Development of a small-size parabolic trough collector: design and experimental test

The thesis deals with the design, the realization and the experimental test of an innovative small size parabolic trough collector (mPTC) suitable to produce heat at medium temperature (100−250°C) with higher efficiency than the solar thermal collectors. Starting from the study of the state of the art, the design phase for the small parabolic trough collector has dealt with the numerical modelization of the parabolic collector from the optical, thermal and structural point of view. A termofluidodynamic FEM model of the receiver tube has been developed and a parametric analysis has been carried out to optimize the components of the receiver tube. Furthermore, the numerical model has allowed to obtain useful details for the realization of the prototype and for the design of the test rig such as the optimal mass flow and the rise in temperature along the collector. A structural finite element model has been realized in order to compute the thermal stress on the absorber tube. Following the indications of the numerical models the prototype of an innovative parabolic trough collector has been realized. The m-PTC collector is characterized by extremely small size since it has been designed to be suitable for the integration on the roofs of industrial environments where the space for installation of solar collectors is in general limited. An indoor test rig has been realized to test the thermal performances and to verify the good quality of the receiver tube. The test rig allows the measurement of the heat losses of receiver heating up the absorber tube through the Joule effect. In order to fully characterize the collector, a test rig for outdoor test has been designed. The test bench has been carefully projected in order to satisfy the requirements imposed by the standard test on solar concentrating collectors. The measurement instrumentation has been properly selected in order to minimize the uncertainty on the final variables to be obtained, the useful thermal power and the efficiency of the collector. Tests have been run for different inlet temperatures of the fluid and different conditions of irradiance. An accurate analysis of the measurement uncertainties has been conducted. The data have been fitted through a multiple linear regression based on weighted least squares obtaining the efficiency curve of the collector. The peak optical efficiency of the m-PTC has been estimated to be 69%. The predicted thermal efficiency at fluid temperature of 180°C, is 63% ±4%. The experimental results have been compared with the numerical outcomes of the termofluidodynamic FEM model that has been validated. The yearly performances of the m-PTC have been evaluated through dynamical simulations with TRNSYS. It has been compared the m-PTC with an evacuated collector and a linear Fresnel collector for four different locations and different levels of inlet temperature.