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Cutting Coal Use with Sunshine

How solar-thermal energy could shrink coal plants’ carbon footprint.
February 10, 2009

Feeding heat from the sun into coal-fired power stations could turn out to be the cheapest way to simultaneously expand the use of solar energy and trim coal plants’ oversize carbon footprints.

No sun? No problem: These parabolic mirrors gather heat energy for a 150-megawatt hybrid solar/natural-gas power plant under construction south of Cairo. During the day, solar heat will displace a fraction of the natural gas required to drive the plant’s turbines. At night, natural gas alone will assure continued power generation.

At least that’s what the Electric Power Research Institute (EPRI), a nonprofit organization backed by the electricity industry, is hoping. Last week, the institute launched a nine-month, $640,000 study to pin down the scale of the opportunity and the engineering challenges involved with making these seemingly disparate technologies work together. The study will examine the potential use of solar-thermal technology at a pair of coal-fired power stations, in New Mexico and North Carolina.

Combining solar power with fossil fuels is not a wholly new idea: over half a dozen new and existing natural-gas power stations are being designed or adapted to incorporate solar-thermal technology, which involves capturing heat generated using fields of mirrors and heat-collection tubes.

Retrofitting existing power plants is a low-cost option for solar-thermal projects because the steam turbines that are needed come for free. Such is the case at a giant natural-gas- and oil-fired power plant operated by the utility Florida Power and Light (FPL) in Martin County, FL, where construction of a solar-thermal collector field of 180,000 mirrors covering roughly 500 acres began in December 2008. Steam turbines can comprise 30 percent of the cost of a stand-alone solar-thermal plant.

FPL’s solar field will provide up to 75 megawatts of the Martin County plant’s 3,705-megawatt capacity by feeding solar-generated steam into the plant’s steam turbines. This solar energy is just sufficient to replace the steam currently generated using relatively inefficient “duct” burners that employ extra gas to increase the heat fed into the steam turbines during spikes in power demand.

Purpose-built hybrid solar/natural-gas power plants, such as those being constructed by Flagsol GmbH in Egypt and by Spanish solar-power developer Abengoa in Morocco and Algeria, should boost efficiency even more. Heat from the solar collector fields will be blended with heat from the gas turbines to produce hotter steam. At retrofitted gas plants or stand-alone solar-thermal plants, steam generated directly from solar collectors tops out at 400 °C. At a purpose-built hybrid plant, this heat can generate 500 to 550 °C steam when combined with the heat already used to power the steam generator, meaning more efficient operation.

But the overall efficiency of retrofitted hybrid solar-gas plants is still limited. That’s because a gas steam turbine that has been modified to accommodate waste heat plus solar heat will suffer an efficiency penalty from running at partial load whenever the sun goes down. This is part of the reason why none of the solar-gas hybrid plants under construction rely on solar for more than 15 percent of their power.

In contrast, coal-fired power plants do not suffer from this efficiency cap because they already produce electricity primarily using a steam turbine. As the sun waxes and wanes, the coal feed to the boilers can be adjusted to keep heat production steady and the steam turbine running at full tilt.

Paul Nava, a managing director of Flagsol GmbH, a solar engineering firm based in Cologne, Germany, that is commissioning a 50-megawatt solar-thermal power plant called Andasol 1 in Andalucía, Spain, says that large coal plants could easily absorb 200 to 400 megawatts of solar-thermal power, rivaling the largest stand-alone solar-thermal power projects under construction and dwarfing photovoltaic installations by an order of magnitude. And thanks to coal’s carbon intensity, the emissions benefit will be higher. FPL estimates that fuel combustion displaced by its solar collectors at Martin County will be equivalent to system-wide C02 emissions of 2.75 million tons over their 30-year lifetime–the equivalent of removing more than 18,700 cars from the road each year. But the same solar collector field on a coal plant should displace about double that much CO2.

Cara Libby, EPRI’s hybrid solar-thermal project manager, says that reducing carbon output is the motivation behind the nine-month feasibility study. The idea is to define a low-cost option for EPRI’s industrial members to meet renewable portfolio standards implemented by many states, and to prepare for federal carbon regulations expected to put a price on every ton of CO2 that their plants release.

There are two large caveats, however. Most power plants–natural gas, coal, or otherwise–will not have the combination of strong sun and flat, open ground required to host a solar-thermal collector field. “What is normally underestimated is the distance to the solar field,” says Nava. “You see proposals where there is an area and it is maybe two kilometers away. That long-distance heat transfer would be quite costly.” He says that even those plants with the right sun and space will not move forward until governments put a firm price on carbon emissions from coal–something that few politicians have been willing to do–to justify trading cheap coal for more costly solar-thermal energy. “The integration is very easy,” says Nava. “It’s just a regulatory and political issue.”

Solar-thermal developers say that more such projects, including large schemes involving coal plants, could help drive the nascent technology forward. “Solar collector fields are in a very early stage of development,” says Nava. “If there are more projects around, that will definitely increase the resources.”

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