Salty solar plant in Spain stores sun's heat night and day

The plentiful sunshine of southern Spain is being harvested to generate electricity day and night

DRIVING through the baking landscape of Almería, it is no mystery why this Spanish province is home to a novel type of power station that generates electricity by harnessing the heat of the sun.

For over 20 years, the Plataforma Solar de Almería, sited on an almost rainless plain in the south of the province, has been at the forefront of research into solar thermal power generation. Helped by Spain's sunny climate and generous government subsidies, this has led to the construction of 10 solar thermal plants across the country in the last three years alone. Some 50 more are planned.

Within the centre, parabolic dishes lie strewn about like huge discarded toys, but the site is dominated by a giant white tower. Thousands of mirrors, known as heliostats, surround it, catching sunlight and focusing it onto a receiver on top of the tower. This concentrated sunlight produces superheated steam that drives a turbine to generate electricity.

Till now, the mainstay of solar thermal power has been the parabolic trough system, in which carefully shaped parabolic mirrors direct solar energy onto glass tubes containing a heat-absorbing fluid. One of the drawbacks of such installations is that to keep costs down they need large areas of flat ground.

With solar towers this is unnecessary. The heliostats can hug the land at different levels and be individually calibrated to beam their rays to the receiver atop the tower.

Another advantage of towers is that they can operate at high temperatures. The heat-absorbing liquid used in the trough system is an oil that can only cope with temperatures up to 400 °C. With the tower there is no need for an intermediate fluid, and steam passing though the receiver is heated directly to around 550 °C. The higher temperature means the heat energy can be converted to electricity more efficiently.

However, because the towers produce steam directly, they cannot store the heat they collect and so stop generating electricity once the sun sets. A new Spanish project, the Gemasolar tower near Seville, may have solved this problem. The 19-megawatt tower will be the first in the world to use a mixture of molten salts to transfer heat from the receiver on top of the tower to a heat exchanger where steam to drive the turbines is generated.

The salt mixture, made up of sodium and potassium nitrates, can operate at the high temperatures generated in a solar tower's receiver. Because the hot molten salt can be stored until the heat it contains is needed, the Gemasolar plant is expected to be able to run for 15 hours without sunlight. The best parabolic trough plants can only manage about half that time.

If all goes well when Gemasolar launches next year, Spain should be able to profit from its scorching climate for some time to come.