No Grid Power = No Solar Power

[AndyH]

I don't care how many solar panels are on the grid - I want to know how many people will be able to keep their food from rotting in the refrigerator next time a superstorm rolls through. And that's the point of the thread, I think...

If that is what you want to know then you need to include all the people who have fossil fuel generators as backup. Much cheaper to buy a ICE generator and convert it to natural gas. Still has maintenance costs though.

No I don't, because I'm not talking about generators. One does not automatically require the other.

I thought the Point was No Grid no Power..

Yes, the point is 'no grid no power' - and I'm talking about how to use the PV with the grid down or non-existent - I'm not talking about letting the grid-tied system go to sleep so that I 'get' to keep adding another five gallons of gas to the generator. I think Queen missed the context before you piled on , though I may be wrong...


Can anyone guide me to charge controller info that says they intentionally drop away from MPPT and move into an inefficient place on the E/I curve? I've read through some MPPT code but don't have much experience with this - but I've not seen or read anything that suggests this is true. The only instance I know about where panels are allowed to intentionally warm is when a charge controller has a 'snow melt' function. Thanks!

 

[Smidge204]

And if the batteries get full because you are so good at conserving? That's what Smidge204 is getting at. Your system has to have a way to stop charging the batts when they are full, the grid is down and the sun's still shining.

That is exactly what I was trying to get at. The root of the misunderstanding was some phrasing that implied his system never throttled or shut off, so I was trying to make things more explicit. Thanks!

Can anyone guide me to charge controller info that says they intentionally drop away from MPPT and move into an inefficient place on the E/I curve? I've read through some MPPT code but don't have much experience with this - but I've not seen or read anything that suggests this is true. The only instance I know about where panels are allowed to intentionally warm is when a charge controller has a 'snow melt' function. Thanks!

I've also not found any info on inverters using MPPT algorithms to purposefully decrease output - it seems the kind of thing that would be technically possible though. Best I know of is AC coupling inverters purposefully producing AC waveforms with out of range frequency (60.6Hz) to trick downstream solar inverters to shut off.
=Smidge=

 

[RegGuheert]

Can anyone guide me to charge controller info that says they intentionally drop away from MPPT and move into an inefficient place on the E/I curve? I've read through some MPPT code but don't have much experience with this - but I've not seen or read anything that suggests this is true. The only instance I know about where panels are allowed to intentionally warm is when a charge controller has a 'snow melt' function. Thanks!

I cannot think of a source, but if I do find one, I will provide it.

But the point is that is the only thing an MPPT controller can do. The I-V characteristics of a PV cell (or array) are dictated by physics: it's design, the temperature and the light striking the panel. The ONLY thing any controller can do is operate on that curve. The old PWM controllers either operated at open circuit or at the battery voltage. The MPPT controllers allow movement along the curve. In order to regulate charging current, you simply move to the operating point which provides the desired current.

 

[planet4ever]

Your system has to have a way to stop charging the batts when they are full, the grid is down and the sun's still shining. You have to do one of the following:

1. Shut down the panels entirely until the battery level drops enough to start charging again. (It puts extra cycles on the battery, but doable if the system doesn't run in the off-grid mode very much).
2. Throttle the inverter output to match what you're currently drawing.
3. Dump the excess generation into a dump load.

Dave's options make perfect sense to me. Excuse my ignorance, but what I don't understand is why people seem so hung up on needing a dump or MPPT deoptimization. Why not simply disconnect the panels for a while? It seems so obvious and so simple. Surely they are not going to burn up or short out if they are just sitting there with no place for the electricity to go. I know when my system was installed the panels were up on the roof for a week before the utility authorized the inverters to be turned on.

Ray

 

[RegGuheert]

...but what I don't understand is why people seem so hung up on needing a dump or MPPT deoptimization.

Like it or not, this is exactly how MPPT controllers work. Grid-tied inverters DO "disconnect" the panel when the grid goes down. But they also sometimes deoptimize their operation to prevent them from exceeding their own maximum output power.

Charge controllers RARELY need to reduce the charging current to zero, even when the batteries are full, because they still need to provide the current required to operate the load. A well-designed charge controller provides two main functions: 1) It maximizes the energy harvest from the PV array and 2) charging the batteries without overcharging them. Switching off the array only suits these two purposes if the batteries are fully charged and there is no load on the system.

 

[DaveEV]

Does the Sunny Boy inverter have a control input so that it can be commanded (by the Sunny Island, or whatever) to "throttle back" (reduce) its power output to any level less than its full (depending upon the amount of sunshine) output (that it normally would be sending to the house and the grid?

Yes, the Sunny Island will shift the frequency slightly around 60 Hz which tells the Sunny Boy to adjust it's output accordingly. If the batteries are full, it will shift the frequency up a bit, if the batteries are empty, it will shift the frequency down a bit. The Sunny Boy inverter (when programmed to respond to frequency shifts like when paired with a Sunny Island) will respond accordingly.

Yes, that's one way to do it. The other, perhaps more old-fashioned way is to turn on a dump load (eg resistive heater) that just consumes the excess power.

Dump loads are another way of making the most of your power - electric water heaters are often used as dump loads.

If you want to get really fancy, you could combine dump loads and inverter throttling if you have the appropriate dump loads that do something useful.

 

[AndyH]

Can anyone guide me to charge controller info that says they intentionally drop away from MPPT and move into an inefficient place on the E/I curve? I've read through some MPPT code but don't have much experience with this - but I've not seen or read anything that suggests this is true. The only instance I know about where panels are allowed to intentionally warm is when a charge controller has a 'snow melt' function. Thanks!

I cannot think of a source, but if I do find one, I will provide it.

But the point is that is the only thing an MPPT controller can do. The I-V characteristics of a PV cell (or array) are dictated by physics: it's design, the temperature and the light striking the panel. The ONLY thing any controller can do is operate on that curve. The old PWM controllers either operated at open circuit or at the battery voltage. The MPPT controllers allow movement along the curve. In order to regulate charging current, you simply move to the operating point which provides the desired current.

Thanks Reg - that's my understanding as well.

 

[AndyH]

Your system has to have a way to stop charging the batts when they are full, the grid is down and the sun's still shining. You have to do one of the following:

1. Shut down the panels entirely until the battery level drops enough to start charging again. (It puts extra cycles on the battery, but doable if the system doesn't run in the off-grid mode very much).
2. Throttle the inverter output to match what you're currently drawing.
3. Dump the excess generation into a dump load.

Dave's options make perfect sense to me. Excuse my ignorance, but what I don't understand is why people seem so hung up on needing a dump or MPPT deoptimization. Why not simply disconnect the panels for a while? It seems so obvious and so simple. Surely they are not going to burn up or short out if they are just sitting there with no place for the electricity to go. I know when my system was installed the panels were up on the roof for a week before the utility authorized the inverters to be turned on.

Ray

You nailed it, Ray. While dumps are necessary to provide a load so that turbines won't over-speed and self-destruct, they're not 'necessary' for PV, which can simply be disconnected from the load.

 

[Smidge204]

If you want to get really fancy, you could combine dump loads and inverter throttling if you have the appropriate dump loads that do something useful.

I wouldn't really consider it a "dump load" if it does something useful

With shutting the PV off, you have to rely 100% on battery (again, assuming no grid!). That does cause extra cycling and possibly more wear and tear on the batteries. This tactic is absolutely in use right now and I have no problem with it, TBH.

MPPT tweaking will get you a long way but I'm not certain it would be enough by itself. It's not clear if any system currently uses this strategy but I don't see why it couldn't be employed.

A dummy load would be able to absorb any and all excess power, in whatever fraction of the actual load. Something like a water heater is nice, but you want hot water anyway, presumably, so there would not be much room for dumping excess watts there. (Maybe a second water heater but I'm not sure how you'd effectively plumb that up.)

Just seems that for the cost, you might do better saving your batteries and run a heater and fan outside or something.
=Smidge=

 

[RegGuheert]

MPPT tweaking will get you a long way but I'm not certain it would be enough by itself. It's not clear if any system currently uses this strategy but I don't see why it couldn't be employed.

There are many thousands of MPPT charge controllers in use today. They have been installed in systems for well over a decade from several different manufacturers.

For a larger system (>1kWp) it is cheaper to install a more advanced charge controller than to install the additional PV panels needed to account for the efficiency lost by a traditional controller. The wild card is reliability, but that improves steadily with time.

 

[Smidge204]

There are many thousands of MPPT charge controllers in use today. They have been installed in systems for well over a decade from several different manufacturers.

I'm aware of MPPT inverters.

What I'm not aware of is MPPT inverters that deliberately deoptimize panel production in order to deliberately decrease output to match load. That's what we're talking about here. All of the MPPT inverters I know of only track the maximum power point and not a specified power point.
=Smidge=

 

[DaveEV]

What I'm not aware of is MPPT inverters that deliberately deoptimize panel production in order to deliberately decrease output to match load. That's what we're talking about here. All of the MPPT inverters I know of only track the maximum power point and not a specified power point.

As I mentioned previously, the Sunny Boy when coupled with a Sunny Island will do just that. I'm certain that others will do the same thing as well.

 

[RegGuheert]

I'm aware of MPPT inverters.

I'm referring to charge controllers, not inverters. Without a battery, you are constrained by the requirement that the load can NEVER exceed the power capability of the array, which is a reasonable constraint for a very limited set of applications such as pumping water into a storage tank.

What I'm not aware of is MPPT inverters that deliberately deoptimize panel production in order to deliberately decrease output to match load. That's what we're talking about here. All of the MPPT inverters I know of only track the maximum power point and not a specified power point.

All Enphase microinverters regulate their power to a specified point when the PV panel reaches the inverter's output limit. You can see this by clicking on the link to our array in my signature and clicking on today's production or another day's production when the system was maxed out. You will see that all of the inverters regulate their power at 199W (or thereabouts).

I will add that it is actually much simpler to regulate to a particular output power than to find the MPP. The reason is that if you are at the correct output power, you know that. However, you can never be sure you are at the maximum power point, so you must always hunt for it to be sure you are there.

 

[RegGuheert]

[You can see this by clicking on the link to our array in my signature and clicking on today's production or another day's production when the system was maxed out. You will see that all of the inverters regulate their power at 199W (or thereabouts).

I'm sorry, that link will not give what I said. The link in my signature points to the new Enlighten website which is not overly informative. You will need to use the link to the old Enlighten page for our system in order to see the microinverters regulating at 199W. (Note for anyone who may read this post in the future: This limiting only happens during certain parts of the year. You will not see this in the summertime, for instance. Also, this link is due to disappear in the not-to-distant future.)

 

[Smidge204]

As I mentioned previously, the Sunny Boy when coupled with a Sunny Island will do just that. I'm certain that others will do the same thing as well.

I took a brief look earlier today and couldn't find anything about that in the literature. Do you happen to know where that function might be described?

I'm referring to charge controllers, not inverters.

They are, for purposes of conversation here, equivalent. Both MPPT capable charge controllers and inverters will limit the effective voltage of the panels to get maximum power... the fundamental difference between the two is the output.

All Enphase microinverters regulate their power to a specified point when the PV panel reaches the inverter's output limit.

Yes, maximum power, which is not necessarily matched to the load.

What I'm looking for is manufacturer's data explaining that if the load is less than what the PV system is capable of creating at any given instant, the inverter/charge controller will deliberately use the MPPT algorithm to "de-tune" from maximum power to required power.

Specifically I'm interested in the limits in which this scheme can operate - how far down can you dial your PV array with this method? If I have a system capable of 4kW on a lazy afternoon but nobody is home and the whole house is drawing ~200 watts, what happens? Edit: Remember, no grid so there's nowhere for that extra power to go. Assume any battery capacity is topped off, too, and that I have plenty of hot water :lol:
=Smidge=

 

[RegGuheert]

Specifically I'm interested in the limits in which this scheme can operate - how far down can you dial your PV array with this method? If I have a system capable of 4kW on a lazy afternoon but nobody is home and the whole house is drawing ~200 watts, what happens? Edit: Remember, no grid so there's nowhere for that extra power to go. Assume any battery capacity is topped off, too, and that I have plenty of hot water :lol:

They can regulate all the way to zero current. For reference, here is a plot of the I-V characteristic of a photovoltaic module. Notice that there is a monotonic slope between voltage and current as the voltage is increases from the maximum power point to the point where no current is produced. In fact, the slope doesn't even vary much, so the control function is not difficult to stabilize. You can easily design your controller to move anywhere from peak power to whatever level you need to maintain the battery at the desired output voltage. If the battery is full and no longer drawing current at this level, that is the same as tracking the load.

 

[Smidge204]

RegGuheert

We seem to be talking past each other. Please assume that I know how MPPT works and what is theoretically possible.

What I'm looking for is what's really available on the market, with documentation.
=Smidge=

 

[RegGuheert]

We seem to be talking past each other. Please assume that I know how MPPT works and what is theoretically possible.

Actually, you've asked specific questions and we have answered your specific questions. This was your previous question:

Specifically I'm interested in the limits in which this scheme can operate - how far down can you dial your PV array with this method? If I have a system capable of 4kW on a lazy afternoon but nobody is home and the whole house is drawing ~200 watts, what happens? Edit: Remember, no grid so there's nowhere for that extra power to go. Assume any battery capacity is topped off, too, and that I have plenty of hot water :lol:

...which I answered.

What I'm looking for is what's really available on the market, with documentation.

Midnite Solar Classic Controller is the current best, IMO. Outback Power Flexmax 60 is its predecessor, designed by the same engineer, IIRC. Xantrex XW-MPPT80-600.

 

[AndyH]

RegGuheert

We seem to be talking past each other. Please assume that I know how MPPT works and what is theoretically possible.

What I'm looking for is what's really available on the market, with documentation.
=Smidge=

Cross-talk seems to go with this forum from time to time.

In the past 15 or so years of exploring off-grid equipment, I've yet to see a solar charge controller, either commercial or homebrew, that intentionally moves to an inefficient location along a panel's current/voltage curve. Seems that the folks that design such equipment are most interested in maximizing harvest when power is needed.

Here's one engineer's take on a small MPPT charge controller - hardware and software:
http://www.timnolan.com/index.php?page=arduino-ppt-solar-charger
Here's the theory article:
http://www.timnolan.com/uploads/Arduino Solar/Peak Power Tracking article.pdf

Here's an MPPT unit that is not a charge controller. It was designed to run a motor directly. I use one as the 'front end' for a custom LiFePO4 battery charger.
http://www.solarfreaks.com/fatmax-high-power-maximum-power-point-tracking-unit-t142.html
http://www.solarmppt.com/index.php?main_page=index&cPath=1

Beyond that, I don't know.

 

[AndyH]

They can regulate all the way to zero current.

Bluntly to the point - I think we all agree that an MPPT charge controller can be designed to slide around the E/I curve - but in the real world they don't unless they searching for the max power point. I think it's this second part we're trying to disprove.