Semi-Large Scale System Logistics

JTCampbell1 · May 2018

Hello!
I am designing a system that currently is really confusing the hell out of me, and would love any advice/help!
I am trying to design a hybrid solar system to cover the needs of a 4 story orphanage in India. We have identified that we would need 22 mono-crystalline panels that (at their peak) produce 370 watts, 38.4v, and 9.63a. So, we decided to make a system that connects series of 2 panels in parallel to reduce costs slightly. This would at the end be producing 76.8v and 105.93a. However this is where it gets tricky. I am trying to identify the right wiring i would need for this system. I would ideally have a fuse box at the end of the parallel system then have it either go straight into hybrid inverter or into a MPPT charge controller attached to a string of batteries that an inverter is connected to. Here are the main problems in summary:
What wires should i use for the system? (obviously there are many different ones)
Should i revise any part of my system?

Thanks!

BB. · May 2018

Back up a moment. What are your loads (watts peak, watt*hours per day). What Amp*Hour and voltage battery bank?

How far from array to battery charger and battery bank?

What Brand and models of mppt charge controllers and ac inverters look good for you?

Bill

mcgivor · May 2018

@JTCampbell1 said

This would at the end be producing 76.8v and 105.93a. However this is where it gets tricky.

The 11 panels in series would be 422.4V ( corrected form previous 76.8V thanks to @littleharbor2 for spotting the error) but the current would remain at 9.63A. Series voltage adds current remains the same, parallel the current adds, voltage reman the same. The two strings of 11 panels would be 76.8V at 19.26A, understandable mistake.

littleharbor2 · May 2018

Sure that's not 422.4 volts @19.26 amps @ 11s2p? or as OP said 76.8v and 105.93a @ 2s11p?

mcgivor · May 2018

Got me, the voltage would be 422.4V with 11 panels in series.

Estragon · May 2018

Like Bill said, more info required.

ASSuming:
- the panels will be roof mounted
- the combiner will be close to controller, maybe a mechanical penthouse
- the battery bank is in the same room
- the bank is 48v nominal
- 38.4 is Vmp
- proposed wiring is 11 strings of two

Each string will go (typically with 10ga MC4 extention wire) to a combiner box where each string positive is wired to ~15a breaker. There will need to be multple controllers to handle the current, so the combiner would be set up to make multiple parallel connections, so 6 strings on one and 5 on the other, or maybe 4, 4, & 3 to 3 controllers. The wire size would be determined by distance and current to keep voltage drop down, and breaker sized properly for chosen wire. The 2 or three controllers would be wired to +ive and -ive common bussbars, then a single heavy wire to bank, via properly sized breaker.

If this is a hot part of India, strings of 2 could be a problem. 38.4 will likely be Vmp at STC (25°C). In normal operation the panels will likely be much warmer, and string voltage lower (possibly too low for proper mppt operation). Strings of 3 or more would be better.

A lot depends on stuff we don't know though, like array distance,;bank voltage etc.

JTCampbell1 · May 2018

Estragon said:

Like Bill said, more info required.

ASSuming:
- the panels will be roof mounted
- the combiner will be close to controller, maybe a mechanical penthouse
- the battery bank is in the same room
- the bank is 48v nominal
- 38.4 is Vmp
- proposed wiring is 11 strings of two

Each string will go (typically with 10ga MC4 extention wire) to a combiner box where each string positive is wired to ~15a breaker. There will need to be multple controllers to handle the current, so the combiner would be set up to make multiple parallel connections, so 6 strings on one and 5 on the other, or maybe 4, 4, & 3 to 3 controllers. The wire size would be determined by distance and current to keep voltage drop down, and breaker sized properly for chosen wire. The 2 or three controllers would be wired to +ive and -ive common bussbars, then a single heavy wire to bank, via properly sized breaker.

If this is a hot part of India, strings of 2 could be a problem. 38.4 will likely be Vmp at STC (25°C). In normal operation the panels will likely be much warmer, and string voltage lower (possibly too low for proper mppt operation). Strings of 3 or more would be better.

A lot depends on stuff we don't know though, like array distance,;bank voltage etc.

These assumptions are correct. I apologize for not giving more information in the beginning. There would be a relatively short distance from the roof to the rest of the system. However, I feel like people are missing that these panels are connected in series of 2 that are then connected in parallel. Also this part of India gets a fair amount of daylight hours, but has an average temperature around 25°C (it never gets above 30). Also, I would use adapters between the panels to link them in parallel, and would end with two wires in the end. Is this a bad system and that is why you dont mention that?

mcgivor · May 2018

When panels are to be in parallel, or series/parallel, each string of panels, or individual panel in parallel, should be protected with a fuse or circuit breaker if the number of string/panels exceeds two. This so achieved with a combiner where all the individual strings feed into a circuit breaker or fuse before being commoned up to a larger conductor which will carry the now higher current to the charge controller. The attached pdf is very informative with regards to the subject.

Estragon · May 2018

In addition to fusing each string, you also have to determine whether strings of two will generate high enough voltage for the mppt controllers. Although ambient temps may rarely exceed 30°C, the dark panels will be much warmer in full sun. If the panels your're considering have a spec for Vmp at normal operating cell temp, you could use that. If not, there should be a spec for a temp coefficient to adjust voltage for an operating temp of 45°C.

The rule of thumb for min voltage on mppt controllers is 130% of charging voltage. My 48v bank has an absorb voltage set to 58.8v (some brands recommend higher), for example, so 58.8 x 130% = 76.4v. That's about what a string of two of your proposed panels produce at 25°C. In the warming sun, they make not produce high enough voltage. Even in minus 30°C weather, my panels get warm enough to melt snow in full sun, and will drop in string voltage from ~120v at dawn to ~105v in full sun. In a string of two, they wouldn't be able to charge properly, especially in warm summer weather. Your panels have a slightly higher Vmp (STC), and may have a different temp coefficient, so you will have to adjust the numbers a bit, but I suspect strings of two may be an issue.

On the flip side, you need to make sure the Voc at record LOW temp is well below the max voltage allowed for the controller. Many are 150v max, so strings of three would work well. Some are lower though, so you'll need to do the arithmetic if considering a lower max voltage controller.

Some controller makers (eg Midnite Solar) have string size calculators on their websites to assist with this.

JTCampbell1 · May 2018

What do you think about this? I know the scale is wrong and what not, but i feel like the idea is still there. I wasn't exactly sure where to include the breakers in this. Should I put it after each series connection? Also what size charge controllers would you recommend?

Dave Angelini · May 2018

This kind of screams for a High voltage Mppt. Much easier, less loss, less parts...

Horsefly · May 2018

I agree with @Dave Angelini. With that many panels, using a HV MPPT can greatly simplify your wiring and get you down to a single charge controller.

JTCampbell1 · May 2018

Horsefly said:

I agree with @Dave Angelini. With that many panels, using a HV MPPT can greatly simplify your wiring and get you down to a single charge controller.

okay then it would look something like:

What size charge controller would you recommend?

Dave Angelini · May 2018

I would use the Schneider Mppt-80-600 and complete the system with all Schneider equipment. There soon will be a second choice with Morningstar finally building an inverter for their hi voltage mppt. Even Outback will soon have one in their Skybox. The op can drop me an e-mail also. Building a system with one brand gets you compatibility from networked equipment. Monitor the system from anywhere with the internet.

JTCampbell1 · May 2018

How does this look?

JTCampbell1 · May 2018

Dave Angelini said:

I would use the Schneider Mppt-80-600 and complete the system with all Schneider equipment. There soon will be a second choice with Morningstar finally building an inverter for their hi voltage mppt. Even Outback will soon have one in their Skybox. The op can drop me an e-mail also. Building a system with one brand gets you compatibility from networked equipment. Monitor the system from anywhere with the internet.

I dont think my voltage fits into this category.

Dave Angelini · May 2018

Why not? You will have to have an even number of panels. Keep one for a spare or buy one....

JTCampbell1 · May 2018

Dave Angelini said:

Why not? You will have to have an even number of panels. Keep one for a spare or buy one....

If I change to series of 3 like the diagram then I will have 21

mcgivor · May 2018

Using 2 600v controllers with 10 panels in series on one and 11 on the other would simplify things and reduce wiring size. Something like this https://www.morningstarcorp.com/products/tristar-mppt-600v/
Using the string calculator a ballenced system can be achieved http://string-calculator.morningstarcorp.com/
The Morningstar is one example, Schneider is another,there may be others, but Morningstar has the string calculator which simplifies posting links, I've no affiliations with the company.

Dave Angelini · May 2018

I do have an affiliation with Schneider Electric solar both in business and stock!

They also make the hi voltage mppt in India at a large plant near the OP. You can use 1 controller or 2. If you use only one it will limit its output at 80 amps and the strings would need to be the same voltage. Mcgivor is the way to go for longevity but since solar rarely hits over 75% and for offgrid it really is about having the power on marginal days one works just fine with 7kw. I have done it many times!

JTCampbell1 · May 2018

Dave Angelini said:

I do have an affiliation with Schneider Electric solar both in business and stock!

They also make the hi voltage mppt in India at a large plant near the OP. You can use 1 controller or 2. If you use only one it will limit its output at 80 amps and the strings would need to be the same voltage. Mcgivor is the way to go for longevity but since solar rarely hits over 75% and for offgrid it really is about having the power on marginal days one works just fine with 7kw. I have done it many times!

Does Schneider have a hybrid inverter that could do this job and be an inverter? if so which one would you recommend for the diagram i posted above? How could i get this in India?

Dave Angelini · May 2018

You want me to search in India where you live? I think you can do that. Look for Schneider electric XW+. It is available in 6.8kw and 5.5kw.

Here is a link to their web site and it will help you find dealers in India.
https://solar.schneider-electric.com/tech-support/

here is a link to the XW+ from the above site. I assume India is 230VAC?
https://solar.schneider-electric.com/product/conext-xw-hybrid-inverter/

Semi-Large Scale System Logistics

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