Researchers at the University of Pretoria’s (UP) Department of Mechanical and Aeronautical Engineering have made use of the moon’s rays to assist them in their goal to improve solar energy.

The researchers form part of a collaborative project called Solar Turbo Cogeneration Heat and Power, which plans to commercialise small-scale hybrid systems that use concentrated solar power to generate electricity and process heat for commercial use. 

“Concentrating solar power systems do this by focusing the sun’s rays onto a small solar receiver that can then heat air to be used in electricity generation through electric turbines, for example,” said researcher Casey Roosendaal who co-wrote an article detailing the results of the project along with his colleagues Dr Willem le Roux and Jonathan Swanepoel.

But while worldwide solar capacity is growing every year, only about 1% of this capacity is concentrated solar power, as these systems are relatively expensive compared to other renewable energy sources such as electric solar photovoltaic panels, Roosendaal explained. “One of the most important issues faced today with regard to solar concentrators is the trade-off between cost and optical accuracy. One thing that makes concentrating solar power systems still relatively expensive is the optical systems used to concentrate the sun’s rays. High-accuracy, low-cost solar concentrators need to be developed to achieve any kind of meaningful concentration efficiency.”

To address this issue, investigations into vacuum membrane solar concentrating dishes with a novel design were done at UP. The design aims to reduce the high construction costs of solar concentrators by using low-cost, off-the-shelf satellite dishes (commonly known as “DStv dishes” in South Africa), with a focus on small-scale systems. The satellite dish is fitted with a reflective polymer membrane that has a thin layer of vapour-deposited aluminium or silver on the front, creating a reflective mirror surface.

“Using a very slight vacuum between the dish and the membrane, these mirror membranes can be drawn in to form an approximate parabolic surface,” Roosendaal said. “This allows any incoming sun rays to be concentrated into a small focal point, which can then be used to heat water to create steam and generate electricity.”

The system was tested using a unique approach by substituting the sun’s rays for those of the moon, which, because they are much dimmer, allowed a normal camera to be used to photograph the concentrated rays on the solar collector. The moon’s rays have many properties in common with the sun – they are made up of a similar spectrum and approach Earth with almost the same angular diameter, which is critical for approximating the sun.

These tests showed that, even when individual dishes are misaligned, up to 88% of the light on the dish was focused into the receiver. These results show promise for further development, which would yield cost-effective, high-performance optical systems for concentrated solar applications, with a larger dish already being developed for commercial operation.