The numerical study and optimization of combined laminar natural convection and surface radiation heat transfer in solar cavity receiver with plate fins is presented in this paper. Minimizing heat loss in cavity receivers is seen as an effective way to enhance the thermal performance and the use of plate fins has been proposed as a low cost means to minimize heat loss. Firstly, the influence of operating temperature, emissivity of the surface, orientation and the geometric parameters on the total heat loss from the receiver was investigated. It was observed that convective heat loss is largely affected by the angle of inclination of the receiver, the presence of fins and the number of fins in the receiver. As for the radiation heat loss it was observed that it is mainly influenced by the properties of the cavity receiver surface. The radiation heat loss was found to be constant at all the angles of the receiver. Significant reduction in natural convection heat loss from the cavity receiver was accomplished by using the plate fins whereas radiation heat loss was marginally reduced by about 5%. Secondly, the optimization was conducted to obtain the optimal fin geometry and lastly, the overall thermal efficiency of the receiver was presented at different operating temperatures. The overall cavity efficiency marginally increased by approximately 2% with the insertion of fin plates although the convective heat loss was suppressed by about 20%. This is due to the fact that radiation heat loss dominates at high operating temperatures compared to convective heat loss.
This study details the numerical modeling and optimization of natural convection heat suppression in a solar cavity receiver with plate fins. The use of plate fins attached to the inner aperture surface is presented as a possible low cost means of suppressing natural convection heat loss in a cavity receiver. In the first part of the study a three-dimensional numerical model that captures the heat transfer and flow processes in the cavity receiver is analyzed, and the possibilities of optimization were then established. The model is laminar in the range of Raleigh number, inclination angle, plate height and thickness considered. In the second part of the study, the geometric parameters considered were optimized using optimization program with search algorithm. The results indicate that significant reduction on the natural convection heat loss can be achieved from cavity receivers by using plate fins, and an optimal plate fins configuration exit for minimal natural convection heat loss for a given range of Raleigh number. Reduction of up to a maximum of 20% at 0 o receiver inclination was observed. The results obtained provide a novel approach for improving design of cavity receiver for optimal performance.
A parabolic dish/cavity receiver configuration is one of the solar thermal systems used for light-heat conversion at high temperature. Such systems are subject to continuous changes in ambient conditions such as wind, solar insolation and ambient temperature. These environmental variations, as well as changes in receiver inclination angle, affect the overall receiver performance leading to energy loss. Natural convection contributes a significant fraction of the energy loss and hence a thorough understanding of its characteristics is essential to effectively minimise it in order to improve the system efficiency. A three-dimensional numerical investigation was conducted on a modified cavity receiver to quantify the convective components of the total heat loss and to determine the effects of the operating temperature, receiver inclination angle and aperture size on the heat loss. The effects of the variation of air properties were accounted for by using polynomial relationships for density, specific heat capacity at constant pressure, dynamic viscosity and thermal conductivity in the simulation. The calculated natural convection heat loss showed a non-linear dependence on the inclination angle and aperture size. Visualisation results such as temperature contours were also presented to gain an insight into the effects of natural convection.
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