Modelling of a Free Piston Linear Expander for Organic Rankine cycle Based Waste Heat Recovery Application

The automotive industry, a significant contributor to energy consumption and greenhouse gas emissions, presents a prime opportunity for transformative advancements. Organic Rankine cycle (ORC) systems have emerged as a beacon of promise for harnessing waste heat recovery (WHR) from automotive exhaust, thereby enhancing energy conversion efficiency. However, their suitability for mobile applications, especially in the miniaturized (<20 kW) power range, hinges on the availability of cost-effective expansion machinery – a challenge we address head-on. This paper introduces an innovative solution: the design and empirical validation of a linear free piston expander with reduced weight and packaging volume, specifically tailored for small-scale ORC-WHR systems. Leveraging Mathworks Simscape, expansion characteristics, valve dynamics, leakage losses, and thermal inefficiencies were thoroughly examined. Rigorous empirical validation, originally conducted with air, is reported in this paper. While we acknowledge a marginal decrease in efficiency compared to turboexpanders, the unique design affords seamless integration with pumps and high-temperature heat exchangers. An expansion efficiency of 66.35% was achieved at an operating frequency of 15.9 hz and a pressure ratio of 16, resulting in an expander output of 10.46 kW. With its potential to revolutionize waste heat recovery for 10 kW systems, this innovation holds the key to both combatting climate change and advancing energy efficiency on a global scale.