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Time to concentrate

 

BELÉN GALLEGO
Founder and Director of CSP Today

Concentrating sunlight to generate energy has been around for millennia. Archimedes, it is claimed, used polished shields to set an invading Roman fleet on fire in 212 B.C.E. And in the fifteenth century Leonardo Da Vinci used large-scale solar concentrators to weld copper. But it is only in the last 200 years that scientists began toying with building solar collectors to generate heat other for than lighting fire — and real progress on concentrated solar power (CSP) only really began toward the end of the twentieth century.

The wheels were first set in motion back in the 1860s, when French inventors August Mouchet and Abel Pifre constructed the first solar engines, but 130 years elapsed before the first of these was connected to a grid. An impressive 8-storey parabolic mirror capable of producing 1 megawatt (MW) was unveiled in France in 1969, but it took a further 16 years for the first CSP plant to come online — in California.

Compared to other renewable energies, like wind and solar photovoltaics (PV), the CSP industry has been slow to get out of the starting blocks. It has been held back by the sheer size and investment requirement characteristic of utility-scale power, perceived investor risk coupled with a weak policy framework to support new technologies, and an unstable economic environment.

Now there are some 1.8 gigawatts (GW) of CSP projects in the global pipeline, with a further 14 GW planned in 16 countries. This may pale in comparison to wind’s expected 2010 global capacity of nearly 200 GW, but CSP has a key advantage over both it and PV: it can be coupled with up to 12 hours of thermal storage (using molten salt, stone and air, or phase change materials). This enables CSP to be dispatched to the grid as and when needed, making it a baseload energy that can compete with fossil fuels and nuclear power.

CSP currently sits at the beginning of its cost curve, which is headed one way — down. Comparing the cost of building a 100 MW CSP plant with 6 hours storage ($0.14 cents per kilowatt-hour (kWh)), with that of building a nuclear plant (a conservative $0.17 – 0.22 cents per kWh) shows it to be cheaper — as well as quicker and cleaner — to deploy. Indeed, if the $557 billion given annually in subsidies to fossil fuels were removed, some concentrating solar power technologies would already be cheaper than coal, and cost competitive with natural gas.

CSP also has industrial applications, as it can replace natural gas boilers traditionally used for heavy industry applications like enhanced oil recovery and can power desalination plants. At the end of its life, the entire plant can be dismantled in a matter of months and — whereas nuclear plant decommissioning costs range anywhere between $100 million to $17 billion — CSP’s are offset by the value of the reclaimed scrap metal.

So how does it work? Like conventional power plants, CSP powers a steam turbine to generate electricity — but by using sunlight. Proven technologies include the parabolic trough, power tower and linear Fresnel systems, which either heat oil to a temperature of up to 370° C in a closed loop to produce steam, or directly produce steam to temperatures up to 500° C.

Power towers already produce direct saturated steam at around 250° C, while a pilot in Israel — the precursor to a planned 370 MW plant in California — produces superheated steam up to 550° C. Fresnel direct steam generators can produce steam at temperatures of 450° C.

Another contender, dish Stirling, does not generate steam to drive a turbine, but uses a mirrored parabolic concentrator dish to concentrate sun onto a receiver, or power conversion unit. This tracks sunlight and heats a gas to temperatures over 600° C to power a Stirling engine that generates electricity.

Dependency on water for cooling plant is a major stumbling block for power generation. A water-cooled nuclear plant requires 720 gallons per megawatt-hour; a coal-fired one up to 520 gallons per megawatt-hour. Most CSP technologies perform better than nuclear and some are on a par with coal. However, CSP relies on near perfect direct normal irradiance (DNI), generally only found in desert-like regions. So obtaining water can be tricky and spark opposition from local people. This has recently given play to more expensive dry cooling technologies. Dish Stirling, however, has a zero water requirement — other than for washing the mirrors.

CSP’s dependence on perfect DNI means it is geographically restricted to the earth’s sunbelt regions, ruling out its use in cooler latitudes. But deserts receive more energy from the sun in six hours than the world’s people consume in a year.

So harnessing this solar resource is vital for Middle Eastern and North African countries like Lebanon and Morocco, which import roughly 97 per cent of their energy. Much of Western Europe will also become increasingly dependent on energy imports as North Sea oil and gas reserves dwindle and so has a vested interest in getting North Africa’s CSP capacity up to speed quickly. Europe represents such a vast energy export market that North African countries could become economic powerhouses.

This would require an investment of up to 200 billion euro in transmission, but the return on investment is guaranteed. Furthermore, it would not only open up a vast new energy market, but enable balancing between renewable energies and thus resolve the despatchability issues that are the Achilles heel of wind and PV.

Fossil fuel import dependents are often locked into long-term purchase agreements, and here bridging technologies like hybrid CSP come into their own. Bolting-on CSP plants to existing coal-fired plants in order to augment steam production would enable existing generators to economize on fossil fuel reserves and create much-needed interim demand for relatively new and expensive CSP technologies.

CSP can be used to augment coal and gas-fired power stations as society transitions to clean, renewable energy, and to replace the fossil fuel reliance of today’s heavy emitters. In the longer term, stand-alone CSP plants could provide 100 per cent clean, sustainable, and renewable baseload power. It makes the future look much brighter.

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