Monday 23/6/2008 Page: 10
WIND power works, and will work better in the future. But wind is only an interim stop on the way to a world where electricity no longer relies on fossil fuels. The ultimate goal is to harvest the sun's energy directly by intercepting sunlight, rather than by waiting for that sunlight to stir up the atmosphere and sticking turbines in the resulting airstreams.
Fortunately, inventors love that sort of problem. Ideas they have come up with range from using the sun to run simple heating systems for buildings, deploying "reverse radiators" painted black, to the sharpest cutting edge of that trendiest of fields, nanotechnology, to ensure that every last photon is captured and converted into electricity. The most iconic form of solar power, the photovoltaic cell, is currently the fastest-growing type of alternative energy, increasing by 50% a year.
The price of the electricity it produces is falling, too. According to Cambridge Energy Research Associates (cERA), an American consultancy run by Daniel Yergin, a kWh of photovoltaic electricity cost 50 cents in 1995. That had fallen to 20 cents in 2005 and is still dropping. Not RE
These two very different approaches illustrate an unresolved question about the future of energy: whether it will be generated centrally and transported over long distances to the consumer, as it has been in recent decades, or generated and consumed in more or less the same place, as it was a century ago.
A hot tin roof
The idea of solar cells is to keep things local. They are like wind turbines, only more so, in that even a single solar panel can produce power immediately. Put a few on your roof and, if you live in a reasonably sunny place, you can cut your electricity bill. Indeed, you may be able to sell electricity back to your own power company. The problem is that at the moment you may need to take out an overdraft to pay for the solar panels, and you will not get your money back for a long time.
Many engineers, however, are working to change that. One of them is Emanuel Sachs of MIT. Some engineers look for big, exciting technological improvements in the way solar cells work, but Dr Sachs prefers incremental change. As he sees it, it is such change that drives Moore's law, that well-established description of the rapid improvement in the power of computer processors. Moreover, the analogy is appropriate. Traditional solar cells are made of silicon, like computer chips, and for the same reason.
They rely on that element's properties as a semiconductor, in which negatively charged electrons and positively charged "holes" move around and carry a current as they do so. In the case of a solar cell, the current is created by sunlight knocking electrons out of place and thus creating holes. Dr Sachs's first contribution to the incremental improvement was a technique called the string ribbon, which halved the amount of silicon needed to make a solar cell by drawing the element (in liquid form) out of a vat between two strings. That invention was marketed by a firm called Evergreen Solar.
His latest venture, a firm called 1366 Technologies (after the number of watts of solar power that strike an average square metre of the Earth's surface), aims to follow this up with three new ideas that should, in combination, bring about a 27% improvement in efficiency. He and his colleagues have redesigned the surfaces of the silicon crystals on a nanoscale in order to keep reflected light bouncing around inside a cell until it is eventually absorbed. They have also managed to do something similar to the silver wires that collect the current. And they have made the wires themselves thinner as well so that they do not block so much light in the first place.
Dr Sachs says that these innovations will bring the capital cost of solar cells below $2 a watt. That is closing in on the cost of a coal-fired power station: a gigawatt (one billion watt) plant costs about $1 billion to build. The price, of course, is a different matter. As Paula Mints of Navigant Consulting, a firm based in Palo Alto, California, points out, price is set by market conditions. These-particularly the generous subsidies given to solar power in some European countries-have kept prices well above costs in recent years. Nevertheless, as chart 4 shows, the price of solar cells has fallen significantly, too. Other researchers back a newer technology known as thin-film photovoltaics.
Thin-film cells can be made with silicon, but most progress is being made with ones that use mixtures of metals, sometimes exotic ones, as the semiconductor. These mixtures are not as efficient as traditional bulk-silicon cells (meaning that they do not convert as much sunlight to electricity per square metre of cell). But they use far less material, which makes them cheaper, and they can be laid down on flexible surfaces such as sheets of steel the thickness of a human hair, which gives them wider applications. At the moment, the commercial leader in this area is a firm called First Solar, which uses cadmium telluride as the film.
But First Solar is about to be given a run for its money by companies such as Miasole, a small Californian firm, that have gone for a mixture of copper, indium, gallium and selenium, known as CIGS. This mixture is reckoned to be more efficient than cadmium telluride, though still not as good as traditional silicon. And it has the public-relations advantage of not containing cadmium, a notorious poison-though First Solar's films carefully lock the cadmium up in a way that renders it harmless.
At the moment thin-film solar cells are being packaged and sold as standard solar panels, but that could easily change. First Solar applies its films to glass, but Miasole's boss, Joseph Laia, points out that his steel-based products are flexible and lightweight enough to be used as building materials in their own right. Greener-than-thou Californians who wish to fall in with their governor's plan for a million solar roofs, announced in 2006, currently have to bolt panels onto their houses-an ugly, if visible, show of their credentials. If Mr Laia has his way, they will soon be able to use sheets of his company's selenium, known as CIGS-covered steel as the roofing material itself.
Supporters of solar thermal energy tend to look askance at solar panels. Cadmium telluride and selenium, known as CIGS may be cheaper than silicon, but glass and steel, on which solar thermal relies, are cheaper still. The technology's proponents think big: square-kilometres big. They want to fill the deserts with steel and glass mirrors and use the reflected sunlight to boil water and generate electricity, then plug into the long distance Dc networks developed for wind power to carry the juice to the cities.
Desert song
Those who worry about the political side of the world's dependence on oil will be less than delighted to find that one country thinking seriously about such systems is Algeria. With the power-hungry markets of Europe to its north, across the Mediterranean, and a lot of sunshine going to waste in the Sahara desert to its south, Algeria's government is looking for ways to connect the two. It is now building an experimental solar thermal power station at Hassi R'mel, about 400km south of Algiers, which if all goes well will open next year.
In April work started on a similar project at Ain Beni Mathar, in Morocco, and others are in the pipeline elsewhere in north Africa. Fortunately for people like Mr Woolsey, the ex-cIA man, America has deserts of it own which are about to bloom with mirror-farms too.
There are four competing designs: parabolic- trough mirrors, parabolic-dish mirrors,"power towers" which use an array of mirrors to focus the sun's rays on to an elevated platform, and Fresnel systems, which mimic a parabolic trough using (cheaper) flat mirrors. All either heat up water to make steam, which drives a generator, or heat and liquefy a salt with a low melting point such as sodium nitrate that is used to make steam.
All four of these designs are now either operating commercially in the deserts of south-west America or are undergoing pre-commercial trials. Although the total capacity at the moment, according to CERA, is a mere 400 megawatts, this will grow tenfold over the next four years if all projects now scheduled come to fruition, and probably a lot more after that. Moreover, those plants that melt a salt are able to divert part of the heat they collect into a thermal reservoir that can keep the generators turning at night. The main objection to solar power-that it goes off after sunset- is thus overcome.
From little acorns
The engineers clearly think they can deliver the technology. But can the technology deliver the power? A back-of-the-envelope calculation suggests that it can. Two years ago a task force put together by the governors of America's western states identified 200 gigawatts-worth of prime sites for solar thermal power within their territory (meaning places that had enough reliable sunshine, were close to transmission lines and were not environmentally or politically sensitive). That is equivalent to 20% of America's existing electricity generation capacity: not a bad start.
Robert Fishman, the boss of Ausra, an Australian-American company based in Palo Alto, California, reckons that his firm's Fresnel arrays combined with its proprietary heat-storage system can produce electricity for 8 cents a kWh. That matches GE's wind turbines, and mass production should bring it down further. It is not cheaper than "naked" coal (Ausra will benefit from various state governments' requirements that their power utilities buy renewable power)-but if there were a carbon tax of $3o a tonne, or a requirement to capture and bury CO2, Ausra would be able to match the coal-fired stations' prices.
The most intriguing technology of all, though, belongs to Sungri, a firm based in Los Angeles. This uses mirrors to concentrate sunlight, but focuses it on a solar cell rather than a boiler. The system is said to turn 37% of the light into electricity. In April the firm claimed it would be able to produce electricity for the magic figure of 5 cents a kWh.
That claim has yet to be put to the test, and should be viewed with some scepticism until it has been. But it is a good indication of the way the field is going. solar power now seems to be roughly where wind was a decade ago. At the moment it contributes a mere 0.01% to the world's output of electricity, but just over a decade of 50% annual growth would bring that to 1%, which is where wind is at the moment.
If Sungri is to be believed, and the point where RE is indeed <>
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