Choosing Solar

[gary955]

We need to become a little more self sufficient on power and are looking toward some solar. Our boat is a small vintage harbour tug and to preserve her profile and classic appearance the installation needs to be as discrete as possible. I think we have room for approx 300-350w of panels on the wheelhouse roof. The problem is that despite being roof mounted there will be a number of things putting parts of the panels into shade, including the navigation lights, the signal mast and the radar scanner.

I noticed that CIS thin film panels work well in low light, does this include partial shade as well? We are not able to orientate the panels toward the sun, they will be mounted flat, so will the thin film technology be a benefit for our application. We would have room for 2 x 165w panels but both would be partially shaded most, if not all, of the time.

Our  alterative would be 7 x 50w small polycrystalline panels. These are 12v but I assume that they could be wired in series and then run through a MPPT controller to charge our 24v bank? With 7 panels I think that at least 3 panels would be completely unshaded and in direct sunlight at all times and the others partially shaded. I've been told that a mono or poly crystalline panel will not produce any power when partially shaded unless they include bypass diodes fitted between cells or groups of cells. How do I determine the presence of such diodes if not indicated in the manufacturers or suppliers literature?

I realise that this is not an ideal installation and we are unlikely ever to see the rated power from the array, what we’re looking for is the best compromise under the circumstances.

[Alan de Enfield]

Talk to these guys - very friendly, approachable and knowledgeable.

http://www.bimblesolar.com/

I'd guess they have supplied more Solar to boats then any other supplier - they seem to be recommended whenever the subject is broached.

I have ordered from them three times.

[Detling]

if you put panels in series the current is limited by the lowest one, if one is in shade it will drastically reduce the current, even with bypass diodes. The best is to put panels in paralle,l but as you are looking at 12 volt panels in low light you may not get sufficient volts for the mppt to work. What I have done for my 6, is to have 3 strings of 2 panels in series, so that way the volts rise up in low light and if one pair is shaded it doesn't limit the others, seems to work well.

[Tony Brooks]

Unless you are on the equator you are unlikely to get anywhere near the panels rated output. I agree that you need to talk to suppliers about the internal diodes but suspect most nominally 12V panels have them these days.

I suspect your idea of sets of series panels paralleled up with a MPPT controller would maximise the output, especially if you can site the series blocks so only one block is shaded at a time. Unfortunately you can not get 7 to make 24V series blocks.