Electrical production of a small size Concentrated Solar Power plant with compound parabolic collectors

The use of the solar energy for electricity or useful heat generation has been extensively investigated as an alternative to fossil fired energy conversion. Particularly in the last decade, many studies have been carried out on Concentrated Solar Power (CSP) which was developed worldwide with Spain acting as the leading country in this field. Concentrating solar energy requires complex mirror systems which continuously move to track the sun. In comparison with flat mirrors, Parabolic Through Collectors (PTCs) have allowed to reduce costs, but they still remain quite an expensive solution. Instead, compound parabolic collectors (CPCs) are able to collect a higher fraction of both the direct and the diffuse radiation, although they have a lower efficiency at high temperature. Moreover, at least within certain limits, they do not require a tracking system. Their employment is therefore suited for the collection of medium temperature heat (up to 200°C) and is useful for the reduction of the installation cost of Concentrated Solar Power (CSP) heating/cooling and energy generation systems. Small size plants (10-50kW) were studied in this paper since they are more likely to be realized due to their smaller initial investment cost and to the capability of being installed on the roof of existing buildings. While the Organic Rankine Cycle (ORC) solution is well established to be the optimal for small size, distributed generation plants, the technology of the expansion device is still to be defined for the investigated installed power range. Accordingly to previous studies, an expansion device based on the Wankel mechanism was employed. Based on these considerations and prior to more detailed analyses, a study of the annual energy production of a small scale ORC power plant using CPCs as a heat source and a volumetric machine as an expansion device was carried out. The influence of the thermodynamic cycle parameters, the working fluid, the concentration and the tilt angle of the collectors on the electrical energy production were taken into account. The thermal module power output, the expansion device isentropic efficiency and the overall efficiency were evaluated by means of a numerical model developed within the simulation tool AMESim v.12.0. The aim of this work is to provide a contribution in the assessment of the optimal configuration of such kind of plants in terms of collectors concentration and tilt angle on one hand, and thermodynamic parameter of the thermal module on the other. The annual electricity production was used as a criterion of comparison among the various parameters combinations. The number of operating hours per year was also taken into account for the sake of ensuring a regular production of energy. A selection of commercial solar tubes for the realization of the solar field was carried out and the optimal configuration for both the solar field and the thermal module was found. The results of this study are encouraging and constitute the basis for the development of future analyses.