Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

326 Fluid Mechanics, Thermodynamics of Turbomachinery
lift. Drag machines such as those developed in ancient times by the Persians were of
rather low efficiency compared with modern turbines (employing lift forces) and are
not considered any further in this chapter.
The design of the modern wind turbine is based upon aerodynamic principles, which
are elaborated later in this chapter. The rotor blades are designed to interact with the
oncoming airflow so that an aerodynamic lift force is developed. A drag force is also
developed but, in the normal range of pre-stall operation, this will amount to only about
1 or 2% of the lift force. The lift force, and the consequent positive torque produced,
drives the turbine thereby developing output power.
In this chapter, the focus of attention is necessarily restricted to the aerodynamic
analysis of the horizontal axis wind turbine (HAWT) although some mention is given
of the vertical axis wind turbine (VAWT). The VAWT, also referred to as the Darrieus
turbine, after its French inventor in the 1920s, uses vertical and often slightly curved
symmetrical aerofoils. Figure 10.3a shows a general view of the very large 4.2MW
vertical axis Darrieus wind turbine called the Eolé VAWT installed at Cap-Chat,
Quebec, Canada, having an effective diameter of 64m and a blade height of 96m.
Figure 10.3b, from Richards (1987), is a sketch of the major components of this aptly
named eggbeater wind turbine. Guy cables (not shown) are required to maintain the
turbine erect. This type of machine has one distinct advantage: it can operate consistently
without regard to wind direction. However, it does have a number of major
disadvantages:
(i) wind speeds are low close to the ground so that the lower part of the rotor is rather
less productive than the upper part,
(ii) high fluctuations in torque occur with every revolution,
(iii) negligible self-start capability,
(iv) limited capacity for speed regulation in winds of high speed.
According to Ackermann and Söder (2002), VAWTs were developed and produced
commercially in the 1970s until the 1980s. Since the end of the 1980s research and
development on VAWTs has virtually ceased in most countries, apart from Canada (see
Gasch 2002, Walker and Jenkins 1997, Divone 1998).
Large HAWTs
The HAWT type is currently dominant in all large-scale applications for extracting
power from the wind and seems likely to remain so. The very large HAWT, Figure
10.4a, operating at Barrax, Spain, is 104m in diameter and can generate 3.6MW.
Basically, a HAWT comprises a nacelle mounted on top of a high tower, containing a
generator and, usually, a gearbox to which the rotor is attached. Increasing numbers of
wind turbines do not have gearboxes but use a direct drive. A powered yaw system is
used to turn the turbine so that it faces into the wind. Sensors monitor the wind direction
and the nacelle is turned according to some integrated average wind direction. The
number of rotor blades employed depends on the purpose of the wind turbine. As a
rule, two or three bladed rotors are used for the generation of electricity. Wind turbines
with only two or three blades have a high tip–speed ratio but only a low starting torque
and may even require assistance at startup to bring it into the useful power producing