10-290W Panel OffGrid System

quique · April 2013

Hi,

I want to design a system with 10 solar panels of 290W each. The panels have these specs:

290 Watts
Watts (PTC): 264.3 Watts
Max Power Voltage (Vmpp): 36.3 Volts
Max Power Current (Impp): 8 Amps
Open Circuit Voltage (Voc): 44.7 Volts
Short Circuit Current (Isc): 8.51 Amps
Max System Voltage: 600 Volts
Series Fuse Rating: 15Amps
Module Efficiency: 15.11%

Im suggested to use 2 combiner boxes of 6 breakers each plus 10 breakers of 15Amps.

Im stuck on why I should use:

2 combiner boxes of 6 plus 10 breakers of 15Amps

instead of

1 combiner boxes of 6 plus 6 breakers of 20 Amps?

I have a Xantrex c60A 12/14v Charge Controller. Here is the design. Attachment not found.

solar_dave · April 2013

Re: 10-290W Panel OffGrid System

Your starting at the wrong end, first you need to know your loads, then you can design a battery/inverter to support those loads, then you can get a charger regimen via solar with a generator backup to support the battery bank.

quique · April 2013

Re: 10-290W Panel OffGrid System

This is the system the technician designed for me, or rather, suggested we design. I just want to get your opinions on the design. I am assuming the requirements have been met according to the loads

NorthGuy · April 2013

Re: 10-290W Panel OffGrid System

quique wrote: »

This is the system the technician designed for me, or rather, suggested we design. I just want to get your opinions on the design. I am assuming the requirements have been met according to the loads

If you used one MPPT controller (instead of 2 PWM) connecting your panels in 5 series of 2, you would need one combiner box with 5 breakers/fuses and would save quite a bit of money on MC4 cables etc.

If I were you, I would make sure that the system is properly sized for your loads. That's the most important thing.

quique · April 2013

Re: 10-290W Panel OffGrid System

Sorry, im a noob at this, but wouldnt that result in 120V of PV array?

Attachment not found.

NorthGuy · April 2013

Re: 10-290W Panel OffGrid System

quique wrote: »

Sorry, im a noob at this, but wouldnt that result in 120V of PV array?

No, no. You connect 2 solar panels in series ("+" of one panel to "-" of the other). Each series gets VOC of 45 * 2 = 90V and Vmp 36 * 2 = 72V. Imp of the series is still 8A. You get 5 of these series and you combine them in the combiner box

You then need to use an MPPT controller (not PWM!!!), which adjusts the voltage to what batteries need. MPPT is also much more efficient than PWM, so you get more energy.

BB. · April 2013

Re: 10-290W Panel OffGrid System

What is your battery bank voltage? And what charge controller (a Xantrex Cxx series is PWM and could work for a 24 (1-panel in series) or 48 volt battery bank (2 panels in series) if he Vmp~36 volts is correct for your panels (and if your Cxx supports a 48 volt battery bank).

The whole series/parallel panel setup depends on both the charge controller chosen and the solar panels that will be used.

For a 2.9 kWatt array, you should be looking at 24 or 48 volt battery bank, and probably a "full sized" MPPT type charge controller (i.e., a $500-$600 80+ amp MPPT charge controller).

-Bill

quique · April 2013

Re: 10-290W Panel OffGrid System

NorthGuy:

So with this design:

Attachment not found.

The 5 series would wire up in parallel to produce 90V and thus my PWM would need to be of at least 90V and 40A...but with an MPPT, i just have to worry about the Amps?!

I did not know that.

BB:

I have not yet designed the Battery Bank. It would be 24V though, just like the CC.

mikeo · April 2013

Re: 10-290W Panel OffGrid System

You are getting closer, but not quite, the MPPT controller will take that 72 volts (your Vmp) at 40 amps and convert it down to over 100 amps at your 28 volt battery float voltage. The MPPT controllers are rated on max output amps. Depending on your climate you would need at least an 80+ amp MPPT controller or multiple controllers. On the other hand, with a 48 volt battery bank you could handle it with a 60 amp MPPT controller. In reality your panels will rarely output 40 amps as the total system is only about 60 percent efficient with all losses. The 100 amp hour battery system in your specs seem very small for this size of system which could support a 1000 amp/hour battery bank.

inetdog · April 2013

Re: 10-290W Panel OffGrid System

quique wrote: »

The 5 series would wire up in parallel to produce 90V and thus my PWM would need to be of at least 90V and 40A...but with an MPPT, i just have to worry about the Amps?!

I did not know that.

BB:

I have not yet designed the Battery Bank. It would be 24V though, just like the CC.

If you have enough panel to produce 100 amps, that will totally fry your 100AH batteries unless they are AGM and specifically rated by their manufacturer for 1C or higher charging rate.

As was stated earlier, you have enough panels in your design to handle a 1000AH battery without a problem.

If you do go with a small battery bank, and also want to be able to supply power from the panels to heavy loads during sunlight hours, you will need to use a CC with a separate battery shunt so that the CC can limit the current supplied to the battery to 10-15 amps and the rest to the inverter or other DC loads. I do not know of any CCs that can do this while in bulk mode though.

NorthGuy · April 2013

Re: 10-290W Panel OffGrid System

quique wrote: »

The 5 series would wire up in parallel to produce 90V and thus my PWM would need to be of at least 90V and 40A...but with an MPPT, i just have to worry about the Amps?

There are two different kinds of controllers:

PWM controllers must have panel voltage matched to battery voltage. C-60 is a PWM controller. With this you stuck with 10 panels in parallel. Your combined array will have Vmp of 36V and Imp of 10 x 8 = 80A. PWM controllers are very inefficent. They just chop off the excess voltage. You have 36V * 80A = 2880W of power. PWM will chop it down to battery voltage (e.g. 27V), so, you'll get only 27V * 80A = 2160W. Everything else is simply waisted.

MPPT controllers may have panel voltage anywhere above battery voltage up to the controller limit. With 5 parallel strings of 2 panels each you get your array with Vmp of 2 x 36 = 72V and Imp of 5 x 8 = 40A. MPPT will take all this energy and convert it to battery voltage with modest losses of about 5%, so you'll get 2880W x 95% = 2736W. That is like having 2 extra panels compared to PWM controllers.

MPPT controllers are more expensive, but it's very well worth it.

If you haven't designed your battery bank yet, I would suggest going with 48V. You will have less losses than with 24V and will need smaller controller.

quique · April 2013

Re: 10-290W Panel OffGrid System

So you mean the MPPT would take 72 V at 40a and change it to 28 V at 100 A?

How does it know to change it to 28 V at 100 A instead of 19 V at 150 A? Does it depend on the battery bank voltage?

As for the battery bank, I have not yet designed it. I was only using those numbers as hypotheticals. However, how would you suggest I wire the battery bank? I understand 48 volts makes it more efficient?

NorthGuy · April 2013

Re: 10-290W Panel OffGrid System

quique wrote: »

So you mean the MPPT would take 72 V at 40a and change it to 28 V at 100 A?

How does it know to change it to 28 V at 100 A instead of 19 V at 150 A? Does it depend on the battery bank voltage?

You tell it what is the nominal voltage of your batteries along with other charging settings and it automatically adjusts.

quique wrote: »

As for the battery bank, I have not yet designed it. I was only using those numbers as hypotheticals. However, how would you suggest I wire the battery bank? I understand 48 volts makes it more efficient?

Charger controller losses and cable losses will be smaller for 48V bank.

The most important is to figure out the correct size of the bank (in Amper-Hours). It depends on your loads (that's where the design should start).

BB. · April 2013

Re: 10-290W Panel OffGrid System

Just to start, a PWM charge controller is, essentially, a computer controlled "On/Off" switch. If your battery bank needs more charging current, the switch which "cycles" tens to hundreds of time a second, simply stays on "longer" during the cycle. As the battery fills, the controller starts reducing the percentage of "on time" (pulse width modulation). If the solar array and battery bank are not "matched" for voltage (and current), the PWM controller will be inefficient and/or can overheat and die.

An MPPT charge controller is the DC version of a computer controlled AC Variable Transformer. Internally it does use PWM and other things to control the energy flow--But it can efficiently "down convert" from the solar array to the battery bank (within limits).

The MPPT controller takes high voltage/low current and down converts it to low voltage/high current very efficiently (~95% efficiency).

Ignoring losses, the MPPT controller is a constant power device... Vpanel*Ipanel=Vbatt*Ibatt. (remembering that Power=Volts*Amps).

Vbatt is set by the battery voltage. And, for the first approximation, the input is set by the solar array's Vmp... The computer (and internal buck mode digitally controlled power supply) does the math to get the conversion "right".

Rather than trying to understand the whole system at once, it is better to do this in stages.

First, define your loads. Then use the loads to define the battery bank (AH and Voltage). Next, use the battery bank size, and daily loads (in Watt*Hours) to define the Wattage of the solar array.

With the above information, then we can pick the hardware to run your system.

After the first time through, it will all make a lot more sense to you.

If you wish--We can talk more about the controllers now, or later... Either is fine. But to talk about both your system design and controllers at the same time can get a bit confusing.

-Bill

vtmaps · April 2013

Re: 10-290W Panel OffGrid System

quique wrote: »

This is the system the technician designed for me, or rather, suggested we design. I just want to get your opinions on the design. I am assuming the requirements have been met according to the loads

You are wise to come to this forum for advice because you really need it. Apparently your technician should also be getting some advice. Why do you trust him/her to have correctly evaluated your loads? You have been warned several times in this thread that proper design requires an analysis of your loads... peak loads, average loads, and even the time of day that you run your loads. (for example: my battery bank is smaller than it might otherwise be because I only do laundry when the sun is shining. Therefore I don't have to store energy in my batteries for laundry.)

If you were buying a pickup truck, you would also need an evaluation of your loads... if you buy a half-ton truck when you need a one-ton truck (or vice versa) you have made an expensive mistake. The purpose of your off grid system is to power your loads. If you don't analyze your loads carefully you might buy the wrong system.

--vtMaps

quique · April 2013

Re: 10-290W Panel OffGrid System

You guys are awesome!

I do understand the loads sizing idea. I have installed systems before including the one at my house, with which I have asked for advice 2x on this forum already. I am starting to take courses oriented to certification. The truth is that the reason my technician didnt calculate the loads for the system I proposed is that its not a real system. A client down here asked me to quote them individual materials. Honduras is quite different than the US. People here dont trust vendors because they always think they are out to get them (or stick them with things they dont need). I normally do an onsite inspection, of course. But this client basically asked me for a price list. He didnt even tell me what they were going to run, he just said "Oh, just think of the business as a small home office". Well, thats very vague. I dont want to lose the client, so I decided to shoot for a more or less typical system in order to simply give him prices in the hopes that he might at least call me in to see the site once he has prices from all competitors.

Thats the way things work here. People always ask for things the wrong way, they first ask for prices on materials to compare. Its not until they see big price differences, or worse, after theyve bought into something that failed!, only then do they call to ask about value added service

However, in designing the system, I did see things I had not used before because the one at home is just a 2 -210W panel system with 1 cc. I do understand im going about the exercise wrong but I did learn a lot.

Since I read the book I bought at SEI for the PV101 course Ive learned a lot. Im starting the course next week and I do expect to learn a lot more. Since that book Ive also started rewiring my house and system to make sure it works well.

I just wanted to clear that up so that you know that I appreciate your help and that you be assured that I am taking your advice

.

Thanks again, Ill review what you've suggested so far and post back before moving forward.

westbranch · April 2013

Re: 10-290W Panel OffGrid System

quique wrote: »

You guys are awesome!

I do understand the loads sizing idea.
A client down here asked me to quote them individual materials.
I normally do an onsite inspection, of course. But this client basically asked me for a price list. He didnt even tell me what they were going to run, he just said "Oh, just think of the business as a small home office". Well, thats very vague. I dont want to lose the client, so I decided to shoot for a more or less typical system in order to simply give him prices in the hopes that he might at least call me in to see the site once he has prices from all competitors.

You are progressing nicely, but I think there is a trap here.
You really need to sit down with the client and use some real info from Him as to the usage, otherwise you may do exactly that you predicted above... you will spec. out the WRONG equipment

HTH

quique · April 2013

Re: 10-290W Panel OffGrid System

I totally agree wb. I hope to get a chance to inspect the site soon.

10-290W Panel OffGrid System

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