【单选题】A large horizontal axis wind turbine operating at the design wind speed will take about half the energy from the wind. Why can it not take all the energy from the wind?
A.
Most of the air escapes through the space between the blades
B.
Some of the wind bounces back off the blades and is unable to pass through the machine
C.
The wind must be left with enough kinetic energy to exit the turbine
D.
The wind turbine has the same thermodynamic efficiency as a combustion engines
【单选题】The velocity of the apparent wind can be more than the true wind,and come from the same direction,if certain conditions are present.One condition is that the ______.
A.
Ship's speed must be less than the true wind velocity
B.
True wind must be from dead astern
C.
True wind velocity must be faster than the ship's speed