[May 2006]  Three questions haunt the decision makers when it comes to which technology used to generate electricity. How predictable is the fuel price in the future? How secure is the supply of fuel? How quickly will the power plant pay for itself?

Solar electricity has no problems with the first two questions but trips up badly with the third. There are instances when PV works economically especially cladding of buildings which would need to be clad anyway. However estimates of payback usually vary between long and very long.

A determined and patient band of engineers in the USA have been steadily gnawing away at this problem for the last 20 years and have finally cracked it. The first 20,000 25kW Stirling Energy generators are being made now for a solar farm in the Mojave Desert. 

Stirling solar generators during their long durability test track the sun at the test facility in New Mexico

Large sun-tracking mirror arrays focus sunlight onto one end of a stirling engine. The engine works by shuffling gas from the hot end to the cold end and back and extracting energy from the resulting pressure changes. A bit like an automobile engine with a generator bolted to it. In 1816 the Rev Robert Stirling filed a patent for the Stirling engine and 190 years later it has finally come of age as an efficient economic solution to one of the world’s most sticky problems. Trial units of this device have recorded the highest efficiency for turning sunlight into electricity of any solar generating technology. It is also cropping up in domestic combined heat and power units and coolers (oh yes, it’s reversible too!).  

Here’s the important bit. Now that the design has been developed the system is inexpensive and uses automotive type technology to produce the parts. Looking at a payback (depending on electricity prices) of 5 -10 years, comparable to coal, gas and better than nuclear alternatives*; this first 500MW looks to be just the start.

There is talk of a 140km square area of the United States that would supply all of the electricity and automotive hydrogen that they need. America is excited by this not so much for the environmental impact but more for its potential to free it from dependence on Middle East oil!

Why aren’t these units being ordered for the fens and plains of England? Well, sadly the English weather and latitude shift the economics in favour of harvesting our more than adequate wind, wave and tidal resources. Let’s hope it takes us less than 20 years!

Michael Reid
Milestone New Product Development Ltd

* Proviso: This comparison is based on the average of a number of sources, which don’t all agree. However every hike in fuel prices tips the balance further to the benefit of these fascinating machines.

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