Photovoltaics
Lecture 11 – Solar Modules 1
Photonics - Spring 2019
dr inż. Aleksander Urbaniak
source: wikipedia
source: Fraunhoffer Institute annual report
Production of solar modules [%]
Global annual production [GWp]
source: Fraunhoffer Institute annual report
Market share
source: wikipedia
source: Fraunhoffer Institute annual report
Production by type
source: Fraunhoffer Institute annual report
Production of thin films
source: Fraunhoffer Institute annual report
Global Cumulative Installed PV
source: Fraunhoffer Institute annual report
Production
source: enfsolar.com
source: wikipedia
Global Cumulative Installed PV
CdTe all
Interdigitated back contact solar cells (IBC) - both contacts at the back
- no shading effects - high quality materials
- electron – hole pair separation at the back
HJT / IBC
source: https://pvlab.epfl.ch/heterojunction_solar_cells
Silicon Heterojunction Technology (HJT)
Solar Modules
• require little maintenance
• water rinse 2-3 time/year
• typically no moving parts
• typical 20-30 year warranty
• 90% of initial eff. for 10 years
• 80% of initial eff. for 25 years
Losses due to
• the interconnection of mismatched solar cells
• the temperature of the module
• failure modes of PV modules
source: wikipedia
Solar Modules
• harsh environment impact
• mechanical damages
• corrosion of wires
For historical reasons, typical c-Si modules have strings of 36 cells connected in
series, yielding a V
mpunder operating conditions of 17-18V. This enables charging
of a typical battery (≥15V). As grid-tied systems become more common, this
voltage constraint is reduced.
Low-iron glass ensures good transmission of light.
• Ethyl Vinyl Acetate (EVA) flows at intermediate temperatures, encapsulating the cells.
Thin film modules often use polyvinyl butyral (PVB), which is less reactive and has lower permeability than EVA.
• Tedlar (Polyvinyl fluoride) forms an impenetrable back layer.
• Aluminum frame provides rigidity.
• Junction box provides electrical connections.
source: pveducation.org, wikipedia
Module structure
Packing density
• Higher packing fraction lowers glass, encapsulant costs per watt peak.
• Lower packing fraction increases optical concentration.
• The "zero-depth concentration effect" in modules with sparsely
packed cells and a white rear surface.
source: pveducation.org
N is the number of cells in series;
M is the number of cells in parallel;
IT is the total current from the circuit;
VT is the total voltage from the circuit;
I0 is the saturation current from a single solar cell;
ILis the short-circuit current from a single solar cell;
ISC total = ISC × M IMP total = IMP × M VOC total = VOC × N VMP total = VMP × N
Packing density
−
−
= 0 exp 1
T Ak
N qV I
M I
M I
B T
L T
One cell: V ≈ 0.6V, I ≈ 30 mA/cm2
https://www.pveducation.org/pvcdrom/modules-and-arrays/module-circuit-design
• mismatch losses are caused by the interconnection of solar cells or modules which do not have identical properties or which experience different conditions from one another.
• the output of the PV module is determined by the solar cell with the lowest output.
• when part od a module is shaded the power being generated by the non-shaded solar cells can be dissipated by the lower performance cell
• highly localized power dissipation and the resultant heating may damage the module.
Mismatch losses
Mismatch losses
source: pveducation.org
Differences in any part of the IV curve between one solar cell and another may lead to
mismatch losses at some operating point
Shading
The output of a cell declines when shaded by a tree branch, building or module dust.
The output declines proportionally to the amount of shading. For completely opaque objects such as a leaf, the decline in current output of the cell is proportional to the amount of the cell that is obscured.
Shading
source: pveducation.org
Cells in series
source: pveducation.org
Hot spot heating
source: pveducation.org
Bypass diode
source: pveducation.org
Parallel cells
Modules are paralleled in large arrays so the mismatch usually applies at a module level rather than at a cell level.
source: pveducation.org
Arrays
Potential mismatch effects in larger PV arrays.
Although all modules may be identical and the array does not experience any shading, mismatch and hot spot effects may still occur
source: pveducation.org
• A mismatch in the series connected modules will cause current to flow in a by-pass diode, thereby heating this diode, lowering its resistance → increased current → heating
→ ….
• If the diodes are not rated to handle the current from the parallel combination of modules, they will burn out and allows damage to the PV modules to occur
By-pass diodes
source: pveducation.org
Blocking diodes
source: pveducation.org
Temperature effects
• encapsulation decrease heat dissipation
• Voc(T)
• thermal expansion → degradation
Loss mechanisms depend on the thermal resistance of the module materials, the emissive properties of the PV module, and the ambient conditions.
A PV module exposed to sunlight generates heat as well as electricity.
For a typical commercial PV module operating at its maximum power point, 10 to 15% of the incident sunlight is converted into electricity.
The factors which affect the heating of the module are:
1. the reflection from the top surface 2. The operating point
• The operating point and efficiency of the solar cell determine the fraction of the light absorbed by the solar cell that is converted into electricity
3. absorption of sunlight by the PV module not covered by solar cells 4. absorption of low energy (infrared) light in the module or solar cells;
5. the packing density of the solar cells.
Temperature effects
Temperature effects
area ty
conductivi thermal
length A
k l
P T heat
=
=
=
T hA Pheat =
h - convection heat transfer co-efficien
(
cell4 surroundin4 g)
heat T T
P = −
A PV module will be typically rated at 25 °C under 1 kW/m2. However, when operating in the field, they typically operate at higher temperatures and at somewhat lower insolation conditions.
source: Electricity from photovoltaic solar cells:
Flat-Plate Solar Array Project final report. Volume VI: Engineering sciences and reliability, Ross, R.
G., Jr. and Smokler, M. I. (1986) Electricity from photovoltaic solar cells: Flat-Plate Solar Array Project
Nominal operating cell temperature (NOCT)
NOCT is defined as the temperature reached by open circuited cells in a module under:
1. Irradiance = 800 W/m2 2. Air Temperature = 20°C 3. Wind Velocity = 1 m/s
4. Mounting = open back side.
insolation T NOCT
Tcell = air + − 80
20
Thermal expansion
- αG and αC are the expansion coefficients of the glass and the - D is the cell width and C is the cell centre to centre distance.
