Solar PV Modules-University of Delaware
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Photovoltaic Modules
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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
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Connecting solar cells
We need to understand how the different connectionsbetween 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?
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Connecting Solar Cells
Series connection increases voltage
Parallel connection increases current
This is for identical solar cells, what happens when they arenot identical depends on the connection
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Connecting Solar Cells
What a solar cell does depends on its bias condition
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Simplest thing to consider is when we have two identicalsolar 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 voltagesfor a single solar cell will be < 0.6 V.
Solar Cells in Series
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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
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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 involtage there is no problem
When the mismatch is in current then we have a much biggerproblem since in series we want current constant through allof the solar cells
So in series connected solar cells the current for the chain isset by the current of the worst performing cell, this is bad butit 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
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Series Mismatch
Can get a serious mismatch for nominally identical cells whenone or more is shaded
What actually happens when this is the situation?
Need to consider the current match condition and the I-Vcharacteristics for the solar cells
Current mismatch is worse than voltage mismatch
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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 lesspower
For current mismatch we see a more drastic effect
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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 therequired current
At higher currents output is pinned by the ISC of the bad celltherefore power reduction is severe
Power is being dissipated in bad cell
Situation is most severe if we have a short circuit over thechain of cells
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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 cellis reverse biased (since we are in short circuit)
Easy way to find the ISC of the chain is shown above, wherewe 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 thebad cell and the reverse voltage across the bad cell may beclose to VOC of the good cells
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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?
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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 circuitvoltages are same
Power is reduced slightly compared to independently biasedcells but effect is minimal
Mismatch in voltage is more drastic
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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 stopsgenerating power and now dissipates
Overall VOC of the cells is reduced to something between thehigh and low values
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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 axisi.e. put I into the equation
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Mismatch
In practice we have nominally identical solar cells so why isthere mismatch?
Shading, degradation of cells etc.. Mean that in practice wecan have mismatch
Parallel connection is less sensitive to mismatch as it is avoltage mismatch that creates bigger problem and the VOCscales logarithmically
In series, the current, which scales linearly, is the biggerproblem
First conclusion is to connect mainly in parallel In reality most cells are connected in series (remember we
need to boost the voltage)
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Connection for a Module
Most often for a module we have 36 solar cells connectedin series
Reason is, we will typically get 17-18 V output voltagewhich makes it compatible with 12 V application
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Hot Spot Heating
If we have current mismatch for series connected solar cellsthen power can be dissipated in bad cell with a maximum
occurring when the chain is short circuited good cells biasthe bad cell so large amount of power dumped into bad cell
This is called hot spot heating
Can severely damage the module
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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 butwe need to have series connected cells
Can we prevent this situation developing? To a certain extentbut when in the field expect the unexpected
The big problem is that we can have say 9 good cellsdumping power into 1 bad cell
Can we stop this happening?
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ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Hot Spot Heating
Bad cell is in reverse bias, therefore is dissipating powerfrom the good cells
Problem is that we are locked into the bad cells I-V curve forconducting current
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Bypass Diode
Put bypass diode in parallel to cell with opposite polarity
Diodes switch on when voltage across bad cell reaches turnon voltage
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Bypass Diode
To understand its operation look at I-V curve for a solar cellwith a bypass diode
The presence of the bypass diode limits the voltage acrossthe cell in reverse bias to pass a certain current and henceless power is disspiated
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Bypass Diode
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Bypass diode
Ideally, we have a bypass diode for each cell, in practice wehave strings of cells with a bypass diode for the string
This works to protect our cells in the module and beingeconomic
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Mismatch for Modules
We can connect modules (strings of series connected solar cells inseries or parallel
Similarly to connecting solar cells we do have some problems associatedwith connected modules
If connected in series and onemodule is open-circuited theneffectively get no power from the
connected modules Can use similar ideas to thoseused for solar cell connections bypass diodes
Want to bypass the bad module inthis case what about if we have aparallel connection?
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Mismatch for Modules
We actually pick up a bonus fromthe bypass diodes already in themodules
These diodes are in effectconnected in parallel to the stringsof cells
Get a bypass effect for free!
Does have a drawback, however Running current through a diode
heats it, reducing resistance andsaturation current
Causing more current to flowthrough the heated diode and cancause breakdown and heatingdamage to module
Diodes must be rated to take totalpossible current of entire parallelarray
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Blocking Diodes
When we have modules connectedto some type of charge storage (saybatteries) we want to prevent the
charge coming back Include a blocking diode
Blocking diode prevents backcharging by a battery array at night in other words the diode prevents
charge coming back from the batteryto the module
Should have a blocking diode foreach module means the diodesdont have to be rated so high
Also prevents one module sendingcurrent through the other when wehave shading
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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 forfront
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
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Module Structure
Rear surface we want:
Stops water (liquid and vapour)getting to the cells
Low thermal resistance
Sometimes have bifacial designmeaning rear must also beoptically 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 gatherand perhaps enter module
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Packing Density
How much of the area of the module is covered by solarcells?
Shape of cells determines maximum packing density Things like offcut also influence packing density
Obviously want to maximize packing density but sparselyset out cells can get a boost
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Every little helps.
We get a very slight boost from the rear surface of themodule so called zero depth concentrator effect
Some of the light incident between the solar cells isscattered in such a way that it reaches active regions of themodule we get more power
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Heat Generation
Since module is exposed to sunlight itgenerates heat as well as electricity
Typically module is converting only 10-15% of the incident power to electricity,remaining power can be largely heat
Some factors include
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
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Heat Loss
Three main ways for heat to belost from the module
Convection
Conduction Radiation
The operating point is the
equilibrium between the heatgenerated and the heat lost bythese mechanisms
If we can enhance these
losses then the operating pointwill be a lower temperature better efficiency
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Heat Loss
Convective: usually done by transferring heat to the wind
Conduction: driven by temperature gradient diffusing heat to othermaterials in contact with module
Radiation: heat is emitted due to temperature of module being higherthan the surrounds
ThAPheat
=
A is area, Tis temperature
h is heat transfer coefficient
heatPT =A
l
k
1
=k
is thermal conductivity is thermal resistance
( )44ambsc
TTP = is Stefan-Boltzmann constant is emissivity of surface
Nominal Operating Cell
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Nominal Operating CellTemperature
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:
Typically ranges between 33 C and 58 C
SNOCT
TTAircell 80
20+= Sis irradiance
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Nominal Operating Cell Temperature
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Thermal Stress
Thermal expansion is another important effect of heating ofmodules
Spacing between cells tries to increase by:
Thermal cycling of module interfaces can also lead to de-lamination
( ) TDCCG
= D is cell width, Ccentre to centre distanceG, C are expansion coeffs for glass and cell
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Electrical & Mechanical Insulation
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 for2500-10000 cycles (~160 km/h)
Hail Impact Damage: 2.5 cm diameter atterminal velocity 23.2 m/s (~80km/h)
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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
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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 surfaceby 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 tomove?)
D d i d F il
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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 interconnectstouching
D d ti d F il
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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
D d ti d F il
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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 de-lamination, cracking or electrochemical corrosion
Module glass breakage: thermal stress, wind, hail,
handling, vandalism (of course)
D d ti d F il
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Degradation and Failure
Module de-lamination: caused by reductions in bondstrength, by moisture or photothermal aging and stress,induced by differential thermal and humidity expansion.
Hot spot failures: as seen previously, caused bymismatched, cracked or shaded cells
Bypass diode failure: usually due to overheating, often due
to undersizing. Minimized if junction temperatures
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Summary
We have seen the major issues in connecting solar cellstogether 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 beenintroduced
Effects of temperature and some design features thatdetermine operating temperature were looked at
Common degradation and failure modes for moduleshave been discussed as well as ways to alleviate