In fall and winter, diffuse radiation has a major influence on the yield of a PV system. This article explains what is meant by diffuse radiation and how high it is in the USA.
What is diffuse radiation?
Diffuse radiation is solar radiation that does not reach the earth's surface directly from the sun. The sun's rays are absorbed and scattered by clouds, air molecules, aerosols and atmospheric particles before they reach the earth. This scattering causes the sun's rays to reach the earth's surface at different angles.
Together with direct radiation, diffuse radiation forms global radiation.
What is the difference between diffuse radiation and direct radiation?
In contrast to diffuse radiation, direct radiation reaches the earth's surface directly from the sun without scattering and is more intense and concentrated.
How is diffuse radiation measured?
Pyranometers are used to measure global radiation, which results from the combination of direct and indirect solar radiation. By using a shadow ring or disk to block the direct solar radiation, the pyranometer can measure the diffuse solar radiation that is scattered by air molecules, particles and clouds.
How high is the proportion of diffuse radiation in the USA?
Diffuse radiation accounts for 50% of global radiation in the USA. In summer, direct radiation predominates, while in winter it is mainly diffuse radiation that reaches the earth.
When the sky is clear, the global radiation in the USA is around 1,000 W/m², most of which consists of direct radiation. With increasing cloud cover, the proportion of diffuse radiation increases.
| Weather Conditions | Summer | Winter |
|---|---|---|
| Clear sky | 1,000 W/m² | 500 W/m² |
| Light cloud cover | 600 – 800 W/m² | 300 – 400 W/m² |
| Light to moderate cloud cover | 300 – 600 W/m² | 150 – 300 W/m² |
| Heavy cloud cover | 100 – 300 W/m² | 50 – 150 W/m² |
What factors determine diffuse radiation?
The proportion of diffuse radiation is determined by two influencing factors. In the long term, the location is decisive, because the global radiation is stronger the closer you get to the equator. In the short term, diffuse radiation is determined by clouds, air molecules, aerosols and atmospheric particles.
What influence does diffuse radiation have on the performance of photovoltaics?
Overall, diffuse radiation leads to a lower yield of the PV system compared to clear, sunny days with a high proportion of direct radiation. However, in temperate climates where it is often cloudy, the output can still be significant throughout the year.
Below are some points that illustrate this influence:
Reduced intensity: diffuse radiation is less intense than direct solar radiation. This means that the energy hitting the photovoltaic modules is lower, resulting in lower electricity generation;
More even distribution: On the other hand, diffuse radiation is more evenly distributed and reaches the modules from different directions. This can be beneficial for installations that are not optimally oriented towards the sun;
Better performance with certain technologies: Some photovoltaic technologies, such as thin-film cells, can respond better to diffuse radiation than conventional crystalline silicon cells. This means that these technologies perform relatively better on cloudy days.
Cooler temperatures: Cloudy days are often cooler, which can improve the efficiency of photovoltaic modules, as high temperatures affect their performance.
Due to the relatively high diffuse radiation, photovoltaic systems in the USA are often designed to make the most of these conditions. This is done by selecting suitable technologies or adjusting the tilt and orientation of the solar modules.
