Solar Afloat #2 60A PWM or MPPT?

Highland_Fling · July 2010

To keep things simple - here we go again - only really one question on this post even if i want multiple answers

As an upgrade to my present trickle charge solar system i am looking at initially two then four Kyocera KD-185-GX-LPU panels as the final solution.

However this means i need to upgrade the controller to a Morningstar Tristar MMPT 45 or 60. I think to get some headroom i would purchase the bigger 60 amp controller.

BUT looking at the difference in cost between a Morningstar PWM 60 and a MMPT 60 i am wondering/questioning if i should buy the MMPT version.

Will i really get a better bang for my buck?

Is this a cost effective move?

Will it pass a cost benefit analysis examination seeing as there is a significant cost difference between the PMW and MMPT versions?

thanks

David

Kamala · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

NAWS makes the following recommendation on their page offering the Kyocera KD-185-GX-LPU panel:

"This is a 23 volt panel for grid tie or for use with an MPPT battery charge controller"

BB. · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

If you have the "right panels" (such as 17.5 volt Vmp for a 12 volt battery bank), are in a temperate climate (not to hot, not too cold), and the wiring runs are short from panels to charge controller--A good PWM system is difficult to ignore.

MPPT are great for large systems and/or longer wire runs (without spending a fortune for lots of heavy copper cables) from the solar array to the charge controller (always keep controller to battery and battery to inverter wires short and heavy) because you can run higher Vmp Array voltage efficiently--and you can use (typically cheaper) > 100 Watt panels designed for Grid Tied systems for battery based off-grid systems.

There are some secondary advantages--1. In sub freezing weather and with snow on the ground--you can get more power from your panels. And 2. in very hot weather and cool batteries Vmp falls down to near Vbatt-charging--so it can be difficult to fully charge a battery bank with "12 volt panels and a PWM controller... Neither situation fits your setup.

Basically, the losses from PWM vs MPPT are (simple first order losses):

14.5 volt charging / 17.5 volt Vmp panel = 0.83 eff for a PWM controller
14.5 volt charging / 23 volt Vmp panel = 0.63 eff for a PWM controller
A MPPT controller will be around 0.95 efficient for larger arrays (i.e., don't use 100 watts of panels on a 60 amp massive MPPT charge controller--there are other losses).

So, for a 60 amp MPPT charge controller, it is best if you have at least ~400 watts of solar panels or the internal losses (switching power supply) will eat up a fair amount of your energy.

Sort of make sense?

-Bill

democopy · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

BB. wrote: »

If you have the "right panels" (such as 17.5 volt Vmp for a 12 volt battery bank), are in a temperate climate (not to hot, not too cold), and the wiring runs are short from panels to charge controller--A good PWM system is difficult to ignore.

MPPT are great for large systems and/or longer wire runs (without spending a fortune for lots of heavy copper cables) from the solar array to the charge controller (always keep controller to battery and battery to inverter wires short and heavy) because you can run higher Vmp Array voltage efficiently--and you can use (typically cheaper) > 100 Watt panels designed for Grid Tied systems for battery based off-grid systems.

There are some secondary advantages--1. In sub freezing weather and with snow on the ground--you can get more power from your panels. And 2. in very hot weather and cool batteries Vmp falls down to near Vbatt-charging--so it can be difficult to fully charge a battery bank with "12 volt panels and a PWM controller... Neither situation fits your setup.

Basically, the losses from PWM vs MPPT are (simple first order losses):
14.5 volt charging / 17.5 volt Vmp panel = 0.83 eff for a PWM controller

14.5 volt charging / 23 volt Vmp panel = 0.63 eff for a PWM controller

A MPPT controller will be around 0.95 efficient for larger arrays (i.e., don't use 100 watts of panels on a 60 amp massive MPPT charge controller--there are other losses).

So, for a 60 amp MPPT charge controller, it is best if you have at least ~400 watts of solar panels or the internal losses (switching power supply) will eat up a fair amount of your energy.

Sort of make sense?

-Bill

Interesting information Bill, Thanks

So, if you have 3/100w 12v Panels, is it just a trade-off and more money going for the MPPT Sunsaver 15L?

Is it going to give you 15-25% more, most of the time?

BB. · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

No--At lower power levels (more losses from MPPT electronics) and on hotter days (depressed Vmp voltage)--the "increase" will be less.

Don't get me wrong--I think well designed MPPT controller are really neat... However, is it 2-3x the price of a PWM controller as neat? For smaller systems, perhaps not.

Look at Page 61 of the PDF Manual for the Morning Star 45/60 Amp MPPT solar charge controllers... There are specific ranges where the losses are minimized and other areas where the losses are higher. [link fixed. -BB]

You can see, below 100 watts, the percentage based losses increase dramatically (as a ratio of input power--not that unit will melt near 50 watts:roll:). And this is a very well designed set of MPPT type controllers.

-Bill

democopy · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

BB. wrote: »

No--At lower power levels (more losses from MPPT electronics) and on hotter days (depressed Vmp voltage)--the "increase" will be less.

Don't get me wrong--I think well designed MPPT controller are really neat... However, is it 2-3x the price of a PWM controller as neat? For smaller systems, perhaps not.

Look at Page 61 of the PDF Manual for the Morning Star 45/60 Amp MPPT solar charge controllers... There are specific ranges where the losses are minimized and other areas where the losses are higher. [link fixed. -BB]

You can see, below 100 watts, the percentage based losses increase dramatically (as a ratio of input power--not that unit will melt near 50 watts:roll:). And this is a very well designed set of MPPT type controllers.

-Bill

Link doesn't work.

Looking for it now.

So the value of MPPT Sunsaver 15L should do 15% more on 300w PV, correct? Just paying 1.5 times the money for it.

BB. · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

Sorry,

Here is the Morning Star MPPT 45/60 amp manual link:

http://www.morningstarcorp.com/en/support/library/TSMPPT.IOM.EN.04.pdf

I will fix the other links.

-Bill

BB. · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

Regarding 15% more... In cold weather--Yea, they can do it.

However... $80 cheaper PWM controller and you can get you almost 30 watts more in solar panels (if you can find the right "mix" of panels at the right price).

But, you also need to look at the fact you are going to lose some real power around solar noon because 300 watts worth of panels will exceed the 15 amp capability of the 15 amp controller.

However, depending on what panels you are looking at, the cheaper panels are many times those where Vmp <> 17.5 volts or so (standard "12 volt" battery charging panel). So, cheaper higher voltage panels and you spend that savings on a MPPT controller...

You almost have to put 2-5 configurations (different panels, charge controllers, etc.) in a spread sheet and see what works out for $$$/Watt pricing with the functions that you want.

Note that the 15 amp MPPT controller has no display--so you need to purchase one of those or a laptop interface. And MorningStar sells PWM controllers both with, and without digital displays (and a couple remote panel options).

It is really tough to generalize.

In the end, it is almost impossible to measure a 10% difference in output between two different setups without lab equipment... So, I would not pay a whole bunch of money based on a 10% difference. Weather, mounting, how you choose to operate/charge your battery bank, etc. can all swamp a 10% difference too.

MPPT controllers are great in temperature extremes (very cold weather, more power) or very hot (increase Vmp >> 17.5 volts so that very hot panels can still charge/equalize a battery bank) and where you need long wire runs from Panels to charge controller.

The other advantages (logging, computer interfaces, Internet server in the 60 amp MPPT MorningStar Controller) are all sort of up to you.

In any case, a couple other options to look at--Remote battery temperature sensor--Mandatory for the MS 15 amp MPPT controller. Highly recommended for any charge controller (especially if battery temperatures change over the day/season).

And, something that is really nice, and only the MS 45/60 amp MPPT controllers have (that I am aware of--others may too), is Battery Voltage Sense Leads... Allows the controller to measure exact battery voltage without worrying about voltage drop in the Charge Controller to Battery Bank heavy cabling. Very cool when you are trying to measure battery voltage to better down to 0.01 volts for proper/faster charging (yea--I am a computer system design engineer--why do you ask

:roll:).

-Bill

democopy · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

BB. wrote: »

Regarding 15% more... In cold weather--Yea, they can do it.

However... $80 cheaper PWM controller and you can get you almost 30 watts more in solar panels (if you can find the right "mix" of panels at the right price).

But, you also need to look at the fact you are going to lose some real power around solar noon because 300 watts worth of panels will exceed the 15 amp capability of the 15 amp controller.

However, depending on what panels you are looking at, the cheaper panels are many times those where Vmp <> 17.5 volts or so (standard "12 volt" battery charging panel). So, cheaper higher voltage panels and you spend that savings on a MPPT controller...

You almost have to put 2-5 configurations (different panels, charge controllers, etc.) in a spread sheet and see what works out for $$$/Watt pricing with the functions that you want.

Note that the 15 amp MPPT controller has no display--so you need to purchase one of those or a laptop interface. And MorningStar sells PWM controllers both with, and without digital displays (and a couple remote panel options).

It is really tough to generalize.

In the end, it is almost impossible to measure a 10% difference in output between two different setups without lab equipment... So, I would not pay a whole bunch of money based on a 10% difference. Weather, mounting, how you choose to operate/charge your battery bank, etc. can all swamp a 10% difference too.

MPPT controllers are great in temperature extremes (very cold weather, more power) or very hot (increase Vmp >> 17.5 volts so that very hot panels can still charge/equalize a battery bank) and where you need long wire runs from Panels to charge controller.

The other advantages (logging, computer interfaces, Internet server in the 60 amp MPPT MorningStar Controller) are all sort of up to you.

In any case, a couple other options to look at--Remote battery temperature sensor--Mandatory for the MS 15 amp MPPT controller. Highly recommended for any charge controller (especially if battery temperatures change over the day/season).

And, something that is really nice, and only the MS 45/60 amp MPPT controllers have (that I am aware of--others may too), is Battery Voltage Sense Leads... Allows the controller to measure exact battery voltage without worrying about voltage drop in the Charge Controller to Battery Bank heavy cabling. Very cool when you are trying to measure battery voltage to better down to 0.01 volts for proper/faster charging (yea--I am a computer system design engineer--why do you ask :roll:).

-Bill

Bill, Thanks for that explanation. I sent you a PM

BB. · July 2010

Re: Solar Afloat #2 60A PWM or MPPT?

LL,

I should also add that solar charge controllers, in general, are not that accurate in power/current reporting.

They don't have to be for them to work correctly (they are looking for differences, not absolute values for MPPT function).

So, a current shunt with the appropriate meter can do way better than the 5% to 30% measurement error that is present in many off-grid solar controllers (even the good ones).

(Rogue's 30 amp MPPT controller should be one of the more accurate ones).

-Bill

Solar Afloat #2 60A PWM or MPPT?

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