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Solar Power When The Sun Doesn't Shine

It is often said that solar power plants can't 'do baseload' - that they only provide power when the sun shines. Not so: Richard Keech and Matthew Wright* explain how solar thermal plants work and how the Spanish now get solar power around the clock using advances in thermal storage.

*Executive Director,
Beyond Zero Emissions

The talk about climate change has seen a renewed interest in power. After all, electricity generation is the largest single man-made source of greenhouse gas emissions, mostly from the burning of fossil fuels, usually coal or gas.

But sustainable alternatives exist – one of the most exciting is Concentrating Solar Thermal (CST) power [sometimes called Concentrating Solar Power (CSP)] .

Traditionally, solar electricity generation has been only when the sun shines. Hence the great appeal of new CST plants which can store their heat and generate power even at night.

Solar resource

At ground level, the power of the Sun on a one meter square surface, at right angles to the Sun’s rays is about 1kW.

Excluding cloud effects, this gives an average of about 6kWh/day for every square meter in sunlight. If you do the numbers this represents a phenomenally large resource.

Australia’s total current electrical peak generation capacity (about 49GW) is equivalent to what falls as sunlight on an area of about 8km x 8km (at noon at Southern Australian latitudes) or about 0.001% of the Australian landmass.

When you take into account typical sunlight patterns, typical plant efficiency and layout, you would still need less than 0.05% of Australia’s land area to generate equivalent power.

To put the required land area in perspective, it would fit six times into Anna Creek, Australia’s largest cattle station. It is clear that in a country like Australia, the solar resource greatly exceeds our energy needs.

CST technologies

Before considering how solar plants can run at night, let’s review the underlying technology of CST. They have in common the basic principle of capturing solar energy to heat water to generate steam (see box below‘not all steamed up’). This steam powers a turbine, which in turn spins an electric generator to create AC power. From the point at which the steam is generated, a CST plant is similar to coal, gas or nuclear in its operating principle.

A solar plant is distinguished by how that steam is generated in the first place. To capture solar energy, mirrors reflect the sun’s rays to a central collection point. Different arrangements of mirrors exist. Broadly speaking these are: troughs, power towers, linear fresnel and dishes.

Trough technology

In a trough configuration, long lines of mirrors with a parabolic cross section focus solar radiation on a pipe. A fluid pumped through the pipe to pick up the solar energy is heated to around 400°C. The fluid is usually a high-grade synthetic oil which does not boil or degrade at high temperatures. These oils are only suitable up to about 400°C.

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