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PV panels in series or parallell on yacht?
By la7qz, Section Solar Posted on Sun Apr 20th, 2008 at 07:48:06 PM MST
What is the best setup when one or more panels may be shaded at times?

Hi
The "normal" way to set up PV panels on a yacht with a 12V system is to connect all panels in parallel and then run them through a 12V regulator. In some cases when daily loads are higher than PV output, the panels are run with no regulator and the batteries are topped off with the main engine or generator when needed. I've run like this for years, and never had a problem with overcharging before I installed the wind turbine.
A sailing yacht, and particularly a two-masted boat like mine is a bit of a special case compared to a land based installation because it's virtually impossible to install the panels so that no panel is shaded by parts of the rigging etc. In other words, as the boat swings at anchor, or is sailing along, the shade of the masts, rigging wire, running rigging and sails will pass over one or the other of the panels and reduce their output. This is different from a land based installation where all panels on the same rack can usually be expected to get the same amount of sun or shading, at least for most of the time.
My question is simply: Would I get an increase in efficiency by wiring the panels in series and using an MPPT controller? What happens to the output of two panels in series when one is shaded? I'm presently running three 55W panels, but intend to install one more, giving me two panels each side of the boat. Would it be an idea to install them as two pairs in series? I.e. the two port ones in series and the two starboard ones in series and then wire the two pairs in parallel giving two 24V arrays going into the MPPT controller?
Another question on the topic of MPPT controllers. MPPT controllers are said to increase charging efficiency. Is it MORE efficient to use an MPPT controller than to connect the panels direct to the batteries with no regulator, or just more efficient than a "normal" regulator? I somehow find it hard to believe that I will get more amps into my batteries through the controller than I am getting with a direct connection.
The wind turbine will have its own Tristar 60 and 600W 12V water heater element (arriving on Tuesday). I now have a Steca 20A controller for the PV panels, but will probably disconnect it once I get the Tristar 60 and dump load installed, as I'd rather heat water with any excess solar power than just let it go to waste. I'm also planning to build a solar water heater on the coachroof. More about that in another thread later.
Owen

PV panels in series or parallell on yacht? | 5 comments (5 topical, 0 editorial)

Re: PV panels in series or parallell on yacht? (3.00 / 0) (#1)
by Flux on Sun Apr 20th, 2008 at 02:17:03 PM MST
(User Info)

In your case I think panels in parallel will be the best solution. You are right that the shading of one panel in a series string will cause a serious reduction in output.

For small installations I doubt that mppt is worthwhile anyway. For a single 12v panel The matching most of the time is fairly good. The time when you may gain from mppt is on a bright sunny cold day with a low battery. If the panel is aiming for 20 plus volts and the battery is below 14 there is a fair mismatch. If the mppt device is sufficiently efficient then you should gain. Most of the time with a hot panel and a reasonably charged battery the match is so good that no mppt is going to gain you more than its own losses.

Mppt scores on larger installations with panel voltages higher than the battery voltage. You can then get a good match without having to use the correct number of cells to suit a given battery voltage.

Without mppt there will be very little difference with or without a charge controller ( a good controller should only add one fet drop and that should be very low. Unless you reach a state of over charge there is no need for a controller but it is a good idea in most cases unless it is constantly monitored.

You will gain with a mppt controller if the energy gained from the better match is not negated by the controller losses. Some of the basic mppt devices may have an efficiency of perhaps 90%, at this level you will hardly ever see an improvement on a single 12v set up, hot days will be worse than without it and only on the sunniest cold days will you be in with a chance. If you have one of the very best controllers with higher efficiency you may see better results except on very hot days.

With higher voltage strings of panels then even a 90% unit may prove worthwhile but you need a reasonable sized set up to get better value for money than buying another panel.

I never thought about this boat problem with inevitable shading but it certainly would seem virtually to rule out any likely gain from mppt.

Flux

Re: PV panels in series or parallell on yacht? (3.00 / 0) (#2)
by la7qz on Sun Apr 20th, 2008 at 03:40:25 PM MST
(User Info) http://home.no.net/naomij

Thanks for the reply Flux.

Confirmed my gut feeling.

It's always nice when someone tells me I don't need another expensive bit of kit. :)

Owen
If you're not living on the edge, you're taking up too much space.
[ Parent ]

Re: PV panels in series or parallell on yacht? (3.00 / 0) (#3)
by FuddyDuddy (long.shanks@comcast.net) on Sun Apr 20th, 2008 at 10:11:35 PM MST
(User Info)

HomePower magizine has had several articles about using diodes in parallel with PV cells to help prevent some of the losses when shading occurs. They have to be large enough to pass the current from a string (series strings) for the unshaded ones. You might check their website (homepower.com)....
FuddyDuddy

Re: PV panels in series or parallell on yacht? (3.00 / 0) (#4)
by la7qz on Fri Apr 25th, 2008 at 06:28:48 AM MST
(User Info) http://home.no.net/naomij

I can not find this information on the homepower website. I may have to pay to view one of the online issues. Since I don't want to look at all of them, could you please point me in the right direction of where to start looking?

Or... even better, could someone explain to me how this is done? I have a hard time figuring out how a diode in parallel with a panel would accomplish anything other than bypassing the whole panel whether it is shaded or not. There must be some important information missing here.

Owen
If you're not living on the edge, you're taking up too much space.
[ Parent ]

Re: PV panels in series or parallell on yacht? (3.00 / 0) (#5)
by kurt on Fri Apr 25th, 2008 at 09:18:13 AM MST
(User Info)

it is my understanding that bypass diodes are included in most modern panels but they are not effective at 12v only at higher voltages Google solar panel bypass diodes you will get lots of hits and lots of explanations.

http://www.reresource.org/

IRC

PV panels in series or parallell on yacht? | 5 comments (5 topical, 0 editorial)

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