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Renewable energy: Electric dreams

08 October 2008

From New Scientist Print Edition.

Helen Knight

This special issue of the New Scientist about renewable energy contains two sections about things related to the Desertec concept.

Sunny side up

There's much more to solar power than photovoltaic cells

Photovoltaic cells are currently the fastest growing energy technology, with production increasing by around 48 per cent each year. By 2015 the price of electricity from PV cells is expected to match that of conventional energy generators. (For a detailed account of the state of the art in photovoltaic cells see New Scientist, 8 December 2007, p 32.)

But photovoltaic cells aren't the only way to capture the power of the sun. Large-scale concentrating solar power (CSP) systems are all the rage in the energy-hungry US. Last June, the 64-megawatt (MW) Nevada Solar One CSP plant switched on near Boulder City. Since then, over 1.6 gigawatts of new CSP capacity have been announced in neighbouring California.

In the past year or so, the US Bureau of Land Management has received more than 30 planning requests to develop large-scale CSP plants across the US. The situation is similar in Europe, where around a dozen plants are under construction with at least 24 more proposed in Spain alone.

Sound economics lie behind this enthusiasm. At the moment, electricity generated by large-scale solar concentrator systems costs around 12 US cents per kilowatt-hour. Though this is around four times the price of electricity from a coal-fired power station, it's half the price of electricity produced by photovoltaic cells. What's more, this technology offers an advantage that could prove decisive in the longer term: the ability to store energy for hours or days at a stretch.

Rather than converting sunlight directly into electricity, a CSP system uses arrays of mirrors to focus sunlight onto tubes filled with water or oil. The fluid is heated under pressure to around 400 °C and is then circulated to a steam turbine to generate electricity. By replacing the water or oil with molten salts, typically a mix of sodium nitrate (NaNO3) and potassium nitrate (KNO3), and storing this hot mixture in insulated tanks, it is possible to use energy collected during daylight hours to generate electricity at times of peak demand - day or night.

"With this system you can make electricity when you want," says Massimo Falchetta, an engineer at the Italian National Agency for New Technologies, Energy and the Environment, in Rome. That means the energy can be sold for a higher price than stuff from wind generators or PV cells.

Solar concentrators are nothing new. The Solar Energy Generating Systems (SEGS) plant has been operating in California's Mojave desert since 1985. Made up of nine energy farms capable of generating a total of 354 MW, SEGS is the largest solar concentrator system in the world.

During the 1980s and 1990s, US engineers tested various designs, including power towers - mirrors arranged around a vertical pipe system - and systems with molten salts. However, the US Department of Energy halted research in 2000 after the US National Research Council suggested that any further gains in performance would be insignificant.

Development continued in Europe, and later this year the first commercial molten salt-based solar collector system is due to be switched on at Guadix in Andalusia, Spain. Andasol-1 has over 500,000 square metres of parabolic mirrors and will generate 50 MW of power. With large storage tanks for the salt solution, it will be able to continue generating electricity for more than 7 hours after sunset.

In April, the Electric Power Research Institute in California released a report suggesting that adding up to 9 hours of energy storage with molten salts to a solar concentrator plant can reduce the cost of its electricity by up to 13 per cent.

This cost could fall further if new experimental fluids containing nanoparticles outperform salts, says Mark Mehos, who manages the solar thermal power programme at the National Renewable Energy Lab in Golden, Colorado. "It's early days but this has the potential to be revolutionary," he says.

Ben Crystall

Edison's revenge

When electricity has to travel thousands of miles you need a different kind of grid

Sometimes newest isn't best. A technology dismissed as obsolete a century ago could turn out to be the key to building a power grid fit for delivering electricity generated from renewable sources.

Nearly all of the world's power lines carry alternating current (AC). The reasons for this go back to an epic argument in the late 19th century between two of the biggest names in the history of electricity: Thomas Edison, the inventor of the light bulb, and the engineer Nikola Tesla. Edison argued that direct current (DC) was the right way to transmit power over long distances, because with AC this can only be done if the voltage is stepped up to lethal levels. He even built the first electric chair to demonstrate his point. Edison lost the "war of currents" because Tesla's AC transmission system proved more practical, and so it remained through the 20th century.

But Edison may yet be vindicated. Unlike conventional power plants, which can be built close to where their electricity is needed, renewable energy sources are not always near population centres, so this power must be transmitted over long distances. Over a distance of 1000 kilometres, AC transmission lines become increasingly inefficient, losing over 10 per cent of the energy pumped into them, whereas a high-voltage DC line would lose just 3 per cent. When you factor in conversion from DC to the AC supply that consumers need, the additional losses are 0.6 per cent at most. In all, at distances of about 800 kilometres and above, DC transmission becomes cheaper than AC.

This has led supporters of renewables to call for a new generation of high-voltage DC "supergrids" linking regions rich in wind, solar or other renewable energy sources with populated areas thousands of miles away.

In May, the environmentalist Robert Kennedy Jr called on the next US president to build a high-voltage DC grid to transport electricity from the "wind corridor" that stretches from the Texas panhandle to North Dakota, and solar power plants in the Southwest, to all major US cities.

In Europe, Gregor Czisch, an energy systems expert at the University of Kassel in Germany, has calculated the costs and benefits of a supergrid stretching from western Siberia to Senegal and providing 1.1 billion Europeans with 4 million gigawatt-hours of electricity a year from renewables. The grid would link onshore and offshore wind farms, hydropower resources and solar concentrator power plants with major European cities. The sheer extent of a grid like this would do a lot to smooth out the energy supply, Czisch says.

Not only would the electricity this provides be clean, it would also be reasonably cheap. At 2007 prices, a European supergrid would deliver wholesale electricity at a cost of ¤0.047 per kilowatt-hour, says Czisch, compared with ¤0.06 to ¤0.07 per kilowatt-hour for electricity from gas-fired power plants.

Helen Knight

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