18 December 2012
The Norwegian company Yara International has identified a new grade of potassium calcium nitrate with promising thermal properties and innovated a second-generation molten salt specifically for heat storage and transfer applications at concentrated solar power (CSP) plants.
“As Nitrate expert, we had demands from the CSP market to improve the current molten salt technology that is widely used today,” says Emilio Iglesias, sales manager of nitrates at Yara. He says the prices of potassium and sodium nitrate have been rising every year, and the few nitrates suppliers worldwide who can cover this demand are controlling the market.
What is more, binary salts have technical limitations, in particular if the salt from the mines has high impurities and is not homogeneous. “So we started to focus our research on the additional benefits the market wanted, like cost reduction, lower melting point and lower corrosion,” Iglesias says. Now, two years later, this Northern European provider of solutions for sustainable agriculture and the environment introduces a breakthrough CSP heat storage material, a new potassium calcium nitrate.
“What is unique about our energy storage solution is the lower melting point,” Iglesias says. In fact, the melting point is 91°C lower than that of conventional molten salts used in CSP applications. This has three benefits, as the expert explains: 1) a plant can absorb more energy with less molten salt, leading to capital expenditure savings; ?2) the lower melting point helps the plant to prevent blockage caused by molten salt solidification, which can be very costly in terms of plant downtime and repairs; and ?3) This new melting point opens a new path to research and facilitates new technical trends in the market, as molten salts could now be used as heat transfer fluid (HTF) in parabolic troughs (PT) and Fresnel lenses.
“Companies are developing tube collectors that will use molten salts instead of thermal oil,” Iglesias reports. And this is good news for CSP, because thermal oil is expensive, not environmentally friendly, the temperature roof limit is 400°C and it requires a (often expensive) heat exchanger between the oil and the molten salt. “The broader temperature range (131–560°C) of the new ternary salts using potassium calcium nitrate allows for new lower working temperatures and provides a higher turbine yield when working at higher temperatures,” Iglesias explains.?? “Our ternary salt proposal with this new synthetic potassium calcium nitrate is less corrosive than the binary. This means a longer lifetime for the plant and less operational expenses over the years.”
Why potassium calcium nitrate? “Among all the calcium nitrates we have in house, this is very pure, chlorides- and ammonia-free, and contains potassium nitrate, which will reduce the required amount in the ternary mixture,” Iglesias says. “Plus, it is very cost attractive.”
According to Yara, the product is ready for the market and production is in place. Says Iglesias: “We are in advance negotiations with companies in the segment to introduce this invention in their facilities as soon as possible. Also, we are now giving all the technical information to all the suppliers that will develop components to be used with this new salt, such as receptors, storage tanks, pumps, etc.”
Yara’s new heat storage material certainly promises important improvements, from cost reduction and yield improvement to better corrosion performance and a longer lifetime of the plants. But Iglesias also is hoping for the new synthetic potassium calcium nitrate to have a more profound impact on the CSP market: “This new invention brings some fresh air and impulse to a market that desperately needs to be catalyzed. It opens the possibility for new research, brings new suppliers to the sector and energizes the market.”
Looking ahead, Yara’s goal for the immediate future is more tangible: “We have a lot of new ideas in mind. As nitrates experts with more than 100 years of experience in nitrogen-based applications, we think there is a lot of room for improvement in the use of molten salts. But our first target in the short term will be to equip the first CSP plant with this new molten salt,” Iglesias concludes.
Written by Sandra Henderson, Research Editor, Solar Novus Today