A Liquid-Piston Steam Engine

Abstract

Reduction of global carbon dioxide emissions is one of the most critical challenges for realizing sustainable society. In order to reduce carbon dioxide emissions, energy efficiency must be improved. Waste heat recovery with external combustion engine is expected to be one of the promising technologies for efficient energy utilization. However, the temperature of waste heat is getting lower with the progress of energy technologies. For example, in Japan which is known as one of the most energy-efficient countries in the world with advanced technologies such as cogeneration and hybrid automobiles, total amount of disposed heat below 300 °C is as much as 10% of the total amount of primary energy supply. Conventional external combustion engines, such as Stirling, thermoacoustic1 and steam engines2 show significant decrease in their efficiency at low temperatures below 300 °C. Utilization of high-temperature heat sources, however, requires relatively expensive materials and advanced processing technologies to achieve high reliability. In order to overcome these issues, a novel liquid-piston steam engine is developed, which achieves high efficiency as well as high reliability and low cost using low temperature heat below 300 °C. Present liquid-piston steam engine demonstrated a thermal efficiency of 12.7% at a heating temperature of 270 °C and a cooling temperature of 80 °C, which was about 40% of the Carnot efficiency operating at same temperatures. The liquid-piston steam engine operated even with wet steam, without requiring steam to be superheated. This low temperature operation yielded relatively little deformation of components, which leads to high reliability of the engine. In addition, present liquid piston engine can achieve both high efficiency and low cost compared to conventional external combustion engines, because it has only one moving part whereas both Stirling and Rankin engines have at least two moving parts. The developed liquid piston engine is thus expected to possess large possibility of recovering energy from waste heat.