Off-design analysis of ORC and CO2 power production cycles for low-temperature surplus heat recovery

Abstract

In process industry large amounts of energy are rejected to the ambient. Recovery of this surplus energy is a wide topic. Among the strategies for energy recovery, production of electricity is very interesting, due to the versatility of this form of energy. For the relatively low temperature heat, which is most commonly encountered for industrial surplus heat, the Organic Rankine Cycle is a well established technology. The transcritical Rankine Cycle recently received special attention due to its performances for energy recovery from low temperature sources. CO2 is a natural candidate as working fluid for this technology. It combines high performance, low cost, low toxicity, is non-flammable and has no environmental impact. This paper focuses on the off-design operation of Rankine cycles and compares the behaviour of transcritical CO2 cycles and an ORC cycle with R-123 as working fluid. The main observation is that the ORC is very sensitive to reduction in available heat, and will with only small changes get droplets in the inlet of the expander. This shows that it is reasonable to operate the ORC with some degrees superheat, to have a buffer. Superheating the outlet 5 to 10 K has only a small effect on the cycle performance. However the gained robustness is also relatively small. With small increments in the available heat source, the CO2 cycle also seem to have a marginally better response without control of the process, which indicates that it is more robust and less in need of detailed control.