CA1168421A - Method and apparatus for storing thermal energy - Google Patents

Abstract

ABSTRACT
A method and apparatus are disclosed for storing thermal energy. A
hydroxide of calcium, magnesium or barium is converted in a first container to the oxide and steam by being exposed to solar radiation. The steam is passed to a holding container and condensed. In another container capable of communicating with said first container is disposed a solution of an alkali metal hydroxide.
When the solar radiation discontinues this latter container is connected to said first container and, at the temperatures obtaining, the vapour pressure above the alkali metal hydroxide is sufficient to cause water vapour to flow into the first container and reconvert the alkaline earth metal oxide to hydroxide, at the same time concentrating the alkali metal hydroxide solution. When solar radiation recommences the process is repeated eventually affording very concentrated or practically anhydrous sodium hydroxide. When heat is required in colder weather the water in the holding container is contacted with the concentrated alkali metal hydroxide resulting in an exothermic reaction which can be heat exchanged in conjunction with a water filled central heating system.

Description

The invention relates to a method for storing thermal energy, in which a hydroxide, for example calcium, magnesium or barium hydroxide is decomposed by the addition of heat into an oxide and steam and these substances are separat-ed and subsequently brought together again. The invention is also concerned with an apparatus suitable for carrying out the method.
Methods and apparatuses of this kind are described in United States Patents 3973552 and 4054126, in which a hydroxide of calcium, magnesium barium is broken down into oxide and steam by the passage therethrough of a very hot heat-conducting gas. Later on, during the winter, when heat is required, the same heat-conducting gas is passed, at a low temperature, together with steam, through the oxide. Hydroxide is formed again from the oxide, the heat-conducting gas heats up and may be passed to a heat-exchanger where it releases its heat to the water in a hot water central heating unit.
This method has the disadvantage that the containers in which the hydroxide, and the oxide subsequently formed therefrom, are stored, must also be heated to the necessary decomposition temperature of 300C. The amount of heat to be recovered from one kilogram of calcium oxide is about 20 times less than the heat obtained by the combustion of one kilogram of petroleum. The density of calcium oxide powder may vary very widely, between about 0.4 and 0.8 kg/dm3.
Since the container is only about 40 to 50% full, a container with a capacity of at least 50 m3 will be needed to store the same amount of heat recoverable from 1 m3 of petroleum. If the whole container must be heated to 300C, heat losses from the surface of the container will normally be great. It appears unlikely that such heat losses could be reduced at a reasonable cost.
It ~s the purpose of the invention to provide a method for storing thermal energy~ and an apparatus for carrying out the method, in which the amount of material used and the heat losses are reduced.

According to the invention, this purpose is achieved in that by means of the oxide, water is removed from a first, water-containing, hygroscopic sub-stance, e.g. a hydrate or a solution of sodium or potassium hydroxide or calcium chloride; after reconversion of the oxide thus formed, this procedure is repeat-ed, by adding further heat to the oxide, until the hygroscopic substance reaches the form of a highly concentrated solution, a solid hydrate, or an anhydrous con-dition; and in that, for the purpose of recovering heat, the dehydrated hygro-scopic substance is caused, by water and/or steam, to react exothermally.
This invention therefore provides a method of storing and subsequently utilizing thermal energy, which comprises:
(a) heating calcium, barium or magnesium hydroxide to above 300 with concentrated solar radiation to decompose the hydroxide into the oxide and steam and collecting and condensing the steam;
(b) thereafter removing water vapour from an aqueous hygroscopic substance comprising sodium or potassium hydroxide or calcium chloride and contacting said water vapour with said oxide to reconvert it to the hydroxide;
(c) repeating alternately steps (a) and (b) to progressively concen-trate said hygroscopic substance to attain a highly concentrated hygroscopic solution or a solid hygroscopic substance; and (d) when required, subsequently regenerating thermal energy by contact-ing said concentrated hygroscopic substance with water derived from said collected and condensed steam from step (a) so as to cause an exothermic reaction.
In a second aspect, this invention provides an apparatus for storing and subsequently utilizing solar energy according to the method of the invention, which apparatus comprises:
(a) a first container for a heat-decomposable hydroxide;
(b) means for directing solar radiation onto a heat receptor located in said first container and capable of being contacted b~ hydroxide therein so as to decompose said hydroxide to oxide when solar radiation is incident on said receptor;
(c) a second container in selective communication with said first container for collecting and condensing water vapour evolving from decomposition of said hydroxide;
(d) a third container in selective communication with said first container and adapted to contain an aqueous solution of a hygroscopic substance, the third container being divided into a large storage container connected to the first container and a smaller heat producing container connected to the second container and cooperating with a heat exchanger for the release of heat, the two part-containers being connected to each other in both directions by lines which can be shut off, the arrangement being such that on discontinuance of solar radiation communication is established between said first and thi-rd containers and that water vapour above said aqueous solution passes to said first container to contact said oxida therein and reconvert it to hydroxide, ~e) a heat-exchanger disposed in the system between the first and second containers, heat from steam formed in the first container being trans-ferred in said heat-exchanger to the hygroscopic solution in the third container, the steam developed in the latter container being adapted to pass to the second container, and (f) the Eirst container being arranged in a cooling-fluid circuit which comprises an additional heat-exchanger in which heat taken from coolant in the first container is transferable to the hygroscopic solution of the third container, the steam developed in the latter being transferable to the second container.
The main advantage of the invention is that only a small container need be heated to store the heat. The means for this is the transfer of the ~ ~r,;~.?
~ - 2a -$~3~

hygroscopic condition from one substance to another, e.g, from calcium oxide to some other hygroscopic mass, e.g. a sodium hydroxide solution containing little water or solid sodium hydroxide.
The apparatus proposed~ according to the invention, for carrying out the new method Collsists of a first container in which the hydroxide may be broken down by the heat ~rom solar radiation, a second container in which water may vapourize, thus withdrawing the heat of evaporation from the environment, the said apparatus being characterized by a third container containing a hygroscopic solution, or the hydrate of a hygroscopic substance, the area there-in above the solution being connected alternately to the first and second container, and by a heat~exchanger between the said third container and a heat-consuming heating device.
~ n one preferred embodiment of the invention, the solid hygroscopic substance, or the highly concentrated hygroscopic solution, is taken in small batches from a large stock and is caused to react exothermally with water or steam. In the apparatus provided for the purpose, the third container is divided into a large storage container to be connected to the first container, - 2b -&~

and a smaller heat-producing container whic~ is to be connected to the second container and which co-operates with the heat-exchanger for the release of heat.
The two containers are connected together, in both directions, by lines adapted to be closed. The particular advantage of this arrangement is that, for the release of heat, only a small container, instead of a large one, is heated, where-by insulation is simplified and heat-loss is reduced to a minimum.
According to another preferred embodiment of the invention, the batches of solid hygroscopic substanc0 are removed by passing an unsaturated solution through the stock, the highly concentrated solution thus formed being caused to react exothermally, in another location, with water and/or steam. If, for the purpose of dissolving the solid hygroscopic material, water were used instead of the hygroscopic solution provided, a very large part of the stored heat would be released already in the storage tank, and without unusual architectural precau-tions, this heat would have to be regarded as lost. In normal designs, only the heat produced in the small heat-producing container is usable. Since a moderate-ly concentrated solution is used to convert the solid hygroscopic mass, the ef-ficiency is substantially higher than if ~he solid mass were dissolved in water.
The invention is explained hereinafter in greater detail in conjunction with the embodiments illustrated in the accompanying drawings, in which:
2Q Figure 1 is a diagrammatical representation of a basic design of an apparatus~ for the storage of thermal energy;
Figure 2 is a flow sheet of an improved installation;
Figure 3 a, b and c are vertical and horizontal sections through, and a plan view of, container 10 of Figure 2;
Figure 4 is a cross-section through container 30 of Figure 2;
Pigure 5 ~s` a modi$ication o$ container 30;
~igure 6 i~ a $10w sheet of another installation;

Figure 7 is a flow sheet of a simplified des;gn of storage installation;
and ~ igure 8 is a flow sheet of a still further heat storage installation.
The apparatus according to Figure 1 comprises a container 10 contain-ing a hydroxide, e.g. calcium, magnesium, or ~arium hydroxide and a cen~ral re-ceptacle 5 which is empty. The hydroxide is arranged around the said receptacle and lies against the external surface thereof. The top of container 10 has a circular aperture sealed off with a glass plate 24 and a silicone rubber seal 26.
When the sun shines, convergent rays pass from a concave mirror 16 to a convex mirror 18, the curvature of which is such that the reflected rays are approxi-mately parallel with each other. The said rays impinge upon glass plate 24 and pass through the aperture iTI the top of container 10, into the interior of receptacle 5.
The receptacle, and the hydroxide lying thereagainst (in this case calcium hydroxide) heat up. At a temperature of less than 400C, calcium oxide is formed and the water released passes into the gaseous state. It is easy to make container 10 of a si~e such that the amount of calcium hydroxide therein is somewhat larger than the amount which is decomposed by radiation on a very sunny day~. The water formed in container 10 passes, in the gaseous state, through a

2~ pipe 8, through a valve 9 shown open, into a second container 20 where it is condensed. Towards evening, when solar radiation is so weak that decomposition no longer takes place, valve 9 is closed and another valve 13, located in a line 22 running to a third container 30, is opened. Container 10 is now connected, through line 22 to container 30. Both containers are airless. Container 30 has a large base, is lo~ in height, and is half-filled with water in which sodium hydroxide is dissolved. At the temperatures obtaining, this solution has an appreciable water-vapour pressure and steam therefore flows into container 2'~

10 where it immediately comBines with the calcium oxide, thus producing hydroxide.
Steam thus flows constantly into container 10 and the concentration of the sodium hydroxide solution therefore increases. Heat is produced in container 1OJ where-as in container 30 the solution cools down. The speed a-t which this takes place depends upon how rapidly the heat formed in container 10 is carried away and how rapidly the removal of heat in container 30 is compensated for. It is desirable for the dimensions of containers 30 and 10 to be such that, by the end of the night, the calcium oxide in container 10 is fully com~erted into calcium hydrox-ide. As soon as hot sunshine begins to convert the hydroxide again into steam and calcium oxide, valve 9 is opened and valve 13 is closed, and the previous cycle is repeated. The water formed passes to container 20 and, towards evening, the calcium hydroxide is conver~ed into calcium oxide. During ~he night, after valve 9 is closed and valve 13 is opened, hydroxide is formed again and the solu-tion in container 30 is further concentrated. Since this is repeated every night, by the time winter comes the solution in container 30 has reached a very high degree of concentration. It is substantially better if the concentrating process is continued until solid hydrates crystallize out of the sodium hydroxide~
or until a solid mass of hydrates and/or an almost anhydrous sodium hydroxide is o6tained. This mass, or the concentrated solution, will be ab~reviated herein-after to hygroscopic storage substance.
As it becomes colder operation o~ the heating system is needed, valves 9 and 13 are closed and valve 19, in a connecting line between containers 20 and 30, is opened. The highly concentrated solution in container 30 has a low vapour pressure as compared with the pure water in container 20. The water therefore vaporizes and condenses in container 30. The hygroscopic storage substance in container 30 heats up and releases its heat to the water in an underlying contain-er ~0 acting as a heat-exchanger, since the bottom of container 30 also consti-tutes the top of container 40. Water from a water heater, cooled down by cir-culating through radiators, passes through a pipe 47 into container 40, where it is heated, and flows through a pipe 49 back to the radiators where it releases its heat in order to heat the rooms.
In order to compensate for possible leaks in the piping or containers, a vacuum pump 90 is provided to eliminate any air penetrating into the system.
A second container of calclum oxide or calcium hydroxide may be provided, and this may be subjected to solar radiation alternately with the first. When cal-cium hydroxide decomposition takes place in the first container, and the water thus formed condenses in container 20, the other container may remain connected to container 30, so that water may also be expelled from the hygroscopic storage substance during the day-time. This is a great advantage if the said substance is already highly dehydrated and further dehydration progresses only sluggishly.
In the winter time, therefore, the unit draws its heat partly from the outside, thus operating like a heat-pump, since the heat required for vaporizing the water in container 20 is taken from the environment. This heat may be sup-plied from the ground through the wall of container 20, or other means may be used for the purpose~ For example, a system of pipes may be arranged in the in-terior o~ the container, through which outside air is passed as long as it is warmer than the water in the container.
Naturally, the system can also be used to provide a supply of warm water in summer time, for example a domestic hot water supply~
It is, of course, also possible to use lenses in place of the mirrors shown in Figure 1.
In the installation described hereinbefore, the conversion of solar radiation i~nto heat is carried out in an area which contains no air but contains more or le~s steam, depending upon operating conditions, and at a location as z~

close as possible to the centre o the calcium hydroxide mass to be decomposed.
If the conversion of solar radiation into heat were to take place on the outside of container 10, the temperature of the container would be higher than that of the mass of calcium hydroxide inside, so that a large part of the heat produced would be lost to the exterior. In contrast to this, in the design of container 10 according to the invention, there is a reverse temperature drop, and the heat-loss by radiation is therefore less, especially if receptacle 5 is bottle shaped.
The point at which the rays enter container 10 cannot be merely an aperture since air would enter and interfere with the vaporizing process. The point of entry must pass light, not air, and is therefore in the form of a glass plate 24.
It is also desirable for the mass to be placed in contain0r 10, whether it be calcium oxide or calcium hydroxide, to contain a small amount, about 1%, of NaOH. It is believed that this increases the reactivity of the calcium oxide, especially since, in preparatory chemistry, so-called soda lime, and not pure calcium oxide, is used for drying gases.
In the embodiment according to Figure 1, the sodium hydroxide may also be replaced by another hygroscopic substance, for example potassium hydroxide or calcium chloride.
The use of calcium chloride for eliminating water from the hygroscopic storage substance permits uery extensive dehydration and has the following advan-tages. The whole mass of hygroscopic storage substance may be subjected simul-taneously to the dehydration process at normal ambient temperature. The result-ing crystals (for example of NaOH.6}l20 or NaOH.4H20) have large surfaces where further dehydration can take place. In addition to this, the crystal structure is loosened still further by the loss of water. However, if a sodium hydroxide solution were required to be concentrated in a vacuum, without the use of calcium oxide, by direct heating by solar radiation, the resulting steam would have to be ~ `

condensed at a~out 20CJ and the resulting water would have a vapour pressure of 17.5 torr or 2337 pascals~ Since the vapour pressure of the solution to be con-centrated must be higher, the temperature must be above 60C if the solution con-tains 50 parts of NaOH per 100 parts of solution. Since the large amounts of NaOH require correspondingly large containers, there would be a considerable loss of heat from the walls thereof. Moreover, the surface where dehydration can take place would still be only the flat surface of the liquid, i.e. that of the solu-tion or molten hydrates. The solution could certainly be pumped into a smaller container for concentration in order to reduce heat loss, but this would be dif-flcult at concentrations of more than 54 parts of NaOH per 100 parts of solution, because the hydrates crystallize out.
It would be possible for the concentrated sodium hydroxide solution, or calcium chloride solution, in contact with the steel plate o-f the container, to develop a small amount of hydrogen in the course of time. This could be eli-minated with the vacuum pump 90 in Figure 1, since hydrogen would also interfere with the vaporization process. However, it is also possible to line container 30 with polyethylene or polypropylene foil, or with some other synthetic material resistant to alkali, thus preventing the formation of hydrogen and inhibiting slow corrosion. It is also possib]e to accommodate the sodium hydroxide solution 2a or the calcium chloride solution in smaller individual containers made of poly-~tyrene which are open at the top and are arranged within large steel container 3a. The latter may also be made of sheet copper. The sodium hydroxlde solution may remain in direct contact with the sheet copper without causing corrosion, since the interior is airless and is thus free of oxygen. The heat required in summer time in eliminating water from the hygroscopic storage substance may be supplied to container 30 by fitting to the outside of the container wall pipes in which water circulates, the said water picking up heat from the ambient air in ~ r~,f,~.~

a heat-exchanger and releasing it to the wall of the container. At the start of the heating periodJ the water would be allowed to drain from this system, provid-ing the said container with good thermal insulation from the surrounding earth.
Instead of a single container 30, two may be used, one of which con-tains sodium hydroxide and the other caclium chloride, both containers being arranged in a larger steel tank. In the event of major structural damage, by fire for example, bursting of the containers would be less of a threat to the environment, since the two substances would mix, forming sodium chloride and cal-cium hydroxide which are substantially less aggressive.
lQ The necessary parts of the apparatus may be arranged in various ways in the complex to be heated. In the case of a residence, the optical system, with container 10, may be installed on a flat roof. Known guide-mechanisms for solar-energy collectors may be used to adjust the mirrors to the motion of the sun. Containers 30 and 40 could be accommodated in an intermediate storey only about 1 m in height between two inhabited storeys in a residence. In this case, heat which does not pass to the central-heating system and would normally be lost, would reach the rooms to be heated through the floor of the upper storey or the ceiling of the lower storey. Container 20 may be located in the ground under the building to be heated, in the ground in an adjacent lot, or above ground.
2a. The valves would, bf course, be actuated by an automatic system.
Furthermore, it would naturally be better for container 40 to enclose most of container 30 instead of contacting it only on one side, an arrangement used merely to slmplify the drawing, since this would improve the transfer of heat and reduce heat-losses.
~i~gure 2 shows an improved design of heat-storage unit, Figure 3 shows details of container 10, and Figure 4 details of container 30. The solar radi-ation is focussed by a concave mirror or a condensing lens9 passing through quartz-glass sheet 105 (~igures 3 a, b, c) into the interior of container 10.
Sheet lQ5 of quartz-glass is secured by a silicon-rubber sealing ring 106 to the top 107 of a straight length o~ pipe 108. The inside of the lower part of the pipe is coated with a thick layer la9 of copper which may be applied by plating.
The interior o~ pipe 108 is preferably airless and free of water-vapour, communi-cating through aperture 110 with the space between covers 162 and 192. The pipe 108 is surrounded by eleven annular zones 111 to 121 arranged one above the other.
The top cover of each zone has an aperture carrying a short, upwardly directed, conical length of pipe, for example zone 111 has an aperture 131 and a length of pipe 141. Up to about 90% of the free interior space of the zone is filled with calcium hydroxide to which is added some lumps of iron oxide. Zones 111 to 121 are sealed off airtightly from the surrounding area by cylindrical wall 150, base 151, and cover 152. Cylindrical wall 160, base 161, and cover 162 enclose the cylindrical area formed by the zones. The space between walls 150 and 160, and covers 152 and 162, is divided into two symmetrical halves by a partition 232.
Inlet pipe 222 opens into front half 234 and outlet pipe 224 into rear half 236.
The said partiti~n does not continue into the space between bases 151 and 161.
Cylindrical walls 170 and 180, together with bases 171 and 181, form an insulated vessel, a so-called Dewar flask, as do cylindrical walls 190 and 200 with covers 2Q 192 and 202. A pipe connector 240 is fitted to cylindrical wall 200. At this locati~on, cylindrical walls 180, l90 and 2~0 are pierced by holes, the center-lines of which coincide with the centreline of connector 240. The latter is con-nected to a vacuum-pump ~not shown), so that a vacuum may be maintained in spite of minor leakage. The bases of the stages are coated with a thick layer of copper by surface-welding. 122 is a silicon-rubber seal.
The solar rays entering pipe 108 produce considerable heat therein, this heat being distributed vertically by layer 109 of copper and horizontally by the copper coating on the bases of the ~ones. In Figure 3 this copper coating is shown, for simplicity, only for one stage 112. The calcium hydroxide heats up and ~orms, in the airless space at about 320C, calcium oxide and steam.
This passes through the short-conical pipes, outlet pipe 204, and valve 206 (Figure 21 into contalner 2Q, where it condenses. This container may be buried in the ground. At this time, valves 57 and 205 are closed. Towards evening, when the rays of the sun become so weak that decomposition terminates, valve 206 is closed and valve 205 ls opened. Container 10 is now connected to container 30 through line 207. Both containers are airless. Container 30 contains a large number of smaller tanks, each of which is open at the top. For the sake of clarlty, only four small tanks 31, 33, 35, 39 are shown in Figure 4. The hygro-scopic mass is distributed into these containers. If the unit has not yet stored any heat, this mass constitutes a hygroscopic solution, in this case a sodium hydroxide solution. At the temperatures obtaining, this solution has an appre-cia~le water vapour pressure and steam therefore flows into the spaces between the 70nes in container 10, where it immediately combines with the calcium oxide and produces hydroxide, and steam therefore flows continuously, and the concen-tration of the sodium hydroxide solution therefore increases progressively. Heat is produced in container 10, whereas the solution cools down in container 30.
2Q Fan 80 is switched on and produces a flow of air. This passes through line 220, into front part 234 ~defined by the partition) of the space between 150 and 160, in a downward direction. The flow of air arrives between bases 151 and 161 and ascends again in the rear part 236 (Figure 3 b) of the space between walls 150 and 160. Along this path, it has picked up heat produced by the formation of calcium hydroxide. The said ~low of air reaches outlet pipe 224 and flows thrQugh line 226 into container 60. ~alves 221 and 61 are open, valves 223, 62 and 65 are closed. In container 60, the air sweeps over the outer wall of container 66 and heats it. ~ithin this container, pump 64 passes the hygroscopic solution over slightly inclined sheet metal plates, so that the solution forms a large free surface favouring evaporation. The heat picked up in container 66 expels the steam from the solution. The steam then condenses in container 20 and the concentration of the solution in container 66 lncreases. As soon as the degree of concentration is adequate, valve 61 is closed, ~alve 65 is opened, and pump 34 is switched on, causing the solution to flow through line 37, from above, into container 30. Valve 65 is then closed, pump 34 is switched off, and valve 62 is opened. A dilute solution 1OWS from container 30 into container 66. This solu-lQ tion is similarly concentrated and pumped ~ack into container 30. When, in the course of time, the solution in container 30 is almost saturated by removal of water, further concentration therein is no longer possible since, upon flowing ~ack through line 36, a highly concentrated solution cools down, precipitates solid hydrate, and may block up the line. In this case~ the flow of air must carry away the heat produced by the calcium hydroxide, expelling it to atmosphere through valve 223.
The heat released during condensation of the steam formed during de-composition of the calcium hydroxide may be used to concentrate unsaturated solu-tion. Two methods of operation are possible. The first is as follows:
~alve 57 is opened, valves 205, 206 and 55 are closed,and ~alve 52 is opened unt~l a certain amount of dilute solution has entered container 56, and is then closed again. Pump 54 is switched on, so that the solution flows over the built-~n, slightly inclined, sheet metal plates, forming a large free sur-face. The s~e~ released during decomposition of the clacium hydroxide in con-tainer 10 condenses on the outer walls of container 56, heating said container and the solution therein. Water is expelled in the form of s~eam~ which flows through valve 53, now-open, and condenses in container 20. As soon as the calci-um hydroxi~de has~b~een decomposed, valve 51 is closed. When, in the course of time, the solution in container 30 is almost saturated by removal of water, valve 57 must be closed and valve 2Q6 opened, so that the steam passes directly to con-tainer 20 where it condenses.
Slnce the steam generated by decomposition of the calcium hydroxide must heat the sodium hydroxlde solution to about 60 to 80C, the temperature at which it condenses is correspondingl~ high, as is its vapour pressure. Therefore the temperature at which the calcium hydroxide decomposes also increases by about 50 to 100C, i.e. to about 380C. This dlsadvantage may be overcome by the sec-la ond method of operation which is as follows:
The valve setting is the same, except that valve 53 is open. The steam originating with hydroxide decomposition reaches heat-exchanger Wa l through valve 57 and is cooled by the outer walls of container 56 from about 300 down to about 80C. Only the heat transferred by this cooling is used here to expel the steam from the solution. The said steam flows through valve 53 into container 20, where it first condenses. This is a very large container, and the temperature thereof is increased only slightly by the relatively small amount of steam. The steam vapour pressure is therefore lower, as is the temperature at which the calcium hydroxide decomposes.
~or the purpose of removing hea~, valves 42, 52 and 62 should be closed.
~ump 34 is switched on, valve 45 is opened, and solution from heat-producing con-tainer 46 is-pumped into container 30. Dissolving the solid hygroscopic mass converts it into an almost concentrated solution. The opening of valve 42 allows it to flow into container 46 after pump 34 is switched off and valve 45 is closed.
Pump 44 i~s switched on and the concentrated solution flows to the interior of heat-producing container 46 over a plurality of slightly inclined surfaces, the said solution thus forming a large, free sur~ace favouring condensation. Valve 41 is openedJ valve 42 is closed, and steam passes ~rom container 20 into con-tainer 46 where it condenses, dilutes the solution, and heats up. The heat is transferred, through the outer wall of container 46, to the water of a cen~ral heating system flowing past said wall. The hot water supply pipes bear the reference-numerals 47 and 49. Container 30 must be very large since it must accommodate a large heat-storage mass in the form of hydroxide, hydrate or solu-tion. The amount of solution in container 40 is, on the other hand, very small.
After being pumped into container 30, this small amount would normally seep into the large hygroscopic mass and no further solution would emerge. In order to avoid this, container 30 may be divided into a number of containers, for example a small, a med~um, and a large container. ~irst the mass in the small container is used, then that in the medium container, and finally that in the large con-tainer. When the large container begins to be used, a substantially larger amount of solution, produced by use of the medium container, is available.
Another way o~ overcoming this difficulty is to connect line 37 to the underside of the bottom of container 30, with a ball made of steel, copper, or an alkali-resistant plastic lying upon the point afentry of the line into the tank.
It should be covered by a rubber element in the form of a bell which is very wide at the bottom. Any solution in the container presses the outer edge of the rubber bell tightly against the bottom of the container. The remainder of the rubber element is not pressed against the bottom of the container, but retains its shape, being supported by the ball. A small amount of solution under the rubber element can drain into the line under the ball, since the latter does not seal cff the end of the line. Pump 34 should be capable of raising the pressure of the solution to a value of one atmosphere. Now when, after storing heat for a long time, the solution in container 30 has been converted into a thick layer of s~lid hydrates, and it is required to remove heat, switching on pump 34 produces ,;

a pressure of one atmosphere under the rubber element. If the diameter of the ~ottom of the rubber element is about 0.60 m, the force acting upon the layer of hydrate thereabove is 0.3 x 0.3 x 3.14 x 104 kg = 2826 kg. The layer of hydrate is lifted slightly off the container-bottom and solution is pumped in. In order to allow the solution to drain away again, a hydraulic cylinder may be arranged to raise the edge of the rubber element.
Heat-exchanger ~a 1, men~ioned above is formed by containers 50 and 56, heat-exchanger ~a 2 ~y container 40 and heat-producing container 46~ and hea~-exchanger ~a 3 by containers 60 and 66.
An airless vacuum shoul-d be maintained in all containers and pipe-lines, except in piplines 220 and 226 and the area in container 10 connected thereto. To this end, a vacuum pump 90 is connected through a valve 91 to line ~2 and can be put into operation when required. Pipe connector 240 is also con-nected to this vacuum pump.
Containers, pipelines, valves and other fittings may be made of steel or copper. The walls of container 10 exposed to heat should be made of an appropriate heat-resistant steel, e.g. a stainless steel.
In heat-producing container 46 it is advantageous to convert a solution of 0.52 parts by weight of NaOH, by steam absorption, into a solution of 0.42 parts by weight of NaOH, and to replace this with a new solution of 0.52 parts by weight.
It might be thought~ at first s~ight, that there is really no point in dehydrating the dilute "used" solution, during heat storage, down to a solid hydrate, if this is converted, prior to the actual heat-producing process, into a solution of 0.52 parts by weight of NaOH. Instead, the solution could well be produced in the concentration required. Actually, however, much storage space is saved for the dilute solution and less sodium hydroxide is needed, as may be gathered from the following.
In uslng the method according to the invention, it is possible to pro-duce 1 kg of concentrated solution, with 0.52 parts by weight of NaOH, from 0.629 kg ofi a solu*ion c~ntainlng 0.42 parts by weight of NaOH to which 0.3708 kg of sodium hydroxide hydrate is added by introduction into container 30.
When heat is removedJ steam absorption produces from this 1.238 kg of a solution with 0.42 parts by weight of NaOH. Of this, 0.629 kg are used to produce 1 kg of new solution of 0.52 parts by weight of NaOH, by dissolving 0.3708 kg of sodium hydroxide monohydrate containing 0.2557 kg of NaOH.
However, if the heat storage unit dehydrates the dilute solution only to a concentration of 0.52 parts by weight of NaOH, 0.52 kg of NaOH must be available for each kg of solution with 0.52 parts by weight of NaOH converted by steam condensati n into a solution of 0.42 parts by weight.
Thus an installation of equal storage capacity requires twice the amount of sodium hydroxide and twice the container capacity to store the dilute solution, if the latter is dehydrated only to a concentration of 0.52 parts by weight of NaOH, instead of to solid monohydrate. The explanation of this remark-able fact is that a part of the solution consumed goes through the heat-develop-ment process twice.
That part of the dilute solution not required to produce new concen-trated solution may be allowed to drain into container 70, for storage, by open-ing valve 77.
The foregoing calculation is based upon the assumption that new con-centrated solution is produced only periodically, when the solution in heat producing container 46 has reached a certain degree of dilution. It would appear to be more satisfactory for new concentrated solution to flow continuously from container 30 into contai~ner ~6. In this case, however, it would appear to z~

be indispensable for the hPtter solution, arriving from heat producing container 46, to transfer its heat through a heat-exchanger 126, to the cooler solution arriving from container 30. It is desirable for this heat-exchanger to operate on the counterflow principle.
In Figure 2 9 container 225 is connected to outlet pipe 224 through a valve 227. It contains water but all air has been evacuated. During the phase în which calcium oxide is converted into calcium hydroxide, said valve 227 may be adjusted to allow the water to reach outlet pipe 224 very slowly, for example drop by drop, and to flow by gravity into the space between covers 162 and 152 and cylindrical walls 160 and 150. Normally, it vapori~es here very quickly, so that the covers or walls are wetted by a light spray for only a short time. The steam passes through line 226 into the heat-exchanger formed by containers 60 and 66, where it condenses and releases heat to the latter. At this time valve 61 is open and valves 221, 62 and 65 are closed. As indicated by dotted lines, valve 223 is connected to vacuum pump 90 which eliminates air from the pipes and containers thus connected to each other. This arrangement is an alternative to the use of fan 80 which uses air to transfer heat.
In Pigure 2, when pump 34 is in operation, solution is allowed to flow thr~ugh line 37J from above, into container 30. As shown in Figure 4, smaller tanks 31, 33, 35 and 39 arranged therein at an angle to each other are accommodated in such a manner that the solution flows first into the upper con-tainer. The solution then overflows from each tank into the one below and, from the lowermost tank, to the bottom of the main container 30. This arrangement ensures uniform distribution of the solid hydrates into the smaller tanks when heat is stored during the summer months. Pump 3~ may then be used to pump solu-tion rom container 70 into container 30 over long intervals of time. In this way~ all of the tanks are uniformly filled to overflowing.

In view o$ the large investment involved, the installation may also be made smaller, so that only a fraction of the heat required in the winter is stored iT). the summer. This means that, in the course of the year, the heat storage is filled and emptied many times, i.e. its storage capacity is used several times a year. The unit thus operates more economically when it is smaller and is supplemented by additional conventional heating.
The designs described hereinbefore involve very high investment costs, since the very large steel containers needed for the accommodation of the hygro-scopic substance, the water, and the resulting solution are very costly, much more so than their contents. The sheet metal must be very thick, since these containers- are exhausted and must withstand atmospheric pressure. Substantial savings are possible by dividing container 20, which holds the water, into a small and a large container. Only the small tank need be exhausted and made of suitably thick sheet. The interior of the large container communicates with the atmosphere through a hole in the wall, and may therefore be made of thinner sheet. The two containers are joined together by two lines each fitted with a pump. The smaller container comprises a level-sensor using a control device to switch on one pump or the other whenever necessary, in order to keep the water at a spec~fic level. In this case, one pump moves water from the small to the large 2Q container, while the other ~pump moves water from the large to the small contain-er. The same applies to container 70. The large tank may be eliminated if the necessary water is taken from the mains. In this case, the steam formed by de-composition of the calcium hydroxide, and the water formed by condensation of the s*eam, may be released directly to the exterior instead of lnto container 20.
Thus the only function of container 20 is to produce the steam which is condensed in heat producing container 46 in order to produce heat, and to take the neces-sary heat of evaporation from the environment, as is done by a heat pump.

-According tG the ~ethod so far described, only a part o the dilute solution formed in heat producing container 46 is raised to a higher concentration.
According to the ~ollowing method, all of the dilute solution thus formed may be raised to a higher concentration, without allowing a part of it to drain away in-to container 70. In this case, however, a hygroscopic mass is not added; in-stead water is removed in the form of steam.
Referring to Figure 5, the apparatus required for this purpose consists o a container 300 in which the hygroscopic substance or the solution is stored as in container 30 described above, and which also communicates with a container 332 containing another solution of another hygroscopic substance iJI water, ~he substance or solution in container 300 communicating with the solution in con-tainer 332 through the vapour phase, and the arrangement of the containers being such that a good heat-condu~tlng connection exists between the contents of con-tainer 30Q and the solution in container 332.
The purpose of this good heat-conducting connection is that heat pro-duced by condensing steam, which heats the contents of container 300, passes to the solution in container 332, since the latter would otherwise cool down too much as a result of water being removed in the form of steam which consumes heat.
Container 300 in this embodiment comprises a large number of small containers 344 arranged one above the other and carrying sodium hydroxide or sodium hydroxide hydrate. For the sake of clarity, only three of these are shown in Figure 5. The two upper containers 344 each have a short vertical tube 336' passing through a hole in the container bottom to which it is welded. The lower edges of the tubes are lower than the lower edges o the containers, so that when the sod~um hydroxide solution enters the upper container through line 37, the solution overflows through the said tube into the underl~ing container. Contain-er 300 also comprises a container 332 which is arranged above containers 344 and contains a solution of lithium bron~de. Heat producing container 46 also contains this soluti~on and additionally pump 334 (see ~igure 6) causes said solution to circulate slowl~ from container 332, through pipe 336, valve 42, heat-exchanger 126, heat producing container 46, ~alve 45, additional pump 334, heat-exchanger 126, and pipe 337, back to container 332. Pump 44 produces a faster circulation of the solution in heat producing container 46, so that the internal obli~ue sur-faces are wetted as well as possible with liquid and form a large, free surface favouring condensation. Apart from the now separate circuit through container 46, lines 36, 37 and 207 perform the same function as in the embodiment illus-trated in Pigure 2.
~hen the unit is not in use, all valves are closed. When heat is needed, valves 41, 41' and 42 are opened and pumps 334 and 44 are switched on.
Steam passes from container 20, through valves 41, 41' and pipe 48, into heat-producing container 46. The lithium bromide solution has a low vapour pressure, a~sorbs steam, ~ecomes diluted, heats up and releases heat through the walls of container 46, to the water of a central heating unit circulating in container 40.
This produces a slight decline in the concentration of the solution. The slow circulation of the solution produced by additional pump 334 ensures that the con-centration is maintained in spite of the constant dilution. In container 300, 2Q the solution flows over the large bottom of container 332. Water is removed therefrom in the form of steam, since the vapour pressure thereof is higher than that of the sodium hydroxide, the hydrate, or the solution in containers 344.
The steam is absorbed therein and heats up. Heat passes through the bottom of containers 344, the wall of container 300, and the bottom of container 332, into the lithium bromide solution. This replaces the heat lost from the lithium bromlde solution by the formation of steam.
~ufficient solution must be available in heat producing container 46 to allow pump 44 to pass enough liquid over the sloping surfaces. On ~he other hand, the level of the liquid must also not be too high, i.e. the sloping sur-faces should not be below the surface of the liquid. One simple solution is to set up the container in such a manner that the level of the liquid in container 332 is at the same height as the correct level of liquid in container 46.
Calcium chloride rnay be stored on the bottom of container 300, below containers 344, in order to reduce the danger from accldents, as indicated here-inbefore.
The unit described operates well at outside temperatures of 0C or less.
On the other hand, nith outside temperatures of 5 or 10C, the dimensions of the very costly container, holding the dilute s~dium hydroxide solution and water, may be greatly reduced, thus lowering investment costs. This improvement is based upon the concept of the invention that, in diluting the saturated sodium hydroxide solution, the water used goes through two evaporation processes and draws heat from the environment each time.
~igure 6 shows a possible design. When the unit is producing heat, steam passes through open valves 371 and 41, from container 20, through line 378, into container 376 containing a concentrated solution of lithium bromide, and condenses therein. This supplies heat to the lithium bromide solution, and this heat is just as rapidly released through the outer walls o~ container 376 to the water from a hot water syst0m circulating in container 370. Containers 376 and 370 thus constitute a heat exchanger Wa 4. Pump 374 provides a constant circulation of lithium bromide solution in container 376, over a plurality of slightly inclined surfaces forming a large condensation surface. Pump 314 also provides a slow circulation of lithium bromide solution from container 376, through pipeline 317, into container 310, and through pipeline 316 back to con-tainer 376. In this connection, the solution arriving from container 376 is ~ ~$~

cooled down, in heat-exchanger 406, almost to the temperature obtaining in con-tainer 310, whereas the solution returning from container 310 to container 376 is heated approximately to the temperature obtaining in container 376. In con-tainer 310, as much water is removed from the solution as was added thereto in container 376 by condensation. This water changes to steam, flows in this form through pipeline 307, valve 305, and pipe 48, into container 46 which contains saturated or highly concentrated sodium hydroxide solution, and is condensed therein. The solution heats up and releases this heat, through the outer walls of heat producing container 46 to the water of a central heating system circulat-ing in container 4Q. Th.e dilute solution arising here is passed by pump 334 in-to container 30Q. Of the central heating system water cooled down in the radia-tors, about one half passes through pipe extension 49 into container 40 and through pipe extension 370 into container 370, returning to said radiators through pipe extensions 47 and 377. The lower space marked 340 in Figure 5 should contain calcium chloride or magnesium chloride since, if the containers burst, these substances would unite with the sodium or potassium hydroxide in contain-ers 344 thereabove and would be converted into substances less harmful to the environment.
The water which dilutes the hygroscopic solution circulating in heat producing container 46 has thus gone through two evaporating processes, one in container 20 and the other in container 310. The additional equipment for this purpose consists of at least two containers, namely a container Bz 2 which sup-plies steam, instead of container 20, which is condensed in container 46, and thus aasorbs heat from the cold environment, the earth, o~ the atmosphere, the said container Bz 2 being connected to container 46 through the vapour phase by a pipeline, and another container Bz 1 which absorbs the steam delivered by container 20 and releases the resulting heat, through heat-exchanger Wa 4, to the heat-consuming heating unit, the said container Bz 1 being connected through the vapour phase by a pipeline to container 20 which contains the necessary water.
One major advantage is that the solution circulating between containers 376 and 310 does not come into contact with the sodium hydroxide solution. Since the quantity required is small, it is possible to select, from among the relative-ly~costly hygroscopic substances, those which, even at the low temperatures ob-taining in container 310, pos~ibly minus 5C, are highly soluble i~n water and form a solution with a very low vapour pressure. For instance, lithium chloride may be used instead of lithium bromide. An exact knowledge of the satisfactory concentration of the lithium bromide solution is not necessary,- since it adjusts itself. Containers 2n and 310 absorb heat from the environment, as indicated by strokes 311 and 21 representing cooling vanes.
Figure 7 illustrates a substantially simplified design. All valves in the unit are assumed to be closed. When the calcium chloride in container 10 is decomposed by solar radiation, valve 451 is opened. The steam formed flows through a pipeline 416 in the interior of container 400, releases heat to the sodium hydroxide solution surrounding the pipe, condenses, and collects in con-tainer 450. This is emptied from time to time by closing valve 451 and opening valve 453. In this case pipeline 416 performs the same function as heat-exchanger Wa 1 in Figure 2.
~hen decomposition of the calcium chloride by the heat of the sun terminates, valve 451 is closed and valve 405 is opened. Steam then flows from container 30 to container 10, where it combines with the calcium oxide to form calcium hydroxide. The heat released passes gradually through the walls of con-tainer 10 into the solution in container 400. The wall of container 10 thus per-forms the same function as heat-exchanger Wa 3, fan 80 and the relevant air-cooling s~stem in the design illustrated in Figure 2. The heat transferred to the solution in container 10 expels the steam from the solution after valve 51 is openedl the said steam passing through pipeline 460 into container 20 and con-densing therein. Pump 474 passes the solution in container 400 over sloping sheets, thus producing a large surface favouring vaporization. As soon as the solution in container 400 has reached an adequate concentration, valve 51 is closed, valve 434 is opened, and the solution is allowed to drain away through pipe 437 into container 30. Valve 434 is then closed, valve 472 is opened and additional solution to be concentrated is passed by pump 414 from container 30 to contalner 400. However, when the calcium hydroxide has ceased to decompose, la valve 410 may also be opened in place of ualve 405. In this case, the calcium oxide draws the steam off through the solution in container 400, concen~rates it, and forms calcium hydroxide. Since this was heated during the preceding decom-position of calcium hydroxide, a substantially higher degree of concentration can be achieved. Pipeline 437 should be heat-insulated and heatable by electri-city, so that it does not become too cool and blocked by solidified solution.
When heat is required valve 42 is opened. The remainder of the unit connected thereto operates exactly as described in connection with ~igure 2.
Sheet 105 of quartz glass, adjacent cylindrical part 23, and base part 25, are in the form of an integral, airtight, hollow element made of quartz 2Q glass, preferably exhausted. It is furthermore desirable, in the interests of optimal utilization of solar radiation, for the diameter of the lower part of the cy~linder to be larger than the inlet-aperture. The focussed rays enter the quartz glass cylinder and impinge upon the layer of calcium hydroxide adjacent thereto in container 10. Some of the rays penetrate to a certain depth into this layer, before being converted into thermal energy, and the heat-transfer therefore initially proceeds more rapidly. It is desirable for container 10 and the amount of calcium hydroxide therein to be so large that only a part of the calcium hydroxide can be decomposed by incident solar radiation in the course of a day, so that the undecomposed portion may act as a heat-insulating layer. The use of a lithium bromide solution in the design according to Figure 6 has the advantage that this solution can still exist as a solution even at a very low vapour pres~sure, whereas at such a vapour pressure and at the same temperature, a sodium hydroxide solution cannot persist in the liquid phase, but forms solid hydrate. The eircuit described therein, through containers 322 and 46, with the aid of pump 334, would therefore no longer be operable at a low vapour pressure.
The hygroscopic substance, caused to react exothermally according to la the method of the invention, should be such that heat is released upon forming a solution by the addition of water or at least steam. Hygroscopic substances which dissolve in liquid water and reduce the temperature are of course less suitable. Heat must at least be released when the solution is formed by the addi-tion of water present in the gaseous phase.
~ith the design according to Figure 7, the following operating method is also possible. All valves are assumed to be closed. Container 30 should have a small aperture somewhere near the top so that atmospheric pressure obtains also in the interior of the container. Now when, as described above, the solution in container 400 has reached an adequate concentration, it can no longer be allowed ~o drain away, as before, through pipe 437 into container 30. The air-pressure in container 30 would drive the solution back. Valve 435 is therefore opened, while valve 434 remains closed. At the same timej pump 436 is switched on and passes concentrated solution from container 400 to container 30. The pump is then switched off and valve 435 is closed. Valve 472 is opened and pump 414 pumps additional solution to be concentrated out of container 400. If this method of operation alone is used, there is no need to exhaust container 30, which ma~ thereore be made of thinner metal which will be substantially less expensive. It is also unnecessary to fit smaller containers 31, 33, 35, 39 in container 3~ as- in the arrangement of Figure 4. Furthermore, container 30 may be so small that it can accommodate only the product obtained by removing water from the sodium hydroxide solution. The dilute solution produced by removal of heat in winter occupies a larger space and, after valve 439 has been opened, lt is passed by pump 438 into container 70 which also need not be exhausted. When heat is heing stored during the summer, valve 433 is opened and pump 440 passes solu-tion from container 70 into container 400 where the solution is concentrated and, after valve 435 has been opened, is passed ~y pump 436 into container 30.
Heat producing container 46 is of almost the same design as the device located in container 4Q0 for concentrating the hygroscopic solution. The latter may~also be used for producing heat, thus dispensing with components.
Figure 8 illustrates a design of this kind. Container 470 encloses most of container 400 and with it constitutes a heat exchanger. It contains the water from a heating installation which can absorb heat, through the walls of container 40Q, from ~he solution contained therein. When the sun shines and valve 51 is open, steam flows from container 20 into container 400 and condenses there when the installation needs more heat for heating purposes than can be supplied by solar radiation and the solution in container 400 cannot be main-tained at a specific temperature. If the heat produced by solar radiation is more than is required, the temperature in container 400 rises and the flow of steam through valve 51 reverses its direction. Steam is no longer condensed and solution is no longer diluted in container 400. The said solution releases s~eam and becomes concentrated.
In Figure 7, the connection between container 400, through pipeline 46Q, to container 20 is not absolutely necessary. It makes it possible to use the heat of the steam expelled from the calcium hydroxide upon condensation.

On the other hand, in the design according to ~igure 8, said connection is es-sential since in thIs case it is used to place the steam formed in container 20 in communication ~ith solution, so that it condenses and releases heat.

Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of storing and subsequently utilizing thermal energy, which comprises:
(a) heating calcium, barium or magnesium hydroxide to above 300°
with concentrated solar radiation to decompose the hydroxide into the oxide and steam and collecting and condensing the steam;
(b) thereafter removing water vapour from an aqueous hygroscopic substance comprising sodium or potassium hydroxide or calcium chloride and contacting said water vapour with said oxide to reconvert it to the hydroxide;
(c) repeating alternatively steps (a) and (b) to progressively concentrate said hygroscopic substance to attain a highly concentrated hygro-scopic solution or a solid hygroscopic substance; and (d) when required, subsequently regenerating thermal energy by contacting said concentrated hygroscopic substance with water derived from said collected and condensed steam from step (a) so as to cause an exothermic reaction.

2. A method according to claim 1, wherein the reaction takes place in a closed system under partial vacuum.

3. A method according to claim 1, wherein the solid hygroscopic substance or the highly concentrated hygroscopic solution is taken in batches from a large stock and is caused to react exothermally with water or steam.

4. A method according to claim 2, wherein partial amounts of the solid hygroscopic substance are removed by passing an unsaturated solution through the stock, the more highly concentrated solution thus formed being caused to react exothermally with water or steam in another location.

5. A method according to claim 4, wherein, after the exothermal reaction, the weak hygroscopic solution is at least partly reconcentrated by absorption of solid hygroscopic material.

6. A method according to claim 1, wherein the heat released during cooling and/or condensing of the steam arising during decomposition of the hydroxide is used for heating and concentrating the hygroscopic solution.

7. A method according to claim 1, wherein the heat arising during exothermal conversion of the oxide into the hydroxide is collected and used to concentrate the hygroscopic solution.

8. A method according to claim 5, wherein the residual heat in the weak hygroscopic solution is used to heat the concentrated solution.

9. A method according to claim 1, wherein the hydroxide is mixed with a neutral substance with good heat radiation properties.

10. A method according to claim 1, wherein the solar rays are deflected into a closed space almost completely surrounded by the hydroxide/oxide.

11. A method according to claim 10, wherein the amount of hydroxide is limited approximately to the maximal amount which will be converted into oxide in one day.

12. A method according to claim 1, wherein by means of the dehydrated first hygroscopic substance, water is removed from a solution of a second hy-groscopic substance, and, for the purpose of recovering heat, the dehydrated second hygroscopic substance is caused to react exothermally with steam.

13. A method according to claim 12, wherein at the temperature and vapour pressure at which a solution of the first hygroscopic substance would already form a solid phase, the second hygroscopic substance would still persist as a pure liquid phase.

14. An apparatus for storing and subsequently utilizing solar energy according to the method of claim 1, which apparatus comprises (a) a first container for a heat-decomposable hydroxide;
(b) means for directing solar radiation onto a heat receptor located in said first container and capable of being contacted by hydroxide therein so as to decompose said hydroxide to oxide when solar radiation is incident on said receptor;
(c) a second container in selective communication with said first container for collecting and condensing water vapour evolving from decomposition of said hydroxide;
(d) a third container in selective communication with said first container and adapted to contain an aqueous solution of a hygroscopic substance, the third container being divided into a large storage container connected to the first container and a smaller heat producing container connected to the second container and cooperating with a heat exchanger for the release of heat, the two part-containers being connected to each other in both directions by lines which can be shut off, the arrangement being such that on discontinuance of solar radiation communication is established between said first and third containers and that water vapour above said aqueous solution passes to said first container to contact said oxide therein and reconvert it to hydroxide, (e) a heat exchanger disposed in the system between the first and second containers, heat from steam formed in the first container being transferred in said heat-exchanger to the hygroscopic solution in the third container, the steam developed in the latter container being adapted to pass the second container, and (f) the first container being arranged in a cooling-fluid circuit which comprises an additional heat-exchanger in which heat taken from coolant in the first container is transferable to the hygroscopic solution of the third container, the steam developed in the latter being transferable to the second container.

15. An apparatus according to claim 14, wherein the arrangement is such that the steam condenses in the heat-exchanger.

16. An apparatus according to claim 14, wherein two first containers are provided which cooperate alternately with the third container and the solar radiation.

17. An apparatus for carrying out the method according to claim 10, which comprises a first container in which hydroxide can be decomposed by solar radia-tion and a second container in which water can evaporate, the heat of evaporation being drawn from the environment, and further comprises a third container which contains a hygroscopic solution or the hydrate of a hygroscopic substance, the space in the third container above the solution therein being connected to the first container; a fourth container containing hygroscopic solution, the space in the fourth container above the solution therein being connected to the second container, a heat-exchanger between said fourth container and a heat-consuming heating unit, a pipeline system permitting a circulation of hygroscopic solution between said fourth container and a fifth container, the space above the solution in the said fifth container communicating with the space above the material in the third container, and means being provided for the exchange of heat between the third and fifth containers.

18. An apparatus according to claim 17, wherein the hygroscopic substance contained in the solution used in the circulation persists as a pure liquid phase at the temperature and vapour pressure at which the hygroscopic solution used in the third container forms a solid phase.

19. An apparatus according to claim 14, wherein the third container is divided into a larger storage container and a smaller container to be connected to the first contalner, at least a part of the outer surface of the walls of said first container being located within said smaller container.

20. An apparatus according to claim 19, wherein a heat-exchanger is arranged between the first and second containers heat from the steam formed in the first container being transferable to the hygroscopic solution from the third container the steam developed in the latter being transferable to the second container and said heat exchanger being located within the smaller container.

21. An apparatus according to claim 14, wherein the third container is divided into four sections, namely a stock container for the hygroscopic storage substance, a storage container for the dilute solution arising in increasing quantities as heat is removed, a container containing hygroscopic solution in which the space above the solution is to be connected through the gaseous phase to the first container and a heat-producing container in which the space above the solution is to be connected to the second container and which co-operates with a heat-exchanger for a heating installation, the last-two mentioned contain-ers being exhausted, while the stock container and storage container need not necessarily be exhausted.

22. An apparatus according to claim 14, wherein the third container is divided into three containers, namely a stock container for the hygroscopic storage substance, a storage container for the solution arising in increasing amounts in the heat-producing container as heat is removed, and a container which contains hygroscopic solution and in which the space above the solution is to be connected through the gaseous phase with the first container and with the second container, it being possible to open and close these connections in-dependently of each other.