Methods for storing and releasing thermal energy, associated reactor and application to the interseasonal storage of solar heat

Abstract

The invention concerns a method for storing or releasing thermal energy by chemical reaction according to which a flow of heat-transfer gas is circulated through a layer (10) of a first hygroscopic salt A, then through a layer (20) of a second hygroscopic gas B, the gas being reactive with salts A and B, the thermodynamic equilibrium curve of salt A being located further to the left than that of second salt B in a pressure-temperature phase diagram, the circulation of the flow of gas causing a dehydration or hydration reaction of both first salt A and second salt B.

Claims

1. A process for storing thermal energy by chemical reaction wherein a flow of heat transfer gas is circulated through a layer of a first hygroscopic salt A then through a layer of a second hygroscopic salt B, the gas being reactive with the salts A and B, the thermodynamic equilibrium curve of the salt A being located further to the left than that of the second salt B in a pressure-temperature phase diagram, the circulation of the flow of gas giving rise to a dehydration or hydration reaction both of the first salt A and of the second salt B; wherein the first salt A is selected from the double sulfate of aluminum and potassium (KA1(S04)2), strontium hydroxide Sr(OH)2, sodium sulfide Na2S and ammonium aluminum sulfate NH4A1(S04)2, while the second salt B is respectively selected from strontium bromide (SrBr2), sodium sulfide (Na2S), and nickel sulfate (NiS04).

2. The process for storing or releasing thermal energy as claimed in claim 1, wherein the flow of gas is a flow of moist air.

3. The process for storing and releasing thermal energy as claimed in claim 1 wherein the circulation of the flow of moist air that gives rise to the dehydration reaction takes place, through the salts, in the same direction as that which gives rise to the hydration reaction.

4. The process for storing and releasing thermal energy as claimed in claim 1, wherein the circulation of the flow of moist air that gives rise to the dehydration reaction takes place, through the salts, in the opposite direction to that which gives rise to the hydration reaction.

5. An application of the process as claimed in claim 1 to an interseasonal storage of solar heat.

Description

DETAILED DESCRIPTION

(1) Other advantages and features of the invention will emerge more clearly on reading the detailed description of the invention given by way of illustration and non-limitingly with reference to the following figures, among which:

(2) FIG. 1 is a schematic cross-sectional view of a stack of two layers of different salts according to one example of the invention;

(3) FIG. 2 schematically illustrates the pressure-temperature equilibrium or moisture content-temperature equilibrium diagrams of the system of moist air and two different salts according to one example of the invention;

(4) FIG. 3 is a perspective and partial cross-sectional view of an example of a fixed bed reactor according to the invention;

(5) FIGS. 4A and 4B illustrate, in schematic cross-sectional view, a stack of two layers of different salts, inserted between which is an open separating structure according to one example of the invention.

(6) FIG. 1 illustrates the implementation of the process for storing or releasing thermal energy by chemical reaction according to the invention. A layer 10 of a first hygroscopic salt A is surmounted by a second layer 20 of the second hygroscopic salt B.

(7) The equilibrium curve of the first salt A is located to the left of that of the second salt B in a pressure-temperature phase diagram. In other words, for an identical partial pressure of water, the equilibrium temperature of the salt A is lower than that of the salt B. In yet other words, for an identical water content, the equilibrium temperature of the salt B is higher than that of the salt A.

(8) The salt A may advantageously be an inexpensive salt such as potash alum while the salt B may be a more expensive salt such as strontium bromide.

(9) The stack of the two layers 10, 20 of the two different salts A, B may be produced in a fixed bed reactor as described in detail below.

(10) According to the invention, in wintertime, a flow of moist air is circulated which will hydrate the first salt A, then the second salt B. Use is preferably made of the moisture naturally contained in the air but the flow of moist air may also be generated by evaporation of water from a reservoir provided for this purpose.

(11) The reaction between the water vapor contained in the moist air and the first salt A produces heat. Next, the moist air, partly dehydrated and heated by its passage through the layer 10 of the first salt A, passes through the layer 20 of the second salt B. The reaction between the water vapor contained in the moist air and the second salt B again produces heat.

(12) FIG. 2 illustrates this operation: it shows the change in the thermodynamic conditions of the air at points 1, 2 and 3 which correspond respectively to the temperature at the inlet of the layer 10 of the first salt A, at the outlet of the layer 10 of the first salt A, and at the outlet of the layer 20 of the second salt B.

(13) The values w represent the water (absolute humidity) losses of the moist air during passage through the salts A then B. Thus, during the respective hydration of salt A then of salt B, the inlet temperature T.sub.1 increases to the temperature T.sub.2 in order to reach in the end the temperature T.sub.3. This temperature increase T.sub.3T.sub.1 therefore corresponds to a heating power that may be used for example for heating a building or producing domestic hot water.

(14) In summertime, for storing thermal energy, a flow of air is circulated that will dehydrate the second salt B then the first salt A. The flow of air may circulate in the opposite direction to that which has just been described for the hydration, that is to say firstly passing through the layer 20 of the salt B then the layer 10 of the salt A. Alternatively, it may circulate in the same direction as that which has just been described for the hydration, that is to say firstly passing through the layer 10 of the salt A then the layer 20 of the salt B.

(15) In order to pass through the two salts A and B in series according to the invention, the two layers 10, 20 may be superposed in one and the same fixed bed reactor. It is also possible to put each of the two layers 10, 20 in a separate reactor of fixed bed type or of circulating fluidized bed type and to connect the two reactors in series. One advantage of using two reactors that are separated from one another is that it is possible to use only one thereof, that is to say to hydrate only the salt having the lowest equilibrium temperature, when the heat recovery requirement is lower.

(16) Represented in FIG. 3 is a fixed bed reactor 3 containing several stages 30 suitable for the implementation of the invention. The reactor 3 comprises an inlet opening 31 through which a flow of moist air may circulate and penetrate via the lower slot 32 of each stage 30 and thus circulate in a distributed manner in each stage 30.

(17) The flow of air that has passed through the two layers 10, 20 of the two salts A and B at each stage 30 then leads via the upper slot 33 in order in the end to leave via the outlet opening 34 of the reactor 3.

(18) On each of these stages 30, it is possible to deposit a stack of a layer 10 of the first salt A on a layer 20 of the second salt B. Each of the two layers 10, 20 is preferably of uniform height. The total height of two superposed layers 10, 20 is between, for example, 5 and 10 cm. The respective amounts of each salt A, B are calculated as a function of the following parameters: reaction kinetics of the materials, energy densities of the reactants, amount of energy that it is desired to extract from each of the reactants, pressure drops of the total layer, temperature and moisture conditions at the inlet, desired temperature conditions at the outlet, and technical and economic optimization.

(19) Between a layer 10 of a first salt A and layer 20 of a second salt B, it is possible to insert a preferably metallic, separating fabric 4. The diameter of the yarn or yarns of the fabric and also the mesh openings of the fabric are suitable both for easily allowing air to pass through and for preventing the mixing of the two salts A and B with one another. The metallic fabric 4 is not held mechanically in the reactor in order to allow the swelling and deswelling of each salt undergoing reaction independently. Represented in FIGS. 4A and 4B are the respectively dehydrated and hydrated configurations of the layers 10 of the salt A and of the salt B. Thus, the hydration of the salts A and B makes them swell freely without there being any mechanical stresses generated on the separating fabric or screen 4.

(20) By way of example, a metallic fabric having an individual yarn diameter equal to 0.040 mm, with a nominal mesh opening equal to 0.071 mm, is perfectly suitable for separating a layer of potash alum from a layer of strontium bromide.

(21) Although described in connection with a flow of moist air, the invention may just as well be carried out with a flow of a mixture of a gas having the role of heat transfer fluid and of a gas having the role of reactant with the salts A, B.

(22) Similarly, although the invention has been described with two salts A, B having different equilibrium curves being placed in series, the invention may just as well be carried out by placing a higher number of salts, for example three different salts, in series.

(23) The invention is not limited to the examples which have just been described; it is possible in particular to combine together features of the examples illustrated within variants that are not illustrated.

REFERENCES CITED

(24) [1]: Solar heating and cooling by a thermochemical process. First experiments of a prototype storing 60 kWh by a solid/gas reaction by Mauran, S., Lahmidi, H., Goetz, V., Solar Energy, Vol. 82, Issue 7, July 2008, pages 623-636; [2]: Evaluation par analyse thermique diffrentielle des chaleurs de dshydratation de plusieurs aluns de chrome, aluminium et fer[Differential thermal analysis evaluation of the heats of dehydration of several alums of chromium, aluminum and iron], by Mireille HARMELIN, Journal of Thermal Analysis, Vol. 1 (1969), pages 137-150.