Community-Scale Wind Project in Mountainous Colorado

Case Study 2: Community-Scale Wind Project in Mountainous Colorado

Scenario A rural co-op in San Juan County, CO (elevation ~2,600 m) seeks a single-turbine installation to offset 30% of local municipal load (~1.2 GWh/yr). Site constraints include steep terrain (18° slope), limited road access (max transport width 3.5 m), FAA lighting waivers, and low air density. Turbine must be Class III (IEC) rated for turbulent, complex flow and fit within a 1.5-acre cleared pad.

Given Data

• Air density: 0.91 kg/m³ (calculated from elevation and temperature profile, verified via onsite barometer)
• Average wind speed: 6.4 m/s (10-year LiDAR campaign at 80 m, corrected for terrain complexity)
• Power coefficient: 0.36 (conservative value accounting for turbulence-induced losses and lower-Reynolds-number blade performance)
• Capacity factor: 0.26 (derived from long-term power curve simulation with TurbSim + FAST, including wake and shear effects)
• Target AEP: 360,000 kWh/year (30% of 1.2 GWh municipal load)

Calculation Using the same AEP formula:

A = AEP / (0.5 × ρ × v³ × Cp × CF × 8760)

Substituting:

• AEP = 360,000 kWh/yr = 360,000,000 Wh/yr
• ρ = 0.91
• v = 6.4 → v³ = 262.144
• Cp = 0.36
• CF = 0.26
• 8760

Denominator: 0.5 × 0.91 × 262.144 × 0.36 × 0.26 × 8760 ≈ 0.5 × 0.91 × 262.144 × 0.36 × 0.26 × 8760 = 99,274.2

A = 360,000,000 / 99,274.2 ≈ 3,626.3 m²

Rounded per tool spec: 3,626.30 m²

Corresponding rotor diameter: D = 2 × √(3626.3 / π) ≈ 2 × √1154.3 ≈ 2 × 33.98 ≈ 67.96 m

This falls within feasible range for mid-size turbines (e.g., GE Cypress 3.0–3.6 MW platform with 135–141 m rotors is oversized; instead, Nordex N149/4.0–4.5 MW fits best with D = 149 m → A = 17,350 m² — too large). Engineers downselected to the Enercon E-138 EP5 (D = 138 m, A = 14,957 m²) but found it excessive. Final iteration used tool’s ‘feasibility filter’ to identify smallest commercially available turbine meeting A ≥ 3,626 m² and transport/logistics constraints: the Vestas V117-3.45 MW (D = 117 m, A = 10,752 m²) exceeded requirement by >2×, but its 3.5-m transport width and modular nacelle allowed site access. AEP modeling confirmed 412,000 kWh/yr — 14% above target — acceptable given low-cost oversizing.

Result and Decision Vestas V117-3.45 MW installed at 85 m hub height. Final AEP measured in Year 1: 408,500 kWh — validating tool output and turbulence-adjusted inputs. Project achieved PPA rate of $0.032/kWh, 22% below regional utility tariff.

Lesson In high-altitude or complex-terrain sites, air density and capacity factor dominate sizing outcomes more than wind speed alone—neglecting either leads to severe underperformance. Always calibrate Cp and CF using site-specific CFD or validated mesoscale-to-microscale modeling, not generic defaults.