Building integrated PV - technical issues - part 1

BUILDING INTEGRATED PV
TECHNICAL ISSUES

http://www.solar-tec.com

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BUILDING INTEGRATED PV-TECHNICAL ISSUES

The presentation gives a short overview of the
technical issues to be considered in designing a
building integrated PV system.
Brief overview of photovoltaic materials and
modules is given.

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Photovoltaic modules
There are a wide variety of modules.

Photovoltaic modules should not be confused with solar thermal panels (used
to heat water or air for water and space heating).

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PV module construction
The photovoltaic module has a sandwich structure.
As a standard there is a glass sheet over the crystalline
silicon PV cells embedded in a resin.
At the back there is a tedlar backing sheet.

Other materials in use:
• Front sheets can be glass or other
plastics for flexibility or impact resistance.
• The PV cells can be made from a variety
of semi-conductor materials
• The backing sheets can be glass to
provide a partially transparent module, as
well as metal or plastic. www.eco-manager.com

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PV module terminology
Photovoltaic cell is the basic block used to make a module.
The array is made up from the required number of modules.

cell

modules (panels) array

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Semiconductor materials - crystalline
Modules are either crystalline or thin film.
The most of the PV modules are made of silicon.
Other semiconductors can also be used e.g. gallium arsenide
which offers higher efficiency at higher costs is used for space
applications.
Crystalline modules use cells made from a
crystalline semiconductor. Normally a large
silicon crystal is manufactured and then is cut
into slices to make cells.
• Monocrystalline silicon, slices of a
single crystal. Efficiency 12-15%
• Polycrystalline silicon, slices of a
ingot of crystals of silicon. Efficiency 11-
14%
Monocrystalline silicon cells and modules
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Semiconductor materials - thin film
Thin film modules are made of layers of semiconductor
material deposited in thin layers on glass, stainless steel or
plastic. They are cheaper than the monocrystalline silicon
cells because less semiconductor material is required and
more suitable for automated production methods however
they are also less efficient.

The most common material is
amorphous silicon. The same material
normally is used in watches, calculators,
etc. but it can also be used for larger
modules. Efficiency varies between 5%
and 7%. Thin film modules made with
cadmium telluride (CdTe) and copper
indium diselenide (CIGS).
amorphous silicon modules

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Design
The main issues in design of a building integrated PV
system:

• What size of system is required?
• Where on the building it could be installed?
• What type of modules would be appropriate?
• How to maximize the energy production of the
system?
• How to fix the system on the building?
• The electrical design of the PV system.

All these issues need to be considered to come up with the most
suitable design for a building integrated PV system.

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Where to install the modules?
How much space is needed?
The first important thing is the approximate size of the PV system
required, i.e. the scale of the system, and the possible areas on
the building where it could be installed.
Things to be known:
• The maximum available surface on the roof, facade or
other areas suitable for installing of a PV system;
• The amount of energy required from the system;
• The funding of the system (some funding programs have
minimum sizes of system for funding which may dictate
the minimum system size);
• The visibility of the PV system – desired or not.

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Suitable areas
• Almost any type of building surface - roof, facade,
sunshades or atriums and etc.
• The surface must be with an appropriate orientation and tilt
to receive as much sunshine as possible and be strong
enough to bear the weight of the PV modules.
• The surface area must be available to provide the required
power output.

Average area required to install 1 kWp of PV modules:
• 8 m2 - for monocrystalline silicon
• 10 m2 - for polycrystalline silicon
• 20 m2 - for amorphous silicon

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Energy production

To analyse in advanced the energy production from a PV
system multiply:

(irradiance on the array plane) x (size of system in kWp) x (performance ratio)

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Outline design
Base on the issues mentioned above a designer can define an outline
design of the PV system, with a choice on:
• the type of modules required,
• the size of the system required and the place on the building where
to be installed.
Ex a m p le :
O ne build ing o wne r m a y ha ve a la rg e a re a a va ila ble a nd m a y wis h to
p ro d uc e the m a x im um p o s s ible a m o unt o f re ne wa ble e le c tric ity
p ro d uc tio n but to be unc o nc e rne d a bo ut the vis ibility o f the s y s te m .
A the r build ing o wne r m a y wis h to g e ne ra te re ne wa ble e le c tric ity a nd
no
to m a ke a vis ible s ta te m e nt o f his e nviro nm e nta l c o m m itm e nt. I this
n
c a s e a fa c a d e s y s te m m a y be m o re s uita ble a nd m a y re q uire c us to m
m a d e s e m i-tra ns p a re nt m o d ule s .

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Maximum energy production
Maximum energy production from a PV system maximizes the
cost effectiveness of the installation and the amount of the
conventional energy displaced and hence CO2 emissions
avoided.

• Orientation and tilt of the PV array reflect to the amount of
solar energy collection and respectively to the total annual
amount of electricity production;
• Minimize the shade on the modules. If some shade cannot be
avoided the good electrical design can minimize their effect
on the energy output of the system;
• Allow ventilation behind the modules so that they don’t get too
hot. Module efficiency drops with increases in temperature.

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Optimal orientation and tilt
The orientation and tilt of the modules reflect to the total
amount of solar radiation received and therefore to the amount
of total annual electricity production.

In the northern hemisphere the best orientation is south.
In the southern hemisphere the best orientation is north.

The optimal tilt angle (deviation from the horizontal) is derived from the
degree of latitude of the building location.

If it’s considered the direct solar radiation only the optimal tilt angle for
maximum energy production over the year would be equal to the latitude
of the location.

In many locations, a major part of the incident irradiation comes as diffuse
radiation from other directions than the sun. This moves the optimal tilt
angle towards the horizontal so that the modules “see” more of the sky.
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Optimal orientation and tilt

To optimize energy production in winter time can be used a
steeper tilt angle.
To optimize energy production in the summer time can be used
a shallower tilt angle.

The optimal orientation dependent on local weather conditions
and topography.
i. e . m o rning fo g c o m m o n o rie nta tio n is s lig htly we s t o f s o uth. I
f
the re is a la rg e m o unta in, o r ta ll build ing e a s t o f the s ite the
o p tim a l o rie nta tio n o f the PV m o d ule s is g o o d to be s lig htly we s t
o f s o uth.

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Acceptable orientation and tilt
Often is not possible to positioned the PV modules at the optimum
orientation and tilt.
The range of orientations and tilts that provide acceptable levels
of solar energy capture are presented of the diagrame bellow.

The diagram illustrates the
percentage of the optimum
energy capture that can be
expected for a range of
orientations and tilts.

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solar radiation and building orientation in Europe

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Shading
Shading on the array occure a great difference to the annual amount
of energy production.
Due to the electrical characteristics of PV modules even a small
amount of shade can cause a disproportionally large effect.
The shading reduces the output of the shaded cells. The shaded cells
show an increased resistance to the flow of electric current which
reduces the flow of electric current through all the cells joined to that
module.
If some shade cannot be avoided it effects can be minimized by good
electrical design.
It is worthwhile designing so as to avoid even small areas of shade
such as that cast by vent pipes or chimneys.

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Shading
Po s s ible c a us e s o f s ha d ing to be c o ns id e re d a nd a v o id e d .

A) Shading influence due to other buildings

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Shading

B) Shading influence due to roof obstacles

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Shading

C) Shading influence due to trees

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Shading
D) Shading influence due to windows

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Ventilation
The efficiency of PV cells decreases with temperature increases.

For crystalline silicon cells the efficiency decrease is almost linearly by
0.4% for every degree rise in temperature.
For amorphous silicon cells the effect is less depending on the specific
production process.

High module temperatures could cause problems for the roof materials.

Two types of the most offten problems:
• the roof material could melt
• the difference in the coefficient of expansion between the PV and the
roof might induce stress that causes tears, leaking or breaking of the PV
laminate.

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PV efficiency and temperature

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Ventilation

The temperature of the PV modules depends on how well
they can dissipate the heat.
If the PV is insulated at the rear side, it can occure only
lose heat at the front side.
If an air gap is provided at the rear of the module it allows a
convective air flow and lowers the PV temperature.
The optimum air gap is 15cm.

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PV ventilation & installation

no efficiency about 5% about 10%
loss efficiency loss efficiency loss
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Building energy consumption

The design of building integrated PV systems should be
considered to minimize the energy requirements of the building.
The investment would be better spent in improving the energy
efficiency of the building if the energy consumption of the building
is known. The best BIPV designs consider all aspects of building
energy use in an holistic way.

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PV design installation

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Installation method
The choice of installation method have to be base on the
usual issues for buildings i.e. strength, corrosion resistance,
ease of installing and maintaining, wind loading, snow loading,
fire resistance, etc.
It’s important to check that any methods and products chosen
meet the local building regulations.

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Electrical design
There are 3 main areas of electrical design of building integrated
PV systems:
• System design (selection of inverters, etc)
• Array wiring
• Interconnection to the utility grid

Designers need to take into account the national requirements
and standards and the characteristics of the products being
used.

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System components
The main component is the inverter which converts the dc power,
generated from the PV array, into ac power.

The system requires fuses, wiring, junction boxes, isolator switches,
earthing and 2 electric meters to measure the electricity flow into and out
of the building.

A choice needs to be made between a centralized inverters, string
inverters or module integrated inverters.
A conventional PV system has an array of modules connected to a
centralized inverter which feeds power into the building distribution
board.

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Central inverter

Central inverter

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System components

The most of the building integrated PV systems have used string
inverters rather than a single centralized inverter. i.e. a number
of small inverters are installed, one for each string of modules in
the array.

Because of a standard range the string inverters can be used for
any size of system standards. This cost less money in compare
with the centralized inverters. In this case the amount of dc wiring
is kept to a minimum.

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String inverters

String inverters

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Module inverters

Another approach is to have one small inverter for each PV
module.

Module inverters www.eco-manager.com

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Electrical design – array wiring
An array is wired up to join modules in series and parallel to
produce the required current and voltage.

In case the modules are connected in a string - the same
output current as a single module but the output voltage is
the total ammount of the individual module voltages.

A number of identical strings can be joined in parallel to
produce an array. In this case the total current is the sum of all
the string currents.

There cannot be any number of modules in an array, there has to
be a multiple of the number of modules in a string. The resulting
array has to have an output current and voltage within the
acceptable input range for the inverter being used.

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Electrical design – array wiring
If part of the array is likely to be shaded at certain times of day it is
best to arrange the array so that the minimum number of strings are
affected.

A shaded module in a string will reduce the output of the entire
string.

If there was a vertical strip of shade down one edge of the roof
every morning the strings should be wired to run vertically rather
than horizontally.

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Local utility grid
In grid-connected systems the PV operates in parallel with the
grid.
It is not necessary to store energy e.g. in batteries which are
expensive, bulky and have limited lifetimes.
Technically connection to the grid is extremely straightforward.
Any system connected to the building electricity distribution
system is connected to the local grid and excess power will
automatically flow out of the building into the grid.
If the building loads require more power than is being supplied by
the PV it will flow from the grid.
The local electricity company has various requirements that a PV
system must comply with.

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Contact details
t. +359 885222471, +359 882909105, +359 888435561
e. office@enbc.eu
http://www.enbc.eu

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Building integrated PV - technical issues - part 1

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