Help me use a solar panel and splice it all together!
August 19, 2013 1:34 PM   Subscribe

I'm in over my head on a DIY project involving electrical terms I don't understand. I want to use a solar panel to power both a water pump and a computer fan, and need to know if this is possible with the components I have, and if possible how can I splice/connect these non-matching wires?

I'm trying to make a version of the swamp cooler described here. It requires a computer fan to move air and a water pump to move water. This tutorial says to buy a solar-panel-powered water pump and splice the wires from the fan to the wires from the pump, thus powering both electrical devices. I don't know how to splice!

Here's a picture of the devices I acquired. I'm concerned that I've put money into a solar panel that might not be powerful enough to meet my needs.

Solar panel: Max power 12W, open circuit voltage 21V, short circuit current 0.773A, max power voltage 18V, max power circuit:0.66A; water pump: input DC 4.5-10V; computer fan: DC 12V 3A. I'm worried about the computer fan's amp number being higher than the short circuit current number for the solar panel. But I haven't studied how amps, watts, volts, etc., relate to one another or work since ninth grade, and it's over my head. How can I assess if this panel is powerful enough to powerful enough both devices?

If the panel is powerful enough, how do I splice the wires together safely? Links to online tutorials would be great.
posted by croutonsupafreak to Technology (9 answers total) 4 users marked this as a favorite
Simple power (in watts) is voltage times current at the operating point:

P = V x I

Your panel delivers 12 W, but your fan will need about 36 W. There's your first problem.

I can't tell you anything about your pump because you haven't provided a rated current. Is the pump a variable speed pump? (I'm assuming it is, since a voltage range is given.)

In any event, your solar panel is woefully undersized. Keep in mind that outputs for solar panels are usually in Wp (watts-peak), which is the absolute maximum power assuming standard testing conditions ("STC", irradiance about 1000 W/m^2 and with a panel temperature of 25 °C). You would be have to be either in the desert or extremely lucky to get those conditions with any consistency in practice. Even then, most panels will underperform their datasheet rating.

Now, if you had a real solar panel, like this one, say, you'd be talking (195 Wp!). But I suspect that if you're trying to build a swamp cooler from instructions you found on the Internet, this will be outside your budget.
posted by rhombus at 1:55 PM on August 19, 2013

The solar panel provides a max of 12 watts, at 18volts.

Volts times amps = watts.

Your fan uses 36 watts. (12v * 3A) This is three times what the panel can put out. The water pump.... you didn't list a draw for that, but lets say it's 3 amps at 10V, then it's 30watts.

So, you need 66watts to drive this whole thing at full power. But your panel only proves 12.

It might work OK, but probably not.

As for splicing wires, there are a number of techniques - the most basic is to just strip the insulation off the wires and twist them together. Put some tape on them to keep the separated. There is no harm in doing it backwards - the fan will blow the other way.

This seems like a fun little project and although it might not work as you expect, you may learn something. Hook it all up and see what happens. The voltages and current you are working with are harmless enough, so... have fun!
posted by Pogo_Fuzzybutt at 2:00 PM on August 19, 2013 [1 favorite]

Are you sure that's not 0.3 amps on your fan? There's no way that tiny thing pulls 36 watts.
posted by LowellLarson at 2:02 PM on August 19, 2013

Response by poster: Cool. So if I strip and twist the wires and it doesn't work, I might have better luck with a wimpier (lower amp, perhaps) fan? This is the fan and this is the solar panel/pump combo, if it helps.
posted by croutonsupafreak at 2:05 PM on August 19, 2013

Response by poster: It may be time to cannibalize an abandoned desktop computer to see if its fan will do the job. Hmmm.
posted by croutonsupafreak at 2:06 PM on August 19, 2013

Wow. I was wrong, it does pull 36 watts. My best guess is that it will run just fine, but slower, on less than that. I don't actually know what will happen if you wire both the fan and the pump to the solar panel. My best guesses in no particular order are: 1) it'll work, 2) it won't make enough power to drive both, or 3) the 20 volt output from the panel will fry the water pump. The most similar thing I have done is to direct wire an auto radiator fan to a 15 watt panel. That works well, speed varies widely depending on sunlight, but it takes good direct sun to get it started spinning.
posted by LowellLarson at 2:13 PM on August 19, 2013

Response by poster: Well, stripped the wires on both the pump side of the solar panel-pump combo and on the fan, and twisted them all up, and nothing happened. Then untwisted the fan and just had the solar panel connected to the pump, and now that doesn't work any more either. So I'm in worse shape than when I started.
posted by croutonsupafreak at 3:32 PM on August 19, 2013

Intuitively, I think you would be better off using the solar panel to charge a battery to run the fan and water pump. That way your swamp cooler would work when the sun doesn't shine. Downside is that you should really use a charge controller (~$35) to prevent overcharging the battery.

This article on swamp coolers has a section on Battery Options and Solar Energy that may be useful. It also points to a $25 solar powered fan that would solve part of your problem at least.
posted by Rob Rockets at 4:36 PM on August 19, 2013 [1 favorite]

The bigger fans often run fine at a lower voltage (substantial reduction in watts required as well) but can be risky if there's a stall.

There is absolutely nothing wrong with underpowering most of these sorts of fans as long as you are mindful of what happens when it goes too slow and stalls (or isn't able to start and stalls), both of which might be bad in unattended operation. A stall may put a motor in a state where it is consuming a lot more power, causes it to heat up, burn up, and/or may not be able to start itself at voltages that would normally cause it to start functioning. Your panel isn't capable of the more damaging of those sorts of things because there just isn't much power to begin with, but if and when you try something else, "good stuff to know."

Solar panels can be finicky about what happens when you abuse their outputs. Whoever has designed that panel/pump package has probably relied on the relative wimpiness of the panel to offset the fact that it is putting out more voltage than the pump was designed for. While I can't guarantee it, past experience with similar kludges suggests that if the panel can never put out more watts than the pump wants, and the voltage output isn't too far out of spec, it'll be fine. So my guess is that the panel is just barely big enough to cause the pump to work appropriately.

As others have suggested, consider a battery. You can then treat the problem as two separate issues: a 12 volt powered swamp cooler, which can be powered by an AC wall wart that outputs 12V DC or a deep cycle or SLA battery of some sort. 12 volts is quite common. Then you need a solar battery charger for a 12V battery to handle solar charging.
posted by jgreco at 7:41 AM on August 20, 2013

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