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Hybrid Thermal and Compressed Air Energy Storage System (HT-CAES): Thermodynamic Analysis and Thermoeconomic Optimization


Global warming concerns, volatile oil costs, and government incentives are leading to increased interest in the adoption of renewable energy sources. However, the integration of renewable sources in our existing infrastructure is challenging, as renewable generation is unpredictable and intermittent by nature. Energy storage compensates for the inherent intermittency of renewable energy sources, by storing energy during surplus power production periods and discharging the stored energy during low production periods. Compressed Air Energy Storage has received much attention as a viable solution due to its economic feasibility, low environmental impact, and large-scale capability. However, conventional CAES systems rely on the combustion of natural gas, require large storage volumes, and operate at high pressures, which possess inherent problems such as high costs, strict geological locations, and the production of greenhouse gas emissions. Through this research, a novel and patented hybrid thermal-compressed air energy storage (HT-CAES) design is investigated as a possible solution. The HT-CAES system allows a portion of the available energy, from the grid or renewable sources, to operate a compressor and the remainder to be converted and stored in the form of heat, through joule heating in a solid-state sensible thermal energy storage medium. The hybrid design has the beneficial effect of mitigating the shortcomings of conventional CAES systems and its derivatives by eliminating combustion emissions and reducing storage volumes, operating pressures, and costs. Therefore, the hybrid system provides flexibility of adjusting to a myriad of storage volumes based on available geological restrictions. Additionally, The hybrid system possesses a wide range of possible operations, without a compromise in its storage capacity, which may prove useful as we move towards a sustainable future.

An ideal HT-CAES system is investigated and the thermodynamic efficiency limits within which it operates have been drawn. 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 not theoretically 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 work 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.

Subsequently, a realistic and irreversible hybrid configuration is presented that incorporates two stages of heating through separate low-temperature and high-temperature thermal energy storage units. A thermodynamic analysis of the HT-CAES system is presented along with parametric studies, which illustrate the importance of the operating pressure and thermal storage temperature on the performance of the storage system. Realistic isentropic component efficiencies and throttling losses were considered. Additionally, two extreme cavern conditions were analyzed and the cyclic behavior of an adiabatic cavern was investigated. An optimum operating pressure resulting in maximum roundtrip storage efficiency of the hybrid storage system is reported. Additionally, a modified hybrid design is investigated that includes a turbocharger in the discharge process, which provides supplementary mass flow rate alongside the air storage. This addition has the potential of drastically reducing the necessary storage volume and pressures, thus further increasing the operational flexibility of the system. The results of this work provide an efficiency and cost map of the HT-CAES system versus both the operating pressure and the distribution of energy, between thermal and compressed air storage. The results of this work illustrate and properly quantify a tradeoff that exists between the HT-CAES system cost and performance. Both roundtrip energy and exergy efficiencies are quantified, presented, and compared. Lastly, a local optimum-line of operation, which results in a local maximum in efficiency and a local minimum in cost, is presented.

The HT-CAES system is also investigated and optimized based on a minimum entropy generation criteria. Regenerative and non-regenerative configurations are examined. It is illustrated that an HT-CAES system designed based on a minimum entropy generation objective may be at a lower energy and exergy efficiency, and lower output power, than otherwise achievable. Therefore, in the case of a hybrid energy storage system, minimization of entropy generation does not always coincide with minimization of energy losses. Only under certain conditions does the point of minimum entropy generation coincide with maximum energy efficiency. Specifically, this occurs only when the input energy, thermal energy storage mass, specific heat, and temperature swing, are a constant. Similarly, only in the specific case where the total input exergy is a constant, does minimum entropy generation coincide with maximum exergy efficiency. Lastly, an exergy analysis of the hybrid system is presented. The calculated and normalized exergy destruction maps provide a means of comparing the component exergy destruction magnitudes for assessing and pinpointing the sources of largest irreversibilities. In addition to the exergy destruction, the exergetic component efficiencies are also presented and compared. Both component exergy destruction and their exergetic efficiencies demonstrate that the largest source of avoidable exergy destruction results from the irreversibilities associated with throttling and the irreversibilities associated with mixing losses within the air storage medium.

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