Exergy, economic and pinch analyses of a novel integrated structure for cryogenic energy storage and freshwater production using ejector refrigeration cycle, desalination unit, and natural gas combustion plant

Abstract

This study aims to investigate an innovative hybrid structure of electricity storage at off-peak hours and its application at on-peak hours. In this paper, a novel hybrid system for energy storage and freshwater production using air compression and liquefaction system, ejector refrigeration cycle (ERC), thermal multi-effect desalination (MED) system, and natural gas combustion unit (GCU) is developed. The wasted heat of the liquefaction cycle is stored at off-peak hours to provide heat to the ERC and thermal desalination system and preheat the inlet streams to the turbines at on-peak hours. Also, wasted refrigeration of liquid air stream and refrigeration resulting from liquefied natural gas (LNG) regasification operations are stored at on-peak hours and are utilized in compressed air liquefaction at off-peak hours. This integrated structure produces 83.12 kg/s liquid air and 38.91 kg/s freshwater at off-peak hours with 58.24 MW power consumption. During on-peak hours, the liquid air along with 2.681 kg/s LNG enters the turbines after preheating and combustion and generated 122.6 MW power. Round-trip and storage efficiencies are obtained as 64.91% and 75.96%, respectively. Also, the exergy efficiencies of the energy storage system at off-peak hours, energy production system at on-peak hours and the whole system are estimated as 78.13%, 75.85%, and 65.92%, respectively. The results of the exergy study illustrate the reactors (43.37%), heat exchangers (26.76%), and throttle valves (10.58%) have the highest contribution to exergy destruction in the proposed combined structure, respectively. This study provides the refrigeration cycle integration with process core as hot and cold composite curves and pinch analysis. Pinch investigation is conducted on the system multi-stream exchangers and the heat exchanger networks are extracted. The economic analysis illustrates that the investment return period, the levelized cost of the product, and net annual benefit in the hybridized system are 4.877 years, 0.0919 US$/kWh, 78.38 MMUS$/year, respectively. The results of sensitivity analysis indicate the round-trip efficiency increases up to 65.32% by raising the pressure of the on-peak power generation cycle.