Small-scale ORC-based unit for domestic micro-cogeneration operating in the temperature range of the solar thermal flat panels

Abstract

CO2 emissions related to the energy demand in commercial and residential buildings account for over 15% of worldwide greenhouse gases emissions. This issue requires the development of new solutions, technologies, and energy management strategies to reduce the environmental impact of this sector. Simultaneous heat and power production, namely cogeneration, is a proven method for this purpose. Among the different power systems, Organic Rankine Cycle (ORC) plants allow the production of mechanical and electric energy using hot sources at low temperature levels. This feature allows to combine this technology with solar collectors, making it possible to exploit the thermal energy exceeding the thermal needs when the solar irradiation is high. In this paper, an experimental characterization of a Solar Organic Rankine Cycle (SORC) is presented. The hot source of the power plant is reproduced by water heated up to 120 °C by two 12 kW electric resistances and stored in a buffer tank with a capacity of 200 L. The 1 kW ORC unit is equipped with a scroll volumetric expander operating off-grid. The main control variable of the unit is the pump speed which is varied in each operating condition to guarantee a superheating degree at the expander inlet. Since the variability of the solar source is a critical issue in these applications, off-design operating conditions have been tested, showing the capability of the plant to be also operated far from the design point. The maximum efficiency of the unit reached 4.7 % while the highest net power was 400 W. A further test conducted to evaluate the transient performances of the unit demonstrated its small inertia and the capability of the expander to produce power right after the unit activation.