A better steam engine: Testing of the Katrix rotary lobe expander for distributed concentrating solar combined heat and power systems

We present analysis and design aimed at the development of a small-scale solar system that would compete with available distributed technologies for heat and electricity generation. In developing such a system we focus on the small-scale expander because no suitable product of less than 10kW has yet been commercialized for this application. The thermodynamic cycle operates as follows: Water (or another working fluid), is pumped to high pressure (∼1000 kPa), heated in a concentrating solar collector to greater than 473K, then expanded producing work on a shaft attached to a generator. The cooled fluid exits the expander and is condensed back to its original state. Ideally, the cold reservoir, to which the working fluid dumps energy, is some useful thermal load, hence completing the combined heat and power Rankine cycle. The test system consists of a compressed air expander (a prototype designed by Katrix, Inc. of Australia) connected to an induction motor driven by a variable frequency drive (VFD) enabling expander testing at varying shaft speeds. Results of the expander testing are a qualified success. We report isentropic efficiency (80-95%) and pressure ratio (6-11) at the tested speeds, both of which are impressive for a device this small and simple. Future refinements could lead to a new class of expanders with unequaled low-cost and highperformance for use in solar combined heat and power.