There Ain’t No Such Thing As A Free Energy Lunch


A South Korean research team announced they have developed a technology that converts body heat to energy: “What if you never had to charge your wearable tech — because it was able to harvest your body heat to run?” One of the biggest problems with wearable technology, such as smart watches, is their limited battery power.

But basic analysis shows that the math doesn’t work: when using their own numbers, a standard smartwatch would take almost four months to obtain a full charge!

Credit where credit is due: the team at SKAIT University, led by Professor Byung Jin Cho, has done some very good work. They have developed a thermoelectric (TE) generator that is both flexible and relatively high power: here is their paper. Kudos to them for producing a prototype device with a good bending radius and an ability to produce around 40 milliwatts (mW) of power from a TE patch 10 cm by 10 cm, assuming skin temperature of 31C and room temperature of about 21C. Unfortunately there are several issues that will likely limit the applicability of these technologies

The first problem is that smartwatches are NOT that large – in fact, looking at the photo on the top of this post you can see that the TE patch for a smartwatch is roughly 2.5 cm long and about 0.4 cm wide, giving it a total area of 1 cm2. Since the 10×10 cm patch produces 40 mW from an area of 100 cm2, we can assume that the patch that fits in a smartwatch band would produce about 0.4 mW.

I travel the world talking about wearables, and the smartwatch I most frequently see on people’s wrists is the Samsung Galaxy Gear. The latest version has a battery rated at 300 milliamp hours (mAh) and 3.7 volts (v). Normally, charging a battery requires a higher voltage, but using those numbers we can just use an online calculator to figure out how many milliwatts it would take to fully charge that battery. (Watts = Volts x Amps) = 300mAh x 3.7 = 1110 milliwatts.

Given that our smartwatch sized TE charger is producing 0.4 mW, and assuming  perfect charging conversion, the watch is switched off and using no power, and no self-discharge from the battery over time (which are unrealistic assumptions, but let’s give them the benefit of the doubt) it would take the TE patch 2,775 hours to charge a Samsung Gear2. Or 115.6 days!

This is true of almost every headline you will ever read about energy ‘harvesting.’ There are always those who are interested in getting energy for free; whether through thermoelectric effects, converting motion into electricity through piezoelectric generators, or from ambient RF energy. They all work – there’s no question about that. But the energy generated tends to be very very small: milliwatts to microwatts, sometimes even nanowatts! And the batteries in our smartwatches and smartphones and so on need MUCH more power to be charged. I wrote about this for Deloitte as part of our 2012 TMT Predictions: “Ambient radio frequency power harvesting: a drop in the bucket.”

I had help writing that Prediction from my Deloitte colleague Kelly McDonald and from my friend Brian Piccioni, technology analyst and author of The Geek’s Reading List. Both of them reviewed my thinking on this blog post, and raised some additional points.

The thermoelectric effect for generating electricity has been known since 1821, and is based on taking advantage of temperature differences, known as ∆T (Delta T or DT.) Most TE generators in use today harvest very large ∆Ts, usually 100° C or more. And while that kind of differential is common on a natural gas or oil pipelines, the human wrist is a much less harvestable environment.

Although our body temperature is about 37C, our skin is typically ~31C. Next, we tend to spend most of our time in environments where the air temperature is fairly close to our skin temperature, usually no more than 10C different. Although I guess you could get a much higher ∆T if you wore your TE-charging smartwatch outside in Winnipeg in January when it is -40C, but getting frostbite seems a poor trade-off for a few milliamps of charge! (And if you lived in Singapore, your smartwatch charger would produce no current whatsoever when you were outside.)

Next, our bodies produce a startling amount of heat all the time. So although the bottom of the TE generator resting against your skin would be at 31C and the average room temperature might be 21C (for a ∆T of 10C,) your body heat raises the air temperature above your skin by a few degrees, reducing the ∆T and extending charge time. (Unless you wave your arm around constantly to air cool the top of the TE generator.) To maintain a constant ∆T, TE generators often make use of large heat sinks or radiators, not something you can easily incorporate into a small piece of wearable technology.

Finally…we wear clothes. Whether a jacket when it is cold out, or even a light long-sleeved business shirt, ANY garment that traps warm air between your skin and the upper surface of your smartwatch will reduces the temperature differential to the point where charging would take years, not months.

That creates a further problem. As pointed out above, a TE generator that is small enough to fit on a smartwatch doesn’t produce enough power to charge the battery in a reasonable timeframe. So why not just make the TE patch larger, such as the 10 cm by 10 cm suggested in the article? First, running a wire to the watch would be difficult, but much more importantly there are no large 100 cm2 bits of the human body that are regularly exposed to air temperature! Placing a TE patch on your back does no good if you’re wearing a shirt and narrowing (or eliminating) the ∆T.

Fun headline, nice science, but not a practical solution for charging a smartwatch. And that’s not just for 2014: the laws of physics mean that this technology with its various limitations will NEVER be useful for most wearables. There is such a thing as free energy…just not very much of it.

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