11 march 2009

From AC to DC: Going green with supergrids

by David Strahan, New Scientist 11 March 2009 , issue 2699.

.....Although DC lost out to AC in the early days of electrification, high-voltage direct current (HVDC) has long had a niche role - transmitting large amounts of power over long distances because it is more efficient than conventional AC lines. Now it is also set to become a key link for the growing number of renewable-energy generators, particularly offshore wind farms. This is leading many in the energy industry to take a fresh look at DC.

Some engineers are thinking big. Their calculations suggest that continent-wide HVDC "supergrids" could help smooth out the variable levels of power created by many far-flung renewable generators to make a fully dependable supply. Supporters say this will eventually mean that coal, gas and nuclear power could be ditched, with renewables replacing them within a couple of decades.

Elements of such a supergrid will soon begin to materialise in Europe, and a proposed €1.2 billion ($1.5 billion) subsidy could help develop these links across the region. Meanwhile, in the US, President Obama's $150 billion energy plan includes a target of 25 per cent renewable electricity by 2025, implying massive investment in high-voltage lines, many of which are likely to be HVDC. At the same time, tests on new superconducting HVDC cables suggest that a grid incorporating this technology could act as a mammoth energy store, helping buffer consumers and utilities against the vagaries of the weather (see "Supercooled grid"). "Whichever way you look at it, there is no doubt that HVDC's time has come," says Graeme Bathurst, safer, lower voltages. That required transformers, which existed for AC networks, but not for DC.

....Despite this victory, DC is far more efficient: at the same voltage, it suffers much lower transmission losses than AC. This is because in a DC line the direction of the current is constant, whereas in an AC line it reverses 100 or 120 times a second. This induces small currents in the transmission line insulation, and this energy is then lost as heat. Because of this, HVDC has long enjoyed a niche role transporting large amounts of power efficiently over unusually long distances. One of the earliest big projects was a 600-megawatt link built in 1965 in New Zealand to connect the North and South Islands, which was later upgraded to 1200 megawatts.

In the past decade, the length and capacity of new HVDC projects has risen fast. This is particularly true in China, where lines are being built to transmit hydroelectric power from the interior to consumers on the coast. The Swiss-based engineering firm ABB has been commissioned to build a 2000-kilometre link from the Xianjiaba dam to Shanghai that can carry 6.4 gigawatts - equivalent to the output of three big power stations.

When the current starts to flow in 2011, ABB says it will deliver major
environmental benefits. Gunnar Asplund, research and development manager for HVDC at ABB, says that huge amounts of power can be transmitted along a single line of HVDC pylons, whereas an AC link would need three abreast. The alternative to transporting hydroelectricity long-distance would have been to build more coal-fired power stations near Shanghai, which Asplund estimates would have put an extra 40 million tonnes of carbon dioxide into the atmosphere each year.

Another major advantage of HVDC is that it can transmit electricity over much greater distances underground and underwater than AC. This is because AC produces powerful alternating electric fields that cause large additional energy losses if the line is buried or submerged. For DC this "capacitance" effect is negligible. That makes HVDC essential for subsea interconnectors like the 600-kilometre NorNed cable between Norway and the Netherlands that opened last year, as well as for connecting remote offshore wind farms.

Thinking big

Those long-distance links are nothing compared with the plans of Desertec, an organisation founded by the Club of Rome - a Swiss-based sustainability think tank - and the National Energy Research Center in Amman, Jordan. Since 2003, Desertec has been arguing for remote electricity generation based largely on concentrating solar power (CSP) in North Africa and the Middle East. CSP is relatively expensive, but has one big advantage: some of the heat captured during the day can be stored in molten salts and used to generate electricity overnight. Desertec says this technology alone could supply 17 per cent of Europe's power by 2050, imported via 20 to 40 long-distance HVDC lines. Other supporters of the concept, however, say that HVDC could deliver even more - a wholly renewable electricity supply.

The basis of that supply is probably the most ambitious plan for HVDC: the supergrid concept now gaining support in Europe and North America. The idea itself is not new - it was first proposed by the architect and designer Buckminster Fuller in the 1950s - but only now is it becoming a practical possibility because of advances in HVDC technology.
The problem with renewable electricity is that the wind doesn't always blow, nor the sun always shine, at the one spot where you build your renewable energy generator. Yet the wind is always blowing somewhere, just as the sun is always shining on half the globe. So with a large enough grid, variations in generation should even out, giving a reliable supply.

The huge potential of this has been demonstrated by Gregor Czisch, an energy system consultant who has made the first quantitative study of how to build an economically viable, wholly renewable electricity supply for Europe and its neighbours (see map). To do this, Czisch used a technique called linear optimisation, originally developed to solve complicated logistical problems in industry and commerce. It took Czisch years to gather the necessary information, including detailed weather and electricity consumption data for the whole area and investment costs for the main renewable technologies.

Czisch then plugged this data into a program to devise the cheapest electricity supply system that could satisfy demand entirely from renewables. He allowed it to decide which forms of generation should be sited where, as well as plan the routes and capacity of the HVDC lines. The results were astonishing. Not only could the electricity demand of more than a billion people be supplied solely from renewables throughout the year, it wouldn't break the bank.

The numbers look daunting: the project would cost more than €1.5 trillion, of which €128 billion would go on the lines and equipment for the supergrid itself, and around €1.4 trillion on renewable-generating capacity. To put this in context, the International Energy Agency forecasts that the global power industry will have to invest $13.6 trillion on fossil-fuel-based power generation by 2030. Under Czisch's plan, investment in clean technologies would displace spending on dirty ones so costs would not escalate.

One of the advantages of the supergrid is that renewables can be sited where wind and sunlight are best for generating electricity, which will bring economic efficiencies as well as electrical ones. What's more, the supergrid itself represents only a small proportion of the total investment, so the extra cost of the grid makes little difference to the overall price of electricity. Czisch calculates the system could deliver electricity for less than 4.7 euro cents per kilowatt-hour - roughly the price of German wholesale electricity in 2005.

One of the advantages of the supergrid is that renewables can be sited where wind and sunlight are best for generating electricity

In Czisch's main study, the bulk of the energy would come from onshore wind, the cheapest form of renewable generation, with powerful summer winds in Morocco and Egypt complementing winter gales around the North Sea. Most of the rest would come from existing hydropower in the Nordic countries and the Alps, which would be turned on only when the other sources failed to match demand. In an alternative scenario, Czisch found that European demand could be satisfied entirely from renewables without imports from Africa, but at a slightly higher cost.

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