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Discussion Starter #1
Hey guys/gals!!

I'm a teacher and our students are competing in a engineering competition this spring. They are working on a photovoltaic cannon. I'm "just" a 4th grade teacher, but I love science stuff, so I'm calling on you all for help. The object is to propel a ping pong ball as far as possible using only 30 seconds of light. The size restrictions are 30 cms cubed. There can be no other type of potential energy involved (wound-up springs, pressurized tubes, etc).

They got a good solar panel that can produce about 7 volts but only about 400 milliamps. They are working on making a motor wind up a spring with the 30 seconds of light energy, but there is not enough electricity to do so.
They are also trying to make the motor pull down some type of trebuchet (catapult).
They are also trying to hook the motor to a fan that shoots the ball out a tube of some sort. This is what they are currently working on.
Is there some type of electronic device that stores voltage until a certain voltage is reached...like some type of capacitor/relay thing?!? So you could keep building electricity then after 20ish seconds, release it all at one time somehow?!?

Any other ideas?
 

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Proud American
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You can use a rechargeable battery or better yet a capacitor to release the electron flow ( amperage) after it stores for 30 seconds...
 

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You're only getting 84 Joules of energy from that solar cell.

What type of lights are being used? Are they LED, CFL, or incandecent? I doubt they're CFLs b/c those take time to get to full brightness and it would be a significant percentage of the given 30 seconds.

If it's LEDs, you're vey limited on what you can do beyond a solar panel. But good thing with LEDs is that they are VERY directional so you can maximize the energy converted.

If it's an incandecent light bulb (I'm going to assume this is the case), there's a BUNCH of things you can do. The disadvantage is that light is emitted in a spere around the entire bulb.

First, what's the power rating on the light bulb? Also, what's the efficiency rating of the solar panel (most likely it's in the range of 10% or less)?

Personally, I would use the solar cells for light as PART of the project. HEAT is another big byproduct of incandecent lights. Using a liquid w/ a low boiling point can you can easily convert that liquid into pressurized gas.

But that might be a little advanced for 4th graders. If it is an incandecent light, you can use aluminum foil to reflect more light towards the solar collector. More light means you'll get more energy.

If you're wanting to store energy, use a capacitor (rechargeable battery won't have enough energy). Are you allowed to put it on a manual switch? Or do you start the test and everthing must be automatic (no touching anything)?
 

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I would try to use a capacitor to store as much juice as possible, then see how big of a solenoid you can trigger. Use the solenoid to physically push the ball out of the tube. Size the tube a bit larger than the ball so there is no friction or pressure differential build up. You could use a trunk pull solenoid (if you generate enough juice) and then you could play with lever arms to take use of the pulling power, but counter act the short stroke.
 

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Discussion Starter #6
Thanks for the replies. I'm the fourth grade teacher, but the competition is for the high school kids.

I'm not sure what type of light they get for 20 seconds, but I can find out.

The tough part is the size restriction. 30 cm cubed. Aound 7" only in a cube...pretty small.

From the previous competitions, if you can get the ball out of the cube after 30 seconds, you are doing pretty good :)
 

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Discussion Starter #9
So we worked on the cannons again tonight.

I have to specify that the cannons will be set on a table 3 feet off the ground. A measuring device will be on the floor and measure how far they go.

One group was working on a fan powered one. That one could get about 16" distance from the tape measure on the floor.

The other group had a pulley on a motor pulling a string hooded to a square piece of wood mounted with a pivot...pretty much a catapult. To build up speed, there was slack in the string. Once the string became tight, it snapped the piece of wood and whacked it off its little stand. It ended up going about 50".

The only problem is that we are not storing up any energy. We are hooking the solar panels directly to the motor. No capacitor, not fancy anything...
 

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So we worked on the cannons again tonight.

I have to specify that the cannons will be set on a table 3 feet off the ground. A measuring device will be on the floor and measure how far they go.

One group was working on a fan powered one. That one could get about 16" distance from the tape measure on the floor.

The other group had a pulley on a motor pulling a string hooded to a square piece of wood mounted with a pivot...pretty much a catapult. To build up speed, there was slack in the string. Once the string became tight, it snapped the piece of wood and whacked it off its little stand. It ended up going about 50".

The only problem is that we are not storing up any energy. We are hooking the solar panels directly to the motor. No capacitor, not fancy anything...
Are you allowed to push stuff off the table? If so, go with the trebuchet. That's 3' of potential energy. Use the light, to drive a motor, that will push the counter weight off the table.
 

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King Trashmouth
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From my experience, engineering projects are all about bending the rules.

That said...I'd say your biggest thing to look into is about "storing" energy. See what you can do about loopholes such as storing potential energy by means of a screw drive to compress a spring, dropping weights, or using an RC circuit. Also look into the size constraint, does that mean nothing operating in it can go outside that dimension? We engineers are pretty evil when it comes to this stuff.
What are constraints on your mechanism being "cocked" or consumables?

But here's more outlandish ideas-
I like the phase change idea. LED's and R141 anyone?
I'd like to see an RC circuit that discharges to a voltage transformer and then a linear solenoid. Pretty evil stuff.
The problem with rotary motors would be their need to spool up. See if you can get a long armed wheel spinning before it strikes the ball.
For fans, the key would be harnessing the flow. Think converging diverging nozzles.

Granted I know high schoolers aren't going to be that great with exponential equations and compressible flow.

Anecdote time.
In high school, one of our projects was to launch a dog retrieval dummy 60yds. We had a weight and size restriction, and had to be able to reload in a certain time frame. One group got a bunch of surgical tubing and made a slingshot. Another made a catapault.

...and I built a shoulder fired rocket launcher. Basically an AT4 that fired dog retrieval dummies. Even painted it camoflauge. Put in all the work as far as trajectories and air flows. :potstir

The result? Other groups barely achieved the 60yds. The rocket launcher? Just over 200yds. Might have also dented some aluminum bleachers in the process. :llama

The lesson? Find loopholes, and go for overkill.:cool:
 

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put a 1 liter or 2liter bottle at the base and find a way to harness the solar to a motor to drop mentos in diet coke and screw the lid back onto the bottle and you will send that ball flyingggggg, the cannon however may go flying with it....haha
 

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Discussion Starter #13
There can be no potential energy stored at all. Also, nothing except the ping pong ball can leave the 20cm cube.

According to the teacher that went with last year's class, the winner's ping pong ball rolled out of the cube and they won...yippie :eek:

With our going 50ish inches, we should win. Next year, we will work on storing the electricity, then releasing it somehow
 

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King Trashmouth
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If there's no energy stored per se, assuming you can't use capacitors...
...which I wouldn't consider a capacitor a storage device like a battery, but hey, like I said, loopholes.
I would say use a small lightweight DC fan into a converging diverging nozzle to maximize flow velocity, and then route it into a tube with the ping pong ball.
Just my thought.
 
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