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All About Solar Irradiance

Mpfana Manu is a mechatronic engineer who's working in green energy electronics products.

Solar irradiance

Solar irradiance

Solar irradiance is a very important quantity that is studied extensively especially in the design of solar technologies. In this article, we're going to learn what solar irradiance is. We're also going to look at what and how it is affected by several environmental factors.

What is solar irradiance

It can be defined as the solar energy you get per unit area. The energy is a combination of the strength/intensity of the sunlight received and the hours the sunlight falls at a particular place. The units for solar irradiance are watts per square meters (W/m2).

Solar panels are designed with a quoted solar irradiance of 1000W/m2. This is not an average, it's only a chosen value so that standard comparisons can be performed. There are only two types of solar irradiance which are diffuse and beam irradiance.

  • Diffuse refers to the irradiance that is reflected from the ground and surrounding objects.
  • Beam refers to the irradiance derived from the radiation that is coming directly from the sun.

What affects solar irradiance?

Solar irradiance is not uniform throughout. It differs from place to place and from time to time. The main factors that affect the amount of irradiance at a place are as follows.

  • Ozone layer thickness
  • Zenith angle
  • Seasons
  • Distance travelled through the atmosphere
  • Haze(dust and vapour)
  • Extend of cloud cover


Ozone layer thickness

The layer constitutes of an allotrope of oxygen called ozone (O3). It's 15-40km above the ground. Radiation fom the sun is a mixture of many electromagnetic waves with different wavelengths.

The waves with longer wavelengths, such as the ultraviolet waves, are absorbed by the O3. So the thicker the ozone layer, the more radiation that is absorbed. When more radiation gets absorbed by the ozone layer, less solar power reaches the ground and consequently, the solar irradiance will be lower.

This might sound bad for solar power generation but it's good for many other reasons. More radiation hitting the ground introduces new nasty problems that are not even related to solar power. Problems like global warming and skin cancers. Yes, it can get that horrible if the O3 doesn't absorb enough radiation from the sun.


Zenith angle

The sun is spherical, such that the radiation is emitted in all directions. So radiation that is directed to our little planet falls on it at different angles. The zenith angle is the angle between the incident radiation from the sun and the normal at point where the radiation hits.

Zenith angle

Zenith angle

Both the ground and the atmosphere reflect some of the radiation back up into the sky and into space. But the amount of reflected radiation is proportional to the zenith angle. The greater the zenith angle, the more solar energy that is reflected back into the sky/space.

The earth is always rotating, so the zenith angle is always changing. And consequently, the solar irradiance is always changing. In polar regions where the sun moves along the horizon and not overhead, the zenith angle is largest. That is why polar regions have the least average solar irradiance compared to other places.


Seasons

The most significant seasons that affect solar irradiance are summer and winter. Seasons are related to the zenith angle. In summer, the sun will be farthest away from the earth and more diverging radiation hits the earth at smaller zenith angles. This means less of the radiation is reflected and more reaches the ground, resulting in increased solar irradiance.

In winter, the sun is closest to the earth and more diverging radiation hits the earth at greater zenith angles. So much of the radiation is reflected back into space, resulting in decreased solar irradiance.


Distance travelled through the atmosphere

The atmosphere is an air cocoon of our precious planet. It's composed of stratified segments of gases and minute solid particles. Since the earth is not uniform, the atmosphere is not uniform either. It is thicker at one place and thinner at another.

Radiation can be reflected, absorbed or refracted. So when the atmosphere is thick, the distance travelled by the radiation is longer and it gets reflected, absorbed and refracted even more. And less and less of it reaches the ground, thus reducing the solar irradiance.


Haze and the extend of cloud cover

Both haze and cloud cover affect the amount of solar irradiance the same way. These make up the lowest layer of the atmosphere where much of the harmful radiation would have been absorbed/reflected in the upper layers.

They reduce solar radiation the same way as any shading does. Clouds are more of reflectors and haze absorbs radiation. The more cloud cover and haze, the less solar irradiance reaching the ground.

How to measure solar irradiance

The solar irradiance at a place can be measured using a pyranometer or a pyrheliometer. These instruments differ in their principle of operation. They're normally mounted together with solar panels to monitor and assess the solar irradiance and the electricity generated.

You can also get the value of the average solar irradiance of a place from NASA's site. NASA has satellites and stations which do the hard work for you. They have been recording and collecting data on the solar irradiance of many places for many years. Now they're a huge and valuable database of meteorological information.

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