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.

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.

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.

Vacuum Tube Solar 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. This means that vacuum tube panels perform well even in cold weather when flat plate collectors are performing poorly due to heat loss (during high Delta-T conditions). In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the evacuated tube this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during storage and operation, thus helping to maintaining the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition.

Heat Pipe

To extract the heat from the tube, a special type of "heat-pipe" is used to absorb the energy and transfer it to the solar panel manifold. Heat pipes are not exclusively found in solar panels but are commonly used in laptop computers and air-conditioning systems. The principle behind heat pipe's operation is very simple and surprisingly efficient.

A heat pipe is simply a copper tube with a small amount of heat conducting fluid inside, and the air removed. When heated (even by a small amount) the fluid inside changes state from liquid to gas.

At sea level, water boils at 100C, but if you climb to the top of a mountain the boiling temperature will be less that 100C. (This is why tea tastes terrible when you go on a skiing holiday, due to the lower boiling temperature of water at altitude your tea cannot diffuse properly).

Based on this principle; by evacuating the heat pipe, we can achieve the same result. The heat pipes used in Wimex solar collectors have an operating point of only 30C. So when the heat pipe is heated above 30C some of the heat conducting fluid vaporizes.

Payback for Solar Panels

A 6m2 flat plate facing due south at 30° + 300 litre solar panel installation will produce about 2000 kWh of energy in a year, replacing an uninsulated cylinder with a high efficiency correctly installed cylinder will save another 2000 kWh Giving total savings of 4000 kWh This is the equivalent of about 500 litres of oil if a system efficiency of 75% is assumed. (normally heating water from a boiler is only about 55% efficient).

Normally a vacuum tube panel with a 300 litre cylinder is sized to produce about 2000 kWh, depending on the roof orientation and type of panel (Sydney or single walled), a smaller panel is generally selected.

The payback can also be thought of in terms of the amortised cost of this oil over a number of years assuming a particular energy inflation rate. Energy inflation and future energy prices are very difficult to estimate, so an easier way for customers to grasp savings, is that they are effectively buying 500 litres of oil per year during the life time of the panel for the upfront cost. So regardless of future energy prices, the following benefits exist;

• More hot water is available
• Better energy rating for the dwelling / Better house value
• Lower co2 emissions
• Fuel cost savings, up to 25%

National Strategy

It is estimated that water heating equates to about 1/3 of the total heating energy a house requires. A solar system typically supplies between 50% and 70% of the water heating energy, giving overall heat savings of up to 25%.

It is generally agreed by experts that huge energy inefficiencies exist within the national housing stock making it a very attractive target to reduce national energy use.

In 2006 residential thermal energy accounted for 17.5% of total energy use. It is estimated that water heating made up 1/3 of this. In other words, 5.8% of the total national energy requirement is used to heat residential water.

Carbon Taxes

It is only a matter of time before carbon levies are introduced. The following table gives tax rates based on different levels of a carbon levy on common fuel types.

Energy and Oil

As can be seen from the table above Heating oil contains about 10.6 kWh per litre and produces 3.02 kg of CO2 when burned.

A lot of people are surprised to find that a litre of oil weighing about 900g is transformed into over 3kg of CO2, this is because as oil is burned the chemical reaction takes Oxygen from the air (a heavy molecule) and combines with the carbon (a light molecule) so the net result is the high weight of CO2.