Solar energy

Party time!

Dec 30th 2008
From Economist.com

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.

Greenhouse gases

Eating carbon

Nov 13th 2008
From The Economist print edition

There is a type of rock with a voracious appetite for carbon dioxide

ONE way of helping to reduce emissions of carbon dioxide into the atmosphere is to pump the gas into underground caverns or old oil fields. But there is also a rock that is happy to gobble it up, and according to the latest research its appetite for the greenhouse gas is not only massive but could also be increased by a little human intervention.

The rock is peridotite, which is one of the main rocks in the upper mantle, an area that provides a girth below the Earth’s crust. The rock occurs some 20km or more down, although in areas where plate tectonics have forced up some of the mantle, peridotite reaches the surface. This happens in part of the Omani desert which Peter Kelemen and Juerg Matter, both from Columbia University, New York, have studied for years.

Geologists have long known that when peridotite is exposed to the air it can react quickly with carbon dioxide to form carbonates like limestone or marble. Some people have looked at the idea of grinding up peridotite and using it to soak up emissions from power stations, but the process turns out to be expensive, partly because of the costs of transporting all the rock. The transportation would also create emissions. In Proceedings of the National Academy of Sciences, Messrs Kelemen and Matter suggest an alternative: pumping the gas from places where it is produced and into underground strata of peridotite.

The team has shown that the Omani peridotite absorbs tens of thousands of tonnes of carbon dioxide a year, far more than anyone had thought. By drilling and fracturing the rock they believe they can start a process to increase the absorption rate by 100,000 times or more. They estimate this would allow the Omani outcrop, which extends down some 5km, alone to absorb some 4 billion tonnes of carbon dioxide a year, which is a substantial part of the annual 30 billion or so tonnes of the gas that humans send into the atmosphere, mostly by burning fossil fuels.

With such rocks situated in an area of the world where an increasing amount of energy is produced and consumed, it potentially provides a convenient carbon sink for the region’s energy industry, say the researchers. Peridotite can also be found at the surface in other parts of the world, including some Pacific islands, along the coasts of Greece and Croatia, and in smaller deposits in America. Nor is it the only rock with carbon-eating potential. The researchers are now looking at volcanic basalt in a new project in Iceland.