13 pv modules

ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner

Photovoltaic Modules


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


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?


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


Connecting Solar Cells

• What a solar cell does depends on it’s bias condition


• 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.

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



• 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 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


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


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


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


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


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?


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


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


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


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 VOCscales 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)


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


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


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?


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


Bypass Diode

• Put ‘bypass’ diode in parallel to cell with opposite polarity

• Diodes switch on when voltage across bad cell reaches turn on voltage


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


Bypass Diode


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


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 onemodule is open-circuited then effectively get no power from theconnected modules

• Can use similar ideas to thoseused for solar cell connections –bypass diodes

Want to bypass the ‘bad’ module in this case – what about if we have aparallel connection?


Mismatch for Modules

• 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 totalpossible current of entire parallelarray


Blocking Diodes

• 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


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


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


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


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


Heat Generation

• Since module is exposed to sunlight it generates 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


Heat Loss

• Three main ways for heat to be lost from the module– Convection– Conduction– Radiation

• 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


Heat Loss

• Convective: usually done by transferring heat to the wind

• Conduction: driven by temperature gradient ‘diffusing’ heat to other materials in contact with module

• Radiation: heat is emitted due to temperature of module being higher than the surrounds

ThAPheat ∆=A is area, T is temperatureh is heat transfer coefficient

heatPT Φ=∆Al

k1

=Φ k is thermal conductivityΦ is thermal resistance

( )44ambsc TTP −= εσ σ is Stefan-Boltzmann constant

ε is emissivity of surface


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:

• Typically ranges between 33 C and 58 C

SNOCTTT Aircell 8020−

+= S is irradiance


Nominal Operating Cell Temperature


Thermal Stress

• Thermal expansion is another important effect of heating of modules

• Spacing between cells tries to increase by:

• Thermal cycling of module interfaces can also lead to de-lamination

( ) TDC CG ∆−= ααδ D is cell width, C centre to centre distanceαG, αC are expansion coeffs for glass and cell


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 for 2500-10000 cycles (~160 km/h)

Hail Impact Damage: 2.5 cm diameter atterminal velocity 23.2 m/s (~80km/h)


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


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?)


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



• 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



• 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…)



• 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


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

13 pv modules

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