In photovoltaics, diffuse radiation (or diffuse sky radiation) refers to the portion of sunlight that does not reach the Earth's surface directly, but is deflected through reflection, refraction, and scattering by atmospheric particles such as clouds, fog, dust, or gas molecules. In short, it is non-directional, evenly distributed light that, unlike direct radiation, does not cast harsh shadows.
In a nutshell: Diffuse radiation is the reason why a solar module can still generate electricity even on a completely cloudy or foggy day. Together with direct sunlight, it forms global solar radiation. In Central Europe, diffuse light actually accounts for more than half of the total annual solar radiation.
Physical Origin and Meteorological Relevance
When sunlight hits the Earth's atmosphere, it interacts with the elements present there. Under a clear sky, directional direct radiation dominates. However, as soon as clouds, water vapor, or aerosols block the path, the light is scattered millions of times in all directions.
Understanding this type of radiation is essential for evaluating the energy performance of photovoltaic systems in regions like Germany, Austria, or Switzerland. Since weather in these locations is frequently changeable or overcast, roughly 50% to 60% of the annual global radiation yield comes from the diffuse component. Therefore, a solar system must be capable of efficiently utilizing this weaker, scattered spectrum of light.
Impact on System Planning and Orientation
The presence of a high proportion of diffuse radiation significantly influences the geometric design of solar installations:
- Reduced dependence on orientation: Because diffuse light reaches the modules equally from all directions in the sky, the yield of less favorably aligned systems (e.g., east/west roofs or facades) drops less sharply under cloudy conditions compared to pure direct radiation.
- Flatter tilt angles for flat roofs: Under purely diffuse radiation conditions, flat module tilts (e.g., 10° to 15°) are often advantageous, as this allows the modules to "see" the maximum portion of the visible sky horizon and thereby capture more non-directional photons.
Technological Optimization in Solar Modules
Since diffuse radiation has a different spectral composition (shifting toward bluish wavelengths) and a significantly lower intensity, modern manufacturers rely on advanced cell architectures.
In particular, innovative N-type TOPCon technology and Heterojunction cells (HJT) are characterized by excellent low-light performance. They possess higher quantum efficiency in the relevant spectral ranges and can convert the energy of diffuse photons into electricity much more effectively than older P-type PERC generations. Furthermore, bifacial (two-sided) solar modules also contribute to yield increases, as they capture additional diffuse radiation reflected from the ground or surrounding environment (albedo effect) via the back of the module. The precise calculation of these radiation components as part of a yield simulation is the responsibility of the planning specialist company.