Tracked Array & Fixed Array through One Controller

nigtomdaw

Hi,

I dont see a problem with what I am proposing but would would like my peers opinions. I currently have two solar arrays going through two separate MPPT Charge Controllers. The maths is OK to put them through one controller. One array is tracked, single axis the other is fixed

Here is the maths

Fixed Array 12 x 200 wp panels VOC 40 volts currently wired in 6 strings of 2 panels at VOC 80v to a FM80 @ 48v
Single Axis Array 6 x 162 Wp Panels VOC 28.8 volts currently wired in 2 strings of 3 panels at VOC 86.4v to a Midnite Classic @ 48v

I have other plans for the Classic and can see no reason why I cant wire the six 162 wp panels one each in series to 2 of the 200wp panels.
Giving 6 strings of 3 panels each string being 40v + 40v + 28.8v = 108.8v VOC with a total watt peak of 3,373 . comfortably below the FM80 amp rating @ 48v

The distance between the tracked array and fixed array is 15 meters about 48 feet. The fixed array is only 3 meters from combiner to Controller So apart from 90 meters of 6mm cable its doable.

The fact that one panel in each string is tracked and the other two are fixed should make no troubles for the MPPT controller as the VMP and VOC will be
identical in each string.

Just hope Im not missing something

Nigel

Cariboocoot

Re: Tracked Array & Fixed Array through One Controller

Two separate arrays require two separate controllers.
A "separate array" is one with a different angle (on either axis), or one whose electrical specifications are otherwise different from the other.
In this case you have one fixed and one tracking, so that counts as different angles. If you don't use separate controllers the MPPT function can be confused by the unequal insolation on the panels, causing it to choose a less-than-perfect power point.

Not saying it won't work, but it won't be ideal. How far off it would be is unpredictable.

nigtomdaw

Re: Tracked Array & Fixed Array through One Controller

But each string is identical so at all times the VMP will be identical in each string the insolation affects the current not the voltage you get max VOC before you even see the sun and I thought the MPPT was voltage specific, where would I loose efficiency

Cariboocoot

Re: Tracked Array & Fixed Array through One Controller

MPPT controllers don't "run" panels at their rated Vmp/Imp. They adjust the "loading" of the array to maximize output for the conditions at the battery terminals. So you may have an array of panels with a Vmp of 35 and Imp of 7 according to the panel specs, but the controller will effectively change the "load" on them so that they may run at a higher Voltage and lower Imp. It's not the same as a PWM controller that just takes whatever the panels put out and passes it on (albeit intermittently to control Voltage level at the battery).

I think Solar Guppy did some tests on this with GT inverters and found that two arrays facing in two different direction (East & West) connected to one inverter screwed up the MPPT function to the point where there was no gain from having the second array. I'm not saying it would be as bad as that, but it will not allow the inverter to pick the right power point for either one of the arrays because of the output of the other.

nigtomdaw

Re: Tracked Array & Fixed Array through One Controller

Thanks Cariboocoot, I can see the extremes of the east west arrays being a problem and ill hold my finger off the trigger for now, but considering that the sun moves through out the day and the tracker adjust minimally every 10 minutes or so (my tracker tracks by time not light sensors, records sun up and sun down daily and divides it up) and the MPPT unit refreshes its MPPT point every 10 minutes (this is a complete guess) Taking an 8 hour sun day as an average
My tracker moves through 90 degrees in that day. So every hour it moves 11.25 degrees, it does this in 10 minute stages ie 6 times an hour thus every 10 minutes the array moves 1.875 degrees. Hardly noticible to the eye when your watching it. If the MPPT tracking resets its power point exactly in the middle of this 10 minute tracking window I will be out of alignement by 1.875 º for five minutes.
I dont think that the one PV panels minute voltage difference because of the 1.875º change will have any effect on power harvest to the batteries with the MPPT value selected by the controller till its next sweep.

Cariboocoot

Re: Tracked Array & Fixed Array through One Controller

The basic problem is that the fixed array will have an output curve that is roughly a function of the sun's position in the sky, whereas the tracked array will be closer to its peak potential at all times as it follows the sun's position. The larger the arrays, the bigger the difference of course. One array of 2400 Watts and the other of 972 could be quite a difference, but likely the larger array will "win out" over the smaller.

BTW, the Voc isn't actually relevant except for determining if the array may exceed the controller's input maximum. You really do need to determine the Vmp of the two arrays as you want to parallel them. If there's too much difference, that could waste more power than even the differing angles would.

On the up side, the MidNite Classic will probably handle this better than an old Outback MX60 as I think the Classic doesn't do "sweeps" but rather readjusts continuously. I don't recall if the FM80 sweeps or is continuous.

Dave Angelini

Re: Tracked Array & Fixed Array through One Controller

The real problems will be with clouds!

nigtomdaw

Re: Tracked Array & Fixed Array through One Controller

Effectively there is only one array in which one panel in each string is always facing the sun

vtmaps

Re: Tracked Array & Fixed Array through One Controller

nigtomdaw wrote: »

Effectively there is only one array in which one panel in each string is always facing the sun

Why is that any different than having a string in which one panel is partially shaded? I think that if your scheme were to work, it would be better to have the tracked array in parallel with the fixed array (assuming that the voltages in each parallel array were equal). In your case you would have to have both types of panel in each array.
--vMaps

stephendv

Re: Tracked Array & Fixed Array through One Controller

vtmaps wrote: »

Why is that any different than having a string in which one panel is partially shaded?

Because the voltage of a partially shaded panel changes, but the voltage of a panel facing the sun or pointed slightly away from the sun doesn't change much. I think the biggest losses with this setup will be in summer when the static array will be fully shaded in early morning and late evening. The rest of the time, both will have Vmps of within 5% of each other, so in the worst case the controller can only make a maximum error of 5% in choosing the voltage.

petertearai

Re: Tracked Array & Fixed Array through One Controller

I think the fm 80 continually adjusts . Essentially that is the difference from the outback mx.

Ralph Day

Re: Tracked Array & Fixed Array through One Controller

Does the cc "see" arrays or just it's input current and voltage? People put mppt charge controllers on full tracking systems, how is that any different from partial tracked/fixed arrays?

I've had 2/3rds fixed and 1/3 tracking in my system since inception. I've never noticed any extra sweeps compared to when there's little tracking happening (the tracking was manual before auto tracked).

nigtomdaw

Re: Tracked Array & Fixed Array through One Controller

Ok Thank you all for your answers, I also posted on Midnite forum when I wasnt getting to hear what I wanted, but Im smart enough to know there are those out there that are smarter.
Thus I have my answer leave alone.
It was only that the classic was having an easy life playing around with about 20 amps of delivery. I will leave the fixed array on the FM80 and I have another tracker to put up same as the first which will accomodate 1200 to 1400 watts with new panels giving the New Classic 150 about 2400 watts to earn its keep with. Then I have the best Controller with the same amount of PV as the fixed array but on two single axis tracker units. Sounds Win Win to me, wheres the cement mixer.

Once again thankyou to all contributors Nigel

65DegN

Re: Tracked Array & Fixed Array through One Controller

Putting a 200 watt panel in series with a 165 watt panel will derate your 200 watt panels by limiting their current to the max output near the 165 watt panels output.
Too bad you didn't get a Midnite 200 V controller.
I can say from experience that a single controller will do a good job tracking the outputs of a combined fixed and tracked array's as long as the various panels power curves are not significantly different.
I have two strings of 7 (ancient) Arco 16-2000 modules in series that are fixed, in parallel with two strings of 3 SolarWorld 175's in series that are tracked.
Turning off the individual breakers to the FM80 there is typically little to no difference between the output for each array running by itself or the two arrays running in parallel.
In other words if I get 1000 watts from the SW175's by themselves and 500 watts from the Arco's by themselves, in parallel they will produce 1500 watts.
There is little difference even if each array is individually MPPT tracked first.
I am not recommending a combination of panels that results in this high of an input voltage. It started as an experiment to attempt to minimize voltage drop over the 325' distance. About once a month it hits 153 volts. Its been running this way for about 3 years.
I may use an aux output to trip a relay that puts a small load on the input from the array if the voltage exceeds a preset limit, maybe 135 V. The load could be calculated to prevent the output from exceeding 140 V under the coldest conditions. Once the controller begins to load the array the relay could open.
This might affect MPPT tracking slightly but I suspect not significantly.