Thermoelectric Generators use a temperature difference between two objects or environments to create voltage. Inside a TEG module, thermocouples made of semiconducting materials like bismuth telluride are connected in series. One side of each couple absorbs heat from a hot source while the other side dissipates heat to a colder sink. https://prachicmi.livepositively.com/thermoelectric-generator-an-efficient-way-of-converting-waste-heat-into-electricity/new=1