Solar Choices

[RainDog]

Originally Posted by TacomaSailor

How does an MPPT controller make more current available by changing the voltage if there is a constant amount of energy (electrons) delivered from the solar panel?

I am no EE, but here is how I understand it.

You solar panel produces power at a specific voltage. Let say for the sake of argument it is 20 volts and it is a 200 Watt panel. Lets say your battery needs 15 volts. A PWM controller works by basically unplugging your solar panel many times a second to limit the voltage to 15 volts. This means you solar panel is only "on" 75% of the time to limit the voltage. So the "extra" power is never produced in the first place.

A MPPT controller uses the Amps = Watts / Volts to convert the 20 volts @ 10 AMPs to 15 volts @ 13.3 APMS. That Wattage never changes, but the circuitry in the controller changes around the Amps and Volts to keep the total the same.

These numbers of course assume no loss anywhere in the system, but are idealized to make the math easy.

[Tootsie]

Here is a clip- out from another blog. Maybe this clarify;

"Maybe a little math fun will help you out. Let’s consider 2 systems. One with a MPPT controller, and another with PWM or linear controller. Both will use a 18 volt 100 watt panel which produces 5.55 amps at peak power output.

System 1 uses a linear controller. With a linear controller the input current = output current. So 5.5 amps in and 5.5 amps out. This is the key difference you want to note. So assuming it is mid day sun your panel is producing 18 volts at 5.55 amps. At the controller we loose 1 volt on the wiring so the input to the controller is 17 volts at 5.55 amps or 93.5 watts input. On the output on the battery side the output is 13 volts at 5.55 amps, or 72 watts output to the battery. You just lost 28% of your power.

Ok same scenario using a MPPT controller. Solar panel is generating 18 volts at 5.55 amps or 100 watts. At the input the controller sees 17 volts @ 5.55 amps or the same as the PWM at 93.5 watts. Now comes the difference, the MPPT controller output is at the same 13 volts but the current is at 7 amps or 91 watts. With a MPPT controller you only loose 9 % compared to 28% using a linear controller.

Ok now another trick with MPPT we are not stuck to using a inefficient low voltage panels, we can step the voltage up to say 100 volts, so at 100 watts means 1 amp of current on the same length of wire, So now at the input of the MPPT controller running a much lower input current the input is now 98 watts, and the output is 95 watts. Only a 5% total loss compared to 9 and 28% loss. "

[deckofficer]

Originally Posted by onestepcsy37

tacoma sailor - i used to be even simpler. apparently its ok to go directly from the solar panel (12volt) to the battery bank, as long as you are not handing the batteries more than 1.5% of their capacity.

so with my one 135 watt kyocera (7.5 amp output) and a 550 amp battery bank i just hooked it up directly. ran that way for maybe four years. no problems. but now i want to add a second kyocera 135 and the output will exceed the 1.5% limit so i just installed a morningstar 20 amp PWM controller. things are getting complicated....

I did this too 25 years ago on a motorhome. I did have a switch in line from the panels ((2) 68 watt) to turn off the panels when I'm not using the RV. You can't leave them hooked up for long periods when not using any energy or you will boil your batteries dry.

If you leave your boat for a month and forget to switch off the panels, your batteries will he history. I would rather not trust my memory.

[noelex 77]

Originally Posted by Tootsie

Dear all,
For a non- electrician like me, I wish to try and agree on what the basic benefit of an MPPT controller is:
My understanding is like this;
The required voltage to charge a 12 V battery bank is 13,6 Volts (not sure about the exact decimal here)

Assume you have a solar panel with an output of 19,6 Volts.
Without an MPPT controller, 19,6 minus 13,6= 6 volts, which will be "lost" i.e. not utilized. That is significant.

With an MPPT controller, the extra voltage will be transformed into more amps of charging.
19,6 volts is 44 % more than 13,6.

I need to check why we end up with only 5-15 % gain as some in this group are claiming...
I know I have seen this calculation somewhere.

The principal you are using is correct, but the devil is in the detail.
If we use the popular Kyocera panels as an example:

Voc is 21.7v but the Vmp (the voltage where the controller will operating) is 17.4v.
Unfortunately these are measured with a panel temperature of 25c. This is only achieved in the lab by using flashes of light. In the real world the black solar panels located behind glass heat up considerably. In sunny conditions where the solar panels will be producing lots of power the cell temperatures are typically about 30-45c. The temperature coefficient of the Kyocera panels is 0.0932 V/C. So at 35c the Vmp under realistic conditions is: Vmp=16.5v.
With good wiring with only 3% voltage drop this makes the actual Vmp at the controller

Voltage at the controller=16v

This assumes the best case of no shadows at all on the solar panels and small shadows will drop the voltage considerably.

The gain of MPPT comes from converting this 16v to the lower battery voltage.
We are concerning ourself with the battery voltage under charge before the float voltage is reached (when charging at float power is being thrown away so the MPPT function is irrelevant).
The battery voltage overnight might be 12.3v, but this will rise rapidly under charge, most rising to the 14.7v bulk voltage. The voltage is obviously variable depending on SOC etc, but if want to determine the average gain the result needs to weighted to when the solar the solar panels are producing their maximum output ( because this is when most of the solar power is produced).

If we use 14v as a suitable number we come up with a theoretical gain from a perfect MPPT regulator of: 14% if we use 13.8v the gain is 16%

This assumes the solar panel produces the same current at 16v as it does at 14v in practice the panel will produce a bit more current at the lower voltage. It also assumes 100% voltage conversion, in practice a few percent is lost and 100% correct tracking. It also assumes no small shadows (from the back stay etc) that will reduce the solar panel voltage.

The final factor to consider is the self consumption of the MPPT circuitry. This is considerable if the tracking is to be accurate. Large controllers (like the Outback and Midnite) consume about 0.4A when operating normally. When solar output is very low (at night) they will go to sleep, but still consume some power so they can wake up 0.1A is typical. Thus 4-5 AHrs are consumed by these controllers. These controllers are only used on large arrays the self consumption of smaller models is less because the handle smaller currents. Some MPPT controllers are very simple and don't consume much power, but their tracking is worse which reduces the gain. Non MPPT regulators also have some self consumption, but this is much less.

Under some conditions , such as when the battery voltage is low (because the discharge is greater charge rate and/or the SOC is low combined with cold temperatures and no shadows) gains of 30% or more are possible but this will not be seen on average. It is also important to be wary of the meters on MPPT controllers that show input and output currents they nearly always exaggerate the gains that are actually produced.

The 5-15% average gain from MPPT is a realistic estimate.

[CarinaPDX]

Good explanation. You do clearly state as you go along that you are making optimistic assumptions, which I would agree with, and you end up with about 10%. I think the 15% number is not very likely in the real world for modest sized installations. I have been saying 5-10% and I think I'll stick with it.

Most of the controllers I have seen output to a single battery bank, which is usually not the situation aboard a boat. I don't like using a manual switch to select the bank to charge (solar or alternator or wind) as it is just too easy to forget to switch it when appropriate. I have used diodes (Schotky) to split the power, but that requires remote voltage sense to work properly (and will still be less than optimal for the non-sensed battery); there is also a power loss due to the voltage drop across the diode (which is why I use the more efficient Schottky diodes). Battery combiners also use power to close the relay. So unless your controller supports 2 battery banks there will be further losses. Something else to think about.

Greg

[Dsanduril]

Originally Posted by TacomaSailor

What do I have wrong about energy and voltage?

Is it that the solar panel only flows the maximum possible number of electrons (current or amps) at the maximum open circuit voltage? I=VR and the R stays the same (wires and internal battery) in a given solar situation.

Is that why my 125 watt panels produce only 8.2 amps at 14.4 V (118 watts)?

This is why I think when looking at solar panels in our applications you really need to look at Imp (current at maximum power). Below Vmp solar panels have a very flat current curve, they will deliver essentially the same current at 1V as at their Vmp (which is dependent on how many cells are in series in the panel).

The Kyocera 140, as an example, delivers 7.91A at Vmp and 8.68A at short circuit (essentially zero Volts). When you are charging a battery from one of these panels the battery is essentially the "voltage regulator". If your battery is at a SoC where 13.6V at 8A is happy, then you'll be running you panel at 13.6V. Your panel might be capable of the exact same current (or very close to it) at 17.7V. So, you are losing the power between 13.6 and 17.7V because the battery, as the "voltage regulator" is pulling the voltage down (not the clearest explanation, but all my brain can come up with at the moment).

The solar panel, below Vmp, doesn't care what the voltage is, it will happily operate at any voltage between 0 and Vmp producing essentially the same amount of current. Something has to tell it at what voltage to operate. That could be your battery in a direct system (which a PWM controller is when you can use all the power), the battery state of charge determines the voltage. When you try to operate a panel between its Vmp and Voc (open circuit voltage) you will rapidly lose current (and power), the current curve has a very sharp "knee" near Vmp and current drops from Imp to zero. For the same Kyocera panel, for instance, the Vmp is 17.7 (at which it produce 7.91A) and the Voc is 22.1V at which is will stop producing any current.

With an MPPT controller (in theory, and excluding losses) that 7.91A at 17.7V would be converted to 9.9A at 13.6V. Think of the DC-DC converter kind of like a heat exchanger, you can put a small number of high energy electrons through one loop, and on the other loop you can extract a larger number of lower energy electrons. In this case you would gain almost 25%, but that is based on panel STC numbers. The sun simply isn't that bright for most of the day, the panel temperature is almost never at STC, etc. Of these, the panel temperature is the killer, available voltage drops as the temperature goes up. The Kyocera example panel drops from Vmp of 17.7V at 25C to 16.0V at the NOCT (normal operating cell temperature) of 45C. Just looking at the Vmp between the "rating" condition and the "normal" condition, your panel will only generate about 90% of its rated power under "normal" conditions (and that doesn't even get into sun intensity). For MPPT gains that starts to look more like 17% under ideal, "normal" conditions and that doesn't include any of the losses introduced by the MPPT circuitry.

[CarinaPDX]

When using a non-MPPT system below the Vmp point the current is approx. flat as noted above. While the battery may be at 13.6V (to use the above example) the panel will actually be a volt or two higher due to the voltage drops across the wiring, controller, and any diodes. As long as the voltage at the panel is below Vmp the current will not be affected by these drops in any significant way. (Much of the small current loss actually occurs on the knee just below mppt, so it is a little desirable to work a volt or two lower.) So let's say you have a 2V total drop in voltage from all sources, and the panel is operating at 15.6V. If an MPPT controller is used, and it works perfectly, the panel will operate at 17.7V, or 2.1V higher. If we use the previous 8A that yields a potential power gain of 16W, assuming the same losses (including within the controller). But the MPPT controller should have larger losses. If the system were extremely efficient and the total voltage drop were 1V then we are talking about 24W max. In practice few installations attain this level of efficiency; with a PWM controller there is usually no need to try as long as Vmp is around 18V as the current delivered to the battery is the same within a wide range of losses.

Greg

[goboatingnow]

Ill add my tuppence here cause there's some major confusion

A solar panel does not have a linear relationship between current and voltage. Ie a panel has a specific power ( watts) output at a given voltage and that output power is different at another operating point ( voltage) hence a 100W panel is only 100W at a particular voltage.

Hence 100w , say at 10V is 10 amps , but at 5v it may only still produce 10 a , in effect it's a 50 w panel at that operating point. If it was linear, there'd be no point to mppt.

Because of the non-linearity there is a peak power point on the curve, a place where the output voltage and current result in the greatest power. This is Vmp x Imp, ( these are specified for the panel )


If you get that you'll see what mppt does

Now forget battery voltage for a minute. The operating point of the panel is determined by the current drawn from the panel. PV panels are primarily current sources not voltage sources.

So imagine placing a resistor across the output of the panel, for a given resistance there would be a current lets call it Iop1 ( operating point 1) and by ohms laws there would be a corresponding Vop1. Ie the panel operating voltage. Hence the panel is delivering Iop1x Vop1 watts of power.

For argument lets say that was 15v and 10A. Giving a panel output of 150w

Now say you halved that resistance. You might expect the current to double. But it doesn't. What you see basically is that Iop2 = Iop1. And that Vop2 is half of Vop1. Hence at this operating point what the panel is delivering has changed

Ie the output is still 10A but the voltage is now 7.5V. ( the load resistance halved remember) . Hence the panel is now only producing 75W of power.

So what mppt convertor does is vary its load resistance to determine the optimum power ( ie VxI) point of the panel.


A completely secondary bit is then to convert the voltage using a DC to DC convertor so that the output voltage is suitable for charging. But that has nothing to do with mppt.

Furthermore " good" mppt tracking consumes no ( or very little) power at all. I completely disagree that " good" mppt tracking consumes lots of power, there is no correlation.

The looses in commercial mppt controllers are ( a) switching efficiency and ( b) quiescent current and also factors like low load switching performance and high load, near saturation, performance.

Note that Vmp does not change much with illumination levels . But does change with shading.

If a panel Vop ends up near its Vmp then mppt adds little , the further the Vop strays away from Vmp , the more valuable a mppt system is. Letting Vop exceed Vmp results in really poor output.

Hydro generators , air generators both have non linear power curves and equally benefit from mppt

dave

[CarinaPDX]

Originally Posted by goboatingnow

Ill add my tuppence here cause there's some major confusion

It seems to me that everyone is saying pretty much the same thing, including your post. Slightly different takes on it, but all is good. So what confusion?

Originally Posted by goboatingnow

Now forget battery voltage for a minute. The operating point of the panel is determined by the current drawn from the panel. PV panels are primarily current sources not voltage sources.

Again, a different take by a different simplification. Since the panel is essentially a current source in the desired voltage range, as everyone has said, the voltage is primarily determined by the battery's response to the panel's current, and that current is primarily a function of the panel.

Originally Posted by goboatingnow

Furthermore " good" mppt tracking consumes no ( or very little) power at all. I completely disagree that " good" mppt tracking consumes lots of power, there is no correlation.

Nobody said that. The point was that "good" controllers track better, and will be closer to the theoretical potential; it's all downhill from there.

I guess I am reacting here to your argumentative wording when no argument exists. I do appreciate your comprehensive knowledge of boat electronics, among other things - I just think it could go down a little easier at times.

Greg

[goboatingnow]

Originally Posted by CarinaPDX

It seems to me that everyone is saying pretty much the same thing, including your post. Slightly different takes on it, but all is good. So what confusion?



Again, a different take by a different simplification. Since the panel is essentially a current source in the desired voltage range, as everyone has said, the voltage is primarily determined by the battery's response to the panel's current, and that current is primarily a function of the panel.



Nobody said that. The point was that "good" controllers track better, and will be closer to the theoretical potential; it's all downhill from there.

I guess I am reacting here to your argumentative wording when no argument exists. I do appreciate your comprehensive knowledge of boat electronics, among other things - I just think it could go down a little easier at times.

Greg

Sorry if my posts come across with the wrong tone, I'm no good at pleasantries in the written form. I read emails I write , they sound fine and its not the first time some find them a bit too direct !!


As to good tracking , there was a post that said good mppt requires more power , that's simply not true.

The voltage of a panel in an mppt situation is decided by the mppt load impedance . The battery voltage is not relevant. People need to break the link between mppt and battery charging

Yes there is much confusion , several posts while trying to explain mppt , got the description wrong electrically, even if at a 50000 foot view they were mostly right. I don't do 50000 foot views. ( the wife blames the higher functioning autism !!)

Again a global all thread apology for email tone, I'm completely email tone deaf. !!!!! ( but I mean well!)

David

[s/v Jedi]

Dave is right in his explanation (it's different from what has been discussed by others), but I think it's a bit too complicated material for non-EE's.

The correct term for what a MPPT controller does is "impedance matching". There's libraries of books written on the subject…

I'll point to the core of the issue again: a solar panel has a power-output curve which is not linear. It will only output it's full rated power at one specific voltage, which isn't the battery charge voltage. The controller will "track" the panel output by varying it's load on the panel and comparing the power transfer, seeking the sweet point of maximum transfer. And it will do so periodically to cope with changing conditions.

I have a 660W array and went without a controller for many years. A year ago I installed an Outback MPPT controller and my power production went up more than 25%. Significant, as I would have to buy 200W of extra panels (that don't fit anywhere anymore) to match that.

MPPT is good and best is big panels with a separate controller for each panel (which only makes sense with big panels). Genasun makes good ones for that.

[goboatingnow]

Originally Posted by s/v Jedi

Dave is right in his explanation (it's different from what has been discussed by others), but I think it's a bit too complicated material for non-EE's.

The correct term for what a MPPT controller does is "impedance matching". There's libraries of books written on the subject…

I'll point to the core of the issue again: a solar panel has a power-output curve which is not linear. It will only output it's full rated power at one specific voltage, which isn't the battery charge voltage. The controller will "track" the panel output by varying it's load on the panel and comparing the power transfer, seeking the sweet point of maximum transfer. And it will do so periodically to cope with changing conditions.

I have a 660W array and went without a controller for many years. A year ago I installed an Outback MPPT controller and my power production went up more than 25%. Significant, as I would have to buy 200W of extra panels (that don't fit anywhere anymore) to match that.

MPPT is good and best is big panels with a separate controller for each panel (which only makes sense with big panels). Genasun makes good ones for that.

Some people have a way with words

Dave

[Silverbullet]

I have to say we dont really know how quirky the Lithium Ion chemistry is at this point, at least I dont and I have over 20 years experience with solar. Maybe Im just old fashioned but I tend to go with what is simple, proven and what I know works. Im going to wait out the jury on the new storage batteries. You might want to ask the manufacturer how sensitive the new batteries are during heavy seas events. If the event some of you havent heard the latest, solar pv panels are going for near .50 per watt right now. Trace and Outback are the premier systems IMHO.

[CarinaPDX]

Dave- I probably overreacted - sorry.

What you and Jedi have added is how an MPPT works; I have just taken it as a given that it does work at Vmp. Looking at one MPPT manual (Morningstar) the efficiency at 16.5 Vmp is between 97-98% efficiency, dropping to 92-93% at 98 Vmp. I take that to mean that the greater the DC-DC converter voltage the greater the loss, somewhat reducing the advantage of the higher voltage.

I have only looked at systems in the few hundred watt area, and the 5-10% efficiency gains aren't enough to justify the additional cost of the MPPT controller when compared to putting the money to another panel.

Jedi's 25% gain is very impressive (as is the massive 660W of panels!). I think it would be interesting to break out what is happening there. Were the panels' Vmp higher than 19V? Or are they operating at 24V or higher? What was the loss due to cable resistance? There is more to that story than simply a more efficient controller.

All good fun...

Greg

[RainDog]

Originally Posted by s/v Jedi

MPPT is good and best is big panels with a separate controller for each panel (which only makes sense with big panels). Genasun makes good ones for that.

I cannot find a Genasun controller that works with a panel over 140 Watts. Am I missing them? I am looking for a controller to work with a single 220 Watt panel.