Smallest number of panels in a string and the effects of shading on HIT panels

[mbuckhurst]

We're busy collecting quotes at the moment, we've got an awkward install in that we have very little space on the roof for PV, and slight shading issues, I'm trying to understand the pros and cons of what we might be able to achieve.

At the moment it's looking like we can fit no more than 5 panels on a roof, that is partially shaded by a small chimney, one quote has suggested this would be too few for a single string on an inverter, which struck me as odd - I'm not sure if this was the case of a national installer not wanting challenging work, but if this was the case could we not use more smaller panels?

Also, the shading issue, how big a problem is it, if we go for say HIT panels, I understand the output my be limited for the duration of the shade, but can the panels be damaged by the shading, or do the bypass diodes effectively mitigate that? In reality the shade only occurs in the early morning, so not at the point of peak output.

Also, we have a flat roof, again suffering from a similar shading issue, but at a size of roughly 4.5m by 3m, is there any reason we couldn't use a dual input inverter combined with the roof, to get closer to the ideal generation capacity. I was thinking at a shallow angle if might be possible to fit a 9x9 array of HIT panels, but this would require 3 strings of 3, or 1 of 4 and 1 of 5, is this possible.

I don't have any major concerns about cost, but would like to maximise the output from the system, to at least offset my current energy costs.

thanks

mike

 

[Worcester]

At 4.5 x 3 once again you're going to be lucky to get much more than 4 panels on it , depending upon exact dimensions and layout,

at those sizes, you are also struggling to get an effective inverter.

Your best bet may be to go for micro-inverters where you install one per panel, shading problem solved!

However the sanyo's aren't comaptible with them

We are one of the certified Enecsys micro-inverter installers, so can give you some advice on what panels are compatible etc.

 

[Graeme Harrold]

Your best bet is going to be micro inverters as mentioned above. Cant see you getting any more than 6 panels, 8 at a push. With shading issues, stringing the panels through a single inverter will potentially cause problems. Remember peak production is between 11am and 3pm (roughly), so if your shading is outside of this, the problem is not as bad. Only your onsite installer can advise fully.

 

[jamesstephenson]

Considering the shading issues and space problem it is better to look for micro-invertors. Apart from safety, it has so many other advantages compared to the string invertor. Its better to get it checked with an expert onsite. He can suggest you the perfect solution after checking the exact dimensions and layout.

 

[mbuckhurst]

Considering the shading issues and space problem it is better to look for micro-invertors. Apart from safety, it has so many other advantages compared to the string invertor. Its better to get it checked with an expert onsite. He can suggest you the perfect solution after checking the exact dimensions and layout.

Wow forgot about this thread, I can now report back.

We did talk to a load of companies, most refused to consider microinverters, most tried to push their own particular solution, despite a single string clearly not working in this scenario. At least a couple admitted they had piles of a particular panel they were struggling to clear, and very few considered fitting them to our flat roofs due to strength - this is despite the fact our joists are close together and a flat roof spreads the load quite effectively.

Then the government changed the rules and suddenly we had to get it done in less than a month, fortunately the installer who'd given us the most comprehensive solutions and had always been happy to consider micro-inverters, was also the one with the quote almost ready, it was completed in a couple of days and we signed the contract.

Apart from delivery problems of the micro-inverter from Enecsys, everything went quite smoothly, and we completed in time. We have 4 sets of panels 5 on our primary pitched roof, 2 sets of four on the other flat roofs one a single story, so it does suffer more shading than the other. And an extra panel sitting up at nearly 90 degrees to the panels on the upper flat roof, it turned out we had a small pitch far enough away to squeeze it in and actually hit the maximum numbers we were allowed under the FITS. All panels are Sanyo and all have beaten the specified generation capacity by more than 10%, although they were expensive.

The purpose of system was to cover all our energy bills, ironically I was made redundant a week after signing the contract so the requirement was even more important, well they achieved that with plenty to spare, between them last year all gas and electricity, plus my water bills were covered by the money from the panels.

On the micro-inverter numbers, it is clear from the individual panels, the system simply wouldn't work without, even if we treated the 5, 4 and 4 panels as separate strings, there's enough shading to stop them all being effective except with micro-inverters, I'd calculate that we'd get less than half the energy even with the most effective combination of strings, and that would have required quite complicated wiring and 2 inverters, so completely negating the benefits of price of string inverter vs micro. Individual panels were still generating with several mm of snow on them, last week.

All in after 12 months of running, I'm very happy, just hope for a better summer in 2013 to achieve a bit more electricity.

mike