Energy harvesting from low-grade heat through supercapacitors

Climate change is the defining issue of our time. The current energy system is not sustainable and major transitions are needed. In 2021, The International Energy Agency (IEA) reported that over 60% of the total primary energy demand was wasted as heat. Studies have estimated that 63% of this waste heat is low-grade thermal energy i.e., at a temperature below 100°C. Improving energy efficiency by focusing on waste heat recovery is widely recognized as a crucial tool to mitigate the increasing global energy demands and reduce GHG emissions. Low-grade thermal energy harvesting poses significant challenges to traditional thermoelectric systems. This thesis work reports the implementation and analysis of a method for the conversion of low-grade heat into energy. It is based on a thermocapacitive cycle involving charging, heating, discharging, and cooling within a temperature range of 0°C to 80°C through the use of a supercapacitor. A voltage output is recovered after every cycle. Unlike previous methods, this technique does not require a fixed temperature gradient. Instead, it operates in a homogeneously changing temperature. It relies on the temperature dependence of the cell voltage of a charged supercapacitor. The mechanism is attributed to potential variation due to thermal energy variation at both electrodes. This study carries out a systematic investigation of supercapacitors comprising activated carbon based electrodes and different electrolytes, namely the ionic liquid [PYR14] [TFSI], the organic salts TEA BF4 and TEA TFSI in propylene carbonate. In order to understand the role of each component of the device and to improve the overall performance of the manufactured device as well as the extractable power, several characterizations have been performed: electrolyte characterization such as conductivity and viscosity measurements; electrochemical characterization of supercapacitors. In conclusion, the high tunability of the system proposed makes it very promising to low-grade thermal energy harvesting research, due to the implementation of sustainable materials and cost-attractive technologies.