Energy, Exergy, Economic and Environmental Analyses of Air-Based High-Temperature Thermal Energy and Electricity Storage: Impacts of Off-Design Operation

The present study presents a comprehensive assessment of impacts of the off-design operation of an air-based high-temperature thermal energy and electricity storage system on its energy, exergy, economic, and environmental aspects. Here, the effects of load variations on the mass flow rate, pressure ratio, and isentropic efficiency of the turbomachinery are considered to give the most accurate possible picture of the techno-economic aspects of the performance of the system. The results of such an assessment will be extremely useful in achieving the optimal performance of the energy storage system while working parallel with solar and wind power plants. The results prove that the system will present the high overall energy and exergy efficiencies of 91.5% and 88.16% when working at full load all the time. These indices, however, will be as low as 67.83% and 65.88% at an annual average operation load of 70% and even further lower to 34% and 32.73% at 40% load, respectively. The payback period of the system will be decreased from 11 to 23 years when the operation load falls from 100% to 80%. The environmental effects of such an energy storage unit for an energy market like Denmark (for instance) will be about 6355, 3227, and 823 tonnes of reduced equivalent carbon-dioxide when working at 100%, 70%, and 40% loads, respectively.