Wind Energy: a thorough Examination of Economic Viability



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Wind speed is partly a function of height and generally weaker near the ground due to friction between earth’s surface and air flow. So placing turbines on hills and on large towers gives access to higher wind speeds. A taller tower not only makes it possible to reach faster winds but also accommodate a bigger rotor for a larger swept area.


All these factors have driven manufacturers to make ever bigger turbines. The turbines of the mid-1990s swept ten times the area of earlier machines (Gipe, 2004.) The size of wind turbines has doubled approximately every 4~5 years (Wizelius, 2007.) Turbines with an installed generator capacity of 5 to 6 MW and a diameter of 110-120 m diameter are running as prototype, see figure 3.
Figure 3 the development of wind turbine size.

Source: Gijs van Kuik et al., 2006.
Intermittence
Wind speed variation has system-wide effects for the electricity generation sector. Wind speed can decrease or increase by a factor of two very rapidly. Each time this happens, generation from a ‘wind carpet’ – namely, the total number of turbines in a relevant geographical area – decreases or increases by a factor of eight. Fluctuation in wind availability leads to sudden drop-outs and surges in electricity supply, requiring ‘up regulation’ and ‘down regulation’ by conventional generating plants (Szarka, 2007.) The bigger the ‘wind carpet’, the more pronounced these effects are. This creates the problem of ‘intermittence’, which has two different components. The first is the total absence of wind energy – and therefore of generation – during high pressure events. The second is rapid up or down variation in wind speeds and power output. The first can be predicted by weather forecasts with increasing accuracy. Predictions of the second are improving – due to better methodologies and tools (epically under short time-frames) – but will always remain a problem because wind speed variation is inherently a stochastic phenomenon (Szarka, 2007.)
Operation
Turbines produce direct current (DC) or alternating (AC) power, depending on the generator. (In our case the prime mover is the rotor.) However, neither way is 100 percent efficient at transferring wind power. The rotor will deliver more power to the generator than the generator produces as electricity. This leads to another fundamental consideration on the size of wind turbines. The size of a generator indicates only how much power the generator is capable of producing if the wind turbine’s rotor is big enough, and if there’s enough wind to drive the generator at the right speed. Thus further confront the fact that a wind turbine’s size is primarily governed by the size of its rotor (Gipe, 2004.)
Wind speeds are crucial for generating utility-compatible electricity. To adapt wind speed variation, electrical generators can be operated either at variable speed or at constant speed. In the first case, the speed of the wind turbine rotor varies with the wind; in the second, the speed of the wind turbine rotor remains relatively constant as wind speed fluctuates.
In nearly all small wind turbines, the speed of the rotor varies with wind speeds. This simplifies the turbines’ control while improving aerodynamic performance. When such wind machines drive an alternator, both the voltage and frequency vary with wind speed. The electricity they produced isn’t compatible with the constant-voltage, constant-frequency AC produced by the utility. Electricity from these wind turbines cannot be used in most our daily equipments. The output from these machines must be treated or conditioned first, usually equipped with features to produce correct voltage and constant frequency compatible with the loads.
Although nearly all medium-size wind turbines, such as the thousands of machines installed in California during the early 1980s, operated at constant speed by driving standard, off-the-shelf induction generators, a number of manufacturers of megawatt-size turbines today have switched to variable-speed operation4. Many of these use a form of induction generators, which may improve aerodynamic performance.
Estimating output
Annual energy production (AEP5) is calculated by applying the predicted wind distributions for a given site to the power performance curve of a particular wind turbine. The site wind distributions are normally based on a Rayleigh distribution6 that describes how many hours (or probability) each year the wind at a given site blows at a particular wind velocity (Figure 4(a)).
The second step in this process requires the power curve for the chosen turbine. Figure 4 (b) is an example of a power curve for a 1.5-MW turbine that is characteristic of current technology.

Figure 4 (a) (b) Power curve method of calculating annual energy output


What may not be stated upfront is that wind speeds of 10-25 m/s are necessary to reach a 1,500 kW output. The power output of a turbine for wind speeds must be determined specific to a specific site. Wind turbine developers can properly install a turbine that is well-suited for each site.
The product of first two curves will be a curve such as that shown in Figure 4 (c). By integrating the area under this curve, it is possible to determine the annual energy production. For a more accurate calculation it is necessary to account for both the mechanical and electrical power conversion efficiency, which varies at different turbine power level losses as described in the operating characteristics above, and the projected machine availability.


Figure 4 (c) Power curve method of calculating annual energy output


Conclusion
Wind turbine technology is in good health: the availability of turbines is ~98%, which means that during 2% of the time they cannot produce due to maintenance or failures. In general, elementary design rules dictate that the bigger the turbines are to deploy as much as possible.

In United States, the overall potential is vast. Wind power energy has been estimated as one of the country’s most abundant energy resources. About one-fourth of the total land area of United States has winds powerful enough to generate electricity as cheaply as natural gas or coal at today’s prices. The wind energy potential in all of the top states – North Dakota, Kansas, South Dakota, Montana, and Nebraska – is principle sufficient to provide all the electricity the country’s current uses (Table 2.) In fact, 20% of the land areas in these Midwest and the Rocky Mountain states belong to NREL Class 4, which are sufficient to make your wind energy business profitable.


However, few of those potential good sites with a high wind speed have been fully developed yet. In what follows, we will point to how wind energy development is likely to be cost-effective. Albeit the one-time investment on turbines is high, the margin cost per output will be relative small. In a certain future, with growing large turbine machine, supporting technologies (HVDC system in next section) and governmental incentive instruments (subsidies and/or tax credits), electricity produced by giant wind turbines in Midwest will be market competitive and become candidate solution to the country’s energy independence.



Table 2 Comparison of wind energy potential vs. installed capacity in selected contiguous states (GWh/year)

Source: Makhijani, 2007. http://www.ieer.org/carbonfree/ and EPA.


State

Wind potential

Installed capacity

North Dakota

1,210

146

Kansas

1,070

227

South Dakota

1,030

97

Montana

1,020

1

Nebraska

868

31

Wyoming

747

643

Oklahoma

725

353









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