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The desire to increase power production through renewable sources introduces a number of problems due to their inherent intermittency. One solution is to incorporate energy storage systems as a means of managing the intermittent energy and increasing the utilization of renewable sources. A novel hybrid thermal and compressed air energy storage (HT-CAES) system is presented which mitigates the shortcomings of the otherwise attractive conventional compressed air energy storage (CAES) systems and its derivatives, such as strict geological locations, low energy density, and the production of greenhouse gas emissions. The HT-CAES system is investigated, and the thermodynamic efficiency limits within which it operates have been drawn. The thermodynamic models considered assume a constant pressure cavern. It is shown that under this assumption the cavern acts just as a delay time in the operation of the plant, whereas an adiabatic constant volume cavern changes the quality of energy through the cavern. The efficiency of the HT-CAES system is compared with its Brayton cycle counterpart, in the case of pure thermal energy storage (TES). It is shown that the efficiency of the HT-CAES plant is generally not bound by the Carnot efficiency and always higher than that of the Brayton cycle, except for when the heat losses following compression rise above a critical level. The results of this paper demonstrate that the HT-CAES system has the potential of increasing the efficiency of a pure TES system executed through a Brayton cycle at the expense of an air storage medium.

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Efficient, large-scale, and cost-effective energy storage systems provide a means of managing the inherent intermittency of renewable energy sources and drastically increasing their utilization. Compressed air energy storage (CAES) and its derivative architectures have received much attention as a viable solution; however, optimization objectives for these systems have not been thoroughly investigated in the literature. A hybrid thermal and compressed air energy storage (HT-CAES) system is investigated that mitigates the shortcomings of the otherwise attractive conventional CAES systems and its derivatives—shortcomings such as strict geological locations, low energy densities, and the production of greenhouse gas emissions. The HT-CAES system allows a portion of the available energy to operate a compressor and the remainder to be converted and stored in the form of heat through joule/resistive heating in a high-temperature, sensible, thermal energy storage medium. Internally reversible and irreversible HT-CAES system assumptions were investigated, in addition to regenerative and non-regenerative design configurations. Several system optimization criteria were examined—including maximum energy efficiency, maximum exergy efficiency, maximum work output, and minimum entropy generation—with a focus on whether the latter may lead to conclusive design guidelines in a real system. It is shown that an HT-CAES system designed based on a minimum entropy generation objective may operate at a lower energy and exergy efficiency as well as lower output power than otherwise achievable. Furthermore, optimization objective equivalence is shown to be limited to certain design conditions.