Solar Panel

Our Solar Panel training and resource guide has been developed as a companion site and on-line resource site for both plumbers undergoing training for for BPEC, SEAI, and Action Renewables (UK and ROI) and homeowners looking to research and compare different types of solar systems. www.solarbook.co.uk has been developed by

Fergus Wheatley B.Sc(eng) DIP(EE) MIEI

Lecturer and Consultant in Solar Thermal Energy Systems

Managing Director of SmartPower

Kevin Murray MEng MSc PGDip PGCE

Lecturer in Renewable Energy Technologies

We both have spend over five years in the developing and delivering courses for installers and designers of renewable energy technologies, primarily solar thermal, solar photovoltaic and micro-hydro.

We are both accredited trainers and assessor for these technologies with a range of awarding bodies.

This information is provided free of charge and without a login, so we would appreciate it if you supported our advertisers. If you want to advertise on this website then please contact us at the email adress given above.

Why Solar is so damn good, (or background Solar facts)

The sun shines for 8760 hours every year. Just 50 mins of this energy is the equivalent of the total energy requirement for the entire human race in 2011.

When we burn fossil fuels, we are literally burring fossilised plants. Coal, Oil, Gas and Peat are all the remains of plants which captured energy over millions of years from the sun.

To put the amount of vastness of solar energy into perspective once again. The large yellow cube below represents the amount of energy received by the sun each year. The small blue cube represents the energy contained in all the fossil fuel mankind burns each year, which is just a tiny portion of the solar energy arriving (without a fuel bill) each year.

A typical 3 bed semi detached house with a 40 sq meter footprint in Manchester, UK will typically receive 37,000 kWh annually of annual energy. Burning 3,500 litres of oil would release the same amount of energy. A 6 sq meter solar panel typically generates between 2,000 kWh and 2,500 kWh of energy each year, so this in itself is only a tiny portion of the solar energy available on the roof.

Solar Intensity or Solar Energy per m² is also known as Solar Isolation

The largest radiation readings are over the equatorial zone because the Sun's rays are on average more concentrated.

As we move north and south towards the poles, the suns rays hit the Earth's surface at and angle and must travel through more atmosphere and therefore have lower energy values.

From the diagram above, it can be seen that for a given m² of sunlight coming towards the earth, the actual surface area that this m² covers when it reaches in the tropics is more concentrated than an equivalent m² reaching a more northerly or southerly area. This means that the further you are from the equator, the more dissipated and therefore weaker the sunlight is. This effect becomes more pronounced as locations closer to either pole.

In addition, at locationas away from the equator, the amount of air, clouds & dust that the incident solar radiation must pass through also becomes greater, so that the energy becomes more dissipated. Because the each hemisphere tilts away from the sun in winter, and tilts towards the sun in summer, this effect changes dramitically between summer and winter.

How can we measure this effect?

In winter on a cloudless day and directly facing the sun at midday (in Manchester), we will measure an energy level of about 400 Watts/m² in contrast to a summer level of about 800 Watts/m². However because the sun is low in the sky in winter, the 400 Watts is "shared" or spread out over more surface area, meaning that the average value is lower again. The energy received per m² on a level surface in a mid-England December is about one tenth of the summer energy. 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

Energy Conversion

There are several units used to measure solar insolution 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 Isolation levels for the UK:

Central Australia = 5.89 kWh/m²/day - Very High

London, UK = 2.56 kWh/m²/day - Moderate

Devon, Uk = 2.56 kWh/m²/day - Moderate

East Midlands, UK = 2.56 kWh/m²/day - Moderate

Glasgow, UK = 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 with changing cloud cover.

Direct and Diffuse Radiation

Diffuse radiation is caused by deflecting direct radiation;

• Rayleigh scattering - particular frequencies being attenuated by specific air molecules (e.g. CO2) - (
• Mie scattering - aerosols, dust & pollution)
• Water Vapour & Cloud cover

The proportion of diffuse to direct radiance has been measured over many years and found to be between 50% and 60%, with proportionally less direct radiation in the winter. The following graph and table gives the average daily Global radiation kWh/m² (Diffuse + Direct) measured in 2005. In this year October happened to be a much sunnier month than normal.

 
As was seen before there is about 10 times more solar energy available in summer than winter, this is one of the factors that makes designing solar systems interesting!

We overcome this difference to some extent, by angling the solar panel towards the sun, the solar panel can then take advantage of the "extra" direct solar radiation it can capture. The actual amount captured is equivalent to the size of the shadow of the panel on flat ground. Angling the panel however does not cause any significant increase from diffuse radiation, diffuse radiation is actually maximized by having the panel "flatter". There is a trade off between maximizing the two types of energy, but experimental and simulation results show that having a panel angled between 20° to 60° to the horizontal offers very little loss in output. The authors have come across "clever" designs that insist that solar thermal panels must be angled at say 37.5° but the output difference between a "normal" roof angle of 28° and that of an expensive bespoke mounting system is typically less than 1%. So be warned!!! It is much more important to keep pipes well insulated and pipe runs short.

Solar Panel Types One of the most common questions asked by students is "Which solar panel type is the best"? Without wishing to be vague, the most accurate answer is that "It depends". We will be looking at the characteristics of various panels and attempting to match panels with their best applications. For the purposes of the exam, there are 5 types that we need to be aware of; High Efficiency Vacuum Tube Sydney Type Vacuum Tube Selective Flat Plate Collector Non-Selective Flat Plate Collector Unglazed Collector (open swimming pool type) Each of these classes of solar collectors will be dealt with in more detail later, but within this page we will simply give a quick overview. As thermal and optical efficiencies are discussed, differences and applications. will become even more apparent. Common (High Efficiency) Tube A clear single tube contains a selective absorber plate. Once assembled, the tube is evacuated down to a very low pressure level. The virtually perfect vacuum eliminates the convection heat losses, the conduction heat looses can only take place at the bottom and top of the tube, so these are virtually eliminated also. Radiation heat losses are in turn minimized through the use of selective coatings which as we have seen earlier hinder the emission of infra-red heat radiation. This makes this the vacuum tube very efficient at retaining heat and hence it is very good for high temperature applications. Sydney Tube Each Sydney tube consists of two glass tubes made from borosilicate glass. The outer tube is transparent, the inner tube is coated with a selective coating (Al-N/Al) which absorbs the solar radiation and turns it into heat. The top of the two tubes are fused together and the space between the two layers of glass is evacuated, giving a the tube vacuum jacket. The insulating properties lie between a good flat plate collector and the high efficiency vacuum tube. Flat Plate Collectors Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with "selective coatings," these coatings absorb UV and visible radiation, but are poor emitters of longer wave infrared so that they retain heat much better than ordinary black paint. Absorber plates themselves are made from copper welded to a copper pipe. Sometimes non-selective coatings may be employed to increase heat losses at high temperatures and prevent a solar panel from boiling, The Solartwin system which deliberately uses a non-selective coating to prevent overheating. Non-selective panels were more common in the past because of the increased cost of selective coatings. They are relatively uncommon in the marketplace, but may enjoy a renaissance as a low cost non-stagnating panel. However deliberately decreasing the efficiency of the panel should be compensated for by increasing its collecting area. Unglazed Collectors For low temperature applications. such as outdoor swimming pool heating, especially in mainland Europe, plastic mat open collectors have been in use successfully for decades. Because they have no glass cover, they offer excellent optical efficiencies, but retain heat poorly and their efficiency drops off quickly as the panel temperature rises above its surroundings. Payback for Solar Panels A 6m2 flat plate facing due south at 30° + 300 litres 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 4000kWh. 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). In some cases savings have been found to be much greater during the spring and autumn, the authors have come across many instances of poor zone controls where the entire house is heated to ensure that the hot water requirement. In these cases, we would recommend that proper zones controls are installed with the cylinder change. 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 end-users to understand savings, is to assume that they are effectively buying 500 litres of oil per year during the life time of the panel for the once-off upfront cost. Regardless of future energy prices, the following benefits of solar thermal 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. (This figures will rise further as houses become better insulated and have better heating/boiler controls). A solar system will typically supply between 50% and 70% of the water heating energy, giving overall heat savings of up to 25%. It is generally agreed by experts that massive energy inefficiencies exist within the housing stock making it a very attractive target to reduce national energy use. Carbon Taxes 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 litres and produces 3.02 kg of CO2 when burned. Most of our students are surprised to find that a litres of oil weighing about 900g is transformed into over 3kg of CO2. But it actually makes sense, when oil is burned a chemical reaction combines Oxygen from the air (a heavy molecule) and combines this with the light carbon molecule, so the result is a high weight of CO2.