The American inventor, who made the incandescent light bulb viable for the mass market, also built the world's first electrical distribution system, in New York, using "direct current" electricity. DC's disadvantage was that it couldn't carry power beyond a few blocks. His Serbian-born rival Tesla, who at one stage worked with Edison, figured out how to send "alternating current" through transformers to enable it to step up the voltage for transmission over longer distances.

Edison was a fiercely competitive businessman. Besides staging electrocutions of animals to discredit Tesla's competing system, he proposed AC be used to power the first execution by electric chair.

But his system was less scalable, and it was to prove one of the worst investments made by financier J. Pierpont Morgan. New York's dominant banker installed it in his Madison Avenue home in the late 19th century, only to find it hard to control. It singed his carpets and tapestries.

So from the late 1800s, AC became the accepted form to carry electricity in mains systems. For most of the last century, the power that has reached the sockets in our homes and businesses is alternating current.

Now DC is making a comeback, becoming a promising money-spinner in renewable or high-security energy projects. From data centers to long-distance power lines and backup power supplies, direct current is proving useful in thousands of projects worldwide.

"Everyone says it's going to take at least 50 years," says Peter Asmus, a senior analyst at Boulder, Colorado-based Pike Research, a market research and consulting firm in global clean technology. But "the role of DC will increase, and AC will decrease."

FROM CLOUD TO MICROGRID

The main factor driving demand is the need to conserve energy and produce more of it from renewable sources. Alternating current is generated by rotating engines, but renewable sources such as wind and solar produce DC power. To use it, because of the way our buildings are wired, we first convert it to AC.

Another thing that's happened since Edison's time is the advent of the semiconductor. Semiconductors need DC power, and are increasingly found in household appliances. These have to convert the AC supply back to DC, which is a waste of energy and generates heat. In the early years of industrialization this wasn't an issue, but today it's important, especially in the huge and fast-growing business of cloud computing.

The companies that handle our information traffic are racking their brains to boost efficiency and cut carbon emissions from their plants. Pike Research expects the green data center business to be worth $41 billion annually by 2015, up from $7.5 billion now. That will be just under a third of all spending on data centers.

Finnish information technology company Academica, for instance, has a data center in a granite cave beneath Helsinki's Uspenski cathedral. It uses Baltic sea water to cool the plant and feeds surplus heat to the city's homes. IBM has designed a solar array to power its Bangalore data center. Microsoft has filed a patent application for a wind-powered data center.

Direct current may be one way to increase efficiency and reduce emissions. Right now, outside a handful of universities, it's not the first thing people are thinking of because there are more basic things to do, says Eric Woods, Research Director for Smart Industry at Pike. But for companies on the leading edge, "it's sort of coming out of the research ghetto."

Pike has not put a figure on how big the DC component of the green data center market will be. Swiss-Swedish engineering firm ABB, a big DC advocate, says about 35 percent of demand for green data centers will come from the United States, 30 percent from Europe, and the rest spread globally.

Every day, says ABB, we all send more than 300 billion emails and 250 million tweets globally. The centers to handle all this data are growing by 10 percent each year and already consume 80 million megawatt-hours of energy annually -- almost 1.5 times the amount of electricity used by the whole of New York City. They're also responsible for about 2 percent of global carbon emissions.

DC power could help. At low voltages it has long been used in data centers but will be "game-changing" at higher voltages, ABB says.

Beyond its potential in data centers, DC power's ability to run on renewable energy sources makes it interesting for important plants that need to operate in "island mode" -- independent of the grid -- in case of a supply failure. Building systems with small, self-contained electricity distribution networks known as microgrids is of particular interest to governments and militaries who worry about terrorist attacks.

"In our view the market (for microgrids) is about to take off," said Pike Research's Asmus, who also sees demand for microgrids in countries that aren't densely covered by AC grids, such as Australia and India, and in developing countries looking to replace costly and wasteful diesel generators.

SMART GRIDS

And it's not just "island mode." Thanks to power electronics - semiconductor switching devices - DC can now be transmitted at high voltage over very long distances, longer than AC. It can be easily used in cables, over ground or under the sea.

High voltage direct current (HVDC) systems are the backbone of plans for smart grids, or supergrids, which aim to channel energy from places where power sources such as sunlight and hydropower are abundant to countries where it is scarce.

Siemens, which vies with ABB for market leadership in HVDC transmission, says demand is increasing fast. "By 2020, I'm expecting to see new HVDC transmission lines with a total capacity of 250 gigawatts. That is a dramatic increase," says Udo Niehage, CEO of the Power Transmission Division in Siemens' Energy Sector. "In the last 40 years, we've only installed 100 gigawatts worth of HVDC transmission lines."

Emerging markets have been the main drivers. ABB has installed a 2,000-km line in China that operates DC power; a 2,375-km HVDC project under construction in Brazil will be the world's longest transmission line when it comes online in 2013.

But Europe is also important. HVDC is now used in a power connection between Britain and the Netherlands. The island of Majorca, whose tourists push up power demand every summer, was hooked up to the Spanish mainland in September. The HVDC system can transmit 30 to 40 percent more energy than with conventional overhead lines carrying alternating current.

Jochen Kreusel, the head of ABB's Smart Grid program, says smart grid demand will put Europe at the forefront of HVDC growth over the next 10 years. "At the moment, based on the number of projects, I'm quite sure it's the strongest market," he said. Pike in November 2010 estimated HVDC investment would reach $12.1 billion by 2015.

The bulk of this DC know-how is currently with European companies, although Chinese firms are joining in. Besides ABB, Siemens and France's Alstom are the main players.

NOT THERE YET

There are plenty of obstacles to all these developments. People in some places worry about the environmental damage from laying new grids, others point to a lack of standards and say DC still has technological limitations that need to be fixed.

Public fears about the potential danger of high voltage cables could also be an issue, especially in the United States where standard voltages are already much lower than in Europe. There are practical limitations, such as a shortage of cable-making capacity.

If the economic climate does not improve, cash may also be a constraint. Countries such as Spain and the Netherlands have already cut subsidies to renewable energy projects. ABB's Kreusel says the economic crisis will have an impact on the market, but he still expects DC to become "an evolutionary add-on" to AC grids over the next 20 years.

How would Edison see all this? He might even have foreseen it. "I'd put my money on the sun and solar energy," he reportedly told his associates Henry Ford and Harvey Firestone in the 1930s. "What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that."

(Edited by Simon Robinson)

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