Effects of snow on solar cell roofs
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. Trying to solve the problem by devoting parts of the roof to snow accumulation will lead to decreased energy generation (due to less solar cell area) and new strains on these parts of the roof from moisture, freezing, thawing, etc.
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 roofs with solar photovoltaic (PV) installations. As can be seen from this photo, some PV surfaces can still hold snow at steep angles. This 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 adhere to a surface if it is very cold before the snow lands on it (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.
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)
Panels mounted at a relatively low angle are more easily blocked by snow, as can be seen in this example from the UK. Despite the clear sunny sky, no electricity was being generated.
Designers of PV roofs must be aware of other potential new hazards, such as snow and ice falling and being a risk for people passing beneath. The EurActive Roofer project considered a number of possible solutions. Electrical heating cables are a possible and acceptable solution in certain circumstances. However, such a solution is not considered viable by many people because the increased energy consumption is likely to exceed that generated by the panel. Mechanical solutions, similar to window wipers, were also considered but dismissed as a viable preventative measure. The only effective solution (but one with other environmental hazards) is manual removal followed by application of a de-icing chemical, like that used on car windscreens.
The EurActive Roofer project also looked at the effect of wind-blown snow on two roof elements:
- Shafts to provide fresh air or passive stack ventilation for a building's occupants.
- 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:
Block off as much snow and rain as possible; to prevent it penetrating the air intake - for example, make 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.
Ventilation for integrated PV panels
Use special tiles at the base of an array of panels, which should generally follow the design guidance for wind-driven rain.
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 while they're 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. Where the underside of an integrated panel is exposed to an attic or living space posed slightly more issues. However, these panels perform similar to double-glazed roof lights and, apart from the need to ensure 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's likely that more integrated PV systems will be above unheated spaces than is the case for roof lights, so adequate ventilation should be maintained to minimise any risk of condensation.
We would like to thank SINTEF (Norway) and the other partners in the EurActive Roofer project for their assistance with this page. The information on this page is for guidance only.
The EurActive Roofer project ran from 2005 to 2008 and was supported by the European Union's programme for Horizontal Actions involving Small and Medium-sized Enterprises (SMEs).