Wind power Study Guide

📖 Core Concepts Wind Power – conversion of kinetic energy of moving air into electricity using wind turbines. Capacity Factor (CF) – actual annual energy ÷ (nameplate capacity × hours in a year); typical wind CF = 35 %–44 %. Power‑Wind Speed Relation – wind power is proportional to the cube of wind speed: $P \propto v^{3}$. Doubling $v$ ⇒ eight‑fold power increase. Betz Limit – theoretical maximum extraction = $\frac{16}{27}\approx 59\%$ of the wind’s kinetic energy; modern turbines achieve 70–80 % of this limit. Onshore vs Offshore – offshore turbines have higher CF, lower visual impact, but higher capital cost; offshore accounts for ≈10 % of new capacity. Variability & Integration – wind output varies hourly‑daily‑seasonally; requires storage or dispatchable generation for reliability. Curtailment – intentional reduction of output when transmission or grid limits are reached. Generator Types – variable‑speed machines with partial (doubly‑fed) or full‑scale converters provide low‑voltage ride‑through. 📌 Must Remember 2024 Global Stats: 2 500 TWh wind electricity, >8 % of world electricity; >800 GW installed capacity. CF Range: 35 %–44 % (typical); 20 % of annual electricity can be accommodated with minimal grid trouble. Power‑Cube Law: $P \propto v^{3}$ (doubling wind speed ⇒ ×8 power). Betz Limit: $16/27 \approx 59\%$; modern turbines reach 70–80 % of Betz. Turbine Spacing: ≈ 7  rotor diameters (7 D) (5–9 D to reduce wake losses). Weibull Shape Factor: ≈ 2 for most sites (used for wind‑speed statistics). LCOE (Onshore): $26–$50 /MWh vs $45–$74 /MWh for new gas plants (2021). Energy Payback Time: 1 year; ERoEI ≈ 20–25. Floating Offshore: enables deployment in deep water (>60 m). Transmission Choice: AC for near‑shore farms; HVDC for long‑distance offshore links. 🔄 Key Processes Estimating Available Power Measure/forecast wind speed $v$. Apply $P = \tfrac{1}{2}\rho A C{p} v^{3}$ (where $\rho$ = air density, $A$ = rotor swept area, $C{p}$ = power coefficient ≤ Betz). Wind‑Farm Layout Design Set turbine spacing ≈ 7 D (adjust 5–9 D for wake mitigation). Use wake‑steering: yaw upstream turbines to deflect wakes, improving downstream output. Pitch Control Operation Monitor wind speed. Increase blade pitch (feather) when $v$ exceeds rated speed → limit power, protect turbine. Variable‑Speed Generator & Ride‑Through Convert mechanical speed fluctuations into electrical frequency via power converters. Maintain grid connection during short voltage dips (low‑voltage ride‑through). Curtailment Decision Detect transmission bottleneck or grid frequency issue. Issue curtail command → reduce turbine output or stop rotation. Storage Dispatch Short‑term (hours): batteries → smooth intra‑day variability. Mid‑term (days‑weeks): pumped‑hydro or compressed‑air. Long‑term (months): hydrogen, seasonal reservoirs. 🔍 Key Comparisons Onshore vs Offshore CF: offshore higher (≈45 %+) vs onshore 35‑44 %. Cost: offshore more expensive (≈2× onshore CAPEX). Visual impact: offshore lower. Generator Types Doubly‑fed (partial converter) – cheaper, limited full‑range control. Squirrel‑cage induction – simple, robust, no converter needed for low‑speed operation. Synchronous (full converter) – full control, best for grid support. Transmission: AC vs HVDC AC: suitable ≤ 80 km, lower conversion cost. HVDC: lower losses for long distances, essential for far‑offshore farms. Horizontal‑Axis vs Vertical‑Axis Turbines Horizontal: dominant market share, higher efficiency. Vertical: niche, lower efficiency, limited size. ⚠️ Common Misunderstandings “Higher wind speed always better” – power follows $v^{3}$, but turbines have cut‑out speeds (≈25 m/s) to avoid damage; beyond that they feather blades. “Betz limit = 100 % efficiency” – the limit is ≈59 % of kinetic energy; real turbines capture a fraction of that. “Wind is always intermittent” – output peaks at night & winter, complementing solar’s daytime/summer profile. “Curtailment wastes all generated energy” – it is a grid‑protective action; some curtailed energy can be stored or redirected. 🧠 Mental Models / Intuition Cubic Law – think of wind power like a volume of water: double the speed triples the flow and cubes the energy. Farm as a Forest – each turbine casts a “shadow” (wake) that slows wind for those behind it; spacing reduces overlap. Capacity Factor ≈ “Average Fill Level” – a CF of 40 % means the turbine runs at its rated power roughly 40 % of the time (or equivalently at 40 % of full power continuously). 🚩 Exceptions & Edge Cases Floating Offshore Turbines – allow deployment in deep water where fixed foundations are impossible. Very High Wind Sites – require aggressive pitch control and may operate at lower CF due to frequent cut‑outs. Complex Terrain – can reduce CF below 35 % despite strong average winds because of turbulence. HVDC Threshold – benefits appear when line length > 80 km; shorter links remain cheaper with AC. 📍 When to Use Which Onshore vs Offshore – choose onshore when land is cheap and grid is nearby; pick offshore for high‑speed, stable winds and when land constraints exist. HVDC vs AC – use HVDC for transmission distances > 80 km or when connecting large offshore farms; otherwise AC is simpler. Generator Type – select full‑converter synchronous for grids needing strong ancillary services; doubly‑fed for cost‑sensitive projects with moderate grid support needs. Storage Option – batteries for hour‑scale smoothing; pumped‑hydro or compressed‑air for daily‑to‑weekly; hydrogen for seasonal storage. 👀 Patterns to Recognize Night‑Winter Peaks – wind output often spikes at night & in winter → look for complementary solar data. Weibull Shape ≈2 – indicates a fairly symmetric distribution; deviations suggest site‑specific anomalies. Curtailment Correlation – appears when transmission line rating is reached or grid frequency deviates; often shown as a sudden drop in generation despite high wind speeds. Wake Loss Signature – downstream turbines produce systematically lower CF than upstream ones, especially when spacing < 7 D. 🗂️ Exam Traps Confusing Capacity Factor with Penetration – CF is a turbine‑level efficiency metric; penetration is the share of total electricity supplied by wind. Misreading Betz Limit – some questions present “maximum possible efficiency”; the correct answer is ≈59 % (Betz), not 100 %. LCOE vs Capital Cost – LCOE includes operation & maintenance over lifetime; a low LCOE does not imply low upfront CAPEX. Assuming All Offshore is Cheaper – offshore turbines have higher CAPEX and transmission costs; only higher CF can offset. “All turbines are horizontal‑axis” – while true for utility scale, vertical‑axis turbines exist for niche applications; answer choices may test this nuance. --- Study this guide, focus on the bold numbers and the cause‑effect relationships (cube law, wake effect, CF), and you’ll be ready to spot the tricks on the exam.