Analysis and comparison of dynamic behavior of heat exchangers for direct evaporation in ORC waste heat recovery applications from fluctuating sources

In this paper, a systematic analysis and comparison of the dynamic response of two types of ORC evaporators for direct heat transfer between an exhaust gas and the organic working fluid is performed. Based on detailed dynamic models and simulations, maps are built to highlight in a generalized, compact and systematic way the dependence of the thermal response time of the evaporators on the geometric design and boundary conditions and its implications on the weight, volume and pressure drops. The analysis and application of the methodology to diesel engines long haul duty trucks, shows that fin and tube evaporators with large tube diameters and small cross-sectional areas of the exhaust side are the preferred option for high thermal inertia design, while louver fin multi-port flat tubes evaporators with large port diameters are better for fast response, despite the high pressure drops.