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Thread: Solar panel max voltages

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  1. #1
    Join Date
    May 2012
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    Oslo area
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    Default Solar panel max voltages

    This subject actually belongs in more specialized fora, I know. But there are some clever people in this group, maybe someone knows some facts. Any references to good spcific answers on solar fora are welcome. But discussions of solar performance are usually not based on different panels in a system.


    I like to use flexible solar panels on the boat, they are light and a catamaran usually has some space where they can be located. You can buy cheap ones and change them more often or buy really good ones at 3-4 times the price. I have done both. Now one of the cheap ones needs to be replaced. As time passes, solar panels slowly develope. The newer ones tend to have higher max power voltages (Vmp). All my solar panels are connected in parallel to one MPPT regulator. There is one panel of 16V Vmp. I am looking at replacing another and have found an interesting candidate that has Vmp 20.5V. The last figure is pretty high, most I see are 17-18V. Will this be a problem? Will the 20.5V panel kind of block out the others when operating in full sunlight? Or will the other panels "drag down" the 20.5V panel to supply a variable average voltage to the MPPT? Information on blocking diodes or not is hard to find regarding cheap flexible panels, so I presume they will normally not have such. And as far as I know diodes lower the efficiency of solar panels anyway.


    The solar panels are installed in various positions on the boat, so all of them will never have optimum sunlight exposure at the same time. Thus the voltages provided by the individual panels will probably vary a lot also as the situation is today. So I am tempted to believe that the 20.5V panel will not be a problem, at least most of the time.

  2. #2

    Default Re: Solar panel max voltages

    Mixing panels is OK but not very efficient. However, unlike a land based grid where all the panels can be oriented the same as you identified in a boat situation it inevitably means that some panels will be shaded and or oriented differently from others.

    The best approach is to separate the similar panels and locations into groups and provide each group with its own controller. So in your case the 16V and 20.5 V panels should be on different circuits with separate controllers. Also if some of the 16V panels are on the bimini and the other 16V panels are on the bow (just examples) then they too should be on separate controllers, The same goes for the 20.5V panels in different locations. Otherwise you will suffer large losses with the weaker panels dragging the stronger ones down.

    A few other points for 12V battery banks MPPT controllers only really come into there own with high voltages typically >24V or when panels are wired in series, When multiple different panels are grouped as described above or <24V panels are in parallel then cheaper PWM controllers work almost as efficiently.

    So for example if you are charging a 12V system and you have two panels wired in series (2x18=36V or 2x20.5V=41V) then connect these using a MPPT controller. If you then had an additional 18V and 2x16V in parallel then connect the 18V to a PWM and the 2x16V to another PWM controller. 5 panels and 3 controllers.

    So you have to weigh up the cost of additional controllers against that of the panels. Also wiring in parallel increases the wire size and therefore cost and weight compared to wiring in series that reduces the wire size and cost weight.

    Hope this helps

  3. #3
    Join Date
    May 2012
    Location
    Oslo area
    Posts
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    Default Re: Solar panel max voltages

    Hi, I do agree and I am fairly familiar with what you post. My original concern was if the high voltage panel would block out the others, but as you remark (and I have read something the same in other sources) the real problem is that low voltage panels will drag system voltage down. And you are right, more regulators should ideally be used. If looking for perfection, one regulator of identical type for each panel.


    A little on the side, but still, regulators do also need and consume power. I do not know how much, but I guess many regulators will be a factor. They are powered on 24/7.



    Regarding MPPT regulators, yes, they are more effective with higher voltages. Still, manufacturers claim they can exploit the higher voltage that solar panels can deliver. And with reference to my original issue, the voltages on new panels meant for 12V systems have increased in later years.


    My conclusion for now is that I can use a Vmp 20.5V panel, the only problem in my system being that I will probably not get the full potential out of it. But it probably will not be worse than the one I am replacing. And good regulators do cost. Separate wiring can also be a real pain to install neatly inside and outside of a boat.

  4. #4

    Default Re: Solar panel max voltages

    I had a look for some further discussions on mixed solar and came up with this site. It shows how to calculate the inefficiencies in easy to understand terms.

    https://solarpanelsvenue.com/mixing-solar-panels/

  5. #5
    Join Date
    May 2012
    Location
    Oslo area
    Posts
    53

    Default Re: Solar panel max voltages

    Really nice information, thanks!

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