Modelling and Optimization of Small Scale Solar Organic Rankine Cycle for Islamabad, Pakistan

In recent decades, energy demand has grown exponentially. Fossil fuels, which today meet the bulk of global energy demand, can be gradually replaced by renewable energies, thus limiting its dangerous consequences such as climate change, environmental pollution, depletion of natural resources, etc. Solar energy can play an important role in the satisfaction of energy demand, especially in a heavily sun-ridden country. The current study focuses on the modelling and optimization of Organic Rankine Cycle (ORC) based on different organic fluids operating in a temperature range below 50-100 °C. The ORCs are a good choice to produce small-scale energy due to the lower temperature range of 50 to 99 °C. They are therefore simple and inexpensive. Flat plate (FPC) or evacuated tube solar collectors (ETC) can also provide the desired energy. In this work, two system configurations are analyzed. In configuration-I (C-I), the water from the collector outlet moves to the hot water storage tank (HWST) connected in series, while in configuration-II (C-II), HWST is not used. Therefore, the hot water from the solar collector outlet enters directly into the auxiliary heater (which will be lit if necessary, otherwise) and the water return from the heat exchanger will become the input of the solar collector. Working fluids suitable for a solar-powered ORC at a temperature of 100 °C or lower are selected using predefined criteria such as higher fluid densities, maximum cycle efficiency, safety and environmental data, a moderate temperature and inexpensive and uncomplicated equipment. The R125 and R245ca were found to be good fluids due to the minimum collector area for the desired yield and maximum efficiency, respectively. For the R125, the minimum required collector area is estimated to be 50 m² for the ETC and 68.14 m² for the FPC. For these areas, the optimized size of the HWST is estimated at 1350 L. System configurations are modelled and simulated in TRNSYS for the entire year, from January 1st to December 31st, to investigate optimal collector tilt, the smallest collector area for maximum solar fraction, and solar collector thermal efficiency. Monthly solar collector efficiency is calculated for both configurations. The results of the simulation showed that C-II gives a comparatively higher solar collector thermal efficiency and solar fraction. For both collectors, the maximum seasonal solar fraction is obtained at an inclination of approximately 14°. A thermal efficiency of evacuated tube solar collector is comparatively higher for C-II than that of C-I and one observes the same trend for FPC. In addition, the thermal efficiency of the ETC at 50 m² is higher than that of the FPC at an area of 68.14 m².