By Nancy Spring
Senior Editor
Solar thermal installations experienced a 15 percent growth rate last year and many experts think that will increase in the next few years — but don't count fossil fuels out yet. Combining solar thermal and fossil fuel energy into a hybrid power plant shows promise.
Speaking at the Renewable Energy World Conference and Expo, Kelly Beninga, global director of renewable energy at WorleyParsons Group, said 5,000 MW of hybrid concentrating solar power (CSP) are currently under development at WorleyParsons comprising five integrated solar combined cycle (ISCC) natural gas projects and three projects that integrate solar with coal.
"We are technology neutral," said Beninga.
Integrating CSP cycles with Rankine cycle coal plants is beneficial in several ways, said Beninga. The cost of electricity is lower, the system is more efficient than a stand-alone CSP and it can come online more quickly. Solar steam can work in parallel with or instead of duct burners and some steam can go directly into the turbine, he said. Hybrid plants don't have to be new: retrofitting existing power plants to work with CSP is an option too, often only requiring some drum modification.
There are issues, such as the limit to how much solar energy the system can absorb, which according to Beninga is about 15 percent to 20 percent, and determining the renewable content of the plant's output for renewable energy credit (REC) accounting, but combining fossil fuel-fired plants with CSP is a good alternative to thermal energy storage, said Cara Libby, project manager, Electric Power Research Institute (EPRI). "It's a good way to firm the output of the plant and make it more dispatchable."
Libby, whose work is focused on solar project feasibility assessment, said EPRI is involved in several solar integration studies, including NV Energy's 1,100 MW natural gas combined cycle (NGCC) Chuck Lenzie Plant and Dynegy's NCGG 570 MW Griffith Plant. She said other hybrid solar options show promise, too, like low temperature geothermal resources.
EPRI and a number of utilities are now studying the potential to add solar power to existing power plants in order to help cut their greenhouse gas emissions. EPRI is working with Tri-State Generation & Transmission Association Inc. and Progress Energy to evaluate the potential to add solar thermal energy systems to the utilities' power plants in Prewitt, N.M., and Roxboro, N.C.
EPRI says its concept involves building fields of mirrors adjacent to a power plant to focus the sun's heat and boil water into steam. The steam from these solar thermal fields would be integrated into the steam cycle of the fossil-fueled power plant to either reduce its use of fossil fuel or to increase the plant's power production. Solar thermal research engineers at Sandia National Laboratory and the National Renewable Energy Laboratory (NREL) will help to analyze the performance of the hypothetical solar energy systems.
Combining solar thermal and fossil-fuel energy in one system is not a new concept. EPRI says most of the solar energy generating system (SEGS) power plants in California use natural gas as a backup energy source. Although the SEGS plants initially operated 25 percent of the time on fossil fuel, according to NREL their annual output from natural gas has been cut back to "a few percent."