From New Scientist, 11 October 2008
electricity has to travel thousands of miles you need a different kind of
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
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
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.