The US Department of Energy (DOE) has announced a $72 million project to further the development of concentrated solar power (CSP). The goal is to build an integrated test facility that can use concentrated solar energy to heat a working fluid to greater than 700°C and deliver it to a steam turbine system to produce electricity. Current commercial CSP systems work in the 300°C to 550°C range. Increasing the temperature to 700°C promises greater efficiency in thermally driven solar power systems.
Most people are familiar with the rooftop, photovoltaic (PV) solar power systems that are becoming popular around the country. The large black glass and aluminum rectangle structures of a PV collector contain hundreds of crystalline silicon solar cells. They produce electricity when exposed to light. But that’s not the only way to produce electrical power from sunlight.
Some Like It Hot
Light beams can be concentrated by refraction through a lens (a magnifying glass), or through curved mirrors, to produce extremely high temperatures. Commercial scale CSP facilities like the recently completed Noor I power plant in the Sahara Desert in Morocco use 500,000 crescent-shaped mirrors that track the sun across the sky. Noor I can produce up to 160 megawatts while operating at almost 400°C. Similarly, Ivanpah, the world’s largest CSP facility—located near the California and Nevada border—has 300,000 mirrors that concentrate the sun’s energy onto three 459-foot-high towers. Power production from the 3,500-acre Ivanpah site is rated at 377 megawatts with operating temperatures as high as 565°C.
|Ivanpah in California is the largest concentrated solar power (CSP) facility in the world, capable of producing 377 megawatts of electrical power. (Image source: Cliff Ho, DOE)|
CSP has the potential for higher efficiencies than the 25% a commercial PV solar cell can produce. “Compared to PV, a much larger percentage of the solar energy that’s collected is actually converted to heat (about 90%),” said Avi Shultz, Acting Program Manager for the Concentrating Solar Power team at the Solar Energy Technologies Office (SETO) of the DOE, in an interview with Design News. “But one of the biggest loss mechanisms is the turbine thermal-to-electric conversion efficiency. Current plants operating steam cycles at 565°C have net thermal-to-electric efficiencies of approximately 41%. By going to these higher temperatures, we’re hoping to enable approximately 50% net power cycle efficiency,” he added.
The problem with operating a CSP plant at higher temperatures comes in the transfer of the heat energy from the collection point to the steam system that generates the electricity. Current CSP systems use molten nitrate salts to transfer the heat. But at temperatures higher than 565°C, these salts become chemically unstable and can no longer be used.
The DOE development project’s goal is to find a new method or material that will allow higher temperature CSP operation. “What we are really looking at in this program is the entire thermal transport systems from the solar receiver—where the fluid is heated up—to the thermal energy storage, to the heat exchanger, where the heat is transferred from that heat transfer media to the fluid that is used in the power cycle,” Shultz told Design News.
Three teams have been selected to investigate the technologies needed to operate a CSP plant at 700°C. Each team will test one of three possible technologies:
- Brayton Energy, Hampton, New Hampshire: $7.6 million — gas phase system
- Sandia National Laboratories, Albuquerque, New Mexico: $9.5 million — falling particle system
- National Renewable Energy Laboratory (NREL), Golden, Colorado: $7 million — liquid (molten salt) system
Over a two-year period, each team will develop and test materials, concepts, and critical components using its assigned thermal transport technology. Each will also create detailed plans to build an integrated CSP facility. At the end of the two-years, one of the technologies will be chosen to move ahead with the construction of a $25 million pilot facility over a period of three years.
In addition to the three major teams, eight additional research universities and organizations will develop component-level technologies to help support the work of the three teams. Ideally, the additional teams will work closely with the three major teams so that their ideas and components will be incorporated into the final pilot facility.
Each of the three thermal transfer technologies uses a different transfer media. NREL will examine new salt formulations that can withstand temperatures in excess of 700°C. Brayton Energy will examine gaseous phase materials that can withstand the high temperatures of the new system. Sandia’s project differs from the other two, as its concept is to allow high-temperature-resistant ceramic beads to fall through the beam of highly concentrated solar energy. The beads heat up quickly and can then transfer their heat to either a storage medium or the steam generating system.
PV Versus CSP
Obviously, CSP requires a larger scale facility than PV systems, which can be easily scaled and distributed across a neighborhood of rooftops. This is part of what makes CSP more expensive than PV. But where PV requires battery storage to be most effective, storing the heat collected by the thermal transfer media is relatively easy. “You can de-couple collection of solar energy during the day from generation of electricity whenever you want it,” said Shultz.
The CSP project announced is part of a bigger effort on the part of DOE to drive the cost of solar energy down. For example, the target for the cost of PV is $0.03 per kilowatt-hour (kWh) by 2030. The CSP target is $0.05/kWh by 2030. But the PV costs do not include storage, where the storage and ability to generate electricity when the sun isn’t shining is built into the thermal CSP system. Shultz notes that if a plant has 12 or more hours of thermal energy storage capacity, then it can operate on the order of 70% of the time. But a PV without storage can only operate 20-25% of the time. “What we see, if we hit these targets, is that CSP seems to have a lot of value,” said Shultz.
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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