Enhanced heat exchanger layout for optimum energy performance in solar thermal ORC-based unit

Abstract

The paper presents an extensive modeling activity for optimum design of the Heat Recovery Vapor Generator, HRVG, in an ORC-based unit, bottoming a solar thermal collector for domestic hot water production. Sliding vanes rotary machines-equipped ORC units are widely acknowledged among the most suitable options for low grade (100-120°C source temperature) heat recovery, due to their technological advantages (low cost, low maintenance) and higher energy merit (flexibility and applicability to a variety of upper thermal sources). In such plants, the energy performance boost calls for the exergy losses reduction at the heat recovery vapor generator, via the improved matching of the heat exchange curves. The paper investigates the potential gain on exergy and energy efficiency of two different techniques, i.e. the evaporation split over two pressure levels for pure R245fa and the use of R245fa/R227ea zeotropic mixtures, as the working fluid. Optimum evaporation pressures, pinch point temperature difference, mass flowrates and superheating degree on both loops are selected for each case study and the system energy performance assessed, hence providing a reliable criterion for optimum design of small-scale plants.