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4. Thermo-chemicalheatstorage 4.1. Principles One of the most novel approaches to the problem of storing thermal energy from a solar collection field, or from transforming excess renewable power (wind turbines, photovoltaics) into heat [14], is the use of reversible chemical reactions. Unlike sensible or latent thermal energy storage, mostly limited in time due to heat losses, chemical energy storage enables to bridge long duration periods between supply and demand, hence making it particular- ly suitable for a large scale electricity generation. In such a system, as outlined in Fig. 22, excess energy is converted into high grade heat at the absorber containing the reactant during the charging cycle: the high grade heat is converted into chemical energy by pro- moting an endothermic reaction whereby reaction products are either stored for re-use in the reverse reaction, or exhausted. At the time of energy demand by the central power generation plant or other users, the high grade heat is regenerated through the reverse exothermic reaction cycle and is converted into electricity and low grade heat (CHP concept) through e.g. a standard steam-driven turbo- alternator or through a heat carrier powered Stirling engine. A large number of reversible chemical reactions can be used for thermo-chemical energy storage, each within a given range of equi- librium temperatures and heats of reaction [157–163]. Table 16 selects some of the possible reaction pairs, within the target tem- perature range of 300 °C to ±1000 °C. Numerous low temperature reversible reactions are not included. Clearly, the Mn2O3—MnO pair does not match the proposed temperature range, but can similar- ly equivalent to oxides, be used in the reaction of MnO and H2O to produce H2 [164–168]. The selection of the optimum system for high temperature ap- plications 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), and previously discussed in Section 1. At the present stage of development, there is a lack of comparative data for such a complete analysis. It is however important that an early guide as to the viability of each system is obtained. The present section hence examines thermodynamic and engineering criteria to provide a sci- entific comparison among the systems listed before [169,170]. The analysis will moreover provide guidance as to the important as- sessment criteria of new candidate reaction systems. 4.2. Thermodynamicassessment 4.2.1. Gibbs’freeenergyandworkrecovery A reversible reaction system suitable for high temperature energy storage, and subsequent steam turbine or Stirling engine use for work recovery, should absorb heat at temperatures below 1000 °C in order to lessen problems of material limitation and heat losses, and should furthermore be capable of delivering heat to the turbine inlet at typ- ically 500–600 °C [168]. As initially presented by Williams and Carden [171], the thermodynamic criteria for the preliminary screening of candidate reactions are assessed from available thermodynamic equi- librium data. Williams and Carden [171] proposed to use two parameters for the initial screening of reversible reactions. The first is the “equilibrium” temperature, T*, i.e the temperature at which neither forward nor reverse reactiions are thermodynamically fa- voured. T* is derived by applying the definition of Gibbs’ free energy to the condition of T*. ΔG(T*,P)=0 (7) where ΔG is Gibbs’ free energy change for the reaction, i.e. Table 16 Possible reaction pairs [157–163]. 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 5Mn2O3 ↔ 5Mn3O4 + O2 Mn2O3 ↔ 2MnO + 1/2 O2 Teq (P = 1 atm) (°C) 259 303 479 490 573 839 870 90 6 1586 ΔHr at Teq (kJ/kg) 1396 1126 1288 868 137 1703 844 185 1237 with ΔH and ΔS respectively the reaction enthalpy and entropy at pressure P and temperature T*. The second screening parameter is the maximum work recov- ery efficiency, ηwmax, defined as the ratio of the maximum available or ideal work to the actual work, and obtained from standard chem- ical thermodynamics [42]. The “lost work” is only zero, when a process is completely re- versible. For irreversible process, the energy that becomes unavailable for work is positive, and ~TsΔS. The engineering significance is clear: the greater the irreversibility of a process, with a greater the rate of entropy generation corresponding with a greater amount of energy that becomes unavailable for work. The maximum work recovery efficiency is hence: H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 23 Fig. 22. Schematics of the thermal energy storage by reversible chemical reactions. ΔG = ΔH − TΔS, in this case ΔH (T *, P ) T*= ΔS(T*,P) (8) max Ws ΔG (Ts , P ) ηw =W =ΔH(T,P) (9) where Ts represents the ambient or the sink temperature. The at- tainment of the maximum work implies that the endothermic heat exchange operation must be reversible and complete, whilst also the exothermic reaction must proceed reversibly. The reaction must ideal sPDF Image | Thermal energy storage: Recent developments
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