THE ADVANTAGES OF AMORPHOUS SILICON
PHOTOVOLTAIC MODULES IN GRID-TIED SYSTEMS
1
1
2
1
Kai W. Jansen , Suparna B. Kadam , and James F. Groelinger
Energy Photovoltaics, Inc., Princeton, NJ 2Kadam Consulting, E. Brunswick, NJ
ABSTRACT
Amorphous silicon (a-Si) technology developed by Energy
Photovoltaics, Inc. (EPV) has significant performance and
cost advantages over traditional crystalline (c-Si)
photovoltaic modules. Testing conducted in warm sunny,
as well as cooler and cloudier, climates has shown that aSi modules typically outperform c-Si modules on a
normalized energy basis. The higher performance arises
from a more favorable power loss temperature coefficient,
and a band gap that facilitates the conversion of blue-rich
light. Also, a-Si benefits from a relative increase in power
during the spring and summer months due to the thermal
annealing of metastable defects. Tandem junction a-Si
modules produced by EPV capture these benefits at the
lowest reported module cost/watt in the industry. As a
result, the energy production cost is lower for an EPV a-Si
PV system than for a similarly rated c-Si system. The
projected cost reductions for an EPV system, using
technology improvements already demonstrated on a
small scale, indicate that future EPV systems in Europe
will have an installed cost no greater than that of c-Si and
an energy cost that is ~20% lower.
crystalline (sc-Si) and multi-crystalline (mc-Si) modules
range between 11-17%. In comparison, the efficiency of
a-Si modules is roughly half, with averages of 6-8% [1].
However, in spite of the lower conversion efficiency,
amorphous silicon technologies have better real world
efficiency in terms of electricity production per installed
watt. Generally this is not recognized because modules
are rated at Standard Test Conditions (STC) of 1000
2
W/m , 25ºC, and AM1.5. However, a module deployed in
the real world generally will be exposed to these
conditions for only a brief amount of time during its life,
and therefore STC ratings are of limited use in evaluating
the economic performance of modules and systems.
Independent studies evaluating side-by-side energy yield
(generated energy per rated peak watt of power) of
various PV technologies show notable performance
advantages of a-Si technology. A study conducted by
Jardine and Lane, 2002, demonstrated double junction aSi modules leading the group in both a hot, sunny climate
(Spain) and a cool, cloudy climate (UK) (see fig. 1) [2].
2000
Spain
UK
INTRODUCTION
THE PERFORMANCE ADVANTAGE
Traditionally, crystalline silicon technology has been the
preferred choice of the PV marketplace due to its higher
energy conversion efficiencies. Efficiencies for both single
kWh/kWp
Amorphous silicon (a-Si) thin-film photovoltaics comprise
about 4% of the total photovoltaics market, which is
dominated by a variety of crystalline silicon (c-Si)
technologies. The low market penetration of a-Si is
generally considered to be a result of its relatively low
energy conversion efficiency and perceived high per-watt
installed cost. However, a closer look at the relative
advantages of a-Si reveals that it is a highly competitive,
and in some instances, an economically superior choice.
Amorphous silicon offers several attractive benefits in
terms of energy production, cost, BIPV suitability, and
environmental attributes.
In the current market
environment, a lifetime economic analysis shows an
Energy Photovoltaics, Inc. (EPV) a-Si system to be 6%
better than the average crystalline system in a sunny
climate. With anticipated improvements, the financial
savings are projected to approach 20% and more.
1600
1200
800
400
0
a-Si Double
CIS
a-Si Triple
mc-Si
sc-Si
CdTe
Fig. 1 Annual energy yield in Spain and the UK for several
PV technologies (Jardine and Lane, 2002)
Similar results were obtained on a smaller scale in a sideby-side test of standard EPV-40 and c-Si modules in
Florida. On a hot summer day, the EPV modules
produced over 30% more energy per rated watt than a
typical c-Si module, particularly during the hottest part of
the day as shown in Fig. 2.
Normalized Output (W/Wp)
1.00
>30% m ore
energy from EPV
0.80
EPV a-Si
c-Si
0.60
Diffuse Light Conditions
0.40
0.20
0.00
5:00
light due to its higher energy gap. In the Sandia study,
they reported a ~6% increase in the performance of a-Si
modules from winter to summer due to spectral effects,
excluding the effects of annealing and temperature. At the
same time, c-Si module performance decreased by ~3%
winter to summer, solely due to the changing spectral
content of the incident sunlight [3].
7:00
9:00
11:00
13:00
15:00
17:00
19:00
Tim e of Day
Fig. 2 Typical EPV a-Si and c-Si energy generation on a
clear summer day in Florida
High Temperatures and Irradiance
Amorphous silicon modules have this performance
advantage over c-Si as a result of fundamental differences
inherent in the a-Si semiconductor. Most importantly, a-Si
dominates in warm, sunny conditions due to its lower
power-loss temperature coefficient.
The temperature
coefficients of a-Si are typically half those of c-Si or CIS at
-0.2%/K vs. -0.4 to -0.5%/K, resulting in half of the power
loss at elevated temperature [1]. At the Spain test site, the
majority of energy was produced at high irradiances
between 500 and 900 W/m2, which correspond with high
temperatures.
At such high irradiances, modules
temperatures can regularly exceed 60ºC, at which point a
40 watt a-Si module will experience a decrease of 2.8
watts in its rated power. However, a similarly sized c-Si
module would lose at least twice that amount of power, 5.6
watts, and have a significant decrease in its power
generating capacity during the period of peak sunlight.
This effect is largely responsible for the difference in
power production between EPV a-Si and c-Si shown in fig.
2.
In addition to the more favorable temperature coefficient,
numerous studies have shown that high module operating
temperatures actually improve the performance of
stabilized a-Si modules. This results from the annealing
and removal of selected light-induced defects inherent in
a-Si. Studies at Sandia National Laboratories have shown
that the effect of annealing for an outdoor a-Si array is an
increase in power of about 7% from winter to summer,
independent of other effects [3].
The spectral matching of the a-Si cell response with the
solar spectrum also gives it an advantage anytime the sun
is at its brightest – at midday or in the summer – which is
also the times that electricity is most highly valued. At
these times, sunlight is rich in blue light. Accordingly, a-Si
has a higher spectral response to blue light than to red
A-Si modules also perform better during cloudy sky
conditions, as shown in the PV Compare UK study where
the majority of the energy was delivered at low irradiances
between 100 – 500 W/m2 [2]. Under these overcast skies,
light is more diffuse and richer in blue illumination. Since
there is a better match with the spectral distribution of
outside illumination, the a-Si modules have a competitive
advantage in such a climate.
THE ECONOMIC ADVANTAGE
In the past, various cost analyses have been used to
justify the selection of a c-Si PV system over an a-Si
system. Under the traditional evaluation of cost on a
dollar per rated-Watt basis, systems using c-Si usually
have a lower installed cost, especially for large arrays. A
more appropriate measure of the economics, however,
considers the levelized cost of energy generated from a
PV system over its lifetime. A cost analysis that is based
on energy generation and not on rated power is more
appropriate since a PV system operator is remunerated for
the energy that is produced and not for the rated power of
their system. To illustrate this difference, two similar
large-scale arrays are considered. One array is based on
published data for a 1 MW c-Si system in Germany [4],
and the other is a similarly designed, hypothetical array
using the lowest cost commercially available a-Si
modules, the EPV-40, manufactured by Energy
Photovoltaics, Inc.
Total Installed Costs
EPV modules have a distinct cost advantage over
crystalline modules because of their low materials cost
and EPV’s unique low cost manufacturing process. In the
current module market, EPV modules, at selling prices in
the range of $2.80/Wp, are roughly 20% less expensive
than average crystalline modules at $3.50/W p. However,
the lower efficiencies of thin film technologies result in the
need for more modules and land area to deliver the same
rated power as a crystalline PV system. This increases
the wiring and structural mounting costs for thin film.
Consequently, the higher balance of system and O&M
cost offsets the savings in module cost. As a result, a
crystalline PV system has a total installed cost advantage
of roughly $0.50/W, which is about 10% lower than the
cost of an EPV-based system (see Table 1).
Generated Electricity Costs
Currently, a-Si systems, including those built with EPV
modules, have a higher total installed cost. However, they
consistently demonstrate that they will generate more
electricity during their lifetimes across a range of climates
[2]. In Spain or an equivalent area, the lifetime energy
yield is 15% higher. In the UK or similar climates, an EPV
system is expected to produce 12% more electricity (see
fig. 3).
'000 kWh/kWp
40
EPV - Spain
Crystalline - Spain
30
35
EPV - UK
Crystalline - UK
30
20
21
19
10
1
5
9
13
Year
17
21
25
Fig. 3 Estimated cumulative energy production of a-Si and
c-Si arrays in Spain and the UK (based on data from
Jardine and Lane, 2002).
Consequently, a large 1MW EPV system currently would
have a slight economic advantage over a crystalline
system after the total installed costs and the lifetime
energy production are accounted for, as Table 1 shows.
As PV technology continues to improve and market
conditions normalize, the economic benefits of a-Si are
expected to become even greater.
Most of the improvements necessary to significantly lower
energy generation costs have already been demonstrated
in pilot production.
In recent conferences, EPV
announced process improvements demonstrating a 20%
increase in performance with a parallel reduction in cost
[5]. As Table 1 shows, by roughly doubling the current
2
module size to 1.6m and raising the maximum system
voltage to 1000V, a 22% cost advantage can be obtained
without any further technology improvements. In addition,
significant cost reductions will be achieved by expanding
production from the present capacity of a few MW/yr to 2050 MW/yr and beyond. These advances will have the
effect of lowering the installed cost/W of EPV a-Si relative
to c-Si modules, while maintaining the energy
performance advantage of the a-Si technology. Based on
the potential of these improvements, it is fully expected
that a system designed with the next generation of EPV aSi modules will have an installed cost/W that is similar to
present-day c-Si systems, and perhaps lower. With
comparable installed costs/W, EPV-based systems are
expected to see at least a 15-20% lifetime energy cost
advantage relative to crystalline systems.
Table 1. Cost Comparisons for 1 MW EPV and c-Si PV Systems
System Elements
EPV
EPV
1.6 m2
module
EPV
1.6 m2 and high
voltage module
Modules
$2.80
$2.60
$2.60
$3.50
/W
Balance of System
Mounting
DC wiring
Inverter/power conditioning
Other (fences, trenches, etc.)
$2.13
$1.38
$0.15
$0.51
$0.08
$1.55
$0.88
$0.11
$0.51
$0.05
$1.42
$0.88
$0.11
$0.38
$0.05
$1.05
$0.53
$0.11
$0.38
$0.04
/W
/W
/W
/W
/W
O&M
Total Installed Cost
PV Electricity Costs
UK
Spain
$0.40
$5.33
$0.40
$4.55
$0.40
$4.42
$0.30
$4.85
/W
/W
$0.250
$0.151
$0.214
$0.129
$0.207
$0.126
$0.260
$0.162
c-Si
/kWh
/kWh
Avoided Peak Power Costs
In locations with high air conditioning (A/C) use on hot
summer days, the generation profile of PV has a strong
alignment with peak electricity demands, which are both
driven by the sun’s intensity. During these peak load
periods, the cost to produce and deliver electricity is the
highest since many utility grids are congested and do not
have the generation and distribution capacity to meet the
demand.
Accordingly, electricity prices during these
periods can significantly exceed the system average. For
these locations, PV – and a-Si in particular – provides the
additional economic benefit of generating electricity that
offsets costlier peak power. The 2-tiered commercial Time
of Use rate structure for Progress Energy in Florida is
shown in Fig. 4 along with the normalized output for EPV
and c-Si test arrays over the course of 5 months (June
through October 2005), deployed at the Florida Solar
Energy Center (FSEC). Both systems provide about half
of their power during the peak load period, but the EPV
system produces 20% more energy during the highest rate
tier, resulting in a corresponding higher overall economic
benefit.
Normalized Output (kWh/kWp)
$0.50
EPV a-Si
80
c-Si
20% more Peak
energy delivered
by EPV
Electricity Rate
60
$0.40
$0.30
40
Peak
$0.22/kw h
20
$0.20
$0.10
Of f Peak
$0.09/kw h
0
4:00
6:00
$0.00
8:00
10:00
12:00 14:00 16:00
Tim e of Day
18:00
20:00
Fig. 4 Normalized cumulative output of EPV a-Si and c-Si
modules tested at FSEC (June to October 2005), shown
with the local Time of Use rate structure.
SUMMARY
Although crystalline silicon PV is the dominant technology
choice today, amorphous silicon technology is very cost
competitive. In spite of lower efficiencies and the need for
more land area, EPV a-Si generates lower cost electricity
than crystalline technology. The three contributing factors
for higher performance – low temperature coefficient, blue
light absorption, and thermal annealing – help a-Si
outperform other technologies under a variety of climates.
In addition, there is significant value in generating more
electricity during peak demand periods in regions with high
summer A/C usage and constrained utility capacity.
These performance characteristics, combined with its
aesthetics, make a-Si a natural fit for architectural building
integrated applications as well. These multiple benefits
demonstrate that a-Si is a compelling and economically
advantageous alternative to conventional crystalline PV.
REFERENCES
[1] PHOTON International February 2005. “Market survey
solar modules 2005.”, pp. 48-67.
[2] C. Jardine and K. Lane, "PV-COMPARE: Relative
Performance of Photovoltaic Technologies in Northern and
Southern Europe," Proceedings of the PV in Europe
Conference, October 2002, Rome, Italy, pp. 1057-1060.
[3] D. King, J. Kratochvil, and W. Boyson. “Stabilization
and Performance Characteristics of Commercial
Amorphous-Silicon PV Modules”, Proc. 28th IEEE PVSC,
2000, pp. 1446-1449.
[4] M. Bachler and C. Bindel. “Cost Comparison of Large
Scale Crystalline and Thin Film Systems.” Proc. of the 20th
European Photovoltaic Solar Energy Conference,
Barcelona 2005, pp. 3134-3138.
[5] K.W. Jansen, H. Volltrauer, A. Varvar, D. Jackson, B.
Johnson, L. Chen, J.A. Anna Selvan, A.E. Delahoy,
“Advancements in a-Si Module Manufacturing at Energy
Photovoltaics, Inc.”, Proc. of the 20th European
Photovoltaic Solar Energy Conference, Barcelona, 2005,
pp. 1553-1556.
ACKNOWLEDGEMENTS
The authors would like to thank Dr. Neelkanth Dhere and
his associates at the Florida Solar Energy Center (FSEC)
for providing performance data. We are also grateful to
Dr. Bolko von Roedern and his colleagues at NREL for
supporting the FSEC study.