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.

Thanks to Al Fin for the tip.

Hagelstein says, “There’s a gold mine in waste heat, if you could convert it. A lot of heat is generated to go places, and a lot is lost. If you could recover that, your transportation technology is going to work better. There’s no reason, in principle, you couldn’t get another order of magnitude or more improvement in throughput power, as well as an improvement in efficiency.”

Now the gold mine analogy is a truth, but Hagelstein is leaving out massive stores of heat or thermal power, from electrical power stations, industrial furnaces and others.  The amount of heat released could equal as much as 75 percent, or three times more energy could optimally be available than what is being put to work now.  A breakthrough at reasonable cost could triple the available energy for people to use.  Harvesting the energy in lost heat would set up decades of energy abundance for the future.

Successful technology would also relieve the expense of getting rid of excess heat.  Imagine an air conditioner that delivered power to the grid instead of drawing from it.  The potential is just stunning.

As part of Wu’s doctoral thesis the pair aimed to find how close realistic technology could come to achieving the theoretical limits for the efficiency of such conversion.  When this work began around 2002, Hagelstein says, such devices  “clearly could not be built. We started this as purely a theoretical exercise.” But developments since then have brought it much closer to reality.

Theory says that such energy conversion can never exceed a specific value called the Carnot Limit, based on Carnot’s 19th-century formula for determining the maximum efficiency that any device can achieve in converting heat into work. Current commercial thermoelectric devices only achieve about one-tenth of that limit.

In experiments involving a different new technology, thermal diodes, Hagelstein worked with Yan Kucherov, who is now a consultant for the Naval Research Laboratory, and other coworkers to demonstrate efficiencies as high as 40 percent of the Carnot Limit. The team’s calculations show that this new kind of system could ultimately reach as much as 90 percent of that ceiling.

Quantum Dot Mini Rainbow. Click image for more info.

Hagelstein’s team carried out their analysis using a very simple system in which power was generated by a single quantum-dot device — a type of semiconductor in which the electrons and holes, which carry the electrical charges in the device, are very tightly confined in all three dimensions. By controlling all aspects of the device, they hoped to better understand how to design the ideal thermal-to-electric converter.

A key to the improved throughput was reducing the separation between the hot surface and the conversion device. A recent paper by MIT professor Gang Chen reported in an analysis showing that heat transfer could take place between very closely spaced surfaces at a rate that is orders of magnitude higher than predicted by theory.  The new report takes that finding a step further, showing how the heat can not only be transferred, but also converted into electricity so that it can be harnessed.

Hagelstein says that with present systems it’s possible to efficiently convert heat into electricity, but with very little power. It’s also possible to get plenty of electrical power, what is known as high-throughput power, from a less efficient, and therefore larger and more expensive system. “It’s a tradeoff. You either get high efficiency or high throughput,” says Hagelstein. But the team found that using their new system, it would be possible to get both at once, he says.

The team’s paper has appeared in the Journal of Applied Physics 106, November 13, 2009 / DOI:10.1063/1.3257402

Hagelstein says a company called MTPV Corp. (for Micron-gap Thermal Photo-Voltaics), founded by Robert DiMatteo, is already working on the development of, “a new technology closely related to the work described in this paper.”

DiMatteo says the work described in this paper “is potentially a major finding.”  He hopes eventually to commercialize Hagelstein’s new idea. In the meantime, he says the technology now being developed by his company, which he expects to have on the market next year, could produce a tenfold improvement in throughput power over existing photovoltaic devices, while the further advance described in this new paper could make an additional tenfold or greater improvements possible.

While it may take a few years for the necessary technology for building affordable quantum-dot devices to reach commercialization, Hagelstein says, “there’s no reason, in principle, you couldn’t get another order of magnitude or more,” improvement in throughput power, as well as an improvement in efficiency.

This writer cannot make clear enough that heat is the energy to capture.  From solar radiation on to every burn man has at his disposal, the amount of energy about is far beyond what can be used with the equipment and tools at hand now.  Efficient heat harvest would take energy off the list of world concerns, and put this writer out of business.  So be it.  The breakthrough might be the MIT solution, if so, congratulations and get on with it!