Numerical Modeling and Experimental Validation of Heat Transfer Characteristics in Small PTCs with Nonevacuated Receivers

The development of small-sized parabolic trough collectors (PTCs) for processing heat production at medium temperatures (100–250 °C) represents an interesting approach to increase the utilization of solar thermal technologies in industrial applications. Thus, the development of simplified models to analyze and predict their performance under different operative and climatic conditions is crucial for evaluating the application potential of this low-cost technology. In this paper, we present a numerical method that by combining three-dimensional finite element simulations (implemented with COMSOL Multiphysics software version 6.1) with a one-dimensional analysis (based on a MATLAB script) allows for the theoretical determination of the power output of a small-PTC with a nonevacuated tubular receiver operating at a medium temperature. The finite element model considers both the nonuniformity of the concentrated solar flux on the receiver tube (evaluated using Monte Carlo ray-tracing analysis) and the establishment of natural convection in the air gap between the glass envelope and absorber tube. The model calculates, for several values of direct normal irradiance (DNI) and inlet temperatures, the thermal power transferred to the heat transfer fluid (HTF) per unit length. The data are fitted using the multiple linear regression method, obtaining a function that is then used in a one-dimensional multi-nodal model to estimate the temperatures and the heat gains along the receiver tube. The outputs of the model are the outlet temperature and the total thermal power transferred to the HTF. In order to validate the developed methodology for the assessment of the heat transfer characteristics in the small-PTC with a nonevacuated receiver, an experiment at the ENEA Trisaia—Solar Thermal Collector Testing Laboratory was carried out. This work compares the theoretical data with those acquired through experimentation, obtaining a good agreement, with maximum differences of 0.2% and 3.6% for the outlet temperatures and the power outputs, respectively.