PV Mate Photovoltaic Modules for the ... - arena.gov.au · simulation mechanism for ......

PV-Mate Photovoltaic Modules for the Australian

Environment—Final Report

Compiled by: Dr Andrew Thomson, Ms Ingrid Haedrich, Jiadong (Harry) Qian and Dr Marco Ernst

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CONTENTS 1 PV-Mate Aims and Objectives ......................................................................................................... 3

1.1.1 Aims: ............................................................................................................................... 3

1.1.2 Objectives and metrics: ................................................................................................... 3

2 PV-Mate Objectives Outcomes ....................................................................................................... 4

2.1 Baseline assessment of Tindo module technology ................................................................. 6

2.2 Quantification of operating conditions specific to Australia that will effect PV performance

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2.2.1 World wide deployment of PV ........................................................................................ 9

2.2.2 Climate Classification and Comparison of PV Regions Dominating PV Installations .... 10

2.2.3 Quantification of conditions specific to the Australian climate .................................... 11

2.2.4 Cell to Module yield and minutely modelling of the impact of the Australian

Environment on Module Performance ......................................................................................... 12

2.3 Source and test new components for module manufacture ................................................ 13

2.3.1 Conventional module design ........................................................................................ 13

2.3.2 Silicon Module Concepts Presented in Industry ........................................................... 17

2.3.3 New components and component testing ................................................................... 22

2.4 Design advanced modules with improved economics.......................................................... 32

2.4.1 Summary of Improvements to multi-crystalline modules ............................................ 33

2.4.2 Improvements in mono-crystalline PERC modules ....................................................... 35

2.5 Fabrication of advanced mini-modules and testing ............................................................. 39

2.5.1 Double glass modules ................................................................................................... 39

2.5.2 Enhanced UV stability ................................................................................................... 39

2.5.3 Mini-module NOCT tests ............................................................................................... 40

2.6 Fabrication of full area modules ........................................................................................... 41

2.7 Co-develop a new module product for Tindo Solar specifically for the Aus. environment .. 42

3 Summary and conclusion .............................................................................................................. 42

4 PV-Mate Lessons Learnt ................................................................... Error! Bookmark not defined.

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1 PV-Mate Aims and Objectives The following outlines the objectives and aims of the PV-Mate project as set out in the project plan

for the ARENA R&D1 application in 2014.

1.1.1 Aims: PV-MATE aimed to design PV modules to maximise their effectiveness in Australia, proving modules

tailored specifically for Australian conditions, so that the yield is maximized and the modules survive

the harsh conditions in the Australian environment.

1.1.2 Objectives and Metrics: Specifically we set out to achieve the objectives listed in Table 1-1, including the added objectives

from a project variation which arose late 2017. All of the project objectives were achieved and are

evidenced in this report and the appended documents. The project had knowledge sharing and

education outcomes which are evidenced by public reports/publications, conference presentations,

and Lessons Learnt Reports.

A summary of the attached materials and relevance to the PV-Mate project is given in Table 1-2,

Table 1-3, Table 1-4, and Table 1-5.

Table 1-1: Project objectives and metrics. The objectives and metrics 1 to 7 are from the project plan, 8 is from the project variation.

Index Objective Specific Metrics Status and Relevant Sections/Attachments

Objective 1 Baseline Tindo module technology versus literature with WP1

Determine CTM loss, operating temperature, mini-module degradation

Complete See Section 2.1

Objective 2 Quantify operating conditions specific to Australia that will effect PV performance

Australian UV spectral component from Aus. locations, model NOCT in different regions.


Objective 3 Source and test new components for module manufacture

UV, and humidity degradation testing of: ARC on glass, financially viable


Objective 4 Design advanced modules with improved economics

Reduce cell to module loss by 40% relative on mini-module scale


Objective 5 Fabrication of advanced mini-modules and testing

Fabricate double-glass modules with 40% reduced cell to module loss, same NOCT as reference, same or better degradation rates.


Objective 6 Fabrication of full area modules

Improved performance, in terms of power and cost


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Objective 7 Co-develop a new module product for Tindo Solar specifically for the Aus. environment

Full-module passes internal testing, and extreme UV exposure with a 5% increase in power, and with potential for cost reduction of 5%.


Objectives arising from a contract variation and project variation

Objective 8 Develop a simulation mechanism for determining hot-spot temperatures

Validate simulations of half-cell and full cell module hot-spots under different shading events


The project also had a commitment to training and knowledge sharing more broadly for Solar PV topics

Objective 9 Training and Knowledge

Publications and reports Complete, see attached reports and papers

Table 1-2: Index title and relevance of Lessons Learnt Reports (LLR) accompanying this final report and their relevance to the project.

Lessons Learnt Reports (LLR)

Index Title Relevance

LLR1 Wp validation and breakdown of module gains and losses Objective 1

LLR2 Satellite versus ground mounted climate prospecting for PV applications

Objective 2

LLR3 Temperature losses of different silicon PV technology around Australia

Objective 2

LLR4 Cell-To-Module yield: A framework–for better performance prediction

Objective 1

LLR5 Suitability of ultra-violet light transparent encapsulant for use PV modules

Objective 3

LLR6 Hot-Spot Cell Temperature in Half-Cell Photovoltaic Modules Objective 8 and Objective 3

LLR7 Output loss analysis for bifacial modules by rear surface non-uniform irradiance

Objectives 3 and Objective 2

LLR8 Impact of the optical properties of a backsheet material on the power output of a solar module

Objectives 3

LLR9 Impact of shading and cell variance on module power Objective and 8 Objective 4

LLR10 CTM analysis Objective 4

LLR11 Impact of cell texture on module output Objective 2

LLR12 Electroluminescence based hot spot temperature simulation of PV modules

Objective 8

Table 1-3 Index title and relevance of Project Publications (PP) accompanying this final report and their relevance to the project.

Project Publications (PP)


PP1 Prospecting solar energy in Australia: accounting for temperature losses

Objective 2

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PP2 Location specific PV yield and loss simulation based on module stack and layout

Objective 2 and Objective 4

PP3 Impact of PV module configuration on energy yield under realistic conditions


PP4 Comparison of ground-based and satellite-based irradiance data for photovoltaic yield estimation

Objective 2

PP5 A PV Module Current Mismatch Simulation Model and Application to Bifacial Modules

Objective 6

PP6 Impact of Damp Heat and Ultraviolet Radiation on Common Solar Module Encapsulant Materials

Objective 3

PP7 Cell to module (CTM) ratios for varying industrial cell types Objective 1, Objective 4, and Objective 6

PP8 PV Module Recycling: Mining Australian Rooftops Objective 9

PP9 The Design and Installation of Solar Home Systems in Rural Cambodia

Objective 9

PP10 Photovoltaic Microgrid Business Models for Energy Delivery Services in Camps for Displaced People

Objective 9

PP11 New program for uncertainty in PCD measurements using Monte Carlo method compatible with Sinton WCT-120 lifetime tester.

Objective 9

PP12 Analysis of a Novel Approach for Calculation of J0 and τSRH from Photoconductance Measurements

Objective 9

PP13 Transparent Long-Pass Filter with Short-Wavelength Scattering Based on Morpho Butterfly Nanostructures

Objective 9

PP14 (Under review)

Two-Dimensional Electroluminescence Based Hot Spot Temperature Simulation for c-Si Photovoltaic Modules

Objective 8

PP15 (Under review)

Comparison of Half-Cell and Full-Cell Module Hot Spot Induced Temperature by Simulation

Objective 8

PP16 (Under review)

Tandem hot-spot cell simulation

PP17 (Under review)

Impact of texture on cell yield

Table 1-4: Index title and relevance of Project Associated Reports (PR) accompanying this Final Report and their relevance to the project outcomes.

Project Associated Reports (PR)


PR1 Improvement Diffused-Region Recombination-Current Analysis Objective 9

PR2 Batteryless Off-grid Voltage Source Inverter for Main Power Application

Objective 9

PR3 Accurate Outdoor I-V Measurement of c-Si PV Module Objective 2

PR4 Slicing Multi-Crystalline Silicon Solar Cells To Mitigate Resistive Losses

Objective

PR5 Improved Calculation of the J0 Parameter from QSS and Transient PCD Measurements

Objective 9

PR6 Harnessing Solar: A Solution for Rural Cambodia’s Energy Needs Objective 9

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PR7 Hydrogenation of As-Grown Defects in Crystalline Silicon Objective 9

PR8 Degradation Analysis of c-Si Solar Modules and Common Encapsulant and Backsheet Materials

Objective 3

PR9 Development of Pico & Micro Solar Power System Objective 9

PR10 Measurement and modelling the irradiance on the rear of a bifacial PV module

Objective 4

PR11 An Analysis of Manufacturing Techniques and Damp Heat Degradation Mechanisms for Crystalline Silicon PV Modules

Objective 3

PR12 Evaluation of most reliable J0 calculation method from PCD and QSSPC measurements, including uncertainty analysis

Objective 9

PR13 UV degradation effects on c-Si cells and EVA Objective 3

PR14 Systems design framework for a solar car PV array: ANU World Solar Challenge entry, 2017


PR15 Analysis of Electrical & Aerodynamic Power Performance of ANU Solar Vehicle

Objective 9

Table 1-5: Index title and relevance of Presentations and Posters (PAP) accompanying this Final Report and their relevance to the project outcomes.

Presentations and Posters (PAP)


PAP1 Impact of PV Module Configuration on Energy Yield Under Realistic Conditions


PAP2 PV module performance, reliability and the impact of installation Objective 9

PAP3 Reliability testing for the Australian market Objectives 2 and Objective 4

PAP4 A Method to Simulate the Hotspot Temperature of Partial Shaded PV Modules

Objective 8

PAP5

2 PV-Mate Outcomes Here we summarise the outcomes of our R&D into the PV-Mate objectives.

2.1 Baseline Assessment of Tindo Module Technology Tindo Solar was founded 2011 by Adrian Ferraretto and three partners. Tindo is currently the only

photovoltaic module producer in Australia. At the start of the project in mid-2014 Tindo produced

multi crystalline p-type 60 cell modules with a power output of 240 - 255 W. This is evidenced by an

excerpt from their 2014 data sheet as depicted in Figure 2-1.

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Figure 2-1: Excerpt of Tindo Solars 2014 data sheet. (Top) a picture of Tindo’s Karra 245, and (Bottom) the module IV parameters.

At this time the modules utilised 17.4% efficient multi crystalline solar cells, with 3 busbars with a

black backsheet. From the production data we have the power histograms and box and whisker

plots from a sample of Tindo’s 2014 production that use a fixed solar cell variety and efficiency bin.

From this statistical analysis we see that the average power panel produced was 244.6 W, where our

analysis would predict 244.7 W. This simple comparison of average production Wp and predicted Wp

suggest our CTM results procedure is working very well. Further we note that the variance in module

power (which we quantify here to be twice the standard deviation) is 1.4% which is less that the

module tester uncertainty.

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Figure 2-2: (left) Histogram of module power ratings of modules produced at the initial stages of the project. (right) Box and Whisker statistical plot corresponding to the data presented in the histogram on the right hand side. The data was taken from 6622 module produced in 2014 when this project was conceptualised and agreed upon for the ARENA R&D round 1.

The cell to module power loss chart is depicted in Figure 2-3where we see the gains and losses of

each of each cell to module optical and resistive effect outlined in the Lessons Learnt Report 1. In

late 2014 when this project was conceptualised Tindo’s module design was lagging behind the

industry standard as can be seen in on page 23 of the ITRPV 6th edition (2015) report. The results of

the ITRPV suggest that the industry was achieving closer to 98% cell to module ratio, where Tindo

was about 1.6% behind the industry average.

Figure 2-3: Relative cell to module power gains and losses (left), and absolute power gains and losses for Tindo module technology employed at the start of the project.

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2.2 Quantification of Operating Conditions Specific to Australia Effect PV

Performance Here we provide a simple review of the installed PV markets to date to explore operating conditions

specific to Australia that impact performance. The index codes of attached documents relating to

this Objective are: PP1, PP2, PP3, PP4 LLR2, LLR3, LLR7, LLR11, PR3, PR14, and PAP1.

2.2.1 Worldwide Deployment of PV From Figure 2-4 we see that Europe, Asia and America have dominated the cumulative installed PV

charts. Hence, to identify operating conditions that impact PV performance specific to Australia, we

will consider Australia’s climate with reference to the regions where there has been significant

cumulative deployment.

Figure 2-4: (left) a bar chart depicting regional cumulative installations in PV over the past 8 years, where we see Europe dominated the PV markets until around 2014. (right) A pie chart depicting the breakdown of installations by region in 2016.

The total installed PV in Australia is depicted in the APVI chart in Figure 2-5. From the start on of the

PV-MATE project the installed PV has increase by 70% form 3.8 GW to 6.5 GW. In 2018 it is now

predicted that solar could double the Australia renewable generation (Zhou, 2018).

Figure 2-5: Chart of cumulative Australian PV Installations, taken from http://pv-map.apvi.org.au/analyses.

A large proportion of this installation in Australia is in small scale residential systems, currently 5.2

GW. Now it is becoming clear, large scale projects are becoming financeable, and the proposed sites

for utility scale sites are no longer centralised in capital cities, but are situated close to connection

points in the network where there are good solar resources with low loss factors to a point of use.

http://pv-map.apvi.org.au/analyses

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2.2.2 Climate Classification and Comparison of PV Regions Dominating PV Installations Australia has a large and diverse climate. Climate is typically classified via a Koppen climate

classification or now more commonly the modified Koppen climate classification, the Koppen-Geiger

classification. We map the Koppen-Geiger climate classifications for different regions that have

dominated PV cumulative installations in comparison to Australia in Figure 2-4.

From consideration of the data in Figure 2-4 we can see that PV modules pre 2008 were primarily

deployed in Western Europe, where they have cool temperate climates, with some warm

temperature climates in Spain and Italy. Post 2008 and installations grew in the USA and Asia, there

is considerable diversification of climates that typical PV installations are likely exposed to. In

comparison to Australia, we see that excluding the temperate and equatorial coastal regions, a large

proportion of the country is dominated by arid climates (BSh, BSk and BWh). While there are arid

climates the USA, there is very little hot arid steppe regions (BSh) and arid hot desert regions (BWh).

Figure 2-6: Maps of the Koppen-Geiger climate classifications for: Europe (a), Norther America (b)), Asia (c)) and Australia (d)). See subplot a) for legend data generated Kottek, Grieser, Beck, Rudolf, and Rubel (2006), then revised by Rubel et al. Rubel, Brugger, Haslinger, and Auer (2016) the data can be downloaded for viewing in mapping software such as Google Earth from (Climate Change & Infectious Diseases Group, 2017).

However climate, as described by the Koppen-Geiger classification does not adequately describe the

impacts of the environment on PV module operation. This is because the Koppen-Geiger climate

classification is based on average temperatures and rainfall, factors that greatly impact the local

biome. In general, similar Koppen climates occur on similar latitudes, and receive similar solar

irradiance which drives the terrestrial temperature and rain fall. For PV performance, the irradiance

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and quality has the greatest impact on production, temperature and rainfall are secondary, but

important effects. In time, we will see purpose built PV climate classification, where the mapping

enables selection of modules that mitigate location specific risks, and development of improved

contractual understanding of performance in different regions. There are many interesting works on:

degradation (Dirk C Jordan & Kurtz, 2013; Dirk C Jordan, Kurtz, VanSant, & Newmiller, 2016; Dirk C.

Jordan, Silverman, Wohlgemuth, Kurtz, & VanSant, 2017; Kambe et al., 2013; Leske, 2016; Miller et

al., 2015; Peike et al., 2013), soiling (Costa, Diniz, & Kazmerski), temperature and spectrum (Morley,

Ghiotto, Boppana, Frederick, & Reynolds, 2017; Skoplaki & Palyvos, 2009), which could be brought

into this.

To demonstrate where a simple example of where the Koppen classification is not suitable for PV we

compare insolation for temperature regions. Qualitatively comparing like for like temperate regions

in Europe (Milan), Asia (Guang Zhao ) and America (Atlanta), there are temperate regions of NSW

that receive 40% to 100% more direct radiation (these figures were found in (NASA, 2018)). These

temperate regions in Australia are important as they are heavily populated regions of Australia’s

eastern sea board. The Arid climates in Australia (labelled BSh, BSk and BWh) can also be found in

parts of the USA. Regions like Arizona have similar climates and insolation compared to Australia.

The reasons for the higher direct insolation are subtle however are well discussed in the work of

Gies et al. (2004). They note that the Earth is approximately 3% closer to the Sun in January (the

southern summer)and coupled with atmospheric conditions (low prevalence of could cover and low

fraction of aerosols), give the Australian continent on averages a higher direct normal irradiance

component.

The high direct normal irradiance exacerbates the exposure to UV radiation. Further the depletion of

the ozone effects further increased UV exposure. In total, Gies et al. (2004) suggests these effects

combine to UV radiation levels 12%-15% for geographical locations in the southern hemisphere

compared to similar regions in the northern hemisphere. While it appears that the ozone depletion

is now being reversed (Várai, Homonnai, Jánosi, & Müller, 2015) it is unlikely that it will be fully

reinstated for another 50 years (Newman, Nash, Kawa, Montzka, & Schauffler, 2006).

As noted in the in Section 2.2.2 module most regions in Australia receive high levels of direct

irradiance. This is sunlight that is largely unscattered. The spectrum of this unscattered light has

higher UV components.

2.2.3 Quantification of conditions specific to the Australian climate Hence in this review when considering climates specific to Australia, although the bulk of the

Australian land mass is made of Arid regions, we focus on temperate regions with high direct

irradiance as being locations specific to Australia.

For establishing conditions unique to Australia, in this work we focus on the hot arid steppe climates

for two reasons:

1. It is the second largest climatological region in Australia.

2. It is a region where most of the new large scale PV plants will be installed by ARENA,

primarily owing to the fantastic solar resources.

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2.2.4 Cell to Module yield and minutely modelling of the impact of the Australian

Environment on Module Performance Through the PV-Mate project we have developed a procedure that enables estimation of module

performance in a given location. The software used accounts for:

Irradiance:

o Angle

o Intensity

o Wavelength

Temperature:

o Wind speed

o Module thermal construction

o Ambient

Electrical losses:

o Non ideal voltage loss

o Resistive losses

Hence it can provide relative yield for modules, based on the material components. In our work so

far we have utilised BOM weather data, over 400 GB of minutely data entries from 16 weather

stations located in Australia, where historical data from the previous 20 years was employed to

mitigate the impact of yearly variations. A map of the sites studies is given in Figure 2-7.

Figure 2-7: Map of BOM weather stations where we have employed our yield analysis.

From the analysis we can breakdown the energy gains and losses owing to the module construction.

This analysis is similar to the cell to module loss we presented in LLR2, LLR4, PP1, PP2, PP3, PP4, and

PAP1, expanded to account for the aforementioned regional specific effects. From this analysis we

have the energy breakdowns for modules, two examples are plotted in Figure 2-8. Clearly one of the

most significant losses is that of temperature.

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Figure 2-8: CTMy energy gains and losses for two example modules using the weather data from Alice Springs.

We compare the performance of 4 module designs, accounting for local effects for 16 Australian

sites.

2.3 Source and Test New Components for Module Manufacture This section of the report gives background on module design and components, and then presents

the work undertaken in this project, trialling new module materials.

2.3.1 Conventional Module Design PV cells are packaged to from modules, where the cells are strung together (typically in three series

connect strings of 20 to 24 cells with bypass diodes) and encapsulated in a sandwich of glass, ethyl-

vinyl acetate (EVA), cell, EVA and backsheet, as depicted in Figure 2-9. The purpose of the module

components are as follows: the glass provides a rigid transparent cover that prevents impact

damage; the EVA provides mechanical and thermal stability, and optical coupling; the rear backsheet

restricts moisture and chemical ingress, and protects the cells from mechanical impact; and a

junction box to allow electrical interconnection. The module must be resistant to environmental

exposure and ideally allow maximum light into the solar cells while shedding as much heat as

possible.

This combination of materials has not altered significantly since the mid-1980s following the Flat-

Plate Solar Array Project, an 11 year long project (Cuddihy, Coulbert, Gupta, & Liang, 1986), that

recommended the aforementioned material combinations, after testing a plethora of combinations

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and the impact on durability and lifetime. An excellent summary of the technological progress in this

area is described by Green (2005).

Figure 2-9: Diagram of a c-Si module, indicating the purpose of each sub component.

In the PV-Mate project we have not deviated from the conventional flat-plate module design as

depicted in Figure 2-9. We have considered how the design can be further improved. We now review

literature regarding module component development trends that have been observed and in some

cases presented in literature. The following sections provide a brief literature review of PV module

components.

Glass and front covers

There have been tremendous advances in the development of optical coating for solar glass in the

past (Deubener, Helsch, Moiseev, & Bornhöft, 2009; Pan, Zhang, Xu, Feng, & Verlinden, 2015).

Optical coatings on glass reduce front reflection by between 1.5% and 2.8% depending on the

coating technology and the coating material and the processing (Ballif, Dicker, Borchert, & Hofmann,

2004). Importantly, the AR coatings have been shown to improve yearly yield, as the technologies

have been found to improve yield by 3.4%-3.7% as they modify incident angle losses and thermal

energy balances (altering the cell temperature) in the module.

A critical issue with these coatings is their durability in the field. Initially, the value of antireflection

coatings was considered by some to be limited, as in some cases the films would wear off. However,

the loss in performance was acceptable with regards to the power warranties of the product

(Köntges et al., 2014). This sentiment has changed as the technology has evolved, where we see

most modules are currently produced with the coatings (See Figure 11 of International Technology

Roadmap for Photovoltaic, 2017), however there is still some work to do as the expected lifetime of

the coatings is in the order of 10 years (See Figure 12 of International Technology Roadmap for

Photovoltaic, 2017).

Further, issues of soiling prevention have been hot topics for coating producers, where many

producers claim the optical layers have surface functionalities that promote cleaning or indeed

Cover glass(Protection + lets sunlight through)

Encapsulation foil(Protection against moisture + UV)

Solar cells(Converts sunlight into electricity)Consider bi-facial cells

Back sheet(Electrical isolation + weather proofing)

Junction box(Electrical interconnection)

Aluminium frame(Structural support + attachment point)

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mitigate soiling. This is an exhaustive topic and an excellent review of soiling and technical solutions

can be found here (Costa et al.).

As part of the project we investigated graded anti-reflection coatings developed by Brisbane

Materials (Work described in LLR12). While the technology was novel, its performance was not

significantly better than that currently in production.

Encapsulant

Encapsulant is the material that binds the module together. Typically they are a thermal cross-linking

material but in some cases they are thermo-setting and are not cross-linked (M. Kempe, 2011).

There are a plethora of material properties that are important for an encapsulant layer to maintain

in order for them to work effectively. It is important that the layers have a high optical transmission

in order to minimise the module optical losses. In addition, they must have reasonable mechanical,

electrical and chemical properties so they inhibit corrosion and do not cause too much thermo-

mechanical fatigue.

The encapsulant is a critical element for module durability and performance. Issues of encapsulant

browning, and reduction of transmission are important (M. D. Kempe, 2010; McIntosh, Powell,

Norris, Cotsell, & Ketola, 2011; Pern, 1996), but seem to be largely mitigated (Dirk C. Jordan et al.,

2017). Whereas a larger proportion of failures stem from cell breakage and causing hot-spots,

something that future encapsulant developers may work toward mitigating. One trend that is

currently observed is the shift to UV transparent EVA which will be discuss in the latter sections of

this report (Malte R Vogt et al., 2016; Malte Ruben Vogt et al., 2017; Robert Witteck et al., 2017).

Cells

The solar cell is the active region of the PV module that generates electricity. It is the most critical

element of the PV module, where almost all of the product value is leveraged. While there have

been significant improvements in cell performance over the past 20 years the cell design remains

relatively unchanged. All of the processes have been thoroughly improved and the cost reduced

such that cells with efficiency in the order of 18% at a cost of 25c/Watt are common place. A number

of manufacturers have developed high-efficiency n-type cell technology (International Technology

Roadmap for Photovoltaic, 2017). The key cell design that the industry is transitioning to is the

passivated emitter and rear (PERC) design of Blakers et al. (Blakers, Wang, Milne, Zhao, & Green,

1989). This is being achieved in industry thanks to the development of passivating films that were

first presented in research in 2007 by Hoex et al. (Hoex et al., 2007), and an extensive research and

development drive by the industry. The processing technology to deploy these layers has finally

matured to the point where they are being deployed readily throughout industry, presently and over

a 10% share of the of the market with projects to grow to 50% by 2024 (See Figure 41 in

International Technology Roadmap for Photovoltaic, 2017). PERC solar cells operate at higher

efficiencies and also reflect more sub-band illumination, which results in the cells running cooler,

and has quantifiable performance improvements (Malte Ruben Vogt et al., 2017).

Cell size significantly jumped in the 2000s where 5” wafers were replace with 6” (156 mm)

technology. Recently there has been some movement for producers to incrementally increase the

cell size where 156.75 mm cells are relatively common, some producers are even offering 161 mm

wafers. One challenge of working with such large wafers, and particularly as the cell absorption

increases, is cell current density and total cell current increases therefore increasing the resistive

(I2R) losses. With a conventional module, designs lose in the order of 4% power from the cell

connections. To combat this, producers have looked at the half cell module. This is where the 6” cell

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is segmented into two pieces (R. Witteck et al., 2016). Such a design halves the current, and can

effectively quarter the resistive losses. One trend that has not played out in recent years is the

further reduction in wafer thickness, for the past 5 years or so the wafer thickness has been

maintained at 200 um or so. It appears that the benefits of using less silicon are currently

outweighed by the additional costs associated with yield reductions and/or increased wafer handling

costs. There are a number of promising research initiatives that are in progress to solve these issues.

They seem attractive as presently the wafer thicknesses employed in industry do not limit cell

efficiency. This is evidenced by modelling (Kowalczewski & Andreani, 2015) and some outstanding

cell efficiency results achieved on the 45 to 165 um thick range (Yoshikawa et al., 2017). However,

the cost of solar grade silicon has plummeted in the past decade, hence the market drive for thinner

cells has diminished. This is particularly true as the conventional module design, as depicted by

Figure 2-9 may simply not be suitable for cells significantly thinner than 200 um.

Bifacial cells are modules of interest to producers because diffuse light can be collected from the

rear of the module, which in excellent conditions can boost power production by 20%. There are a

number of complex issues with market acceptance of this technology that are being dealt with by

researchers and particularly by the IEA PVPS Task Group 13 (International Energy Agency:

Photovoltaic Power Systems, 2018). For a long time technology developers have sought to have the

value of the bifacial power recognised (Stein, Burnham, & Lave, 2017), so that the Wp rating of the

module reflects the added generation from bifacial technologies in the field. Anecdotally, cell and

module producers still mistrust these processes. My opinion is that technically bifacial modules

present an excellent opportunity to improve PV power plant capacity factor and utility. The key

barriers for acceptance seem not to be economic from the technological point of view but are more

systemic. If the banks and developers do not buy into the technology, large volumes sales willonly

occur when additional cost for bifacial is negligible based on the front measured STC Wp.

Cell stringing and interconnection

The interconnection of cells, has been rapidly improved over the course of the project. There was a

significant shift in cell interconnection technology with the multi-wire interconnections (Braun,

Hahn, Nissler, Pönisch, & Habermann, 2013; Green, 2011). The apparent benefits of multi-wire

technology are slowly being eroded as the number of conventional busbars increases. At the start of

the project, 3 busbars were the industry standard, now 4 busbars are common and many producers

are working on 5 and 6 busbar cells. Increasing the number of busbars increases the complexity of

the string and soldering process, however they mitigate resistive losses, and have the potential to

prevent or mitigate degradation due to cell cracking. Multi-wire cell designs show impressive results

for reduced silver consumption (Braun et al., 2013), however pressure on this aspect of design is

subdued owing to a weak silver price (macrotrends, 2018), compared to the prices circa 2011.

There have been a few producers touting structure ribbons or painted wiring such that they improve

light collection form the busbars (Holst et al., 2016; Muehleisen et al., 2016). While the efficacy of

such products have been demonstrated in literature (Zhang et al., 2016),we are not aware of the

commercial deployment of these products.

Backsheet and Backcover

The back cover of a module must electrically isolate the module circuitry, provide mechanical

protection for the cells so they are not damaged too readily in transport and limit chemical ingress

such that degrading chemical reactions do not progress too rapidly. Most common module back

17

covers are made from polymer layers comprised of PET, and TedlarTM a proprietary fluorinated

polymer that has been employed in industry for the past 4 decades (Cuddihy et al., 1986).

A number of different backsheet materials were tested at NREL, (G Jorgensen, Terwilliger, Glick,

Pern, & McMahon, 2003). They measured the WVTR of different materials, and then measured the

adhesion strength of different material combinations after accelerated testing with UV and damp

heat. TedlarTM PET and EVA layers, the industry standard performed well with UV and were

outstanding with damp heat exposure.

When considering low WVTR materials, obviously glass is excellent. However Jorgensen’s results

suggest caution as they suggest that the glass backsheet promoted aluminium corrosion, where they

believe that compounds formed during damp-heat exposure could not escape (G Jorgensen et al.,

2005). There were more findings from this work (GJ Jorgensen, Terwilliger, Kempe, & McMahon,

2005). Metrology has been developed for backsheet adhesion measurements at NREL (Tracy, Bosco,

Novoa, & Dauskardt, 2017) and standards on adhesion have been quantified (Bosco, Kurtz, &

Eafanti, 2017). This is an expanding area of research, where researchers are attempting to better

predict the durability of material combination.

Final word on materials

Development and deployment of new module materials is a continuous process, as the growing 100

GW/pa industry develops (Finance, 2017). It is an extremely competitive market where all producers

have to reduce costs in the hopes of making a profit. To give context to how the market has

developed, the relative costs of module components are compared in Table 2-1 from 2012 (Goodrich

et al., 2013) and the end of the project. From these material costs we see that cell costs dominate

the module costs. With further analysis we can determine the cost modularisation also in Table 2-1.

Currently all the predictions are for low costs as the industry develops. Where there is still plenty of

room to move for efficiency improvements and then development of cost reductions of these

processes.

Table 2-1: Summary of component costs in 2012 and 2018. The 2012 data was taken from (Goodrich et al., 2013), the 2018 costs taken from ENF solar’s module material directory (ENF Solar, 2018), the comparison is not ideal as the Goodrich’s data looks at production in the context of production in the USA, however the raw materials are a reasonable comparison.

Unit 2012 US$/W 2018 US$/W

Module $ 0.85 $ 0.33

Sub components

Cell $ 0.77 $ 0.22

Glass $ 0.11 $ 0.023

EVA $ 0.06 $ 0.012

Backsheet $ 0.06 $ 0.012

2.3.2 Silicon Module Concepts Presented in Industry Here we review and discuss the current module concepts under development by the solar industry.

These are summarised in Table 2-2. These are the modules observed at recent trade shows, research

visits and conferences over the course of the project. The different module features according to

different design categories can obviously be combined achieving different features. We will now

explain the different module designs and examine different claims on the module performance.

The 6” cell, 60 cell module wafer is the work horse of the PV industry. With typical Wp ranging from

240 Wp to 340 Wp (ENF Solar, 2018), the module are comprised of 3 20-cell strings with 3 by pass

18

diodes. With typical dimensions 1 m × 1.7 m. In 2016 they made up 69% of the market, however

they are being superseded by the larger models the 72 and the 96-cell modules, with power ratings

and ranges increasing by 20 and 40% respectively. ITRPV predict that the by 2020 there will be 50/50

split between 60 and 72-cell modules, and 96 cells will comprise 4%-5% (International Technology

Roadmap for Photovoltaic, 2017). The desire to transition to larger modules comes from both the

system developer and the manufacture. Developers pay per panel install, so providing the module

weight does not become impractical large modules increase productivity per lift/install/clamp

tighten. They mean less module connections and a lower level of wiring complexity which reduces

cost.

Issues with the deployment of larger modules are the weight, and wiring design. In general we find

that the continuous reduction in glass thickness that the industry has transitioned to has made the

deployment of 72 cell modules more feasible.

Almost all cells produced by the industry are p-type, and most are multi-crystalline (International

Technology Roadmap for Photovoltaic, 2017). This is a phenomenal achievement by the industry to

continually develop the materials and the processing such that multi-crystalline cells can achieve

comparable performance compared to mono-crystalline modules.

19

Table 2-2: Common module concepts under investigation, the description of the design change, qualitative technical impacts and datasheets and references.

Feature Common Configuraitons Description Technical Impact Datasheets/References

Size 60 cell 75 cell 96 cell

Different module sizes are employed using between 60, 72 and 96 6” solar cell.

Can result in cheaper costs. Lower balance of systems costs, lower balance of plant, and slightly lower production costs. Could have reliability issues with more cells per by pass diode, and higher static and dynamic load issues

(Paso Valley Solar, 2018; Q-Cells, 2018; Yingli Solar, 2018)

Mono/Multi-Crystalline

Mono Multi/Poly

Either mono crystalline or poly/multi crystalline wafers employed in production of cells

Not much product differentiation. Mono cells tend to be higher efficiency, but this is largely equalised in modularisation. Performance is slightly different, but is small.

(Trina Solar, 2018a, 2018b)

Aluminium BSF/PERC

Aluminium BSF PERC

The rear of the solar cell is contacted in a more complex manner in PERC solar cells to increase voltage.

A significant increase in efficiency. Lower balance of systems costs are traded for increase cell fabrication costs. The performance of PERC should be better in the field for yield but may suffer from different degradation mechan

(Canadian Solar, 2018a, 2018b; Hannebauer, Dullweber, Baumann, Falcon, & Brendel, 2014)

Mono-facial/Bi-facial

Mono-facial Bifacial

The cell backside is optically transparent which enables useful absorption from the rear of the device.

Higher yield installed compared to like for like mono facial, yield increases in the order of 5% to 20% seem are achievable. This advance is traded off against non-standard cell processing.

(Trina Solar, 2018c, 2018d)

20

Polymer backsheet/ Glass backsheet

Polymer Glass backsheet

The rear polymer backsheet is replaced with a glass backsheet

Modules are heavier, but are touted as being more durable. Cells are in the neutral stress zone. Similar costs per module. The industry standard is 2 times 2.5 mm sheets of glass

(Trina Solar, 2018e)

Ultra-thin glass, with white ceramic inserts double glass

([email protected], 2012)

Thinner glass sheets are employed, the glass sheets may be chemically tempered.

The white ceramic inserts negate the need for white EVA which is challenging to laminate.

(Almenden Solar, 2018)

Half Cell

Cells are segmented, generally the cells are configured in two parallel strings resulting in similar IV parameters compared to conventional modules.

Improves performance for additional cell handling. The resistive losses are reduced and optical coupling is improved. Secondary benefits such as cooler hot-spot temperatures seem plausible.

(Zhang et al., 2016), PP3

21

Multi wire

(Söderström, 2014)

The solar cells are connected by many metal wires >10/cell, compared to the metal ribbon connections, typically 4-5/cell

Lower cell shading and improved optics due to the shape of the wires. Improved current transport. Complexity of stringing increase, and some of the processes are patented. Secondary effects like mitigation of the impact of cracks are plausible.

(Söderström, 2014; Walter, Tranitz, Volk, Ebert, & Eitner, 2014)

22

2.3.3 New Components and Component Testing The key components that we are considering for advance module fabrication in this project were 2.0

mm tempered glass, 12 different encapsulant materials, PERC solar cells, segmented solar cells,

busbar tape, and 3m reflector tape.

2.3.3.1 Backsheet Reflectance and Gap

At the start of the project, Tindo used a black backsheet. In the early stages of the project this was

swapped for the white backsheet. Our project discussion on cell to module losses and module design

prompted this change. Using Tindo’s production data this resulted in a 2.5% improvement in their

module power. This change has been implemented into current production. This improvement was

adopted in the early stages of the project and we demonstrate the impact of backsheet on the

current Tindo module design as they produce runs of black modules for special orders. We

demonstrate this with 2017 production using 18.6% efficient solar cells, the benefit of the white

blacksheet is depicted in Figure 2-10. We note some customers prefer the aesthetics of the black

backsheet over the performance gain.

Figure 2-10: Box plot comparing the impact white backsheet on module STC power. We see on average a 2.5% increase in module power from 263.7 to 270.2, with 18.6% efficient solar cells.

Following the move from black to white backsheet and CTM analysis of their Karra 250 module to

improve the module power output at Tindo, we demonstrated the impact of an increased cell gap.

Initially Tindo employed a 2.5 mm cell to cell gap, now, following PV-Mate recommendations Tindo

have increased the cell gap to 4 mm. Our modelling showed that an increase of the cell gap from 2.5

to 4 mm would result in a 0.9% increase in Isc as depicted in Figure 2-11 a). On the production line,

we found excellent agreement with our mini-module test result.

To determine the improvement realised in production, two groups of modules were compared, both

fabricated from 18.0% efficient multi-crystalline solar cell, the first group, with a sample size of 3743,

260

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White BlackBacksheet Color

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23

had Tindo’s standard 2.5 mm gap, the second group, with a sample size of 2652, had a gap of 4 mm.

These modules were produced in 2015/2016. The STC power results of these modules can be seen in

Figure 2-11 b). From this boxplot, we can see the average power has increased from 257.6 to 262.4

W, a relative increase of 1.71%. We extend the results further to include production results for cells

with efficiencies 17.8% and 17.9% in Figure 2-11 c). Similar analysis of large amounts of data (sample

size above 2000 modules for each group), finds a similar improvement in the ranges 0.7% to 1.7%.

We note that the measurement uncertainty is significant compared to the gain observed, hence to

be confident that this improvement is transferred to the module level, tests such as this, with good

statistics give excellent confidence in the results. The absolute power results from Figure 2-10 and

Figure 2-11 b) should not be directly compared as they used groups of input solar cells with different

efficiencies (18.6% vs 18.0%, a 0.6% absolute difference but a 3.3% relative difference). Later when

the design improvements are summarised we control for the impact of cell efficiency.

a) b)

c)

Figure 2-11: a) ANU tests of the impact of cell gap on module Isc and hence performance for varying cell gap. b) Boxplot comparison of modules produced with 18.0% efficient solar cell, with 2.5 mm and 4.0 mm gaps. C) Average perfromance imporvement due to the increased cell gap across 3 cell effiicency bins, where the data was taken from approximately 10,000 production modules.

y = -0.0002x2 + 0.0067x - 4E-06

8.44

8.49

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17.7 17.8 17.9 18 18.1

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Cell Group Efficiency (%)

24

2.3.3.2 2.0 mm tempered glass supplied by Asahi and 2.8 mm glass on production module

There were a number of challenges sourcing thin tempered glass. No international supplier aside

form Ashahi were offering 2.0 mm tempered solar glass. However, upon testing and installation into

the production line full area module testing, it was clear that the glass was not sufficiently

toughened, it was so-call annealed or semi-tempered glass. This was evident when the glass

shattered upon loading into the glass handling robot at Tindo’s production facility in Adelaide. It

shattered creating large shards of glass.

Further shipping and handling of a module with a glass backsheet, cracked, this may be preventable

if the module is adequately framed, but further tests are required. We have been pursuing the

2.0 mm glass as it will result in a module weight similar to that of conventional modules.

In addition, we also trialled the Ashahi Leoflex glass. Leoflex is a toughened glass product that ranges

in thicknesses in the order of 1.1 to 1.8 mm thick. We found that when applied to the mini-module,

the optics was comparable to conventional glass products. However the residual stresses from the

lamination caused the glass to shatter. Since these tests it appears the product has some degree of

commercial success (Light Joule, 2017), however the project did not have the resources to further

develop this work. We note there have been recent announcements of glass production lines that

can produce fully tempered 2.0 mm glass, which may be a new pathway for further developing this

technology in the future (Kenning, 2017).

2.3.3.3 Encapsulants

We have degradation tested 10 encapsulants for advanced modules. The encapsulantsare listed in

Table 2-3.

Table 2-3: List of encapsulant tested for use in advanced modules.

Product Manufacture Functionality Comment

EVASKY Bridgestone Standard UV blocking

EVASKY-experimental Bridgestone UV transparent Not currently offered commercially

EVASKY-white Bridgestone White Not currently offered commercially

520.68 Vista Standard UV blocking

520.78 Vista UV transparent

E2FN SKC Standard UV blocking

E2FT SKC UV transparent

HFT01 Novopolymer Standard UV blocking Non-conventional, designed for double glass

HFU01 Novopolymer UV transparent Non-conventional, designed for double glass

HFW01 Novopolymer White Non-conventional, designed for double glass

To further improve the performance of Tindo’s module we investigated the performance of UV

transparent EVA in the modules. In this case, we sampled different encapsulant materials from a

range of suppliers, and their impact on mini-module short circuit current (a factor which in this case

is proportional to power output). In this work, we measured it was possible to achieve a 1%-3%

improvement in Isc with reference to the non-UV transparent materials, the results of this summary

25

are presented in Figure 2-12. UV transparent material was trialled in full sized modules. It was

employed in 4 double-glass modules, 2 of which used multi-crystalline cells and 2 which used mono-

crystalline PERC cells.

Figure 2-12: Summary of the impact of UV transparent materials on Isc improvements for multi-crystalline silicon mini modules produced in this project.

All materials were tested in either conventional or double glass mini-module setups. They were

laminated with a conventional lamination procedure. This resulted in the following.

All new encapsulants that were laminated with a backsheet modules worked perfectly

without visual defects.

There were a number of visual defects with the double glass modules.

To elaborate on the issues with double glass module lamination it was necessary to make full size

modules, which are depicted in the photographs in Figure 2-13.

a) b) c)

Figure 2-13: Photographs of full size double glass modules fabricated in our last trip to Tindo Solar,

0.0%

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26

To improve the double glass lamination procedure we reduced the lamination pressure, this seemed

to mitigate the overflow of EVA on top of the cells and interconnections, however on detailed

inspection, counting the number of white EVA overflow occurrences there was little difference

between the laminations.

We find that the double glass lamination leads to a spreading of the cells during lamination, more so

than is for the standard lamination, as depicted in Figure 2-14, where the distance between cells was

measured at 15 points over the entire module. The cell separation is increased for the double glass

modules; on average by 1 mm. Improvement of the lamination to mitigate cell separation is ongoing.

Figure 2-14: (Left) sketch of a 60 cell module with fixed cell spacing, (middle) contour plot of measured cell separation after lamination for the standard module, (right) contour plot of cell separation for the double glass module.

The EVA samples were subject to extreme UV exposure, to assess if they were UV stable. They were

exposed to 6 weeks of UV in a Q-sun Xe-1. This is roughly equivalent to 6 times the IEC61215

exposure dose. The results of this UV exposure were that all the samples with UV blockers did not

degrade whereas the samples that had no UV blockers did suffer some reduction in transmission.

The results for all the materials are plotted in Figure 2-15. Summary results are presented in Table

2-4. We find there is a 1.6% to 4.3% reduction in transmission for the UV transmitting samples, and

for the standard samples mixed results but all of a smaller magnitude.

Position (cm)

Posit

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))

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27

Figure 2-15: (Top) Transmission of different encapsulant materials as cured and (Bottom) after UV exposure. While the differnt functionalities of the encapsulant brands is not labeled it can be observed by the cutoff.

Table 2-4: Relative changes in transmission after UV exposure, equal spectral weighting 250 to 1150 nm. .

Standard UV transparent WTE

Bridgestone 0.81% -1.87% 2.23%

SKC 1.05% -4.35%

NP -0.35% -1.63% 15.23%

Vista -0.03% -1.77%

Similarly the same tests were performed for damp heat exposure. In this case the samples were

exposed to 85 °C/85% relative humidity for 7 weeks or roughly 1000 hours as is the IEC61215

standard. In this case there was no real trend between encapsulant variety, results were general

more severe that the UV exposure which is typical. Interestingly some UV transparent samples

degraded significantly and some were improved. This observation warrants further investigation.

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BridgestoneBridgestoneBridgestoneSKCSKCNPNPNPVistaVista

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Figure 2-16: (Top) Transmission of different encapsulant materials as cured and (Bottom) after DH exposure. While the differnt functionalities of the encapsulant brands is not labeled it can be observed by the cutoff.

Table 2-5: Relative changes in transmission after DH exposure, equal spectral weighting 250 to 1150 nm. .

Standard UV transparent WTE

Bridgestone -1.47% -1.23% 8.31%

SKC -2.52% -3.45% NP -1.47% 1.65% 6.20%

Vista -2.79% 2.74%

10 encapsulants were tested for use in improved modules. They were degradation tested with an

extreme UV dose, and exposed to damp heat for 7 week. Significant reductions in transmission were

observed upon UV exposure for samples without UV blocking. The results for DH exposure were

mixed. Interestingly some samples without UV blocking degraded and some improved. The results of

UV exposure run contrary to the work of Miller et al. where they show a material without UV

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29

blocking not degrading while samples with UV blocking degrade (Miller et al., 2015), hence it is clear

that the mechanisms of the degradation is complicated and affected by the specific material

composition and treatment and not only the presence of UV blocking.

2.3.3.4 Busbar Tape

The project explored the application of busbar tape to improve the reflectivity of the solar cell

ribbon. This solution was not implemented on the full-size module; it was however demonstrated on

the mini module level. The total area or the ribbon atop the cells for Tindo’s 3 busbar design was

3×1.2 mm × 156 mm or 5.6 cm2 or 2.3% of the total area of the cell. When we applied the busbar

tape to the cells this resulted in a 1.7% increase in the multi crystalline cells Isc but for the tests with

the mono-crystalline cells only a 0.2% increase was observed. Hence, without larger statistics

through the application to production modules we conservatively assume that the improvement in

the order 0.2% is feasible. Mini-modules fabricated with and without busbar tape are depicted in

Figure 2-17 a) and b). The impact of busbar tape on Isc and relative module performance is given in

Figure 2-17 c) and d).

30

Figure 2-17: Photograph of mini-module produced with a) and without b) white busbar tape. Measurements of the impact of busbar tape on module Isc c) and as summary of the relative gain for both multi and mono crystalline modules d).

2.3.3.5 PERC Solar Cells

We have examined the use of more transparent EVAs. When used in combination with passivated

emitter and rear contact (PERC) cells, we can achieve a 1.5%-3% gain in performance. The PERC

module improvement is detailed in Section 2.4.2.

8.1

8.2

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BBT CONTROL

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a) b)

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31

2.3.3.6 4 Busbar Solar Cells

We also considered the use of cells with additional busbars, as depicted by mini-modules

photographs in Figure 2-18 a) and b). At the start of the project, three busbars in the modules were

commonly employed. We considered the value of increasing the number of busbars from 3 to 4. We

worked with Tindo, on the selection of a new stringer tabber1 that was suitable to string 4 busbar

cells. First, we computed the reduction in resistive losses that should be observed in production, for

18.0% efficient cells we found this would lead to a 2.3 W or 0.9% improvement in module power as

depicted in Figure 2-18 where plot c) and d) compare the CTM chart for an 18% efficient cell with 3

or 4 busbars respectively. After the tabber was upgraded and 4 busbar modules were adopted into

production we found that the average power of 3 and 4 busbar modules was in approximate

agreement with the computed improvement. In production average power was found to increase

from 261.7 to 263.5 W, indicating a 0.7% improvement. This improvement was realised in

production, on 1000s of full size modules.

1 A stringer tabber is the tool used to wire together solar cells. It soldiers ribbon (flat tin coated copper wire) to

the top and bottom of cells, connecting the cells in a string, top to bottom, to build voltage akin to a battery pack.

32

Figure 2-18: Photographs of 3 and 4 busbar mini moduels, a) and b) respectively. CTM calculations for full-sized module with 3 and 4 busbar cells, c) and d) respectively. Plot e) depictes the change in moduel power measured in production for 3 and 4 busbar modules.

2.4 Design Advanced Modules with Improved Economics

a) b)

c) d)

6.63.4 0.0 4.5

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We define the design of modules with improved economics in two ways,

1. Reducing the Wp module price ($ per Wp)

a. Which we achieve by either reducing the material costs or increasing the module Wp

rating, the latter we achieve primarily by increasing the cell to module power ratio

2. Reducing the Wp rooftop system price

a. This second metric is more complicated as the balance of systems materials and

labour must be accounted for.

In this section we outline our module designs and then evaluate the impact on the module and

system price in $/Wp. We perform our analysis for two types of modules, the multi-crystalline

module and the PERC high efficiency.

To separate the improvement afforded by the PV-Mate research from the improving cell efficiency,

we control for improved cell performance. Since the start of the project, the average cell efficiency

has increased from 17.4% to 18.2% (a 4.6% relative gain in cell power) resulting from changes in cell

design, materials and processing. These cell improvements are not achievementsof this project but

for the cell production industry. Hence, these effects are controlled for and removed from our

summary of module improvements. We note however that increasing cell efficiency makes the task

of achieving a good cell to module ratio harder as has been described in literature (Haedrich, Eitner,

Wiese, & Wirth, 2014). This is largely due to cell improvements that increase the cell current, reduce

the module optical gains and increase the connector resistance losses.

2.4.1 Summary of Improvements to multi-crystalline modules In our Milestone 7 summary report we presented multi-crystalline module improvement summary,

which summarises all key cell to module improvement measures trialled in the project. Each of the,

improvements were tested and different levels either:

On mini modules, typically for fast tests on any new concept or material

On full area modules, for promising changes, and

In production, for changes that were adopted by Tindo.

The Milestone 7 report related the power improvement to 17.8% efficient cells, which were

employed in 2015. This summary is now presented in Table 2-6where the improvements were

determined from design of experiment (DOE) tests, simulations, in production and with full area

module test production.

Table 2-6: Summary of module changes implemented and tested in the PV-Mate project on multi-crystalline modules.

Change Performance gain summaries from DOE measurement

Pmax | CTM Simulation ηcell = 17.8%

Improvement measured with production data sample size > 1000

Measured on full sized modules not in production

Baseline 0.% 250 W | 96.4% 251 W 0.0%*

Backsheet 2.5% 255 W | 97.9% 257 W +2.4%

Spacing 0.9% 257 W | 98.8% 260 W +3.6%

3 to 4 Busbars 0.9% 259 W | 99.6% 261 W +4.0%^

2.8 mm glass

UV through EVA 1.5% 262 W | 101%

Double Glass UV through EVA

265 W +5.6%

Busbar tape 1.6% 265 W | 102%

34

Total inferred effects

+7.4% +5.6% CTM 6% increase in Pmax

+4.0% +5.6%

*Inferred from modules produced with cells 18.6% efficiency, converted to 17.8% by Pmax = E(Pmax) × 17.8/18.6 ^Inferred from modules produced with cells 18.0% efficiency, converted to 17.8% by Pmax = E(Pmax) × 17.8/18.0

While the analysis presented in Table 2-6 suggests an improvement in the cell to module ratio of

between 4% and 7% is possible, not all of these modifications are currently achievable (at least in

Tindo’s factory) at negligible costs. The production of double glass modules without new plant is not

possible, and the busbar tap applicator needs to be developed.

To demonstrate the improvements of the new module design, we compute the cell to module

power ratio for the baseline design as described in Section 2.1, however we update the simulation to

use cells with efficiencies that were being deployed in 2017. When we do this we find the cells have

a similar cell to module power ratio as plotted in Figure 2-19, but the power absolute power is

significantly higher as the cells are 18.6% efficient compared to 17.4%. The improvements developed

specifically from this project and adopted by Tindo owing to the improved economics are the

replacement of the black backsheet with a white backsheet, and the increase of the gap between

cells from 2.5 to 4.0 mm. This resulted in a 2.8% improvement in the cell to module ratio, at no

additional cost.

35

a) b)

c) d)

Figure 2-19: a) and b) Relative cell to module power gains and losses at the start and end of the project respectively, and c) and d) absolute power gains and losses for Tindo module technology employed at the start and end of the project computed using modern using the same solar cells as were employed at the end of 2017.

2.4.2 Improvements in mono-crystalline PERC modules Through this project and our collaboration with Tindo Solar we have worked on strategies for

improving module performance, specifically for the Australian environment. Considerable efforts

have been put to reducing cell to module losses, or CTM measured under standard test conditions.

Indeed by increase increasing the cell gap and better making use the module area, we have achieved

reduce improve the CTM for the multi cells, this has been proved in production as well as in the

laboratory. The evolution of module CTM is presented in Figure 2-22, for this summary, only results

recorded on full area 60 cell modules are presented. The positive CTM for the multi-crystalline

module was achieved with a double glass module and an EVA without UV blocking. The module

design achieved a 4%, improvement in CTM. The impact of an improved gap design in production is

presented in Figure 2-23, which presents a summary plot of the impact of changing cell gap, where

19,000 panels were compared.

To achieve a 5% gain in standard operating test condition performance we have developed a half-

cell module. Here the cells are cut in two; hence a 60 cell module becomes a 120 cell module. To

Start of project.

Start of project.

End of project.

End of project.

36

maintain similar operating voltages and currents as conventional modules we form two parallel

strings of cells together. Forming a module like this has a two-fold effect on performance; first the

current in the cell connectors is halved. Second, the generation current in the cells is enhanced due

to the higher edge to cell gap ratio. Simply put, the gaps between the cells contribute to a modules

current generation, light reflected from the gaps can be redirected on the cells and can contribute to

the current generation. From our tests, we find that the gap between the cells enhances the 2.4%

for a normal module and 4.8% for a half cell module.

Large high-power PERC modules are becoming more common, but cost-effective integration is

challenging. Tindo does not produce mono-crystalline full area modules commercially, our research

and development of these modules using full size PERC mono modules is depicted in Figure 2-20.

The Pmax of the module ranged from 284 W to 298 W, with input cell efficiencies 20.8% to 21.3%.

Recent trials with 4 busbar cells have performed poorly. Through the PV-Mate project we have

improved the CTM from -4% to -1% a 3% enhancement. We note these CTM figures are far lower

than for multi crystalline modules; this can largely be attributed to the cell reflectance in air. Multi-

crystalline solar cells have a higher initial reflectivity of approximately 5% compared to 2%. The

higher reflectivity enables better encapsulation performance by capturing reflected light; hence

multi-crystalline modules have a lower initial performance but a higher performance when

encapsulated. This effect for all modern cell types is thoroughly investigated through the PV-Mate

project and the results are currently being published.

Figure 2-20: Summary of CTM results for PERC modules produce in the PV-Mate project.

One clear trend from industry is the movement to half cut PERC solar cells. In the PV-MATE project

we demonstrated the improvement that can be achieved with half cut modules through a mini-

module CTM experiment. In this case we employed 20.8% efficient mono-PERC solar 4 busbar cells,

with and without UV transparent EVA. Our results are depicted in Figure 2-21. There were

considerable current cell gap current gains from 5.3% to 10.1%, this leads to a 2.5% improvement in

module power.

-8%

-7%

-6%

-5%

-4%

-3%

-2%

-1%

0%

4bb 4bb 4bb 4bb 4bb 3bb 3bb 3bb UVT 3bb UVT

Cel

l to

Mo

du

le p

ow

er

Module Specifics

37

Figure 2-21: Summary of the percentage increase in cell Isc from 4 mm of cell spacing for both full and half solar cell.

Of course, the primary benefit from using half-cell modules is the reduction in resistive losses. This

effect reduces the resistive losses from 11.1 W to 2.8 W leading 2.8% improvement in the module

power. Hence, on the mini module level we have demonstrated that with 20.8% efficient mono PERC

solar cells, by going to a half cell design the power can be improved by 5.2%. Our experimental

validation demonstrates that a 300 W module can be achieved, without the use of a UV transparent

material.

Figure 2-22: Evolution of CTM for multi and mono-crystalline silicon modules, though out the project.

102.0%

103.0%

104.0%

105.0%

106.0%

107.0%

108.0%

109.0%

110.0%

111.0%

Full Half

Fbs

-5.00%

-4.00%

-3.00%

-2.00%

-1.00%

0.00%

1.00%

2.00%

3.00%

Dec-14 Apr-15 Jul-15 Oct-15 Jan-16 May-16 Aug-16

CTM

Project duration

Multi

Mono

Linear (Multi)

Linear (Mono)

38

Figure 2-23: Plot of measure module power versus cell efficiency, for the 2.5 mm and 4 mm cell gaps. The data represents some 19,000 panels from Tindo solar’s production line.

The design changes we are considering, and are investigating for: STC power, yield and durability

are:

Double glass modules

Non-UV blocking EVA front sheet, with UV blocking rear sheet.

PERC solar cells

Half cells

By-pass diode free modules

Light reflecting cell interconnection ribbon

Consideration of bifacial modules and non-ideal effects.

The double-glass and different encapsulants were discussed in Section 2.3.3, and the impact of half

cells and impact of going encapsulant free was discussed in Section 2.2.4. Although without specific

shading assumption we cannot quantify the absolute loss from shading of a module design, we have

investigated the whether it is beneficial for energy yield to remove by pass diodes. This depends on

the cell breakdown, and if the cells damaged by revers breakdown. These issues are particularly

interesting for the rooftop market where shading is prevalent, and the used of micro-inverters are

more common.

To quantify the impact of the half-cell module on yield, we average the energy output from the four

module designs presented in Section 2.2.4 the relative changes compared to a standard module are

presented in Table 2-7. Using a UV transparent EVA could improve the yield for the EVA modules by

a further 1%, however we will carefully consider the impact of degradation in our on-going works.

Table 2-7: Comparison of relative yield averages for the 16 sites discussed in Section 5.5

Cell Size Encapsulant Yield

PERC Full EVA 0.0%

PERC Full Air -3.9%

252

254

256

258

260

262

264

266

268

17.7 17.75 17.8 17.85 17.9 17.95 18 18.05 18.1

Mo

du

le P

ow

er

Cell efficiency (%)

2.5mmgap

4mm gap

39

PERC Half EVA 5.3%

PERC Half Air 1.1%

2.5 Fabrication of Advanced Mini-Modules and Testing

2.5.1 Double Glass Modules Double glass modules were produced for this project. The weight of the modules was identified as a

key issue for the technology. For the glass to be effectively tempered, it requires thermal annealing

and cooling such that there is a large temperature gradient. This is only possible for glass thicknesses

2.8 mm and over. The tempering technology may improve such that thinner sheet can be considered

fully tempered; however the 2.5 mm glass used commonly by producers of double-glass modules is

semi-tempered. The project investigated the use of chemically tempered glass. Instead of thermal

tempering, the aluminosilicate glass is heated in a potassium containing salt which induces ion

exchange, where the differing ion sizes induce surface stresses like the thermal annealing procedure

but applicable to thinner glass sizes. Four full size double glass modules were produced during the

project. We employed 2 mm thick chemically tempered glass imported from Japan. When handling

the glass, it was found that: the conventional robots used in Tindo Solar lay-up, were not suitable for

moving the glass (the thin glass sheets require special handling to prevent shattering). Hence it was

concluded that the double glass modules produced from chemically tempered glass were not

suitable for Tindo Solar’s production line despite our findings that they had good CTM power

performance, operating temperatures and resistance against accelerated degradation. The idea

should be revisited when the glass technology improves.

2.5.2 Enhanced UV Stability Both conventional and double-glass mini module degradation testing was performed on the current

materials and UV transparent material. All mini-modules were found to pass extended UV and damp

heat tests. The results of which are given in more detail below.

In this project 8 different encapsulants were compared for their UV stability. The EVA samples were

subject to extreme UV exposure. They were exposed to 6 weeks of UV in a Q-sun Xe-1. This is

roughly equivalent to 6 times the IEC61215 exposure dose. All materials passed and the reduction in

transmission was less than 5%demonstrating that all the materials are suitable for enhanced UV

stability as depicted in Table 2-4. The impact of damp heat was also investigated as presented in

Table 2-9

Table 2-8: Relative changes in transmission after UV exposure, equal spectral weighting 250 to 1150 nm.

Standard UV transparent

Bridgestone 0.81% -1.87%

SKC 1.05% -4.35%

NP -0.35% -1.63%

Vista -0.03% -1.77%

Table 2-9: Relative changes in transmission after DH exposure, equal spectral weighting 250 to 1150 nm.

Standard UV transparent

40

Bridgestone -1.47% -1.23%

SKC -2.52% -3.45%

NP -1.47% 1.65%

Vista -2.79% 2.74%

To extend this testing we applied both damp heat and UV testing to the same samples. With this

procedure DH1000 was applied first, followed by UV equivalent to 2 times IEC61215. All materials

demonstrate excellent UV and damp heat stability as is summarised Table 2-10.

Table 2-10:1000hrs DH testing, and then 1000DH + 2 weeks (3 IEC) of UV testing. Average transmission change.

Company Standard EVA UV through EVA

Relative change in solar and IQE weighted transmission

DH DH+UV DH DH+UV

Bridgestone -0.71% -0.66% -0.14% 0.27%

Vista Solar -1.3% 2.0% -18% 16%

SKC -1.1% 2.1% -0.53% -0.01%

Novopolymer 0.04% 0.8% -1.5% -0.04%

Mini modules were tested for extreme damp heat, over 3 times IEC61215. The performance of all

modules was similar irrespective of whether a UVT material was employed, or whether a

conventional module or a glass-glass module structure was employed. Some of the degradation

observed was greater than 5%, no module was significantly different all modules and materials

performed similarly. Extreme UV exposure was applied to 3 types of modules. 3 times IEC61215 for

double glass modules and 17 times IEC61215 for the conventional module with the UV transparent

front EVA. Essentially the advanced module materials are unaffected by UV and damp heat, results

summarised in Table 2-11.

Table 2-11: Calibrated percentage changes in DH exposed PV module characteristics

Damp Heat UV exposure

Structure Front EVA

Rear EVA

PMAX 1900 hrs

PMAX

3250 PMAX 18 times IEC61215

PMAX 3 times IEC61215

Glass-backsheet STD STD -6.0% -3.0 - -

Glass-backsheet UVT UVT -2.5 -5.1 +1.56

Glass-glass STD WTE -2.5 -5.2 - +2.25

Glass-glass UVT WTE -3.4 -5.5 - -0.32

2.5.3 Mini-Module NOCT Tests We investigated the temperature performance of double glass modules as it is a perceived challenge

for the technology. Heuristically it seems reasonable that cells laminated between two sheets of

glass approximately 2 mm thick would stagnate or operate at a higher temperature compared to

conventional modules which have a 0.45 mm thick polymer backsheet. Importantly, the thermal

conductivity of glass is 4-5 times higher than polymer backsheets; hence through modelling NOCT

tests we demonstrated that the operating temperature for double glass modules can be lower than

41

that of conventional modules. We found that modelled NOCTs under cloudy conditions to be 49.5

and 49.2°C for standard and double glass respectively and our measurements agreed with this.

a)

b)

Figure 2-24: a) measured operating module temperature in cloudy conditions. b) Modelling of layer temperatures under NOCT conditions.

Fabrication of Full-Area ModulesTable 2-12 designs.

1) Our fist optimised module is the multi crystalline 60 cell module. This module features 4 busbar

solar cells, with a white backsheet and a cell gap of 4 mm. For now, the use of a UV transparent

EVA, has not been employed as it is: Tindo has not arranged a supply contract for UV transparent

EVA. The 1000s of these modules that have been manufactured have produced 4% more power

than the module design employed during the start of the project.

2) Our second module optimisation is the double glass multi crystalline and mono PERC double

glass modules. Both modules employ a UV transparent EVA, and used thin annealed glass.

3) Our third optimum module employs half- PERC cells, validated on the mini-module level, results

in a 300 W PERC module with a positive CTM.

Table 2-12: Full area module performance results.

1) Conventional multi-crystalline silicon, 4 busbar cells, 4 mm

cell gap, white backsheet

0

5

10

15

20

25

30

35

0 100 200 300 400 500 600 700 800

TM

od

ule

-T

Am

bie

nt

(°C

)

Global tilted irradiance (W/m2)

Standard

Double glass

48

48.5

49

49.5

50

50.5

51

51.5

NO

CT

(°C

)

Standard

Double glass

42

Using 18% efficient multi crystalline 4 busbar solar cells

263.5 W 262.8 +0.27%

In production 263.5 W 262.8 +0.27%

2) Double glass

a. multi PERC with UVT EVA

Using 17.8% efficient multi crystalline 3 busbar solar cells

Pmax PV-Lab Pcell CTM

Mod 1 (1608354) 265 W 260 +1.9%

Mod 2 (1608354) 264 W 260 +1.5%

b. Double glass mono PERC with UVT EVA

Using, 20.8% efficiency 200 mm diameter 4.97 W cells 3 busbar cells:


Mod 1 (1608382) 295 W 298.2 -1.0%

Mod 2 (1608383) 294 W 298.2 -1.4%

3) Full-cell mono PERC


Mod 1 (1608771) 297 W 302.4 -1.8%

Mod 2 (1608770) 298 W 302.4 -1.4%

2.6 Co-develop a new module product for Tindo Solar specifically for the Aus.

environment As part of the project, we have trialled double glass modules, where we have found them

challenging and without an obvious pathway for market acceptance. We have co-developed PERC

modules with Tindo, providing them with modelling results, access to alternative EL and IV testing

equipment and cell to module analysis. Now that Tindo understand the PERC technology, and the

market, they are considering converting their production to PERC cell technology in 2018.

2.7 Simulation Mechanism for Determining Hot-Spot Temperatures This Section details our work on hot-spot simulation, and the application to half-cell modules. Our

objectives were to:

1. Develop a simulation mechanism for determining hot-spot temperatures

2. Validate simulations of half-cell and full cell module hot-spots under different shading

events

We submit 4 attachments which detail our methods and results for these objectives. For 1 and 2 we

attach a Lessons Learnt Report (LLR14) which details our simulation methodology, and validation on

full and half-cells. We also attach a paper draft PP14, it is undergoing revisions before resubmission

to journal. For 2 specifically we also include LLR6 which presents the impact of half cells on module

hot-spot temperature, and we include a draft paper PP15 which is undergoing a final review for

Progress in Photovoltaics.

In these attachments our novel methods for hot-spot simulation which use reverse-bias

electroluminescence imaging is demonstrated. It is then used to demonstrate that half-cell modules

have lower hot-spot temperature, an important consideration when mitigating the impact of hot-

spots on module durability.

3 Conclusion It has been a privilege to partner with Tindo, PV Lab, ARENA, PV Lighthouse and ANU on this project.

The outcomes of the work started in PV-Mate is ongoing and will be continued by the PhD students

43

Ingrid Haedrich and Jiadong Qian who have made excellent contributions. Australia is entering a new

and very practical phase where the large-scale deployment of solar modules is becoming and will

soon be common place. Researchers, Developers and Engineering Procurement and Construction

Companies can gain insight in from this into module technologies, and consider using the tools

develop and described in the attached publications to understand the implication of different

elements of module design on the plant performance

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