Pwm or mppt

mountainman · April 2020

With a 1kw array and a 24 volt 208 ah
4 GC batteries in series.

I can't decide between using 3 in parelell
345 watt q cell panels. 38 vmp 47 voc and 9 imp.
With a TriStar pwm controller.

Or 4 250 watt Reno sola 2s2p 30 vmp 37.5 voc and 8.30 imp.
With a Morningstar mppt.

On mppt a single 40' run of 10 awg ~2℅
On pwm 3 40' runs of 10 awg ~2℅
I already have 150' of cable
Pros/cons thoughts anyone?

mike95490 · April 2020

My gut feeling is MPPT if you can swing the $ for a decent one.

Here's my math with your systems:

345 watt q cell panels. 38 vmp 47 voc and 9 imp.

With a TriStar pwm controller.

is 28V @ 27A ( 756 watts) (in PERFECT lighting conditions)

Or 4 250 watt Reno sola 2s2p 30 vmp 37.5 voc and 8.30 imp.

With a Morningstar mppt.

is 28V @ 34.6A ( 970 watts) (in PERFECT lighting conditions, 3% MPPT loss)

mcgivor · April 2020

mike95490 said:

My gut feeling is MPPT if you can swing the $ for a decent one.

Here's my math with your systems:
345 watt q cell panels. 38 vmp 47 voc and 9 imp.
With a TriStar pwm controller.
is 28V @ 27A ( 756 watts) (in PERFECT lighting conditions)

Or 4 250 watt Reno sola 2s2p 30 vmp 37.5 voc and 8.3With a Morningstar mppt.
is 28V @ 34.6A ( 970 watts) (in PERFECT lighting conditions, 3% MPPT loss)

The results would differ depending on the ambient temperature, the following is from Morningstar, an excerpt of the full text here https://www.morningstarcorp.com/whitepapers/traditional-pwm-vs-morningstars-trakstar-mppt-technology/

PWM Over MPPT:

The preceding discussion of PWM vs. MPPT may cause some to wonder why a PWM controller would
ever be chosen in favor of an MPPT controller. There are indeed instances where a PWM controller can
be a better choice than MPPT and there are factors which will reduce or negate the advantages the
MPPT may provide. The most obvious consideration is cost. MPPT controllers tend to cost more than
their PWM counterparts. When deciding on a controller, the extra cost of MPPT should be analyzed
with respect to the following factors:

1. Low power (specifically low current) charging applications may have equal or better energy harvest
with a PWM controller. PWM controllers will operate at a relatively constant harvesting efficiency
regardless of the size of the system (all things being equal, efficiency will be the same whether using
a 30W array or a 300W array). MPPT regulators commonly have noticeably reduced harvesting
efficiencies (relative to their peak efficiency) when used in low power applications. Efficiency
curves for every Morningstar MPPT controller are printed in their corresponding manuals and
should be reviewed when making a regulator decision. (Manuals are available for download on the
Morningstar website).
2. As explained in the Environmental Considerations section, the greatest benefit of an MPPT regulator
will be observed in colder climates (Vmp is higher). Conversely, in hotter climates Vmp is reduced. A decrease in Vmp will reduce MPPT harvest relative to PWM. Average ambient temperature at the
installation site may be high enough to negate any charging advantages the MPPT has over the PWM. It would not be economical to use MPPT in such a situation. Average temperature at the site
should be a factor considered when making a regulator choice (See Appendix).
3. Systems in which array power output is significantly larger than the power draw of the system loads
would indicate that the batteries will spend most of their time at full or near full charge. Such a
system may not benefit from the increased harvesting capability of an MPPT regulator. When the
system batteries are full, excess solar energy goes unused. The harvesting advantage of MPPT may
be unnecessary in this situation especially if autonomy is not a factor.

BB. · April 2020

If you have hot ambient temperatures and a cool FLA battery bank, the Vmp~38 volt array can get down towards ~31 volts Vmp-array-hot (104F with cells being ~36F higher than ambient under full sun). And you want to charge your battery somewhere around 29.5 volts or so.... The Vmp/Imp/Pmp curve is relatively flat, so it is not like the output goes from 100% to 0% at that point, but does fall a bit.

That means possibly somewhat less harvest during the middle of a hot day (you have a couple of volts of wiring drop, a bit of drop across the PWM controller). Not the end of the world--But does reduce harvest some (of course, middle of summer with a large array--You probably have more harvest than you would need anyway).

If you have longer wire runs, higher voltage on the Varray configuration makes wiring much easier and cheaper--Figure out your array to controller distance, and the wire AWG needed... It generally works out that you can get away with upwards of 6 AWG smaller wire diameter with a "higher Vmp-array" working voltage.

As I always suggest, do a paper design with both options and see what works out best for you (cost, wiring issues, features like Internet/remote management that can be available on larger/more expensive MPPT controllers, etc.).

-Bill

BB. · April 2020

Forgot to add... MPPT controllers will safely and reliably "throttle" the output curent... So you can "overpanel" an MPPT controller (~1.3x larger array) and all will be OK.

PWM controllers cannot throttle their output like a MPPT controller can, so you should not overpanel a PWM controller.

As always, check the manual for details.

-Bill

mountainman · April 2020

Thanks all . In my area summers 104f.
These panels are local to me.
4 250 watt panels cost .24/watt $240
3 345 panels 1035 watts are .50/watt $525
With a Morningstar 30 mppt I can over panel
800x1.3 1040 watts
Morningstar 30 mppt $382 from naws
My local dealer has a 45 amp Tristar pwm $75.
Pwm system cost~ $600
Mppt system cost ~$622
plus mppt shipping tax

mountainman · April 2020

@mcgivor information on the Morningstar
Pwm vs mppt.
From my understanding if you are using
10-15℅ of your battery mppt has less advantage over pwm. Compared to daily using 30-40℅.

And if you live in a cold area mppt is better.

If its warm to hot all year a properly designed Pwm system would be cheaper. And work just as good or better than mppt.

It states for 24 volt charging no less than 30 vmp in the highest temp.
And 34-36 vmp would be the most efficient.

And For long distance arrays from the controller Mppt is cheaper than pwm because you have to use big expensive wiring.
The larger the array the more mppt would Be the best cost effective choice.

mike95490 · April 2020

Regardless of temperatures or where you live, panel/battery voltage mismatch is wasted power.

Higher voltage strings loose less power in small copper wires. There are a lot of factors to consider on sizing a sytem

mountainman · April 2020

@mike95490 agreed there are a lot of factors. Higher voltage is more efficient in wiring. But the higher the difference between array and bus voltage the less efficient the controller becomes.
It's not a one size fits all.
the biggest challenge of solar is in winter short days and frequent overcast skies with my 3.59 January sun hours I need all the help I can get.
The cost between mppt And pwm is not a factor at least not in my situation.
So I'll spend the money on mppt and cheaper Grid tie panels.

Pwm or mppt

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