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Photovoltaic Panel Array Tilt and Emergency Generation Capability in Salt Lake City, Utah

Several photovoltaic (PV) installations are currently being planned in the Salt Lake City area, which are meant to have emergency generation capabilities. The arrays are to be grid-connected in order to defray electrical costs over the course of regular business, and are planned to be connected to battery back-up systems that serve mission-critical departments, providing supplementary power in the case of a prolonged power outage. This dual-use model is becoming popular, supplementing emergency power capacity in a very cost-effective way. A trend in rooftop PV installations has been the use of lower array tilt angles. Lower tilt angles allow for closer placement of panels, commonly referred to as “ground placement ratio” or “ground coverage ratio” (GCR), as panels are less likely to shade each other. Lower tilt angles are also preferred for aesthetic reasons, as rooftop arrays are less visible from street level. This option is also the most expensive, as measured by cost / kWh. Manufacturers and developers promote this type of design because it allows more panels ($$) to be fit on a customer's rooftop. It is well known that reduced tilt angles decrease the amount of energy generated by an array, but planners should be especially careful to consider the effect of tilt angle on emergency power generation. Although emergencies are by-definition unforeseen, two of the most common scenarios for electric grid failure in Utah involve seismic activity and severe winter storms (Zwern, 2006). As few possible causes for grid failure are seasonal, it may be supposed that the probability of an emergency PV generation event is at least as high in the winter as at other times of the year. Winter PV generation in Utah is much lower than the annual average that is commonly used by developers and planners. Because the sun is at lower angles in the winter, the winter array angle is more important than the annual average would suggest. The Department of Energy NREL PVWatts program (NREL, 2011) illustrates the effect of array tilt on system output for a 1 MW system located in Salt Lake City:   Although the effect of tilt on annual output is quite low, the effect on winter output is quite high. The December – February outputs are summed below:

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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