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Old 01-09-2014, 08:29   #16
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Re: What Type of Controller(s) Should I use for these Solar Panels?

I'm curious how a single MPPT controller is going to track 2 different sets of panels aimed in opposite directions?

Do you feel both sets of panels are going to produce the same output even under those conditions?
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Old 01-09-2014, 09:07   #17
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Re: What Type of Controller(s) Should I use for these Solar Panels?

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I'm curious how a single MPPT controller is going to track 2 different sets of panels aimed in opposite directions?

Do you feel both sets of panels are going to produce the same output even under those conditions?

For parallel connection (which I think is superior), the output does not have be identical, or even close what is important is the Vmp.

With panels experiencing different conditions a single controller will select the Vmp that gives the optimum combined output of the two panels.

A single controller for each panel will give better tracking, but the self consumption of sophisticated controllers is quite high which reduces the benefits, and there are some practical disadvantages.
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Old 01-09-2014, 09:36   #18
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Re: What Type of Controller(s) Should I use for these Solar Panels?

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The output does not have be identical, or even close what is important is the Vmp.

With panels experiencing different conditions a single controller will select the Vmp that gives the optimum combined output of the two panels.

A single controller for each panel will give better tracking, but the self consumption of sophisticated controllers is quite high which reduces the benefits, and there are some practical disadvantages.
If one is going to compromise on the combined output of the 2 sets of panels by not getting the maximum power point of either set of panels, then there isn't much reason to spend the extra to buy an MPPT controller.

Midnite solar doesn't publish power consumption specs for the 30 A Kid controller, but the Outback 60 A controller is published to consume less than 1w in standby and to be up to 98.1% efficient @ 60A, 48V. That's pretty good.

Assuming 2 x 30A controllers consumed 1w ea and were pretty close to 98% efficient, I'd still go with 2 x 30 A controllers for the better tracking for each set of panels. I also like the idea of redundancy, in case of solar controller failure. Better to harvest half than none at all.
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Old 01-09-2014, 10:26   #19
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Re: What Type of Controller(s) Should I use for these Solar Panels?

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If one is going to compromise on the combined output of the 2 sets of panels by not getting the maximum power point of either set of panels, then there isn't much reason to spend the extra to buy an MPPT controller.
No the gain is only lost when the combined Vmp is identical to the battery voltage. (Ignoring the higher self consumption of. The MPPT circuitry)

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IOutback 60 A controller is published to consume less than 1w in standby and to be up to 98.1% efficient @ 60A, 48V. That's pretty good.
The problem is that it will not be in standby when its working.

The Outback 60 self consumption when working is about 0.5A (6w).

I agree redundancy is an advantage of two controllers (although you can manage without a controller in an emergency if you are careful)
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Old 01-09-2014, 12:37   #20
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Re: What Type of Controller(s) Should I use for these Solar Panels?

Something is not adding up here. The original installation may have been screwed up, and its best to start with the basics.

From the panel labels, you should have 8 panels with 22volts open circuit voltage and about 5 amps output apiece. These panels were designed to charge 12v batteries.

My first step would be to check each panel. Disconnect the boat wiring and put a separate wire to the plus and minus terminals. Put a voltmeter across the two wires and it should read at least 20 volts in the sun. Then short the two wires together and read the current with your clamp-on ammeter. The shorted current should read about 5 amps on each panel.

Assuming the panels check out, tackle the wiring and controllers next. As you probably realize by now, the original controllers were simple devices, intended to work with panels in parallel. If the panels were wired in series, they would be wasting 75% of the solar power, and you would be seeing 10 amps instead of 40. If the controllers were wired in series, they would fry the batteries on a 12v system.

You can either wire the panels in parallel with PWM controllers or in series with MPPT controllers, or some combination thereof. Parallel is easier and cheaper. If you use PWM controllers, you have to make sure that you don't exceed their current rating. If you use MPPT controllers, you have to make sure that you don't exceed both their current and their voltage rating.

If you have trouble understanding parallel versus series, you need to get a little help locally.
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Old 01-09-2014, 17:30   #21
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Re: What Type of Controller(s) Should I use for these Solar Panels?

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No the gain is only lost when the combined Vmp is identical to the battery voltage. (Ignoring the higher self consumption of. The MPPT circuitry)
This part I still don't understand.

MPPT controllers periodically "sweep" and determine the optimum voltage to harvest max. power from the solar panels. It varies by panel, but more importantly, varies based on solar intensity, thus the need for them to sweep periodically, say every few minutes or faster.

If one set of panels is directed at the sun and the Vmp @ 10:00 is 18.75V and the other set at that same instant is 16.50V, it seems like the controller will try to average them out and as a result, not get the optimum Vmp for either one.

The effect could be even greater earlier and later in the day when Vmp on one set may be 14.8V and the other set is 18.92V. Neither set of panels is getting any benefit of MPPT out of this scenario. Both are producing less than max. output.

Am I missing something?
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Old 01-09-2014, 23:58   #22
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Re: What Type of Controller(s) Should I use for these Solar Panels?

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Am I missing something?
What you are saying is fundamentaly right but the differences you are imagining in Vmp are much greater than are seen in practice.
Vmp does not vary much with light intensity.

Have a look at this graph for a Kocera panel:



The difference in Vmp between the top graph at at very bright 1000Wm2 and the bottom graph at a dull 200wm2 is less than 0.8v. In practice it will be slightly less than this due to temperature differences (at high illuminates the panel will heat up more reducing the Vmp).

The large sort of differences in Vmp you are quoting are not seen in practice.

If you draw a line down from the Vmp of top graph and operate the the panel exposed to 200Wm2 at this voltage, as would happen with a single controller, you can see the loss of power is very tiny less than 1W.

The Vmp in the panel specifications is for a very unrealistic cell temperature of 25c in practice the Vmp will be around 16v. You will not see 18v+ apart from very briefly if the sun comes out from heavy overcast, or in very cold temperatures.

If we take a typical realistic scenario where panel has a Vmp of 16v and the other a Vmp of 16.5v. To calculate the loss is quite complicated but we can make certain assumptions that make the maths easy. If we assume the solar controller uses the lower Vmp (instead of a compromise) and we assume the current does not rise as the voltage is lowered. These assumptions exaggerate the difference considerably, but even so, the loss in output of single controller verses one for each panel is only 1.5%. Given the higher self consumption and standby current of two units it is likely that that a single controller would be superior in these conditions.

Greater differences in Vmp between panels than 0.5v can, and do occur mainly with isolated shadows. I think we are both in agreement that a single high quality controller for each panel will generally give a higher output, but the magnitude of the difference is not great. In practice the difference is often between a single high quality controller, or multiple lower performance, cheaper units, when this is taken into account the difference becomes very small, or can swing in favour of the single unit.

Multiple controllers, including one for each panel are certainly worth considering. Two controllers one controlling the panels on the port side and one on the starboard side can work especially well, but often a single unit is the most practical option. With all these options it is worth remembering that we are only making very small differences that effect the last few percent of output.
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Old 02-09-2014, 04:53   #23
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Re: What Type of Controller(s) Should I use for these Solar Panels?

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... and its best to start with the basics.

From the panel labels, you should have 8 panels with 22volts open circuit voltage and about 5 amps output apiece. These panels were designed to charge 12v batteries.

My first step would be to check each panel. Disconnect the boat wiring and put a separate wire to the plus and minus terminals. Put a voltmeter across the two wires and it should read at least 20 volts in the sun. Then short the two wires together and read the current with your clamp-on ammeter. The shorted current should read about 5 amps on each panel.
+1. The panels are getting on a little. At that age some panels might well be kaput. No point putting in a really nice controller if only a fraction of its capacity will be used.

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