Solar facts and figures

The sun provides more than 10,000 times the energy needed by the 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 graph shows the amount of energy generated by the sun each year. As you can see, the annual worldwide energy consumption is just a tiny tiny fraction in comparison to the solar energy incident on the earth.

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.

Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.65	1.18	2.18	3.42	4.17	4.65	4.73	3.66	2.76	1.57	0.78	0.46

 
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.

Simulator outputs at different panel angles

Despite the massive differences between summer and winter energy levels, a significant correction can be made by the angling of the panel so that equivalent winter shadow lengths are as long as possible. The solar panel is then receiving all the energy in the shadow area.

The graphs below show the expected monthly outputs of the equivalent of 1m² Wimex® vacuum tube panel at different incidence angles over the course of a year.

Placing the solar panel flat on the ground, leads to an output which is negligible in winter, however any angle over 60° gives a excellent winter output (70° is optimum), by further increasing the angle up to 90° reduces spring output, but also significantly reduces summer excess heat.

These outputs can be scaled, e.g. at 60° 5 square meters of panels will give 105 kWhr a month in January, (equivalent to 3.4 kWhr per day, or the immersion heater on for just over 1 hour).

However a note of caution must be introduced at this point, these output figures depend on heating a tank up from cold (all solar panels are more efficient at lower temperatures) and these are average figures. Naturally enough, on dark and dull days, any solar panel will struggle to produce anything worthwhile.

How Vacuum Tube Solar works

Each evacuated tube consists of two glass tubes made from strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating (Al-N/Al) which features excellent solar radiation absorption and minimal reflection properties.

The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes.

A vacuum is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, it won't lose it!! The insulation properties are so good that while the inside of the tube may be 150C, while the outer tube is cold to touch.