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Shading has more impact on solar PV than orientation or pitch

Posted by Paul Hutchens on 9 March 2011 at 9:15 am

Many of our clients ask about the orientation and pitch of solar panels - it appears that this has been well publicised. However, the effects of orientation and pitch are less than most people think. Moving the orientation from south to south-west, for example, reduces efficiency by less than 5% as does reducing the pitch from 30 degrees to less than 20 degrees.

What does make a more significant difference is shading to the solar panel; particularly hard shading from a tree, chimney, flue or another building to a solar PV array. In the worst case scenario, this can prevent the system from working at all - a bit like putting a resistor in a circuit!

As a result hard shading should be avoided at all if possible or, at least, limited to early morning/late evening and preferably in the winter only.

Recent evidence from one of our installations has demonstrated that hard shading can create the bizarre situation where solar energy yield is higher in cloudy conditions than in full sunlight. This appears to be because the hard shading "shadow" is limiting the flow of electricity through the array - even though more energy has been created!

On one occasion a 15% level of shading on an array reduced the output to less than 5% of what would otherwise be expected.

There are solutions to this - apart from avoiding shading in the first place. The main one is to split the array into two or more "strings"; each of which carries an independent parallel flow of electricity. That way, even if one panel or string is shaded, the other will continue to produce useful energy.

We continue to monitor our installations and research the market in order to find better solutions where they exist.

About the author: Paul Hutchens is founder and director of Eco2Solar, which installs solar systems around the UK.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

10 comments - read them below or add one

Ernest Grodner

Ernest GrodnerComment left on: 4 September 2014 at 8:36 am

Hi, If you look for help in shading analysis please use my free app 'ScanTheSun' for Android phones and tablets which allows to:

- calculate the solar energy reaching the collector
- find the energy loss due to shadow casted by objects in the vicinity
- plan the collectors mounting (selection of the best part of the roof, proper panels orientation) even in a complicated urban areas
- calculate the hours of water warming or electric energy production during the day
- prepare a complex documentation of insolation and efficiency of solar installations

Here is a ScanTheSun screen recorded video:
https://www.youtube.com/watch?v=GHKAhpQv1Vs

and another video with additional explanations
https://www.youtube.com/watch?v=GHKAhpQv1Vs

For more details on ScanTheSun please refer to Google Play

P { margin-bottom: 0.21cm; }

https://play.google.com/store/apps/details?id=com.scanthesun&hl=en

 

Ernest Grodner, PhD, University of Warsaw

 

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Dr Chris

Dr Chris Comment left on: 26 October 2011 at 8:21 pm

Further to my comment on 22nd June: I took the plunge and installed  a pv system which would be in partial shade before 9am. My SMA inverter with Optitrac Global Peak turned on did just what the 'Made for Shade' technical briefing says it should do, and was producing roughly an extra half to one kW/h of power per day before 9am in August. There has been no obvious loss of power for the rest of the day - the system is 30% over estimate in the first three months of operation.

It may be significant that the installation firm, though very competent in other respects, had never implemented this feature before - I expect this is typical! Well, we're all learning...

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 21 July 2011 at 10:07 am

HenGus

I think you are right to ask the questions. This shading at 100 foot tall is likely to cause a problem and should be considered in the energy yield (and in terms of whether the installation should go ahead or whether the tree should be lopped).

I do not believe that shading from an evergreen tree during autumn, winter and spring is insignificant!

The SAP calculation may or may not be pessimistic but that should not be the basis of the proposal - this is probably contrary to MCS guidelines anyway.

If you are not getting satisfactory answers then I think you need to think carefully about the installer that you are considering.


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HenGus

HenGusComment left on: 21 July 2011 at 9:20 am

I have just had a proposal from a company for a 2115Kwh PV solar system. When I reviewed the proposal - which followed an on-site technical survey - it quickly became apparent to me that a large conifer tree (about 60ft in front of the house and over 100ft tall) had not been considered when calculating the predicted panel output. When I raised this as a concern, the response that I got was, at least to me, surprising. I was told that the output calculator would show a reduction in output of up to 20%; however, this was mitigated by two factors: one, the tree would not obscure the panels during the summer months and, two, the SAP predicted output figures in the proposal were pessimistic. I have to ask what faith can a potential purchaser have in these oral statements when a Company's Ts and Cs contain a Standard Energy Prediction Disclaimer? Equally, if an installer ignores shading then the payback calculation cited in the proposal is undoubtedly flawed. This is the message that I am getting from the posts below.

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 27 June 2011 at 7:26 pm

Hi PV Novice

The use of different strings will provide separate electricity flows and, although they will aggregate at the inverter, the point is that each string has the oportunity to provide a contribution separately. The problem with a particular string is that it carries a flow of electricity - a bit like a hosepipe carries water. Shading is a bit like putting your foot on the hosepipe; although there is water flowing up to that point, it cannot get past. Multiple strings at least ensure that there is more than one hosepipe carrying smaller amounts of water flow.

So the representative that you spoke to appears to be incorrect!

There are other options such a the Solar Edge system which has a voltage optimiser to keep the flow open or micro inverters which treat each panel separately. See my blog os 19th April for more information.

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Slar pv novice

Slar pv noviceComment left on: 27 June 2011 at 7:28 am

I am looking to invest in a solar pv system and I have already discovered that the more I learn about solar pv, the more I realise that I don't know!

In Paul's original entry on this blog he states that one way to reduce the impact of shading is to divide the array into multiple strings, presumably attached to the same inverter.

I recently spoke to a UK representative of a leading inverter manufacturer (not SMA, but I will not disclose the name of the company) and he advised me that, even if I divided the array into two or more strings,  when these strings came together at the inverter, then the power output of the whole array would be degraded to the level of the shaded panel(s).

Is this correct? - or does it depend on the make & model of the inverter used? Any information that would help clarify my inderstanding of this topic would be gratefully appreciated. I have partial shading at certain points of the day and would like to ensure that any installed system obtains maximum efficiency.

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Dr Chris

Dr Chris Comment left on: 22 June 2011 at 3:15 pm

If SMA are right about the effectiveness of Optitrac Global Peak, then the shadow problem should be more or less history by now! How strange then, that a year after it became standard on Sunny Boy transformerless inverters, I cannot find anything on the web to suggest that it works - indeed, I can't find anything to suggest that it even exists other than stuff which has obviously been cut and pasted from SMA sites. So please, before I fit anything, could somebody with experience of using Optitrac Global Peak tell us whether it is any good at overcoming shading problems?

Thanks!

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ElectrAssure Solar

ElectrAssure SolarComment left on: 2 April 2011 at 1:50 pm

See the 'Made for Shade' technical briefing from SMA here http://download.sma.de/smaprosa/dateien/7418/GlobalPeak-UEN101210.pdf

It gives simple guidelines on how to deal with shade along with the statement 'The impact of shadows is over-estimated'!

SMA claim that the 'Global Optitrack Peak' function in the firmware of the SMA TL series inverters can minimise the effect of shading.

Has anyone seen evidence of the effectiveness of this function?

Does any other manufacturer have a similar solution? 

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Explore Solar

Explore SolarComment left on: 23 March 2011 at 5:47 pm

I agree shading is a big issue. You can also consider micro inverters, which we are now installing. To find out more please go to our web site

http://www.exploresolar.co.uk

 

 

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Engensa ltd

Engensa ltdComment left on: 9 March 2011 at 11:36 am

This is a very good post.  Shading can cause real problems in solar installations.  Using a regular inverter indeed limits the current in each string to that of the lowest performing panel.  

We install many systems using a new type of inverter system called SolarEdge which gets around the problem of shading by operating each panel individually.  This means that a shadow will only affect shaded panels.  SolarEdge allows you to monitor the performance of each panel individually and ensures that each module operates at its maximum power point.

There's some more info about SolarEdge on my blog: 

http://engensa.com/blog/uncategorized/a-uk-first-engensa-and-solaredge-install-revolutionary-new-solar-pv-technology/

Dr Toby Ferenczi 

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