Science & Technology | Power transmission
Where the wind blows
July 26, 2007
A grandiose plan to link Europe's electricity grids may recast wind power from its current role as a walk-on extra to being the star of the show
PLUG in your toaster—or your television or your vacuum cleaner—and the electricity that surges through it is an alternating current. The question of whether the world would be powered by direct current (DC), in which electrons flow in one direction around a circuit, or by alternating current (AC), in which they jiggle back and forth, was decided in the 1880s. Thomas Edison backed DC. George Westinghouse backed AC. Westinghouse won.
The reason was that over the short distances spanned by early power grids, AC transmission suffers lower losses than DC. It thus became the industry standard. Some people, however, question that standard because over long distances high-voltage DC lines suffer lower losses than AC. Not only does that make them better in their own right, but employing them would allow electricity grids to be restructured in ways that would make wind power more attractive. That would reduce the need for new conventional (and polluting) power stations.
Wind power has two problems. You don't always get it where you want it and you don't always get it when you want it. According to Jürgen Schmid, the head of ISET, an alternative-energy institute at the University of Kassel, in Germany, continent-wide power distribution systems in a place like Europe would deal with both of these points.
The question of where the wind is blowing would no longer matter because it is almost always blowing somewhere. If it were windy in Spain but not in Ireland, current would flow in one direction. On a blustery day in the Emerald Isle it would flow in the other.
Dealing with when the wind blows is a subtler issue. In this context, an important part of Dr Schmid's continental grid is the branch to Norway. It is not that Norway is a huge consumer. Rather, the country is well supplied with hydroelectric plants. These are one of the few ways (but not the only way, see article) that energy from transient sources like the wind can be stored in grid-filling quantities. The power is used to pump water up into the reservoirs that feed the hydroelectric turbines. That way it is on tap when needed. The capacity of Norway's reservoirs is so large, according to Dr Schmid, that should the wind drop all over Europe—which does happen on rare occasions—the hydro plants could spring into action and fill in the gap for up to four weeks.
Put like this, a Europe-wide grid seems an obvious idea. That it has not yet been built is because AC power lines would lose too much power over such large distances. Hence the renewed interest in DC.
Westinghouse won the battle of the currents in the 1880s because it is easier to transform the voltage of an AC current than of a DC current. High voltage is the best way to transmit power (the higher the voltage, the smaller the loss), but high voltage is not usually what the user wants. Power is therefore transmitted along high-tension AC lines and then “stepped down” to usable voltages in local sub-stations.
Edison was right, however, to argue that DC is the best way to transmit electricity of any given voltage. That is because the shifting current of AC runs to earth more easily than DC does. To avoid this earthing, AC lines have to be built a long way from the ground—and the higher the voltage, the farther away they need to be. At 400 kilovolts, a standard value for long-distance transmission, an alternating current 30 metres (100 feet) from the ground has a fortieth of the loss of a similar cable at ground level. But even at this height an overhead DC line will beat an AC line at distances more than 1,000km (600 miles), while ground-level DC will beat AC at distances as short as 30km.
Dr Schmid calculates that a DC grid of the sort he envisages would allow wind to supply at least 30% of the power needed in Europe. Moreover, it could do so reliably—and that means wind power could be used for what is known in the jargon as base-load power supply.
Base-load power is the minimum required to keep things ticking over—the demands of three o'clock in the morning, or thereabouts. At the moment, this is supplied by traditional power stations. These either burn fossil fuel and thus contribute to global warming, or use uranium, which brings problems such as how to get rid of the waste, as well as political opposition.
Though wind power has its opponents, too, its environmental virtues might be enough to swing things in its favour if it were also reliable. Indeed, a group of Norwegian companies have already started building high-voltage DC lines between Scandinavia, the Netherlands and Germany, though these are intended as much to sell the country's power as to accumulate other people's. And Airtricity—an Irish wind-power company—plans even more of them. It proposes what it calls a Supergrid. This would link offshore wind farms in the Atlantic ocean and the Irish, North and Baltic seas with customers throughout northern Europe.
Airtricity reckons that the first stage of this project, a 2,000 turbine-strong farm in the North Sea, would cost about €2 billion ($2.7 billion). That farm would generate 10 gigawatts. An equivalent amount of coal-fired capacity would cost around $2.3 billion so, adding in the environmental benefits, the project seems worth examining. Such offshore farms certainly work. Airtricity already operates one in the Atlantic, and though it currently has a capacity of only 25 megawatts, increasing that merely means adding more turbines.
Nor is this the limit of some people's vision. The Global Energy Network Institute, based in San Diego, California, reckons high-voltage DC lines could be used to bring solar energy to market from places such as the Sahara. Wind and geothermal power could be gathered from as far afield as South America and Siberia. Such a globalised market has its attractions. Whether the world is ready for the Organisation of Electricity Exporting Countries to take over from OPEC, though, remains to be seen.
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