Facilites @ the Solar Energy Group

The Solar Energy Group is in the final stages of characterising a high flux Fresnel parabolic solar concentrator that will be used as a platform for the experimental evaluation of devices and surfaces under realistic conditions.

The test facility is a 2-axis tracking parabolic Fresnel concentrator. A special consideration in this project has been the particular application of the concentrator as a test facility for solar energy conversion materials and devices. Rather than trying to maximize the total solar energy collected at the receiver, the design process has focussed on the requirements of a well defined source of high-intensity illumination and sufficient flexibility in test parameters. The concentrator is comprised of 6 spherical mirrors (expandable to 18), each with an aperture area of 0.20 m2, giving an aperture area of 1.2 m2 (maximum of 3.6 m2).

The mirrors have a radius of curvature of about 4.8 m and a rim angle of 7◦ (as seen from the focal point). The mirrors are bonded structures comprising an anodised aluminium reflective surface mounted on a lightweight aluminium honeycomb structure, enclosed by aluminium sheets at the back and along the sides. A hard oxide layer (approximately 2 μm) protects the softer aluminium reflecting surface in the outdoor environment and reduces the deterioration of the reflecting surfaces.. The mirrors are attached to a 3-metre parabolic dish frame, and the focal length of the system is 2.4 m. The placement of the mirrors is 6 on an inner ring, expandable to 12 on an outer ring.

By positioning the absorber plane about the focus, fluxes ranging from 200-1000 suns are available. A chiller is employed to maintain a constant temperature for devices at the focus between 5-80 oC to an accuracy of 1 K. The concentrator facility also has a full range of weather and solar monitoring and video characterisation equipment to measure the fluxes and solar conditions in real time.

Testing program

1. Further characterisation: The first priority of the experimental facility is to refine the theoretical models simulating both the flux and the PV performance to get greater correlation with observed data under real conditions. While a strong correlation already exists, to optimise a system of commercial modules the greater the performance gained the greater the results.
2. Homogenous flux testing One method to obtain a uniform flux distribution is to shape the receiver in such a way that an even flux will fall on its surface [16]. Absorbers will be designed for the University of Sydney’s solar concentrator and the surface will be tested for even illumination.
3. High performance cooling Thermal management of the absorbers forms the operating basis for efficient solar thermal collectors and is an important issue for PV systems. The University of Sydney is applying its considerable experience in heat transfer simulation and measurement of thermal absorbers to the problem of cooling PV under concentrated sunlight. Active and passive cooling mechanisms are being examined.
4. Performance and degradation of selective surfaces The concentrator gives the capability of undertaking high solar flux degradation testing of candidate selective surfaces for which it is important to separate thermal and solar flux degradation mechanisms. Although thermal degradation is routinely examined in laboratory ovens, there are little data in the literature on the prolonged response to high solar fluxes.
5. Performance and degradation of photovoltaic cells Testing solar cells under high concentration is more complex than under low concentration. Accelerated degradation tests reported in the literature involve either intense light illumination or forward biasing of the cell. In this project, the performance will be measured under high concentrations as a function of temperature and flux. The project will test a range of PV-cells. To reduce reflection losses, common concentrating PV-cells have a patterned surface consisting of an array of fine inverted pyramids. The optical performance of the cell is very sensitive to the shape of the pyramids and the angle of the incident light. A variable angle device will be built to allow the performance of the cells to be examined for a range of incidence angles.
6. Performance of beam splitter materials The project will be testing a specially constructed beam splitter designed by the University of Sydney and manufactured by Avtronics Ltd. of Sydney. The beam splitter characteristics and performance will be examined using techniques as for the investigation of selective surfaces discussed above.