Modeling of Beam Down Solar Concentrator and Final Optical Element Design

Abstract

The Beam-Down Optical Experiment (BDOE) is a unique Concentrated Solar Power (CSP) pilot project currently operated by Masdar Institute and located at the Masdar Institute Solar Platform at 25.44204 N 54.61677 E in Abu Dhabi. The plant is a point focusing Fresnel system of 33 alt-az tracking heliostats which uses a second complement of fixed tower-mounted mirrors to redirect the solar radiation back down towards the ground, hence the name "Beam Down". The benefits of this plant potentially include a more thermally efficient solar receiver located at ground level for easier operations and maintenance and decreased parasitic energy consumption and thermal and fluid pumping losses. To this end the Masdar Institute/MIT collaborative project Concentrated Solar Power on Demand Demonstration (CSPonD Demo), is developing a direct absorption molten salt solar receiver combined to the thermal energy storage system. This thesis was part of this project. The main objective of the thesis was the investigation of a final optical element (FOE) design that, coupled with the BDOE, provides high concentration and optical efficiency for the CSPonD and various other types of receiver. The first step in FOE design is developing an accurate, flexible, and well documented model of existing BDOE optics. A Monte Carlo raytracing model of the BDOE has been developed using a structured model configured, where appropriate, with the aid of scripts that produce Scheme descriptions of optical components in precise locations and orientations. Optical losses by cosine factor, blocking, shading, central reflector spillage and absorption and optical imperfections of the heliostat and central reflector facets are characterized. It is shown that for a molten salt direct absorption receiver a conical final optical element (FOE) is slightly better than a compound parabolic concentrator and that a faceted cone is almost as good. The reasons for these unexpected result are: 1) The optical configuration of the Masdar BDOE provides a nearly radial and somewhat converging angular distribution at the FOE inlet. 2) Better angular distribution incident on the molten salt surface (FOE outlet), and also a uniform distribution at the outlet of the cone. The property of uniform FOE outlet flux helps to prevent hot spots that can cause molten salt degradation. CPC and conical surfaces are reasonable but not necessarily optimal profile choices for the FOE design. A near optimal profile-of-revolution FOE that is better than either the CPC or the cone is developed for the MI-MIT’s CSPonD Demo molten salt receiver.

Date of Award	Aug 2015
Original language	American English
Supervisor	Peter Armstrong (Supervisor)