One solution for exercise and powering the home?

November 21, 2005 11:11 AM

I'd like to help power the house and exercise, both solved with one solution, and need some suggestions.

For example: if I set up a massive and ultra-heavy flywheel (half-ton-plus, maybe 8-ft-plus diameter) in my garage, that I could start rotating by pedaling (albeit very slowly at first), could I conceivably get it spinning so fast, perhaps by gear changes on a 10-speed or similar, that I could walk away for hours at a time and it still generate elecricity to power a home-capacity battery from which to draw power?

posted by vanoakenfold to technology (21 comments total)

No, you can only get out as much energy as you put in to the system. Flywheels will smooth out the power output, not increase it.
posted by b1tr0t at 11:16 AM on November 21, 2005

from what I remember as a kid at those hands-on science museums, the power output from a single human won't be very significant. I can't imagine you'd power any more than a 60W lightbulb for a directly proportional period to your saddle-time.
posted by kcm at 11:21 AM on November 21, 2005

Related question: any specific recommendations for hooking a stationary bike up to power a television? I've googled it, but there doesn't seem to be any clear answer or how-to guide out there...
posted by diastematic at 11:27 AM on November 21, 2005

Riding a bicycle - hard - can generate around 100 Watts. So unless your house energy consumption is less than 100 Watts, you're unlikely to ever have any energy to store.
posted by jellicle at 11:30 AM on November 21, 2005

As b1tr0t has already pointed out, a flywheel isn't going to help you - the laws of thermodynamics simply aren't in your favor.

What might be possible, however, is to hook a pedal powered generator up to a battery of some kind (I'm not an electrical engineer, so I could be very wrong here). Storing energy over time might let you power a small appliance (a television or computer, say) for a reasonable period.

Ultimately, it's not going to appreciably impact your energy bill. But it might provide some psychological comfort. I imagine it'd be quite satisfying to watch a DVD on a system totally removed from the grid, knowing that the electricity spinning the disc came directly from your body.

In fact, talking about it has me wanting to do just that!
posted by aladfar at 11:30 AM on November 21, 2005

Here's a page of various pedal-powered projects. This page gives some instructions on how to build a general-purpose 12V pedal-powered generator. Unfortunately, most of the images seem to be broken.
posted by chrismear at 11:34 AM on November 21, 2005

Sorry but I just have to ask, where in the world did you get the idea that a human could power a household? Or anything even remotely close to that? Let along walk away for "hours at a time" from? I'm honestly curious, not trolling.
posted by Cosine at 11:39 AM on November 21, 2005

Riding a bicycle - hard - can generate around 100 Watts.

Quick, someone tell Lance. Trackies can generate 1KW for brief periods, trained road cyclists can easily do 500W. Sustained periods - i.e. hours - of 300W are no problem.
posted by fixedgear at 12:05 PM on November 21, 2005

I saw this at the county fair a few months ago:



My wife thinks I need a Geek-a-Cycle when I play World of Warcraft. Actually, a treadmill would be more appropriate. Maybe as I ran on the treadmill my character could run across The Barrens.
posted by jdroth at 12:10 PM on November 21, 2005

The flywheel idea can work, but using it to charge a battery is silly because the flywheel IS your battery. You only get the energy out of it that you put in (minus friction and conversion losses), but if you wanted to walk away for a few hours, that's fine, you just need to put in enough energy before that to last those hours. Eg, (assuming no losses) if you want it to run a 100W application, and continue to power it for 3 hours after you stop cycling, then you'd need to cycle at, say 300W for two hours beforehand, thus generating 600Watt-hours in two hours, to be expended over a period of 5 hours, giving you 120Watts per hour.

But a lead acid battery and mains inverter is going to be a much cheaper and easier way to try out the concept.
posted by -harlequin- at 12:25 PM on November 21, 2005

As previous posters point out, a big not probable on the household flywheel. However i do have a customer who uses a small battery, charge control and exercise bike to power his 12 volt tv. He feels watching tv and doing nothing makes him fat, so if he wants to watch he has to pedal ( he rides 1 hour for 2 hours of tv).
As a aside, small flywheels spun up by small solar panels are starting to replace batterys in a few roadside callboxes and caterpillar has some flywheels for computer room ups systems.
(on preview, geek a cycle! Profit!)
posted by blink_left at 12:26 PM on November 21, 2005

b1tr0t: But does a flywheel (perhaps I am using an inaccurate term) of such weight not continue spinning -- given, gradually slower and slower -- even when I've gotten off the bike, thus be as if I were still on the bike and just pedaling gradually slower, the heavier the wheel the longer the overall time until stopped? Meaning that the momentum would keep generating, gradually less, power to the total? If you put in enough power to tip over a delicately balanced but heavy rock, does it not generate greater amounts of energy/momentum rolling down the hill... or do I have my illustrations mixed?

The idea was to store it on a battery (not be a direct feed) for sometime for such times as power outages when no one has any and I will have some accummulated over a long period for basics like lights or fans. I am not familiar with how electricity is stored, per se, to note.

Cosine: it was a question, asking if, not asserting that it could be done.

I may have a misconception of power/electricity in general, especially in regard to kWh, etc -- that it can be accumulated in small quantities, like pennies to make a dollar, over time to eventually make a large output (like filling a dam with a trickle of water, to suddenly pull the plug much later and have it spew out. Do I instead need the battery charged at high input in order to produce high outputs?
posted by vanoakenfold at 12:27 PM on November 21, 2005

But does a flywheel (perhaps I am using an inaccurate term) of such weight not continue spinning -- given, gradually slower and slower

Yes, but it's not only the friction in the flywheel that slows it down--taking energy from the spinning wheel itself slows it down.

Imagine that you had a perfectly frictionless flywheel. You get it up to a certain speed. Now, as long as you don't draw any energy from it, it will continue spinning forever and ever. But, once you hook it up to a generator, to draw electrical energy from it, whatever you get out of it in electrical energy will be lost in rotational energy--it slows down, and once you've extracted all the energy you initially put into it, the flywheel has stopped completely. Ultimately, you can't get any more energy out of it than you put into it.

If you put in enough power to tip over a delicately balanced but heavy rock, does it not generate greater amounts of energy/momentum rolling down the hill

Yes, but that's because the heavy rock already had energy in it by virtue of being delicately balanced on a hilltop. If you find a rock on its side at the bottom of a valley, that doesn't help you. You could carry it to the top of a hill, but that too takes energy--as much energy (or more, when you consider friction) as you would get out of it when it tumbles to the bottom of a hill.

The proper analogy to having a rock precariously balanced at the top of a hill, in this case, would be if you happened to find a flywheel that was already spinning when you found it. The stationary flywheel is analagous to the rock at the bottom of the hill.
posted by DevilsAdvocate at 1:04 PM on November 21, 2005

A flywheel can be used for energy storage, and in fact there are some very high-tech flywheels used for backup power; it would be inefficient to use it in-line with a battery, which is just another form of energy storage.

Building a rig that stored a useful amount of power would be a challenge. I haven't run through the numbers, but my guess is you'd need a stationary bike with about 30 distinct gear ratios in order to spin up enough momentum to be worth using.
posted by adamrice at 1:29 PM on November 21, 2005

I may have a misconception of power/electricity in general, especially in regard to kWh, etc -- that it can be accumulated in small quantities, like pennies to make a dollar, over time to eventually make a large output

That's basically right, as long as you apply it to energy, and not to power. It's important, in this case to understand the difference. Watts (W) are a unit of power, as are kilowatts (kW); 1kW = 1000W. Energy, measured in watt-hours (Wh) or kilowatt-hours (kWh) is power times time. (Other units are also used to measure power or energy, but we need not worry about them here.)

If you burn a 60-watt light bulb for 1 hour, you've used 60 Wh of energy. If you burn the same 60-watt bulb for 1000 hours, it uses 60000Wh, or 60kWh, of energy.

You're right that you can save up small amounts of energy bit-by-bit, and then use a large amount of energy all at once, like saving up pennies and spending dollars. But the important thing to remember is that it's energy which must balance out in the end, not power.

If you generate 200 watts by cycling, and you cycle for 3 hours, you've generated 600Wh. You now have 600Wh you can use to power something else. If everything in your house uses 700W (<= actual average use for my apartment, based on my November bill), you've generated enough energy to power your house for 6/7 hours, or about 51 minutes.

The important thing to remember is you can save up small amounts of energy, which can add up to big amounts, but it's energy you're saving up, not power. To go back to your saving-up-dollars analogy, power would be analagous to your annual income, but energy would be analagous to how much money you have. If you have a job which pays $120K/year, but you only work 1 month and then quit, you get $10,000. Not $10,000 per month for the rest of your life, but $10,000, period. Once you've spent that $10,000, you're broke, unless you go back to work.
posted by DevilsAdvocate at 1:32 PM on November 21, 2005

Cosine:

The Matrix?

Seriously though, I imagine you could definitely power a home by bicycle generator, but the home would need to be designed (or retro-fitted) for ultra-low power consumption. I know of a solar powered bach off the grid that would be doable, mainly because to the two big power sinks (hot water, oven) are gas rather than electric. Everything else is ultra-efficient, and the place is small. A few hours cycling would run it for a day, assuming you can generate 100-200Watts.
posted by -harlequin- at 3:05 PM on November 21, 2005

This is simply not possible to do. Lance armstrong outputs about 500 watts at the wheel, and most people can't do this. Homes use far more then 1kw. You'd be lucky to power a standard PC.
posted by Paris Hilton at 9:29 PM on November 21, 2005

You need to go to the closest science museum. They usually have an exhibit somewhere that has a stationary bicycle hooked up to a generator and light bulb. You get on there and pedal, and the harder/faster you pedal the brighter the bulb gets. And it's normally a standard 60 Watt incandescent bulb that everyone is familiar with.

Boy let me tell you, unless you are really in shape it is hard to make much light for more than a few minutes. Yeah yeah, lance armstrong can output 300Watts, but that's completely irrelevent because he is so vastly superior to the regular joe in terms of musculature and endurance. Your normal person would be hard pressed to keep a 60W bulb fully illuminated for more than 10 or 20 minutes.

Forget about powering a PC or a (CRT) television, both of which require hundreds of watts. It's just not possible. You might have a chance if you were talking about a small LCD from a portable DVD player or a miniture 12" TV. But a normal TV takes considerable power.
posted by Rhomboid at 11:04 PM on November 21, 2005

I understand that your everyday will slow the flywheel down, but won't the slowing of the wheel by the generator, in addition to the normal friction that would slow it unattached, still add up more, albeit it gradually less and less? I'm not suggesting the flywheel remain spinning at the same speed during the away-time.

Rephrasing the analogy of the rock: There's a huge sphere of rock on the pavement, and for each 5-yard increment that I roll it, I get number of points in a game. It would take a long time to get it rolling, but once I've gotten it rolling fast enough, I can at some point quit pushing it and let it coast, still racking up points, albeit at a gradually slower accumulation until finally no additional points once it stops. If I had an ultra-heavy flywheel with sufficiently oiled bearings or whatnot on the axle, the wheel would still generate current (like points) as it slows although I am not pedaling (pushing the sphere). That was my purpose in having the flywheel, so that when I stopped pedaling that it wasn't immediately ceasing the supply of current, that there was still some residual power being racked up as it gradually slowed that I could walk away from "for hours", not that it would continue at the same speed.

But instead of having to slowly get the rock started each time, I could keep bumping it while it was still rolling and keep up the pace of points accumulation, akin to having the ultra-heavy flywheel spinning very fast and having the bike in a better gear so that I still rack up the same level of output by pedaling slower or periodically to get it back up to speed, so that sustaining the output requires less effort once it's high enough than it did to get it up to that point from being at rest.

DevilsAdvocate: your remarks have been very helpful in my understanding. Perhaps in regard to the money analogy, that having 60Wh available, like having $60k, means I can only burn off 60W for an hour, not but at a rate of 60W per hour continually, whereas having $60k would allow me to buy a $60k car once, but not repeatedly since it would be already spent, do I follow? Does this also mean that if possessing 60Wh, I could burn off 15W for 4 hours, or 4x 15W bulbs for 1 hour?

All the bulbs in my house are those 7-year 14W swirly-shaped bulbs -- if I were to rack up a mere 60W per hour for 1 hour per day (adding 60Wh each day) [or 30W in 2 hours, etc] by exercising with the cycle, could I then theoretically (if I could find such a battery) store up 365 x 60Wh in a year and have 21,900Wh, or 21.9kWh, and be able to burn four of my 14W bulbs (or say, 15W to make it even) around the clock for some 15 days in case of some major power outage?

Rhomboid: My father used to have one of those very devices that powered a bulb by pedaling on the bike, which I was thinking about which gave me the idea. With the flywheel, though, the bulb would stay lit (although grow dimmer, but still have some light supplied) and it would take less effort to keep the bulb lit.

Kinda makes me appreciate the work and generator effort involved with supplying the grid with power that the city does! Whew!
posted by vanoakenfold at 9:49 AM on November 23, 2005

Yes, everything you said about kilowatts and kilowatt hours in your last post is correct.
posted by chrismear at 12:12 PM on November 23, 2005

Rephrasing the analogy of the rock: There's a huge sphere of rock on the pavement, and for each 5-yard increment that I roll it, I get number of points in a game. It would take a long time to get it rolling, but once I've gotten it rolling fast enough, I can at some point quit pushing it and let it coast, still racking up points, albeit at a gradually slower accumulation until finally no additional points once it stops.

yes, but the "free" rolling time you think you are getting is proportional to the time that it took to get rolling in the first place. You aren't getting anything for free, you are just shifting effort and progress around. In your example, you make most of the effort in the first and second third of the time-scale, but you get most of the progress in the second and third thirds of the time scale.

As others have mentioned, flywheels in this case function exactly like batteries. For most applications, batteries are much easier to use and don't involve the danger of keeping a very heavy object spinning quickly in your home.
posted by b1tr0t at 7:18 PM on November 27, 2005