Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 18

as micro wind turbine [41]. Micro wind turbines are especially suitable in loca-
tions where the electrical grid is unavailable. They can be used on a per-structure
basis, such as street lighting, water pumping, and residents at remote areas, par-
ticularly in developing countries. Because micro wind turbines need relatively low
cut-in speeds at start-up and operate in moderate wind speeds, they can be exten-
sively installed in most areas around the world for fully utilizing wind resources
and greatly enhancing wind power generation availability.
Small wind turbines usually refer to the turbines with the output power less than
100 kW [42]. Small wind turbines have been extensively used at residential houses,
farms, and other individual remote applications such as water pumping stations,
telecom sites, etc., in rural regions. Distributed small wind turbines can increase
electricity supply in the regions while delaying or avoiding the need to increase the
capacity of transmission lines.
The most common wind turbines have medium sizes with power ratings
from 100 kW to 1 MW. This type of wind turbines can be used either on-grid
or off-grid systems for village power, hybrid systems, distributed power, wind
power plants, etc.
Megawatt wind turbines up to 10 MW may be classifi ed as large wind turbines.
In recent years, multi-megawatt wind turbines have become the mainstream of the
international wind power market. Most wind farms presently use megawatt wind
turbines, especially in offshore wind farms.
Ultra-large wind turbines are referred to wind turbines with the capacity more
than 10 MW. This type of wind turbine is still in the earlier stages of research and
development.
5.1.4 Direct drive and geared drive wind turbines
According to the drivetrain condition in a wind generator system, wind turbines
can be classifi ed as either direct drive or geared drive groups. To increase the
generator rotor rotating speed to gain a higher power output, a regular geared
drive wind turbine typically uses a multi-stage gearbox to take the rotational speed
from the low-speed shaft of the blade rotor and transform it into a fast rotation on
the high-speed shaft of the generator rotor. The advantages of geared generator
systems include lower cost and smaller size and weight. However, utilization of a
gearbox can signifi cantly lower wind turbine reliability and increase turbine noise
level and mechanical losses.
By eliminating the multi-stage gearbox from a generator system, the genera-
tor shaft is directly connected to the blade rotor. Therefore, the direct-drive con-
cept is more superior in terms of energy effi ciency, reliability, and design
simplicity.
5.1.5 On-grid and off-grid wind turbines
Wind turbines can be used for either on-grid or off-grid applications. Most
medium-size and almost all large-size wind turbines are used in grid tied applica-
tions. One of the obvious advantages for on-grid wind turbine systems is that there
is no energy storage problem.