Abstract
In the present paper, the use of radiating gas instead of air inside the cavity of compound parabolic collectors (CPSs) is suggested and verified by numerical analysis. The collector under study has a simple cone shape with flat absorber which is filled with a participating gas such as carbon dioxide instead of air for the purpose of increasing the thermal performance. In numerical simulation, the continuity, momentum and energy equations for the steady natural convection laminar gas flow in the CPC’s cavity and the conduction equation for glass cover and absorber plate were solved by the finite element method (FEM) using the COMSOL multi-physics. Because of the radiative term in the gas energy equation, the intensity of radiation in participating gas flow should be computed. Toward this end, the radiative transfer equation (RTE) was solved by the discrete ordinate method (DOM), considering both diffuse and collimated radiations. The approximation was employed in calculation of the diffuse part of radiation. It was observed that the gas radiation causes high temperature with more uniform distribution inside the cavity of collector. Also, numerical results reveal more than 3% increase in the rate of heat transfer from absorber surface into working fluid and hence a desired performance for the collector because of the gas radiation effect. Comparison between the present numerical results with theoretical and experimental data reported in the literature showed good consistency.
Highlights
Today, using renewable energies has attracted a great number of researchers; because the prediction for global population and their energy demand is important in near future
The effect of gas radiation in enhancing heat transfer in plane solar air heaters was investigated [13, 14]. To materialize this aim in Compound Parabolic Concentrators (CPCs), the current study introduces the use of radiating filling gas such as carbon dioxide, water vapor and N2O instead of air inside the cavity of compound parabolic collector
The central zone of the CPC cavity is in relatively low temperature and increasing in gas temperature can be seen as we moves toward the absorber and side walls which are under the incidence of Sun heat flux
Summary
Today, using renewable energies has attracted a great number of researchers; because the prediction for global population and their energy demand is important in near future. Even with the current rate of energy consumption, there is a deep gap between total demand and provided renewable energy sources. On this subject, the use of solar energy for many engineering applications such as air and water heaters has a major role. Among the different types of solar concentrators, Compound Parabolic Concentrators (CPCs) have attracted a large attention. This type of solar collectors has naturally a low heat loss characteristic property. This property makes it more efficient than other common collectors; so it is a good design for medium and high temperature applications. Since CPCs are designed as stationary collectors and are capable to provide a wide range of temperature from (T < 100 oC) to (T > 250oC); they are cost effective and capable of being used in many thermal systems such as solar desalination [1]
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