HOW TO TURN WIND INTO ELECTRICITY?
Wind energy is intermittent, highly variable, and site-specific, exists in three dimensions, and is the least dependent upon latitude among all renewable resources. The power density (in W/unit area) in moving air (wind) is a cubic function of wind speed and therefore even small increases in average wind speeds can lead to significant increases in the capturable energy.
Wind sites are typically classified as good, excellent, or outstanding, with associated mean wind speeds of 13, 16, and 19 mph, respectively.
Aeroturbines employ lift and/or drag forces to convert wind energy to rotary mechanical energy, which is then converted to electrical energy by coupling a suitable generator. The power coefficient Cp of an aeroturbine is the fraction of the incident power converted to mechanical shaft power, and it is a function of the tip speedto- wind speed ratio λ as shown in Figure 60.3. For a given propeller configuration, at any given wind speed, there is an optimum tip speed that maximizes Cp.
Several types of aeroturbines are available. They can have horizontal or vertical axes, number of blades
ranging from one to several, mounted upwind or downwind, and fixed- or variable-pitch blades with full blade control or tip control. Vertical-axis (Darrieus) turbines are not self-starting and require a starting mechanism.
Today, horizontal-axis turbines with two or more blades are the most prevalent, and considerable work is underway to develop advanced versions of these. The electrical output Pe of a wind-electric conversion system (WECS) is given as
where ηg and ηm are the efficiencies of the electrical generator and mechanical interface, respectively, A is the swept area, K is a constant, and v is the wind speed incident on the aeroturbine.
There are two basic options for wind-electric conversion. With varying wind speeds, the aeroturbine can be operated at a constant speed by blade-pitch control, and a conventional synchronous machine is then employed to generate constant-frequency ac.
More commonly, an induction generator is used with or without an adjustable var supply. In this case, the aeroturbine will operate at a nearly constant speed. Alternatively, the aeroturbine rotational speed can be allowed to vary with wind to maintain a constant and optimum tip speed ratio, and then a combination of special energy converters and power electronics is employed to obtain utility-grade ac.
The variable-speed option allows optimum efficiency operation of the turbine over a wide range of wind speeds, resulting in increased outputs with lower structural loads and stresses. All future utility-grade advanced turbines are expected to operate in the variable-speed mode and use power electronics to convert the variable-frequency output to constant frequency with minimal harmonic distortion.
Large-scale harnessing of wind energy will require hundreds or even thousands of WECS arranged in a wind farm with spacings of about 2 to 3 diameters crosswind and about 10 diameters apart downwind. The power output of an individual WECS will fluctuate over a wide range, and its statistics strongly depend on the wind statistics.
When many WECS are used in a wind farm, some smoothing of the total power output will result, depending on the statistical independence of the outputs of individual WECS. This is desirable, especially with high (>20%) penetration of WECS in the generation mix. While the output of WECS is not dispatchable, with large wind farms the possibility of assigning some capacity credit to the overall output significantly improves.
Although wind-electric conversion has overall minimum environmental impacts, the large rotating structures involved do generate some noise and introduce visual aesthetics problems. By locating wind energy systems sufficiently far from centers of population, these effects can be minimized.
The envisaged potential for bird kills turned out to be not a serious problem. Wind energy systems occupy only a very small fraction of the land. However, the area surrounding them can be used only for activities such as farming and livestock grazing.
Thus, there is some negative impact on land use. Today, the cost of energy delivered by wind plants rivals those obtained from some nonrenewable sources. By 1990, wind became the most utilized and competitive option among all the solar energy technologies for the bulk power market at a cost of generation of about 8¢/kWh (or roughly 7¢/kWh in 1987 dollars).
Ongoing research and development work in new design tools, advanced airfoils, site tailoring, operating strategies, array spacing, and improved reliability and manufacturability is expected to bring the cost of energy further down by a factor of 2 to 3.
At around 1600 MW, nearly 90% of all the WECS installed in the world are in California. They are expected to generate nearly 3 billion kWh of electricity per year to the state’s utilities to which they are interconnected.
Although their lack of control and the intermittent nature of wind-derived energy are not embraced enthusiastically by electric utilities, this gap is expected to be bridged very soon with appropriate computer controls and operating strategies. Wind energy is already an economical option for remote areas endowed with good wind regimes.
The modularity of WECS, coupled with the associated environmental benefits, potential for providing jobs, and economic viability point to a major role for wind energy in the generation mix of the world in the decades to come.
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