There is a mirrored venue on the southern edge of Albuquerque, New Mexico. In the center of this venue is a 61-meter tower. This is a national solar thermal test facility operated by Sandia National Laboratories. Here, researchers are developing hotter, cheaper and more efficient solar thermal power generation technology.
The role of the hundreds of mirrors here is to focus the sunlight on the receiver above the tower. In a traditional solar thermal system, water, molten salt, or other liquid is contained in the receiver to heat the turbine to generate steam that can drive the turbine generator. But this receiver is not ordinary: its interior is a continuous "ceramic particle curtain." Do not underestimate these artificial particles, which look like black gravel, and can easily heat up to 100 ° C above their maximum liquid temperature compared to traditional liquids. Therefore, these particles are crucial for increasing the efficiency of solar thermal power generation and reducing the cost of power generation and energy storage.
These particles are but one of three technologies that make solar thermal power generation cheap and sustainable. Last January, the National Renewable Energy Laboratory published a demonstration roadmap for the next generation of solar thermal power generation. They include Falling Particles, a higher temperature molten salt system, and a gas-based heat exchange fluid, all three highly promising technologies. Their goal is to reduce the cost of solar thermal power to 6 cents per kilowatt-hour by 2020, in accordance with the launch schedule announced by the U.S. Department of Energy in 2011.
Figure | Technicians John Kelton and Daniel Ray are examining drop particle receivers at the National Solar Thermal Test Facility In September of this year, the U.S. Department of Energy announced that it will invest 62 million U.S. dollars in these three technological directions, greatly stimulating the enthusiasm for solar thermal power generation, which is no longer "hot".
Researchers from Sandia Laboratories, National Renewable Energy Laboratory, Savannah River National Laboratory, and Brayton Energy recently confirmed with the MIT Review that they have submitted an investment application to the Department of Energy . The deadline for project concept papers for investment applications is early November, and the full application report is due by mid-January 2018.
"Everyone in the field of solar thermal power sees this as a unique research opportunity," said CliffHo, a Sandia lab engineer and lead researcher for the Falling Particle Project.
Compared with the rapid development of solar photovoltaic power generation in recent years, the biggest advantage of solar thermal power generation is that thermal energy storage easier and cheaper than electricity. Therefore, a solar thermal power plant can increase or decrease the amount of power generated as the demand for electricity changes. In the evening, stored heat energy can also be used to generate electricity. If there is no expensive large-scale battery, the photovoltaic system can not achieve such a way of generating electricity.
However, the biggest problem with solar thermal power generation is the high cost of construction and operation. Built in 2014 in the Mojave Desert, California, the Iván Solar, with 170,000 mirrors, costs $ 2.2 billion. However, since its inception, the Ivanpah plant has been entangled in a variety of things, including high costs, low electricity generation, a fire and the threat of a public utility board closure. In fact, as early as the 1980s, LuzInternational had built 9 solar thermal power plants using early technologies in the same desert. Unfortunately, after the end of government supportive policies, they all went bankrupt because of high operating costs.
According to Lazard's average energy cost analysis released in December last year, the cost per MWh of solar thermal power plants with storage facilities is 119 to 182 U.S. dollars. In contrast, natural gas combined cycle power plants cost between US $ 48 and US $ 78 per megawatt hour. According to a report released by the National Renewable Energy Laboratory in 2015, the latter cost only 1/8 of the cost per kilowatt.
Researchers at the U.S. Department of Energy have demonstrated that to increase the efficiency of a solar thermal power plant, it is necessary first of all to transform it from a conventional steam turbine to a supercritical carbon dioxide Brayton cycle, in which the carbon dioxide has both liquid and Gas characteristics, which will greatly improve its energy conversion rate.
A paper published in Science in May of this year shows that the Brayton cycle of supercritical carbon dioxide will be 30% more efficient than conventional steam turbines. The problem is that this new type of energy cycle requires temperatures of at least 700 ° C and above, and heat exchange systems that can operate at such high temperatures can reach their full potential.
The three technologies given by the National Renewable Energy Laboratory are all designed to increase the temperature of the heat collection. Each has its own potential and flaws. Molten salt is already avail- able technology, but more durable sealing materials, pipes, and pumps will be needed if you want to use molten salts at higher temperatures. Gas technology can use gases that are relatively easy to manage, such as carbon dioxide and helium, but researchers need more research to minimize energy consumption when gas is circulated.
Sandia Lab's Falling Particle Receivers are built on the closest workable design prototypes of the three technologies. Engineers racked it up on the central tower of the national solar thermal test facility in July 2015.
The particles it uses are mainly composed of alumina and iron oxide. When they fall from the focused sunlight, they are transported back to the top by an elevator and continue to circulate. CliffHo, a Sandia lab engineer, said they had achieved over 900 ° C.
At this stage, the receiver is not connected to any other device. However, Sandia's team is working with private contractors to develop a heat exchanger that can exchange particulate heat for pressurized carbon dioxide flow in a closed loop.
In the meantime, another team at Sandia Labs is developing and testing a supercritical carbon dioxide cycle. However, CliffHo's team opted to develop a dedicated recycling system for their solar thermal power plant. They plan to complete the heat exchanger manufacturing by March next year and complete the manufacturing of the carbon dioxide recycling system shortly thereafter. If everything goes according to plan, their entire system will be fully tested next summer.