Eco-Gym
Can we somehow transform and take advantage of all that energy wasted inside gyms across the country?
Have you ever had an idea that seemed absolutely brilliant at first, only to have it fall apart bit by bit as you start analyzing all that it entails? First, you share it with a couple of people around you and their initial reaction is great - until you talk to that one person who has a bit of insight and can throw a stick or two into your wheel. Anyway, let us start with some background to all of this, and maybe it will start to make sense.
I grew up in a cold and dark place known to most people as Sweden. As a child, I rode my bike everywhere - the bus stop to go to school, soccer practice, the golf course, work, random frozen lakes to play hockey, you name it. As Sweden gets dark very early in the day during the colder parts of the year, it is rather important that you have a bike light to avoid collisions with badgers, moose, hedgehogs, and the occasional penguin. Back then, battery lights were for the rich kids, so I had a dynamo-powered one. To those of you who have never come into contact with bike dynamo generators, it's a little apparatus that transfers some of the kinetic energy of your rotating tire(s) into AC electricity by using magnets and other fun stuff. So essentially, moving bike + a little bit of extra resistance = electricity, that in this case is used for lighting so you won't run over a beaver or small child in the dark.
A young Erik riding on 'The Exterminator' (not equipped with a dynamo generator).
Living in Boulder County, it is difficult to not notice the general obsession of fitness here. People bike, rock climb, participate in recreational sports, and spend countless hours in gyms. This is where all the useless information above comes into the picture. If you are a gym rat interested in getting cardiovascular exercise, you are probably going to spend significant time on an exercise bike, an elliptical rider, or a similar piece of machinery. By increasing time spent as well as general resistance, you increase the level of exercise you are getting. However, it is likely that all your spent energy is 100% wasted - apart from the improvement of your fitness level, that is. Let us now imagine that your exercise equipment's resistance was provided by some kind of generator instead of magnets or a tightened belt. That instead of using electricity, you were generating it.
So that is my idea in a nut shell. You have a gym where you pay a normal membership fee, but you also have an account that keeps track of your accumulated electricity. This account can be used to provide a rebate on your personal electricity bill, or maybe you want to donate the electricity you have generated to help cover a charity's electric costs. Either way, you can probably see how this could be an additional factor of motivation of even making it to the gym in the first place. If you have solar panels on your house that generate excess electricity, power companies will now buy this from you (at a heavily discounted rate, of course), so why would they not want to buy electricity created by your sweat? In a place like Boulder where the Prius reigns supreme among cars, you can imagine the potential to get the people to go to YOUR gym if you have a good reason to stick the magic word 'Eco' in there somewhere.
Alright, so now that the idea is presented I am certain that a lot of you have either stopped reading, or are itching to start commenting on all the inherent flaws of it. So before those of you remaining jump at it, I will present the ones already helpfully supplied by friends and co-workers, or that popped into my own feeble mind.
- The amount of power generated is likely to be quite low. In all reality, what kind of loss are we even looking at in terms of efficiency?
- Gym equipment goes through pretty heavy wear and tear and is constantly vibrating and moving. A generator attached to it would have to be incredibly robust and be able to handle a lot of abuse.
- Modifying existing gym equipment will not be a trivial task. How do you even completely change out the current system that provides resistance and replace it with generators that can gradually change resistance and power output?
- How do you collect the generated electricity and store it? What kind of systems can handle the low effect output likely to come from these generators and transform it into something useful?
- How do you set up a system that keeps track of gym members' generated electricity and that lets them follow their progress?
- Cost. 'Nuff said.
So why did I even write about this if the idea is inherently flawed and limited in the first place? Well, I read everything that comes through our homepage posts; crazy projects where people have proven the skeptics wrong and solved problems thought not practical to bother with. Is this really all that different? It seems that despite a fairly long list of tough-to-solve problems, including ones this article may have completely overlooked, the idea still has some potential for success. Can I challenge any of you to make an attempt at this? After all, I am a computer scientist, and although curious about DIY Electronics, I do not possess the electrical engineering skills required to take this anywhere. Maybe this idea is nothing new, has anybody been to a place where it is already implemented?
Why does the end product have to be electricity? Keep the energy mechanical, and you can do a lot of stuff, much of it similar to devices that were replaced by labor-saving devices.
Clothes washers, bellows, grain processing, textile manufacturing, water pumping. If the process ever had livestock, water, wind or people providing mechanical energy, why not put people back into the process?
I can see it now: People donning yokes to pull a plow through a field as exercise. The next day, they show up to pump water into a municipal water tank. Sounds like work to me. Maybe we just need to add work back into our lives instead of driving to a gym to ride in place.
It's funny how rarely we think about this. We remove work from our lives every way we can, then try to artificially put it right back. Come to think about this, we do that with food, too. Weird. We have problems :/
Hi everyone! I've been following the sparkfun blog for awhile now, but this is my first time commenting. :)
I read this thinking it was an awesome idea, on top of me looking for a new project.. plus I have an idle CNC mill hanging around.. so I decided to run the numbers.
It doesn't look good. Assuming you generate about 0.5hp or around 375 watts, (a very high estimate) and the gym runs for 10 hours and has 10 people in it at all times. This is ignoring efficiency, and assumes the gym will have 10 people in it at all times. This is just a rough guess, but I think it might not be too far off, as not many people will be there 9-5, but weekends may have lots of people, etc.
From my friend WolframAlpha:
375Watts * 10 * 10Hours =
37.5 kW h (kilowatt hours)
= energy released by combustion of one gallon of gasoline
Before all the losses generating electricity, storing it, inverting it...
Now, this is just a guess, a prototype would obviously give you more realistic numbers.
I have to echo this. They installed a power generating gym at my school.
http://media.barometer.orst.edu/media/storage/paper854/news/2009/02/19/News/Dixon.Generating.Energy.From.Exercise-3637874.shtml
It did not live up to expectation. They have a computer with a monitor hooked up to show the instantaneous power output. With a full room of people cranking, away 120 watts are typical output. That doesn't even cover the computer. Let alone lighting, A/C, etc.
It wouldn't really be that hard from a technical standpoint. The excercise machine isn't a harsh environment at all compared to automotive, mining, or aerospace applications. What you'd need is to have the pedals hooked up to a high sailency ratio PM machine (to help voltage regulation over a wide speed range) or an induction machine in the 500W to 1kW range (standard human averages about 250W while biking but can push a lot more for a hill). This would hook into an back-to-back inverter with a decent vector control for the bike side and a standard unity power factor grid side. The grid controller would just maintain DC-link voltage while the bike side would take a power or torque command from the exercise controls.
I couldn't have said it better...
PLS let me add that the technique is sometimes called "regenerative braking" widely used in industrial applications, elevators and in hybrid cars breaking.
The beauty of it is that it feeds the energy back into the grid actually making your meter run backwards :-)
So it really doesn't matter how little or how much does it push back... eventually the cost will break even.
Hej Erik! Det var så tråkigt för mig att läsa din artikel - trodde jag var den enda svensken som kommit på denna idé ;) Hur som helst så är det en fenomenal idé och tänkt vilken potential detta kan ha i Sverige med både hälsotrenden och miljömedvetenheten som boomat på senaste. Det är ju verkligen ett winning concept, ända problemet ligger väl fortfarande i att teknologin inte är tillräckligt utvecklad än när det kommer till att generera el effektivt från gym-redskap. Hur som helst så funderar jag på att skriva min Bachelor Thesis på marknads-potentialen det kan ha i Sverige. Hör gärna av dig till mig så att jag kan fråga ut dig lite mer om dina tankar och idéer! ywe@student.cbs.dk
So, apparently they're researching it at Berkeley: http://hpg.berkeley.edu/ where they estimate their gym produces about 1 kWh (which is 0.7% of their required energy)
Also, they have gyms set up in Hong Kong: http://www.inhabitat.com/2007/03/08/human-powered-gyms-in-hong-kong/ (Watch out, the articles uses watts per hour...)
I posted about this topic a while ago in my blog.
http://k0lee.com/2008/01/electricity-from-human-power/
The executive summary is thus:
A person can generate about $.06 of electricity by exercising at a respectable rate (75W) for 8 hours a day. However, you'd have to double your caloric intake to maintain your weight if you did that much exercise. Do you think you could double your food intake for an extra 6 cents a day? And how would you feel about making an effective wage of less than a penny an hour?
I have already seen something similar on the net (where???).
It was an hotel which had exercise bikes connected to electrical generators.
The idea was that you bike for a while and get a free meal (or discount) at the hotel restaurant.
I don't remember if their were directly using the electricity generated or selling it to their power company.
I looked into this a few years ago. I envisaged a system akin to regenerative braking in cars to apply a varying load on the user. (another post above suggests this). Like so many projects, it never came to fruition, partly because all the solar/electric car engineers I know are far to busy to help me design a measly 200W exercise bike!
As many, many people have pointed out, the best we can expect is 200W per person, so maybe the ipod/phone charger is a better idea (and certainly a great way to raise awareness of the project). Low-current hardware like this can be hacked up by just about anyone (dynamo, recifier diodes, a 7805 regulator and a couple of smoothing caps) and doesn't require grid connection so is a lot cheaper and and simpler to implement.
The electrical power output is only half the story though. How efficient is a human body? How many watts of heat does a body generate during exercise? In summer in Australia, air-conditioning to cool all those sweaty bodies forms a significant proportion of a building's electricity consumption.
What about a system that helps with the climate control, but remains disconnected from the main grid? each machine could dump power into an array of peltier units, and change the polarity based on whether heating or cooling was required. It probably doesn't have the same marketing appeal as an ipod or phone charger though.
I would recommend looking into using a CVT (continuously variable transmission) to transfer power from the equipment to the generator/alternator. This would give you variable resistance for the person exercising without them having to run the machine faster to generate more energy.
You can also vary the physical resistance by changing the load current. A short circuited generator makes a very effective brake, iirc. Coming up with a way to vary the load current without creating additional losses could be very tricky, though.
I don't know how much energy you could get out of it, but the more efficient your generator is, all the better, imho.
I have an exercise bike attached to a small DC motor which is attached to a enphase inverter which is attached to my solar array. The enphase inverters are "micro inverters", you use one per solar panel. So I put an extra one in to the bike.
Hers the problem from the follow the money perspective. Cost of the inverter and motor: $250. I don't ride it much in the summer (I commute to work by bike instead)... but, if you ride it for 30 minutes (pushing pretty hard) you will generate about 40-50Watt Hours of electricity. That means I'm generating about 80-100 watts. (powering just ONE light bulb)
Here is the kicker, 1 Kwh (1000 watt hours) will cost you from the power company about .10 (yes 10 cents). So recover of the $250 I spent building it? just about never. But if was a fun project.
Years ago in prehistoric times (1986)I had a customer
who had a startup (in Boulder). He had a design
for an exercise bike in which the main resistance element was a generator to which the bike pedals were
attached. The drag was varied by kicking power resistors
in to increase drag and kicking them out to reduce drag.
One of the biggest problems we had with the design was
that the heat from the power resistors had to be transfered to the surrounding air and shielded from
the riders legs. They got hot enough to burn skin.
We used to joke that we should just hook the bike up to a battery and run the computers or the lights.
I think your idea is great. I think it would be possible to power an electric water heater. Thereby you can at least make your own warm water for the shower afterwards. And a minimum of problems with regulation as well as storage:)
Snygg m?ssa f?r ?vrigt, den fick en gammal bergabo att hajja till... (Swedish comment regarding the hat)
Berg? Liten v?rld, detta. Tydligen finns det folk hemmavid som f?ljer SparkFun :)
(Further commenting on his hat comment)
In Germany there are a couple of Gyms offering the possibilty to charge your MP3 player or iPhone with the power you generate.
they call it: "Green Gym - Watts take away"
My cousin linked me your post since we discussed this exact same idea a year or so ago and I've been crazy about it since.
The building has to be custom built/rebuilt. the whole place wired for DC, lots of big heavy rows of copper in the floor,with custom terminals on the floor for the generator hookup. Battery bank in the back and solar panels on the roof.
As for the equipment, don't use anything new, been done. The answer is old heavy wheeled equipment.
http://www.instructables.com/id/Turn-an-exercise-bike-into-an-energy-bike/
Don't feel like giving the rest of the answers away but definitely hit me up if you want to know more. Even have the whole tracking users's wattage etc.. :)
I read somewhere that the average person can do about 1/10 hp, or about 75 watts. An Athlete is more like 1/4 hp.
Are we sure we want to go down this road? Does anyone remember the movie Soylent Green?
--tr
I'm keen to help build one, how do I help/get involved.
Let's suppose that a child could generate sufficient energy to power a small television (perhaps 100-150 watts). Now when the child asks for permission to watch television, the parents can say "Fine. Watch as much as you like. But you have to supply the power!".
Results:
1. The child will be more selective about what he or she chooses to watch.
2. Watching television is now good for the child's physical health.
3. The same generating device can be used to power a radio or a lamp during a power outage.
4. A hybrid bike/desk could be designed that permits people to power their computer monitors while also using the keyboard and mouse in an ergonomic manner. I'm not sure this would be possible, but if it worked, it would help office workers to get some much-needed exercise while working.
I'm a huge fan of this idea. Maybe that could be a solution to the rampant growth of childhood obesity here in the U.S. ;)
1.yes but when you combine 200 people all on different machines, 200 watts turns into 40000 watts, 40 kilowatts, thats a hell of alot.
2.exmple-automotive alternator
3.simple, have a system like your bike dynamo, little wheel touching main wheel.
4.who needs to store it? have the dynamo(s) feed a switched mode psu of 90+ efficiency with an adjustable current limiter which feeds a building universal inverter in sync with the grid. you would litteraly be hooked up to the grid directly through the current limiter(so you dont end up having a 10 giga watt load on your machiine). to adjust difficulty you adjust the current limiter, more current, more effort
5.scan your gym id linked a credit card in the machine before a workout. it would be liked to the power co. of your choice, and would keep track of your monthly output and at the end of the month, they would subtract 90-95% of it from your powerbill, and 5-10% from the fitness center's bill
6.talk to the power co about your idea, set up a deal through the power co and a fitness center, they could communicate through the internet and use your existing membership account
We actually did this once at my university my professor had a bike hooked up to a windmill and we measured the power that was generated. It was pretty cool to see.
Reminds me of http://www.worldchanging.com/archives/000446.html
You can also see some non-rotational generation referenced here: http://trackernews.net/search/?s=generator
I like the wisdom of start small, get some experience, make little mistakes, reassess your goals, take the next step. And document as you go- who knows who will stand on your shoulders one day.
good idea, I've actually been thinking about this one for a while, though mainly the generator part. here are some answers to your above questions...
1. depends on the generator and person, although the average output is likely to be around 40 watts.
2. there are already many tough gens out there, from car alternators(with permanent magnets)to water turbines.
3. experiment and gain some welding skills (soldering on a large scale) when you find something that works, great!
4. inverter (and battery) or the 12V lights that will be put in this gym, (and battery)
5.RFID cards that the gym members keep in their pockets. when they get on the machine it reads their card then updates and displays their status.
6. mini gym at sparkfun, if you guys hosted free day, you can probably spend the $300 per machine. (if you gain your welding skills aka make your own machine from scratch)
The money part could be sticky too. Instead of trying to get rebates from the power company, why not use the electricity generated to power the building that houses the gym, and then give people discounts on their gym membership based on their power generation? People could use their rfid gym cards to log into the devices.
One thing you might want to consider though is how this is probably an unbalanced system unless you can find a way to generate power from free weights. I wouldn't like paying full price for a gym membership when other people get discounts or other compensation for using machines I don't use.
There is some debate as to whether cardio exercise is the best (healthiest) kind of exercise to do.
A friend of mine worked for these guys:
http://www.futurespark.com.au/
... who built a bicycle trailer that feeds power into the grid. Fun, if only to gain an appreciation for just how much work is required to generate a miserly amount of electricity. I can probably glean some technical details if you're interested (shoot me an email).
some expensive exercise machine already have magnets in them for Resistance
GO FOR IT.
The university I work for does this, on a somewhat limited basis. Several of their treadmills have been modified. The rear roller has a PTO which is hooked to a generator. The user then generates the overhead lighting for their workout by, well, working out!
In Boulder the building is most likely consuming energy for heating a large part of the year. During that time you're probably better off leaving the energy from people working out as heat; it isn't waste.
The problem is that human beings really aren't capable of producing much power.
Even a relatively fit person would be very hard-pushed to produce more than 250-300W over a 1 hour period which, at most electricity rates, is just a few cents.
Over the period of a year one might just about generate enough electricity to pay for a beer :-)
If you could harness the energy when I pump iron, you could power the universe.
Seriously.
Hi Erik!
I had the exactly the same idea as you got. I told my family about it and they said it was a great idea.
The funny thing is that im from Sweden aswell.
Start developing together? I live in Gothenburg.
My electronic skills are pretty descent.
Regards Niklas Cooke
A friend of mine and I rigged an old excerbike to a heavy flywheel that spun an alternator. We used to use it to charge car batteries, though two people going for an hour each day would charge a dead battery in about two weeks. LOL
I remember reading that it is already done in china. I tried to find the article but no luck. It was a asian country and they where harnessing power from the user to power the lighting and other stuff.
here are other links
http://blog.makezine.com/archive/2008/08/green_exercise.html?CMP=OTC-0D6B48984890
http://www.humandynamo.net/
http://www.rerev.com/default.html
I don't know if this would produce enough electricity to be worth the power company's time. Lance Armstrong can produce about 500 watts. If you lined up 10 Lance Armstrongs and got them to pedal for eight hours, that would be 40 kWh. That's less than five bucks worth of power in an average market.
How about a better model: let people plug in their phones and laptops and iPods and charge them up? That way you don't have to deal with bureaucracy of the power company and people can get an immediate, personal benefit. Plus they can go the rest of the day with that smug, holier than thou feeling, knowing their phone is "green" powered.
I've always envisioned a gym where you've got a row of TVs that light up when more people are putting their part in (probably would have to be artificial fading in), or some operational thing in the gym realizes its full potential dependent on people working out. I don't think that generating electricity is a bad idea, but the fact that so little is to be made, monetary reimbursement may be a little to much as the reward wouldn't be so substantial. Maybe LED lights glow a little brighter, or make it into a game where people get competitive and stats/graphical queues are shown on TVs. Maybe you get a cut in your health insurance, since they can actually track you being proactive for your health.
As far as tracking people's progress, I'd opt for RFID for ease of use and low maintenance. I think you've got a good idea and I hope you get something going here. I'd like to go to a green gym someday.
Kristoff
Penguins in Sweden, really?
A typical jog on a treadmill at 5 degree slope for an hour might require about 600 kJ of physics work by a person. One should ask how much of that could we reasonable extract. Even if we could extract it all, the power output by the person would be about 170 Watts. Multiply this by a dozen people, and we get close to 2 kW. Not much for anything major, but it could power some computers or TV monitors.
There are plenty of penguins in Sweden - within the safe boundaries of parks and zoos, though :)
But I agree, it's not really feasible at this point to extract any significant amounts of power even from a lot of people. One idea would be to have a gym leaderboard of most electricity generated too, since you keep track of it anyway.
There is already a company in Seattle producing electricity generating cardio fitness equipment: http://www.resourcefitness.net/
There is also a gym in Portland, OR that runs off this equipment: http://albertapdx.thegreenmicrogym.com/. It must be somewhat successful because they are opening another gym in the city.
Someone is applying for a Pepsi Refresh grant with a similar idea:
http://www.refresheverything.com/fuelyourfitnessatrensselaer
It's a good idea. It would even be nice to see that be a part of the design of fitness equipment as opposed to retroactively added.
Interesting idea...., ya know if you hooked up a smaller altenator (say for a high end gokart) it would cause less resistance then a full sized car altenator, then u hook up a gear ratio on it [for example, u have a verry small gear 2-3" on the altenator, and then attached by a belt around the bike wheel thingy, for every one rotation on the bike wheel, you get many times that on the smaller gear, in-effect spinning it much faster then one is peddling.]
I would love to see if you try this lil R&D project, let us know if you end up doing it!
P.S. it seems like alot of the people commenting on this post solved most of the problems :S
OK like I've sorta been there and done that. I'm a ham operator and our radio club worked the ARRL field day contest. This is a radio activity where you set up a station out in the middle of nowhere and attempt to simulate emergency communications with the outside world. This involves portable antennas, generators, etc... In the 70's the ARRL added bounus points for power from renewable sources (wind, solar, waterpower, etc). So I helped to rig up a bicycle powered generator. We pulled the tire off of a bicycle and put a large v belt around the rim of the wheel to drive an auto generator. It was rigged up with a couple of chairs, angle iron, hose clamps, ubolts and duck tape. We all took turns peddeling out power to drive a 2 meter rig and made maybe a dozen contacts before we all plotzed.
I think the value of this idea is not in the energy that is produced but in the amount of energy that might be saved. We waste a lot of energy. People do not seem to have any sense of how much energy they waste. Lights are left on for hours when rooms are empty. Fans run for hours when no one is home. TVs play to empty rooms. Either people do not mind spending their money on wasted energy or they do not mind wasting someone else's money. The harvesting of exercise equipment energy will never generate more than a small fraction of one percent of the energy we use each day. However, if we realize just how much "work" is involved in generating the energy we use, we would be much more aware of our energy use and would likely stop wasting so much.
I completely agree Jim. How much work does it take to make the light that I've left on work? I have a little bit of an idea. Also want to watch TV then pedal for it.
To all those suggesting hooking generators to exercise bikes, I'd like to offer a better idea:
Simply offer incentives to people for riding their regular bicycles to destinations that they would have normally driven their cars to.
Energy saved riding a bike that would have been consumed by an automobile is essentially the same as generating new energy.
Nothing says you can't do both, though. Plenty of companies offer incentives to employees who take the bike (or walk) instead of driving. It's a great idea, but I still don't feel it negates the opportunities for saving/generating energy in other areas of life - even if you're just pedalling to charge your cell phone, like a few people have suggested now.
Hi Erik,
I've just graduated from Cal Poly in California, and some of my fellow students and our professors are working on a project just like this. As far as I know, they are attempting to take our university gym off the grid. You can see the current progress at http://courseware.ee.calpoly.edu/~dbraun/srprojOLD.htm#EHFEM, and hopefully get some inspiration to continue your work or join forces with them!
http://bits.blogs.nytimes.com/2008/12/30/ringing-in-2009-with-people-power/
Didn't they have exercise bikes to charge batteries for new years, for the ball drop?
Yes, this is a perfectly practical means of using energy - just not in a 24hr line-delivered civilization. At Regis Mountain School (AZ) - way off grid, an exercise bike is used to grind grain.
I seriously had an idea like this at one time! Awesome!
I Think the issue would be getting the power to the grid, You would have to have some nice converters that can take a VERY irregular voltage/current and turn it into clean AC.
Is there a break out board for that?
hahaha I dont know if there is, but hay people invent things every day right? y not invent it!
Some time back, I saw an article where a guy hooked up his TV to a generator on an exercise bike. If his kids wanted to watch TV, they had to pedal on the exercise bike. That guy was a genius.
It is a fine idea and has potential. I've worked on a similar project using playground equipment to generate power. More on that here (http://allthingsradical.com/).
During my research there were no power producing gyms in existence.
However, there are individuals who use modified bicycles as generators (http://bit.ly/1Q49K).
In addition, there are places in Africa that implement playground equipment to pump water (http://bit.ly/dhlAVX).
Since it also falls into the too complicated for ME to attempt it, I'll throw out my variation of this that a friend of mine and I talked about a couple of years ago. What if instead of attempting to produce electricity which is really difficult, expensive and very inefficient to store, you used the bike electrical power to generate hydrogen which could then be used in a hydrogen fuel cell. Just a thought...
Just like solar cells or wind, use the gym equipment to charge a battery bank, and then an inverter can supply power back to the grid.
Have the operator "login" to the equipment so power generation can be recorded.
Every Lifecycle or spinner type stationary bike I used has a generator in it. That's what powers the electronics in the bike. Look around the bike, there's no power cord coming out of it and you need to start pedaling before the displays will even light up. Since most of these bikes use magnetic resistance for the load, also powered from the generator (or it's the generator itself), there may not be much of a surplus of generated electricity to tap into.
step 1) Permanent magnet dc generator with capacitor buffered output
step 2) switch mode power regulation (sometimes approaches 95% efficiency or greater)
step 3) optional temporary storage for when output is really low (think one or two exercisees), something like lead acid batteries
step 4) utility certified inverter
steps 3 and 4 can serve a whole gym, but each exercise unit would require it's own generator and power regulation. Things like rowing machines could spin a flywheel for added efficiency and regularity.
Wired Mag had pretty much this idea in one of their artifact from the future pages a few years back.
Reminds me of this:
http://news.cnet.com/8301-11128_3-10406025-54.html
Certainly possible. It would be a little tricky to deal with the fact that the voltages you generate will vary widely with what kind of equipment you are using and how rapidly you are using it. This is not a deal breaker, it would just likely cut your efficiency unless you get really creative.
Great idea, Erik!
My $0.02:
1 - This is why you test first. Get an old exercise bike or something, wire it up and see how much you generate.
2 - Your car's alternator is a pretty robust, constantly vibrating generator, isn't it? There's got to be a source for these somewhere... Also, if it's cheap enough (inexpensive DC motors, for example), it might not hurt if you have to replace them more often.
3 - You might not need to change the whole system - just hook the generator up to the main drive wheel and let it spin. Keep all the existing electronics and resistance mechanisms in place - just let the generator harvest whatever energy you generate. That way all you need to develop is a "black box" of some sort that can bolt onto an existing piece of equipment and generate electricity.
4 - Hasn't this already been done? Wind power, solar, generators... These have already been used to return power to the grid. Once you know how much electricity you produce on the bike, it shouldn't be hard to find a system that can feed it back to the grid.
5 - That's easy. Give your members a card with a magnetic strip on it, and install a card reader on each machine. Swipe your card, workout, then move on. Same system casinos use to track players. Bonus: Buy the parts from SparkFun!
6 - Build a prototype first, then worry about full implementation cost. That's what R&D is for.
Like the ESPN commercial of Lance powering the building! I always thought that there should be a way to use the (even small) amounts of energy generate by exercise equipment to at least power some of the workout buildings! At the thousands of TVs we watch...