Thread by @anderssandberg

#FridayPhysicsFun – Coolest fact I learned this week: under some conditions light-emitting diodes can be more than 100% efficient, and act as refrigerators.

Normally energy conversion introduces losses: there is a production of entropy turning high-quality energy (e.g. mechanical motion, electricity) into disordered low-quality (e.g. heat), and turning low-quality into high-quality is less efficient.

This is why normally any device promising more than 100% efficiency is fake. Thermodynamics does not allow it.

Except that LEDs, when run on a low voltage, can produce more light energy than they take from the electricity supply. What is going on?
journals.aps.org/prl/abstract/1…

LEDs work by electrons in the current meeting holes (absences of electrons); as they fall into them, they release light. More current, more holes meeting electrons, and more light (and heat, which tends to degrade the LED).
en.wikipedia.org/wiki/Light-emi…

If the LED is warm thermal excitations help electrical field generate luminescence by forming more electron-hole pairs than would otherwise have happened. Electrical work pumps heat into light, making the LED act as a small refrigerator!
physics.aps.org/articles/v5/s30

It is not a very effective refrigerator. But it does remove heat in the form of disordered light. Yes, light can transport heat, or rather, entropy. In efficiency calculations one needs to take all flows into account.