Performance evaluation of a thermally concentrated solar thermo-electric generator without optical concentration

Abstract

Recent research has shown the use of solar thermoelectric generators (STEG) to be an alternative method of harnessing solar energy. They comprise of a spectrally selective absorber coupled to thermoelectric materials to generate power as governed by the Seebeck effect. Since output power generated is dependent on thermal gradient in the STEG, it is important to maximize this parameter. This can be done by increasing energy absorbed from the sun and minimizing energy lost from the system. Herein, the former has been achieved using thermal concentration by area while the latter has been addressed by use of a vacuum enclosure. The effect of varying enclosure pressure on convective heat loss from the solar selective substrate (α=0.90, ε=0.15, area: 22.4cm×12cm) was studied. It was observed that at 100mbar enclosure pressure, convective heat transfer coefficient as low as 0.6W/m2°C (cf., 10W/m2°C at atmospheric pressure) was obtained. This resulted in the rise of substrate temperature from 97°C at ambient pressure to 142°C at 100mbar enclosure pressure, an observation validated by simulations. Further study was conducted where open circuit voltage (OCV) and output power were observed and efficiency was calculated at different enclosure pressures. In these tests the effect of using a large area absorber was also studied. It was observed that decreasing the enclosure pressure increased the OCV, output power and efficiency of the STEG setup. It was also noted that using a large surface area absorber further increased the OCV and output power of the setup. During the course of the study, a maximum OCV of 823mV, output power of 44.2mW, and an efficiency of 0.82% were achieved by the setup at an enclosure pressure of 100mbar and thermal concentration ratio of 22. The results show that the present setup may be used as an alternative to optically concentrated STEGs which require a power consuming solar tracking system.