Organic Rankine Cycle, variable speed, radial inflow turbine
This paper analyzes, given the large variation in turbine discharge pressure with changing ambient temperatures, whether variable-speed radial-inflow turbine operation has a similar benefit for Organic Rankine Cycle (ORC) power recovery systems as variable-speed centrifugal compression has for chiller applications. The benefit of variable-speed centrifugal compression over fixed-speed operation is a reduction in annual electricity consumption of almost 40 %. Air-conditioning systems are by necessity always designed for the highest possible ambient temperature. This is not necessary for ORC systems. Lower cost ORC systems are obtained when the design point is selected at moderate ambient temperatures. However, these systems show reduced power output at higher ambient temperatures. The more expensive ORC systems designed to achieve full power at higher ambient temperatures will produce constant power independent of ambient temperature but consume more heat and require a control mechanism to prevent overloading the turbine at lower ambient temperatures. The benefit of variable-speed ORC turbine operation over fixed-speed is an increase in annually averaged ORC power output and/or a reduction in annually averaged thermal heat input demand. However, the variable-speed benefit is as, will be explained in the paper, inherently smaller for ORC systems than for centrifugal chillers.