The document discusses modeling energy losses from snow on photovoltaic (PV) systems using the System Advisor Model (SAM). It describes a snow loss model implemented in SAM that estimates snow coverage over PV modules based on factors like system tilt, irradiance, temperature and snow depth. The model was validated against measurements from two PV systems, showing much better accuracy when including snow losses. National modeling for the U.S. estimated average snow losses by region and their correlation with total snow depth. Future work may consider snow losses for tracking systems or improving accuracy at shorter timescales.

1. NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Modeling Energy Losses Due
to Snow on PV Systems
4th PV Performance Modeling
and Monitoring Workshop
Janine Freeman
October 22, 2015

2. 2
Why Include a Snow Model? System Advisor Model
Image Credits: http://www.energymanagertoday.com/cleaning-the-snow-off-solar-modules-097476/
Becker, Gerd, et al. "An approach to the impact of snow on the yield of grid connected PV systems." Bavarian Association for the Promotion of Solar Energy, Munich (2006).

3. 3
Snow Loss Model by Marion et al
• Estimates the amount
of snow still covering
a row of modules
• Sliding is the
dominant removal
process
• Model requires:
• System tilt
• POA Irradiance
• Ambient temperature
• Ground snow depth
• Some module/string
layout parameters
System Advisor Model
Image credit: “Measured and modeled photovoltaic system energy losses from snow for Colorado and Wisconsin locations”, Bill Marion et al, Solar Energy 97 (2013) pg 112-121

4. 4
Implementation in SAM
Also accessible through SDK as part of pvsamv1 or as
separate compute module for post-processing
System Advisor Model

5. 5
Demonstration for Two Systems System Advisor Model
Annual Error:
W/o Snow Model: 9.9%
With Snow Model: -1.9%
Annual Error:
W/o Snow Model: 7.3%
With Snow Model: -0.1%
Figure 2. Results from the validation study using Forrestal system in
Washington, D.C. and the RSF2 system in Golden, Colorado

6. 6
Modeled Snow Losses for the U.S. System Advisor Model
Figure 4.
Results from a
national study
modeling PV
production losses
due to snow
coverage using
both a fixed-tilt
tilt-equals-
latitude and a tilt
equals 20° tilt
system design,
the 1961-1990
NSRDB data set,
and the newly
implemented
snow model in
SAM
Note: Tabulated results by location available as an appendix to the full report

7. 7
Snow Loss Estimates for the U.S. System Advisor Model
Figure 5. General trends in average snow losses as a percentage of annual energy production
Note: Like-colored regions have similar loss percentages and are labeled in the figure. The specific
region around Nevada and the Four Corners states is special (indicated by an *) in that high altitude
regions, such as Flagstaff, Arizona and Ely, Nevada should be considered to be in the next higher tier of
snow losses. This plot is a broad enough generalization that it may apply to either a tilt=latitude or a
tilt=20° system.

8. 8
Snow Loss Estimates By Snow DepthSystem Advisor Model
Figure 6. Correlation between the sum of the hourly snow depth array and the resulting
percent loss for each year of each location in the tilt-equals-latitude national study

9. 9
Other Considerations/Future Work
• Model application on AC vs. DC side
(demonstrated < 2% difference)
• Temporal resolution of snow depth data (daily,
hourly, subhourly)
• Snow losses for one-axis tracking systems
• How to improve model accuracy at shorter
timescales? (monthly, hourly)
System Advisor Model

10. Thank you! Questions?
Full report:
http://www.nrel.gov/docs/fy15osti/64260.pdf

11. Supplemental Slides

12. 12
Snow Model Flow Chart System Advisor Model
No
Yes
No
SAM calculates
expected energy
production during
current time-step
without accounting
for snow
Go to next time-step
Begin
Was a new snow-fall
detected during this
time-step?
Was there a non-zero
snow coverage during
the previous time-step?
Do not deduct from
previously calculated
energy production
Set snow coverage on
PV array to 100%
Go to next time-step
Decrease snow coverage
by the snow sliding
amount (calculated from
tilt angle)
Maintain the previously
calculated snow
coverage
Deduct the appropriate
amount of energy from
the previously calculated
energy production
according to current
coverage
Do the current Temp.
and POA allow for snow
sliding?

13. 13
PV Array Diagram System Advisor Model
Figure 1. Simplified diagram of a PV array

14. 14
Modifications to Marion’s Model
• Check to prevent snow coverage from going
below 0%
• If measured snow depth is zero, snow
coverage also reset to zero
• Snow depth threshold: 1 cm
• Snow delta threshold: 1 cm
• Extend to accept hourly/sub-hourly snow
depth data
System Advisor Model

15. 15
Snow Model Applied to 1-Axis TrackingSystem Advisor Model
Figure 3. Results from applying the implemented snow model to the Mesa Top one-axis
tracking array in Golden, Colorado

16. 16
Tabulated Monthly Errors System Advisor Model