Active Roofs

Snow, Condensation

The EurActive Roofer project also included work packages looking at snow and ice, condensation effects and maintenance and safety issues. Most of the last two categories would not apply in the UK, other than to note that PV is a low maintenance product, and that safety issues would be dominated by working at heights. Electrical safety was outside the scope of the project but again tends to follow national practices. UK installations should be installed by a competent person, and an electrical safety certificate issued on completion. For the majority of systems that are grid-connected, compliance to the G83 code would also be necessary.

Snow and Ice

Traditionally, roofs have been designed to keep snow on top, and to minimise the chance of it slipping or sliding down. The main European Standard on the subject, EN1991-1-3 therefore concentrates on the load to be borne by the roof. However active solar roofs with PV or solar thermal panels need to be cleared of snow as soon as possible if the panels are to function properly. This is especially important at high altitudes when snow can be found for several months of the year and not just during the shortest winter days. Other active roofs, such as roof windows, may also require as little snow as possible on top of them.

Solar cell roofs covered by snow during long periods in winter will suffer from a substantial decrease of both energy and cost effectiveness, at the time of the year when the energy is most needed. Devoting parts of the roof for snow accumulation will, in addition to decreased energy generation due to less solar cell area, lead to new strains on these parts of the roof, both with respect to building physics problems like moisture, freezing, thawing, etc. and with respect to structural building and roof properties. Some typical roof problems, caused by snow and ice, which may af­fect active roof installations, are shown below.


The images above show three Norwegian examples of snow hazards: from left they are snow hanging from roof and covering the glass facade of a school building; large icicles covering the glass facade at a shopping centre and large icicles in front of the glass facade of a school building, covering the windows and representing a hazard to the chil­dren. (All photos courtesy of SINTEF)

Both new material surface technology - to reduce the snow coefficient of friction below 0.05 - and new architectural roof design can play important roles in the task of avoiding snow from staying on the active roof installations. This task will also become more important with increased use of building integrated photovoltaic (BIPV) systems. However, as can be seen from the photo of snow on a standalone PV panel at a very steep angle, some PV surfaces can still hold snow at steep angles. The issue is compounded by there being 80 recorded different types of snow and a wide range of temperatures at which snow customarily falls in different European countries. Snow is more likely to be able to adhere to a surface if it is very cold before the snow lands on it (and in the photograph shown, the air temperature was -19°C prior to the snowfall), or if there is already a thin sheet of ice on it.

Designers of active roofs must be aware of other potential new hazards, such as snow and ice falling from an active array (such as a façade), representing a risk for people passing be­neath. The EurActive Roofer project considered a number of possible solutions. Electrical heating cables may also be regarded as may also be regarded as a pos­sible and acceptable solution in certain circumstances. However, such a solution is not considered as viable by many, due to the increased energy consumption, which is likely to exceed the PV generated by the cleared panel. Mechanical solutions, similar to window wipers, were also generally dismissed as a viable preventative measure. The only effective solution - but with other environmental hazards - is likely to be manual removal followed by application of a de-icing chemical, as is used on car windscreens.

A second part of the work package considered the effect of wind blown snow on two active roof elements: shafts to provide fresh air or passive stack ventilation for a building's occupants, and ventilation provided for integrated PV panels. In all cases the best solution was to provide simple external shielding, supported by drainage to safely remove any snow or wind-driven rain that had penetrated inside the building:

• Shielding - block off as much as possible of the snow and rain from penetrating the air intake, e.g. by making a shield where air enters only upwards from below and into a larger space and thereby with reduced air velocity, see right.
• Drainage - make a watertight system which drains away any water coming into the ventilation system.

Condensation

This work package from the EurActive Roofer project identified that there were no unique issues surrounding condensation on solar panels, either photovoltaic or thermal. It did note that most panels are mounted above the surface, and are designed to allow airflow behind, to help cool them in use. However this airflow should also help prevent any condensation build up, and reduce the risk of water damage or marking from drips from the lower edge of the panel.

Integrated panels, where the underside of the panel is exposed to an attic or living space, posed slightly more issues. However essentially they perform little differently to double glazed roof lights, and apart from the need to ensuring that any condensation that does build up is not allowed to get into contact with the electrical connections to the PV, few special considerations were identified. It is likely that more integrated PV systems will be above unheated spaces than is the case for roof lights, and so adequate ventilation should be maintained to minimise condensation risk.

NEF would like to thank SINTEF (Norway) and the other partners in the EurActive Roofer project for their assistance with the snow and ice part of this page. The information given is for guidance only.