Patent Application
20200224072
The present invention relates to a method of thermo-chemical energy storage by executing reversible chemical reactions in order to store heat energy in the form of chemical energy.
Thermochemical energy storage, i.e. the storage of thermal energy in the form of chemical energy, is a method of energy storage comprising cyclization of at least one chemical compound between the statuses of at least one reversible equilibrium reaction, which has been known for decades, but has only been intensively researched for a few years. U.S. Pat. No. 4,365,475, for example, reveals the combination of two equilibrium reactions for the purpose of thermochemical energy storage, namely the alternating reversible endothermic formation of the ammine complexes CaCl2.8NH3 and ZnCl2.NH3.
Among other complexes of alkaline earth and transition metals, combinations are known as systems specifically using ammine complexes, in which the ammine complexes alternate between two different coordination states. Examples are a combination of the two chlorides CaCl2 and FeCl2, with which the following reactions are carried out according to U.S. Pat. No. 4,319,627:
CaCl2.8NH3CaCl3+4NH3
FeCl2.2NH3+4NH3FeCl2,6NH'
or another combination of calcium chloride, namely CaCl2 and MnCl2, which according to Li et al., AlChE J. 59(4), 1334-1347 (Apr. 2013), undergo the following reactions in a similar way to the above combination with iron chloride:
CaCl2.8NH3CaCl2.4NH3+4NH3
MnCl2.2NH3+4NH3MnCl2.6NH3
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 following equation:
CoCl2.2NH3+4NH3CoCl2.6NH3
However, the energy storage density of the above-mentioned systems is rather low in most known cases, and the corrosiveness of some of the salts used often poses a device-related problem. Additionally, also the transport and storage of the metal salts used will cause issues, since the temperatures reached during the exothermic reaction are often close to or even higher than the melting points of these salts or complexes, respectively, so that at least some of the salts will melt, thus leading to agglomeration.
The present inventors proposed a solution to this problem in their pending Austrian Patent Application AT A 327/2016, which discloses a process in which ammine complexes of transition metal salts are formed and decomposed according to the following reversible sum reactions:
[Me(NH3)n]X+ΔHRMeX+nNH3
or in alternative notation:
MeX.nNH3+ΔHRMeX+nNH3,
wherein Me represents at least one transition metal ion and X represents one or more counterions in an amount sufficient to neutralize the complex, according to their valences and that of the transition metal ion, wherein one or more transition metal salts are used supported on a carrier material which is inert to the reaction.
In contrast to the systems cited at the beginning, this only involves the ammine complex formation reaction of the at least one transition metal salt instead of a combination of two parallel reactions complementing each other chemically or thermodynamically. This means that there is no switching back and forth between the 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 discovered that transition metal ammine complexes have very high enthalpies of formation. This could lead to the problems mentioned above in connection with the at least partial melting of salts or complexes. However, the invention solves this problem by depositing the metal salts onto a carrier, thus achieving a “dilution” of the salts at the same time, so that melting processes and the associated agglomeration of the salts are precluded. The transition metal salts supported on the carrier remain easy to handle even at higher temperatures. If a particulate carrier is used, the material remains free-flowing and can therefore be easily transported and stored.
The disadvantage of this procedure is that, as mentioned above, especially without the use of a carrier material, but sometimes also despite its use, excessive heat development can occur, which causes at least partial melting of the transition metal salt, which can lead to the formation of aggregates or lumps or to blockage of the pores of a large heat storage medium during cooling.
The objective of the invention was therefore the development of an alternative process to solve these problems.
The present invention achieves this objective by providing a method of thermochemical energy storage by carrying out reversible chemical reactions for storing heat energy in the form of chemical energy in one or more ammine complexes of transition metal salts of the formula [Me(NH3)n]X, wherein Me is at least one transition metal ion and X is one or more counterions in an amount sufficient for charge equalization of the complex, using the following chemical equilibrium:
[Me(NH3)n]X+ΔHRMeX+nNH3,
which, in comparison with the above procedure, is characterized by the fact that said heat storage is performed by endothermic cleavage of the NH3 ligands from the ammine complex and/or that the heat release is performed by exothermic loading of the transition metal salt with the NH3 ligands in at least two steps at different temperatures.
In the course of further research, the inventors have found that, in the ammine complexes of some transition metal salts, it is possible to cleave off the NH3 ligands from the central atom, or to link them to it, individually or in pairs at different temperature levels and thus to achieve a ligand-free transition metal salt in at least two steps “cascading” from the fully coordinated ammine complex to the ligand-free transition metal salt or vice versa.
This allows for a more finely tuned adaptation of the process to the respective conditions, i.e. to a current excess or requirement of heat, by targeted loading of the heat storage medium with discrete amounts of heat, while simultaneously separating a respective part of the NH3 ligands of the ammine complex, or the release of only a part of the heat stored in the heat storage medium by linking only a part of the maximum coordinateable ligands to the central atom at different respective temperatures.
For example, the tetrammine complex of CuSO4 releases its four NH3 ligands at temperatures starting at about 80° C. (1 ligand), about 170° C. (2 ligands) and about 310° C. (1 ligand), respectively, as will be shown in the following examples. In the event that there is not enough storable thermal energy (such as waste heat) available to heat the heat storage medium to over 300° C., it only needs to be heated to a temperature of >80° C. or >170° C. in a first step, whereby only one ligand or three ligands is/are cleaved off, respectively, and the amount of heat required to achieve this is stored in the form of chemical energy in the heat storage medium. The remaining ligand(s) can then be cleaved off at a later point in time when sufficient storable heat is available. In doing so, the coordination sites of the heat storage medium can also be replenished with NH3 ligands in the meantime by releasing heat before the ligands are cleaved off again.
The reverse option is at least as important. For example, the heat stored in the ligand-free CuSO4 is released with simultaneous stepwise regeneration of the tetrammine complex by contact with an ammonia-containing gas mixture at the following temperature levels: about 250° C. (1 ligand), about 140° C. (2 ligands) and just under 70° C. (1 ligand). If only a part of the stored heat is required, only a part of the coordination sites of the heat storage medium needs to be filled with ligands by directed cooling the hot CuSO4 down to one of the two higher temperatures, and the rest of the stored heat can be released at a later point in time by completely filling all coordination sites with NH3.
In the process of the present invention, a transition metal salt is preferably used, with which both the loading with heat and its re-release can be carried out stepwise, which applies particularly to the CuSO4 mentioned.
In general, however, a salt of Cu, Ni, Co or Zn is preferably used as the transition metal salt, since the suitability of these metals for the process according to the invention has already been confirmed, which of course does not rule out the possibility that the inventors' current ongoing research will yield further suitable metals. A salt of Cu or Ni, especially Cu, is used even more preferentially, since their temperature levels can be controlled more precisely than those of the other preferred metals.
Furthermore, a sulfate or chloride, or even more preferably a sulfate, of the transition metal is preferably used in the process according to the invention, since the suitability of these anions has already been confirmed, whereas no ammine complex formation at all could be observed for the anions carbonate, silicate and phosphate in the inventor's first experiments. Due to the volatility of the chloride during the thermal decomposition of the complexes, sulfate is particularly preferred, especially CuSO4, as was already mentioned above.
In further preferred embodiments of the invention, the transition metal salt is used supported on a storage material which is inert to the reaction, thus further reducing the risk of agglomeration of the heat storage medium and improving its handling. In addition, however, the temperature levels at which the NH3 ligands are cleaved off from the respective ammine complex of the transition metal salt sometimes shift considerably. Without wanting to be bound to any specific theory, the inventors assume that the degree of this shift will depend on the specific heat capacity of the carrier material.
The inventors have found that not every carrier material is suitable for the cascading implementation of the process according to the invention, which is why the carrier material is preferably selected from silica, sepiolite, Celite, vermiculite and activated carbons, of which silica, sepiolite or Celite, in particular sepiolite, are used even more preferably, since by using these preferred carrier materials the temperature levels of the heat storage or heat release, respectively, can be controlled better compared to other cases, and particularly with sepiolite, it was found that it is not only particularly suitable for the process of the invention, but that it was also decisively more cost-effective when obtained commercially as compared to silica or Celite, for example.
When using a carrier material, the transition metal salt is preferably used thereon at a weight ratio between salt and carrier material of at least 1:1, or even more preferably at least 3:1, in order to enable a high amount of heat to be storable per weight unit of the combination. Even more preferably, the transition metal salt on the carrier material is used in a weight ratio between salt and carrier material of between 4:1 and 6:1, in particular in a weight ratio of about 5:1. If the amount of transition metal salt is to be kept as low as possible while at the same time retaining a high amount of storable heat, a ratio of only 1:1 can be selected.
The invention will be described in more detail below on the basis of concrete exemplary embodiments with reference to the enclosed drawings, the latter of which show the following.
All reagents used in the following examples, i.e. transition metal salts and carrier materials, are commercially available and have been used without further purification.
Preparation of the Ammine Complexes
MeX+nNH3[Me(NH3)n]X+ΔHR
Me=Cu2+, Ni2+, Co2+, Zn2+
X=SOx2− or 2Cl−
n=4 (for SO42−) or 5 (for Cl−), respectively
The respective anhydrous transition metal salt from commercial sources was converted to the ammine complexes in a laboratory fluidized bed reactor using an excess of NH3.
In this way the following ammine complexes were produced:
Synthesis Example 1: [Cu(NH3)4]SO4
Synthesis Example 2: [Ni(NH3)4]SO4
Synthesis Example 3: [Co(NH3)4]SO4
Synthesis Example 4: [Zn(NH3)4]SO4
Synthesis Example 5: [Cu(NH3)5]Cl2
The ammine complexes prepared above were then heated in an Al2 O3 jar using a STA 449 C Jupiter® DSC TGA instrument from Netzsch under an atmosphere of N2 at a heating rate of 10 K/min and cooled in N2 atmosphere and thermo-gravimetrically analyzed. The introduction of ammonia was started immediately after reaching the maximum temperature. The TGA curves (uncorrected) plotted in this case are shown in
It can be seen that in the case of [Cu(NH3)4]SO4, the four NH3 ligands are cleaved off in three steps during heating, where first one ligand is cleaved off, then two ligands are cleaved off together, and finally one ligand is cleaved off again. During the subsequent cooling by introducing ammonia-containing carrier gas, the original tetraammine complex is also restored in steps, the re-attachment of the four NH3 ligands ocurring according to the same (reverse) scheme 1-2-1.
These results clearly show that both the heat storage and the subsequent heat release using this ammine complex as heat carrier can be carried out in two or three steps, wherein the cleavage and the re-uptake of the NH3 ligands can be carried out according to any of the schemes 1-2-1, 1-3, 3-1, 1-2 or 2-1, depending on the temperatures to which the ammine complex or the ligand-free transition metal salt is exposed or how quickly the heating or cooling takes place. The temperature differences between the individual stages in both processes, i.e. cleavage and uptake of NH3 ligands, amount to several dozen Kelvin each, so that a temperature in between can be selected easily.
It can be seen that in the cycles of heat storage and release, combinations of two different schemes for the two partial steps are also possible, which increases the flexibility in the choice of temperature enormously compared to the state of the art.
Here it can be seen that the cascading process is not stable across cycles, i.e. in the first cycle, a cleaving pattern of 2-1-1 can be seen, while in the second cycle, only a ligand cleavage of 2-2 will occur. While a cascading uptake of the NH3 ligands will occur during the first cooling in NH3, this process will only occur continuously during the second cooling.
It can be seen that the four NH3 ligands from the cobalt complex are cleaved off in two steps from two ligands each, the temperature difference amounting to around 40° C., such that a maximal temperature in between may be easily selected so that only two out of four ligands will be cleaved off, if desired, if there is not enough storable heat available. The ligand re-uptake however will only take place in one single step, starting from approximately 79° C., such that by using this transition metal salt-ammine complex only the stepped variant of heat storage used in this experiment will be available.
It can be seen that in the heat storage process, the four ligands from [Zn(NH3)4]SO4 are cleavable in four steps, especially since the temperature differences between these steps each amount to around 40° C. and 60° C., respectively. The ligand re-uptake occurs in four steps, the first NH3 uptake starting at approximately 250° C.
Here it can be seen that the cleavage of the 5 NH3 ligands occurs in two steps according to the scheme 3-2, while the NH3 uptake occurs continuously and thus in a non-cascading manner.
In order to investigate whether heat storage or release can also be carried out in a cascading manner using ammine complexes bound to carrier materials, the ammine complex that performed best in the above examples, i.e. tetraammine copper sulfate [Cu(NH3)4]SO4, was tested supported on different carrier materials.
For this purpose, 50 g of the respective carrier material were immersed in a saturated CuSO4 solution for varying periods of time and thus impregnated. The respective product was then separated, rinsed with 200 ml deionized water and dried at 60° C. for 12 h and then at 350° C. for 6 h. The respective CuSO4 contents were determined by X-ray fluorescence analysis.
Subsequently, the carriers impregnated with CuSO4 were gassed with NH3 in the laboratory fluidized bed reactor to produce the tetraammine copper sulfate complexes, which were subsequently subjected to similar heating/cooling cycles and treatments as described above with simultaneous TGA analysis. In addition, the amount of thermal energy storable in the materials was determined by DSC analysis.
In analogy to their earlier experiments described in the patent application cited at the beginning, the inventors first tested zeolite as a carrier material. For this purpose, two different loads of zeolite with copper sulfate were prepared, for which the carrier was subjected to one impregnation cycle or three impregnation cycles in CuSO4 solution, respectively, and the impregnated carriers were then directly reacted with NH3 in the DSC TGA instrument to form the tetraammine complex in order to measure the heat flows. The heat quantities that could be stored in the impregnated substrates were calculated to be 143.2 and 215.3 kJ/kg, respectively. For comparison: In pure [Cu(NH3)4]SO4, 1772 kJ/kg are storable, and in pure zeolite 64.43 kJ/kg are storable, as shown graphically in
In an analogous manner to the above experiment using zeolite, activated charcoal (grain size 1-5 mm, untreated) was impregnated with CuSO4 solution in one or three cycles, respectively, and reacted in the DSC TGA instrument with NH3 to give the tetraammine complex. This resulted in storable heat quantities of 225.3 or 505.7 kJ/kg, respectively, the value for activated charcoal itself being 71.53 kJ/kg, as shown graphically in
Due to the sufficiently large differences between the four stages of ligand cleavage, this material can be used in practice for cascaded heat storage. However, due to the lacking possibility of also carrying out the heat release in a cascaded way, this material is not preferred according to the present invention.
In an analogous manner to the above tests, vermiculite (grain size 1-3 mm) was loaded with copper sulfate in one or three impregnation cycles, respectively, which was reacted with NH3 to form the tetraammine complex. This resulted in quite acceptable values for the storable heat quantities of 362.3 and 570.7 kJ/kg, respectively, the value for vermiculite itself only being 5.9 kJ/kg, as shown graphically in
Since the temperature difference between the two stages was around 100° C., but the heat release could not be carried out in a cascaded manner here either, this material can also be used in practice, but is not preferred according to the present invention.
Celite was loaded with copper sulfate at a weight ratio between CuSO4 and carrier of 10:1, which was again reacted in NH3 to form the tetrammine complex, resulting in an excellent calculated heat storage capacity of 1136 kJ/kg.
Due to the relatively high temperature differences between the stages and the relatively high heat storage capacity, this material is quite well suited for practical application and therefore preferred according to the present invention.
In an analogous manner to the above experiment with Celite, silica (particle size 60 μm) was also loaded with copper sulfate at a weight ratio of 10:1, which was reacted in NH3 to form the tetrammine complex, resulting in an even better calculated heat storage capacity of 1263 kJ/kg.
In an analogous manner to example 9, sepiolite was loaded with copper sulfate at a weight ratio of 10:1, which was reacted in NH3 to form the tetrammine complex, resulting in an equally excellent heat storage capacity of 1227 kJ/kg.
The TGA curve in
Due to the lower costs compared to Celite and silica, however, sepiolite is currently the carrier material of choice for practicing the method according to the invention.
Consequently, further experiments were conducted with sepiolite as a carrier, in which the ratios 5:1, 2:1, 1:1 and 1:2 were investigated in addition to the already tested ratio of 10:1 between ammine complex and carrier material.
Thus, it may be deducted that, with regard to heat storage capacity, the preferred ratio between copper sulfate and sepiolite should be at least 3:1, even more preferably between 4:1 and 6:1, and especially about 5:1. With regard to the costs of the heat storage medium, however, a ratio of 1:1 can be considered sufficient in practice in many cases.
| Number | Date | Country | Kind |
|---|---|---|---|
| 3452017 | Aug 2017 | AT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/072067 | 8/14/2018 | WO | 00 |
