A fire is usually the last thing you would do when trying to keep something cold. But a new DIY cooling system from Greenhill Forge turns that logic on its head, using heat, water and road salt to cool without a traditional electric compressor.
The experimental setup, shown in a YouTube video, cooled the section to about 5 °C (41 °F), while the ambient temperature was about 16 °C (61 °F). That’s a significant temperature drop for a system that runs primarily on fire, although it’s still a prototype and not a proven replacement for a home refrigerator.
The project explores an old technical idea with a potential modern use. Cooling that can be used in locations where electricity is scarce or unreliable.
How fire and salt keep you cool
The system uses a technique called absorption cooling, a process that has been around for more than a century. Instead of relying on an electrically powered compressor to circulate refrigerant, it uses heat to drive a chemical process that enables cooling.
Greenhill Forge’s design consists of two main sections: a generator and an absorber. The generator contains a calcium chloride solution and distilled water. When a fire heats the mixture, water vapor separates from the solution and travels through a pipe to a condenser, where it cools and returns to liquid form.
The liquid water then passes through a small adjustable valve into an evaporator that works under a strong vacuum. At the system’s very low pressure, about six millibars, water can evaporate at temperatures well below its usual boiling point. This evaporation absorbs heat from the environment, creating the cooling effect.
But there’s a catch. As water vapor accumulates, the pressure increases and ultimately limits further evaporation. To prevent this, the system sprays concentrated calcium chloride solution into the absorber. The solution absorbs the vapor, helping to maintain the low pressure required for cooling. In fact, the fire provides the heat that drives the cycle, while the brine helps maintain the conditions that allow the water to evaporate at low temperatures.
An Arduino monitors the system
The device is also a comprehensive metalworking project. Greenhill Forge made much of it from stainless steel sheets and tubes, welding the housings and silver brazing smaller tube connections. The contractor formed the cooling coil by filling a pipe with water, freezing it, and bending it around the frozen core to prevent the pipe from collapsing.
Before filling, the client checked the system for leaks and evacuated it with a vacuum pump. Maintaining vacuum is critical as even small leaks can affect the low pressure evaporation process.
An Arduino-based controller monitors temperatures and pressures, adjusts pump speed, and triggers safety alarms. The facility still requires electricity for its circulation pumps and cooling fans. The pumps alone consume around 30-50 watts.
This means that the refrigerator is not completely without power. However, a small solar panel or battery could potentially meet the modest power needs, while a fire provides the majority of the thermal energy.
Could fire-powered cooling work off-grid?
Initial test results are promising, but the temperature readings require context. According to Hackaday’s report, the system reached about 5 °C (41 °F) with an ambient temperature of about 16 °C (61 °F). That shows a cooling effect, but it doesn’t show how well the prototype would perform in hotter conditions, how much food it could keep cold, or how consistently it could maintain safe storage temperatures.
The technology itself is not new. Absorption refrigerators have long been used where traditional electric compressors are impractical. Greenhill Forge’s project instead explores a DIY implementation using accessible materials and a heat source such as firewood.
As development continues, similar systems could be useful in remote workshops, cabins or other locations where fuel is easier to come by than reliable grid power.
