Recently a friend sent a piece on an energy harvesting device that coverts some of your body's waste heat into electricity. The description was sparse, but it mentioned the average person produces about one hundred watts of waste heat and went on to say that is enough to power a laptop.
umm.. Both true, but there are some problems ...
The average adult metabolizes about 100 watts on average to stay alive. That's about the same as eating 2000 calories of food a day. The average adult has about 1.7 square meters of skin area.1 This means we need to get rid of about 0.006 watts per square centimeter of skin area. Six milliwatts of power per square centimeter. The photo of the device showed a ring like affair with some fins - maybe two square centimeters of radiator area. Let's round that to about ten milliwatts for the device - hardly enough to deal with a laptop, which are on the order of 20 watts or even a smartphone which are a few watts. That assumes all of the waste heat can be harvested with one hundred percent efficiency. Here things begin to go down hill quickly.
A thermoelectric converter on human skin has a theoretical efficiency of about 5% at room temperature.2 This reduces the six milliwatts per square centimeter down to 0.3 mw/cm2. Even if you covered your entire body, you'd only harvest five watts and you'd be shivering. And that's unrealistically optimistic as these devices rarely have more than single digit efficiencies on top of their thermal efficiency in this temperature range. Even optimistically we're down to about 0.03 mw/cm2 or 30 microwatts/cm2. Something like an armband might have 100 cm2 giving us 3 milliwatts.
This isn't reason to abandon hope. There are hardware devices with sub-milliwatt power requirements. Small energy harvesting devices that harvest mechanical or solar energy also exist and make it practical to put tiny sensors with tiny transmitters nearly anywhere as long as your receiver is nearby. The micro part of the Internet of Things (which has its own issues). It turns out this is a rich area for college engineering projects.
But what about larger power demands where it's inconvenient to change batteries? Spacecraft that travel past the orbit of Mars generally rely on radioisotope thermoelectric generators (RTGs) because the Sun is too dim to make solar power practical. An RTG puts a thermoelectric converter on something hot - a radioactive material that decays rapidly. Ideally it's an alpha emitter so you don't need heavy shielding. There are a number of candidates, but Plutonium-238 (Pu-238) is the most common American choice with a half life of about 88 years. Not only do these generators produce electricity, the extra heat is used to warm the spacecraft's warm. The current generation of Martian rover is RTG powered.
There are a number of RTG stories.3 I don't want to go on forever, but the nuclear powered pacemaker and artificial heart are among the more interesting.
A problem with early pacemakers was longevity. Batteries had to be replaced every eighteen months or so necessitating expensive and risky surgery. In the late 60s and early 70s two types of tiny radioisotope generators were manufactured for use by several pacemaker manufacturers. They had enough shielding to be very safe and would last longer than even very young patients. They were also very cost effective and a few thousand patients received them. Then, in the mid 70s, the practice suddenly halted. The worry was devices might not be removed before cremation from patients who had died. This would destroy the shielding and turn an area in and around the crematorium into a radioactive hot zone requiring evacuation and a very expensive cleanup.
The radioactive heart implant never made it into a patient. To make enough power to run the heart, the RTG generated about 50 watts of waste heat adding to a normal person's 100 watts. It wouldn't be bad if it could be spread over a skin sized area, but that would be impractical. It would be like holding an incandescent light bulb. One of those interesting ideas that you wonder about afterwards...
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1 The approximation medical researchers usually use is from Mosteller: (height[cm] * weight[kg]/3600)0.5 a
2 The theoretical maximum efficiency of a heat engine is 1 - Tc/Th where Te is the temperature of the environment - the air around the device in this case - and Th is the temperature of the radiator - the skin temperature. We have to use an absolute temperate scale so Th = 310°K and Tc ~ 295°K. So about 4.8% efficient.. let's call it 5, although it is probably half that with poor coupling to the skin and air.
3 One could go on for a long time .. the CIA's RTG in the Himalayas is particularly interesting. And then there's the Apollo 13 RTG that required last minute maneuvers as the limping spacecraft approached the Earth. The very hot RTG is somewhere on the floor of the Pacific.
hacking creativity and collaboration
A recent Bell Labs reunion celebrating the 50th anniversary of Unix gave me a chance to visit with old friends I hadn't seen in a long time. Some of the people had moved into industry (Google was a popular landing place) and others found university posts. I found myself thinking about the differences between the evaluation process of major research universities and the old Bell Laboratories.
In a run that lasted about five decades Bell Laboratories - call it Bell Labs Classic - was arguably the most applied research organization in the world. There were certainly warts and missed opportunities and it effectively came to an end about five years after the mid 80s breakup of AT&T, but it was an astonishing place while it lasted. I'm continually been struck by differences between that institution and almost everything else I've run into.
In universities hiring and promotion (tenure and beyond) is strongly connected to ten or so recommendation letters from leaders of the field in different universities. The authors are almost always experts in the same subspecialty as the junior researcher. Bell Labs was different. There was an internal ranking that extended throughout the research organization. A department head would rank their people and the results were merged with the rankings from the other department heads in the same center. These rankings would then go through the same process for the next two levels up until a ranking of the entire institution existed. An expert in a field might receive fantastic marks inside their department, but do poorly further up in the process as others might be unaware of them. Interdisciplinary work would stand out and the highest ranked people tended to have one solid speciality along with work in centers far removed from their own.
I entered the Labs as an experimental particle physicist. There were a number of physics groups, but no particle physics. I was initially assigned to an applied physics group and found myself working with a number of other groups as I scrambled to find my way. In the process I made connections inside and outside the center and found some encouragement to trade help with others. All researchers were given the title of MTS - member of the technical staff. It resulted in a certain equality that made approaching others easy.
Most researchers weren't interested in management, but there was one small step up. If you were in the top ten percent or so for a number of years you became a distinguished member of the technical staff and given more freedom. In my case I was initially given a day a week and resources to do anything I wanted. Those projects were all collaborations far from my home organization.
Guaranteed funding for pure and applied research went away about five years after AT&T's breakup and folks had to attach their work to business units. It’s what killed Bell Labs, but realistically it was the only logical path for the company at the time. Bell Labs Classic could only exist as part of a regulated monopoly, but that’s another story.
Some universities are playing around with this kind of ranking to stimulate interdisciplinary work. I don’t think it works in industrial applied research.. at least not at scale, but it might be worth considering. Of course there are different mechanisms in other organizations - places like Pixar - but the difference between Bell Labs Classic and research universities is quite possibly the result of collaboration encouraged by a simple ranking process.
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