Study on off-design performance of transcritical CO₂ power cycle for the utilization of geothermal energy

Exploiting renewable energy could greatly alleviate current severe energy situation. Transcritical CO₂ (tCO₂) cycle system as a type of competitive and promising energy converter could utilize low temperature geothermal energy effectively. In this paper, the methodology for the off-design operation of tCO₂ system has been proposed and the quantitative performance analysis is carried out for the utilization of geothermal energy. With the aim at obtaining better thermodynamic and economic performance, the optimal power cycle parameters can be determined by the non-dominated sorting genetic algorithm-II (NSGA-II) in the design stage. The models of exergoeconomic analysis and main system components including turbine, pump and heat exchangers are established. The sliding pressure control strategy is applied to respond to the varieties of heat source and heat sink conditions in the off-design stage. Results show that there exists an optimal value of geothermal resource mass flow rate to maximum the thermal efficiency. The pump rotational speed increases linearly with the increasing geothermal resource temperature and geothermal resource mass flow rate while the pump rotational speed is gradually sensitive to the increase of heat sink temperature. The guideline listed in the table for pump could provide effective reference for practical operation.