# Sweat electricityMay 9, 2006 12:56 PM   Subscribe

Why does the equipment at the gym that uses rotational resistance not have generator feeding electrical power back into the power grid?

The workout summary of both the exer-cycle and the elitiptical trainer say I'm burning approx. 200 watts during usage, couldn't that be utilized? It seems like a considerable amount of energy wasted, if you look at all the machines that are almost constantly in use.
posted by Keith Talent to Technology (48 answers total) 1 user marked this as a favorite

Hell Yeah!! I wonder why they don't power the lights at the gym with all that juice coming off of the elipticals / treadmills at 6 PM. They could even wire up each machine to "your" light. So as you run, the light gets brighter.

I am tempted to give up my day job to pursue building a system that does this. It's probably trivial to design the feeding of the batteries and stuff. But it's probably a bit harder to design something to bolt onto the equipment, which means you'd need to design your own, which means it'll never happen.
posted by zpousman at 1:01 PM on May 9, 2006

Every so often you read about this - I read about a laundromat which had exercise bikes kitted out this way. I believe it defrayed something like 1% of the total energy usage, or some similar marginal amount.
posted by ikkyu2 at 1:03 PM on May 9, 2006

Yes, the amount of power generated is quite insignificant.
posted by keswick at 1:06 PM on May 9, 2006

Response by poster: It say's the output is 200 watts. Is this somehow different than a 60 watt bulb? Could I not power 3 60 watt bulbs for instance? If so, the amout of power being wasted by the fifty lifecycles is very significant.
posted by Keith Talent at 1:10 PM on May 9, 2006

I'm burning approx. 200 watts

What? Wattage is a measure of power, energy (in this case, joules) per unit time (seconds). Can you really sustain 200 watts? It's possible, if you're fit. Lance Armstrong can do approximately 500 for an hour. Regardless, it's not a lot of power. How many people are in a gym, cycling for what period of time? How many kilowatts do they run, on average.

Add to that the fact that the energy generation is erractic (how many people are cycling at noon? how many a 7pm? 4am?) and the amount of energy you could feasibly store and get back after all the inherent ineffeciencies in doing so is pretty negligible.
posted by phrontist at 1:16 PM on May 9, 2006

It say's the output is 200 watts. Is this somehow different than a 60 watt bulb? Could I not power 3 60 watt bulbs for instance? If so, the amout of power being wasted by the fifty lifecycles is very significant.

Yes, that is correct. You could power three sixty watt lightbulbs, assuming you could sustain 200 watts. Problem is, you probably cycle for less than a few hours, and energy store isn't very efficient. And the energy isn't wasted per se, it's heating the room!
posted by phrontist at 1:18 PM on May 9, 2006

How many kilowatts do they run, on average.

By which I mean, how many kilowatts does the gym consume.
posted by phrontist at 1:19 PM on May 9, 2006

The figure I've seen is that humans can generate around 125 watts for a long period of time, bicycling.

So let's say there are eight bikes fitted out like this. They generate one thousand watts continuously (always in use) for the twelve hours/day that your gym is open, or twelve kilowatt-hours of juice. That has a street value of about one dollar. So eight bikes being pedaled continuously cuts your electricity bill by a dollar each day, say \$363 dollars/year if you're closed Christmas and New Year's.

How many years will such a bike last, being used continuously? Five? Probably less. So over five years the eight bikes earn you \$1800. Now, how much does it cost to fit out your gym with the switching power supply, batteries, inverters, and all the other kit needed to amalgamate pedal power and the local electric utility? A lot more than \$1800, maybe \$10K minimum. Not even counting the extra cost of the bikes, which would be considerable. And you have to replace them every five years.

So that's why: not economically viable.
posted by jellicle at 1:25 PM on May 9, 2006 [1 favorite]

Response by poster: Yes, that is correct. You could power three sixty watt lightbulbs, assuming you could sustain 200 watts. Problem is, you probably cycle for less than a few hours, and energy store isn't very efficient. And the energy isn't wasted per se, it's heating the room!

Even more reason then, as they're running AC full tilt to keep the room from becopming too hot.

And I might not be on the cycle that long, but the cycle is virtuually in constant use from 6 AM to midnight, seven days a week.
posted by Keith Talent at 1:30 PM on May 9, 2006

Well, it is a little more complicated than just hooking a bike up to a generator. For instance, you'd need a load distribution system. Otherwise, every time someone dropped out of the grid or a new light switched on you'd feel a change in resistance of your bike. I bet people would get pretty annoyed.

Other than that, I think jellicle shows convincingly that the math doesn't work.
posted by sbutler at 1:38 PM on May 9, 2006

The argument that the supply is erratic is not a strong one. People put PV cells on their roofs even though the sun isn't always shining. The electricity can be pumped into the grid and sold to the electric company either way.

I suspect the reason this isn't done is because the initial investment wouldn't be recouped. For example: suppose you had a gym with 50 generator bikes that were being used for a total of 12 hours a day and producing 200 W each, continuously. That's 120 kWh per day for the gym as a whole, which probably works out to a retail electrical cost of about \$130/month. The wholesale rate at which you'd sell to the power utility would (optimistically) be half that. So, you'd be able to make back \$65/mo or \$780/yr.

If we estimate the marginal cost of adding generating capacity to exercise equipment is \$50/unit (I think this number is low, but what the heck), it would cost an extra \$2500 for the gym, and would take about 4 years to recoup. That's actually not too bad, but for this to work, every rig needs to be in service for at least 4 years and have \$0 maintenance costs associated with the generator. But for the same amount of money you could put more insulation in the attic and probably save more energy that way.
posted by adamrice at 1:38 PM on May 9, 2006

On postview, what jellicle said.
posted by adamrice at 1:39 PM on May 9, 2006

Even more reason then, as they're running AC full tilt to keep the room from becopming too hot.

Without data, I'm willing to bet that most of the heat comes from the people, not the bike. So you'd still have to run the AC.
posted by sbutler at 1:41 PM on May 9, 2006

I read about a laundromat which had exercise bikes kitted out this way.

That was probably from this askme, in which the feasibility of the enterprise is also discussed.
posted by Espy Gillespie at 1:44 PM on May 9, 2006

oops, forgot to preview.
posted by Espy Gillespie at 1:45 PM on May 9, 2006

People put PV cells on their roofs even though the sun isn't always shining.

Notice how effective that is.
posted by phrontist at 1:48 PM on May 9, 2006

A cool and fun gimic would be for people to come into the gym with their phones, ipods, palm devices all out of power and plug them into the exercise equipment they use and not stop pedaling until they are fully charged.
posted by any major dude at 2:07 PM on May 9, 2006

I'd rather see the TV's be human-power-dependent. That way they'd be off most of the time (except for those rare occasions when a horde of bionic marathoners *really* wanted to see Antiques Roadshow).
posted by garfy3 at 2:56 PM on May 9, 2006

Can you really sustain 200 watts? It's possible, if you're fit. Lance Armstrong can do approximately 500 for an hour.

It appears I can sustain almost or around 300 watts (on my way to work, cycling up a long hill at a rate that is sustainable long term for me, I made some observations of how long it takes me to lift my weight through a set height) and while I'm fit, I'm not super fit or anything, I don't work out or race, I just cycle-commute to work at a comfortable speed.

I've heard many wattage figures attributed to Lance Armstrong, they all seem to differ widely, Here for example, his sustained output is given at 600W, and is only a few percent above his competitors, so I don't think 200W sustained is unrealistic - it's a mere third of what an athlete does.
posted by -harlequin- at 3:05 PM on May 9, 2006

The treadmills are already powered, so the generator is already installed. Running doesn't have the efficiency of cycling, so it would no good for running lights, but that doesn't mean you can't have fun, such as wiring the motor of one treadmill directly to the motor of another, then each treadmill will power the other, making the fitter person work harder, but at the same time making them push the slower person to run faster, until they reach an equilibrium of misery :-)
posted by -harlequin- at 3:12 PM on May 9, 2006

garfy3:

I'd rather see the TV's be human-power-dependent.

There is a TV powered by a handcrank in the Vancouver science museam. Unfortunately, since it's only a little one-arm handcrank generator, it's a similarly tiny TV, but that could be a feature in itself:

[insert Drill Sergeant]
"You maggots want to watch the big screen TV?! You're too weak to watch the big screen TV! You're going to watch the tiny black and white TV until you're man enough to handle the big screen TV!"
posted by -harlequin- at 3:18 PM on May 9, 2006

Remember that 200 watts in doesn't translate to 200 watts out. Even if there was a generator which pumped power back into the wall, most of that 200 watts is going to be disappated in the room as heat, because of those pesky Laws of Thermodynamics.

In fact, the efficiency is going to be terrible, because you've got to convert to AC and you've got to be in phase with the wall. That means multiple transformation stages, each one of which loses some energy as heat. If you got 25% of that 200 watts back into the wall, I'd be really surprised.

Which means that the capital cost is increased (and so is the maintenance cost) and the monetary payback is decreased, making the economic case even less appealing.
posted by Steven C. Den Beste at 3:21 PM on May 9, 2006

Steven:

The altenators in cars are up to 90% effecient.
posted by -harlequin- at 3:32 PM on May 9, 2006

I think it's already been explained, but the reason this isn't done is because it is much cheaper and easier for the gym to just buy power at ten cents per kWh from the power company. Power generation is one of those things that scales very well, and when you are generating hundreds of megawatts you can get much better scales of efficiency than you can with a dozen bikes putting out a spotty 200W for 20 or 30 minutes at a time.

In order for this to be workable, you'd need a large bank of inverters, as well as a large bank of storage cells (lead acid batteries.) You can't just hook the output directly to the line because then the lights would dim and brighten constantly, and most AC appliances expect a steady voltage, which is not what you get from the output of a human that is exercising. The initial investment and upkeep on this equipment is not cheap. You face the same problem when you equip a home with solar panels -- the payback time on these systems is typically 10 to 20 years at least, and that is with *regular* sunlight for N hours a day in sunny locales. A person riding an exercise machine is a lot less constant and reliable, and I don't think it's in the gym's best interest to take on this long term investment for what probably amounts to a thousand dollars or less per year in energy bills.
posted by Rhomboid at 4:45 PM on May 9, 2006

A few basic facts:

1)Unlike natural gas or home heating oil, electricity CANNOT be stored. You've got to use it right then and there.

2)Something has to run the turbine (or generator that you'd use on a small scale). Many big turbines are powered by natural gas these days (hence rising costs for natgas also increase the price of electricity). Does the generator have to run on gasoline? So you're paying to power the turbine/generator to create a very modest amount of electricity.

3)Reliability is a key part of electricity generation. As someone pointed out, the erratic nature of folks using the machines at the gym defied reliability. So Jim decides he's had enough of the exercycle for today and there go the lights. :)

You are sort of touching on a topic called distributed generation (DG).
posted by bim at 5:03 PM on May 9, 2006

Gym Power
posted by dead_ at 5:10 PM on May 9, 2006

Very nice find, dead_.
posted by bim at 5:14 PM on May 9, 2006

Incidentally, if the goal is energy efficiency, and the reason it's not effective is that humans operate on such tiny amounts of energy compared to our machines, it's interesting to turn that around - I just looked at comparitive efficiencies, and if you stop spending your time at the gym, and pedal instead of drive to work, the same amount of energy that would get you to work once in the car will get you there every day (including weekends) for two months on the bike.

So the way the gym could become a really massive producer of energy possible is by shutting the doors and closing it down :-)
(because that way it's as if it's producing nearly 60 times the energy that the bikes absorb!)

(Please no-one misinterpret this and get nasty about imagined cycle advocacy - this thread about the energy outputs got me interested in how those figures rated compared to other things, and this post is just humorously sharing what I found.

Bim:

None of those points are relevant to generators in gyms feeding power back into the grid - a person switching on their dryers makes a far bigger change that someone getting on or off their bike, and the grid deals with it just fine - it's just noise. It's trends that are orders of magnitude greater that need to be managed.
posted by -harlequin- at 6:00 PM on May 9, 2006

bim, you can store electricity. That's what a capacitor does for an example. The main reason that gyms don't use human power is that the amount of power generated would be insignificant compared to the cost of the equipment or the relief it would make on the power bill. Wattage or calorie readings on exercise equipment is horribly inaccurate as well. Your 200 watts of output makes some assumptions about body weight, efficiency and most likely makes huge concessions for vanity. I'm going to be on my Nordic Track in about 10 minutes. I'll be on it for about an hour and it'll say I burned somewhere around 800 Calories (from memory). Screwing around I've kept up that pace for a few hours but I don't really believe that I've burned around 2400 Calories or 2/3 pound of fat.
posted by substrate at 6:19 PM on May 9, 2006

Lots of these machines DO take advantage of the power output, albeit not by feeding back into the power grid. Many elliptical trainers (especially the newer Precor ones) use the power from exercise to charge the battery that operates the digital interface on the machine. A few years ago, most of these machines had plugs, but these days, they leverage the user's own ouput.

So it does happen. Just not on the scale you might expect.
posted by yellowcandy at 7:41 PM on May 9, 2006

A back of the envelope proposition..

You could develop an alternator and controller to provide pedling resistance for a marginal cost increase of about \$100 per bike. You would use those to drive a line interconnect inverter like the Sunny Boy 2500U (at \$2,250 on ebay). - the point being, it is agency approved for direct connection to the utility line, you just plug it into the wall!
Note: You can buy a full peddle generator for \$500 retail - Windstream Mk III Human Power Generator.

Now comes one of two major outstanding questions.. Unlike the part you hook up to the bike, which you need to put on every bike, you only want enough line interface inverter capacity for some optimum level of generation which maximizes your return on investment. When the power generation is above the optimum level you just dump the excess - it costs too much to add line interface capacity to capture the power during peak generation periods. The BBC article naively uses 2 hours/day x 40 bikes, but if 25% of the bikes are used for at least 6 hours per day you would be much better off designing around that capacity. So for arguments sake lets use those numbers (25% for 6 hours/day).
Note: Remember, you can still capture power when fewer bikes are in use. To make this calculation simple lets say the place is open for a total of 14 hours/day, and in the other 6 hours/day of operation you get 10% of the bikes running.

The other major outstanding question is, how much output power do you expect from the average exerciser. For arguments sake Lets say 125W (the BBC suggests 350W, others here are thinking a little under 200W, the Windstream MkIII link says 125W). After efficiency considerations we will call this 100W.
Note: Considerations are the human output efficiency, the bicycle attached alternator efficiency, and the line interface inverter efficiency. Using the MkIII number we only need to consider the inverter efficiency, which will be above 80%.

If we hook up 100 bikes to a 2.5kVA inverter it will cost about \$12,500. The electricity produced will be 10 x 100W x 12h + 25 x 100W x 6h = 27kWh/day, or a little less than 10,000 kWh per year - about \$1,000 worth each year.

Costs are about 2-4x too high for this scheme to pay out. On the other hand, I'd be willing to bet that my calculations are very conservative. Add to that the marketing cache and it might make sense to try this.

Anybody with money want to back a start up?
posted by Chuckles at 8:43 PM on May 9, 2006

Corrections (argh!!!):

To make this calculation simple lets say the place is open for a total of 14 hours/day, and in the other 6 hours/day of operation you get 10% of the bikes running.

That should be the other 8 hours of operation.

10 x 100W x 12h + 25 x 100W x 6h = 27kWh/day, or a little less than 10,000 kWh per year - about \$1,000 worth each year.

That should be 10 x 100W x 8h + 25 x 100W x 6h = 23kWh/day, or about 8,000 kWh per year - about \$800 worth each year.

I'd be willing to bet that my calculations are very conservative. Add to that the marketing cache and it might make sense to try this.

The calculations are conservative, but in addition, the load estimates are also very much unknown. Finally, even if the load numbers are correct, you could save lots of money by only hooking up the most popular bikes - something I wanted to avoid because it is hard to control behavior.
posted by Chuckles at 8:52 PM on May 9, 2006

electricity CANNOT be stored. You've got to use it right then and there.

you're kidding, right?
posted by juv3nal at 2:25 AM on May 10, 2006

I thought that electricity could be stored in batteries, (i.e. car batteries).
posted by sic at 2:59 AM on May 10, 2006

you're kidding, right?

No.
posted by Yelling At Nothing at 2:59 AM on May 10, 2006

Yelling at Nothing is entirely correct. Bim is also correct. Indeed, batteries cannot store electricity, they store energy. Chemical energy in fact. The key is converting it back and forth without losing too much in the process. Self-discharging also becomes a problem, as well as the lifecycle of the batteries, basically creating even more maintenance costs.

Perhaps some sort of lithium battery application like the type used in hybrids (supplemented by the energy generated by braking friction) could be used.

Don't give up on this idea, however. 10 years ago solar energy was impractical for most, due to the high cost of photovoltaic cells. But new production methods and lower prices are making it more feasible. Hopefully someone can find creative solutions to all the problems people have mentioned.
posted by tweak at 3:15 AM on May 10, 2006

Indeed, batteries cannot store electricity, they store energy. Chemical energy in fact.

Ok, technical point ceded, but I don't see how that's an important distinction to make when the comparison is with natural gas and home heating oil which are similarly stored chemical energy.
posted by juv3nal at 3:43 AM on May 10, 2006

I agree the pedantic technical point misses the larger, actually important question, juv3nal, I just wanted to end that little bit of shit-slinging with a quick explanation. Bim and Yelling At Nothing did not volunteer one.
posted by tweak at 3:57 AM on May 10, 2006

Capacitors store "electricity".
posted by Ethereal Bligh at 4:56 AM on May 10, 2006

I'm not talking about battery in your car or your ipod or some other minimal usage of electricity. We were talking about "the grid" at one point and I repeat (perhaps with clarification):

UTILITIES DO NOT STORE ELECTRICITY.

Natural gas is socked away before the winter months. Home heating oil can be stored of course. You will NEVER hear of your local power company (i.e. part of "the grid") storing electricity. It's totally unfeasible or perhaps we should say non-cost effective.

"Power plants do not store electricity, so it is constantly being made and then distributed. Utility companies have to anticipate demand for electricity and distribute it to consumers."

"Electricity is difficult to store. Battery technology does not allow efficient storage for any but minimal uses. The only effective way to store electricity is to pump water up hill and keep it there until it is needed, but such opportunities are limited. The only effective supply response to peak demand is idle capacity, or reserve margins."
posted by bim at 5:21 AM on May 10, 2006

And if we could "store" electrcity on a grand scale, we wouldn't have problems in the hot summer months from big time air conditioning usage resulting in blackouts or rolling blackouts and the like.

Just ask New York City. This is a problem every summer. :)
posted by bim at 5:27 AM on May 10, 2006

ok, what about storing the energy as kinetic energy (like a big fast fly wheel) and just tapping it when you want some juice?
posted by Tryptophan-5ht at 5:32 AM on May 10, 2006

Flywheel? The DOE is interested. So is NASA.
posted by bim at 6:36 AM on May 10, 2006

Capacitors store "electricity".

Capacitors self-discharge at a rate that makes them impractical for large-scale storage:

"Moreover, the larger the capacitor, the larger the rate of charge leakage." The linked article talks about mobile phones, but the geeneral issues of battery vs capacitor storage applies to any system.

Look, if there was a decent electricity storage mechanism, power companies would be using it and skip having to watch system load every second of the day to change generator output.

And regardless of how you store it, there's always conversion losses.
posted by GuyZero at 6:48 AM on May 10, 2006

There is positively no need for storage in this application! It is very likely that demand would far outstrip generation during all hours of operation.
posted by Chuckles at 6:58 AM on May 10, 2006

Yes, storage is irrelevant to this application. On the topic though, a primary means by which power stations store energy for the grid is lake levels - turn off the generators, and the lake levels rise, storing more and more energy until you want to use it. In this way, solar power (ie rainfall) is collected during winter and stored until summertime.
posted by -harlequin- at 10:19 AM on May 10, 2006

Harlequin -- For the record, hydropower (a type of renewable energy) is great, but they're just aren't enough lakes to get the job done. In fact, hydropower is a very small portion of our energy production in the U.S.

Coal and natural gas are the main fuels for producing electricity. And for many years we tried to get away from coal due to bad pollution problems caused by it. And now natural gas prices have gone up significantly. That's why winter heating bills have risen for both folks that heat by natural gas and electricity (since natural gas is used in the generator plants).
posted by bim at 5:17 PM on May 10, 2006

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