4E (Energy, Exergy, Economic, Environment) analysis and optimization of a multigeneration system integrating two energy storage systems for remote islands

Remote island areas, due to their unique geographical locations, possess abundant renewable energy resources. However, fluctuations in these resources make it essential to integrate energy storage systems to enhance energy reliability and improve the energy consumption situation for residents. Therefore, this paper presents a multi-generation system based on underwater compressed CO2 energy storage and the Carnot battery, which supplies electricity, thermal energy, and freshwater. The system includes an underwater CO2 storage subsystem, a Carnot battery subsystem, and a reverse osmosis desalination subsystem. Performance is evaluated across energy, exergy, economic, and environmental aspects, with key factors such as heat storage temperature, underwater pipeline diameter, compressor pressure ratio, and CO2 mass flow rate analyzed. Results show that increasing thermal storage temperature and pipeline diameter improves overall system efficiency, and round trip efficiency rises from 52.70% to 67.54% as temperature increases from 95 degrees C to 135 degrees C. However, a higher compressor pressure ratio and CO2 mass flow rate reduce performance. Under optimal conditions, the system achieves a round-trip efficiency of 66.46% and a levelized cost of electricity of 0.4134 $/kWh. A multi-objective optimization shows that a round-trip efficiency of 63.38% and a reduced electricity cost of 0.3874 $/kWh are achievable.