Concentrating Solar Power

The Oil Drum: Europe
By Gerry Wolf
Posted by Chris Vernon
May 31, 2007

Introduction

It is said that, in 212 BC, Archimedes used polished bronze shields to focus sunlight, trying to set fire to wooden ships from the Roman Empire that were besieging Syracuse. Although we don’t know whether he succeeded, the Greek navy recreated the experiment in 1973 and managed to set fire to a wooden boat at a distance of 50 metres. In the 16th century, Leonardo da Vinci proposed the use of concave mirrors to concentrate sunlight to heat water.

It was not until the late 19th century and early 20th century that the idea of capturing solar energy with mirrors was tried on an industrial scale in California, Egypt and other places with lots of direct sunshine. But the era of cheap fossil fuels snuffed out these early developments and it was the mid 1980s before serious attempts were made again to apply the technique of ‘concentrating solar power’ (CSP).

The basic idea is to arrange mirrors so that they concentrate sunlight into a relatively small area and then use the resulting heat to raise steam to drive turbines and generators, just like a conventional power station. Direct sunlight is needed and CSP works best when sunshine is plentiful, as it is in hot deserts. In those kinds of conditions, CSP is currently the most cost-effective way of capturing solar energy but this might change in the future with further developments in photovoltaics (PV).

A nice feature of CSP is that it is possible to store solar heat in melted salts (eg nitrates of sodium or potassium) so that electricity generation may continue at night or on cloudy days. This is currently a lot cheaper than flow batteries or other technologies for bulk storage of electricity. Because CSP plants are so similar to conventional power stations, it is also possible to use gas as a stop-gap source of heat when there is not enough sun. With heat storage and hybridisation with gas-firing, CSP plants can provide base load, intermediate load and peaking power according to need.

There are several variations on the scheme that I have outlined, including systems that use heat to drive a Stirling engine and generator, and hybrid schemes that use mirrors in conjunction with PV.

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