--- title: "Horizontal and vertical axis wind turbines" subject: "Energy Resources" chapter: 4 key: "semester_6/energy_resources:hawt-vawt-types-characteristics" source: https://www.bctengineer.com/q/semester_6/energy_resources/hawt-vawt-types-characteristics site: BCT Engineering --- # Horizontal and vertical axis wind turbines Energy Resources · Chapter 4: Wind energy - Asked in: year not given, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/energy_resources/hawt-vawt-types-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-4 ## Question Differentiate between horizontal axis and vertical axis wind turbines. Name the types of VAWT and explain the power coefficient versus tip speed ratio characteristics of different rotors. ## Short answer HAWT shafts are horizontal, need yaw, high efficiency; VAWT shafts are vertical, accept wind from any direction, generator at ground. VAWT types: Savonius (drag, high starting torque, λ≈1, Cp≈0.15–0.2) and Darrieus (lift, not self-starting, λ≈4–6, Cp≈0.35–0.40). Cp–λ curve: each rotor has one optimum λ; multi-blade rotors peak at low λ, three-blade HAWT peaks at λ≈6–8. ## Answer **Tip speed ratio** is $\lambda = \dfrac{\omega R}{v}$, where $\omega R$ is the blade tip speed. The curve of $C_p$ against $\lambda$ shows how a rotor type behaves. #### HAWT versus VAWT | Point | HAWT | VAWT | |---|---|---| | Shaft | Horizontal, parallel to wind | Vertical, normal to wind | | Wind direction | Needs yaw mechanism | Accepts wind from any direction | | Generator and gearbox | On tower top, heavy | At ground level, easy service | | Efficiency ($C_p$) | High, 0.40–0.50 | Lower, 0.15–0.40 | | Self-starting | Yes | Darrieus is not self-starting | | Tower and wind speed | Tall tower, uses higher wind | Short tower, lower wind speed | | Blade loading | Gravity load varies with rotation | Pulsating torque, fatigue | | Use | Utility-scale (kW to MW) | Small, urban, low-wind sites | #### Types of VAWT - **Savonius:** two or three half-cylinder cups; a drag device; high starting torque, low speed, $\lambda \approx 1$, $C_p \approx 0.15$–0.2; used for water pumping and ventilation. - **Darrieus:** curved or straight blades (H-rotor) with aerofoil section; lift device; $\lambda \approx 4$–6, $C_p \approx 0.35$–0.40; needs starting help. #### $C_p$-$\lambda$ characteristics ```text Cp 0.5| ___ 3-blade HAWT | / \ 0.4| / _.-Darrieus | / / \ 0.2| Savonius \ | _/_.-'''-. +---------------------- lambda 1 2 4 6 8 ``` - Multi-bladed American windmill: peak at $\lambda \approx 1$, high torque, low $C_p$ (about 0.3) and good for pumping. - Three-blade HAWT: peak $C_p$ at $\lambda \approx 6$–8, high speed, suitable for electricity. - Each rotor has one optimum $\lambda$; away from it $C_p$ drops, so variable-speed operation keeps $\lambda$ near the optimum. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which turbine type needs a yaw mechanism? A. VAWT B. HAWT C. Savonius D. Darrieus Answer: B. A VAWT accepts wind from any direction. (Key point: Yaw) 2. Which describes the Savonius rotor? A. Drag device with high starting torque, λ ≈ 1, Cp 0.15-0.2 B. Lift device, λ 4-6, Cp 0.35-0.40 C. Drag device, λ 6-8, Cp 0.5 D. Lift device, not self-starting, λ 1 Answer: A. It is used for water pumping and ventilation. (Key point: Savonius) 3. Which is true of the Darrieus rotor? A. It is self-starting with high torque B. It is a drag device with λ ≈ 1 C. It has Cp 0.15 only D. It is a lift device with λ ≈ 4-6 and is not self-starting Answer: D. It needs starting help. (Key point: Darrieus) 4. What is the tip speed ratio, and why does it matter? A. λ = v/ωR; Cp is independent of λ B. λ = ωD; Cp rises forever with λ C. λ = ωR/v; each rotor has an optimum λ at which Cp is highest D. λ = Cp/v; it is fixed Answer: C. Variable-speed operation keeps λ near the optimum. 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