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.
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.