Micro-Scale Waste Heat Recovery from Stationary Internal Combustion Engines by Sub-Critical Organic Rankine Cycle utilizing Scroll Machinery

The movement towards renewable energy sources has created new challenges for energy production and has promoted the concept of distributed energy production. High efficiency Generators for Small Electrical and Thermal Systems (GENSETS) provide a viable solution to residential-scale power production at low cost. In an effort to improve the energy production efficiency of small scale internal combustion engine generators, a waste heat recovery (WHR) bottoming cycle can boost the operating efficiency of such systems by up to 7%. Organic Rankine Cycle (ORC) technology is a popular method of achieving WHR from low temperature heat sources. However, the application of ORC systems using the high-temperature exhaust stream of ICE generators as heat input is currently unexplored. Thermodynamic modeling of a sub-critical ORC system proved its viability as a bottoming cycle for ICE generators. The simulations also enabled the selection of the optimal working fluid for the specified application given the system restrictions. The refrigerant R245fa was selected for its high-temperature chemical stability and thermal efficiency. Detailed modeling that reflected the selected ORC components then provided insight into system performance with a range of ICE generators and further proved the system’s viability. The high temperature application required the creation of several prototype components to create an efficient ORC WHR system. By utilizing a novel evaporator heat exchanger, it is possible to establish a cycle with high superheat in an effort to minimize exergy destruction in the evaporation heat transfer process. Additionally, the sub-critical architecture of the ORC benefits from custom scroll-type expander and pump that match the designed pressure and volume ratios. However, the prototype ORC components required further development and testing and therefore, an experimental test stand was constructed to perform component evaluation and system performance measurement. Using the test stand, it was possible to evaluate two different types of scroll expander: the Oldham ring orbiting scroll, and the spinning scroll. A performance comparison is presented and the strengths and weaknesses of each are considered. Additionally, a performance comparison between the working fluid R245fa and its HFO replacement R1233zd(E) is presented in an effort to evaluate the feasibility of a working fluid change. The work performed on evaluating the different scroll machines presents a good first step in the successful development of an ORC bottoming cycle for ICE generators. Through further development, ORC WHR will become more feasible and assist in the goal of distributed energy production