Solar Energy Panels And Metal Roofing

NZMRM has been involved in discussions about solar water heating panels before and has contributed to the MBIE G12/AS2 and “Solar Water Heaters - Guidance for suppliers, installers and building consent authorities – December 2009”. We have commented on the issues with these in at least one Scope article. 

Our misgivings about the installation of solar water heating panels onto our roofing products are several – creation of unwashed areas under the panel; unplanned loading without adequate support; penetration quality; maintenance traffic; and not least that most installations parallel to the roofing (or the COP- preferred method of insertion similar to that of skylights) nearly always result in less than optimal orientation to the sun.

These issues have apparently seen some disillusion with solar water heating, but at the same time we have seen a significant increase in the use of photo-voltaic (PV) cells both on and off the roof.  This article is to discuss the different issues that arise from the use of PV materials on the roof. Globally we have seen a steady increase in development of all forms of non-fossil-fuel energy generation ranging from wind farms and solar panels (both water and PV) through to tidal energy generators.  Most of these have some sort of disadvantages, such as expense or energy not being available when you need it, or energy storage.

NZ of course relies heavily on renewable hydro-power to generate electricity and less dependent on fossil energy than many countries which use coal, gas or nuclear based energy generation.  Reducing the cost of energy and avoiding creation of more energy infra-structure is still a desirable goal, both nationally and individually.   Nationally, even if we do use hydro-power extensively, the transmission across quite long distances (e.g. lower South Island to Auckland) creates power losses which add to the cost of generation significantly.  Individually the cost of solar power remains constant at the payback over the fixed period of 25 years (or whatever period is used for the payback calculation) and after this is free until the cells finally stop working.

Growth in photovoltaic electricity generation

One of the most significant increases in “alternative” energy generation has been in the use of photo-voltaic cells to directly generate electricity.  In a rather chicken and egg fashion manufactured prices have tumbled as usage has gone up and the development of methods of using this generation method  (which on the face of it only works during daylight hours) have also increased in availability and sophistication.

Use of pv technology ranges from huge “solar farms” covering many hectares, observed in Greece, Australia and elsewhere and rumoured for the Sahara Desert, down to a few panels fitted on house-roofs to generate top-up energy for on-grid locations, or a lot of panels for total power off-grid in more remote locations, or specific non-grid applications e.g. motorway phones, or electric fences. The majority of these are rigid silicon micro-crystalline arrays protected by glass and mounted in flat rigid panels.

On-roof installations

Looking at domestic (or limited application commercial) installations fitted onto the building, the same issues exist as apply to solar water heating panels but generally to a lesser extent.  PV panels are generally lighter, can be oriented better to the sun, and are in fact less dependent on orientation anyway as they generate to an extent even in diffuse light.   Penetrations are a cable rather than a pipe.

Obviously some issues remain.  Fitting close to and parallel with the metal roof can still create an unwashed roofing situation.  Some installations where the entire roof appears to be covered by panels would have to raise some questions about what happens to the roof, if there is one.  Panels still need to be washed to remove dirt which otherwise reduces the efficiency (and this again raises questions about whole cover installations).  Fortunately the relatively light weight means the panels can be mounted on rails leaving adequate clearance from the roof material, and also that properly braced they can be oriented better to the sun.   They can be mounted on tracking devices to allow optimum exposure to the sun, although this is less important than for water heating, and is not very useful in New Zealand where the light is more diffuse.  Design life of these panels is said to be 25 years, but of course as they improve in technology and output the cost-effectiveness increases and payback improves, or alternatively smaller panels can be used to provide the same output. 

Laminated panels

Another, newer alternative is for amorphous silicon panels directly laminated to the roof cladding.  This is less efficient and more expensive per kilowatt generated but virtually invisible on dark roofs and without the need for any rails or brackets, and is simply installed as the roof cladding (apart from the wiring).  Several NZMRM companies in NZ are looking at this process, and at least one is producing roof panels of this type (see Dumpling Hut pictures).  This obviously results in no unwashed areas but does have the disadvantage of not being able to be oriented for optimum generation (although this is apparently less important than it is for solar water heating panels).  There are no penetrations (typically connections are under the ridge cap), and no wind uplift issues, as there are for any panel supported off the roof.    The life span of these panels, unprotected by glass from sun and rain is not known but likely to be shorter than that of rigid panels, possible as little as 10 years.   They will require washing as do the glass panels, and this would seem to risk foot traffic damage.  How these will be processed at the end of life is not yet clear.  So, a work in progress, but obviously one of importance to roofing manufacturers as they are an inherent part of the roof cladding, not an extra (although still needing to be installed by a registered – and experienced – electrician).   

PV coatings

Finally, and still in the future, the ultimate metal cladding product may be photo-voltaic cells based on dye technology (Dyesol) which are claimed to have the possibility of being applied by factory coil-coating, like colour finishes.  The basic cell technology exists (and has done for some years) but the application method is still coming, as is any possible installation method.  Corus Steel (part of Tata) has been working on this in Wales (so far unsuccessfully).   We have even seen a couple of claims that spray-on solar paint is being developed! 

Orientation

As we understand it, pv panels are less susceptible to poor orientation reducing total annual output than are water heating panels. In addition the  ability to raise or orient the rear of the panel, which is easier and safer with pv cells, makes this more likely to happen, although again, few of the installations seen seem to have taken advantage of this.

What to do with the power?

So, having got our roof generating electricity from one of these technologies in amounts depending on the number and type of panels and their orientation, and the amount of sunlight impinging on them at any time, what do we do with it?  

There are basically two different strategies – for on-grid and off-grid.  Both require the high voltage DC generated to be converted to 230 v AC for use in the house. 

For both you need a conversion method and this in on-grid applications is normally an inverter which converts DC to AC.  For off-grid you normally need and use battery storage to provide power during darkness, and this is a significant investment which also creates some safety hazards.

Looking at on-grid, your pv electricity is fed into your domestic power supply and is used to provide power for appliances used during sunlight hours (or water heating).  Any excess of electricity generated over what is used may be fed into the grid through a meter.  Any shortfall, and all power used during darkness, is drawn from the grid.

Depending on the country and the power supplier you can be paid more than you are charged for grid power, or less, or nothing, or a variable amount.

Major users

In Germany which has been probably the largest and most enthusiastic proponent of pv power this was seen as a way of avoiding building more (nuclear) power stations and a 2:1 payment was started (i.e. at the beginning you get paid twice for your pv power what the grid power is charged at, gradually reducing over the plan period).  In 2013, 50% of all the power in one summer weekend was produced in this way; 24 GW was being produced; and $100 billion Euros was invested in private power generation.   Whatever may happen in future all this generating capacity still exists and has made a permanent change in the power generation model in Germany. 

Minor users

In small quantities such as we currently see in NZ, distributed photovoltaic power generation has little effect on the overall power generation sustainability because much of NZ’s power is derived from renewable sources, not fossil or nuclear fuels.   However distributed generation can reduce the power losses during distribution which are more significant in NZ because of the large distances and small customer base, in any case distorted by one third of the population living in Auckland, far from any generator. 

The various NZ power companies have different payment strategies.

Payback considerations

The payback on panels is at its simplest based on what you generate at grid rates. This means the payback is how long it takes to save the capital cost at grid rate.   When you put power into the grid the payback is longer because the return is at less than grid rate..  The Herald article suggests that an approach with optimum cost benefit is to determine your daytime power usage and create an installation for no more than this, or not much more.  This will ensure you use all your generation to save money and don’t invest capital into a worse payback.  Of course once the payback period you allowed for in your calculations (typically 25years) is completed, your solar power is then free until the panels stop working.

Roofing considerations

(Finally, you might think). The installation and maintenance considerations are similar to those we have previously published regarding solar water heaters mounted on the roof, but with some differences.

These comments cover separate rigid panels, not integrated laminated panels (or indeed paint), for which some of the issues are different.

Mounting   

Even the relatively light weight of the pv panel should be still supported by the roof support and the fixing should go through the roof cladding into a purlin, or purlin/rafter, which can properly and rigidly support the weight of the panels, not just being fixed to the roof cladding itself.

What is equally important is withstanding wind uplift forces which will try to rip the panel off the roof. This will be more important for panels oriented off the roof plane for greater efficiency.

The not-uncommon method used for solar water heating panels, of just screwing a bracket into the roof with a leg at each corner, can damage the roof cladding in several ways, and if the panel has only one support at each corner this may well not coincide with the structural members underneath. For this reason the support rail is preferred, as shown in G12/AS2 etc.

This allows the support bolts/screws to be mounted over the support members underneath the roof.

Mounting the panel from support rails more easily creates the 100mm minimum manual washing clearance required between the roof and the panel for regular maintenance. How this is maintained in whole-of-roof coverage installations remains to be seen, but as suggested above, for on-grid operation this amount of power may not be economical anyway.

Rails and supporting members must be made of materials/finishes of good durability, but which will be compatible with the roof.  For most metal roofs this means hot dip galvanised or zinc coated and painted.

Roof washing

NZMRM have always emphasised the importance for a metal roof that it is either rain-washed or can be manually washed to remove corrosive deposits, to extend its life.  Corrosion failure caused by the roof cladding not being washed is not covered by the warranty.  Experience shows that a minimum gap of 100 mm between panels and roof is necessary to allow manual washing (which can be done at the same time as panel maintenance, or more frequently if necessary).  Panels raised to improve orientation will normally allow adequate rain-washing.

Orientation

The rail mounting also allows the rear of the panel to be elevated to provide better pitch orientation.  In fact the horizontal orientation can also be improved from a rail, not just the pitch.

Method of support fixing 

As mentioned above, the fixing should be into the purlin, or the side of the rafter. 

G12/AS2 suggests several methods for this, of which this is one, which shows the use of a boot flashing on the crest of the roof cladding, so that the support bolt/stud is securely fixed and its penetration of the roof cladding is water-tight.

Metal tiles use small section battens and supports should always be fastened to the rafters not just to the battens.

Forces.  

PV panels do weigh less and can be supported from a purlin not a rafter, if necessary.

Penetration 

Putting a pipes or cable through any type of roofs, when installed after the roof, is typically the point at which common sense and respect for other people’s work seems to be abandoned.  Penetrations for electric cables for pv panels is much simpler than that for water-pipes but still require doing properly through a boot penetration flashing (or as for laminated panels, under the ridge capping.)  It is necessary to avoid damaging the roof coating and to remove any swarf created by drilling or cutting.

The method of putting a pipe or cable through a metal roof is well documented in E2/AS1, the NZMRM COP and elsewhere.

Laminated panels

As these cover most of the flat section of the roof profile and so much of the above installation detail for glass panels does not apply.  The collection cables (which of course are one per roof pan) run under the ridge cap and are collated inside the roof.

This type of panel cannot be installed as a retrofit except by a total re-roof.

Insertion into the roof

The Code of Practice in 2.6.9 shows a method of installing a panel by inserting it into the roof as a penetration in a similar manner to a skylight or powered ventilator, and this is also possible for rigid pv panels.  This does of course prevent any other orientation than that of the roof and in this sense is similar to the laminated panel.

Maintenance

All pv materials require the removal of aerially deposited dirt in order to keep working efficiently.  As for washing of roofs the frequency of this depends on how much air-borne dirt occurs, how often it rains and on the pitch of the panel.  At some point though, access will be needed for cleaning, and in order to avoid damage to the panel or the roof some form of access way may be needed. The glass panels can apparently stand being walked on.  Laminated panels may need more care to avoid damage than glass.  However access is gained under current H & S regulations any access to the roof other than by the owner requires edge protection.

Safety.

Live power

At this point one might note that all of these products are generating electricity at apparently about 500v DC as soon as they see sunlight (or indeed any light).   

This would seem to mean that they are permanently “live” as are the cables that come from them until they are fed into some switchgear.  This obviously poses some hazards during (and after) installation and only registered electricians experienced in this work should be employed.    Damage during cleaning might also create problems. 

The interface between roofer and electrician for laminated roof panels (and painted roofs if they come) will need to be managed with safety in mind.

Note also that lower voltage to power any sort of device, means higher current and for (typically off-grid) low voltage DC, installations need to be done with heavier cable than normal domestic installations to avoid overheating.

Final thought

This is obviously a coming thing and we need to be aware of all the issues in thinking about generating electricity from the roof.    In my last article I asked why solar panels aren’t mounted on the ground to avoid penetrations to the roof and easy access for cleaning.  This actually related to solar water heating panels, but maybe the same applies to pv panels.

Sources

• SEANZ (Sustainable Electricity Association NZ) presentation to NZMRM conference 2013

• Acceptable solution G12/AS2 (MBIE) and Guidance document

• NZMRM COP V 2.2

• NZ Herald “Elements” July 2014

• Sustainable Business News