  We present a systematic optimization of nighttime thermoelectric power
generation system utilizing radiative cooling. We show that an electrical power
density over 2 W/m2, two orders of magnitude higher than the previously
reported experimental result, is achievable using existing technologies. This
system combines radiative cooling and thermoelectric power generation and
operates at night when solar energy harvesting is unavailable. The
thermoelectric power generator (TEG) itself covers less than 1 percent of the
system footprint area when achieving this optimal power generation, showing
economic feasibility. We study the influence of emissivity spectra, thermal
convection, thermoelectric figure of merit and the area ratio between the TEG
and the radiative cooler on the power generation performance. We optimize the
thermal radiation emitter attached to the cold side and propose practical
material implementation. The importance of the optimal emitter is elucidated by
the gain of 153% in power density compared to regular blackbody emitters.
