Integrating Solar Thermal Capture with Compressed Air Energy Storage. World-changing research www.nottingham.ac.uk. Dept. of Mechanical, Materials and ...
Integrating Solar Thermal Capture with Compressed Air Energy Storage MC Simpson, SD Garvey, AJ Pimm, B Kantharaj, B Cárdenas, JE Garvey Introduction With intense pressure to decarbonise electricity systems, there are strong drivers for renewable energy generation. For intermittent renewables, energy storage is typically required to help synchronise production and consumption. An important class of technologies integrates both generation and storage within one system. This can bring capital cost savings from shared power conversion equipment, and reduced losses from additional energy transformations [1]. A ready example is natural hydroelectric power, in which the dam permits storage of potential energy until such time as it is needed. Concentrating solar plants can operate similarly by storing high temperature heat, for example in molten salt. The system described here is an extension to a compressed air energy storage plant (CAES). Through the addition of pre-heating before compression, the exergy stored per unit of pressure storage can be increased. The pre-heating stage also permits integration of solar thermal energy capture.
Air charging Inlet air enters the system and passes through the lower three layers of thermal store, heating up as it does so. The pre-heated air is then compressed and inter-cooled, releasing heat at high temperature, which is then stored in the top thermal store. The pressurised air is cooled to ambient and stored. Fig. 1: Air charging
Solar thermal charging Heat from solar collectors, e.g. parabolic troughs, is circulated through the lower three layers of thermal store, charging them up. This occurs independently of air charging.
Approach Thermodynamic modelling of the system is being undertaken, with the aim of establishing a practical configuration and the associated exergetic efficiency. Further work will then seek to determine a cost- and performance-optimised system.
System operation The major components of the system are a pressure store (e.g. a salt cavern), four layers of thermal store at increasing temperatures, solar thermal collectors, and compression and expansion machinery. The system has three different operating modes: Air charging, Solar thermal charging, and Discharging. Air charging and solar thermal charging need not take place at the same time or rate.
Results
Fig. 2: Solar thermal charging
Discharging cycle Air is released from the high pressure store and extracts heat from all four layers of thermal store. However, because the number of compression stages exceeds the number of expansion stages, excess heat remains in the thermal stores. Therefore, water is pumped to high pressure and also extracts heat from each thermal store, before mixing with the air prior to expansion in a single stage turbine. A heat pump is used to transfer the heat of vaporisation.
Dept. of Mechanical, Materials and Manufacturing Engineering
Modelling of a reversible system demonstrated the potential increase in total exergy stored for a given pressure store size. The maximum pressure and temperature of the system were maintained Fig. 4: Total exergy stored against pre-heat constant. Preliminary results have been obtained for the non-ideal air charging cycle, simulating 80 bar air storage with three compression stages, one expansion stage. Pre-heating to 327°C was employed, well within the scope of parabolic trough outputs. The exergy stored as heat is 1.7 times that stored as pressurised air, meaning that for a given cavern size, the system stores 2.7 times the exergy of a conventional (diabatic) CAES plant.
Further work
Fig. 5: Air charging T-S diagram with irreversibilities
Ongoing thermodynamic modelling is defining the discharge cycle, incorporating the air-steam mixture, heat pump and exhaust to atmosphere. Cost and economic value modelling will be carried out to determine the system’s ability to compete in grid-scale generation and storage markets.
References Fig. 3: Discharging
1. Garvey SD et al., On generation-integrated energy storage. Energy Policy, vol. 86, pp.544-551. 2015.
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