pvlib.atmosphere.windspeed_powerlaw#

pvlib.atmosphere.windspeed_powerlaw(wind_speed_reference, height_reference, height_desired, exponent=None, surface_type=None)[source]#

Estimate wind speed for different heights.

The model is based on the power law equation by Hellmann [1] [2].

Parameters:

  • wind_speed_reference (numeric) – Measured wind speed. [m/s]

  • height_reference (float) – The height above ground at which the wind speed is measured. [m]

  • height_desired (float) – The height above ground at which the wind speed will be estimated. [m]

  • exponent (float, optional) – Exponent based on the surface type. [unitless]

  • surface_type (string, optional) –

    If supplied, overrides exponent. Can be one of the following (see [1]):

    • 'unstable_air_above_open_water_surface'

    • 'neutral_air_above_open_water_surface'

    • 'stable_air_above_open_water_surface'

    • 'unstable_air_above_flat_open_coast'

    • 'neutral_air_above_flat_open_coast'

    • 'stable_air_above_flat_open_coast'

    • 'unstable_air_above_human_inhabited_areas'

    • 'neutral_air_above_human_inhabited_areas'

    • 'stable_air_above_human_inhabited_areas'

Returns:

wind_speed (numeric) – Adjusted wind speed for the desired height. [m/s]

Raises:

  • ValueError – If neither of exponent nor a surface_type is given. If both exponent and a surface_type is given. These parameters are mutually exclusive.

  • KeyError – If the specified surface_type is invalid.

Notes

Module temperature functions often require wind speeds at a height of 10 m and not the wind speed at the module height.

For example, the following temperature functions require the input wind speed to be 10 m: sapm_cell(), and sapm_module() whereas the fuentes() model requires wind speed at 9.144 m.

Additionally, the heat loss coefficients of some models have been developed for wind speed measurements at 10 m (e.g., pvsyst_cell(), faiman(), and faiman_rad()).

The equation for calculating the wind speed at a height of \(h\) is given by the following power law equation [1] [2]:

(1)#\[ WS_{h} = WS_{ref} \cdot \left( \frac{h}{h_{ref}} \right)^a\]

where \(h\) [m] is the height at which we would like to calculate the wind speed, \(h_{ref}\) [m] is the reference height at which the wind speed is known, and \(WS_{h}\) [m/s] and \(WS_{ref}\) [m/s] are the corresponding wind speeds at these heights. The exponent \(a\) [unitless] depends on the surface type. Some values found in the literature [1] for \(a\) are:

Values for the Hellmann-exponent#

Stability

Open water surface

Flat, open coast

Cities, villages

Unstable

0.06

0.10

0.27

Neutral

0.11

0.16

0.40

Stable

0.27

0.34

0.60

In a report by Sandia [3], the equation was experimentally tested for a height of 30 ft (\(h_{ref} = 9.144\) [m]) at their test site in Albuquerque for a period of six weeks where a coefficient of \(a = 0.219\) was calculated.

It should be noted that the equation returns a value of NaN if the reference heights or wind speed are negative.

References

On this page
View on GitHub