"The cure for boredom is curiosity. There is no cure for curiosity" - Commonly attributed to Ellen Parr. It explains why I've never been bored.

Saturday, June 06, 2015

More on LightSail Energy

Image credit: Cody Pickens, Wired.co.uk
It's difficult finding post topics if I insist that they meet certain criteria: can't take so long to research and write that my family/work/school life is disrupted; must be something about which I have some level of knowledge and the ability to do sufficient research and investigation to post intelligently; must be something that I think my (very few) readers will find to be interesting; and must not simply parrot something someone else said (other than the clear links to other sites with, perhaps, a comment).

But it's clear that LightSail Energy and, in particular, Danielle Fong are of interest to the sort of person who may follow my blog. I posted an article about Ms. Fong and her firm a few months ago and was surprised to receive a Tweet from her. In the interchange that followed, she was kind enough to agree to show me LightSail's facility should I be in her area. As it happened, this week I had a meeting with another firm (to be described in a subsequent post) at UC Berkeley. LightSail is located in Berkeley and Ms. Fong, despite being jet lagged, agreed to show me around.

Because her Company is in a developmental stage and I was simply a visitor, I felt that it would have been inappropriate to photograph the facility or record Ms. Fong's replies to my many questions so readers will need to take my word for what I saw and heard. I mentioned to her that I would be completely respectful of any proprietary information, she said that she was ok with my mentioning pretty much anything (though, on two occasions, she asked that I be circumspect). I want to be, if anything, overly cautious in this regard.

My first impression was that LightSail's facility looked very much like a prototyping facility. They have a large machine shop with CNC machining equipment, a robust metrology lab, assembly areas, and design facilities. There's certainly no hint of "vaporware."

Image Credit: Progressmedia.ca
Since I have a machine shop in my garage and my Company has a large amount of material testing equipment, I was surprised that, while seeing the facility and the components being assembled was fascinating, Ms. Fong's answers to my endless questioning were even more interesting.

As one can find from various biographical sketches (Fong's autobiographical sketch is here), her educational background is that of a physical scientist and LightSail certainly incorporates this background into its business. But, as our conversation continued, I was compelled to ask how much of her working time is spent on: politics; fundraising, mechanical engineering issues, science, and management. Her answer, unsurprisingly, was that "it depends" but I got a strong impression that it's less science and engineering these days than it is management and fundraising.

I asked Fong about her plans with respect to commercializing the technology. It's clear to me that LightSail can't be a manufacturer, at least in their present facility. She said that they do have the ability to produce a limited number of systems, using components from various vendors and others that they produce internally. I'm rather skeptical of that aspect.

In discussing field implementation, Fong mentioned that LightSail has three sites currently in their backlog. One of them has been mentioned in other news articles, she didn't name the others. I'll certainly be looking forward to following the development of these projects.

Fong responded to my questions about some somewhat negative press regarding some layoffs at LightSail by stating that the need for these layoffs was more operational than financial in nature. She believes that LightSail is a stronger and more efficient firm as a result. As a partner in a mid-sized business, I can certainly acknowledge that this is plausible.

I asked Fong about the merger between two firms, General Compression and SustainX, that could be regarded as competitors in the area of compressed air energy storage (CAES). She generally disregarded the viability of SustainX's system (though SustainX also claims to use water during the compression process to attempt to approximate isothermal compression, one key to LightSail's technology). Among other things, Fong mentioned the sheer size of the SustainX system as being non-viable. Reading between the lines of the various articles discussing the proposed merger, it appears as though Fong is correct as it seems that SustainX will not move forward with above-ground storage (current CAES systems store air in underground caverns, typically salt domes, and General Compression follows this model).

On the political and policy front, surprisingly to me, Fong was not enthusiastic about LightSail's participation in California's energy storage mandate. She is of the opinion that the commercial marketplace is the best filter for storage technologies. Fong mentioned to me that someone from the Department of Energy asked her what policy initiatives might jump start a sound energy path for the U.S. Fong answered that immigration reform would top her list, pointing out the very strong representation by immigrants in successful startups. She pooh-poohed tax reform.

Fong mentioned on several occasions that many of LightSail's technical personnel come from a background in auto racing. While I was surprised to hear this, in thinking about it further, it makes sense. Racing is a world of extremely demanding performance of mechanical components under great pressure and where innovation in mechanical design can win the day (a good driver and lots of money help too). Fong introduced me to a couple of these mechanical technicians in front of a huge valve (perhaps 60 centimeters in diameter) that was likely a titanium alloy with a titanium nitride coating.

I asked Fong about the time frame her firm's investors anticipate for a so-called "liquidity event," i.e., for them to see a financial return on their investments. These investors include Vinod Khosla's "Khosla Ventures," Bill Gates (needs no hyperlink), Peter Thiel, Total Energy Ventures, and now, apparently, Chinese VC firm Haiyin Capital, and others. She replied that they all have a relatively long investment time frame and that LightSail does not feel pressured to deliver an immediate return on investment (though she did mention a couple of very specific time frames).

Finally, we spoke in general about the need for storage and the wastefulness of burning fossil fuels (ultimately limited in supply, fracking, sands, shales, etc. notwithstanding) to generate electricity when various renewable sources can ultimately meet our needs. LightSail's technology, along with many other developing storage technologies, can make this possible by overcoming the inherent intermittency of wind and solar to enable these sources to provide "base load power" and reliable peaking power (though some will argue that the need for base load power is a myth). Following the progress of LightSail Energy will be fascinating, combining my interests in energy, thermodynamics, entrepreneurship, and finance.

Note for the video below: LightSail's technology involves removing the heat (poor thermodynamic phraseology, I know) during compression of air with a water mist and reusing the heat either during expansion or for building heating, process heat, etc. Danielle Fong seems pretty "rad" to me, so "Cool It" by She's So Rad seemed a perfect fit.

Sunday, March 22, 2015

Your car goes 3.5 m.p.h.?

John Michael Greer, the Grand Archdruid of the Ancient Order of Druids in America, publishes a weekly post at his site, The Archdruid Report. Greer is a much deeper thinker than I, particularly with respect to the interplay between past, present, and future. He describes his area of thought as the "history of ideas," and his grasp is broad and deep, regardless of whether you agree with him or not. A steady diet of The Archdruid Report would certainly depress me, though Greer does not strike me as depressed. He's certainly "cocksure" though, in the sense of certainty of his conclusions.

Nevertheless, I read his posts from time to time and invariably find them thought provoking. Today, in reading his post entitled "Peak Meaninglessness," I found him citing Ivan Illich from "Energy and Equity" (available as a free pdf download) contending that
"Illich’s discussion focused on automobiles; he pointed out that if you take the distance traveled by the average American auto in a year, and divide that by the total amount of time spent earning the money to pay for the auto, fuel, maintenance, insurance, etc., plus all the other time eaten up by tending to the auto in various ways, the average American car goes about 3.5 miles an hour: about the same pace, that is, that an ordinary human being can walk."
Is it true? If it is, does it have any meaning? Since, for reasons that should be obvious, I'm not interested in using my personal financial details for such a calculation, I'll posit an American household earning the median income of $52,000/year. The household consists of a husband, wife, and two children. The husband works 2,000 hours/year and earns $40,000 and the wife works 1,000 hours/year and earns $12,000. As an aside, this family doesn't live in Southern California. The average hourly earning is thus about $17.30/hour. Of course they'll only bring home, at best, perhaps $14/hour after taxes.

I'll assume two cars traveling a total of 26,000 miles per year with an average of 1.2 people in the vehicle for 31,200 passenger miles per year. The vehicles average 25 m.p.g. and gas costs $3.80/gallon, so they spend about $3,950/year on gas.

One car is a relatively new (say, two years old and purchased for $29,000 on a five year loan at 5% API with 20% down) five passenger sedan. The monthly payment is around $440, or $5,280/year. The other is a minivan on a three year lease with monthly $3,000 at signing and $300/month lease payments, or $3,600/year. They paid $8,800 up front to have the vehicles but, since interest rates on money market investments are close to zero, the opportunity costs are quite low, so I'll use the up front cash divided by the respective term in months. This adds about $2,160 to the annual total. The total to finance the cars is $11,040/year.

They take the vehicles in for scheduled maintenance a total of eight times per year and spend $300 each time (sometimes less, sometimes more depending on the service required by the schedule). The total is $2,400/year.

This family has decent driving records and no teen drivers so the annual insurance premium is about $1,500.

The grand total (leaving out car washes, aftermarket accessories, etc.) of annual expenses is $18,890. Wow, that IS a lot of money! Now, this $18,890 takes 1,350 hours (45% of their working hours) of this family's time to earn. So the final result is in: 26,000 miles/1,350 hours is about 19.25 miles per hour. This is about 5.5 times faster than Illich's estimate. So the answer to the first question, "is it true?", appears to be "no."

The second question is not so easily answered. The entirety of this family's lifestyle revolves around the vehicles. Without them, it's unlikely (though certainly not impossible) that the $52,000 would be earned. And, while a vehicle undoubtedly constrains them financially, it also enables them to do many things that would otherwise be difficult or impossible. A vehicle-free lifestyle is certainly possible (I've lived such a lifestyle at various times and for various reasons), but this family has decided that the tradeoff is worth it. I WILL say, however, that they'd have been much better off with different vehicle choices. Another way of saying this is that I believe I've made assumptions that are generous to Illich's claim as repeated by Greer.

Eating energy

In round numbers, there are 7 billion people eating on planet Earth each day. It's pretty clear that my diet here in Southern California is very different from that of a subsistence farmer in Namibia or a subsistence fisherman (or fisherperson) in Madagascar. It's also true that, as a fully grown adult of age 60, I probably consume more calories than my one year old grandson. But I estimate that my daily intake is on the order of 2000 kilocalories per day of food energy and I don't think that long term survival for an adult is possible on much under 1000 kilocalories per day. Of course, not all humans are adults and estimates of population in each quinennial group are available from the UN. Still, for rough estimates, 7 billion people consuming 1000 kilocalories per day will work.

Thus, in raw terms, this equates to humanity ingesting food energy at the rate of 7*10^12 kilocalories per day or about 10 "quads"/year (for some reason, lots of analysis of world primary energy is done in quads, where a quad is 10^15 or a quadrillion btu(a so-called "short scale" quadrillion)). Of course, the amount of chemical energy in our food as measured by bomb calorimetry exceeds this number since we cannot oxidize 100% of the mass that we ingest, the unburned residue leaves us in ... various ways... ahem. And we certainly don't ingest 100% of the food plants we eat. Further, many of us eat the meat of animals who have ingested the plants, or even the animals who have eaten the animals who...

But, in my simplistic model world, I'm going to estimate that 20% of the mass of a food plant is edible, that we burn 50% of it for energy, that meat represents 20% of the kilocalories consumed by humanity, and that the "hit" on losses due to an animal intermediary is the square of the losses inherent in eating plants directly. Thus, 10*0.8*P+100*0.2*P=C where P is the available kilocalories of "primary burnable energy" ("PBE")and C is kilocalories ingested as metabolizable food energy. So we have that 28 kilocalories of PBE are required for every dietary kilocalorie in this model.

So, we're now talking about 28*10 or 280 quads per year in photosynthetically created PBE. Depending on the plant species involved, the efficiency of using solar energy to convert carbon dioxide and water to biomass is in the range of 3% to 6%. I'll use 5% since it makes the arithmetic easy, and thus 5,600 quads of solar energy per year are needed to feed us. Let's move to SI units: the 5,600 quads are 5.9*10^9 terajoules and 5,600 quads/year are 187 terawatts.

This energy comes, of course, from the sun. There are approximately 14 million km^2 or 14*10^12 m^2 of arable land on our planet so, on average, each arable square meter must be responsible for converting (5.9*10^9 terajoules)/(14*10^12m^2)=422 megajoules/year or an average rate of 13.4 watts of incoming solar energy into ingested food energy.

And, as we see in the graphic at right, this is something like an order of magnitude away from the total incoming energy absorbed by the surface of the Earth. While I've looked at other articles that come to different conclusions about solar energy embodied in our food, I'd be shocked if I were off by an order of magnitude. The lesson? There's not a lot of spare capacity in our system for squandering our biota's ability to feed us.


Sunday, March 01, 2015

Rob's aphorisms

There have been a few seemingly simplistic or tautological things that I've incorporated into various situation analyses.
  1. If something's wrong, something's wrong. This is a conclusion reached jointly by my closest friend Dr. Captain Right Reverend Frank Hanna (Frank is second from right in this photo from 2000 of the Northern Arizona University Department of Geography and Public Planning), may he rest in peace. It came to us during flying together but has wide applicability in business, at home, and elsewhere. For example, I'm flying at 11,000 feet density altitude, power is at 27" manifold pressure and 2,400 r.p.m. I usually get 142 knots indicated airspeed in this situation, today it's 132. "Huh, oh well, la de da." 10 minutes later: "Oh $*)^%*, I forgot to put the flaps up." If something's wrong, something's wrong.
  2. If it did, then it can. Frank was a professor of geology and took students on field trips, sometimes to map geological features. Students would look at, measure, and map some feature, scrunch their brow and say "but can it do that"? Answer: If it did, then it can.
  3.  If they are, then they do. I got my start in the business I'm now in via performing inspection of various facets of building construction. I'd see some very shady things apparently involving various quid pro quos (quids pro quo?) between contractors and construction managers (who ostensibly represent the interests of the owner of the development). A newer inspector would say "Do they really do that"? If they are, then they do.
  4. Everything is dramatically more difficult and complex than you thought it would be, even when you allow for things being dramatically more difficult and complex than you thought they would be. This applies to everything from installing drywall or toilets to welding to applying conservation of momentum to a dynamics problem. Self explanatory.
Now, along with always assuring that you're wearing clean underwear (about which I wrote a song decades ago, motivated by a friend who said she couldn't bring herself to commit suicide because she didn't want to be found in her unkempt apartment), applying these should come close to assuring a smooth sail through life's various meanders.

You're welcome.

Sunday, January 18, 2015

California's storage mandate solved!

This post won't be the first time I've expressed my annoyance with the California Public Utilities Commission over their storage mandate of 1.325 "gigawatts of storage" (scare quotes because this is already mixing apples and oranges - it's analogous to saying "65 miles per hour of distance") by 2020.

The linked post gives my more detailed thinking, but this one is just a quickie to bring home the inadequacy of only giving the rate of delivery of electrical energy in the mandate with no reference to the period for which this energy could be delivered. It's obviously (as you'll see) an extreme, but I can meet the mandate as stated for a bit under $2M!

How? A photographer's flash unit takes electricity and stores it in a capacitor. When it's triggered, it quickly (quickly is the key here) discharges the capacitor through a flash tube. Here, we see the Elinchrom Pro HD 1000 flash unit. It can discharge 1000 W-s (photographers use "watt-seconds" but a watt-second is just another name for a joule) through the tube in 1/1430 second. This rate is 1000 watt*second/(1/1430 second)=1.43MW (megawatts). To get to 1.325 GW (gigawatts) I need 1,325/1.43=927 such units. I can purchase each for $1,049.88 ($964.88 if I order no later than January 31) for a total of $973,238.76. Of course, that doesn't include shipping and some rewiring, so double it.

Now, these units are set to power flash tubes but there's no requirement for that. They simply store energy and discharge it quickly. So, wired in parallel, they could supply the 1.325 gigawatts for about 2/3 of a millisecond. Problem solved!

Of course, the total energy delivered is a bit lacking. It's the 1000 joules*927 Elenchrom flash units=927,000 joules or a bit over a quarter of a kilowatt hour. Still, it meets the mandate and I bet I can have them delivered this year, beating the deadline by some five years.

It's rather shocking to me that our esteemed CPUC has managed to put this mandate in
place with no reference as to capacity. I will agree that rate is important (as I mentioned in the post linked above) but it's foolish to not also include capacity. However, I'm speculating that they aren't even aware of the problem. They are, after all, political appointees and, among the five of them, only Carla Peterman appears to have any sort of energy related experience or education. Mostly, it's attorneys. This is, after all, California!

If you know anyone from Southern California Edison, Pacific Gas and Electric, or San Diego Gas and Electric, please don't tell them. I want to get a good earnest money deposit on my simple solution to their storage mandate woes.

Sunday, January 11, 2015

OK, enough negativity

There's no doubt that I've spent a lot of keystrokes pointing out things that won't work or won't live up to their hype, from solar chargers to harvesting of kinetic energy. And, while I have posted favorably or at least neutrally on a few things, I seem to have had more fun debunking than bunking.

So I'd like to spend a bit of time discussing what appears to me to be a promising technology and one that, if it fulfills its promise and comes to fruition, would be a potential game changer.

It's no secret that I'm of the opinion that, whether for the reason of the finite amount of fossil fuels available to be exploited ("peak oil") at prices that can sustain an economy or for the reason that the extraction and burning of fossil fuels for energy results in the release of massive amounts of CO2 and other greenhouse gases, we need to reduce and ultimately eliminate our reliance on fossil fuels for other than production of products needing their chemistry (almost everything really, from pharmaceuticals to plastics to fertilizers).

Thus, I want to see the increasing penetration of renewable energy for all purposes (including transportation). For a couple of the leading candidates, wind and solar, intermittency is a big issue (though others will argue that it's not such a big deal). This is the case not only because intermittent sources will, on occasion, generate more energy than can be used and not enough at others, but also because the grid relies on very fine adjustments of both energy and frequency.

So, what is required to solve this issue? Without a doubt, the ability to store solar or wind generated electricity at times when supply exceeds demand would be outstanding. It would also enable renewable energy to be supplied at a steady rate.

There are a variety of technologies for grid scale storage of electrical energy:

I want to look at compressed air energy storage and a startup called "LightSail Energy." Their concept is to utilize a variant of compressed air energy storage ("CAES"). The idea is that, typically, compressing air heats the air and, when the air is stored, in cooling the air
loses much of its total energy. LightSail adds a fine mist of water during compression. The water (with its very high specific heat) absorbs the heat of compression and the hot water is stored for process heat or whatever. Then, when the air is utilized, heat (either from the original heating or elsewhere), is added so that the escaping air, which cools upon expansion, has increased energy to be utilized in the piston compressor/expander. So the idea is to use the renewable source to compress air, remove the resultant heat with a water mist, store the hot water for use in a heat exchanger, add heat when the air is discharged through a turbine piston expander. A schematic drawing from a LightSail patent application is to the right.

It should be noted that there are other firms out there using other media to capture and reuse the heat of compression of air in energy storage systems, one such is Energy Storage Power Corporation who uses oil for this purpose. A schematic diagram of their process is at left.

LightSail's Chief Scientist and one of its founders is Danielle Fong. Fong is a prodigy who dropped out of junior high school at age 12 to attend college, from which she graduated at 17. She was accepted into a Ph.D. program at Princeton but dropped out (or took leave) to move into something with more immediate applicability in the energy field than plasma physics.

There are a LOT of videos out there with Fong carrying the flag for LightSail (see here, here, here, here, and here for examples). She's photogenic, is "aww shucks" humble, wears interesting clothes, and what media outlet doesn't love an attractive female child prodigy Ph.D. candidate dropout who's founded a startup that can be stated in the blurbs to have the potential to "change the world" (and, as can be seen in the intro photo at the top left, I'm not immune)? One thing I respect about Fong is that, in articles about her firm, she'll address naysayers directly and without obfuscation in the comments.

LightSail claims, I would assume potentially, a round trip thermal efficiency of 90% and shows a working 200 bar, 250 kW prototype (200 bar is about 20 mPa (megapascals) (and, by the way, can one nest parentheses in textual material?)) at a one-way efficiency of 71%. 

LightSail's Technology page (interestingly, the landing page for the site) describes smaller "Power Units" of 250 kW power and "Storage Units" of 750 kWh capacity, clearly highlighting the distinction between rate of energy delivery (250 kW in the "Power Units") and storage capacity (750 kWh "Storage Units"). When I simply assume an ideal gas and evaluate the work that can be done by isothermally expanding [edit: a cubic meter of compressed air] from 300 bar (about 30 mPa) that LightSail is aiming for and ignoring the work against the atmosphere, about 48 kWh is available and so 30kWh certainly seems realistic if a close approximation of isothermal expansion can be achieved. Long term storage, though, will require lots of thought and engineering with respect to the thermal energy carried off by the mist.

According to Fong, LightSail's storage technology is capable of storing and delivering 30 kWh (kilowatt hours) per cubic meter of storage space. Thus, we can infer that the 750 kWh Storage Unit will occupy 25 cubic meters. For reference, a standard shipping container (to the extent that such a thing exists) has a volume of 38.5 cubic meters. Ok, how much is 750 kWh? At my house, my family of four uses electrical energy at the rate of, on average, about 2 kW (embarrassingly enough) or about 1,460 kWh/month. So these 750 kWh would last me a bit over half of a month at my house. My Company's headquarters laboratory facility uses, on average, just short of 750 kWh/day. That facility has about 15 people working there. I don't have (and LightSail probably does not have, though I can't imagine that they haven't made some estimate) any figures on what such a Storage Unit will cost.

The California Public Utilities Commission (CPUC) has mandated that the State bring 1.325 gigawatts of storage on line by 2020. So, at 30 kWh/m^3 storage capacity, how much volume would be needed to meet this mandate? Infuriatingly, there's no way to know! California uses electricity at an average rate of about 34 gigawatts, so 1.325 gigawatts represents the ability to deliver a bit under 4% of the State's electrical energy but there's no way to know for how long. The state has said 1.325 gigawatts but did not (so far as I can find) mandate capacity so there's no telling how long the systems to be implemented could deliver these 1.325 gigawatts. An hour? A day? I've posted on this frustrating situation before.

Such details aside, LightSail has discussed storing the compressed air in underground caverns for utility scale storage and that's a VERY big step up from a 750 kWh Storage Unit. But with funding from Vinod Khosla's VC firm, Bill Gates, Peter Thiel, and Total Energy, there's a fair amount of confidence in LightSail.

A deeper excursion into the thermodynamics of this system will follow and I believe that that will shed light on possible integration of such a system into our generation/transmission/distribution system. My initial feeling is that it's likely to be most useful for such things as frequency regulation rather than solving the intermittency problem thwarting confidence in the wide scale implementation of solar and wind generated energy.

I'll acknowledge that the only connection between this video and LightSail is the word "Sail." But I just wanted to use any excuse to embed this song. It's hard to believe that it's the Beach Boys.

Friday, January 09, 2015

Media Notices America's Grievous Shortage of Laws | Coyote Blog

Media Notices America's Grievous Shortage of Laws | Coyote Blog:

'via Blog this'

Yup. My only addition is that California is doing its best to remedy this travesty of having literally minutes go by without some governmental agency, ANY governmental agency, making a business decision for us.

Thursday, January 01, 2015

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.