Renewable Energy Questions/Discussion > Solar (heating or electric)

Intro and questions, if I may

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ksouers:
Hello everyone.
I’ve been lurking around for about a week and decided to join your friendly little group. I have a question or two, but first I’d like to introduce myself.

My name is Kevin. I’ve been using solar on my sailboat for many years. I have settled on a single panel set up that works quite well for me. I’m also interested in wind power and I’ve lurked on the “other” site off and on for quite some time (years). When I was much younger, in the 70’s and 80’s, I had dreams of living off-grid with only wind power and passive solar for my basic needs. As in many things in life, that dream never fully developed.

I’m older now and preparing for retirement, though that is still many years away. I’ve starting taking some of my energy needs off-grid a portion at a time, starting with the garage and my hobby machine shop. I’m starting with what I’m familiar with, solar, and I’ll eventually add a small (micro?) wind turbine or two. I have a terrible site for wind power; the lot is on a hillside surrounded by lots of mature trees and tract houses. Small wind is about the only way I can go, and summer here usually means NO wind at all (except for the twisty ones). I’ll be satisfied with a few hundred watt/hours or so a month from a small turbine. I have no illusions about taking the house completely off-grid. Just the satisfaction of knowing at least my hobbies are completely independent of the utility companies is enough for me. Perhaps later I can find a halfway decent wind site to retire on.

Now on to the questions:
I’ve taken some of the “surplus” equipment I’ve had laying around from the sailboat projects and applied them to the garage project; a 25 watt panel, a 5 watt panel and a Blue Sky MPPT charge controller. I also have a 1000 watt Harbor Freight inverter that is used a few times a year in the car for another hobby. The battery bank is two group 27 batteries in parallel. Yesterday I added an 85 watt panel for a total of 115 watts, and just like the wind dying when a new turbine is flying, the skies clouded up shortly after the new panel was hooked up to the array. I’ll add another 85 watt panel before summer and perhaps another before the end of the year.

 The inverter has been idling continuously for several days (also cloudy) ready to run the garage door opener and lights with the 30 watts array keeping the battery voltage just above the 50% mark through the clouds. Yesterday was supposed to be clear in the morning, and it was for a while, the battery bank was showing 12.19 volts just before sunrise. It looked like a great test day for the new panel. As the clouds started moving in early I did see a steady charge of 1.5+ amps with several minutes of 3.5-4 amps during breaks in the clouds. Still, the batteries were only charged up to 12.33 volts. I would have expected a better charge than that, even on a mostly cloudy day. This is what I would expect just from the 30 watts array.

As it’s a 12 volt system all the panels are hooked up in parallel with individual leads to the charge controller. Is this perhaps not a good thing? Should I take the smaller panels out of the array and just run the 85? Or were my expectations too high? Should I give it more time before making a judgement?

I know you guys can’t judge the sunshine we had here yesterday, but I expected a little better performance from a potential 115 watts of panels. I didn’t get to watch the controller all day, I did have work to do, so the cloud cover could have been worse than I observed, perhaps I just happened to see the better parts. I’m more worried about the mish-mash of panels and possibly causing some damage.

Whew! That’s a rather long winded setup for just a couple questions! Sorry about that.

Thanks,

Kevin

bj:
   There are many here better qualified to answer you Kevin, so for now, just let me say
   welcome!

WooferHound:
Hey There and welcome
You seem to have a good start on a decent system and you're gonna have fun with it.
You left out an important piece of information. To answer you properly I would need to know your Battery Capacity.
I have a 150 watt solar power system (see link in sig). Each individual panel passes through a blocking diode to prevent power from backfeeding the panels when they aren't producing. Your charge controller may be doing this function for you. These diodes also help prevent the panels from backfeeding themselfs if some of them are shaded and others are not.

My batteries are 225 amphours. I use the 12 volt power directly off the batteries without an inverter. My loads are rather small and I max out at around 4 amps when I have a lot of stuff turned on, so My batteries will last over 10 days without Sun before I start to get worried about the voltage going down too low. But when the Sun returns after that amount of time, it takes a few days to pull the battery voltage back up to a 14.7 volt full charge. Also it seems that even in full Sun, the voltage will rise steady to a point and then stop rising and remain steady and unchanging even with the Sun beaming. It rises slowly through the 13 volt range and then it seems to rapidly rise past 14 volts and on up to a full charge. After a full charge and at night, the batteries will settle back down to 12.6 or 12.7 volts without any load on them.

So what is your battery size ?
How much power does your inverter pull when you aren't using any power from it ?

ChrisOlson:

--- Quote from: WooferHound on February 01, 2012, 10:47:57 am ---So what is your battery size ?

--- End quote ---

Group 27's are 95 ah.

Edit: I might add that to match solar capacity to battery capacity, you really only need to replace what you take out.  But assuming a daily load that draws the batteries to 50% SOC every day, the usual recommended solar capacity would be 10% of the amp-hour rating of the bank.  Or for for 190 ah, 19 amps @ 12 volt nominal would be about 225 watts of required solar capacity to get the bank thru bulk and absorb during the available daylight hours.

If there are normal loads on the bank during the charging process, those normal loads must be added to the above so the solar panels can carry both the normal loads and handle the charging requirements during available daylight.

You would typically want about 1% of the ah capacity, or about 1.9 amps, to maintain float.  1.9 amps @ 13.2 volts is 25 watts.
--
Chris

ksouers:
Thanks for the warm welcome, guys.

Woofer: The charge controller does keep the batteries from back feeding into the panels. The batteries are 90 ah, I think. A little on the smallish side for group 27's. I need to get some T-105's, but this is what I had on the sailboat and will serve until they are dead or repurposed elsewhere. The inverter pulls 1.4 amps when idling.

Chris: Thanks for confirming that. I knew it intuitively from experience but seeing someone else explain it helps a lot.

My main concern was the mix of different sized panels. I've never run panels in parallel before, on the boat each battery had it's own panel charging it while it was off line. I'm concerned about the wide disparity between these panels causing some damage. The open voltage between the panels is within half a volt of each other, so logically I didn't think there'd be a problem. But I don't have the experience to believe it to be so. I'd rather be using all matched panels, but that will happen in time.

I don't have diodes on the individual panels. I've read where people recommend they be used, but I've also read reliable sources that say they are not necessary. I just don't know how much, if any, a lighted panel will back feed into a shaded panel. And yes, I do have that situation at my site with trees casting shadows in the morning.

While I'm thinking of fixing those things that are not great, can I mix T105's with the group 27's? I know they'll have to be in pairs. 4 T-105's at one shot is not going to be cheap, was thinking about doing it in pairs, though.

Update:
Today was mostly clear, only some sparse high cirrus. I checked on the batts when I got home and voltage was 12.87 and still had .7 amps going in. I'll check again after they've had time to rest, but I already know it'll be a solid 12.7 volts. The inverter was turned off. I had to go to the office today so didn't get to adjust the panels, just set them facing South. Yes, a tracking system is already on the list. Also on the list is a smaller standby inverter for the door opener.

Now that I've got a good charge I can see if this is going to run the lathe :)
Baby steps, it's all baby steps.

Thanks,
Kevin

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