So I am going to one of these huge music festivals next month. 4 days of the hottest Tennessee sun you can find. Every year we go as a big group. And I've been taped by our peoples to build an Air conditioner.

So the plan was to Get an evaporator coil and blow some air through with fans powered by the sun. SO i find a Huge Coil and have it all ready to go, Pick out some fans that will work and then look to buy a solar panel and it is way out of budget. Then one of the members of our group surprises me and buys a panel.


So I have This
http://www.amazon.com/Silver-Frame-Mono-crystalline-battery-Charger/dp/B000HJXQD2/ref=pd_sim_auto_77

Witch should be able to power 2 of these
http://67.222.45.157/~conceptw/catalog/images/THBPF01G box front.png

I hope. HELP!!!!


My math for the whole project is something like this.

DC input for the fans is
12 V
.7 A

From what I calculate this gives us a system where one fan is
17 ohms

and will require
8.4 Watts

so a 20 watt panel running at 80% efficiency plugged directly into 2 fans will work??

BUT the data on the amazon page says that "The working voltage for charging is 17.2V" so how am i going to hook up these fans to work with that number?

From what i under stand a motor will run at what ever voltage you give it? or should will i fry a little motor in 4 days off 20 watts?


IDK guys I'm really good at building stuff but trying to get this to work right in my brain seem to be the stumbling block. Please help, I lurked on here all day yesterday and was not able to find anything similar to my problem.

I don't need a permanent solution or really even a good solution. just some thing that will last for 4 days.
Should i stick a 12v battery in the mix just to regulate the push from the PV panel ?


I"ll share some pics of the construction of the unit after work.

 

It would be better to charge a battery with the solar panel and take power from the battery the same time.

However, if you want connect one fan on the solar panel and measure the voltage across the motor. If it is 12V then the motor won't burn, otherwise try both motors connected in parallel. If the output voltage is 12V then they will be fine otherwise remove both and buy a voltage regulator.

 

You won't automatically push 20 watts through the fans. The 17.2 volts will push whatever power the fans will take from a source at that voltage. Actually, as the current through the fans increase, the voltage from the solar-cells will decrease, and the power to the fans will not be a direct function of the resistance you calculated.

There is a voltage-current curve for the fans (that you may be able to find) that you could correlate with the voltage-current curve for the solar-cells and find the operating point graphically, but it might just not be worth the trouble.

What you might do is put something to block the sun from the panel completely, hook it to the fans, then expose the cells gradually till the voltage goes up to 12V. This is probably the easiest way. But the time of day, and every cloud will get a corresponding response from the fans until you change things to match.

If you are also using batteries to buffer the power (you probably should), you can control the battery charging rate the same way. Batteries will allow you some leeway... If the sun angle changes, or clouds, or people pass in the way, the batteries will give you plenty of time to make adjustments. They will also allow you to just set the charge rate slightly higher than what the fans use, and just let the batteries dissipate the excess charge (as heat, thus the word slightly).

How are you powering the compressor?

 

How was the 20 W calculated? Multipling open ckt. V by short ckt. I gives an inflated power , if at peak power with an 18V panel, should have about 13V @ 1.7A. Two fans might be OK, at least long enough to take measurements. Are you going to run ice water thru coil? Might use smaller fans and give ea. member a fan, and drink the ice water.

 

How was the 20 W calculated?

The mfg. info said 20W, which would probably be "best case" (Mountain top, in the desert, on the equator, at noon, cold cells, etc).

 

Ok so first off, What was promised.

 

Ok. So Yes it will be cooled by gravity and icewater. Ice will be easier to get then clean cups at this kind of festivity.

Everybody has said use batteries. Ok. that will be in the plan somehow. What about a slightly sulfated lawnmower battery. I'll dig it out and see what it might do. Just has to be a buffer right not a permanent solution to anything.... ??? 4 days ???

As far as voltage regulation. Both ideas may be able to be used in this situation... I will have an inline meter of some sort. weather i need to regulate V ? or A ? All of the people around this thing will want it to run at the optimum it will be like a people powered sun tracking system.
...
OR what about a more precise design??? like an actual variable voltage controller... I know there is some 3 prong thing i could wire up to a variable resistor, But will that last under possibly constant changing in V?


We do have some luck (at least i think so) I like about 3 hours away from this place and right in line with it as far as sunshine is concerned. The guy that got it for us. he lives up north but it will be shipped to me opon arrival... So this will get tested in a few scenarios (and i like taking pics)


Oh and the event is is less then a month.

Thank you guys for helping me.

 

If you put an in-line linear regulator to control the current or voltage, any extra power is gotten rid of by the regulator by dissipating it as heat! Are you sure that is what you want?

If you put the regulator outside where it is hot, you will have to use a big heat-sink, or melt some extra ice, or maybe even put an extra fan to blow on the heat-sink.

If you put the heat-sink in the cool air from the radiator, it will be easier to keep it from over-heating, but at the expense of using up some of the cool.

There are some switch-mode things you could do, and we could spend the next few weeks talking you through it, and in the end it might work.

Seems too easy to only create the energy you need, by blocking off a portion of the cells. With a battery buffering the extra power, you probably could get away with just always running the solar-cells wide open. Even at full power its only about 1-1/2 amps. Subtract what the fans are using and it's almost nothing. A two amp charger for a car battery is called a "trickle charger" and can safely be left on continuously on a large battery. For a lawnmower battery, you may want to disconnect it if the fans aren't being used.

 

Should have looked into 'Swamp Cooler', would have saved you from using ice...

 

Sometimes, when seeking the simplest solutions it would behoove us to look back in time when when Vacuum Tubes didn't even exist yet. What would those early electronics experimenters do?? This is what I think they would have done. It is not a perfect voltage regulator but it should do the job. The hysteresis of the the relay will provide crude but effective charging cycles. The hysteresis of a relay can also be manipulated.

 

Here's another version of the last circuit I posted. This circuit can change the apparent hysteresis of the relay.

 

Ok This bird ain't dead yet. Have all of the pieces together and almost ready to go. Tests of the solar panel and motors will begin sunday. But today i'll post up what i found inside of the fans (for posterity sake)

 

Prediction: You will not need speed switch, nor will you have to woory abouy burning out fans, both will operate at around 70% of rated output in full sun. OK to connect small 12V battery in parallel with panel, with fans running. Wish you luck.