Production of Very Low-Temperature Refrigeration in a Thermochemical Device

Abstract

The invention relates to a thermochemical device and to a method for producing refrigeration at very low temperature.

Description

The present invention relates to a thermochemical device for producing refrigeration at very low temperature.

A system composed of a thermochemical dipole using two reversible thermochemical phenomena is a known means for producing refrigeration. The thermochemical dipole comprises an LT reactor, an HT reactor and means for exchanging a gas between LT and HT. The two reactors are the site of reversible thermochemical phenomena chosen such that, at a given pressure in the dipole, the equilibrium temperature in LT is below the equilibrium temperature in HT.

The reversible phenomenon in the HT reactor involves a sorbent S and a gas G and may be:

• • a reversible adsorption of G by a microporous solid S;
  • a reversible chemical reaction between a reactive solid S and G; or
  • an absorption of G by a saline or binary solution S according to the scheme:

“sorbent S”+“G”⇄“sorbent S+G”.

The reversible phenomenon in the LT reactor involves the same gas G. It may be a liquid/gas phase change of the gas G or a reversible adsorption of G by a microporous solid S¹, or a reversible chemical reaction between a reactive solid S¹ and G, or an absorption of G by a solution S1, the sorbent S1 being different from S. The refrigeration production step of the device corresponds to the synthesis step in HT:

“sorbent S”+“G”→“sorbent S+G”.

The regeneration step corresponds to the decomposition step in HT:

“sorbent S+G”→“sorbent S”+“G”.

The production of refrigeration at a temperature T_F in a dipole (LT, HT) from a heat source at the temperature T_c and from a heat sink at the temperature T_o implies that the thermochemical phenomenon in LT and the thermochemical phenomenon in HT are such that:

• • during the step of producing refrigeration by the dipole, the exothermic consumption of gas in HT takes place at a temperature close to and above T_o, which creates a pressure in the dipole such that the equilibrium temperature in the LT reactor is close to and below T_F; and
  • during the step of regenerating the dipole, the endothermic release of gas in HT is carried out at the temperature T_c which creates a pressure in the dipole such that the temperature at which the exothermic consumption of gas in LT is carried out is close to and above T_o.

The thermochemical phenomena currently used enable refrigeration to be produced at a negative temperature in LT, but they do not fulfill the above criteria with the objective of producing refrigeration at very low temperature (T_F typically from −20° C. to −40° C.) for long-lasting foodstuff preserving and freezing applications from a heat source, the thermal potential of which is around 60 to 80° C., the heat sink generally composed of the ambient medium being at a temperature T_o of around 10° C. to 25° C. These phenomena either require, during the regeneration, a temperature T_c well above 70° C. to operate with a heat sink at the ambient temperature T_o, or they require a heat sink at a temperature below T_o if a heat source at T_c=60-80° C. is used.

For example, to produce refrigeration at −30° C. using a heat source at 70° C., when LT is the site of an L/G phase change of ammonia NH₃, and HT is the site of a chemical sorption of NH₃ by a reactive solid S: if S is BaCl₂, a heat sink at 0° C. would be needed for the LT reactor during the refrigeration production step, whereas if S is CaCl₂, a heat sink at −5° C., that is to say at a temperature well below T_o, would be needed during the regeneration step.

Solar energy or geothermal energy are advantageous heat sources, but they supply heat at a low temperature level which is not, in general, above 60-70° C. when a low-cost collection technology is used, such as for example the flat collectors conventionally used for producing domestic hot water. The use of these types of energy consequently does not enable the intended aim to be achieved.

The inventors have now found that it was possible to produce refrigeration at a temperature T_f below −20° C. from an available heat source at a temperature T_h between 60 and 80° C. and from a heat sink at the ambient temperature T_o varying from 10° C. to 25° C., by combining two dipoles Da and Db: the dipole Db being able to be regenerated with an available heat source at the temperature T_h and a heat sink at the ambient temperature T_o, but requiring a heat sink at a temperature below T_o produce refrigeration at the desired temperature T_f; the dipole Da being able to be regenerated with an available heat source at the temperature T_h and a heat sink at the ambient temperature T_o.

The object of the present invention is consequently to provide a method and a device for producing refrigeration at a temperature T_f below −20° C., from an available heat source at a temperature T_h of around 60-80° C. and a heat sink at the ambient temperature T_o of around 10° C. to 25° C.

The device for producing refrigeration according to the present invention comprises a refrigeration producing dipole Db and an auxiliary dipole Da, and it is characterized in that:

• • Da comprises an evaporator/condenser ECa and a reactor Ra connected by a line enabling the flow of gas, and Db comprises an evaporator/condenser ECb and a reactor Rb connected by a line enabling the flow of gas;
  • ECa contains a gas Ga and Ra contains a sorbent Sa able to form a reversible phenomenon Ga, and ECb contains a gas Gb and Rb contains a sorbent Sb able to form a reversible phenomenon with Gb; the gases and the solids being chosen so that, at a given pressure, the equilibrium temperatures of the thermochemical phenomena in the reactors and the evaporators/condensers are such that T(ECb)≦T(ECa)<T(Rb)<T(Ra) during the refrigeration production step;
  • the thermochemical processes implemented in the dipole Db are such that this dipole may be regenerated from a heat source at the temperature T_h and a heat sink at T_o, and produce refrigeration at the temperature T_f with a heat sink at a temperature below the ambient temperature T_o;
  • the thermochemical phenomena in the dipole Da are such that this dipole may be regenerated from a heat source at the temperature T_h and a heat sink at the temperature T_o; and
  • the dipoles are equipped with means that enable ECa and Rb to be thermally coupled during the refrigeration production step.

In the remainder of the text, the expression “the elements” of a dipole will be used to denote both the reactor and the evaporator/condenser of the dipole.

As an example of thermochemical phenomena used in the present invention, mention may be made of the L/G phase change of ammonia (NH₃), of methylamine (NH₂CH₃) or of H₂O in the evaporators/condensers. For the reactors, mention may be made of:

• • a reversible chemical sorption of NH₃ by CaCl₂, by BaCl₂, by PbBr₂ or by NH₄Br or of NH₂CH₃ by CaCl₂;
  • an adsorption of water by a zeolite or a silica gel;
  • the adsorption of methanol (MeOH) or of ammonia in active carbon; and
  • the absorption of NH₃ in a liquid ammonia solution (NH₃.H₂O).

The thermal coupling between ECa and Rb may be carried out, for example, by a coolant loop, by a heat pipe or by direct contact between the reactors ECa and Rb.

In a preferred form of the device of the invention, each of the elements EC is composed of an assembly comprising an evaporator E and a condenser C connected together and with the reactor of the same dipole by lines equipped with valves enabling the flow of gas or of liquid.

The method for producing refrigeration at the temperature T_f from a heat source at the temperature T_h and a heat sink at the ambient temperature T_o consists in operating the device according to the invention from an initial state in which the dipoles Da and Db are to be regenerated (that is to say, that the sorbents are found in the reactors Ra and Rb respectively in the form “Sa+Ga” and “Sb+Gb”), the two elements of a given dipole being isolated from one another, said method comprising a series of successive cycles made up of a regeneration step and a refrigeration production step:

• • at the beginning of the first step, which is the step of regenerating the device, the two elements of each of the dipoles are connected and heat at the temperature T_h is supplied to each of the reactors Ra and Rb for the decomposition reactions in Ra and Rb, the gas Ga and the gas Gb released being transferred respectively toward the evaporators/condensers ECa and ECb in which they condense, the heat of condensation being extracted in the heat sink at T_o; and
  • during the second step, which is the refrigeration production step, Rb and ECb are connected, which causes the spontaneous endothermic evaporation phase in ECb (refrigeration producer) which releases Gb in gas form, said gas flowing into the reactor Rb in which the exothermic absorption of Gb by the sorbent Sb takes place; the heat released in Rb is transferred toward ECa to cause the release of the gas Ga that flows into Ra to be absorbed by the sorbent Sa exothermically, the heat released in Ra being extracted toward the environment at T_o.

In this method, the dipoles Da and Db operate in phase.

The various steps may be carried out continuously or on demand. At the end of the regeneration step, it is sufficient to isolate the elements of one and the same dipole from one another, to keep the device in the regenerated state. To produce refrigeration, it will suffice to connect the elements of each dipole. The regeneration of the device is carried out either immediately at the end of a production step, or subsequently.

The method may be implemented permanently if the heat source is permanently available at the temperature T_h, for example if it is geothermal energy. The operation will be in batch mode if the heat source is not permanent, for example if it is solar energy whose availability varies throughout a day.

In a first embodiment, the thermochemical phenomena are chosen so that T(ECb)<T(ECa)<T(Rb)<T(Ra) in the refrigeration production phase. In this case, Ga and Gb are different.

The method of producing refrigeration according to the first embodiment is illustrated in FIGS. 1 and 2, which represent, in the Clausius-Clapeyron plot, the thermodynamic positions of the two dipoles, respectively for the regeneration step (FIG. 1), and for the refrigeration production step (FIG. 2). The straight lines 0, 1, 2 and 3 represent the equilibrium curves respectively for the L/G phase change of the gas Gb, the L/G phase change of the gas Ga, the reversible phenomenon Gb+SbB⇄(Gb, Sb) and the reversible phenomenon Ga+Sa⇄(Ga, Sa). On the right-hand part of FIG. 1 and the lower part of FIG. 2, the gas flows are depicted by simple arrows, and the heat flows are depicted by thick arrows.

During the regeneration step, heat at the temperature T_h is supplied to Rb (point Db on the straight line 2) which releases gaseous Gb that will be condensed in ECb (point Cb on the straight line 0) while releasing heat at T_o. At the same time, heat at the temperature T_h is supplied to Ra (point Da on the straight line 3) which releases gaseous Ga that will be condensed in ECa (point Ca on the straight line 1) while releasing heat at T_o.

During the refrigeration production step, the evaporation of Gb in ECb (point Eb on the straight line 0) extracts heat at T_f from the medium to be cooled and therefore produces refrigeration at this temperature. The gaseous Gb thus released is transferred by chemical affinity into Rb to be absorbed by Sb while releasing heat at a temperature below T_o (point Sb on the straight line 2). The heat released by the sorption step in Rb is transferred toward ECa to produce, by evaporation, the release of gaseous Ga (point Ea on the curve 1), Ga being transferred into Ra for the exothermic sorption in Ra (point Sa on the curve 3), releasing heat into the environment at T_o.

In a second embodiment, the dipoles Da and Db of the device according to the invention involve the same working gas G, so that, for a same working pressure, T(ECb)=T(ECa)<T(Rb)≦T(Ra). In this case, the two dipoles contain the same gas G.

According to a first variant of this second embodiment, the reactors Ra and Rb contain sorbents whose thermodynamic equilibrium curves are close to one another, that is to say the deviation observed between the equilibrium temperatures for a same pressure do not exceed 10° C. According to a second particularly advantageous variant of the second embodiment, the reactors Ra and Rb contain the same sorbent S, which corresponds to T(ECb)=T(ECa)<T(Rb)=T(Ra).

The method of this second embodiment is characterized in that the second step comprises two phases: during the first phase, the elements of the dipole Da are isolated from one another, and ECb and Rb are connected, which causes the release of Gb in ECb and the exothermic synthesis in Rb, the heat released in Rb being transferred toward the reactor ECa. When the pressure in ECa is such that it enables operation of the dipole Da with the heat sink at the ambient temperature T_o, the second phase begins by connecting the elements of the dipole Da, which causes an endothermic evaporation in ECa and a concomitant exothermic sorption of Ga in Ra. Thus, Rb is cooled further, which allows production of refrigeration at T_f.

The Clausius-Clapeyron plots are represented in FIG. 3 for the first step, and in FIG. 4 for the second step. FIGS. 3a and 4a correspond to the first variant, FIGS. 3b and 4b correspond to the second variant of the second implementation mode. FIGS. 3c and 4c represent the gas flows (simple arrows) and the heat flows (thick arrows).

In the second variant of the second embodiment, in the regeneration phase, the points ECa and ECb on the Clausius-Clapeyron plot are merged, as are the points Ra and Rb. In the refrigeration production phase, the points ECa and ECb are found on the same equilibrium curve, as are the points Ra and Rb.

FIG. 5 is a schematic representation of a device according to the present invention. In accordance with FIG. 5, the dipole Da comprises a reactor Ra, a condenser Ca and an evaporator Ea. Ra and Ca are connected by a line equipped with a valve 1a, Ca and Ea are connected by a simple line. Ra and Ea are connected by a line equipped with a valve 7a. Ra is equipped with heating means 2a and means 3a for removing heat. Ca is equipped with means 4a for removing heat. The dipole Db comprises a reactor Rb, a condenser Cb and an evaporator Eb. Rb and Cb are connected by a line equipped with a valve 1b, Cb and Eb are connected by a simple line. Rb and Eb are connected by a line equipped with a valve 7b. Rb is equipped with heating means 22. Eb is equipped with means 5b for extracting heat from the environment. Ea and Rb are equipped with means 6 enabling the exchange of heat between them.

The valves 1a, 1b, 7a and 7b may be, in another embodiment, simple valves whose operation (opening and closing) is only carried out by the play of slight pressure differences resulting from the physicochemical processes implemented in the dipoles. The use of valves enables the device to self-adapt its operation to the temperature conditions imposed by the heat source and the heat sink without external intervention. The flow direction of each valve is represented in FIGS. 5, 6, 7, 8 and 9.

Ra is the site of a reversible chemical sorption of the gas Ga on the solid Sa, Ca and Ea being the site of a condensation/evaporation phenomenon of the gas Ga. Rb is the site of a reversible chemical sorption of the gas Gb on the solid Sb, Cb and Eb being the site of a condensation/evaporation phenomenon of the gas Gb.

The parts of the device that are active during the regeneration step are represented in FIG. 6. The valves 1a and 1b are opened, the valves 7a and 7b are closed, and the heat transfer means 6 are deactivated. Heat at the temperature T_h is supplied to Ra and to Rb respectively by the means 2a and 2b, which triggers the release of gas Ga and of gas Gb which flow into the respective condensers Ca and Cb in which they condense. The heat released by the condensations is removed by the means 4a and 4b, then the liquid forms of Ga and Gb flow respectively into Ea and Eb.

The parts of the device that are active during the regeneration step of the device are represented in FIG. 7. The valves 1a and 1b are closed, the valves 7a and 7b are open. The connection of Eb with Rb causes an evaporation of Gb (producing the refrigeration at T_f) which is transferred into Rb where it is absorbed exothermically by Sb, the heat released being transferred via 6 toward Ea to trigger the evaporation of Ga and the synthesis in Ra. At the end of this refrigeration production step, the device must be regenerated. The regeneration may be immediate or take place later.

FIGS. 8 and 9 represent the state of another embodiment of a device according to the invention, during the regeneration step (FIG. 8) and during the refrigeration production step (FIG. 9). In this embodiment too, the evaporators/condensers EC are each divided in two as an evaporator E and a condenser C. Here, the thermal coupling between Rb and ECa is carried out by direct contact between Rb and the evaporator Ea of the element ECa, so that, during the refrigeration production step in the dipole Db at T_f, the heat released in Rb is transferred directly into Ea. The means 6 that can be seen on the device from FIG. 5 have thus been replaced by the direct contact between Rb and Ea. In this embodiment, the device contains, in addition, a reservoir 8 between Ca and Ea, said reservoir being connected to Ea on the one hand by a valve 9, and on the other hand by a purge line 10. Thus, during the refrigeration production step, the evaporator Ea may be supplied with liquid Ga by the valve 9. During the regeneration step, the evaporator Ea may be purged of the excess of liquid Ga by the purge line 10.

EXAMPLES

The implementation of the method of the invention for producing refrigeration at T_f according to the first embodiment, in which the thermochemical phenomena are chosen such that, for a same working pressure:

T(ECb)<T(ECa)<T(Rb)<T(Ra)

may be illustrated by the following thermochemical phenomena:

dipole Da
reactor Ra
4NH2CH3 + (CaCl2•2NH2CH3) ⇄ (CaCl2•6NH2CH3)
ECa
NH2CH3 liquid/gas state change
dipole Db
reactor Rb
8NH3 + BaCl2 ⇄ (BaCl2•8NH3)
ECb
NH3 liquid/gas state change.

FIG. 10 represents the Clausius-Clapeyron plots of this embodiment.

The implementation of the method of the invention for the production of refrigeration at T_f according to the first variant of the second embodiment, in which the thermochemical phenomena are chosen such that T(ECb)=T(ECa)<T(Rb)<T(Ra), may be illustrated by a device in which the two dipoles are the site of the following thermochemical phenomena:

dipole Da
reactor Ra
NH3 + NH4Br ⇄ (NH4Br•NH3)
[this process could be replaced by
2.5NH3 + PbBr2•3NH3 ⇄ (PbBr2•5.5NH3)]
ECa
NH3 liquid/gas state change
dipole Db
reactor Rb
8NH3 + BaCl2 ⇄ (BaCl2•8NH3)
ECb
NH3 liquid/gas state change.

FIG. 11 represents the Clausius-Clapeyron plots of this embodiment.

The implementation of the method of the invention for the production of refrigeration at T_f according to the second variant of the second embodiment, in which the thermochemical phenomena are chosen such that T(ECb)=T(ECa)<T(Rb)=T(Ra), may be illustrated by a device in which the two dipoles are the site of the same thermochemical phenomenon as follows:

Reactors: 8NH3 + BaCl2 ⇄ (BaCl2•8NH3)
EC:       NH3 liquid/gas state change.

FIG. 12 represents the Clausius-Clapeyron plots of this embodiment.

Claims

1. A device for producing refrigeration at a temperature Tf below −20° C., from an available heat source at a temperature Th of around 60-80° C. and a heat sink at the ambient temperature To of around 10° C. to 25° C., which comprises a refrigeration producing dipole Db and an auxiliary dipole Da, characterized in that: Da comprises an evaporator/condenser ECa and a reactor Ra connected by a line enabling the flow of gas, and Db comprises an evaporator/condenser ECb and a reactor Rb connected by a line enabling the flow of gas;ECa contains a gas Ga and Ra contains a sorbent Sa able to form a reversible phenomenon with Ga, and ECb contains a gas Gb and Rb contains a sorbent Sb able to form a reversible phenomenon with Gb; the gases and the solids being chosen so that, at a given pressure, the equilibrium temperatures of the thermochemical phenomena in the reactors and the evaporators/condensers are such that T(ECb)≦T(ECa)<T(Rb)<T(Ra) during the refrigeration production step;the thermochemical processes implemented in the dipole Db are such that this dipole may be regenerated from a heat source at the temperature Th and a heat sink at To, and produce refrigeration at the temperature Tf with a heat sink at a temperature below the ambient temperature To;the thermochemical phenomena in the dipole Da are such that this dipole may be regenerated from a heat source at the temperature Th and a heat sink at the temperature To; andthe dipoles are equipped with means that enable ECa and Rb to be thermally coupled during the refrigeration production step.

2. The device as claimed in claim 1, characterized in that the thermochemical phenomena in the reactors/condensers are chosen from the L/G phase change of ammonia (NH3), the L/G phase change of methylamine (NH2CH3) and the L/G phase change of H2O.

3. The device as claimed in claim 1, characterized in that the thermochemical phenomena in the reactors are chosen from the reversible chemical sorptions of NH3 by CaCl2, by BaCl2, by PbBr2 or by NH4Br; the chemical sorption of NH2CH3 by CaCl2; the adsorption of water by a zeolite or a silica gel; the adsorption of methanol (MeOH) or of ammonia in active carbon; and the absorption of NH3 in a liquid solution of ammonia (NH3.H2O).

4. The device as claimed in claim 1, characterized in that each of the elements EC is composed of an assembly comprising an evaporator E and a condenser C connected together and with the reactor of the same dipole by lines equipped with valves enabling the flow of gas or of liquid.

5. A method for producing refrigeration at the temperature Tf from a heat source at the temperature Th and a heat sink at the ambient temperature To consisting in operating the device as claimed in claim 1 from an initial state in which the dipoles Da and Db are to be regenerated (that is to say, that the sorbents are in the reactors Ra and Rb respectively in the form “Sa+Ga” and “Sb+Gb”), the two elements of a given dipole being isolated from one another, said method being characterized in that it comprises a series of successive cycles made up of a regeneration step and a refrigeration production step: at the beginning of the first step, which is the step of regenerating the device, the two elements of each of the dipoles are connected and heat at the temperature Th is supplied to each of the reactors Ra and Rb for the decomposition reactions in Ra and Rb, the gas Ga and the gas Gb released being transferred respectively toward the evaporators/condensers ECa and the ECb in which they condense, the heat of condensation being extracted in the heat sink at To; andduring the second step, which is the refrigeration production step, Rb and ECb are connected, which causes the spontaneous endothermic evaporation phase in ECb (refrigeration producer) which releases Gb in gas form, said gas flowing into the reactor Rb in which the exothermic absorption of Gb by the sorbent Sb takes place; the heat released in Rb is transferred toward ECa to cause the release of the gas Ga that flows into Ra to be absorbed by the sorbent Sa exothermically, the heat released in Ra being extracted toward the environment at To.

6. The method as claimed in claim 5, characterized in that the thermochemical phenomena are chosen such that Ga and Gb are different.

7. The method as claimed in claim 5, characterized in that the dipoles Da and Db of the device involve the same working gas G.

8. The method as claimed in claim 7, characterized in that the second step comprises two phases: during the first phase, the elements of the dipole Da are isolated from one another, and ECb and Rb are connected, which causes the release of Gb in ECb and the exothermic synthesis in Rb, the heat released in Rb being transferred toward the reactor ECa; when the pressure in ECa is such that it enables operation of the dipole Da with the heat sink at the ambient temperature To, the second phase begins by connecting the elements of the dipole Da, which causes an endothermic evaporation in ECa and a concomitant exothermic sorption of Ga in Ra.

9. The method as claimed in claim 7, characterized in that the reactors Ra and Rb contain sorbents whose thermodynamic equilibrium curves are close to one another, that is to say that the deviation observed between the equilibrium temperatures for a same pressure does not exceed 10° C.

10. The method as claimed in claim 7, characterized in that the reactors Ra and Rb contain the same sorbent S.