Efficiency in nanometre gap vacuum thermionic refrigerators

The performance of vacuum thermionic refrigerators with emitter-collector separations of the order of a few nanometres is examined. The importance of the spectrum of transmitted electrons on device behaviour is highlighted. We find that for room temperature refrigeration applications, radiation losses are not negligible when the device is designed for high efficiency. A trade off between currents below and above the Fermi level is found to occur, with the optimal result not necessarily being achieved with minimum emitter-collector separation.