See gallery: How to catch the Sahara’s sun for Europe
EVERY DAY, the sun pours more energy onto the surface of our planet than we use from all sources in an entire year. It is an inexhaustible powerhouse that has remained largely untapped for human energy needs. That may soon change in a big way. If a consortium of German companies has its way, construction of the biggest solar project ever devised could soon begin in the Sahara desert. When completed, it would harvest energy from the sun shining over Africa and transform it into clean, green electricity for delivery to European homes and businesses.
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Prospects for the project, called Desertec, have blossomed over the past year, and this month 20 major German corporations are expected to announce the formation of a consortium that will provide the €400 billion needed to build a raft of solar thermal power plants in north Africa. They include energy utilities giants E.ON and RWE, the engineering firm Siemens, the finance house Deutsche Bank and the insurance company Munich Re.
The current plan, outlined by the German Aerospace Centre (DLR) in a report to the federal government, envisages that the project will meet 15 per cent of Europe’s electricity needs by 2050, with a peak output of 100 gigawatts – roughly equivalent to 100 coal-fired power stations. Preliminary designs in the German report show electricity reaching Europe via 20 high-voltage direct-current power lines, which will keep transmission losses below 10 per cent (żěè¶ĚĘÓƵ, 14 March, p 42). Trans-Mediterranean links will cross from Morocco to Spain across the Strait of Gibraltar; from Algeria to France via the Balearic islands; from Tunisia to Italy; from Libya to Greece; and from Egypt to Turkey via Cyprus.
“It is claimed that the project could meet 15 per cent of Europe’s electricity needs by 2050”
Desertec would take its place in a wider European supergrid that conveys power generated from wind turbines in the North Sea, hydroelectric dams in Scandinavia, hot rocks in Iceland and biofuels in eastern Europe. Adding solar thermal capacity would help ensure a steady supply of green electricity.
But is this really the best use of such a colossal amount of money? Critics are lining up to point out the project’s shortcomings. They say it could make Europe’s energy supply a hostage to politically unstable countries; that Europe should not be exploiting Africa in this way; that it is a poor investment compared to covering Europe’s roofs with photovoltaic (PV) solar panels; and that, while deserts have plenty of sun, they lack another less obvious but equally indispensable resource for a solar thermal power plant – water. Is Desertec really the model of future power generation, as its promoters would have us believe, or is it politically misconceived and a monumental waste of money?
Canned heat
Unlike PV panels, which convert sunlight directly into electricity, solar thermal electricity generation plants first trap solar energy in the form of heat, and use this heat to generate electricity just as a conventional power plant does. Solar thermal plants come in four main varieties. Three use mirrors that concentrate sunlight to heat oil, water or a molten salt, which is in turn used to generate steam that drives a turbine. The mirrors can take the form of parabolic troughs or an array of flat reflectors that redirect sunlight onto pipes suspended above them, heating the fluid that they contain. In the Mojave desert in California, an interlinked system of nine solar thermal plants which use trough mirrors has been generating up to 300 megawatts of electrical power for more than two decades.
Alternatively, a field of mirrors can focus sunlight onto a central ceramic heat absorber mounted on a tower. A prototype plant in Spain has 21,000 square metres of glass mirrors that heat the absorber to over 1000 °C and generate 1 megawatt. In the fourth type, a dish focuses heat on a Stirling engine which generates electricity by exploiting the expansion and contraction of a gas in a sealed piston chamber as it is heated and then allowed to cool (see diagram).
Solar thermal energy is now coming to the fore, as it proves itself to have several advantages over PV. Among these is its ability to produce electricity in power-station quantities, without the complex organisation that distributed generation entails. What’s more, it can feed electricity into the grid at night as well as by day. This is done by storing the heated fluid in an insulated container and releasing it hours later when the energy is required. Storing energy from PV panels would require a new generation of high-capacity batteries – still a research project in its infancy for the scale needed. The clincher is cost. Building a power-station-scale solar thermal installation costs only a fraction of PV generators with the same output. As a result, an army of new solar thermal plants are being planned for the US, China, Australia and Israel.
PV cells have one clear advantage, however. Solar thermal requires direct sunlight, and so a cloudy day will slash power output to near zero, whereas PV cells will generate at least some power until night descends. But the intensity of sunshine blasting the Sahara desert more than compensates for this. Every year, each square metre of the Sahara receives more heat from the sun than would be obtained by burning two barrels of oil. Desertec reckons that a patch less than 600 kilometres across could meet the entire world’s electricity needs today and that all of Europe’s electricity could be made in an area 250 kilometres across.
In December, world leaders are likely to agree in Copenhagen, Denmark, on the need to cut carbon dioxide emissions by up to 80 per cent by 2050, so technologies to deliver these cuts are needed. Until recently, PV panels had been seen as the favourite route to emissions-free electricity. They are modular, compact and generate electricity even on a cloudy day. Because the panels are costly, supporters of PV have advocated putting panels on roofs, and generating electricity that will make the buildings concerned self-sufficient – and with luck provide some extra that can be fed back into the grid.
Out in the Sahara, however, another problem has to be solved. Like a regular coal or oil-fired plant, a solar thermal generating station requires large amounts of cooling water to condense the steam after it goes through the generator’s turbines, and there are inevitable losses from evaporation. The solar trough plant in the Mojave desert consumes around 3000 litres of water for every megawatt-hour of electricity it produces, and others are likely to need similar amounts. That’s a lot of water to find in a desert. A typical Saharan solar farm would be expected to deliver abount 120,000 megawatt-hours of electricity per year per square kilometre. That equates to some 350 million litres of water,or enough to flood its area to a depth of 35 centimetres – not much less than would be needed to irrigate a crop of wheat.
There is water beneath the Sahara. The giant Nubian aquifer is probably the world’s largest, containing an estimated 60,000 cubic kilometres of water. But this is a non-renewable resource: it is a deep fossil reserve that has been there for thousands of years and will not be replenished by rains. Moreover, the expense and effort required to tap it are huge. Libya has spent $27 billion of its oil revenues over the past two decades building a system to deliver this water to coastal farms for irrigation. The country pumps about 2 cubic kilometres (2 trillion litres) of water a year to the surface and sends it down a network consisting of 3500-kilometres of massive 4-metre pipes that make up the so-called Great Man-made River. Costs on anything like that scale would make Desertec uneconomic.
In a , the Department of Energy concluded in February this year that water is shaping up to be a major constraint on harnessing solar power in desert areas. The US National Park Service agrees. In April, it warned that solar thermal projects proposed for the Mojave desert could destroy its limited underground water reserves.
Switching to air cooling for thermal solar power stations would cut water demand by up to 90 per cent, but brings problems of its own. Air cooling is less efficient than water cooling, so the installations would require more land and more mirrors, adding to the capital and running costs. David Mills, whose company Ausra in Palo Alto, California, wants to build an air-cooled solar thermal plant in the Nevada desert, says air cooling involves about a 10 per cent penalty in terms of cost or performance.
The DLR’s report does not directly address Desertec’s potential water requirements. Instead, it focuses on the fresh water the plants could produce by using part of their output to desalinate seawater – a factor that Desertec’s backers hope will persuade north African governments to allow their land to be used to generate electricity for export. Let us use your sun to generate power, the argument goes, and you can share some of that energy to desalinate seawater to irrigate crops that will help feed your growing populations. The German report estimates that north Africa’s demand for water will increase by two-thirds in the coming 40 years – far beyond available supply. The region is already over-pumping underground reserves by 35 trillion litres a year. By 2050, Desertec’s solar thermal plants could almost double the region’s supply of fresh water.
Whatever part desalination plays, it will not be feasible to build solar thermal plants close to the sea, as clouds lurk there. Maps in the German report show that along much of the Atlantic and Mediterranean coastlines of north Africa, solar radiation levels are below the threshold of commercial viability, which the report’s authors have set at 2 megawatt-hours per year from every square metre.
Some of the opposition to Desertec comes from an unexpected quarter. Hermann Scheer, the German member of parliament who masterminded the programme that has put solar panels on 100,000 of the country’s roofs, declared Desertec to be an unnecessary and expensive distraction that would divert investment from projects in Germany itself. Europe could generate as much solar energy as it needs domestically with more rooftop PV panels, Scheer says. “It is strange how even Greenpeace has not yet understood this,” he adds. “By the time solar power from north Africa can be supplied for the prices Desertec has promised, solar power generation will happen at a noticeably lower price here at home.” Desertec supporters argue that the two types of system complement each other. In particular, stored heat at solar thermal plants can deliver electricity through the night, when output from Europe’s photovoltaic panels dies.
“Europe could generate all the domestic energy it needs through rooftop photovoltaic solar panels”
If the argument comes down to cost, how do the numbers stack up? In terms of capital cost, large-scale solar thermal installations are the clear winner. Building the plant now operating in Spain has cost €1670 for each megawatt-hour of electricity it produces each year. Jeremy Leggett, who heads Solar Century, a London-based company which sells PV technology, says that installing PV panels would eat up more than double that, at €4000 per megawatt-hour per year. But capital cost is not the end of the story. While a solar thermal power plant requires a round-the-clock crew, PV installations pretty much run themselves. What’s more, PV power plants can grow piecemeal: they can start generating power for the grid from the day the first panel is installed, while solar thermal mirrors are useless until the entire power station is completed. For Desertec, there is also the small matter of getting Desertec’s electricity from Africa to Europe. With this many variables in the equation it becomes hard to make realistic comparisons between the available options.
To fill the gap, Anthony Patt of the International Institute for Applied Systems Analysis, a think tank in Laxenburg, Austria, is conducting a detailed feasibility study of solar thermal power generation. He reckons that Desertec is unlikely to be competitive with coal-fired power generation for at least two decades, during which time it will swallow between €20 and €50 billion, similar in scale to Germany’s expected subsidy for PV cells over the same period.
And then there are the tricky political issues that arise from a plan that will exploit land in Africa, and possibly Asia too, to generate electricity for Europe. Desertec says that two-thirds of the potential solar resource in north Africa and the Middle East lies in Algeria, Libya and Saudi Arabia. Europe, like the US, wants to reduce its reliance on energy imported from distant lands with an unpredictable political future. Why create a new hostage to fortune? The stage is set to recreate an uncomfortable parallel with western dependency on oil from Saudi Arabia, Iran and Iraq.
Ifeanyi Amajuoyi, who runs the Nigerian renewable energy company Nkubadorf, has set up an organisation called Desertec-Africa to promote the cause of exploiting African sunshine for the benefit of Africans. He plans to begin campaigning for governments in Africa to ensure the energy from Desertec stays in Africa.
While the technical and political hurdles facing Desertec are considerable, the rewards if it succeeds could be huge. It will elevate renewables into one of the dominant energy providers for Europe, and help make fossil fuels a thing of the past. If it fails, the red faces won’t be from desert sunburn.
