Thermal energy storage using supercritical fluids

Abstract

A thermal energy storage system is described employing latent heat storage of a supercritical fluid instead of typical phase change materials. Two fundamental thermodynamic concepts are invoked. First, by using the latent heat of liquid/vapor phase change, high energy density storage is feasible. Second, by operating the thermal energy storage system at a higher pressure, the saturation temperature is increased to operate at molten salt temperatures and above. Beyond the two-phase regime, supercritical operation permits capturing and utilizing heat taking advantage of latent and sensible heat, both in the two-phase regime as well as in supercritical regime while at the same time, reducing the required volume by taking advantage of the high compressibilities.

What is claimed is: 
     
         1 . A thermal energy storage apparatus, comprising:
 a pressurized storage vessel;   a storage fluid within the pressurized storage vessel; and   at least one heat exchanger coupled to the pressurized storage vessel and in contact with the storage fluid to occasionally transfer heat energy between the storage fluid and at least one working fluid;   wherein the storage fluid changes from a two-phase state to a supercritical state as the heat energy is transferred from the at least one working fluid to the storage fluid and changes back to the two-phase from the supercritical state as the heat energy is transferred from the storage fluid to the at least one working fluid.   
     
     
         2 . The apparatus of  claim 1 , wherein the at least one heat exchanger is disposed within the pressurized storage vessel. 
     
     
         3 . The apparatus of  claim 2 , wherein the at least one working fluid comprises a heat source working fluid and the at least one heat exchanger within the pressurized storage vessel receives the heat source working fluid to transfer the heat energy from the heat source working fluid to the storage fluid. 
     
     
         4 . The apparatus of  claim 3 , wherein the heat source working fluid is coupled to a solar thermal plant. 
     
     
         5 . The apparatus of  claim 2 , wherein the at least one working fluid comprises a heat sink working fluid and the at least one heat exchanger within the pressurized storage vessel receives the heat sink working fluid to transfer the heat energy from the storage fluid to the heat sink working fluid. 
     
     
         6 . The apparatus of  claim 5 , wherein the heat sink working fluid is coupled to a steam generator generating electrical power. 
     
     
         7 . The apparatus of  claim 1 , the storage fluid comprises a polyaromatic hydrocarbon (PAH). 
     
     
         8 . The apparatus of  claim 7 , wherein the PAH is selected from the group consisting of naphthalene, methylnaphlene, dimethylnapthalene, and biphenyl. 
     
     
         9 . The apparatus of  claim 1 , wherein the storage fluid is selected from the group consisting of glycerol, pthalic anhydride, benzoic acid, 3,4 xylenol, iodobenzene, and an ionic fluid. 
     
     
         10 . A method of storing thermal energy, comprising:
 storing a storage fluid within a pressurized storage vessel; and   occasionally transferring heat energy between the storage fluid and at least one working fluid with at least one heat exchanger coupled to the pressurized storage vessel and in contact with the storage fluid such that,
 the storage fluid changes from a two-phase state to a supercritical state as the heat energy is transferred from the at least one working fluid to the storage fluid, and 
 the storage fluid changes back to the two-phase from the supercritical state as the heat energy is transferred from the storage fluid to the at least one working fluid. 
   
     
     
         11 . The method of  claim 10 , wherein the at least one heat exchanger is disposed within the pressurized storage vessel. 
     
     
         12 . The method of  claim 11 , wherein the at least one working fluid comprises a heat source working fluid and the first heat exchanger within the pressurized storage vessel receives the heat source working fluid to transfer the heat energy from the heat source working fluid to the at least one working fluid. 
     
     
         13 . The method of  claim 12 , wherein the heat source working fluid is coupled to a solar thermal plant. 
     
     
         14 . The method of  claim 11 , wherein the at least one working fluid comprises a heat sink working fluid and the second heat exchanger within the pressurized storage vessel receives the heat sink working fluid to transfer the heat energy from the at least one working fluid to the heat sink working fluid. 
     
     
         15 . The method of  claim 14 , wherein the heat sink working fluid is coupled to a steam generator generating electrical power. 
     
     
         16 . The method of  claim 10 , wherein the storage fluid comprises a polyaromatic hydrocarbon (PAH). 
     
     
         17 . The method of  claim 16 , wherein the PAH is selected from the group consisting of naphthalene, methylnaphlene, dimethylnapthalene, and biphenyl. 
     
     
         18 . The method of  claim 10 , wherein the storage fluid is selected from the group consisting of glycerol, pthalic anhydride, benzoic acid, 3,4 xylenol, iodobenzene, and an ionic fluid. 
     
     
         19 . A thermal energy storage apparatus, comprising:
 a pressurized storage vessel means for storing a storage fluid means for storing heat energy; and   at least one heat exchanger means for occasionally transferring the heat energy between the storage fluid means and at least one working fluid means for transferring the heat energy, the at least one heat exchanger means coupled to the pressurized storage vessel means and in contact with the storage fluid means;   wherein the storage fluid means changes from a two-phase state to a supercritical state as the heat energy is transferred from the at least one working fluid to the storage fluid and changes back to the two-phase from the supercritical state as the heat energy is transferred from the storage fluid to the at least one working fluid.   
     
     
         20 . The apparatus of  claim 19 , wherein the storage fluid means is selected from the group consisting of glycerol, pthalic anhydride, benzoic acid, naphthalene, 3,4 xylenol, iodobenzene, methylnaphlene, dimethylnapthalene, biphenyl, and an ionic fluid.