Panels mounted at 10° only have 2/3 the output of panels mounted at 40° over the winter season. It is important to keep in mind that the best-case, 40° tilt output is already less than 70% of the output that a planner using annual output might expect. Compounding this reduced winter emergency capability is the effect of snow cover on panel output. When arrays are covered by snow, output falls dramatically. Surprisingly-thin layers of snow can reduce output to -0-. Arrays mounted at steeper angles have an advantage, as they tend to shed accumulated snow. The obvious solution, cleaning the snow from the panels manually, may sound reasonable to someone who is not trying to control a city-wide emergency. Research is currently under way at NREL and at St. Lawrence College and Queens University in Canada, but currently-available data is very limited. One single-winter study compared two arrays in Minnesota at 40° and at 23° (NREL, PVWatts/Changing Parameters, 2011). With 155 days below freezing and 50 inches snowfall, Minneapolis may be comparable to Salt Lake, with 122 days below freezing and 58 inches of snow. Factoring the snowfall effects observed in the Minnesota study provides the following results:
Low-tilt arrays are far more likely to be covered by snow than high tilt-arrays when a winter emergency occurs. Clearly, the effects from year to year will vary considerably; compare Salt Lake City's Dec 2010 precipitation (3.4″) to Dec 2011 (.03″). Adjusted for the probability of snow cover, arrays mounted at low angles may have less than 1/2 the winter output provided by steeper arrays. Until further research results are available, today’s planners should carefully balance aesthetic and practical considerations when making decisions about array tilt for PV systems with emergency missions. NREL. (2011). PVWatts. Retrieved from NREL:PVWatts: http://www.nrel.gov/rredc/pvwatts/grid.html NREL. (2011). PVWatts/Changing Parameters. Retrieved 2011, from NREL PVWatts: http://www.nrel.gov/rredc/pvwatts/changing_parameters.html#dc2ac Zwern, S. A. (2006). Evaluating the Risk of a Severe Winter Storm Causing a Critical Risk Event in Park City. Park City.
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