Characteristics of Small ORC System for Low Temperature Waste Heat Recovery

Abstract

This paper describes fundamental characteristics of a small organic Rankine cycle (ORC) system to be used for power generation from low temperature heat sources such as waste heat and solar energy. The aim of the study was to develop an ORC system with a small power output of less than 1 kW with a hot source with temperature ranging from 60 to 100°C and a cold source with temperature ranging from 10 to 30°C. An ORC system with a potential to produce a turbine/expander power of 250 W was built and its fundamental characteristics were elucidated. A turbine/expander was not actually installed but was simulated by controlling two expansion valves. First, steady-state energy balance of the system was examined and the required turbine/expander efficiency was estimated in consideration of pump power of the working fluid. Then, the relationship between the expansion ratio and thermal efficiency was elucidated. The most important result of the study was that for maintaining high thermal efficiency in the case that the temperature difference between hot and cold sources varies during operation, it is indispensable to employ a variable expansion mechanism by which the expansion ratio of the turbine/expander can be adjusted to fit the optimal ratio at the operating temperature level.