MX60 Clarification

Re: MX60 Clarification

Dennis,

The MX60 is somewhat conservatively rated by OutBack to handle a PV array rated at ~1,600 W STC when charging a “24 V” battery bank. Adding the additional three KC-130 modules will increase your array’s STC power rating to 1,290 W – well within the MX60 capabilities.

Its power handling capabilities for 12 V and 48 V systems are ~800 W STC and 3,200 W STC, respectively.

It’s generally OK to mix ’n’ match PV modules from various manufacturers into a single large PV array feeding a single controller. Overall performance can be compromised depending on individual module specs. Although you’re planning on an array of three different modules sizes, you’re also planning on three identical modules within each of the four series strings, so that will help minimize any problems.

Here’s what I believe are the important specs for your various modules:

Siemens SR90: 90 W STC, 17.0 Vmp, 5.4 A Imp each; 270 W STC, 51.0 Vmp, 5.4 A Imp in a series string of three modules (voltages add for modules in series, and series string current = current of lowest rated module.)
Kyocera KC-120: 120 W STC, 16.9 Vmp, 7.10 A Imp each; 360 W STC, 50.7 Vmp, 7.10 A Imp in a series string of three modules
Kyocera KC-130: 120 W STC, 17.6 Vmp, 7.39 A Imp each; 390 W STC, 52.8 Vmp, 7.39 A Imp in a series string of three modules

PV module cells heat up when exposed to the Sun, so their voltage drops. Accordingly, ~88% of STC Vmp is a good working voltage. The working Vmp’s for your various strings will be ~44.5 V, ~44.5 V, and ~46 V. When all of the series strings are connected in parallel, the array will operate at the lowest string voltage. So, look for the array to operate at ~44.5 V -- perhaps a bit lower in summer and a bit higher in winter -- when the controller is in bulk stage.

The total array current at ~mid-day on a sunny day will be the sum of each string current: 5.4 A + 5.4 A + 7.10 A + 7. 39 A = ~25 A. While the KC-130 modules will operate at somewhat less than optimal due to the lower voltage of the other strings, their slightly higher current rating will result in performance that’s a bit better than that of the KC-120’s.

In sum, the MX60 will be able to comfortably handle your planned 1,290 W STC array and 24 V battery bank, its DC-DC conversion feature will take the ~44.5 V x ~25 A in and convert it to ~29 V x 36 A out (bulk stage; assumes ~94% efficiency), and its MPPT feature will make the most of environmental conditions in producing the greatest amount of power available from the array.

HTH,
Jim / crewzer

Re: MX60 Clarification

Dennis,

Glad to help. Yeah, it does get a bit crowded in the ol' noggin on occasion, but I'm slowly learning to just remember what info I need, where to get it, and how to interpret it rather than just memorize a bunch of stuff and not understand it. I also find it much more satisfying to explain and justify my assessments and recommendations to the forum. In doing so, I hope that other members will be able to understand how to analyze and tackle their individual design challenges.

I'd recommend L-16 batteries over the T-105's. Why? A smaller overall "footprint" for the batteries, fewer battery interconnect cables (lower probability of failure, less maintenance), fewer cells to maintain (12 x 3 = 36, vs 16 x 3 = 48).

However, L-16's might need a special charging algorithm. See the following reference:

http://www.morningstarcorp.com/products/TriStar/info/TS_Manual.pdf (page 48)

HTH,
Jim / crewzer

Re: MX60 Clarification

Dennis,

No, you won't need a separate charge controller. The MX60's target voltages for the absorb-, float-, and EQ stages can be user-set to the values you'll need. The point of the reference above is that it's different from the settings recommended by Trojan for their batteries.

Regards,
Jim / crewzer