Research on utilizing low-grade heat from sources such as industrial waste streams, geothermal activity, and solar heating has focused on using solid-state thermoelectrics and Stirling engines to harvest low-grade waste heat as electrical energy. However, the researchers note, despite much progress over the past decades, current thermoelectric energy conversion technology is not very cost-effective and is constrained by physical and material limitations, while Stirling engine technology is disadvantaged by high initial cost and problems with long-term reliability.

Thermo-electrochemical cells (otherwise known as thermogalvanic cells or thermocells) that utilize the temperature dependence of electrochemical redox potentials (i.e., the Seebeck effect) to produce electrical power may become an attractive alternative for harvesting low-grade heat, given their simple design, direct thermal-to-electric energy conversion, continuous operation, low expected maintenance, and zero carbon emission.

...Thermocells using aqueous potassium ferrocyanide/ ferricyanide redox solution have been studied by many groups because this redox system reversibly exchanges one electron per iron atom and produces a large reaction entropy, yielding Seebeck coefficient (>1 mV/K) and high exchange current. However, to obtain efficiencies of reasonable interest noble metals such as Pt are usually required as electrode materials in thermocells, and this restricts commercial viability. Also, the best prior-art thermocells typically have efficiencies of~0.40% of Carnot efficiency (when efficiency is correctly evaluated, as discussed below). In fact, it was previously predicted that a power conversion efficiency of 1.2% of the Carnot efficiency would be difficult to achieve.

—Hu et al.

Hu et al. developed carbon nanotube (CNT)-based thermocells that utilize the ferri/ferrocyanide redox couple and electrodes made from carbon-multiwalled nanotubes (MWNT) buckypaper and vertically aligned MWNT arrays. The buckypaper is made by a filtration process that is analogous to that used for making ordinary paper.

They found that the performance of MWNTs as thermocell electrodes supersedes that of conventional electrode materials, including platinum foil and graphite sheet. With a hot-side temperature of 65 °C and a temperature difference of 60 °C, they achieved a maximum output power of 1.8 W/m² in a stagnant cell, corresponding to an efficiency relative to the Carnot cycle efficiency of 1.4%.

They developed different designs of the thermocells to demonstrate energy harvesting in several practical scenarios.

With improvements in cell design and optimization of MWNT properties and electrode structure, thermocell efficiency is likely to increase. Thin coin-like thermocells were fabricated and operated for three months to provide essentially constant power output.

In such configurations, direct synthesis of MWNT forest electrodes was shown to provide improved thermal contact that contributed to a 30% increase in efficiency as compared to buckypaper electrodes that required secondary attachment to the package substrates. The performance of MWNT-based thermocells was shown to be scalable and amenable to complex systems.

With the cost of MWNTs decreasing, thermocells with the performance reported here may develop into an economical solution for harvesting untapped supplies of low-grade heat. Moreover, the enhanced thermocell performance demonstrated in this study using MWNT electrodes suggests that other nanostructured electrode materials might also be applied to significantly enhance the efficiency of thermocell devices.

—Hu et al.

Resources

• Renchong Hu, Baratunde A. Cola, Nanda Haram, Joseph N. Barisci, Sergey Lee, Stephanie Stoughton, Gordon Wallace, Chee Too, Michael Thomas, Adrian Gestos, Marilou E. dela Cruz, John P. Ferraris, Anvar A. Zakhidov and Ray H. Baughman (2010) Harvesting Waste Thermal Energy Using a Carbon-Nanotube-Based Thermo-Electrochemical Cell. Nano Lett., Article ASAP doi: 10.1021/nl903267