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This site is being developed as the on-line companion site for solar installers undergoing training for for BPEC, SEAI, and Action Renewables (UK and ROI). We offer solar consultancy, training and course material development.
Fergus Wheatley B.Sc(eng) DIP(EE) MIEI
Lecturer and Consultant in Solar Thermal Energy Systems
Managing Director of Solar & Renewable Technologies
Kevin Murray MEng MSc PGDip PGCE
Lecturer in Renewable Energy Technologies
Experience over the last six years developing and delivering courses for installers and designers of renewable energy technologies, primarily solar thermal, solar photovoltaic and micro-hydro.
Accredited trainer and assessor for these technologies with a range of awarding bodies.
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 Ireland 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.
Installation Standards are vital for the long-term health of the market.
During the oil shock of the 1970’s Greece and Portugal both embarked on a Solar programme. Both countries have similar populations and climate patterns. Greece maintained and enforced installation standards, while Portugal was not so strident. Today less than 1% of homes in Portugal have solar installed while 30% of the homes in Greece are equipped with a Solar panel. Allied Solar helped set up the Sustainable Energy Association to help make sure industry standards don't slip.
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.
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:
The raw energy conversions are:
Average annual insolation levels for Ireland:
Central Australia = 5.89 kWh/m²/day - Very High
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;
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.
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.
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. More information can be found here; Detailed graphs and solar panel output comparisons.
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;
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.
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.
Solar panels can be used to supplement wood, oil or gas burners in providing space heating.
Allied Solar has joined SEA Ireland an industry body for ethical and professional renewable energy businesses.
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