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26 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 expression. Values are given in Table 18, with A determined for T>Teq +100K: For the given reversible reactions, the maximum conversion is given by Eq. 13 above and the reaction stoichiometry. To achieve 99% of the maximum conversion in a first order reaction, Eq. 11 de- termines the required reaction time above Teq below 300 s for all of the reactions studied. This is however tentative only, pilot/large scale experimentation is certainly required. Recently, the German Aerospace Center (DLR) had been also testing TCS heat storage materials at laboratory scale. In order to investigate TCS at a larger scale, a test bench as well as a reactor containing around 20 kg of reaction material was built and brought into operation. Due to the good availability at low cost and its favourable temperature range for CSP plants, previous work at DLR focused on the reversible dissociation reaction of calcium hydrox- ide [175–177]: CaO(s) + H2O(g) ↔ Ca(OH)2(s) + ΔHreaction The dehydration was performed at 450 °C and the rehydration at about 550 °C. Reversible conversion of 77% of the material with no degradation effects after 10 cycles was determined. Within their results they evidenced losses of chemically stored thermal energy, but they concluded that these could be estimated by implementing a loss term into their simulation and accounted for around 2 kW during the first minutes of the reaction. Addition- ally, the good agreement between experimental and simulated results showed that the reaction kinetics determined in micro-scale mea- surements as well as the known bed properties offered a sufficient representation of the effective conditions within the bed. Anyhow, research is still needed in order to develop TCS at an industrial level. The selection of the optimum reaction system for high temper- ature applications is governed by both the overall thermodynamic performance, and by chemical engineering criteria (kinetics, design factors, etc.) which determine practicability and cost of operation. In a complete analysis, these factors are normally combined in terms of global cost-effectiveness, expressed as the levelized cost of elec- tricity (LCOE), as explained in Section 1. At the present stage of development, there is a lack of comparative data for such a com- plete analysis. It is however important that an early guide as to the viability of each system is obtained, and this is provided by ther- modynamic calculations and kinetic measurements. From the previous and preliminary assessment, TCS certainly offers a potential for medium to high temperature heat storage. The reactions are fast and generally reversible (except Mn2O3 and Co3O4). The reaction rate itself can be considered as moderate (≤300 s), and is thereafter not a limiting factor. The heat penetration inside the chemicals’ packed bed is governed by conduction, where the con- duction approach of PCM storage, as developed in Section 3.3 (with or without inserts), will determine the rate of heat penetration, and hence the overall charging/discharging rate of the TCS system. Additional pilot-scale experiments with Co3O4 and doped Co3O4 were examined by Tescari et al. [178] and the reaction concepts were numerically modelled by commercial CFD software. Adding up to 10 wt% of non-reactive oxides to the Co3O4 is shown to prevent con- solidation and enhance the reversibility of the Co3O4/CoO redox reaction. 5. ContainmentofLHSandTCSmaterials 5.1. Preliminaryselectionconcernsandcriteria 5.1.1. Pressureuponphasechangeorchemicalreaction Phase change materials and thermochemical storage (TCS) systems certainly offer a significant potential towards thermal energy storage. Of course, the materials used need to be properly con- tained in e.g. a spherical capsule, heat exchanger tube or sandwich plates, or in any other container that meets the required need of being closed and able to withstand the high temperatures envis- aged, to resist possible chemical attacks by the contained materials [179–184], and to resist the pressure build up during the phase change (expansion for solid to liquid phase) or during the release of gases and vapours during the thermochemical transformations. This containment is hereafter referred to as “encapsulation”. Table 18 First-order reaction results for different reactions. Fig. 28. The BET isotherm linear plots of Mn2O3 (a) and Mn3O4 (b), respectively. Reaction Mg(OH)2 ↔ MgO + H2O MgCO3 ↔ MgO + CO2 Ca(OH)2 ↔ CaO + H2O CaMg(CO3)2 ↔ MgO + CaO + 2CO2 CaCO3 + H2O ↔ Ca(OH)2 + CO2 CaCO3 ↔ CaO + CO2 2Co3O4 ↔ 6CoO + O2 6Mn2O3 ↔ 5Mn3O4 + O2 EA Teq (P = 1 (kJ/mol) atm) (K) 198 532 52 576 131 752 219 763 185 846 228 1112 189 1115 254 1179 A (s−1) 4.26 × 1016 86.3 2.09 × 106 1.15 × 1012 2.89 × 108 7.03 × 107 1.41 × 106 2.60 × 108PDF Image | Thermal energy storage: Recent developments
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