This popular article oversells it by about 3 orders of magnitude.
Roughly, the effect is that moisture causes a nickel hydroxide material to expand. You can drive the moisture out with light or heat and it contracts. In a humid environment, you can cycle it. The fastest cycle time they show graphs for is 0.1 Hz (a cycle every 10 seconds.) They use a thin film of about 1 micrometer -- presumably a thicker film would take a lot longer to change the moisture level.
They don't try to measure the efficiency, but usually anything that involves temperature change is pretty inefficient.
Roughly, the effect is that moisture causes a nickel hydroxide material to expand. You can drive the moisture out with light or heat and it contracts. In a humid environment, you can cycle it. The fastest cycle time they show graphs for is 0.1 Hz (a cycle every 10 seconds.) They use a thin film of about 1 micrometer -- presumably a thicker film would take a lot longer to change the moisture level.
They don't try to measure the efficiency, but usually anything that involves temperature change is pretty inefficient.