A model for a flat-plate solar thermoelectric generator (STEG) with sandwich-like structure is established in this paper. The influence of the thermal concentration ratio, length of the thermoelectric legs and other geometrical factors on the performance of the STEG are investigated by numerical calculations. The results indicate that the maximum conversion efficiency and output power per unit mass can reach 5.5% and 6.5 W/kg, respectively, for Bi2Te3 based STEGs working in the Earth's orbit. For PbTe based STEGs working in the orbits of Venus and Mercury, the maximum conversion efficiency can reach 4.7% and 5.8%, respectively. It is also shown that, under high intensity radiation, the conversion efficiency of a flat-plate STEG can be maintained at a relatively stable level by adjusting the device geometry, which is precisely the advantage of STEG when compared to photovoltaic devices. (C) 2016 Elsevier Ltd. All rights reserved.