15

Why don't they make playgrounds generate electricity (at least, not on a large scale)? It's a pity that all that energy of children horsing around goes to waste instead of being harnessed. You could incorporate a generator into a swing, for example.

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36

Because a playground equipped with generators would probably not be much fun.

  • The merry-go-round would grind to a stop the moment it is no longer being pushed.
  • The swings would not swing; they would glide back to their resting place then sit idle.
  • The jungle gym would require children to follow a prescribed exit that could capture their stored gravitational energy.

Even if children could be convinced to exploit themselves for child labour, there is still the problem that all of their rambunctious energy originates as food energy. Food is not free to purchase, nor is it free to produce, so any kind of power generation based on food and human kinetics tends to have poor economics.


But since we're here, let's think about how much energy could potentially be captured.

A child age 8 expends about 1000 kcal/day as the basal metabolic rate (BMR). A very active person (such as a playful child) might have a physical activity level (PAL) around 2.0, which means that 1000 kcal/day are available to perform mechanical work. The human body is about 25% efficient at converting food energy into mechanical work. Turbine generators are around 80% efficient.

Therefore, one playful child could produce about 200 kcal (2.3 kWh) of usable mechanical energy each day. Where I live, that's about $0.14 worth of electricity. This clearly won't be a good investment.

7
  • 6
    But they eat anyway Jul 27 at 20:10
  • 22
    @SergeyZolotarev: the financial payback period on the equipment would be functionally infinite. The biggest swing set I recall seeing has maybe 10 swings; assuming 10 children swing 24 hours a day (which is generous), that's $1.40 in electrical production; if the generator bits add $1000 to the cost, that'll take 714 days - almost 2 years - to pay off. In that time, the kids will probably have broken the generator so that the swings are more fun to swing on. And, $deity help you -if-when somebody gets shocked and sues for millions.
    – minnmass
    Jul 28 at 4:08
  • 1
    Actually, on the surface, a 2 year payback does not seem terribly uneconomical: playground equipment tend to have lifespans measured in decades rather than years. But obviously they won’t be used 24x7; plus, as you say, they wouldn’t be fun to use and/or the generator components will break much sooner than playground equipment otherwise would.
    – eggyal
    Jul 28 at 7:21
  • 7
    Don't forget the embedded energy in the generators. There's a good chance that it would never repay the energy involved in making it in the first place.
    – pjc50
    Jul 28 at 9:11
  • 4
    @SergeyZolotarev: That BBC article was a little short on actual info, so I searched around a bit for info on piezoelectrics. I see a lot of huge promises around "experiments" (e.g. in Israel), and some actual results of such installations lighting... basically, the installation. Since the advent of LED's, light is very cheap (power-consumption wise); from lighting a lamp to making a contribution to the power net is a huge step. Also, a rolling vehicle is a huge amount of energy compared to a child on a swing. (Try having one crash into the other and see which is sent flying...)
    – DevSolar
    Jul 28 at 15:40
14

A better target would be gym exercise machines

  • Indoors, not exposed to elements, controlled environment so less chance of theft and vandalism.
  • Customers stay on machines for a prolonged period instead of kids who would play on 1 thing for 5 minutes or less before moving on to the next one
  • Arrays of identical machines can be connected to same grid tie inverter, cheapening costs
  • Gym already has electrical supply and on-site consumption so no need to lay much extra cabling.
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  • 9
    Also users find drag or otherwise lost energy desirable, unlike on a swing; when e.g. using an elliptical machine, stair-stepper or treadmill you WANT there to be resistance. Jul 30 at 3:32
  • Even for gym equipment it’s not really worth it. Humans can sustain between 100 to 400W of extended power output, depending on fitness level, mass, duration etc. That’s barely enough to power the lights above the machine and its electronics+display+fan. It’s not even enough to power the air conditioning or heating.
    – Michael
    Jul 30 at 20:59
11

Playground equipment is very expensive and not typically something that local councils have large budgets for. Hooking it up to the electrical grid for minor amounts of electricity generation would increase the cost substantially and require an ongoing electrical maintenance contract.

5

There are two main areas to consider here:

  1. Safety.
    Anything put near children has to be safe, and mains voltage and generators generally aren't. Damaged wires could electrocute somebody, a generator rotor could kick back, injuring a kid.

  2. Durability
    A regular playground will be used in every possible unintended way, and there will be often damages. Not a good place to put an expensive equipment.

You could put a rugged bicycles as "charge your phone" stations novelty, but don't expect any huge output.

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  • 1
    I'd say the energy generation rate is a rather important thing to consider, but only mentioned in a side clause of your answer. If kids could generate GW of power I'm sure the safety and durability concerns could be overcome.
    – Rick
    Jul 30 at 16:52
2

They do.) Just to show how small power such generator can produce.

In the picture bicycle generators that convert mechanical power into different types of power are shown, they're located near Municipality of Jerusalem. Bicycle generators near Municipality of Jerusalem

2

My brother is a bicycle fanatic, a former engineer, and a current nurse. According to his calculations ... Lance Armstrong (with some chemical enhancement) is likely the single greatest generator of electricity in the human race. According to brother's calculations ... Lance could roughly keep a 60 to 75 watt bulb (incandescent, better with LED) going during his maximum cycles. Realizing the best kids are going to be about half of that ... this is something only government would invest in ... because the costs would be 100x or more over the benefit. Even in a gym's spin class (far better source than a playground) ... you might only spend 20x as much as the benefit you would get.

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  • 1
    An elite cyclist can output around 400W of mechanical power for a whole hour. A good electric generator can convert more than 90% of that into electricity. But it doesn’t change the conclusion that it’s just not worth it.
    – Michael
    Jul 30 at 21:03
  • Please check your numbers and consider adding references to your sources. Lance Armstrong is known to produce over 400 watts sustained. Even a beginner cyclist can produce 75 watts.
    – Nic
    Aug 6 at 15:17
1

The power produced would be so small scale and have so poor capacity factor that it isn't worth the effort.

Firstly, you need investments. You can't make power without equipment. You need a generator. A wind turbine for example has a generator that makes power at 30% capacity factor.

A playground? If we assume it's used 5 hours per day (and produces always power at the same rate), that's only about 20% capacity factor, worse than wind turbines. So your power production scheme can't really compete with wind turbines but requires the same investment for generators that run at a very poor capacity factor.

Also children aren't machines that produce power at exactly the same rate. I suppose the power produced would fluctuate a lot and to allow generating power during the peaks at peak power, you need excess capacity. So the 20% capacity factor at non-fluctuating power output actually turns into something like 10% capacity factor. That's worse than solar power.

Also your power production scheme that can't compete with wind or solar based on capacity factor also can't compete with quantity. A child may produce perhaps 100 watts and at 10% capacity factor that's 10 watts average. To put that into perspective, the human population uses around 5000 watts per capita and there have been unsuccessful proposals to reduce that to 2000 watts per capita. Considering that perhaps 15% of the population are children of the age that produce the 10 watts average, it means you get 1.5 watts per capita. Try satisfying your power need of 5000 watts per capita from that and you see why it won't be successful.

0

Because it's useless. A typical bike generator outputs 100 watts, If you pedal for an hour a day, 30 days a month, that's (30 hours x 100 watts) = 3000 watt-hours, or 3 kWh. Enough to power four lightbulbs..... for 7 hours.

1
  • or 4 LED bulbs for 70 hours - get with the times
    – user253751
    Jul 30 at 8:48

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