Of course keeping it directed at the sun may well be the Achilles heel of this design. Another toss off brainstorm that then needs a stroke of genius to make it practical. The trick is to have the focal point oriented at the sun and able to adjust continually.
Dec 30th 2008
Metallised balloons may be the best way to make solar electricity
Cool Earth SolarBoffins preparing to make light
SOLAR cells are expensive, so it makes sense to use them efficiently. One way of doing so is to concentrate sunlight onto them. That means a smaller area of cell can be used to convert a given amount of light into electricity. This, though, brings another cost—that of the mirrors needed to do the concentrating. Traditionally, these have been large pieces of polished metal, steered by electric motors to keep the sun’s rays focused on the cell. However, Cool Earth Solar of Livermore, California, has come up with what it hopes will be a better, cheaper alternative: balloons.
Anyone who has children will be familiar with aluminised party balloons. Such balloons are made from metal-coated plastic. Cool Earth’s insight was that if you coat only one half of a balloon, leaving the other transparent, the inner surface of the coated half will act as a concave mirror. Put a solar cell at the focus of that mirror and you have an inexpensive solar-energy collector.
Cool Earth’s balloons are rather larger than traditional party balloons, having a diameter of about two-and-a-half metres (eight feet), but otherwise they look quite similar. The solar cell apart, they are ridiculously cheap: the kilogram of plastic from which each balloon is made costs about $2. The cell, whose cost is a more closely guarded secret, is 15-20cm across and is water-cooled. That is necessary because the balloon concentrates sunlight up to 400 times, and without this cooling it would quickly burn out.Like a more conventional mirror, a solar balloon of this sort will have to be turned to face the sun as it moves through the sky, and Cool Earth is now testing various ways of doing this. However, the focus of the light on the solar cell can also be fine-tuned by changing the air-pressure within the balloon, and thus the curvature of the mirror.
The result, according to Rob Lamkin, Cool Earth’s boss, is a device whose installation costs only $1 per watt of generating capacity. That is about the same as a large coal-fired power station. Of course, balloons do not last as long as conventional power stations (each is estimated to have a working life of about a year). On the other hand, the fuel is free. When all the sums are done, Mr Lamkin reckons the firm will be able to sell electricity to California’s grid for 11 cents a kilowatt-hour, the state’s target price for renewable energy, and still turn a tidy profit.That belief will soon be put to the test. Cool Earth plans to open a 1-megawatt plant this summer. If it works, more will follow and, in the deserts of California and elsewhere, it will be party time for solar-energy enthusiasts.