GB2312500A - Apparatus and materials for thermal storage - Google Patents

Abstract

A thermal storage system comprises first and second heat exchangers (16a, 16b) connected by a closed loop (14) containing a fluid medium in which are suspended particles comprising a material that undergoes a reversible phase change on thermal input/output, the system comprising also a chamber (10) containing the fluid and a relatively high particle concentration, means (12) e.g an anger for metering the particles from the chamber into the loop, and means (13) for returning particles from the loop to the chamber. Such a thermal storage system may comprise a chamber containing a heat exchanger and, surrounding the heat exchanger, a concentrated suspension of a hydrophilic material having a water content of 85 to 98% by wet weight. A method for preparing particles of a hydrophilic polymer, comprises spraying monomer onto a moving substrate (21), irradiating the monomer to form a continuous sheet of polymer (23) on the substrate, stripping the sheet of the substrate (25), and rendering the sheet into particles.

Description

APPARATUS AND MATERIALS FOR THERMAL STORAGE Field of the Invention This invention relates to apparatus and materials for use in thermal storage.

Background of the Invention GB-A-1587725 describes a thermal store, in which heat is transferred in a fluid between a mobile suspension of suspended particles comprising a material that undergoes a reversible phase change with release or absorption of heat.

The suspending fluid may be a gas or liquid. For low temperature storage, it is proposed that the fluid may be an oil, the particles a hydrophilic material, and the phase-change substance water. It is suggested that the concentration of particles in the suspension should be as high as possible, to maximise the effect obtained by the phase change, but that a high concentration may present difficulties, in maintaining the suspension and in pumping it.

The specific apparatus that is disclosed in GB-A1587725, comprises a storage vessel filled with a hydrocarbon in which particles of an acrylonitrilevinylpyrrolidone (AN-VP) polymer, hydrated with water, are suspended. In use, the suspension is circulated, using a pump, from the top of the vessel to its base. A heat exchanger in the vessel is operated to cool the water in the hydrated polymer; the water freezes and its latent heat of fusion is extracted. The stored matter can then be used to cool water or air in a circuit passing through the vessel and to an external output, e.g. the air in an air conditioner.

The theory underlying the disclosure of GB-A-1587725 is correct, but its practical expression has been limited.

This may be due in part to the difficulty, mentioned above, of maintaining a sufficiently high concentration of suspended particles. Economical production of suitable materials may also have been a problem.

Summary of the Invention According to a first aspect of the present invention, a thermal storage system comprises first and second heat exchangers connected by a closed loop containing a fluid medium in which are suspended particles comprising a material that undergoes a reversible phase change on thermal input/output, the system comprising also a chamber containing the fluid and a relatively high particle concentration, means for metering the solids from the chamber into the loop, and means for returning the solids from the loop to the chamber.

This storage/metering device avoids the difficulty of simultaneously storing and pumping a suspension having the same solids concentration for each purpose when ideally the storage function requires high solids concentrations (e.g.

up to 85% by volume) while the circulation application requires a lower solids concentration (e.g. from 5 or 10%, up to 30% by volume).

According to a second aspect of this invention, the hydrophilic material in a thermal store is designed to have the maximum possible water content. It has been found that a suitable material can be made having a water content of 85 to 98% water by wet weight, e.g. by radiation polymerisation of a monomer water solution, so producing a pre-hydrated material. The resulting polymer gel is very soft and elastic, and is well suited to providing a packing round the heat exchanger/evaporator of, say, a domestic refrigerator. It is suitable either for slow speed circulation or for static use when it operates to prevent density-induced stratification of the phase-change material (water) during freezing and thawing. Non-uniform freezing and thawing, a process which reduces the efficiency of conventional uncirculated water/ice stores, is thus reduced or eliminated.

According to a third aspect of this invention, the same or different hydrophilic material, which is suitable for use in a circulating pumpable thermal storage system, is made by spraying monomer onto a moving substrate, irradiating the monomer to form a continuous sheet of polymer on the substrate, stripping the sheet of the substrate and rendering the sheet into particles by, for example, grinding.

Description of the Invention The principles underlying the present invention, and materials that may be used, as medium, particulate matrix and phase-change material, are described in GB-A-1587725, the relevant content of which is incorporated herein by reference. By contrast to the disclosure therein, the present invention provides a relatively low concentration of suspended material in circulation and a relatively high concentration in the specified chamber. The pipes providing the specified closed loop may provide the major volume of the system, and moving suspended particles through the loop will be difficult, if they are present in high concentration; that problem is overcome by means of the present invention.

According to the present invention, particles in circulation are introduced as desired, by metering means such as an auger which may also serve to circulate the relatively concentrated suspension in the store. Such means, preferably together with valves which control the pressure differential across the mixing chamber, allows control of the system, depending on the rheology of the particle-fluid system. Some factors which apply are (by way of illustration only): (i) Control of hydration to achieve partial hydration: partially-hydrated hydrophilic material placed in oil and sealed into the apparatus remains partially hydrated and free from agglomeration or clogging which may occur if the material is fully hydrated, due to the formation of ice "bridges" between the particles.

(ii) Choice of hydrophilic material to control spread of freezing temperature: it is known that certain hydrophilic materials, e.g. acrylonitrile-vinylpyrrolidone copolymers, exhibit the property of suppressing freezing until a temperature below O"C has been reached. This effect can be controlled by the choice of comonomers used in the polymerisation of the hydrophilic copolymer, and by the control of the ultimate water uptake of the polymer when fully hydrated. Suitable choice of material and freezing temperature can be used to optimise the efficiency of a cold storage system and match the cold store to the application and to the type and performance specification of a refrigerator heat pump.

(iii) Choice of hydration solutions with non-zero freezing/thawing characteristics: hydrophilic materials have been shown to be hydratable in a number of salt solutions, including NaCl and CaCl2, which depress the freezing temperature of aqueous systems, and it has been established that the effective freezing temperature of the hydrophilic material hydrated in the solution is also depressed. This is a second way in which the storage temperature can be matched to the application and to the properties of the heat pump. It has also been shown that a number of materials which form aqueous solutions increase the freezing temperature, e.g. to above 0 C. An example of such a material is sodium disulphide (20% Na2S in water has a freezing temperature of 210C) or sodium hydrosulphide (NaSH). This enables the storage temperature to be above OOC.

In a particularly simple embodiment of the invention, particles in circulation are distributed between return to the chamber and further circulation, by density separation.

For this purpose, the closed loop suitably has an inlet to, and an outlet from, the upper part of the chamber.

Material to be recirculated is suitably introduced into the loop from the lower end of the chamber. The distribution effect may be enhanced by the provision of a cyclone, or a suitable impeller having flexible vanes. A combined liquid/gas entrainment pump may be used, for circulation.

The use of, say, a liquid cyclone, to actively separate the hydrated hydrophilic materials from the fluid rather than rely upon density separation, enables the system to service a local circulation system operating at low solids concentrations, e.g. 10 to 30%, while allowing the storage chamber to operate with a higher solids concentration, for example of more than 30%, e.g. at least 40%, say 50 to 80% solids by volume. Further, it is possible to have rheological control of the support liquid, by the inclusion of gelation agents, e.g. Cab-O-Sil, to control flow and settlement, by controlling the yield stress in the fluid.

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 view of a thermal storage system embodying the first aspect of this invention; Figure 2 is a schematic view of a preferred embodiment of part of a thermal storage system; and Figure 3 is a schematic view of apparatus for use in the third aspect of the invention.

Figure 1 shows a central chamber 10, having a neck 11 at its lower end, in which a rotatable auger 12 (shown in dotted lines) is disposed. An (optional) liquid cyclone 13 is also shown, near the top of the chamber. The chamber 10 is in communication with a closed loop 14 having a point of connection 15 to a pump (not shown). The loop passes through first and second heat exchangers 16a,16b shown respectively on the left and right-hand sides of the loop.

In use of the device shown in Fig. 1, an oil medium is used, by way of example, in the loop. Depending on the input and output temperatures, the oil medium may contain 30% solids, in that arm of the loop in heat exchanger 16a, and a low concentration of solids, i.e. nearly clear oil, in the other arm, in heat exchanger 16b. A solids concentration gradient or the cyclone 13, if present, causes a predominant flow as indicated by arrows, and thus provides a greater concentration of particles in the chamber 10 than in circulation in the loop 14.

The circulating store illustrated in Fig. 2 has many of the same components, with the same reference numerals, as the embodiment of Fig. 1. Slurry returns to the character 10 at 14a, and clear fluid passes out to a primary circulating pump (not shown) at 14b. Clear fluid returns via a valve 17a, and pumped slurry, at a controllable solids concentration, passes through a valve 17b. The rotatable control device 12 is positioned in what is effectively a missing chamber 18. The chamber 10 includes supply devices 19.

The apparatus of Fig. 2 has been operated, using a primary chamber whose right cylindrical part is 300 pm in diameter and 900 pm high. The control device is rotatable at 1-10 Hz.

In experiments, it was found that the "transport" section of the apparatus could be controlled within the range 0-40t by adjusting the rate of rotation of the metering screw. The slurry then passed through the external circuit and returned to the top of the storage tank where the solids were separated from the oil, and the cycle was repeated. A non-specialist pump could be used to provide the primary circulation effect in the external circuit, and the effective energy content of the slurry could be controlled by changing the concentration of solids. The system should be relatively easy to scale up to commercial size because of the independence of the storage function (operating at a high concentration of material) from the rheology of the pumped mixture in the external circulation system.

It was found that the addition of two valves (17a and 17b) adjacent to the mixing chamber provided an additional "rapid response" control of the mixing process, by allowing the pressure difference across the mixing chamber to be varied and thus the concentration of solids in the circulating flow to be controlled, independently of the rate of rotation of the screw.

Using this apparatus, it was found that the store could be maintained at an effective solids concentration of > 508 while the circulation system operated at 0-40%, providing separate thermal and rheological control of the processes at each stage. In a commercial device, it is probable that some positive means of separating the solids from the oil on the return limb may be desirable, and a form of liquid cyclone is recommended for this purpose so that the range of flow rates used in the external circuit can be increased without detriment to the separation process.

The particles pass downwardly through the lower neck of the chamber, and are urged through the neck by the auger, at a rate controlled by the shape and rate of rotation of the auger and any applied pressure differential. It is desirable that the particles should be of a high tear strength material. If a hydrophilic material is used, AN-VP is preferred.

Figure 3 illustrates how suitable hydrophilic materials can be prepared cheaply on a continuous basis.

Liquid monomer is sprayed by means (not shown) at 20, in the direction indicated, onto a belt 21, e.g. of polyethylene or PTFE, mounted on rollers 22a, 22b driven by means (not shown) in the direction indicated. The spraying may be conducted, if necessary, in a controlled (N2) environment. A layer 23 of material is formed on the belt, and is irradiated by means of a light source represented by hoods 24. If W radiation is employed, e.g. Hg discharge lamps, then a W-sensitive initiator such as DMPA is used.

The resulting polymer is removed continuously from the belt using a stripping blade 25. It can then be ground to size, for use.

In experiments using the apparatus shown in Fig. 3, a rough strip of polymer, 1-2 Am thick was obtained. This was well suited to grinding in a hammer mill.

This process has potential for the production of hydrophilic materials for non-ophthalmic medical applications, e.g. in skin care and wound care. In particular, a high-strength, high purity cross-linked material, e.g. based on acrylonitrile and vinylpyrrolidone can be produced, in powder form.

Claims (11)

1. A thermal storage system comprising first and second heat exchangers connected by a closed loop containing a fluid medium in which are suspended particles comprising a material that undergoes a reversible phase change on thermal input/output, the system comprising also a chamber containing the fluid and a relatively high particle concentration, means for metering the particles from the chamber into the loop, and means for returning particles from the loop to the chamber.

2. A system according to claim 1, wherein the metering means comprises a auger that may also serve to stir the suspension in the chamber.

3. A system according to claim 1 or claim 2, wherein the means for returning particles comprises a cyclone.

4. A system according to any preceding claim, wherein the particle concentration in the loop is up to 30% solids by volume, and the particle concentration in the chamber is more than 30% solids by volume.

5. A system according to any preceding claim, wherein the particles are of a hydrated hydrophilic material.

6. A thermal storage system comprising a chamber containing a heat exchanger and, surrounding the heat exchanger, a concentrated suspension of a hydrophilic material having a water content of 85 to 98% by wet weight.

7. A system according to claim 6, which additionally comprises means for circulating the suspension.

8. A system according to any of claims 5 to 7, wherein the hydrophilic material is an acrylonitrilevinylpyrrolidone copolymer.

9. A refrigerator comprising a system according to any preceding claim.

10. A method for preparing particles of a hydrophilic polymer, which comprises spraying monomer onto a moving substrate, irradiating the monomer to form a continuous sheet of polymer on the substrate, stripping the sheet of the substrate and rendering the sheet into particles.

11. A method according to claim 10, wherein the polymer is an acrylonitrile-vinylpyrrolidone copolymer.