Direct radiation has a significant influence on the efficiency and yield of a photovoltaic system. In this article, you can find out exactly what direct radiation is and how high its proportion is in the USA.
What is direct radiation?
Direct radiation is sunlight that reaches the earth's surface directly from the sun without any detours or scattering. It produces clear, sharp shadows and is an essential component of natural sunlight.
Direct and diffuse radiation make up global radiation.
hat is the difference between direct radiation and diffuse radiation?
While direct radiation reaches the earth's surface directly, diffuse radiation is only scattered and absorbed. As a result, diffuse radiation is weaker in intensity.
How is direct radiation measured?
Direct radiation is measured using a pyrheliometer. This instrument is designed to measure the intensity of direct radiation. The pyrheliometer has a small aperture that is aligned with the sun. It therefore only captures direct sunlight.
How high is the proportion of direct radiation in the USA?
Only about half of the global radiation in the USA consists of direct radiation. The proportion of direct radiation is directly related to global radiation: as global radiation increases, so does the proportion of direct radiation. In summer, the proportion of direct radiation is greater than in winter, when more diffuse radiation reaches the earth's surface.
In the USA, global radiation reaches an average of 1,000 W/m² on clear days, with most of the radiation being direct. With increasing cloud cover, the proportion of diffuse radiation increases.
| Weather Conditions | Summer | Winter |
|---|---|---|
| Clear sky | 1,000 W/m² | 500 W/m² |
| Slight 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 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 the USA than in the north. Direct radiation is negatively influenced by clouds, air molecules, aerosols and atmospheric particles.
What influence does direct radiation have on the performance of photovoltaics?
Direct radiation is mainly responsible for the power generation of a PV system. It is at a favorable angle to solar modules, causing the solar cells to generate more energy.
Here are some points that show the influence:
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 irradiation angle: Direct radiation reaches the solar modules at the ideal angle, which favors power 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. The direct radiation causes heat to build up on the back of the solar modules. This heat build-up reduces the efficiency of solar cells by 0.4% per degree Celsius.
With the ideal inclination and orientation of the solar modules, you can make the most of the proportion of direct radiation in the USA and achieve high yields. These vary depending on the application and local conditions.
