**2. Utilization of wind energy**

The utilization of wind energy can be dated back to 5000 B.C. when sail boats were propel‐ led across the river Nile. It was recorded that from 200 B.C. onwards wind was used as an energy source to pump water, grind grain, and drive vehicles and ships in ancient China and Middle East. The first documented windmill was in a book *Pneumatics* written by Hero of Alexandria around the first century B.C. or the first century A.D. [52]. Effectively, these wind mills are used to convert kinetic energy into mechanical energy.

The use of wind energy to generate electricity first appeared in the late 19th century [35] but did not gain ground owing to the then dominance of steam turbines in electricity genera‐

tion. The interest in wind energy was renewed in the mid-1970s following the oil crises and increased concerns over resource conservation. Initially, wind energy started to gain popu‐ larity in electricity generation to charge batteries [17] in remote power systems, residential scale power systems, isolated or island power systems, and utility networks. These wind turbines themselves are generally small (rated less than 100kW) but could be made up to a large wind farm (rated 5MW or so). It was until the early 1990s when wind projects really took off the ground, primarily driven by the governmental and industrial initiatives. It was also in 1990s there seemed a shift of focus from onshore to offshore development in major wind development countries, especially in Europe.

Offshore wind turbines were first proposed in Germany in 1930s and first installed in Swe‐ den in 1991 and in Denmark in 1992. By July 2010, there were 2.4 GW of offshore wind tur‐ bines installed in Europe. Compared to onshore wind energy, offshore wind energy has some appealing attributes such as higher wind speeds, availability of larger sites for devel‐ opment, lower wind sheer and lower intrinsic turbulence intensity. But the drawbacks are associated with harsh working conditions, high installation and maintenance costs. For off‐ shore operation, major components should be marinized with additional anti-corrosion measures and de-humidification capacity [24]. In order to avoid unscheduled maintenance, they should also be equipped with fault-ride-through capacity to improve their reliability.

**Figure 1.** Ever-growing size of horisontal-axis wind turbines [36].

Over the last three decades, wind turbines have significantly evolved as the global wind market grows continuously and rapidly. By the end of 2009, the world capacity reached a total of 160 GW [7]. In the global electricity market, wind energy penetration is projected to rise from 1% in 2008 to 8% in 2035 [45]. This is achieved simply by developing larger wind turbines and employing more in the wind farm. In terms of the size, large wind turbines of the MW order began to appear in the EU, the US and now in China and India. Typically, the large installed wind turbines in utility grids are between 1.5-5MW whilst 7.5 and 10 MW are under extensive development, as shown in Fig. 1. Nowadays, modern wind turbines are re‐ liable, quiet, cost-effective and commercially competitive while the wind turbine technolo‐ gies are proven and mature. At present, technical challenges are generally associated with ever-growing wind turbine size, power transmission, energy storage, energy efficiency, sys‐ tem stability and fault tolerance.

**Figure 2.** The world's energy potential for land-based wind turbines (estimated energy output in kWh/kW from a wind turbine that is dimensioned for 11 m/s) [36].

Currently, wind power is widely recognized as a main feasible source of renewables which can be utilized economically in large quantity. A world map for wind energy potential is il‐ lustrated in Fig. 2. Taking the United Kingdom for example, the usable offshore wind ener‐ gy alone is enough to provide three times more than the required electricity consumption in the country, given sufficient support. However, wind power fluctuates by its nature and such applications demand high reliability and high availability while the market is still look‐ ing to reduce weight, complexity and operational costs.
