Life-time Energy Yield of PV Modules

Jiang Zhu, Ralph GottschalgApplied Photovoltaics Research Group

Centre for Renewable Energy Systems Technology (CREST)

Wolfson School of Mechanical, Electrical and Manufacturing Engineering

Loughborough University

Outline

• Key impact on life-time energy yield- degradation of PV modules- reliability and durability

• Power degradation ≠ energy degradation- stresses- device responses

• Need for LEP and ALET- modelling- accelerated testing

• Summary

‘Value’ of life-time energy yield

0.5

0.6

0.7

0.8

0.9

1

1.1

YieldVariability

• The generated kWh equate to income

• Variability in energy yield comes out as a ‘performance risk’

• Typical module energy related impacts• Rating: ~±3%

• Device specific Power-to-Energy: ~±5%

• Degradation: 0.1-3% per year (>30% over life-time)

• System effects may be more important but data not available

• Looking at these impacts: degradation poses the biggest risk

Reliability and warranties

• Graph to right: Literature

analysis of reported

degradation rates.

• PV warranties are sort of

linear (linear with

exception of first year)

• Everything below the line

is a warranty case.

• But no consideration of

how this gets there.

Warranty assumption

Clear impact on RoI: ~30% of

systems will fail

In terms of LEP: only relevant

until ‘legal dust’ settles

Importance of durability

• Durability is the shape of the degradation

• Considering three shapes • Exponential (decay)

• Linear (first order fatigue)

• Inverse exponential (multi-stage, activation)

• Simplified calculations for 20 years

• Difference is nearly ±7.5% on LTE

• Enough to change investment decisions!

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

Rela

tive P

ow

er

Lo

ss

Years of Operation

Exponential Ageing

Linear Ageing

Inverse Exponential Ageing

0.85

0.9

0.95

1

1.05

1.1

E L IRela

tive E

nerg

y

Gen

era

tio

n

Ageing shape

Beyond current certification

• Certification testing in power is currently being done to

enable the manufacturer to sell product

• The EPC thinks passing this is sufficient due diligence

• Financier mistakenly believes this ensures that the

modules (or any other component) will achieve the

promised lifetime/degradation warranties

• Not testing for life-time energy yield poses a significant

risk to the RoI for the owner – i.e. end user

Take-away

Degradation poses the biggest risk

Durability is important for user

(but not really the manufacturer)

Life-time energy prediction (LEP) should be done

as part of due diligence

Stress factor and degradation pathways

Safety

Performance

InsulationVisual

Appearance

Optical

Absorption

Electrical

Conversion

Electrical

Properties

Optical

Properties

Ensemble

Chemistry

Thermo-

elec

Properties

Thermo-

mech

Properties

Atmospheric

ConditionsLightElectri

c field

Impact of degradation on power

0 5 10 15 20 25 30-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

V (V)

I (A

)

0 5 10 15 20 25 30-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

V (V)

I (A

)

0 5 10 15 20 25 30-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

V (V)

I (A

)

0 5 10 15 20 25 30-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

V (V)

I (A

)

Degradation in Isc Degradation in Rs

Degradation in Rp Degradation in Voc

Degradation in Isc (photocurrent)

Browning

cracking

Degradation in shunt resistance

PID

Hot spots

Pressure (flexible)

Degradation in series resistance

Corrosion

Loss of contacts

Degradation in Voc (diode)

Chemical changes

PID

Impurities

• Different degradation mechanisms affect behaviour differently

• Same 20% power loss, but different degradation modes

Impact of degradation on energy

Isc Rs

RpVoc

• Same 20% power loss, but different degradation modes

• Different impact at different G-T levels

Example: Significant different energy yields

Temperate climateTropical climate

• Module dependent

• Site dependent

Tropical Location Temperate

-20% PMPP -20%

-17.60% RS -8.30%

-27.10% RP -53.30%

-20.20% IPH -20.40%

-23.60% Diode -21.10%

How could a LEP model look like?

Microclimatic Data

Energy Rating Parameters

Loop over all time points.Done?

Calculate Energy

Modify ER Parameters

Still within warranty?

Energy sum Lifetime

N

N

What does this mean for LEP

• Need to understand relevant

failure modes to assess

• Need to understand realistic

environmental stresses

• Need to understand life-time of

device in realistic environments

• Uncertainty of test needs to be

smaller then that of the effect to

be observed

• Need for energy rating data

• Need to understand progression

of ageing in dependence of

stress

Standard Life-time

(reliability) testing.

Only minor additions to

a life-time test

‘just energy rating’

ALET - drive same mechanism and reduce

uncertainty

• Testing needs to be in an acceptable time

frame

- ‘real life testing’ does not work

- testing uncertainty lowers with increasing

testing time

• Acceleration-Greed results in more

expensive tests as the wrong failure mode

may be accelerated

• Appropriate number of testing samples

- current testing of two repeats tends to

underestimate failure rate

• Compromise: acceleration – uncertainty –

sample number0%

5%

10%

15%

20%

25%

30%

35%

10% 15% 20% 25% 30% 35% 40%

Pro

ba

bilit

y o

f fa

ilin

g a

ce

rtif

ica

tio

n t

es

t

Sample failure rate of distribution

testing: 2 samplesfail: one failure after retesting

Adhesive and cohesive failure

Progression of degradation

• Energy modelling requires • Irradiance and temperature response

appropriate for the model used

• Potentially changes in SR, AoI and Tmod if relevant

• The specifics of energy modelling measurements need to match up specifics of the model used. However, IEC61853 delivers data which allows extraction for all models available.

• LEP requires the same for a given time point (i.e. the current state of the device)

• ALET is thus a life-time test where an Energy Rating instead of a power rating is carried out at pre-set control points

Module temperature [oC]

Eff

ective irr

adia

nce [

W/m

2]

0.10.1

0.20.2

0.3

0.3 0.3 0.3

0.4

0.4 0.40.4

0.5

0.5 0.50.5

0.6

0.6 0.60.6

0.7

0.7 0.7 0.7

0.8

0.80.8

0.8

0.9

0.9

0.90.9

1

1

11

1

1.1

1.1

1.1

1.11.1

1.2

1.2

1.2

1.2

15 20 25 30 35 40 45 50 55 60

100

200

300

400

500

600

700

800

900

1000

1100

Module temperature [oC]

Eff

ective irr

adia

nce [

W/m

2]

0.10.10.10.2

0.2 0.2 0.2

0.3

0.3 0.3 0.3

0.4

0.4 0.40.4

0.5

0.5 0.50.5

0.6

0.6 0.6 0.6

0.7

0.7

0.7 0.7

0.8

0.8 0.80.8

0.8

0.8

0.9

0.9 0.9 0.9

0.9

0.9

1

11

1

1

1.1

1.1

1.1

1.1

1.2

1.2

1.2

1.2

15 20 25 30 35 40 45 50 55 60

100

200

300

400

500

600

700

800

900

1000

1100

1st year

5th year

Take-away

There should be a combined Life-time and ALET

testing protocol

Energy degradation is different from power

degradation and the difference could be significant

Acceleration – Uncertainty – Cost of samples

compromise

Summary

• ALET is a life-time testing + test routine and should be done jointly with LTT

• LEP/ ALET will be an important sales tool in a subsidy free market

• Life-time testing ensures low number of warranty cases, i.e. avoids unhappy customers

• LEP ensures RoI, i.e. creates happy customers or a commercial USP

• ALET will be costly but• Could be done by the manufacturer?

• Should be part of normal QA?

• Well worth the costs in terms of business development

• In the long term – cost-risk-benefits will be in favour of LEP

Thank you for your attention!