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Mixing Solar with Coal to Cut Costs

A new strategy could reduce coal plant emissions and cut the cost of solar power.

A project that will add solar power to a coal-fired power plant could reduce the amount of coal required to generate electricity and dramatically cut the cost of solar power.

Solar vision: A rendering of a proposed solar array at the site of a coal plant near Grand Junction, CO.

The approach, announced by Abengoa Solar, based in Lakewood, CO, and Xcel Energy, Colorado’s largest electrical utility, would make it easier for utilities in sunny states like California to meet impending state renewable-energy requirements.

The project will involve building an array of parabolic mirrors designed to concentrate heat from the sun, and using that heat to help make the steam that drives the coal plant’s turbines and generators, making electricity. Such mirrors have been used for decades to generate electricity at stand-alone concentrated-solar power plants, also called solar-thermal plants, which are currently the cheapest source of solar power. But pairing the concentrating mirrors with a coal power plant offers a way to make this type of solar power even cheaper because a large part of the cost of a solar-thermal plant is the equipment for converting heat into electricity. The Abengoa Solar project will use existing boilers, turbines, generators, and so on, reducing this cost.

“The thing that’s attractive about this is you only have to buy the solar field portion of the plant, which is 50 to 60 percent of the cost of the plant,” says Hank Price, director of technology at Abengoa Solar. That could effectively make solar-thermal power about 30 to 50 percent cheaper, according to various estimates. That would equate to a range of about six to 12 cents per kilowatt-hour, which is competitive with many conventional sources of electricity. “It’s potentially the most cost-effective way to get significant solar power on the grid,” he says.

In the new project, because parabolic troughs don’t generate sufficiently high temperatures, the heat they produce won’t be fed directly into the turbines. Instead, it will be used to preheat water that will be fed into the coal plant’s boilers, where coal is burned to turn the water into steam. Ordinarily, this preheating is done by siphoning off some coal-generated steam. Under the new design, more of that steam can be directed to the turbine to generate electricity. The net effect is that less coal is used to generate a given amount of electricity, and the augmented system reduces carbon dioxide emissions as much as a stand-alone solar-thermal plant with the same size array, but at a much lower cost, says Craig Turchi, a senior engineer in the concentrated-solar power program at the National Renewable Energy Lab in Golden, CO.

This will not be the first time solar thermal and fossil fuels have been paired. Several solar-thermal plants have been built with natural gas as a backup energy source so that, among other things, output can be maintained on cloudy days. But integrating solar thermal with existing power plants has a couple of advantages. The first is the cost savings that comes from using existing turbines and generators. A second advantage is the potential for operating at higher efficiencies. The turbines and generators in solar-thermal plants are optimized to run at the temperatures generated by parabolic mirrors (at least in current designs), which are lower than those generated in fossil fuel-powered plants–about 400°C versus 500°C or higher. Using the higher-temperature turbines in coal plants results in higher efficiency–about 45 percent of the energy in the heat generated by the coal and solar concentrators combined is converted into electricity, as opposed to only 38 percent of the heat with a typical solar-thermal plant.

Another approach to integrating solar power with fossil-fuel plants is being taken in projects in North Africa, two of which are being built by Abengoa Solar’s parent company, Abengoa, based in Spain. At these projects, solar-thermal heat is fed into combined-cycle natural-gas-powered plants designed with solar input in mind. One turbine is turned by gases that expand as the natural gas is burned, and the heat generated by burning natural gas is used to make steam that drives a second turbine. The solar-thermal heat is used to help generate this steam, improving the overall efficiency of the plant.

Although these approaches offer a way to reduce the cost of solar power, their scope will be limited. At the most, the contribution from solar power at existing plants will probably be no more than 10 to 15 percent of the electricity produced, Turchi says. For the Colorado project, which is a relatively small demonstration project, the share will be more like 3 percent, although this can be increased by installing a larger array of mirrors. What’s more, the only coal plants that can be augmented by solar are those in sunny areas with enough nearby land to accommodate the mirror arrays.

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