Electric heating apparatus

Abstract

An electric heating apparatus 10 comprises a thermal energy storage with a core 14 of an operating temperature from 100 to 950 °C and comprising phase change material (PCM) 6, ideally a composite PCM

Description

An electric heating apparatus is disclosed. More particularly, an electric heating apparatus comprising a thermal energy storage configured for heating a space such as one or more rooms in a building or a group of buildings is disclosed. Electric heaters comprising a thermal energy storage are commonly referred to as electric storage heaters. The electric storage heaters are designed to store thermal energy by charging a thermal energy storage medium with thermal energy by heating the medium. The charging can be provided using electric heater elements. Subsequently the stored thermal energy is progressively discharged to heat a space such as a room, office, etc. Electric storage heaters are typically thermally charged when the unit cost of electricity is relatively low, for example at night when electricity demand is reduced, and / or when electricity is readily and / or economically available, e.g., from solar power, wind power and so on. The thermal discharge, which does not require significant consumption of electricity, can occur during the subsequent daytime period or another appropriate period so that the electricity can be used in efficient and optimised manner. Conventional electric storage heaters use bricks composed of a refractory material to store thermal energy. These conventional electric storage heaters can have high manufacturing cost for a given thermal storage capacity. The energy storage density, expressed in Watt-hours per kilogram, is low, for example typically lower than 130 Wh / kg. A typical known commercial domestic storage heater with a storage capacity of about 10 kWh has a weight of at least 80 kg. Such a high weight increases the cost of manufacturing, transportation and delivery, and increases installation costs, and can also restrict locations where the storage heater can be installed. It is also known to persons skilled in the art of thermal storage to use a phase change material (PCM) as a thermal energy storage medium. For example, US 10203165B discloses a climatic apparatus for storing thermal energy using a phase change material on a metal wool support where a latent heat storage uses a material which, under the effect of heating or cooling, undergoes a phase transition, said phase transition taking place with the absorption, on heating, or the restoration, on cooling, of a latent heat of transition. The PCM used is one of an aqueous solution, an alkane, a polyol, or a salt. The PCM may contain nanoparticles and microparticles. The temperature range of the material is from -35 °C to 200 °C. There exists a need in the art for an electric storage heating apparatus which can exhibit enhanced performance without compromising safety and efficiency, in particular for storage heaters used in domestic residences, office premises and the like. The present invention solves one or more of the issues in the art by providing an electric heating apparatus comprising a high temperature thermal energy storage and an integrated heat exchanger for controllably transferring thermal energy from a high temperature fluid in a first fluid circulation loop into a second fluid that can be circulated in a second fluid circulation loop for use in heating at least one space such as a room in a building. For example, a central heating system for heating a domestic house or a flat, an office space or the like can be provided. Accordingly, in one aspect there is provided an electric heating apparatus according to claim 1. Optional or preferred features are defined in dependent claims 2 to 25. hi other aspects there is also provided a central heating system for heating at least one space according to claim 26 or 27. In yet another aspect there is also provided method of heating at least one space according to any of claims 28 to 31. A computer program product may also be provided for controlling the operation of at least a part of the herein described electric heating apparatus and for use in implementing the claimed method. The computer program can comprise artificial intelligence I machine learning algorithm(s) to assist in optimisation and control of the system. Various examples how of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the principal structure and function of an electric heating apparatus for heating a building in accordance with an example; Figures 2 is a schematic front-side perspective view, partly in phantom, of a core and surrounding part of a housing suitable for use in the Figure 1 apparatus; Figure 3 is a schematic end view, partly in phantom, of the core and housing illustrated in Figure 2; Figure 4 is a schematic contour map of the temperature of a block of phase change material of Figures 2 and 3 after thermal charging, the core being composed of an assembly of such blocks; Figure 5 is a schematic diagram of the principal structure and function of an electric heating apparatus for heating a building in accordance with another example; Figure 6 is a P-H diagram for operation in accordance with Figure 5 example; Figure 7 is a schematic diagram of the principal structure and function of an electric heating apparatus for heating a building in accordance with yet another example; Figure 8 is a flow chart in accordance with a method in accordance with the invention; and Figure 9 is a graph showing the relationship between temperature and time, for both the phase change material and the electric heater elements, during thermal charging of an electric heater in accordance with an example. Examples of integrated high temperature phase change material (PCM) based central electric heating apparatuses are described next with reference to the Figures. It is noted that in the presentations are schematic and in most real life implementations the heating units 10, 50 and 70 would be located in the one or more buildings 24 to be heated. It is also noted that the following description gives an exemplifying description of some possibilities to practise the invention. Although the specification may refer to “an”, “one”, or “some” examples or embodiment(s) in several locations of the text, this does not necessarily mean that each reference is made to the same example of embodiment) s), or that a particular feature only applies to a single example or embodiment. Single features of different examples and embodiments may also be combined to provide other embodiments. The herein disclosed integrated PCM based heating units can be used in place of a conventional boiler unit to heat at least one space such as one or more rooms in one or more buildings 24. The phase change material is provided in a core of a thermal energy storage heater part of the unit. The phase change material suitable for the core has a composition which absorbs or releases heat isothermally, or substantially isothermally, by transitioning, in a respective transition direction, between a first phase state and a second phase state at a predetermined transition temperature. The temperature can change substantially when temperature is different from the transition temperature and the phase change material exhibits behaviour of a sensible heat storage material. For example, the first phase state is a solid state and the second phase state is a liquid state, and the predetermined transition temperature is the melting temperature, Tm, of the phase change material. A suitable phase change material can have a latent heat of from 100 to 800 kJ / kg, typically from 120 to 300 kJ / kg for the transition between the first and second phase states at the predetermined transition temperature. The predetermined transition temperature of the phase change material can be within the range from 100 to 950 °C. In accordance with a preferred examples the temperature range is from 200 to 850 °C. PCM of this range can provide advantage through reduction of the weight of the system. In accordance with specific examples the range of predetermined transition temperature of the phase change material is from 200 to 850 °C, 300 - 700 °C, 300 - 500 °C, or 400 to 600 °C. In accordance with yet another specific application the range can be from 450 to 550 °C. In accordance with a particular example illustrated in the graph of Figure 9 the transition temperature is in the order of 500 °C. It is noted that different phase change materials or composite phase change materials can have different operating temperature ranges. Also, in view of the lower bounds of the temperature ranges of PCM materials it shall be appreciated that all materials can be used between the room temperature and their maximum operating temperature whereas the ranges given for the material can denote the effective operational range thereof. The core 14 can have an energy storage density from 140 to 300 Wh / kg including sensible heat, for example from 160 to 250 Wh / kg including sensible heat. In accordance with an example the energy capacity can be from 2 to 50 kWh. For larger buildings and multibuilding systems sharing one storage unit the upper limit can be greater, tens or even hundreds of MWhs. For example, the capacity for such a unit can be as high as 500 MWh, 700 MWh or even higher. In one non-limiting example, the phase change material (PCM) comprises at least one inorganic salt or a mixture of a plurality of inorganic salts. The inorganic materials, typically alkali metal salts, may be selected from the group consisting of nitrates (e.g. NaNOa, KNO3 and LiNO3), nitrites (e.g. NaNO2 and KNO2), carbonates (e.g. Na2CO3, LiaCOa, K2CO3), chlorides (e.g. KC1, NaCl), bromides (e.g. KBr, LiBr, NaBr, Li2Br), fluorides (e.g. LiF, KF, NaF), sulphates (e.g. Na2SO4 and K2SO4), and hydroxides (e.g. NaOH, KOH, LiOH). When a mixture of PCMs is provided, for example when the PCMs are inorganic materials, the phase change material may comprise a binary, ternary or quaternary eutectic mixture of individual phase change material components. Figure 1 which shows a first schematic example of an integrated heating unit 10 comprising an electric storage heater part 13 and a gas to liquid heat exchanger part 20. The electric storage heater part 13 comprises a storage core 14 containing phase change material (PCM) 6. Other main components of Figure 1 shown schematically comprise housing 11 and insulation 12 encompassing the components of the electric storage heater part 13 and a gaseous fluid to liquid heat exchanger part 20. The heat exchanger part 20 comprises at least one heat exchanger 21. The housing and insulation can be provided by any appropriate elements, materials, and structures. Heater elements are also provided in the core. Preferably, heater elements are in direct contact with, or are immersed in, the phase change material of the core 14, which enhances heat transfer rate between the heater elements 40 (shown in Figure 3) and the phase change material and hence provides better charging process response and control. In Figure 1 the housing surrounds the core 14 and defines a cavity within which the core 14 is disposed. In this example the housing 11 includes integrated thermally insulating material 12 which at least partly surrounds the phase change material 6. The housing 11 can define a system of air channels connecting the core 14 and the heat exchanger 21 thereby allowing for circulation of fluid in the first loop 15 as indicated by the arrows in Figure 1. The first loop 15 has paths or channels 36 extending through the core 14 between manifolds 28, 29 defined by the housing at either side of the core 14. That is, the housing defines an input manifold 28, on an input side of the core 14, which receives fluid coming from the heat exchanger 21, and an output manifold 29, on an output side of the core 14, from which the fluid is guided towards the heat exchanger. As shown in Figure 1, the plurality of channels 36 extend through the core 14 between the input and output manifolds 28, 29 to form a thermal energy receiving part of the first loop 15. The shape and positioning of the channels and manifolds can differ from that shown in the example. The housing can also define a bypass conduit 17. In Figure 1 example gaseous first fluid is recirculated in a first fluid circulation loop 15 within the integrated heating unit 10 to transfer thermal energy from the storage core 14 to a second fluid circulated in a second fluid circulation loop 22. The first fluid can comprise gas such as air which will be used as an example of suitable gases in the following. Air is preferred for reasons safety, environmental friendliness and being free and readily available but other gaseous fluids such as nitrogen, helium or CO2, or their mixtures are also possible. The second loop is a part of a central heating system of a building 24. The second fluid circulated in the second loop is for transferring heat to one or more spaces of the building 24 by one or more radiators 23 in a per se known manner. The second fluid may comprise water, a suitable mixture of water and chemicals such as inhibitors or any other fluid suitable for circulation in a central heating system. Heat transfer occurs from the high temperature gaseous fluid such as air in the first loop 15 to fluid such as water in the second loop through the gas-liquid heat exchanger 21. The temperature of the fluid circulated in the second loop 22 is maintained on a lower level than is the temperature of air in the first loop 15. By means of this the high temperature part of the operation is contained within the integrated and insulated heating unit 10. Despite the high temperatures involved in the core 14 and the first loop 15 good controllability and safe operation can be achieved through separation of the two circulation systems. Control of the flow and temperature of the air in the first loop 15 within the integrated storage heater unit 10 is provided, at least in part, by a control valve 18 associated with the bypass conduit 17. The valve can be operated by an appropriate actuator connected via a suitable connection 34 to a control unit 32. Operation of the valve 18 can be used to control the amount of air circulated via the channels 36 in the core 14. The bypass conduit 17 connects a first junction 30 and a second junction 31 of the first loop 15 to define an air path external of the core 14. The valve 18 can be used to control the mixing at the second junction 31 of heated air that has passed the core with cooler air from the heat exchanger stage which has been allowed to circulate via the bypass, as shown by the arrows. The control system can further comprise fan or blower 19 circulating air in the first loop 15 towards the heat exchanger 21 and then towards the core 14 and the bypass conduit 17. Operation of the blower 19 can also be controlled by the control unit 32. Opening the valve 18 allows more air pass through the bypass 17 and consequently less air moves towards and through the core 14, and vice versa. Dimensioning of the two available air flow paths between junctions 30 and 31 can be such that opening of the valve 18 to a fully open position allows most, or substantially most of air flow via he the bypass conduit 17. The air flow can also be controlled by controlling the operation of the blower. By means of controllably circulating and mixing the air the temperature of air returning to the heat exchanger can be lowered, maintained, or raised. The control unit 32 can comprises comprise any appropriate data processing computing apparatus configured to implement at least some of the herein described control features. The data processing apparatus can comprise at least one processor, at least one memory and other internal circuitry and components necessary to perform the tasks. The at least one processor may be configured to execute an appropriate software code to implement one or more of the herein described aspects. The software code may be stored in the at least one memory. The controller may be provided with appropriate interface for connection to a data network, for example the Internet and / or Intranet. The at least one memory may comprise at least one ROM and / or at least one RAM. The controller unit 32 may comprise other possible components for use in software and hardware aided execution of tasks it is designed to perform. A computer programme at the control unit for controlling the operation of the system and / or a computer programme for generating controlling software code for the control unit can comprise artificial intelligence I machine learning algorithm(s) to assist in optimisation and control of the system. Training of the algorithm can be based on use data from the system and / or data collected from a plurality of heating systems. In the example of Figure 1 manifolds 28, 29 are schematically shown to have substantially constant depth profiles. Various alternative manifold designs are possible. The contour and dimensioning of the manifolds can be designed to optimise the air flow distribution into and through the channels 36 of the core. For example, the input side manifold 28 can progressively decrease in depth from the junction 30 side end to second end of the manifold 28. In contrast, the output manifold 29 may have a constant depth. In a further alternative the input manifold progressively decreases in depth and the output manifold progressively increases in depth in a direction of flow of air. The control can be based at least in part on feedback 26 from various sources associated with the second loop, for example one or more temperature sensors 25 in the building 24. By controlling the rate of air flow along the bypass conduit 17 the ratio of the flow rate of air along the air paths via the core 14 and the bypass 17 the heat output at the house 24 can be variably controlled in response. Additional control can be provided by the control of the speed of air flow and pressure of air circulated in the first loop 15 through control of the blower 19. In addition, the fluid flow in the second loop 22 can be used for the control. In the example of Fig. 1 operation of pump 27 and / or valve 37 arranged on the second loop 22 can also be controlled by the controller 32. The control system provides a simple and reliable control on the amount of thermal energy transferred from the first loop 15 to the fluid in the second loop 22 and therefrom to the building 24. The location of the blower 19 can be different from that shown. For example, the blower can be located after the heat exchanger and before the bypass conduit 17 connecting the first and second bypass junctions 30, 31. Also, in the illustrated example, the bypass junctions 30, 31 are located at lower corners of the electric storage heater element. However, in alternative embodiments, the junctions and the bypass channel may be located at an alternative position, for example at a side edge of the electric storage heater or at the side or below the heat exchanger. Different valve mechanisms can be used to control the ratio of air flows through the bypass conduit 17 and the core 14. Examples comprise gate valves and ball valves. The air flow control valve mechanism can be positioned differently from that shown in Figure 1. For example, an air flow valve can be positioned at, or close to, the first junction 30 or the second junction 31. More than one valve can be provided to control flow of air in the first loop 15. The opening of the air flow valve can be controlled between fully open and fully closed based on control signal from controller 32. The controller 32 can be configured to control the amount of thermal energy transferred to the second fluid by controlling the opening of the valve via control connection 34. The controller may also control operation of the blower 19 via control connection 33. The determination of appropriate control instructions for the valve 18 and / or blower 19 can be based at least in part on temperature information receiving through connection 25 from the building 24 to be heated. The information may be provided by one or more appropriate temperature sensors 25 arranged to measure the temperature in spaces(s) heated by radiator(s) 23. Other information, for example information associated with optimum and / or safe operating temperatures and / or pressures in the first loop and / or second loop and / or outside temperature information, temperature and / or pressure difference information and so on may also be taken in account in the control. Temperature sensors may be provided anywhere in the heating system to measure temperatures at relevant locations. For example, temperature of the air in any part of the first loop 15 may be monitored and used as a basis of the control. An example for the thermal energy storage part of the integrated electric heating unit 10 comprises a core comprising composite phase change material (CPCM). Composite phase change materials can exhibit a good cost-performance ratio. In particular CPCM can provide a core that exhibits highly efficient thermal storage, with good thermal conductivity, and good physical and chemical stability. Composite phase change material can also be manufactured in a cost-effective manner. The CPCM can be formed into a desired shape for enclosure withing the unit and for accommodating the fluid channels and electric heater elements. A composite phase change material (CPCM) can comprise an inorganic material as a phase change composition, a structural material for structurally shape-stabilising the phase change material, and a heat transfer enhancement material dispersed in the phase change material. The structural material of CPCM can comprise, for example, an alkaline earth metal oxide, such as MgO. Examples of the heat transfer enhancement material include graphite, carbides, metals, or metal oxides or a mixture of any two or more thereof. Figures 2 to 4 show an example where the air channels 36 form an array of parallel air channels. The air channels 36 can be horizontally oriented and mutually spaced in a height direction of the core 14 formed from CPCM. The air channels 36 can be centrally located across a width direction of the core 14. At least some, and in the illustrated example each, of the air channels 36 are located between, and laterally spaced from, a plurality of heater bores 38 which are located, in the width direction (of the core 14, on respective opposite sides of the air channel 36 and are spaced along the height direction of the core 14. Figure 3 shows how the air channels 36 are centrally located between, and laterally spaced from, first and second pairs 38a, 38b; 38c, 38d of the heater bores. The first and second pairs 38a, 38b; 38c, 38d of the heater bores are spaced from each other in the height direction (H) of the core 14 and in each of the first and second pairs 38a, 38b; 38c, 38d the heater bores are spaced from each other in the width direction (W) of the core 14 and are located on respective opposite lateral sides 5a, 5b of the core 14. Consequently, in this example each air channel 36 is located at a geometric centre of a quadrilateral polygon having a shape and dimensions defined the positions of four heater bores 38. However, other configurations for positioning the air channels relative to the heater bores may be employed. Figure 3 also shows an electric heater element 40 that is received with and extends along each respective heater bore 38, so that a plurality of the electric heater elements 40 are positioned within the core 14. The heater elements heat the core 14 to charge the core with stored thermal energy by raising the temperature as well as transitioning the phase change material from the first phase state to the second phase state. It shall be appreciated that heat can be stored in sensible and latent forms. If the core is at a temperature below the PCM melting point, only sensible heat is stored, and the core is partially charged. The core is fully charged if the temperature is above the melting temperature of the PCM, with both sensible and latent heat. For clarity of illustration, a pair of opposed electric heater elements 40 are shown in one pair of heater bores 38 in Figure 3, but as described above each heater bore 38 can receive a respective electric heater element 40. As shown in Figure 2 the core 14 can be formed as a continuous body 42 with only the air channels 36 and heater bores 38 extending through the continuous body 42. The continuous body 42 can be assembled from a plurality of blocks of phase change material 6. Since the phase change material 6 transitions from one phase (often solid phase) to another phase (either solid or liquid phase) during thermal charging. For solid-liquid phase transition, each block comprises a body of the phase change material 6 within an outer case which seals the phase change material 6 within the block to avoid leakage of the phase change material 6 during successive thermal charging / thermal discharging cycles. When CPCM is used, the outer casing can be used. This, however, is not essential as CPCM provided a shape stable structure. Figure 4 illustrates temperature distribution of the example of Figures 2 and 3 where the construction of the core is in the form of the continuous body formed from assembled CPCM blocks, with only the air channels and heater bores extending through the continuous body. The temperature contour is shown for a single block 125, which comprises composite phase change material, after thermal charging. It may be seen that the configuration of the air channel 36 and heater bores 38a, 38b; 38c, 38d, provides the advantage that the core is uniformly heated and any temperature difference between the hottest and coldest parts of the core is only about 30 °C when the core is fully heated to a temperature of more than 500 °C. The arrangement of the air channels and heater bores in a continuous body of the core comprising phase change material provides enhanced, more uniform and more efficient thermal energy storage by the core. In operation the integrated electric heating apparatus can be switched between a thermal charging configuration where no, or substantially no, first fluid is circulated via the core and a thermal discharging configuration where at least some first fluid such as air is circulated via the core. In a thermal discharge phase, air flowing through the air channels 36 is heated by the core 14 and returned to the heat exchanger 21. In the thermal charging configuration air flows via the bypass conduit 17. In the thermal discharging configuration the heat exchanger 20 can receive a mixed air flow from the air output manifold 29 and the bypass conduit 17. The ratio of the mix can be controlled by the controller 32. The valve 18 may be controllably disposed at an intermediate position which only partly closes the airflow via the bypass 17 so that the core can be simultaneously thermally charged and thermally discharged. The degree of closure / opening of the valve 18 can be varied across a desired range. The range can be continuous or indexed, to enable the thermal storage / thermal output to be varied as desired. The valve mechanism can be configured to be continuously open by at least a minimum threshold amount, so that the core 14 is capable of continuous thermal discharge, at least at a minimum output level, if required. The valve 18 is controlled by an actuator 35 and is configured to be operated between a first position in the thermal charging configuration and a second position in the thermal discharging configuration to expose the heat exchanger 20 to air heated by the core 14. As described above, the valve may be controllably operable to be movable between the first and second positions and to be disposed at intermediate positions to permit simultaneous thermal charging and discharging. Also, the valve may be controlled to permit at least a minimum thermal discharge to be continuously conducted. The controller 32 is connected (as shown schematically by lines 26, 34, and 33 in Figure 1) to the temperature sensor 25, the air flow control valve 18 and the air blower 19. The controller may also be connected to the flow control system of the second loop such as the pump 27 and / or valve 37. The connections can be wired or wireless connections. The controller 32 can be configured to set the output temperature of the central heating system to a desired temperature. The controller 32 receives a temperature signal from the temperature sensor 25 and sends an actuation signal to one or more valves of air flow control valve mechanism and / or the flow control system of the second loop. The controller 32 also operates the air blower 19. Accordingly, the temperature at the building 24 can be thermostatically controlled during thermal discharge. The controller can comprise appropriate software and hardware components to provide the necessary functions. For example, at least one processor and memory can be provided. Figure 5 shows another example of an electric heating apparatus 50 which is configured to use a working fluid suitable for heat transfer through liquid - vapor cycles. A working fluid is typically a gas or liquid that is used to primarily transfer heat into or out of a region of interest by conduction, natural convection, forced convection, and / or radiation. In the example of Figure 5 the working fluid comprises water or a mixture comprising water circulated in a first fluid circulation loop 51 extending via a core 52. A pump 53 is provided to circulate the working fluid in the first fluid circulation loop 51. The core comprises phase change material (PCM) or composite phase change material (CPCM) contained within the casing of the apparatus. The electric heater elements for heating the core are not shown for simplicity. Exemplifying general details of core materials and heater elements are already explained above, and therefore this detail not repeated here. In this example an evaporator 55 is provided within the core 52. Working fluid such as water is pumped through the high temperature phase change material (PCM) or composite phase change material (CPCM) core where it will be heated, evaporated, and superheated due to heat exchange. Superheating refers to a process where additional heat is added to 100% vapor working fluid to increase the temperature thereof, this increase in temperature being called superheat. For example, the operating temperatures can be up to ~950°C for the core and a temperature ranging from below 90°C, to ~130°C, and even to hundreds of degrees Celsius for the fluid such as superheated water in the first fluid loop. The fluid temperature for domestic heating systems can typically be below ~90°C. However, for, e.g., district heating system the primary loop taking heat from the core the fluid temperature can be up to 130°C, whereas the second loop to the house is often up to 90°C. The superheated water in form of steam is circulated from the core to heat exchange stage provided by a condenser 56. The condenser 56 then cools the steam down through condensation. The large volume change that occurs in the evaporator 55 is balanced out by the large volume reduction in the condenser 56. An example of water evaporation - condensation process is shown in the Pressure Enthalpy (P-H) diagram of Figure 6. A P-H diagram describes the relationship of pressure, temperature, and enthalpy of a water evaporation condensation cycle. In the Fig. 6 cycle a fluid (using water as an example) circulates between the condenser 56 of Fig. 5 where heat is released through condensation and the evaporator 55 where heat is absorbed from the core 52. As illustrated by the rectangular, low temperature low pressure water out of the condenser 56 (bottom left corner) is pressurised by pump 53 to a high pressure (top left comer) before entering the evaporator 55. In the evaporator 55 water is heated up by the core 52 to a preset (high) temperature (top right comer). High pressure and high temperature water then returns to the condenser 56 where temperature and pressure reduces (bottom right corner), completing the cycle. The evaporator and condenser arrangement of Figure 5 can make the electric heating apparatus more compact and may provide more efficient heat transfer. The heat transfer in the condenser from the first fluid to second fluid in a second loop 58 for heating the building 24 is more efficient than that of the evaporator. This is so because condensation occurs on one side of the condenser wall and forced convection on the other side of the condenser wall which both have high heat transfer coefficient. Instead of this, in the evaporator the heat transfer on the core is more limited, which helps to minimise the risk of pressure buildup in the system. A valve 54 is provided for controlling the flow and pressure in the first loop 51. The actuator of the valve is connected to a control unit 60. In the example of Figure 5 the first loop control valve 54 is assembled between the pump 53 and the evaporator 55. Other valve locations are also possible. The valve 54 can be configured to operate between open and closed positions based on predefined values of relevant temperatures in the system and load in the pipework. The actuator of the valve 54 can be controlled based control signals from the control unit 60. The control unit can control the actuator of the valve based on temperature information in a similar manner as explained above with reference to controller 32 of Figure 1. Valve 54 controls mainly the first fluid in loop 51 whereas valve 57 is arranged to control second fluid in the second loop 58. Both valves can be controlled based on information from one or more thermometers in the house 24. The combined operation and control of the valves 54 and 57 can be used to control heat exchange between the two loops. Pressure relief valves and expansion vessels (not shown) can be used to ensure safety. Pump 59 can be provided for controlled circulation of fluid in the second loop. Figure 7 shows another example of a heating apparatus 70 comprising an evaporator 75 and a condenser 76. A difference to Figure 5 is that natural convection is used to enable heat transfer between the thermal energy storage side at the core 72 and the load side at the condenser 76. Use of natural convection can remove the need for a pump for liquid circulation in the first loop 71. Figure 7 arrangement may have a more limited heat transfer rate (heating power) in the discharging process than what is achievable by the Figure 5 arrangement. On the other hand, a less complex and more cost-efficient heating system may be provided. Figure 7 also shows a pump 79 in the second loop 78 for circulating heated water in the second loop to the radiator(s) 23 at the house 24. Although not shown, a similar pump can also be provided in the second loops of Figures 1 and 5. Figure 8 is a flowchart for a method of heating at least one space such as at least one room in a building. The method comprises charging at 100, by at least one electric heater, a thermal energy storage of an electrical heating apparatus to an operating temperature within the range of 100 to 950 °C. The thermal energy storage comprises a core of phase change material (PCM) having a composition which absorbs or releases heat, with the heating process isothermally, or substantially isothermally, during phase transition, in a respective transition direction, between a first phase state and a second phase state. A first fluid is circulated at 102 in a first fluid circulation loop extending within the core and connected to a heat exchanger. Thermal energy is transferred at 104 from the PCM of the core to the first fluid in the first fluid circulation loop such that the first fluid is heated to a substantially high temperature. The first fluid is then circulated to a heat exchanger for transfer of thermal energy at 106 from the first fluid of substantially high temperature to a second fluid in a second fluid circulation loop. The second fluid can then be circulated at 108 to heat the at least one space. The temperature of the second fluid circulating in the second fluid circulation loop is controllably maintained at a suitable lower and safe level for circulation in the second loop. The level can be considerably lower than the temperature of the first fluid. The flow and / or temperature and / or pressure of the first fluid in the first fluid circulation loop can be controlled during the operation based at least in part on information associated with the second fluid circulation loop. For example, information from a temperature sensor in a space to be heated, information of operation of the second loop such as temperature and / or pressure information of the fluid in the second loop can be taken into account. Information relating to operation of the first loop may also be taken into account in the control. In accordance with an embodiment the core comprises composite phase change material (CPCM). Referring next to the graph of Figure 9 showing an example of the relationship between temperature and time during thermal charging of an electric storage heater. In particular, an upper plot shows the increase in heater temperature (i.e., the temperature of the electric heating elements) and a lower plot shows the increase in core temperature (i.e., average temperature of the phase change material) over time during charging. These two plots are closely aligned and have substantially the same heating rate up to a temperature of over 500 °C. This shows that the phase change material is being uniformly heated by the electric heating elements. At a temperature of about 500 °C, the temperature change of the phase change material, and correspondingly the electric heating elements, is significantly reduced. This shows that the phase change material has reached the phase transition temperature (Tm) and is transitioning from a solid phase to a liquid phase, during which thermal energy is absorbed by the phase change material in the form of latent heat. The total charging time of the apparatus containing this particular phase change material is about 210 minutes. Testing has shown that desired thermostatically controlled output temperatures can be reliably achieved and maintained. The electric heating apparatus has shown to be capable of increasing or decreasing the temperature in the space to be heated upon demand. These thermostatically controlled output temperatures were achieved over an extended discharge period, throughout which the temperature of the phase change material was decreasing from an initial temperature of about 500 °C to a final temperature, after the discharge period of about 8 hours, of about 100 °C. Testing has shown that while the output temperature of air can readily be varied, the temperature of the external casing of the heating apparatus can be maintained at a substantially constant temperature of no more than about 30 °C. Accordingly, the integrated electric heating apparatus can be provided with sufficient thermal insulation so that the external casing is maintained at a constant safe temperature, despite the electric heater containing the PCM core can be operated at a very high temperature, for example of about 500 °. Electric storage heating apparatuses as described above can have a light weight and cost for a given thermal storage capacity and output. For example, for a typical storage capacity of about 10 kWh for a domestic storage heater, the electric storage heaters according to the invention typically have a weight of about 60 kg. By using a phase change material, which stores additional thermal energy in the form of latent heat as compared to a solid refractory material, the weight required for a given thermal energy storage capacity is reduced. Furthermore, the amount of a typical composite phase change material required for such a domestic storage heater has a lower cost than the cost of the corresponding amount of solid refractory material for the same given thermal energy storage capacity. Therefore, the present invention can provide electric storage heaters, using a phase change material as a thermal energy storage medium, which can exhibit enhanced performance, including a thermostatically controllable constant output temperature over an extended discharge period and a higher energy storage density. Low manufacturing complexity and cost may be provided. It is noted that although the above detailed examples have been described with reference to certain processes, applications and apparatuses there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention. In particular, the different embodiments have been described as examples. Different features from different embodiments may be combined. For example, core structures described with reference to examples where the first fluid comprises gas such as air can be used in embodiments where the first fluid comprises working fluid such as water or mixtures of water. The foregoing description provides by way of exemplary and non-limiting examples a full and informative description of exemplary embodiments of the invention. However, 5 various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. Various modifications to the illustrated embodiments as described hereinabove will be apparent to those skilled in the art and are intended to be included within the scope of the present invention. 10

Claims

1. An electric heating apparatus comprising:a thermal energy storage comprising a core configured to provide an operating temperature within the range of from 100 to 950 °C, the core comprising phase change material (PCM) having a composition which absorbs or releases heat, with heat absorption or release process isothermally, or substantially isothermally, during phase transition, in a respective transition direction, between a first phase state and a second phase state;at least one electric heater element positioned within the core to charge the core with thermal energy by transitioning the phase change material from the first phase state to the second phase state;a first fluid circulation loop arranged in part within the core for transfer of thermal energy from the core to a first fluid in the first fluid circulation loop such that the first fluid is heated to a substantially high temperature;a heat exchanger connected to the first fluid circulation loop for transferring thermal energy from the first fluid of substantially high temperature to a second fluid in a second fluid circulation loop, the heat exchanger being connected to the second fluid circulation loop arranged to circulate the second fluid to at least one space to be heated, the temperature of the second fluid output from the heat exchanger being lower than the temperature of the first fluid; anda control system configured to control the flow of the first fluid in the first fluid circulation loop based at least in pail on information associated with the second fluid circulation loop.

2. An electric heating apparatus according to claim 1 comprising a housing surrounding the core, the first fluid circulation loop and the heat exchanger.

3. An electric heating apparatus according to claim 1 or 2 wherein the first fluid comprises gas.

4. An electric heating apparatus according to claim 3 wherein the gas comprises air.

5. An electric heating apparatus according to claim 3 or 4 wherein the first fluid circulation loop comprises:a bypass conduit connecting a first junction before the core and a second junction after the core to define a gas flow path external of the core; anda flow control valve mechanism for variably controlling gas flow rate along the bypass conduit to variably control the ratio of gas from the core to gas bypassing the core.

6. An electric heating apparatus according to any of claims 3 to 5 further comprising an air blower for circulating the gaseous fluid in the first fluid circulation loop.

7. An electric heating apparatus according to claim 5 or 6 when dependent on claim 5 configured to control the flow of the gas between a thermal charging configuration, in which there is no circulation, or substantially no circulation, of the gas within the core, and a thermal discharging configuration, in which a mixed gas flow from the core and the bypass conduit is enabled.

8. An electric heater according to claim 7 configured to be switchable to an intermediate configuration so that the core can be simultaneously thermally charged and thermally discharged, and the degree of closure and opening of the flow control valve mechanism can be varied across a desired range to enable control of the thermal output as desired.

9. An electric heating apparatus according to claim 1 wherein the first fluid comprises a working fluid suitable for an evaporation - condensation process, the core comprises an evaporator for superheating the working fluid in the first fluid circulation loop and the heat exchanger comprises a condenser and for transferring thermal energy from the superheated working fluid to the second fluid in the second fluid circulation loop.

10. An electric heating apparatus according to claim 9 wherein the working fluid comprises water.

11. An electric heating apparatus according to claim 9 or 10 wherein the condenser is configured to transfer thermal energy more efficiently from the first fluid to the second fluid than what the evaporator is capable of transferring from the core to the first fluid.

12. An electric heating apparatus according to any of claims 9 to 11, wherein the first fluid circulation loop comprises a pump.

13. An electric heating apparatus according to any of claims 9 to 11, wherein circulation of the working fluid in the first fluid circulation loop is arranged by means of natural convection.

14. An electric heating apparatus according to any foregoing claim comprising at least one valve for variably controlling temperature and / or pressure in the first fluid circulation loop.

15. An electric heating apparatus according to any foregoing claim comprising a plurality of electric heater elements positioned within the core for heating the core and wherein the first fluid circulation loop comprises a plurality of fluid channels positioned within the core for receiving heat from the core, wherein the electric heating elements and fluid channels form an array of parallel fluid channels and electric heater elements.

16. An electric heating apparatus according to any foregoing claim, wherein the first fluid circulation loop comprises a first manifold on input side of the core and a second manifold on output side of the core, and a plurality of fluid channels extending through the core between the first and second manifolds.

17. An electric heating apparatus according to any foregoing claim wherein the core is formed as a continuous body.

18. An electric heating apparatus according to claim 17 wherein the continuous body is assembled from a plurality of blocks of phase change material.

19. An electric heating apparatus according to any foregoing claim wherein the phase change material has a latent heat of from 100 to 800 kJ / kg, optionally from 120 to 300 kJ / kg, for the transition between the first and second phase states at the predetermined transition temperature.

20. An electric heating apparatus according to any foregoing claim wherein the core has an energy storage density of from 140 to 300 Wh / kg, optionally from 160 to 250 Wh / kg.

21. An electric heating apparatus according to any foregoing claim wherein the core has an energy capacity of from 2 to 100 kWh, preferably 2 to 50 kWh.

22. An electric heating apparatus according to any of claims 1 to 20 wherein the core has a substantially high energy capacity, preferably up to 700 MWh.

23. An electric heating apparatus according to any foregoing claim wherein the predetermined transition temperature of the phase change material is within the range of from 200 to 850 °C, optionally within one of the ranges from 300 to 700 °C, 300 - 500 °C, 400 to 600 °C, or 450 to 550 °C.

24. An electric heating apparatus according to any foregoing claim wherein the phase change material is a composite phase change material which comprises an inorganic material as a phase change composition, a structural material for structurally shape-stabilising the phase change material; and a heat transfer enhancement material dispersed in the phase change material.

25. An electric heating apparatus according to claim 24 wherein the structural material comprises an alkaline earth metal oxide and the heat transfer enhancement material comprises graphite, carbide, metal or metal oxide, or a mixture of any two or more thereof.

26. A central heating system comprising an electric heating apparatus according to any preceding claim, and further comprising the second fluid circulation loop and at least one radiator.

27. A central heating system according to claim 26 wherein a temperature sensor is controllable to vary the output temperature of the at least one radiator through control of amount of thermal energy transferred via the first fluid circulation loop to the second fluid in the second fluid circulation loop.

28. A method for heating at least one space, comprising:charging, by at least one electric heater element, a thermal energy storage of an electrical heating apparatus to an operating temperature within the range of 100 to 950 °C, wherein the thermal energy storage comprises a core of phase change material (PCM) having a composition which absorbs or releases heat, with heat absorption or release process isothermally, or substantially isothermally, during phase transition, in a respective transition direction, between a first phase state and a second phase state,circulating a first fluid in a first fluid circulation loop arranged in part within the core and connected to a heat exchanger;transferring thermal energy from the core to the first fluid in the first fluid circulation loop such that the first fluid is heated to a substantially high temperature;transferring thermal energy from the first fluid of substantially high temperature to a second fluid in a second fluid circulation loop, the heat exchanger being connected to the second fluid circulation loop;circulating the second fluid to the at least one space, the temperature of the second fluid circulating in the second fluid circulation loop being lower than the temperature of the first fluid; andcontrolling the flow of the first fluid in the first fluid circulation loop based at least in part on information associated with the second fluid circulation loop.

29. A method according to claim 28 comprising transferring thermal energy from the core to gas, preferably air, circulated in the first fluid circulation loop.

30. A method according to claim 28 comprising superheating, by an evaporator, working fluid circulated in the first fluid circulation loop and transferring thermal energy by condensation from the superheated working fluid to the second fluid.

31. A method according to any of claims 28 to 30 wherein the phase change material comprises composite phase change material (CPCM).

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