Better Recovery of Energy from Waste Heat

A research team from Australia, India, and the US has created working thermocells capable of harvesting electrical energy from waste heat below 130 C, using carbon multi-walled nanotube electrodes.

These electrodes provide high electrochemically accessible surface areas and fast redox-mediated electron transfer, which significantly enhances thermocell current generation capacity and overall efficiency. The team showed efficiency of thermocells with MWNT electrodes to be as high as 1.4% of Carnot efficiency—3-fold higher than for previously demonstrated thermocells.

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.

...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%. _GCC

In other news, Raytheon has awarded a second-phase work order to Cyclone Power Technologies:

Pursuant to the work order, Cyclone will be developing preliminary designs, specifications and test parameters on a compact 10 hp (7.5 kW) external combustion engine for use in various power applications.

The proposed Cyclone engine would be designed to run on both traditional fuels and a monopropellant called Moden Fuel, which can combust in the absence of oxygen. (Moden Fuel was originally developed by James R. Moden, Inc. to power US Navy torpedoes.)_GCC

The US defense department is also looking into using the Cyclone engine to power autonomous robots that would be capable of powering themselves from biomass and other waste carbons.