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.
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