In a thermoelectric generator system with specific semiconductor material, for example bismuth-telluride alloy, thermal to electricity conversion efficiency is a function of operating temperature difference. As equation 2 shows, an increase in temperature difference provides a corresponding increase in available heat for conversion as dictated be the Carnot efficiency, so large temperature difference are desirable. However in most renewable heat sources like solar heating system and geothermal systems the maximum temperature is less than 150 cํ so many interests have been shown on low temperature conversion which there is still due to almost free energy source.