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Forum Index : Solar : PWM vs MPPT

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Disco Stu
Newbie

Joined: 13/12/2009
Location:
Posts: 16
Posted: 11:41am 16 Oct 2010
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hi people

just wondering if it is worth the additional expense of going to mppt, apart form the well noted benifits like upping the voltage in your panel string to reduce your wire size and increace distance from the battery bank, and the apparent power claims some think up to 30% although most seem to claim 15% however this falls away in hot conditions.

Does anyone have any actual comparisons of pwm vs mppt logged under actual conditions monitoring the tempreature at the same time?? there are alot of writeups claiming that they harvest more power in cold conditions but what is the figure when hot? is it worth while?? there is one guy in youtube with his meters showing the difference when he switches from pwm to mppt, but doesnt say anyting about how hot it is or other things like that.

Reason im asking is that with the dollar at its current rate the american gear ie Outback is becoming more and more viable but the battery bank monitoring gear that you buy to suit these systems is quite a significant amount again, to get it to monitor as simply as the PL40 with a simple shunt kit, im a great fan of Plasmatronics gear for this monitoring ability and simplicity (and one of the few things aussie made still!!).

Could anyone comment who knows more about this stuff or has fist hand experience with this and if the extra cost is justified??

thanks for your help
Stu
 
RossW
Guru

Joined: 25/02/2006
Location: Australia
Posts: 495
Posted: 02:41am 17 Oct 2010
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Stu,
I have 5 seperate arrays, each of 6 * 100W modules, and one of 6 * 80W

I purchased a FlexMax 80 and run each array as a "nominal" 100V array, each with their own metering and their own cable from the arrays to the power room.

In summer there is a reasonable increase in recovered power - I'd estimate about 20% would be close. In winter it's a little more.

It does however save quite a lot of copper losses (I only ran my arrays in 4 sq mm cable, and they're about 25m runs (so 50m of cable).

You can do some of your own figuring though - a module can produce a certain current at maximum. Lets take my panels for example, it's near enough to 6 amps. To get maximum power from the panels, you want to take power at the max power point, which will be around 17 volts, depending on temperature. Lets just fly with 17V for now.

17V @ 6A = 102 watts.

Now lets say you're using a simple voltage-limiting charge controller. Your batteries are low after a nights use and are down to 12.0V

The maximum you can get out of your panels is still 6 amps, but now their output is dragged down to 12.0V, so 12*6 = 72 watts. Thats all thats doing useful work for you.
Thats near enough 30% down on what you could have, or put the other way, you could get 41% MORE POWER if you used a MPPT (neglecting losses, which are fairly small)

As your battery terminal voltage rises, this "wasted potential" gets less, and the gains from MPPT are less, but even with "floating" cells at say 13.8V, thats only 83W compared to a potential of 102, so again you could get 23% more useful power simply by maximising the matching between what the panels are making and how your batteries want to use it.

I run a 48V system and used to have my arrays connected in 4 series panels, and noticed a substantial increase in daily recovered kWh when I reconfigured to 6 series and used the MPPT.

Tracking the arrays to follow the sun provides a substantial (additional) benefit, but thats another story.
 
GWatPE

Senior Member

Joined: 01/09/2006
Location: Australia
Posts: 2127
Posted: 03:45am 17 Oct 2010
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Hi Stu,

In Ross's case, his panels are around NOM 3.5kW peak power. So the 30% is equivalent to about 1kW of panels.

The cost of the MPPT is around AU$1,000. I doubt you will pick up 1kW of panels for this price. These FlexMax80's that Ross mentioned record power harvest data and give LCD info on real time performance. Many other brands exist and many are available on EBay.

Shop around, and be careful with maximum voltage and current ratings.

Gordon.

become more energy aware
 
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