The advantages of solar panels are well-known. This sustainable solution lowers inhabitants’ energy consumption and greenhouse gas emissions, while simultaneously increasing the building’s value. The benefits of green roofs and cool roofing are equally clear-cut. But how about combining solar panels and green or cool roofing on a single building?
Solar Panel Efficiency
Ambient Temperature
The summers are getting hotter, which seems like the perfect scenario for solar panel owners. Yet the opposite is true. Scientific research has shown that solar panels produce the most energy under outdoor temperatures of around 26 °C. On very warm days, however, surface temperatures on roofs between 70 and 80 °C are no exception, especially on bituminous roofs. Whenever the temperature of the solar panels’ surroundings reaches these levels, the panels actually decrease in efficiency because of the excessive heat.
Combined With A Green Or A Cool Roof
Green roofs have excellent heat-absorbing properties, as dew and rain cause the plants to ‘perspire’, which effectively lowers the ambient temperature and optimises solar panel efficiency. The combination of a green roof and solar panels can lead to a 4 to 5% efficiency gain. The combination with a cool roof is even more effective to lower both the roof temperature and the temperature within the building itself. Membranes with white reflective slates (CRW membranes) reduce the temperature of the roof surface to 40 – 50° C, instead of 70° C on black slates. When a reflective coating such as ALSAN 970 CR is installed, the temperature can drop down to 35 °C. The Solar Reflectance Index (SRI) of ALSAN 970 CR is very high: 108. Bituminous membranes with reflective slates reach a value of +/- 74.
The SRI’s of some of our other cool roof products are:
- PVC FLAGON SR ENERGY PLUS: 97
- TPO FLAGON EP/PR ENERGY PLUS: 99
- Sopralene Optima 4 GF C3 FR CRW: +/- 70
- DuO High Tech 4 CRW/F C180 FC: +/- 70
Another advantage reflective roofs have over green roofs in terms of efficiency is that the latter always entail the risk of plants shading the solar panels. In other words, vegetation has to be selected with great care in order to avoid this problem. And lastly, as solar technology continues to evolve and improve, more and more solar panels capture both the reflections coming up from the cool roof as well as the sunlight coming down upon them.
Order Of Installation
When installing a green roof, make sure to keep the Green Roof Code in mind. In this Code, the Green Roof Organisation (GRO) lists all best practices relating to green roof design, specification, installation and maintenance, and includes reference to the relevant European and British Standards. Careful design consideration needs to be adopted on a project-specific basis to ensure that the installation sequence does not compromise the solar or green roofing.
Depending on the green roof build-up and the solar panels used, you need to be aware of weight loading – especially in a refurbishment scenario. SOPREMA can not only help you make the necessary calculations; we can also provide a combination of green roof, cool roof and solar panels that meets all your project requirements. Feel free to get in touch with our experts for more information or assistance.