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.
It’s always worth doing the maths on these proposals. Recently here in England, Bristol City Council published a satellite-generated map of city roof tops that were suitable for the installation of solar panels — along with a total of how much electricity could be generated yearly if all the appropriate sites were covered. While the total seemed impressive, when you worked out how much energy it was per-person per-day it amounted to enough to boil the water to make every Bristolian one daily cup of tea! So not really worth the all the effort, at least in a place at 51 degrees latitude and a cloudy climate.
How long must a tile real world output to repay the energy cost of its creation, placement, and maintentance? Gleise 710 is a K7V orange dwarf star whose Oort cloud will smash the Earth in 1.36 million years, arxiv:1003.2160. We do not have forever.
If it will never repay its own energy cost, it’s crap.
One thing anthropologists do in studying various cultures is measure how much energy is expended to collect or hunt food vs. the energy gained from eating that food. (If it takes you 1,000 Calories to chase down and spear a 500 Calorie meal, you should have been lazy.) With the advent of gasoline-powered agriculture and ranching, the food energy >> gathering energy rule from anthropology has been flipped backwards to a stunning degree. You can find varied estimates of the ratio, but we’re certainly talking an order of magnitude difference for average foods. For particularly inefficient offenders, it can go over two orders of magnitude difference. Maybe a hint that we’re doing something a little odd?
Nice bit of Maths – always nice to see high brow content 🙂 – but I don’t see why you compare the energy of the food eaten to the electricity produced? surely the people are not going there just to generate electricity? Or eating more because the new floor takes more energy to walk on? The work is being done whether you harness it or not?
Would the comparison between costs to produce and fit a standard tile not be the more relevant one?
In a hypothetical future where flooring like this now costs the same in currency and energy to produce and fit as standard flooring, so we can have no other type flooring anywhere, then surely it would be a no brainer Irrelevant of the energy costs of our food?
Have I missed something?
Steve-
I make the comparison because it’s more work being done, and that extra energy will need to be replaced by the people doing the physical work, even if it is minuscule on a per-person basis. Collectively they will need to eat a dollar-menu cheeseburger (or equivalent) as a result, so you need to compare the carbon impact of that food to the penny’s worth of electricity you’ve generated if you want to know if it’s green, or self-sustaining.
But how about this idea : generating electrcity from gym members. They go to gyms to willingly expend energy. And quite a bit as well (much more than footfals). I wonder why gyms don’t wire up their machines to harvest the energy?
beautox-
Larger power output is available, as you point out, since an individual output exceeding 100W is feasible on a device, so maybe you can possibly exceed 1 kWh per day, which is $0.12. That’s upwards of $50 a year. So yes, scaling up a factor of 10 puts you in a regime where it’s possibly cost-effective.
And gyms have done this, which I’ve discussed. http://blogs.scienceforums.net/swansont/archives/2858 But I’ll point out again there is a difference between harnessing wasted energy and using this as a primary source. The latter is not green.