Solar panel voltage matching question

[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)

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.

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.   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]

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)

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.

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.

Correct.

 

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

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.

[Red Squirrel]

Good to know, so yeah in my case strings of 2 or even 3 panels would work fine.  Maybe 3 would be even better?  Of course each string has to match so I pick a VMP and stick with it for the whole system.  Guessing I'd want diodes for each string as well.  Of course that's details I'd figure out once I pick a system.  I would most likely try to find a solar dealer that can explain all of this and sell me all the accessories I need.  I'd need racking, connectors, boxes etc... 

And yeah my example with 1000w was assuming a proper battery bank, and not accounting for other losses.  Idealy I'd probably want to size the system with assumption that I'm getting half, because the rated wattage of a panel is under extreme ideal conditions.   Then take other losses into account too.

[Red Squirrel]

Another solar related question, figured I'd use the same thread.   When you have multiple strings of panels spread out over different areas, they will get different sun exposure, angle etc and thus, probably also each have a different maximum power point.   With that said, is it best to have one small charge controller for each string so they can each achieve maximum power per string, or do you just stick them in parallel and the MPPT controller will just kind of find the average maximum power point for the whole system?    Or do lot of charge controllers actually have capability to add separate strings that are handled independently?  Been pondering on where I could put panels other than the south facing roof, and I could stick some strings in places that are less optimal, but still get some sun, but I would not want those strings to basically affect the main ones.   I imagine each string would also have it's own diode to prevent the higher output strings from feeding the lower output ones as well? 

[Someone]

Another solar related question, figured I'd use the same thread.   When you have multiple strings of panels spread out over different areas, they will get different sun exposure, angle etc and thus, probably also each have a different maximum power point.   With that said, is it best to have one small charge controller for each string so they can each achieve maximum power per string, or do you just stick them in parallel and the MPPT controller will just kind of find the average maximum power point for the whole system?    Or do lot of charge controllers actually have capability to add separate strings that are handled independently?  Been pondering on where I could put panels other than the south facing roof, and I could stick some strings in places that are less optimal, but still get some sun, but I would not want those strings to basically affect the main ones.   I imagine each string would also have it's own diode to prevent the higher output strings from feeding the lower output ones as well?

You've got it almost all worked out well. Think of the panels as current sources where the current is proportional to insolation, you don't need blocking diodes when paralleling identical strings as they are all pushing current out and won't present a load while illuminated. A MPPT or just peak voltage regulator is fine for identical parallel strings even if they are each differently illuminated, though MPPTs generally aren't available in big current ratings for parallel strings.

[mtdoc]

  When you have multiple strings of panels spread out over different areas, they will get different sun exposure, angle etc and thus, probably also each have a different maximum power point.   With that said, is it best to have one small charge controller for each string so they can each achieve maximum power per string, or do you just stick them in parallel and the MPPT controller will just kind of find the average maximum power point for the whole system?

I think you mean multiple Arrays instead of strings.   With different arrays (each with one or more strings) that have different sun exposures fed in parallel into the same MPPT controller you will lose some efficiency - how much depends and may or may not justify using more than one controller.  Also parallel strings, whether part of the same array or not should all have approximately the same (withing say 5-10%) the same Vmp.

, though MPPTs generally aren't available in big current ratings for parallel strings.

I guess it depends on what you mean by "big" .  For example the Midnite Classic 150 will put out up to 96A  for a 12V system (1380 W) or up to  86A for a 48V system (5022 W).

[cravenhaven]

The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

[Seekonk]

The MPPT point for a panel will change with the irradiance

Well, indirectly.  It changes with the temperature of the panel.

[Red Squirrel]

The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

[mtdoc]

The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

The charge controllers would each be programmed with the same bulk, absorb, and float voltages (assuming LA batteries) as recomended by the battery manufacturer. Ideally each CC will have its own temp probe at the battery bank. This will usually work just fine, with each controller transitioning from bulk to absorb to float at approximately the same time. Midnite's have a "follow me" option that allows multiple CCs to be networked via  RJ45 cables and then share one temp probe and coordinate charging stages exactly. One will act as the master and the others as slaves.

[Someone]

The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

Look at how much the peak power point will vary with temperature/insolation, going from a single voltage on all strings to individual MPPT on all strings will only gain a few % in increased generation. Then you can compare the costs of adding additional MPPT and cabling compared to the additional energy produced, or adding more panels to the strings.

[Red Squirrel]

The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

Look at how much the peak power point will vary with temperature/insolation, going from a single voltage on all strings to individual MPPT on all strings will only gain a few % in increased generation. Then you can compare the costs of adding additional MPPT and cabling compared to the additional energy produced, or adding more panels to the strings.

Yeah true guess if it's not that much of a difference it may not be worth going through the trouble.  I could probably also put arrays on relays, have the relay of the less optimal one turn on when there is enough sun.   

This is more of a what if scenario anyway, chances are good I'd start with just the roof top array.    What I've been thinking about as well is I need to build a shed, I might make it use solar power for lights etc and it will make a decent solar project as a start to get an idea of what is involved.

[Someone]

The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

Look at how much the peak power point will vary with temperature/insolation, going from a single voltage on all strings to individual MPPT on all strings will only gain a few % in increased generation. Then you can compare the costs of adding additional MPPT and cabling compared to the additional energy produced, or adding more panels to the strings.

Yeah true guess if it's not that much of a difference it may not be worth going through the trouble.  I could probably also put arrays on relays, have the relay of the less optimal one turn on when there is enough sun.   

This is more of a what if scenario anyway, chances are good I'd start with just the roof top array.    What I've been thinking about as well is I need to build a shed, I might make it use solar power for lights etc and it will make a decent solar project as a start to get an idea of what is involved.

Disconnecting panel strings when they are only illuminated even from diffuse light would just reduce production capacity, the MPPT would find the peak of all the strings together and none would be "taking" power away unless they were in complete darkness (measure the output under moonlight if you don't believe it!). It's very hard to come up with situations where reverse leakage will be a problem especially if the string voltages are well above the battery voltage and using a buck regulator.

[Red Squirrel]

The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

Look at how much the peak power point will vary with temperature/insolation, going from a single voltage on all strings to individual MPPT on all strings will only gain a few % in increased generation. Then you can compare the costs of adding additional MPPT and cabling compared to the additional energy produced, or adding more panels to the strings.

Yeah true guess if it's not that much of a difference it may not be worth going through the trouble.  I could probably also put arrays on relays, have the relay of the less optimal one turn on when there is enough sun.   

This is more of a what if scenario anyway, chances are good I'd start with just the roof top array.    What I've been thinking about as well is I need to build a shed, I might make it use solar power for lights etc and it will make a decent solar project as a start to get an idea of what is involved.

Disconnecting panel strings when they are only illuminated even from diffuse light would just reduce production capacity, the MPPT would find the peak of all the strings together and none would be "taking" power away unless they were in complete darkness (measure the output under moonlight if you don't believe it!). It's very hard to come up with situations where reverse leakage will be a problem especially if the string voltages are well above the battery voltage and using a buck regulator.

I was thinking it might drive the MPPT curve down for the rest of the panels but guess I'm probably just overthinking this.

[Someone]

Disconnecting panel strings when they are only illuminated even from diffuse light would just reduce production capacity, the MPPT would find the peak of all the strings together and none would be "taking" power away unless they were in complete darkness (measure the output under moonlight if you don't believe it!). It's very hard to come up with situations where reverse leakage will be a problem especially if the string voltages are well above the battery voltage and using a buck regulator.

I was thinking it might drive the MPPT curve down for the rest of the panels but guess I'm probably just overthinking this.

There is no need to imagine anything when you have data available, strings need to be extremely dark to produce no energy into a parallel array. Attached is an example IV curve floating around the internet annotated with the voltage of the peak power point of the fully illuminated case, and it crosses the 0A axis well below any of the other traces, you could expect that a panel at just 5% illumination of the peak would still be contributing energy. But as panels heat up the peak power point voltage drops which would include more energy from the less illuminated panels.

It all falls apart when you have partial shading of significant parts of a panel/string, then you might consider shorter strings and higher paralleling or optimisers/micro inverters with MPPT per panel. Or positioning the system so it won't be shaded.