Standalone hybrid generation system for the remote area of Thar, Pakistan
The swept area,
A depends on the dimensions of the rotor. For a horizontal
axis turbine of rotor
diameter d, the swept area can be given by [11]:
2.12
For a vertical axis turbine of maximum rotor width
wr and
rotor height hr, the swept area can be
approximated by [11]:
2.13
The air density ρ depends on pressure and temperature. It can be expressed as [11]:
2.14
Where
P is the air pressure,
T is the absolute temperature and
R represents for the gas constant. The
gas density is about 1.225 kg/m3 at sea level, 1-atm and 289K temperatures; but it has less value at
high elevation [11, 22].
The volumetric flow rate is (A.
v) and the mass flow rate of the air in Kg/s (ρ.A.
v) resulting in the
above expression for the mechanical power in the moving air. The potential of wind power at a given
site is expressed in terms of wind power per square meter area of the swept rotor blades, referred to as
specific power and is given as:
2.15
This power that is in the upstream wind [11] varies with air density linearly
and cube of the wind
speed. The extracted power by the blades is, therefore, less than the above valued as some power is
left in the downstream air that continues to move at reduced speed. The actual mechanical power
extracted by the rotor blades is [11]:
2.16
Where
v and
vo are the upstream and downstream wind velocities at the
entrance and exit of rotor
blades respectively and the mass flow rate of air,
mfr is given as a function of the average speed.
2.17
With some algebraic manipulation, the power extracted by the rotor blades
can be expressed as a
fraction of upstream wind power as follows [11, 22, 23].
2.18