Wind Energy Basics

Power from the wind o The kinetic energy of wind is harvested using wind turbines to generate electricty. o Among various renewable energy sources, wind energy is the second most technologically advanced renewable energy source; hydropwer is the first. o Wind blows in every cover of the earth; however, it does not blow constantly. o In addition, it must maintain a certain speed to be effective for running a wind turbine and generating electricity.

Global Wind Power Cumulative Capacity Figure 1

China Leads the World in Wind Capacity Total Installed Generating Capacity (MW) Figure 2

Regional suitability for onshore wind power generation Source: Eltamaly et al. 2012, Rehman et al. 2003, Apricum analysis. Wind speeds measured at 10 m height. Figure 3 :

Wind Turbine Components Figure 4

The Mechanics of a Wind Turbine

Energy and power from wind The kinetic energy of wind is converted to mechanical or electrical energy using wind turbines. The amount of energy captured by the rotor depends on the density of the air, the rotor area and the wind speed. No other factor is more important to the amount of power available in the wind than the speed of the wind. Power is a cubic function of wind speed 20% increase in wind speed means 73% more power. Doubling wind speed means 8 times more power.

Calculation of Wind Power R Swept Area: A = πR 2 Area of the circle swept by the rotor (m 2 ). Wind Power P w = ½ρAV 3 Figure 5 Figure 6

Example: V = 10m/s; A = (2 m) 2 = 4 m 2  = 1.2 kg/m 3 Theoretical Maximum Betz Limit: 59.3% of the theoretical is the maximum amount extractable by a.wind energy conversion device (WEC) Practical Maximum

1.The electrical power output : Where Cp is the efficiency coefficient of performance when the wind is converted to mechanical power. η m is mechanical transmission efficiency and η g is the electricity transmission efficiency. The optimistic values for these coefficients are: Cp = 0.45, η m = 0.95 and η g = 0.9, which give an overall efficiency of 38%. For a given system, P w and P e will vary with wind speed. Calculation of Electrical power output and capacity factor for a wind turbine 2. The capacity Factor: The capacity factor of a wind turbine is the actuel energy output for the year divided by the energy ouput if the turbine operated at its rated power output for the entire year.

The power curve is an important item for a specific wind turbine. The wind power curve also shows the relationship between wind speed and generator electrical output. Idealized wind turbine power curve Figure 7

1. Cut-in wind speed When the wind speed is below the cut-in wind speed (VC) shown in the Figure 7, the wind turbines cannot start. Power in the low speed wind is not sufficient to overcome friction in the drive train of the turbine. The generator is not able to generate any useful power below cut in speed. 2. Rated wind speed We can see from the Figure 7 that as the wind speed increases, the power delivered by the generator will increase as the cube of wind speed. When the wind speed reached VR the rated wind speed, the generator can deliver the rated power. If the wind speed exceeds VR, there must be some methods to control the wind power or else the generator may be damaged. Basically, there are three control approaches for large wind power machines: active pitch-control, passive stall-control, and the combination of the two ways. 3. Cut-out or furling wind speed Sometimes, the wind is too strong to damage the wind turbine. In the Figure 7 this wind speed is called as cut-out or the furling wind speed. Above VF, the output power is zero. In terms of active pitch-controlled and passive stall-controlled machines, the rotor can be stopped by rotating the blades about their longitudinal axis to create a stall. However, for the stall-controlled machines, there will be the spring-loaded on the large turbine and rotating tips on the ends of the blades. When it is necessary, the hydraulic system will trip the spring and blade tips rotate 90◦ out of the wind and stop the turbine. Idealized wind turbine power curve

Carnage! Figure 8

Factors affecting wind power 1. Wind statistics Wind resource is a highly variable power source, and there are several methods of characterizing this variability. The most common method is the power duration curve. Another method is to use a statistical representation, particularly a Weibull distribution function. Long term wind records are used to select the rated wind speed for wind electric generators. The wind is characterized by a Weibull density function. 2. Load factor  There are two main objectives in wind turbine design. The first is to maximize the average power output. The second one is to meet the necessary load factor requirement of the load. The load factor is very important when the generator is pumping irrigation water in asynchronous mode.  Commonly assumed long-term average load factors may be anywhere from 25% to 30%. 3. Seasonal and diurnal variation of wind power It is clear that the seasonal and diurnal variations have significant effects on wind. The diurnal variation can be reduced by increasing the height of the wind power generator tower. In the early morning, the average power is about 80% of the long term annual average power. On the other hand, in early afternoon hours, the average power can be 120% of the long term average power.

4. Impact of tower height: Wind speed will increase with the height because the friction at earth surface is large. The rate of the increase of wind speed that is often used to characterize the impact of the roughness of the earth’s surface on wind speed is given as: Factors affecting wind power where v is the wind speed at height H, v o is the nominal wind speed at height H o, and α is the friction coefficient. This can be translated into a substantial increase in power at greater heights. Table 1.1 gives the typical values of friction coefficient for various terrain characteristics.

5. Wind turbine sitting The factors that should be considered while installing wind generator are as follow: (1) Availability of land. (2) Availability of power grid (for a grid connected system). (3) Accessibility of site. (4) Terrain and soil. (5) Frequency of lighting strokes Once the wind resource at a particular site has been established, the next factor that should be considered is the availability of land. The area of the land required depends upon the size of wind farm. In order to optimize the power output from a given site, some additional information is needed, such as wind rose, wind speeds, vegetation, topography, ground roughness, etc. In addition other information such as convenient access to the wind farm site, load bearing capacity of the soil, frequency of cyclones, earthquakes, etc., should also be considered. Factors affecting wind power

Optimizing rotor diameter and generator rated power Figure 9 shows the trade-offs between rotor diameter and generator size as methods to increase the energy delivered by a wind turbine. In terms of Fig. 9(a), increasing the rotor diameter and keeping the same generator will shift the power curve upward. In this situation, the turbine generator can get the rated power at a lower wind speed. For Fig. 9(b), keeping the same rotor but increasing the generator size will allow the power curve to continue upward to the new rated power. Basically, for the lower speed winds, the generator rated power need not change, but for the high wind speed area, increasing the rated power is a good strategy. Figure 9: (a) Increasing rotor diameter gives the rate power at lower wind speed, (b) increasing the generator size increases rate power

Modern Wind Turbines Turbines can be categorized into two classes based on the orientation of the rotor. Most are horizontal axis wind turbines (HAWT), but there are some with blades that spin around a vertical axis (VAWT). Examples of the two types are shown in Figure 10. Figure 10: Horizontal axis wind turbines (HAWT) are either upwind machines (a) or downwind machines (b). Vertical axis wind turbines (VAWT) accept the wind from any direction (c).

Vertical Axis Advantages – Can place generator on ground – You don’t need a yaw mechanism for wind angle Disadvantages – Lower wind speeds at ground level – Less efficiency – Requires a “push” (generally, an external power source is required to start the rotation) Horizontal Advantages – Higher wind speeds – Great efficiency Disadvantages – Angle of turbine is relevant – Difficult access to generator for repairs Modern Wind Turbines

Small (  10 kW) Homes Farms Remote Applications (e.g. water pumping, telecom sites, icemaking) Large (250 kW - 2+MW) Central Station Wind Farms Distributed Power Intermediate ( kW) Village Power Hybrid Systems Distributed Power Horizontal-Axis Wind Turbines

Wind Turbine Size-Power Comparison Figure 12

Various components of a wind turbine Figure 13

Comparison Between Turbines Wind turbines may be compared against each other by comparing their efficiency coefficient of performance (CP) against tip-speed-ratio (TSR). The efficiency coefficient of performance, also known as the power coefficients, is defined by : For a given windspeed, rotor efficiency is a function of the rate at which the rotor turns. If the rotor turns too slowly, the efficiency drops off since the blades are letting too much wind pass by unaffected. If the rotor turns too fast, efficiency is reduced as the turbulence caused by one blade increasingly affects the blade that follows. The usual way to illustrate rotor efficiency is to present it as a function of its tip-speed ratio (TSR). The tip-speed-ratio is the speed at which the outer tip of the blade is moving divided by the windspeed: where rpm is the rotor speed, revolutions per minute; D is the rotor diameter (m); and v is the wind speed (m/s) upwind of the turbine.

A plot of typical efficiency for various rotor types versus TSR is given in Figure 14. The American multiblade spins relatively slowly, with an optimal TSR of less than 1 and maximum efficiency just over 30%. The two- and three-blade rotors spin much faster, with optimum TSR in the 4–6 range and maximum efficiencies of roughly 40–50%. Also shown is a line corresponding to an “ideal efficiency,” which approaches the Betz limit as the rotor speed increases. The curvature in the maximum efficiency line reflects the fact that a slowly turning rotor does not intercept all of the wind, which reduces the maximum possible efficiency to something below the Betz limit. Typical efficiency for various rotor types Figure 14: Typical efficiency for various rotor types