Direct measurement of the genuine efficiency of thermogalvanic heat-to-electricity conversion in thermocells

Harvesting wasted thermal energy could make important contributions to global energy sustainability. Thermogalvanic devices are simple, chemistry-based devices which can convert heat to electricity, through facile redox chemistry. The efficiency of this process is the ratio of electrical energy generated by the cell (in Watts) to the quantity of thermal energy that passes through the cell (also in Watts). Prior work estimated the quantity of thermal energy passed through a thermocell by applying a conductive heat transfer model to the electrolyte. Here, we employ a heat flux sensor to unambiguously quantify both heat flux and electrical power. By evaluating the effect of electrode separation, temperature difference and gelation of the electrolyte, we found significant discrepancy between the estimated model and the quantified reality. For electrode separation, the trend between estimated and measured efficiency went in opposite directions; as a function of temperature difference, they demonstrated the same trend, but estimated values were significantly higher. This was due to significant additional convection and radiation contributions to the heat flux. Conversely, gelled electrolytes were able to suppress heat flux mechanisms and achieve experimentally determined efficiency values in excess of the estimated values (at small electrode separations), with partially gelled systems being particularly effective. This study provides the ability to unambiguously benchmark and assess the absolute efficiency and Carnot efficiency of thermogalvanic electrolytes and even the whole thermocell device, allowing ‘total device efficiency’ to be quantified. The deviation between the routinely applied estimation methodology and actual measurement will support the rational development of novel thermal energy harvesting chemistries, materials and devices.