In theory, solving the world’s energy problems should be pretty straightforward. Locate a piece of sun-drenched land about half the size of Texas, find a way to capture just 20 per cent of the solar energy that falls there and bingo – problem solved. You have enough power to replace the world’s entire energy needs using the cleanest, most renewable resource there is.
For years, supporters of solar power have heralded every new technical breakthrough as a revolution in the making. Yet time and again it has failed to materialise, largely because the technology was too expensive and inefficient and, unlike alternatives such as nuclear and wind power, no substantial subsidies were available to kick-start a mass transition to solar energy. This time things are different. A confluence of political will, economic pressure and technological advances suggests that we are on the brink of an era of solar power.
The first PV cell was created by Bell Labs in 1954, the efficiency with which a cell can convert light into electricity has been the technology’s Achilles’ heel. The problem is rooted in the way PV cells work. At the heart of every PV cell is a semiconducting material, which when struck by a photon liberates an electron. This can be guided by a conductor into a circuit, leaving behind a “hole” which is filled by another electron from the other end of the circuit, creating an electric current.
Each semiconducting material has a characteristic “band gap” – an energy value which photons must exceed if they are to dislodge the semiconductor’s electrons. If the photons are too weak they pass through the material, and if they are too energetic then only part of their energy is converted into electricity, the rest into heat. Some are just right, and the closer the photons are to matching the band gap, the greater the efficiency of the PV cell.
Bell Labs discovered that silicon, which is cheap and easy to produce, has one of the best band gaps for the spectrum of photon energies in sunlight. Even so, their first cell had an efficiency of only 6 per cent. The past decade has seen a sea change as inexpensive cells with an efficiency of 20 per cent have become a commercial reality, while in the lab efficiencies are leaping forward still further.
Last year, Allen Barnett and colleagues at the University of Delaware, Newark, set a new record with a design that achieved 42.8 per cent energy conversion efficiency. Barnett says 50 per cent efficiency on a commercial scale is now within reach. […]
“In some places, the cost of solar-generated electricity is close to that of electricity from conventional sources”
George W. Bush has acknowledged this new dawn, setting aside $168 million of federal funds for the Solar America Initiative, a research programme that aims to make the cost of PV technology competitive with other energy technologies in the US by 2015. Rogol thinks Bush’s target is achievable. He says the cost of manufacturing PV equipment has fallen to the point where, in some places, PV-generated electricity could already be produced for less than conventional electricity. Manufacture PV cells at $1 per watt of generating capacity and the cost should be competitive everywhere.
Perhaps surprisingly, given its less than cloudless skies, one of the countries leading the solar revolution is Germany. In November 2003, amid rising oil and gas prices and growing concern over global warming, its parliament agreed a feed-in-tariff, which guarantees a market for solar power. Anyone who produces electricity from solar power can sell it to the national grid for between ¬0.45 and ¬0.57 per kilowatt-hour, which is almost three times what consumers pay for their electricity, roughly ¬0.19 per kilowatt-hour.[…] Today there are over 300,000 PV systems in Germany, mostly on the rooftops of homes and small businesses, and Germany is the world’s fastest-growing PV market. It has 55 per cent of the world’s installed base of PV panels and can generate around 3 gigawatts of electricity from solar energy, equivalent to between three and five conventional power stations.
Last year, following in Germany’s footsteps, Italy and Spain launched their own tariff programmes, while the California Solar Initiative earmarked $2.8 billion for cash incentives that will subsidise new PV installations to the tune of up to $2.50 per watt, with the aim of creating 3 gigawatts of capacity by 2016. By the end of 2008, 20 nations will have similar tariff programmes for solar power, Rogol predicts.
The hope is that by spurring demand, these subsidies will also stimulate PV research and manufacturing technology, driving down costs. This may help speed the development of existing PV technologies, but could also drive the industry down a blind alley, as silicon PVs may soon reach their theoretical efficiency limit of about 30 per cent. Yet according to Martin Green at the University of New South Wales, Australia, it should be possible to create cells from other materials with a 74 per cent efficiency limit.
New Scientist, no 2633
December 8th 2007
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