Albedo refers to the reflectivity of a surface - the fraction of sunlight that a surface reflects back into the atmosphere. It is expressed as a percentage, with 0% representing a completely absorptive surface and 100% representing a perfectly reflective one. Albedo plays a crucial role in the energy production of vertical bifacial solar panels, which generate electricity using sunlight captured on both sides of the panel.
Surfaces with higher albedo values reflect more light onto vertical bifacial panels, increasing energy yield. Conversely, low-albedo surfaces absorb most of the sunlight, reducing the potential for energy generation from reflected light. The effect is especially strong when the sun is positioned so the light goes parallel to the solar panels and virtually all the light hits the roofing surface, which happens in the middle of the day if the installation is oriented with azimuth east-west.
Why White Roofs Are Better for Energy Production
White or cool roofs have significantly higher albedo values, typically reflecting 70-85% of sunlight. This higher reflectivity increases the amount of sunlight reaching the vertical bifacial panels, boosting their energy output. For example, a white roof can improve energy production by 20-35% compared to a dark roof, depending on factors like the panels' azimuth (orientation), height of the panels, distance between panels, and the building's location.
Typical Albedo Values for Flat Roofs
Black or dark bitumen roofs with mineral slate cover typically have an albedo value between 5% and 10%, meaning they reflect only a small fraction of sunlight. Grey bitumen slate or polymer membrane roofs have slightly higher albedo values, ranging from 15% to 40%, depending on the color of the membrane. White or cool roofs are the most reflective, with albedo values of 60% to 80%, making them highly effective for enhancing solar panel efficiency.
Green roofs, which are covered with vegetation, usually have albedo values between 15% and 25%. Finally, snow-covered roofs, which have very high reflectivity, has albedo values of 80% to 90%, particularly during the winter months when snow coverage is most prevalent.
Roof type | Albedo |
|---|---|
Black bitumen membrane | 5-15% |
Grey polymer membrane | 15-40% |
White roofing membrane | 60-80% |
Green roof | 15-25% |
Professional albedo measurements demand substantial measurement equipment and competence, however, a simple estimation can be done with a free Android application called Albedo: A Reflectance App and using a photography grey card that can be bought for about 10 € online.
Albedo Effects of Snow and Green Roofs
Snow: Snow-covered roofs, with their very high albedo (80-90%), provide excellent reflectivity during winter months, significantly enhancing the energy output of bifacial panels. However, snow's seasonal nature limits its year-round impact.
Green Roofs: Green roofs have moderate albedo values (15-25%). Although less reflective than white roofs, they still contribute to energy gains compared to dark surfaces while offering the additional benefit of lower operating temperature.
Our Albedo Measurements and Experiments
Over Easy Solar has conducted extensive experiments over the last three years using high and low albedo roofing membranes in various locations, including our R&D test plant outside Oslo, Norway. These experiments demonstrate that even slight increases in albedo can yield significant energy production gains for vertical solar panels.
Simulating the Albedo Gain for Vertical Bifacial Solar Panels
While common simulation software like PVSyst and PVSol can estimate energy yields, they often fail to account for the complexities of albedo and reflected light accurately. Advanced ray-tracing software like Bifacial Radiance provides precise results but is time-consuming and impractical for most rooftop projects.
To address this, Over Easy Solar has developed a proprietary simulation model. This model quickly and accurately estimates energy yields for vertical panels based on location, azimuth, and albedo, making it a practical tool for real-world applications.
An Example of Albedo Gain on a Rooftop Solar Installation in Southern Europe or Northern US
We here compare data from two installations at approximately 40 degrees latitude, representative of Madrid or New York, on a sunny day in September 2024. One is on a low-albedo roof (ca 5%) and one on a high-albedo roof (ca 60%). The installations don’t have the same azimuth (orientation). The important is to observe how the “valley” and peak production differs between the installations.
The graph below illustrates simulation from our proprietary simulation model of the energy output of a 1 kWp vertical bifacial solar installation at the same latitude with various azimuths, comparing high (80%) and low (5%) albedo conditions.
Simulation of albedo gain at 37 degrees latitude for two different orientations of vertical solar panels
How to Achieve High Albedo and Energy Yield on a Rooftop
There are several ways to enhance the roof’s albedo:
Use a white roofing membranes (cool roofs): Products like Protan Cool Roof membranes offer excellent reflectivity and durability.
Paint the roof white: An affordable and straightforward solution that has been shown to give durable and good results if the right paint product is chosen.
White mineral chippings or pebbles: Suitable for gravel and bitumen roofs, but may not give the durability desired as chippings may wash away and pebbles may be affected by being covered in leaves and organic material.
Green roofs: While not as reflective as white surfaces, they still offer better albedo and also a lower operating temperature which also boosts the energy yield.
Choosing a high albedo roof solution can increase energy production by 20-35% compared to dark roofing. Contact us to discuss how albedo can help to support energy production, considering your rooftop’s location, azimuth, and current albedo properties.
