Lil' 12V solar lighting system

aranderson · March 2012

Greetings-

We recently purchased a yurt rental business in n. Utah. Primary use is winter recreation. So far, we had to haul in 5 gallons of white gas to both yurts for the lanterns, and I'd like to replace the highly flammable liquid lighting with a solar system.

So far, I found LED lights that should provide enough lighting, at about 10 Watts. Figure lights are on 5 hours a day, 50 WH.
My neophyte questions:
Would a 15 W panel recharge a battery in a sunny day?
Could I use NiMH batteries to store the energy? I have seen D NiMH that boast 10 Ah. Would 8 work for this application?

I have a decent understanding of max. discharge rates and such, but can't find much info as to why NiMH batteries shouldn't be used with solar. I'd consider just SLA, but the yurts are 4 miles in, and it gets to ~15F during the winter.
Thanks! Alex

westbranch · March 2012

Re: Lil' 12V solar lighting system

Welcome, a few (more) questions for you to answer first.

Ac or DC?
what voltage do you want to use ?
how many hours strong daylight per day?
what reserve capacity would you need ie 7 , 8 hrs / day of use?

how far from the panels to the battery?
how far from the battery to the light(s)?

there is more but this will start things rolling

hth

aranderson · March 2012

Re: Lil' 12V solar lighting system

westbranch wrote: »

Welcome, a few (more) questions for you to answer first.
Ac or DC?
what voltage do you want to use ?
how many hours strong daylight per day?
what reserve capacity would you need ie 7 , 8 hrs / day of use?

how far from the panels to the battery?
how far from the battery to the light(s)?

there is more but this will start things rolling

hth

Hi There,
DC, 12 V seems appropriate for some LEDs for a 16 foot diameter yurt.
5-6 hours strong daylight per day
Reserve, about 8 hours
Panels to battery, 20 feet (7m)
battery to light, 15 feet (4.5m)

Just a simple system, so that folks that rent the yurts don't have to deal with flammable white gas and we don't have to haul it in.

Cariboocoot · March 2012

Re: Lil' 12V solar lighting system

Beware of two things: "rental" and the inevitable increase in loads. You calculate 50 Watt hours per day, but how can you guarantee renters won't leave the lights on all the time? And once you have those lights in you just might want "a few" more. Next thing you know it's 500 Watt hours in loads.

That said, Westbranch's concerns are also valid: if the yurts are far apart and you try to centralize the power system you'll have trouble keeping losses to a minimum with long runs of 12 VDC wiring. But if you can provide each with its own self-contained system it will work.

Can you use NiCads with solar? Yes, but ... The "but" being that you need a charge controller that meets the profile for the NiCads. Will a 15 Watt panel recharge a battery? Yes, providing the battery is small. That size panel in reality will put out 1 Amp at best. So it would work for quite small batteries. 10 Amp hours would probably be the top limit, and you likely would have trouble keeping that charge in low-light conditions (bad weather, winter).

Obviously you have specialized parameters here. If you could give us some more info regarding the actual physical layout and usage as per WB's request I'm sure we can figure out a few options for you.:D

aranderson · March 2012

Re: Lil' 12V solar lighting system

Thanks for the response.
Yeah, renting is a little nervewracking, but it's the only way we could afford a vacation property!
Your same concerns about the load freaks me out about the white gas lanterns. I took up a gallon of fuel, and a week later it was gone since perhaps the schnapps made the guests forget to turn off the lantern.
The yurts are about 5 miles apart - I'm looking for two systems. Folks around here like to jack panels off of the forest, so I'd put the panel up in an aspen about 15 feet, running to a battery in perhaps a cooler in the yurt, then LED lights running along the roof rafters. The yurts are 16' diameter, top of roof ~8'. They are situated on south facing ridges in aspen groves, so they get great winter sunlight.

Cariboocoot · March 2012

Re: Lil' 12V solar lighting system

Ah, you posted while I was posting.

So, each unit will have its own system. That is good. 50 Watt hours per day each expected usage. At 12 VDC that would be slightly more than 4 Amp hours, meaning a minimum of 8 Amp hours. You want 8 hours reserve? How about one day's worth? Then you'd be at a 16 Amp hour battery, normally discharged only 25% leaving 25% in reserve. For that you'd need 1.6 Amps @ 12 Volts or 19 Watts, less typical panel derating would be around 24 Watts. A quick look at Voltage drop says 12 gauge would handle 2 Amps max current over your 35 feet combined distance. Not bad.

Depending on the distance between yurts, it might be more economical to run two off one 35 Amp hour AGM battery recharged from one 60 Watt panel.

I told you there'd be options.

Cariboocoot · March 2012

Re: Lil' 12V solar lighting system

How many yurts are we looking at here? Just two five miles apart?
The safety concerns are there, but clearly electric light is better than a burning flame in this application!
As for theft, how about a portable system you can take with you when you go? Or are you renting out even when you're not there? That's always problematic. Mounting panels in trees is not a good idea, because trees move around in the wind and drop things. We're looking at small panels here, so it wouldn't be difficult to get it up on its own pole.

aranderson · March 2012

Re: Lil' 12V solar lighting system

Hi Coot-
Two yurts. Yep, the white gas lanterns scare me. The operation has been going 25 years, but I don't want to see them burn down.
We'd need to leave the system in place, and actually, theft isn't a big deal except in the summer, which is when we could pull the system, or just the panel, down.

westbranch · March 2012

Re: Lil' 12V solar lighting system

have a look for this type of light, one per Yurt should work...

http://www.bulbamerica.com/osram-sylvania-8w-120v-e26-g25-dimmable-led-light-bulb.html?mr:referralID=f641329c-6ed7-11e1-a6d5-001b2166c62d

btw these are pricey

Cariboocoot · March 2012

Re: Lil' 12V solar lighting system

Let me run this system idea past you and see what you think:

35 Amp hour AGM: http://www.solar-electric.com/unba35amseag.html $60

SunGuard controller: http://www.solar-electric.com/sg-4.html $26

55 Watt solar panel: http://www.solar-electric.com/so55wamusomo.html $169

That's about $300 for the major components, which you probably just choked on.

It would provide about 105 Watt hours @ 12 VDC and only 25% DOD, which is double your expected 55, meaning it would be about four days worth total. That's about twice what you were after, and probably three times the price.

Going down to a smaller system can actually be more expensive, because the cost per Watt on small panels is ridiculous and likewise the value in smaller batteries and controllers.

BTW, NiCads also like to be completely discharged before recharging. I'm not saying they won't work, but they might be disappointing.

So here's another idea: battery only in the rental yurt(s) with a longer expected running time and a "central recharging station".

BB. · March 2012

Re: Lil' 12V solar lighting system

To address the lights left on issue--You might want to use a simple spring wound twist type timer from the local electrical supply house/hardware store (you have to watch DC current--A standard AC switch will usually switch much smaller DC currents than the AC rating--DC arcs are much harder to suppress and can quickly erode switch contacts)...

You can also look at landscape lighting controllers. Program to allow the lights to be on only during dark (get a large enough panel to run the lights all night--if it is cost effective).

In any case, you probably will be lucky to get 1-2 seasons from small lead acid batteries. They do not last long anyway, and you have the rental issues.

-Bill

aranderson · March 2012

Re: Lil' 12V solar lighting system

Actually, not bad considering we spent $50 on white gas per yurt in one year. And yes, I'd prefer cheaper, but...

So you'd recommend a panel with that much output?

I like the fact the battery only weighs 12kg, as I will be packing it in on my back.

BB. · March 2012

Re: Lil' 12V solar lighting system

Based on what you asked for:

50 Watt*Hours * 1/0.61 DC power system eff * 1/5 hours min sun per day = 16.4 watt panel (Vmp~17.5 volts)

A 20 watt panel is ~$91 plus shipping and insurance (be sure to get delivered to your door pricing--Shipping solar/glass panels can be expensive).

Max Rated Power (Pmax)
20 Watts

Voltage at Max Power (Vmp)
17.2 Volts

Current at Max Power (Imp)
1.17 Amps

Open Circuit Voltage (Voc)
21.7 Volts

Short Circuit Current (Isc)
1.25 Amps

Module Efficiency
10.0%

Length x Width x Depth (inches)
21.7 x 13.8 x 0.98

Weight
6.17 lbs.

25 Year Limited Warranty

Battery size... Assume 2 days no sun and 50% maximum discharge:

50 WH * 1/12 volts * 2 days * 1/0.50 max discharge = 17 AH minimum

One thing to watch with batteries--Is the larger the capacity, the more solar panel you should have to meet the minimum charging current requirement (we use 5%-13% as a rule of thumb here)...

35 AH * 17.2 volts Vmp panel * 0.05 rate of charge = 30 watt recommended minimum panel
35 AH * 17.2 volts Vmp panel * 0.10 rate of charge = 60 watt "nominal" panel
35 AH * 17.2 volts Vmp panel * 0.13 rate of charge = 108 watt "max cost effective" panel

AGM's can work better at the lower charge currents (5%) range. Flooded cell should probably be higher (more losses, need to "stir" the electrolyte--more important on much larger batteries).

If you think people are going to use more power (external Porch Light, cell phone chargers, etc.)--You probably should aim for the larger (towards 60 watt panel on a 35 AH battery bank).

The above are "rough numbers"... I just carried out the extra digits so you can reproduce my math (and catch my errors

).

-Bill

PS: I should add--The more days per year the camps are occupied--The more solar power makes sense... If this is a summer weekend type camp--justifying solar becomes more difficult. If this is a 9 months a year occupation--Solar can make a lot more sense.

westbranch · March 2012

Re: Lil' 12V solar lighting system

thinking about the backpacking of the batteries....I would not want to pack an open top FLA on a pack-board, they can leak and H₂SO₄ is corrosive to just about anything... Sealed, AGM or Gel would be OK

BB. · March 2012

Re: Lil' 12V solar lighting system

And GEL batteries should have a maximum of ~5% rate of charge (the batteries can make "gas bubbles" in the gel electrolyte--damaging battery capacity.

-Bill

jleblan1 · March 2012

Re: Lil' 12V solar lighting system

I set-up a small 12v lighting system. I found a charge controller online that can be set to kick on the lights when the panels quit producing a charge after the sun goes down, and can be digitally set to a certain ammount of hours to run, that way the tennants couldnt leave the lights running. I have found its easiest to stay in the 12v domain for general lighting. I have 2 15 watt panels, a 8.5ah battery, and 5 2watt 12v led bulbs running 5 hours per night. While ive only had the system set-up a few weeks, ive been more than pleased......

here are some photos.......

https://www.facebook.com/media/set/?set=a.10150588048812469.402635.504672468&type=3&l=093cc6edd7

shift · March 2012

Re: Lil' 12V solar lighting system

Nice work! its looks good! i'm using the same controller for my "light timer". Do you have a window or hole on that box for it light sensor to work properly?

A side question - What would happen you had to large of an amp hour battery bank (l parallel array) and the rate of charge was down below 5%? Would it still charge just at a lower rate or would it not have enough current to properly charge the batteries?

niel · March 2012

Re: Lil' 12V solar lighting system

jleblan1,
that will work as long as you are not leaving all lights on all night long. 10w of draw at 12v is .83a and if over 8hrs would translate to 6.67ah drawn for about 78.4% dod. that's not good for the battery and all night long would strain to have the pvs charge the battery back to full at times too.

jleblan1 · March 2012

Re: Lil' 12V solar lighting system

niel wrote: »

jleblan1,
that will work as long as you are not leaving all lights on all night long. 10w of draw at 12v is .83a and if over 8hrs would translate to 6.67ah drawn for about 78.4% dod. that's not good for the battery and all night long would strain to have the pvs charge the battery back to full at times too.

I have the timer on the controller set for 5 hours per night. Technically each light uses 1.8 watts, for a 3.75ah draw per night, but i like to round up to 2 watts. Ive also got a 12ah battery on the way to give me more DOD headroom.

As for the light sensor, the charge controller uses the PV panels as a light sensor, when it dosent detect any power from the panels it knows the sun has gone down. It has a 10 minute delay to not be tricked by a passing cloud or lightning.

westbranch · March 2012

Re: Lil' 12V solar lighting system

I don't recall where you are sited but how much twilight (hrs) do you get as this might affect the delay time needed.

I have a similar (same?) CC but have not used the 'night light ' feature as yet... satisfied with the output so far on keeping the campers battery up to snuff. The C will have a small parasitic load too..

brbpab94 · March 2012

Re: Lil' 12V solar lighting system

Where did you buy the controller and lights?

jleblan1 wrote: »

I set-up a small 12v lighting system. I found a charge controller online that can be set to kick on the lights when the panels quit producing a charge after the sun goes down, and can be digitally set to a certain ammount of hours to run, that way the tennants couldnt leave the lights running. I have found its easiest to stay in the 12v domain for general lighting. I have 2 15 watt panels, a 8.5ah battery, and 5 2watt 12v led bulbs running 5 hours per night. While ive only had the system set-up a few weeks, ive been more than pleased......

here are some photos.......

https://www.facebook.com/media/set/?set=a.10150588048812469.402635.504672468&type=3&l=093cc6edd7

aranderson · March 2012

Re: Lil' 12V solar lighting system

Yes, I'd prefer to not have acid leaking down my back. My limited knowledge about batteries still makes me wonder why using 8 D nimh batteries, rated at 10 Ah, wouldn't work, be very safe, and easy to install and replace. What don't I know (about batteries, not the whole list). I thought nimh have amazing discharge and recharge properties.

BB. · March 2012

Re: Lil' 12V solar lighting system

You can try them... The problem is recharging NiMH properly and quickly requires a different sort of charge controller than that used for Lead Acid batteries. NiMH batteries get very hot (when quick charging) and actually drop their charging voltage when near full.

If you can find a good charge controller that will take PV Solar input power--Then I would try it (all things, including costs, being equal).

An "automatic" low tech charging can be done at C/10 type charging currents (1/10th the rated AH capacity of the cell). Basically, too little current to overheat/overcharge the batteries (no charge controller. Just right sized solar panel + blocking diode + NiMH D battery pack).

So, for a typical ~10AH (10,000 mAH) battery bank, using 10 cells in series with ~14-16 volts charging (a Vmp~17.5 volt panel should be OK):

10 AH * 1/10 rate of charge * 17.5 volts Vmp = ~17.5 Watt "12 volt" solar panel maximum

Note that charging efficiency and losses for NiMH cells are variable--but you can start with an assumption of 66% charging efficiency.

1 amp rate of charge * 5 hours of sun * 0.66 eff = 3.3 AH per day @ 12 volts of "useful" power per string of "D" NiMH batteries

Also be aware that NiMH can only be operated to around -20C (-4F). Lead Acid can be stored (when fully charged) well below -40C/F.

And with (virtually) any rechargeable battery chemistry--If you draw down a bank below ~20 to 0% state of charge (leave the lights and draw battery bank dead), one or more cells may get "reversed" charged and permanently damaged.

-Bill

jleblan1 · March 2012

Re: Lil' 12V solar lighting system

brbpab94 wrote: »

Where did you buy the controller and lights?

Charge controller on e-bay

LED lights from a RV supply company near by. Each one has 2 cree xp-g LED emitters and is rated at 160 lumen and 1.8 watt draw. Rated to run from 6v to 24v. No brand was listed, and they just came in non-descript white boxes.

mike95490 · March 2012

Re: Lil' 12V solar lighting system

Many charge controllers will work as lighting controllers too. Use one of those, and a switch in the yurt, to turn the light off. Run it all in conduit, to keep folks from "tapping in" to power their stuff.

Lil' 12V solar lighting system

Comments

Categories