Optical Analysis of a Window for Solar Receivers Using the Monte Carlo Ray Trace Method

Concentrated solar power (CSP) systems use heliostats to concentrate solar radiation in order to produce heat, which drives a turbine to generate electricity. We, the Combustion and Solar Energy Laboratory at San Diego State University, are developing a new type of receiver for power tower CSP plants based on volumetric absorption by a gas-particle suspension. The radiation enters the pressurized receiver through a window, which must sustain the thermal loads from the concentrated solar flux and infrared reradiation from inside the receiver. The window is curved in a dome shape to withstand the pressure within the receiver and help minimize the stresses caused by thermal loading. It is highly important to estimate how much radiation goes through the window into the receiver and the spatial and directional distribution of the radiation. These factors play an important role in the efficiency of the receiver as well as window survivability. Concentrated solar flux was calculated with a computer code called MIRVAL from Sandia National Laboratory which uses the Monte Carlo Ray Trace (MCRT) method. The computer code is capable of taking the day of the year and time of day into account, which causes a variation in the flux. Knowing the concentrated solar flux, it is possible to calculate the solar radiation through the window and the thermal loading on the window from the short wavelength solar radiation. The MIRVAL code as originally written did not account for spectral variations, but we have added that capability. Optical properties of the window such as the transmissivity, absorptivity, and reflectivity need to be known in order to trace the rays at the window. A separate computer code was developed to calculate the optical properties depending on the incident angle and the wavelength of the incident radiation by using data for the absorptive index and index of refraction for the window (quartz) from other studies and vendor information. This method accounts for regions where the window is partially transparent and internal absorption can occur. A third code was developed using the MCRT method and coupled with both codes mentioned above to calculate the thermal load on the window and the solar radiation that enters the receiver. Thermal load was calculated from energy absorbed at various points throughout the window. In our study, window shapes from flat to concave hemispherical, as well as a novel concave ellipsoidal window are considered, including the effect of day of the year and time of the day.