Solar Panel Design Question

[Glennman]

I've been having a hard time trying to determine the configuration (series/parallel) that I need with the panels and coupling that with the voltage of the finished system. I can fit ( 305 watt/39 volt open circuit panels on my roof, so I am hoping to get the max usage possible from them. I have 4 of them now and plan to get 4 more. I've read online the numerous configurations that are possible, but I cannot get any ideas on what to do with what I have. I'm hoping to capitalize on the knowledge everyone has here. Being that I have yet to pick up 4 more panels, I am not locked into anything yet (I can limit myself to 5 panels or 6 or 7 or 8, etc. depending on what is ideal for 39v and 305 watt panels). I know that I will need to design the controller, inverter and batteries accordingly, but I cannot even seem to get past the panels themselves.

If I was to install a 24 volt system, what would be the best series/parallel configuration for 39v o.c. 305 watt panels?
If I was to install a 48 volt system, what would be the best series/parallel configuration for 39v o.c. 305 watt panels?

I understand that I am designing backwards and one needs to know the loads on the system. However, I can get to all that eventually. For now I need to know how to get to either 24v or 48v with these panels. Anyone able to weigh in? Thanks!

[windtrader]

You have several options on configuring solar panels. The critical component is the solar charge controller as it is the interface between the panels and the battery, inverter.


Put some numbers into this calculator to get an idea of what Victron offers to match your design.


https://www.victronenergy.com/mppt-calculator

[bobofthenorth]

There's a lot of moving pieces in that decision:
- your controller will have a maximum voltage it can handle so pick the controller after deciding on the array layout
- more panels in series yields higher voltage @ lower amperage which means smaller wire gauge required
- one shaded panel in a series string will limit the amperage through the entire series array

Personally I lean towards more panels in series to reduce the wire gauge required. 

If you get three people discussing solar arrays you'll have at least four competing opinions as to the best solution.

[richard5933]

I set up out system in a combined series/parallel configuration but not like the typical one.

We currently have three 180w folding panels which we ground deploy. I use them in a series configuration for two reasons. Main reason is that it allows me to use a smaller gauge wire to connect it to the bus, and to use a 50-ft cable for this so the panels can be in the sun while the bus is in the shade. If we're depending on solar we're likely not running a/c, so I wanted to be able to maximize shade.

The other reason is that it allows me to easily use one, two, or all three panels depending by simply adding them to the chain.

The solar charge controller was sized so that at a later time I can add a similar array to the roof, which will also be set up in series to minimize wire size needed to be snaked through the chases. This roof top array will be connected to the controller in parallel with the ground array.

Whatever you decide to do, it might be a good idea to figure out your future expansion plans for solar and install a controller with the needed capacity now. No point is spending money for a controller and doing the install only to have to rip it out when you enlarge the system next year.

[David Anderson]

I am not a solar guy, but some busnut posted this guy's link here on the MAK board to his blog about solar.  He is super technical but a wealth of experience in success and failures. 

Grab a cup of coffee and read it.  It is pretty long.

https://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

David

[Iceni John]

Because my eight 255W panels are hinged off a central walkway between my two roof hatches, each array of four panels will usually be producing a slightly different power than the other array due to the different angles of each array.   For this reason I run each array to its own charge controller that in turn charges its own bank of batteries:  in effect I have two completely separate independent systems running in parallel.   Redundancy is good!   I wired each array's four panels in parallel, so if (when?) a panel is shaded or covered in bird poop or leaves it won't drag down that entire array's output.   While it's a good idea to maximize voltages through the cables for home systems due to their larger size and longer cable runs, on any bus the entire installation is necessarily much smaller, making any savings by using thinner cables an insignificant drop in the ocean when compared to the entire system's cost!   (However, having remote panels may make higher voltages worthwhile.)   Each of my panels produces 8.5 amps, so a 4AWG downfeed cable for each array to its CC is plenty large enough:  I ran both positive and both negative cables inside a roof rib to protect and hide them.

Another thing to consider:  the greater the voltage difference between the input and output voltages of an MPPT charge controller, the lower the CC's efficiency and the greater its heat output.   Morningstar suggests having no more than a 2:1 ratio between in and out voltages, at which the CC's efficiency is in the very high 90s percent; bigger ratios make the electronics work much harder and produce more heat, which will shorten the CC's life.

While many folk like the fan-cooled CCs such as Outback's or Midnite Solar's, they can have a major drawback in an RV or bus PV system.   The fans can introduce a lot of dust or moisture into the CC's interior if used in dusty or humid environments such as most of the US Southwest or South, and this again can shorten the CC's life.   The single biggest cause of failure of fan-cooled CCs is the fan itself!   Convection-cooled CCs such as my two Morningstar TS-MPPT-60s will probably last longer than fan-cooled CCs when used in RVs and bus conversions, plus they're totally silent in operation.

HandyBob is well worth a read, but so is the collective wisdom and experience on the Northern Arizona Wind & Sun forum:  if the experts there recommend doing or not doing something, trust their advice!

Don't forget to include an easy and safe way to wash your panels on the bus roof.   This was one reason I built my walkway there, and I installed two quick-connect water outlets under the walkway so it's very easy and completely safe to wash the panels with a washdown brush and a short curly hose.

John

   

[Glennman]

I know my question is very loaded. The 2:1 ratio is in line with what I have read, for the most part. One guy suggested I install 9 panels so that I can have 3 sets of 3 panels in series/parallel. In other words, I would have 3 panels in series at a time, and those 3 groups would run parallel to get to 3, 117v arrays. Then size the CC accordingly. The way I understand it then is that if I install a 48v system, I would be at a ratio of about 2.5:1. Then if I have some shaded panels, I would have a better chance of having at least 117v in one or 2 of the arrays. Does this make some sense? This is getting pretty complicated, but the more I ask, the more I gain some understanding. Things are starting to come together a bit. I plan to install the panels for now, then decide on the configuration once I get it figured out. I really appreciate everyone's help in this. Very valuable information.

[jozef]

Glennman:

In your original request, you state that you will install 4-305 watt solar panels initially and then consider adding 4 more and that these panels have an open circuit voltage of 39 volts.  You are considering either a 24 volt or 48 volt system.

The 305 watt panel at 39 open circuit volts will have a optimum operating voltage of about 32 volts.  This is too low to charge a 24 volt battery bank with a MPPT charge controller.  If I was doing this solar system I would only consider a 24 volt bank.  I would then wire the four panels in a series/parallel system.  This would give you an optimum operating voltage of about 64 volts (78 open circuit volts) and a current of about 16 amps.  I would then feed the solar panel output to a Victron Energy Smart Solar MPPT 100/20 charge controller (or equivalent).  The 100 volt rating of this charge controller is higher than the 78 open circuit volts but this voltage is at standard operating temperature.  At colder temperatures, voltage on the solar panel increases and will approach 100 volts.  Please note that a solar panel will only output its optimum voltage and current for a short period of time when the sun is at its highest point in the sky.  Through most of the daylight hours the solar panels will be operating below optimum voltage and current but if sufficient voltage exists above the battery charging voltage then the charge controller will still be feeding some amps into the batteries.  This factor is more important than the loss of MPPT efficiency at peak output and max voltage since most of the time the solar panels are operating at lower voltage and amp output

When adding the additional 4-305 watt panels I would duplicate the initial setup and have two MPPT charge controllers.  This adds some redundancy and very much minimizes output loss due to partial shading of the solar panels.  To allow for this I would have two wire pairs run from the roof to the charge controller location.  Also note that the MPPT should be as close to the batteries as possible.

[Glennman]

Thank you Jozef... So with your advice, I could install my 4 panels, wire 2 panels in series and run those parallel with the other 2 series panels giving me 2 arrays at 64/78v (78 being o.c.). Then as I add additional panels, add in sets of 2 (and 12 volt to 24 volt in series batteries, which could also be added in sets of 2). Install 2 sets of wires from the roof to two MPPTs and install those close to the batteries. This would be set up for 24 volts. That's the first I've heard of installing the MPPT's close to the batteries. That makes perfect sense (learning more with every inquiry). I like that because it is cheaper to add in sets of 2 than in sets of 4! 8 panels would be most I could install anyway, as I would run out of roof area. Thanks again.

[windtrader]

Check out SBMS0. It's been working great and is based on an entirely different design. Cost is very low and simple to install and expand. It is a combined battery charge controller and integrated BMS too.

[freds]

IMHO mounting panels is a lot of work.

So even if you have to spend more money; get the highest wattage panels that you can.

In our installations you will never get full output from the panel due to the mounting angle. So it is basically impossible to over panel even if it is more than your solar charge controller's can handle.

I went overboard on my installation and mounted ten panels on the top of my bus. Which I was able to do as I mounted them above the air-conditioners.





This brings up the next point the larger commercial panels have to be shipped on a double pallet so you most likely will have to go pick them up at shipping depot and it typically cost just as much to ship a couple of panels as it does ten. So hey look around and talk to friends to see if they would like to get some also.

I got my panels from Santan solar in AZ which were used and about 80% of the wattage of top of the line panels at the time.

I wired all my panels in parallel in case of shading and I have a 24V house battery pack. If I would have had a 48V pack I would have put two panels in series and paralleled the groups.

There's more details on my build thread:

https://www.busconversionmagazine.com/forum/index.php?topic=34318.msg404307#msg404307

[Glennman]

That's awesome Fred. I do recall looking at your system back in the day when you posted all that, but I forgot it was there. I almost bought a bunch of those huge panels but ended up with 305 watt 39"x65" panels (approximately). I'm sure I could get 10 or maybe even 12 up there, especially if I eliminate the roof hatch at the front. I'm keeping the rear roof hatch so that I can get up there and service the panels.