Patent Application
20240084184
The present invention relates to a process for the reversible thermochemical storage of energy and release of energy. Further, the invention relates to a system comprising a thermochemical energy storage system.
In thermochemical energy storage systems, heat is stored by endothermic reactions and released by exothermic reactions. Such thermochemical energy stores use reversible reactions according to the scheme A+B→C (+D) for energy storage and the reverse reaction C+(D)→A+B for energy release.
Compared with heat storage systems using water as the energy carrier, thermochemical energy storage systems have the advantages of higher energy storage density and the possibility of long-term storage; compared with latent heat storage systems, the higher energy storage density is a significant advantage. Major criteria that currently limit the practical use of thermochemical energy storage systems are, on the one hand, the high cost of chemical substances that enable meaningful energy storage and/or, on the other hand, the maximum number of storage cycles (i.e., the number of reversible conversions of the chemical substances in the reaction system). As the number of storage cycles increases, a reduction in the quality of the substances is observed, caused, for example, by abrasion, corrosion, sintering or aging.
The low number of storage cycles is the decisive limiting factor here. Therefore, thermochemical energy storage is currently still limited to laboratory scale and prototypes.
AT 518 448 B1 describes a thermochemical energy accumulator which uses the reversible reaction equilibrium between boric acid and anhydride of boric acid (borontrioxide) according to the reaction scheme
2H3BO3⇄B2O3+3H2O
as a system for storing energy (boric acid/boron(III) oxide system).
While the cost of the substances required in the boric acid/borontrioxide reaction system is relatively low, the maximum number of storage cycles is low because agglomeration of the substances can be observed. Agglomeration negatively affects the reaction kinetics, so that the maximum number of storage cycles and the storage capacity are reduced. AT 518 448 B1 attempts to remedy this problem by using a fluidized bed reactor in the energy storage system. In the fluidized bed reactor, the substances are in a floating bed, which on the one hand reduces agglomeration and on the other hand accelerates the kinetics of the reaction. Agglomeration is not completely prevented in AT 518 448 B1. Although the number of cycles in the boric acid/boron(III) oxide reaction system can be increased with the fluidized bed reactor, agglomerates can also be observed after a few cycles in the fluidized bed reactor. The number of cycles gained does not outweigh the higher costs for the fluidized bed reactor itself and the complex control of the reaction process in the fluidized bed reactor.
The object of the present invention is to provide a thermochemical energy storage device and a process, based on the reversible reaction system boric acid/boron(III) oxide, which allows a higher number of storage cycles at low operating costs and easy handling.
This task is solved by a
Orthoboric acid is present as a powder, which is suspended in a suspension medium. A suspension medium is a liquid in which boric acid is not soluble and in which boric acid can be suspended.
The inventors have found that the agglomeration tendency of the reversible reactions of orthoboric acid to metaboric acid or boric oxide (boric acid/boric oxide system), according to the reactions
2H3BO3⇄B2O3+3H2O
or
2H3BO3⇄2HBO2+2H2O and then
2HBO2⇄2B2O3+H2O
or
2H3BO3⇄2HBO2+2H2O and
2HBO2⇄½H2B4O7+½H2O and
½H2B4O7⇄B2O3+½H2O
or the reverse reactions is greatly reduced if the reactions are carried out in a suspension medium, resulting in a higher maximum number of storage cycles. Thus, the storage capacity also remains substantially constant throughout the number of storage cycles. In this process and in the processes described below, the dehydration of an acid (H3BO3) to the corresponding anhydride (HBO2, H2B4O7 or B2O3) or the hydration of an anhydride (HBO2, H2B4O7 or B2O3) to the corresponding acid (H3BO3) is used to store or release energy. In handling, it has been shown that acid/anhydride systems are much more prone to agglomeration than, for example, systems based on the intercalation of water of crystallization, e.g. in salts, or the sorption of water on the surface. All the more surprising were the findings in the boric acid/boron oxide system in suspension, according to which agglomeration is considerably reduced by a suspension medium, so that the maximum number of storage cycles can be increased by a factor of 10 to 100.
The details described below, such as material quantities, temperatures, pressure, process conditions or details of the equipment, can also be applied to reversible thermochemical energy release, which is described below.
Suitable suspension media include refined rapeseed oil, mineral oil-based thermal oil, silicone-based thermal oil, bio-oil and the like.
Preferably, the mass ratio of orthoboric acid to suspension medium used for thermochemical energy storage at the start of the reaction is 1 (H3BO3) to 0.6 to 1.2 (suspension medium), preferably 1 (H3BO3) to 0.9 to 1.0 (suspension medium), preferably about 1:0.83. If the amount of suspension medium in relation to H3BO3 becomes too large, the storage capacity decreases; if, on the other hand, the amount of suspension medium in relation to H3BO3 becomes too small, isolated agglomerates may occur. In the context of the patent application, the mass ratio is understood as the ratio of mass to mass.
The energy density, corresponding to the storage capacity of the energy storage medium, is 2.2 GJ/m3 for boric acid in relation to boric oxide. In suspension, the energy density decreases to 1.32 GJ/m3 in a thermal oil as suspension medium at a 1:0.83 mass ratio.
Preferably, the suspension medium with the orthoboric acid suspended therein is agitated, especially preferably stirred, during the reaction. This results in better heat distribution in the suspension and improves the reaction kinetics.
In one embodiment, the water formed is removed from the suspension during the course of the reaction. This shifts the reaction equilibrium to the side of the metaboric acid and the boron oxide and the back reaction to orthoboric acid is prevented.
The temperature range for loading (energy storage) is preferably 110° C. to 200° C., particularly preferably between 135-165° C., and loading can be carried out at, for example, about 1.0135 bar. In order to shift the reaction equilibrium towards the metaboric acid side or boron oxide side, it has been found advantageous if the pressure is below 1.0135 bar. Preferably, the pressure is below 200 mbar, preferably below 100 mbar, for example at least 1 mbar.
Typical charging but also discharging times are about 0.5-1 hours. This results in theoretical power densities of 0.367 MW/m 3 (for a discharge time of 60 minutes) and 0.733 MW/m 3 (for a discharge time of 30 minutes) for a suspension in a suspension medium at a 1:0.83 mass ratio.
Accordingly, the invention also relates to a
The invention also relates to a for the reversible thermochemical storage of energy and release of energy,
The processes according to the invention are based on the reversible reactions of orthoboric acid to metaboric acid to boric oxide and vice versa. For the storage of energy, starting from orthoboric acid either metaboric acid (which is a multistep process according to Huber et al. “The multistep decomposition of boric acid,” Energy Sci Eng. 2020; 00:1-17) and/or boric oxide is produced by loss of water. Release of energy release utilizes the reverse reaction of boric oxide and/or metaboric acid with water to form orthoboric acid:
2H3BO3⇄B2O3+3H2O
2H3BO3⇄2HBO2+2H2O
2HBO2⇄2B2O3+H2O
2H3BO3⇄2HBO2+2H2O
2HBO2⇄½H2B4O7+½H2O
½H2B4O7⇄B2O3+½H2O
2H3BO3⇄B2O3+3H2O
2H3BO3⇄2HBO2+2H2O
2HBO2⇄2B2O3+H2O
2H3BO3⇄2HBO2+2H2O
2HBO2⇄½H2B4O7+½H2O
½H2B4O7⇄B2O3+½H2O
The conversion of H3BO3 into B2O3 and vice versa proceeds in several steps via metaboric acid. Metaboric acid is characterized here via the molecular formulae HBO2 and H2B4O7, but may also have other molecular formulae between H3BO3 and B2O3. Thus, there are other structures of metaboric acids that are also covered in the sense of the invention. The starting point for the processes is either orthoboric acid or boron oxide. For energy storage, however, there may then also be the formation of metaboric acid, which is no longer or no longer completely converted into boric oxide. It has also been found in the context of the invention that the reaction equilibrium orthoboric acid−water⇄metaboric acid+water, without boric oxide being generated, is even less prone to agglomeration.
Preferably, the mass ratio of boron oxide to suspension medium used for thermochemical energy release at the start of the reaction is 1 (B2O3) to 0.34 to 0.68 (suspension medium), preferably 1 (B2O3) to 0.51 to 0.56 (suspension medium), preferably about 1:0.47, with the proviso that between 0.70 to 0.85, preferably 0.75 to 0.80 g of water is added per 1 g of B2O3.
The amount of water added should be approximately stoichiometric (0.776 g H2O per 1 g B2O3) to allow virtually complete conversion of B2O3 to H3BO3. Significantly larger amounts of water would lead to a solution, which is undesirable; too small amounts of water will lead to an incomplete reaction and thus to an incomplete utilization of the storage capacity.
The discharge time or energy release time can be about 0.5-1 hours to allow good energy dissipation. The water supply should also be regulated accordingly.
Discharge (energy release) can take place at approx. 1.0135 bar, for example. In order to achieve higher temperatures, it has proven advantageous if the pressure is above 1.0135 bar. Preferably, the pressure is at least 5 bar, particularly preferably at least 8 bar, for example up to 10 bar.
Additional additives, emulsifiers and/or foam inhibitors can be added to the suspension medium. Examples of such additives include quartz sand, non-ionic surfactants (e.g. alcoholic ethoxylates, alcoholic polyglycol ethers, fatty acid alcohols).
In the process according to the invention, the use of a suspension medium reduces the energy storage density. However, this disadvantage is more than offset due to the agglomeration prevented by the suspension medium and the maximum number of storage cycles gained as a result. In addition, process control is simpler and the storage and release of energy can easily take place at separate locations. For example, at the charging unit, suspension with B2O3 is removed after the charging process and exchanged for suspension with H3BO3. At the unloading unit, suspension containing B2O3 is used and, after unloading, the suspension now containing lower-energy H3BO3 is removed. Another advantage of the suspension medium is the improved heat transfer to the orthoboric acid.
Furthermore, the task is solved by a system for the thermochemical storage of energy and release of energy, in particular for carrying out one of the processes as mentioned above, with at least one suspension reactor, wherein an energy source is associated with the suspension reactor, wherein an agitation device is provided in the suspension reactor, wherein an outlet for water vapor from the suspension reactor and a water reservoir connected to the outlet is present, wherein a feed line is provided into the suspension reactor which is connected to the water reservoir, wherein a heat exchanger is provided at the suspension reactor, wherein the heat exchanger is connectable to a consumer
The agitation device can be, for example, a stirrer.
The system can further comprise a control device which is designed in such a way that the amount of energy delivered to the heat exchanger at the suspension reactor can be controlled.
In addition, a gas supply line may be provided for the suspension reactor to discharge water generated in the reactor.
Since the water produced is gaseous, a heat exchanger can be assigned to the gas supply line in order to heat supplied gas—preferably nitrogen—or to reduce the relative humidity.
A heat exchanger may be associated with the outlet, with which the gaseous water is condensed.
Furthermore, a device for controlling the flow rate can be assigned to the supply line. This makes it possible to control the amount of heat delivered.
With a control device designed in such a way that the amount of energy delivered to the heat exchanger at the suspension reactor can be controlled via the flow rate control device, the amount of heat delivered can also be controlled.
In one embodiment, two suspension reactors are provided, which are connected via at least one bypass line, whereby means are provided for transporting the contents of one suspension reactor into the other suspension reactor. This would allow energy to be stored and consumed in parallel if both processes are desired in parallel at certain times.
Furthermore, an energy source and a consumer can be provided.
The invention is explained in more detail with the aid of the figures and figure descriptions.
This can be done, for example, by pumping, by applying a vacuum, via nitrogen gassing or the like. In order for these schematically illustrated reactions to take place in the processes according to the invention, they can take place in thermochemical reactors, as shown in the following figures.
The reaction in reactor 1′ is preferably carried out in such a way that a stoichiometric amount of water is added, i.e., 3 moles of H2O are added to 1 mole of B2O3. In addition, a metering device may be provided to meter the water flow and thus time the water supply so that the heat release is continuous. The water supplied to reactor 1′ may be the stored water released in reactor 1. If the stored water is used, a closed system can be used in which no feed is required and a stoichiometrically correct ratio between B2O3 and H2O is automatically present. During the reaction in reactor 1′, it is also envisaged that the suspension is agitated, for example by stirring.
Reactor 1 can be operated with suspended orthoboric acid where large amounts of heat are generated, e.g. in industrial plants or at solar collectors. After reactor 1 is loaded with B2O3, the mixture of suspension medium and boron oxide can be removed and transported to a location where the energy in reactor 1′ is to be released again (e.g. in a single building heating system or a district heating system).
The system according to
| Number | Date | Country | Kind |
|---|---|---|---|
| 50039/2021 | Jan 2021 | AT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AT2022/060024 | 1/26/2022 | WO |
