Greenhill Forge Shows How Fire and Road Salt Can Power a Real Refrigerator

Most refrigerators depend on an electric compressor to move heat around, but Greenhill Forge built a working cooler that gets its energy from a wood fire and a solution of common road salt instead. Water serves as the refrigerant. When water boils it soaks up heat from whatever is around it, and the same principle holds here. The difference is that the system keeps part of the water under a strong vacuum so the liquid can boil at a temperature close to freezing rather than at the usual 100 degrees Celsius. That low-pressure boiling is what produces the cold.

The fire heats up the tank containing the salt solution, which is calcium chloride and distilled water, the same thing used to treat ice roads in the winter. The water vapor that accumulates in the solution rises up a length of tubing and into a condenser, where it cools back to liquid water. A simple valve allows the liquid to slowly trickle into a second tank that has been pushed down to approximately six millibars of pressure. At such pressure, the water rapidly evaporates, drawing heat away from a heat coil that can be placed within a box or next to an air conditioning fan. However, the vapor does not stay in the tank for long; instead, a thin spray of the super-concentrated salt solution pours down from a perforated head and absorbs the vapor in a flash, helping to keep the vacuum tight and allowing the operation to continue without interruption.

A small electric pump moves the salt solution back and forth between the heated tank and the spray area, and a few fans and sensors draw power in the background, but the good news is that it’s enough to run off a small solar panel or battery. Of course, the majority of the machine’s energy comes from the fire. Greenhill Forge built the majority of the machine out of stainless steel sheet and tubing, and they used a method to bend the cooling coil by freezing water within it first to keep it from collapsing. The temperature and pressure sensors monitor everything, and a little Arduino board adjusts the pump speed and alerts you if anything goes wrong.

During early testing, the coil dropped to a frigid 5 degrees Celsius, while the air around it was a pleasant 16 degrees. That’s still a little warmer than you need for a domestic fridge, but the builder believes they can shrink the gap further by enhancing the insulation, flow rates, and vacuum performance. The good news is that when everything is finished, the system will be freon-free; there will be no need for high-pressure refrigerant lines or a large electric motor, as anyone can put up a setup like this as long as they have a consistent heat source and a bag of calcium chloride on hand.
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