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Preprint: Heidari, N., Gwamuri, J., Townsend, T.,Pearce, J.M. (2015). Impact of Snow and Ground Interference on Photovoltaic Electric System
Performance. IEEE Journal of Photovoltaics 5(6),1680-1685, (2015). doi:10.1109/JPHOTOV.2015.2466448
Abstract— Assessing snow-related energy losses is necessary for
accurate predictions of photovoltaic (PV) performance. A PV test
platform with seven portrait-oriented modules placed at four tilt
angles (0, 15, 30, and 45 degrees) was installed in Calumet,
Michigan to measure the energy loss in this snowy climate. As a
best-case snow-shedding configuration, three modules were
elevated high enough to prevent surface interference. The opposite
effect of maximum surface interference was introduced by
mounting the other four modules at ground level. The platform
was monitored for one year beginning in October 2013. The
snowfall that winter was 5.3 m (209 inches). Snow-related annual
energy losses ranged from 5% to 12% for the elevated,
unobstructed modules, with the steepest tilt angle experiencing the
least amount of energy loss. For the obstructed modules, there was
proportionately less angular dependence on lost energy and
annual energy losses ranged from 29% to 34%. This relative 3- to
6-fold increase in lost energy when ground interference is present
points out the importance of prompt snow clearing for portrait-
oriented PV. Depending on the breadth of an inverter’s operating
voltage limits, these results suggest that landscape-oriented array
layouts and perhaps snow-clearing mechanisms may be
advantageous in snowy climates.
Index Terms— Electricity; Energy loss; photoelectricity;
photovoltaic cells; power systems; solar energy
I. INTRODUCTION
Solar photovoltaic (PV) technology is a technically viable
and environmentally beneficial solution to society's future
electrical needs [1,2]. However, to optimize both the
environmental [3-5] and economic [6-9] outcomes, including
financing of PV systems, [10,11] accurate prediction of system
yields is critical and requires in-depth accounting of all loss
mechanisms [12,13]. PV technology is increasingly being
deployed in areas with low annual irradiation, like Northern
Europe [3], and in regions that regularly experience snowfall,
such as Germany, Japan, Canada, and the northern U.S. The
accumulated snow on the modules affects the performance of
the system and decreases the output power [14]. Previous
studies indicate that annual snow losses for a low tilt angle
system can easily reach or exceed 15%. Even for an
unobstructed, higher-slope (28 degree) roof mount system in
N. Heidari is in the Department of Electrical & Computer Engineering,
Michigan Technological University, MI (e-mail: heidari@mtu.edu)
J. Gwamuri is in the Department of Materials Science & Engineering,
Michigan Technological University, MI (e-mail: jgwamuri@mtu.edu )
T. Townsend is in the Renewables Advisory – Solar department, DNV GL,
San Ramon, CA (e-mail: Tim.Townsend@dnvgl.com )
Germany, the losses, while smaller, could still range from 0.3-
2.7% [15-22]. As more of the better sites for PV are claimed
(e.g., unobstructed rooftops and high-rack ground mounts),
lesser-grade sites such as those subject to frequent snows will
become more popular. To quantify the snow loss effect, a test
site has been designed and deployed in a heavy snow location
to investigate PV electricity generation losses as a function of
tilt angle and snow sliding obstruction geometry.
II. METHODOLOGY
A system has been developed to investigate the effects of snow
on the performance of photovoltaic modules as a function of tilt
angle and degree of ground interference. The system was
deployed at the Keweenaw Research Center (KRC) located in
Calumet, MI USA, and webcam archived images are freely
available to the public [23]. The study has been conducted on
seven 140 W Kyocera (KD140) poly-Si modules. Four modules
are mounted at ground level at angles of 0, 15, 30, and 45
degrees, with no ground clearance. Three other modules are on
a raised rack with a ground clearance of 1.5 m, at angles of 15,
30, and 45 degrees. To simulate the effect of the modules being
in a large array, a 30 cm border of blue metal shielding was
applied around each module. The layout of the test site is shown
in Fig 1. It should be pointed out that only one module at zero
degree tilt angle is needed, as the same snow losses are expected
for a well-bordered horizontal module that is either elevated or
mounted close to the ground.
Each PV module was monitored for temperature (T) with an
accuracy of +/-0.9 °C and for short-circuit current (Iexp(T)) with
an accuracy of +/-1%. All measurements were carried out at 15-
minute intervals. Solar irradiance (Psun) was measured using
four LI-COR Li-200SA pyranometers mounted on individual
panels for each angle. The pyranometer uncertainty is ±5%
[24,25]. The parameters
P25
̊
C
,
I
exp25'C
,
T
25'C
, and TC are
obtained from the module manufacturer’s data sheet [26]. The
short-circuit current data for the seven PV modules was
recorded for the year starting in October 2013 and analyzed for
the entire year and also over a subset of it encompassing just the
Fall 2013 and Spring 2014 snow season (November 2013 –
May 2014). The total snowfall recorded for this period was 209
inches [27]. The 2013-14 snowfall of 209 inches was 15%
above the past 5-year average, but 4% below the 40-year
J.M. Pearce is in both the Department of Electrical & Computer Engineering
and Department of Materials Science & Engineering, Michigan Technological
University, MI Corresponding author: 601 M&M Building, 1400 Townsend
Drive, Houghton, MI 49931-1295 (e-mail: pearce@mtu.edu).
Impact of Snow and Ground Interference on
Solar Photovoltaic Electric System Performance
N. Heidari, J. Gwamuri, T. Townsend, J.M. Pearce, Member, IEEE
Preprint: Heidari, N., Gwamuri, J., Townsend, T.,Pearce, J.M. (2015). Impact of Snow and Ground Interference on Photovoltaic Electric System
Performance. IEEE Journal of Photovoltaics 5(6),1680-1685, (2015). doi:10.1109/JPHOTOV.2015.2466448
average. The recorded amount can be viewed as typical, since
it fit well within the long-term standard deviation of ±25% for
annual snowfall at this location as seen in Table I.
TABLE I
HISTORIC SNOWFALL RECORD FOR KEWEENAW RESEARCH CENTER
Year
Annual snowfall
(inches)
Comment
2013-14
209
15% > 5-yr avg.
4% < 40-yr avg.
5-year avg
181 ±34
±19% variation
@1σ
40-year avg
218 ±54
±25% variation
@1σ
40-year low:
2011-12
132
39% < 40-yr avg.
40-year high:
1978-79
356
63% > 40-yr avg.
Townsend and Powers developed equations to calculate
monthly energy loss as a function of climate and array geometry
[15]. Others, including the authors above, have attempted to
correlate snow-related energy loss on shorter hourly or daily
time scales, with significant limitations. The focus of this study
is on monthly results, though individual measurements are
recorded on 15-minute intervals. Here power is not directly
measured, but is estimated. This is done by relying on
pyranometer and temperature data in the first case, and in the
other case, by relying on short-circuit current and temperature
data. In both cases, the field measurements must be
supplemented with manufacturer’s specifications for the
module and the pyranometers (in this case, Kyocera 140 W
poly-Si type modules, with 36 cells in series, and LI-COR Li-
200SA pyranometers).
The power from each snow-exposed module at each 15-minute
interval was calculated as:
=()(())
(( )) (1)
By considering the flash-tested power rating of the modules and
the irradiance obtained from the pyranometers at each angle, the
potential clean-module power that can be extracted at each tilt
angle is then determined using equation (2) below:
= (
1+( )) × (())
(2)
In the equation above, either the reference irradiance of 1,000
W/m2 or the measured irradiance should be corrected for
temperature. As the reference irradiance is normally viewed as
a constant, here the measured irradiance is adjusted to see what
it would have been under the same conditions that the reference
irradiance was measured at, 25°C. The module power, however,
is sought for the field temperature, not the reference
temperature, so that temperature adjustment is the same as the
one done for the snowy module. The energy loss due to snow is
calculated as the difference in energy without snow, Pc, versus
the energy obtained from snow covered modules, Pm:
()=(
×)(
×) (3)
A. Abbreviations and Acronyms
α: Temperature Coefficient of Current, module [1/C]
: Temperature Coefficient, pyranometer [1/C]
C: Temperature Coefficient of power, module [1/C]
Eloss: Energy loss [kWh]
Iexp(T): Short-circuit current measured at experimental
temperature [Amps]
Iexp25'C
: Short-circuit current at Standard Test Condition
[Amps]
Pc: Power that can be extracted from each (hypothetical) clean
module (without snow) [Watts]
Pm: Calculated output power of snow-exposed module (at
various angles and heights) [Watts]
Psun: Irradiance obtained by pyranometer (at various angles)
[Watt/m2]
P
25
̊
C
: Maximum power of flash-tested modules at STC,
nominally 140 W [Watts]
T : Experimental Temperature [Celsius]
T
25'C
: Temperature at Standard Test Conditions (STC) 25oC
[Celsius]
t : Time Stamp, (15 minutes was used for this test) [Hours]
Fig. 1. 7-module snow test platform at the Keweenaw Research Center
III. RESULTS AND DISCUSSION
Fig. 2 below shows the yearly energy loss for the seven
module positions, while Fig. 3 presents the same results, but
limited to the snow season months of November-May. Fig. 2
shows that as the tilt angle increased from zero to 45 degrees in
the unobstructed cases, the yearly energy loss decreased from
34% to just 5%. However, in the case of obstructed modules,
that trend is not apparent, as the losses appear to be similarly
clustered in the 29-34% range. At low angles, the obstructed
case results are similar to the unobstructed case, and at zero tilt,
are deemed to be identical. Measurement error obscures some
of the true trend, but even at 45 degrees, the loss percentage
remains very high. Even though snow readily slides off the
Preprint: Heidari, N., Gwamuri, J., Townsend, T.,Pearce, J.M. (2015). Impact of Snow and Ground Interference on Photovoltaic Electric System
Performance. IEEE Journal of Photovoltaics 5(6),1680-1685, (2015). doi:10.1109/JPHOTOV.2015.2466448
steeply tilted module, if it remains piled up at the base it will
still cause a near-100% loss of power as long as even one cell
in the 36-cell string is shaded.
Fig. 2. Yearly energy losses due to snow for obstructed and unobstructed
photovoltaic modules located in Calumet, MI.
Energy loss for the 45 degree unobstructed module was
5.2%, while the loss for the obstructed module with the same
tilt angle was 31.3%. This represents a six-fold worsening of
lost energy (a 26.1% absolute annual loss) for this extreme
obstruction geometry. At the 30-degree tilt, the loss
magnification is less severe, just a three-fold difference, but still
represents an additional absolute annual loss of 18.8% (10%
and 28.8% for the unobstructed and obstructed cases,
respectively). Results for the 15-degree tilt were about the same
as for the 30-degree situation, and the worst-case loss of 34%
occurred for the zero-tilt case, as expected.
Fig. 3 shows that energy losses during the snow season
follow the same relative trend as for the yearly energy losses
depicted in Fig. 2. However, on a fractional basis, the energy
losses in the snow season (November - May in the Upper
Peninsula of Michigan) are higher due to the reduced winter
solar resource. The error for the measurements was small and
can be attributed to dust or snow accumulation on the
pyranometer. This error would bias the reported energy losses
to be slightly lower than they actually were. Finally, daily
energy losses in kWh from mid-October 2013 to mid-October
2014 for each module have been plotted, and the results for the
worst case (obstructed with the tilt angle of 45), and the best
case (unobstructed with the tilt angle of 45) are presented in
Figs. 4 and 5, respectively.
Fig. 3. Snow season (November to May in the Upper Peninsula of Michigan)
energy losses for obstructed and unobstructed solar photovoltaic modules in
Calumet, MI.
Fig 4. Daily energy loss, in kWh, for the obstructed module with the tilt angle
of 45 degree throughout the year in Calumet, MI.
Figures 4 and 5 show the daily effect of module snow cover
on PV system electricity generation capacity. It can be observed
by comparing the obstructed and unobstructed modules of the
same angle that the power for the unobstructed module (Fig. 5)
rapidly returns to full power after each snowfall event. Fig. 4,
on the other hand, shows significant and persistent losses
through the entire snowy season, especially in February and
March, as the module rarely became clear enough to deliver full
power.
Fig 5. Daily energy loss, in kWh, for the unobstructed module with the angle
of 45 degree
Since the accumulation of snow on the modules has such a
strong effect on the performance of the modules [14], the proper
assessment of energy losses have become important for
improving the electrical performance of the system and
understanding its economic performance [21]. Many of the
studies that have investigated snowfall have been in less snowy
areas or have been conducted during low-snow periods. For
example, in [21], 70 modules with different technologies
oriented at different angles were monitored for two winters near
Toronto, Ontario, Canada to obtain energy losses. Yearly losses
ranged from 1-3.5%, which is small in comparison to the
present study, which finds yearly losses ranged from 5-34%,
Preprint: Heidari, N., Gwamuri, J., Townsend, T.,Pearce, J.M. (2015). Impact of Snow and Ground Interference on Photovoltaic Electric System
Performance. IEEE Journal of Photovoltaics 5(6),1680-1685, (2015). doi:10.1109/JPHOTOV.2015.2466448
indicating that energy losses in the Michigan UP region are
significantly higher than other regions with typically half of the
normal snowfall, like Toronto. It should be noted the years for
the Ontario study were abnormally warm and had little snow.
Townsend and Powers conducted a study in Truckee,
California, which receives about 200 inches of snow annually.
Data were obtained at three tilt angles for pairs of modules, one
of which was kept clean and one which was allowed to gather
and shed snow as prevailing weather conditions dictated. The
snow-exposed modules were mounted about 0.5 m above grade,
so pile-up interference happened often, but not always [15].
Data were also available for a similarly oriented system located
just 3 km away, but which had no ground interference. The
annual losses were found to be 6% for the no-interference, 35-
degree tilt system. These losses increased to 13%, 17%, and
26% for the test station’s angles of 39, 24, and 0 degrees,
respectively. These values are in agreement with the results
from the current study. Other studies found that annual snow
losses for a low profile system can easily reach or exceed 15%.
Even for an unobstructed, higher slope (28 degree) roof mount
system, the losses, while smaller, could still range from 0.3-
2.7% [15-22].
In order to improve performance of the PV systems in snowy
climates like this region, obstructed PV systems should not be
used, unless heavy snow-related losses are viewed beforehand
as acceptable or a method of clearing them is deployed. This
will, for example, limit some rooftop applications, as in this
area they can result in a yearly energy loss of 30% even for high
tilt angles. The results of this study also indicate that the
physical orientation and electrical stringing of the modules is
important. For example, when the modules are partially covered
with snow (as the obstructed modules are on the right of Fig. 1),
the current was decreased by a much larger percentage than the
percent of module area that the snow covered. In this study, the
modules were all oriented in portrait layout, resulting in the by-
pass diodes becoming ineffective during periods of partial
shading. This affected the output power of modules giving the
worst case scenario of power loss of up to 92% (or more,
depending on the thickness of snow cover) [28]. This problem
could be partly alleviated by orienting the modules in landscape
format. Landscape format enables bypass diodes to nominally
skip one-third of the obstructed cells at a time, and if the micro-
inverter or string inverter can operate at a reduced dc input
voltage, the system can operate. However, this study was not
constructed to quantify this more shade-tolerant geometry and
future work is needed to quantify this loss. [29].
At the KRC site, the pyranometers were heated to help melt
off the snow, and technicians were available to clean the
pyranometers after a snow event. The data were screened to see
if there were occasions when the pyranometers were covered
with snow. This was done by comparing readings of sun-facing
pyranometers (located at 0, 15, and 30, and 45 degree modules)
with the readings of a downward-facing pyranometer (from the
back of the elevated 45 degree module). The main purpose for
this pyranometer is to serve as a quality check against irradiance
measurements made on the sun-facing front sides of the frames.
The rear-facing module is shielded from snow and during
daylight hours should always read a small positive number. If
the down-facing pyranometer had a higher value than the
readings of a sun-facing pyranometer, it was likely because the
sun-facing pyranometer was being affected by snow. In order
to compare the readings, whenever the reading of the sun-facing
pyranometer was less than 90% of the reading of the
downward-facing pyranometer, then the sun-facing
pyranometer was assumed to be covered with snow. The hours
that pyranometers were covered with snow in the winter were
found to be 2, 13.5, 12, and 1.5 hours for pyranometers located
at the 45, 30, 15, and 0 degree modules respectively. There were
also times that pyranometers were covered with snow for 15
minutes or half an hour during snow fall events and were then
either cleaned or self-cleaned by wind, but these brief, low-to-
zero irradiance periods were of trivial numerical impact on the
analysis and were not considered among the total hours that the
pyranometers were deemed to be covered with snow.
Future work is also necessary for investigating how snow
affects low-concentration systems, which have been proposed
[30]. In these setups, a reflector is attached from the back top of
one row of modules to the front bottom of the next. Normally
all reflectors are flush. If the reflectors are utilized at the back
of the modules to enhance system electrical generation [31,32],
they will need to be spaced out far enough from the modules to
allow for snow to slide off.
Finally, all systems could benefit from an active method to
clear snow. Thus future work is needed to quantify the
effectiveness of different forms of cleanings such as melting off
the snow [33,34], chemical coatings [35], using squeegees [36],
and tennis balls divots to catalyze clearing [37] to reduce the
snow-related energy losses. This work involves experimental
and economic analysis of these snow removal techniques.
IV. CONCLUSIONS
In this paper, energy losses of PV systems with different
architectures and tilt angles were quantified for a test site
located in Calumet, Michigan. It was seen that energy losses
due to snowfall are significantly dependent on the tilt angle and
the degree of ground interference. The study indicated that
annual energy losses decreased dramatically, from 34% to 5%,
as tilt angle increased from zero to 45 degrees for unobstructed
systems. For obstructed systems, annual energy loss hovered in
the 30-34% loss range, regardless of tilt angle. This suggests the
role of ground interference varies from no impact on flat-tilt
systems to a six-fold worsening for 45-degree tilt systems. This
one-year study (2013-14) showed that snow-related energy
losses ranged from 5% to 12% for three elevated modules and
from 29% to 34% for comparably tilted modules mounted next
to the ground. The 2013-14 snow season was slightly higher
than the 5-year average preceding it but slightly lower than the
40-year average, so future losses for this location should be
roughly comparable to this study’s findings. Current results
serve to inform PV system design and optimizations in this and
other locations with similar weather patterns. The study is still
ongoing to better home in on long-term expected performance.
It was found that proper assessment of energy losses due to
snowfall can significantly improve understanding of system
economics and also highlight possible ways to improve
performance by actively cleaning the modules. It can be
surmised that in this region, higher tilt angles of around 45
degrees and unobstructed snow shedding geometries are
Preprint: Heidari, N., Gwamuri, J., Townsend, T.,Pearce, J.M. (2015). Impact of Snow and Ground Interference on Photovoltaic Electric System
Performance. IEEE Journal of Photovoltaics 5(6),1680-1685, (2015). doi:10.1109/JPHOTOV.2015.2466448
recommended in order to keep annual energy loss to an
acceptable level.
ACKNOWLEDGMENTS
The authors would like to thank the generous support of the
Keweenaw Research Center and DNV GL for materials and
labor to perform this work.
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Preprint: Heidari, N., Gwamuri, J., Townsend, T.,Pearce, J.M. (2015). Impact of Snow and Ground Interference on Photovoltaic Electric System
Performance. IEEE Journal of Photovoltaics 5(6),1680-1685, (2015). doi:10.1109/JPHOTOV.2015.2466448
Negin Heidari received her bachelor’s
degree in Electrical Engineering from
Azad University of Kerman.
She is currently a Master student at
Michigan Technological University.
Her research activities include the
investigation of impact of snow and
ground interference on the performance of PV systems, liability
of greenhouse gas emissions, energy production calculation for
large scale PV systems, modeling and simulation of micro-
grids, wind turbines, and batteries.
Jephias Gwamuri was born in Lupane,
Zimbabwe, in 1970. He received his
B.S. degree in Physics and Electronics
from ISP “Enrique Jose Varona”, Cuba,
and his M.Sc in Lasers and Applied
Optics from NUST, Bulawayo in
Zimbabwe in 2004. He is currently
pursuing his Ph.D. degree in material
science and engineering at Michigan Tech, Houghton, MI.
From 2006 to 2012, he was a lecture at the National
University of Science and Technology, Bulawayo, Zimbabwe.
Since 2012, he has been a PhD research fellow with the Material
Science and Engineering Department, Michigan Technological
University, Houghton. His research interests include novel thin
film PV devices, renewable energy technologies, energy
efficiency and sustainability.
Mr. Gwamurir was a recipient of the SPIE best student
poster presentation and OSA student award. He also received
the Fulbright Science and Technology award – PhD fellowship
in 2012.
Tim Townsend received his B.S. in
thermal & environmental engineering
from Southern Illinois Univ.-Carbondale
and his M.S. in mechanical engineering
from the Univ. of Wisconsin-Madison.
He performed O&M and monitoring for
over a decade at the landmark PVUSA
utility-scale PV demonstration project in
Davis, CA, authoring several conference
papers stemming from that project. He
has provided PV training for over 2,000 journeyman
electricians, and contributed (as principal author) the “PV
Systems” chapter of the Handbook of PV Science and
Engineering (2nd ed., Wiley & Sons, 2011). In 2005, he
established today’s common style for PV Independent
Engineering (IE) analysis and reporting, including applying
PVsyst as the preferred modeling platform for aiding investor
decision-making. Part of his recent and ongoing IE work has
included designing a PV snow loss experiment at Truckee, CA;
this resulted in the first and still sole model for predicting
monthly PV snow loss [15] and was the impetus for this paper
with MI Tech.
Joshua M. Pearce received his Ph.D. in
materials engineering from the
Pennsylvania State University.
He then developed the first
Sustainability program in the
Pennsylvania State System of Higher
Education as an assistant professor of
Physics at Clarion University of
Pennsylvania and helped develop the
Applied Sustainability graduate
engineering program while at Queen's University, Canada. He
currently is an Associate Professor cross-appointed in the
Department of Materials Science & Engineering and in the
Department of Electrical & Computer Engineering at the
Michigan Technological University where he runs the Open
Sustainability Technology Research Group. His research
concentrates on the use of open source appropriate technology
to find collaborative solutions to problems in sustainability and
poverty reduction. His research spans areas of electronic device
physics and materials engineering of solar photovoltaic cells
and systems, and 3-D printing, but also includes applied
sustainability and energy policy. Dr. Pearce is the author of the
Open-Source Lab: How to Build Your Own Hardware and
Reduce Research Costs (Elsevier, 2013).
