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.