Carbon nanotube based thermo-electrochemical cells for harvesting waste heat

Abstract

Harvesting energy from low-grade waste heat has received much attention due to the world`s growing energy problem. Critical needs for harnessing waste heat are to improve the efficiency of thermal energy harvesters and decrease their cost. Thermal electrochemical cell (also known as thermogalvanic cell or thermocell) that provides electrical power originating from the temperature dependence of electrochemical redox potentials is becoming attractive alternative for low-grade heat recovery. A thermocell consists of two electrodes operating at different temperatures and placed in contact with redox-based electrolyte. The inter-electrode temperature difference causes difference in the redox potential of the electrolyte, generating electrical power. The thermally generated potential derives electrons in the external circuit and ions in the electrolyte, thus electrical current and power can be delivered.

A large source of waste heat can be found in power plants or various industrial facilities where large amounts of waste heat are lost through numerous pipes that carry hot fluid. To effectively harvest waste thermal energy from these arbitrary shaped heat sources, plastic thermocells with all pliable materials, such as polyethylene terephthalate (PET), fabrics, and wires were developed. The plastic thermocells are flexible enough to be wrapped around cylindrical shapes and to be wearable on the human body. The electrical energy generated from waste pipe heat using a serial array of the thermocells and voltage converters can power a typical commercial light emitting diode (LED). Also, the thermocell charges up a capacitor when worn on thermocell embedded T-shirt by a person.

Thermocells have major advantages of simple design, direct thermal-to-electric energy conversion, zero carbon emission and low cost, however they presently have no commercial applications because of their low energy conversion efficiencies and low areal output power of thermocells. Therefore, a significant efficiency increase is required for thermocells to become commercially attractive. The deployed optimization strategies to improve thermocell efficiency involve use of CNT aerogel sheets as electrodes, removal of low activity carbonaceous impurities that limit electron transfer kinetics, decoration of CNT sheets with catalytic platinum nanoparticles, mechanical compression of nanotube sheets to tune conductivity and porosity, and the utilization of cylindrical cell geometry. The output power density generated by thermocell with the optimized aerogel sheet reached 6.6 W/m2, which corresponds to a Carnot-relative efficiency (ηr) of 4.2%. To date this ηr is the highest reported value in thermocells.