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Know It All: Renewable Energy -- Natural energy

Published in Geare magazine, Issue #42, 2007

Climate change seems to be on everyone's lips at the moment. The UN's International Panel on Climate Change has recently, as I write, released its summary of the Fourth Assessment Report (the report itself is yet to be published, and given past performance it remains uncertain how accurately the summary summarises). This warns yet again that things are getting hotter, and that we done it.

We done it through pumping carbon dioxide into the atmosphere and this, it says, traps in heat making us get warmer. The carbon dioxide comes from burning things, like petrol in our cars and coal in our power stations. Since in these environmentally conscious times nuclear appears to be a big no-no, the most commonly proposed replacements for our coal-fired power stations are wind and solar energy.

Personally, I love the idea of getting something for nothing. With all that energy just pouring down from the sun, why not use it?

Unfortunately, both wind and solar have major problems.

Ever read a story of the sea-faring ways of our ancestors, sailing to the four corners of globe on their multi-masted ships? Or watched Master and Commander? Remember how those sailing ships used to become becalmed, often for days on end?

That's a problem with wind power. The wind blows according to its own schedule, and this does not often line up nicely with our power demands. If you are going to use wind power for a significant proportion of your power requirements, then you have a choice. You can be prepared to accept black-outs, or at least brown-outs, at frequent, but random, times. Or you can ensure that your wind generating capacity is backed up by a near equivalent capacity for producing reliable power.

That makes wind power very expensive. Coal and gas fired power stations pay a lot for their fuel, but the plants are relatively cheap to build. Nuclear stations pay hardly anything for their fuel, but they cost a lot to build. The huge windmills on their 'farms' cost nothing for fuel, and are fairly expensive to build, but also have to be backed up for their quiet times. If all new power stations were to be wind-based, then you can be certain that within a few years the brown-outs will commence.

Solar energy shares with wind the characteristics of being free to run, expensive to build, unreliable in operation, and expensive in its use of land.

How much land? If you were able to place the solar panels in orbit so that they received the full output of the sun, unfiltered by the Earth's atmosphere, then they would receive nearly 1,400 watts on each square metre, or roughly the same as the power used by a single bar radiator. Unfortunately, down here, the atmosphere does intrude, so the very best you can get is around 1,000 watts per square metre.

But nothing is perfect, and that includes solar energy generation. The very best photo-voltaic technologies (ie. traditional solar panels) top out at around twenty per cent efficiency. Large scale generators use solar thermal technology, where mirrors focus light on a boiler, from which steam drives a turbine in a similar way to a normal coal-fired power station. These range in efficiency from around twenty to forty per cent.

Let us be generous. If your high efficiency one square meter solar collector is placed on the equator and the clouds stay away, you can hope to get some 400 watts out of it in the middle of the day, considerably less in the afternoons and mornings, and none at all in the night. Australia's total electrical generating capacity in 2004 came to around 50 gigawatts. To deliver that output, you would need an area of land, 125 square kilometres in size, completely covered with solar collectors. And that would only be for a short time around the middle of a nice day in summer. In winter, peak power usage occurs around 6pm, and this can be up to fifty per cent higher than the average level, at a point in the year and a time of the day when the sun is pretty much useless.

To accumulate sufficient energy during the reasonably high intensity daylight hours to cover our full electricity requirements, you'd need much, much more than 125 square kilometres, probably in excess of ten times that amount.

Except that there's a further problem: there is as yet no effective way of storing that much energy. So there is no point having more solar capacity than you need, because you can't do anything much with the excess anyway.

© 2002-2009, Stephen Dawson