Hot water after a shower, warm air from ventilation, the heated walls of an engine — all this is heat below 100°C. Usually, it dissipates without doing any useful work. Now it can be captured and turned into electricity.

Inside the new device is a wire made of a nickel-titanium alloy. This material remembers its shape: when heated, it contracts; when cooled, it returns to its original shape. If you stretch such a wire like a string and then heat and cool it, it will twitch like a muscle. This motion generates electricity.

Previously, such devices wasted a lot of energy stretching the wire before each cycle. The authors came up with a system where one wire helps stretch another, like two climbers taking turns pulling each other up a cliff. And a water flow that washes across the wire allows the temperature to change quickly, regardless of the wire's length.

As a result, the device delivers 366 milliwatts per cubic centimeter of active material. That's 1.7 times more than the best previous thermoelastic device, and better than all known thermomagnetic and pyroelectric generators. In terms of power per dollar, it even beats thermoelectric generators in this temperature range.

So the heat we're used to throwing away becomes a source of electricity. Just stretch a smart wire and let it work like a muscle.