Heat transfer analysis of low thermal conductivity solar energy absorbers

Abstract

Polymers have been proven to be high potential low-cost materials for the design and mass production not only for ordinary solar water heaters but also for very simple large size, modular solar collectors, suitable for easy erection of large solar heating plants. Their major drawback for solar–thermal conversion applications is their low thermal conductivity, which prohibits their use unless an appropriate absorber design is employed. The low thermal conductivity of polymers has imposed the need of a particular absorber design, which is basically composed of a pair of dark, closely spaced parallel plates at the top of which solar radiation is absorbed, forming a thin channel for the flow of the heat transfer fluid. The aim of the present work is to investigate the particular limitations of this polymer plate absorber design, for a wide range of collector loss and convective heat transfer coefficients between heat transfer fluid and absorber plate. The aim is also to calculate the particular collector efficiency factors and conditions under which the associated collector performance parameters should be modified to account for the finite absorber plate conductance. This conductance was proven to be another decisive absorber design parameter, improper selection of which may probably lead to strong deterioration of the collector efficiency.