Wind Measurement Mast

Overview

A meteorological (met) mast is a specialized tall tower instrumented with anemometers, thermometers, humidity sensors, and data loggers, measuring atmospheric conditions at multiple heights. The primary application is wind resource assessment prior to wind farm development, establishing site-specific wind speed, direction, and turbulence profiles to estimate annual energy production (AEP) of turbines.

A typical mast reaches 80–120 m height, with anemometers at 40 m, 60 m, and 80 m. Each anemometer records 10-minute wind speeds, from which software calculates vertical wind shear (typically power law exponent 0.2–0.3), turbulence intensity, and directionality. A 12-month measurement campaign costs 80,000–200,000 USD and reduces uncertainty in AEP forecasts from ±20% (from models alone) to ±10% (with measured data).

Met masts also support academic turbulence research, environmental monitoring (particulates, temperature gradients), and validation of numerical weather prediction models. An international network of 5000+ permanent met sites provides open data for climate and renewable energy research.

How it works

The Foundation is a reinforced concrete pad 4 × 4 m wide and 1.0 m deep, reinforced with rebar and 12 M24 anchor bolts. The Tower Section lattice or monopole is assembled in 10–20 m sections, bolted together and anchored to the foundation. Guy Wires (three or four stainless steel cables) extend from the tower at 60° angles, anchored to concrete deadmen 50–100 m away, stabilizing the mast against wind sway and resonance.

The Anemometer Boom extends 2–3 m horizontally from the tower face, supporting multiple Anemometer cup or propeller anemometers and Wind Vane direction sensors. Cup anemometers rotate in the wind; a reed switch or photo-gate counts rotations per minute, converting to wind speed (m/s). Propeller anemometers measure both speed and direction directly. Anemometers are typically mounted at 40, 60, and 80 m to capture vertical wind profile (shear).

All sensor wires are routed via the Boom Cable Tray and tower interior down to the Data Logger enclosure, which samples analog inputs (0–10 V from anemometer pickup frequencies) at 1 Hz and averages to 1–10 minute intervals, storing data on an internal SD card.

The Data Logger runs continuously from a Logger Battery (lithium or lead-acid, 100+ Ah) charged by a Solar Panel (100–150 W) via a Logger Charger (MPPT controller). This configuration provides >3 months autonomy without solar, sufficient for cloudy seasons or equipment downtime.

A Modem (cellular 4G or satellite) enables real-time data transmission to a cloud server, allowing remote diagnostics and early warning if sensors fail. Low-power modes reduce consumption to <2 W during idle periods.

The Lightning Protection ground rod and continuous ground cable protects sensors from lightning strikes common on tall towers. Surge Suppressor devices on the anemometer inputs clamp transients to <500 V, protecting logger circuits.

Maintenance access is via a Climbing Ladder with a Fall Arrest Cable and Fall Arrest Trolley for safe descent and mid-climb rest. A technician can reach any sensor height in <2 hours for cleaning (removing bird nests, dust) or calibration check.

Wind Shear & Vertical Profile

Wind speed increases with height due to surface friction. The vertical profile follows a power law:

$$v(z) = v_{ ext{ref}} left( rac{z}{z_{ ext{ref}}} ight)^{alpha}$$

where α is the shear exponent (typically 0.2 for calm water, 0.35 for rough terrain). A site with anemometers at 40 m (v=7 m/s) and 80 m (v=8.5 m/s) yields α≈0.2, implying smooth terrain (possibly offshore). A 50% steeper profile (α≈0.3) indicates roughness (forests, urban).

For wind energy, higher shear is favorable, taller turbines access faster winds. A 10 m/s site with α=0.2 is wind-rich; the same wind speed at α=0.35 indicates terrain-constrained flow unsuitable for utility-scale turbines.

Turbulence Intensity

Turbulence intensity (TI) is the ratio of wind speed standard deviation to mean:

$$ ext{TI} = rac{sigma_u}{langle v angle}$$

Typical values: 5–8% (smooth water), 12–20% (forested/urban). High TI increases turbine fatigue loads; sites with TI>25% require special low-speed turbine designs or are avoided.

Site Assessment Workflow

1. Pre-survey: Identify potential site, check permits, power grid connection.
2. Mast installation (2 weeks): Foundation, tower assembly, sensor mounting, test run.
3. Data collection (12 months): Continuous measurement, monthly data backup and QA checks.
4. Post-processing (4 weeks): Validate data (removing sensor failures, spikes), calculate AEP using industry software (WASP, Windographer, OpenWind).
5. Feasibility report: Estimated AEP, uncertainty ranges, turbine recommendations.

Data Quality & Validation

Anemometer calibration uncertainty is ±3–5%. Cup anemometers drift 1–2% per year; annual recalibration is recommended. Propeller anemometers are more stable but direction-sensitive (±20° cone).

Lightning strikes often corrupt anemometer output (pin-locked at max scale). Data screening removes periods with TI>50%, sustained calm (<0.5 m/s), or stuck sensors. A well-executed campaign recovers 90+ percent of valid data.

Economics

• Capital: 1000–2000 USD/m height (smaller masts 40 m = 40–80K USD; taller 120 m = 120–240K USD)
• Operating: 5–10K USD/year (rent of land, maintenance, data analysis)
• Value: Reduces AEP uncertainty from ±20% to ±10%, justifying 5+ MW projects; ROI typically <6 months on large wind farms

Met masts remain essential despite advances in remote sensing (lidar, sodar). Lidar provides superior vertical resolution at lower cost but requires clear weather; met masts provide ground-truth data for model validation regardless of weather.