A Flexible Thermal Model for Solar Cavity Receivers Using Analytical View Factors

Abstract

Abstract This paper presents advances to a thermal model for a cavity-type receiver that will be integrated into NREL’s System Advisor Model (SAM) software. Traditional concentrated solar power towers make use of an external cylindrical receiver where all active surfaces are fully exposed to the environment, resulting in significant convective and radiative losses. Cavity-type receivers promise to mitigate these losses by instead accepting solar flux through an aperture. In order to allow detailed resolution of the temperature distribution across the cavity, it is necessary to create refined meshes for different cavity geometries and determine the view factor accurately and quickly between any two elements in the mesh. To accomplish this, an analytical function is written to precisely calculate view factors between arbitrary planar polygons without requiring the use of computationally expensive Monte Carlo ray tracing. These view factors are modified using the F-hat method and used as the basis for a two-band radiation heat transfer model. Heat transfer fluid routing is handled through an elemental connectivity matrix, which specifies the elemental fluid temperature variation from inlet to outlet and allows the cavity mesh to interact with the fluid elements. The model is solved iteratively for panel and then fluid temperatures in order to account simultaneously for all energy transfers (convective, long wavelength, short wavelength, and fluid). This approach offers a computationally efficient but still detailed simulation of cavity receiver configurations making it suitable for use in an annual-hourly time series simulation tool such as SAM.