Patent Publication Number: US-2019178584-A1

Title: Method for thermo-chemical energy storage

Description:
The present invention relates to a method for thermo-chemical energy storage by carrying out endothermic chemical reactions for the storage of heat energy in the form of chemical energy in one or more chemical compounds for later re-release in the form of heat energy by carrying out chemical equilibrium reactions of ammine complexes of transition metal salts. 
     PRIOR ART 
     Thermo-chemical energy storage, i.e. storage of heat energy in the form of chemical energy, is a method of energy storage by cycling at least one chemical compound between the states of at least one reversible equilibrium reaction, said method having been known for decades, but only been subject to more intense research in the last few years. For example, U.S. Pat. No. 4,365,475 discloses the combination of two equilibrium reactions for the purpose of thermo-chemical energy storage, namely the alternating reversible endothermic formation of two ammine complexes, CaCl 2 .8NH 3  and ZnCl 2 .NH 3 . 
     In general, the reactions suitable for thermo-chemical energy storage can be divided into two categories, namely the category of “sorption processes”, where the primary valences of the chemical compounds involved remain unchanged and coordinate bonds are formed only via secondary valences, as is the case with the above cited reactions to form ammine complexes, but also, for example, to form hydrates and other solvates such as hydrates, and the category of “chemical reactions”, where the primary valences are changed. In both cases, mainly metal salts are used due to the relatively high number of coordination sites and oxidation states, respectively. 
     Systems from both categories described in the subject literature are, for example, BaO/BaO 2  (Fahim et al., Chem. Eng. Journal 27(1), 21-28 (1983)), CuO/Cu 2 O (Chadda et al., Int. J. Energy Res. 13, 63 (1989)), PbO/PbCO 3  (Kato et al., Prog. Nucl. Energy 32(3-4), 563-570 (1998)), MgO/Mg(OH) 2  (Aphane et al., J. Therm. Anal. calorim. 96(3), 987-992 (2009)), CaO/Ca(OH) 2  (Schaube et al., Thermochim. Acta 538(0), 9-20 (2012)), and various salt hydrates such as MgSO 4 /MgSO 4 .7H 2 O (Ferchaud et al., JPCS 395(1), 12069 (2012)). They are still at a very early stage in their development, though, and for most of them a practical or economic application is still years away. 
     In addition to the above pair of CaCl 2 .8NH 3  and ZnCl 2 .NH 3  known from U.S. Pat. No. 4,365,475, other combinations which, however, alternate between different co-ordination states of ammine complexes throughout, are known as systems using ammine complexes. Examples include a combination of the two chlorides CaCl 2  and FeCl 2 , which are used according to U.S. Pat. No. 4,319,627 to carry out the following reactions: 
       CaCl 2 .8NH 3   CaCl 2 .4NH 3 +4NH 3    
       FeCl 2 .2NH 3 +4NH 3   FeCl 2 .6NH 3    
     or a further combination using calcium chloride, namely CaCl 2  and MnCl 2 , which is according to Li et al., AIChE J. 59(4), 1334-1347 (April 2013), undergo the following reactions in a manner analogous to the above combination with ferrous chloride: 
       CaCl 2 .8NH 3   CaCl 2 .4NH 3 +4NH 3    
       MnCl 2 .2NH 3 +4NH 3   MnCl 2 .6NH 3    
     As a further example of ammine complexes of a transition metal chloride, Aidoun and Ternan, Appl. Therm. Eng. 21, 1019-1034 (2001), disclose the use of cobalt chloride according to the equation below: 
       CoCl 2 .2NH 3 +4NH 3   CoCl 2 .6NH 3    
     In most known cases, however, the energy storage density of the systems named above is rather low, and the corrosivity of some of the salts used often presents a problem with respect to the equipment. Additionally, transport and storage of the metal salts used cause problems, because the temperatures reached during the exothermic reaction are frequently near or even above the melting point of the salts/complexes, so that at least a part of the respective salts will melt, which leads to agglutination. 
     Furthermore, for all known systems for thermo-chemical energy storage, the quality of the thermal storage material used decreases as the number of cycles increases, so that it either has to be exchanged or, in some cases, cleaned expensively. Such a quality decrease can either result from incomplete conversion, e.g. due to formation of a nonreactive layer at the surface of the thermal storage material, or from changes in the mechanical properties of the solid particles, e.g. by abrasion, sintering, etc., both being equatable with a decrease in thermal storage capacity, i.e. energy storage density. For example, Ishitobi et al. (J. Chem. Eng. Japan 45(1), 58-63 (2012)) have demonstrated a decrease by 30% after 105 cycles for the system MgO/Mg(OH) 2 . 
     Therefore, against this background, the aim of the invention was to at least partly solve the above problems. 
     DISCLOSURE OF THE INVENTION 
     The present invention achieves said aim by providing a method for thermo-chemical energy storage by carrying out reversible chemical reactions for the storage of heat energy in the form of chemical energy in one or more chemical compounds for later re-release in the form of heat energy using chemical equilibrium reactions, wherein equilibrium reactions of ammine complexes of transition metal salts are carried out for the storage and re-release of the energy, characterized in that 
     a) ammine complexes of transition metal salts are formed and decomposed according to the following reversible total reactions: 
       [Me(NH 3 ) n ]X+ΔH R   MeX+ n NH 3  
 
       or in an alternative notation: 
       MeX. n NH 3 +ΔH R   MeX+ n NH 3  
 
     wherein Me represents at least one transition metal ion and X represents one or more counterion(s) in a quantity sufficient for charge equalizing the complex, according to the valence thereof and that of the transition metal ion; and
 
b) one or more transition metal salt(s), carried on a carrier material that is inert with regard to the reaction, is/are used.
 
     Contrary to the systems cited in the introduction, according to the present invention, only the ammine complex formation reaction of the at least one transition metal salt is carried out, rather than a combination of two parallel reactions complementing each other chemically or thermodynamically. This means that the reaction does not alternate between different coordination numbers of the ammine complexes, but rather the entire enthalpy of formation of the ammine complexes is recovered during the exothermic reaction. The inventors have found out that transition metal ammine complexes have very high enthalpies of formation. While this could result in the above problems related to at least partial melting of salts and complexes, respectively, the invention solves said issue by applying the metal salts onto a carrier, whereby simultaneously a “dilution” of the salts is achieved, which precludes melting processes and an associated agglutination of the salts. Rather, the transition metal salts applied onto the carrier remain easy to handle even at higher temperatures. For example, when using a particulate carrier, as is preferred according to the invention, the material remains free-flowing and thus easily transportable and storable. As a consequence thereof and due to the fact that even when chlorides or other salts of volatile acids are used, according to the present invention—in contrast to the prior art using non-immobilized salts—hardly any irreversible decomposition of salts occurs on the carrier, the working life of the material serving as an energy carrier is considerably increased. 
     The at least one transition metal is preferably selected from Mn, Fe, Co, Ni, Cu, and Cd, because ammine complexes of salts of these metals have particularly high enthalpies of formation during the exothermic reaction, i.e. the “reverse reaction” in the above reaction equation, thus releasing high amounts of heat per mass unit, and they are relatively inexpensive, too. 
     In particularly preferred embodiments, the at least one transition metal is selected from Cu and Cd, most preferably Cd, as these two metals when used in the inventive method have advantageous heat balances relative to the other transition metals. On the one hand, when complexed with ammonia, copper salts show very high enthalpies of formation, thus releasing very high amounts of heat even after “dilution” and application onto the carrier. On the other hand, quite surprisingly, ammine complexes of cadmium salts may be decomposed even by simple purging with air, optionally while heating them to relatively low temperatures in order to increase the already high reaction rate. Therefore, when using cadmium in a cycle of the above forward and reverse reactions hardly any heat needs to be invested. Rather, a considerable amount of heat may be recovered, making cadmium salts, especially CdCl 2 , also suitable in a method for recovering thermal energy from chemical energy, even without applying them onto a carrier. 
     According to the present invention, preferably a sulfate ion SO 4   2−  or two chloride ions Cl −  are used as the counterion(s) X, as these salts are both well researched and available without limitations, and have high enthalpies of formation of their ammine complexes. Additionally, unlike the chlorides, the sulfates do not undergo irreversible decomposition even when heated to very high temperatures exceeding 600° C., whereas the chlorides tend to have higher enthalpies of formation than the sulfates when complexed with ammonia. Without wishing to be bound by theory, both of the above also seem to be due to the fact that the chlorides are capable of coordinating five ammonia molecules, with the sulfates being able to coordinate only four thereof. 
     Therefore, in particularly preferred embodiments, the sulfate or chloride of copper or cadmium, or a mixture of two or more thereof is used as the transition metal salt, CuSO 4  and CdCl 2  being particularly preferred. In some cases, the use of mixtures of two or more transition metal salts on the same or on separate carrier(s)/carrier particle(s) may be preferred in order to control, for example, the amount of heat released during the exothermic reaction. For manufacturing and cost reasons, embodiments are preferred where each salt is applied on separate carriers/carrier particles. As a rule, embodiments where only a single transition metal salt is used are particularly preferred. 
     The carrier material as such is not particularly limited, as long as it is chemically and thermally inert under the reaction conditions chosen. For reasons of availability and costs, it is preferably selected from vermiculite, porous aluminium silicates, and zeolites, more preferably from zeolites and expanded vermiculite, zeolites being particularly preferred due to the wide range of usable modifications thereof. 
     Furthermore, in preferred embodiments a particulate carrier material having a grain size of 0.1 to 5 mm, more preferably 0.5 to 3 mm, most preferably 1 to 2 mm, is used because, for one thing, particulate material can be handled more easily than, for example, pellets or even larger pieces of the carrier. Thus, for example, bulk material is easily transportable via chutes, and finely divided, e.g. powdery, material having grain sizes below about 2 mm may be conveyed using a blower and may be collected using funnels. Most importantly, however, powdery material may be reacted in fluidized-bed reactors, which reduces equipment use, increases turnovers and prolongates the durability of the material. Moreover, regenerated material may be sized using sieves in order to create reproducible reaction conditions. Furthermore, fine grains have a relatively large surface area and thus may be loaded with higher ratios of transition metal salt. 
     According to the present invention, the carrier material preferably is loaded with about 10 to 70 wt % of the transition metal salt, expressed as the weight of the transition metal salt based on the carrier weight. The load depends, among other things, on the enthalpies of formation of the respective ammine complexes, on the equipment configuration, such as the intended use of the exothermic reaction, and on the nature and amount of waste heat available for the decomposition of the complexes. With a load considerably higher than 70 wt %, there could still be a risk of agglomeration by melting processes, as in such cases the particle mostly consists of the salt, whereas loads below 10 wt % may be uneconomical. The latter also applies to the preparation of very high loads, e.g. higher than 80 wt %. So far, loads between about 35 and about 65 wt %, more preferably between about 40 and about 60 wt %, have been shown to be preferred according to the invention. 
     In a second aspect, the present invention also discloses the use of ammine complexes of transition metal salts for thermo-chemical energy storage by carrying out chemical equilibrium reactions of the ammine complexes of transition metal salts for the storage and re-release of energy, characterized in that 
     a) ammine complexes of transition metal salts are formed and decomposed according to the following reversible total reactions: 
       [Me(NH 3 ) n ]X+ΔH R   MeX+ n NH 3  
 
       or in an alternative notation: 
       MeX. n NH 3 +ΔH R   MeX+ n NH 3  
 
     wherein Me represents at least one transition metal ion and X represents one or more counterion(s) in a quantity sufficient for charge equalizing the complex, according to the valence thereof and that of the transition metal ion; and
 
b) one or more transition metal salt(s), carried on a carrier material that is inert with regard to the reaction, is/are used.
 
     The preferred options for said inventive use are, of course, the same as disclosed for the above method according to the first aspect of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in more detail by way of exemplary embodiments and with reference to the attached drawings, which show the following: 
         FIG. 1  shows a photograph of the fluidized-bed reactor used in the examples and reference examples. 
         FIG. 2  shows the temperature profile of the ammine complex formation reaction of Comparative Example 1. 
         FIG. 3  shows the temperature profile of the ammine complex formation reaction of Example 1. 
         FIG. 4  shows the temperature profile of the ammine complex formation reaction of Comparative Example 2. 
         FIG. 5  shows the temperature profile of the ammine complex formation reaction of Example 2. 
         FIG. 6  shows the temperature profile of the ammine complex formation reaction of Example 3. 
         FIG. 7  shows the temperature profile of cycles of the ammine complex formation reaction and ammine complex decomposition reaction of Example 7. 
     
    
    
     EXAMPLES 
     All reagents used in the examples below are commercially available and were used without further purification. The zeolite Y used had a particle size of about 2 mm, and the expanded vermiculite had a particle size of about 2 to 3 mm. Both carriers as well as all transition metal salts were obtained from Sigma Aldrich. 
     The x-ray fluorescence (XRF) analyses for determining the carrier loads obtained in the synthesis examples were performed on an Axios XRF device from Panalytical, and simultaneous thermal analysis (STA) measurements for determining the thermal storage capacity were performed on a Jupiter® STA 449 F1 from Netzsch. 
     Synthesis Example 1 
     In a typical loading experiment, 50 g of zeolite Y, which had previously been dried for 6 h at 400° C., were immersed into a saturated CuSO 4  solution for 2 h and thus soaked therewith. The bluish product was then separated, flushed with 200 ml of deionized water, and dried for 12 h at 60° C. and thereafter for 6 h at 350° C. A CuSO 4  content of 39.7 wt % was determined using XRF analysis. 
     Synthesis Example 2 
     In a manner analogous to Synthesis Example 1, zeolite Y was soaked with a saturated CuCl 2  solution. A CuCl 2  content of 44.3 wt % was determined using XRF analysis. 
     Synthesis Example 3 
     In a manner analogous to Synthesis Example 1, zeolite Y was soaked with a saturated CdCl 2  solution. A CdCl 2  content of 59.5 wt % was determined using XRF analysis. 
     Synthesis Example 4 
     In a manner analogous to Synthesis Example 1, instead of zeolite Y, expanded vermiculite was soaked with a saturated CuSO 4  solution. A CuSO 4  content of 58.4 wt % was determined using XRF analysis. 
     Synthesis Example 5 
     In a manner analogous to Synthesis Example 4, expanded vermiculite was soaked with a saturated CuCl 2  solution. A CuCl 2  content of 57.8 wt % was determined using XRF analysis. 
     Synthesis Example 6 
     In a manner analogous to Synthesis Example 4, expanded vermiculite was soaked with a saturated CdCl 2  solution. A CdCl 2  content of 61.3 wt % was determined using XRF analysis. 
     Examples 1 to 6 and Comparative Examples 1 to 9 
     Both pure powders of the transition metal salts and the carriers prepared in the synthesis examples that had been loaded with the respective transition metal salt were used as thermal storage media in an inventive method for the storage of thermal energy. 
     To this end, initially, the respective metal salt powder (Comparative Examples 1 to 4) and the loaded carrier material, respectively, of Synthesis Examples 1 to 6 (Examples 1 to 6) were provided at room temperature in a fluidized-bed reactor, and the solid material was then fluidized with a stream of 10 I/min of NH 3  as the reactive carrier gas for 15 min, and was simultaneously converted to the respective ammine complex, with the temperature developing inside the reactor due to the exothermic reaction being measured continuously by means of temperature sensors at different heights of the reactor.  FIG. 1  shows a photograph of the fluidized-bed reactor used. Before fluidizing, the bulk particle bed in all cases reached a height just above the lowermost thermal sensor. 
     In  FIGS. 2 to 6 , the temperature profiles of the ammine complex formation reactions measured for Examples 1 to 3 as well as Comparative Examples 1 and 2 are shown, wherein T1 shows the measured curve of the lowermost temperature sensor and T4 shows that of the uppermost temperature sensor, respectively. As can be seen, in each case, the highest temperature was measured at the lowermost thermal sensor, because the fluidized particle bed came into contact with the reactive gas NH 3  before reaching a higher height, and the bed material was mixed immediately due to fluidi-zation, which is why such temperatures could not be reached at higher heights. Thus for purposes of comparison, the temperature corresponding to the uppermost of the four temperature curves of the lowermost thermal sensor T1 is to be used throughout. 
     A comparison of  FIG. 2  and  FIG. 3 , i. e. between pure CuSO 4  powder (without a carrier) of Comparative Example 1 and CuSO 4  on zeolite Y of Example 1, shows that the maximum temperature reached using the salt carried on zeolite, i.e. 215° C., was considerably lower than that reached using the pure CuSO 4  powder (about 330° C.). The same follows from a comparison of  FIGS. 4 and 5 , in which the curves of pure CuCl 2  powder (without a carrier) of Comparative Example 2 and CuCl 2  on zeolite Y of Example 2 are shown. When reacted with NH 3 , the pure powder causes a temperature increase inside the reactor to about 365° C., whereas the “dilution” on the zeolite carrier reached a temperature of “only” about 203° C. Nevertheless, the amount of heat thus released is, of course, still usable for various applications, i.e. by separately storing defined amounts of transition metal salt and ammonia and mixing them, if required, in order to release the heat. 
     In  FIG. 6 , the temperature profile for CdCl 2  carried on zeolite when reacted with ammonia is shown, where a temperature of about 74° C. was reached, while the temperature increased to about 174° C. with pure CdCl 2  powder (data not shown). 
     In Table 1 below, the temperatures reached in the examples and comparative examples are listed. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Maximum 
               
               
                   
                   
                 temperature 
               
               
                 Example 
                 Heat storage medium 
                 [° C.] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Comparative Example 1 
                 CuSO 4   
                 330 
               
               
                 Example 1 
                 CuSO 4  on zeolite Y 
                 215 
               
               
                 Comparative Example 2 
                 CuCl 2   
                 365 
               
               
                 Example 2 
                 CuCl 2  on zeolite Y 
                 203 
               
               
                 Comparative Example 3 
                 CdCl 2   
                 147 
               
               
                 Example 3 
                 CdCl 2  on zeolite Y 
                 74 
               
               
                 Example 4 
                 CuSO 4  on vermiculite 
                 245 
               
               
                 Example 5 
                 CuCl 2  on vermiculite 
                 262 
               
               
                 Example 6 
                 CdCl 2  on vermiculite 
                 65 
               
               
                 Comparative Example 4 
                 CdSO 4   
                 68 
               
               
                   
               
            
           
         
       
     
     Furthermore, the energy and mass increases of the materials of Examples 1 to 3 and of Comparative Examples 1 to 3 were determined using simultaneous thermal analysis (STA), wherefrom the amounts of energy released and thus specifically storable were calculated, obtaining the values cited in Table 2 below. For Comparative Examples 5 to 9, values obtained from literature were used. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Storable 
               
               
                   
                   
                 energy 
               
               
                 Example 
                 Reaction/material 
                 [kJ/kg] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Comparative 
                 CuSO 4  + 4 NH 3     CuSO 4 •4NH 3   
                 1622 
               
               
                 Example 1 
               
               
                 Example 1 
                 CuSO 4  on zeolite Y 
                 217 
               
               
                 Comparative 
                 CuCl 2  + 5 NH 3     CuCl 2 •5NH 3   
                 2217 
               
               
                 Example 2 
               
               
                 Example 2 
                 CuCl 2  on zeolite Y 
                 188 
               
               
                 Comparative 
                 CdCl 2  + 5 NH 3     CdCl 2 •5NH 3   
                 1425 
               
               
                 Example 3 
               
               
                 Example 3 
                 CdCl 2  on zeolite Y 
                 178 
               
               
                 Comparative 
                 MnCl 2 •2NH 3  + 4 NH 3     MnCl 2 •6NH 3   
                 296 
               
               
                 Example 5 
               
               
                 Comparative 
                 FeCl 2 •2NH 3  + 4 NH 3     FeCl 2 •6NH 3   
                 319 
               
               
                 Example 6 
               
               
                 Comparative 
                 CoCl 2 •2NH 3  + 4 NH 3     CoCl 2 •6NH 3   
                 329 
               
               
                 Example 7 
               
               
                 Comparative 
                 NiCl 2 •2NH 3  + 4 NH 3     NiCl 2 •6NH 3   
                 362 
               
               
                 Example 8 
               
               
                 Comparative 
                 CaCl 2 •4NH 3  + 4 NH 3     CaCl 2 •8NH 3   
                 229 
               
               
                 Example 9 
               
               
                   
               
            
           
         
       
     
     As can be seen, the transition metal salts applied onto carriers release comparable amounts of heat that are only slightly below those becoming available by cycling ammine complexes of transition metal salts between different coordination numbers according to the prior art. At the same time, the salts applied onto carriers according to the present invention are resistant to agglomeration due to melting processes. 
     The ammine complexes obtained in the above reactions with ammonia, either as a powder or on a carrier, were subsequently thermally decomposed by heating using an air current having 400° C. in the fluidized-bed reactor and noting the decomposition temperatures measured at the lowermost thermal sensor, which are listed in Table 3 below. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Decomposition 
               
               
                   
                   
                 temperature 
               
               
                 Example 
                 Reaction/material 
                 [° C.] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Comparative 
                 CuSO 4 •4NH 3     CuSO 4  + 4 NH 3   
                 &lt;400 
               
               
                 Example 1 
               
               
                 Example 1 
                 CuSO 4 •4NH 3  on zeolite Y 
                 &lt;400 
               
               
                 Comparative 
                 CuCl 2 •5NH 3     CuCl 2  + 5 NH 3   
                 &lt;300 
               
               
                 Example 2 
               
               
                 Example 2 
                 CuCl 2 •5NH 3  on zeolite Y 
                 &lt;300 
               
               
                 Comparative 
                 CdCl 2 •5NH 3     CdCl 2  + 5 NH 3   
                 &lt;400 
               
               
                 Example 3 
               
               
                 Example 3 
                 CdCl 2 •5NH 3  on zeolite Y 
                 &lt;400 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 3, application onto a carrier material does not significantly change the decomposition temperatures. 
     Example 7 
     Surprisingly, the inventors have found in the experiments using cadmium chloride that its pentammine complex could be decomposed not only thermally, but also by simple purging with air at room temperature, so that no high amount of energy was required for regenerating the uncomplexed salt. 
     Subsequently, in the fluidized-bed reactor, cycle experiments were performed in which NH 3  and air at room temperature were alternately used for fluidizing the bed material, i.e. CdCl 2  on zeolite Y as a carrier, and simultaneously for forming and irre-versibly decomposing the ammine complex according to the reaction equation below: 
       CdCl 2 +5NH 3   CdCl 2 .5NH 3    
       FIG. 7  shows the temperature profile of these cycles. 
     Cadmium chloride therefore constitutes a particularly promising material for future use as a thermo-chemical heat storage medium. 
     Further examples, including with CdSO 4  and other transition metal sulfates, are currently subject to further experiments by the inventors. 
     The invention thus provides a method through which relatively large amounts of energy in the form of chemical energy may be stored in transition metal salts applied onto a solid carrier, and may be spontaneously re-released by being reacted with ammonia without the risk of agglomeration due to partial melting or of irreversible decomposition of the salts.