The Paint Branch Lecture: A New Type of Heat Engine, Using LEDs as Refrigerators
Very efficient light emitting diodes (LEDs) actually become colder as they operate because the emitted light carries away entropy. This surprising effect requires superb LED efficiency, which is achieved by using 2D photonic crystal structures to extract the luminescence. 2D photonic crystals have likewise been employed in photovoltaic cells to trap incident light. We now know that the photovoltaic cell and the LED are reciprocals of one another. The maxim that “a great solar cell has to be a great LED” has led to solar cells with record efficiency.
What if the electrical output of a photovoltaic cell drives an LED, and the LED light in turn drives the photovoltaic cell? You might fear that it would become a perpetual motion machine. Instead it becomes a heat engine in which a small amount electricity can efficiently provide refrigeration, or conversely a small temperature difference can generate electricity. Such an electro-luminescent heat engine, in which photons are the working fluid, can be more efficient than thermo-electrics, in which electrons are the working fluid.