Utilities traditionally have met renewable portfolio standards with power purchases from IPPs. But new approaches are allowing utilities to build their rate bases with investments in solar...
Hybridizing fossil plants with solar thermal technology.
be for the host unit. The temperature of the solar steam depends on the type of solar technology used. For example, while steam temperatures in a coal-fired turbine can reach 1,100 degrees F or more, parabolic mirror-based concentrator solar systems using synthetic oil heat transfer fluids, can’t exceed a flashpoint of 750 degrees F. However, if solar tower technology is employed, the molten salt heat transfer fluid can reach 850 degrees F or higher. And linear Fresnel concentrated solar lines using water as the transfer fluid can reach temperatures exceeding 540 degrees C (1,000 F), according to Areva Solar. Each of the technologies has different advantages and price considerations, all depending on what the host unit needs most.
Tower-based solar hybrids are the most tested in the United States thus far. “Power tower performance today is better than trough performance,” says Mike Gradiola, GE Energy’s general manager of concentrated solar power. “We’re talking with a number of U.S. folks about solar-fossil hybrids. We’re less than a year into our business play, and tower-based concentrating solar power is in the discussions now.”
Linear solar heat designs are touted as the least expensive per-thermal unit generated. Areva already has a long target list of U.S. clients for its solar hybrid Fresnel technology. “I’m optimistic about the potential for this type of solar hybrid; there are 60 to 75 U.S. fossil-fueled plants in good solar isolation areas that could take advantage of this technology,” says John Robbins, director of North American sales for Areva. “Apart from fossil fuel plants, we’re talking to utilities and independent power producers about biomass, geothermal and waste-to-energy plants, if they have steam turbines. Utilities understand steam, so it’s a good fit, and marrying the two provides good dispatchable power.
“Solar booster projects like this are gaining momentum in the United States and around the world as a way to leverage existing power infrastructure to provide needed energy with no new emissions,” Robbins adds.
For each solar thermal approach, technology developers are trying to drive costs downwards. The most promising idea involves energy storage systems. Several competing energy storage technologies are largely at the demonstration stage among U.S. utilities, but the use of molten salt as a heat transfer fluid and storage medium is well researched.
“The current cost estimate for installing molten salt tower-based concentrating solar hybrid capacity is about 15 cents per kWh, compared to standard PV a couple or pennies cheaper at the utility scale,” Turchi estimates. “But with gas prices as low as they are, there have to be other considerations.”
Turchi co-authored a 2011 NREL study that examined the efficiency gain from a solar hybrid plant in combination with molten salt storage. The study showed strong benefits: “In the preliminary analysis, it is shown that a single 40-MW aeroderivative gas turbine mated with a 100-MW parabolic solar trough plant can be more efficient than two separate power plants.”
Similarly, modular solar hybrid technology designs—which are scalable to virtually any size—can help reduce costs and maximize project bankability. One U.S. company offering a modular approach