I am a grower who is finally taking brave excursions into the world of electricity and electronics. We are building a number of greenhouse structures with thermal mass storage that will need electronics to turn on fans or water pumps when the temperature reaches a certain temperature. I
have found and ordered a few fans and thermostats, and am wondering the best way to run them. We are looking at solar, as the fan will basically be needed only when the sun is shinning, but I presume the thermostat will need constant power, so we would need a small 12v battery to run this. They will run at under 0.5 Amp (1/2 amp) per hour. So that should be 6W of power an hour? If we get a 10W solar panel, would it be enough to keep the fan running? I am guessing we would need a separate solar panel/circuit for the battery to be recharged than for to run the fan/water pump. If we get a little 1.5W panel for the battery, is it bad for it to have excess power constantly flowing into it?
What kind of fan are you using? A half amp 12v fan doesn't move much air so I am assuming these are smaller spaces.
A 10w solar panel in a good position should be able to power a .5 amp 12v fan.
They can all be on the same circuit. A solar panel connected to a charge controller connected to a battery. Then from the charge controller a snap switch connected to the fans. The battery is there to buffer the charge on partly cloudy days or hot days in the spring and fall. It is bad to have power constantly flow into a battery. The charge controller will stop charging a full battery and stop providing power when the battery is low.
Here is a $15 charge controller. I have had one of these running continuously in a project for two years now.
If you have water pressure solenoid valves can be powered by the same battery. Without water pressure there are 12v DC pumps that can be powered on the same system. This is one I've used. It will pump a gallon a minute with 3 meters of head at 4.2 watts. (Still assuming this is a small space.)
I haven't looked into over the counter timers or controllers for pumps. If all you want to do is fill a tank that gravity feeds the plants that would be easy enough with a float switch. If the pump only runs occasionally a 15w panel should cover the fan and pump but a larger charge controller might be needed.
I like snap-switches for controlling fans. They open at a set temperature. They don't require electricity to operate.
I like the hydraulic openers that will do things like open a window when the temperature reaches a certain point, and close it when it cools down. A simple pressure switch could be installed on the window frame to operate fans/etc when the window opens.
For electric heating, I really like the thermostats made for operating baseboard heaters.
Batteries are like swimming pools. You can add to them and take from them at the same time...
I have used snap switches for several applications like this, fans.
They are inexpensive, straightforward and widely available.
Make sure the switch is rated to carry the watts of the device that is operated by the switch. If they are not heavy enough, the electric baseboard heater thermostats referenced above are excellent and heavier duty.
I have found that, in the long term (more than a year) they are not super reliable. This is not a deal breaker and there are ways to counteract the problem.
If the application is not life or death (for plants) critical, just have a spare or two, they are cheap. And check regularly that they are working as expected.
If the application IS a life or death (for plants) situation, you can run them in parallel or in series, depending on what you want to accomplish.
Let's say if a fan fails to come on when it's supposed to, you're going to cook $2,000 worth of bedding plants. NOT GOOD!
You would put two snap thermostat switches in parallel. That way, if either one turns on, the fan turns on and the plants are saved, despite one switch failing in the off position.
Or, let's say it's super important that, at a certain temperature, a pump MUST turn off or something gets flooded. Then you would put two snap switches in series. If either switch turns off, the pump WILL go off, even if the other switch is stuck on.
Annie Hope wrote: ... So that should be 6W of power an hour? If we get a 10W solar panel, would it ...
The suggestions above sound great.
I have one small thing to add, kind of nitpicky, though. You will start to understand your system and needs better if you remember that Watts are RATE, not a lump sum. So yes a 6W device should run off a 10W panel while it is getting good sun. It can run at 6W for a minute, an hour or a year. There is no such thing as "Watts per hour."
Sorry about the nitpicking but it will help your clarity.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
I wouldnt skimp on pv watts unless your battery is small or the load just does not justify it. 20w is the line where module prices generally start to get reasonable as far as $/watt. If the fan is 6 watts running and you need power aside from that, more pv is likely a good thing. Altestore has 20w poly modules with glass covers and aluminum frames for $20 from time to time.
I would add a caution, based on my own experience with a solar greenhouse. I have gone a bit beyond what you are doing, and set up large tanks of water along the North wall as thermal mass, with solar-powered pumps to pump the water through salvaged car radiators in the peak of the greenhouse, and fans to blow the warm air through the radiators. This is all powered by a 90 Watt, 12 Volt solar panel. Turns out a 12 Volt panel actually puts out 19.5 volts, (in order to have a higher voltage than a 12 volt battery so it will actually charge). As best I can determine, the little centrifugal "solar" pumps widely available from China on eBay for 10 or 15 dollars, can run directly off the solar panel. But the over-size 12 volt computer cooling fans I used to blow the air through the radiators burned out in under a minute when connected directly. It is essential to incorporate a voltage regulator in the circuit, (available for $2 apiece from DxExtreme (in China))
I had an additional requirement, as I need the pumps and fans to run only when the temperature in the peak of the greenhouse is higher than the temperature of the water in the tanks. For this, I needed a differential thermostat, which I purchased from a British shop, REUK. And, yes, this does need a steady voltage to drive it, which can be provided either by an A-C transformer, or from a battery charged by the solar panel - but with a charge controller between the panel and the battery to avoid over-charging the battery.