Thermodynamic analysis of two micro CHP systems operating with geothermal and solar energy

Micro combined heat and power (CHP) plants operating through an organic Rankine cycle (ORC) using renewable energy are analyzed. The reference system is designed to produce 50 kWe. The heat sources of the system are geothermal energy at low temperature (80–100 C) and solar energy. Two different system layouts, a single and a double stage arrangement, are presented. The first uses a solar field composed only by evacuated solar collectors, and work is produced by a single turbine. In the double-stage system, a field of evacuated solar collectors heats the working fluid up to an intermediate temperature. After this first stage, only a part of the working fluid flow rate is heated in a second solar field, composed of directsteam parabolic through collectors (PTCs), up to the maximum temperature of the cycle. The mechanical work is then produced in two turbo-expanders arranged in series. For the investigations, different working fluids (e.g. R134a, R236fa, R245fa) are considered. The results of the simulation in terms of efficiencies, heat and electricity production and the main characteristics of the system (i.e. heat exchanger surface, solar collector area) are presented and discussed.