Human energy harvesting--a very silly idea

"Power to the people" was a popular rallying cry among anti-establishment activists in the 1960s.

"Power from the people" appears to be the latter-day equivalent.

The theory behind the slogan is that humans move around a lot, and the only result of all this motion is that the humans end up in a different place.

According to some, this isn't good enough.

The MIT News reports that two MIT graduate students in architecture have proposed to extract energy from the motion of humans through public spaces such as train stations:

A responsive sub-flooring system made up of blocks that depress slightly under the force of human steps would be installed beneath the station's main lobby. The slippage of the blocks against one another as people walked would generate power through the principle of the dynamo, a device that converts the energy of motion into that of an electric current.

But if there's enough motion to provide harvestable energy, there's enough motion for the humans to notice. Ever walked along a pedestrian suspension bridge that bounced under your feet? It takes more energy to walk on such a surface than it does on a rigid surface.

Where does that energy come from? From you, of course. It's like carrying a parasite that takes a little bit of your energy. In fact, this approach is also called parasitic power generation. By keeping the parasite fed, you get a little more tired and you eat a little more food. In effect, you become a highly inefficient motor that runs on food.

Food calories are inefficient to produce. A wheat field is a giant biochemical solar panel that turns a small part of the sun's energy into chemical compounds that you can eat.

And then those compounds have to be kept cool and transported large distances, then cooked and eaten. By comparison, traditional electric power generation is hugely more efficient.

So when you see celebrity Ed Begley Jr. using a stationary bicycle to turn a generator to power his toaster, remember that this is a crime against the environment--not environmentalism.

The same goes for parasitic energy generation--it creates exceptionally expensive energy. Nevertheless, there are places where this approach is entirely appropriate.

Self-winding watches, for example, have been around a long time--first as purely mechanical devices, and currently as what are referred to as "automatic quartz" watches, which generate the small amounts of electricity necessary to power a quartz movement. That makes sense because little power is required, and parasitic power generation is arguably more convenient than changing batteries.

According to a story in EE Times Europe, Zarlink Semiconductor and a coalition of several other companies are developing what they call self-energizing implantable medical micro systems (SIMMs) such as pacemakers, where replacing batteries would otherwise require surgery. This sounds like a good idea to me, especially if the generated energy can be stored in supercapacitors that have much longer service lives than batteries.

One potential application that keeps coming up in the context of energy harvesting is military electronics, especially among front-line soldiers. An article on Physorg.com describes how a soldier walking with a 100-pound backpack could generate 45.6 milliwatts of power through shoulder straps made with polyvinylidene fluoride, a piezoelectric material.

But...seriously, 45.6 milliwatts?

A soldier walking with a 100-pound ruck is probably using over 800 watts of power just humping the thing around. Extracting another 20th of a watt from the soldier's effort probably isn't going to make his life any worse, but what's the point?

If you're going to add any technological complexity to a soldier's load, make sure it generates enough power to make a difference. I don't think 45.6 milliwatts is going to make much difference. A single CR123 lithium battery will produce almost that much power for four days in a row, and if it fails, the soldier's backpack won't fall off. Carrying around a flexible solar panel might work even better, though of course there still has to be a battery in the system.

Or give the guy a mil-spec version of the "yo-yo" pull-string generator developed for the One Laptop Per Child (OLPC) project. A minute with such a device will generate more energy than seven hours of marching with that ridiculous backpack idea.

I have no problem with the idea of "appropriate technology," but as an engineer, I really must insist that technology should be technically appropriate, too.