ADVANCEMENTS IN THE DEVELOPMENT OF ‘‘ATAMO’’: A SOLAR MODULE ADAPTED FOR THE

CLIMATE CONDITIONS OF THE ATACAMA DESERT IN CHILE

Enrique Cabrera1, Andreas Schneider1, Jorge Rabanal Arabach1,

Pablo Ferrada2, Raul R. Cordero3, Edward Fuentealba2, Radovan Kopecek1 1International Solar Energy Research Center -ISC- Konstanz, Rudolf-Diesel-Str. 15, 78467 Konstanz, Germany

2University of Antofagasta, Av. Angamos 601, 1270300 Antofagasta, Chile 3Universidad of Santiago de Chile, Av. Bernardo O’higgins 3363, Santiago, Chile.

Corresponding author: Phone: +49 7531 3618356; Fax: +49 7531 3618311; Email: enrique.cabrera@isc-konstanz.de

ABSTRACT: Standard testing conditions as IEC and UL were not developed for applications in climates such as the

Atacama Desert, which is well known for extremely high irradiation levels (also UV), large temperature gradients,

corrosive environment, partial high humidity (“camanchaca”) and a really fine dust (“chusca”). Therefore the PV

system performance and longevity is strongly affected, and the standards are only partly applicable to determine the

module stability against climatic impacts. In the present study, we investigate the performance of different setups of

half-size and full-size cell modules under accelerated indoor test conditions, performed at ISC Konstanz, Germany

and later installed at the Solar Platform of the Atacama Desert (PSDA) in Yungay, Chile. We fabricated mini-

modules employing variations of glass thickness, cell configurations (p- and n-type), 3 types of encapsulants (EVA,

low UV light cut-off EVA and thermoplastic material), 3 different backsheets (white standard, transparent and desert

type) and glass-glass configurations. After 6000 hours of damp heat (DH) exposure the groups perform differently

due the encapsulated materials used. Best results are observed with thermoplastic material, regardless of the

backsheet used. The humidity freeze 41 (HF) and UV62 test reveals a negligible degradation for all module

configurations. Thermo-cycling 200 (TC) shows less relative Pmpp losses when glass on the rear side is used, while

half-size cell modules are more sensitive for thermal stress than full-size cell modules, showing more defects such as

cracks and metallization damages. Finally, preliminary outdoor measurements carried out at the PSDA and in

Konstanz reveal that the ARC glass helps to improve the performance ratio (PR) up to 1%. The full-cell size modules

show a strong PR decay above 800 Wm-2 whereas for half-size cell modules with ARC the specific yield is up to

2.3% higher compared to standard glass and up to 4.3% higher than for full-size cell modules above 900 Wm-2.

Keywords: PV module, Accelerated Environmental Indoor Test, Desert Module, Defects, Silicon Solar Cell

1 INTRODUCTION

Current PV modules are developed, produced,

characterized and tested according to international norms

(IEC and UL) in order to help the final solar products

passing environmental tests and reaching long lifetimes

with guaranteed power output. These standards are

derived partly from long-term data analysis of solar

modules installed in Europe and North America, which

reveal the typical damage and defects associated to the

environments in these regions.

In the last years, Chile entered to the PV market with

a large number of new installations. Chile's cumulative

installed PV capacity stood at 582 MW as of the end of

July 2015 and, in contrast, Chile's pipeline of approved

PV projects totaled 9.6 GW, according to the latest report

from CIFES [1]. The main driver for PV installations in

Chile is the high electricity price, cost-effective PV

modules and high radiation levels. Chile has declared

itself to be a country with huge potential for PV due to

the combination of large irradiation levels, long sunhours

and low temperatures, resulting in an energy potential of

more than 1800 kWh/kWp for c-Si technologies [2]. The

decline of the prices for solar modules combined with the

attraction to Chile as the right place for investors caused

a first Chilean boom for PV applications, positioning

Chile into the second place of the top ten most attractive

PV markets and market growth expectations [3]. Thus,

Chile became one of the first countries where PV energy

is economically integrated to the energy grid without any

subsidies, as it was predicted in [4]. However, in relation

to the quality of solar modules and performance, almost

no scientific work has been carried out for the Atacama

Desert conditions. The long exposure time and high

intensities compared to Europe will have a strong effect

on the lifetime, degradation and power achieved by the

modules.

Consequently, PV modules must be evaluated for

quality standards adapted to Chile with the aim of

reaching the same lifetime found in Europe or North

America. Constituting materials, characterization and

evaluation methods have to be improved. To proceed,

knowledge and experience on the damage and defect

patterns of PV systems in the Atacama Desert are

required.

In the present study, we investigate the performance

of different setups of half-size and full-size cell modules

under accelerated environmental indoor test conditions,

performed at ISC Konstanz, Germany and later installed

at the Solar Platform of the Atacama Desert (PSDA) in

Yungay, Chile, in order to find the best material

combination for the so called AtaMo; a solar module

adapted for the climate conditions of the Atacama desert

in Chile.

2 EXPERIMENT

One cell modules based on 6-inch p-type mc-Si and

bifacial 6-inch n-type pseudo-square Cz-Si solar cells

with standard 3 busbar metallization patterns and a

variation of different glasses, encapsulants and

backsheets were processed. The ribbon soldering process

on the front and rear side was performed manually by

hand or automatically by a stringer machine using the

same type of ribbon and flux. Several glasses with

different thicknesses were employed, meaning 1.0, 1.5,

2.0 and 3.2 mm glass. In particular, for the latter one

antireflection coating layer (ARC) was also incorporated.

For the encapsulation material ethylene vinyl acetate

(EVA), thermoplastic materials (TM) and low UV light

cut-off EVA (U) was used. For the backsheet material

standard (white), transparent and desert (which is

optimized in terms of abrasion and thickness for desert

regions) type sheets, as well as glass were employed. The

pressure and temperature for the lamination process was

adapted for the glass-glass module configuration when

necessary.

Electroluminescence (EL) measurements were

carried out to control the soldering and lamination

process as well as to investigate the environmental indoor

test performances. Peel tests were carried out to monitor

the soldering process, IV characteristics were measured

by a class AAA solar simulator with an illumination area

of 22 × 22 cm2 at STC conditions before and after

lamination to determine the cell to module losses (CTM)

and to quantify the environmental indoor test results.

Thus, in total, 37 groups were fabricated from which 14

groups are based on mc-Si cells for DH, HF and Ultra-

Violet (UV) degradation test and 23 groups are based on

bifacial n-type cells for TC test and outdoor

measurements at the Solar Platform of the Atacama

Desert (PSDA) in Yungay, Chile and at ISC Konstanz in

Germany.

In relation to the quality of solar modules and

performance, no scientific work has been carried out for

accelerated environmental indoor test conditions taking

into account the Chilean climatic condition for the

Atacama Desert. Therefore, in the presented study we

extended the application time and cycles for the

IEC61215 tests. DH test ranging from 1000 to 6000

hours was carried out on 10 groups (each containing

between 2 to 4 one mc-Si cell modules) according to

Table I. One part is currently being tested at ISC

Konstanz whereas the other part was sent to PSDA in

order to investigate the performances at outdoor

conditions. HF test was performed up to 41 cycles on 8

groups marked with bold in Table I (one module per

group).

Table I: Experimental setups for DH and HF test

Glass thickness

[mm] Backsheet (BS) Encapsulation

1.5 Desert (D) E

1.5 Standard (S) E

2.0 Desert (D) E

2.0 Standard (S) E

3.2 Desert (D) E

3.2 Standard (S) E

3.2 Desert (D) TM

3.2 Standard (S) TM

2.0 Transparent (T) E

3.2 Transparent (T) E

Another 4 groups of mc-Si cell modules were

produced for UV degradation (2 modules per group).

Low UV light cut-off EVA (U) encapsulation material

and glass with ARC are incorporated. All groups were

sent to the University of Santiago de Chile (USACH) for

UV exposure test and then received back to investigate

potential UV degradation. Due to the high UV-B

radiation in the Atacama Desert [5] as well as mean

temperatures between 10 and 20°C for cold months and

20–30°C for warm months [6], the UV chamber was

operated only with UV-B and at room temperature. The

UV-B dose was selected to correspond to 1 day of

irradiation in the Atacama Desert during 30 min of UV

chamber operation. In order to achieve a deeper

understanding of UV degradation, spectral photometer

measurements of reflectivity (R) and transmissivity (T)

versus wave length from 300 to 1200 nm were carried

out. The structure of the laminated modules were based

on front flat 3.2 mm solar glass followed by EVA (E),

thermoplastic material (TM) or low UV light cut-off

EVA (U), separated by a bifacial n-type cell and finished

by another encapsulation layer of the same kind as the

front material and a rear flat 1.5 mm solar glass, see

Table II. The R and T measurements were performed

outside the cell area (2 modules per group).

Table II: Experimental setup for UV exposure test

Glass thickness [mm]

Backsheet (BS)

Encapsulation

3.2 Standard TM

3.2 Standard U

3.2 Standard E

3.2 ARC Standard E

For TC test, 15 module groups based on 19.6%

efficient bifacial half- and full-size cells were fabricated

based on glass-glass and glass-transparent backsheet

(TBS), see Table III for details.

Table III: Experimental setup for TC200 test

Front side Rear side Bifi cell Encapsulation

3.2 Glass TBS FULL TM

3.2 Glass TBS FULL E

3.2 Glass TBS HALF TM

2.0 Glass TBS FULL TM

2.0 Glass TBS FULL E

2.0 Glass TBS HALF TM

2.0 Glass 2.0 Glass FULL TM

2.0 Glass 2.0 Glass FULL E

2.0 Glass 2.0 Glass HALF TM

1.0 Glass TBS FULL TM

1.0 Glass TBS FULL E

1.0 Glass TBS HALF TM

1.0 Glass 1.0 Glass FULL TM

1.0 Glass 1.0 Glass FULL E

1.0 Glass 1.0 Glass HALF TM

Additionally, 8 groups of glass-glass and glass-TBS

modules featuring 20% efficient n-type solar cells were

manufactured in order to investigate the bifaciality

concept installed at the PSDA in Yungay, Chile, and later

on at ISC Konstanz in terms of energy yield performance.

For each group the bifaciality factor is at least 91%. For

outdoor characterization half- and full-size cell modules

were fabricated using a glass thickness of 3.2 mm with

and without ARC, see Table IV. For the setup in

Konstanz, a white cover (with an albedo of more than

90% reflectivity) is used in order to improve the rear

irradiation from the ground. An average summer day was

selected, while no white cover is used at the PSDA. IV

measurements were performed every 5 min using an IV

curve outdoor tracer. Resistance thermometer PT100

sensors were used to measure the ambient temperature

and working temperature of the module. A silicon cell

photodiode was used for measuring the global irradiance

(GTI) at the array plane.

Table IV: Experimental setup for outdoor measurements

Front side Rear side Bifi

cell Encapsulation

3.2 Glass TBS FULL E

3.2 Glass TBS HALF E

3.2 Glass 3.2 Glass FULL E

3.2 Glass 3.2 Glass HALF E

3.2 A

Glass 3.2 Glass FULL E

3.2 A

Glass 3.2 Glass HALF E

3.2 A

Glass

3.2 A

Glass FULL E

3.2 AGlass 3.2 A

Glass HALF E

3 RESULTS

3.1 CTM analysis

3.1.1 mc-Si cell modules

The mc-Si full-cell modules fabricated for DH and

HF tests (see Table I) reveal less CTM Isc losses for

thinner glass, as Figure 1 shows, due to less absorption in

the glass substrate. In terms of lower CTM Pmpp losses,

the best material combination is 1.5 mm glass, EVA as

encapsulant and desert backsheet (DBS).

Figure 1: Relative CTM Pmpp and Isc losses of modules

before DH and HF tests

For the CTM analysis, the measured Isc is the critical

size to validate the performance, where 3% gain in Isc is

observed by the flat glass with ARC and EVA as

encapsulant (A-E), as it is expected. Note that this gain in

current means less relative Pmpp losses and also less

relative efficiency losses, see Figure 2.

Figure 2: Relative CTM losses of modules before UV

exposure test

3.1.2 Half-size and full-size bifacial n-type cell modules

Fifteen bifacial n-type cell modules were fabricated

and classified into 5 groups according to the embedding

material configuration, meaning glass-glass or glass-TBS.

In Figure 3 the relative FF losses for these 5 groups is

displayed, divided in 3 sub-groups delimited by dotted

lines (as a guide for the eye). The red column

corresponds to EVA (E) and the blue columns to

thermoplastic material (TM). Finally, the difference

between the blue columns corresponds to the full-size

cell module design, while half-size cell modules

correspond to the blue columns with patterns. It should

be mentioned that no half size-cell module were

fabricated with EVA.

The CTM analysis shows relative FF losses of

approximately 4% in average for the front and rear side.

No micro-cracks were observed by EL after soldering or

after the lamination process. Less FF losses for half-size

cell modules with identical connector tabs were observed

due to a reduction of series resistance and output current,

if compared to the full-size cell configuration. Thus, half-

size solar cells generating half the current significantly

reduce the series resistance losses to a quarter compared

to full size cells as it is reported in previous studies [7, 8].

Comparing the half-size cell module configurations,

higher relative CTM FF losses are observed for 1.0 mm

glass and TBS. Thus, higher effects of thermal stress

meaning higher relative Pmpp losses after TC test are

expected for this group as a consequence.

Figure 3: Relative CTM losses of modules before TC

environmental test

3.2 DH test

Figure 4 shows the relative FF losses of the modules

during DH testing for the individual groups. From the

beginning of the test up to 2000 hrs no significant losses

are measured. After DH2000 long-term exposure, a

degrading effect caused by the high humidity and

temperature on the samples is observed. While standard

backsheet resulted in high relative FF losses after

DH3000, desert and TBS exhibited a much more stable

behavior. The largest degradation of this test is observed

from DH3000 to DH4000, where groups based on EVA

showed around 50% of relative FF losses. This is the

point where thermoplastic material makes the difference.

From DH4000 to DH6000, delamination, corrosion and

discoloration as a result of humidity penetration become

stronger pronounced and lead to more losses in Isc than

Impp resulting in less relative FF losses. After DH6000,

the best weatherability and performance was found for

thermoplastic material (TM). This phenomenon is shown

by the red curves in Figure 4.

Figure 4: Relative FF losses after DH6000 for different

solar material configurations with focus on the

encapsulant. Black curves denote to EVA as

encapsulation material, while red ones refer to

thermoplastic material (TM)

In parallel, EL characterization was performed

applying the same current and exposure time in order to

obtain comparable pictures. From Figure 5, a strong

humidity penetration effect can be observed at the edge

of 3.2G-SBS-E after DH4000, where delamination and

corrosion are present. Humidity diffused through the

edges and across the ribbons causes this effect. In the

case of 3.2-DBS-TM, after DH6000 the module remains

electrically active achieving around 10% relative FF

losses.

Figure 5: EL picture during DH test intervals

3.3 HF test

Figure 6 shows excellent performance of the modules

during HF testing, giving certainty on the ability of

samples to withstand the effects of high temperatures

combined with humidity for all tested groups. After HF41

long-time exposure, no significant changes are observed.

The little variation is within the measuring error range.

Figure 6: Relative FF losses vs. HF cycle number with

focus on the encapsulant

3.4 UV exposure test

In order to identify materials able to withstand high

doses of ultra-violet (UV) radiation, UV lab tests were

performed considering the UV dose in the Atacama

Desert. The UV chamber located at USACH in Chile was

operated only with UV-B due to its high contribution in

comparison with UV-A measured at real conditions in the

Atacama Desert [5] and at room temperature. After UV41

long-time exposure, accounting for 7 years real world

(37.3 kWh m-2) exposure, no significant changes are

observed. It should also be mentioned that some cell

cracks are observed due to the transport from Chile to

Germany, which do not affect the measurement results

and are within the error range, see Figure 7. After 62 days

UV exposure, with a UV-B dose of 56.3 kWh m-2,

corresponding to approximately 10 years in the Atacama

Desert, results are still within the error range.

Figure 7: Relative FF losses vs. UV degradation test

In order to investigate the reason for the low UV

degradation performance, spectral photometer

measurement for reflectivity (R) and transmissivity (T)

versus wavelength λ were carried out for the 3

encapsulants: EVA (E), thermoplastic material (TM) and

low UV light cut-off EVA (U). Taking into account only

the UV-B range, in our case, from 300 to 315 nm and

introducing the effective R and T [9] by integrating the

curve and weighting it by the AM1.5 solar spectrum

(only for the UV-B range), more transmissivity of UV-B

is expected for the modules using TM compared to U and

E, where TM permits to transmit 32.55% of UV-B

irradiance, 0.08% with U and 0.04% with E, respectively.

The reflectivity values remain also on the same level, see

Table V.

Regarding the dose of applied UV-B in comparison

with the IEC61215 definition, a higher total UV

irradiation was used. This may lead to faster degradation

which can occur in combination with UV-A, since even

UV-A has longer wave-lengths meaning less energy than

UV-B, largest percentage of UV-A can penetrate into the

glass-encapsulant system and it may be enough to change

the composition of the encapsulant. Thus, UV-A opens

the way for UV-B penetration and more damage could be

detected.

Table V: Effective R and T values for the experimental

setup for encapsulant E, TM and U

Spectrum

300-315 [nm]

E

[%]

TM

[%]

U

[%]

T eff 0.04 ± 0.0 32.55 ± 0.88 0.08 ± 0.03

R eff 4.81 ± 0.3 5.72 ± 0.05 4.49 ± 0.07

3.5 TC test

For relative Pmpp losses after TC200, slightly higher

ability of the modules based on EVA as encapsulant than

TM to withstand the effect of thermal stress as result of

extreme temperature change ((–40±2) °C to (85±2) °C) is

observed, no matter which kind of glass on the front and

glass or TBS on the rear side is used, compare blue with

red columns in Figure 8.

The best performance for full-size cell design is

achieved by 1.0 mm glass-glass with 0.23% relative Pmpp

losses together with 2.0 mm glass-glass module with

0.38%, both of them with EVA as encapsulation material.

For half-size cell design, best results are measured for 1.0

mm glass-glass with 1.65%. Note that, groups with rear

glass show less relative Pmpp losses in comparison with

TBS, since glass-glass is more robust in terms of less

thermal expansion and contraction than glass-TBS. In

particular, the worst result is observed in half-size cell

modules with 1.0 mm glass on front and TBS on rear

where relative Pmpp losses reach more than 20% (see

Figure 8 -column 12), which showed high relative CTM

FF losses in comparison to the half-size cell counterparts

(Figure 3).

Figure 8: Relative Pmpp losses after TC200

Figure 9 shows EL images before and after TC200.

Cracks underneath the ribbons and metallization damages

due to finger and busbar interruptions were found,

evident from the dark areas, and are well correlated with

the relative Pmpp losses in Figure 8 such as described in

[9]. The defects were not visible before starting the TC

test and are more relevant on half-size cell modules,

which are more sensitive for thermal stress than full-size

cell modules as expected. Thus, as the cracks appear on

almost all half-size-cell modules, the power losses

increase.

Figure 9: EL images before and after TC200 for the best

(column 8) and worst (column 5) performing full-size

cell, and the best (column 15) and worst (column 12)

performing half-size cell module with EVA

3.6 IV Outdoor performance

Energy yield studies and outdoor measurements were

performed at the PSDA (see Figure 10) and at ISC

Konstanz.

Figure 10: Testing under outdoor conditions at PSDA

Figure 11 shows the specific performance ratio (PR)

versus the irradiance for half- and full-size cell modules

with and without ARC glass performed at ISC Konstanz.

The level of density of the PR values is defined by grey-

back areas, while the blue curves represent the tendency

line between all the PR values. ARC glass improves the

PR up to 1% (4% for irradiance above NOCT) which in

combination with half-cells improves the specific yield

up to 2.3%. Full-size cell modules show a strong PR

decay in comparison with half-size cell modules,

especially for irradiances above 800 Wm-2.

Figure 11: PR at different irradiance levels under real

conditions at ISC Konstanz

Figure 12 shows the PR versus the irradiance and the

specific yield for irradiances larger than 900 W/m-2.

Results were carried out at the PSDA. Preliminary results

show an average PR of 110% for half- and 107% for full-

size cell module. More precise and longer investigations

of PR in bifacial standard sized modules showed an

average PR of 100% for modules measured in the El

Gouna desert in Egypt [11]. This confirms the high PR

values. Current studies developed at the coastal zone of

the Atacama Desert reveal an average of maximum PR of

75% for a thin film plant, 78% for a mono-Si plant [6]

and 70% for a mc-Si plant [12]. This huge difference is

due the bifaciality gain, meaning the contribution of the

rear side in the PR calculation. Half-size cell modules

performed up to 2.3% better in specific yield and 2.7%

better in specific performance ratio than full size

modules, mainly driven by less electrical losses due to

strongly reduced current output coming along with higher

voltage compared to full-size cell modules.

Figure 12: Testing under real conditions at PSDA

4 CONCLUSION

Accelerated environmental indoor test was carried

out extending the application time in order to determine

the best combinations of materials for a PV module for

the Atacama Desert. Preliminary results after DH6000,

TC200, UV62, HF41 and IV outdoor measurements

carried out at PSDA and at ISC Konstanz suggest thinner

glass-glass module (1.0 to 2.0 mm thickness) which

shows better resistivity against thermal stress compared

to glass-TBS. Thermoplastic material shall be used as

encapsulant due to its high robustness against long-term

humidity exposure and higher transmissivity expected in

the UV-B range. Finally, outdoor results indicate that

half-size cell modules have a better performance in the

field due to lower electrical power losses at high

irradiation conditions. However, they are more sensitive

for extreme temperature changes than full-size cell

modules as proven in TC test, which means, more cracks

underneath the busbars and more metallization damages.

5 OUTLOOK

For the indoor tests, more cycles in HF, UV and TC

are necessary in order to confirm our preliminary results.

An extended long term outdoor characterization (at

PSDA) has to be performed in order to compare the

indoor performance behavior under real outdoor

operating conditions.

5 ACKNOWLEDGEMENTS

This work was supported by the German Ministry of

Education and Research (BMBF) under contract no.

01DN14005 (SolarChilD). This investigation is

supported by the University of Antofagasta, Universidad

of Santiago de Chile and the Chilean Solar Energy

Research Center (SERC Chile) under the framework of

the BMBF project SolarChilD.

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