# Superior Energy Yields of *UNI-SOLAR^®^* Triple Junction

# Thin Film Silicon Solar Cells compared to Crystalline Silicon Solar Cells under Real Outdoor Conditions in Western Europe 

M. van Cleef , P. Lippens, J.Call ^\#^

Bekaert ECD Solar Systems Europe N.V.- BESS EUROPE

Karreweg 13, B-9870 Zulte, Belgium

Ph: +32-9-3385926, fax: +32-9-3385911, e-mail
maarten.vancleef@bekaert.com

^\#^ United Solar Systems Corp.

1100 West Maple Road, Troy, MI 48084, USA

Ph: +1-248-362 4170, fax: +1-248-362 4442, e-mail: jcall@uni-solar.com

> ABSTRACT: For many years, amorphous silicon photovoltaic modules have
> had difficulties to establish themselves in the grid-connected
> PV-market. Causes for this lack of market acceptance of first
> generation amorphous silicon PV modules were their relatively low
> conversion efficiencies, unstable power and not well understood
> outdoor characteristics. Various manufacturers of amorphous silicon
> modules have resolved these initial problems by enhancing the
> efficiencies of their cells, while at the same time improving the
> long-term stability of the products. Still, the outdoor behaviour of
> amorphous silicon modules is not well understood by many users, even
> by insiders of the PV-community. Results of this study will show that
> new generation multi-junction amorphous silicon modules, and in
> particular *UNI-SOLAR*^®^ modules based on the Triple Junction solar
> cells, perform excellent under western European climatic conditions,
> with yields and performance ratios significantly higher than all
> present crystalline silicon technologies. This effect is especially
> pronounced under low light conditions and under non-ideal
> orientations. The enhanced outdoor performance, up to 20 % higher on a
> yearly kWh/kWp base, can be attributed to the higher sensitivity for
> low light conditions and for diffuse light, better performance at high
> temperatures and improved shadow tolerance of *UNI-SOLAR*^®^ modules.
> Keywords: Monitoring -- 1: Multi-junction Solar Cell -- 2: Performance
> -- 3

1.  **INTRODUCTION**

Photovoltaic modules are sold with their peak power performance (Wp)
tested under laboratory conditions (STC): i.e. under a very high and
direct irradiation (1000 W/m²), module temperature of 25°C, and only one
type of solar spectrum (AM1.5: mainly direct irradiation). Under real
outdoor conditions, this peak power is seldom achieved, since module
temperature usually is more in the range of 40-60°C under illumination
(especially true for modules that are building integrated), the hours of
1000 W/m² irradiation is only about 1 % of total sun-hours and the
spectral and specular content of the solar spectrum change continuously
with varying climatic conditions. Diffuse light for instance dominates
when the sky is clouded or during mornings and evenings. In Northern and
Central Europe, the majority of solar irradiation comes from diffuse
light (more than 50 % of all solar irradiation), and even in Madrid, the
diffuse part is still 33 %. Customers buying photovoltaic modules are
interested in high-energy yields per purchased Watt peak-power over a
certain time period of a PV-system in true outdoor conditions. This is
especially relevant in countries where there are high feed-in tariffs
per kWh produced available such as in Germany or Spain. In order to
predict what will be the energy output of a PV-module or a PV-system
with a certain nominal power, it is necessary that behaviour of the
modules under various climatic conditions is well known.

We will demonstrate in this article that *UNI-SOLAR*^®^ modules produce
more energy than could be expected from their nominal power and STC
efficiency. *UNI-SOLAR*^®^ modules actually perform better than all
crystalline silicon and other thin film counterparts under real outdoor
conditions in Western-European climates (higher kWh energy production
per kWp purchased). It will be shown that this enhanced performance is
especially pronounced under low light conditions, non-ideal
orientations, and in conditions were module temperatures are high (e.g.
in BIPV systems).

2.  ***UNI-SOLAR^®\ ^*TRIPLE JUNCTION CELLS**

All *UNI-SOLAR^^* photovoltaic modules use the unique Triple Junction
solar cells of United Solar System Corp. This Triple Junction technology
provides unprecedented levels of efficiency and stability for amorphous
silicon solar cells (stabilised aperture area cell efficiency of 7.0-7.5
%). Each cell is composed of three semiconductor junctions stacked on
top of each other. The bottom cell absorbs the red light, the middle
cell the green/yellow light and the top cell absorbs the blue light.
This spectrum splitting capability is one of the keys to higher
efficiencies and higher energy output, especially at lower irradiation
levels and under diffuse light. The cells are produced in a unique
roll-to-roll vacuum deposition process on a continuous roll of stainless
steel sheet, employing only a fraction of the materials and energy of
the production of standard crystalline silicon solar cells. The result
is a flexible, light weight solar cell. The solar cells are encapsulated
in UV-stabilised and weather-resistant polymers. The polymer
encapsulation includes EVA and the fluoro-polymer TEFZEL^^ (a DuPont
film) on the front side. The resulting modules are exceptionally
durable. By-pass diodes are connected across *each* cell, allowing the
modules to produce power even when partially shaded. *UNI-SOLAR^^*
modules are sold with a 20-year power warranty on 80 % of nominal power.
Figure 1 demonstrates the excellent stability of *UNI-SOLAR^^* modules
with Triple Junction Solar Cells over a 3-year period (Triple Junction
technology was introduced at the end of 1997). As can be seen in figure
1, small seasonal variations occur in the conversion efficiency. This
periodic variation is the result of two competing processes during
summer months: a deterioration of the semiconductor quality at higher
operating temperatures and the annealing at those higher temperatures of
light induced defects in the material, created by the
Staebler-Wronski-effect, which repairs light induced defects again.

Fig. 1 -- Outdoor cell efficiency of a *UNI-SOLAR^^* module measured by
Forschungszentrum Jülich from February 1998 to February 2001. Data by
courtesy of FZ Jülich

3.  **MODULE MEASUREMENTS**

Several European test centres (e.g. ECN in the Netherlands and TISO in
Switzerland) have recently performed comparative studies on the outdoor
performance of PV-modules from different manufacturers and with various
technologies (mono- and poly-c-Si and various thin film technologies).
In the ECN study \[1\], the outdoor sensitivity to various illumination
levels has been measured and the temperature behaviour of the modules
has been determined. This ECN-study shows that *UNI-SOLAR*^®^ PV modules
perform more than 40 % better at low light conditions (50 - 100 W/m²),
than all present crystalline technologies (figure 2). Whereas for all
c-Si modules and other thin film technologies the outdoor efficiency
rapidly decreases below 300 W/m², the efficiency of *UNI-SOLAR*^®^
modules shows a constant increase up to values as low as 50 W/m². The
outdoor efficiency at 100 W/m² is more than 25 % higher than the
STC-efficiency. In Northern and Central European climates, where low
light conditions and diffuse light prevails, this enhanced low light
sensitivity results in higher yearly energy output when normalised to Wp
purchased power.

![](media/image1.wmf)Fig 2 -- Influence of irradiance on the relative
outdoor module efficiency of various module types measured by ECN (see
ECN report \[1\]). Module efficiency is normalised to the STC-value at
1000 W/m².

![](media/image2.wmf)

![](media/image8.wmf)Another important aspect is the temperature
dependence of the module characteristics. Crystalline silicon modules
show a linear decrease of power output for increasing module temperature
with a temperature coefficient for maximum power of about - 0.4 to - 0.5
%/°C. This means that at high ambient temperatures under full
illumination (typical module temperature of 60°C), a crystalline silicon
module of 100 Wp will exhibit an effective power of just 83-86 Watt,
which means a power loss of about 15 %. For amorphous silicon modules
this power loss at high operating temperatures is much less. The
temperature behaviour of *UNI-SOLAR*^®^ modules is also not linear.
Outdoor measurements at TISO in Lugano \[2\] have shown that the power
output of *UNI-SOLAR*^®^ panels displays no temperature dependency for
illumination levels below 600 W/m². For higher illumination levels
(900-1100 W/m²) the effective power starts to drop only at ambient
temperatures above 25°C and only very slowly (figure 3). Note that an
effective power of 61 Watt was measured at TISO for a *UNI-SOLAR*^®^
US-64 panel of 64 Wp nominal power at 1000 W/m² illumination and at an
ambient temperature of 30°C (a loss in power of only 5%).

Knowing the outdoor characteristics of the various modules under various
climatic conditions, an annual energy production yield can be determined
for a certain PV-module. This has been done by simulations for a typical
Dutch climate (ECN-study \[1\]) and by measurements over a 15-month
period for a southern Swiss climate (TISO-study \[2\]). Table I shows
the values of the DC-yields and performance ratios of the ECN study,
while Table II exhibits the same values for the TISO measurements. It is
striking to see that these independent results (modules were anonymously
acquired on the free market) demonstrate that for both climatic
conditions all amorphous silicon modules perform better than their
crystalline silicon counterparts. The *UNI-SOLAR^®^* modules in these
tests have the highest yield figures and performance ratios, 12 to 15%
above the best c-Si module. This can be explained by their excellent low
light level efficiency compared to the STC-efficiency and their
insensitivity to temperature influences. Though the absolute value of
the DC-yield must be taken with care (these are only single module
measurements and do not include statistical variations of production
processes, nor mismatch losses in string connections, and inverter
losses), it does say however something in a relative sense.

Fig 3 -- Influence of ambient temperature and irradiance of a
*UNI-SOLAR*^^ US-64 module as measured by TISO from March 2000- May
2001. Data of TISO \[2\].

Table I - Simulated yearly DC-yields and performance ratios (PR) for
various modules determined by measured data of temperature and
illumination behaviour of modules under Dutch outdoor conditions. See
ECN report \[1\].

  ---------------- ------------ ------------ ------- ------------ ----------------- --------
  Manufacturer and Solar Cell                STC                  DC--yield         PR (%)
  Type             Technology                power                (kWh/kWp/y)       
                                             (Wp)                                   

  *UNI-SOLAR*^®^                a-Si TJ              32           1164              95
  US-32                                                                             

  FEE A13P                      a-Si SJ              12           1084              88

  BP Solar MST 43               a-Si DJ              43           1001              81

  BP Solar BP585                mc-Si                85           977               80

  Kyocera KC-60                 pc-Si                60           964               79

  Siemens Solar                 mc-Si                55           963               79
  SM-55                                                                             

  ASE ASE-100                   pc-Si                100          966               79

  Shell Solar                   pc-Si                73           961               78
  RSM75                                                                             

  Siemens Solar                 CIGS                 30           930               76
  S-30                                                                              
  ---------------- ------------ ------------ ------- ------------ ----------------- --------

Table II -- Measured DC-yield and performance ratio (PR) for various
modules in Lugano (CH) over the period March 2000 - May 2001 (cycle 7).
Data courtesy of TISO.

  ------------------ ------------- ------------ ----------------- --------
  Manufacturer and   Technology    STC power    DC--yield         PR (%)
  Type                             (Wp)         (kWh/kWp/y)       

  *UNI-SOLAR*^®^     a-Si TJ       64           1341              93
  US-64                                                           

  Siemens Solar ST40 CIGS          38           1264              88

  Duna Solar DS40    a-Si DJ       40           1246              86

  ASE 100GTFT        mc-Si         100          1186              82

  Kyocera KC-60      pc-Si         60           1151              80

  Siemens Solar      mc-Si         55           1146              79
  SM-55                                                           

  Shell Solar RSM50  pc-Si         49           1139              79

  Photowatt PW100    pc-Si         91           1131              78

  Eurosolare M500A   pc-Si         55           1069              74

  BP Solar BP275F    mc-Si         75           1065              74

  GPV GPV75P         mc-Si         75           1036              72

  BP Solar MSX83     pc-Si         83           1024              71

  Atlantis Sunslates mc-S          12,5         1004              70

  Atersa APX90       mc-Si         90           892               62
  ------------------ ------------- ------------ ----------------- --------

4.  **PV-SYSTEMS WITH *UNI-SOLAR^®^* MODULES**

In order to get more statistically relevant data, larger PV-systems over
longer time frames have been monitored. As is very often the case with
all PV-systems, the ideal orientation and installation is very difficult
to implement due to all kinds of practical restrictions which are either
site specific or have to do with the amount of compromises the architect
and/or building owner is willing to accept. Anyway, even taking into
account those restrictions, excellent data have been achieved with
several *UNI-SOLAR*^^-PV-systems. Table III summarises the results of
*UNI-SOLAR*^^-PV-systems in several western European locations.

Table III -- Measured AC-yields of various *UNI-SOLAR*^^-PV-systems in
the Netherlands, Switzerland and the UK. Data courtesy of Ecofys (NL),
Glattwerk (CH), and Solar Century (UK).

+---------------+-------------+-----------+-------------+-------------+
| Location      | Product     | Total     | Monitoring  | AC-yield    |
|               |             | power     | period      | (kWh/       |
|               |             | (kWp)     |             | kWp/y)      |
+---------------+-------------+-----------+-------------+-------------+
| Eindhoven, NL | SSR-128/    | 3.456     | 11/98-11/99 | 863         |
|               | SSR-64      |           |             |             |
+---------------+-------------+-----------+-------------+-------------+
| Dübendorf, CH | US-64       | 6.912     | 1/99-1/00   | 916         |
|               |             |           |             |             |
|               |             |           | 1/00-1/01   | 898         |
+---------------+-------------+-----------+-------------+-------------+
| Richmond, UK  | SHR-17      | 1.088     | 3/99-4/00   | 827         |
+---------------+-------------+-----------+-------------+-------------+

The system in Eindhoven in the Netherlands consists of a standing seam
metal roof on a private house in which *UNI-SOLAR*^^ PV-Laminates of
128 Wp and 64 Wp are integrated. The roof has an inclination of 15° and
is oriented towards the southwest (215° with respect to north). A
chimney and a few trees give some shadowing, especially during winter
months. Ecofys, that has monitored the system over more than a year
\[3\], has calculated that the orientation of the system gives rise to 7
% less insolation than the optimal equivalent. The system has been
grid-connected in June 1998. The 64 Wp panels have been series connected
in pairs so as to form 128 Wp units as well (in total there are 27 units
of 128 Wp). Each such unit has an NKF-OK4 inverter of 100 Wp.
Unfortunately, these inverters were under-dimensioned and several of
them switched off when a high DC-current from the PV-panel came in (at
high insolation levels). After switching off, they did not switch on
again so that substantial energy yield from the modules has been lost.
Therefore, the inverters have been replaced by new ones on Feb. 02,
1999. Fortunately, some individually monitored 128 Wp panels had
inverters, which have remained intact throughout the testing period and
therefore, it is possible to make a correction on the recorded AC-energy
yield. Without such a correction, the system has generated 803 kWh/kWp
over the one year period between November 1998 and October 1999. With a
correction for the defective inverters, the system achieves 863 kWh/kWp
in the same one-year period. Remark that poly-c-Si systems in resp.
Amersfoort (School 'De Border') and Utrecht have generated 781 kWh/kWp
resp. 744 kWh/kWp in that very same year. The Ecofys-study \[3\]
concludes that this *UNI-SOLAR*^^ PV-system performs very well, remains
stable and has generated about 10-15 % more energy than a
standard-system based on c-Si, even though the in plane insolation is 7%
lower than for the optimal orientation.

The system in Dübendorf (some 10 kms north of Zürich, Switzerland)
consists of framed *UNI-SOLAR*^^ US-64 modules installed on the flat
roof of Glattwerk AG, a local utility. Total rated power of the system
is 6.912 kWp and the series-strings are connected to 2 ASP-inverters.
The system was grid connected in November 1998 and data for both the
calendar years 1999 and 2000 are available. Glattwerk AG has conducted
the monitoring. In January 1999, the PV-system has been disconnected
from the grid for about two weeks in order to do some finishing works on
the cabling and the other two weeks of that month, the modules were
covered with snow. The data have therefore been corrected (1999 only)
with an energy output equal to the average between the December and
February months in order to take the lost January month into account.
Not only do the data for this system show fairly high energy yields,
they also show that the modules remain stable over at least two years.
Remark that the flat roof, on which this system is installed, receives
quite some shadow from a neighbouring, much taller building. It is
estimated that this shadowing accounts for minimum 10 % of lost
insolation. In ideal conditions in Dübendorf, *UNI-SOLAR*^^ modules
would generate at least 1000 kWh/kWp/year. Remark that the average
annual energy yield of all systems in Switzerland over the years 1996,
1997 and 1998 was 858 kWh/kWp \[4\]. This is considerably less than for
the Dübendorf-system.

Fig. 4 A private house in Richmond (London) covered with *UNI-SOLAR*^^
PV shingles on the southern and northern slopes of the roof. Photo
courtesy of Solar Century.

On a private house in a suburb of London (Richmond), a 1.632 kWp
*UNI-SOLAR*^^ PV-shingle roof has been installed in March 1999. It is
the first PV-installation in the UK to be completely made using
electricity generating shingles. The PV-system consists of two separate
parts: 64 SHR-17 PV-shingle units are installed on the southern face of
the roof (total rated power 1.088 kWp), while 32 SHR-17 PV-shingle-units
were placed for testing purposes on the northern slope (total rated
power 0.544 kWp). The roof inclination is 40°. The PV-shingle units are
connected in 6 series strings of 16 units, which are distributed over
three SMA Sunnyboy SWR700 inverters. In the period March 1999 until
March 2000, the southern slope system generated a total of 900 kWh,
corresponding to a specific AC-yield of 827 kWh/kWp for the full year.
This yield is very high for UK standards, especially if one considers
the location of the installation: an urban environment close to a large
airport (London-Heathrow) that causes serious air pollution. A typical
c-Si PV-system under the same condition would generate in London no more
than 700-720 kWh/kWp per year \[5\], some 15% less. It is interesting to
note that the northern face installation also had a high yield: 528
kWh/kWp for the same period. This yield is still some 64% of the one of
the southern face installation despite the fact that the insolation on
that plane is more than 45% less than on the southern slope. This is a
clear indication that *UNI-SOLAR*^^ PV-products perform well under
non-ideal orientations.

5.  CONCLUSIONS

The gathered data have shown that new generation multi-junction
amorphous silicon solar modules, and in particular *UNI-SOLAR*^^
PV-products using Triple Junction solar cells, show good stability and
excellent outdoor performance. These PV-modules actually perform better
in real outdoor conditions than all competing c-Si and other thin film
silicon products. The higher energy production of *UNI-SOLAR*^^
PV-products, up to 20% kWh per kWp purchased, can be understood by their
excellent conversion efficiency at low light levels and their relatively
insensitivity to ambient temperature.

![](media/image9.jpeg){width="2.5368055555555555in"
height="2.046527777777778in"}ACKNOWLEDGEMENTS

The authors are indebted to Jaap Eikelboom of the Energie Centrum
Nederland (ECN), Nerio Cereghetti and Domenico Chianese of the Ticino
Solar group (TISO) of the University of Applied Sciences of Southern
Switzerland (SUPSI), Edith Molenbroek of Ecofys, Andreas Mück of the
Forschungszentrum Jülich (FZ Jülich), and Daniel Davies and Clive
Weatherby of Solar Century, for kindly providing us the monitoring data.

REFERENCES.

\[1\] Characterisation of PV-Modules of New Generation, J. Eikelboom, M.
Jansen, ECN-report No ECN-C --00-067, 2000 (downloadable at www.ecn.nl)

\[2\] Energy Production and Power Loss in PV Modules, N.Cereghetti, D.
Chianese, A. Realini, S. Rezzonico, G. Travaglini, paper B10, 16.
Symposium Photovoltaische Solarenergie, Staffelstein (D), 2001, p.
426-432

\[3\] Eindrapportage monitoring van het 3.5 kWp a-Si PV-syteem te
Neunen, Ecofys-report Nr. E 2087.3, 2000

\[4\] C. Meier, L. Konersmann and W. Blum: 'Photovoltaikstatistik 1998'
in Bulletin SEV/VSE 10/99, p. 11-13.

\[5\] Photovoltaics in Buildings -- A Design Guide, Report No ETSU
S/P2/00282/REP by Max Fordham & Partners in association Feilden Cleggs
Architects 1999, p12.