Development of a model-based control system for a small-scale solar-driven Organic Rankine Cycle based power unit

The integration of small-scale Organic Rankine Cycle power unit equipped with Thermal Energy Storage tank and solar thermal collectors represents one of the most attractive technological alternatives for the combined production of heat and power in residential sector. Despite these benefits, the power unit is almost always in severe off-design conditions since the high fluctuation of solar source and the variable temperature inside the tank during the operation. This requires the adoption of a reliable control system. In literature, few studies on the development of control strategies for so small power units are available. To fill this gap, in the present work an innovative control procedure is proposed based on the integration of a Feedforward and a Feedback procedure. Such model is validated against experimental data collected on a fully instrumented test bench. Once validated, it was demonstrated that even under severe variation of hot source (flow rate and temperature) the control unit provides a power in range of 0.5-1.1 kW for the quite entire time interval. This occurs considering that the hot source of the recovery unit is decreasing when the unit operates. Moreover, control system results effective also in case when the unit is fed by the Thermal Energy Storage tank in absence of solar power energy refurbishment. In this harsh condition, the controlled power unit presents a larger operating robustness and an increase of 7.3% of the whole electrical energy is yearly produced by the unit, with respect to a baseline case.