Multiple Expansion ORC for Small Scale – Low Temperature Heat Recovery

ORCs are widely recognized as one of the most suitable solution for energy recovery, if the temperature of the heat source is of about 200°C, or lower. In case of heat sources of about 100 kW or smaller, the more common solutions prescribe a simple cycle and a single stage expander, in order to reduce complexity and costs. Scroll expanders, derived from scroll compressors, are expected to be available at very low unit costs. The drawbacks of this kind of solutions, originally designed for automotive, or HVAC applications, are mainly two: the low fixed volumetric expansion ratio and the small volumetric flow rate, that are not always well-suited for the requirements of power production. In this paper, different ORCs with multiple expansions are evaluated with the aim of achieving a better exploitation of small scale-low temperature waste heat sources. The comparison takes in consideration different possible solutions for the multiple expansions, with internally recuperated and not-recuperated cycles, whilst the data describing the actual behaviour of compressors derived scroll expanders have been previously obtained by a test rig, set up at the University of Trieste, using R245fa as working fluid.