article · Case Studies in Thermal Engineering
Carnot batteries offer a low-cost electrical energy storage option that avoids geographical constraints and power-capacity coupling while assisting multi-vector energy management. A study evaluated four hybrid configurations powered by solar photovoltaic electricity and geothermal heat, using thermodynamic, exergoenvironmental, and exergoeconomic assessments alongside multi-objective optimisation. The assessed systems integrated Brayton, heat pump, flash, and organic Rankine cycle (ORC) units. Supplying geothermal energy improved the environmental and thermodynamic performance across the configurations. Charging via a flash-heat pump cycle combined with discharging via an ORC unit produced the most optimal heat-to-power efficiency. Conversely, discharging with a regenerative-Brayton cycle resulted in higher investment cost rates compared to ORC units. A system combining a regenerative heat pump with a regenerative-Brayton cycle achieved the largest gain from geothermal integration, demonstrating a 5.3-fold improvement due to increased thermal energy capture.
Balancing power grids as renewable power grows demands flexible, cost-effective energy storage that does not rely on specific landscapes like pumped hydro. By combining solar electricity and geothermal heat within thermal storage cycles, this work identifies system layouts that optimise efficiency and economic cost. These insights support the long-term transition towards reliable, integrated clean energy networks capable of managing multiple energy vectors simultaneously.
This work informs power utility planners, storage technology developers, and geothermal operators seeking scalable alternatives to chemical batteries. The findings provide design guidance on selecting cycle components for hybrid solar-geothermal storage systems. Because the results stem from thermodynamic simulations, economic modelling, and multi-objective optimisation, the concept represents early-stage conceptual research that requires component-level testing and pilot demonstration before moving towards commercial application.
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Among the different electrical energy storage technologies, the Carnot batteries are promising options with low specific cost that do not suffer from geographical limitations and power-capacity coupling. In addition to power balancing, this approach can also be unique for multi-vector energy management. A comprehensive evaluation (thermodynamic design and exergoenvironmental and exergoeconomic evaluations), comparison, and multi-objective optimization of four Carnot battery configurations based on solar-electric energy and a geothermal source is presented. Geothermal energy can simultaneously improve the thermodynamic and environmental performances of the Carnot battery. The main structure of all configurations is based on electrical energy obtained from PV and captured thermal energy from a geothermal source. The four Brayton, heat pump, flash, and organic Rankine cycle (ORC) units are periodically integrated. The outcomes point out that the discharging process is based on an ORC unit and a flash-heat pump cycle (F-HPC)-based charging process makes more optimal heat-to-power efficiency. Moreover, the Carnot battery based on the regenerative-Brayton cycle (R-BC) unit has a higher investment cost rate compared to the ORC unit (in the discharging process). When integrating the geothermal, the third configuration (R-HPC/R-BC) experiences the greatest improvement (5.3-fold) due to the increase in thermal energy received from the geothermal source.
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DOI: 10.1016/j.csite.2024.104031
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