Solar facts and figures

The sun provides more than 10,000 times the energy in fuel used by the entire human race.

Almost all energy ultimately comes from the sun. Coal, Oil, Gas and Peat are all fossil fuels. These are the remains of plants which captured energy over millions of years from the sun.

The large yellow cube below represents the amount of energy received by the sun each year. The small blue cube represents the amount energy contained in the fossil fuel mankind burns each year. As can be seen, the annual worldwide energy consumption is just a tiny tiny fraction of the solar energy incident on the earth.

In fact a 40 sq meter roof in the UK will typically receive in excess of 37,000 kWh annually. This is the same amount energy as is contained in 3,500 litres of oil.

The reserves of other energy sources are also dwarfed by the amount solar energy arriving on the planet each year. Unfortunately the energy is dispersed over half the Earth's surface at any time and in varies in intensity from one location to another and due to weather conditions. In comparison fossil fuels have the advantage of very high energy densities. Just consider how far a car can travel on a litre of petrol or that only a relatively small volume of oil is required to heat a house over the course of a year. The convenience and constant availability of fossil fuel systems are a constant challenge to the designers of renewable energy systems as they have to try to adapt a variable supply of energy to fairly rigid demand profiles. In solar systems, the design must be sized sufficently to concentrate the solar energy for later use and adapt to daily and sessonal variations in sunshine.

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

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.