From New Scientist, 11 October 2008

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