IMPROVED COOLING MEDIUM FOR USE IN A THERMAL ENERGY STORAGE SYSTEM

Abstract: The present invention provides a cooling medium for use in a thermal energy storage system comprising water, a guest molecule and a surfactant having a critical micelle concentration in an amount less than about twice said critical micelle concentration. Preferably the critical micelle concentration is less than about 1x10<-3>M and more preferably between 1x10<-4>M and 1x10<-6>M. A thermal energy storage unit which uses said cooling medium and a process for using said thermal energy storage unit are also disclosed.

Description: IMPROVED COOLING MEDIUM FOR DSE IN A THERMAL ENERGY 1TORAGE SYSTEM This application is a continuation-in-part of Application Serial No. 722,428, filed on June 27, 1992. Background of the Invention 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 cooling medium. The removal of heat to drive the decomposition causes the surrounding area to become cooler. U.S. Patent No. 4,540,501 discloses a thermal energy storage system which uses clathrates as the cooling medium. Clathrates are hydrates which use a non-stoichiometric number of water molecules per guest molecule. The guest molecule fills the interior of the lattice, stabilizing the clathrate. This stabilization allows the water lattice structure to form at temperatures significantly higher than the temperature of ice formation (0°C) . The guest molecule must be highly insoluble in water, and must have a molecular size which is less than 7 A. The halogenated hydrocarbons which are used as the guest molecules are not water iscible. Clathrates will not form unless the guest and host (lattice) compounds are in contact. In an attempt to bring the guest molecule and water into closer contact, various surfactants have been added. U.S. Patent No. 4,540,501 discloses using a nonionic fluorosurfactant having the chemical formula F(CF 2 CF 2 ) 3 - β CH 2 CH 2 θ(CH 2 CH,0) g . 11 H when the guest molecule is a refrigerant chosen ro brominated, chlorinated and luorinated hydrocarbons including CC1 2 F 2 , CC1 3 F, CBrF 3 , CHC1 2 F, CHCIF 2 , CH 2 C1F and CH 3 CC1F 2 . U.S. Patent No. 4,821,794 discloses the use of Zonyl® Florosurfactants in amounts between 1 to 5000 ppm generally, and the use of Zonyl* FSN with trichlorofluoromethane in the amount of about 200 to 300 ppm. Zonyl® Fluorosurfactant and Unidyne DS-401 have been added to water-l,l,l-tetrafluoroethane clathrate forming thermal energy storage medium in Formation of Gas Hydrate or Ice by Direct-Contact Evaporation of CFC Alternatives. F. Isobe and Y.H. Mori, Int. J. Refrig., vol. 15, No. 3 (1992), pgs. 137 - 142. Proc. Inter. Soc. Enerσy Convers. Enσ. Conf.. Akiya et al., 1991, 26th(6) 115-119 used two unspecified surfactants in concentrations up to 500 ppm to enhance the rate of formation of the clathrate from a water - 1,1-dichloro-l-fluoroethane cooling medium. However, for the known guest molecules relatively large quantities of surfactant have been used (up to and in excess of 1000 ppm) and some of the guest molecule will associate with the surfactant instead of forming a clathrate with water. This decreases the efficiency of the thermal enerσy storage system. Prior to the present invention there has been no teaching in the art of how to select a surfactant for a particular cooling medium or how to determine the amount of surfactant which will insure optimum mixing with a minimum of guest molecule association. Furthermore, many of the guest molecules presently being used are CFCs such as trichlorofluoromethane (CFC-11) . The use of these compounds is becoming disfavored because of the detrimental effect to the ozone layer. Thus it is a goal of the present invention to find a cooling medium which poses less of a threat 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 proposed as the guest molecule in clathrate formation according to the present invention. Description of the Figure FIGURE 1 shows the relationship between the surfactant concentration and the surface tension of water for the surfactant DRSC®. Figure 2 shows the relationship between surfactant concentration (DRSC«) and the interfacial tension for 1,1-dichloro-l-fluoroethane/water solution. Detailed Description of the Invention The present invention provides a cooling medium for use in a thermal energy storage system comprising water, a guest molecule and a surfactant having a critical micelle concentration in an amount less than about twice the critical micelle concentration. Preferably the critical micelle concentration is less than about lxlO "3 and more preferably between lxlO"*M and 1 10 " ^. A thermal energy storage unit which uses the cooling medium and a process for using the thermal energy storage unit are also disclosed. The guest molecules of the present invention may be any compound'capable of forming a clathrate with water. Suitable guest molecules generally have an average diameter of less than about 7 A. Preferably, the guest molecule is a refrigerant selected from the group consisting of hydrochlorofluorocarbons, hydrofluorocarbons, and mixtures thereof. Examples of preferred hydrochlorofluorocarbon guest molecules include l-fluoro-l,l-dichloroethane, and chlorodifluoromethane. Examples of preferred hydrofluorocarbon guest molecules include 1,1,1,2- tetrafluoroethane, 1,1,1-trifluoroethane, difluorormethane, pentafluoroethane, and 1,1- difluoroethane. The configuration of the thermal energy storage system of the present invention is the similar to that of 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. Accordingly, emulsions of water and the guest molecule are highly desirable. Clathrates of the present invention are formed from the guest molecule, and water. Depending on the size of the guest molecule between 5 to 17 water molecules per guest molecule are needed to form a clathrate. Preferably the amount of each the guest molecule and water is at least equal to the ratio necessary to form clathrate. More preferably, an excess of water is used to maintain a slurry, and ensure continuous and efficient heat transfer. For example, where HCFC- 141(b) is used, 20 moles of water is used for each 1 mole of HCFC-141(b). The concentration of free surfactant in water affects the properties of the water and particularly the surface tension, which is shown for DRSC® in Figure 1. For multiphase systems, such as the guest molecule/water mixtures, which are used in thermal energy storage systems the interfacial tension between the guest molecule and water may be measured. The effect of DRSC* at varying concentrations on a 1,1- dichloro-1-fluoroethane/water is shown in Figure 2. The properties of both solution change rapidly between about 25 ppm and about 125 ppm. Beyond that range of concentration, properties change more gradually. This narrow concentration range in which properties change rapidly is called the critical micelle concentration or cmc. After the cmc has been exceeded water surface tension decreases only slightly by adding more surfactant. Because water-guest molecule mixing increases as the surface tension of water decreases, maximum water-guest molecule mixing is achieved near the cmc. Moreover, after about 200 ppm (twice the cmc) of the DRSC® surfactant has been added there is virtually no change in either the surface or interfacial tension. Thus, regardless of what surfactant is used, the optimum concentration of surfactant necessary to provide maximum mixing is not greater than about twice the critical micelle concentration, preferably less than about 1.5 times the cmc and most preferably less than about the cmc. By limiting the amount of surfactant used to less than about twice the cmc it is possible to use small amounts of surfactants, especially if the critical micelle concentration is small (less than about 10 "3 M) . Each surfactant has a unique cmc, which depends upon its structure. Generally surfactants with longer hydrocarbon chains have lower critical micelle concentrations. The lower the cmc, the less surfactant is necessary to achieve maximum mixing. Thus, preferably surfactants of the present invention have critical micelle concentrations which are below 1x10 ' 3 M, and preferably between lxl0 " *M and .1x10 " ^. Critical micelle concentrations for many surfactants are listed in "Critical Micelle Concentrations of Aqueous Surfactant Systems'* by Mukerjee and Mysels [Nat. stand. Ref. Data Ser. , Nat. Bur. Stand. (U.S) 36, Feb. 1971. Many ways of determining the cmc of surfactants are described by Mukerjee et al. Moreover, because the present invention provides a large number of suitable surfactants, surfactants which produce only gradual changes in surface or interfacial tension over varying surfactant concentrations, and thus have poorly defined cmcs are not preferred. An example of a surfactant having a poorly defined cmc is Zonyl® FSN. Most preferably, a surfactant having a cmc less than lxl0"*M is used in an amount about equal to or slightly in excess of the cmc. By choosing surfactants having low cmcs and limiting the amount of surfactant to the cmc, inefficiencies due to guest molecule association with the aggregated surfactant and competition between the surfactant and guest molecule may be minimized. When any non-polar substance is in contact with water, the water molecules become arranged or organized in a cluster around the non-polar moiety. It is believed that clathrates are formed by the crystallization of this cluster. Similarly, when a surfactant is present, water molecules cluster around the surfactant, forming surfactant aggregates. This competition between the potential guest molecules and surfactant molecules for water molecules decreases the amount of clathrate which can be formed with a given amount of water molecules. Thus it is preferable to minimize the amount of surfactant used. In some situations, it may be desirable to exceed the cmc in order to acheive a particular effect. When the cmc is low, the concentration of surfactant may still be quite low-as compared to conventional surfactant concentration even though it is above the cmc. Thus, the competition between the surfactant and the guest molecule is proportionately less severe, even at concentrations which are above the cmc. Thus, surfactants with critical micelle concentrations below about lxlO"*M are preferred. An example of a surfactant species which has been found particularly effective in enhancing emulsion formation where HCFC-141(b) is used as the guest molecule is the surfactant DRSC« (alkyl dimethyl benzyl ammonium salt of octaphenyl phosphoric acid, commercially available from Allied-Signal, Inc.). The physical properties of DRSC* are shown in Table 1, below. The cmc for DRSC® is between about 50 ppm and about 125 ppm, which was determined by measuring the surface tension of water as increasing amounts of DRSC® were added. Thus, less than about 200 ppm of the DRSC® is required to insure emulsion formation between water and the chosen guest molecule. Preferably less than 100 ppm DRSC® is used. The losses of the guest molecule (HCFC-141(b)) due to association with the surfactant decrease as the amount of surfactant used is decreased, thereby increasing the efficiency of clathrate formation, and the thermal energy storage system. 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. Emulsions formed according to the present invention are stable at room perature, and remain emulsified for as long as two days with minimum drainage. 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 tempera*-nre of formation for the clathrate is reached. Ti sressure and temperature are maintained until all of tϋώ 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. Example A solution of DRSC® having a concentration of 25 ppm was made by adding 0.025 ml of DRSC® to 1 liter of water. 300 ml of the surfactant solution was poured into a 500 ml jar, and 30 ml of 141(b) was added. The jar was capped and shaken vigorously for l minute. An emulsion formed in the jar, which was stable and remained emulsified for two days without noticeable drainage. The sealed jar was placed in a freezer at 40°F. A considerable amount of snowflake-like crystals (the clathrate) was observed in the jar after 1.0 hour. The jar was left in the freezer overnight. By morning crystals had formed in the jar, indicating that clathrate had formed. Accordingly, DRSC®, which has a low cmc, is a suitable aid for clathrate formation, forming clathrate at low surfactant concentrations. Because only a small amount of surfactant is required (twice the cmc or less) , there is less surfactant to associate with the guest molecule (here HCFC-141(b)) , and thus the clathrate formation process, and the thermal energy storage system are more efficient.

Claims: I CLAIM: 1. A thermal energy storage system having a crystallizer compartment containing a clathrate forming cooling medium, a means for circulating the cooling 5. medium through a heat exchanger and a means for lowering the temperature in said crystallizer compartment; the improvement comprising: using as said clathrate forming cooling medium a mixture comprising water, a guest molecule and a 0 surfactant in an amount less than twice said surfactant's critical micelle concentration. 2. The thermal energy storage system of claim 1 wherein said surfactant is used in an amount up to said surfactant's critical micelle concentration. 3. The thermal energy storage system of claim 1 wherein said critical micelle concentration is between lxlO'*M and lxlO _6 M. 4. The thermal energy storage system of claim l wherein said guest molecule is selected from the group consisting of hydrochlorofluorocarbons and hydrofluorocarbons. 5. The thermal energy storage system of claim 4 wherein said guest molecule is selected from the group consistingof 1,1-dichloro-l-fluororethane, l-fluoro-l,l- dichloroethane chlorodifluoromethane, 1,1,1,2- tetrafluoroethane, 1, 1, 1-trifluoroethane, difluoromethane, pentafluoroethane and 1,1- difluoroethane. 6. The thermal energy storage system of claim 1 wherein the surfactant is alkyl dimethyl benzyl ammonium salt of octaphenyl phosphoric acid. 7. The thermal energy storage system of claim 6 wherein the amount of said surfactant used is less than 200 ppm. 8. A process for thermal energy storage wherein a cooling medium is induced to form a clathrate, and heat is removed from the surroundings to melt the clathrate; the improvement comprising: using as said cooling medium a mixture comprising water, a guest molecule and a surfactant having a critical micelle concentration less than lxlO "3 M in an 5. amount less than twice said critical micelle concentration. 9. The process of claim 8 wherein said surfactant has a critical micelle concentration between lxlO " M and lxlO _s M. 0 10. The process of claim 9 wherein said surfactant is alkyl dimethyl benzyl ammonium salt of octaphenyl phosphoric acid. 11. The process of claim 18 wherein the amount of said surfactant used is less than 200 ppm. 12. A cooling medium for use in a thermal energy storage system comprising water, a guest molecule and a surfactant having a critical micelle concentration in an amount is less than twice said critical micelle concentration. 13. The cooling medium of claim 12 wherein said critical micelle concentration is less than lxl0 '3 M. 14. The cooling medium of claim 13 wherein said critical micelle concentration is between lxl0 ' *M and lxlO _6 M. 15. The cooling medium of claim 14 wherein the surfactant is alkyl dimethyl benzyl ammonium salt of octaphenyl phosphoric acid.

Inventors: LI CHIEN CHI, THOMAS RAYMOND HILTON PERCIVAL, WILSON DAVID PAUL, LI, CHIEN, CHI, THOMAS, RAYMOND, HILTON, PERCIVAL, WILSON, DAVID, PAUL
Applicants: ALLIED SIGNAL INC, ALLIED-SIGNAL INC.
Application year: 1992
Main classification: F25D3
IPC codes: C09K5, F25D3