400w system - panel options?

[FlyFishn]

All,

I bought a charge controller over the weekend - works with both 12v and 24v systems, mppt, and is rated to 400w in a 12v nominal voltage system.

I was told the MPPT controller works best above 36 volts panel voltage. I am getting mixed info on the top end - I think it is 100v open panel voltage and 75v under load, but I am not 100% sure on that. As it was described - the MPPT aspect works all the way through the range on the upper end, but on the low end it does not work well below 36 volts. The controller is said to work with down to 17 volts panel voltage for a 12v system.

I would like to maximize the power production with the controller. It was a bit of an impromptu purchase - I didn't go in to it with specs in mind, I just looked at the opportunity to get it and it fit a lot of ideas I've had in the past. So I am sort of going about the backasswards I think, but none the less I'd like to maximize what I have.

That is where I have my questions.

With the 400 watt limit and the voltage in mind of "no less than 36 volts" the direction I am pointed towards is linking panels in series to get the voltage up.

This is the panel I am looking at - Bioenno BSP-100.

Another source for that one shows the voltage at 18v. I'd think the 18v is more realistic - only in ideal peak conditions would it get any higher, if that.

If I ran 2 of those in series, or 2 in parallel then those pairs in series, that would get me 36-42 volts in peak sun. Or, if I ran 4 in series that would be 72-84 volts (again, not sure how realistic it is to get above 18v or so per panel).

In low light conditions the option of running more panels in series to get the voltage up seems to make a lot of sense. With 4 panels in the realm of possibilities here - is that enough? Or would I have more options if I had smaller panels - like 50w panels instead of 100w? Would 2x 200 watt panels be worth a consideration - especially if the open panel voltage is a lot higher like in the 30-40v range?

One of the other benefits to running higher voltage is lower amperage at the same wattage. Lower amps = less line loss to the charge controller. So what makes sense to "get there"? Wherever "there" is - panel count vs system input voltage?

For what it is worth, this is the controller I got Bioenno sc-122430ne.

I think they have changed their model around a little bit. Mine has different specs than the manual you can download (400w panel capacity at 12v, 800w panel capacity at 24v).

 

[bigrockcandymountain]

I would just go with a modern 72 cell 400w panel.  The voltage will be about 40v under load. 

[FlyFishn]

Another tidbit is the intended system is to be portable - the panel(s) won't be permanently installed, though I may make a frame I can set them on to better hold them in-use.

Do you have an example of said modern 72 cell 400w panel?

[bigrockcandymountain]

https://store.santansolar.com/product/new-canadian-solar-390w-144-half-cell-bifacial-solar-panel/

They will be big and heavy and not ideal for portable but they are usually the cheapest per watt.

  I guess the answer to your question is with a mppt controller with 100v max input, you can make almost anything work.  I would stay in the 30v to 60v under load for charging 12v batteries, but it should work. 

2x 200w panels in series would probably be my choice for portable.  Parallel might be just as good or better.  It's hard to say exactly how the mppt will track. 

[OperaHouse]

A panels MPP voltage is based on temperature, not light level thru the useful range.  Extra panels in series may help with shade when the other panels force a very shaded one to go into bypass. Many low cost controllers can get lost in tracking if the panel voltage goes over 2 1/2 times the nominal battery voltage in partial shade conditions. I can not stress how much panel performance will drop with only thin line shading.

[FlyFishn]

Interesting replies.

Let me ask this then -

Is it reasonable to plan for lower light conditions to try to get more panel wattage under less-than-ideal conditions? In this question - that would mean more peak wattage than the controller can handle.

The vendor I got the controller from suggested fusing both the panel end and battery end with appropriate size fuses. If the above is in the realm of reason (more panel wattage at peak than the controller is rated to) - the fuses would blow.

In this line of thought - it would be conceivable to build a current sensing circuit that would send power to a dump load to drain the excess, when there is excess, so as to not overload the controller.

Yes, dumping power in ideal conditions is a shame. However, in the studies I've done on the alternative energy subject over the past several years - it comes with the territory. In order to have sufficient energy generation in normal conditions you are inevitably going to have a (significant) surplus in peak conditions. If you plan a system to provide your power needs at peak you will significantly short-change yourself on power when production is low. Yeah there are site studies you can do to maximize the efficiency/matching of the conditions, but you can only get so "ideal". You will inevitably have more power in peaks some times.

In the dump load idea - it would allow more power in lower intensity sun conditions. That would be a good thing.

[OperaHouse]

Peak power is at most 2 hours a day.  I have special shading conditions and run4,000W into a 500W controller. Controller seldom gets anywhere near that.  Some controllers will limit power and there are no problems doing that. I also run some east facing panels. These panels actually produce the most power at any one time.  This is power when it is actually needed to recover in the morning. I don't waste any power as any momentary excess power goes into heating water.  I have a friend that does the same thing with only 650W of panels at his camp which heats 6 gallons of water. As this camp is only used on weekends, any extra power goes to a space heater after the water tank is up to temperature.  Most people have no idea of how much power they waste in their solar systems.