Development and Testing of a Free Piston Linear Expander for Organic Rankine Cycle Based Waste Heat Recovery Application

orc2019 Tracking Number 144

The global community has agreed to limit global warming well below 2oC (COP21) . Automotive internal combustion engines (ICE) generally convert 40% of fuel energy into useful power and discharge the remaining to environment. Disruptive ideas are needed to achieve the set goals for improvement of energy conversion efficiency. Organic Rankine cycle systems appear to be favourable for waste heat recovery from the automotive exhaust, but the technology readiness levels are still low for mobile applications particularly due to non-availability of suitable and cost-effective expansion machines in mini-scale (<20kW) power scale. The work is focussed on the development and experimental testing of linear free piston expander. Free piston expanders are suitable and have smaller leakage losses. The expander also has a passive inlet port which controlled by bounce pressure due to small recompression and thus complex valve mechanisms can be avoided. The generator also encompasses a linear electrical machine on the piston of the expander to generate electricity directly. Multi-physics models were used to understand the gas expansion characteristics, valve dynamics, leakage losses and heat losses using Mathworks Simscape. The models helped to size the equipment (ports/plenum/chambers etc.) and also provided the control schemes for the control expansion machine The work also presents details of adaptation/retrofitting of an organic Rankine cycle (ORC) rig originally used for testing of a turbo-generator. The currently, the proposed expander is based on R1233zde so the balance of plant was re-iterated along with the specific expander integration equipment details. The expander was tested at 8 bar inlet pressure and 1 bar discharge pressure, the machine under investigation is the downsized version to generate 2.5kWe power. The results concluded that the proposed expander is an effective solution for waste heat recovery applications for 10kWe scale output. Despite, lower efficiencies compared to a turbo-expander, the design and shape permits integration of pump and high-temperature heat exchanger to the expander to form a compact unit, thus saving cost and weight.