Supplying energy with footsteps yet again

It just won't go away. On my Galaxy Note 3 (which I mostly really like) I have Flipboard. Generally speaking, I like that as well. When I'm biding my time someplace like a Doctor's office, waiting to meet someone, etc., I'll often flip through it. This evening, lo and behold, I found an article from CNN entitled "Harvesting energy from human footsteps?"(which I've since looked up on the web to refer to here, though it's labelled "CNN" but the site is "Click on Detroit).

It features Pavegen, a Company I've looked at before. The concept is the installation of tiles that, when stepped on, deflect and thereby produce electricity via the piezoelectric effect. Does such an effect exist and can electricity be produced in this fashion? Yes and yes. Can the electricity generated thereby be useful in some circumstances? Again, yes. Is this an efficient and effective way to "Transform Tomorrow?" That would be a "no."

Amazingly, Pavegen's CEO, Laurence Kemball-Cook, actually says (and the article duly repeats) that the Pavegen tiles can "produce up to 7 watts of energy with each step." Of course, watts are not units of energy, they are a rate of doing work or converting energy. Kimball-Cook repeats that and, in this TEDx talk, even says "eventually, our vision is that this technology power our cities." This is, frankly, ridiculous and I intend to ridicule it.

The claim is that a single step generates up to "7 watts of energy" (despite the unit confusion, we can infer what they are really claiming if we make some estimates). I went to Vernier's site where  student laboratory measurement, data gathering, and analysis equipment is sold and interesting experiments are described. One of the "Innovative Uses" experiments described was called "Walking Biomechanics Using a Force Plate." There, sufficient data is provided (I'll leave out the calculation details unless requested to add them) to determine that the claim of "up to 7 watts" is not ridiculous, though I'd estimate that a bit under 5 watts is more likely a good average. But we don't care about watts, we care about energy so, based on the data at the Vernier site, the approximately 700 Nt. applied through 0.005 meters over a period of about 0.74 seconds results in kinetic energy delivered to the tile of about 3.5 joules/footfall. This is somewhat confirmed by Pavegen in their graph (at left) of their own office where we can infer that a bit under 250,000 steps produced a bit under 500 watt hours. This translates to about 7.2 joules/footfall. Close enough.

Since there are 3.6 million joules per kilowatt hour, we can say that it would take right at a million steps to generate a kilowatt hour of energy, and that assumes that the piezoelectric system is 100% efficient at converting kinetic energy to electrical energy. Actual data on this efficiency is sparse at best, particularly for those, such as I, who are unwilling to spend $30 to retrieve an article from behind a paywall that may or may not have the information that I want in order to complete a blog post. Nevertheless, I was able to find this document, where it appears that a conversion efficiency of maybe 2.4% can be achieved (yes, you read that correctly).

Thus, our 3.5 joules/footfall is reduced to 0.088 joules/footfall, requiring something on the order of 41,000,000 footfalls to generate a kilowatt hour. I have to say that it seems likely that I'm missing something here. Possibly, 2.4% conversion efficiency is much lower than what these tiles achieve, though I can't find any data that indicates that significantly higher efficiencies have been demonstrated in a laboratory environment. And, in the Wikipedia article on Pavegen, it's stated that "the exact technology is a secret, but PaveGen officials have said it involves the piezoelectric effect and induction by copper coils and magnets."

So it's possible that Pavegen is using something more efficient, so I'll use an upper bound of 3.5 joules/footfall as described above. Here, in an article from Urban Times with a healthy degree of skepticism and yet also open-mindedness, figures from the City of Westminster regarding Oxford Street are cited. Looking there, it's stated that there's an estimated 4.3 million people per week using the street. Urban Times discusses 10 tiles on Oxford Street. So 4.3 million/7 is 614,000 people per day. If they each hit all 10 tiles, the upper bound would be 614,000*10*3.5=21,500,000 joules or just shy of 6 kilowatt hours.

Quoting from the Urban Times article, Pavegen states that

"the pressure of a single footstep creates 4 to 8 watts. It was also calculated that assuming 8 watts is created per step, “during peak hours, one tile produces 12kWh ideally [and] during the off hours, it produces 5kWh ideally” and that “floor tiles, during peak hours will be stepped on 926-1889 times per hour… and 0-719 times during off hours… which works out to about 56 kWh per weekday”.

I can't find that quote from Pavegen, but if it's an accurate quotation, it would imply about 57 million footfalls per weekday. Hmm, I doubt it.

Then, there's the fact that any energy harvested by the Pavegen system is parasitic in nature, in that it's generated by a human converting food energy to kinetic energy. Whatever energy is harvested and wasted by a Pavegen tile is energy above and beyond what the people walking on the tile would otherwise have to exert. It doesn't come from nowhere and it isn't free.

There is likely a place for human generated electricity. As Kemball-Cook points out, there's no need to have the users of gyms merely generate heat on their treadmills, stair climbers, elliptical exercisers, etc. Such folk are going there for the express purpose of burning food energy without a destination or other goal. Such energy might just as well be harvested and, in many cases, it is.

These distractions that allow politicians to have green photo ops and people to get a warm fuzzy feeling about saving the Earth from some oversold "green energy" scheme do much more harm than good. The drastic changes in how we need to live in order to achieve equilibrium with what our planet and our sun can sustainably provide are only put off when people buy into such schemes.

Let Steve Earle describe Pavegen.

Ack! ANOTHER "capture the energy of walking/driving" system

I really don't want to turn this blog into a debunking site but some things just must be said. Here I described a completely impractical system for "capturing energy from pedestrians." And now we find an article from Science Daily about Mexican entrepreneur Héctor Ricardo Macías Hernández, pictured at left, who's developed yet another system for capturing energy from passing traffic - vehicular or pedestrian.

It apparently consists of a traffic wearing surface that sits five centimeters above street or sidewalk level. Passing traffic squeezes a bellows, compressing air into a tank (the linked article says "...where it is compressed..." but I don't imagine that that's accurate) from which it is expanded into a turbine to generate electricity.

Think about it. Your engine (either that of your vehicle or of your metabolism) is squeezing air into a tank. This work will reduce your gas mileage (or use your food energy) as your vehicle or your feet do the work of compression. There is no free lunch here. I'm surprised to see it in Science Daily which, although it is sometimes prone to exaggeration, usually doesn't publish nonsense.

There are no figures given, either in the Science Daily article or the articles linked from there, so I don't know what kind of traffic would be claimed to generate what kind of power. But I do know that, whatever the amount generated, it would be more efficient to burn natural gas in a turbine. Both vehicular internal combustion engines and human metabolisms are inefficient and compressing air is a lossy process. I'd love to see figures for this but it's yet another candidate for my prospective Greenwashing Hall of Fame. Yes, I know that the term "greenwashing" is typically applied to deceptive ad campaigns but I think it's equally applicable to deceptive products.

Update: In thinking about it, I suppose that one could concoct a scenario wherein a developing country with few energy resources would rather have the "rich" who own cars spend some of their energy (i.e., gasoline or diesel) purchase on providing energy for township than purchase natural gas, oil, or coal and then charge the poor residents for the electricity or pay for the fuel with taxes. But even there, better a gasoline tax with the proceeds used to pay for more efficient energy generation.

Greenwashing Hall of Fame nominee

The Greenwashing Hall of Fame doesn't exist (to my knowledge) but it ought to. I've posted on a potential candidate or two myself. I may set it up and solicit nominees through my blog. The one I have in mind today was brought to my attention by BBC World Service on Sirius XM in my car. It was, seemingly, an advertisement for Pavegen Systems thinly disguised as a news feature (though I'm sure BBC doesn't think so).

Pavegen manufactures tiles that use piezoelectricity (I think it's safe to assume, though the site doesn't say) to "harvest" electricity from motion. They have a variety of posts for tiles installed in offices, at rail stations, schools and colleges, etc.

Specifications are given nowhere on the site, so I've had to use information from secondary sources (though I should be able to take a look at real-time generation at an office during regular hours by going here).

Looking at a couple of videos it looks like a tile deflects maybe a quarter of an inch (call it 0.006 meters) under the weight of a normal size adult male, say 76 kilograms and a weight of 750 newtons (round numbers here, they're just rough estimates). So the work done by 750 newtons through 0.006 meters is 4.5 joules. I'll assume that the tiles are 67% efficient at turning this work into electricity, making 3 joules available per step.

The source linked in the previous paragraph says that, at the West Ham Station installation, each tile got about 5 steps per minute and that a tile will produce about 75 watt hours of electricity "on a good day."  The information is confusing because it says that 50 steps per minute yields 6 watts which, in 24 hours, would yield 144 watt hours. But never mind. OK, back to the calculations. 3 joules times 5 steps per minute/60 second per minute is 0.25 watts. So, each hour would deliver 0.25 watt hours. Doing this for 24 hours would yield 6 watt hours. 50 steps per minute would yield 60 watt hours, close to the 75 cited above.

Anyway, the 6 watt hours would cost me 0.006 kilowatt hours * $0.16/kilowatt hour, or $0.00096 (96 thousandths of a penny). Though the price of a tile isn't given, it's pretty certain to take a very long time to pay off financially at a penny every 10 days or so.

I take something like 5000 steps per day, and would generate 5000*3=15,000 joules, or 3.6 kilocalories (that is, food type calories) were every step on a Pavegen tile. I step about 2.5 feet per step, so 5000 steps is 12,500 feet or about 2.4 miles. Here, we read that a 180 pound man burns about 100 kilocalories per mile as he walks, so I'd burn about 240 kilocalories (in other words, 240 kilocalories of food would be eaten) to walk this distance. I'm sure it's not exact, but here we read that, in order to feed me with 240 kilocalories, 2,400 kilocalories of fossil fuel is used.

So, putting it together, if the Pavegen system harvested the energy from every step of my walking, 2,400 kilocalories would go to produce 3.6 kilocalories of electrical energy. Of course, this isn't fair to Pavegen since I'd also, presumably, have gotten where I was going. On the other hand, I paid for the food to do that walking.

In the end, generating electricity through human effort is just not very efficient in comparison with other methods. If we're using muscle power for exercise and the effort would otherwise simply heat the environment, there may be a place for it (though I doubt it would ever pay off financially) but as a green power source, sorry, it's no sale.