1. Photovoltaic Systems Engineering
SEC598F18
Photovoltaic Systems Engineering
Session 07
PV Systems Components
Solar Modules – Part 2
September 12, 2018

2. Session 07 content PV System Components Solar modules – operation
Solar modules – reliability, lifetime, etc.

3. Learning Outcomes Introduction to module science and technology
Consideration of module performance, reliability, lifetime, and decommissioning

4. PV Systems: Solar module operation
Deutsche Gesellschaft für Sonnenenergie, Planning and Installing PV Systems

5. PV Systems: Solar module operation, cont.
Reverse
Bias
Forward
Bias
PVCDROM

6. PV Systems: Solar module operation, cont.

7. PV Systems: Solar module operation
But consider the case of two diodes in series
PVCDROM

8. PV Systems: Solar module operation
Under all conditions for two solar cells connected in series (open circuit, short circuit, or in-between) and

9. PV Systems: solar module operation
For the special case of the two solar cells in series at open circuit, and

10. PV Systems: Solar module operation
For the special case of the two solar cells in series at short circuit, and

11. PV Systems: Solar module operation
Graphical solution of current flowing through two mismatched diodes in series (at short circuit)

12. PV Systems: Solar module operation
Now consider the case of ten diodes in series
PVCDROM

13. PV Systems: Solar module operation
At short circuit, and

14. PV Systems: Solar module operation
Graphical view of 9+1 solar cells in series

15. PV Systems: Solar module operation
Graphical solution of current flowing through ten mismatched diodes in series (at short circuit)

16. PV Systems: solar module operation
Use of a bypass diode
PVCDROM

17. PV Systems: solar module operation

18. PV Systems: solar module operation
(a) From the figure above, the sunny module short circuit current is roughly 5.8A and the shaded module short circuit current is approximately 3A. The open circuit voltage for both modules is close to 20V. But when the two modules are combined in series and operated in short circuit mode, then:
The voltages for the two modules are equal and opposite:
Therefore the power dissipated in the shaded module is given by:
This power is delivered to the shaded module from the sunny module, which is generating +60W. Since they are operated in short circuit, the pair generates net zero power!

19. PV Systems: solar module operation
(b)

20. PV Systems: solar module operation
(b) All three bypass diodes in the shaded module will be activated and forward biased, and each diode then will have a typical silicon diode voltage drop of 0.7 V. Therefore the three bypass diodes have a voltage drop of -2.1 V. We are still operating in short circuit conditions, so the sunny module has a voltage drop of +2.1 V.
At a voltage of 2.1 V, the sunny module will be generating a photocurrent a bit less than the short circuit current of 5.8 A, but let’s still call it 5.8 A. So the sunny module will generate a power of:
Therefore, in short circuit mode, the shaded module dissipates this same amount, 12.2W. It is substantially lower than in part (a)

21. PV Systems: solar module operation
Bypass diodes across groups of solar cells. The voltage across the unshaded solar cells depends on the degree of shading of the poor cell
PVCDROM

22. PV Systems: solar module operation - heat generation
For a typical commercial PV module operating at its maximum power point, only 10 to 20% of the incident sunlight is converted into electricity, with much of the remainder being converted into heat:
the reflection from the top surface of the module
the electrical operating point of the module
absorption of sunlight by the PV module in regions which are not covered by solar cells
absorption of low energy (infrared) light in the module or solar cells
the packing density of the solar cells
PVCDROM

23. PV Systems: solar module operation - thermal considerations
It is desirable for modules to operate at as low a temperature as possible
Solar cell output is increased at lower temperatures
Thermal cycles and stress are reduced
Degradation rates increase with increasing temperature
S.R.Wenham et al., Applied Photovoltaics

24. PV Systems: solar module operation - thermal considerations
The normal operating cell temperature (NOCT) is defined as the temperature reached by open-circuited cells in a module under these conditions
Irradiance on cell surface = 800 W/m2
Air temperature = 20o C
Wind velocity = 1 m/s
An approximate (empirical) expression for cell temperature (Celsius) is
S.R.Wenham et al., Applied Photovoltaics

25. PV Systems: solar module operation - degradation and failure modes
Front surface soiling
Dust, dissolved solids, bird droppings
Cell degradation
Decreased adherence or corrosion of contacts (increase in series resistance
Metal migration through or around pn junction (decrease in shunt resistance)
Deterioration in anti-reflection coefficient (reduced light absorption)
Deterioration of semiconductor material through atomic level processes (reduced conversion efficiency)
Module degradation
Discoloration of encapsulating materials
S.R.Wenham et al., Applied Photovoltaics

26. S.R.Wenham et al., Applied Photovoltaics
PV Systems: solar module operation - degradation and failure modes, cont.
Open-circuited cells
Cracked cells
Thermal stress
Hail, gravel, other projectiles
Damage during assembly
Short-circuited cells
S.R.Wenham et al., Applied Photovoltaics

27. S.R.Wenham et al., Applied Photovoltaics
PV Systems: solar module operation - degradation and failure modes, cont.
Open-circuited modules
Short-circuited modules
Module glass breakage
Module delamination
Hot-spot failures
Bypass diode failures
Encapsulant failures
S.R.Wenham et al., Applied Photovoltaics

28. New developments in module technology
Never-ending efforts to make solar module cheaper – reduced solar cell costs, reduced manufacturing costs
Increased effort to reduce the amount of steel and aluminum in module construction
Glass-on-glass modules are looking more favorable
Bi-facial modules, which allow light capture/absorption from both sides, are gaining favor
S.R.Wenham et al., Applied Photovoltaics

29. References for Solar Modules
Deutsche Gesellschaft für Sonnenenergie, Planning and Installing PV Systems, Chapter 2
S.Bowden and C.Honsberg, PVCDROM
S.R.Wenham, M.A.Green,M.E.Watt, R.P.Corkish, A.B.Sproul, Applied Photovoltaics, 3rd Ed., Earthscan, 2011,