SYSTEM AND METHOD FOR THERMOCHEMICAL STORAGE OF ENERGY

Abstract

The present invention discloses a closed system for thermochemical storage comprising at least one water condenser and at least two thermochemical modules, wherein a first thermochemical module comprises a first thermochemical material and a second thermochemical module comprises a second thermochemical material, and wherein the at least one water condenser and the thermochemical modules are connected so that water vapour can be exchanged individually between any two selected from the list consisting of the at least one water condenser and the at least two thermochemical modules. A method for desorption in the system according to the invention is also described. In this method, the first thermochemical module is used as a condenser to dry the second thermochemical module.

Claims

1. Closed system for thermochemical storage comprising at least one water condenser and at least two thermochemical modules, wherein a first thermochemical module comprises a first thermochemical material and a second thermochemical module comprises a second thermochemical material, and wherein the at least one water condenser and the thermochemical modules are connected so that water vapour can be exchanged individually between any two selected from the list consisting of the at least one water condenser and the at least two thermochemical modules.

2. The system according to claim 1, further comprising a connection system wherein each condenser and each TCM module has a connection through a valve with a central tube.

3. The system according to claim 1, wherein the first and the second thermochemical material are different materials.

4. The system according to claim 1, wherein the thermochemical material is selected from the group consisting of zeolites, silica gel, hygroscopic salts, metal organic frameworks (MOF), carbon, and aluminum phosphates.

5. The system according to claim 4, wherein the hygroscopic salt is selected from the list consisting of chlorides, sulfates, iodides, nitrates, sulfides and its hydrates.

6. The system according to claim 1, comprising further a third thermochemical module comprising a third thermochemical material.

7. Method for desorption in a system for thermochemical storage according to claim 1, comprising a step wherein the desorption in the second thermochemical module is realized using at least the first thermochemical module as a condenser.

8. The method according to claim 7, comprising a further step, wherein the desorption in the first thermochemical module is realized using the water condenser.

9. The method according to claim 7, wherein the system comprises a third thermochemical module and the desorption in the third thermochemical module is realized using either of the first thermochemical module or the second thermochemical module as a condenser or using the water condenser.

10. The method according to claim 7, wherein the system comprises a third thermochemical module and the desorption in the third thermochemical module is realized using both the second and the first thermochemical modules as a condenser.

11. The method according to claim 7, wherein the thermochemical material is selected from the group consisting of zeolites, silica gel, hygroscopic salts, metal organic frameworks (MOF), carbon, and aluminum phosphates.

12. Method for sorption in a system for thermochemical storage according to claim 1, comprising a step wherein the sorption in the second thermochemical module is realized using at least the first thermochemical module as an evaporator.

Description

EXAMPLES

Example 1 (Comparative)

[0041] Singe Stage Thermochemical Storage with a Zeolite Used as TCM

[0042] In this example, Zeolite 13X is used as TCM and water as a sorbate. The system used is depicted in FIG. 1A.

[0043] The water vapour diagram is presented in FIG. 2. In this diagram, several curves are present. The most left curve corresponds to the pure water vapour diagram, which corresponds to the conditions in the water condenser. Other curves correspond to the water vapour in a system with Zeolite Z13X with different water loadings B—particularly, from left to right, the curves are shown for B=0.26, 0.24, 0.22, 0.20, 0.18 g/g, etc.

[0044] The TCM module loaded with the zeolite is desorbed at T.sub.D=90° C., which is a typical temperature when solar collectors are used to supply heat. The released water vapour is condensed in the water condenser at T.sub.C=30° C., p.sub.C=42 mbar. This corresponds to B=0.22 g water/g zeolite.

[0045] For the sorption reaction, water vapour is evaporated in the water condenser at T.sub.E=10° C., p.sub.E=12 mbar, and the zeolite is allowed to adsorb water vapour at T.sub.S=40° C., leading to B=0.26 gw/gz. The difference in water loading ΔB=0.04 gw/gz, which indicates how much heat can be stored. This difference of 0.04 gw/gz corresponds to storage density Q/M=0.12 GJ/ton zeolite, or Q/V=0.084 GJ/m.sup.3 zeolite.

[0046] For further desorption of the zeolite, to lower B values, a higher T.sub.D is necessary, which is indicated by the arrow in FIG. 2.

Example 2 (Comparative)

[0047] Higher Thermochemical Storage, One Stage

[0048] In this example, the purpose was to obtain higher output temperatures with Zeolite 13X as TCM with thermochemical storage in one stage. The vapour pressure diagrams are shown in FIGS. 3A and B.

[0049] Typically, the temperature needed for domestic hot water (DHW) is 60° C., while for space heating (SH) −40° C.

[0050] As can be seen from the figures, these conditions were possible to achieve with dry Zeolite 13X (see FIG. 3A, maximum sorption temperature 340 K=67° C.), but not with half-dry Zeolite 13X (see FIG. 3B, maximum sorption temperature 327 K=54° C.).

[0051] The 60° C. heat should be stored separately, or in stratified boiler (but not for long, as stratification disappears by heat conduction).

Example 3

[0052] Two Stage Thermochemical Storage

[0053] In this example, two identical Z13X modules M.sub.1, M.sub.2 loaded with the same amount of Zeolite Z13X as the TCM are used as depicted in FIG. 1B. The modules and the water condenser are connected through a central tube as explained herein-above. The water vapour diagrams are shown in FIG. 4.

[0054] In a first step, both modules are dried at T.sub.D=90° C. with water condenser operating at T.sub.C=30° C. After that the valve between M.sub.1 and M.sub.2 is closed. M.sub.1 is cooled down to 30° C., while M.sub.2 is kept at 90° C. The heat released in M.sub.1 can be regained by heat exchange, e.g. with another module (<50% for concurrent and <100% for countercurrent heat exchange).

[0055] Subsequently, the valve is opened and an pressure equilibrium takes place, which leads to the drying to M.sub.2 with M.sub.1. ΔB.sub.2=−ΔB.sub.1=−0.045 at p.sub.eq2=p.sub.eq1=10 mbar. Finally, M.sub.2 reaches water loading B=0.175 gw/gz. In total ΔB=0.085 gw/gz is stored, which corresponds to storage density Q/M=0.255 GJ/tz.

Example 4

[0056] Three Stage Thermochemical Storage

[0057] In this example three identical modules M.sub.1, M.sub.2, M.sub.3 loaded with the same amount of Zeolite Z13X as the TCM are used. The modules and the water condenser are connected through a central tube as explained herein-above. The water vapour diagrams are shown in FIG. 5.

[0058] All the modules are first dried at T.sub.D 90° C. with the water condenser at T.sub.C=30° C. After that, modules M.sub.2, M.sub.3 are dried with M.sub.1 to B=0.175 gw/gz at p.sub.eq2=p.sub.eq1=10 mbar, as in Example 3. In a further step, M.sub.3 is dried with M.sub.2 which leads to ΔB.sub.3=−ΔB.sub.2=−0.065 and equilibrium when p.sub.eq3=p.sub.eq2=2 mbar. Finally, M.sub.3 reaches the water loading B=0.11 gw/gz. In total ΔB=0.15 gw/gz is stored, which corresponds to storage density Q/M 0.45 GJ/tz.

Example 5

[0059] Different TCM Materials: Zeolite Z13X and SG125

[0060] In this example, two TCM modules are used with different materials. One module uses Zeolite Z13X and the other one Silicagel Grace 125 of equal mass. The module with SG125 is used here as a condenser for the zeolite module. The modules and the water condenser are connected through a central tube as explained herein-above.

[0061] The vapor pressure diagrams are shown in FIG. 6, wherein the left diagram is for Zeolite Z13X and the right diagram for SG125.

[0062] As can be seen from FIG. 6, SG125 absorbs more H.sub.2O at lower T.sub.C with lower increase of p.sub.eq. When the Z13X module is dried with the SG125 module, ΔB.sub.2=−ΔB.sub.1=0.06 gw/gz. In two stages 0.1 gw/gz˜0.30 GJ/tz.

Example 6

[0063] Different TCM Materials: Na.sub.2S and SG125

[0064] In this example, a TCM module with hygroscopic salt Na.sub.2S is dried using a silica gel containing module. The modules and the water condenser are connected through a central tube as explained herein-above. The vapor pressure diagrams are shown in FIG. 7, wherein the left diagram is for Na.sub.2S and the right diagram for SG125.

[0065] A peculiarity of vapour pressure diagrams for salts able to form hydrates is that there are less curves with different water loadings, since the hydration states are limited to the number of existing hydrates. In this case, the diagram for SG125 present nearly a continuum of vapour pressure curves, while the diagram for Na.sub.2S only shows three curves corresponding to the formation of hydrates Na.sub.2S.9H.sub.2O, Na.sub.2S.5H.sub.2O and Na.sub.2S.2H.sub.2O. The most left curve shown is for pure water vapour.

[0066] This system allows to carry out desorption of Na.sub.2S at 70° C. instead of >90° C. with e.g. SG125.

[0067] As an example of a suitable configuration, a single, large SG125 condenser can be used for large stock of compact Na.sub.2S modules.

Example 7

[0068] Multiple Stages of MgCl.sub.2

[0069] In this example, the same thermochemical material MgCl.sub.2 is used in different TCM modules. The modules and the water condenser are connected through a central tube as explained herein-above. FIG. 8 shows water vapour pressure curves for water (most left curve) and different hydration states of MgCl.sub.2. (from left to right—formation of MgCl.sub.2.6H.sub.2O, MgCl.sub.2.4H.sub.2O, MgCl.sub.2.2H.sub.2O hydrates).

[0070] The method of the invention can be realized as a two stage desorption of MgCl.sub.2.(6.fwdarw.4.fwdarw.1)H.sub.2O at T.sub.D=120° C. instead of 150° C. This can also be performed as a two stage desorption of MgCl.sub.2.(6.fwdarw.4.fwdarw.2)H.sub.2O at T.sub.D=100° C. instead of 130° C. Another option is a three stage desorption MgCl.sub.2.(6.fwdarw.4.fwdarw.2.fwdarw.1)H.sub.2O at 100° C.

[0071] Therefore, a lower desorption temperature can be used than when one stage desorption is performed (see “single stage reference” in FIG. 8). This is particularly important for MgCl.sub.2, which is unstable at higher temperatures.

Example 8

[0072] In this example a sorption method is illustrated that allows to achieve a higher temperature T.sub.S, which in turn can be used for higher heat needs such as DHW (60° C.). In this example use is made of TCM modules M.sub.1 and M.sub.2 and a water condenser, which works as a water evaporator. The modules and the water condenser are connected through a central tube as explained herein-above.

[0073] FIG. 10 shows separate steps of this method. In the first step (left picture) the TCM modules M.sub.1 and M.sub.2 are hydrated by water vapour supplied from a cold water evaporator (T.sub.E=10° C.). During hydration heat is released, which brings the TCM modules at the temperature T.sub.S 40° C. In a second step (picture in the middle), one TCM module M.sub.1 is brought in contact with a warm water evaporator (which can be the same of different as the cold water evaporator) and heats the evaporator up to T.sub.E 40° C. In the next step (right picture), the warm water evaporator is contacted with the second TCM module M.sub.2 to hydrate that module further, which leads to the heating of that module to T.sub.S=60° C. This temperature is sufficient for typical needs for hot water (DHW).

[0074] FIG. 11 shows the water sorption diagrams in case zeolite Z13X (FIG. 11A) or SG125 (FIG. 11B) is used as the thermochemical material. FIG. 11A shows that for Z13X it is possible to reach T.sub.S′=340 K with water loading B=0.26. FIG. 11B shows that for the silica gel T.sub.S′=333 K can be reached with water loading B=0.16.

[0075] This example shows that with two TCM modules it is possible to reach a higher sorption temperature, which can be sufficient for DHW needs.