Next Generation Energy

The buzzwords of the day: TE with high TZ.

The world doesn't need a major technology breakthrough to cost-effectively cut carbon emissions in half by midcentury. Indeed, most such breakthroughs would be difficult to deploy fast enough and on a large enough scale to make a significant difference in that timeframe. Other key medium-term technologies, like low-cost solar photovoltaics, don't require breakthroughs as much as they need steady technological advances, economies of scale, and continued experiential learning from increased market sales.

Sure, we are going to need big-time advances to give us new low-carbon technologies for widescale deployment in the second half of this century to have any hope of getting back to 350 ppm -- but is there any genuine breakthrough that could make a serious difference fast enough to matter by 2050? Such a technology would have to be compatible with the existing energy system. Ideally, it would take advantage of major existing inefficiencies or flaws in our current energy system. It would have to be a technology that could be scaled to many different applications.

Only one long sought for technology I can think of, a true clean energy dark horse or holy Grail, fits the bill: thermoelectric (TE) materials and devices, which directly convert temperature differences to electric voltage and vice versa.

Thermoelectric devices are based on the fact that when certain materials are heated, they generate a significant electrical voltage. Conversely, when a voltage is applied to them, they become hotter on one side, and colder on the other. The process works with a variety of materials, and especially well with semiconductors -- the materials from which computer chips are made.

Why does the ability to turn low-level heat into electricity matter? Because the energy system throws away vast amounts of energy as waste heat. Heck, the energy now lost as waste heat just from U.S. power generation exceeds the energy used by Japan for all purposes.

And that doesn't even include the massive amount of waste heat from much smaller scale engines, like those in your car, where some 80% of the fuel's energy is lost. Wouldn't it be great to capture some of that waste heat and use it for electricity -- in plug-in hybrids, for instance? Imagine if you could design a TE device right into a microchip, to take waste heat and generate more power for you laptop? And what about the potential of high-efficiency, solid-state heating and cooling devices? Or, as M.I.T. noted recently:

The same materials might also play a role in improving the efficiency of photovoltaic cells, harnessing some of the sun's heat as well as its light to make electricity. The key will be finding materials that have the right properties but are not too expensive to produce.

And, of course, a larger scale system could take the waste heat that needs to be rejected from baseload solar (a concentrated solar thermal electric system) and use it to increase efficiency and power output.

Okay, if TE devices are so great, why aren't they everywhere already? After all, the key underlying scientific principles of TE were first discovered nearly 200 years ago.

But [TE] always had one big drawback: it is very inefficient. The fundamental problem in creating efficient thermoelectric materials is that they need to be very good at conducting electricity, but not heat. That way, one end of the apparatus can get hot while the other remains cold, instead of the material quickly equalizing the temperature. In most materials, electrical and thermal conductivity go hand in hand. So researchers had to find ways of modifying materials to separate the two properties.

This looks like a job for nanotechnology. Critical work in the early 1990s by MIT Institute Professor Mildred S. Dresselhaus and others has lead to a tremendous resurgence in TE devices:

The key to making it more practical, Dresselhaus explains, was in creating engineered semiconductor materials in which tiny patterns have been created to alter the materials' behavior. This might include embedding nanoscale particles or wires in a matrix of another material. These nanoscale structures -- just a few billionths of a meter across -- interfere with the flow of heat, while allowing electricity to flow freely. "Making a nanostructure allows you to independently control these qualities," Dresselhaus says.

For those interested in a more technical discussion, I'd strongly recommend a major review article in the latest issue of Science by Lon Bell: "Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems" (subs. req'd). Bell explains "TE devices are solid-state heat engines. Unlike today's air conditioners, which use two-phase fluids such as the standard refrigerant R-134A, TE devices use electrons as their working fluid."

Bell explains the one term of art, ZT, that is worth being able to drop in semi-technical discussions to impress your friends and wow your colleagues:

A figure of merit, ZT, expresses the efficiency of the p-type and n-type materials that make up a TE couple. The parameter Z is the square of the Seebeck voltage per unit of temperature, multiplied by the electrical conductivity and divided by the thermal conductivity, and T is the absolute temperature. In today's best commercial TE cooling/heating modules, ZT is about 1.0, and in air-conditioning applications is about one-quarter as efficient as a typical conventional system, such as one that uses R-134A. Ideal TE system efficiency increases nonlinearly with ZT, so that to double efficiency, ZT has to increase to about 2.2. To achieve a fourfold increase (to equal the efficiency exhibited by today's two-phase refrigerants), ZT would need to increase more substantially to about 9.2.

As noted, it has really been the emergence of nanotechnology that has led to the resurgence of interest in TE by industry, academia, and government :

In 1993, the U.S. government's Office of Naval Research and Defense Advanced Research Projects Agency asked interested researchers to propose pathways to improve ZT for cooling and heating applications. A specific interest was to determine whether the then-emerging nanotechnology and its potential quantum-scale synthesis could lead to new superior TE materials. In 1993, Hicks and Dresselhouse published a theoretical model predicting the effect on ZT of confining electrons to two-dimensional quantum wells. They calculated that the Seebeck coefficient could be increased and the thermal conductivity could be suppressed. The promise of this concept and other ideas from within the TE community led the U.S. government to fund several innovative approaches in the mid-1990s. This initiative set in motion a substantial increase in both theoretical and TE-material developmental research.

By 2001, Venkatasubramanian of Research Triangle Institute announced achievement of a room-temperature ZT of about 2.4 for a nanoscale structure made by alternating layers of two TE materials that both enhanced the Seebeck coefficient and suppressed thermal conductivity. The next year, Harman of Lincoln Laboratory published results claiming a ZT of up to 3.2 at about 300°C for a material with nanoscale inclusions that dramatically reduced thermal conductivity. In 2003, Kanatzidis at Michigan State University led a team in the development of a complex bulk tertiary material with a ZTof at least 1.4 at 500°C. Recently, Heremans at Ohio State University and an international team claimed reaching a ZT of 1.5 at 500°C.

Despite these promising results, efficiency gains at the device level have yet to be demonstrated. The scaling of the nanomaterials has proven to be quite difficult and is still in the development stage. The bulk material has yet to be made commercially available.

So we still need a major breakthrough to get commercial products. Still, I have talked to serious companies actively pursuing TE materials and related devices. The potential opportunity is simply too large to ignore:

Until recently, TE technology has languished despite the astonishing gains made in electronics, photonics, and other solid-state fields. Now, 15 years after U.S. government initiatives spurred resurgence in TE research, substantial progress is evident. More-efficient thermodynamic cycles and designs that reduce material costs are coming into commercial production.

If the final enabling advancement, higher ZT in TE materials, is realized, gas-emission-free solid-state home, industrial, and automotive air conditioning and heating would become practical. In power generation, fuel consumption and CO₂ emissions would be reduced by electric power production from vehicle exhaust. Industrial waste-heat recovery systems could reduce emissions by providing supplemental electrical power without burning additional fossil fuel.

The question is, Is TE technology on a path to overcome the historic limitations of low efficiency and high cost per watt of power conversion that have limited its applications in the past? If so, TE solid-state heat engines could well play a crucial role in addressing some of the sustainability issues we face today.

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