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Last Updated: Friday, 14 April 2006, 13:50 GMT 14:50 UK
Gym power
Bright ideas from Magazine readers

There must be a national effort to bring about a "green revolution", says the government. But what simple things can we all do to save the Earth?

Green Light is a series of bright ideas from Magazine readers to help save the world
Everyone could help save the planet by making just a few changes in our household energy use. So the Magazine invited you to tell us how you think we could all be greener and more energy efficient.

Gym power

Melissa is just one of the many to make a similar proposition:

    What about creating green gyms where people pay their memberships as normal, but then use swipe cards before and after they get on the machines. The difference here is that the machines would all be linked to a large central generator, so if you are really working out you will be generating much more energy than is needed to power the machine display.

    The swipe cards would monitor how much time you spent on each machine then - depending on the efficiency of the machine - calculate whether you were a net user or contributor of energy to the building. At the end of the year, people get a rebate on their membership depending on how much energy their use of the treadmills and cardio machines has generated for the gym.

    This could become really sophisticated. If you have generated more energy than the average gym user uses (divide the heating and lighting costs of the gym by the number of users) then next year's membership could be free, or it could even operate on a week by week basis so that if you generated more energy than your last visit, your next trip to the gym is 20% cheaper. You'll be slimmer, the gym fees will be cheaper and the gym could potentially generate enough energy to supply electricity to nearby buildings as well as run its own energy needs.

Expert verdict

Graeme Bathurst, of independent energy consultancy TNEI, offers this very considered analysis:

    Technically, yes this is possible. However, without meaning to sound cynical, most things are possible technically. Whether they are cost effective or technically sensible however is another matter. In short, the key issue in this case is that humans don't use very much energy.

    The average diet is 2000 calories a day so if all of that were converted to electrical energy then a single person could run a 100 Watt light bulb for about 22 hours. The problem of course, as all those who have exercised know, is that humans get hot. The energy burnt is not converted directly into mechanical movement and so it is not possible to harness the full energy usage.

    As an example of this, a vigorous 10 minute work-out on a rowing machine for the average person will burn about 100 calories, or in other words, enough electrical energy to run the same 100 Watt light bulb for one hour. Assuming though that at most we can harness 50% of the energy usage, this translates to an average power of 350W, which is probably the same as the average power of the advanced rowing machine that you are currently using!

    Taking the sweaty exertions from 40 machines in a gym covering the usual suspects of rowing, cycling, running and walking over the peak two hour activity period, then this could generate in the order of 25kWh of electrical energy. This is assuming of course that all the individuals concerned were genuinely attempting to workout. At an electricity retail price of 7p/kWh, this will allow the gym to recoup the princely sum of 2/day, or annually about 500. So, is this cost effective? Probably not.

    However, 25kWh is also the average daily consumption of several houses and so is not an inconsiderable quantity of energy. Energy scavenging and self-power devices are topics that are developing a considerable level of technical and commercial interest and so there may be some mileage in a gym type application particularly with the green or sustainable marketing angle.

    If nothing else, the 'gymee' could take heart from the fact that their excursions have provided sufficient energy to recharge their ipod for the arduous journey home

Thanks for all your suggestions for Green Light over the past two weeks. This concludes the series - sorry if we couldn't include yours.

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