SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 1

High performance double-glass bifacial PV

modules through detailed characterization

Yong Sheng Khoo, Jai Prakash Singh, Min Hsian Saw

Solar Energy Research Institute of Singapore

National University of Singapore

29th September 2016

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 2

Outline

Introduction

Loss characterization in double-glass bifacial PV modules

Optical loss

Resistive loss

Approaches for high performance double-glass bifacial module

development

Half-cut cell and multi-busbar cell modules

Bifacial modules with IR reflective coating

Bifacial modules with selective reflective coating

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 3

Introduction

Power

Module Performance

Cost

Reliability

Levelized cost of electricity (LCOE)

Cost/kWh

𝐿𝐶𝑂𝐸 =

𝐿𝑖𝑓𝑒𝑐𝑦𝑐𝑙𝑒 𝑐𝑜𝑠𝑡

𝐿𝑖𝑓𝑒𝑡𝑖𝑚𝑒 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛=𝐶0 +

𝑀𝑡1 + 𝑖 𝑡

𝑛𝑡=1

𝐸𝑡(1 − 𝑑)

𝑡

1 + 𝑖 𝑡𝑛𝑡=1

𝐶0: initial capital cost 𝑀𝑡: annual O&M cost

𝐸𝑡 : annual energy produced

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 4

LCOE study of PV in Singapore Grid parity can be achieved either with aggressive price or innovative

technology strategy

GHI = Global Horizontal Irradiance, PR = Performance Ratio

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 5

Double-glass bifacial PV modules

Diffuse light

Direct light Bifacial module

LCOE can be reduced through

Higher energy yield (10-20% gain is achievable in outdoor

conditions by using Albedo from surroundings)

Improved reliability

Promising future module technology

illumination

EVA

glass

glass

EVA

bifacial cell

front side

rear side

illumination

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 6

Module structures using bifacial cells

air

glass

EVA

bifacial solar cell

backsheet

1 2Glass/backsheet

Power gain under

STC

Sellable module

power (STC

measurements)

Double-glass

More energy generation

in outdoor conditions by

using Albedo from

surroundings

illumination

EVA

glass

glass

EVA

bifacial cell

front side

rear side

illumination

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 7

Outline

Introduction

Loss characterization in double-glass bifacial PV modules

Optical loss

Resistive loss

Approaches for high performance double-glass bifacial module

development

Half-cut cell and multi-busbar cell modules

Bifacial modules with IR reflective coating

Bifacial modules with selective reflective coating

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 8

Optical loss: bifacial cell transmittance

Reflection (1-air-glass, 2-glass-encapsulant, 3-encapsulant-cell)

and Absorption (4-glass, 5-encapsulant): same as standard

glass/backsheet modules

Transmission (6-through bifacial cell and rear glass)

glass

encapsulant bifacial cell encapsulant

1 2 3

4

5

glass

6

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 9

Significant amount of near infrared light passes through bifacial

cells.

Double-glass structure shows a loss of ~ 1.30% compare to the

glass/backsheet structure under STC measurements.

EVA

glass

glass

(b)

EVA

bifacial cell

front side

rear side

Optical loss: bifacial cell transmittance

J. P. Singh, et al. "Comparison of Glass/glass and Glass/backsheet PV Modules Using Bifacial Silicon

Solar Cells," IEEE Journal of Photovoltaics, vol. PP, pp. 1-9, 2015.

-0.45

0.87

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

glass/backsheet

rela

tive c

hange in I

sc [

%]

double-glass

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 10

Double-glass structure:

No current contribution due to

the cell-gap region

Glass/backsheet structure:

Current gain due to the static

concentration effect of the

light incident on the cell-gap

region

Optical loss: cell-gap area

encapsulant

rear cover

air

front glass

encapsulant

bifacial cell

air

glass

EVA

bifacial solar cell

backsheet

1 2

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 11

Double-glass bifacial

modules show 3-4%

power loss compared to

glass/backsheet modules

The loss depends upon

the cell-gap

Optical loss: cell-gap area

J. P. Singh, et al. "Comparison of Glass/glass and Glass/backsheet PV Modules Using Bifacial Silicon Solar

Cells," IEEE Journal of Photovoltaics, vol. PP, pp. 1-9, 2015.

0 5 10 150.98

1.00

1.02

1.04

1.06

1.08

1.10

no

rma

lise

d m

od

ule

cu

rre

nt

cell-gap [mm]

double-glass

glass/backsheet

glass/backsheet_simulated

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 12

Resistive loss in stringed bifacial cells

Mainly caused by the current

flowing through the ribbons

Impact of resistive losses is

more on bifacial modules

Current flow pattern is

different for monofacial and

bifacial cells

High current generation due

to albedo

Higher resistive loss

compared to monofacial

S. Guo, et, al, "Two-dimensional current flow for stringed PV cells and its influence on the cell-to-

module resistive losses," Solar Energy, vol. 130, pp. 224-231, 2016

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 13

Outline

Introduction

Loss characterization in double-glass bifacial PV modules

Optical loss

Resistive loss

Approaches for high performance double-glass bifacial module

development

Half-cut cell and multi-busbar cell modules

Bifacial modules with IR reflective coating

Bifacial modules with selective reflective coating

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 14

Reducing resistive losses

Halved-cell modules

Multi-busbar modules

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 15

Reducing resistive losses

Combining the half-cut and multi-BB concepts, ~ 4% power gain is

achievable compared to standard 3-BB full-cell modules.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

1

2

3

4

5

6

4-busbar half-cut

5-busbar half-cut

6-busbar half cut

7-busbar half-cut

3-busbar half-cut

3-busbar full-cell (base-line)

rela

tive

po

we

r g

ain

[%

]

busbar width [mm]

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 16

Improving optical performance: reduce module transmittance loss Minimize the losses which

occurs due to bifacial cell

transmittance

Solution

Using IR reflective coating on the

rear side glass

Reflective coating only reflects

IR and is transparent to other

spectrum

Maintain the bifacial performance

EVA

glass

glass

(b)

EVA

bifacial cell

front side

rear side

IR reflective

coating

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 17

Reflectance measurement of IR reflective coatings

Evaluation of IR reflective coatings: Reflectance

300 400 500 600 700 800 900 1000 1100 12000

20

40

60

80

100

refle

cta

nce

[%

]

wavelength [nm]

coating type A

coating type B

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 18

EQE measurements with and without reflective coating

Current gain contributed by IR reflective coating is calculated using

the bifacial cell transmittance, cell EQE & reflectance of coating

Evaluation of IR reflective coatings: External Quantum Efficiency (EQE)

950 1000 1050 1100 115020

40

60

80

100

no IR-coating

coating type A

coating type B

glass/cell/glass transmittance

wavelength [nm]

exte

rna

l qu

an

tum

eff

icie

ncy [%

]0

5

10

15

20

25

tra

nsm

itta

nce

[%

]

300 400 500 600 700 800 900 1000 1100 12000

20

40

60

80

100

no IR-coating

coating type A

coating type B

glass/cell/glass transmittance

wavelength [nm]

exte

rnal qu

an

tum

effic

iency [%

]

0

5

10

15

20

25

transm

itta

nce

[%

]

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 19

Current gain with IR reflective coating

*double-glass (without coating) is the reference module

calculated current gain in the range of 1 %

0.930.98 1.01

1.07

coating type A coating type B0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

cu

rrent gain

[%

]

EQE-calculated

reflectance-calculated

max. theoretical current gain = 1.36 %

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 20

Improving optical performance: reduce cell-gap loss Minimize the losses which

occurs due to double-glass

design

Solution

Using reflective coating on the

rear side glass

To maintain the bifacial

performance, reflective coating is

applied in the cell-gap region

only

encapsulant

rear cover

air

front glass

encapsulant

bifacial cell

reflective coating

reflective coating

(cell-gap)

IR reflective

coating (cell)

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 21

Current gain from cell-gap area with reflective coating

Current gain for different cell-gap is calculated using the EQE-line

scan and cell-gap area information

*double-glass (without coating) is the reference module

0 2 4 6 80

1

2

3

4

5

6

full-size cell module

half-cut cell module

curr

ent ga

in [%

]

cell-gap [mm]

EQE scan

cell-gap area solar cell

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 22

Current gain from cell-gap area with reflective coating: experiments Mini-module with (9-half-cut cells) were fabricated to measure the

current gain for different cell-gaps.

*double-glass (without coating) is the reference module

0

1

2

3

4

5

6

1.78

2.87

4.18

cell-gap=2mm, string-gap=3mm

cell-gap=3mm, string-gap=5mm

cell-gap=5mm, string-gap=7mm

rela

tive

pow

er

ga

in [%

]

3 mm

5 mm

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 23

*Baseline: standard double-glass, bifacial, 3-busbar, full-size

cell module

Cell-gap= 3 mm, string-gap= 5 mm

Power gain for various module designs

baseline mod-A mod-B mod-C mod-D0

2

4

6

8

10

12 IR-reflective coating

reflective coating on glass

multi-busbar

half-cut

rela

tive

po

we

r g

ain

[%

]

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 24

Double-glass bifacial PV module made @ SERIS

Half-cut

Multi-busbar

Selective

reflective coating

(rear side)

rear

front

SERIS is a research institute at the National University of Singapore (NUS). SERIS is sponsored by the National University of

Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). 25

Thank you for your attention!

More information yongshengkhoo@nus.edu.sg