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March 2009

Power plants burn coal, sunshine

With turbines already in place, the idea is to surround a coal-fired plant with solar reflectors that supplement the heat and steam that course through the system.

The Electric Power Research Institute, a nonprofit organization backed by the electricity industry, 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.

Parabolic trough power

Technology Review reported 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.

Combining solar power with fossil fuels is not a wholly new idea: Over half a dozen new and existing natural gas power stations are adapting or incorporating 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 already there and thus, 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 in Martin County, Fla., where construction of a solar-thermal collector field of 180,000 mirrors covering roughly 500 acres began in December. Steam turbines can comprise 30% of the cost of a stand-alone solar-thermal plant.

Purpose-built hybrid solar/natural-gas power plants, such as Flagsol GmbH is building 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 blend 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°C to 550°C steam when combined with the heat already used to power the steam generator, meaning more efficient operation.

However, the overall efficiency of retrofitted hybrid solar-gas plants is still limited. That is because a gas steam turbine that they modify 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 relies on solar for more than 15% 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 compensate and adjust 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, said 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.

Nicholas Sheble (nsheble@isa.org) writes and edits Automation Update.