critique my planned system for Uganda

System · August 2011

Thanks in advance for your input

I am moving to Uganda for a couple years to volunteer with my wife for a small clinic in a rural area. Our electric needs will be for an off grid system to power an aquaponics system (pumping water from a 200 gallon fish tank to hydroponic grow beds)

the system will power 2X 6.6watt 12v dc water pumps 24/7
plus a couple small led UV lights approx 1 watt (attract bugs for fish food) running 2 hours a day in evening
thats it.

Masaka, Uganda has a lowest rated solar insolation of 4.6 per day in the rainy months and highest 5.5 during the other 9 months a year.
I want 5 days of battery backup for really rainy times since if the system runs out completely I could lose 100 tilapia in a matter of a couple hours due to oxygenation needs.
I will put my panels at 90 degrees from vertical (aka horizontal, because we will be directly on equator) and I will wash weekly with water and a cloth to remove dust as it will surely land on the level surface.

I figured this as my system major components:
2X unisolar 68 watt flex panels in parallel 15ft from batteries (I can bring these in my luggage easier than other panels, I am aware of issues with these panels)
300 amp hour battery bank 12v (will buy in Uganda)
solar boost 2000e charge controller
will run with a 15amp circuit breaker between panels and batteries and also between batteries and the pumps and lights on the fish system.
planning to wire from the batteries X 20 to 30 feet to garden with 10 gauge wire then attach pumps and lights in parallel.

Batteries and controller will be in the home (isolated from living area) and with ambient range from about 60f to 95f) should be pretty stable. no chance of cold weather or serious heat. Elevation is 3500 feet if that matters
Please let me know if this seems like good planning and what I forgot to tell you all.

Joe

Cariboocoot · August 2011

Re: critique my planned system for Uganda

Welcome to the forum Joe.

First up, let's look at the load: two 6.6 Watt pumps is 13.2 Watts. Running 24 hours per day is 316.8 Watt hours. That certainly can be done. It would need approximately 26.4 Amp hours @ 12 VDC which would mean a minimum battery size of about 60 Amp hours. You want to run it for up to five days without sun (I understand the difficulties of generators in locations like that) so that would be 300 Amp hours. Just the battery size you picked: good job!

To recharge that you're going to want to try for 30 Amps of peak charge current so they'll come up quickly. That works out like this: 30 Amps * 14.4 charging Volts = 432 Watts.
Now usually we have to apply some derating to panels as they don't produce their "nameplate" numbers on average. In your case you've got 3500 feet of elevation working for you which means the panels will probably perform close to 80% efficient. But you've also got equatorial heat working against you. Going with the typical 77% efficiency just to be safe that would be 561 Watts of array. Your panels @ 68 Watts would come close with eight of them: 544 Watts. Two panels alone wouldn't recharge the battery: it might reach 3% charge rate, but that isn't really enough. Four would be a bare minimum, and then you'd need luck on your side (good efficiency).

I don't know the exact specs of the Unisolar panels (Vmp, Voc, Imp, Isc) so I can't advise on how they'd best be configured. Not sure about that charge controller either. Maybe someone else knows these.

niel · August 2011

Re: critique my planned system for Uganda

the vmp is 16.5v and the imp is 4.13a for the 68w laminate. for the costs of these 2 pvs you could put it to a more efficient pv that would be around 200w and these larger more efficient pvs would be a better way to go imho.

coot is right that you would lack the proper charging ability with those 2 68w pvs for that much battery capacity and being that is insufficient that means the controller would be too. i'd say 3 pvs at about 190w or better and a good downconverting mppt controller that will handle the power. now if you come across a bargain on pvs that may be 180w or 185w you may get away with using these as most of the extra ah in the battery is in the form of backup so a slightly lower un-ideal charge rate can be utilized knowing that it may take longer to bring the batteries up to a full charge should the extra capacity be used from the battery bank. another cloudy period soon after the 1st one that drained the reserve it will not handle well and could plummet the state of charge lower than 50%. as such getting a bit more in pv may allow a slightly faster recharging of the battery bank.

mike95490 · August 2011

Re: critique my planned system for Uganda

I'd suggest you need small backup generator and fuel with stablizer. 5 days is a long time to plan around, 3 days is normal, and you can fire up the genset to preserve batteries is critical if you want 7 year lifetimes.
Best to bring spares too, if this is super critical. Spare panel, spare controller, inverter.....

System2 · August 2011

Re: critique my planned system for Uganda

Okay thanks for all the input. I was hoping to get by on 2 panels, but that seems too low. Not sure that buying 2 more would be reasonable as the cost becomes very prohibitive due to needing a charge controller. Maybe I can get some extra panels there in Uganda before the rainy/cloudy season hits while I am there. I know mix and match is not ideal but hey this is Africa right? the rainy/cloudy days won't be until next march I believe.

I would also be able to use one pump only during cloudy times as the water temp will drop and O2 sats will be better. maybe that can compensate also.

I will keep you guys updated with what happens.

Joe

BB. · August 2011

Re: critique my planned system for Uganda

A decent PWM charge controller should be in the $50-$100 range. And would be very helpful for longer battery life.

Besides killing batteries from over discharge, over charging is probably the next best way to kill the batteries.

Also, I would suggest a simple and relatively inexpensive DC current clamp (plus digital multi-meter functions) to take with you. Should be very useful to diagnose problems with your solar system and other local gear.

Also, if you get flooded cell batteries, you should have a hydrometer too... Or probably several. The good ones are made from glass and are easy to break.

Batteries need lots of care to prevent an early death.

Deep Cycle Battery FAQ
www.batteryfaq.org

-Bill

critique my planned system for Uganda

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