Chapter 1
Overview

1.1 Wind Energy and Wind Energy Resources

1.1.1 Basic Concepts

Wind energy is the energy of moving air. In a broad sense, wind energy is derived from solar energy. The radiant energy from the sun is constantly transmitted to the earth's surface. Solar radiation does not heat every part of the earth's surface evenly resulting in differences in temperature and pressure and forming the wind.

According to aerodynamic theories, the moving air has energy and the wind energy flowing through the section perpendicular to the wind speed A (m2) per unit time, namely, the wind power [1], is (Formula (1.1))

where E is wind energy with the unit W (kg·m2·/s3); m is air mass (kg/m3); v is air flow speed, namely, wind speed (m/s); A is the area of the section the air passing through that is perpendicular to the direction of the air flow (m2); ? is the density of air (kg/m3).

Wind power is proportional to the square of the wind speed while wind energy (kinetic energy) is proportional to the third power of the wind speed. That is to say, if wind speed doubles, wind power output increases eight times.

Wind energy is a renewable energy source. From a long-term perspective, wind energy is inexhaustible. Meanwhile wind energy is a form of process energy that cannot be directly stored. Instead it has to be converted into other forms of energy in order to be stored.

According to different requirements, wind energy can be converted into a great variety of energy forms, including mechanical, electrical, thermal energy, and so on, in order to achieve pump water irrigation, power generation, sail-assisted navigation, and other functions.

Wind energy resources are kinetic energy resources created by the movement of the air across the surface of the earth. The formation of wind energy resources is affected by multiple natural factors, especially the climate, terrain, and sea and land location. Wind energy is widely distributed in space and meanwhile it is unstable and discontinuous. Since wind is very sensitive to the climate, it is variable and varies from region to region and season to season.

There is an abundance of wind energy resources in nature. According to the World Meteorological Organization (WMO), global wind energy totals 3 × 1017 kW of which 2 × 1010 kW is exploitable wind energy, 10 times more than the total amount of exploitable hydro energy on the earth [1]. The amount of technologically exploitable wind energy resources totals about 53 TW·h/year (1 TW = 1015W = 1012kW), equivalent to more than two times the world's total electricity demand in 2020 [2]. China is rich in wind energy resources: the total reserves of wind energy resources at an altitude of 10 m above the land are estimated to be 3,226 GW; the reserves of the exploitable onshore wind energy are 253 GW and the reserves of exploitable offshore wind energy are 750 GW, totaling 1,000 GW [3, 4].

The potential for wind energy resources in a certain area of the earth is expressed by the wind energy density and available hours in this area.

Wind energy density is the kinetic energy of the moving air perpendicularly passing through the unit section per unit time, namely, the wind power density. If the area A = 1 in Formula (1.1), then the wind energy density (Formula 1.2) is shown as

Wind energy density is also changing with time and with the change of wind speed. The average value of the wind energy density over a certain period of time (e.g., one year) is called the average wind energy density that is shown in Formula (1.3):

where is the average wind energy density; T is a certain period of time; v(t) is the wind speed changing with time; dt is the duration of a certain wind speed within T. If in the wind speed measurement the wind speeds v1, v2, ., vn and their corresponding durations t1, t2, ., tn within T can be directly obtained (or after data processing), then the average wind energy density can be calculated by Formula (1.4):

In the actual use of wind energy, wind turbines only work within a certain range of wind speeds. The wind energy density within a certain range of wind speeds is regarded as effective wind energy density. In China the range of wind speeds corresponding to the effective wind energy density is 3-20 m/s [1, 5].

The air density (?) can be calculated by a great variety of formulas that vary in complexity, parameters, and accuracy. Usually the more parameters the formula has, the more accurate it is. It's suggested Formula (1.5) should be used to calculate the values in reference [1].

where ? is the average air density (kg/m3); p is the average air pressure (hPA); e is the average water vapor pressure (hPa); t is the average Celsius temperature (°C).

The air density varies with altitude. At an altitude of below 500 m, that is, at a normal temperature and under standard atmospheric pressure, the air density is 1.225 kg/m3. If the altitude is above 500 m, the relationship between the air density and the altitude can be calculated according to the experience of China's meteorological stations (Formula (1.6)) [5]:

where ?z(kg/m3) is the air density at an altitude of z(m).

Wind speed and wind direction are two important factors in the utilization of wind energy. In order to estimate wind energy resources, we must measure the daily and annual wind speed and wind direction and understand their changing laws. The wind direction in a certain area of the earth is, first of all, related to the atmospheric circulation. Besides, it is also related to the geographical location (its distance from the equator and the south and north poles) and the earth's surface (ocean, land, valley, etc.).

The fundamental basis for the calculation of wind energy resources is the hourly wind speed that can be calculated in three ways: ? average the measured hourly wind speeds; ? average the wind speeds measured in the last 10 minutes of each hour as required in China; ? average the several selected instantaneous wind speeds in each hour.

Wind speed varies with height. From the surface of the earth to the upper air layer at an altitude of 10,000 m, the moving of the air is affected by factors such as the eddy, viscosity, and surface friction. The higher it is above the earth's surface, the higher the wind speed is. In engineering the index method is usually used to express the change of the wind speed with height (Formula (1.7)):

where h, h1 are different heights from the earth's surface; v1 is the wind speed at a height of h1 above the earth's surface; v is the wind speed at a height of h above the earth's surface that is to be calculated; index n is related to the surface evenness (roughness), the stability of the atmosphere and other factors, ranging from one-half to one-eighth and being one-seventh in areas with normal stability. China's meteorological departments measure the wind speeds at various heights and calculate the average value of n to be between 0.16 and 0.20 that can be used to estimate the wind speed at different heights. Obviously, the higher the wind turbines are placed, the more wind energy they can capture.

Wind direction is usually expressed using 16 directions. The diagram based on the frequency of winds blowing from different directions is called wind direction frequency rose diagram. Shown in Figure 1.1 is the wind rose diagram that displays the average wind direction and corresponding average wind speed at Lvsi Ocean Wind Measurement Station (see Table 1.1 for the corresponding data). It can be seen that according to the average annual frequency of wind direction measured over many years, the prevailing wind directions are N, ESE, and SSE and the corresponding frequency is 9%; the secondary prevailing wind directions are NNE, ENE, and E and the corresponding frequency is 8%; the wind direction NW has the greatest average wind speed, 8.1 m/s, followed by the wind direction NNW, which has the average wind speed of 7.9 m/s. It shows that prevailing wind directions are different from strong wind directions, but generally wind directions from NNW to SSE have higher frequency and wind speed, with the frequency and average wind speed being 72% and 6.7 m/s respectively. The wind rose diagram can accurately display the distribution of wind frequency in a certain area so as to determine the overall arrangement of wind turbines in a wind farm (WF) and facilitate WF micro-siting. It plays an important role in the initial design of WF construction and wind power forecasting.

Figure 1.1 Wind direction frequency rose diagram and corresponding average wind speed rose diagram.

Table 1.1 Average perennial wind direction frequency (fw) and corresponding wind speed (v)