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Old Posted Oct 27, 2009, 1:50 PM
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Solar superpower: Should Europe run on Sahara sun?

http://www.newscientist.com/article/...ef=online-news




26 October 2009 by Fred Pearce

EVERY two weeks, the sun pours more energy onto the surface of our planet than we use from all sources in an entire year. It is an inexhaustible powerhouse that has remained largely untapped for human energy needs. That may soon change in a big way. If a consortium of German companies has its way, construction of the biggest solar project ever devised could soon begin in the Sahara desert. When completed, it would harvest energy from the sun shining over Africa and transform it into clean, green electricity for delivery to European homes and businesses.

Prospects for the project, called Desertec, have blossomed over the past year, and this month 20 major German corporations are expected to announce the formation of a consortium that will provide the €400 billion needed to build a raft of solar thermal power plants in north Africa. They include energy utilities giants E.ON and RWE, the engineering firm Siemens, the finance house Deutsche Bank and the insurance company Munich Re.

The current plan, outlined by the German Aerospace Centre (DLR) in a report to the federal government, envisages that the project will meet 15 per cent of Europe's electricity needs by 2050, with a peak output of 100 gigawatts - roughly equivalent to 100 coal-fired power stations. Preliminary designs in the German report show electricity reaching Europe via 20 high-voltage direct-current power lines, which will keep transmission losses below 10 per cent (New Scientist, 14 March, p 42). Trans-Mediterranean links will cross from Morocco to Spain across the Strait of Gibraltar; from Algeria to France via the Balearic islands; from Tunisia to Italy; from Libya to Greece; and from Egypt to Turkey via Cyprus.

Desertec would take its place in a wider European supergrid that conveys power generated from wind turbines in the North Sea, hydroelectric dams in Scandinavia, hot rocks in Iceland and biofuels in eastern Europe. Adding solar thermal capacity would help ensure a steady supply of green electricity.

But is this really the best use of such a colossal amount of money? Critics are lining up to point out the project's shortcomings. They say it could make Europe's energy supply a hostage to politically unstable countries; that Europe should not be exploiting Africa in this way; that it is a poor investment compared to covering Europe's roofs with photovoltaic (PV) solar panels; and that, while deserts have plenty of sun, they lack another less obvious but equally indispensable resource for a solar thermal power plant - water. Is Desertec really the model of future power generation, as its promoters would have us believe, or is it politically misconceived and a monumental waste of money?

Canned heat

Unlike PV panels, which convert sunlight directly into electricity, solar thermal electricity generation plants first trap solar energy in the form of heat, and use this heat to generate electricity just as a conventional power plant does. Solar thermal plants come in four main varieties. Three use mirrors that concentrate sunlight to heat oil, water or a molten salt, which is in turn used to generate steam that drives a turbine. The mirrors can take the form of parabolic troughs or an array of flat reflectors that redirect sunlight onto pipes suspended above them, heating the fluid that they contain. In the Mojave desert in California, an interlinked system of nine solar thermal plants which use trough mirrors has been generating up to 300 megawatts of electrical power for more than two decades.

Alternatively, a field of mirrors can focus sunlight onto a central ceramic heat absorber mounted on a tower. A prototype plant in Spain has 21,000 square metres of glass mirrors that heat the absorber to over 1000 °C and generate 1 megawatt. In the fourth type, a dish focuses heat on a Stirling engine which generates electricity by exploiting the expansion and contraction of a gas in a sealed piston chamber as it is heated and then allowed to cool (see diagram).

Solar thermal energy is now coming to the fore, as it proves itself to have several advantages over PV. Among these is its ability to produce electricity in power-station quantities, without the complex organisation that distributed generation entails. What's more, it can feed electricity into the grid at night as well as by day. This is done by storing the heated fluid in an insulated container and releasing it hours later when the energy is required. Storing energy from PV panels would require a new generation of high-capacity batteries - still a research project in its infancy for the scale needed. The clincher is cost. Building a power-station-scale solar thermal installation costs only a fraction of PV generators with the same output. As a result, an army of new solar thermal plants are being planned for the US, China, Australia and Israel.


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