Damien Buie[13], David Mills, Anne Gerd Imenes, Solar Energy Group, School of Physics, Building A28, University of Sydney, Australia 2006, and Philipp Schramek, Muhlbergstr.26, 82319 Starnberg, Germany

This paper presents a methodology to create a receiver surface in a concentrating system that will be illuminated with an homogenous solar radiation flux. By means of a theoretical simulation, a receiver surface in a paraboloidal dish concentrator was generated where variations in the illumination across the receiver surface was not greater than ±5%. The optical efficiency of the generated surface however, was not ideal, intercepting only 74% of the total reflected insolation. This poorer than ex­pected optical efficiency can be overcome with a more robust mathematical model for generating the receiver surface.

Introduction

Homogeneous flux distributions in the focal region of concentrating solar power systems greatly improve both the performance and longevity of solar receiver modules. Photo­voltaic (PV) receivers for concentrating systems have limitations on both the thermal load and solar fluxes incident on their surface. Where the solar illumination on the PV cell’s surface is non-uniform the problem is exacerbated as it causes significant local heating and higher ohmic drops across the cell, the result of which is a decrease in efficiency [1,2, 3]. These local hotspots can also permanently damage expensive high concen­tration PV cells.

The flux distribution incident on a thermal absorbers can, in some cases, also benefit from an homogenous illumination profile. Hot spots can cause rapid degradation in se­lective surface coatings on thermal receivers and the effect of extreme fluxes within vol­umetric receivers are still being researched [4]. It is necessary to point out however, the performance gain achieved by even illumination over thermal receivers is of far less im­portance than the requirement for even illumination over photovoltaic receivers.

There exists a large number of methods to create an homogenous flux incident onto a re­ceiver, each with its own advantages and limitations. Ries [5, 6] among others described methods to directly tailor optical surfaces such that a flux distribution can be created in an imaging plane. Ries also illustrated that the shape of the flux distribution could be defined by correctly designing either the reflector or lens. This method is ideal for applications with a static illumination field, minimising optical losses by using only one concentration surface such as in trough or dish concentrators.

A second method of creating homogenous flux distributions, is to use secondary opti­cal components between the concentrator and the receiver modules, such as compound parabolic concentrators (CPC) or kaleidoscopic distributors [7]. This methods creates the desired flux distribution at the cost of optical efficiency, which is reduced due to the ad­ditional optical components in the system. These systems do however, have the advan­tage of a possible use in dynamic solar applications such as power towers, where the sun moves relative to the optical components. Combinations of both designing the concentrat­
ing component and secondary concentrators have also been successfully demonstrated [8] adopting both the pros and cons of each system.

Finally, homogenous fluxes have been generated by the micro-alignment of individual heliostat’s aiming points in large Fresnel concentrators [4]. This type of system is ideal in terms of optical considerations for large concentrators as it introduces no extra opti­cal surface. The system does however have the inherent problem of dealing with both the large amounts of residual errors that exist in the accuracy of all of the optical com­ponents, from the tracking accuracy of the heliostat to the calculated position of the sun, and the computer power required to optimise the flux distribution in real time. In spite of this, an homogenous flux distribution on a planar surface has been successfully demon­strated at the Plataforma Solar de Almeria (PSA-CIEMAT) [9].

An alternative approach to the above mentioned methods is to shape the receiver so that an even flux will fall on its surface [10]. By investigating the solar fluxes about the focal region and using a detailed optical model, this paper describes one method to create such surfaces for a single theoretical application. For this particular paper we chose to design an evenly illuminated surface for a simple case: a paraboloidal dish concentrator with an aperture and focal length of 3 m.