Generating electricity at the gym?

The other day a friend and I were mulling over this question: “Why doesn’t the cardio equipment at the gym capture the energy from all the people working out?” It’s something I’ve thought idly about before but figured it would be fun to do some calculations on the subject. (Yes, sometimes my idea of fun is outside the norm.)

Before I even got around to posting on the subject though, the subject surfaced again when I saw some folks generating electricity on some stationary bicycles at the Seattle Green Festival this past weekend. OK, I get it, it’s time to do the calculations!

First let me make sure I’m answering the right question. The answer to the question “COULD the cardio equipment at the gym capture the energy….?” is certainly yes. People have designed and built attachments that you can hook your bicycle up to to convert it to, in essence, a stationary exercise bike that generates electricity (like I saw this past weekend). Some of the cardio equipment I have seen in the gym doesn’t plug in to an outlet, the electrical power for the display and the controls is generated by the person exercising on the machine. (Interestingly, when I looked up this type of machine on the web, it was referred to as “self-powered;” this phrasing to me seems to marginalize the rather important role we, as the people actually exercising, are performing; but, I’m willing to let it go and move on with my life.)

So we know that electrical energy can be generated fairly simply in at least some types of common cardio equipment. Why are these “self-powered” machines built? It’s not to sell electricity back to the utility, clearly, as there is no electrical cord leading into or out of the machines. A self-powered cardio machine will save some small amount of electricity, in that you don’t have to provide any electric power to it, but I’m guessing that a self-powered bike or elliptical machine or whatever is attractive because you can get away with having an exercise room without a power outlet every four feet on the floor.

Another related question we can get out of the way right away is “Could we save the planet/stop global warming/charge our plug-in hybrids by capturing this energy?” I think the answer to that is almost certainly no. The energy we, as exercisers, spend at the gym comes from our food intake. We can’t hope to get perfect efficiency out of our food calories converted into electricity this way (or any way, really), that is, for every calorie of food we burn on the treadmill we’ll get substantially less than one calorie of electrical energy out. And it gets worse, because the food calories that we eat take energy to grow, and the higher on the food pyramid you go to get your food calories, the more energy went into growing that food. That’s a whole other subject. My point is that producing electricity in this way is not an electricity source with a free fuel supply like wind or solar. Generating electricity from exercise is partially capturing a waste stream, and that’s all to the good as that energy is normally just wasted as heat; just don’t expect it to save the planet.

To get back to our original question of “Why doesn’t the cardio equipment capture the energy?,” let’s walk through some sample calculations of how much energy you could produce on a machine.

Let’s get the units out of the way. A convenient unit of energy for our purpose will be the killowatt-hour (kWh), because it’s the unit that utility electricity is metered and sold in. Watts are used in reference to athletes’ output, too, so that will be helpful…an athlete outputting 500 watts for one hour will have generated 500 watt-hours or 0.5 kWh. The equipment I use at the gym lists my output in calories per hour…that’s energy/time, so multiplying by time will give me calories which I can convert to kWh. Whew, glad that’s over with.

When I’m chugging along on the elliptical trainer, I usually have a power output of about 900 calories/hour, which works out to about 1000 watts. So I think to myself, “hmm, not bad…that’s a lot of power!” But when I look online, I see that wattage output for athletes covers a wide range, with peak outputs on the order of 1000 watts. So, am I to believe that I’m outputting about 1000 watts during a low-intensity cardio workout when top athletes can peak at around the same value? It would be nice to believe that, but it’s clearly not the case. I think the difference is this: they are probably doing 1000 watts of work, while I am expending 1000 watts of effort, but actually doing much less than that in useful work. The athlete outputting 1000 watts is probably burning much much more than that in his/her power expenditure. One tip I had that my energy usage was consumption, and not output, came from the cardio machine itself. When I start the workout it prompts me to enter my weight, among other things. The higher the weight you put in, the more calories are apparently burned at a given workout intensity. If the machine were really measuring power, it wouldn’t care what your weight was; it would just measure the speed of the flywheel or friction disc or whatever is inside there, that would be a direct measure of the work done.

The guy riding the bike at the Seattle Green Festival said he was producing about 200 watts. I find that number pretty believable for the output I’m producing on a cardio machine as well, which would mean an efficiency of about 20% in converting my food energy into electrical work. That’s a believable number…most of the effort I’m expending is going to heating up my body and moving my muscles in ways that don’t perform useful work; and an electrical generator in a cardio machine would probably not be very efficient either, losing a fair bit of mechanical motion to generating waste heat.

Let’s say an average schmoe like me working out on cardio equipment can produce about 200 watts of electricity. Working out for an hour, then, would produce 0.2 kWh. The average retail price of electricity in the U.S. is about $0.09/kWh (found that here…http://www.ppinys.org/reports/jtf/electricprices.html). So an hour of average exercise might produce about $0.09/kWh x 0.2 kWh = about 2 cents.

How much would some cardio equipment produce in a gym over a year, then? Let’s say the gym is open from 6 AM to 10 PM and figure out the dollar value of the electricity in a best case situation: the equipment is in constant use: 16 hours/day x 365 days/year x 0.2 kWh/hour x $0.09/kWh = $105/year. Keep in mind that that’s undoubtedly high, the equipment is certainly not being used every minute the gym is open. A more realistic assumption would be less than 30% utilization, bringing it down to $30/year or less.

Well, but that still adds up, right? Yes, it would over time, but there are a few things working against implementing this kind of thing. Where are you going to put the power? The electric utility doesn’t want you just shoving electrons into the wall outlet…the utility doesn’t want your gym members electrifying the grid when there is a power outage, as this could harm repair workers. You’d need electronics that would isolate the grid from this secondary power source in the event of a utility outage. These things exist, they are used in solar installations all the time, but they cost money to buy and install. Also, you’d need an inverter to convert the generated electricity from DC to AC, and to synchronize it with the grid’s frequency. Again, these exist and are commonly used in the solar biz, but they cost money. Another option to somewhat avoid the above issues would be to use the electricity to charge batteries for local use, but that comes with its own infrastructure costs. HOWEVER, if you were off-grid in a cabin or something, a piece of generating equipment like this could be extremely useful! One way to look at it is to figure your cost of electricity at your cabin or wherever…if you’re not hooked up to the grid, your cost/kWh from a gas/propane/diesel generator is going to be far, far higher than $0.09/kWh. Having an exercise bike that you ride for an hour a day, producing 0.2 kWh, would let you run two 13 watt CFL lights for 7 hours each day. Use LED lights instead and you’ll do even better. That’ll improve your quality of life in that cabin.

Back to the case of the gym, though. What if all the cardio equipment in the room generated electricity, and was plugged up to a common inverter and disconnect? That would surely save a lot on infrastructure costs. For the sake of discussion, let’s assume you can make a generating piece of cardio equipment for the same cost as a regular one (no idea if that’s true or not), and that minimal additional wiring in the room is needed…you just take all the AC circuits offline that were originally used to power the equipment, and rewire them in a junction box somewhere to feed the inverter, which feeds into the grid. Let’s say the inverter costs a couple of thousand dollars.

Now, how big is the workout room? Well, my gym is pretty big, and I estimated the number of cardio machines in the cardio room today. There were five rows of machines, each row with about 20 machines in it, for a round 100 cardio machines. Let’s say the machines are used 30% of the time, and are used by average schmoes like me outputting 200 W of electricity while exercising. The gym is open let’s say 50 weeks a year to account for holidays and is open 16 hours a day. Our new annual output is…
100 machines x 30% usage x 16 hours/day x 350 days/year x 0.2 kWh/hour = 33,600 kWh/year;

At $0.09/kWh, that works out to $3024/year.

If your costs were only a couple of thousands of dollars for hardware, that could be a pretty reasonable return. It depends a lot on the assumption of how often the machines are used, and if there is significant additional cost associated with making the machines such that they can generate electricity.

Where does that leave us? Well, with all the assumptions I’ve had to make it’s really hard to say for sure, but I think such a scheme is possibly reasonable, though it’s fairly small potatos in the scheme of things. On the other hand, if you are looking at it from the perspective of someone who wants to have one machine to supplement their electricity in an off-grid use, or to provide some minimal back-up electricity for a light or two, an electricity-generating piece of cardio equipment could be really useful.

Something I like about this type of calculation is that it gives you a good sense of perspective. In this case, it helps me appreciate how much electricity does for us. The average U.S. home uses about 10,000 kWh of electricity each year (2001 data, see here: http://www.eia.doe.gov/emeu/reps/enduse/er01_us.html#Electricity). So, the workout room from our example above could power about three average homes. So if I rearrange the calculations a bit, I figure that it’d take 20 people exercising about 8 hours a day (outputting 200 W), every day, to power a home continuously. Think about that and realize what a remarkable worker electricity is for us!