How to design a solar power system and where to find info?

[MrCurry]

I read some of the previous threads on solar and they didn’t really answer my query so thoughts might get away with starting a new one!

I don’t know much about solar power setups, I’ll put that out there first, but I’m having trouble finding good info which can help me figure out how to design a system for my application, which I’ll describe below. This is all probably a bit specialist, so what I probably need is a pointer to a solar power web forum filled with geeks who might relish this challenge. I’ve tried a few Facebook groups and got nowhere.

I want to run a ventilation fan for at least 18 hours a day. The fan runs on DC, between 16 to 28V, and right now runs on a DC power supply at 20V, 250mA (5 watts). I need it to run for 8 hours or so on batteries after the solar panel is no longer providing power. I don’t know how to approach sizing the solar panel, batteries and charge controller for this. My best attempt is to say 5W for 8 hours is 40Wh of battery capacity, which could be a 2Ah 20V battery pack. To charge this within, say, 6 hours of sunlight needs a solar panel which can provide about 15W (5W to keep the fan running and 10W to charge the batteries), and maybe I should double that to 30 or 40W to take into account lower production on cloudy days. So the solar charge controller needs to handle 2A (40W/20V)?

If anyone can correct my sums I’d be very grateful!

Apart from “what parts do I need for this and how do I put them together”, I guess my biggest head scratcher right now is how do you deal with the different amount of solar power available through the year? In winter here in Sweden it’ll be getting dark from 3pm and during the day the strength of the sunlight will be low. In summer it’s like a bleeding greenhouse and doesn’t get dark until midnight. So, do you design the system to be big enough to cope with lower levels of winter sunlight, then what do you do with all that extra power in the summer to stop it frying your batteries? Where does the extra power go? Or do you size things to work ok in the summer and have a mains powered backup system which kicks in and takes over in winter when there’s not enough sunlight to charge the batteries. How does that work? Where do you connect your backup power supply and what device controls switching over to it - needs to be automated.

All the solar charge controllers I’ve seen have connections for the solar panel, the batteries and a dummy load. Where and how would you connect a mains PSU for taking over when the batteries run down (in winter)?

 

[two sheds]

Sorry not to answer your questions but:

The fan runs on DC, between 16 to 28V, and right now runs on a DC power supply at 20V, 250mA (5 watts).

Why not keep going with this? Any solar system is going to be a lot more expensive and so have near infinite repayment times if it's working ok now.

 

[Saul Goodman]

What will you be powering your grow lights off?

 

[MrCurry]

Sorry not to answer your questions but:



Why not keep going with this? Any solar system is going to be a lot more expensive and so have near infinite repayment times if it's working ok now.

You’re probably right about the payback time. I guess it’s a mix of reasons. Part wanting to lower our carbon footprint, part OCD about wanting to keep improving (lowering) our electricity bills each year and running out of “low hanging fruit” to keep making improvements with, so looking to more elaborate things like this solar fan setup.

What will you be powering your grow lights off?

Hehe, it’s not a fan for weed growing. Just general house ventilation. We have a central extract fan and duct system into the bathrooms and kitchen. Last year I swapped out the old fan for a modern brushless DC motor variant, which is why it’s only 5W.

 

[Nylock]

If you're looking to run year-round and independent of the grid, you need a set-up that is designed to work on your worst performing days and have a battery bank that's big enough to cope with the extra charge on your best days. Have a mains inverter connected to the bank and use its output for charging devices when you are over-producing (or have something else that uses electricity on your bank to use as a power sink (if you have a pond, put the water pump on it for instance)). Or have your panel on a tilt/pan system that just moves it out of the direct sunlight when the batteries are charged. It looks like you could get away with a leisure battery setup (24V/110AH*), charge controller and 40W panel... If you want to use an inverter, a 500-750w true sine wave one would do the job for charging several devices with...

Alternatively you can do a grid-fallback system where you charge your batteries from your panels and, once they are charged, dump the surplus into the grid via a grid-tied inverter... But those setups are very expensive and are only really effective at a scale bigger than your current needs...

*if you are running lead acid type batteries then you will always need at least twice what your maximum requirements are as these batteries start to suffer if you drop below 50% charge on them. The more often you dip below 50% the more damage they accrue...

 

[MrCurry]

Thanks Nylock I think you’re right, a grid-tie system is going to be out of the question on cost grounds. I guess I’m hoping to pick up a cheap panel and one of the temptingly-cheap eBay charge controllers and then make a bank of some old lead acid car batteries, which I can probably score for next to nothing. I will keep in mind the point about avoiding deep discharge.

I’ll have to think some more about what excess load I could give the system during summer, because we don’t seem to use much leccy through the summer here. I’d also like to search out whether I can do the opposite of sizing the system at a compromise which works most of the year and then only in the dark winter, switch over to running my fan from the mains again. This seems to me a way to minimise the cost of the whole setup, as my batteries, solar panel and charge controller could be that much smaller than if it’s all sized to work ok with the brief hours of weak winter sun. In any case, I probably need some kind of mains-powered fallback to cover the eventuality of the solar panel being covered in snow for a few weeks, as could easily happen.

 

[Nylock]

If you're going to score some lead acid batteries MrCurry , go for leisure batteries (like you would have in a caravan or camper). Car batteries are designed for short duration high current use (starting your car), not for sustained discharge. They have thin plates and no endurance. They'd be a false economy...