Solar Thermal-Based ORC Power Plant for Micro Cogeneration - Performance Analysis and Control Strategy

The paper deals with the performance assessment of a small scale cogeneration system for building applications, featuring an Organic Rankine Cycle-based plant bottoming a solar collector array for combined heat and electricity generation. A sliding vanes rotary expander and a water cooled condenser are employed in the recovery section. A comprehensive MATLAB (R) model accounts for the dynamic of each component, as both a stand-alone device and a plant-integrated unit: a parametric study is presented and an off-design analysis is performed to properly assess the performances of both the heat exchanger and the expander. Heat availability to the ORC heat exchanger is evaluated, based on solar availability, thermal losses in the pipes and plant requirements, in terms of operating temperature and pressures, having the collection area, the mass flowrate for the fluid in the solar collector branch and the fluid type in the recovery section as main variables. Due to the need for DHW production, a storage unit for hot water is present, upstream the recovery branch: dependently on the ability the fluid at the collector outlet has to meet the ORC requirements for proper operation (about 110 degrees C), the ORC evaporator is fed and the recovery section enabled. Both continuous and unsteady operation underwent an in-depth analysis, as well as the benefits associated with different discharge times for the storage unit: dependently on whether the electrical output or the thermal one need to be maximized, a different control logic for the whole system comes out (e.g. either a flash or a progressive tank discharge). The virtual platform allowed the setting-up of a pilot plant, for direct performance assessment and model validation. (C) 2018 The Authors. Published by Elsevier Ltd.