solar pv modules-university of delaware

7/30/2019 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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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

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