Solar Insolation Levels

Insolation means the amount of energy reaching the earths surface per square meter. (kW/m²) The largest radiation values are over the equatorial zone because the Sun's rays are more concentrated.

Towards the poles the rays hit the Earth's surface more obliquely and are more diffuse and therefore have lower radiation values.

From the diagram above, it can be seen that for a given segment of Insolation, the area that is covered in the tropics is much smaller than at the poles. In other words, the same amount of energy that hits the Earth's surface at the poles is much weaker and more dissipated than at the equator. The amount of air clouds & dust that the radiation has to pass through is greater the further you move away from the equator. This will result in more of the insolation being reflected by the atmosphere (due to cloud cover, particulate matter in the atmosphere etc.) at the poles. Because the northern hemisphere tilts away from the sun in winter, and tilts towards the sun in summer, this effect changes between summer and winter.

How big is this effect?

On a cloudless day, directly facing the sun at mid day, mid winter insolation levels are about ½ of summer levels. However because the sun is low in the sky, the available energy is spread over more ground, so each sq meter of ground receives much lower energy in the winter.

Nasa Website

http://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi?uid=3030

Met Eireann

http://www.weather.ie/climate/monthly-data.asp

What units are used to express Insolation levels?

The values are generally expressed in kWh/m²/day. This is the amount of solar energy that strikes a square metre of the earth's surface in a single day. Of course this value is averaged to account for differences in the days' length. There are several units that are used throughout the world.

The conversions based on surface area as follows:
1 kWh/m²/day = 317.1 btu/ft2/day = 3.6MJ/m²/day

The raw energy conversions are:
1kWh = 3412 Btu = 3.6MJ = 859.8kcal

Average annual insolation levels for Ireland:

Central Australia = 5.89 kWh/m²/day - Very High
Dublin, Ireland = 2.56 kWh/m²/day - Moderate

Solar Energy and Expected Heat Output.

The average monthly solar irradiance is an important value for designing solar systems. Over the course of the month, these values vary significantly from day to day due to changing cloud cover.

Direct and Diffuse Radiation

The values in the tables below for solar radiation are for the global or total radiation that will fall on a horizontal area of 1 square metre. The Global radiation is composed of two components, the direct radiation or sunshine that comes in astraight line from the sun through the atmosphere to the ground and the diffuse component that arrives after being reflected of clouds or the ground and scattered by dust or water vapour or air molecules in the atmosphere. Ireland and Britain have an Atlantic maritime climate that often yields high relative levels of diffuse radiation, particularly on our overcast days. The climate also delivers many days with broken sunshine, when overcast conditions give way to fairly intense bursts of sunshine, where radiation values can go over 1200 W/m². The rain that often accompanys these conditions has fortunately little effect on the performance of solar panels, much less than it has our view of what constitutes a 'good day'.

Diffuse radiation is caused by deflecting direct radiation;

• Air molecules - (Rayleigh scattering)
• Dust Particles - (Mie scattering)
• Cloud Cover

Over many years the average proportion of diffuse to direct radiance has been found to be between 50% and 60%, with much higher values in the winter. The following graph and table gives the average daily Global radiation kWhr/m² (Diffuse + Direct) measured in Dublin Airport in 2005. Notice that October was a much sunny month than normal that year.

 
As can be seen there is about 10 times more solar energy in summer than winter, this is one of the factors that makes designing solar systems challenging!

However, by angling the solar panel towards the sun, the solar panel can take advantage of the "extra" direct solar radiation equivalent to the size of the shadow of the panel. Angling the panel however does not cause any significant increase from diffuse radiation.