AN ELECTRICITY generating station in southern Spain powered by heat from the sun is signalling a new dawn for an almost forgotten source of renewable energy. Trials on a prototype solar thermal power plant in the desert landscape of Almer’a have shown it to be cheaper, more efficient and able to operate on a bigger scale than rival photovoltaic technology.
Solar thermal energy was briefly fashionable after the oil price shocks of the 1970s. But the fashion waned when photovoltaic (PV) cells, initially developed for space travel, came into favour for what visionaries foresaw as the “solar century” ahead.
But a few died-in-the-wool solar thermal power advocates refused to give up, and they now believe their time has come. Only thermal power can concentrate solar energy sufficiently to generate electricity on the scale of commercial power stations, they say.
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The technology owes its change of fortunes to a combination of factors. To meet targets set by the European Union, the Spanish government has introduced financial incentives for electricity generators to use solar power. At the same time, advanced ceramics have become available that can withstand the high temperatures required for efficient solar thermal power generation.
Last month the EU Research Directorate held an open day for its SolAir project in arid Almer’a, where the spaghetti westerns of the 1970s were filmed. “It’s the sunniest place in Europe,” says Diego Martinez, director of SolAir. “We get 3000 hours of sun here every year.”
The sun is collected by some 300 units called heliostats, each one with 70 square metres of glass mirror. The heliostats track the sun as it crosses the sky, reflecting its rays up to a ceramic heat absorber with an area of only a few square metres mounted on a 100-metre tower. The surface of the silicon carbide absorber reaches a temperature of 1000 °C, while air blown through its honeycomb structure is heated to 680 °C. The hot air then passes down the tower to a heat exchanger, where it generates steam that drives a conventional turbine generator delivering up to 1 megawatt of electrical power (see Graphic).
Previous solar thermal energy systems, such as one in the Mojave desert, California, heated fluid, typically oil, running through a pipeline in front of the mirrors. But the maximum temperature possible with this arrangement was only 400 °C. The higher temperatures achieved by super-concentrating the solar energy mean that the Spanish plant can operate more efficiently.
The plant’s silicon carbide ceramic heat absorber is uniquely capable of operating at the high temperatures generated by its mirror arrays. The same material can also be used to store up heat energy, so the generators can keep running when it is cloudy or at night, although the cost of the ceramic makes it prohibitively expensive to store more than an hour or two’s worth of heat.
After the trials at the demonstration plant end this summer, the commercial phase can begin. In one of its final acts before being ousted in last month’s general election, the Spanish government set premium rates for the purchase of renewable energy of around €0.18 per kilowatt-hour. This is expected to be enough to make solar thermal electricity commercially viable in the country’s sunny south.
“In 5 to 10 years there should be several plants across Europe, each 15 to 20 times larger than the demonstration plant, and together generating hundreds of megawatts,” says Manuel Romero of Spain’s ministry of science and technology. These plants are expected to produce energy at around one-third the price of PV.
And this could be just the start. “I think the main market is outside Europe,” Romero says. He sees solar thermal energy generation taking off in the world’s desert belts. Arrays of heliostats covering a patch of the Sahara about 500 kilometres across could, in theory, meet the world’s entire demand for electricity.
Already there are plans for generators powered by African sunshine to sell electricity to Europe. “Algeria wants to build plants close to the coast and export power to Europe. Egypt is talking about doing the same thing,” says Romero.
Renewables are go
Europe’s solar thermal success formed part of a trio of announcements from EU researchers in late March on new forms of renewable energy. Within the past year, Denmark’s Wave Dragon wave-energy generator has started feeding power into the national electricity grid. And in France, the country’s 17 years of experiments in generating electricity from hot dry rocks 5 kilometres below the surface of Alsace has passed a key milestone with the completion of the boreholes (New Scientist, 3 April, p 23).
The EU has set a target for its 15 current member states of generating 22 per cent of electricity from renewable sources by 2010. That is a substantial increase from the present 14 per cent, most of which comes from hydroelectric dams, which have little potential for expansion. The EU’s head of renewables research, Wiktor Raldow, said the target effectively requires a tripling of the proportion generated from non-hydro renewables, from 3 to 9 per cent.
He called this target “ambitious but realistic”. But in the same week, the European Renewable Energies Federation warned that Europe’s efforts to spread green energy were stalling, and that without a new push, the EU is unlikely to meet its target.
