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.