Published on April 5th, 2016 | by Roy L Hales1
What Thermoelectric Power Generators Must Overcome
April 5th, 2016 by Roy L Hales
Originally published on the ECOreport.
A new National Renewable Energy Laboratory (NREL) study found that a “finely tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat.” This is significant because “more than half of the energy consumed worldwide is rejected primarily as waste heat.” Andrew Ferguson, a research scientist in NREL’s Chemical and Materials Science Center and a co-lead author of the paper, described some of what thermoelectric power generators must overcome.
“A big challenge for organic thermoelectrics in general, including SWCNT ( Single Wall Carbon Nanotubes) based thermoelectrics, is the n-type leg, which is required for typical thermoelectric generator designs. N-type doped organic semiconductors are typically less stable than their p-type counterparts. This challenge also exists for any electronic applications that may use organic semiconductors, including applications like field-effect transistors for digital logic.
“Another big challenge is rigorously comparing the transport properties within one plane of the material and providing ZT (the typical thermoelectric performance figure of merit) values where the thermal transport is measured in the same plane as the electrical conductivity and thermopower. We have done that in this study, and we are now working on ways to further decrease the in-plane thermal conductivity.
“The thermoelectric generator device architecture is another challenge for organic thermoelectrics. SWCNT-based devices may be even more challenging in this regard, since they have highly anisotropic carrier transport. There will be a lot of great research in the coming years on how to engineer the appropriate architecture for different types of real-world organic thermoelectric devices. Within this overarching challenge, there are a number of separate aspects to consider:
“organic thermoelectric materials are typically fabricated as thin films, requiring novel solutions to afford efficient device architectures,
“the unusual device architectures require elegant strategies to fabricate the multiple n- and p-type legs required in a thermoelectric generator, and to deposit the electrical contacts to these legs,
“generating high-efficiency devices in unique form factors. The latter goal represents one of the advantages of organic semiconductors, in terms of making flexible devices in unique form factors that aren’t really achievable for rigid inorganic thermoelectrics, but it is also an aspect that is still relatively uncharted territory.”
There are already prototype thermoelectric generators using single wall nanotubes.
“The majority of these, except for one example, use SWCNT films that have both semiconducting and metallic SWCNTs, and have power factors that are at least an order of magnitude lower than we find in our study. The one example using pure semiconducting SWCNTs has films that have power factors that are roughly 1/3 of our values. So, the technology is still at the prototype stage, but our fundamental results can help to inform strategies for dramatically improving the performance of these prototypes.”
He did not want to “put an exact time frame on when devices will be in the market,” but did say they are “working on a number of strategies to further improve the thermoelectric figure of merit by increasing the power factor and decreasing the thermal conductivity.”
There are still many discoveries ahead about the potential of both single wall carbon nanotubes and organic semiconductors.
Andrew Ferguson and Jeffrey Blackburn, a senior scientist and manager of NREL’s Spectroscopy and Photoscience group, were the lead authors of “Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties,” which appears in the journal Nature Energy.
Their work is a collaboration between NREL, Professor Yong-Hyun Kim’s group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink’s group at the University of Denver.
Image: Solutions containing carbon nanotubes of different thermoelectric materials. Photo by Dennis Schroeder / NREL