How Do You Move Heat With No Moving Parts? Boil It and Let It Condense

The device is a sealed tube containing a small quantity of a working fluid and nothing else, with the air removed. At the hot end the fluid boils, absorbing a large quantity of heat as it changes state, and the resulting vapour fills the tube and travels to the cold end, since vapour moves from higher pressure to lower and boiling raises the pressure locally. At the cold end it condenses back to liquid, releasing exactly that heat. The liquid then returns to the hot end along a wick lining the wall, drawn by capillary action, and the cycle repeats continuously. Nothing is pumped and nothing moves except the fluid itself.

The advantage over conduction through copper is enormous and follows from what is being transported:

• •Changing state absorbs far more energy than warming a material does
• •So a small mass of vapour carries a large quantity of heat
• •Vapour travels the length of the tube almost instantly
• •The temperature difference between the ends stays very small
• •Effective conductivity exceeds that of solid copper by a factor of hundreds
• •The device is also far lighter than the equivalent solid conductor

Several distinct failure modes cap how much heat a given device can carry. The wick can fail to return liquid fast enough to match the boiling, which dries out the hot end and stops the cycle abruptly. Vapour travelling at high speed can entrain the returning liquid and carry it backwards. Gravity matters, since a device relying on capillary action works in any orientation while one relying on gravity to return the liquid works only with the hot end below. The fluid must be chosen for the operating temperature, with water suiting ordinary electronics, ammonia suiting spacecraft and liquid metals suiting very high temperatures. And any trace of non-condensing gas accumulates at the cold end and blocks it.

A variant arranged to work in one direction only is a useful device in its own right and is what the pipeline supports use. If the wick is removed and the liquid returns by gravity alone, the device carries heat upwards efficiently and carries none downwards, since the liquid cannot climb back to a hot end that is above the cold one. That makes a thermal one-way valve with no moving parts and no control system. Buried vertically with the hot end in the ground and the cold end in the air, it extracts heat from the ground whenever the air is colder and does nothing whenever the air is warmer, which keeps ground frozen through a summer without any power at all.

The device is far more widespread than its obscurity suggests. Laptop and games console cooling uses flattened versions carrying heat from the processor to a fan at the edge of the case, which is why a thin machine can cool a component that would otherwise need a large heatsink directly on top. Spacecraft use them extensively to move heat from instruments to radiators, since there is no air to carry it away and no gravity to rely on. The Alaska pipeline uses several thousand large vertical ones to keep the permafrost frozen around its supports, extracting heat in winter and doing nothing in summer, which prevents the thaw that would collapse the structure. Solar water heating and several industrial processes use them.

Fluid boils at the hot end, the vapour travels to the cold end and condenses, releasing the heat, and capillary action returns the liquid along a wick, all inside a sealed tube with nothing pumped. Changing state carries far more energy than warming metal does, giving effective conductivity hundreds of times that of copper. Laptops, spacecraft and the supports of the Alaska pipeline all depend on them.