The five main types of roof suitable for solar photovoltaics (PV) are where the roof:
- Slopes and 'traditional' bolt-on solar panels are used.
- Slopes and includes solar panels that are integrated modules and replace the tiles (generally using façade technology).
- Slopes and includes solar slates or solar tiles.
- Slopes and includes continuous thin film technologies.
- Is flat and includes 'traditional' mounted modules.
Traditional bolt-on panels
The most common way of placing PV onto roofs is by using rigid panels 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 panels 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 panels that can be fixed to roofs in this way. This illustration was provided by Schüco.
This photographs shows the installation of panels on the roof at the National Energy Centre in Milton Keynes. In the left section of the photo, the panels are being lifted onto the roof, and this gives an indication of their size. The middle and right sections show the fixing system, which includes rails to keep the panels 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.
Roof integrated modules
In this method, the panels replace the roof covering, rather than sitting above it. In many cases the panels 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 panels to ensure watertightness if they are being fitted into a tiled or slate roof. However, 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 panels as can be seen on the house shown here in Milton Keynes. These might use amorphous silicon and might 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. However, each has an electrical connection at the rear that enables them to be joined into strings for connection to an inverter. At the moment, this tends to be a more expensive way of incorporating PV into a roof, although there are a number of suppliers 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
Panels do not have to be snug to the roof surface, and standard PV panels can instead be placed into rigid frames that are mounted on the surface of a flat roof. This has the added advantage that the angle of the panels can be optimised for solar gain, without having to change the building's orientation. Panels 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 be done by either using heavy concrete cradles for the PV or using a lighter framework and weighting it with stones or concrete blocks. This photograph shows the EurActive Roofer project team inspecting a flat-roof PV installation on the Centraal Bibliotheek in Amsterdam.
Panels installed as an add-on
A potential sixth way of installing PV panels is by attaching them mechanically to buildings as an add-on. However, because there is little consistency in this method, it is 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 panel, 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 panels on a vertical façade, shown here.
We would like to thank the partners in the EurActive Roofer project for their assistance with this information. It is 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. The EurActive Roofer project ran from 2005 to 2008 and was supported by the European Union's programme for Horizontal Actions involving SMEs.