Shading of PV arrays

[billi]

thanks   that helps  

and i had a look on the SMA website , there seems do be some news about shading  named "OptiTrac Global Peak" to reduce efficiency loss due to   shading   .

So the new GTI models  will work better

http://www.sma.de/en/products/knowledge-base/optitrac-global-peak.html


Billi

[StBarnabas]

EA
thanks. I think using current sources makes shading and bypass diodes easier to understand. I am going to be annoying here (sorry) I have drawn a few diagrams but stored on the work compute. Will post this Monday.
Sean

[Outtasight]

Something you have to be careful of in selecting the blocking diode is to make sure that its reverse breakdown voltage is safely above the maximum array voltage.  AIUI, you'd ideally like a diode with a small forward bias voltage drop (Schottky?) but these tend to also have low reverse breakdown voltages.

What I'm not sure about is which panels have bypass diodes and whether they're really put in one per cell as I've shown in the diagram or if a bunch of cells are bypassed together.

You can get 150V rated Schottky diodes now but they leak a bit in the reverse (a few 10's of mA, getting worse with high temperatures).

The bypass diodes usually are just one for every 18 cells so a typical 17.5V 36 cell module has two bypass diodes.  An outfit called Solar Integrated use modules made from Unisolar triple junction amorphous cells and are made with one bypass diode per cell for the highest resistance to shading.

Cheapo low power panels sometimes omit them altogether.  The last 40W panel I bought from eBay doesn't have any but the used 40W Kyocera ones I bought do.

[StBarnabas]

thanks for this. My limited understanding of bypass diodes is that there are normally only one or two per panel – though obviously one per cell would be better.
(Edit Thanks Outta – for clarifying this)
PV cells are neither current sources nor voltage sources but a strange hybrid of both. My understanding is gradually improving but unless I can simulate things and get the model to agree with experiment I am not happy and not there yet.
My very limited understanding of bypass diodes is by approximating PV as a current source, When panels are unshaded then they will all produce equal currents. A shaded panel however will current limit the rest of the string hence bypass diodes are used. The downside is that the panel is effectively switched off. The diagram below should give a crude idea as to how they may work. But this is a “straw man” so am hoping for a better explanation!
(Edit 13:56 8/3/10 - sorry arrow missing from diagram)

[johnxgoodall]

Thanks to all who posted above though I confess that as a layman much of it is too technical for me. I asked the company who told me they would put arrays on different areas of my roof with differing orientations, how their panels would handle the fact that they would receive shading at different times of the day, as well as shading from the house next door. Here is a copy of his reply - I would be very grateful for any comments.


if the light is not direct ie when it is diffuse, then you can place the panels flat on the ground or face north or any direction you like and they will receive the same amount of light so the effect mentioned can't happen and that is something like 70/80% of the time.
 
We get most diffuse light in the lower sunlight months ie from end of September to end of March when in those 6 months you only are chasing 25% of the annual amount of enegy available.
 
In summer when you get most direct sun it rises rapidly in the sky so if it is a clear day at noon all Panels receive light whatever way the are facing. so only at the early part of the day and towards the end will there be marked differences between Panels output some which facef southwest and others south east.
 
Of course it is the time between say 9am to 3pm when the majority of sunlight is available, so those early and late differences are when ther is very little energy available anyway, so we are rapidly cutting down the potential times when the effect might play a role in reducing the total production.
 
We then have a further reduction in possible effect in that most PV Panels have bypass diodes on them, which reduces the effect of shading considerably by stopping any Panels receiving sun from pushing power into those that are shaded.
 
Also, there are some Panels which work much better in shade than others.  These are thin film Panels, which means low efficiency thin film can work as well as some silicon PV Panels in low light or shading, but there is one other Panel which has this characteristic, which is of course Sanyo Hybrid.
 
These have 2 layers of thin film of Amorphous Silicon on the top and bottom of the Monocrystalline middle layer, which is the negative layer, with the thin films being the positive layers and of course the bottom layer as it covered by the top two layers is actually working in shade, as the light is working on both the two p-n junctions unlike most Cells which only have one.
 
Therefore if you have Sanyo Hybrids you further miytigate the effect.
 
Finally, the Inverter we use, Fronius IG, actually is clever enough to work out when there is differences in power and their tests show only a 1% drop off of power output from that Inverter cf a multiple String one, under the conditions where there is a difference in power output of strings, which is not that often and can be mitigated by using Sanyo Panels.

Thanks for any help you can give me

[Outtasight]

I agree with some of what they said but I'd never recommend installing a panel in the UK on a North facing slope!  In the winter the sun gets down to 15^o above the horizon, low enough that on a (admittedly rare) clear sunny December day you'd get almost nothing from a panel on a North facing slope as the roof apex will shade the whole side of that roof (and the panel with it) and diffuse light from clouds amounts to (in my experience) less than 10% of peak Amps output.

Likewise, it's not a good idea to have fixed arrays laying flat because although they will get the most diffuse light that way (as they can see a 180 degree view of the sky from horizon to horizon), in winter the direct light will be hitting the panel at 15^o, a very shallow angle and the power received by a panel is proportional to the area it presents to the light source.  When the angle that the light hits the panel is at 0^o (directly edge-on) then the collection area is also zero so the output is zero (ignoring diffuse light reflected from other directions).

It doesn't take much direct sunlight to out-generate a dull day.  As the current output is about 10:1 stronger in direct sunlight (even in winter) you collect as much energy in one hour of clear sunlight falling nearly square-on to a panel than in ten hours of overcast diffuse light.

It's true that in a high enough voltage string, shading on a few cells will not cause the whole string to stop working but it depends on the type of shadow.  The cells are usually arranged in a snake pattern with one bypass diode per 18 cells (one for the left hand side block and the other for the right hand block along the long edge of the panel).  A shadow on one cell can cause the bypass diode to conduct for the whole block of 18 cells (bypassing them).  On a 200W panel that means you've lost half the Volts of that panel and half the power in Watts of that panel.  So long as the inverter can still work at the lower Voltage, it will do so but with the loss of 100W of output.

The worst case shadow is a small one that covers a few cells in both blocks (typically a wall edge shadow creeping up the length of the panel).  Now only the bottom 4 cells out of the 36 are shaded but both bypass diodes cut in and the entire panel is bypassed (losing the full panel voltage and 200W of power).  That's why I had to bury my overhead cables as they were casting a long thin line shadow that moved across the surface of all three blocks of series pairs of panels I had below the wire.  With mine being a low voltage system, any shadow on a single cell can cause the 18 cell block to drop out of the string and then the whole pair drops out of charging as it may only be 5 Volts above the battery Voltage when fully lit.  Bypassing one block loses 8.5V...

Their assertion that a "good" single tracker inverter can equal the performance of a multi tracker type holds water if the strings are balanced and the diversity in peak power Voltages is small (same number and power of panels in each string and similar orientation with no shading). In that case a multi tracker has little advantage and may even lose out due to higher losses from having to waste power on activating the two trackers!  For grossly imbalanced arrays (different number of panels in each string or frequent bad shading events that cause whole panels to bypass) a single tracker will definitely lose out.

[johnxgoodall]

Thanks very much for that. My intention is to install arrays on up to four pitches of roofs with three orientations - two are south south east (4 panels on each) and two south  west (4 on the first and perhaps 6 on the second.  Shading from the house next door hits the south south east at around 4.30 pm (as of one day last week) and the first south west at around the same time. All of these are more or less all shaded within a half an hour. The second south west one, is in full sun up to 5.00 (again as of last week) then loses it within 20 mins or so. But this one does  not start to get the sun until around 9 am. Does this help in assessing likely loss of electricity generation?

[Outtasight]

A bit tricky to figure out without a drawing of the plan but the issue seems to be the split in numbers of panels (8 in one orientation and 10 in another).  If they were all in the same orientation and the inverter could handle the Voltage, I'd have put them into a 18 module string and let the bypass diodes do their thing.

If the inverter can't handle 315VDC (assuming 17.5V per panel) then the next best configuration would have been 2x9 at 157V.  The orientations SSE and SW aren't so different and if there had been no shading issues you could have dispensed with the blocking diodes (but they are still recommended by the module makers in case of some other source of shading or a fault in one string).  In your case 2x9 doesn't work because you've only got 8 on one orientation so using the 9th from the other orientation will mean the 9th panel will always have different power to the other 8 and so cause losses (either in itself or the other 8 ) even if none of them are shaded.

Grouping the 8 and 10 panels into two strings in parallel into a single tracker is the least favourite option as you'll get exactly the twin peaks syndrome of my chart because the strings will be different voltages and powers by design (irrespective of lighting conditions).  My father-in-law's system is such and uses a voltage converter-combiner in front of the main inverter (effectively a dual tracker).

Doing anything with the individual groups of 4/6 is unlikely to help as the inverter probably won't work on just 70V.

You might think about putting the 10 group on the side that gets the most sunlight hours though throughout the year to maximise output.

I have most of my array pointing to the equinox angle(ish) so that it's balanced for summer and winter power but a small 80W string is almost vertically mounted on the wall to boost power in mid-winter on those days when we do get some direct sun.  In the summer they'll not do much with the sun directly overhead but then the rest of the array is pumping out loads of power, needless to say they were very cheap panels so I don't begrudge them being just for "winter boost".

[johnxgoodall]

Thank you so much for your advice. I have been offered two types of panels by this company - Sanyo and Yingli - here is what they say about them:

We believe we should only offer PV Systems with Panel efficiency of 14% and over, so our Customers can get the best possible returns, from PV Systems using the Yingli 230wp Panel at 14.1% efficient, being a star at an amazing price, next Solar Century’s new 180wp all black Mono at 14.3% efficiency and the Sanyo 215wp Hybrid at 17.1% being the most cost effective Sanyo, with 240wp hex cell 17.3% also available.
We can also get the very effective Kyocera Polycrystalline Panels, but the price we would have to install at would be quite high, so the Yingli is our favourite Polycrystalline Panel.
To get the most 41.3p’s for Systems up to the Governments 4Kwp limit for that max payment, the Sanyo 215wp is the best Panel as 18 Panels is a 3.87Kwp System. Sanyo Hybrid Panels are the best available where there is in low light, shading, or high temperatures.

The other company that I am dealing with has offered me Mitsubishi PV-185MF5 panels which they quote as having 13.5% efficiency. They said they could get me Sanyo if I wanted but in their opinion, the extra output would not be worth the extra money they would cost. Do you have any thoughts on the optimum panel?

[EccentricAnomaly]

johnxgoodall's potential supplier wrote:

... We believe we should only offer PV Systems with Panel efficiency of 14% and over, ...

This makes sense if you're limited by roof space or installation of a larger area would be expensive (more scaffolding or whatever).  In general, though, it's watts/pound that matter, not watts/square metre.  If a less efficient but bigger panel is cheaper and practical to install then it should not be rejected arbitrarily.  The photovoltaic efficiency of roof slates not covered by panels is 0%, though a later upgrade may be an option.

[AidyB]

I'd be looking to put eight panels in each string on a single inverter.  I heard the same thing from Fronius regarding the 1% difference, and it doesn't take much to figure out the first company is a solar century reseller.  If you tell me where you are in the country I can probably guess who you've talked with.

I don't particularly like the explanation given by the company above, it is too positive and doesn't really portray a balanced argument.

I would assume that 1kWp of hybrid panel will outperform 1kWp of standard monocrystalline panel by 10%.  There isn't a difference between 1kWp mono and 1kWp polycrystalline, but personally I much prefer the look of the mono's and they are now very close in cost so we don't even sell poly's anymore (though as I say, there is nothing wrong with them).

If the Sanyo costs more than 10% more than the equivalent kWp in other panels it is only worthwhile if you really need the extra electricity.  Personally I'd go for a solid crystalline panel at the moment.  We favour Sharp because we can trust the brand and offer them at good prices, but we've used Kyocera, Mitsubishi, Schueco, and loads of others as well.

I think the second company comes across as more honest, but obviously think you should get us to quote.   ;-)

You won't go wrong asking for advice from this board though.

[Justme]

To get the most 41.3p’s for Systems up to the Governments 4Kwp limit for that max payment, the Sanyo 215wp is the best Panel as 18 Panels is a 3.87Kwp System.

Not sure that that is right.

My understanding of something that Ted said was that its the inverter size & settings that will decide if you are under the 4kw limits. So size up the array & then set the inverter to only invert 4kw's. As panels are cheap having more panels than the inverter will use in summer will help increase the winter production.

We have a similar set up on our off grid set up. The solar charge controller can only use 80amps & our array can produce about 100 (with the bat at 12v & full sun) but as that does not happen that often but low levels of light & high bat bank voltages do so we get more power in winter when we need it most. The excess does not need "dumping" it just gets unused.

[johnxgoodall]

I have now had a detailed proposal by a third company who told me originally that there would be no problem fitting arrays to different roofs with different orientations but have now changed their mind and recommend one array only. Here's what he says

We propose that an 8 panel system be installed entirely on the South West facing roof to achieve an
optimum output. The reason for this is that the South Eastern roof is likely to only be able to fit about 5
panels. None of the inverters available will work with 5 panels, 6 is really the minimum.
If you connect all 13 panels together you will need to match the Inverter to 13 panels but in the
morning when the westerly side is in shade you will only have 5 southerly panels in the sun and at the
end of the day when only the westerly panels are in the sun. At these times the system will probably
generate nothing or very little.
It is possible to install a parallel system, but we will need to be careful estimating the energy yield. On
these split systems it is best to have an inverter matched for each section so you maximise the output
of both the south and west systems but you will need at least 6 on each side.
For these reasons I have put together a breakout of the expected levels of retun for an 8 panel South
West roof mounted Solar PV system.

I now have quotes offering
1) Estimated generation of 1180 kwh/year for an outlay of a little under £8300 for an     8 panel array on one roof
2) Estimated generation of 3250 kwh/year for an outlay of £14500 for a 16 panel array on four roofs

Clearly the second one on the face of it offers a better return – but how confident can I be that it will be achieved?

[billi]

None of the inverters available will work with 5 panels, 6 is really the minimum.

why ?   too low volt ?

Billi

[linesrg]

Good Evening All,
I'm kind of dragging my 'Interesting lesson' thread over to this thread but I think there are some 'issues' I'm facing which are relevant to this thread i.e. shading and orientation.

I decided to expand my tracker based 1.28kW to 2.56kW assuming that I would see an appropriately increased output. Granted my panels are not yet roof mounted and due to the nature of the buildings there is some shading in the morning and afternoon I am seeing a whole lot less than I would like. Additionally my panels are orientated SSW.

If you have read my thread you'll be aware I'm examining all possible reasons for the currently low output I'm getting. I have now put all 3 lots of 16 series linked panels into seperate SMA 2500 inverters (two bought cheap on eBay for £900 a while back). I have checked the parameters and they are all the same. The tracker mounted 16 are winning hands down.

At 1230 today my Efergy meter told me we were making 2.5kW when the sun was shining which given the time of year isn't bad for a 3.8kW system really so I don't think there is anything fundamentally wrong with the equipment although I still want to do some cross connecting of strings and inverters to be sure.

Currently my view would be to be extremely wary of shading and the direction the panels are facing. Whilt I have yet to reach definitive conclusions I am already resigned to building a second tracker. The good news there is that the existing Lorentz electronics can be used to drive any number of trackers so all I have to do is provide the hardware.

Regards
Richard