Mar
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Dawn Of The Thermo Cell
March 2, 2010 | 4 Comments
A thermocell is a device that captures heat energy and converts it to electricity, an idea with the potential of doubling or tripling the available power supply if applied to the various thermal heat engines that are used to generate electricity. Success at high efficiency, scale and price would have a dramatic effect of power costs worldwide. As the numbers below show, applications could be widespread. This is definitely a field to watch closely.
An international team of researchers from the US (link to University of Texas Dallas), India and Australia demonstrated thermo-electrochemical cells in practical configurations from coin size and shaped cells to cells that can be wrapped around exhaust pipes that harvest low-grade thermal energy (temperature below 130 °C), using relatively inexpensive carbon multiwalled nanotube (MWNT) electrodes. Applications can be anywhere heat is a source and electricity is a part of the energy use.
Thermocell Installed Over Pipe. A thermocell is wrapped around a stainless steel pipe to generate electrical power.
The team’s work paper was published online February 19th in the ACS journal Nano Letters.
International Team's Thermocell Function. Click image for the largest view.
The thermocell is a structure that has an anode that operates in the heat source and a cathode that operates in cooler or cold source. The team’s anode and cathode provide high electrochemically accessible surface areas and fast redox-mediated electron transfer. The surfaces the team has designed significantly enhance 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 times higher than previously demonstrated thermocells. They are getting somewhere with a good jump.
So far low efficiencies and costly electrode materials have limited harvesting of thermal energy as electrical energy using thermo-electrochemical cells. With the cost of MWNTs decreasing, MWNT-based thermocells may become commercially viable for harvesting low-grade thermal energy. One part of the team’s astonishing result is from efficiency further improved by directly synthesizing MWNTs as vertical forests that reduce electrical and thermal resistance at electrode/substrate junctions.
The team developed the carbon nanotube -based thermocells utilizing 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. That common process for making thermocells is very encouraging.
They found that the performance of MWNTs as thermocell electrodes supersedes that of conventional electrode materials, including expensive 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, justifying the efficiency claim of Carnot cycle efficiency of 1.4%. Cheap enough – this could make great sense. The temp zone is below what is being seen for binary generator sets. From the paper (a pdf file link):
“…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.
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.
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.”
This is just Version 1 of the latest thermocell leap. The list of possible applications just boggles ones’ thoughts. With a starting point of only 65 °C and a temperature difference of 60 °C the team’s efforts are addressing a huge store of energy with higher temperature application research surely in mind.
Thermocell may well have a role in cogneration. In an ideal energy production, all the heat would go out in work. That level of overall efficiency would change the entire field of view in energy production and use. The team’s work is an effort worthy of congratulations.
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At the top of the article:
…an idea with the potential of doubling or tripling the available power supply if applied to the various thermal heat engines that are used to generate electricity.
There is a role for such devices, mostly as sources of small amounts of power to run things like wireless sensors. But don’t expect significant amounts of energy from them, aside from something that is already known to be wasteful. Another good application I have seen is as a power generator for the fan on a natural gas furnace. With a device like this, the furnace does not need electricity from outside to run.
The absolute maximum amount of energy that can be extracted from heat is:
(Th-Tl)/Th
where Th is the high temperature and Tl is the low temperature. Both temperatures are absolute temperature, either kelvins or degrees Rankin. Ideally, the Carnot cycle can achieve this limit.
Take the example cited for 130°C (403K) as the hot end. Assume 25°C (298K) for the low temperature end. Run the numbers, and you find that no more that 26% of this heat can be converted into electrical energy. Of course, real-world conversion efficiency will be lower, most likely a lot lower.
The very best thermo-mechanical systems (large combined cycle power plants — gas turbine exhausting into a boiler for a steam turbine) presently achieve a hair over 60% conversion efficiency based on the non-condensing heat value of natural gas. The efficiency is “only” 60% due to practical limitations, not because the designers are stupid. It is quite remarkable to do this well.
If heat is harvested somewhere to improve on efficiency already that high, one needs to make sure not to screw up efficiency of the base plant.
1.4% efficiency – this isn’t a very useful conversion rate.
I would like to exchange links with your site newenergyandfuel.com
Is this possible?
I’m totally unschooled in this stuff, but what are the applications for air conditioning? If the objective was to get rid of the heat and the amount of electricity generated was just a side-benefit, would this be useful?