residual pressure reduction.

[0153] When it is desired to discharge heat from the heat storage reservoir 103, liquid heat transfer fluid is supplied through output inlet 110 to the top of the heat storage reservoir 103 such that it contacts the hot solid granular material 108. Once the liquid heat transfer fluid contacts the hot granules 108 of the reservoir 103, the liquid heat transfer fluid will heat up and / or evaporate, absorbing energy from the hot solid granular material 108 and, if evaporation takes place, causing a significant increase in volume. Where evaporation occurs, this volume increase will cause the evaporated heat transfer fluid to escape the heat storage reservoir through output outlet 118 from where it may pass to output heat sink 134. Where evaporation does not occur, the heated heat transfer fluid may be drawn out of the heat storage reservoir in a suitable manner through output outlet 118 from where it may pass to output heat sink 134. For example, the heated heat transfer fluid may be passed to a heat exchanger system to transfer its heat to another process such as the water or steam in a steam turbine or steam generator or the pressure fluid in an organic Rankine cycle system. Once the heat has been extracted from the heated heat transfer fluid and it has (if evaporated) returned to the liquid state, the cooled liquid heat transfer fluid may be recirculated to the heat storage reservoir 103 again through output inlet 110 to transfer further heat from the heated solid granular material 108. Once the temperature of the solid granular material 108 in the heat storage reservoir reaches the boiling point of the heat transfer fluid, in order to extract further heat a lower boiling point heat transfer fluid must then be employed, in the same way and for the same reasons as a succession of increasing boiling point heat transfer fluids was used during the charging of the heat storage reservoir. Again, the reduction of residual air pressure during the discharging of heat from the heat storage reservoir 103 is found to improve the heat transfer rate, as explained above for charging of the heat storage reservoir, where the heat transfer takes place at least partially by phase change of the heat transfer fluid. Similarly to the situation for charging the heat storage reservoir, the residual pressure reduction system is preferably operated during the entire cycle of introducing cool liquid heat transfer fluid to the heat storage reservoir in order to prevent any small leaks in the heat storage reservoir causing air or gases to enter the system and thus increasing the residual air pressure and reducing the internal heat transfer rate over time. However, it is contemplated as an alternative that the residual pressure is reduced by operation of the residual pressure reduction system prior to introduction of cool liquid heat transfer fluid to the solid granular material, and that the residual pressure reduction system ceases to operate before cool liquid heat transfer fluid is introduced into the heat storage reservoir. In the event that the reduced residual air pressure is maintained in the heat storage reservoir during the storage period, it is possible that the residual air pressure will be sufficiently low for the benefits of the invention in terms of heat transfer to be obtained without further operation of the residual pressure reduction system immediately prior to discharging heat; however, this will depend on the degree of fluid-tightness of the heat storage reservoir and the duration of the storage period. It is likely that an additional residual pressure reduction step would be required immediately prior to, and / or during, discharge of heat from the heat storage reservoir.

[0154] Figure 4 shows an embodiment of the construction of the heat storage reservoir 103, shown as a plan view of the heat storage reservoir 103 taken as a cross section through the plane A-A indicated in Fig 3. Where the same elements are shown as in Fig 3, these are allocated the same reference numerals.

[0155] Starting from the centre of Fig 4, a volume of sold granular material 108 is placed within cylindrical inner container 106, and cylindrical outer container 102. Outer container 102 may be formed of any material suitable to provide a substantially fluid-tight container, such as concrete or a metal such as steel. On the inner surface of the outer container 102 is insulation 104, which may suitably be ceramic bricks or other non-absorbent insulation material. The inner container 106 is formed from a plurality of steel panels 140, which are curved to follow the general curvature of the cylindrical form of the heat storage reservoir 103, and which overlap one another such that each panel has one edge that is on the inner side of the inner container 106 and one edge that is on the outer side of the inner container 106. In Fig 4 are shown seven such panels 140, but it will be understood that any suitable number of panels in a similar arrangement may be used. The plurality of panels 140 are themselves surrounded by plural parallel wires 150, spaced along the vertical axis of the inner container 106, of which one is shown in Fig 4, each wire being joined into a closed loop by a tensionable fastener 160. The wire 150 (and the parallel wires not shown) are each tightened around the overlapping panels 140 which causes the overlapping steel panels 140 to move into close contact with one another and together to form a generally cylindrical inner container 106. The panels 140 are held in their intended spacing by the volume of solid granular material, such that tightening the wires 150 will not make the inner container 106 excessively small.

[0156] The steel panels 140 may suitably be panels of a thickness of from 1 mm to 4 mm, such as 2 mm. In order to save on cost, weight and materials, the steel panels may be perforated, for example by using a steel plate into which slits have been cut in an offset pattern and the plate pulled in the plane of the panel normal to the direction of the length of the slits, such as to form an openwork mesh panel. In the event that perforated panels are used they should of course have perforations or openings that are smaller than the size of the smallest solid granules to be contained in the inner container 106.

[0157] The wire 150 should be of at minimum a similar thickness to the thickness of the panels 140, up to a maximum of 10 times the thickness of the panels 140, preferably five times the thickness of the panels 140. Thus, the range of suitable wire thicknesses might suitably be from 1 to 40 mm, such as 20 mm or 10 mm. The use of a thicker wire allows the pressure applied by the wire to the panel to be distributed over a wider area, and thus to be less likely to deform the panels 140.

[0158] In use, on charging the heat storage reservoir with heat, the steel panels 140 comprised in inner container 106 expand as their temperature rises. This creates tension in the wire 150 and parallel wires (not shown). The expansion also allows the arrangement of the solid granular material 108 to change to reflect the larger diameter of the inner container 106 resulting from the expansion. In general, this will result in the granular material settling somewhat into an arrangement taking up a greater diameter than prior to charging. When discharging the heat from the heat storage reservoir, the panels 140 will tend to contract as their temperature lowers. In the inner container 106, the individual panels 140 will contract, but their overlapping arrangement and the slightly larger size of the inner container 106 caused by the settling of the solid granular material 108 means that the panels 140 will slide in relation to one another together to form a complete container 106 having a slightly larger diameter than the original diameter, and slightly less overlap between the panels, in which there is a greater tension than originally in the wire 150 and the parallel wires (not shown). Over a number of charge-discharge cycles, a final dimension will be reached in which the wire 150 is at its maximum tension and does not permit further expansion on heating, such that no further shifting of the solid granular material 108 can occur and no additional strain is placed on inner container 106. Outer container 102 is under no strain as a result of heating and cooling the heat storage reservoir, as it is not in contact with the inner container 106.

[0159] Fig 5 shows an alternative embodiment of the construction of the heat storage reservoir 103, shown as a plan view of the heat storage reservoir 103 taken as a cross section through the plane A-A indicated in Fig 3. Where the same elements are shown as in Fig 3, these are allocated the same reference numerals.

[0160] Starting from the centre of Fig 5, a volume of sold granular material 108 is placed within cylindrical inner container 106, and cylindrical outer container 102. Outer container 102 may be formed of any material suitable to provide a substantially fluid-tight container, such as concrete or a metal such as steel. On the inner surface of the outer container 102 is insulation 104, which may suitably be ceramic bricks or other non-absorbent insulation material. The inner container 106 is formed from a plurality of steel panels 141 and 142, which are curved to follow the general curvature of the cylindrical form of the heat storage reservoir 103, and which overlap one another. Outer panels 141 are curved such as to reproduce a slightly larger radius of curvature than inner panels 142. Panels 141 and 142 are positioned alternately such that outer panels 141 are outside inner panels 142 and the panels overlap one another, with both edges of the outer panels 141 being outside the inner panels 142, in relation to the centre of the inner container 106. Outer panels 141 if extended to touch one another would form an outer cylinder and inner panels 142 if extended to touch one another would form a smaller inner cylinder. In Fig 5 are shown three such outer panels 141 and three such inner panels 142, but it will be understood that any suitable number of panels in a similar arrangement may be used. The plurality of panels 141 and 142 are themselves surrounded by plural parallel wires 150, spaced along the vertical axis of the inner container 106, of which one is shown in Fig 5, each wire being joined into a closed loop by a tensionable fastener 160. The wire 150 (and the parallel wires not shown) are each tightened around the outer panels 141 which causes the overlapping steel panels 141 and 142 to move into close contact with one another and together to form a generally cylindrical inner container 106. The panels 142 are held in their intended spacing by the volume of solid granular material, such that tightening the wires 150 will not make the inner container 106 excessively small. The steel panels 141 and 142 may suitably be panels of a thickness of from 1 mm to 4 mm, such as 2 mm. In order to save on cost, weight and materials, the steel panels may be perforated, for example by using a steel plate into which slits have been cut in an offset pattern and the plate pulled in the plane of the panel normal to the direction of the length of the slits, such as to form an openwork mesh panel. In the event that perforated panels are used they should of course have perforations or openings that are smaller than the size of the smallest solid granules to be contained in the inner container 106.

[0161] The wire 150 should be of at minimum a similar thickness to the thickness of the panels 141 and 142, up to a maximum of 10 times the thickness of the panels 141 and 142, preferably five times the thickness of the panels 141 and 142. Thus, the range of suitable wire thicknesses might suitably be from 1 to 40 mm, such as 20 mm or 10 mm. The use of a thicker wire allows the pressure applied by the wire to the panel to be distributed over a wider area, and thus to be less likely to deform the panels 141 and 142.

[0162] In use, on charging the heat storage reservoir with heat, the steel panels 141 and 142 comprised in inner container 106 expand as their temperature rises. This creates tension in the wire 150 and parallel wires (not shown). The expansion also allows the arrangement of the solid granular material 108 to change to reflect the larger diameter of the inner container 106 resulting from the expansion. In general, this will result in the granular material settling somewhat into an arrangement taking up a greater diameter than prior to charging. When discharging the heat from the heat storage reservoir, the panels 141 and 142 will tend to contract as their temperature lowers. In the inner container 106, the individual panels 141 and 142 will contract, but their overlapping arrangement and the slightly larger size of the inner container 106 caused by the settling of the solid granular material 108 means that the panels 141 and 142 will slide in relation to one another together to form a complete container 106 having a slightly larger diameter than the original diameter, with slightly less overlap between the panels, in which there is a greater tension than originally in the wire 150 and the parallel wires (not shown). Over a number of charge-discharge cycles, a final dimension will be reached in which the wire 150 is at its maximum tension and does not permit further expansion on heating, such that no further shifting of the solid granular material 108 can occur and no additional strain is placed on inner container 106. Outer container 102 is under no strain as a result of heating and cooling the heat storage reservoir, as it is not in contact with the inner container 106. Figure 6 shows a vertical cross section through the inner container 106 of Fig 3. The overall form of the inner container 106 is that of a series of cylinders, each formed of multiple panels 140 as previously described, and with the cylinder of largest diameter at the base of the container, with successively smaller diameter cylinders stacked above. The inner container 106 is shown filled with granular material 108. The inner container 106 is formed of three vertical tiers of panels 140a, 140b and 140c. Each tier of panels together forms a cylindrical figure that is of a dimension arranged such that the lower end of an upper tier overlaps the upper end of the adjacent lower tier in order that the tiers stack above one another. The upper tier is smaller in diameter than the lower tier by an amount sufficient for the lower edge of the upper tier to sit within the upper edge of the lower tier, with sufficient overlap that the upper tier will not move sideways or slip out of engagement with the lower tier. In Fig 6, the upper tier is shown supported by the granular material 108 filling the lower tier; however, a bracket, hinge or other support may be provided in the inner face of the panels 140a to support the panels 140b of the tier above, and on the inner face of panels 140 b to support the panels 140c of the tier above, and so on. Each panel 140a, b or c is in the form of a segment of the periphery of the cylindrical tier to which it belongs, and, as previously described, partially overlaps adjacent panels in the same tier, such that together the panels form the periphery of the cylindrical tier to which they belong. The use of a succession of tiers of panels to form the inner container 106 allows for modular construction to reach a desired height for the container, in which a first tier is assembled from panels 140a, the wire loop 150a placed around the panels, the granular material filled into the container, and then the next tier assembled from panels 140b in the same way. Each panel 140 is provided with one or more brackets 170 to support the wires 150. While the Figure shows a single bracket per panel, it will be understood that the number of brackets will reflect the number of parallel wires 150 used and that the brackets need not be placed vertically in alignment, but may be placed as is convenient to determine the positioning and spacing of the wires 150 and to ensure their contact with the panels 140. Each bracket 170 is arranged to support the wire 150 from underneath, and to enclose the wire by providing a vertical projection substantially parallel to the face of panel 140, but has an open upper face. Thus, when constructing the inner container 106, a closed loop of wire 150 can be lowered over the arranged panels 140, tightened from a single tensioning point to close to the correct diameter and lowered into the bracket. This is much easier to accomplish than threading the wire through closed eyes or other designs of brackets having a closed upper face, and allows the heat storage reservoir to be constructed with a single access point to the inner container 106 at which point the tensionable fasteners 160 for the wires 150 would be provided, rather than needing access to multiple points to thread wires through closed eyes. In the event that wire 150 were to break, the vertical projection of the bracket would to some degree contain the wire from springing outwards and reduce the force impacting the inner face of outer container 102 or the insulation 104. In addition, it is again easier to replace a broken wire by lowering of a closed loop of wire 150 into brackets having an open upper face than to thread a wire through multiple closed loop eyes.

[0163] Figure 7 shows an alternative embodiment of the construction of the heat storage reservoir 103, shown as a plan view of the heat storage reservoir 103 taken as a cross section through the plane A-A indicated in Fig 3. Where the same elements are shown as in Fig 3, these are allocated the same reference numerals.

[0164] Starting from the centre of Fig 7, a volume of solid granular material 108 is placed within cylindrical inner container 106, and cylindrical outer container 102. Outer container 102 may be formed of any material suitable to provide a substantially fluid-tight container, such as concrete or a metal such as steel. On the inner surface of the outer container 102 is insulation 104, which may suitably be ceramic bricks or other non-absorbent insulation material. The inner container 106 is formed from a plurality of gabions 510, which are of generally cuboidal form, with faces made of steel mesh, and the inner volume filled with rocks or stones of a size larger than the apertures of the steel mesh. For example, the mesh size can suitably be 76.2 mm x 76.2 mm. The tensile strength of the steel mesh is suitably in the range 540-770 Nmnr2, with a wire diameter of 5 mm. Each gabion may be of dimensions 1m x 1m x 1m. The gabions 510 are arranged in layers, one of which is shown in Fig 7. Each layer comprises a plurality of gabions 510 each placed with one inner face 520 at a tangent to the radius of the cylinder of the inner container 106, and with the upright edges of that inner face 520 abutting the upright edges of the corresponding inner faces of the two neighbouring gabions, such that these inner faces together form a polygonal ring of inner faces of gabions to define the cylindrical inner container 106. A single gabion 510 without a filling is shown in Fig 8. It can be seen from this Figure that each face of the gabion 510 is formed from a wire mesh. In order to stabilize the structure, steel braces 560 connect opposite faces of the gabion 510 at intervals: in the Figure two braces are provided along the midline of a pair of opposite faces at one third and two thirds of the way along the midline. These prevent the gabion faces from bowing outward under the pressure of the filling material. A detailed view of two adjacent connected gabions 510 is shown in Fig 9. The inner faces 510 of the adjacent gabions are connected by helical steel binders 580 wound around the wires of the mesh that constitute the two edges of the faces 520 to be connected. As can be seen in both Fig 7 and Fig 9, the outer face 530, which is the opposite face to each inner face 520, of each gabion is spaced apart from the outer faces 530 of the neighbouring gabions. Each such space is filled in with a mesh panel 540 that abuts the upright edges of the outer faces 530 of two neighbouring gabions. As shown in Fig 9, the upright edges of the outer faces 530 are connected to the mesh panel 540 with helical steel wires, as for the inner faces 520 of the gabions. Each gabion is filled with a solid granular material such as rocks or stones with a size large than the apertures of the mesh. The wedge shape 550 thus formed between the gabions may also be filled with rocks or stones of a size larger than the apertures of the mesh. The abutting horizontal edges of neighbouring gabions or mesh panels are connected to one another by means of c-rings 570, shown in Figs 8 and 9, provided at intervals along the relevant edges, with the c-rings surrounding the two wires constituting the edges of the faces or mesh panels to be connected. The c rings are of a suitable strength to maintain connection between the adjacent gabions during heating and cooling of the inner container 106 in use, for example having a pullapart strength of at least 2 kN.

[0165] The plurality of gabions 510 may optionally themselves be surrounded by plural parallel wires 150, spaced along the vertical axis of the inner container 106, of which one is shown in Fig 5, each wire being joined into a closed loop by a tensionable fastener 160. An example of a suitable fastening arrangement is shown in Fig 10. Fig 10 A shows the successive stages of fastening a loop and eyelet 590 in one end of wire 150 by clamping the wire loop together by screw fasteners 595 in three positions, one adjacent the end of the wire 150, one adjacent the eyelet 590 to hold it in the desired position, and a third fastener positioned between the first two. Fig 10 B shows the two ends of the wire threaded through interlocked eyelets 590 and each tensioned and locked into position in the manner shown in Fig 10 A with screw fasteners 595 (only those adjacent the eyelet 590 shown). The wires 150 can be supported in position on the gabions by means of c rings surrounding the wire 150 and part of the mesh outer face 530 of the gabion 510 to which the wire is to be attached. The wire 150 (and the parallel wires not shown) are each tightened around the outer faces 530 of the gabions 510 which ensures the inner faces 520 of the gabions 510 remain in close contact with one another and together to form a generally cylindrical inner container 106, even if the expansion of the inner container results in settling of the volume of solid granular material 108. Successive layers of gabions are stacked vertically and connected to one another as described for the adjacent gabions in each layer, in order to build up the cylindrical inner container 106 to the desired height.

[0166] The wires 150 are suitably of a range of thicknesses from 1 to 40 mm, such as 20 mm or 10 mm. The use of a thicker wire allows the pressure applied by the wire to the outer faces 530 of the gabions 510 to be distributed over a wider area, and thus to be less likely to deform the outer faces 530 of the gabions 510.

[0167] In use, on charging the heat storage reservoir with heat, the mesh panels of the gabions 510 comprised in inner container 106 expand as their temperature rises. The solid granular material 108, and the fill of the gabions, expands to a significantly lesser extend than the mesh panels of the gabions.

[0168] Where there are no wires 150 provided surrounding the gabions, the gabions are held together in place by means of the adjacent inner mesh faces 520 connected by the helical steel binders 580 wound around the wires of the mesh that constitute the adjacent edges of the faces 520. Effectively, this provides a succession of connected wires that are a part of the mesh of the gabions, which provide the tensionable binder to limit the expansion of the inner container 106. The expansion of the circumference of the gabions and thus the inner container is limited to the expansion of the individual wires making up the inner faces 520 of the ring of gabions plus their helical binders 580. This is insufficient to allow significant settling of the solid granular material 108 within the container 106, and thus limits the stresses placed on the inner container 106 by cycles of heating and cooling. Thus, when the heat storage is again cooled, contraction of the gabions can bring the circumference of the inner container 106 back to the original size as the solid granular material has not appreciably shifted in to a new arrangement on heating the storage.

[0169] Alternatively, wires 150 may be provided surrounding the gabions 510 as shown in Fig 7, or may be provided to the inside of the gabions 510 along the inner faces 520 or threaded through the middle of the gabions. In these cases, charging the heat storage reservoir with heat creates tension in the wire 150 and parallel wires (not shown). The expansion may also allow the arrangement of the solid granular material 108 to change to reflect the larger diameter of the inner container 106 resulting from the expansion, if the wire 150 is not tensioned on construction of the container to limit this. In general, this will result in the granular material settling somewhat into an arrangement taking up a greater diameter than prior to charging. Where the wire is tensioned during construction, the expansion of the inner container 106 will not permit significant settling of the solid granular material 108. When discharging the heat from the heat storage reservoir, the mesh panels of the gabions 510 will tend to contract as their temperature lowers. Where the wire 150 is pre-tensioned, the contraction will bring the size of the inner container 106 back to its original size. Where it was not pre-tensioned, the settling of the solid granular material 108 to a new position will define a new size for the inner container 106 with increased tension in wire 150, which will be the size to which the container 106 returns in subsequent cycles of charging and discharging with heat.

[0170] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

[0171] All patent and non-patent references cited in the present application are also hereby incorporated by reference in their entirety.

Claims

Claims1. A heat storage reservoir comprising an outer container and an inner container for containing heat storage material, the inner container comprising a panel that does not change in size in response to expansion or contraction of the base of the outer container on heating or cooling, the panel being slidably mounted on the base of the outer container, and the side walls of the inner container being mounted on said panel.

2. The reservoir of claim 1, wherein the panel is supported by a bed of particulate matter disposed on the base of the outer container.

3. The reservoir of claim 2, wherein the bed is filled with oil to lubricate the movement of the panel and particulate matter.

4. The reservoir of claim 2 or claim 3, wherein the particulate matter comprises one or more of: smooth rocks or stones; bearings; and rollers.

5. The reservoir of any of claims 1 to 4, wherein the panel comprises a plurality of base plates, wherein the base plates are able to move relative to each other to accommodate thermal expansion or contraction of the base such that stresses arising from a difference in the thermal expansion or contraction of the base relative to the peripheral side wall are not coupled to the side wall.

6. The reservoir of claim 5, wherein the plates are steel.

7. The reservoir of claim 5 or 6, wherein the plates are between 10mm and 20mm thick.

8. The reservoir of any of claims 5 to 7, wherein the plates are between 0.5m and 1.5m in length and / or width.

9. The reservoir of any of claims 5 to 8, wherein the base plates partially overlap each other.

10. The reservoir of any previous claim, wherein the plates are welded or otherwise mechanically fixed to elements of the side walls.

11. The reservoir of any previous claim, wherein the panel is perforated to allow heat transfer fluid to drain from the reservoir.

12. The reservoir of claim 11 , wherein the holes are smaller than the particulate matter.

13. The reservoir of any of claims 1 to 5, wherein the panel is a ceramic plate.

14. The reservoir of any of claims 1 to 13, wherein the side wall of the inner container comprises: a plurality of wall elements arranged such that each wall element abuts or partially overlaps adjacent wall elements to form a continuous peripheral side wall for containing the granular material; at least one hoop tension element binding together the wall elements by which radially outward movement of the wall elements due to expansion of the granular material is resiliently resisted in use.

15. A heat storage reservoir comprising a container having at least a base and a peripheral side wall for containing a granular heat storage material, the base comprising: a plurality of base plates that support the peripheral side wall, the base plates being supported by a bed of particulate matter such that the base plates are able to move relative to each other to accommodate thermal expansion or contraction of the base such that stresses arising from a difference in the thermal expansion or contraction of the base relative to the peripheral side wall are not coupled to the side wall.

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