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oz etzioni

new york, NY, United States

Member since September 08, 2008

  • Pros answering my questions

    Well-being, Industrial Design

    After sending my sketches and explaining my electrical dilemmas to a professional electrical engineer, this is the answer i received, which helped me a lot to proceed with my electrical system planning:

    "Hi Oz!

    First of all , sorry for the delay…I read your presentation…

    Some general numbers regarding energy:

    A person on an average exercise is burning lets say approx 800 calories (It's actually Kcal…which is 1000 calories) 800 Kcal is 3349600 Jaul .

    Lets say that you can extract from the person only 50% of the energy due to body efficient (also produce heat…like any other engine) That means that with an average exercise you can extract 1674800 jaul….since 1 watt is 1 jaul/sec this is equivalent to using a 465 watt equipment for an hour…..or lighting up 100Watt bulb for 4.5 hours….

    This what the numbers says…in real life , if you connect a generator to a bike or a cross trainer you will need to work very hard to produce even 250 watt….this is because the 50% efficiency of our body is very optimistic and probably not realistic….also the transmission from rotational movement to electric current is suffering from loss.

    Anyway , building a bike that can transfer part of the pedaling energy to electricity is feasible and actually pretty lowtech…

    You need a:

    1.DC generator/Alternator – it's a generator with a magnet core that does not need an excitation current..meaning can start producing voltage immidiattly.

    2.A voltage regulator – since RPM changes all the time , you need to regulate the voltage to be always few voltage above the battery you want to charge..otherwise you get a reverse current…of course the voltage regulator shuts down when you are in a certain minimal RPM. As the RPM raises above the minimum, the voltage remains the same but the charge current raise…also this circuit should also limit the current so it would not be to hard to pedal….if a battery is drained and you connect it to a generator via a voltage regulator…it will request an unlimited current…meaning pedaling is very very hard….so if you limit the charge current you let the person to ride the bike from A to B and extract only a certain percentage from the energy produced by the pedaling

    3.battery – the highest energy density (energy per weight) today for rechargeable batteries are the lithium type.they are also the most expensive….please note that there are also safety consideration regarding these batteries , when shortcutting due to damage they tend to burn and king of explode due to uncontrolled temperature rise…don't mess with them…even the hybrid car industry after millions of dollars of research is very carefull about these batteris….I think that a small lead battery (like of a motorbike) is sufficient for the use……on the other hand there is a company called A123systems (they have a website that developed a unique Nano technology for Lithuim battery that says that even in 250 Deg…which is much more then the temp that lithium explode…their batteries behaves stable (no burned or exploded)….they sell battery packs for Hybrid cars , electric trains , and power tools (saws , drillers …) , the have packs that can feet your needs..

    you need to develop a rigid transmission from the wheels or pedals to the generator (without rubber bands and staff so it will last long enough.

    Regarding actual design and engeneering…I think that you need to take it to a power electronic engineer …they can fit very quickly the DC alternator you need…the voltage regulator circuit and the battery…please remember that the electric regulating circuit must allow you to change the current limitation (by a rheostat or something) . you don't want to design something and end up with a system that is too hard to rotate (to pedal…)

    I found a nice web site where they built a generator connected to a bike (static bike)

    You can see from this website , and I think it's make sense , that you can extract 20-30watt from a person that rides a bike…more then that it will be hard to pedal and not fun at all to get from point A to point B….if a person take a bike for an average of 1 hour so the battery size (in units of energy) should be accordingly 20-30 watt/hour……a 20-30 watt/hour is enough energy to light the new efficient bulbs which are equivalent in lighting to the old 100watt tungestan bulb (regular Libon bulb) for one hour…"

  • 1947_lamp_l_177_

    Oz, So, a person pedaling a bike could illuminate a 30 watt light bulb without noticeable strain? I have trouble believing that. I used to have a little generator on my bike as a kid, and I remember that it would light up a little head lamp and a tail light, but these were very low power, and the brightness went up and down in proportion to how fast I was going. But, I guess there was no battery to store power. Steven

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