Design of the lay-out of a PTC solar field for a SHC plant at the Misericordia of Badia a Ripoli : A method for the optimization of the energy collection

Cooling demand is rapidly increasing in many parts of the world and the Solar cooling technologies, which use solar thermal energy provided by solar collectors to drive cooling machines can significantly contribute to a rising share of renewable heat sources in the building sector and a sustainable energy development in Europe. However, while larger solar cooling systems have been successfully demonstrated, smaller systems have not yet entered the market due to various technical and economical reasons. Until today, there is a lack of small scale units, fully automated and autonomous package-solutions for residential and light commercial or industrial applications, low temperature cooling systems. Within the FP7-ALONE project, financed by the EU, the Department of Energy “S.Stecco” is responsible for the installation at the Misericordia of Badia a Ripoli (Florence-Italy) of a small scale solar heating and cooling (SHC) plant which foresees the coupling of a parabolic trough collectors (PTC) solar field of about 108 m2 collecting area, with an ammonia-water (NH3-H2O) absorption chiller with a nominal cooling capacity of 17kW, driven by direct steam. The positioning of PTC and the design of the lay-out of the solar field are the key-point for the selection of the best positioning of the PTCs, taking into consideration the possibilities offered by the site for the installation of all components of the SHC System. A method for the optimization of the lay-out of a PTCs solar field is outlined in this paper by the definition of the “Solar Field Efficiency” (ηsf) as a function of the thermal efficiency of collectors, optical losses due to shading effects between rows and the collectors’ geometry: in fact, the solar incidence angle causes an optical loss along the longitudinal axis of the PTCs collector, as a function of the focal length and the diameter of the absorber tube. A TRNSYS model has been developed in order to estimate the “ηsf” and the seasonal energy yield. It implements the characteristic curves for a PTC collector and the evaluation of the longitudinal optical losses with the main data from the solar field: latitude, tracking axis orientation, number or rows, distance between rows, and number of collectors. The method have been tested using a reference area for the installation, then, it has been applied to the available areas at the Misericordia Site. Different lay-outs were analyzed and the seasonal energy yield were evaluated.