Where Are the Hybrid Solar Plants?

When a coal plant converts to gas, why not build solar too?


It was an opportunity lost. In February this year, Public Service Co. of New Mexico agreed to shutter two coal-fired generators totaling 868 megawatts (MW ) at the San Juan Generating Station, and replace them with gas-fired boilers. It was part of a deal with the U.S. Environmental Protection Agency (EPA) to reduce emissions. It was disappointing that no solar technology was included in the redevelopment plans, scheduled for completion in 2017.

Solar and gas make good companions. Solar generators operate intermittently, producing power most prolifically when the sun is brightest. But utilities need plants that can operate on demand. Energy storage can be added to solar plants to extend their operation to non-producing hours, but it adds to the cost of an already relatively expensive technology. Even with storage, auxiliary generation is needed to fill in on cloudy days.

solar thermal and natural gas energy plantfpl

FPL’s Martin Next-Generation Solar Energy Center connects
a field of solar thermal mirrors with an existing combined-cycle
natural gas power plant. Photo: FPL

In a solar-natural gas hybrid plant, the gas-fired generator effectively replaces storage. Gas-heated boilers operate more flexibly than coal generators, and from an engineering point of view, gas systems are relatively easy to meld with solar systems.

San Juan, located in the sunny Four Corners desert region, would have been an ideal location for a concentrating solar power system. CSP is best suited to a gas-boiler hybrid, because both systems produce heat for the same set of steam turbines and condensers. San Juan also has adequate land area for a solar field.

Natural gas produces about half the carbon emissions of a coal plant and far fewer of those that are suspected of affecting human health, such as nitrogen and sulfur oxides and mercury. According to the EPA, coal-to-gas plant conversions are responsible for a 200 million-ton decline in carbon dioxide emissions in the United States in 2012. U.S. CO2 emissions are at about a 20-year low. The United States is one of only a handful of countries where emissions have declined while over- all global CO2 emissions have increased to record levels.

Coal-to-gas plant conversions would be even more cost-effective and carbon-efficient if they included solar integration. According to the U.S. Energy Information Agency 15 gigawatts (GW) of power generation will be retired in the next two years. Many of them will be in locales where a solar hybrid plant is a technical option. Assuming that solar technology can be integrated in half of these plants, with each producing half of the generating capacity, the potential new solar installation capacity through 2015 is nearly 4 GW, about 2 GW annually. That is a 37 percent increase in annual solar capacity growth, based on current installation rates.

CSP for hybrid gas plants can take the form of parabolic troughs or power towers. In both cases the heat-transfer fluid goes through a heat-exchange boiler to produce steam for the turbine that drives the generator. Heat to this boiler can also come from thermal storage or from burning natural gas. The solar system is the primary source of heat, and fossil fuel is used when the solar heat is insufficient, such as at night. The design goal is to integrate the largest possible solar field, often with a small amount of thermal storage for operational stability. Economics and available solar resource are the key drivers in the design. These in turn are influenced by geography and climate — CSP needs clear sky conditions in order to focus sunlight accurately.

Another potential hybrid design involves a large PV field paired with one or more gas-turbine combustion engines. The combustion turbine — essentially a large jet engine turning a generator — can be brought into operation very quickly: Ramp rates from idle to full power are measured in just a few seconds, which makes them ideal companions for PV, which can ramp down in the same time frame due to clouds. While gas turbine systems are effective for short-term use, they are not economical for producing large amounts of electricity over long periods of time. Thus, there is a tradeoff between the cost of the PV field and the size of the combustion turbine.

The window of opportunity for solar hybrid plants is lucrative but narrow. Coal plant operators face growing pressure to meet increasingly strict emission requirements. The Obama administration has recently added additional inducements. But most plant operators have neither the experience nor understanding of how hybrid plants work. The challenge for the solar community is to identify these conversion projects as early as possible and to present the solar options to both the utility operators and to the appropriate regulators — while the projects can still be grandfathered in before state solar incentives expire.

Burning gas is not an ideal solution, but the gas-solar hybrid electric plant is certainly an improvement over today’s coal generators.

Dave Menicucci (dfmenic@unm.edu) is retired from Sandia National Labs and heads Building Specialists Inc., an energy research company.


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