Power from Pedaling

Cyclist82 said:

My question is: "Is there any cost effective means to store the energy regardless of how hard a person is pedalling?" Or am I just being stupid and there is an easier solution. Any suggestions would be helpful.

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Use a bigger battery that can withstand the highest output from the generator, which is only going to be small anyway.

Could you rephrase that question?

What do you mean by regardless of how hard? As in, if you are worried about the excessive voltage generated in particular cases, the most common method is to use a large capacitance to store the initial energy surge(Take care of the rating, capacitors are quite sensitive to overvoltages) and then use a voltage controller which will gradually release the power to the battery.

Mind you this plan is not if you pedal all the time, it can be implemented only when using regenerative braking(mentioned before)

I am a bicyclest. I am old and out of shape but I still ride bikes. I am told the average person in good condition can pedal about 1/4 hp. Buy a generator rated about 1/4 hp or smaller and gear it high enough the person cranking the pedals is pedaling about 85 to 90 RPMs. Skinny light weight legs can crank faster and easier that heavy legs. I have never been a fast pedaler I like to crank about 80 RPMs.

If you want to do this test for 7 to 8 hours then you can do it on paper. You can calculate a 1/4 HP generator running for 8 hours. How much power will it produce? What size battery will it charge? That can be calculated but you still need to build it and test it to see if a person can really do it.

If I were going to build this I would put an 8 speed sprocket set on the generator with a derailleur just like a real bicycle. The preson doing the pedaling can start off in low gear and shift gears to pick up speed. I would also put a flywheel on the generator to maintain a constant RPM.

Still don't get you sorry.

Do you need a mechanism which will make it easier to pedal and generate power at the same time? If so, then you are definitely going to lose out on the amount of power you generate.

Sorry to say, but this is almost impossible.

Hi,

Thanks for all of the comments. Sorry I was out of town and was not able to clarify my problem earlier. I will try to clarify now.

Setup:

I have a bicycle attached to a resistance trainer (with the resistance unit removed). This makes the bike stationary. The bike's rear wheel spins a cylinder which turns a motor/generator shaft. The motor then connects to a charge controller and then a battery. The cylinder has a diameter, d, and is a design variable. For every one turn of the rear wheel, the cyclinder spins 2.1/d times. The output voltage of the motor/generator is given by

V = 3.23*v/d

where V is the voltage in (Volts), v is the bike's "velocity" (mph), and again d is the diameter of the cyclinder. In order, to get a "good workout", I would ideally like the resistance to vary similar to that of the road. Typical resistance units have a parabolic power curve given by something similar to

P = 300*(v/25)^2

where P is the wanted power curve (Watts) and v is the "bike's velocity." Since the mechanical power needed to overcome resistance of the motor and bicycle resistive forces are small, I count them as negligilbe. All of the power should be electrical. The electrical power is given by

Pe = V^2/Req

where Pe is the electrical power (Watts), V is the voltage (Volts), and Req is the equivalent resistance (ohms). The Req will come from the resistance of the battery and charge controller (or whatever else is connected electrically to the motor).

Average avid cyclists can average about 250W with peak workout powers of about 500Watts (or at least that is what I would like to design the system for these specs). This where I get stuck.

Problem:
A person producing 0~500Watts (with the velocity given by v = 25*P/12), voltages also create a wide range (0~sqrt(500*Req)Volts). My charge controller only wants voltages in the range of (10~15Volts) otherwise it shuts off. The power being overcome when the controller shut off is pretty much only the small mechanical resistive forces. So when the controller is off, I am not getting a workout. Secondly, my energy is not being stored at this point. I would like to have my energy stored and get a workout in as much of the velocity range (0~32.3mph) as possible.

Any suggestions/corrections to my thoughts would be greatly appreciated.

Thank you

Hmm..

The best solution which comes to my mind is to use a gear system to reduce the amount of voltage produced. Since the Voltage produced by a generator is directly proportional to the speed, you could try using a 4:1 gearbox. I think you might get a surge in the current though, so make sure your wires are thick enough and your battery can handle it.

Can someone else confirm this?

Mithrandir2008 said:

Hmm..

The best solution which comes to my mind is to use a gear system to reduce the amount of voltage produced. Since the Voltage produced by a generator is directly proportional to the speed, you could try using a 4:1 gearbox. I think you might get a surge in the current though, so make sure your wires are thick enough and your battery can handle it.

Can someone else confirm this?

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No, reducing speed will reduce BOTH voltage and current - however, I'm dubious about his imagined power output anyway, I would suggest he connects it up and measures what he gets. If he uses a car battery I doubt he could generate enough power to cause it any problems.

Cyclist82 said:

A person producing 0~500Watts (with the velocity given by v = 25*P/12), voltages also create a wide range (0~sqrt(500*Req)Volts). My charge controller only wants voltages in the range of (10~15Volts) otherwise it shuts off. The power being overcome when the controller shut off is pretty much only the small mechanical resistive forces. So when the controller is off, I am not getting a workout. Secondly, my energy is not being stored at this point. I would like to have my energy stored and get a workout in as much of the velocity range (0~32.3mph) as possible.

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What kind of batt were you charging here?
You could build a DC/DC converter to make ~12v regardless of speed, but you may need to replace your charge controller entirely. See, a normal lead-acid, nimh, or lithium charger has a state to it. If you stop pedaling for a bit, it may reset the state and thus keep erroneously entering the bulk charge state initially. Also the charger will operate at varying power levels based on the battery's scheduled plan, which won't agree with pedal output. So extra pedaling over what the charger needs won't charge the batt faster, it'll only reduce the resistance as you speed up (rotational work is rpm times torque, so for a fixed load, more speed is less resistance). That may not be optimum since you'd rather charge based on what you can actually produce.

To overcharge a typical 40 AH car battery or even a deep cycle battery that is new or very near new you would have to supply around 16 - 18+ volts at 20+ amps for many hours to even begin to start to do damage.

I seriously doubt you and five of your fittest friends could do that even if you took turns! I would be skeptical that you could even get the battery to even be slightly warm to the touch in that time frame.
I rather doubt most people could even fully recharge a standard issue 18 volt 2.3 AH cordless drill power pack in under an hour without numerous stops along the way and without much complaining as well!

People just dont put out any where near the energy that most people think they do. If you can get someone who can even do 100 watts per hour for more than 2 hours non stop they are in extraordinary cardiovascular condition!

Amazing just how efficient biologicals are, huh?

Just a quick question.. have you actually tried peddaling a bicycle with a dynamo attatched?

There is a reason why cyclists prefer to use battery powered lights, can you figure out what it is?

I don't know enough about batteries. If I use a regular car battery connected directly to the generator with a blocking diode, will the battery be damaged by the low voltages? Also will the battery still be charging during the low voltages? Is there a particular battery that I should use?

I do believe my power estimates are different do to the set of cyclists we are discussing. The cyclist I am more designing for are the amateur racers. In a recent 10km indoor time trial, the best cyclist averaged 383Watts for 15 minutes. In fact, about 43 out of the 57 people averaged over 200Watts. The results can be seen at <http://www.w8ds.com/RaceResults.php?RaceDate=2009-12-13>. As for longer races, many of racers can average over 22mph for 25miles (see results at http://www.speedy-feet.com/races/2009/1018/Crank.htm). The best I can figure (using the bike calculator found at Bike Calculator with a bike weight of 20lbs and a rider weight 180 (my weight)), a speed of 22mph corresponds to about 276Watts for a little over an hour. I do believe that a typical rider can probably only produce about 100Watts. I guess the numbers I was refering to were for a particular subset of riders.

Thanks

LED lights aren't so bad, it's like adding a very minor uphill slant to everything, at least of all the mechanicalys and electrical things are well designed.

The Mad Professor said:

Just a quick question.. have you actually tried peddaling a bicycle with a dynamo attatched?
There is a reason why cyclists prefer to use battery powered lights, can you figure out what it is?

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Well, I figured the problem with those old dynamos we had when I was a kid was also that they're inefficient. Didn't they have gears inside? I never got to take one apart.

But anyhow, yes a person REALLY trying can sustain somewhat over 200W for some time, but not all day- but it sound like QUITE a workout. How's this supposed to work, anyways? No one is going to bike-generate for over an hour a day. It'll kill ya.

Hmm, I do think people would prefer to do a "maximum" workout within an hour as opposed to a more leisurely 1.5hr lower-output workout. But, that's where the charger is useless, because it won't adjust the load to suit the user's output. You have to have a DC/DC converter either way, better to just run it straight into the battery with some basic monitoring and current adjustment capabilities.

Be aware of losses. A FWB rectifier alone has a forward drop of 1.4v and is thus a 10% loss of power to begin with for a 12v batt charger (~14.4v). A synchronous rectifier with switched MOSFET transistors is much more complicated, but it'll provide extremely good efficiency.

Cyclist82 said:

I don't know enough about batteries. If I use a regular car battery connected directly to the generator with a blocking diode, will the battery be damaged by the low voltages? Also will the battery still be charging during the low voltages? Is there a particular battery that I should use?

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No, the battery will not be damaged by the low voltages. It will only charge when the voltage is above the batteries; the higher, the more current. Any voltage below that will cause a negative differential, which would normally cause a negative current except the diode is preventing that. The circuit is pretty much in a "stalled" state at that point and does nothing.

smanches said:

No, the battery will not be damaged by the low voltages. It will only charge when the voltage is above the batteries; the higher, the more current. Any voltage below that will cause a negative differential, which would normally cause a negative current except the diode is preventing that. The circuit is pretty much in a "stalled" state at that point and does nothing.

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Well, without a DC/DC converter, what you'd get is below a certain speed=no load, then at a certain speed, with a strong low-resistance generator, you'd hit a "brick wall" where more work gets more resistance but no more speed.

Which isn't necessarily a bad thing because humans need to pedal at about 90 rpm for trained cyclists, and 60-70 rpm for non-bikers to get optimum leg output. Faster wastes leg energy and less means that the leg will have to pump against too stiff of a resistance load to generate the same power (half the rpm requires twice the torque to generate the same power). Maybe at cool-down, a person should have a reduced load at lower speeds, but at "power", it needs to be ~90 rpm.

A DC/DC can change the generator output to whatever the batt needs. Also, how does a car alternator work? It's always ~13.6v over a huge rpm range! Well, that doesn't have a fixed permanent magnet making the field. It has a separately excited field coil- an electromagnet. The voltage produced is proportional to the magnetic field strength, so it can adjust voltage through that field winding. It takes a number of amps IIRC to run the field winding, but the driver's not a linear reg, it's a DC/DC chopper. The field winding spends much of its time with its terminals shorted together which, in an inductor, causes the current to circulate and only decays as per the I^2*R losses in the winding. As such it doesn't actually require that much power to run.

In fact a car alternator, with a regulator, might be a good starting point for you, although that falls short of the "90 rpm regulation" requirement.

For some more information on pedal power checkout the link below, a few guys have made their own using the F&P washing machine motor.


TheBackShed.com - Pedal Power


Cheers Bryan

I think the way to go is to use a car alternator, I have used an exercise bike and alternator to power a TV, fan and phone charger. It worked pretty well, but it was hard work. Instead of using the alternator to charge a battery, then feed that power into the grid, it would be more efficient to feed the power from the alternator directly into the GTI. I have a 250Watt GTI and tested it's power characteristics - input voltage 14V-28V, 20Amps input max. The GTI ramped up the current it draws slowly and levels off dependant on input voltage. at around 14V it draws up to 2Amps or so, 16V draws 6 or 7 amps etc.

This should work ok with an alternator with just the three phase bridge and no regulator.

When I was in the Antarctic, as part of a medical program each of us had to pedal as fast as we could continuously on an exercise bike for five minutes (to exhaustion).
The best any of us could do was one half horsepower continuous for five minutes.
I am told an Olympic cyclist can reach three quarters of a horsepower for short periods.

The guy that crossed the English Channel in a pedal powered ultralight aircraft (The Gossamer Albatross) could hold three quarters of a horsepower for about twenty minutes while training. And he was not a large man either.
The flight required 0.4 Hp sustained for almost three hours. A huge achievement, very few very highly trained athletes would be capable of.

Gossamer Albatross - Wikipedia, the free encyclopedia

An average untrained unfit and overweight tourist, will be lucky to do even one tenth of a horsepower over half an hour, and probably far less. And he/she would be totally exhausted at the end.

The power you will get will just not be worth the trouble.
I suggest you spend your money on solar photo voltaic panels.

Even a very small solar panel will create far more power than you will ever create on a bike !!
And it will work all day long.

Warpspeed said:

Even a very small solar panel will create far more power than you will ever create on a bike !!
And it will work all day long.

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A very small one certainly wouldn't

An average fitness male can supposedly sustain 60W or so from a bicycle generator - which if he only managed it for an hour a day is going to require a pretty decent size solar panel to beat - and a country like Australia