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The one clean-tech breakthrough that could lead to a core climate solution: Thermoelectricity

By Joe Romm  

"The one clean-tech breakthrough that could lead to a core climate solution: Thermoelectricity"

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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 (see “The breakthrough technology illusion“). Indeed, most such breakthroughs would be difficult to deploy fast enough and on a large enough scale to make a large difference in that timeframe. Other key medium-term technologies, like low-cost solar photovoltaics, don’t require breakthroughs so 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.

http://www.thermoelectrics.com/images/tc_gen.gifOnly one long-sought-for technology I can think of, a true holy Grail of clean energy, 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 materials:

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.

UPDATE: The Science article has an interesting discussion of “whether alternative thermodynamic cycles could be used to improve efficiency”:

In thermodynamic terms, each p and n TE couple is a separate heat engine and, in principle, could operate independently of the other engines that make up a TE device. If each engine could operate optimally (that is, at the ideal temperature and current), system-level efficiency could increase. The analog is to compare the efficiency of two common heat engines that burn oil. In a diesel engine, each cylinder is an independent heat engine, but all cylinders operate at the same temperature and pressure conditions, the diesel cycle. The engine delivers about 30 to 45% efficiency. In contrast, in a turbine engine, such as is used in municipal electric power–generation systems, every stage of the compressor and expansion sections operates optimally for the working-fluid conditions at each point. This is a regenerative Brayton cycle, and efficiency in modern systems is about 60 to 65%, nearly double that of the diesel cycle. We developed a cycle analogous to the Brayton cycle, in which the TE engines are arranged as shown in Fig. 2A.

te-engine.gif

Fig. 2. Thermodynamic cycles. By optimizing each element along the thermal gradient, the engine resembles a gas turbine engine (the high-efficiency Brayton cycle) rather than the less efficient diesel cycle, in which the temperature and pressure conditions of every element (TE junction or combustion cylinder) are the same. This approach is shown for heating and cooling in (A) and for power generation in (B).

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14 Responses to The one clean-tech breakthrough that could lead to a core climate solution: Thermoelectricity

  1. Arthur Smith says:

    Sorry Joe, you can’t beat Carnot.

    Once you’ve turned primary energy from fuel into heat (as in any standard electric power generator) you have already lost roughly 60+% of that energy to a form that cannot be used for work. Thermo-electric devices simply cannot recover that irrecoverable 60+%, no matter what nanotech techniques you apply.

    The thermodynamic efficiency of a thermal power plant is determined by the ratio between the temperatures of the two heat baths it has available, the high-temperature bath being from the primary fuel consumption (usually applied to make high temperature steam), and the low-temperature bath of coolant (usually ambient water). Your thermoelectric solution would have the same two heat baths available, and the same maximum efficiency. No getting around the basic thermodynamics.

    Gas turbines can beat Carnot by turning some of that fuel energy into mechanical energy (the expanding burning gas turns the turbine) rather than heat. Fuel cells can beat Carnot by using electrochemical processes that avoid the thermal step altogether. But thermoelectrics depends on the thermal step, and so is ultimately not going to help, at least with major power generation.

    Now, thermoelectrics can help in situations where you naturally have a high-temperature waste stream and you’re at too small a scale to do something like a steam turbine. For example, pulling heat from automobile exhaust, it could well make sense to do this. Small industrial facilities that generate waste heat similarly.

    But thermoelectrics do not help at the large power-plant scale, sorry.

  2. Rick C says:

    Joe,

    Are these TE devices Peltier chips?

    [JR: Those are used for cooling. They are currently incredibly inefficient, so they only make sense for specialized devices that need small, localized cooling. I would imagine that an improved Peltier chip might be an outcome from this new research.]

  3. Bob Wallace says:

    Speaking of heat…

    I’d like to see more coverage of drill-down geothermal.

    Huge advantages if it works out. It’s 24/7, not influenced by weather changes, can (likely) be install close to point of use, non-polluting, ….

  4. No heat engine “beats Carnot”, certainly not gas turbines.

  5. Joe says:

    Arthur:

    I have talked to companies that are working on converting waste heat to electricity. Some use processes that are not precisely the traditional thermoelectric effect, but I am lumping this heat-to-power stuff all together here.

    I don’t think what you wrote is true. Thermal power plants give off huge amounts of waste heat, sometimes using cooling towers to dissipate. Heat can be turned into electricity through a variety of means, just not efficiently or cheaply today.

  6. Megan Michaels says:

    So I am wondering about algae biofuel? What are your thoughts on this:

    http://www.sapphireenergy.com/

  7. David B. Benson says:

    I believe there is a company no making ‘wind’ powered generators which are designed to be fitted over the exhaust stack of coal reactors. THe idea is that the rising flue gasses provides the ‘wind’.

    If the thermal gradient is low, consider Sterling cycle engines equipped with linear generators.

  8. Brendan says:

    “I don’t think what you wrote is true. Thermal power plants give off huge amounts of waste heat, sometimes using cooling towers to dissipate.”

    If you know thermodynamics and are referring to something else, I’m sorry. Thermal powerplants have to give off lots of waste heat to work. That’s why cogen plants are good, because they take this waste heat and turn it into something useful. So in a sense, these would be a sort of cogen if used in this mannor.

    Do you have any sense of the numbers involved in these? For example, all houses in the winter need insulation. If these materials could provide insulation while turning that temperature difference into energy, it seems like that would be pretty big news. However, if temperature gradients of 40 degrees only provide mW of power, then it isn’t very useful.

    We don’t have to worry for too long, fusion is only 30 years away…

  9. Arthur Smith says:

    Joe, you of all people should know not to be taken in by company hype!

    Here are the basic physical limits.
    When a quantity E of primary fuel energy (say oil), is burned, the energy E becomes thermal energy of the combustion products, raising their temperature to some high value T_high. In order to extract useful work from this thermal energy E, you need some process that takes it from the T_high system and eventually exhausts the energy through a waste stream at some lower temperature T_low. The fraction of the energy E that can be extracted as work, W, is physically limited by the laws of thermodynamics according to the two temperatures:

    W <= (1 – T_low/T_high) * E

    or a maximum efficiency for the system of (1 – T_low/T_high). It doesn’t matter if the process is a steam boiler or a thermocouple, once you’ve burned the fuel to thermal energy the efficiency limit is the same. If T_low is around room temperature (300 K) and T_high is around 500 K (227 degrees C) the maximum efficiency you can possibly get is 40%. That is, as soon as your primary energy E has been burned to thermal energy at 500 K, 60% of that original energy is no longer available to perform work, and at least that 60% is bound to leave your plant as waste heat.

    You can get more work out of the same primary energy by increasing T_high – that’s an element of several proposals for advanced nuclear power reactors, to run them at higher temperatures and thus extract more of the primary energy from the fission fuel. For coal you can do something similar by gasification, which allows more direct use of the combustion products (Kirk above is of course right that these don’t “beat Carnot”, they just beat the normal limits for steam turbine plants by making more direct use of the waste stream at its hottest). But there are capital and maintenance cost issues – high temperature operation is more dangerous and more likely to lead to corrosion and failure, so the efficiency improvement may not be worth the extra costs.

    It’s possible that replacing steam turbines with thermo-electrics could be a cost effective solution, I’ve not seen the numbers on these systems at all. If they could couple to a higher-temperature part of the process they might even have some potential for efficiency improvements. But most of that 60% wasted by power plants is a simple consequence of burning fuel, and thermoelectrics can’t avoid the same fundamental physical limits.

    If we’re talking about waste streams at not much above room temperature, the efficiency limits there are far lower – for a 40 C differential you couldn’t get more than 12% of the energy out as work (or electricity). Imagine a two-step process where the steam turbine extracts the energy at T_high of 500 K, leaving a waste stream at T_low of 340 K. Then add a thermo-electric element between the 340 K waste stream and a 300 K bath (a local river, say). Of the original primary energy E, at most 0.32 E can be extracted by the steam turbine, and 0.68 E goes into the waste stream. The maximum possible efficiency of the thermo-electric system is then a little under 12%, which applied to the 0.68 E waste stream gives us an additional 0.08 E, for a total of 0.40 E energy extracted. But that is exactly the same useful work we could have extracted from the system if we could have had the original process exhaust at 300 K instead of 340 K – all that’s required to do that is the exact same larger volume of coolant water that we’d need to keep the thermo-electric system cool.

    The long and short of it is, there’s no way to trick the thermodynamics into letting you have more useful energy once you’ve burned the fuel; to improve efficiency you need to work on the front-end of the process, not the back-end.

  10. Larry Coleman says:

    Arthur,
    Yes, it is not possible to fool the Second Law. But my interpretation of Joe’s original post is to do exactly what you describe so well: operate between the waste temp and ambient temp, 40 K in your example. The point is that power plants do not exhaust at ambient temperature, much as they would like to, which is why cooling is necessary and also why cogen works. Yes, the efficiency would be low but it is essentially free energy so that’s ok.

  11. Cyril R. says:

    Of course you don’t fool the second law. But you can fool Carnot by not using a heat engine. Carnot is about heat to motion. TE are a fundamentally different concept.

    Now, thermo-electrics do have crap efficiency and they have their own curves which resemble the delta T’s of a heat engine. It’s kind of an academic issue. What’s important, as Joe also points out, is that thermo-electrics just aren’t efficient, and progress is only coming slowly.

    By any reasonable defintions, there have actually been several breakthroughs lately, some of them nano, but the efficiency under low temperature is still crap. The best are perhaps in the 2-3 percent range. This leads to high cost simply because the costs can’t be levelized over a large amount of electricity generated; low efficiency means low kWhs harvested.

    There is interesting work going on in infrared nano-antennas though, which could plausibly lead to efficient (>50%) low temperature to electricity conversion. Call them radiative electrics?

  12. Cyril R. says:

    Oh, and all electric conversion devices have their own second law efficiencies. PV has second law maximum greater than 95% so this is not a serious constraint. Far more relevant are a plethora of practical and technical limits.

    Close to 100% efficiency is theoretically possible with heat engines. Just need an infinite temperature difference. Easy as that :)

  13. Connor says:

    Hey there,

    This link is broken -

    http://climateprogress.org/2008/07/23/energy-efficiency-is-the-core-climate-solution-part-1-the-biggest-low-carbon-resource-by-far/

    Any chance it could be fixed, I am really keen to read it!

    Cheers…

    [JR: My apologies -- All fixed!]

  14. Taylor says:

    This is right here, in the present, not the future.