Difference between Direct and Diffused radiation

Solar Radiation can be classified into two types:

Direct radiation also known as “beam radiation” or “direct beam radiation” is used to describe solar radiation travelling on a straight line from the sun down to the surface of the earth.

Diffuse radiation, on the other hand, describes the sunlight that has been scattered by molecules and particles in the atmosphere but that has still made it down to the surface of the earth.

In simple terms, direct radiation has a definite direction but diffuse radiation follows a much more scattered, uncertain path. Because when the radiation is direct, the rays are all travelling in the same direction, an object can block them all at once. Therefore, it can be noted that shadows are only produced when direct radiation is blocked.

During times when the sky is clear and the sun is very high in the sky, direct radiation is around 85% of the total insolation striking the ground and diffuse radiation accounts for about 15%. As the sun lowers into the sky, the percent of diffuse radiation keeps increasing until it reaches 40% when the sun is 10° above the horizon.

Atmospheric conditions like clouds and pollution also increase the chances of diffused radiation. On an extremely overcast day, pretty much 100% of the solar radiation is diffuse radiation. Technically speaking, the larger the percentage of diffuse radiation, the less the total insolation.

Direct/diffuse ratio varies with latitude and climate

The percentage of the sky’s radiation that is diffuse is much greater in higher latitude, cloudier places than in lower latitude, sunnier places.

Moreover, the percentage of diffuse radiation tends to be higher in the winter than the summer in this higher latitude, cloudier places. The sunniest places, by contrast, tend to have less seasonal variation and therefore more direct radiation. 

As an example, compare London, UK (51° North; wet and mild climate) to Aden, Yemen (19.5° North; dry and hot climate).

In London’s sunniest month (June), the average daily irradiation is about 5.5 kWh/m² and about 50% of that is diffuse. In December, the irradiation is less than 1 kWh/m² and by far the majority of that radiation is diffuse.

In Aden’s sunniest month (May), the average daily irradiation is about 7 kWh/m² and less than 30% of the radiation is diffuse. In December, the irradiation is around 5.25 kWh/m² and about 35% is diffuse radiation.

Direct or Diffused?

It is important to note that when you tilt your solar panels so that the sun rays are hitting it at a 90° angle, you are maximizing the amount of direct radiation that they receive.

However, it is found to be that diffuse radiation is pretty equally distributed throughout the sky and that the most diffuse radiation is gathered when your solar panels are laying down horizontally.

The steeper your solar panels are tilted, the less of the sky they are facing and the more of the sky’s diffuse radiation they miss out on. If, for example, your solar collectors are tilted at a 45° angle, they are facing away from about a quarter of the sky and can only collect about three-fourths of the diffuse radiation in the sky.

However, because direct radiation is much more intense than diffuse radiation, the amount of radiation missed by tilted solar panels is generally more than compensated for by the extra radiation gained by tracking the sun constantly.

Reflected radiation 

This refers to the kind of radiation that has been reflected off the ground. Asphalt reflects about 4% of the light that strikes it and a lawn of about 25%. However, solar panels tend to be tilted away from where the reflected light is going. Moreover,  reflected radiation rarely accounts for a significant part of the sunlight striking their surface.

An exception is in very snowy conditions which can sometimes raise the percentage of reflected radiation quite high. Fresh snow reflects 80 to 90% of the radiation striking it.