Comparative analysis of ORC and condensing heat engines for low grade waste heat recovery

The re-use of low-grade waste heat has the potential to contribute significantly to a better energy efficiency of our economies. There is a resource of around 100 TWhr per year in this area in Europe alone. The technology development in this area is still ongoing. Organic Rankine Cycle systems are considered the most promising technology. However, a nearly forgotten technology, the condensing engine (CE), was recently re-discovered. CEs use water as working fluid, with an operating temperature of 100°C at atmospheric pressure. The water is evaporated, and then condensed in the engine, where the arising vacuum is employed to generate power. Condensing engines were built until the late 19th Century, and then disappeared. Results from tests conducted in 1885 with a 0.735 kW commercial engine showed a mechanical efficiency of 3.7%, with a second law efficiency of 24.7%. For comparison, four typical experimental studies of Organic Rankine Cycle systems with power ratings between 0.5 and 1.4 kW were reviewed. Their thermal efficiencies ranged from 4.2 to 6.8%. The ORC systems’ second law efficiencies ranged from 20 to 35%, with an average of 27.5%. The comparative analysis showed that the CE’s performance is comparable to modern systems. Theoretical work suggests that it has significant further development potential. The CE’s simplicity combined with good efficiency, the use of a very simple working fluid, reduced safety requirements and the development potential makes this a technology which can become important again