Solar 'thermostats' face gas competition

Weekend Australian
1 January 2011 Page: 26

SOMETHING new is headed for the southwest desert in the United States: solar power plants that can make electricity whether or not the sun is shining. Abengoa Solar expects to start construction in six months' time on a plant in Arizona that will store sun-generated heat to provide six extra hours a day of electric-generating capacity. The heat creates steam that is used to turn power turbines. Abengoa's SUS2 billion ($1.966bn) Solana plant is expected to be the first major stored heat plant in the US when it enters service in 2013. Some already exist in Spain and a few more are on the drawing board for Nevada and California.

On December 21, Abengoa Solar, a unit of Spanish utility company Abengoa, cleared a major hurdle when it announced it received a SUS1.45bn US loan guarantee for the 250-MW Arizona project. The Solana plant will be able to meet winter heating and lighting needs by putting electricity on the grid early in the morning before the sun is shining and help satisfy summer cooling demand by producing power after sunset.

The plant, which can power up to 70,000 houses, has signed a 30-year agreement to sell electricity to utility company Arizona Public Service. Such utility-scale solar plants use mirrors to focus the sun's rays on a liquid, contained in tubes, which can be heated to very hot temperatures.

The liquid is used to boil water and create steam. By using a conventional steam-turbine generator, electricity is produced. But the twist is that the Arizona facility will have two giant salt tanks, each 37 metres in diameter and 10.4m deep, that together can hold and store 40% of the heat created by the plant. Such storage technologies are expected to become more commonplace in the US at solar plants, as officials try to limit the release of CO₂ from fossil fuel power plants and make renewable power production more dependable.

Mark Mehos, a solar program manager for the National Renewable Energy Lab in Golden, Colorado, said such molten salt storage systems added about 20% to the construction cost of solar plants, but more than make up for it by boosting a plant's flexibility and productivity. Electricity from solar plants is expensive, especially at a time when natural-gas prices have plunged, making gas-generated electricity cheap by comparison.

Utilities, which are under state mandates to buy more clean power, say solar power may look more economical in the future if fossil fuel prices rise or if a tax is imposed on carbon emissions by power plants. When it comes to renewable energy, solar competes most heavily against wind power. A study by the Lawrence Berkeley National Laboratory in Berkeley, California, in February 2010 found that utility-scale solar plants with storage capacity were three times as costly to build as wind farms without energy storage.

But the study found that solar electricity was more valuable, because its output was more correlated to peak electricity demand. Still, experts say that unless costs come down, the number of solar projects that get built will be limited. SolarReserve, a power development company in Santa Monica, California, is working on two solar projects in Nevada and California that will have even more heat-storage capacity, relative to their size, than Abengoa's project.

These plants will put out 110MW and150MW in electricity, respectively, and will be able to store enough heat to run eight to 12 hours without additional sunlight. SolarReserve expects to have the two plants in service by 2014. Each will cost SUS650 million to SUS750m.

Don Brandt, chairman and chief executive of Pinnacle West Capital Corporation, parent of Arizona Public Service, said heat storage at the Abengoa project made it "an extremely attractive project for us". By 2015, Arizona Public Service wants to get 10% of its electricity from renewable sources, and Abengoa's plant is expected to contribute a third of that. Arizona's statewide goal is 15% renewable energy by 2025.

Mr Brandt said peak electricity demand for his utility typically hits about 4pm in the summer, but "we remain at elevated levels until around 10 o'clock at night" so getting renewable power later in the evening is valuable. The US National Renewable Energy Laboratory, part of the Department of Energy, says molten salt storage is "proven technology". "When you put heat into one of these tanks, you get 95% or 96% of the heat back out again", said Mr Mehos, at the federal energy lab. "It's a nice big Thermos".

Another plus of all three plants was they would produce power that could be tailored to a utility's specific needs, the president of Abengoa Solar, Santiago Seage said. That is an asset to electrical grid operators that like to know they can rely on certain amounts of power flowing on to lines.