What is a good capacity factor for a wind farm?

Onshore wind farms typically achieve capacity factors of 25–45%, with sites in the Great Plains of the US frequently exceeding 40%. Offshore wind farms achieve 40–60% due to stronger, more consistent winds at sea. A capacity factor below 25% generally makes a project economically marginal at current turbine costs, while sites above 40% can deliver competitive LCOE even in higher-cost geographies.

How long does a wind resource assessment take?

A bankable wind resource assessment requires at least 12 months of on-site measurements to capture seasonal variability. Most lenders and investors prefer 24 months of data. The campaign includes installing met masts or deploying LiDAR systems at representative locations within the project area, correlating short-term measurements to long-term reanalysis datasets using MCP (Measure, Correlate, Predict) techniques, and running wind flow models to produce a P50/P90 energy estimate.

What is the difference between P50 and P90 energy production estimates?

P50 is the energy production estimate with a 50% probability of being exceeded — it represents the median expected output and is used for base-case financial modeling. P90 is the estimate with a 90% probability of being exceeded, meaning there is only a 10% chance actual production will fall below this figure. Lenders typically size debt service coverage using P90 (1-year) or P99 (10-year) estimates to ensure conservative financing structures. The gap between P50 and P90 reflects uncertainty in the wind resource assessment and is reduced by longer measurement campaigns and better flow modeling.

What is turbulence intensity and why does it matter for turbine selection?

Turbulence intensity (TI) is the standard deviation of wind speed divided by the mean wind speed, measured over a 10-minute averaging period. High TI increases fatigue loads on turbine blades, the drivetrain, and the tower. IEC 61400-1 defines Normal Turbulence Model (NTM) categories A (high, TI=0.16), B (medium, TI=0.14), and C (low, TI=0.12). Turbines must be selected or certified for the TI expected at the site — deploying a Class B turbine at a Class A site will accelerate fatigue damage and may void the manufacturer warranty.

How does wind shear affect energy production estimates?

Wind shear describes the increase in wind speed with height above ground, typically modeled using a power law (v2/v1 = (h2/h1)^α) where α is the shear exponent. Forested or complex terrain sites may have α = 0.3–0.4, while flat open terrain typically has α = 0.1–0.2. Higher shear means installing taller towers captures significantly more energy — increasing hub height from 80 m to 120 m on a high-shear site can increase AEP by 10–20%. Accurate shear characterization requires measurements at multiple heights, which is a key advantage of profiling LiDAR systems.

Engineering·11 min read·May 1, 2025

💨 Wind Energy Fundamentals: Turbine Types, Capacity Factor, and Site Assessment

A comprehensive engineering guide to wind energy — covering horizontal vs. vertical axis turbines, capacity factor analysis, wind resource assessment methods, wake effects, and IEC 61400 design standards.

📖Renewable Energy Design Reference (Full Access)

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Introduction to Wind Energy Engineering

Wind energy is one of the fastest-growing sources of electricity generation worldwide, accounting for over 2,000 GW of installed capacity globally as of 2024. Unlike fossil fuels, wind is a variable resource — its engineering challenges center on maximizing energy capture, predicting output reliably, and integrating variable generation into the power grid. Understanding wind turbine physics, turbine typology, and rigorous site assessment methods is essential for any renewable energy engineer.

Topics covered

wind energywind turbinecapacity factorwind site assessmentHAWTVAWTIEC 61400wind resource assessmentwake effectBetz limitwind farmanemometerwind shearturbulence intensitypower curve