Scavenging For “Free” Energy Isn’t Necessarily Cheap

5 watt-seconds. 

That’s the miniscule amount of energy released mid-stride upon the ground when the average person is walking.  At least, so it was reported in a business plan I recently read from a college student group to pursue a new technology concept to harvest the otherwise-wasted energy created by foot traffic in dense pedestrian areas such as airports.

It seems like a reasonable number, so let’s work with it.  5 watt-seconds, when continuously applied, translates to 300 watt-minutes, or 18,000 watt-hours, or 18 kilowatt-hours.  At 10 cents per kilowatt-hour, a representative value for electricity from a typical utility, it would be worth $1.80 if someone was to constantly tread upon a gadget that could perfectly capture the kinetic effort being exerted and supply it into the grid.

Over the space of a year, $1.80 per hour could theoretically work up to nearly $16,000.  This sounds like an impressive sum, and could lead someone like a group of hopeful students to think about developing a technology, about a square foot in size, which could underlie tiles, carpeting or a thin-veneer of pavement in busy pedestrian corridors.

Well, there are several simplifying assumptions in the above analysis that are easy to gloss over, but which will likely prove economically fatal to this seemingly-appealing concept.  Where to begin?

First, 100% efficiency capture is impossible.  Indeed, I’d be surprised if 20% capture was realistic, but for the sake of argument, let’s modify our analysis to use 20% as an assumption.  That knocks $16,000 per year down to $3,200 per year.

Second, what square foot of real estate is going to experience constant pressure from landing feet?  The answer: none.  I don’t know of any study of pedestrian traffic patterns, but I’d be surprised if any one spot on a floor receives more than one footfall every five seconds even in the busiest of times.  Using that probably optimistic assumption cuts the economics by another factor of 5, down to $640 per year.

Starting to sound a little sketchy already, but it gets worse:  what fraction of the time is any busy area really busy?  It’s certainly not 24 hours a day, 7 days a week.  It’s easy to overlook how many non-busy hours there are in a year:  fully one-third of hours are between 11 pm and 7 am, and 28% of hours are on Saturdays and Sundays.  In contrast, peak activity times only represent maybe 10% of the hours of the year.

All of this means that the prior assumption of one footfall every 5 seconds, reflective of peak periods, is inappropriate for an annual average.  Although I have absolutely no data to back it up, I’d guess that one footfall every minute is a reasonable annual average (8760 hours in a year) for the very busiest spots on the planet.  That worsens the last estimate of value by another factor of 12, down to $53.33 per year.

Over an entire space upon which this technology would be deployed, this degree of utilization — reflective of the busiest spots — is still far too high.  I would guess that the average traffic location in a venue like an airport would be no more than a quarter of the highest traffic location, which reduces the average value estimate down to little more than $13 per year per square foot.

I still think this is way too high, but I’ll stop here and assume it’s about correct:  5 watt-seconds per footfall is maybe worth about $10-15 per year per square foot of floorspace in a busy venue.

Now, let’s look at the cost side of the equation.

How much will a tiny but robust generating device implanted in a floor covering cost to manufacture in volume?  Let’s assume that the gizmo is made in China, where high-volume electronic equipment can be produced for very low cost.  Based on Chinese wares of similar size/complexity I’ve seen at prior exhibitions of the Consumer Electronics Show every January in Las Vegas, I would estimate that a Chinese manufacturer would be willing to sell such a device for $10 a unit.

But, that’s shipped out the factory door.  Then, there’s transportation to the U.S., which is at least another couple of dollars.  And this doesn’t include the costs of wiring.  And, maybe even more importantly, some energy storage device and power conversion/quality equipment to “smooth out” all of the lumpy jolts and surges of energy production into something the grid can absorb.

All told, the cost of goods, delivered to the site, will be at least $25 per square foot.

Alas, the product cost is just the tip of the iceberg.  More importantly, there’s installation.  In a retrofit situation, the existing floor covering will need to be ripped out, the generation device will need to be installed — and crucially, wired in series and connected somehow to the building’s electrical system — and the floor made usable again.

I’m not a flooring expert, but the labor involved in such an endeavor has to be considerable.  If it’s going to be in a civic context (which it pretty much must be, to get that degree of pedestrian traffic), then the labor is likely to be unionized, at probably $30/hour — maybe more for the electrical work.

With all these considerations, it’s hard to figure how the total cost per square foot of this energy harvesting device, installed, will be less than $100.  I would guess it would be far more than that.  But, even at this cost, it’s about a 10-year payback on deployment cost — assuming no maintenance costs (another optimistic assumption) — relative to the value calculated above.  My gut tells me that the actual payback in a real-world situation would be much longer, maybe 20 or even 30 years — which is probably longer than the expected life of the generator unit itself.

This is not the economic foundation of a successful product concept.

Going through this long example indicates the fundamental commercial challenges associated with energy harvesting — a class of technological concepts to capture energy from incipient sources.  I’m seeing an increasing number of energy harvesting ideas coming across my desk, usually from younger people who wonder why we can’t make use of something that already is occurring and being wasted to gather some “free energy”.  Well, “free energy” isn’t exactly free:  while many of these energy harvesting ideas may be technologically possible, most are uneconomic, some ridiculously so.

Just because something can be done doesn’t mean it should be done.

To the extent that energy harvesting ideas are being pursued — whether it’s capturing the motion of a hiking person for battery recharging (as is being pursued by Tremont Electric with their nPowerPEG device), or cultivating a virus to biologically produce electricity (potentially for micro-devices such as implanted pacemakers) such as recently discovered by researchers at Lawrence Berkeley National Laboratory — the common element underlying potential success is that the value associated with energy being harvested is very high. 

Energy harvesting is unlikely to make economic sense in displacing any electricity generated by powerplants, which is available on the grid for on the order of 10 cents/kwh.  On the other hand, the cost of electricity from batteries typically exceeds $1.00/kwh, so energy harvesting technologies might have a niche where the scavenged energy can replace battery-supplied energy.

To those students who had the idea of emplanting generation devices in the floor to capture the energy of walkers:  I appreciate your creativity, but I would turn your attention elsewhere.  It’s OK; one usually has to walk through several truly wacky ideas before landing on a really good one.

3 replies
  1. RankineCycle
    RankineCycle says:

    Second law of thermodynamics.

    The same reason there aren't water turbines in mountain brooks and drainage ditches. It's like walking around cities and digging through dumpsters the find all of the materials necessary to build a house: theoretically possible but extremely labor-intensive and therefore not worth it.

    All "free energy" systems involve scavenging low-grade, spread-out, dilute forces and upgrading them to produce useful work. Unfortunately, by the time the collection equipment is installed and the dilute joules pass through the Rube Goldberg conversion or upgrade device and increase their entropy as a condition of moving and converting, the theoretical treasure has been lost to the laws of physics (reality).

    There's the saying "Water water everywhere but not a drop to drink". Energy works the same way.

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