Thermodynamic analysis and optimization of a partial evaporating dual-pressure organic rankine cycle system for low-grade heat recovery

The organic Rankine cycle (ORC) has been proven as one of the most effective technologies for low-grade heat recovery. However, its efficiency is limited due to exergy losses caused by the large temperature difference between the heat source and working fluid in the evaporator. To enhance the ORC system efficiency, in this paper, a novel partial evaporating dual-pressure ORC (PEDORC) system is proposed. A mathematical model is developed to evaluate the system thermal characteristics and investigate the effects of key parameters (e.g. evaporating temperature, quality, superheat temperature, energy distribution) on the system performance. Results show that an optimal evaporating temperature in the evaporators exists to achieve the maximum net power output and exergy efficiency. However, the system thermal efficiency increases as the evaporating temperature increases. Furthermore, the best thermal performance occurs when the thermal energy is properly distributed in the evaporators I and II. The performance of the proposed PEDORC system is further compared with the simple ORC (SORC) and basic dual-pressure ORC (BDORC) systems. The results show that the net power output and exergy efficiency of the proposed PEDORC system are increased by up to 27% and 4.6%, respectively, in comparison to the SORC system. By comparing with the BDORC system, the net power output and exergy efficiency of the proposed system are increased by up to 9.2% and 4%, respectively, and the Levelized Cost of Electricity is reduced by up to 4%. These analyses demonstrate that the proposed PEDORC system is an effective means to recover low-grade thermal energy.