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Old 04-17-2011, 05:34 PM   #1
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Re: Solar Gurus?

Have you seen this link before?

I just started looking into solar and found it elsewhere in the forum the other day. Seemed like a really good resource.
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Old 04-17-2011, 08:28 PM   #2
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Re: Solar Gurus?

OK, here goes:

Disclaimer: I do not have a bus (yet), or an off-grid home. I am responsible for two off-grid communications sites with photovoltaic wired in the 1990's. I have had to learn a few things about solar electricity in order to keep them working and deal with contractors.

First, let's understand your proposed panels.

Voc stands for Voltage, open circuit. It is the maximum voltage each panel is rated to put out in "standard" sunlight with no load (nothing but a sensitive volt meter connected). If you look at a photo of that model, you will see it is a matrix of 36 cells (6 wide by 10 high). Each cell in the panel puts out about 0.6 volts, and they are added in series to make the 36.4 volts.

[brain-dead edit: 6 x10 is 60, not 36. 60 x 0.6 volts = 36 volts. duh!]

Isc stands for current (amps), short circuited. This is the maximum current that each panel is rated to put out in "standard" sunlight with the positive and negative wires shorted together through an ammeter shunt. The rating for these is 8.4 amps.

If you get a data sheet for these panels, look at the power curves and you see the rated power curve (blue line) produces the 8.4 maximum amps when loaded down to between zero and up to about 26 volts. With lighter loads, less current is drawn, and the voltage approaches the maximum of 36.4 output volts. I would consider these to be "24-volt" nominal panels, for direct connection to 24-volt systems without MPPT.

Since power (watts) is volts times amps, at short circuit the output is 0 volts x 8.4 amps = 0 watts. At open circuit the output is 36.4 volts x 0 amps = 0 watts. The maximum power is neither of these. According to the spec, maximum power is 28.4 volts x 7.8 amps = 221.52 watts, hence the 220-watt rating.

You can use the open circuit voltage and the short circuit current to figure the panels in series or parallel, and should use these maximums when sizing electronics on the system. You have already used Morningstar's calculator to do this.

I do not know enough to tell you how many panels for how many batteries you should have. Handybob had a figure, and I have seen others here. I myself am searching for a good number that I can trust.

Second, the question is parallel, series, or series-parallel.
You will have to make a decision on this yourself.

If you put 4 panels in series, you get a nominal 96 volts, or 145.6 volts maximum open circuit. [edit: That is at standard insolation and temperature. On a crisp, cold day, it could be even higher.] Some controllers will blow their transistors if the input goes that high. If you touch the hot wiring without putting a blanket over the panels first, you can get a fatal jolt just like with AC wiring. But the current will always be 8.4 amps or less. The size of wiring between the panels and the controller is much less likely to have resistance problems that will burn up current as heat. As far as MPPT efficiency, I think I remember from Outback manuals that the higher the input to output voltage ratio is, the less efficient the controller is.

If you read Handybob, he has great concern about shadowing a portion of any of your panels (turning off a number of cells). If all 4 panels are all in series, and have internal bypass diodes to skip failed cells, then blocking one cell drops the maximum voltage from 145.6 to 145.0 (without the cell) or 144.4 (with diode insertion loss). The MPPT should theoretically change gears and keep on chugging along.

If you put 4 panels in parallel, you get a 24-volt system making up to 33.6 amps. You now need heavy wire, shorter distance, and cleaner connections between the array and the controller. The voltage is closer to battery voltage, so based on my memory of efficiency figures, less power will be lost in the MPPT. You may want to double-check that. With low voltage, there is less danger of electrocution, but greater danger of fire from high current heating a high-resistance connection.

Now we come to shadowing. As I recall, panels may have internal diodes to prevent the electrical "pressure' in the batteries from flowing out through the panels when the sun isn't shining. If so, cover one 0.6-volt cell on one of the four panels, and the up to 36.4 volts from the three panels in full sunlight will cause a "back pressure" that will shut down the flow from the shaded panel operating at less than 35.8 volts, so your 880-watt (maximum) system is now rated at 660 watts.

Finally, why use MPPT?

Looking back at the power curves, if I connect one panel direct across a 12-volt battery, we see that each panel puts out maximum current up to about 26 volts. A battery discharged to 11 volts will short the panel output to 11 volts, so the output is 11 volts x 8.4 amps, or 92.4 watts maximum from a 220-watt panel. When the battery is charged up to 14 volts, the output increases to 14 x 8.4 or 117.6 watts.

So, the deeper your batteries have discharged, the less power a panel will put out to refill it. If shorted by a battery discharged to 10 volts, your 220-watt panel can only manage up to 84 watts with a direct connection.

With an MPPT controller, it acts kind of like a DC-DC converter. Your 220-watt panel runs at 28.4 volts, and considering 90% efficiency, puts (220/11 x 0.9) = 18 amps instead of 8.4 amps into your battery discharged to 11 volts. Theoretically, the power will remain roughly constant but the amperage will go down as the battery voltage comes up, so maximum current is provided to a deeply discharged battery, and less to a battery nearing full charge. This is in addition to the fact that the current would slow down as the "pressure' in the battery begins to equal the "pressure" of the charging system. This is like Handybob's example of pumping up a truck tire.

Handybob says he does not discharge his batteries below 85% full if possible, so the advantage MPPT has on nearly-dead batteries does not help him much. He instead uses a direct-connection controller that throttles back the output when the batteries are full, and instead eliminates inefficiencies wasting the generated electricity in controllers or in long or undersized wiring and terminals.

Anyone considering direct connection control should select a different model photovoltaic array than the one you listed, unless they have a 24-volt system. A panel for direct connection to 12 volts would have a Voc of about 18 volts, and a Vmp of about 15 volts.

OK, so I did not answer your questions, so you do not owe me a beer. But I hope the information gives you a better ability to make your own informed decisions. Good luck and have fun!
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
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Old 04-18-2011, 02:01 PM   #3
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Re: Solar Gurus?

Great post Redbear!
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Old 04-19-2011, 08:08 AM   #4
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Re: Solar Gurus?

Personally, I would call Morningstar as it is their MPPT controller you're planning to use, and pose your question to them. Their controller is what will need to handle all the power from the cells, so it is the potentially limiting factor in the design. They would be in the best position to help you optimize the input to their controller.

I'm curious as to the thought processes that resulted in you deciding on these particular items in your design. I'm still in the knowledge absorbing phase of my design, and any input on the reasoning behind your choices could be helpful.

Thanks, and hope this helps,
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Old 04-19-2011, 11:29 AM   #5
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Re: Solar Gurus?

Originally Posted by Iceni John
... The folk over on the BCM forum are mostly not solar-savvy, so their response to my plans has been less than enthusiastic. I think us skoolie folk are much more willing to try the less-common approaches and to adopt more inventive and imaginative thinking...
It's not just the coach folks but the sticks-n-staples folks are the same... with a few exceptions. Narrow minds, little imagination. Anything out of the "ordinary" boggles their mind (My battery locker is XX" high and can't be changed, so should I slice the 1.5" off the top or the bottom of the battery case????) so all I ever get is... no you can't use deep cycle marine batteries, you HAVE to use golf cart batteries which isn't true.

Over on one of the RV forums, they chewed some poor guy up and down because he was asking questions about the cheap solar setup that Harbor Freight sells. Guy ended up buying the setup and installing it. It was a good low priced entry level setup for learning about solar. I thought it was a decent idea. Personally, unless I win the big lottery, I simply don't have a couple thousand laying around to "experiment" with a PV system. We are converting the bus "out-of-pocket" and we don't have deep pockets!! Apparently you can tie more than one set together (which the guy did... I think he ended up buying three sets over time as they when on sale). For us, since we would rarely boondock for less than a week once a year, solar is not a financially feasible option. But for $200 (or less sometimes), I would think about trying it just to boost the batteries a tad at times.

45 Watt Solar Panel Kit
Solar panel kits are a great way to generate plenty of clean, quiet energy, using solar energy from the sun to run TVs, lights, computers, even recharge 12 volt DC batteries. Setting up Chicago Electric solar panel kits is easy. The solar panel kit comes with three 15 watt solar panels - simply connect the solar panels to your own 12 volt DC storage battery, and then use at least a 300 watt power inverter (sold separately) to power your 120 volt AC appliances anywhere.

* Weatherproof solar power center works under all light conditions
* Includes 3, 6, 9 and 12 volt DC adapter outlets
* Easy-to-read LED charge indicator
* Includes mounting hardware, lights, 12 volt DC socket and battery clamps
* Overweight Item subject to $14.95 additional Freight Charge

Requires 12 volt storage battery and 300 watt power inverter (sold separately).
Maximum current: 3000 mA
15 watts max per panel
Peak voltage: 23.57 volts open current
Panel dimensions: 12.40" x 36.42" x 0.75" each
Shipping Weight: 49.65 lbs.

Three 15 watt solar panels generate plenty of clean, quiet energy, using solar energy from the sun to run TVs, lights, computers, even recharge 12 volt DC batteries. Simply connect the panels from the solar panel kit to your own 12 volt DC storage battery, and then use at least a 300 watt power inverter (sold separately) to power your 120 volt AC appliances anywhere. Some assembly required for solar panel kits.
This post is my opinion. It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
Fulltime since 2006
The goal of life is living in agreement with nature. Zeno (335BC-264BC)
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Old 04-20-2011, 08:56 PM   #6
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Re: Solar Gurus?

Thanks, that does help.

Just one thing that I just caught... if the batteries are 200AH 6V, then wiring two in series gives you 200AH 12V, and then doing the same again for the other two gives you 2 200AH 12V and wiring those in parallel makes it 400AH 12V, not 800AH like you're thinking.

General rule of thumb when wiring DC: series adds the voltage, parallel adds the current.

We also have apple computers (2 desktops, a few laptops). Looking at our needs, I'm going back and forth between one larger inverter and several smaller ones (dedicated per device, and so able to be disconnected when a device isn't needed). The laptops will just use straight 12V adapters, and avoid the MSW/PSW issue altogether.

We're going to stick with flooded cells. Basically, we have a ton of underbelly storage, and we plan to make good use of it. I can't justify the AGM cost when I can simply vent the compartment, and also I want to be able to keep an eye on the specific gravity in the batteries periodically.

I'm sold on Morningstar for our charge controller. I'm still considering the PWM controller based on handybob's feedback, but if my solar array gets large enough, and MPPT might make more sense. He seemed to imply that something around 1000 watts was a good cutover point for making the MPPT worth the extra money. Assuming I'm remembering correctly of course.

The Morningstar PWM controller lets your input voltage be up to 125Voc for example, and costs a bit less than the MPPT version. If the higher voltage from the panel is the sole reason for choosing an MPPT, this gives you a place to potentially save a few dollars.

We plan to live off grid as much as possible. I do plan to carry a small generator (air conditioning comes to mind), but hope not to use it except in extremely hot days. I want to get as much of my energy needs met by solar as possible. We'll be using solar for heating, hot water, and electricity. We'll supplement heating/hot water with propane and diesel boilers, but I would prefer not to have to waste fossil fuels. We'll use propane for cooking, and diesel for moving (obviously), and that's about as much fossil fuel use as I want to do.

Thank you again for the information,
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Old 04-21-2011, 12:50 PM   #7
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Re: Solar Gurus?

Originally Posted by Tesla
About the 12 volt adapters, are you doing DC wiring from your battery bank? Or is it a cigarette-lighter type of thing? I've read that the cigarette-type receptacles are fire hazards in a residential context.

We're going to have DC receptacles (using AC receptacles that are rated for 240 volts, as suggested by Backwoods Solar - this is "to code" as long as you don't actually have any 240-volt wiring in your bus) as well as standard AC receptacles. If there are chargers for our laptops and iPhones that can be used with DC through these 240-volt-rated AC receptacles I'd love to find out about them.
Now it's my turn to have incorrect information.

I've often seen the apple magsafe airline adapter which has a cigarette lighter adapter, and just assumed it would work with a standard cigarette lighter socket. However, that is not the case. This adapter doesn't actually charge for one thing, and for another, needs 15VDC @ 75W (empower airline power specs). There is not an apple approved method for charging and running a macbook (or pro or air) from a DC power source.

Back to the drawing board.

Thanks for making me check my facts again.
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Old 04-27-2011, 11:17 AM   #8
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Re: Solar Gurus?

Hello all,
Usually Im not to active on this site I just rwead the posts. I have lived off grid solar powered for 24 years now and made every mistake you cam make. I now design and install systems for folks who would like them. The biggest mistake we all make is under charging or batteries. We do this by under sizing our system and over sizing our battery bank, some folks call this the shoot ready aim method .

The proper sequnce should be
1.load calculation (both 12 volt and 120)
2.inverter sizing
3.battery bank sizing
4.charge system sizing

Under load calculationsa you need to do a complete analisys of your loads. Refigerator lights fans etc. Their voltage amperage and how many hours per day you will run them

example: 12 volt lights@ 3amps for 6 hours 12x3=36 watts for 6 hours+.216KWHr. for each light you run for 6 hours. in this case (real small) a system that would produce .216KWHr is what you would nee to break even. You would go through each component and figure it out this would help you determine all the rest of you system.
Inverter sizing is not to hard you would need an inverter big enough to start you biggest AC load in most cases this would be you ac unit. Check your spec plate, there should be 2 different amp ratings one for running and another for starting. If the start up amps is not avalible you can usually use 156% of the running amps and yoour safe. lets say start amps is 20x156%=31.2 amps (I prefer to round up for safety sakes) so I would use 40 amps as my start current.
now the problem here is something else is always running so you dont want the surge to trip your inverter off so you still need to caculate the rest of your loads.

Using your 12 volt battery bank a 40amp ac load would require nearly 500amps dc for your surges. so an 880aHr bank would be fine.

Charging this size battery bank requires a charge rate of C10 min. that would be 88adc prefered would be C5 or 176adc. your pannel should be okay but the charge controller wont be, due to current limits.

Something to remember is MPPT chrgers down convert voltage to the battey level, by doing so we get an increased chrge rate (amps) up to the llimit of the cnotroller. Also the higher your input voltage the lower the efficancy of the charger. Here is a link to a web site fourm with some walking geniuses as far as this stuff goes, its also has a string calculator under resources. and no I dont work for them.

They have a whole section dedicated to RV systems and massive amounts of info. I hope I helped and did not confuese.Here is a link to a load calculator to help out.
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Old 04-27-2011, 08:45 PM   #9
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Re: Solar Gurus?

Thanks, Chuck.

By the way, I really like the Alt E Store, and intend to buy from them when I am ready. Others have different favorite suppliers.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
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Old 04-28-2011, 09:02 AM   #10
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Re: Solar Gurus?

Your welcome redbear, I like their educational series and recomend it to folks who want to learn more about how solar operates. Solar has really come a long way since the start aand still can go a lot further.
One mistake we all make is run out and buy expensive batteries, I tey to recomend a less valuble set for the first setup because we all seem to ruin at leats one set while we learn to take care of them. Thats all pat of the learning curve.
Have a good one
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