CHEMICAL PHASE CHANGE HEAT STORES

Abstract: A heat store which comprises a first chamber containing a phase change material and a second chamber for circulating a heat transfer fluid in proximity with the store, the store including a thermally conductive member in thermal contact with the phase change material so as to facilitate heat transfer through the bulk of said phase change material.

Description: CHEMICAL PHASE CHANGE HEAT STORES This invention relates to heat stores which utilize the latent heat of fusion of certain chemicals to store heat and to release it at a different time. Specific applications include the storage of heat produced by cheap rate electricity supplies, waste heat from industrial processes and solar generated heat. Interest has developed in recent years in phase change materials (hereinafter called P.C.M's) because the heat capacity of p.C.M's per unit volume is significantly greater than sensible heat stores, such as fire bricks or tanks of water. In the development of P.C.M. .heat stores two significant practical problems have been encountered in their use. The most effective P.C.M's which have been investigated are hydrates of inorganic salts or eutectic mixtures of such hydrates. These salts are highly sensitive to ingress or loss of water and it is therefore necessary to encapsulate or hermetically seal the P.C.M. within a chamber in the heat store. A secoτ_d problem is to extract the stored heat from the encapsulated P.C.M. at a satisfactory rate and to ensure efficient heat exchange between P.C.M. and a heat transfer fluid, such as air or water. Faced with these two problems the tendency of designers in the art has been to encapsulate the P.C.M. in a large number of sealed containers such as plas.tic pipes, plastic bottles or steel cans, such as aerosol cans, in order to establish a large ratio of surface area to volume, while minimising the risk of degradation of a major amount of the P.C.M. The choice of suitable containers has not been entirely trouble-free since P.C.M.'s expand and contract when undergoing phase change and this imposes a fatigue strain upon small metal or plastic containers. Also the use of some plastics has led to difficulties arising from penetration of water after a relatively small number of phase change cycles. According to the present invention there is provided a heat store which comprises a first chamber containing a phase change material and a second chamber for circulating a heat transfer fluid in close proximity with the store, the store including a thermally conductive member in thermal contact with the phase change material so as to facilitate heat transfer through the bulk of said phase change material. The advantage of the heat store acco'rding to this invention is that since the P.C.M. may be contained in a single chamber or small number of chambers it can be constructed so as to provide sufficient strength and resistance to fatigue strain. At the same time efficient heat exchange is achieved by a thermally conductive member of high surface area, which extends through the P.C.M. This is important since the P.C.M. tends to solidify as a crust which has a lower heat conductivity than the fluid salt. Thus the thermally conductive member enables heat to be exchanged between the fluid P.C.M. in the core of the heat store" and the heat transfer fluid and does not rely upon heat exchange through a solidified crust, as occurs in the prior art heat stores, in which the PCM is divided into a large number of small volumes. Preferably the heat store of the present invention is constructed from metal e.g. from pressed steel which enables a strong compact store to be cheaply manuf ctured. In one preferred embodiment the heat store is manufactured from a pair of spaced, pressed steel radiators which form the second chamber, the two radiators being connected by side, top and base panels and the space between the radiators forming a first chamber for containing the P.C.M. By utilizing standard pressed steel water radiators in the construction of the heat store, a relatively cheap but effective heat store can thus be manufactured. Alternatively, a 'double' steel radiator can be sandwiched between two, spaced single radiators toprovide a construction having chambers for PCM on each side of the double radiator. The chamber or chambers containing the PCM may be sealed after introducing the PCM in molten form. In such a construction the thermally conductive member or members may be metal projections, such as fins, which are attached e.g. by welding, to the walls of the second chamber and extend into the PCM. However, it is possible to avoid the necessity of forming a sealed chamber for the PCM by casting the PCM in - TE OMPI a f lexible bag constructed from a non-permeable membrane and clamping the bag between the members forming the second chambers, such as the radiators. In this case, it is not necessary for the f irst chamber to be sealed. Indeed, i t is not essential in thi s embodi ment f or the f irs t chamber to have side , bottom and/or top pane ls , as long as there i s suf f icient mechani cal support for the bags containing the PCM. Since it is desi rable f or optimisation of heat exchange rates for the membrane to be as thin as poss ible , it is pref erred to provide side and bottom panels , (at least) , between the spaced radiators. In practice the PCM may be melted and poured into f lexible bags which " are positioned between radiators spaced apart by the appropriate distance. Prior to introducing the molten PCM, the thermally conductive member or members is placed in the bag. Thus , after cooling, the radiators can be removed and the bag containing solidified PCM will have a shape corresponding to the space between- the radiators which constitutes the first chamber. When installing the heat store in the desired location, the shaped bag or bags containing solidif ied PCM and thermally conductive member can be readily positioned between a pair of radiators , which are then clamped together so that the undulations in the surf ace of the bags correspond with those on the surf ace of the radi ator and good thermal contact is achieved in use. The thermally conductive member may be, for example, a corrugated metal panel or an expanded metal sheet or mesh, preferably formed from a metal of high thermal conductivity, such as aluminium. Two or more metal panels or expanded sheets may be included in the bag and they may be joined together preferably with spacers to enable the PCM to penetrate throughout the structure. Because most heat sealable plastics become permeable to water vapour at the melt temperatures of the usual PCM's, the bags for containing the PCM's are preferably laminated from a heat-sealable plastics sheet and a non- vapour permeable foil. The foil may be plastics or metal. Multi-layer laminates may be used. For example, we have found that a laminate of polypropylene, aluminium and polyester is satisfactory. The polypropylene is on the inside of the bag and gives the heat-sealing properties while the aluminium foil gives excellent vapour barrier properties and the polyester increases the strength of the laminate. Advantageously, the laminate should be as thin as possible in order to reduce heat transmission losses through the thickness of the laminate. We currently prefer to use a laminate of the kind described above having a thickness of about 4-5 thousandths of an inch (0.1 to 0.125 mm). In order to allow some freedom for expansion the bags are preferably not completely filled with the PCM salt. However, a void space of a few per cent is sufficient to accommodate the expansion of the salt during phase changes. A number of different P.C.M.'s are available and may be used in the present invention. A large number of P.C.M.'s are listed in the article by George A. Lane published in the International Journal of Ambient Energy, volume 1, number 3 published by the Construction Press Limited, July 1980. We currently prefer to use in the heat stores of the present invention, hydrated inorganic salts such as magnesium nitrate hexahydrate and magnesium chloride hexahydrate, the former being particularly suitable because it is non-corrosive towards aluminium or steel. These salts have a relatively high melting point and a high latent heat of fusion. Preferably, a PCM salt is chosen which melts in the region of 70 to 90°C so that water can be used as the heat transfer fluid. As already mentioned previously, the thermal conductivity of the solid P.C.M.'s is generally less than of the molten materials but it is generally desirable in the design of P.C.M. stores to avoid formation of a thick layer of solidified salt during the initial stages of extracting heat from the store. We have found that if a heat transfer fluid having a temperature which is substantially below the temperature of the molten P.C.M. is circulated through a heat store a thick crust of solidified P.CM. tends to form rapidly on the interior of the walls of the P.C.M. container. According to a further aspect of the present invention, therefore, there is provided a method of exchanging heat between a P.C.M. heat store and a load which comprises circulating through the P.C.M. heat store a first heat transfer fluid having a temperature close to the melt temperature of the P.C.M., and exchanging heat between said first heat transfer fluid and a second heat transfer fluid, at a lower temperature, in a heat exchange remote from the heat store. For example, if the first. heat transfer fluid is oil this can be circulated through the heat store at a temperature only a few degrees below the temperature of the molten P.C.M. and this avoids rapid freezing of a crust of solidified P.C.M. on the walls of the P.C.M. container. The resulting hot oil can then be circulated through a remote heat exchanger where a " second heat transfer fluid, such as water, is heated by the oil and is used to heat the load on the system. The same heat transfer fluid can be used in the two heat exchange stages, the choice of heat transfer fluid being influenced by the melting point of the PCM. The method described above may be used with the heat store of the present invention but may also be used with advantage in conventional P.C.M. heat stores. One embodiment of a heat store in accordance with the present invention and examples of heat transfer circuits utilizing the heat store will now be described with reference to the accompaning drawings in which:- Figure 1 is plan view of a heat store, partly broken away to show the interior; Figure 2 is a side elevation of the heat store similarly broken away to show the interior; Figure 3 is a view of the same heat store from one end, also partly broken away; Figure 4 is a sectional view of a modification of the heat store shown in Figures 1 to 3 in which the PCM is contained in a flexible bag; Figure 5 is a diagrammatic representation of a circuit showing the way in which the heat store may be connected to an electrical heater for heating the store and also to a load for transferring the stored heat energy; and Figure 6 is a diagrammatic representation of a preferred circuit for a heat store in which two heat transfer fluids are employed; Figure 7 is a diagrammatic side elevation of a heat store similar to that shown in Figures 1 to 4 arranged to feed a domestic radiator; and. Figure 8 is an end view of the heat/store radiator shown in Figure 7. Referring to Figures ■ 1 and 3 of the accompanying drawings, the heat store comprises a pair of pressed steel radiators which are spaced apart and connected by outer panels 3 and inner panels 4, conveniently welded to the facing walls of the radiators 1 and 2. The chambers 6 formed by panels 3 and 4 communicate with the interior of radiators 1 and 2 so that a heat transfer fluid, such as water, can be circulated through OMPI the entire chamber formed by chamber 6, and the interior of radiators 1 and 2. Panels 3 and 4 are conveniently welded to the radiators 1 and 2 so as to form a sealed internal chamber 7 for containing the phase change material. Attached to the facing walls of radiators 1 and 2 are a series of projections 8 which project into the chamber 7. Projections 8 are in the form of fins of sheet metal such as steel formed by folding steel strip and spot welding the bases of the prism-shaped fins to the walls of radiators 1 and 2. Thus, projections 8 (which constitute thermally conductive members) are hollow and apertures may be provided in the steel of the. fins 8 to enable phase change material to penetrate and fill the spaces 9 within each of the projections 8. Connecting ports 10, 11, 12 and 13 are provided to circulate a heat transfer fluid, such as water, through the heat store. The resulting heat store is shaped to be conveniently located against a wall or in a roof space and will be encased in a heat insulating material such as rockwool or polystyrene beads. It will be appreciated that the projections 9 enable the heat to be transferred efficiently from the centre of the P.C.M. to the radiators and hence distributed through-out the system. The fins or projections 8 may be staggered so that projections 8 from one radiation extend into spaces between projections on the other radiation. Figure 4 shows a modification of the heat store shown OMPI in Figures 1 and 3 and is a sectional view similar to Figure 1. As in the construction already described, a pair of steel radiators 101 and 102 provide the second chambers through which the heat exchange fluid is passed. A first chamber for housing the PCM is formed by clamping the radiators 101 and 102 together using side plates 103 and 104 and bottom and top plates (not shown) which may be of similar construction. One or both of the plates 103 & 104 may be constructed as clamps, e.g. as shown at 104 to ® ensure good thermal contact between a bag 105 containing the PCM and the radiator surfaces. The PCM is contained by a laminated membrane formed into a bag 105 which is pressed between the radiators 101 and 102. One or more thermally conductive members 106 (such as corrugated 5 aluminium sheet are also contained in the bag 105 and serve to facilitate heat exchange throughout the bulk of the PCM 107. A suitable thickness for the aluminium sheet members 106 is 16 gauge (B.S.I). Connections 110,111, 112 and 113 are provided to circulate heat exchange fluid " through the radiators 102 and 103. Units such as shown in Figure 4 may be grouped together, e.g. by placing a number of radiators,such as 5, side by side with PCM containing bags between adjacent radiators. The individual radiators may be piped together in series or parallel or in a combination thereof. The heat store shown in Figure 4 is constructed on site by sandwiching the bag or bags 105, containing solidified PCM and thermally conductive members 106, between radiators 101 and 102 and adjusting the clamps 103 and/or 104 so that the bag is pressed firmly into contact with the inner surfaces of the radiators 101 and 102 and their respective undulations correspond. The bags 105 are pref erably manuf actured f rom a f lexible polypropylene/aluminium/polyester laminate, e.g. that sold by D.R.G (U.K) Limited of Bedminster, Bristol. The laminate comprises a layer of polyester having a weight per unit surface area of 17 grams/square metre, a layer of aluminium foil having a weight per unit surface area of 32 grams/square metre and polypropylene having a weight per unit surface area of 67 grams/square metre. Prior to introducing the PCM the bags are formed as pounches by heat sealing two layers of the laminate along three edges. Bags of suitable size are about 720 mms. square and these hold about 50 kilograms of the PCM. The PCM salt, such as magnesium nitrate hexahydrate is melted by heating to about 95°C and then immediately poured into the bags, which are heat sealed immediately. Magnesium nitrate hexahydrate melts at about 89°C and has a heat of fusion of 163 kj/kg. During cooling to solidify the salt, the bags are held in a dismantlable chamber or container whose shape corresponds to that of the PCM chamber in which the bags will be eventually used. Figure 5 shows a simple circuit for using the heat store in a domestic heating system. The heat store is indicated diagramatically at 20 and is heated by OMPI circulating a heat transfer fluid from an electrical heater 21 working on cheap rate electricity. Pump PI circulates the heat transfer fluid through the heat store 20 to heat the P.C.M. to a temperature at which becomes molten. A second pump P2 is provided to circulate the heat transfer fluid from the P.C.M. heat store to a load 25 such as a domestic radiator circuit. Thus pumps PI and P2 and heater 21 will be controlled by a time clock and programmer so that heat supplied during a cheap rate period is stored in heat store 20 and the stored heat is transferred to the load when desired for use. If desired, a solenoid valve 24 may be included in the primary circuit and set to interrupt the circuit if the temperature rises substantially above the melt temperature e.g. above 95°C. Figure 6 illustrates a suitable circuit for operating the method described above. A heat store 31 is connected to a heater 32 for supplying heat to the heat store using a heat transfer fluid circulating pump PI. ' A second pump P2 is arranged to circulate the heat transfer fluid through a heat exchanger 33 having a primary and a secondary circuit. The heat transfer fluid, which is circulated through store 31, and the primary circuit of heat exchanger 33, is a high temperature fluid such as oil. A second heat transfer fluid is arranged to be circulated through the secondary side of the heat exchange 33 by means of a pump P3 and thence to the load 35. The second heat transfer fluid may be at a lower temperature and would ' conveniently be water. A control valve 34 may 13 be provided to short circuit the load. A solenoid valve 36 may be incorporated in the primary circuit containing the PCM store 31 and set to close at a temperature in excess of about 95°C It will be appreciated that instead of using the heat store as a store of excess heat it could equally be used as a store of "coolness". In such a case, the heater 21 or 32 could be replaced with a refrigerator and the heat exchange fluid, which may be a refrigerant such as a halogenated hydrocarbon such as a "Freon" (Trade Mark of Du Pont) circulated through the PCM store. Obviously, in this case a low temperature melting PCM would be selected and the load on the system would then be an air conditioning circuit. The refrigerator would operate on off-peak electricity and the store of "coolness" released through the air conditioning circuit during peak periods. Referring to Figures 7 and 8, these show an arrangement for using a PCM heat store to feed a radiator 62 directly and thereby provide an individual room heater utilizing "off-peak" generated heat. The PCM heat store 61 is constructed as described in Figures 1 to 4 and is heated by an immersion heater 63 located in the heat transfer fluid. Heater 63 is timed to heat the fluid at a time when economy electricity rates prevail and thereby melt the PCM in the store 61. Whilst heating is taking place, solenoid valve "A" is energised closed, thus preventing heat from being transferred to the radiator 62 attached to the front of the heat store. Radiator 62 is also connected to the heat store by thermostatic valve "B". When heating is completed and the supply is stopped, valve "A" will open and allow the heat transfer fluid to thermosyphon through the radiator and emit heat, if valve "B" will allow this as it is a thermostatic valve. The radiator will then draw heat by thermosyphon whenever valve "B" calls for it. Valve "B" can also be closed manually if heat is not required during normal hours.

Claims: CLAIMS 1. A heat store which comprises a first chamber containing a phase change material and a second chamber for circulating a heat transfer fluid in proximity with the store, the store including a thermally conductive member in thermal contact with the phase change material so as to facilitate heat transfer through the bulk of said phase change material. 2. A store according to claim 1, wherein the said thermally conductive member comprises one or more metal projections extending from a wall of said second chamber into said first chamber. 3. A store according to claim 1, wherein the thermally conductive member comprises a metal member of high surface area immersed in said phase change material. 4. A store according to claim 3, wherein the phase change material and said thermally conductive member are sealed in a membrane which is non-permeable to the phase change material. 5. A store according to claim 4 wherein the membrane is a laminate of a heat-sealable plastics material and non-permeable foil. 6. A store according to any one of the preceding claims which is fabricated from a pair of pressed steel radiators which are spaced apart to define, with connecting panels, an internal chamber for containing a phase change material, the radiators constituting said OMPI second chamber or chambers. 7. A store according to any one of the preceding claims which includes an electrical heater to heat said phase change material to its melting temperature, either directly or by heating the heat transfer fluid. 8. A store according to any one of the preceding claims, which is combined with a space heating radiator in such a way that heat transfer fluid heated by the phase change material can be circulated through the space heating radiator, thereby providing a room heater operable on stored energy. 9. A method of exchanging heat between a PCM heat store and a load which comprises circulating through the PCM heat store a first heat transfer fluid having a temperature close to the melt temperature of the PCM and exchanging heat between said first heat transfer fluid and a second heat transfer fluid, at a lower temperature, in a heat exchanger remote from the heat store.

Inventors: DUTTON NORMAN A, HOWLING RICHARD J, DUTTON, NORMAN, A., HOWLING, RICHARD, J.
Applicants: LINGARD ENGINEERING LIMITED
Application year: 1984
Main classification: F28D20
IPC codes: F24D11, F24H7, F28D20