On the reasonability of heat recovery in isobaric expansion engines

• A method to determine the overall efficiency of isobaric expansion engines is proposed. • A dynamic heat exchanger model is developed and applied to heat recovery description. • Optimal operating conditions for isobaric expansion engines. • Benefits and limitations of heat recovery within isobaric expansion engines are identified. Heat recovery by recuperators is expected to improve the performance of isobaric expansion engines. However, such recuperators include dead volumes reducing the positive effect. This leads to an optimization problem whose solution should yield a reasonable heat exchanger sizing. In this work, the optimization problem is solved by a combination of a thermodynamic model estimating the thermal efficiency and a dynamic heat exchanger model based on heat transfer coefficient correlations. A comprehensive study of the heat exchanger performance and the overall thermal efficiency of an isobaric expansion engine operating at three different temperature levels (50°C, 100°C, and 150°C), is carried out. As heat exchangers the so-called printed circuit heat exchangers are adopted, in view of their promising potential in application with isobaric expansion engines. This study demonstrates that recuperators bring significant advantages, with up to 70% increase in efficiency, mostly when the heat source temperature is 100°C or higher.