“…As mentioned in the introduction, the ideal energy conversion efficiency of photon-enhanced thermionic emission has been predicted to be above 50% [25], and even when loss mechanisms are considered, the efficiency is still in the range of 30-40% [26][27][28]. For the case of NETEC system, its energy conversion efficiency should be defined as the ratio of the electrical power output, P NETEC , to the heat input to the NETEC system, or near-field thermal radiation absorbed by the cathode, Q E→C : Figure 5 shows the efficiency of the NETEC system for different emitter temperatures, T E , and cathode electron affinities, χ C , respectively.…”