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.”