Direct radiation has a significant impact on the efficiency and yield of a photovoltaic system. In this article, you will learn what direct radiation is exactly and how high its proportion is in Germany.
What is direct radiation?
Direct radiation is sunlight that travels directly from the sun to the earth's surface without detours or scattering. It creates clear, sharp shadows and is an essential component of natural sunlight.
Direct and diffuse radiation make the global radiation out of.
What is the difference between direct radiation and diffuse radiation?
While direct radiation reaches the earth’s surface directly, diffuse radiation only scattered and absorbed. This means that the diffuse radiation is weaker in intensity.
Direct and diffuse radiation
How is direct radiation measured?
The direct radiation is measured with a pyrheliometer measured. This instrument is designed to measure the intensity of direct radiation. The pyrheliometer has a small opening that is directed towards the sun. Thus, it only captures direct sunlight.
What is the proportion of direct radiation in Germany?
Only about half of the global radiation in Germany consists of direct radiation. The proportion of direct radiation is directly related to the global radiation: as global radiation increases, the proportion of direct radiation also increases. In summer, the proportion of direct radiation is greater than in winter, when more diffuse radiation hits the earth's surface.
In Germany, global radiation reaches an average of 1,000 W/m² on clear days, with most of the radiation being direct. As the clouds increase, the proportion of diffuse radiation increases.
| weather conditions | Summer | winter |
| clear skies | 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 |
Which influencing factors determine direct radiation?
Direct radiation is primarily determined by the location. Global radiation increases the closer a PV system is to the equator. For example, global radiation is higher in the south of Germany than in the north. Direct radiation is negatively influenced by clouds, air molecules, aerosols and atmospheric particles.
Direct radiation in Germany (Source: dwd.de)
What influence does direct radiation have on the performance of photovoltaics?
Direct radiation is primarily responsible for the electricity generation of a PV system. It is at a favorable angle to solar modules, which means that the solar cells generate more energy.
Here are some points that show the impact:
- Higher intensity: Direct radiation is much more intense than diffuse radiation and is therefore largely responsible for electricity production. The more direct radiation hits the solar modules, the higher the yield;
- Better angle of incidence: Direct radiation reaches the solar modules at the ideal angle, which promotes electricity generation;
- Better performance: Crystalline silicon cells work extremely efficiently with direct radiation. They have a poor low-light ratio. Their performance decreases significantly with diffuse radiation;
- performance losses due to heat: Crystalline silicon cells are sensitive to high temperatures. Direct radiation causes heat to develop on the back of the solar modules. This heat build-up reduces the efficiency of solar cells by 0.41% per degree Celsius.
With the ideal inclination and orientation of the solar modules Make the most of the direct radiation in Germany and achieve high yields. These vary depending on the application and local conditions.
