Theoretical and experimental analysis of the impact of a recuperative stage on the performance of an ORC-based solar microcogeneration unit

Abstract

Microgeneration ORC-based units driven by solar energy, which enable combined heat and power generation (CHP), are a promising solution for decarbonizing the domestic sector. However, the intermittent availability of solar energy, coupled with the variability in user demand for domestic hot water (DHW), can lead the system to frequent off-design conditions and a less reliable energy supply. Consequently, increasing attention has recently been focused on the technological and design solutions for improving plant performance and ensuring its continuous operation. This paper presents the results of an experimental campaign carried out to investigate the possible advantages – in terms of efficiency and savings of thermal energy in the upper source – of introducing a recovery heat exchanger (RHX) in the basic configuration of a solar ORC-based system. Tests were conducted on a fully instrumented ORC-based plant with two 12 kW electric heaters providing the thermal power recovered through the solar collectors. The RHX is introduced into a recuperative branch that can be bypassed by closing a dedicated three-way valve. The study aims to investigate the behavior of the ORC unit in the absence of solar radiation (with the electric heaters switched off) when the recovery unit is powered only by the hot water stored in the Thermal Energy Storage (TES) tank. Another purpose of the present work is to evaluate the benefits introduced by the RHX in reducing the temperature decrease of the TES hot water and, consequently, maximizing the operating time of the ORC-based unit. In order to support the experimental analysis, a comprehensive theoretical model of the unit was developed and validated against experimental data. The model was used as a software platform to optimize the plant design and recuperative branch configuration. The theoretical model was developed in the GT-Suite™ environment thus integrating a mono and zero-thermo-fluid-dynamic approach. In this way, a physical representation of the entire ORC-based unit is performed allowing also to define an optimal control strategy for maximizing plant performance under severe off-design and transient conditions.