GB2183326A - Latent heat thermal storage heaters - Google Patents

Abstract

A storage heater comprises a latent heat store 1, the preferred medium being caustic soda. The store may be surrounded by a fluid 6 which is contained in an outer tank 3 filled to a level 7. Heat is provided by a heating element 2. During heat output the heating element 2 is switched off, the baffles 5 are opened and the fluid 6 rises in natural convection currents, circulating around the divider plates 4 and thus carrying heat away from the heat store 1. During heat input the heating element 2 is turned on and the baffles 5 are closed, resulting in very little heat transfer to the outer tank 3 or the surrounding air. The store may be in direct heat exchange contact with air, either by natural or forced convection. The store may comprise a plurality of separate containers. <IMAGE>

Description

SPECIFICATION Latent heat storage system This invention relates to a latent heat storage heater whereby latent heat is released from a storage medium buy a liquid to solid phase transition and stored by melting the solid phase of the medium.

The advantage of a latent heat storage system over a conventional heat storage system is the controllability of the heat output. Heat istaken up and given off at a constant temperature. The passive heat loss rate is small since the heat is stored at much lower temperatures than in conventional heaters and the differences between store and ambient temperature is therefore small. Consequently, the heating system may be virtually turned off when not required, whereas conventional heaters can only be turned down slightly. This should mean much lower running costs.

Latent heat storage systems which use as a storage medium an aqueous solution of a salt hydrate (NaSO4.H2O) are art already known. However, the known disadvantages of using such salt hydrates as storage media, is that they have low crystallization velocities and a reluctance to nucleate. As a resultof this, when the storage medium is cooled below its melting point, itdoes not reliablysolidify. That isto say, supercooling tends to occur. Large degrees of supercooling are undesirable in storage heater application. Proposals have been made for overcoming this problem, but these often involve additional apparatus being attached to the basic storage heater system. For example U.K. Patent No.

1543655 proposes the combination of a heat pump and a heat exchanger to attackthis problem. Such additions increase manufacturing costs and possibly also running costs, and it would be better if the problems were removed altogether.

One of the objects of the present invention isto provide, as a storage medium for a latent heat storage heater, a substance which substantially does nothavethesupercooling problems that are associated with existing salt hydrate storage media, while retaining the advantages associated with using a latent heat storage medium, most significantly, controllability of heat output. The net result maybe a controllable, yet robust latent heat storage system, susceptible of easy and inexpensive manufacture.

According to the present invention there is provided a storage heater comprising heat-storage means having a material in which the bulk ofthe heat is stored as latent heat, which material, in use, changes phase at a convenienttemperature and which material, when heated to convert it from one of its phases to the other, remains in that other phase for a considerable period oftime and which reverts to its first phase, giving up latent heat as it does so, without exhibiting a significant supercooling effect.

Preferably, the heat-storage means comprises caustic soda (NaOH.H2O) in eutectic solution. It has a high latent heat of fusion, around 446kJ/1 and a melting point of about 61"C. This follows a store comparable in size to a conventional unit, butata maximum operating core temperature of approximately 90"C instead of 750 C. Owing to this low storage temperature, it is feasible to achieve minimal passive heat loss rates. Thus the heating system can be adjusted to have a high heat output when required orvirtuallyswitched offwhen not, the net result being a controllable storage heating system.

Preferably, the heat-storage means comprises a plurality of containers, each holding some of said material.

Caustic soda is cheap, readily available and can be stored in containers of mild steel, stainless steel or plastic. Thus, containers can be made cheaply and moreover, since caustic soda exhibits only a 1.8% reduction in volume in going from its liquid to its solid state, there is no major problem associated with change of volume of the eutectic material upon phase change.

It is proposed that prefilled containers of caustic soda, of up to approximately 25kg, be used for the fabrication of the heater, thus removing the necessity of handling the caustic soda during manufacture of the heater. These containers may be shaped, having grooves to improve both stiffness and surface area, and may also have foil heating elements adhered to their walls. Since the peak store temperature will be around 90"C, cheap insulation materials, such as polyurethane foam, expanded polystyrene and glass blanket may be used.

Preferably, the heat-storage means is surrounded buy a liquid or by air.

Preferably, there is provided means to effect circulation offluid around the heater, thereby to provide a heat transfer.

Heat may be transferred from the heat store using a number of methods. The store may be surrounded either by a liquid or by air and then the heat transferred by natural convection. Alternatively, a fan can be used to blow ambient air over and around the latent heat store.

Embodiments ofthe invention are described more fully below with reference to the accompanying drawings in which Figures 1 {at - (c) represent schematic longitudinal cross-sectional views ofthe heater, with its latent heat store surrounded by afluid; Figure2 is a schematic cross sectional view of a heater which uses natural air convection for heat transfer; Figure3is a schematic cross-sectional viewofa heater which uses forced air convection for heat transfer; and Figure 4 is a schematic cross-sectional view of an arrangement for reducing radiative heat loss.

In one embodiment ofthe invention, shown in Figures 1(a) - (c), there exists a latent heat store 1 the preferred medium being caustic soda (NaOH.H2O), surrounded by a fluid 6which is contained by an outertank3filledto a level 7. Heat is provided bya heating element 2. During heat output (Figure 1 (a)), the heating element 2 is switched off, the baffles 5 are opened andthefluid 6 rises in natural convection currents, circulating around the divider plates 4 in the direction indicated by arrows 8 (a) and thus carrying heat away from the heat store 1.

During heatinput,shown in Figure 1(b),the heating element2 is turned on and the baffles 5 a re closed, giving convection currents as indicated by arrows 8 (b) and resulting in very little heattransfer totheoutertank3orsurrounding air.

In orderto turn the heating system off the heating element2isturned offandthe bafflesSareclosed.

Convection currents are as indicated by arrows 8(c) and there is little heattransferto the airsurrounding the heater.

Figure 1(c) representstheheaterduring quiescence, namely with the heating system turned off.

In another embodiment of the invention, shown in Figure 2, there exist latent heat stores 9, the preferred storage medium being caustic soda (NaOH.H2O), surrounded by air 10. The whole system, that is heat stores and circulating air, is enclosed in an insulated casing, shown in more detail in Figure 4, with air inlets 11 and outlets 12 provided to allow circulation of air around the heat stores 9. The heating elements are included in theheatstores9 and the heateris controlled by the baffles 13.The baffles 13 are open for heat output (heating elements off), closed for heat input (heating elements on) and closed afterthe heating system is turned off(quiescent). During heat output, baffle 13 being open, the aircirculates around the heat stores by natural convection in the direction of arrows 14.

In yet another embodiment of the invention, Figure 3, there exists a latent heat store 15, the preferred storage medium being caustic soda (NaOH.H2O), incorporating a heating element which is surrounded by air 16 and insulated casing 17, of thetypeshown in Figure4.Afan 18drawstheairin through air inlet 19, blows itaroundthe heat store 15 and outthrough airoutlet20 inthe direction of arrows 2, when heat output is required. During heat input and when the heating system is turned off (quiescent), the fan is turned off. Reversing the direction ofthe fan will cause air to be drawn in through 20 and outthrough 19.

Figure 4is a longitudinal schematic cross-section of part of the type of insulation system most likely to be used in heaters ofthe sortshown in Figures 2 and 3. Reference 21 indicates the latent heat store, 23 the air passage, 24 the outer casing and 25 the insulation material, which may be polyurethane foam, expanded polystyrene, glass blanket or some other substance with similar insulating properties. In order to prevent radiative heat loss a reflecting layer of aluminium foil 25 may be placed insidethe insulating material.

Claims (6)

1. Astorage heater comprising heat-storage means having a material in which the bulkofthe heat is stored as latent heat, which material, in use, changes phaseata convenienttemperatureand which material, when heated to convert it from one of its phases to the other, remains in that other phase fora considerable period oftime and which reverts to its first phase, giving up latent heat as it does so, without exhibiting a significant supercooling effect.

2. A storage heater according to Cl aim 1,wherein the heat-storage means comprises caustic soda in a eutectic solution.

3. AstorageheateraccordingtoClaiml orClaim 2, wherein the heat-storage means comprises a plurality of containers, each holding some of said material.

4. A storage heater according to any one of the preceding claims, wherein the heat-storage means is surrounded buy a liquid or by air.

5. Astorage heateraccordingto anyoneofthe preceding Claims, comprising means to effect circulation offluid around the heater, thereby to provide a heat transfer.

6. Astorageheatingsubstantiallyas hereinbefore described with reference to, and as illustrated in, anyone of Figures 1 to 3, either alone or in combination with Figure4,ofthe accompanying drawings.