Photovoltaic Modules
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Photovoltaic Modules
We have seen previously seen the behaviour and design of
solar cells in isolation. In practice they are connected
together and packaged as a module to provide specific
power output and to protect the solar cells from the
elements. We will look in more detail at the following issues
- Connection of solar cells and mismatch between
- Packaging of modules
- Failure modes for modules
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Connecting solar cells
• We need to understand how the different connections
between solar cells affect performance and most critically
what happens when solar cell performance is mismatched
• We will look at whether the solar cells are connected in:
– Series: give greater voltage
– Parallel: gives greater current
• Mismatch between solar cells must be taken into account
when designing a module, how is this done?
• How do we construct a module that will be relatively cheap
but also provides good reliable power?
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Connecting Solar Cells
• Series connection increases voltage
• Parallel connection increases current
• This is for identical solar cells, what happens when they are
not identical – depends on the connection
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Connecting Solar Cells
• What a solar cell does depends on it’s bias condition
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Solar Cells in Series
• Simplest thing to consider is when we have two identical
solar cells connected in series
• Since the cells are in series, the currents will be matched
(not a problem as they are identical), voltages will add.
• Useful for when we want a specific voltage, typical voltages
for a single solar cell will be < 0.6 V.
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Solar Cells in Series
• Recall the I-V characteristic for a solar cell
• Realistic I-V curve tells us that a slightly higher current can
be obtained when solar cell is reverse biased
• This is important when we consider solar cells that are not
identical in performance
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Solar Cells in Series
• Since the voltages add when in series, if the mismatch is in
voltage there is no problem
• When the mismatch is in current then we have a much bigger
problem since in series we want current constant through all
of the solar cells
• So in series connected solar cells the current for the chain is
set by the current of the worst performing cell, this is bad but
it gets worse when we have a short circuit condition
• We can get a situation where the worst performing solar cell
is reverse biased and is dissipating power
• Major cause of cracking and all-around destruction of solar
cells in modules
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Series Mismatch
• Can get a serious mismatch for nominally identical cells when
one or more is shaded
• What actually happens when this is the situation?
• Need to consider the current match condition and the I-V
characteristics for the solar cells
• Current mismatch is worse than voltage mismatch
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Voltage Mismatch in Series
• Voltages add together at each value of current
• At maximum power point the overall power is reduced
compared to identical cells as the bad cell is producing less
power
• For current mismatch we see a more drastic effect
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Current Mismatch in Series
• At low currents no problem as all cells can produce the
required current
• At higher currents output is pinned by the ISC of the bad cell
therefore power reduction is severe
• Power is being dissipated in bad cell
• Situation is most severe if we have a short circuit over the
chain of cells
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
ISC Mismatch in Series
• In order to match the current from the good cells the bad cell
is reverse biased (since we are in short circuit)
• Easy way to find the ISC of the chain is shown above, where
we simply set the V of the good cell to be –V
• We see the ISC for the chain is a little above the ISC for the
bad cell and the reverse voltage across the bad cell may be
close to VOC of the good cells
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Solar Cells in Parallel
• Currents add, voltage is the same across cells in parallel
• Obviously can use parallel connection to boost current output
• But what if the cells are non-identical?
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Current Mismatch in Parallel
• Currents add, so no real problem, as long as open circuit
voltages are same
• Power is reduced slightly compared to independently biased
cells but effect is minimal
• Mismatch in voltage is more drastic
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Voltage Mismatch in Parallel
• At low voltages there is no problem
• When voltage is higher than the VOC of the bad cell it stops
generating power and now dissipates
• Overall VOC of the cells is reduced to something between the
high and low values
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Voltage Mismatch in Parallel
• Can find the VOC for the parallel cells quite easily
• Simply reflect I-V curve of good cell across Voltage axis
i.e. put –I into the equation
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Mismatch
• In practice we have nominally identical solar cells so why is
there mismatch?
• Shading, degradation of cells etc.. Mean that in practice we
can have mismatch
• Parallel connection is less sensitive to mismatch as it is a
voltage mismatch that creates bigger problem and the VOC
scales logarithmically
• In series, the current, which scales linearly, is the bigger
problem
• First conclusion is to connect mainly in parallel
• In reality most cells are connected in series (remember we
need to boost the voltage)
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Connection for a Module
• Most often for a module we have 36 solar cells connected
in series
• Reason is, we will typically get 17-18 V output voltage
which makes it compatible with 12 V application
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Hot Spot Heating
• If we have current mismatch for series connected solar cells
then power can be dissipated in bad cell with a maximum
occurring when the chain is short circuited – good cells bias
the bad cell so large amount of power dumped into bad cell
• This is called hot spot heating
• Can severely damage the module
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Hot Spot Heating
• Hot spot heating is big problem for series connected cells but
we need to have series connected cells
• Can we prevent this situation developing? To a certain extent
but when in the field expect the unexpected
• The big problem is that we can have say 9 good cells
dumping power into 1 bad cell
• Can we stop this happening?
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Hot Spot Heating
• Bad cell is in reverse bias, therefore is dissipating power
from the good cells
• Problem is that we are locked into the bad cells I-V curve for
conducting current
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Bypass Diode
• Put ‘bypass’ diode in parallel to cell with opposite polarity
• Diodes switch on when voltage across bad cell reaches turn
on voltage
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Bypass Diode
• To understand its operation look at I-V curve for a solar cell
with a bypass diode
• The presence of the bypass diode limits the voltage across
the cell in reverse bias to pass a certain current and hence
less power is disspiated
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Bypass Diode
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Bypass diode
• Ideally, we have a bypass diode for each cell, in practice we
have strings of cells with a bypass diode for the string
• This works to protect our cells in the module and being
economic
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Mismatch for Modules
•
•
We can connect modules (‘strings’ of series connected solar cells in
series or parallel
Similarly to connecting solar cells we do have some problems associated
with connected modules
• If connected in series and one
module is open-circuited then
effectively get no power from the
connected modules
• Can use similar ideas to those
used for solar cell connections –
bypass diodes
Want to bypass the ‘bad’ module in
this case – what about if we have a
parallel connection?
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Mismatch for Modules
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We actually pick up a bonus from
the bypass diodes already in the
modules
These diodes are in effect
connected in parallel to the strings
of cells
Get a bypass effect for free!
Does have a drawback, however
Running current through a diode
heats it, reducing resistance and
saturation current
Causing more current to flow
through the heated diode and can
cause breakdown and heating
damage to module
Diodes must be rated to take total
possible current of entire parallel
array
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Blocking Diodes
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When we have modules connected
to some type of charge storage (say
batteries) we want to prevent the
charge coming back
Include a blocking diode
Blocking diode prevents back
charging by a battery array at night –
in other words the diode prevents
charge coming back from the battery
to the module
Should have a blocking diode for
each module – means the diodes
don’t have to be rated so high
Also prevents one module sending
current through the other when we
have shading
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Module Structure
• Need to construct module to stand up to field conditions
• Typical structure is Tedlar (usually white) base, EVA
encapsulant for the cells (top and bottom), low Iron glass for
front
• Want the glass to have:
– Good transmission in the wavelength range of most use to the solar
cells, low reflectivity
– Impervious to water
– Be able to take a hit
• Encapsulant we want:
– Stable at high temperatures
– Optically transparent
– Low thermal resistance
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Module Structure
• Rear surface we want:
– Stops water (liquid and vapour)
getting to the cells
– Low thermal resistance
– Sometimes have bifacial design
meaning rear must also be
optically transparent
• Frame, we need some sort
• of mechanical frame:
– Want lightweight but sturdy –
Aluminum is usual
– Design so there are no pits or
protrusions for water to gather
and perhaps enter module
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Packing Density
• How much of the area of the module is covered by solar
cells?
• Shape of cells determines maximum packing density
• Things like offcut also influence packing density
• Obviously want to maximize packing density but sparsely
set out cells can get a boost
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Every little helps….
• We get a very slight boost from the rear surface of the
module – so called ‘zero depth concentrator effect’
• Some of the light incident between the solar cells is
scattered in such a way that it reaches active regions of the
module – we get more power
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Heat Generation
• Since module is exposed to sunlight it
generates heat as well as electricity
• Typically module is converting only 1015% of the incident power to electricity,
remaining power can be largely heat
• Some factors include
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Reflection from top surface
Operating point of solar cells
Absorption of light not by solar cells
Absorption of infra-red light
Packing density of solar cells
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Heat Loss
•
Three main ways for heat to be
lost from the module
– Convection
– Conduction
– Radiation
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The operating point is the
equilibrium between the heat
generated and the heat lost by
these mechanisms
If we can enhance these
‘losses’ then the operating point
will be a lower temperature –
better efficiency
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Heat Loss
•
Convective: usually done by transferring heat to the wind
A is area, T is temperature
h is heat transfer coefficient
Pheat = hA∆T
•
Conduction: driven by temperature gradient ‘diffusing’ heat to other
materials in contact with module
1 l
Φ=
k A
∆T = ΦPheat
•
k is thermal conductivity
Φ is thermal resistance
Radiation: heat is emitted due to temperature of module being higher
than the surrounds
(
4
P = εσ Tsc4 − Tamb
)
σ is Stefan-Boltzmann constant
ε is emissivity of surface
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Nominal Operating Cell
Temperature
• Module typically rated at 25 C and 1 kW/m2 insolation
• More realistic to consider cell under the following conditions
– 800 W/m2 irradiance on cell surface
– Air Temperature 20 C
– Wind Velocity 1 m/s
– Mounting is open back side
• Cell temperature can be approximated by the following:
Tcell = TAir
NOCT − 20
+
S
80
S is irradiance
• Typically ranges between 33 C and 58 C
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Nominal Operating Cell Temperature
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Thermal Stress
• Thermal expansion is another important effect of heating of
modules
• Spacing between cells tries to increase by:
δ = (α G C − α C D )∆T
D is cell width, C centre to centre distance
αG, αC are expansion coeffs for glass and cell
• Thermal cycling of module interfaces can also lead to delamination
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrical & Mechanical Insulation
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Encapsulation must handle at least the system voltage
Frames must be grounded
Rigid enough for wear and tear at least for installation
Tempered glass due to thermal gradients (cells are hot spots)
Able to take twisting of frame (due to wind)
Australian Standard AS4509-1999
Static load: 3.9 kPa for 1 hour then back
(~ 200 km/h winds)
Dynamic load: 2.5 kPa then back for
2500-10000 cycles (~160 km/h)
Hail Impact Damage: 2.5 cm diameter at
terminal velocity 23.2 m/s (~80km/h)
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Degradation & Failure Modes
• Manufacturers guarantee up to 20 years for a module
• There are a number of degradation and failure modes for the
modules, some reversible, others not
• Failures are almost always down to water ingress or thermal
stresses
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Reversible Degradation Modes
• Main cause of reduced power is soiling of the top surface
by dust or ornithological ablutions
• Can also have some type of shading from say a tree
(maybe we can, gasp, trim it or chop it down, best to
move?)
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Degradation and Failure
• Solar Cells can be degraded permanently by:
– Increase in RS due to corrosion or peeling of contacts
– Decrease in RSH due to metal migration i.e. creates shorts
– Deterioration of AR coating (usually by water ingress)
• Cells can also be short circuited by interconnects
touching
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Degradation and Failure
•
Open circuited cells due to:
– Thermal stress
– Hail damage
– Latent cracks present from manufacture only seen later
•
Can be alleviated by redundant contacts and interconnect busbars
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Degradation and Failure
• Interconnect open-circuits: cyclic thermal stress and
wind loads responsible
• Module open-circuit: typically occur in the bus wiring or
junction box of the module
• Module short-circuit: insulation degradation meaning delamination, cracking or electrochemical corrosion
• Module glass breakage: thermal stress, wind, hail,
handling, vandalism (of course…)
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Degradation and Failure
• Module de-lamination: caused by reductions in bond
strength, by moisture or photothermal aging and stress,
induced by differential thermal and humidity expansion.
• Hot spot failures: as seen previously, caused by
mismatched, cracked or shaded cells
• Bypass diode failure: usually due to overheating, often due
to undersizing. Minimized if junction temperatures <128°C.
• Encapsulant failure: slow depletion, by leaching and
diffusion, once concentrations fall below critical level, rapid
degradation occurs. Browning of the EVA layer, due to
build-up of acetic acid, causes gradual reductions in output,
especially concentrating systems
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Summary
• We have seen the major issues in connecting solar cells
together to form modules
• In particular, the effects of mismatch due to shading etc.
have been looked at
– Series connected: current mismatch is major problem
– Parallel connected: voltage mismatch big problem
• Strategies for overcoming these issues have also been
introduced
• Effects of temperature and some design features that
determine operating temperature were looked at
• Common degradation and failure modes for modules
have been discussed as well as ways to alleviate
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner