Theoretical and experimental evaluation of the working fluid temperature levels in a CPV/T system

Abstract

A linear focus Concentrator Photovoltaic and Thermal (CPV/T) system can match the thermal demands of a user. The evaluation of the cooling fluid temperature levels of a CPV/T system is fundamental to understand if this system is capable of satisfying the typical thermal requirements of a residential user (heating, cooling and domestic hot water). First, an experimental line-focus CPV/T system, realized in the Laboratory of Applied Thermodynamics of the University of Salerno (Italy), has allowed to determine the cooling fluid temperature at the CPV/T system outlet. Successively, the cooling fluid temperatures, experimentally obtained, have been compared with the same temperatures calculated by means of a theoretical model under the same operation conditions. A deviation in terms of the percentage relative error between theoretical and experimental results included between about 0.5% and 5%, has been found. The goodness of the theoretical-experimental comparison in terms of the temperature of the operation fluid at the CPV/T system outlet has represented a fundamental point to evaluate theoretically, by means of the TRNSYS software, the other levels of temperature of an integrated system, constituted by CPV/T system, thermal tank and user, for different temporal scenarios (hourly, weekly, monthly and yearly). The input data of the TRNSYS model are: Direct Normal Irradiance (DNI), Triple-Junction (TJ) cell temperature and environmental conditions. A tank model is also adopted to satisfy the thermal energy demand peaks, and the temperature stratification in the tank linked to the CPV/T system, as function of the height, is obtained in winter and summer. It is important to define these thermal levels to verify if a CPV/T system is capable to satisfy the residential user energy demands or a thermal energy integration is necessary in some periods of the year. A good stratification has been noted in the summer season, with temperature values that are variable between about 40 and 90 °C. From April to October, the tank average temperature is generally resulted about 10 °C higher than the temperature required by the fluid sent to the residential user, and a very low integration is then necessary. It has been verified that the CPV/T system covers a large part of the thermal energy needs of the residential user during the year; the coverage is limited only in the winter months.