20v solar panel powering two 12v fans and a 6v pump (swamp cooler for burning man)

Question

Building a solar swamp cooler for burning man, how can I use a 20v 0.8a (in full sun) 15watt solar panel to power TWO 12v 0.4a fans, AND a 6v 0.1a water pump?

One of the fans and the water pump have their own mini solar panels (that don't work very well), any way I can connect the solar panels in series?

What components would I need (and where would I get them) to make this work? This apparatus would ideally function only in full sun (in a remote Nevada desert)

Sorry, I'm a newb at this, and thanks for any insights!

Answer 1

I wouldn't recommend connecting the panels in series in case the 20V panel pumps too much current through the ones that don't work very well. To decide how to connect them (if it's possible) details of those panels would be required.

However, for the 20V panel I would say you need a switch mode buck regulator that takes the 20V and reduces it efficiently to 12V to power the two 12V motors. At 0.8A total draw from the two 12V motors the power requirement is 9.6W. The panel and regulator should be able to supply 90% of 20V x 0.8A = 14.4W.

This leaves about 4W (conservatively) for the 6V motor which you imply requires 0.6W. You'll need a second step down buck regulator for this device and it sounds feasible.

Switch mode buck regulators - there are tons of them but I can only recommend a design type such as the LT3976: -

For a slightly more efficient regulator there is the LTC3780 which will work providing 12V down below 12V from the panel: -

Both regulators can supply 12V at 5A.

I wouldn't bother thinking about a linear regulator because it will waste about 8W in heat.

It's possible that you can search along these lines and get some kind of module/PCB that you don't have to build. Maybe someone can recommend something?

Answer 2

Some rough power calculations using Ohm's Law (P = IE):

Solar panel, 20V 0.8A: $$20 * 0.8 = 16W$$

Fans, 2 at 12V 0.4A: $$12 * 0.4 * 2 = 9.6W$$

Pump, 6V 0.1A: $$6 * 0.1 = 600mW$$

Total power needed: $$9.6 + 0.6 = 10.2W$$

So far, so good. You have enough power to drive the loads. However, you need to convert 20V to 12V and 6V. Voltage conversions are not 100% efficient, so you will lose some power doing so, but you have about 5W overhead so this should not be a problem.

One way to lower the voltage would be with inexpensive linear voltage regulators, but these are not the most efficient, and many have a maximum input voltage of 18V.

That being the case, I would look for a DC-DC Buck converter. They are more expensive, but offer higher efficiency because they use switching instead of converting excess voltage to heat.

You can build or buy (such as this $5 module on eBay). I am not sure I would trust the eBay units, so I would be interested in building my own. However you might try your luck given the low price.

Keep in mind that the solar panel output will vary (with cloud cover, sun angle, etc.) so you might want to consider using batteries as a buffer to ensure that you aren't trying to operate things with insufficient power (for example, if someone shades the panel). Another option might be to add a circuit which disables the output if the solar panel isn't producing enough. There are lots of resources on the internet about charging batteries from solar panels, so I won't go into detail on that.