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Annual Review of Heat Transfer, Vol. 15, p.131-177 https://doi.org/10.1615/AnnualRevHeatTransfer.2012004651 4. Storage of chemical heat Chemical energy storage systems utilize the enthalpy change of a reversible chemical reaction. The interest in these systems is mainly motivated by the option to store energy at higher densities than other TES types. It is also feasible to store reactants and the product at ambient temperature to avoid thermal losses. Compared to sensible and latent heat systems, thermochemical storage can have different temperature levels for the charge and discharge process. Hence, it is feasible to upgrade and downgrade heat and supply heat at a suitable temperature level. Systems without storage capacities are known as heat transformers and chemical heat pumps. Heat transformers absorb heat at a low temperature level and supply heat at a high temperature level. Chemical heat pumps absorb heat at a higher temperature level and supply heat at a lower temperature level (Garg 1985). Although these systems do not aim for heat storage, there are links between developments in the areas of heat transformation/chemical heat pumps and thermochemical storage. The energy is stored in the form of chemical compounds A and B created by an endothermic reaction and it is recovered again by recombining the compounds in an exothermic reaction (Equation 15). At high enough temperatures, the products A and B are spatially separated. Hr AB A B (15) The heat stored and released is equivalent to the heat (enthalpy) of reaction. The enthalpy of reaction ∆Hr, generally, is much larger than the enthalpy of transition in latent heat storage or the sensible heat stored over a reasonable temperature span. Hence, the storage density, based on solid mass or volume, is much larger for thermochemical storage materials than for latent or sensible heat storage materials (Wenthworth 1976, Mar 1980, Sizmann 1980). Many thermochemical energy storage concepts are in an earlier state of development compared to sensible and latent heat systems. Low temperature sorption systems are one exception. The potential of thermochemical storage was identified early in the evolution of CSP- technology (Ervin 1977, Williams 1978, Brown 1992), but these systems are still in an earlier state and require further research and development. A large number of groups actively investigate solar driven chemical processes. These processes aim for fuel production and useful chemical products. In a general sense, these approaches also form an energy storage system. In the scope of this chapter, solar driven chemical processes are not further considered. Thermochemical storage system can be classified into two major categories. Open type systems exchange gases with the environment. During the charging process gases are released to the environment. During the discharging process a gas from the environment is utilized. Hence, these systems can operate without gas compression and storage and this simplifies the system design. Gases include oxygen, nitrogen, water vapor and potentially carbon dioxide. Open systems may introduce unwanted substances from the environment. Impurities may be dust, sulfur dioxide, carbon dioxide and organic compounds. Such substances may deteriorate the system performance. System designs with filters may avoid such difficulties. Storage of the unpressurized gas phase in a closed type system is usually not feasible because of the unacceptable large gas volume. Commonly, closed type systems compress or condense the gas. Subsequently, the pressurized gas or liquid can be conveniently stored. Alternatively, the gas may be reabsorbed by a second chemical reaction (Sizmann 1980). This discussionPDF Image | Annual Review of Heat Transfer
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