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Thread: How many bicycles does it take to boil a kettle?

  1. #1 How many bicycles does it take to boil a kettle? 
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    Before I start, if you do graciously provide constructive feedback to this request, please bear in mind that I am well out of my depth given my education is not in electronics and this is only a recent interest.

    I really need your help. As the title of the thread suggests, I need to build a prototype installation of 30 bicycles powering a 3000w 240v kettle (yes I am in the UK!).

    The specification of the bicycles is fixed as these have already been assembled. Each bike will have a 350watt 24v DC motor attached. Given that each cyclist will be generating about 120watts baseload, at say 12v, could anyone tell me what size and type of capacitor I would need? Please take into account that

    a) 30 people pedalling simultaneously is I suspect going to generate in excess of 250 amps.

    b) Enthusiasm is going to be high when people start pedalling (although of course we do have control of this) so amps / watts could be much higher.

    c) The capacitor will feed into an inverter to convert the current to 240v 12.5amps AC.

    d) not sure if some sort of diode will be need to prevent feedback?

    e) Cost is not too prohibited at the moment, although lead time is short.

    f) Suppose we only need a few seconds of 'stored' power in the capacitor to ensure smooth delivery.

    Anyone have a cunning plan?

    Thanks
    Colin


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  3. #2  
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    Quote Originally Posted by electricpedals
    Before I start, if you do graciously provide constructive feedback to this request, please bear in mind that I am well out of my depth given my education is not in electronics and this is only a recent interest.

    I really need your help. As the title of the thread suggests, I need to build a prototype installation of 30 bicycles powering a 3000w 240v kettle (yes I am in the UK!).

    The specification of the bicycles is fixed as these have already been assembled. Each bike will have a 350watt 24v DC motor attached. Given that each cyclist will be generating about 120watts baseload, at say 12v, could anyone tell me what size and type of capacitor I would need? Please take into account that

    a) 30 people pedalling simultaneously is I suspect going to generate in excess of 250 amps.

    b) Enthusiasm is going to be high when people start pedalling (although of course we do have control of this) so amps / watts could be much higher.

    c) The capacitor will feed into an inverter to convert the current to 240v 12.5amps AC.

    d) not sure if some sort of diode will be need to prevent feedback?

    e) Cost is not too prohibited at the moment, although lead time is short.

    f) Suppose we only need a few seconds of 'stored' power in the capacitor to ensure smooth delivery.

    Anyone have a cunning plan?

    Thanks
    Colin

    When you put a single diode into the circuit to isolate the "generating motor"/"generator" on the bicycle from the main battery, feeding the converter. So it does not rotate and use power, while not moving fast enough to generate power.

    You will lose some voltage. I would say about a volt. You also need about 13.8-14 plus volts to charge a twelve volt lead battery. To output to a device that needs twelve volts. The length of wire you will need is going to whack your voltage hard. So you may be peddling rather fast even with large wire.

    The problem is that although, you are not making any more watts, by peddling faster to create the higher voltage, you have to create the amps you need at a higher voltage. This adds to the generators load. And it is harder to peddle.

    You could do it with just one heavy duty battery, while the generators are running. However I would put a couple in parallel. If the load amps are going to be high, and 250 amps is very high. A single battery is not made to put out many hundreds of amps. It is better you put a couple in parallel.

    You can get switching devices that will allow, you to use one or both batteries. We use them in marine applications to switch batteries, or work on connections at sea without shutting down the motor. These devices the good ones have resistors inside of them to ease, the load and cut down on arcing during circuit switching or battery change. But you must know whether or not they can switch a load of that size. Some may not.

    If this is just a test you could probably get by with this. But a severe accident is foreseeable or at least possible.
    If you are looking to make something permanent, you would probably want to create a bank of batteries. Designed with resistors, and or fuses/breakers in between each bike generator and the main battery. This will also use up voltage.

    So if there is an accident and there will be. You will not detonate a battery.

    When you put a diode onto a battery. You create a double diode. During an arc, you can see some serious voltage, back to the battery. Enough to pop it. Maybe disintegrate it.

    During charging of powerful batteries there is sometimes an accident. They can be pretty scary.

    In automobiles they often hide little known safety devices in common objects, that we do not see readily. I am sure the solenoid wire, that connects the points inside the solenoid limits current throughput to the starter. There are fuse boxes and fuses between just about everything and the battery. You just do not always realize it.

    I am a lifelong welder, so I know what 400 amps can do. Either to weld or cut. It can hurt you.

    If you are sweating and you touch the hot wire, while the ground wire, that is connected to the bicycles and the ground, is also in contact with the cement. You can also, get a cool tingle in your back. Even from low voltage sources.


    I would spin a high voltage alternator, myself. Run small wire to the rectifier near the storage batteries. And do the rectifying, near the batteries.


    Sincerely,


    William McCormick


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  4. #3 Re: How many bicycles does it take to boil a kettle? 
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    Quote Originally Posted by electricpedals
    Before I start, if you do graciously provide constructive feedback to this request, please bear in mind that I am well out of my depth given my education is not in electronics and this is only a recent interest.

    I really need your help. As the title of the thread suggests, I need to build a prototype installation of 30 bicycles powering a 3000w 240v kettle (yes I am in the UK!).

    The specification of the bicycles is fixed as these have already been assembled. Each bike will have a 350watt 24v DC motor attached. Given that each cyclist will be generating about 120watts baseload, at say 12v, could anyone tell me what size and type of capacitor I would need? Please take into account that

    a) 30 people pedalling simultaneously is I suspect going to generate in excess of 250 amps.

    b) Enthusiasm is going to be high when people start pedalling (although of course we do have control of this) so amps / watts could be much higher.

    c) The capacitor will feed into an inverter to convert the current to 240v 12.5amps AC.

    d) not sure if some sort of diode will be need to prevent feedback?

    e) Cost is not too prohibited at the moment, although lead time is short.

    f) Suppose we only need a few seconds of 'stored' power in the capacitor to ensure smooth delivery.

    Anyone have a cunning plan?

    Thanks
    Colin
    OK, Some years ago I constructed a similar machine, the load was 60watt in the form of a 12" television set. I could pedal and keep the set working for several minutes without any problem, if I increased the load to 100 watts I became exhausted almost immediately

    I am not sure that your 120watt target is realistic, since the efficiency of your generator will come into play here, I was using a car alternator most of which have an efficiency of between 50 and 70%.

    Why do you need a 3KW kettle? my own kettle is around 800 watts.

    Here are some pointers, it takes 1 calorie (4.6 joules) to heat 1cc of water by degree C.

    Suppose you want to raise 1 litre of water by 80 deg C (ie to boiling point from a starting temperature of 20 C). 1000 cc * 80 * 4.6 = approx 400,000 joules or watt seconds.

    THus if you used a generator capable of supplying only 1 watt then it would take 400,000 seconds to boil - this of course assumes ZERO losses (ie heat lost through the vessel).

    If you use a thermos/Vacuum flask and lower a wirewound resister in to it (as well as a thermometer (bring the leads out through a small vent hole drilled through a cork/plastic lid) you could then boil the water using a single 60 watt generator
    you could initiall fill the thermos with hot water and see how long it takes to cool,
    from this you can work out how much energy you need to put in, just to overcome losses (beware the higher the temperature the greater the loss) - read up on Newtons laws of heating and cooling) or jus estimate it

    If it takes 24 hours to cool from 90 to 42 C for a litre in your thermos flask, then the temperature is falling by 2 deg/hour or 9200 watt/seconds of loss which is equivalent to continuously providing about 2.6 watts to keep the temperature steady.

    IN an uninsulated vessel a temp drop of 48 degs might occur in as little as 20 minutes which would then require 200watts just to overcome the loss!!


    THus the insulation of the vessel is very important. THe longer it takes to warm the vessel the more heat will be lost.

    ANother thing BUT make sure you have a qualified electrician to help with this...

    Instead of using an invertor (which will increase the system losses) why not mechanically link all the pedals together and use a single mains generator,
    converting a low cost portable mains generator might be a more efficient alternative (yes, I note you already have the bikes) - another might be to connect sets of generators in series and perhaps then in parallel (if DC) then connecting each generator to the load via a diode.

    HOpe you can pick something useful out of that, If you get areply from Mr Mcormack - ignore it as rubbing a balloon against a diode powered sky hook won't work!
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  5. #4  
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    This sounds like Obama's energy policy.

    I wouldn't bother with the capacitor. The inverter no doubt has features that will limit the output voltage, so you are not going to burn out the heater. The inverter will probably have some limits on the input voltage and will trip off below or above that voltage. If you start out with a discharged capacitor, it will be hard pedaling for a while, as it will appear to be a dead short initially. You won't be running the teakettle until the voltage builds up to the minimum inverter input voltage.

    I'd just give each pedaler a d-c voltmeter and tell them to keep it under, say 14 volts or so. If there are a few pedalers who don't comply, then they will be doing all the work while the others loaf.
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  6. #5  
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    After thinking about this, I think you could run into a problem starting the system up. The bicyclists will have to start pedaling first to bring the voltage up before you can close the inverter output breaker. They will not be putting much pedaling effort into it until you close the breaker. Then the voltage will drop until they can react to the load, and the unit could trip off on undervoltage.

    If you use a capacitor, it would have to be sized to ride through the transient and supply the load for as much time as it takes for the cyclists to react, maybe a couple of seconds, while still keeping the voltage above the undervoltage setting.

    If the bikes have flywheels, that might smooth things out enough to ride through the transient.

    Another way might be to connect some resistors in series with the teakettle (more teakettles?) to reduce the initial load current, then switch them out sequentially building up the load in steps.
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  7. #6 Re: How many bicycles does it take to boil a kettle? 
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    Quote Originally Posted by Megabrain
    If you get areply from Mr Mcormack - ignore it as rubbing a balloon against a diode powered sky hook won't work!
    I believe you will see my reply will actually give you a well rounded, workable solution. I know because I have built similar DC systems. The storage battery cuts down on the amps needed to start pedalling. People can start and stop pedalling, and the system will just continue. Until you get an alarm from the inverter, if you fall way below the input amps needed to charge the batteries for some duration.





    Sincerely,


    William McCormick
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