May 14, 2012

Sun shines on solar power innovation

Construction Corner | Korky Koroluk

Who says you can't have solar power when the sun isn't shining?

Certainly not SolarReserve LLC, which is building a concentrating solar power (CSP) plant at Crescent Dunes in the Nevada desert.

Not BrightSource Energy and Bechtel, who are developing another at Ivanpah, in the Mojave desert in eastern California.

Not the various government agencies that had to approve the project before a shovel went into the ground.

A CSP plant in Spain has demonstrated the viability of the technology by generating electricity for 24 consecutive hours one day last summer. It has followed up that impressive feat by continuing to produce electricity for an average of about 20 hours a day.

The Crescent Dunes plant will produce 110 megawatts. The Ivanpah project is actually three plants on one large site, and will generate 392 MW.

Both are now under construction.

The first phase of the Ivanpah project should be operating next year, with the second and third phases coming on stream over the next five years. The Crescent Dunes plant should be in production by the end of next year.

These are expensive projects. The Ivanpah plants are expected to cost $2.2 billion and are supported by a $1.375 billion loan guarantee from the U.S. Department of Energy. The Crescent Dunes plant will cost about $1 billion and has received a $737 million loan guarantee from the department.

So how will these plants generate solar power when the sun isn’t shining? In a word: salt.

They will use what has come to be called power-tower technology, which stores the sun’s energy as heat in molten salts. When the sun sets, that heat is used to generate steam to drive the plant’s turbines.

The system consists of hundreds of flat glass mirrors, called heliostats, to concentrate the sun’s rays on a receiver atop a tall tower.

The heat is transferred to immense tanks of sodium and potassium salts, which can reach 565°C. That provides plenty of heat to produce steam for the turbines.

Here are some figures from the Crescent Dunes project. The site covers 718 hectares of land. On it, arranged in a circle around the central tower are 17,000 heliostats.

Each one uses computer drives to follow the sun, reflecting its rays onto the collector atop the tower.

Every heliostat is made up of 25 flat mirrors, all on a steel frame atop a 7.5-metre post anchored in a concrete foundation. The electrical connections involve about 320 kilometres of wire and about 16 kilometres of conduit.

The power tower, of reinforced, slip-formed concrete, is 164 metres tall and 18 metres in diameter.

The receiver consists of 14 panels of tubing that carry the salt mixture. There are two huge holding tanks for the salt, with a total capacity of about 150 million litres.

All this takes space, of course.

The circle of heliostats is about three kilometres in diameter. That means you can’t build them just anywhere. Sunny desert or near-desert locations are ideal.

These projects involve large workforces — about 600 construction people at Crescent Dunes, about 1,400 at Ivanpah, so there are economic benefits to the local areas.

Once in production, the jobs will be far fewer, of course, but the plants will emit almost no greenhouse gases. That’s important because Nevada lawmakers have decided that 25 per cent of the state’s electricity must come from renewable sources by 2015.

In California the objective is 33 per cent by 2020.

There will be a price, however.

Power from these plants will be somewhere around 13.6 cents per kilowatt hour. That will be blended into the rates consumers now pay for power from conventional sources, so there will be a rate increase and some think that’s unacceptable. Others look at it differently. To them, it’s simply an investment in the environment.

Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments or questions to editor@journalofcommerce.com.

Print | Comment