Design Help Requested: A Small System for an Informational Kiosk

erix

Howdy Folks,

I'm designing an informational kiosk/shelter for a nature preserve and I'd like to have USB charging capability to it with a small off-grid system. I've done solar lighting projects before but never one that had to provide power. My research brought me to Northern Arizona and this fantastic forum. I've learned a lot in the last couple of days going through this site but I haven't found anything similar to this application.

I did some preliminary math and it looks like my numbers are off by a lot! I'm hoping to get some verification of my math and advice about battery selection and system layout.

For maintenance and mounting convenience I'd like to use a pair of combination duplex outlet + USB port device for people to plug into like this part from Leviton:


It think my system diagram will look like Solar Panel --> Charge Controller --> Battery(s) --> Inverter --> USB outlet.

If I assume each outlet is used to it's maximum - 15A @ 125V - that equates to 1875W potential per outlet, 3750W for the pair! In reality I don't think anyone is going to drag their air compressors or table saws out there so I'll use 250W + 18W per outlet, 5V x 3.6A for the USB ports and 250W for the duplex outlet (based on my laptop's power supply). I should have a potential load of ~550W.

Assuming 6 hours of run time after dark, 550W x 6h = 3300Wh

Assuming 12V system, 3300Wh/12V = 275Ah

Assuming 80% battery efficiency, 275Wh/.8 = 344Ah <-- minimum battery capacity required

Power output of solar panels required, 344Ah x 12V = 4128Wh

Assuming 4 hours of effective sunlight (project is in Western Michigan), 4128Wh/4h = 1032W <-- size of solar panel array required

So now the questions!
1. Does this math look correct? That looks like a whole lot of solar panel power to charge a few phones!

2. My math assumes battery operation only 6 hours at night. But there is potential for the outlets to be used continuously throughout the day - as much as 14 hours. How do you tweak the math to accommodate that? Simply change the 6 hours to 20? How to account for the fact that the batteries will be draining as they are being charged?

3. I'm thinking that AGM batteries would be best suited for this project due to the remote location and minimum maintenance the owners can provide (non-profit, all volunteer). It looks like the 12V selection is AH-limited so a series connection (or series-parallel connection)of 6V would be required. Does that sound right? What are your opinions about maintenance vs.. size and cost of flooded-type and AGM?

4. Inverter selection - there appear to be many choices! Any advice to a manufacturer and model number?

5. Is it worth using an MPPT charge controller for this small array?

Many Thanks for any and all helpful advice/fact checking you can offer!

ok,
erix

westbranch

erix wrote: »

I don't think anyone is going to drag their air compressors or table saws out there

You never know what some people will do, so use a 10 Amp or better yet 5A circuit breaker to protect you setup.

erix

Thanks westbranch, I'll add that to the project.

The Guest cabin system in your sig - is that big enough to handle my requirements? I hadn't thought about NiCd batteries....

westbranch

I do not think it would be big enough for your loads, they are not so meager from what I see, ~3Kwh , for that you need something the size of my big system, with a 3 days of autonomy built in.
I do not see the allowance for only 50% depletion per day of the fully charged battery either..

hth

erix

Thanks and Yikes! Maybe I need to scale back my anticipated loads....

I was looking at similar systems yesterday and found this rig with a pretty complete BoM.
Attachment not found.
This uses a 530W array, (2) 12V batteries for 225Ah, a Morningstar SureSine300, and a Morningstar TS-MPPT-45 and it looks like this inside:
Attachment not found. Attachment not found.
I'm assuming the breakers on the left limit the current to the outlets.

The load is double my design goals (4-duplex + 8 USB) and provides 2 days of autonomy. Presumably I can half the load and reduce the panel size, battery Ah, and controller capacity.

The specs say 75-150 portable charges a day which, at 6.6W (my estimate) and 18 hours of run time is 120Wh and 9.95Ah at 12V.

ok,
erix

westbranch

the suresine is a limiting factor in and of itself, my wife found out that our 600W inverter on the old system, could NOT run her vacuum... as long as you do not hae anything other than the USB and computer chargers you can have a very modest setup...it's the loads that set the design