Xantrex XW MPPT efficiency issue

System

Hi, I am new here and I have a question regarding input and output power of xantrex XW MPPT

I have a system with 2 thin film a-Si panels connected in parallel , 24V AGM battery bank and Xantrex XW60 MPPT charge controller.
The thin-film panels are high voltage: Pmax 100W, Voc 100V, Isc 1.65A, Vmp 76V, Imp 1.32.

As an example in direct sunlight I am getting something like that:

- input 73V, 2,7 A, 197W
- output 24,5, 7,2A, 176W

in low light conditions I am getting:

- input 71V, 06A, 43W
- output 24V, 1,1A, 26W

So here is my problem – input is always higher that output by something around 10W-30W. I know that input and output would be equal only under perfect conditions, however, I think the difference is too large. It’s 10-15% energy loss in direct sunlight and almost 50% in low light conditions.
I suppose that MPPT may have worse efficiency when is converting 73V down to 24V and maybe I should get 48V battery bank so the efficiency will be better, or maybe XW 60A is just too large for only 200W array but I am not sure. Please tell me if and what is going wrong. I appreciate Your help.

Seweryn

Solar Guppy

Re: Xantrex XW MPPT efficiency issue

A couple of things happening

First, all charge controllers have idle losses, typically 4-10 watts depending on the controller. You using a 60 amp charger in a 4 amp setup.

Second, there are tolerances on measurements, a controller that has 1% full scale for 60 amps is 0.6 amps , that could be ~ 14 watts reported variance possible

Lastly, the Xantrex Mppt is not the best choice for large voltage step down as it doesn't have synchronous rectification, so the diode losses add-up in large Vin -> Vout operating conditions . The Outback and MoringStar units are synchronous and the MorningStar Mppt has the ability to power down converter sections for improved low power efficiency

System2

Re: Xantrex XW MPPT efficiency issue

Thank You for Your help. I suspected that among other things this large volatge step down could cause it . I am thinking about changing the battery bank on 48V and adding 8 more modules so this loss will not be significant.

niel

Re: Xantrex XW MPPT efficiency issue

sevvq wrote: »

Thank You for Your help. I suspected that among other things this large volatge step down could cause it . I am thinking about changing the battery bank on 48V and adding 8 more modules so this loss will not be significant.

first, i agree with the steps you plan on taking, but even with more pvs the idle losses stay the same and it just becomes a smaller percentage of the overall power handled so it will remain 4-10w lost.
secondly, the upping of the battery voltage shall narrow the gap between input and output inefficiencies, but that will also still be there. the actual improvement over the other i can't say exactly what it would be even though i believe it will improve.
as was mentioned output will never equal input as that would be an impossibility.

Dave Angelini

Re: Xantrex XW MPPT efficiency issue

FYI my XW-60 is near 96% most of the time that I have bothered to look. This is a 24V battery with an MPPT array of 70vdc. It probably is worse as the array voltage increases. Yesterday I was running my heat pump in the middle of a 98F day pulling 1606 watts out of 2 arrays labled for 2KW. The input power from the arrays was 1686 watts and this was with two controllers. At some point good enough is good enough! All these numbers are from the controllers which may have some error BTW.

To the OP before you rush off and convert to 48v nominal consider your overall efficiency rather than the obvious advantages of 48V nominal. Right now, today the best freezer in the world is 24v, there is only one 48V water pump and it was not available earlier this year. Many 24V pumps, an excellent 24V evaporative cooler is available. Many array trackers run from 24v directly. 24V yard lighting.
There are several 24V refrigerators as well. All solar domestic hot water pumping is either 24vdc or AC. The safety advantage is also there as your system voltage never exceeds 50vdc. You have to do the math because I have seen 24V systems outperform similar 48V. You have to use the inverter conversion loss! It just depends on your needs. I actually have the data and I am working on number 38 this afternoon. If it is in the shade...