By now we’re pretty used to being the product, as many of us participate in online activities like Facebook or Twitter, and/or photo-sharing sites, where we provide the content. (On some of those sites, what we post actually becomes the property of the host. Read carefully!) Here’s another example of being the product:
Award-winning footstep energy to help power shopping centre
and
Pavegen. Renewable energy from footsteps.
Each tile has a capacity of 6 watts, but in order to use the tile’s full capacity, there needs to be a constant flow of about 50 steps / minute.
The reality is that the tiles are seeing about 5 steps / minute, and on a good day, the kinetic sidewalk will generate about 75 watt-hours of electricity. This is equivalent to powering an old 60-watt incandescent lightbulb for about 1 hour and 15 minutes.
Let’s start with the obvious: one could take the view that this is stealing. Someone is taking work you (the actual physics definition of work, at that) and using it without paying you. It’s also being advertised as being green and self-sustainable. It also needs to be cost-effective. Is it?
Let’s run the numbers. The pad flexes ~5mm when you step on it, so that’s about 5 Joules of work for a mass of 100 kg, so that’s roughly in agreement with the 50 steps/min giving 6 Watts, assuming high efficiency. 75 W-h is 270 kJ of energy. At an electricity rate of $0.12 per kWh, this represents a penny of electricity.
A penny.
The device has to be less than 100% efficient and your body’s conversion of food into the energy being harvested certainly isn’t (I’ll assume around 25%), so at 4.18 kJ per Calorie, the people providing this energy collectively burned about 270 Calories, which came from the food they ate. The cost of that food can vary widely, but it’s going to be on order of a dollar, making this system’s cost efficiency about 1%. (This won’t change at higher power production, either) And here’s where (and why) the claims of “green energy” fall apart. Touting human power as green is dubious, because you don’t know where the food came from, but odds are it’s not all that “green”, and to tout this as a replacement — at 1% efficiency — means that the people providing the energy need to have 1/100 of the carbon footprint of the raw electricity. Transporting the food, preparing it, etc. has to be greener than the energy it replaces by a factor of 100, and there’s no way it is. This is a misdirection, moving the carbon footprint issue out of immediate sight, asking us to pay no attention to the carbon footprint behind the curtain. Human power is not green — the only time it works is if you are harnessing energy that would otherwise be wasted, similar to regenerative braking on electric cars.
Is it cost-effective? I couldn’t find a credible price anywhere, save for a promised target of $50 per tile once production ramps up. Installation is probably the largest cost, along with some infrastructure of wiring, batteries and an inverter. At the target traffic load giving an output of 6 Watts, even if the traffic were present all day long, that’s 1 kWh per week per tile. At $0.12 per kWh saved, that’s just barely $6 a year in electricity savings. The tiles were installed at a tube station at the Olympics and generated just 20 kWh from 12 tiles. The olympics ran 16 days (the story says two weeks); it’s ballpark agreement either way. 20 kWh is $2.40 of electricity.
Unless I’m missing something, there’s no way this is cost-effective. You can pay for it out of your advertising budget, raising awareness of, well, something, since it’s not green, which means it’s just a gimmick.
