Researchers in the University of Pretoria’s (UP) Department of Mechanical and Aeronautical Engineering have surprisingly needed to use the moon to help in their goals of improving solar energy. The researchers form part of a collaborative project called Solar Turbo Cogeneration Heat and Power that plans to commercialise small-scale hybrid concentrating solar power systems that use concentrated solar power to generate electricity and process heat for commercial use.
“Concentrating solar power systems do this by concentrating the sun’s rays onto a small solar receiver which can then, for example, heat air to be used in electricity generation through electric turbines, or direct heating of spaces and processes,” 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.
A solar concentrating system being tested at night, comprising of multiple dishes concentrating the moon's rays onto a collector.
But while worldwide solar capacity is growing every year, only about 1% of the worldwide solar capacity is concentrated solar power, as these systems are relatively expensive compared to other renewable energy sources, such as electric solar photovoltaic panels, explained Roosendaal. “One of the most important issues faced today with regard to solar concentrators is the trade-off between cost and optical accuracy. One of the reasons concentrating solar power systems are still relatively expensive is the optical systems used to concentrate the sun’s rays. To achieve any kind of meaningful concentration efficiency, high-accuracy low-cost solar concentrators need to be developed.”
The solar power concentrating system, with numerous small dishes concentrating the sun's rays onto a central collector.
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 side, 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. This allows any incoming sun to be concentrated into a small focal point, which can then be used to heat water to create steam and generate electricity,” said Roosendaal.
The satellite dishes covered in reflective mirror membranes that can be adjusted through a vacuum to focus sun (or moon) rays onto a central collector.
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 moon rays on the solar collector. The moon’s rays have many properties in common with the sun, as the moon’s rays are made up of a similar spectrum as the sun and approach the earth with almost the same angular diameter which is critical for approximating the sun. But because they are dimmer, the concentrated moon rays are not too bright for a normal camera to record.
The researchers at night, with the system concentrating the dimmer moon's rays onto a collector surface.
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.