My colleagues Laurie and Andrew have just recovered from producing NPL’s blockbusting ‘Energy Harvesting’ stand at the Royal Society’s Summer Exhibition. And last week they asked me whether it was possible to have a carbon neutral gym? The idea must have occurred to many people as they sweat and struggle. Surely, they think , all this effort is not entirely pointless? Surely we could capture some this work being done and use it to power the lights? Surely if Seattle can have 20 carbon neutral office blocks, we manage a gym! Well, let’s see.
Let’s look at the power 20 squad members in a Rugby team training together might produce.
- Rowing Machines: If I try hard I can generate 150 W of power on a rowing machine for around 10 minutes. Let’s assume rugby players can generate for 250 W for 15 minutes. If we had 5 machines then we could rotate banks of 5 players through these machines to generate a continuous 5 x 250 = 1250 W – that’s impressive .
- Treadmills: A treadmill actually consumes energy :-(, and a typical device might have a 2 horse power (1500 W) motor. So either we could have 6 rowing machines powering one treadmill, or more sensibly, we could just abandon treadmills and get the team to run around a Rugby pitch.
- Exercise bikes: I don’t have a figure for bikes, but in my experience they are less knackering than rowing so I would guess they can generate less energy. Let’s guess we have 5 exercise bikes and each one can generate 150 W then we could rotate banks of 5 players through these machines to generate a continuous 5 x 150 = 750 W.
- Cross Trainers: I don’t have a figure for cross-trainers (sometimes called elliptical trainers), but in my experience they are less knackering than rowing but more knackering than exercise bikes, so I would guess they can generate power somewhere in between the previous two. Let’s guess we have 5 cross-trainers and each one can generate 200 W then we could rotate banks of 5 players through these machines to generate a continuous 5 x 200 = 1000 W.
So with 5 of each type of cardio machine and no treadmills, 15 rugby players could generate 1250 W + 750 W + 1000 W = 3000W – a useful amount of power. Each person would be working at a rate which would burn roughly 1000 kCal/hour which is pretty intense exercise, so let’s guess that they could do this for one hour.
- Muscle Machines: In these machines pulleys translate the physical motion into a lift of some weights, typically by about half a metre. Let’s assume that each lift is of 40 kg, then each lift requires (m x g x h) (40 x 10 x 0.5) 200 Joules of energy. If we could capture this in a way that generated electricity, and a lift was performed on average every second (assuming several people are exercising simultaneously) then that could generate 200 W of power.
Is it feasible: Given these simple calculations the answer has to be – ‘Yes’. But there are caveats
- In general one can’t capture all the energy people dissipate in these machines. Dynamos are not ideally efficient and the muscle machines would not couple easily to dynamos; some players may not be ideally fit; and we have assumed 100% occupancy of the equipment. So in practice our 3000 W of power would be substantially reduced, but 1000 W would be a fair estimate.
- Human’s generate heat when they exert themselves – about 6 times more than work they produce. So if 3000 W of work is being produced there would be an astonishing 18 kW of heat produced – so some kind of cooling or ventilation would be required – which would take energy. Let’s assume this is negligible and that we cool using a chimney effect.
- The gym would need to have a relatively large floor area which would probably require lighting – which would probably require several hundred watts – but let’s ignore this and assume the training is in daylight.
- Over one hour, the team would have plausibly generated 1 kWh of useful electrical energy, which would have cost at most £0.25.
- Over one day (12 hours) assuming the equipment was highly utilised, this might amount to 12 kWh of energy, which would have cost at most £3.00.
- Over a week (5 days) this might amount to 60 kWh of energy, which would have cost at most £15.
- Over a year this might amount to 3120 kWh of energy, which would have cost at most £780.