Very small scale solar setup questions

Hey gang! I have what is probably a very simple project but I lack any sort of knowledge of solar panels. My goal is to run probably two 12v cpu fans up to 24 hours a day, using solar panels and some sort of battery. Not sure if a full sized car battery would be required but that's fine if that's what I need. This would be on a small greenhouse type situation, and can get 10+ hours of full sun during the day so charging shouldn't be an issue. 

Looking online I am finding mostly RV setups and stuff that is way beyond the simple setup I need. I don't know what size panel I need, or if I will need more than one. I think I need a charge controller, and the fans can run directly off the battery which is being charged by the solar panels through the controller. 

Can anyone point me in the direction of some good reading on this or offer any advice? Thanks in advance!

Do you happen to know the draw of your fans? That's going to drive the whole thing.

When you say "cpu" fan, what size are you talking about?

A full size car battery is certainly not required, but for simplicity, you'll probably want a lead acid battery with a reasonable capacity, so auto or powersports deep cycle batteries are not a bad place to start.

You won't get 10+ hours of full sun unless you are installing this at the equator.  NC is in the upper 4 hour range average (https://www.nrel.gov/gis/solar.html)

You might also want a battery protector that will disconnect the load to the fans before they completely drain the battery.

Sorry I don't have a good link to read-up on this stuff, but I am following along for suggestions.

I'm probably going to munch some units here, I apologize in advance. Watts and amps and volts are all basically intertwined :)

Google tells me the fans will pull about 1.8 watts each. So you need 86.4 Wh (1.8 x 2 fans x 24 hours) to run them for a day. Your solar panel will have to generate that power over a 10 hour period, so you need a 9 W panel. That's not big :)

You'll also need a battery to run the fans in the dark, but again your draw is so small it won't take much of a battery. Minimum would be enough to provide 50.4 Wh (3.6 W x 14 hours of darkness). That's about 4.2 Ah, which is nowhere near car battery size.

Now, that's assuming you get 10 hours of perfect sun every single day and that you're willing to flatline your battery every night, which is not healthy. If these are mission critical fans, then you can oversize your system to allow for cloudy days or the fact that the nights are longer in the winter.

This looks like it would be an easy plug-and-play option that's more than twice as big as needed for your given scenario and it includes a controller that will care for the battery. Throw in a car battery that's tired and you're probably set.

Keith's recommendation is sound, but bear in mind that the rated output of solar panels is at the equator at noon under perfect conditions. My experience is that if you divide the rated capacity by two, you get somewhere near real life. I've also found that two panels in series work better under suboptimal conditions, so when each panel is only giving 10.5 volts (I.e. no charging happening), combined they give 21 volts, which your charge controller can do something useful with. Just make sure your controller is rated for double the max rated voltage of each panel.

Seems like you could do this fine with a simple trickle charge solar battery maintainer and a lead acid battery. 

Tractor Supply sells them cheap

Harbor freitlght has a 25w panel for 65 bucks, then there the 20% coupon. 

I have the 45 watt kit (three 15 watt pannels, controller), purchased a long time ago,use it to keep the rv, lawn tractor, bike..... batteries topped off over the winter. Works ok. 

Thanks for the great info! I am still in the planning stages here, so I haven't actually acquired any of the fans or other pieces, so I am not certain of the draw. I am thinking of the 4" size computer cooling fans (x 2). I do have an older car battery that while well outside of it's warranty period does seem to charge according to the charger, but is no longer in use because I don't trust it to provide the amps needed to start a car. 

The spot I have in the yard to place the greenhouse does get unobstructed sun for most of the day year round, although I realize that the angle of the sun may not be ideal for solar charging, so I don't mind using a larger than "necessary" battery (or multiple panels in series) to ensure that the fans will run 24/7.

Just a throw away thought as I was reading: Might it be better to have two or three smaller panels, so they can be aimed to catch the sun at different times of the day. 

Or, that would not work as well as all three aimed to catch the noonday sun.

I have no idea, but that never stopped me from asking a question.

Ideally, the smallest solar kit with a charge controller you can find and a 7 amp hour (UPS, etc.) battery.  Cheaper, about any solar panel and the 7 amp hour battery and let it potentially overcharge the battery.

To me those fans seem a bit on the small side for a greenhouse unless is just a tiny one the size of a computer to put hot peppers in. 

At the same time the solar powered roof vent fans I was looking at for mine next year are both expensive and overkill. 

So I'm following along for ideas. 

Are you after air circulation or heat/humidity removal? 

Air circulation they might work, but if you're trying to control temperature or humidity, you're going to need something much bigger.

In reply to RevRico :

In my application, the fans will be used to push and pull air through a 4"pvc pipe that runs under a compost pile, in order to utilize the heat generated by the compost. I don't want to put the compost inside the greenhouse (really just two raised beds parallel to each other with a hoop roof covered with plastic). The compost will be at one end joining the two beds, and the pvc pipe run under with outlets into the center walkway area. Since the compost can produce temps well above 80* in the center, I want to see if I can distribute that heat into the greenhouse to keep the temps above freezing. The idea is really just to extend the growing season a bit, I don't anticipate that I will be able to grow throughout the whole winter period. 

I've done this calculation for a hydroponics system I've set up- but have never actually completed it.  

There are a number of solar system calculators out there that are very similar to what Keith has calculated- but add in a few other factors, like how many days of battery only do you want, and factoring historical sun coverage (weather + angles) to help out winter calculations.

Most of them are found using "off grid solar system calculators".

For super small systems, you can get away with a panel + a battery.  But at some point, you should get a charge controller- so that you don't over or under charge a battery.  Those become the interface for both power and charging, which is pretty cool.  And they can be super cheap.

EastCoastMojo said:

In reply to RevRico :

In my application, the fans will be used to push and pull air through a 4"pvc pipe that runs under a compost pile, in order to utilize the heat generated by the compost. I don't want to put the compost inside the greenhouse (really just two raised beds parallel to each other with a hoop roof covered with plastic). The compost will be at one end joining the two beds, and the pvc pipe run under with outlets into the center walkway area. Since the compost can produce temps well above 80* in the center, I want to see if I can distribute that heat into the greenhouse to keep the temps above freezing. The idea is really just to extend the growing season a bit, I don't anticipate that I will be able to grow throughout the whole winter period. 

Wait, are you in my back yard???  I have the same set up, but....

Here in Michigan, I'm still working on getting a hot enough pile to heat the greenhouse.  And until I get that heat, I'm not adding the fan.  At the moment, I'm trying to use convective flow to move the air.  

Good luck!!!  

Good advice so far, you need to really oversize your panels to be happy with your system. Keep in mind that solar panels are essentially the inverse of LEDs. It means that when you have shading on just a part of your solar array it makes every other cell in series with the shaded portion non functional. Jcamper

In reply to Jcamper :

I believe the general recommendation is to at least not run the panels in series, for the exact reason you just mentioned.

EastCoastMojo said:

In reply to RevRico :

In my application, the fans will be used to push and pull air through a 4"pvc pipe that runs under a compost pile, in order to utilize the heat generated by the compost. I don't want to put the compost inside the greenhouse (really just two raised beds parallel to each other with a hoop roof covered with plastic). The compost will be at one end joining the two beds, and the pvc pipe run under with outlets into the center walkway area. Since the compost can produce temps well above 80* in the center, I want to see if I can distribute that heat into the greenhouse to keep the temps above freezing. The idea is really just to extend the growing season a bit, I don't anticipate that I will be able to grow throughout the whole winter period. 

Why not use a thermosiphon to do this passively?

In reply to ProDarwin :

I hadn't come across info on that, but I'll look into it.

EastCoastMojo said:

In reply to ProDarwin :

I hadn't come across info on that, but I'll look into it.

https://grassrootsmotorsports.com/forum/off-topic-discussion/help-me-build-a-thermosiphon-solar-heater-for-my-garage/144999/page1/

Same concept as that, except your heat source is a compost pile.

In reply to ProDarwin :

Thanks for the link. I think I am going to stick with the solar setup plans for now, even if just for the versatility of using it to top off batteries when it's not used in the greenhouse. I like the versatility of solar.

Just a few notes on solar from my off-grid setup I put together this summer.

• No-name panels are fine, no-name controllers are not worth it.  Victron Energy MPPT is the only way to fly for a controller.
• Plan on at least 2x more rated watts than what you need.  Watts are given in perfect world scenario and even on a perfect cool sunny day I only ever saw around 80% of rated output in Wisconsin.

Thats it!  Super simple, get yourself some components and start building.  I love solar.  Its so cool and quiet.  

Following along.

Jcamper said:

. It means that when you have shading on just a part of your solar array it makes every other cell in series with the shaded portion non functional. 

Like those berkeleying Christmas lights that piss everybody off!! 

is there anyway to know the watts per square foot on used unknown brand panels ?

how bad were 10 - 15 year old  solar panels compared to now ?

The math should be straightforward.  You need enough battery for the total wattage (or amperage) of the fans times the number of hours they would run without charging.  Factor in a good margin depending on the type of battery.  Something like a Li-ion you can fully discharge without much consequence.  A traditional liquid acid/lead battery wouldn't last very long with constant full discharging since it allows the acid to eat at the lead without the charge.

So, for 12w worth of fan(s) running 12 hours at night, you would need about 144w/h.  Larger batteries tend to use A/h as a rating, but it's the same basic principle.  In this example, you would be drawing 1A for 12 hours, so it would be a 12 A/h battery.

Depending on the chemistry of the battery, you would want to add capacity.

When you're just doing a simple setup (like hooking the panels directly to the fans), you would want a panel that makes enough oomph to run the fans.  Period.  At night the fans stop.  When you're doing a battery setup with it, it no longer becomes as much about the size of the appliance, it becomes about the size of the battery.  For instance, I have a monster H8 AGM deep cycle battery I use for remote power at the theater.  It is 95 A/h capacity.  Which means, with a 10A (120W) charger, it takes 10 hours to fully charge it.  Getting 10A for ten hours from a solar panel will take up a considerable portion of real estate.  So you'll want to choose a battery that is enough to supply the fans for the time period, but not way overkill.  The panels you then choose to be capable of recharging the batteries (and then some) in the daylight time you have.