Types of Active Photovoltaic Roofs
The EurActive Roofer project identified 5 main types of active photovoltaic roof:
- "Traditional" bolt on modules (on sloping roofs)
- Roof integrated modules (generally using façade technology)
- Solar Slates or Solar Tiles
- Continuous thin film technologies
- "Traditional" modules mounted on flat roofs
Traditional bolt on modules
The most common way of placing PV onto roofs is by using rigid modules that are fixed onto the surface of the roof by a combination of rails and hooks or dedicated fixings. Variations on this method will work with traditional tile, slate or metal roofs. The basic roof integrity is maintained, although there can be problems with penetrations for the DC power cable, and care must be taking if using a hook system not to lift the tiles by more than a few millimetres otherwise the roof is at risk from wind or rain. The modules themselves are normally several millimetres above the surface of the roof, to permit ventilation of the underside which will keep them cooler in operation. Almost all main PV suppliers in the UK can provide modules than can be fixed to roofs in this way; the illustration here (above right) comes from Schüco.
The photographs above show the installation of modules onto the surface of the roof at the National Energy Centre in Milton Keynes. At left, the modules are being lifted onto the roof, and this gives an indication of their size. The two top images show the fixing system, which includes rails to keep the moduls above the surface of the roof, special fixing blocks (which were fitted onto the surface of the roof before the zinc covering was attached) and special locking bolts for security. The lower two pictures show the units in place.
Roof integrated modules
In this method, the modules replace the roof covering, rather than sitting above it. In many cases the modules are fixed directly onto the roof trusses, although they can also be fitted using façade technologies (above, right) or in roof lights (above, left). This can reduce the visual impact, and may reduce costs slightly where a new roof is being fitted. Great care needs to be taken around the edges of the modules to ensure watertightness if they are being fitted into a tiled or slate roof, but as they are fully integrated there are no special considerations that need to be given to wind loading. This method is especially popular where a complete roof is being covered by solar panels, as in the Oxford Solar House (above, centre). It is also widely used where PV and solar thermal panels are being placed side by side, as in the Solar Synergy product sold by PV Systems/Imagination Solar.
Solar Slates and Tiles
This is a relatively recent way of including PV in roofs, and aims at even greater visual integrity than can be achieved through roof integrated modules as can be seen on the house shown here in Milton Keynes. These may use amorphous silicon and be slightly flexible, or may be rigid with polycrystalline cells. Solar tiles (which are typically the same height as ordinary tiles, but the width of 4 to 6 tiles) and solar slates are fitted onto the roof battens in the normal manner by tilers, but each has an electrical connection at the rear that enables them to be joined into strings for connection to an inverter. At the moment these tend to be a more expensive way of incorporating PV into a roof, although there are a number of vendors of products in the UK including Solar Century, Imerys and Redland tiles.
Continuous thin film technologies
Continuous thin films are used on installations where flexibility is helpful. The two leading products are bound to a typical commercial roof covering of a white PVC membrane (Evalon from Alwitra) or pre-bonded onto zinc for a standing seam roof (Uni-Solar from Rheinzink). As with solar tiles, they are supplied complete with electrical connections and are designed for installation by regular roofers.
Traditional modules mounted on flat roofs
Modules do not have to be snug to the roof surface, and standard PV modules can instead be placed into rigid frames that are mounted on the surface of a flat roof. This has an added advantage in that the angle of the modules can be optimised for solar gain, without having to change the building's orientation.
The EurActive Roofer project team inspect a flat roof PV installation on the Centraal Bibliotheek in Amsterdam.
Modules can be fixed to the roof permanently on a steel or aluminium frame (although there is a danger that this will break the waterproof roof membrane) or - where roof loadings are strong enough - can be weighted down on the surface of the roof. This can either be done using heavy concrete cradles for the PV, or by using a lighter framework and weighting it with stones or concrete blocks.
Modules installed as an add-on
A potential sixth way of installing PV products is by attached them mechanically to buildings as an add-on. However because there is little consistency in method, these are generally outside the scope of the EurActive Roofer project. For example, the wind loading is not determined by the shape and dimensions of the PV module, but more by the fixing product, together with the effect the building on which it is mounted. This is clearly the case in the brise-soleil PV system, with additional modules on a vertical façade shown here.
NEF would like to thank the partners in the EurActive Roofer project for their assistance with this micro-site. The information given is for guidance only and should not be used in place of proper engineering calculations in accordance with the relevant British or European Standards.