The new hot ticket

A lack of government support pulled the rug from under the fledgling Concentrating Solar Thermal Power (CSP) industry. Twenty years later, as E&T discovers, the technology is enjoying a renaissance.

Ask people what they think the new age of solar power will look like, and most see images of flat solar panels stuck to roofs, providing heating or hot water. A smaller number will have heard that you can generate electricity from the sun too - through panels that use semiconductor materials (mostly silicon in crystal or amorphous form) to convert light into electricity - so-called solar photovoltaics (PV).

That's probably as far as it goes. Many people remain sceptical that solar power could produce cost-competitive, utility-scale electricity generation.

But that's exactly what the Concentrating Solar Thermal Power (CSP) industry has been striving for many years to achieve, despite a false dawn in the 1980s when CSP's early pioneer (Luz International, which built nine CSP plants in California's Mojave Desert) went bankrupt. This signalled the start of a dormant period as CSP technology gradually faded into the background of the renewable electricity sector, surpassed by robust interest in the more 'glamorous' technology of PV.

CSP basics

As opposed to PV, CSP generates electricity using technology common to most single-cycle or combined-cycle power plants. But, instead of using natural gas or coal as a fuel source, the sun produces the steam that drives the engines or turbines in a plant. The clever part, and this elicits much debate in CSP circles, is how best and most cost-efficiently to capture the sun and turn it into heat energy. Parabolic Trough CSP plants, for example, consist of large parallel arrays of parabolic trough solar collectors, which constitute the solar field. Power Towers consist of a tower surrounded by a large array of heliostats, which are mirrors that track the sun and reflect its rays onto a receiver at the top of the tower.

Other technologies are being developed too - Fresnel mirror CSP systems have long flat mirrors at different angles to focus sunlight on one or more pipes containing heat-collecting fluid, which are mounted above the mirrors. And Solar Dishes consist of a dish-shaped concentrator that reflects solar radiation onto a receiver mounted at the focal point; this receiver can either be a Stirling engine and generator for dish/engine systems, or a type of PV panel that has been designed to withstand high temperatures - in Concentrating PV (CPV) systems.

As the number of megawatts generated through CSP has grown, so the scale and size of the projects planned has increased. There is now around 6GW of CSP capacity in the USA (installed or under contract), and 3.7GW active or planned throughout the rest of the world. Operational CSP installations range from 1MW through to 64MW - Solar Nevada One in the USA.

But there is also an increasing number of utility-scale projects in the pipeline that could result in plants of 100MW plus over the coming years. One example is US utility PG&E's deal with Solel for the 553MW Mojave Solar Park (scheduled for 2011). And Southern California Edison (SCE) recently signed a Power Purchase Agreement with BrightSource Energy to use power from the latter's 1,300MW of planned Power Tower CSP plants, also planned for the USA.

These deals alone suggest that the utilities themselves believe CSP has a bright future.

CSP revival

Currently, Spain and the USA's south west host much of the commercial, large-scale activity in CSP. But technology could also be deployed in countries like Algeria, Morocco, South Africa, Israel, India, and China.

Seville, Spain, has two commercial plants - Abengoa Solar's PS10 and PS20 (11 and 20MW respectively). Recently constructed, when both these plants become fully operational they will provide energy for 200,000 people. PS10 is operational, PS20 is currently in the start-up phase.

In the USA, States like California, Arizona, Nevada, New Mexico, Colorado, some of Utah, and Texas could host a large amount of CSP generation. The south west of the USA, for example, has 87,000 sq miles of land that could be used for CSP, potentially generating a staggering 11,200GW, believes Dr Fred Morse, a specialist in CSP.

Looking beyond 2012, by which time many of the current generation of CSP plants will be operational, there is also talk of CSP plants situated in northern Africa transmitting electricity to Europe via high-voltage direct current (HVDC) cables across the Mediterranean. One Project - the Mediterranean Solar Plan (Med Solar) - aims to have in place 20,000MW of CSP in north Africa by 2020.

This may seem far-fetched, but some scientists believe that less than 1 per cent of the world's deserts, if covered with CSP plants, could produce as much electricity as the world currently uses.

Aside from an increased focus on renewables brought about by climate change, security of supply, and depleting fossil resources, CSP has benefited from the shortcomings of other high-profile renewable electricity technologies. The high cost of PV is one contributing factor that has gone some way to reviving interest in CSP. "Truthfully, without the silicon shortage and the resulting constraints on supply [for PV], CSP might still be struggling," says Paula Mints, a PV expert with Navigant Consulting.

Wind power too, whose installed capacity worldwide now stands at a respectable 120GW, has its own well-documented disadvantages. The biggest of these is its intermittency, combined with a lack of electricity storage, so until there is a commercially viable way of storing wind for use during periods of high demand, wind will always be at a disadvantage when compared with baseload energy technologies.

CSP can be much more predictable and better aligned with peak demand, as Direct Normal Insolation (DNI) - the 'fuel' for CSP, is strongest during the day when people need power.

Thermal storage and utility

Certain CSP technologies, such as Parabolic Trough and the Power Tower, can incorporate thermal storage, in the form of, say, steam or molten salt. Excess heat is pumped into vats of salts, turning them into a molten, lava like consistency. Heat can then be released from the salts when there is no sun to keep the turbines going. The 50MW Andasol 1 in Spain uses fertiliser salts - a mix of sodium and potassium nitrate - as thermal storage.

It is also possible to use gas as a stop-gap source of heat when there is not enough sun. CSP with heat storage and/or hybridisation with gas firing allows CSP plants to provide baseload power, which is extremely valuable to a utility.

Add to that the high cost of technologies such as PV - alongside intermittency concerns of wind - and you find CSP looking a lot more attractive - especially those projects such as Andasol 1 that include thermal heat storage: "Higher prices for crystalline flatplate technologies," says Mints, "drove the price of PV technology up in the short term; this has opened a window through which CSP - with its storage capabilities - climbed through."

While CSP still can't be directly cost competitive with wind, that's actually not always the major issue. Solar thermal electric technologies function like most single-cycle or combined-cycle power plants. And while wind power generation prices (per kWh) may currently be cheaper, another important consideration is the value of the technology to the utility, and the reliability/dispatchability that can be factored in.

"The value of energy is not a straightforward calculation", explains Barbara D. Lockwood, manager - renewable energy at the Arizona Public Service Company (APS): "With solar [CSP] for example, we know that we can count on it and it is producing in the extremely peak parts of the day so it's worth a whole lot more to us. The cost gap is not linear - you can't just subtract the price of wind from the price of solar and say 'it's that much cheaper' and conclude that 'that's what the value is'. The value of wind is generally less than the value of solar because solar produces power during periods of high demand."

Barriers to CSP

Of course, CSP still faces significant challenges. The major one is cost, especially compared with wind power. Costs for CSP, currently around 9-18¢ (US) per kWh - depending on technology, plant size and scale, and whether heat storage is incorporated - are predicted to fall to about 6¢ per kWh over the next decade, thanks to scale and improved technology. But wind is still a cheaper option at around 4-6¢ per kWh (though offshore wind would be more than this).

CSP is also capital intensive, and the credit crunch hasn't helped. However, in an industry that admittedly still needs government support, President Obama has so far been kind to the solar industry in the USA, a fact that Fred Morse acknowledges: "We now have a grant instead of the investment tax credit, and we have a federal loan guarantee, which should help on the debt side. Those are extremely important."

Perhaps an even bigger problem faced by CSP developers is transmission. Plants by their nature are generally built away from large centres of population and this makes transmission a huge challenge that needs to be addressed before CSP really has a chance to shine: "It's only when you can access the sun-rich parts of the country that CSP could start to meet a broader market," says Morse.

But addressing the transmission issue is hugely complex: "It's a problem of three dimensions," he explains. "One is planning; then there is a major issue of siting. And then how do you allocate the cost of this new high voltage that they call a 'backbone' transmission system? Who pays? What if a line goes through a State and none of the power comes off in that State? Do they pay at all?"

Despite a promised concerted effort in the USA under the new administration to address the issue of transmission, Morse's best guess is that no new transmission will open up for at least five years, and this could stifle the conception of new CSP projects and partnerships, at least in the USA.

Finally, CSP also has detractors who justifiably point to the large amounts of water needed as part of the process, though much effort is being undertaken to improve efficiency and reduce the water requirements. Other concerns centre on the effects that large solar power stations have on fragile desert ecosystems, as well as the perception that CSP might turn out to be another case where rich countries take what they need from poor countries, and leave little for them except pollution.

But in a world that needs to wean itself off fossil fuels and put GW-scale alternative energies on the ground, many believe that CSP simply can't be overlooked this time around.

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