GB2455748A

GB2455748A - Elastomeric containment of PCM in latent heat storage device

Info

Publication number: GB2455748A
Application number: GB0724776A
Authority: GB
Inventors: Frederick George Best
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-12-19
Filing date: 2007-12-19
Publication date: 2009-06-24
Also published as: WO2009077765A1; EP2235466A1; GB2468619A; US20110030915A1; CN101932898A; GB0724776D0; GB201012030D0; CN101932898B; GB2468619B

Abstract

A latent heat storage device has an elastomeric container 1 to contain a phase change material (PCM) 2 containing nano-particle conductive powder, which may be carbon or aluminium, arranged in a containment vessel 3. The elastomeric container 1 may have walls arranged to be as thin as possible, maximising heat transfer, whilst being strong enough to retain the PCM, may be formed into square or circular chambers or may be a continuous tube sealed at both ends, folded, and inserted into the containment vessel 3. Heat exchange fluid 6 may flow through the vessel in a closed circuit, passing through gaps between adjacent tubes or folds of the elastomeric container 1, these gaps being restricted by thermal expansion of the PCM thus avoiding overheating. The containment vessel may provide the structural integrity for the device when the PCM has melted, have a sealed lid, insulated panels or walls and a plurality of compartments and different PCMs, the heat exchange fluid 6 being directed to different compartments in turn or selectively. The device may have several vessels 3, which may have different atmospheres, e.g. reduced oxygen.

Description

IMPROVED LATENT HEAT STORAGE DEViCE The invention relates to the efficient storage, with rapid absorption and extraction, of thermal energy.

In recent years the need to store energy, usually electrical, has increased significantly. With this invention it is now possible to efficiently store thermal energy which can be utilised to generate electricity, cooling, heating or for what ever purpose at a later date.

There have been various inventions such as W08900670 (Whitman) which show various ways of storing thermal energy utilising Phase Change Materials (PCM) but they all have fundamental problems. With the wax varieties of PCM the main problems centre on its poor thermal conductivity and relatively large coefficient of thermal expansion.

For Eutectic and Salt Hydrate type PCM's the thermal conductivity is significantly better but corrosion can be a major problem. All of the current PCMs have expansion and contraction issues which make the containment of these materials extremely difficult, especially when trying to get thermal energy in and out of the PCMs quickly and efficiently, using the smallest volume possible. Existing methods have employed strong thick materials; mm diameter balls made out of 3.0 mm thick stainless steel and thick plastic mouldings, most of which have to leave air gaps to allow for some expansion and minirrnse the stresses on the containment materials.

There have been successful demonstrations using Stainless Steel balls and tubing containing PCM. However in order to get the thermal energy into and out of these stores they have had to be very large, or have complex fluid distribution and control systems, so that the slow absorption and extraction

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rates can be accommodated. Some attempts at including conductive rings and elaborate finning on complex heat exchangers have been attempted, especially with the wax variety of PCM. This invention significantly improves the heat transfer rates even if the PCM is not very thermally conductive.

US Patent 6889751 (Lukas) shows a polygon shaped structure as this is well understood as being the best way of getting the maximum volume of material into a known space when using tubes. However the invention does io not take account of the properties of the PCMs being used. Using small bore pipes to pass the heat transfer fluid through and surrounding these tubes with PCM is not the most efficient use of the space, even if the tubes are finned. Alternatively by putting the PCM into the tubes may be an improvement but the ends of the tubes have to be sealed and allow for thermal expansion etc. It is an object of the present invention to increase the density of storage of thermal energy in a given volume and to facilitate the speed of absorption and extraction of that energy.

The invention provides a latent heat storage device comprising a containment vessel, at least one phase change material (PCM), and at least one PCM containment means, the containment means comprising very thin elastomeric material. The thin elastomeric material can be formed into any shape of thin section and providing a very large surface area to volume ratio, for example having many sided or circular chambers, and provided the distance through any section of PCM is small enough to effect rapid melting and freezing of the PCM. In a particularly advantageous form the containment means comprises thin elastomeric material formed as a continuous tube, filled with PCM and sealed at both ends, then folded along its length to occupy the maximum amount of space that is available within the containment vessel. This arrangement allows the amount of sealing required to be minimised and provides a very efficient means of maximising the amount of space used within the vessel by the containment means.

The elastomeric material is selected to have as thin a wall thickness as possible to structurally retain the PCM and at the same time to provide the minimum effect on the transfer of heat to and from the PCM. This enables the device of the invention to have a very rapid response time and to absorb io and provide energy very quickly. Additionally it is effective even for very small temperature differences.

Preferably there are small gaps between the PCM filled elastomeric material to allow heat exchange fluid to pass between said material to facilitate the absorption and extraction of thermal energy.

A number of different arrangements of the containment means and the PCMs within the vessel are possible. Advantageously the device may have a multiplicity of different PCM, with different properties, within the one vessel. The device may also have a multiplicity of different compartments within the one vessel, either with PCMs that are the same or that are different. The compartments may be formed using insulated or non-insulated panels. Advantageously the flow of heat exchange fluid is controlled through the vessel and it may be directed to the different compartments in turn. The flow of fluid can be directed to different parts of the device to accommodate different requirements at different times.

In an alternative arrangement, the device may have a multiplicity of vessels within the one device.

In a preferred embodiment of the invention, the latent heat storage device vessel has a sealed lid. If the device is sealed, then gas or fluid can be injected into the sealed vessel to effect a different atmosphere or environment such as a reduced oxygen atmosphere for the benefit of any heat transfer fluid or other material's needs for a reduction in oxidation.

Advantageously the inert gas may be nitrogen or carbon dioxide.

In another preferred embodiment of the invention, the device has a means whereby heat exchange fluid is supplied to the vessel and removed from the vessel, so as to be a closed circuit such that whatever fluid is supplied is also removed at the same time to avoid overfihling or emptying of the vessel.

In a particularly advantageous embodiment of the invention, the elastomeric contained PCM is allowed to expand and contract according to its nature.

In particular, the expansion and contraction can take place in the top of the vessel and in the heat exchange channels within which the heat exchange fluid flows. This arrangement has the particular advantage in that it can be used to provide an automatic limiting of the flow of fluid as the expansion acts to progressively restrict the flow of heat exchange fluid between the elastomeric tubes. This provides a particularly useful safety mechanism to prevent overheating of the PCM, elastomeric and or other materials used.

In many arrangements of the latent heat storage device of the invention, the vessel is arranged to provide the exoskeleton structural integrity for the elastomeric PCM, once the PCM has melted. If the device has internal compartments or dividers, these can also be used to provide the structural integrity for the elastomeric PCM, once the PCM has melted.

In many arrangements of the invention it will be advantageous for the vessel to be surrounded by insulation. This may be any suitable insulating material including vacuum insulation. Alternatively the device may be surrounded by a secondary insulated tank filled with water or any other suitable fluid, such that any thermal energy that escapes from the inner vessel or vessels will be absorbed and there will be minimal loss to the surrounding atmosphere.

The thermal conductivity of certain PCMs may be improved by adding very io small quantities of very fine powders of suitable conductive material to the PCM. If the particle sizes are small enough then they will remain suspended within the main body of the PCM. They will also have a tendency to get continually redistributed by any convection currents induced by the melting of the PCM.

The invention further provides a latent heat storage device comprising a containment vessel, at least one phase change materal (PCM) and at least one PCM containment means wherein at least one very fine nano-particle conductive powder is added to the PCM to improve the transfer of thermal energy. The addition of very fine nano-particle conductive powders can significantly improve the performance of poorly conducting PCMs.

Suitable examples include, but are not limited to, carbon and aluminium.

The concentrations can vary depending upon the materials used but typically can be anything from 0.5% to 2%; larger concentrations may well reduce the amount of PCM volume and influence the overall performance.

It is now possible to make up new composite materials utilising the properties of the different components to maximise the thermal capacity and heat transfer rates. Although it is possible to improve the situation the current technical specifications are exacting as there is a tendency for these materials to settle or separate out with time and the present invention enables these problems to be overcome.

In another alternative form of the invention, the vessel may be surrounded by secondary layers of different PCM filled elastomeric material.

Advantageously, but not essentially, the PCM of the secondary layer may have a lower phase change temperature than the PCM of the vessel.

if the latent heat storage device of the invention is used with a solar heating io device or other device, it can be arranged so that the heat exchange fluid feed-back temperature is lower than it would otherwise be; so as to maximise the efficiency of the solar heating or other device.

The invention will now be described by way of example only with reference to the accompanying drawings, of which: Figure 1 shows a schematic cross-section of a latent heat storage device according to the invention.

Figure 2 shows a more detailed view of a corner of the device shown in Figure 1.

Figure 3 shows an example of a containment means in the form of a continuous round tube folded into six rods.

Figure 4 shows an embodiment of the invention in form of a "tank within a tank".

Figure 5 shows an embodiment of the invention, wherein separators can be used to direct the flow of heat exchange fluid through different compartments of PCM.

Figure 6 shows an alternative shaped containment means, with detail shown in Figure 6a.

Figure 7 shows a further alternative shaped containment means, with detail shown in Figure 7a.

Figure 8 shows a possible packing arrangement of containment means of the type shown in Figure 3.

The various embodiments of the invention overcome most or all of the hereinbefore mentioned problems.

As shown in Figure 1, a rapid absorption and extraction latent heat storage io device according to the invention comprises a means I for storing a phase change material 2 arranged within a containment vessel 3 (for which insulation is not shown in this example). The containment means 1 is in the form of rods or tubes, which are made out of elastomeric material 4 containing the PCM 2. A heat exchange fluid 6 flows along the length of the rods 1. The PCM 2 may advantageously have a fme nano-particle conductive powder such as carbon or aluminium added to improve its conductivity.

The containment vessel 3, is shown as hexagonal but could be any shape that maximises the storage capacity for round rod-like multiple components 1 but the rods can be any shape provided they offer a thin enough section, for conduction, to enable heat transfer through the whole section in an acceptable time frame. In order to accommodate the expansion and contraction the outer skin of the rods is made from an elastic material 4 which utilises the close proximity of all the adjacent rods to support them when the PCM 2 is in the liquid phase. The rods 1 have a small diameter and are long. Entry/exit pipes 13a allow the heat exchange fluid to be fed into and removed from the vessel 3.

Figure 2 shows the detail of a corner of a containment vessel 3. The heat exchange fluid 6 passes through the spaces in between the rods 1. Tn the configuration shown in Figure 1, it is possible to get about 90% of the volume filled with PCM 2. The tubular elastomeric material 4 used for the containment of the PCM 2 must be thin enough to not take up too much volume and also to conduct thermal energy efficiently. initial prototypes have shown that many kilo watts of thermal energy could be stored and released in only a few minutes.

Figure 3 shows a preferred example of a containment means of the invention. The containment means comprises a continuous tube 7, of preferably circular cross-section, folded into six rods 1. Where the folds take place at the top and bottom of the rods, while the PCM 2 is molten, this area can be shaped to provide the round rod like shape.

In one particular embodiment, the containment means 1 was made from one length of tubing nominally 4500 mm long. In this example there are only two seals 8 needed for each batch of six rods. The containment means 1 can be any desired length but handling and the strength of the tubing will create a practical limit.

In one embodiment of the invention, a small single stack of rods 1 made with about 30 kg of salt hydrate PCM 2, having a phase change temperature of 58°C, flow rates of heat exchanger fluid 6 of over 2 litres/mm at temperatures over 100°C, absorbed all the thernial energy contained within the heat exchange fluid 6 and the exhaust temperature did not start to rise above room temperature until about 75% of the PCM 2 had already reached melting point.

Figure 4 shows an embodiment of the invention in form of a "tank within a tank", where two different types of PCM 2a, 2b are housed within the same tank and separated by separators 9, which in this example are an internal block and an external block where the separators 9 are suitable internal insulation; typically capable of withstanding higher temperatures. Similarly external insulation 10 is also shown. Insulation can be made from any suitable materials including Vacuum Super Insulation.

Figure 5 shows a different arrangement, where separators 11 are arranged io within the containment vessel 3. The separators 11 can be used to direct the flow of heat exchange fluid 6 through the different compartments of the vessel 3 which can utilise different types of PCM 2 having different melting temperatures should this be required. Similarly, as with Figure 4, the separators could be made out of any suitable insulation materials.

Alternative shaped rods of wide flat configurations or circular, as shown in Figures 6 and 7 can also be used. The critical configuration is that the section of PCM 2 between the heat exchange fluid exposed surfaces must be no thicker than that to ensure rapid melting and solidifying. Because of the thin sections of material very large surface areas for heat transfer are possible and it was found to be ideal for the eutectic and salt hydrate materials used in the prototypes constructed. It is still effective with plain waxes but the sections have to be thinner unless fme nano-particle conductive materials are added. In the non-round rod configurations, the channels for the flow of heat exchange fluid 6 will need small separators 12 to ensure the channels remain open until over temperature situations arise.

Similarly, if alternative shaped rods are used, the periphery of the whole block or assembly of materials will need the external surface to be shaped so as to allow heat exchange fluid to flow between the containment vessel 3 and the outer skin 4 of the elastomeric contained PCM 2. Provision for the overall movement heat exchange fluid 6 can be made by the provision of pipes or channels 13, an example is shown in Figure 1.

Figure 6 shows a rod having a rectangular cross-section. In this case, the section depicted is effectively where the round rods depicted in Figure 1 are joined together. With this kind of design it is possible to actually get more PCM into any given volume, but this would be by the diminution of the heat exchange fluid. Consequently higher velocities of flow will be encountered but it is anticipated that over 95% of the volume could be effectively io occupied with PCM.

Figure 7 gives another example of different shaped rods. In this case the vessel 3 is round and the elastomeric rods 14 are similarly shaped and are arranged concentrically with heat-exchange fluid 6 flowing between the layers. With any configuration of this type it will normally be necessary to ensure a none-uniform perimeter of the assembled rods, so as to allow heat exchange fluid to pass between the vessel and the perimeter. In the embodiment shown, this is in the form of a "corrugated" type outer surface 15, which can be seen in more detail in Figure 7a. The flow path could alternatively be accommodated by varying the shape of the vessel but the essential feature is that the rods are still supported and contained by the vessel as it acts like an exoskeleton for the main structure.

It is an additional feature of the invention that it tends to be self regulating; as the currently used PCM expands, it starts to restrict the flow of fluid through the stack of rods. Should the temperature of the heat exchange fluid get too high for the materials contained, then by design, the flow can be cut off completely. In a similar manner if alternative shaped rods (not round) are used then some supporting ribs 12 will be needed to prevent the heat exchange channels from collapsing during normal operation. In principle this invention will facilitate the maximum speed of input and extraction of thermal energy what ever materials are encased within the elastomenc material, it is therefore possible, by balancing the pressures across the elastomeric material, to have open-ended tubing or channels and to use the elastomenc material as a conventional heat exchanger.

Figure 8 shows a packing arrangement of a plurality of tubes 7 of the type shown in Figure 3, each folded into six rods I and close packed within a hexagonal containment vessel 3. A single entry/exit pipe 13b allows the io heat exchange fluid 6 to flow in and out of the vessel 3, as required.

Initial prototypes have been constructed using PCMs that have state changes at 58°C and 89°C but the basic principle will work with any temperature.

Even heat transfer fluid temperature differences of only 4°C, above or is below the phase change temperature, will affect efficient heat transfer in only a few minutes.

In this example the elastomeric material 4, containing the PCMs 2, is provided in the form of an extruded tube but the elastic film surrounding the PCMs can be sprayed on to the PCM material or the PCM material can be dipped into a solution. It is also possible to construct the elastomeric material using the 3D printing technologies so that the complete structure can be made as one unit, or as an assembly of smaller units. a

Claims

1. A latent heat storage device comprising: a containment vessel; at least one phase change material (PCM); and at least one PCM containment means, the containment means comprising very thin elastomeric material.

2. A latent heat storage device according to claim 1 wherein the elastomeric material is selected to have as thin a wall thickness as possible to structurally retain the PCM and at the same time to provide the minimum effect on the transfer of heat to and from the PCM.

3. A latent heat storage device according to claim 1 or claim 2 wherein is the thin elastomeric material is formed into any shape of thin section, filled with PCM.

4. A latent heat storage device according to any one of claims 1 to 3 wherein the thin elastomeric material is formed into rectangular chambers of thin section, filled with PCM.

5. A latent heat storage device according to any one of claims 1 to 3 wherein the thin elastomeric material is formed into circular chambers of thin section, filled with PCM.

6. A latent heat storage device according to any one of the preceding claims wherein the containment means comprises thin elastomeric material formed as a continuous tube, filled with PCM and sealed at both ends, then folded along its length. a

7. A latent heat storage device according to claim 6 wherein the continuous tube is folded and inserted within the containment vessel to occupy the maximum amount of space that is available within the contamment vessel.

8. A latent heat storage device according to any one of the preceding claims wherein there are small gaps between the PCM filled elastomeric material to allow heat exchange fluid to pass between said material to facilitate the absorption and extraction of thermal energy.

9. A latent heat storage device according to any one of the preceding claims wherein the device has a multiplicity of different PCM, with different properties, within the one vessel.

10. A latent heat storage device according to any one of the preceding claims wherein the device has a multiplicity of different compartments within the one vessel.

11. A latent heat storage device according to claim 10 wherein the device has a multiplicity of different compartments within the one vessel where some of the compartments are formed by using insulated panels.

12. A latent heat storage device according to claim 10 or claim 11 wherein the device has a multiplicity of different compartments within the one vessel where some of the compartments are formed by using non-insulated panels.

13. A latent heat storage device according to any one of claims 10 to 12 wherein the flow of heat exchange fluid is directed to the different compartmentS in turn. a

14. A latent heat storage device according to any one of claims 10 to 13 wherein the flow of heat exchange fluid is controlled such that the flow of fluid can be directed to different parts of the store to accommodate different requirements at different times.

15. A latent heat storage device according to any one of the preceding claims wherein the device has a multiplicity of vessels within the one device.

16. A latent heat storage device according to any one of the preceding claims wherein the vessel has a sealed lid.

17. A latent heat storage device according to claim 16 wherein the device has a gas or fluid injected into the sealed vessel to effect a different atmosphere or environment.

18. A latent heat storage device according to claim 17 wherein the device has an inert gas injected into the sealed vessel to effect a reduced oxygen atmosphere for the benefit of any heat transfer fluid or other material's needs for a reduction in oxidation.

19. A latent heat storage device according to claim 18 wherein the inert gas is nitrogen or carbon dioxide.

20. A latent heat storage device according to any one of the preceding claims wherein the device has a means whereby heat exchange fluid is supplied to the vessel and removed from the vessel, so as to be a closed circuit such that whatever fluid is supplied is also removed at the same time to avoid overfllling or emptying of the vessel.

21. A latent heat storage device according to any one of the preceding claims wherein the elastomeric contained PCM is allowed to expand and contract according to its nature.

22. A latent heat storage device according to claim 21 wherein the expansion and contraction takes place in the top of the vessel and in the heat exchange channels within which the heat exchange fluid flows.

23. A latent heat storage device according to claim 21 or claim 22 wherein the expansion acts to progressively restrict the flow of heat exchange fluid to providing a safety mechanism to prevent overheating of the PCM, elastomeric and or other materials used.

24. A latent heat storage device according to any one of the preceding claims wherein the vessel provides the exoskeleton structural integrity for the elastomeric PCM, once the PCM has melted.

25. A latent heat storage device according to any one of the preceding claims wherein the compartments and or other features of the device provide the structural integrity for the elastomeric PCM, once the PCM has melted.

26. A latent heat storage device according to any one of the preceding claims wherein the vessel is surrounded by insulation.

27. A latent heat storage device according to any one of the preceding claims wherein the vessel is surrounded by secondary layers of different PCM filled elastomeric material. a

28. A latent heat storage device according to claim 27 wherein the PCM of the secondary layer has a lower phase change temperature than the PCM of the vessel.

29. A latent heat storage device according to any one of the preceding claims wherein the heat exchange fluid feed-back temperature to any solar heating or other device is lower than it would otherwise be; so as to maximise the efficiency of the solar heating or other device.

30. A latent heat storage device according to any one of the preceding claims wherein the device is surrounded by a secondary insulated tank filled with water or any other suitable fluid, such that any thermal energy that escapes from the inner vessel or vessels will be absorbed and there will be minimal loss to the surrounding atmosphere.

31. A latent heat storage device according to any one of the preceding claims wherein very small quantities of very fine powders of suitable conductive material are added to the PCM to improve the thermal conductivity

32. A latent heat storage device comprising a containment vessel, at least one phase change materal (PCM) and at least one PCM containment means wherein at least one very fine nano-particle conductive powder is added to the PCM to improve the transfer of thermal energy.

33. A latent heat storage device according to claim 31 or claim 32 wherein the powder comprises fine nano-particles of conductive powder.

34. A latent heat storage device according to claim 33 wherein the powder comprises fine nano-particles of carbon or aluminium.

35. A latent heat storage device as hereinbefore described with reference to any one of the accompanying drawings.

36. A method of manufacturing a latent heat storage device as hereinbefore described with reference to any one of the accompanying drawings.