MPPT Con ?

[mbianka]

I agree with Cuthbert's thinking. You will pay more for being an early adopter. I also think is also a lot of solar silicon snake oil being sold here too! I also don't think it is always a good idea to add more complex electronic devices than you really need on a boat. Especially when one is exposed to things like thunderstorms on a regular basis. A lightning strike may blow out both a standard PWM and MPPT solar controller but, the standard one will be cheaper to replace. But, as Cuthbert pointed out competition and technology strides will probably bring down the price of MPPT units in the future or may make them standard features when you need to replace the controller in the future too.

[cal40john]

Originally Posted by noelex 77

I think it has more to do with time of day. The test was stopped at 10am before the panels were receiving enough light to put out their rated current.
I don’t see how the regulator could be charge limiting at say 6:30 am at 1.1A, but an hour later the batteries will accept 4A an hour later again they will accept 8A.
This profile is characteristic of the regulators in boost phase when the batteries will accept all the power the solar panels can put out.

What does the short circuit output tell you? (Right hand column) He didn't get that value by hooking the solar panel to the batteries. Looking at the I-V curves graph I posted, what do you maximize when you short a solar panel? He is giving you the maximum current that the panel can put out in the same light conditions as the panels that are being tested by controllers. In other words the control of the experiment, the no load output. Looking at the I-V curves again, hooking the panel directly to a 12 volt battery will drop the current by a few percent below the short circuit value. (Assuming the battery is not already completely charged.) The PWM is consistently outputting 80%-84% of the SC value at each time of day/light level. If that's the PWM's idea of bulk charge phase I'll buy a manually operated switch instead.

The final control would be to see the current output of a panel directly connected to the battery.

John

[cfarrar]

Skepticism and critical thinking are good, but I suspect this thread ignores some larger facts. MMPT's are commonly used in thousands of home solar arrays. Almost all grid-tie systems use MMPT. Are all those engineers / installers / homeowners wrong? I know an engineer involved with the design of Google's solar arrays (they have a few megawatts). His choice of controller for his own home array? MMPT.

Here's a side-by-side comparison of MMPT and standard controller on

You Tube.

The question is not whether it works, but whether it's practical and economical for a sailboat. The answer is probably: "It depends." My boat has finite room for solar panels, no generator, and it's never dockside. My Blue Sky MMPT controller costs $169, and it's worked flawlessly for 24 months of daily charging. So far, so good.

[witzgall]

If you buy a MPPT cotroller, you are NOT an early adopter. They have come down in price so much (Some of them, anyways) that it really does not make sense to buy anything else, except for the simplest of systems/needs. My Opinion.

Chris

[hellosailor]

Noelex-

"Open circuit voltage, short circuit current, " Neither of which mirrors real-life operation with a battery attached to be charged. Open circuit voltage especially being a total deception, because you may see 14V open circuit drop to 4 volts when a battery is attached. "Ooops".

The 10AM is another good catch, typically solar panels left flat or oriented for noon sun would be something like 30% under their nominal output when the sun is two hours out of position.

My only complaint against MPPTs would be that they are expensive. You need to be able to pay that expense back, in terms of the size and extra power benefit from them, before they make sense.

[noelex 77]

Originally Posted by cal40john

What does the short circuit output tell you? (Right hand column) He didn't get that value by hooking the solar panel to the batteries. Looking at the I-V curves graph I posted, what do you maximize when you short a solar panel? He is giving you the maximum current that the panel can put out in the same light conditions as the panels that are being tested by controllers. In other words the control of the experiment, the no load output. Looking at the I-V curves again, hooking the panel directly to a 12 volt battery will drop the current by a few percent below the short circuit value. (Assuming the battery is not already completely charged.) The PWM is consistently outputting 80%-84% of the SC value at each time of day/light level. If that's the PWM's idea of bulk charge phase I'll buy a manually operated switch instead.

The final control would be to see the current output of a panel directly connected to the battery.

John

The short circuit current Isc is not achievable when the panel is connected to a battery. The regulators are only putting out 80% of this current because they are connected to a battery. A manually operated switch wont help. The Isc current cannot be directly used.
The energy output of the panel is given by the current at load( Ipm) multiplied by the voltage at load (Vpm).
The Ipm will always be lower than the Isc.
The difference between the Ipm and the Isc is not a loss that is introduced because of the regulator. Nor does a difference between Isc and Ipm represent that the controller is regulating or reducing the power it is sending to the batteries.

[Paul Elliott]

Originally Posted by hellosailor

"Open circuit voltage, short circuit current, " Neither of which mirrors real-life operation with a battery attached to be charged. Open circuit voltage especially being a total deception, because you may see 14V open circuit drop to 4 volts when a battery is attached. "Ooops".

Regardless of the methodology used in the test we are discussing, open circuit voltage and short circuit current are valuable parameters that tell us a lot about a solar panel, and these are good parameters to use as controls when comparing conditions and performance. They don't tell the full story, but they are far from being "deceptions". You just have to understand what they are telling you.

In the test under discussion, I wish the battery voltage had been noted. That would have told us a lot.

I think that MPPT controllers can improve battery charging performance. The question is how much and whether it's worth it. Before I head out on another long trip, I will probably (again) replace my cheap non-MPPT controller with a new MPPT one. I can't easily fit any more solar panels, and I don't like running my engine to charge the batteries (I do it, but I don't like it), so MPPT makes sense for me.

[noelex 77]

Originally Posted by cfarrar

Skepticism and critical thinking are good, but I suspect this thread ignores some larger facts. MMPT's are commonly used in thousands of home solar arrays. Almost all grid-tie systems use MMPT. Are all those engineers / installers / homeowners wrong? I know an engineer involved with the design of Google's solar arrays (they have a few megawatts). His choice of controller for his own home array? MMPT. .

One of the overhelming advantages of MPPT regulators is ability to modify the voltage. This result in large savings on cable installation in a domestic situation. The very similar and shade free conditions typically experienced by all the panels is also where MPPT regulators perform at their best.

[cal40john]

Originally Posted by noelex 77

The short circuit current Isc is not achievable when the panel is connected to a battery. The regulators are only putting out 80% of this current because they are connected to a battery. A manually operated switch wont help. The Isc current cannot be directly used.
The energy output of the panel is given by the current at load( Ipm) multiplied by the voltage at load (Vpm).
The Ipm will always be lower than the Isc.
The difference between the Ipm and the Isc is not a loss that is introduced because of the regulator. Nor does a difference between Isc and Ipm represent that the controller is regulating or reducing the power it is sending to the batteries.

Yes I know I-sc will not be used, that is why I said look at the I-V curves, again the I at 12 volts is a couple of percent below I-sc, not 20% down. For the panel to output 20% less current than I-sc, the voltage has to be up around 18 volts.

In the video at the panel PWM is 12.3 volts, 1.7 amps. MPPT is 19.7 volts, 1.45 amps. That's a 15% drop in current from the 12.3 to 19.7 volt points on the I-V curve which has the knee in it. Now look at the rest of the curve from 12.3 volts back to I-sc. You're saying that there is a 20% change in the current on that flat part of the curve?

John

[Paul Elliott]

I wrote this:

Originally Posted by Paul Elliott

And you will notice that in no case do either the PL20 (conventional) or the MPPT-250 regulators put out anything close to the stated short-circuit current of the three paralleled panels. This means that the regulators are in charge-limiting mode and the differences are probably due to the specific setpoints and charging modes for the regulators. This is not a valid test of MTTP efficiency.

I know that the panel's Impp is going to be less than the Isc, so what I should have said is that the regulated current was less than I would have expected, based on the Isc.

Looking at it again, I may have been wrong about that, especially if the panel was operating at a high temperature. I still say that without knowing the battery voltage and charger setpoints the test is difficult to interpret.

I've always felt that a per-panel (or per-string, or per-cell) MPPT controller would be best on a shade-challenged platform such as a sailboat. Of course the cost would be an issue. Look at this company's literature for an example of using multiple MPPT controllers for better performance on a boat: Genasun . I've not used Genasun's products, but they look interesting -- definitely not the cheapest, though.

[Paul Elliott]

Originally Posted by cal40john

look at the I-V curves, again the I at 12 volts is a couple of percent below I-sc, not 20% down. For the panel to output 20% less current than I-sc, the voltage has to be up around 18 volts.

This is going to depend on how hot the panel is. At 75degC, the current drops 20% from Isc at (around) 13V. Panels do get hot baking in the sun. I wish we had more data.

[noelex 77]

I wonder if anyone has experience fitting individual MPPT devices to each panel. This removes one of my main concerns that a single MPPT will not be very effective on a boat with multiple panels because, unlike a domestic situation, often panels are often experiencing different conditions.
The other advantage of this option is there are some relatively low cost MPPT devices that are of very high efficiency (faster tracking, better conversion etc) designed usually by enthusiasts for things like solar cars.
For example here is a discussion on the Fatmax which is available for sale at $139 (Australian dollars)

FATMAX - High Power Maximum Power Point Tracking Unit : Electronics

[cal40john]

Originally Posted by Paul Elliott

This is going to depend on how hot the panel is. At 75degC, the current drops 20% from Isc at (around) 13V. Panels do get hot baking in the sun. I wish we had more data.

You're right, but from 7 am to 10 am the output was 20% down. We're talking about an interesting place that brings the panels up to 170 deg F nice and early in the morning.

John

[noelex 77]

Originally Posted by Paul Elliott

I've always felt that a per-panel (or per-string, or per-cell) MPPT controller would be best on a shade-challenged platform such as a sailboat..

That’s not my understanding. They offer the most advantage when there is a large difference between the solar panel voltage and the battery voltage.
Any shade such as a small shadow from a halyard will significantly reduce the solar panel voltage and reduce or eliminate the MPPT. The inherent losses of the voltage conversion in the MPPT regulators remain.
This makes them less sutable for a shade-challenged platform such as a sailboat.

[Paul Elliott]

Originally Posted by noelex 77

That’s not my understanding. They offer the most advantage when there is a large difference between the solar panel voltage and the battery voltage.
Any shade such as a small shadow from a halyard will significantly reduce the solar panel voltage and reduce or eliminate the MPPT. The inherent losses of the voltage conversion in the MPPT regulators remain.
This makes them less sutable for a shade-challenged platform such as a sailboat.

What I am thinking is that with random shadows, having multiple small, independent panels and MPPT controllers makes sense because the Vmpp of the panel changes with the amount of light and temperature. You should in theory get better performance than you would by just connecting all the panels in parallel. Remember, MPPT doesn't necessarily mean stepping the panel voltage down, in some systems you will use low-voltage panels and step them up to match the battery voltage. This isn't too common, but it has been done (see Genasun: Genasun).

In a random-shading situation connecting panels in series is not a good idea. With a high-voltage string of panels having bypass diodes (such as in a terrestrial power-grid-connected application), some shaded panels can be tolerated, but in typical marine applications we don't have enough voltage to make this work.

In grid-connected systems the use of per-panel "micro-converters" is becoming popular. They claim more efficiency using these distributed MPPT converters than the traditional "big string of panels and a large controller/inverter" systems (there are other benefits, too).

I've spoken with the guy who runs Genasun and he is a big fan of per-panel converters. He sells converters, but it does make sense to me as long as the converter efficiency is there.

Of course, what is possible and what is practical are often quite different. While I am intrigued by methods to optimize solar panel output on my boat, what I have actually installed is quite plain-vanilla. For now.