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Background:
In an automobile, only 1/3 of the potential energy in the fuel is converted into mechanical energy and significant portion of the energy is lost as heat.

There have been previous attempts to recuperate this lost energy. In the early 1990's, Porsche developed automotive thermoelectric generators (ATEG) which didn't go past prototyping stage. Currently, Porsche Motorsports is testing a thermal energy harvesting system in their LeMans series Race car.

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In addition to Porsche's research, GM is in collaboration with Future Tech, LLC. to explore the idea of using themoelectric technology to harvest energy from internal combustion engines. Other automotive manufactures, such as BMW, are also exploring this technology.

Currently the power usage in a

  • Small car is approximately 150 W
  • Full size truck is approximately 500 W

If this technology can successfully be implemented, then components such as the radiator, water pump, and alternator could effectively have reduced workload or removed from the system, thus reducing the load to the internal combustion engine.

Question:
With the growing interest in green technology, are there technology barriers beside efficiency that are preventing the implementation of energy harvesting from internal combustion engines using thermoelectric technology?

References:

Footnote

The suggested duplicate is related, but still distinctly different. The order of magnitude of energy available to recover from an internal combustion engine is significantly greater than within the GPU of a video card. As such, the economies of scale are different and different solutions are therefore possible.

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  • $\begingroup$ That's just a wild guess but I think that harvesting energy actually increases the engine to ambient thermale resistance, thus increasing the engine temperature. I am not sure about the "reduce or remove radiators" part, did you read it somewhere or just guessed it? I'm feeling the exact opposite is true. $\endgroup$
    – Vladimir Cravero
    Jan 31, 2015 at 20:25
  • $\begingroup$ @Trilarion If you have any suggested obstacles, I can update the question with those specific obstacles to help narrow the scope. Appreciate your comment $\endgroup$
    – Mahendra Gunawardena
    Feb 5, 2015 at 11:33
  • $\begingroup$ @MahendraGunawardena Thanks for the nice reply. I would suggest asking new questions for each obstacle, since they are more specialized that way. But I only know that a friend is working on that in a big car company. I may have to ask him what are the problems. I guess that the low efficiency of thermoelectric conversion in general is an obstacle. $\endgroup$
    – Trilarion
    Feb 5, 2015 at 11:37
  • $\begingroup$ it seems bizarre that you've excluded efficiency as a consideration: why else would you look at energy harvesting anyway? $\endgroup$
    – 410 gone
    Feb 19, 2015 at 6:56

3 Answers 3

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As with any new technology, the cost is the big driver here. In addition, these devices produce electricity which is a form of energy that a typical internal combustion automobile can only utilize for ancillary equipment. This would effectively improve fuel efficiency but the gain would be relatively minor.

Engineers are generally reluctant to use expensive new technologies that are relatively untested when existing methods are sufficient for achieving the goal. In this case, most automobile manufacturers strive to produce a cost-competitive product. The people who are most concerned with fuel efficiency will tend to consider a hybrid (or all-electric) vehicle instead of one powered by a standard internal combustion engine. Most other consumers would prefer to purchase a less-expensive vehicle over one that could slightly increase their fuel efficiency.

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There are good reasons why, although most of your links are from several years ago, very little has come of any of them. The economics and the engineering just aren't favourable. Part of the art of an engine is dumping the heat as quickly as possible; furthermore, heat is a low-quality form of energy, so once your energy is heat, you've already lost all your best opportunities to get more work done with it. Let's look at those two things in detail.

getting rid of the heat quickly is the name of the game

The exhaust is designed to take heat and the products of combustion away from the engine. The radiator does a similar job of removing and dissipating the heat.

Dumping heat quickly is crucial in a heat engine, as the efficiency depends on the temperature of the cold reservoir, and the delta to the higher temperature.

So anything you put in the way, such as your proposed energy harvesting device, will slow the rate at which heat leaves the engine. This not only reduces efficiency (it seems bizarre that you've excluded efficiency as a consideration: why else would you look at energy harvesting anyway?) It will also raise the equilibrium temperature of the car's working parts, shortening its life.

If fuel is that valuable that energy harvesting looks attractive, then it's worth making the car more efficient in the first place, so that there was less waste heat: first, reduce the consumption of high-value energy, before trying to recycle low-value energy. And that brings me to ...

energy versus exergy

Heat is, in a large proportion of cases, a waste product. It's almost always the least useful form of energy. That's really what the Carnot efficiency limit is telling you: that to get any work out of low grade heat, you can only do so with very low efficiency; that is, almost all of the heat will stay as heat.

When doing engineering with heat and other forms of energy, it's very useful to build up an intuition to distinguish between energy (the thing measured in joules) and exergy (the thing that gets work done). The form that energy is in, determines how much work it can do. Chemical energy, such as that in fuel, can do huge amounts of work efficiently - it has very high exergy. But the same amount of energy as heat can do much less work - it has very low exergy. Internal combustion engines are so inefficient because they take a high-grade form of energy (chemical), and immediately convert into a low-grade one (heat).

The humble alternator is very good at its job. Solid-state thermo-electric generators simply don't come close, not in cost, not in performance. Using the relative rotation of a conductor and a magnet is a very effective means of turning heat into electricity: that's why it's used in power stations, cars, bike dynamos, and many other uses.

So although energy harvesting looks clever, it's bad engineering: its trying to fix one symptom, rather than address the underlying cause.

If you want more work out of those joules, then get that work done before those joules are in the form of heat.

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  • $\begingroup$ One thing I've long wondered: when a typical car is operating at cruising speed, how much energy is spent pumping the vacuum downstream of the throttle? From what I can tell, not only is 99% of the energy spent drawing vacuum is wasted (most cars harvest a tiny bit of it for things like power brake booster) but engines operate more efficiently at a lower intake temperature, so harvesting energy from the pressure differential would not require the engine downstream to do any more work. $\endgroup$
    – supercat
    Mar 19, 2015 at 17:52
  • $\begingroup$ @supercat 5 years on: Ever driven a vehicle with manifold vacuum operated wipers? :-) Great fund, mostly. I once did furniture delivery work while at university driving an old ?Commer? van. Pouring rain. Motorway. Wipers on fast. Pull out to overtake. Floor throttle. Wipers stop completely in mid screen. Blind. Overtaking then consists of as long as you dare with throttle floored, with an occasional complete back off to allow wipers one sweep, then back on throttle :-) $\endgroup$
    – Russell McMahon
    Apr 3, 2020 at 0:12
  • $\begingroup$ @RussellMcMahon: Wipers don't take much power, of course; I would expect that even with wipers on, most of the energy spent drawing vacuum would be wasted. I remain curious about how much energy that is. $\endgroup$
    – supercat
    Apr 3, 2020 at 14:11
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The use of any idea/technology is dependent on its cost effectiveness. Efficiency is just one part of the analysis. If the modules were free and never went bad, they would certainly be in use. However, high temperature modules are expensive for their power produced; making them currently not cost effective.

EnergyNumbers does a good job of explaining the thermodynamics involved. Because of Carnot you cant harvest significant amount of power in this fashion. This is also why money for peltier modules is better spent on increased engine efficiency/performance instead. However, high temperature waste heat will be there regardless and when thermoelectric becomes cost effective, you will see it in use.

Despite what people may advertise, no idea/technology is "green" until it can pull its own weight in a cost benefit analysis. You have to look at the whole picture; how many dollars and raw materials did it consume to provide this "green" energy.

If you would like to discover it for yourself, here are some high temperature modules you can price and evaluate. Remember the watts listed is the consumed power not the generated power. tetech.com

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