An appraisal of proportional integral control strategies for small scale waste heat to power conversion units based on Organic Rankine Cycles

Abstract

Despite the increasing number of Organic Rankine Cycle (ORC) installations at megawatt scale, the waste heat rejected by industrial processes can vary substantially from a few kW to many megawatts. Hence, ORC units with a power output in the range of tens of kilowatts should be developed to tackle the opportunity for heat recovery and business opportunities that can arise from it. In the current research activity, a dynamic model of a small scale ORC system was developed using a commercial software platform. The unit is equipped with two plate heat exchangers, a centrifugal pump and a radial turbine designed and optimized using an in-house code and a commercial 1D modelling tool. The off design behavior of the ORC system has been characterized by varying the inlet conditions of heat source and sink, and the turbomachine’s revolution speeds. Moreover, the response to transient thermal inputs at different time scales has also been investigated. Finally, four control strategies have been compared from the performance and energy efficiency perspectives. The results show that the turbine based regulation strategies achieve better control performance while pump based controls are able to regulate the system by maintaining the net power output closer to the design point.