How good are modern PV panels in cloudy/shady conditions?

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russbost
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How good are modern PV panels in cloudy/shady conditions?

Postby russbost » Mon Sep 19, 2011 5:53 pm

I've tried all the usual Google searches & all I come up with is that "Yes, solar panels do work in cloudy conditions though not as well as in full sun" there is also a suggestion that they actually might work better in thin cloud where light is spread diversely particularly if your array doesn't face directly at the sun, no idea how true that might be.

So my Q is this, has anyone got any real life experience of how panels will produce under cloud or shade, eg as a % of normal max output.

I'm strongly thinking of going for the government scheme several of you have mentioned in other threads - prices have dropped, I have been offered an MCS installed 4Kw system for £11k, but my system will be in partial shade for a couple or 3 hours in the mornings (only when the sun is lower in the sky) due to surrounding trees; in midsummer, ie peak generating times the roof is hardly ever in shade at all. The trees obviously have less shading effect during Winter months when the leaves drop.

They have said they can split the array so the half of the roof which gets more shade won't cut down the output from the better side, as I understand output is limited by the lowest producing panels?

Any thoughts/observations are welcome - though considering the figures they base their projections on assume %age increases of around 3% pa in power costs & the electricity companies have recently hiked prices by 15% plus, the array would need to have a fairly dire performance for it not to make sense financially! or power costs would need to drop overnight!

Grumpy-b
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Re: How good are modern PV panels in cloudy/shady conditions

Postby Grumpy-b » Tue Sep 20, 2011 11:29 am

Ask them for real data, not projections.
We have a large array at work and its peak output which is about half its rated output has struggled to get to 4kw. Most of the time ie normal cloudy days its way down on output. What is also quite important, and isnt generally addressed by the installers, is the angle of the panels. In our Lattitude this needs to be quite steep, and as such many panels installed on shallow pitched roofs are therfore unable to ever generate their rated output.
My Father inlaw and his Girlfriend have both had small setups installed, neither of which have met expectations.
Ask for local installations and details of the production from these panels.
What also can be a problem is the way in which they install the supports through your existing roof. I do know of one person who was left with a number of damaged roof tiles, which the installer would not replace or make watertight. SO beware of the small print and poor installers.

This is very akin to the sales of Endowment policies and double glazing in the 80s. Its all about them selling you the kit, rather than really delivering the goods. This will never save the earth, and may not return the investment return you expect.

Grumpy-b

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Peter Eggleston
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Re: How good are modern PV panels in cloudy/shady conditions

Postby Peter Eggleston » Tue Sep 20, 2011 8:44 pm

My neighbour recently had solar panels fitted and after a while the supplier came to double check the installation because it was producing way above its expected output. It is however in an ideal location on the side of a valley facing south with no obstruction by trees or shadow. For years people have said that the sun shines harder in our garden.
Peter

Beemer
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Re: How good are modern PV panels in cloudy/shady conditions

Postby Beemer » Thu Jun 20, 2013 12:56 pm

Image

These numbers are affected differently if the cell is heated or cooled, have non-reflective glass and what-not. They love to be cold.
I've been battling out thoughts like, "If a roof has cells on both sides facing East and West, would it be more efficient to have two separate inverters instead of relying on the reverse current diode in each cell and only one inverter?".

With panel pricing now dipping as low as $0.4/W in bulk, they are rapidly becoming cheaper than roof tiles!

I read Amorphous Silicon panels have a lower output in high Sun but work well in cloudy conditions - that might be a relative thing.
Mono-crystalline panels are more efficient than Poly-crystalline in bright Sunshine.. But don't quote me.

What you can quote me on is the quality of the panels. The better ones might not pay for themselves so quickly as cheapies but ought to last longer. In other words, how long is the string?

jonathan jewkes
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Re: How good are modern PV panels in cloudy/shady conditions

Postby jonathan jewkes » Sat Jun 22, 2013 8:49 pm

I have had a 3kW installation for about 18 months now. Clearly the best output is when bright sun is shining straight at them, ie at 1pm (summer time) assuming the roof faces due South. However, earlier in the morning, and later in the afternoon, when the sun is not shining so directly onto the panels, I do definately get more power on overcast days than on bright clear days.
Beware of shading! It is most definately NOT the case that 50% shading means 50% output. I have found that even if only one of my 12 panels is shaded, and that one only 1/4 shaded, the output of the whole array is greatly reduced. Like the above comment, I have had my system divided into two strings, so that I continue to get output from half the panels when others are in shade (but needs a more expensive inverter).
The pitch of the roof will detrmine whether your system does better in summer, or in spring/autumn. In summer the sun is high and so a flatter pitch is good, but in spring and autumn the sun is lower and a steeper pitch would be better. Does anyone know the optimum pitch to get maximum output averaged over the year?
Jonathan Jewkes
Daily EV user for 10years - an enthusiast and also a realist

glyndwr1998
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Re: How good are modern PV panels in cloudy/shady conditions

Postby glyndwr1998 » Sun Jun 23, 2013 10:18 am

Hi,

I have a 4kw system installed, been in now since xmas 2011, just before the 43p tarrif was reduced.

My location is not ideal, i am east west orientation, however, in the summer this is not much of an issue as the sun is high and sets late in the west, i know this as i have a friend whi has a 3.7kw system too and i am not that far behind him on production and he is perectly south facing.

I have 1kw on the East and 3kw on the west with a sma4000TL inverter that has 2 solar trackers. I opted for more in the west as i have a mouintain closer to the east, and my thought process was in the summer he sun carn burn away any clouding in the morning and the west panels will catch that in the afternoon.

I will say i am very pleased with the production. To date since installation i have produced more than 5000 units, you do the maths, it was an excellent investment choice, bearing in mind your money in the glorious banking system we have struggles to get more than 1% return that in iteslf is taxable.

Shading is a big issue, I had a tree behind my garden that was huge, when the sun was dipping over it at about 5.30pm our production instantly dropped from 1.7kw to around 300 watts, thas the effect shading has. That tree has now been trimmed, now get good sun to past 8 o clock in the summer where it is quite flat and still get around 500 watts at this time, cutting the tree back will gain approx 3 additoinal units in a very nice sunny day.

I have seen ads for installatins of a 4kw system for around £6000, for me that is a no brainer, 10% yeild on your money every year.

And no, i am not an installer or seller, (in case your wondering) just like you guys i was looking for the best return for my money, and reduce my soaring energy bills, I wish I coulkd say I done it for ECO purposes, but alas i would be lying if i told you that, it was purely an investment choice, and I am very pleased with the decision i made.

Hope this helps you with your decision.


Beemer wrote:Image

These numbers are affected differently if the cell is heated or cooled, have non-reflective glass and what-not. They love to be cold.
I've been battling out thoughts like, "If a roof has cells on both sides facing East and West, would it be more efficient to have two separate inverters instead of relying on the reverse current diode in each cell and only one inverter?".

jamesingram
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Re: How good are modern PV panels in cloudy/shady conditions

Postby jamesingram » Sat Dec 14, 2013 4:36 pm

Hi , noticed this and thought I'd offer my pennies worth.
I've a 4 kWp sys. and also occasionally fit them for a living, installed around 50+ systems.
In the SE UK my experience is, on a heavily cloudy day you'll get about 10-20% rated output during the peak generation time, around 10-3 in the winter , longer in the summer, of course this all depends on orientation and amount of cloud cover etc.
cheers Jim

tigerbay
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Re: How good are modern PV panels in cloudy/shady conditions

Postby tigerbay » Thu Mar 27, 2014 9:07 pm

russbost wrote:So my Q is this, has anyone got any real life experience of how panels will produce under cloud or shade, eg as a % of normal max output.

They have said they can split the array so the half of the roof which gets more shade won't cut down the output from the better side, as I understand output is limited by the lowest producing panels?


Though I'm kinda new to the whole thing, I've been doing research on the efficiency for fast top-up charging for small-scale EVs on the move and/or stationary with high-efficiency solar energy PV Cells.

As far as I'm concerned, energy companies and solar installers are ripping people off by saying on the doorstep that savings will be made from an array that hasn't even been specially designed and tested for the task of the property or application it is applying - Thousands of pounds are quite literally wasted installing these things for them not to work properly afterwards, hence why if they don't have a guarantee scheme on how much energy can be produced, then best go to another provider.

The issue with panels in shade verses in light is only significant if your panels are in a different position (even a slight difference in the angles each panel is mounted can make a significant impact) to each other and not configured correctly. Whilst blocking diodes prevent backflow of currents from the battery bank and charging circuit, each panel even at a different angle should be fitted correctly with a Bypass Diode - This allows any energy created by the other cells to be unhindered and (most importantly) protects the shaded PV cell from damage from energy trying to force its way through the PV Cell. From what you've described, the company you must be dealing with are worse than amateurs (or a better description if you like: the Halfords of the Solar Energy world - they don't have a clue on anything other than selling you the product by fancy PR and figures that they don't understand)

As for how much they will produce depends on how the solar panel banks are made up and the specifications of each solar cell component. The higher the rated shorted voltage and current of each solar cell, the better it is likely to perform in dull light conditions. The charging controller should also be important to factor in, as many need a fully-charged bank to even operate, so you also have to research the efficiency, voltage and current drawing of the controller. If the solar panels alone are generating far more power by themselves when measured by a multimeter, then find a more efficient solar charge controller, as this is detrimentally decreasing the charging efficiency of the panels.

Finally, you also need to calculate how many panels you required to get the acceptable voltage and current ratio on a dark day, which should also show the efficiency of various types of solar panel as listed in the earlier graphs (simply put, polycrystalline is far more efficient per cell than monocrystalline, etc.) If you can produce the minimum voltage and current required for charging (e.g. for a 12 volt 12amp AGM battery bank, your value would need to be a few volts over 12 volts, with a current that is large enough for the energy you're using out of the system to allow for fast-enough top-ups - this usually is dictated by the tolerances of the battery bank and should be in this case be producing enough to replenish 6 amps of charging power within 2 hours (don't quote me, but this is the basic theory))

Hope this helps :)

jamesingram
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Re: How good are modern PV panels in cloudy/shady conditions

Postby jamesingram » Sun Mar 30, 2014 9:28 pm

If installed correctly split systems should be divide into strings (groups of panels with the same make, size, orientation and inclination)
then put on their own independent inverter (and mppt tracker) or an inverter with dual/multiple mppt trackers for each string.
Any heavy shading on part of each string is a no-no (except as the array comes and falls out of exposure at the start and end of the suns track across it) If heavy shading is likely micro inverters or single panels mppt's to a dumb inverter can be used.
Currently a South facing 4kWp system in the UK will produce approx. 3500-4000kWh per year and cost around £6k to install.

As for fast charging your EV, well if it's clear and sunny you'll get up to 16A produced ( no more for 4kWp sys. as DNO
(district network operator) limits output for simple to register domestic sys. ( ie. inverter output 3.68kWp)
Output from inverter does depend on the solar irradence at the particular moment in time , panel output is almost direct proportional to it
Panels are rated at a test figure of 1000W/m2 solar irradence, (a bright clear day at a set panel temp.) , and all figure quoted relate to this.
Say a 250W panels is rate a 8.2A then at 500w/m2 irradence you'll get 4.1A
then there's system loses etc.
So what sort of current is a EV 's fast charger going to want ?

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Kevin Sharpe
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Re: How good are modern PV panels in cloudy/shady conditions

Postby Kevin Sharpe » Tue Apr 01, 2014 8:18 pm

We have two identical 3.5kWp systems installed on both the SE and NW facing roofs of our little semi near Bath.

In the first year we produced 5801kWh with the SE roof producing ~3202kWh and the NW ~2573kWh demonstrating that deployment on none optimal roofs is worthwhile!

During the summer we often exceed 25A for a couple of hours during the middle of the day and we use that for 'fast' charging the car. Most of the time however we limit that car to just 7A using our EVSE.

Our system cost £16K in July 2012 and had dropped to £12K in July 2013.
Kevin Sharpe - Founder and Patron for UK registered charity Zero Carbon World. Founder and Chairman Mainpine Group. http://about.me/kevinsharpe


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