Thermodynamic analysis of the Compressed Air Energy Storage system coupled with the Underground Thermal Energy Storage

Abstract

Improvement of flexibility is one of the key challenges for the transformation of the Polish Power System aiming at a high share of renewable energy in electricity generation. Flexible and dispatchable power plants will contribute to this ongoing transformation process as they compensate for fluctuations in electricity generation from renewable energy sources such as wind and photovoltaics. In this context, CAES storage tanks are currently the only alternative to storage facilities using pumped-storage hydroelectricity due to the possibility of obtaining the appropriate energy capacity of the storage facility. However, a relative disadvantage of these plants is the heat loss caused by the cooling of the air after compression. The basic elements of the CAES warehouse are: an air compression station, a compressed air reservoir that is also a storage facility (in the existing solutions, these are underground caverns), an expansion station with combustion chambers and gas turbines, and a generator. A key aspect of CAES is the optimal configuration of the thermodynamic cycle. In this paper, the situation of cooperation between the current conventional power plants and wind farms is first analysed, and then, based on thermodynamic models, the process of storing thermal and electrical energy in the CAES system coupled with heat recovery after the gas turbine is analysed. A solution with a ground heat exchanger was also proposed, as the soil, due to its properties, may serve as a thermal energy storage. The paper also analyzes the discharge of the heat storage based on CFD approaches. The ground can be charged during the cooling down of the compressed air. On the other hand, thermal energy was recovered when water flowing to the heat customers was heated. On the basis of non-stationary calculations, the heat stream received from the underground thermal energy storage was estimated.