What is a PERC Solar Cell?

 This article was written by guest blogger, Dr KT Tan, Technical Director at Viridian Solar

What is the fuss about PERC?

Anyone who follows the latest developments in the Solar Photovoltaic market will have come across the technical term  PERC at least once in the past year or so. All respectable suppliers compete to launch the best PERC as a means to secure the premium part of the market share.

PERC stands for Passivated Emitter Rear Cell; the concept was first proposed by the University of New South Wales in a grant report in 1984 (1). As a matter of fact, the modern PERC generally refers to two specific configurations called PERT (Passivated Emitter, Rear Totally-diffused), and PERL (Passivated Emitter, Rear Locally-doped), which have proven to be the most viable solutions amongst other PERC configurations (2).

The PERC concept has taken some time to meet with commercial success, with the first high efficiency solar cells being fabricated in a lab with up to 20% efficiency from the 1990s onwards (3). Obviously at the time, there were other competing technologies, e.g. back contact and HIT. Hence, the PERC concept was only one of many promising ideas to increase solar cell efficiency. It took nearly 30 years for the industry to catch up with the concept and produce efficiencies achieved at the research level.

How does a PERC Solar Cell work?

In a nutshell, a PERC solar cell can be created by adding a rear surface passivation film to a conventional crystalline cell. In practice, it involves two additional steps: first, a rear passivation film is applied. Then, either lasers or chemicals are used to open up tiny pockets in the film through which the rear conducting layer can contact the silicon below the passivation layer. Figure 1 compares the configurations of conventional and PERC solar cells.

Figure 1: Comparison of cell configurations 

The above technique enables the efficiency of the solar cell to be improved in three ways:

1. By minimising surface recombination

The atoms at the surface of a silicon wafer have 'dangling bonds' which can capture charge carrying electrons and pull them back into the silicon crystal structure (a process called surface recombination).  As a result, when an electron reaches the back surface of a conventional solar cell, it is likely to be captured and does not contribute to the current.   However in a PERC solar cell, a passivated film is grown on the back surface of the solar cell and reduces this effect by tying up the 'dangling bonds'.  A charge carrying electron that strays too close to the back surface is allowed to continue on its way and the chance it will reach the emitter and contribute to the electric current produced by the solar cell is increased.  (Figure 2).

Figure 2 - How a PERC cell increases efficiency by decreasing surface recombination.

Longer wavelengths (red light) generate electrons near the back surface, compared to shorter wavelengths (blue light) (4). Since the PERC solar cell helps prevent surface recombination, it will still be able to capture these wavelengths (5). This capability increases the solar cell performance during mornings and evenings when longer wavelengths are present, which leads to the claims of better weak light performance by many manufacturers.

2. By Increasing Internal Reflectivity to Capture More Light

The rear film reflects the light that passes through the solar cell without being absorbed. This provides the light with more opportunity for a second absorption attempt (see Figure 3). In other words, the efficiency of energy conversion is just becoming higher.

Figure 3 PERC cell increases efficiency by reflecting light back through the solar cell

 

3. By Reflecting Counter-productive Wavelengths

Generally, silicon solar cells stop absorbing wavelengths above 1180 nm, instead they are absorbed by the backside metallisation layer and tuned into heat (4). The rear passivated film reflects these counter-productive wavelengths out of the solar cell and hence maintains cooler temperatures. As a result, PERC solar cells are considered to have better heat resistance.

All of the above, if manufactured correctly, will undoubtedly increase the solar cell efficiency. The current commercially available PERC solar cells are in excess of 20% with a record efficiency of nearly 23%. As this technology is piggybacked on conventional silicon solar cells, you can bet the efficiency will not stop here and will keep on improving!

Why has it become the most exciting feature amongst the new PV technologies?

It has been a holy grail in the PV industry to find a product with the highest possible efficiency, while maintaining the low manufacturing cost. To some extent, PERC solar cells provide the answer. Hence, it is not surprising that this technology has a rosy outlook.

From standstill, the PERC commercial production capacity is expected to occupy nearly half of the total solar PV cell capacity by 2020 (see Figure 4) (6).

Figure 4: Global production capacity for PV cells from 2016 to 2020 (Source: Energy Trend)

The main attraction is that the PERC production requires minimum modifications to existing solar cell manufacturing lines. The existing lines can easily be upgraded to produce PERC solar cells without having to invest in large capital expenditures or completely overhaul the entire lines.
In other words, one could increase the solar cell efficiency without having to take huge financial risks. Meanwhile, by recycling most of the existing equipment, one simply makes the money work harder for the original investment. It appears to be a no brainer.

Too good to be true?

Despite the PERC being a thirty year old concept, they have not been tested commercially until quite recently. It was first reported by Ramspeck (7) that PERC cells can exhibit stronger power degradations during the early days through a process called light-induced degradation (LID).

Fraunhofer Centre for Silicon Photovoltaics in Germany subsequently conducted extensive research to find out the degradation mechanism due to illumination (8). They were mainly attributed to chemical and physical root causes, namely Boron-oxygen complex activation and Iron-boron pair dissociation for the former, and elevated temperature for the latter.

To be fair, LID is not a new phenomenon, affecting conventional solar cells too. This type of degradation has simply entered the spotlight with the introduction of mass-produced PERC solar cells.

In response, some cell manufacturers have addressed the issue by adopting small changes to the solar cell process, such as modified process temperatures and different wafer materials (9).

Conclusions

PERC is going to take centre stage in the foreseeable future. This technology will progressively take a bigger market share – the benefits to developers, designers and installers are obvious.

Sooner or later, most of the PV modules will feature this technology. One just has to carefully choose trusted, quality suppliers with proven test records.

This fuss is, after all, worth paying attention to.

References

1. M.A. Green, A.W. Blakers, J. Kurianski, S. Narayanan, J. Shi, T. Szpitalak, M. Taouk, S.R. Wenham and M.R. Willison, Ultimate Performance Silicon Solar Cells, Final Report, NERDDP Project 81/1264, Jan. 82-Dec. 83 (dated Feb., 1984).

2. M.A. Green, The Passivated Emitter and Rear Cell (PERC): From conception to mass production, Solar Energy Materials & Solar Cells, 143 (2015) 190-197.

3. C.J. Chiangand, E.H. Richards, A 20% efficient photovoltaic concentrator module, conferencerecord,21st IEEE Photovoltaic Specialists Conference, Kissimmee, May 1990, pp.861–863.

4. D. de Rooij, PPERC solar cell technology: why will PERC dominate silicon cell technology? 2015 (http://sinovoltaics.com/solar-cells/perc-solar-cell-technology-will-perc-dominate-silicon-cell-technology/).

5. PERC cells: production costs down, efficiency up. May 2016. (https://www.pv-magazine.com/press-releases/perc-cells-production-costs-down-efficiency-up_100024647/)

6. TrendForce Reports PERC Cell’s Global Production Capacity to Reach 25GW in 2017, Resulting in Doubling of Total Annual Output. 19/1/2017 (http://press.trendforce.com/press/20170119-2737.html)


7. Ramspeck, K. et al. 2012, “Light induced degradation of rear passivated mc-Si cells”, Proc. 27th EU PVSEC, Frankfurt, Germany, pp. 861–865.


8. Tabea Luka, Christian Hagendorf & Marko Turek, Multicrystalline PERC solar cells: Is light-induced degradation challenging the efficiency gain of rear passivation? Photovoltaics International, 2016

9. Haase, J. Mono as well as multi PERC cells will get a significant market share, PV Magazine 10/2015, pp.74 -77.

Higher Standards for Housebuilders Do Not Slow Development

The Truth has Been Revealed by Scottish Developers

NOTE - a new blog with updated data is available here

Housing developers say that if you make them build more energy efficient homes, they'll cost more and less houses will be built.

Our politicians have swallowed this argument hook line and sinker time and again.

I've written about this before - the flaw in the argument is the assumption that the developers costs have to rise. They don't. This is because one of the main costs of building the house is what you pay for the land, and if everyone is faced with the same regulations, then the value of the land is driven down and the landowner makes a slightly smaller profit from the deal.

Developers have successfully held up tighter building regulations on numerous occasions.

• The update to building regulations in England in 2012 ended up as only a 7% reduction in carbon emissions (compared to the significant cut required in the original zero carbon homes policy)
• In Scotland in 2012 there was no energy efficiency improvement at all.
• The Housing Standards Review resulted in legislation in 2015 with the intent of limiting local authorities powers to require higher energy efficiency homes through planning (legislation that is still not in force).
• Finally, after 10 years of clear policy direction, one of George Osborne's last acts before disappearing off to take lucrative directorships, was to tear up the Zero Carbon Homes plan (the existence of which had been mendaciously used to justify the changes in the Housing Standards Review).

So it's very interesting to see what's been happening in Scotland. After many years of shadowing Westminster on building regulations (apart from the obvious requirement to go just a percentage point or two lower on carbon emissions to make a point), Scotland really pulled ahead with its changes to regulations in 2015.  The graph at the top of this piece shows the gap opening up.

If developers' claims that higher levels of regulation would stop housebuilding in its tracks were true, you'd expect housebuilding in Scotland to have gone off a cliff.  Has it?

Has it hell.

The Merton Rule is Not Dead, Long Live the Merton Rule!

Can Local Authorities Still Require Energy Efficiency Higher Than Building Regulations?

The largest housing developers would very much prefer it if they were able to build the same house in Aberdeen that they build in Abingdon. If they can do this, then the cost of the architect and engineers to design their standard properties can be spread across more units, and their buying power can be increased by using the same component parts in every house they build. This is one of the ways they out-compete smaller, more local building companies.

For this reason they dislike local rules and regulations that affect the houses they build.

In 2008 UK government enacted the Planning and Energy Act, which among other things clarified that local planning authorities had the legal right to require energy efficiency standards in new homes that exceeded the national building regulations. This approach to pushing developers to build housing with an energy performance beyond the national minimum had become known as the 'Merton Rule' after the local authority in London that had pioneered the approach.

Here's what the Act says:

1  Energy policies

(1)A local planning authority in England may in their development plan documents, and a local planning authority in Wales may in their local development plan, include policies imposing reasonable requirements for—
(a) a proportion of energy used in development in their area to be energy from renewable sources in the locality of the development;
(b) a proportion of energy used in development in their area to be low carbon energy from sources in the locality of the development;
(c) development in their area to comply with energy efficiency standards that exceed the energy requirements of building regulations.

It has been estimated that around 50% of local authorities took advantage of the new clarity to build such requirements into their local plans.

National builders didn't like it. They didn't like it at all. Different local authorities chose to ask for 10% renewable energy on site, 20% renewable energy on site, Code for Sustainable homes level 4.

The national developers were faced with different requirements up and down the country, a situation further complicated for them by the fact that different local authorities enforced their planning requirements with different levels of enthusiasm and competence. In some areas, particularly those areas that combine both high housing need and low house prices, developers might find a planning requirement for renewable energy on new developments to be highly negotiable. Other local authorities, notably in the south, with high house prices that means developers are queuing up to build new homes there, have been far more successful at holding the line on the aspirations of their development plans.

It could be argued that this is exactly as it should be. The UK has very high geographical differences in house prices. Local authorities could set their local planning regulations to achieve the highest energy performance that was consistent with the economics of housing development in their area, specifically whether the value of development land is sufficient to cover the additional costs of more efficient homes.

However, in 2015 things swung back towards the large developers. The government of Cameron and Osborne announced a 'bonfire of regulations' to free business from costly red tape. The Housing Standards Review was formed to look at red tape afflicting the house builders. Developers successfully argued that this patchwork of local planning requirements was 'red tape' and the review concluded that it should be swept away.



The government chose to add the changes to the Deregulation Act 2015. Section 43 of this Act amends the Planning and Energy Act as shown below.

43 Amendment of Planning and Energy Act 2008
In the Planning and Energy Act 2008, in section 1 (energy policies), after subsection (1) insert—
“(1A)Subsection (1)(c) does not apply to development in England that consists of the construction or adaptation of buildings to provide dwellings or the carrying out of any work on dwellings.”

This amendment would remove the ability of local authorities (in England only) to require developers to exceed the building regulations for energy efficiency. Note that sections 1(a) and 1(b) remain, allowing local authorities to continue to require that a percentage of the energy consumption of a new development to be met with renewable or low carbon energy.

The passing of the Deregulation Act is, however, not the last word in this story. If you work in a local authority in England and a housing developer is telling you that you can't impose higher standards than building regulations on a development, they're wrong. The fact is that section 43 has not yet been brought into force, so the original Planning and Energy Act text still applies.

If you look at the Commencement Section of the Deregulation Act, you'll see how the various elements of the Act are to be brought into force.

Some sections come into force on the day the Act is passed in parliament, others some set number of months later. No special mention is made of section 43, so it falls under this provision:

(7)Except as provided by subsections (1) to (6), the provisions of this Act come into force on such day as the Secretary of State may by order made by statutory instrument appoint.

A check of statutory instruments shows that this has not yet happened for Section 43, fully two years after the Act itself was passed by Parliament.

In fact, during a Lords debate on the Neighbourhood Planning Bill, in response to a question by Baroness Parminter, Lord Bourn confirmed that it was the case that local authorities still have powers to require higher building standards:

"The noble Baroness asked specifically whether local authorities are able to set higher standards than the national ones, and I can confirm that they are able to do just that."

So there you have it, the Merton Rule lives on!

 Local authorities still have powers to drive low carbon development in England, and it's just as well because our central government seems to have lost the will to do so. It's down to the sustainability officers and planning officers to enforce their local plans and they have the power to do so.

The Residual Valuation Model

Why Legislators Have Nothing to Fear from Tougher Building Standards

Simple isn't it? A chain of logic that seems irrefutable.

Legislate to make developers build better (low energy) homes and their build costs will rise.
These homes will not command a higher price, because the market is dominated by the price of existing homes for sale.
So the developer will make less profit.
Less homes will be built at a time when the country desperately needs them.

Our politicians have been buying this argument again and again from well-funded lobbyists working on behalf of housing developers.

The flaw in the logic is the assumption that the developers costs have to rise when you increase building standards. They don't. This is because one of the main costs of building the house is what you pay for the land, and if everyone is faced with the same regulations, then the value of the land is driven down and the landowner makes a slightly smaller profit from the deal.

The windfall from selling land to developers is so significant that a small decrease in the value is not going to slow down the market.

Thin Film Solar PV vs Silicon Wafer - Which is Better?

A guest article by Dr KT Tan cuts through the marketing to find out



Figure 1 (Source: Jethro Betcke, Oldenburg University, Germany)

Thin film solar PV was hailed as the next big thing in solar nearly a decade ago. Then, crystalline silicon wafer (c-Si) cells occupied more than 80% of the market share compared to thin film PV (1). There was a high anticipation in the industry for thin film PV to position itself for a run at c-Si and dominate the market for the near future. However, 10 years on, history shows that not only did thin film fail to conquer the market, but its market share has subsequently declined to only 7% (2).

Obviously, one major factor was due to the collapse of the price for c-Si cells, which quickly wiped off the cost advantages of thin film technologies. This blog is not going to discuss the reasons for this distorted market competition, caused mainly by the exponential expansion of production in c-Si cells, but to question and compare the technical merits of thin film PV versus c-Si.

Do thin film PV technologies have an arsenal of special features to outperform c-Si cells? 



Low Light Performance

The first common belief is that thin film solar PV performs better in low light conditions or diffuse sunlight (for example on a cloudy day). But is this true? The fact that this has been heavily promoted by the marketing guys is because these two technologies do have different spectrum responses to solar light. In other words, their ability to convert solar energy to electricity varies at different wavelengths. In general, the average wavelength in diffuse sunlight is shorter (i.e. more blue) that of direct sunlight – so if you have a spectral response peaking at short wavelengths, e.g. thin film amorphous silicon (a-Si), then you would perform better under diffuse conditions than clear sky conditions.

Figure 1 for shows the different spectrum responses of different solar technologies against the power of sunlight of different wavelengths at sea level at mid-lattitudes of Earth (called AM1.5).  Crystalline monocrystalline silicon (labelled m-Si) is compared against different thin film solar technologies based on amorphous silicon (a-Si), Copper Indium Gallium Selenide (CIGS) and Cadmium Telluride (CdTe).

If you look at Figure 1, you probably would have noticed that not all thin film technologies have the same performance response to differing light wavelengths. Thin film CIGS solar panels, for example, have a broad spectrum response akin to mono-crystalline wafer cells (m-Si), so based on this their performance in diffuse lighting conditions would be little different to m-Si.

Amorphous Silicon has a quite significantly different spectral response to crystalline silicon, with a greater response to low wavelength light.  So how do they compare in field trials? Figure 2 illustrates the results of a comparative study between a-Si and c-Si on a cloudy day. On average, the tests show an increase in energy generated of 15% for a-Si at low irradiance levels below 260 W/m2.  (Note: the tests were published by NexPower, a manufacturer of amorphous Silicon panels)

Figure 2 (Source: NexPower In-house test report)

 

However, performing better on cloudy days is of little benefit if it is combined with performing less well on sunny days (when more energy can be collected). If this were the case then the advantage of thin film PV under diffuse conditions might be a complete red-herring created by the marketing gurus.

A recent research project (3) supported by the Deutsche Bunderstiftung Umwelt (German Federal Foundation for the Environment) compared several solar module types (including thin film and c-Si) under North German Climatological conditions in a side by side trial for a year, and it turned out that no significant difference between the performances of the different type of modules could be found .

Shading

Let’s move on to the second common claim, that thin film PV are more immune to shading effects. There is no magic physics in thin film technologies that make them less tolerant to PV’s number one enemy – partial shading, except that the cells in thin film panels are usually very long and narrow (5 to 10mm wide and the whole length of the panel). In this case, the likelihood of total cell shading is diminished, provided that the installer has correctly oriented the solar modules. Most modern thin film solar modules have further split the narrow cell into multiple sections and incorporated by-pass diodes (4). Nevertheless, if they are not oriented wisely to avoid potential shadows, then it is back to square one (See figure 3).

Figure 3a: Correct orientation to shading           Figure 3b: Incorrect orientation to shading
(Source: Technical Note – Optimising Thin-Film Module PV Systems by SolarEdge)

High Temperature

Finally, how about the claims for superior heat resistance of thin film PV? This is perhaps the only undisputable advantage of thin film technologies – intrinsically, they all have a better temperature coefficient compared to s-Ci (5). In other words, their performance does not degrade as quickly as s-Ci when cell temperatures increase above 25oC.  However, as figure 4 shows, different thin film technologies display a wide variation in temperature response.  Amorphous Silicon (a-Si) is least affected by temperature, whereas CIGS solar panels are very similar in performance to crystalline Silicon.

Fig. 4 Variation of Power Output with Temperature for Different Solar Technologies
 Source: Virtuani. A, Pavanello. D and Friesen. G. Overview of Temperature Coefficient of Different Thin Film Photovoltaic Technologies, 25th European Photovoltaic Solar Energy Conference and Exhibition. 2010, Spain.

A comparative study between amorphous silicon and crystalline silicon suggests the benefit can be up to 20% more output on a hot day with an average ambient temperature of 34oC. See Figure 5. (Note: the tests were published by NexPower, a manufacturer of amorphous Silicon panels).

Figure 4 (Source: NexPower In-house test report)

Although the above result may sound impressive, you may be wondering which parts of the world regularly has an average ambient temperature above 30oC. Unsurprisingly, some research bodies in countries likes, Thailand (6) and India (7), have recommended thin film PV for precisely this reason.

In Summary

Bringing all these factors together, a collaborative research project carried out by Universities of Stuttgart and Cyprus compared thin film PV and c-Si by measuring actual performance over many years in Cyprus (8). The data has obviously taken into account all the differences in spectrum responses and temperature coefficients, the results are summarised in Figure 5.  Data for four years is presented from 2007 (labelled a) to 2010 (labelled d). The clear conclusion from this multi-year side by side test is that thin film modules do not outperform crystalline silicon modules.

Figure 5 Muli-Year Comparison of Solar Energy Yield from Different Technologies
(Source: Reference 8 – page 222)

There appears to be no clear technological advantage for thin-film PV against c-Si at present. In order for thin-film PV to experience a revival, there must be other factors involved which would make thin film PV more attractive than crystalline silicon solar PV. 

For example the homogenous appearance of thin film panels may make them look more appealing.
Thin film solar can be printed on any thickness of substrate and combine with other materials to form see-through graphics, stained glass, company logos, and blinds. With the ability of being semi-transparent, they could even mimic the appearance of natural materials, for example wood or marble.

Needless to say, apart from such niche applications, thin film PV also needs to gain more headroom in cost advantage against c-Si to offset a lower overall efficiency. Until then, it seems like c-Si will stay on top for now.

References:

(1) http://www.marketsandmarkets.com/Market-Reports/thin-film-pv-31.html
Renewable Energy Sources and Climate Change Mitigation: Special Report of the Intergovernmental Panel on Climate Change
(2) Photovoltaics Report by Fraunhoher Institute for Solar Energy System. 6 June 2016www.ise.fraunhofer.de
(3) FLINS Project (www.flins-projekt.de) hosted by Universitat Oldenburg, Germany (http://www.uni-oldenburg.de/en/physics/research/ehf/energiemeteorology/research/former-projects/flins/).
(4) Correspondence with NexPower (www.nexpw.com )
(5) Overview of Temperature Coefficients of Different Thin Film Photovoltaics Technologies by Alessandro Virtuani, Diego Pavanello, Gabi Friesen at 5th World Conefrence on Photovoltaic Energy Conversion, Spain (https://www.researchgate.net/publication/256080289)
(6) Investigation on Temperature Coefficients of three types Photovoltaic Module Technologies under Thailand Operating Condition by P. Kamkird, N. Ketjoy, W. Rakwichian and S. Sukchai. Published on Procedia Engineering 32 (2012) 376 – 383.
(7) Variation of Temperature Coefficient of different technology Photovoltaic modules with respect to irradiance by P. Dash and N. Gupta. Published on International Journal of Current Engineering and Technology, Vol. 5, No. 1 (Feb 2015).
(8) Performance of Photovoltaics under Actual Operating Conditions by G. Makrides, B. Zinsser, M. Norton and G. Georghiou (pages 201 to 232). Third Generation Photovoltaics ISBN 978-953-51-0304-2. March 2012.

Bird Proofing for Solar Panels

Bird Mess

I was surprised when posts about ‘bird-proofing’ solar panels popped up in my Linked-in Feed the other day.  “Is this really a thing?” I wondered.  So I contacted the guy from the pest control company that was offering the service to find out more.

thesolarblogger:

“What damage do birds cause to above-roof solar systems like this? Why don't we see more of this kind of thing? Are birds a problem in only some circumstances or places?”

Kelly Farrant, pest control specialist:

“There are several reasons I get asked to carry out this type of work. Pigeons love solar panels as a place to build their nest as they protect them from the elements and is a safe environment for them from predators and humans. The two biggest complaints I get is first of all the noise, with the pigeons waking the home occupants around 3 or 4 in the morning every day with the bird noises echoing through the loft. The second is the mess they make. A heavy infestation can cause a blocked gutter in a week and over flow of bird mess down the side of the house, not nice if you have a white exterior. It also causes a lot of mess around the outside of the house which can also be very slippery when wet. Another concern is the constant collection of broken eggs and dead chick on the floor where they have fallen from the nest. The final reason although I have not heard it happening yet is a concern that the birds will cause damage to the cables. I have lost count of how many solar panel jobs I have completed this year but I would say it is over 40 with my best month being April when we completed 12 in one month. I hope this helps.”

Other LinkedIn-ers from the Pest Control Industry provided more background.

Mark Porter, Area Surveyor at Servest Pest Control:

“agree with the above. The guano deposited tends to build up & blocks up guttering. They also provide ample nesting sites to female birds looking to shelter the young from the cold & wet. with this comes biting insect activity that feed on the young while in the nest. Bird mite soon then become a problem for the resident when the chicks are strong enough to leave.… “

Barry D Phillips, a Pest Control Training Supplier:

"Everything Kelly just said plus the main issue with Gulls is they are attracted to solar panels as they see the reflection of the sky and mistake them for a pond , lake or other water source. They and Pigeons cause additional damage from via their fouling which is highly acidic and can cause erosion to certain materials and paint / or protective coatings. Gulls also like to drop items onto the ground from height so this explains a lot of the bones etc you may have found on some jobs."

So there you have it, yes bird-proofing solar panels really is a thing.

As if rack-mounted solar panels were not ugly enough already, you now need to cover the edges with a metal mesh to protect yourself from an infestation of pigeons or gulls that will keep you up all night with their noisy parties, fill up your gutters and damage your paintwork with all their poop, leave dangerous slip hazards on your garden path and infect you blood-sucking parasites....

Yet more reason to do the right thing and go for good looking roof integrated solar?

Nailed Down

Clearline Fusion integrated solar installed by Go Green Systems.  Image (C) Go Green Systems

Change to UK Roofing Standards with Consequences for Solar

The British Standard for slating and tiling (BS 5534:2014) was updated in August 2014, and after a period of overlap to allow industry to finish jobs already started the previous version was withdrawn in February 2015.

Although the building regulations do not specifically call it up, most manufacturers of tiles and slates make sure that their installation guidelines closely follow the standard.  In addition, architects and designers will use it in their specifications and providers of insurance warranties to housebuilders such as NHBC require that tiles and slates are installed in accordance with the standard (for example, see section 7 of the NHBC technical document). 

The bottom line is that most new roofs in the UK will now be installed to this standard and the changes have some impact on the installation of solar on these roofs.

Fixing of Underlay

The underlay resists a proportion of the pressure difference created across the roof by wind.  If it is insufficiently well secured, it can balloon up and press on the underside of the tiles or slates.  New guidance in the standard describes how the laps strips of underlay should be secured - either by covering the bottom edge of the lap with a batten or sealing between the laps with double sided adhesive tape.

Solar wiring is often passed through into the building by passing the cables up between the laps in the underlay.  However, the double sided tape used to  stick down the laps is so effective it can't be peeled without tearing the underlay.  Installers might be tempted to cut through the underlay to pass cables through instead of taking the more time-consuming option of running them up to the top of the roof and into the building through the ridge vent.

No Tile Without a Nail

BS5534:2014 has an updated wind speed map, in which the wind speeds have increased compared to the previous version. This apparently reflects the greater likelihood of extreme weather events in a warming planet, and aligns more closely with European standards. 

Whatever the reason for this change, the outcome is that where previously only single-lapped tile roofs in highly exposed locations would need to mechanically fix every tile, now pretty much every roof is going to have every single tile nailed, screwed or clipped down to the tile battens.

For a roof that has been laid in this way, the task of retro-fitting an above-roof solar system has just got much more difficult. 

With a loose-laid roof (perhaps with only every fourth course of tiles fixed) you could simply slide tiles up and under the row above to fix brackets to the roof structure below.  This trick is also often used to provide foot-holds on the roof and avoid the risk of cracking tiles by walking around on top of them. 

For a roof built to the new standard, this option isn't available.  Real care is going to have to be taken to avoid the risk of damaging the roof covering so access to the roof is likely to need to be on boards or roof ladders.  Removing and replacing tiles for fixing roof hooks is going to be significantly more time-consuming, perhaps involving ripping nails to remove  a patch of tiles for each roof hook and using adhesive to replace that final tile (since it's impossible to nail).  Installers  may find that removing a large area of tiles and replacing them with a roof integrated solar system is far less trouble.

As the years go by, to stock of homes in the UK that have been built or had their roofs re-laid since the standard was updated will slowly increase.  It will become less and less of a sure thing to assume that the roof of the building you are quoting to install on will have loose laid tiles.  Solar installers should be particularly wary when pricing jobs on homes that have clearly been built in the last couple of years.

Of course for those solar installers that operate in the new build market the impact of the change is immediate.  With a loose-laid roof it was previously possible to install an above-roof solar system on a new home just like a retrofit on an existing one.  Occasionally the roofing contractor would refuse to provide a warranty on a roof if a solar installer had been tramping round on top of it lifting and notching tiles to fix roof hooks behind them.  Then it might be necessary for the build programme to allow time for the solar installer to fix roof hooks after the roofer had battened the roof but before they had tiled it.  This now becomes the only way to work with a nailed roof and clearly requires an additional visit to site for the installer. 

Even then, when you come back to fit the rails and modules above the tiles you need to figure out a way to get the cables through  into the building.  The cheapest option is push the cable up through a notch cut in the back of the tile (hopefully with some protection against abrasion of the insulation such as a flexible plastic conduit).  Alternatives include a flashing with cable glands or a similar cable entry created with lead.  All these options are extremely awkward to install without being able to easily remove the tiles fixed down by the roofer.

Little wonder that with the advent of cost-effective roof integrated solar that has closed the gap in cost with above-roof kits, most solar installers working in the new build sector have decided that roof integration is the only way forward.  Not only are the aesthetics more acceptable to customers it's now just so much less hassle to install.  It needs only a single site visit to fit the system after which the roofer can fit the slates or tiles all around using as many nails as they like!


Acknowledgements:

Thanks to Etienne Hilaire  Avonside Renewables for their advice on the practical impact of the new standard.