The condensing cycle for heat engines with operating temperatures from 50 to 100°c

Abstract

Low grade thermal energy with low temperatures of up to 150° C occurs e.g. as waste heat or waste steam in industry, or in in renewable energy systems such as solar thermal collectors or geothermal systems. The cost-effective utilisation of such low-grade thermal energy still constitutes an engineering challenge. Existing technology such as Organic Rankine Cycle systems is complex, and only cost-effective for power ratings of 500 kW and more. At Southampton University, the condensing steam cycle, which has an operating temperature of 100C and was originally proposed by James Watt in 1782, was re-examined. The theoretical efficiency at 100°C ranges from 0.064 for a simple condensing engine to 0.174 for a steam expansion ratio of 1:8. A 30 Watt model engine confirmed the efficiency increase. Further theoretical work indicates that operating temperatures as low as 50°C with an efficiency of 0.06 could be possible, still using water as working fluid. The condensing engine is a simple machine which could provide a cost-effective solution for low-grade heat conversion.