Instead, the total energy required should be considered across the entire service life of the plant, typically in excess of 30 years. Viewed against this benchmark, which type of power plant has the best environmental credentials?

Answering this question requires the most comprehensive details on the energy consumption of a power plant: from the manufacture of its constituent parts, through its operational lifetime, to its decommissioning and disposal.

This challenge was taken up by energy experts from the DLR Institute of Technical Thermodynamics in Stuttgart and from the Institute for Energy and Environmental Research in Heidelberg.

Their results were published in June 2009 by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, on whose behalf the sustainability of different types of power plants was investigated.

Wind power and solar-thermal power offer high levels of energy efficiency
Wind farms are the most efficient when it comes to matching the total energy required for their manufacture, operation, decommissioning and disposal. Their 'energy-related amortisation period' fluctuates between three and seven months, depending on the choice of location and the efficiency rating of the equipment.

Comparable figures were otherwise only achieved by solar-thermal power stations in sunny locations such as Morocco. Hydroelectric power stations need up to 13 months. Solar (photovoltaic) power stations in central Europe need to generate electricity for up to five years before they deliver an equivalent energy balance.

However, modern thin-layer solar cells are able to produce energy more efficiently, cutting this timescale to just three years.

By virtue of their operating principles, all power stations generating electricity from fossil fuels or uranium are incapable of achieving this type of energy balance. This is because they consume fuels with higher energy content than the electricity they generate. A perpetual motion machine offering efficiency ratings in excess of 100 percent simply does not exist.

The best solution lies in an intelligent mix
Although wind turbines, both on the coast of the North Sea and offshore, and solar-thermal plants in southern regions deliver the best energy balance, it would be unwise to dispense with other forms of electricity generation.

"There is simply no such thing as one single 'best' technology for reliable electricity supply," says DLR researcher Franz Trieb, who also contributed to the BMU report on sustainable energy provision. In order to feed affordable electricity into the power grid with security of supply, a diverse mix of renewably-fuelled power stations would need to be employed.

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