Capacity Factor Sensitivity to Wind Shear Exponent

yield-assessment

offshore-wind

python

How much does the power law shear exponent matter for offshore AEP estimates?

Author

Anemona Team

Published

February 14, 2026

Introduction

Offshore wind shear is typically low (α ≈ 0.10–0.14) but uncertainty in α propagates into hub-height wind speed estimates and ultimately AEP. We quantify this sensitivity here.

Code

import numpy as np
import matplotlib.pyplot as plt

z_ref, z_hub = 10, 120  # reference and hub heights in metres
v_ref = 9.0             # mean wind speed at 10 m (m/s)
alphas = np.linspace(0.05, 0.20, 100)

v_hub = v_ref * (z_hub / z_ref) ** alphas
print(f'Speed range: {v_hub.min():.2f} – {v_hub.max():.2f} m/s')

Code

# Simplified capacity factor proxy (cubic power law, cut-in=4, rated=13, cut-out=25)
def capacity_factor(v):
    if v < 4 or v > 25:
        return 0.0
    elif v >= 13:
        return 1.0
    return ((v - 4) / (13 - 4)) ** 3

cf = np.array([capacity_factor(v) for v in v_hub])

fig, ax = plt.subplots(figsize=(8, 4))
ax.plot(alphas, cf * 100)
ax.set_xlabel('Shear exponent α')
ax.set_ylabel('Capacity factor (%)')
ax.set_title('CF sensitivity to wind shear')
plt.tight_layout()

Conclusion

A change in α from 0.10 to 0.14 translates to roughly 1–2% absolute CF difference — meaningful at project scale. Always validate shear assumptions against met-mast or lidar measurements.