Direct radiation and its impact on photovoltaic power generation

Direct radiation has a significant impact on the efficiency and yield of photovoltaic systems. In this article, you can learn exactly what direct radiation is and how high its proportion is in the United States.

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 important component of natural sunlight.

Direct and diffuse radiation make up global radiation.

What is the difference between direct and diffuse radiation?

Direct radiation reaches the Earth’s surface directly, while diffuse radiation is only scattered and absorbed. Therefore, the intensity of diffuse radiation is weaker.

How to measure direct radiation?

Direct radiation is measured using a pyrheliometer. This instrument is designed to measure the intensity of direct radiation. A pyrheliometer has a small aperture that is aligned with the sun. So it captures only direct sunlight.

What is the proportion of direct radiation in the United States?

Only about half of the global radiation in the United States is direct. The proportion of direct radiation is directly related to global radiation: as global radiation increases, the proportion of direct radiation also increases. The proportion of direct radiation is greater in the summer than in the winter, when more diffuse radiation reaches the Earth’s surface.

In the United States, global radiation averages 1,000 W/m² on a clear day, most of which is direct. As cloud cover increases, 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
Thick clouds	100 – 300 W/m²	50 – 150 W/m²

What factors determine direct radiation?

Direct radiation is primarily determined by location. The closer a PV system is to the equator, the greater the global radiation. For example, global radiation is higher in the southern United States than in the northern United States. Direct radiation can be negatively affected by clouds, air molecules, aerosols, and atmospheric particles.

What impact does direct radiation have on the performance of photovoltaic power generation?

Direct radiation is primarily responsible for the generation of electricity in photovoltaic systems . It strikes the solar modules at a favorable angle, allowing the solar cells to produce more energy.

Here are some points that show the impact:

Higher intensity: Direct radiation is much stronger than diffuse radiation and is therefore responsible for a large part of the electricity generation. The more direct radiation that hits the solar module, the higher the yield;

Better irradiation angle: Direct radiation reaches the solar panels at an ideal angle, which is beneficial for power generation;

Better performance: Crystalline silicon cells work extremely well in direct radiation. Their low light ratios are poor. Their performance drops significantly with diffuse radiation;

Thermally induced performance loss: Crystalline silicon cells are sensitive to high temperatures. Direct radiation causes heat to build up on the back of the solar module. Each degree Celsius of heat buildup reduces the efficiency of the solar cell by 0.4%.

With the ideal tilt and orientation of your solar modules, you can take full advantage of the direct radiation ratio in the US and achieve high yields. These depend on the application and local conditions.