Chapter 5: Renewables Flashcards
What do the terms “wind energy” and “wind power” describe
describes the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. A generator can convert this mechanical power into electricity to power homes, businesses, etc.
How do vertical wind turbines work?
- There is an inflow of wind
- Inflow of wind activates rotor and then blades
- Rotor & blades spin the main shaft and gearbox
- Then the main shaft and gear box spin the generator, resulting in electrical output
Wind power depends on…
- amount of air (volume)
- speed of air (velocity)
- mass of air (density)
Wind power, Theory
kinetic energy = 1/2 m v^2
Power = kinetic energy per unit time
therefore if t = 1 second then, P = 1/2 m v^2
m^. (mass flux) = dm/dt (and from fluid mechanics) = (density) * A * v
Thus P = 1/2 * (density) * A * v^3
A = (pi) * r^2 (rotor swept area)
P is proportional to v^3
Betz limit and power efficiency
Power coefficient, Cp, is the ratio of power extracted by the turbine to the total contained in the wind resource. Cp = P_T / P_W
P_T = power output = 1/2 (density) * (A) * (v^3) * Cp
The betz limit is the maximum possible C_p =16/27 = (0.59)
59% efficiency is the maximum achievable efficiency a wind turbine can do while extracting power from the wind
What is the capacity factor (CF) :
The fraction of the year the turbine generator is operating at rated (peak) power
Capacity factor = average output / peak output = 30 % (approx)
CF is based on both the characteristics of the turbine and the site characteristics (typically 0.3 or above for a good site)
additional losses:
10% (drive train, generator)
10% (inverter and cabling)
HAWT vs VAWT
HAWT : Horizontal-Axis Wind Turbines
VAWT : Vertical-Axis Wind Turbines
HAWT
Advantages vs disadvantages
HAWT: Advantages -
- Variable blade pitch (thus, optimum angle of attack and maximum amount of wind energy).
-The tall tower base allows access to stronger wind in sites with wind shear. (In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%).
-High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation.
Advantages (HAWT) Disadvantages (HAWT)
Disadvantages:
-Transport and installation of heavy masts (ca. 20% of
equipment costs).
-Stronger tower construction is required to support the heavy blades, gearbox, and generator.
-Affecting radar installations.
-Landscape.
-Fatigue and structural failure caused by turbulence (blade passes through the tower’s wind shadow, thus, upwind design, with
the rotor facing the wind in front of the tower); Additional yaw
control mechanism to turn the blades toward the wind.
VAWT
Advantages vs disadvantages
Advantages:
- Omni-directional
- accepts wind from any direction
Components can be mounted at ground level
- ease of service
- lighter weight towers
-Can theoretically use less materials to capture the same
amount of wind
Disadvantages:
- Rotors generally near ground where wind is poorer
- Centrifugal force stresses blades
- Poor self-starting capabilities
- Requires support at top of turbine rotor
-Requires entire rotor to be removed to replace bearings
-Overall poor performance and reliability
HAWT are preferable (more efficient)
Tidal energy
Form of hydropower energy
- The terms “tidal energy” or “tidal power”
describe the process by which energy obtained
from tides is used to generate mechanical power
or electricity. Conversion of the kinetic energy
from tides into mechanical power. A generator
can convert this mechanical power into
electricity to power homes, businesses, etc.
Forms of tidal energy:
Tidal stream generator
Tidal barrage
Dynamic tidal power
Tidal Lagoon
Wave energy
