Solar Thermal Mixes Well with Conventional Baseload Facilities
Solar thermal has a proven track record and a commercial operating history of more than 20 years. Solar thermal power plants can be found in the U.S. Southwest, along with a facility located in Florida. This technology also has a significant presence in Spain, North Africa, Mexico, India and the Middle East.
“Black & Veatch’s solar thermal technology experience dates back to the late 1970s during the oil crisis,” said Kevin Joyce, Black & Veatch Renewable Energy Consultant. “More recently, we have worked on the Martin Next Generation Solar Energy Center for Florida Power & Light, the first and only solar thermal booster project with an existing combined cycle.”
That project highlights one of several advantages solar thermal offers – its ability to work with existing facilities.
“Solar thermal technology fits with the traditional paradigm of using steam cycles to generate electricity,” said Sam Scupham, Kansas City Office Manager for Renewables for Black & Veatch’s global energy business. “Utilities are pursuing this integration as a way to add renewables to their energy mix in a cost-competitive way and to ensure the stability of renewable generation.”
Solar thermal power plants can also be built with storage capability, which enhances the ability to dispatch the plant when energy is needed most, not just when the sun is available. The energy is stored using a storage media – molten salt – which is kept in insulated tanks. “Solar thermal is the only renewable energy technology with a commercially viable solution for integrating bulk energy storage into the plant,” Joyce said. “It gives utilities and grid operators ways to manage the intermittency of demand.”
This storage ability “de-couples,” at least to a degree, the collection of solar energy from producing power, as electricity can be generated in periods of inclement weather, or even at night, using the stored thermal energy in the molten salt tank.
A recently completed 19.9 megawatt (MW) project in Spain demonstrated the potential of solar thermal power with storage. With enough storage capacity to run the plant at full capacity for 15 hours without solar input, this plant will operate 24 hours a day for most of the year. This plant, called Gemasolar, is the first commercial plant capable of around-the-clock operation.
SOLAR THERMAL IN ACTION
In general, solar thermal technologies must concentrate solar energy and deliver it as heat to a central power plant. There are a number of ways to accomplish this. The leading design uses parabolic troughs, which are rows of curved mirrors that reflect the sun’s heat into a pipe containing a heat transfer oil. The oil is piped to a central power plant, where it is used to generate steam. This design has a significant operating history, with more than 300 MW in the U.S. in operation for more than 20 years.
Though troughs are perceived as the lowest risk of all solar thermal technologies, their potential for cost reduction and performance improvement is limited. The oil has temperature limits that restrict the conversion efficiency and cost efficiency of the trough designs. The solar industry is conducting research into alternate fluids to replace the oil, but is also developing alternate solar thermal technologies to address this limitation.
Trough plants use oil and pipes to transfer energy to a central plant. One alternative design eliminates the oil and pipes and instead reflects the solar energy to one point and collects all the energy using one receiver on the top of a tower at the central plant. Plants using this approach are called central receiver or power tower plants.
These plants operate at higher temperatures than troughs and can operate in two ways. The first pumps water into the solar receiver, which generates steam to drive a turbine. This design is simple and relatively inexpensive, but it doesn’t have the thermal inertia or storage potential of other technologies. The second method uses molten salt instead of water, which integrates extremely well to create a storage system. A challenge of this design lies in the need to keep the salt in liquid form, thereby requiring high temperatures to be maintained at all times. Power towers do not have the commercial operating history of trough plants, but examples can be found in southern Spain, and a few are under construction in the U.S. Southwest.
A third type of design is called Linear Fresnel, a linear focusing collector which uses a flat array of mirrors that tilts with the movement of the sun to reflect sunlight to a receiver pipe suspended overhead. Because the receiver doesn’t move and the glass mirrors are flat, this design has the potential to be a lower-cost alternative to troughs. The challenge for this design is getting the temperature high enough in order to realize its full low-cost potential. There are plants demonstrating this technology in Spain, Australia and the United States.
A fourth solar thermal technology uses a parabolic dish to collect solar energy to drive a small stirling engine/generator. This approach is different from other solar thermal technologies because it does not use a centralized power plant. The small generating units, which resemble satellite dishes ranging from 15 to 40 feet tall, reflect light into a receiver, which powers an engine held at the focal point of the dish. This design’s modularity and self-contained nature lends it to remote applications, though it has also been proposed for large, utility-scale applications when deployed by the tens of thousands.
This design’s challenges are mass producing the engines in a cost-competitive way and the maintenance of thousands of engines over a 30-year project life cycle. This design doesn’t realize the benefits of storage or integration with thermal plants. There are dish/engine design demonstration projects in the United States and Spain.
A STRONG FUTURE
As utilities see more renewable technologies on the grid, the benefit of solar thermal’s storage capability will likely become more apparent.
“The economic benefit of solar thermal technology will change in the coming years,” Joyce said. “The true value of these differentiators – its stability, its potential for integration with conventional plants, and its ability to include thermal storage – may not be fully realized until renewable penetration increases and utilities can see the value proposition for energy storage in solar thermal power generation.”