Patent Publication Number: US-4319627-A

Title: Chemical storage of energy

Description:
This is a continuation of application Ser. No. 702,838 filed July 6, 1976, now abandoned. 
    
    
     Energy systems used heretofore usually store the energy at elevated temperatures. These methods suffer from thermal losses so that only a partial recovery of the stored energy can be made. This situation is aggravated when the energy source is intermittent, since auxiliary heating must be supplied almost immediately. If the energy can be stored at ambient temperatures then normal losses are minimized and storage can be maintained indefinitely. By recovering the energy stored at elevated temperatures a high degree of efficiency can be accomplished. 
     The prior art U.S. Pat. No. 3,075,361 discloses a heat transfer system wherein solar heat is employed to endothermically dissociate a liquid hydride into a liquid metal and gaseous hydrogen wherein the dissociated hydride products are transferred to a storage vessel (heat sink) to recombine exothermically. The hydride to metal-gas and return is repeated over and over in the heat cycle. 
    
    
     IN THE DRAWINGS 
     FIG. 1 is a block diagram that illustrates the operation of the present invention relative to the theoretical Carnot cycle performance; 
     FIG. 2 is a diagrammatic representation of the chemical cycle in the Carnot cycle format; 
     FIG. 3 is a representation of the collection and storage mode of the present invention using an ammoniated salt system; and, 
     FIG. 4 is a representation of the regeneration mode. 
    
    
     The present invention is directed to any cyclic energy system which receives energy at a high temperature, stores it at a low temperature, and recovers the energy on demand near the temperature at which the energy was received. This may be a solar energy system, nuclear power plant or the like. The energy storage and recovery systems of the present invention are passive and have no moving parts in the usual sense. The only electrical energy required is for the pump which circulates an exchange fluid through the energy collector and through the space to be heat conditioned. 
     A solar heating system according to the present invention comprises the storage and abstraction of heat of reaction of chemical systems near equilibrium. The preferred chemical systems are ammoniated salts but the invention is not to be construed as being limited thereto. Alcoholates and hydrates, as well as salts of ethers and ketones, may be used. 
     For most efficient operation it is preferred to select two salt systems that have essentially the same equilibrium vapor pressures at two different temperatures. Then by a slight change in temperature the equilibrium can be shifted to either recombine or dissociate a reaction product from the salt, as desired, and thereby utilize the heat of reaction in a storage or release mode. 
     The method of the present invention provides the chemical equivalent of a Carnot heat-pump cycle in that high temperature thermal energy is reversibly stored partially in the form of chemical energy and partially in the form of thermal energy at a lower temperature. Then, on demand, the stored energy, both chemical and thermal, is recovered as thermal energy at the original elevated temperature. The method of the present invention has the advantage of passively operating a reversible thermal cycle without the attendant irreversibilities associated with rotating machinery. The net changes in stored chemical energy during the cycle are equivalent to the work interchanged in a mechanical heat engine-pump cycle. By storing the thermal energy at or near ambient temperature, thermal storage losses can be reduced to negligible values. 
     The novel method of this invention is based on the following key features: 
     (1) High temperature thermal energy is used to decompose a solid compound such as an ammoniated salt at an elevated temperature in a reaction cell 20. 
     (2) The gas evolved, for example ammonia, is reacted with another solid at a lower temperature in a separate storage vessel 10 to form a solid compound, for example, an ammoniated salt. The heat generated by this second reaction may be stored, as in a phase change material, such as lithium nitrate trihydrate or other suitable thermal storage means such as a large body of water and the like, near ambient temperature. The high temperature reaction and the low temperature reaction must be paired so that the vapor pressure of the fluid reactant, i.e., ammonia, at the lower temperature is essentially equal to the vapor pressure of the fluid reactant, i.e., ammonia, in the reaction cell 20 at the elevated temperature. The selection of the particular reaction pair is dependent upon the source and sink temperatures desired. 
     (3) Upon demand, the process can be reversed by lowering the reaction cell 20 temperature with a coolant fluid to a value slightly below the original decomposition temperature. When the reaction-cell pressure drops below the storage vessel pressure, the gaseous reactant can be transferred back to the reaction-cell. During the process, the stored salt in the storage vessel 10 decomposes and recovers the stored, low temperature thermal energy. The vapor reacts in the reaction cell 20 and generates heat at an elevated temperature. This heat of reaction is transferred to the coolant fluid at a temperature only slightly less than the original temperature at which the thermal energy was added to the system. 
     The only inherent irreversibility peculiar to this cycle is, in effect, a net irreversible transfer of thermal energy from the source to the sink temperatures. This sensible heat degradation is a small fraction of the total heat absorbed. In addition, this cycle also shares the usual heat transfer irreversibilities in common with all real thermal cycles. 
     The example set forth in FIG. 1 illustrates the operation of the cycle relative to the theoretical Carnot cycle performance. 
     
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Basis:   Source temperature = 344° K. (160° F.)             
         Storage (Sink) temperature = 294° K. (70° F.)      
Equation (1)  Q.sub.2 /Q.sub.1 ≧ T.sub.2 /T.sub.1 = 294/344 =      
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0.855                                                                     
 
    
     The temperature ratio of Equation (1) represents the minimum fraction of thermal energy available at 344° K. which must be stored at 294° K. 
     Equations (2) and (3) depict the reactions for one system as set forth in FIG. 2 of the drawing. 
     
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REACTION CELL                                                             
Equation (2)                                                              
            Ca . 6NH.sub.3 ⃡ Ca + 6NH.sub.3 ↑           
STORAGE VESSEL                                                            
Equation (3)                                                              
            LiCl . 4NH.sub.3 ⃡ LiCl . 3NH.sub.3               
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            + NH.sub.3                                                    
 
    
     The following data is from International Critical Tables, McGraw-Hill, New York, Vol. 7, page 224, 1930. 
     
                       TABLE I                                                     
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Reaction Cell     Storage Vessel                                          
               ΔH, kcal/        ΔH, kcal/                     
T° K.                                                              
      P, atm   gmole NH.sub.3                                             
                          T° K.                                    
                                P, atm                                    
                                      gmole of NH.sub.3                   
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344   1.4      10.3       294   1.4   8.8                                 
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     It is essential that the heats of reaction be referred to the same vapor pressure. Equation (4) verifies that a reversible chemical cycle is equivalent to a mechanical Carnot cycle operating between the same temperature limits--see Equation (1) 
     
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Equation (4)  Q.sub.2 / Q.sub.1 = ΔH.sub.2 /ΔH.sub.1 =        
8.8/10.3 = 0.855                                                          
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     This cycle can be applied to the storage of thermal energy obtained from a solar collector. For the purposes of illustration, reference is made to FIG. 3 which sets forth the collection and storage mode of a system according to the invention on the basis of a 10 6  Btu of thermal energy leaving the collector in a fluid at 344° K. 
     Table II is illustrative of the calculations required for the quantities of materials used for Equations (2) and (3). 
     
                                           TABLE II                                
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Reactant requirements to absorb 10.sup.6 Btu:                             
Equation (5)                                                              
 ##STR1##                      =                                          
                                 53.94 lb moles NH.sub.3 generated        
Equation (6)                                                              
 ##STR2##                      =                                          
                                 917 lbs NH.sub.3 generated               
Equation (7)                                                              
 ##STR3##                      = =                                        
                                 ##STR4##                                 
Storage Requirements:                                                     
Equation (8)                                                              
 ##STR5##                      =                                          
                                 5955 lbs of LiCl · 4NH.sub.3    
Equation (9)                                                              
 ##STR6##                      =                                          
                                10688 lbs of phase change material with   
                                latent heat of fusion = 80 Btu/lb at      
                                290° K.                            
Total weight in storage vessel                                            
                              = 16,643 lbs                                
Sensible heat in NH.sub.3                                                 
dissipated in storage                                                     
           =    (53.94) (8.56) (1.8) (343-292)                            
                              =  42,386 Btu                               
reaction heat released                                                    
           =    (53.94) (8.8 × 10.sup.3) (1.8)                      
                              =  854,410 Btu                              
Total energy transferred to phase change                                  
                              = 896,796 Btu                               
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     Because the storage temperature of the example of Table II is selected near ambient, thermal losses are negligible. 
     Referring now to FIG. 4, there is illustrated the regeneration mode according to the present invention. This illustration indicates that out of the original 10 6  Btu of energy available at 344° K., 95.8% can be recovered at 340° K. and 4.2% is dissipated at 290° K. This cycle requires no external source of mechanical energy for its operation. 
     The materials chosen above are for illustration only. Other pairs may also be used with equal success. Choice of particular materials should be dependent on cost if all other factors are equal. The following should be considered in selecting a system. 
     1. The kinetics of the forward and reverse reactions of the salts; 
     2. Heat transfer rates; and, 
     3. Aging cycles of materials. 
     Space cooling may be carried out by a pair of systems according to the invention in which the reactions are alternatively run in the forward and reverse directions. 
     For power applications, the following pair of reactions, where equilibrium vapor pressures can be matched over a selected temperature range, are illustrative: 
     
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Source: Temperature range 775° K. to 845° K. (935°   
F. to 1060° F.)                                                    
Equation                                                                  
       2 CuCl.sub.2 ⃡ Cu.sub.2 Cl.sub.2 + Cl.sub.2            
                        ΔH = 42.5 kcal/mole Cl.sub.2                
(10)                                                                      
Sink: Temperature range 277° K. to 300° K. (40° F.   
to 80° F.)                                                         
Equation                                                                  
       Cl.sub.2 . 8H.sub.2 O ⃡ 8H.sub.2 O + Cl.sub.2          
                        ΔH = 17.96 kcal/mole Cl.sub.2               
(11)                                                                      
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     In this application only 36% of the collected energy is stored as thermal energy at ambient temperature. 
     Table III cites two representative experimental points on the vapor pressure curve of the reaction. 
     
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Equation (12)                                                             
           CaCl.sub.2 . 8NH.sub.3 ⃡ CaCl.sub.2 . 4NH.sub.3 +  
           4NH.sub.3                                                      
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     The pressures listed are slightly higher than current values cited in the literature. 
     Table IV cites two representative experimental points on the vapor pressure curve of the reaction. 
     
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Equation (13)                                                             
           FeCl.sub.2 . 6NH.sub.3 ⃡ FeCl.sub.2 . 2NH.sub.3 +  
           4NH.sub.3                                                      
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     The pressures listed are somewhat higher than current values cited in the literature. 
     Table V lists the ammonia evolution rate (mass of ammonia per hour per mass of available ammonia) as a function of temperature at a constant pressure of 760 torr for the reaction. 
     
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Equation (12)                                                             
           CaCl.sub.2 . 8NH.sub.3 ⃡ CaCl.sub.2 . 4NH.sub.3 +  
           4NH.sub.3                                                      
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     The equilibrium temperature at this pressure is 29° C. The data indicates that the ammonia evolution rate is a function of temperature deviation from equilibrium. Additional data indicates that the process is reversible, i.e., CaCl 2 .4NH 3  absorbs NH 3  at a rate that is a function of the temperature (29° C.) in the temperature range below the equilibrium temperature. 
     
                       TABLE III                                                   
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Equilibrium vapor pressure data obtained for reaction                     
CaCl.sub.2 . 8NH.sub.3 ⃡ CaCl.sub.2 . 4NH.sub.3 + 4NH.sub.3   
Temperature, °C.                                                   
                  Pressure, Torr                                          
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29                760                                                     
19                450                                                     
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                       TABLE IV                                                    
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Equilibrium vapor pressure data obtained for reaction                     
FeCl.sub.2 . 6NH.sub.3 ⃡ FeCl.sub.2 . 2NH.sub.3 + 4NH.sub.3   
Temperature, °C.                                                   
                  Pressure, Torr                                          
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245               750                                                     
231               450                                                     
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                       TABLE V                                                     
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Ammonia evolution rates for the reaction                                  
CaCl.sub.2 . 8NH.sub.3 ⃡ CaCl.sub.2 . 4NH.sub.3 + 4NH.sub.3   
vs. temperature (P = 760 Torr)                                            
               Rate of NH.sub.3 Evolution,                                
Temperature, °C.                                                   
               lbs/hr - lb available NH.sub.3                             
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29.0           0                                                          
32.2           .05                                                        
33.8           .34                                                        
36.0           .84                                                        
38.5           1.45                                                       
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     The following reactions were carried out under laboratory conditions to substantiate the operation of the reaction mechanism. ##STR7## 
     Heat was first applied to the &#34;hot&#34; side and ammonia gas was transferred to the cold side, then to reverse the process, the temperature of the &#34;hot&#34; side was lowered and the calcium chloride ammoniate decomposed endothermally, evolving ammonia which was transferred to the hot side and reacted exothermally with the ferrous chloride ammoniate. The weight change in the reaction vessels was used as a measure of ammonia transfer. 
     Temperature and weight measurements made during ammonia transfer reactions confirmed the process. 
     While there have been described what at present are considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention. It is aimed, therefore, in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.