An astronomer famous for designing some of the largest telescope mirrors on the planet now has another trick up his sleeve: he wants to build highly efficient solar collectors that produce electricity on the cheap. An Arizona company he founded, REhnu, claims the new design could drive the price of a solar power farm down to $1 per watt by 2020.
Two decades ago, Roger Angel cast a honeycomb-structure mirror 6.5 metres in diameter at the University of Arizona, using a unique spin-casting technique that rotated a furnace several times a minute, so the molten glass inside formed a parabola close to the mirror's final shape. Five years later, he scaled up to 8.4 metres, and the age of giant Earth-based telescopes was in full swing. Today, four 8.2-metre mirrors designed in this way are used in the Very Large Telescope in Chile. Angel has also proposed making ultra-thin mirrors that could serve as a sunshade to cool the planet.
So far, most solar power farms have used large panels of low-cost photovoltaic cells which reach 10 to 15 per cent efficiency when facing the sun directly. Some solar cells have exceeded 40 per cent efficiency by using multiple layers to capture energy across the solar spectrum, but they are too expensive to use over large areas. REhnu takes a different approach, using mirrors to focus sunlight onto small areas of costly multi-layer solar cells.
Others have tried solar collectors, but have had a hard time keeping costs down. REhnu gets around this by making its mirrors out of thin sheets of cheap float glass, the stuff used in windows. Heating softens the sheets so they slump into a parabolic mold, and once cooled they get a reflective metal coating. A prototype uses a mirror 3.1 metres wide that focuses the sunlight onto a spherical glass receiver which distributes the sunlight to an array of high-efficiency three-layer solar cells.
REhnu is now building a 20-kilowatt prototype with a lightweight frame to track the sun on an test site in the Arizona desert. The developers claim they can get the capital cost down to $1/watt of generation capacity by 2020, and estimate that a solar farm producing a gigawatt of electricity would cover about 15 square kilometres.