Electronics > Power/Renewable Energy/EV's

Solar panel voltage matching question

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Red Squirrel:
For a while I've been toying with possibly setting up a small solar system - going off grid is a dream but reality is it probably won't happen where I am due to size of property.     Now for my actual question, solar panel nominal voltages seem to vary, some of the 280w ones I'm finding are about 30 volts for example, I've seen some that are 36.   This is an odd ball number as from my understanding you typically size a system so that it's 12 , 24 , 36  or 48 volts (among some other higher voltage setups too).  But given solar panel voltages have quite a large range anyway, and the whole point of the controller is to bring that down to a nominal battery charging voltage (ex: 54v for a 48v system) do I just make banks of 2 30v panels in series and call it a day?  The rated voltage is what it will produce in it's maximum power point, but the controller will bring that down to the proper charge voltage anyway right.   Am I understanding that right, or do I really need to ensure to find panels that are rated in a multiple of 12, so that I can match the voltage correctly?   My train of thought is that you kind of want to have a higher nominal voltage than your battery bank nominal voltage so that you can still generate power when they are not running at their peak.  Am I right in this thinking?

So as an example, this panel: https://www.wholesalesolar.com/cms/suniva-solar-panel-specs-3283399253.pdf

With a 48v charge controller, I would have these panels setup in groups of 2 in series, for a nominal 60v going into the charge controller, which would then bring it down to a proper battery charging voltage, ex: 54v.   Each "bank" of two panels would be 280w.  So 4 banks or 8 total panels would be 1.1kw.

There are also prepackaged systems, so I imagine it comes with all proper documentation on proper setup, so I would possibly just go with one, like this:

http://www.wholesalesolar.com/1892640/wholesale-solar/complete-systems/4.2-kw-grid-assisted-solar-system-with-outback-radian-and-15x-suniva-280-panels#components

But piecing it together myself could be nice too as I could oversize some stuff for future expansion.  I'm far from being able to afford this now, so just looking at various options at this point.

DenzilPenberthy:
Basically yes, you're correct.



Have a look at this graph, As you move along the red line, you are changing the resistance of the load attached to the PV panel.

 Top left is Isc which is the short circuit current of the panel i.e. a load of zero ohms. Maximum current but no power because there is no voltage developed across the load (P=IV). Bottom right is Voc (load of infinity ohms) which is the open circuit voltage of the panel. Maximum volts but again no power because there is no current flowing.

The blue line shows how power delivered into the load increases as load resistance increases then decreases again when the load resistance is too great.

If you directly connect your panels to your battery, you will force the panels to operate at the point on the red line which corresponds to your battery voltage. It will work but it is unlikely to be near the maximum power point.

A proper charge controller will be able to present a variable resistance to the panel and adjust itself so that the panel is operating at its maximum power point and convert this power into an appropriate charging voltage for the battery.

mtdoc:
The issue is this:

Most PV panels produced now are 60 cell panels  with Vmp somewhere around 30 V. They are sometimes sold as "24V panels" but they really are not. They are designed to be used in large high voltage string found in grid tie systems (the vast majority of PV systems).

"12V panels" are sold which have a Vmp or 17 V or higher.

There are very few true 24V panels made.

To reliably charge a 24V battery bank you needs a Vmp of at least about 35V.

So what do people with 24V or 48V off grid or grid tied with battery back up systems do?

They combine the common 60 cell panels with Vmp's less than 35 V into strings of 2 or more (for 24V system) or 3 or more (for 48V system) to get the voltage high enough and then use MPPT charge controllers (which buck the string voltages down to the appropriate levels) to charge their battery bank.

Red Squirrel:
Yeah that's kind of what I was wondering, but I figure it should be safe to give 60+ volts to the 48v charge controller right?  Open circuit voltage of those panels for example is 38v, so the charge controller would see 76v max.   Found this in the documentation of the Midnite controller. (not dead set on any equipment just using that one as example)


--- Quote ---The Classic offers some unique opportunities if you are faced with longer than normal wire runs
between the DC source and the Classic. The Classic comes in 3 input voltage ranges letting you design
a DC source at a higher voltage if it is beneficial. For example let‘s say you have a 300 ft run from a
PV array to the Classic you could wire for an open circuit voltage close to 250vdc accounting for the
coldest temperature you will encounter
. This will allow you to run a smaller gauge wire than with a
lower voltage charge controller. The efficiency of a high voltage Classic is less than the lower voltage
versions, so you need to weigh the benefit. If this sounds too complicated use this rule of thumb in
selecting the proper Classic. PV runs up to 100 feet, use the Classic 150. Runs up to 180 feet, use the
Classic 200. Above 180 feet use the Classic 250.
--- End quote ---

So I'm thinking these are probably typically designed to handle quite a large input voltage range, so I guess I would more than likely be fine to connect higher voltage strings.  But guess the closer you are to nominal the more efficient it will probably be right?     Ex: I probably would not want to go with 100+ volts pv if I don't actually need to.

As a side note, I think I was having a brain fart in another statement I made about wattage and series panels, I was thinking if I have two  panels in parallel, then I don't add up the wattage, but I do, right?  So whether my panels are in series or parallel, my system wattage is equivalent to how many panels I have and not how many strings I have.  I kept thinking in terms of amps and not watts.  Basically, 1000w of solar, no matter what the nominal voltage of the system is, should power 1000w of AC 120v right?  (of course there are conversion losses etc)  So now that I realized this brain fart of mine, that changes a lot, I may be able to fit way more watts on my roof than I was originally thinking.  And if I only need 2 panels in a string, that also opens options to stick some more in less optimal areas that may still get sun for a part of the day but not all day.   Next time I'm off I'm going to go measure my roof then compare typical panel dimensions to figure out how much I can put. 

At this point I have zero money to do this, but the more I think about it the more I want to despite it not being that viable here for most of the year, still will produce more than if I don't have any.   Maybe I can throw it in the mortgage. :P  I would imagine that's what a lot of people do when they get these systems.  I pay a lot extra into the mortgage anyway so not like it would throw me behind.  Have lot of time to think about this anyway.

mtdoc:

--- Quote from: Red Squirrel on August 20, 2016, 12:07:39 am ---Yeah that's kind of what I was wondering, but I figure it should be safe to give 60+ volts to the 48v charge controller right?  Open circuit voltage of those panels for example is 38v, so the charge controller would see 76v max.   Found this in the documentation of the Midnite controller. (not dead set on any equipment just using that one as example)
--- End quote ---

For a 48V battery bank the String Vmp should be 70v Minimum .  With long wire runs from array to battery bank the voltage drop can be an issue to consider as well.  Both of my array's have string Vmp's of about 90V

It's better to err on the side of higher string voltage to have headroom for less than optimal conditions - but within reason - i.e a 100 V Voc for a 12 V battery bank will mean significant efficiency losses.  Also keep in mind Voc limits of your controller.

The Midnite Classics are excellent charge controllers - They are very well built and customer service is unbeatable. Their design engineers answer questions on their user forum and may even answer the phone if you call.  I have 2 Classic 150s  in service now.


--- Quote --- So whether my panels are in series or parallel, my system wattage is equivalent to how many panels I have and not how many strings I have.
--- End quote ---

Correct.

 
--- Quote --- Basically, 1000w of solar, no matter what the nominal voltage of the system is, should power 1000w of AC 120v right? 
--- End quote ---

Not exactly. The size of the loads your system can handle depends on the size of your battery bank and the max output of your inverter, not the size of your PV array.

Your PV array should be sized to adequately charge your battery bank. 10% charge rate is a good rule of thumb. So for 48 V 200 Ah battery bank (20 hour rate), you'd want to be sure your PV array/ charge controller can supply the battery bank up to 20 Amps at the appropriate voltage (say 58V during bulk charge fro typical FLA batteries) during peak sun hours.

Don't forget to factor in that a 1000 W STC rated PV array can only be counted on to deliver 70-80% of that power to the batteries in typical full sun conditions.




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