Three teenagers designed a mini refrigerator that cools itself with salt and doesn’t require an outlet. They’re bringing it to hospitals to help transport medical supplies to rural areas without electricity.
Dhruv Chaudhary, Mithran Ladhania, and Mridul Jain live in Indore, India and all have parents working in medical fields. The boys decided to find a salty refrigeration technique after hearing how challenging it was to bring COVID-19 vaccines to rural areas without electricity.
Their invention, which they call Thermavault, won them the 2025 Earth Prize on Saturday. The award comes with $12,500, which they plan to use to build 200 of their refrigerators and send them to 120 hospitals for testing.
They hope their refrigerator can help transport vaccines, other medicines and supplies, and even transplant organs.
“We have been able to keep the vaccines inside the Thermavault for almost 10 to 12 hours,” Dr. Pritesh Vyas, an orthopedic surgeon who tested the device at V One hospital in Indore, says in a video on the Thermavault website.
With some improvements like a built-in temperature monitor, he added, “it will be definitely helpful, definitely useful in the remote places, the villages.”
Finding the right cooling salt
Some salts can have a cooling effect when they’re dissolved in water.
That’s because when those salts dissolve, the charged atoms, or ions, that make them up break apart. That separation requires energy, which the ions pull from the environment, thus cooling the water around them.
Chaudhary, Ladhania, and Jain searched the internet, first compiling a list of about 150 salts that might work, then narrowing it down to about 20 that seemed most efficient.
They then borrowed a lab at the Indian Institutes of Technology to test those 20, or so. To their disappointment, none of the salts cooled the water enough.
They were back to square one. Turns out, they didn’t need the internet after all — their school teacher recommended trying two different salts: barium hydroxide octahydrate and ammonium chloride.
“While we did scour through the entire internet to find the best salt possible, we kind of just ended up back to our ninth-grade science textbook,” Chaudhary said.
The trio says they found that ammonium chloride maintained temperatures of around 2 to 6 degrees Celsius (about 35 to 43 degrees Fahrenheit), which is ideal for many vaccines. Adding barium hydroxide octahydrate to the mix produced sub-zero Celsius temperatures, which is ideal for some other vaccines and sometimes for transplant organs.
Now they had two different refrigeration options.
About three months later, they’d built a prototype and were testing it in local hospitals.
Meet the Thermavault salt refrigerator
The fridge itself is an insulated plastic container with a copper wall lining the inside, where the vaccines or organs would sit. The cooling solution, made by dissolving the salts in water, is poured into a space between the plastic outer wall and the copper inner wall.
Cold boxes and coolant packs are already in widespread use for bringing vaccines to rural areas without electricity. Those carriers typically rely on simple ice packs.
One advantage of the ammonium chloride solution, the trio of teens says, is it’s reusable in the field without electricity. You don’t need a freezer to pull ice from. Rather, you can remove the salt water from the box, boil away the water, and collect the salt in its solid form, ready to dissolve in new water and produce its cooling effect all over again.
Jain said they’re planning to use the prize money to pursue a Performance, Quality and Safety (PQS) certification through the World Health Organization so they can pitch it to Gavi — an international alliance that distributes vaccines.
The Earth Prize program also has a volunteer who can help them pursue a patent, according to a spokesperson.
The Earth Prize casts across the planet for teens who are working on environmental projects and awards one winner from each world region. Chaudhary, Ladhania, and Jain won the prize for Asia. A global winner will be chosen by public vote, which closes on April 22.
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