Photovoltaics

Lecture 11 – Solar Modules 1 Photonics - Spring 2018

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 Vmp under 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; IL is 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

1exp0TAk

N

qV

IMIMIB

T

LT

One cell: V ≈ 0.6V, I ≈ 30 mA/cm2

• 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

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

areatyconductivithermal

length

Ak

l

PT heat

ThAPheat

h - convection heat transfer co-efficien

44

gsurroundincellheat TTP

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.

insolationNOCT

TT aircell

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.

TDCX CG

The spacing between cells tries to increase an amount X:

Double interconnects are used to protect against the probability of fatigue failure caused by such stress

Other issues

• mounting • grounding the frame • heating the frame • other…