  The present article reports a comprehensive energy balance analysis of a
photon-enhanced thermionic emission (PETE) device when it is used for
concentrated solar power (CSP) generation. To this end, we consider a realistic
PETE device composed of a boron-doped silicon emitter on glass and a
phosphorus-doped diamond collector on tungsten separated by the inter-electrode
vacuum gap. Depth-dependent spectral solar absorption and its photovoltaic and
photothermal energy conversion processes are rigorously calculated to predict
the PETE power output and energy conversion efficiency. Our calculation
predicts that when optimized, the power output of the considered PETE device
can reach 1.6 W/cm$^2$ with the energy conversion efficiency of $\sim$18 \% for
100$\times$ solar concentration, which is substantially lower than those
predicted in previous works under ideal conditions. In addition, the
photon-enhancement ratio is lower than 10 and decreases with the increasing
solar concentration due to the photothermal heating of the emitter, suggesting
that PETE may be an adequate energy conversion process for low-to-medium CSP
below $\sim$100$\times$ concentration. These observations signify the
importance of a rigorous energy balance analysis based on spectral and spatial
solar absorption distribution for the accurate prediction of PETE power
generation.
