One-step synthesis of anhydrous Mg(NO3)2-NaNO3-KNO3 molten salt for high-temperature thermal energy storage

The Sun's Sticky Problem and the Magic of Dry Salt

Imagine trying to store the sun's energy like you store money in a piggy bank. When the sun is shining bright, you want to catch as much of that heat as possible and save it for a rainy day—or a cloudy night. This is the big idea behind Concentrating Solar Power (CSP). Scientists use giant mirrors to focus sunlight onto a receiver, heating up a special liquid that flows through pipes. This hot liquid then spins a turbine to make electricity, even after the sun has gone down.

The problem is finding the perfect "piggy bank" liquid. For a long time, scientists have used a mix of two common salts (sodium nitrate and potassium nitrate) because they are cheap and hold heat well. But they have a few annoying flaws: they freeze at temperatures that are too high (making them hard to start), and they start to break down and rot if they get too hot, limiting how much energy you can store. To fix this, researchers tried adding a third ingredient: magnesium nitrate. This is like adding a secret spice to a soup to lower the freezing point. However, the magnesium nitrate they usually buy comes with a heavy backpack of water molecules attached to it (it's a "hydrate"). When you try to melt this wet salt, the water causes a chemical mess, creating corrosive gunk that ruins the mixture and makes it unstable at high temperatures. The challenge has been: how do you take the water off without breaking the salt or making it explode?

The One-Step Dry-Out Trick

In this study, a team of researchers from Qinghai Normal University and other institutions decided to tackle this "wet salt" problem head-on. They wanted to create a super-stable, water-free (anhydrous) version of a three-salt mixture: magnesium nitrate, sodium nitrate, and potassium nitrate. Instead of buying dry magnesium nitrate (which is expensive and hard to handle), they started with the wet, cheap version and figured out a clever, one-step way to dry it out right inside the mixture.

Think of the wet magnesium nitrate crystals as sponges soaked in water. If you just throw them into a hot oven, the water might boil off too fast, causing the sponge to crumble or react badly with the other salts. The researchers designed a specific "baking schedule" to gently squeeze the water out. They placed the wet magnesium nitrate in a special container and heated it up in stages. First, they warmed it to 150°C, then 250°C, and finally 300°C, holding it at each temperature for a specific amount of time. They did this in a vacuum (a space with no air) or under a flow of gas to whisk the water vapor away as soon as it left the salt. This careful, step-by-step drying process turned the wet, unstable salt into a dry, stable powder without letting it turn into the corrosive "gunk" that usually happens.

Once the magnesium nitrate was dry, they mixed it with the other two salts to create their new "MNK" ternary molten salt. They then put this new mixture through a battery of tests to see how it behaved compared to the old, wet version. They used a machine called TG-DSC, which is like a super-smart scale that also measures heat. It heats the salt up slowly while weighing it to see if it loses weight (which would mean it's breaking down or losing water) and measures how much energy it takes to melt it.

The results were quite promising. The new, dry salt was a much better heat storage material than the wet one. First, it didn't start to break down until it reached 457.6°C, whereas the wet version started falling apart at just 404.1°C. This is a huge deal because it means you can heat the salt up much higher, storing more energy without it rotting. The researchers found that the dry salt could safely operate up to 450°C, extending the "safe zone" for solar power plants.

Second, the dry salt melted at a lower temperature. The wet mixture needed to get to 157.5°C before it turned into a liquid, but the dry version started melting at 148.2°C. This makes it easier to start up the system in the morning. Even better, when it melted, it absorbed more energy. The "latent heat of fusion" (the energy it soaks up while melting) jumped from 66.61 J·g⁻¹ for the wet salt to 88.71 J·g⁻¹ for the dry salt. That's a significant boost in how much "battery power" the salt has.

The team also checked if the salt would survive being heated and cooled over and over again, simulating years of day-and-night cycles. After heating the dry salt to 450°C and letting it cool down, they weighed it. It lost almost no mass (only about 2.01% after 300 hours at high heat), proving it was very stable. In contrast, the wet salt lost a lot of mass (8.61%) at the same temperature because the water inside was escaping and the salt was decomposing. They even looked for chemical signs of trouble, like the presence of a corrosive byproduct called magnesium hydroxynitrate. They found this bad stuff in the wet salt samples, but it was completely absent in the dry salt.

The paper concludes that this simple, one-step drying method is a viable way to make high-performance salt for solar energy. By carefully removing the water before the salt melts, they created a material that is more stable, holds more heat, and can survive higher temperatures than the traditional wet mix. While the paper doesn't claim this is the final solution for all solar power, it suggests that this specific dry salt is a very strong candidate for making solar energy storage more efficient and reliable.