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
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.
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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
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.
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.
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
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
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%
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.
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.