Harnessing direct solar energy – a progress report
Key text
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We often hear about solar car challenges and solar heating, but will solar energy ever be a major energy source for
industrial societies?
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Can society meet its energy needs without pollution?
Alternative energy sources are far less polluting than traditional fuels, although they may have other drawbacks. One source of alternative energy is the sun. The Earth is bathed in huge amounts of the sun’s energy. We can use this solar energy directly, simply by capturing sunlight.
Solar energy is our oldest energy source
Humans have always used the sun’s energy directly (eg, for drying clothes and foodstuffs), as well as indirectly to power the agriculture that supplies us with food. This century we have started to use solar energy more effectively, and it is likely that we’ll increase its use in the future.
It’s reliable, free and clean, but...
The great feature of solar energy is the fact that it is likely to continue to exist so far into the future that we can think of it as being unending. It is, therefore, a form of renewable energy. This is a big contrast with non-renewable energy sources, most of which are running out as we use them.
In addition, using solar energy doesn’t cause air pollution or involve damaging the Earth’s surface. It requires no difficult and expensive extraction procedures.
But the main problem is what to do when the sun doesn’t shine. The times when we most need energy – when it is dark or cold – are when sunlight is least available. But there are possible ways around this, so read on...
Compared to fossil fuels, sunlight is a weak energy source
Capturing sunlight is not as easy as it sounds. It is a dilute energy source, spread out over time and space. Earth receives 5.6 x 1018 (5,600,000,000,000,000,000) megajoules of solar radiation each year (Box 1: Eliminating the zeroes), but to make it worthwhile we need to collect it over many hours and across many square metres of ground. We then need to concentrate it so as to make available the sort of power that modern society needs. Sunlight is not as ‘energy-dense’ as oil but this is made up for by the fact that it is present over such a large area.
By careful design and positioning of houses we can use sunlight to warm our homes and our domestic water. This passive solar heating can help us reduce fossil fuel use (and save money) but it’s not enough to replace those traditional fuels entirely.
To be most useful, the energy in sunlight must be converted to another form
Solar energy becomes much more useful when we change it to another form. Light can be changed directly to an electric current by photovoltaic cells. The efficiency with which these convert light to electricity is still too low, and their cost too high, to make them useful for many applications. Furthermore, you need to have a large area of photovoltaic cells to power something like a car – although it can be done, as demonstrated by the entrants in the World Solar Challenge car rally (Box 2: Driving on a sunbeam).
Australian research is forging ahead on reducing the cost and improving the efficiency of photovoltaic cells (Box 3: Light to electricity).
Many places where energy is needed are not very sunny
Unfortunately, we can’t yet power our homes entirely on sunlight. Photovoltaic cells for a house are expensive, and anyway most houses are not in the sunniest part of Australia or the world. The solution is to put the sun’s energy into a form which can be stored and moved around, so that we can collect it in those places where most of it falls and move it to where it is needed.
Steam or hydrogen gas are the best future conversion options
Sunlight can be concentrated by solar collectors – best sited in a desert. These focus sunlight from a large area on to a central vessel in which water is heated to become very high temperature steam. The expanding steam can power a turbine and generate electricity on a sufficiently large scale that it can be sent across a power grid. The world’s largest, free-standing, steerable solar concentrating dish is in Canberra, where it forms part of the Australian National University’s solar research program (Box 4: The Big Dish).
The greatest challenge for solar energy is to power modern society’s transport and industrial needs. Transport fuels must be light but packed with energy. They must also operate when it’s dark, so photovoltaic cells are out. The answer for the future probably lies in hydrogen gas, derived from water split apart using solar electricity or the sun’s concentrated heat (Box 5: Chemical fuels from the sun).
Boxes
1. Eliminating the zeroes
2. Driving on a sunbeam
3. Light to electricity
4. The Big Dish
5. Chemical fuels from the sun
Related Academy Material
Nova
Wind power gathers speed
Generating new ideas for meeting future energy needs
Biomass – the growing energy resource
Fuelling the 21st century
Which way ahead for hydrogen cars?
Interviews with Australian Scientists
Professor Louis Davies (Physics)
Public Lectures
Australia's Renewable Energy Future - Solar photovoltaics: Power source for the future?
Professor Marin Green,
7 October 2008
Australia's Renewable Energy Future - Solar thermal concentrators: Capturing the sun for large scale power generation and energy export
Professor Keith Lovegrove, 7 April 2008
Page updated December 2011.