Greening Coal-Fired Power with Solar
As renewable energy technologies become more commonplace, more questions emerge as to the practicality and other problematic side-effects from otherwise promising approaches. Even solar technologies have come into question with problems related to disposing of spent photovoltaic panels. The Electric Power Research Institute (EPRI) assists the electric utility industry by researching new technol...
As renewable energy technologies become more commonplace, more questions emerge as to the practicality and other problematic side-effects from otherwise promising approaches. Even solar technologies have come into question with problems related to disposing of spent photovoltaic panels.
Solar locating guides calculate available energy based on average "sun hours," or the number of hours per day where irradiance is at least 1 kW per square meter. While the two plants in the study are in much different parts of the country, the solar energy availability is surprisingly similar.
The Electric Power Research Institute (EPRI) assists the electric utility industry by researching new technologies related to the generation, delivery, and use of electric power. Its member utilities are responsible for more than 90% of the electricity generated in the U.S., with international participation from 40 countries. The group has researched many environmental and renewable projects, guiding development from concept through evaluation, texting, and commercialization.
As one of its renewable energy projects, EPRI is in the evaluation phase with two new projects to study solar steam generation that can supplement conventional thermal technologies, allowing utilities to increase output or reduce fossil fuel consumption. The objectives of the studies are to determine the practicality and effectiveness of adding solar steam generation to existing natural gas combined cycle plants and conventional coal-fired plants.
Solar technologies have been a major area of interest for renewable power generation, however the biggest drawbacks have been efficient power conversion. The amount of solar energy hitting the earth is far greater than we could ever need, but we simply do not have practical technology to harness it. Considering that fossil fuels were formed from ancient plant life, all sources of energy on the earth (with the exception of nuclear and geothermal) are ultimately solar.
Photovoltaic vs. thermal
Photovoltaic technologies work, but are not very efficient at this time. Those that are considered cost-effective are typically in the 15% efficiency range, generating about 12 W per square foot, but this number is growing. One concern looking forward is creating hazardous waste with used panels that have run out their effectiveness, due to some toxic substances used in their construction.
Solar technologies are attractive to utilities, because the availability of sunlight typically corresponds with demand for electricity. "Yes, that's correct, roughly," says Cara Libby, manager for EPRI's solar steam projects. "It doesn't quite correspond with the demand peak if it's in the late afternoon, but it's much more on-peak than wind which tends to be highest at night. However, you don't always have the same input of solar power. It's going to be low in the morning, peak sometime in the early afternoon, and then start to decline."
The control strategy of a typical coal-fired plant with supplemental solar power will require agility to keep everything running on an even keel. "Controlling and balancing the changes in solar output with the traditional steam cycle is a very important aspect," Libby adds, particularly when the plant has to track with changes in demand from the grid.
Solar-generated steam has been developed for quite a while and some approaches are considered relatively mature technologies. These studies intend to center on using parabolic trough solar collection, where long pipes are laid into reflective troughs that concentrate sunlight on the surface. Steam generated in these pipes can reach temperatures of 700 °F, so the output is sufficient for many uses in a power plant. The problem is that it is not equal to steam generated by a typical utility boiler, which is more like 1,000 °F and higher.
This means that adding solar steam to the main turbine feed will actually degrade efficiency; however, the effect of that is difficult to predict. "That's the purpose of the study," Libby adds. "We might look at some different scenarios, depending on the needs of the individual utility. Some are looking at this to meet RPS (renewable portfolio standard) requirements. Others want to reduce emissions or back off on their fuel consumption. Some need to determine how much solar steam can be added to the plant, and that depends on the solar technology chosen."
Solar generated steam can be used in many applications in a plant, but may not go directly to the main turbines.
Using lower-quality steam
Since solar steam is not the best for existing turbines, the study will also explore ways that it can be used in other parts of the plant. This will reduce the amount of steam the main boilers generate and still maintain an equivalent level of output. "For coal plants, the most straightforward approach is to do feed water heating," Libby suggests. "In these plants, there are five or six stages for reheating the water. There are other auxiliary components that are steam-driven that might be able to use the solar steam."
EPRI has chosen two locations for the coal-fired plant study that represent much different situations. The first is Tri-State Generation & Transmission Association's 245 MW, Escalante Generating Station in Prewitt, NM. The second is Progress Energy and Southern Company's 742 MW Mayo Plant in Roxboro, NC.
"These projects will demonstrate a near-term and cost-effective way to use large amounts of solar energy at commercial scale to provide clean electric power," says Dr. Bryan Hannegan, vice president of generation and environment at EPRI. "These 'hybrid power plants' will combine the low-cost reliability of existing fossil power plants with the environmental benefit of renewables, and help companies meet federal and state mandates to reduce their emissions of air pollutants and greenhouse gases with renewable energy."
Libby has been working with the plant operators and has seen their interest. "The companies we're working with are really excited about these projects," she says. "There's every indication that they plan to move forward assuming that the economics are attractive and the performance of the plant is unaffected. Utilities have these existing assets that they don't just want to turn off. This is a good option for extending the life while greening those assets. We think this will be the lowest cost approach for a utility company to add solar generation to its portfolio.
"You don't have to build a new power block, you don't have to gain transmission access or all the plant permits that would be required for a greenfield site. So there's a great cost savings in having the existing plant already in place. Retrofits are a lot simpler than developing a project from scratch."
The question going forward will be to see how much steam the site can generate, and what kind of effect it has on the plant's operation. Do the utilities have some break-even point in mind to determine success? Libby answers, "We're really not shooting for any specific number. We're trying to see what's technically feasible. What's the maximum amount of solar that the system can accommodate without doing anything to the reliability or operation and maintenance or damage the equipment?
"We'll be doing modeling to see if the performance of the plant will change with different amounts of solar input, and how that affects the economics. We'll look at things like if there were a carbon tax imposed and how that would change the plant cost structure over its lifetime.
"There are a lot of variables, and the results will be a range of options. Ultimately it's the plant owner's decision if they want to make this capital investment, but it extends beyond dollars. It's a perception thing—if you build a solar thermal plant as part of your coal plant, that gives you a lot of visibility. Each of these companies has different motivations for doing this kind of project, but they're all facing legislation that will call for higher levels of renewables. They're looking at all their options, seeing what they can do and what it costs."
Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com .