An ordinary heat engine is like a mill between the hot and cold banks of a river: the greater the drop, the stronger the current. Here, the drop is tiny, just one or two degrees, but water can still turn the wheel. The secret is that water is made to boil not by heating but by a sudden drop in pressure, so steam bursts out even from barely warm liquid.

The cycle is simple: water is slightly warmed by the environment, then enters a narrow channel where pressure drops sharply. The liquid instantly boils, the steam expands and pushes a piston or turbine. After that, the steam is compressed back into liquid, but without much energy cost, because compression happens in the liquid phase. This way, useful work is extracted from heat that would otherwise just dissipate.

Calculations with real water properties confirm that under ideal conditions the cycle produces positive work and does not violate the laws of thermodynamics. But the energy output is so small that any friction in the mechanism would eat up all the gains. For such an engine to work, parts must move with the precision of a clockwork mechanism.

The idea resembles trying to harvest a field where the ears are almost empty. But even a tiny trickle of energy flowing all around us might one day prove useful — for example, to power sensors far from any outlet.