I like to follow energy-harvesting research developments and actual installations, as there are many creative approaches and useful applications. In many cases, harvesting has solved a power-source problem effectively and with reasonable cost versus benefit.

At the same time, however, I see energy harvesting as often being oversold at best and overhyped at worst. There’s a real glow with the concept of getting something for (almost) nothing that is often associated with it, when the harsh reality is you may be getting very little energy for a much higher cost and complexity than what was promoted. That tradeoff may be acceptable if you are desperate or have no viable alternative, but often that is not the case.

Perhaps the strangest non-conventional harvesting scheme I saw was a specialized coating that could be applied as wall paint (see References 1 and 2). That coating used humidity in the air to harvest energy, with the speculative projections that maybe you could power a house using this paint. Of course, beyond the obvious issues of physical-connection wiring, there was the near-trivial actual available output. The power density output of 0.0001-0.05 watts/meter² was quite modest (to be polite) in both absolute and relative terms and certainly wouldn’t power your house or even a smartphone.

Tailpipe TEG

A good example of a more practical harvesting arrangement is a recent thermoelectric generator (TEG) story I saw in the Wall Street Journal, of all places (Reference 3). A research team at Pennsylvania State University developed a TEG that fits into the exhaust tailpipe of an internal combustion engine (ICE) vehicle and uses the exhaust waste heat to generate up to about 40 watts (Figure 1).

Figure 1 (a) 3D schematic diagram of the TEG system. The geometry of the exhaust gas pipeline can vary. (b) Power (P) and (c) power density (ω) for automobile and high-speed object conditions. Source: Pennsylvania State University

While that’s enough to power or recharge a small electronic device, it’s a fairly modest amount of power in the context of the power of the engine of a car, small airplane, or helicopter. One of their claimed innovations in this implementation is that it is optimized to work better when there is cooling airflow around the moving tailpipe, yielding a larger temperature differential and thus greater output. The design has been modeled, a prototype built and tested, and the collected data is in line with the expectations (Reference 4).

So far, so good. But then it the storyline goes into what I call extrapolation mode, as the “free energy” and “something for almost nothing” aspects start to overtake reality. How much does this harvester cost as a single unit, or perhaps as a mass-produced item? How long will it last in the tailpipe, which is a harsh environment? What’s the effect on engine exhaust flow and back pressure? What’s the developed energy density, by weight and volume?

The WSJ reporter covering this story seemed to be a non-technical journalist who basically repeated what the researchers said—which is certainly a valid starting point—but didn’t ask any follow-up questions. That’s the problem with most energy-harvesting stories, especially the free-heat TEG ones: they are so attractive and feel-good in concept that the realities of the design and installation are not brought up in polite conversation while the benefits are touted.

I’m not saying that this TEG harvesting scheme is of no value. It may, in fact, be useful in specific and well-defined situations. There are many examples of viable waste-heat recovery installations in industrial, commercial, and residential settings to prove that point. But as will all designs, there are hard and soft costs as well as short- and long-term implications that shouldn’t be ignored.

Small-scale TEG

There are also smaller-scale TEG-type harvesting success stories out there. For example, for many decades, gas-fired home water heaters used their own always-pilot light (no longer allowed in many places due to energy mandates) to heat an array of thermocouples. This array then provided power to activate and turn on the gas valve and ignite the gas to heat the water in the tank (Figure 2).

If the pilot light was out for any reason, turning on the gas valve to heat that water would be extremely dangerous. However, the gas-heated thermocouple system is self-protecting and fail-safe: in the absence of that pilot light that ignites the gas and heats the thermocouples, there is no power to actuate the valve, thus the gas flow would be shut off. As an additional benefit, no electrical wiring of any type was needed by the water heater. It was a plumbing-only (water and gas) installation with no external electricity needed.

Figure 2 This schematic of a gas-fired water heater shows the bottom thermocouple assembly whose electrical output controls fail-safe actuation for the gas-flow valve. Source: All Trades Las Vegas

Harvesting hubbub

My sense is that harvesting gets so much favorable attention because it is so relatable and appears to offer no/low-cost benefits with little downside, at least at first glance. There’s little doubt that the multifaceted attraction of TEG and other energy-harvesting approaches attracts a lot of positive attention and media coverage, as this one did. That’s a big plus for these researchers as they look for that next grant.

Engineers know that reality is usually different. When it comes to generating, capturing, and using energy and power, the old cliché that “there’s no such thing as a free lunch” usually applies. The real question is the cost of that lunch.

Have you used TEG-based harvesting in any project? What were the expected and unexpected issues and benefits? Did you stick with it, or do you have to go with another approach?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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References

1. Nature, “Generic Air-Gen Effect in Nanoporous Materials for Sustainable Energy Harvesting from Air Humidity”
2. Nature, Supplement to “Generic Air-Gen Effect in Nanoporous Materials for Sustainable Energy Harvesting from Air Humidity”
3. The Wall Street Journal, April 18, 2025, “The Heat Coming Out of Your Car’s Tailpipe? Some Can Be Turned Into Electricity”
4. ACS Applied Materials & Interfaces, January 7, 2025, “Thermoelectric Energy Harvesting for Exhaust Waste Heat Recovery: A System Design” (behind paywall, but it is also posted here at ResearchGate)

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