By RYAN HEINSIUS • NOV 17, 2017
The Navajo Generating Station is slated to close in 2019, and officials are considering several potential buyers to prevent the shutdown. In July, U.S. Interior Secretary Ryan Zinke and Salt River Project officials met with the U.S.-based company NantWorks. They discussed converting the coal-fired plant into a high-powered solar facility incorporating a new energy storage technique using molten salt. To find out exactly what this technology is, KNAU’s Ryan Heinsius spoke with University of Arizona engineering professor Peiwen Li who specializes in renewables.
The company SolarReserve built the 110 megawatt Crescent Dunes Solar Energy Plant in southwestern Nevada. It creates enough electricity for 75,000 homes and stores thermal energy with molten salt. The plant uses mirrors instead of conventional solar panels and is one of several such facilities in the Southwest.
Peiwen Li: Molten salt keeps as a liquid for a wide range of temperatures, and it stores a lot of thermal energy, so you can store the energy for use during the night or during the bad weather time.
Ryan Heinsius: So, essentially it’s just a big battery.
PL: Correct. To some sense, yes, it’s a battery. It’s a huge tank of salt. It stores energy in the form of thermal energy, not electrical energy. And when you use thermal energy you need to pump up this hot fluid and then go to the power plant to use the thermal power plant to generate electricity.
RH: Maybe you could explain to me why energy storage is such a big problem facing the renewables industry.
PL: Generally, after sunset people need another six hours to have electricity—people go home and they use a lot of electricity. So you need to supply the power if you rely on renewable energy to do that, and then energy storage is so critical, it’s important. And compared to any other energy storage method, thermal storage can be a very good choice because you can store a large capacity of energy. People use batteries but the storage capacity is very limited. So you cannot expect a power plant to have battery to store that huge amount of energy.
Huge tanks of molten salt store thermal energy at SolarReserve’s solar facility in Nevada.
RH: Are there any negative environmental aspects or effects from the use of molten salt energy storage?
PL: All the salt we use there is no toxicity and they are natural materials. For example, potassium nitrate is a kind of fertilizer salt. Sodium chloride and magnesium chloride salt, these are the materials people in the north use to melt snow during winter. Sodium nitrate is also a well-known material. So, they are friendly salts. There is no big concern regarding the environmental impact.
RH: Hundreds of people stand to be laid off if the Navajo Generating Station here in northern Arizona closes. Could this technology potentially save those jobs?
PL: So far, I’ve never heard of any country, or in the U.S., any plan to convert a coal-fired plant into a solar thermal-based power plant, yet. Once it starts running the jobs for maintenance probably will be much less than the coal fired plant, because in a coal-fired plant you have workers to deal with the waste from the power plant and so on. But once the solar thermal plant is constructed it’s really clean and the maintenance jobs are reduced.
The 110 megawatt solar facility in Nevada.
RH: Is solar power using molten salt storage a viable alternative to coal-generated power or other fossil fuels?
PL: Yes, I think it will be widespread particularly in this region where the solar energy resources are rich. For example, California is pursuing this technology very actively; Nevada and also Arizona. So, we might completely give up gas and coal energy resources and we can completely rely on our solar energy.