Irreversibility in the Organic Rankine Cycle for Low-grade Thermal Energy Conversion System

orc2019 Tracking Number 162

Organic Rankine cycles (ORCs) are used in power generation applications for low-temperature heat sources such as waste heat recovery from plants, geothermal binary, hot springs, and ocean thermals. Meanwhile, ORC equivalent technologies are applied in ocean thermal energy conversion (OTEC), which uses the temperature difference between the surface and the depths. ORCs with such low-grade thermals must consider various forms of irreversibility, not just the performance of the turbine/generator and heat exchangers. Typical ORCs are mainly composed of a turbine/generator, a working fluid circulation pump, and heat exchangers. A new performance evaluation concept for low-grade thermal energy conversion (LTEC) is proposed that normalizes the thermal efficiency considering heat leak and equilibrium state as a dead state based on finite-time thermodynamics. In addition, considering the trade-off between the heat transfer performance and the pressure drop due to the flow, a unique performance evaluation method based on the maximization of the net obtainable energy per heat transfer is initiated on a plate-type heat exchanger in LTEC. Here, to keep the cost of systems down, ease of examination, and proper design of the equipment, standardization and calculation tools are useful. In this research, theoretical maximization of the available power employing the irreversibility of LTEC system is conducted employing the parameter analysis of simple Rankine cycle to arise the coefficient of irreversibility. In the results, the coefficient of irreversibility is clarified on working fluids (HCFC245fa and HCFO1224yd(z)), turbine efficiency, and working fluid circulation pump efficiency, respectively.