| The gradual depletion of fossil fuels,the increasingly serious environmental problems and the accelerated rise of electricity demand make people pay more and more attention to the development and utilization of clean renewable energy.However,solar energy,wind energy and other renewable power has problems such as poor stability,periodic fluctuations and so on.Electricity storage is one of the key technologies to solve the unstable fluctuation and intermittency of renewable energy.Therefore,the development of electricity storage technology is of great significance to alleviate energy pressure and promote sustainable development of social economy.Pumped thermal energy storage(PTES)is a new type of large capacity electricity storage mode,which has the advantages of stable operation,high round-trip efficiency,and not restricted by geographical location.Compared with the PTES,thermochemical energy storage has the advantages of high capacity density and high energy storage stability.In this paper,thermochemical storage is integrated into the PTES system,and a new technology for electricity storage-chemical Looping electricity storage(CLES)system is studied.Firstly,the corresponding thermodynamic models considering the temperature difference between charge and discharge were developed.Then,the exergy flow distribution characteristics of the CLES system using Ca(OH)2 and CaO working pair were revealed.The influence of the key thermodynamic parameters on the system’s round-trip efficiency were investigated.Finally,approaches to further improve the round-trip efficiency were briefly discussed.The results show that the existence of the chargedischarge temperature difference will reduce the system’s round trip efficiency.Taking Ca(OH)2/CaO working pair as an example.The charge temperature ranges from 750 to 850K and the discharge temperature ranges from 650 to 750K,the system can achieve a round-trip efficiency of 43.38-61.45%,lower than that of the PTES system under the same charge temperature.The discharge temperature variation will influence the round-trip efficiency more significantly than the charge temperature.Adoption of a turbomachine with high isentropic efficiency,reducing the pressure loss within the reaction vessel and the heat exchange temperature difference within the recuperater,and optimizing the temperature ratio can all effectively improve the system round-trip efficiency.Further considering the outlet temperature limit of the compressor,two Chemical Looping Electric Storage systems using electric heaters are studied:one directly using electric heating,the other further using electric heating after the compressor.Detailed thermodynamic calculation of the thermal performance of the system is conducted.The influence of charge and discharge temperature on the round-trip efficiency and the exergy loss of each part of the system is obtained through sensitivity analysis.Based on this,the ways to further improve the efficiency of the system are discussed.The results show that for the CLES system heated by the electric heater and the compressor,the increase of charge temperature will increase the exergy loss rate of the electric heater and reduce the round-trip efficiency;moreover,the high charge temperature will make the heat pump cycle into a heat machine cycle,or even prevents the system from working.By connecting two reactors in series during charge and in parallel during discharge,the working temperature range and the round-trip efficiency of the system can be improved.For the CLES system directly heating by the electric heater,the efficiency of the system in the low temperature section is very low,and the exergy loss caused by heat rejection to the environment during discharge can reach to 8.9%.During discharge,using multistage compression intercooling or isothermal compression can reduce exergy loss and increase the round-trip efficiency by 8.2%-16.3%. |
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