WO1993004139A1

WO1993004139A1 - Improved thermal energy storage system and process for thermal energy storage and transfer

Info

Publication number: WO1993004139A1
Application number: PCT/US1992/007255
Authority: WO
Inventors: Chien Chi Li
Original assignee: Allied-Signal Inc.
Priority date: 1991-08-27
Filing date: 1992-08-27
Publication date: 1993-03-04
Also published as: AU2517992A

Abstract

An improved thermal energy storage system and a process for thermal energy storage and transfer are disclosed. The cooling medium, a clathrate forming mixture, comprises water, an appropriate guest molecule and at least one help gas. Preferably the help gas has a van der Waals diameter of less than about 5.35 Å, and more preferably is selected from the group consisting of nitrogen, nitrous oxide, hydrogen, oxygen, methane, neon, krypton, argon, ethylene, xenon, ethane, hydrogen sulphide and carbon dioxide. The guest molecule may be introduced into the host molecule solution via an ultrasonic atomizer.

Description

IMPROVED THERMAL ENERGY STORAGE SYSTEM AND PROCESS FOR THERMAL ENERGY STORAGE AND TRANSFER

Background of the Invention

The use of air conditioners during the summer months requires large quantities of energy primarily during the daytime hours when other forms of energy consumption are also high. Peaking generators are required to increase electricity generating capacity so that power loads are met. However, energy consumption decreases dramatically at night, and the peaking generators are not needed. Thus, the expensive peaking generators are run only half the time, decreasing the efficiency of the power facility. To alleviate this problem, thermal energy storage systems, which can utilize off peak, night-time electricity have been proposed.

Thermal energy storage systems contain a cooling medium, which is frozen during the off peak, evening hours. During the daytime, heat from the surrounding area is used to melt the frozen cooling medium. The removal of heat to drive the decomposition causes the surrounding area to become cooler.

The use of ice and water as the cooling medium is an ancient practice. However, the systems using water are inefficient and bulky. Extraction of sensible heat from water is inefficient in comparison with other cooling medium such as clathrates.

Hydrate systems, such as Na₂SO₄*10H₂θ have also been disclosed. However, in operation, the hydrates tend to segregate after a few cycles. In addition such hydrates are inefficient and corrosive to the system.

Gas clathrates made from refrigerants and water have been suggested as cooling media suitable for thermal energy storage systems. Attempts at increasing the efficiency of these units have been limited to increasing the efficiency of clathrate formation. For example, U.S. Patent No. 4,540,501 discloses adding surfactants to the cooling medium to increase clathrate formation. U.S.

SUBSTITUTE SHE€T Patent No. 4,821,794 discloses increasing clathrate formation and system efficiency by intimately mixing the water and guest molecule.

Furthermore, many of the guest molecules presently being used are CFCs such as trichlorofluoromethane (R-

11) . The use of these compounds is becoming disfavored because of the possible detrimental effect to the ozone layer. Halohydrocarbons such as HCFC 141(b) which contain hydrogen, and are believed to pose less of a threat to the ozone layer, and are thus the preferred guest molecules in clathrate formation according to the present invention.

Detailed Description of the Drawing

Figure 1 shows one embodiment of the present invention, a clathrate formation apparatus including an ultrasonic atomizer. Detailed Description of the Invention

The present invention relates to a thermal energy storage system having a clathrate formation chamber containing a clathrate forming cooling medium comprising a guest molecule, water and at least one help gas, means for lowering the temperature in said clathrate formation chamber and means for circulating the cooling medium containing clathrate through a heat exchanger. A process for thermal energy storage and transfer comprising the steps of producing a clathrate slurry from a clathrate forming cooling medium comprising a guest molecule, water and at least one help gas and circulating said clathrate slurry through a heat exchanger is also disclosed. Gas hydrates or clathrates are crystalline solids with icelike lattices formed from hydrogen-bonded water molecules. The lattices contain almost spherical holes which enclose guest molecules, usually of gases and volatile liquids. The guest molecule fills the interior of the cage lattice, stabilizing the ice structure of

_.BST_.TUTESHEET clathrate, and allowing formation at temperatures significantly higher than the temperature of ice formation (0°C) . The structure of the clathrate usually depends upon the size of the guest molecule. Smaller guest molecules (up to about 5.3 A diameter) form Structure 1 hydrates, containing 46 water molecules per unit cell. Each unit cell contains two small and six large cages. Larger molecules (up to about 7A diameter) tend to form Structure II clathrates, having 136 water molecules per unit cell. Each unit cell contains 16 small and 8 large cages. When the guest molecule is too large to fill the small cages, greater stabilization is possible by using "help" gases whose molecules have diameters small enough to enter the smaller cages. As used herein the term gas means a compound which is a gas at ambient conditions. Such stabilization results in even higher formation temperatures. Preferably help gases have van der Waals diameters less than about 5.35 A. More preferably the help gas is selected from the group consisting of nitrogen, nitrous oxide, hydrogen, oxygen, methane, neon, krypton, argon, ethylene, xenon, ethane, hydrogen sulphide, carbon dioxide and air. Most preferably the help gas is nitrous oxide, carbon dioxide or nitrogen. Any appropriate guest molecule may be used. It was heretofore unrecognized that the efficiency of thermal energy storage units could be increased by adding a help gas to the cooling medium. Further, the addition of at least one help gas may increase the rate of clathrate formation in some cases. The cooling media of the present invention may be used in any thermal energy storage system known in the art, such as that of U.S. Patent No. 4,540,501.

A preferred clathrate formation apparatus for use in a thermal ene_.rgy storage system and the process for using the device are best understood by reference to Figure 1.

SUBSTITUTE SHEET The clathrate formation chamber, 1, is filled with host solution comprising water and at least one help gas. Preferably the water/help gas (host solution) is saturated so that pressurization of the clathrate formation chamber is not necessary. The host solution is cooled to about 5°C. by refrigeration coil, 7. The guest molecule is cooled in chamber 2, by refrigeration coil 4 until the guest molecule solution is at the same temperature as the host solution in the clathrate formation chamber. The guest molecule solution is removed from chamber 2 via line 5, and passes through atomizer 6. The atomizer 6, introduces the guest molecule into the clathrate formation chamber 1, as particles with a diameter below about 100 microns. Preferably the diameter of the droplets is between about 20 and about 50 microns. An ultrasonic atomizer is preferred as the atomizer, however any other means for forming a large quantity of droplets of the appropriate size, thereby generating a large surface area may be used.

The droplets of guest molecule mix with the aqueous carbon dioxide solution and form a mixed clathrate which resembles snow-like flakes which have a density close to water. Preferably, the guest molecule is introduced to the clathrate formation chamber until a clathrate/aqueous carbon dioxide slurry is formed. Slurries have the best heat exchange properties, and are thus preferred. Once the desired clathrate slurry is formed, the atomizer 6, is shut off. During the daytime heat from the surrounding area is exchanged via refrigeration line 7, and the clathrate is decomposed.

Any guest molecule which does not form clathrate (or guest molecule which is released as a result of the decomposition of clathrate upon heating) settles to the bottom of the clathrate formation chamber 1, and may be

SUBSTITUTE SHEET recycled to the guest molecule chamber 2, via line 3.

The rest of the configuration of the thermal energy storage system of the present invention may be any configuration known in the art, such as U.S. Patent No. 4,540,501.

To form a clathrate the guest molecule and water must be dissimilar and be in contact with each other. The more intimate the contact, the more efficient the clathrate formation will be. Any suitable surfactant may be used to increase both the contact between the guest molecule and water, and thereby the rate of clathrate formation.

The guest molecule may be any refrigerant having a molecular size which is small enough to be surrounded by host molecules. The guest molecule is preferably chosen frombrominated, chlorinated and fluorinated hydrocarbons including CC1₂F₂ (dichlorodifluoromethane, R-12) , CC1₃F ( tri chl orof1uoromethane , R-ll) , CBrF₃ (bromotrifluoromethane , R-13B) , CHC1₂F ( di chl oro fluoromethane , R-21) , CHC1F₂ ( chl orodi f luoromethane , R-22) , CH₂C1F (chlorofluoromethane, R-31) , CH₂F₂ (difluoromethane, R- 32), CC1₂FCH₃ (l-fluoro-l,l-dichloroethane, HCFC-141b) , CC1F₂CH₃ (l-chloro-l,l-difluoroethane, HCFC-142b) , CF₃CHF₂ (1,1,1,2,2-pentafluoroethane, HFC-125) , CH₃CHF₂ (1,1- difluoroethane, HFC-152a), CF₃CH₂F (1,1,1,2 tetrafluoroethane, HFC-134a) and CF₂HCF₂H (1,1,2,2- tetrafluoroethane, HFC-134) . Most preferably the guest molecule of the present invention is l-fluoro-1,1- dichloroethane (HCFC-141b) or 1,1,1,2 tetrafluoroethane (HFC-134a) . Other compositions suitable as guest molecules may be readily determinable by one skilled in the art using the teaching of the present invention.

While virtually any concentration of help gas will result in formation of a mixed clathrate, saturated

SUBSTITUTE SHEET water/help gas solutions are preferred because the formation chamber does not need to be pressurized to keep the help gas in solution. However, for help gases with low solubilities in both water and the clathrate former (guest molecule) concentrations of help gas in excess of saturation may be required.

An effective amount of guest molecule and water must be present to insure clathrate formation. Preferably, an excess of water is used to maintain a slurry, and ensure continuous and efficient heat transfer. Where HCFC- 141(b) is used as the guest molecule, at least about 20 moles of water is used for each 1 mole of HCFC-141(b). Appropriate preferred ratios for other guest molecules may be readily determined by one skilled in the art using the teaching of the present invention.

Agitation is not required to ensure clathrate formation of the cooling medium of the present invention.

However, agitation may be used to further encourage clathrate formation. The clathrate is formed in a storage tank/crystallizer. The pressure in the crystallizer is decreased by means of a compressor, as described in more detail in U.S. Patent No. 4,540,501, and heat is removed until the temperature of formation for the clathrate is reached. The pressure and temperature are maintained until all of the clathrate is formed. The clathrate is circulated through the heat exchanger via the recirculation loop. Clathrate is circulated through the heat exchanger, decomposed, and the water and guest molecule mixture is returned to the crystallizer.

Various modifications and changes may be made without departing from the true scope of the invention, which is defined by the following claims.

SUBSTITUTE SHEET

Claims

I CLAIM;

1. In a thermal energy storage system having a clathrate formation chamber containing a clathrate forming cooling medium, means for lowering the temperature in said clathrate formation chamber, means for circulating the cooling medium containing clathrate through a heat exchanger; the improvement comprising: using as said clathrate forming cooling medium a mixture comprising a guest molecule, water and at least one help gas.

2. The thermal energy storage system of claim 1 wherein said help gas has a van der Waals diameter of less than about 5.4 A.

3. The thermal energy storage system of claim 1 wherein said help gas is selected from the group consisting of nitrogen, nitrous oxide, hydrogen, oxygen, methane, neon, krypton, argon, ethylene, xenon, ethane, hydrogen sulphide, carbon dioxide and air.

4. The thermal energy storage system of claim 3 wherein said guest molecule is selected from the group consisting of dichlorodifluoromethane , trichlorofluoromethane, bromotrifluoromethane, dichlorofluoromethane, chlorodifluoromethane, chlorofluoromethane, difluoromethane, l-fluoro-1,1- dichloroethane, 1,1,1,2,2-pentafluoroethane, 1-chloro- 1,1-difluoroethane, 1,1-difluoroethane, 1,1,1,2- tetrafluoroethane and 1,1,2,2-tetrafluoroethane.

5. The system of claim 3 wherein the cooling medium further comprises a surfactant.

6. The thermal energy storage system of claim 3 further comprising means to introduce said guest molecule to said clathrate formation chamber as droplets having a diameter less than about 100 microns.

7. The thermal energy storage system of claim 6 wherein said means for introducing said droplets is an

SUBSTITUTESHEET atomizer.

8. A process for thermal energy storage and transfer comprising the steps of: producing a clathrate slurry from a clathrate forming cooling medium comprising a guest molecule, water and at least one help gas; and circulating said clathrate slurry through a heat exchanger.

9. The process claim 8 wherein said help gas has a van der Waals diameter of less than about 5.4 A.

10. The process of claim 9 wherein said at least one gas is selected from the group consisting of nitrogen, nitrous oxide, hydrogen, oxygen, methane, neon, krypton, argon, ethylene, xenon, ethane, hydrogen sulphide and carbon dioxide.

11. The process of claim 10 wherein said guest molecule is selected from the group consisting of dichlorodifluoromethane, trichlorofluoromethane, bromotrifluoromethane, dichlorofluoromethane, chlorodifluoromethane , chlorofluoromethane, difluoromethane, l-fluoro-l,l-dichloroethane, 1,1,1,2,2- pentafluoroethane, 1-chloro-1,1-difluoroethane, 1,1- difluoroethane, 1,1,1,2-tetrafluoroethane and 1,1,2,2- tetrafluoroethane.

12. The process of claim 10 further comprising a surfactant.

SUBSTITUTE SHEET