Thursday, 13 February 2014

In-roof Solar PV - Hot or Not?

Results from a study into heating effects on in-roof solar modules

In-roof solar PV may look great, but what's the trade off on energy performance?
Image: Viridian Solar and Elliott Brothers

Every solar professional knows it.  The power output from a crystalline silicon PV module reduces as it gets hotter.  PV systems installed in-roof suffer from lower ventilation rates on the shade side than modules installed on a rack above the roof covering and therefore produce less energy.

But how much less?  Is it a lot or a little?

How much of the heat is actually lost from the shade-side of a panel, compared to that from the sun-side?

I was recently in a meeting at the Solar Trade Association with a group of the top technical brains from the UK PV industry.  I asked the group to estimate the increase in annual energy yield by switching a crystalline silicon PV module from a sealed in-roof installation to an installation above roof.

The answers ranged from 1% to 14%. 

Conversations with other solar professionals have produced estimates as high as 25%.

Everyone knows it has an effect. But no one seems to know by how much.

Having a quantitative, evidence-based answer to this question is becoming more and more relevant in our industry.  As the solar market matures more and more customers for solar PV want the benefits of reduced energy bills but without compromise to the looks (and potentially re-sale value) of their properties.

In-roof systems offer an alternative that ticks the box on aesthetics for many people at a price they are willing to pay, but just how big is the trade-off on energy yield? 

Now researchers at Viridian Solar, collaborating with the Engineering Department at Cambridge University and Enphase Energy have produced an answer to this question.

The authors are aiming to publish the research in a peer-reviewed journal later this year, but a briefing document has been released summarising the experimental results.


Replacing Opinion with Evidence

The experiment is described in more detail here, but in simple terms it consisted of three steps:

  • Build a test rig with PV modules installed in a range of situations representative of real life construction
  • Understand the relationship between weather conditions and module operating temperature for each type of installation
  • Use the experimentally derived temperature profiles to calculate the annual energy yield for each installation situation.


Test Rig

Clearline PV15 modules were installed in five different ways

1. Free standing on an open framework (rear fully open)

2. Above a pitched tiled roof on a metal framework (open gap between panel and tiles)

3. Integrated in a pitched tiled roof with cold-roof construction behind (batten-space ventilation)

4. Integrated in a pitched tiled roof with warm-roof construction behind (insulation between roof joists)

5. Integrated with a pitched shingle roof with plywood sarking board (module rear un-ventilated)

The images below show the roof build up for two of the pitched roof installations.


Temperature Rise

The graph below shows the temperature response of each of the installation types - the lines show the operating temperature above ambient as the light levels increase.  As expected, the temperature of a module with less ventilation to the shade side rise faster as light levels rise.

For example, at 1,000 W/m2 (a bright sunny day with sun directly onto the module) the rack-mounted module above the pitched roof was 10 degrees C warmer than the free standing module.  The integrated module in the cold roof is a further 9 degrees C warmer than the rack-mounted module.
Clearline PV modules have a power-temperature coefficient of -0.509 %/degree C, quite typical for a crystalline silicon module, so a reduction in temperature of 9 degrees would produce a power increase of 4.5%.
However it's not always sunny, and the sun isn't always directly onto the module.  In fact, in the UK irradiation levels higher than 1,000 W/m2 are very much the exception and not the rule.  At lower levels of light, the temperature difference between different installation types is smaller.

Annual Energy

So, what's the answer?  What was the annual energy benefit for rack-mounted systems compared to the in-roof systems in the experiment?
The temperature characteristics were used with a climate file for Cambridge, UK and the power-temperature coefficient for the modules to calculate the annual energy yield for each installation.
It turns out that a rack-mounted module would yield 3% more energy than a roof-integrated module. 



Clearly, for some situations an extra 0.3% return on investment due to energy yield will matter, but for many domestic customers minimising the visual impact on their building will be more important. 
As an industry at least we now have facts to present to potential customers so that they can make an educated choice.



  1. A really interesting article and study, I look forward to seeing all the results. I would be interested to see if the long term effects of the higher temperatures (thermal cycling) in a BIPV system would accelerate performance degradation. Even if the theoretical performance only drops by 3%, over time this may increase if the PV cells degrade at an accelerated level. Thermal cycling in BIPV systems could also cause defects in cells potentially, again a long term study would allow us to see the affects.

    Steve Coonick

  2. Yes the power output from a crystalline silicon PV module reduces as it gets hotter.For the better performance the crystalline silicon module should be finalized perfectly, so a reduction in temperature of 9 degrees would produce a power increase of 4.5%.

  3. Taking care of this investment, which amounts to a substantial portion of the overall cost and value of your home, will help prevent the need for roof replacement prematurely, but it will not prevent the eventual need.Go to

  4. It proved to be Very helpful to me and I am sure to all the commentators here! SpecifiedBy I appreciate everything you have added to my knowledge base.Admiring the time and effort you put into your blog and detailed information you offer.Thanks.

  5. Useful information shared..I am very happy to read this article..thanks for giving us nice info.Fantastic walk-through. I appreciate this post. Roof Cleaning

  6. What happened to your site Some of its features are not visible on the website. maciej

  7. Notwithstanding, on the off chance that your rooftop isn't genuinely harmed, you can bear to invest some energy doing your exploration and keeping an eye on the potential temporary workers. Decorative Concrete Walkway NH

  8. small angle cut tiles can suffer considerable damage. So, as I learn the right way, and in the hands of the tiles outside the shingles to repair them, and when needed.More about the author

  9. Such roof restorations also involve removal of rusty or loose screws and nails. After these initial steps pressure cleaning of the entire area is done followed by an application of primer metal etch coating. Visit Here

  10. The Council on Foreign Relations, a research organization, has detailed that power utilization and creation have reliably extended.marketplaces

  11. It likewise meddles with the basic trustworthiness of your home. Rooftop reclamation helps in keeping the harms via fixing the tiles. Visit Here

  12. It’s a shame you don’t have a donate button! I’d certainly donate to this excellent blog! I suppose for now i’ll settle for book-marking and adding your RSS feed to my Google account. I look forward to fresh updates and will talk about this site with my Facebook group. Chat soon! Continued 

  13. Be that as it may, similarly as with any part of your home even the most indefatigable property holder who designs normal reviews and support will in the long run need to repair or reestablish their rooftop. Click Here

  14. This comment has been removed by the author.

  15. Reclamation of these rooftops begins with the expulsion of rust wherever present. This is finished by pounding it utilizing a rust convertor, which is very intense. Visit Here

  16. In the event that you can't make the installments also, make them on time, you could wind up losing your home. random sander

  17. I was suggested this website by my cousin. I am now not sure whether this put up is written by way of him as no one else understand such distinct approximately my problem. You’re incredible! Thanks! danesroofing

  18. This is cool post and i enjoy to read this post. your blog is fantastic and you have good staff in your blog. nice sharing keep it up. roofing contractor

  19. Trickle edge isn't costly to incorporate into a rooftop substitution venture and the advantages significantly out way the expense. see here

  20. I am appreciative to the essayist for composing this.
    powerbank mit solar laden

  21. Thanks for Nice and Informative Post. This article is really contains lot more information about This Topic.  Broken Arrow Roof Repair

  22. Light brings a lot of value to our lives every day. In times of darkness it delivers safety and security and we demand it everyday with the simple flick of a string lights reviews

  23. Solar systems utilize an estimation of value related to the power yield per zone. Establishment of the solar system notwithstanding mounting is one of the important phase of the undertaking.

  24. Now, one may wonder, what makes appointing a roof repair contractor such a serious job.flat roof repair