An arrangement and a method for storing thermal energy in the ground

Abstract

The invention relates to arrangement (1) for storing thermal energy in the ground. The arrangement comprises a circuit (2) containing a working fluid, wherein the circuit has a plurality of pipes (3) which pipes are distributed throughout a volume of the ground forming a heat storage. Each pipe is arranged in a channel (4) in the ground for heat exchange between the working fluid and the surrounding ground, and the arrangement (1) further comprises an electrical heating element (6). One said pipe (3) and the electrical heating element (6) are arranged in one and the same said channel (4).

Description

[0001] An arrangement and a method for storing thermal energy in the ground

[0002] TECHNICAL FIELD

[0003] The invention relates to an arrangement and a method for storing thermal energy in the ground.

[0004] BACKGROUND

[0005] There is a need for storing energy. Weather and seasonal changes cause variations in both production and consumption of heat and electricity. Today there are some methods available for storing energy. For example, hot water can be stored in pits, rock shelters and drill holes, and electricity can be stored in batteries or as potential energy by pumping water into reservoirs.

[0006] All these storing methods have one or more drawbacks, such as having low efficiency and high costs, being unsuitable for longer storage periods, providing too low temperature when the energy is to be retrieved and used, etc.

[0007] Thus, there is still a need for efficient storage of thermal energy, particularly for long term storage between seasons, in order to improve the energy supply in today’s society.

[0008] SUMMARY

[0009] An objective of the invention is to provide an arrangement for storing thermal energy in the ground, by which arrangement thermal energy can be stored in a heat storage in an effective way.

[0010] The objective is achieved by an arrangement for storing thermal energy in the ground, wherein the arrangement comprises a circuit containing a working fluid, the circuit has a plurality of pipes which pipes are distributed throughout a volume of the ground forming a heat storage, each pipe is arranged in a channel in the ground for heat exchange between the working fluid and the surrounding ground, and the arrangement further comprises an electrical heating element, wherein one said pipe and the electrical heating element are arranged in one and the same said channel.The invention is based on the insight that by such an arrangement, electrical energy can be efficiently converted to heat energy by means of the electrical heating element, which heat energy in turn can be stored for long periods at high temperatures in the ground in an efficient way. By the invention, an electrical energy surplus can be directly converted into heat in the heat storage. Although, theoretically, the ground can be heated to temperatures up to the melting point of the current bedrock or sediment, as an example only, a typical temperature interval for the heat storage can be 120-600°C, but temperatures above 600°C and up to 1600°C can be used.

[0011] Hereby it is possible to both store heat from high-temperature heat sources and provide thermal energy from the heat storage at high temperatures sufficient for production of district heating, process heating and / or heat or steam for electricity generation.

[0012] Further, the pipe and the working fluid can be preheated by the electrical heating element. The electrical heating element can be used for providing heat both when heat is to be transferred to the heat storage and when heat is to be recovered from the heat storage. For example, when using a molten salt as working fluid, which requires temperatures around 200°C to behave as a low viscosity fluid, charging of the heat storage by the electrical heating element can be complemented by heat from the working fluid, and during recovery of heat from the heat storage the risk of freezing the molten salt can be reduced by providing heat from the electrical heating element.

[0013] In addition, when charging the heat storage by the electrical heating element, the ground can be heated by heating the pipe which in turn heats the surrounding ground, rather than heating the ground directly by the electrical heating element. A relatively high thermal conductivity and a large surface of the pipe will promote the heat exchange with the ground.

[0014] According to one embodiment of the arrangement, the electrical heating element extends along said pipe. Hereby, the heat generated can be effectively transferred to the pipe and further to the surrounding ground. For example, arranging a relatively small diameter heating cable in contact with a large diameter metal pipe having higher thermal conductivity than surrounding filling material, will improve heat transfer since heat is spread more rapidly to the surrounding rock volume due to the larger surface area of the metal pipe compared to the heating cable.According to a further embodiment of the arrangement, the electrical heating element is attached to said pipe. Hereby, the electrical heating element and the pipe can be installed at the same time in a safe way ensuring that the electrical heating element is not damaged. Avoidance of abrasion from the channel walls is facilitated. Further, when using an insulated resistive heating cable for instance, by fixing the electrical heating element to the pipe, it can be ensured that the insulated resistive heating cable does not cross itself, thereby avoiding overheating which otherwise could melt the cable.

[0015] According to a further embodiment of the arrangement, the electrical heating element is an insulated resistive heating cable. Hereby, a cost effective way of heating the ground to high temperatures can be achieved. The cable can be attached to the pipe by cable ties for instance, and be arranged to extend along the pipe. For example, mineral insulated heating cables are resistant to corrosion, have long life of operation and provide high energy output per meter at high temperatures typically 550 to 1000 degree C. These allow conversion of DC or AC current to heat without moving parts that can wear or tear.

[0016] According to a further embodiment of the arrangement, the electrical heating element is a part of said pipe. By integrating the electrical heating element in the pipe, the installation is further simplified.

[0017] According to a further embodiment of the arrangement, at least a part of the electrical heating element is constituted by said pipe. By the use of the pipe as the electrical heating element instead of installing a separate electrical heating element in the channel, the number of components can be reduced and the installation can be simplified. The pipe can be designed and adapted for impedance heating where a low voltage AC is applied to the pipe for generating the heat. Hereby, any additional resistive heating cable or similar can be omitted.

[0018] According to a further embodiment of the arrangement, said pipe is a U-pipe having a first pipe portion and a second pipe portion, the first pipe portion and the second pipe portion extending in the longitudinal direction of said channel such that the working fluid can be circulated while flowing in the first pipe portion in a direction into said channel and in the second pipe portion in a direction out from said channel. Hereby, a spacing between the first pipe portion and the second pipe portion can reduce the thermal short circuiting between the ingoing and outgoing working fluid flow, which means that more energy andhigher temperatures can be extracted. Optionally, the spacing between the pipe portions could be filled with an insulating material to further reduce thermal short circuiting.

[0019] According to a further embodiment of the arrangement, said pipe is a coaxial pipe having an inner pipe portion and an outer pipe portion, the inner pipe portion and the outer pipe portion extending in the longitudinal direction of said channel such that the working fluid can be circulated while flowing in the outer pipe portion in a direction into said channel and in the inner pipe portion in a direction out from said channel. Hereby, installation of a pipe with a relatively large diameter (larger than for a U-pipe for instance) is allowed and thereby a greater working fluid flow can be achieved, which can be important for working fluids with a relatively low heat capacity.

[0020] According to a further embodiment of the arrangement, said pipe is a flexible corrugated metal pipe. Hereby, any thermal expansion of the pipe can be absorbed by minor flexing in each bend of the pipe instead of having to rely on flexible joints of the pipe for taking up the expansion. The flexible corrugated metal pipe is also less sensitive to any movement of the surrounding rock and / or sediment volume due to thermal expansion. The corrugated pipe also has an increased surface area as compared to a conventional pipe which improves the heat exchange. Such a flexible corrugated metal pipe can be provided in lengths which are sufficient for use in the heat storage without the need of welding or screwing pipe portions together.

[0021] According to a further embodiment of the arrangement, the insulated resistive heating cable forms a loop in said channel, wherein the insulated resistive heating cable extends into the channel along an outer surface of the first pipe portion and out from the channel along an outer surface of the second pipe portion, and preferably the insulated resistive heating cable extends along the outer surface of the first pipe portion on a first side of the U-pipe and along the outer surface of the second pipe portion on a second side of the U-pipe which second side is opposite to the first side. Hereby, the U-pipe protects the heating cable from being damaged by abrasion with the channel walls during installation. The U-pipe provides a support structure which makes it possible to install a small diameter heating cable which otherwise could be difficult to place close to the channel walls and without the cable crossing itself. The configuration allows more efficient and equal spread of heat from the heating cable to the surrounding along the entire channel length, due to the high thermal conductivity of the pipe that spreads the heat faster thanany filling material and due to the placement of the U-pipe which can abut against the channel wall.

[0022] According to a further embodiment of the arrangement, the insulated resistive heating cable forms a loop in said channel, wherein the insulated resistive heating cable extends into the channel along an outer surface of the outer pipe portion on a first side of the coaxial pipe and out from the channel along the outer surface of the outer pipe portion on a second side of the coaxial pipe, and preferably the second side of the coaxial pipe is opposite to the first side. Hereby, a relatively large diameter of the coaxial pipe allows a greater number of heating cables to be arranged in the same channel and thus a greater thermal heating capacity per meter of the channel can be achieved.

[0023] According to a further embodiment of the arrangement, the working fluid is a liquid having a boiling point above 120°C, preferably above 150°C, and more preferably above 200°C, at atmospheric pressure. By using a working fluid having a relatively high boiling point, the system can work with a liquid at high temperatures. This in turn makes it possible to both storing heat from high-temperature heat sources and providing thermal energy from the heat storage at high temperatures sufficient for production of district heating, process heating and / or heat or steam for electricity generation. At the same time, the system can work at atmospheric pressure without any pressurization of the working fluid.

[0024] According to a further embodiment of the arrangement, for at least a part of the length of said channel, said channel is filled with a filler material, and the filler material is refractory. This means the filler material can withstand temperatures up to at least 1600°C. The filler material can be either granular (e.g. quartz grains) or a solid material (e.g. calcium aluminate cement). A granular material allows some movement of the pipe and the electrical heating element, which may reduce stress introduced during thermal expansion and contraction cycles. The use of a solid material, such as calcium aluminate cement, will stabilize the channel and prevent movement in fractures of the rock resulting from thermal expansion and contraction of the rock.

[0025] The invention also relates to a method for storing thermal energy in the ground, comprising the steps of heating a volume of the ground forming a heat storage, by an electrical heating element, and transferring heat from the heat storage to an energy carrier for energy supply, by a circuit containing a working fluid, wherein the circuit has a pluralityof pipes which pipes are distributed throughout said volume of the ground, and each pipe is arranged in a channel in the ground for heat exchange between the working fluid and the surrounding ground, wherein the method further comprises heating the ground by the electrical heating element being arranged in one and the same said channel as one said pipe.

[0026] The invention is based on the insight that by such a method, electrical energy can be efficiently converted to heat energy by means of the electrical heating element, which heat energy in turn can be stored for long periods at high temperatures in the ground in an efficient way. By the invention, an electrical energy surplus can be directly converted into heat in the heat storage. Although, theoretically, the ground can be heated to temperatures up to the melting point of the current bedrock or sediment, as an example only, a typical temperature interval for the heat storage can be 120-600°C, but temperatures above 600°C and up to 1600°C can be used.

[0027] Hereby it is possible to both store heat from high-temperature heat sources and provide thermal energy from the heat storage at high temperatures sufficient for production of district heating, process heating and / or heat or steam for electricity generation.

[0028] Further, the pipe and the working fluid can be preheated by the electrical heating element. The electrical heating element can be used for providing heat both when heat is to be transferred to the heat storage and when heat is to be recovered from the heat storage. For example, when using a molten salt as working fluid, which requires temperatures around 200°C to behave as a low viscosity fluid, charging of the heat storage by the electrical heating element can be complemented by heat from the working fluid, and during recovery of heat from the heat storage the risk of freezing the molten salt can be reduced by providing heat from the electrical heating element.

[0029] In addition, when charging the heat storage by the electrical heating element, the ground can be heated by heating the pipe which in turn heats the surrounding ground, rather than heating the ground directly by the electrical heating element. A relatively high thermal conductivity and a large surface of the pipe will promote the heat exchange with the ground.According to one embodiment, the method comprises the step of using the working fluid being a liquid having a boiling point above 120°C, preferably above 150°C, and more preferably above 200°C, at atmospheric pressure. By using a working fluid having a relatively high boiling point, the system can work with a liquid at high temperatures. This in turn makes it possible to both storing heat from high-temperature heat sources and providing thermal energy from the heat storage at high temperatures sufficient for production of district heating, process heating and / or heat or steam for electricity generation. At the same time, the system can work at atmospheric pressure without any pressurization of the working fluid.

[0030] Further advantages and advantageous features of the invention are disclosed in the following description and in the claims.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0033] In the drawings:

[0034] Fig. 1 is a schematic view showing an arrangement for storing thermal energy,

[0035] Fig. 2A shows a cut view along A-A in Fig. 1 , illustrating a channel accommodating a pipe in the form of a U-pipe heat exchanger used in the arrangement,

[0036] Fig. 2B shows a variant of the pipe in Fig. 2A designed as a coaxial pipe heat exchanger,

[0037] Fig. 2C shows a further variant of the pipe in Fig. 2A, wherein the pipe constitutes an electrical heating element,

[0038] Fig. 3 is a cut view along B-B in Fig. 1, illustrating electrical heating elements, and pipes which are fluidly connected to each other in series, wherein each electrical heating element and each pipe is arranged in the respective channel, andFig. 4 is a schematic flow chart for illustration of an embodiment of a method.

[0039] DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0040] Fig. 1 is a schematic view showing an arrangement 1 for storing thermal energy. The arrangement 1 is illustrated in a top view looking at the ground from above.

[0041] The arrangement 1 is a so called underground thermal energy storage. The arrangement 1 comprises a circuit 2 containing a working fluid. The working fluid is circulated in the circuit 2. The circuit 2 is at least partly arranged in the ground, preferably most of or substantially the entire circuit 2 is arranged underground, though for illustration purposes the circuit 2 is shown in Fig. 1. The heat storage utilizes the soil, sediment and / or bedrock, which is found naturally in the ground below the surface of the earth, for storing heat within the ground which surrounds the circuit.

[0042] The circuit 2 has a plurality of pipes 3 which pipes 3 are distributed throughout a volume of the ground forming a heat storage. In Fig. 1, such a pipe 3 is schematically indicated by two small circles. Each pipe 3 is arranged in a channel 4 in the ground for heat exchange between the working fluid and the surrounding ground. Such a pipe 3 arranged in such a channel 4 forms a working fluid collector well. Each channel 4 can be a borehole, preferably a substantially vertical borehole, drilled in the ground for receiving the vertically arranged pipe 3.

[0043] In the example embodiment illustrated in Fig. 1, only one circuit 2 comprising some of the pipes 3 is shown. However, depending on the current application, the number of such circuits used in the arrangement and the number of pipes comprised in the circuit can be varied.

[0044] The working fluid can flow in the pipe 3 of the circuit 2 from a position close to the ground surface, to the bottom of the channel 4 and back up to a position close to the ground surface. In general, there are two main options of creating the desired circuit 2 for the pipe configuration used. According to a first option, only the pipes which are included in a predetermined circuit where the working fluid is to be circulated are fluidly connected to each other, such as in series or in parallel to each other. According to another option,more pipes, or all pipes of the arrangement, are fluidly connected to each other and by means of valves and control equipment the flow can be open or closed between the pipes. Thus, the pipes to be included in a certain circuit can be selected by controlling the valves such that the working fluid is circulated through these selected pipes, which pipes can be connected in series or in parallel to each other. Both these options can of course be combined and used in the same heat storage.

[0045] Further, in some applications, the channels are not vertically arranged. For example, by using a tunnel, shaft or rock shelter, the channels can be drilled in various directions in the ground to obtain the heat storge. The pipes and the electrical heating elements can be installed in channels with any orientation. This is important since it allows installation around tunnels and shafts. The pipes and the electrical heating elements can also be installed in curved channels that could help to reduce the surface footprint if channels are all drilled from substantially the same position at the surface and curved to distribute the channels over a larger volume underground.

[0046] In the example embodiment illustrated in Fig. 1 , the volume of the ground forming the heat storage is defined by the surface area 5 over which the pipes 3 are distributed and by the length of these pipes 3, where the depth of each channel 4 accommodating such a pipe 3 is suitably the same as the length of the pipe 3 or slightly longer.

[0047] The arrangement 1 further comprises an electrical heating element 6. One said pipe 3 and the electrical heating element 6 are arranged in one and the same said channel 4. At least one electrical heating element 6 is included in the arrangement 1 but often a plurality, and preferably a lot of electrical heating elements 6 are used.

[0048] The arrangement 1 may comprise a plurality of said electrical heating element, which electrical heating elements 6 are distributed throughout the volume of the ground forming the heat storage. The number of electrical heating elements is suitably adapted to the size of the heat storage and the number and size of the pipes arranged in the channels in the heat storage. The number and power of the electrical heating elements are also suitably adapted to the working fluid used and the current bedrock or sediment of the ground.

[0049] For example, the electrical heating elements can be distributed within an inner zone in the centre of the heat storage or each electrical heating element is arranged in a local centrepoint or zone of the heat storage. In a case where only one or a few electrical heating elements are used, the electrical heating element is suitably arranged in a central position of the heat storage.

[0050] In the example embodiment illustrated in Fig. 1 , the number of electrical heating elements 6 is the same as the number of pipes 3. One electrical heating element 6 and one said pipe 3 are arranged together in one said channel 4 for each of the channels 4. Optionally, however, a plurality of electrical heating elements can be arranged in one and the same said channel.

[0051] The electrical heating element 6 is shown in the enlarged view of one pipe in Fig. 1. The electrical heating element 6 is arranged in the channel 4 in the ground to heat the ground, thereby storing thermal energy in the heat storage. Such an electrical heating element 6 arranged in such a channel 4 forms an electrical heating well.

[0052] The number of pipes 3 can be 10-10000, preferably 100-5000, and often the number of pipes is in the range 500-3000. Different types of pipe 3 can be used. Some examples are shown more in detail in Figs. 2A and 2B where cut views of a channel accommodating a pipe, an electrical heating element and the surrounding ground are illustrated.

[0053] The size of the channel 4 is suitably in the range 30-250 mm, preferably 50-200, and often 100-150 mm in diameter. The diameter of the pipe 3 can be varied depending on the application and the size of the channel. For example, the diameter of the pipe can be in the interval 30-80 mm.

[0054] The distance between the channels 4 can be selected based on the power of the electrical heating element 6, the temperature gradient in the bedrock, the allowed temperature of the working fluid, etc.

[0055] For example, the distance between two adjacent channels (pipes) can be in the range 0.5-15 meters, preferably 1-10 meters, and often the distance is in the range 2-5 meters.

[0056] The length of the pipes 3 can be 1-1000 meters, preferably 10-500 meters, and often 50-300 meters.As an example only, this means that the heat storage can have a size that covers a ground surface area in the size of 10 000 m2and that the approximate volume of ground forming the heat storage can be in the size of 1 500000 m3.

[0057] As also illustrated in Figs. 2A and 2B, the electrical heating element 6 can extend along said pipe 3. Preferably, the electrical heating element is part of the pipe 3 or the electrical heating element 6 is attached to the pipe 3. The electrical heating element 6 can be a resistive heater converting electrical energy into heat. The electrical heating element 6 is suitably an insulated resistive heating cable, preferably a mineral insulated resistive heating cable. Each electrical heating element 6 is connected to a power source (not shown) for generating the heat.

[0058] Fig. 2A shows a pipe 3 that is a U-pipe forming a U-pipe heat exchanger. The channel 4 is a vertical borehole in the ground. The U-pipe is part of the circuit 2 such that the working fluid can be circulated in the U-pipe. At the same time, heat can be exchanged between the working fluid and the surrounding ground. The U-pipe has a first pipe portion 7 and a second pipe portion 8. The first pipe portion 7 and the second pipe portion 8 extend in the longitudinal direction of the channel 4 such that the working fluid can be circulated while flowing in the first pipe portion 7 in a direction 9 into said channel 4 and in the second pipe portion 8 in a direction 10 out from said channel 4. Of course, the arrangement 1 can be designed such that the flow direction of the working fluid can be reversed for any pipe configuration. The pipe is suitably a flexible corrugated metal pipe. The pipe 3 can be made of stainless steel for instance. When using a flexible corrugated metal pipe, a minor part of the pipe, such as the U-part connecting the first pipe portion and the second pipe portion, can be made from a non-corrugated pipe portion, i.e. from a usual solid pipe portion.

[0059] The insulated resistive heating cable 6 forms a loop in said channel 4. The insulated resistive heating cable 6 can be attached to the pipe by cable ties for instance. The insulated resistive heating cable 6 extends along an outer surface of the first pipe portion 7 on a first side of the U-pipe and along an outer surface of the second pipe portion 8 on a second side of the U-pipe, which second side is opposite to the first side.

[0060] The arrangement 1 has suitably spacers 11 arranged between the first pipe portion 7 and the second pipe portion 8. The spacer 11 is arranged to keep the first pipe portion 7 andthe second pipe portion 8 apart from each other. The spacer 11 can also be arranged to position the first pipe portion and the second pipe portion adjacent to or in contact with an inner surface of the channel 4, i.e. the first pipe portion 7 and the second pipe portion 8 may abut against the inner surface of the channel 4.

[0061] In Fig. 2B, a variant of the pipe 3 is illustrated. The pipe 3 is a coaxial pipe forming a coaxial pipe heat exchanger. The coaxial pipe 3 has an inner pipe portion 12 and an outer pipe portion 13. The inner pipe portion 12 and the outer pipe portion 13 extend in the longitudinal direction of said channel 4 such that the working fluid can be circulated while flowing in the outer pipe portion 13 (outside the inner pipe portion 12) in a direction 9 into said channel 4 and in the inner pipe portion 12 in a direction 10 out from said channel 4. In the same way as described with reference to Fig. 2A, also this pipe 3 is suitably a flexible corrugated metal pipe. The pipe 3 can be made of stainless steel for instance. The direction of flow can be reversed in that the fluid flows downwards in the inner pipe portion and upwards in the outer pipe portion if heat is to be transferred from the working fluid to the heat storage.

[0062] The insulated resistive heating cable 6 may form a loop in said channel 4. Here, the insulated resistive heating cable 6 extends along an outer surface of the outer pipe portion 13 on a first side of the coaxial pipe and along the outer surface of the outer pipe portion 13 on a second side of the coaxial pipe, which second side is opposite to the first side.

[0063] As an option, it is also possible to install an insulated resistive heating cable with dual cores which cores are connected at the bottom of the channel so that the current goes up and down in the same heating cable. In other words, the loop could be arranged internally in a dual core heating cable. For example, when using a U-pipe shown in Fig. 2A, one such dual core heating cable can be arranged on each of the first pipe portion and the second pipe portion.

[0064] According to a variant of the arrangement 1, shown in Fig. 2C, at least a part of the electrical heating element 6’ is constituted by said pipe 3’. In such a case, the pipe 3’ also acts as the electrical heating element 6’ instead of installing a further component, such as a separate electrical heating element, in the channel 4. Such a pipe 3’ can be designed and adapted for impedance heating. During such heating a power source 20 is connected to the pipe 3’, suitably via cable lugs attached to terminal plates 21 of the pipe 3’. Forexample, a power transformer can produce a low voltage AC, in the range 1-30 Volt for instance, applied to the pipe 3’ for generating the heat to be emitted and stored in the surrounding ground, i.e. in the heat storage.

[0065] In all embodiment described herein, the working fluid can be a liquid or gas. When using a liquid, the working fluid in the circuit suitably has a working temperature range from a lowest allowable temperature to a highest allowable temperature for which working temperature range the working fluid is a liquid at atmospheric pressure. By atmospheric pressure means the standard sea-level pressure, one standard atmosphere, equals 760 mmHg.

[0066] The working fluid is suitably a liquid having a boiling point above at least 120°C, suitably above 150°C, and preferably above 200°C, at atmospheric pressure. For many applications, the boiling point of the working fluid can be even higher, above 300°C. Favourable, the boiling temperature of the working fluid is within the interval 200-600°C, and often 300-600°C. For this reason, the working fluid can be an oil or a compound or element in a liquid state, such as a molten salt. Some examples of usable working fluids are vegetable oils, such as refined rapeseed oil, paraffin oils, such as Duratherm HTO and Duratherm HF, silicon oils, such as Duratherm S, and molten salts, particularly ternary mixtures of salt, such as Solar Power Molten Salt.

[0067] A lot of the above-mentioned liquids are also favourable due to the fact that they can work at a relatively low temperatures, where the lowest allowable temperature (pour point) often is within the interval minus 50°C to minus 5°C. This is advantageous due to decreased risk of clogging and freezing when the system is cold. Pumping of the liquid can be performed at low temperatures since the liquid will maintain a relatively low viscosity at low temperatures.

[0068] For high temperature applications where the temperature is in the range 600-1600°C, the working fluid can be liquid metals, liquid glass, hot air or other gases, such as off-gases from industrial processes.

[0069] Fig. 3 is a cut view along B-B in Fig. 1. This view illustrates five pipes 3 which are fluidly connected to each other in series. For illustration purposes, the distances between the pipes 3 have been heavily shortened. Each pipe 3 is arranged in a channel 4. Theworking fluid is circulated in the circuit 2 to the pipes 3 and back again for heat exchanging between the working fluid and the surrounding ground (heat storage) when the working fluid flows in the vertical pipes 3. As illustrated, the pipes 3 together with the electrical heating elements 6 are arranged in the ground. As previously mentioned hereinabove, each electrical heating element 6 is connected to a power source (not shown) for generating the heat. The channels 4 accommodating the pipes 3 are suitably formed by drilling, but may be achieved by any other equipment enabling the pipe 3 to be arranged in the ground.

[0070] Thus, the pipes 3 are arranged in the bedrock or sediment. In case the channels 4 are not completely filled by the pipes 3, for at least a part of the length of the channels, the channels are filled with a filler material 14. This can be a high-conductive filler material for filling the channels 4 and creating the heat conduction properties that are required. The high-conductive material can be for example quartz sand or magnetite sand. In more high temperature applications, the filler material 14 is suitably any kind of calcium aluminate cement.

[0071] In the upper part 15 of each channel, a low-conductive material is however suitably used for lowering the losses to the ground closest to the surface of the earth. The low-conductive material can be for example extruded clay or foam glass. Immediately above the pipes 3 and the bedrock there is a layer of an insulating material 16, such as foam glass or expanded clay. Portions of the circuit 2 that connect the different pipes 3 to each other, are suitably arranged in this insulating layer 16. On top of the insulating layer 16 a filling material 17, such as soil or gravel, can be arranged.

[0072] The arrangement 1 preferably comprises a heat exchanger 18 (see Fig. 1) for transferring heat between the working fluid of the circuit 2 and an energy carrier. The circuit 2 is fluidly connected to the heat exchanger 18 for energy supply from the heat storage. Further, the arrangement 1 suitably comprises additional equipment, such as a pump 19, for circulating the working fluid in the circuit 2. In the example embodiment illustrated in Fig.

[0073] 1, one circuit 2 connected to the heat exchanger 18 is shown. The heat exchanger 18 can be arranged to transfer heat from the working fluid to the energy carrier for energy supply, such as production of district heating, process heating and / or heat or steam for electricity generation.Fig. 4 is a schematic flow chart of an example embodiment of a method. As it regards the arrangement and the components thereof used in the method, see previous description and figures mentioned hereinabove.

[0074] The method for storing thermal energy in the ground comprises the step of heating 100 a volume of the ground forming a heat storage, by an electrical heating element 6.

[0075] The method further comprises the step of transferring heat 200 from the heat storage to an energy carrier for energy supply 300, by a circuit 2 containing a working fluid. As previously described herein, the circuit has a plurality of pipes 3 which pipes are distributed throughout said volume of the ground, wherein each pipe 3 is arranged in a channel 4 in the ground for heat exchange between the working fluid and the surrounding ground. The working fluid used is suitably a liquid having a boiling point above 120°C, preferably above 150°C, and more preferably above 200°C, at atmospheric pressure.

[0076] The method further comprises heating 100 the ground by the electrical heating element 6 being arranged in one and the same said channel 4 as one said pipe 3. As previously described herein, the electrical heating element 6 is arranged together with the pipe 3 in the channel 4 in the ground to heat the ground for storing thermal energy in the heat storage.

[0077] According to a further embodiment of the method, before transferring heat from the heat storage to the energy carrier, in addition to the heating by the electrical heating element 6, the method comprises the step of transferring heat 400 from an external energy source to the working fluid of the circuit, for storing thermal energy in the heat storage, wherein the temperature of the working fluid being increased to above 120°C, suitably above 150°C, and preferably above 200°C by the external heat source. For example, heat can be transferred from an energy carrier to the working fluid by means of a heat exchanger. Such a heat exchanger used for transferring heat from the external energy source to the heat storage can be the same as the heat exchanger 18 used for transferring heat from the heat storage for energy supply, wherein the working fluid flow direction in the circuit has been reversed.

[0078] It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognizethat many changes and modifications may be made within the scope of the appended claims.

Claims

CLAIMS1. An arrangement (1) for storing thermal energy in the ground, the arrangement comprising a circuit (2) containing a working fluid, the circuit having a plurality of pipes (3) which pipes are distributed throughout a volume of the ground forming a heat storage, each pipe being arranged in a channel (4) in the ground for heat exchange between the working fluid and the surrounding ground, the arrangement (1) further comprising an electrical heating element (6), characterized in that one said pipe (3) and the electrical heating element (6) are arranged in one and the same said channel (4).

2. An arrangement according to claim 1, characterized in that the electrical heating element (6) extends along said pipe (3).

3. An arrangement according to claim 1 or 2, characterized in that the electrical heating element (6) is attached to said pipe (3).

4. An arrangement according to any of claims 1-3, characterized in that the electrical heating element (6) is an insulated resistive heating cable.

5. An arrangement according to any of claims 1-3, characterized in that the electrical heating element (6) is a part of said pipe.

6. An arrangement according to claim 1 or 2, characterized in that at least a part of the electrical heating element (6’) is constituted by said pipe (3’).

7. An arrangement according to any preceding claim, characterized in that said pipe (3) is a U-pipe having a first pipe portion (7) and a second pipe portion (8), the first pipe portion and the second pipe portion extending in the longitudinal direction of said channel (4) such that the working fluid can be circulated while flowing in the first pipe portion in a direction into said channel and in the second pipe portion in a direction out from said channel.

8. An arrangement according to claims 1-6, characterized in that said pipe (3) is a coaxial pipe having an inner pipe portion (12) and an outer pipe portion (13), the inner pipe portion and the outer pipe portion extending in the longitudinal direction of said channel (4) such that the working fluid can be circulated while flowing in the outer pipe portion in a direction into said channel and in the inner pipe portion in a direction out from said channel.

9. An arrangement according to any preceding claim, characterized in that said pipe (3) is a flexible corrugated metal pipe.

10. An arrangement according to claims 4 and 7, characterized in that the insulated resistive heating cable (6) forms a loop in said channel (4), wherein the insulated resistive heating cable extends into the channel along an outer surface of the first pipe portion (7) and out from the channel along an outer surface of the second pipe portion (8).

11. An arrangement according to claim 4 and 8, characterized in that the insulated resistive heating cable (6) forms a loop in said channel (4), wherein the insulated resistive heating cable extends into the channel along an outer surface of the outer pipe portion (13) on a first side of the coaxial pipe and out from the channel along the outer surface of the outer pipe portion on a second side of the coaxial pipe.

12. An arrangement according to any preceding claim, characterized in that the working fluid is a liquid having a boiling point above 120°C, preferably above 150°C, and more preferably above 200°C, at atmospheric pressure.

13. An arrangement according to any preceding claim, characterized in that for at least a part of the length of said channel (4), said channel is filled with a filler material, the filler material being refractory.

14. A method for storing thermal energy in the ground, comprising the steps of heating (100) a volume of the ground forming a heat storage, by an electrical heating element (6), and transferring heat (200) from the heat storage to an energy carrier for energy supply, by a circuit (2) containing a working fluid,the circuit having a plurality of pipes (3) which pipes are distributed throughout said volume of the ground, each pipe being arranged in a channel (4) in the ground for heat exchange between the working fluid and the surrounding ground, characterized by heating (100) the ground by the electrical heating element (6) being arranged in one and the same said channel (4) as one said pipe (3).

15. A method according to claim 14, characterized by using the working fluid being a liquid having a boiling point above 120°C, preferably above 150°C, and more preferably above 200°C, at atmospheric pressure.

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