Panel over dimensioning on 12/24v MPPT solar controllers

[Treechange]

Hi, from sunny Australia

My first post here, in such august company, so please be gentle. 

In domestic 240v systems it is common place (at least in Australia, perhaps due to our feed in tariff tariffs and regulatory structure), to 'over dimension' 240v solar inverters.

For example a 1.5kw system may be dimensioned with 2kw solar panels.

The primary outcome of this practice (as the cost of panels continues to fall) is overall improved output (Watts per $). In the 1.5kW example (assuming optimally positioned fixed panels), the solar bell curve is simply held at 1.5Kw, once the inverter reaches maximum output and as a bonus, it's output in times of poor solar radiation is improved. For a capital outlay less than the return, the systems performance may be boosted by 10-15% or more. It is my understanding that power limiting is achieved by the inverter driving 'off' the MPP point, lowering the panels available input energy, so that P(in) = P(out) - inverter inefficiency.

Question?

Is anyone aware of an off the shelf 12/24v MPPT controller(s) that performs in like fashion?

As an example, a 20A, 12v MPPT controller will invariably have 240 - 260 watt specified as the maximum panel size. This happily matches the 12 to 13v x 20a = 240 to 260watt delivered to the battery at maximum panel output (in very ruff roundies).

Are there any readily available 12/24v 20A solar controllers that can be continuously over-dimensioned with say, 400watt panels?

Perhaps most/all MPPT (and PWM) regulators are current limited and over-dimensioning is not an issue?

Anyone know the answer, or can quickly test, to see if over-dimensioning 'lets out the white smoke'?

PS It would be useful in the RV market.

Cheers

John k

PS love the solar vids

[Treechange]

Well I've not got far with my quest.

Whilst current and voltage input protection is tricky on PV regulators, it can be done.

The early model EPSolar Stacer series MPPT 1210RN appears to use some form of input load sensing and is capable of 'infinite' PV panel input. Input load sensing does not appear on the current model and I'm checking with EPSolar as to this models capability.

Note: The 1210RN is confirmed by connecting it to a 36v Li ion battery (PV input) and a 12v load. In this configuration, it regulates nicely at maximum output current (10A) to the battery and 91% efficiency.

The CTEK 20A MPPT D250S dual DC/DC charger is confirmed by Sweden (email) to accept panels from 40w to 450W. Although this controller has a Voc max of only 22v and is unprotected, 450watts represents a 1.73 increase on what would be the accepted practice of 260 watt panels on a 20A controller.

Anyone know of others?

To my thinking, the easy way to manage excessive PV power on an MPPT regulator is to 'steer off' the MPP voltage. But who if anyone does it, and if not, why not?

Perhaps MJLorton would like to comment on the PWM Steca PR1010 and the MPPT Morningstar SS-MPPT-15L?

The Steca and Sunsaver products look well protected on the PV input side of things.

The Sunsaver specs indicate:

Nominal Max. Input Power = 200 Watts @ 12v

That appears a pretty feeble PV rating for a 15A controller.

I don't think the Steca specifies a maximum panel wattage?

[MJLorton]

Are there any readily available 12/24v 20A solar controllers that can be continuously over-dimensioned with say, 400watt panels?

Perhaps most/all MPPT (and PWM) regulators are current limited and over-dimensioning is not an issue?

Anyone know the answer, or can quickly test, to see if over-dimensioning 'lets out the white smoke'?

PS It would be useful in the RV market.

Cheers

John k

PS love the solar vids

Hello John,

Welcome to the forum and thanks for the interesting post.

First off, my first reaction to over-dimensioning a solar array for the rated power of an inverter / charger is that it should not be done....but then I could be wrong!? (after reading your second post I'm now wiser)

I fully understand the idea of having a larger array to cater for lower radiation and our's is spec'ed to a certain extent that way.

Our main inverter / charger will change the mains frequency from 50 Hz to 53 Hz as a means of communicating with the grid tie inverter to "back down" the power production if net load is less than what is available from the array. I assume the grid tie inverter then moves off the MPP to achieve this as you mentioned. The down side I have noted with this increase in mains frequency is that any clocks that are mains powered and perhaps use the frequency as a reference....run fast...

I'm now intrigued with this whole concept and will do some further investigation. I will also ask Victron Energy and see if they will tell me how the grid tie inverter backs down the power production.

As for as my Steca you were asking about...I've just been to their website and seen the following:

"The latest charging technologies, combined with a Steca-AtonIC state of charge determination which has been significantly improved once again, result in optimal battery maintenance and control of the module output of up to 900 Wp which can be connected to it."
http://www.stecasolar.com/index.php?Steca_PR_10_30_en

This is brilliant and confirms what you are saying.

I'll post some more feedback once I have more information.

Cheers,
Martin.

 

[Treechange]

Hi Martin,

Yes I think both Steca and Sunsaver are up to the mark.

Check the image from your Steca url (Panel 720w)?



Thanks for taking up the challenge.

Cheers

John K

PS I suspect for most PWM regulators a pretty safe bet is:

Controller 'Imax load' = Panel 'Imp'

[MJLorton]

Hi John,

I did try and pursue an answer on how a grid tie MPPT inverter backs off power production....and was fobbed off with a statement that those inner workings are "proprietary information"....

I don't really think that information is proprietary...I could be wrong...I think the person I asked did not know the inners workings and that was the easiest answer to give me...

I did remove my Steca from "operation" in the hope of setting up and videoing a demonstration of PWM backing off the "power"...but I've been distracted with other work. I hope to get back to this at some point. I don't think this will demonstration what you are talking about...but might be interesting none the less.

If you have any further light to shed on the topic please post.

Thanks Martin.

[Treechange]

Thanks Martin,

Whilst the initial query was for MPPT regulators; just how the Steca regulates array current higher than it's specified output current would be fascinating.

My attempt to contact and discuss over-dimensioning with 12/24v MPPT regulator suppliers has been unsuccessful.

Their reply is that the maximum input power is specified and that's the end of it. For MPPT regulators the specified input power can be less than the output power.

Cheers

John k