pvlib.snow.loss_townsend

pvlib.snow.loss_townsend(snow_total, snow_events, surface_tilt, relative_humidity, temp_air, poa_global, slant_height, lower_edge_height, string_factor=1.0, angle_of_repose=40, front_side_fraction=1.0)

Calculates monthly snow loss based on the Townsend monthly snow loss model.

This model is described in [1].

Parameters:

• snow_total (array-like) – Snow received each month. Referred to as S in [1]. [cm]

• snow_events (array-like) – Number of snowfall events each month. Snow events are defined as days in the month that have snowfall greater than 1 inch. May be int or float type for the average events in a typical month. Referred to as N in [1].

• surface_tilt (float) – Tilt angle of the array. [deg]

• relative_humidity (array-like) – Monthly average relative humidity. [%]

• temp_air (array-like) – Monthly average ambient temperature. [C]

• poa_global (array-like) – Monthly plane of array insolation. [Wh/m2]

• slant_height (float) – Row length in the slanted plane of array dimension. [m]

• lower_edge_height (float) – Distance from array lower edge to the ground. [m]

• string_factor (float, default 1.0) – Multiplier applied to monthly loss fraction. Use 1.0 if the DC array has only one string of modules in the slant direction, use 0.75 otherwise. [-]

• angle_of_repose (float, default 40) – Piled snow angle, assumed to stabilize at 40°, the midpoint of 25°-55° avalanching slope angles. [deg]

• front_side_fraction (numeric or array-like, default 1.0) – Fraction of monthly energy from front-side insolation. [unitless] Multiplies the calculated loss fraction. For example, use 0.9 when 90% of monthly energy is from the front side of a bifacial system and 10% is from the rear side.

Returns:

loss (array-like) – Monthly average DC capacity loss fraction due to snow coverage.

Notes

This model has not been validated for tracking arrays; however, for tracking arrays [1] suggests using the maximum rotation angle in place of surface_tilt. The author of [1] recommends using one-half the table width for slant_height, i.e., the distance from the tracker axis to the module edge.

The parameter string_factor is an enhancement added to the model after publication of [1], as described in [2]. The definition for snow events documented above is based on [3].

For bifacial systems, [2] recommends including both front-side and rear-side insolation in poa_global. The resulting loss is scaled by the front-side energy fraction front_side_fraction.

References

[1] (1,2,3,4,5,6)

Townsend, Tim & Powers, Loren. (2011). Photovoltaics and snow: An update from two winters of measurements in the SIERRA. 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, USA. DOI: 10.1109/PVSC.2011.6186627

[2] (1,2)

Townsend, T. and Previtali, J. (2023). A Fresh Dusting: Current Uses of the Townsend Snow Model. In “Photovoltaic Reliability Workshop (PVRW) 2023 Proceedings: Posters.”, ed. Silverman, T. J. Dec. 2023. NREL/CP-5900-87918. Available at: https://www.osti.gov/biblio/2229734

[3]

Townsend, T. (2013). Predicting PV Energy Loss Caused by Snow. Solar Power International, Chicago IL. DOI: 10.13140/RG.2.2.14299.68647