Modeling of a novel solar down beam test facility utilizing Newtonian optics
Creators: name: Hoffmann, Ryan J.
Publisher: name: University of Nevada, Las Vegas
Year: 2011

description: As advances in concentrated solar energy progress there will inevitably be an increase in the demand of resources for testing new conceptions. Currently, there are limited facilities available for taking concentrated solar energy concepts from the laboratory bench scale to the engineering test scale. A proposed solution is a scientific and developmental facility that provides highly concentrated solar energy at ground level. The design presented is a solar down beam test facility utilizing a Newtonian optics approach with a flat rectangular down beam mirror to reflect and concentrate the sun 's rays at ground level. 
Literature review suggests a hyperbolic reflector implementation for down beam reflector systems. An alternative Newtonian design uses a planar mirror to direct converging light from the heliostat field to a convenient focus at ground level. A flat mirror has the advantage of relatively cheap construction, and the intrinsic capability to host multiple experiments. Additionally, in comparison to a convex down beam mirror, there is significantly less optical distortion of the solar energy collected on a solar receiver at the top of a tower. The Newtonian design inherently preserves the focus of the heliostat field, mimicking the behavior of tower top geometry. The planar mirror geometry simply reflects the rays to ground level, and thus has little effect on the achievable concentration ratio other than the additional reflective losses. 
The presented work focuses on the optical analysis and modeling of such a novel test facility. System examination was accomplished utilizing the commercially available ray tracing software Advanced Systems Analysis Program (ASAP<sup>®</sup> ). ASAP<sup>®</sup> is able to simulate the interaction of the sun 's rays with the designed optical system by utilizing Monte Carlo ray tracing techniques. This allowed for all optical losses to be simulated within the model; including cosine, blocking, shadowing, attenuation, and reflective losses. 
In order to achieve a high level of concentration with a planar down beam reflector, converging heliostats were utilized. Parallel rays were implemented to identify the focusing conditions of the heliostats. A realistic sun source was developed within ASAP <sup>®</sup> to simulate the maximum 0.5° angular divergence characteristics of the sun 's rays. This allowed for practical concentration ratios to be simulated within the model. Algorithms were developed and implemented in MATLAB<sup>®</sup> to properly simulate the sun 's position as well as the heliostat field layout and orientation for any given hour of the year. A Sun Position Orientation (SPO) algorithm was constructed to determine the relative location of the sun source for any given hour within the ASAP <sup>®</sup> model. A Heliostat Reflection Orientation Position Vector (HROPV) algorithm was developed to properly orientate each heliostat to reflect to the proper aim point (AP) of the heliostat filed. Additionally, a North-South Cornfield Heliostat Field Layout (NSCHFL) algorithm was established to tightly pack heliostats within the utilizable range of the down beam reflector. 
Utilizing a tightly packed North-South (N-S) cornfield heliostat field layout the optical performance of the facility was demonstrated for a day in each of the four seasons, providing a general performance trend over the course of the year. Extensive ray tracing establishes the achievability of the targeted 1MW, 1,000 suns concentration at ground level for several hours of the day over the course of the year. Furthermore, the simulation results demonstrate nearly 2 MW and 4,000 suns concentration is achievable for optimal orientation hours in which the tower, sun, and center of the heliostat field are all co-linear. 
Future system analysis should examine modifications of optical geometry parameters such as heliostat dimensions, heliostat field layouts, and down beam reflector heights as well as orientations. Additionally, the effect of varying focal points needs to be investigated. The implications of heliostat pointing inaccuracies should also be accounted for.