Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 50

A wind turbine is a device for extracting kinetic energy from the wind. By removing some of its kinetic energy, the wind must slow down, but only that mass of air that passes through the rotor disc is directly affected. Assuming that the affected mass of air remains separate from the air that does not pass through the rotor disc and does not mix with it, a boundary surface can be drawn containing the affected air mass, and this boundary can be extended upstream as well as downstream, forming a long streamtube of circular cross‐section. No air flows across the boundary, and so the mass flow rate of the air flowing along the streamtube will be the same for all streamwise positions along the streamtube. Outside the streamtube, air that passes close to the rotor is not retarded in the same way but is subject to both retardation and acceleration associated with the divergence of the streamlines around the rotor. Because the air within the streamtube slows down, but is effectively incompressible at these speeds, the cross‐sectional area of the streamtube must expand to accommodate the slower moving air; see Figure 3.1.

Although kinetic energy is extracted from the air flow, a sudden step change in velocity is neither possible nor desirable because of the enormous accelerations and forces this would require. Pressure energy can be extracted in a step‐like manner, however, and all wind turbines, whatever their design, operate in this way.

The presence of the turbine causes the approaching air, upstream, gradually to slow down such that when the air arrives at the rotor disc its velocity is already lower than the free‐stream wind speed. The streamtube expands as a result of the slowing down and, because no work has yet been done on, or by, the air, its static pressure rises to absorb the decrease in kinetic energy.

As the air passes through the rotor disc, by design, there is a drop in static pressure such that, on leaving, the air is below the atmospheric pressure level. The air then proceeds downstream with reduced speed and static pressure: this region of the flow is called the wake. Eventually, far downstream, the static pressure in the wake must return to the atmospheric level for equilibrium to be achieved. The rise in static pressure is at the expense of the kinetic energy and so causes an additional slowing down of the wind. Thus, between the far upstream and far wake conditions, no difference in static pressure exists, but there is a reduction in kinetic energy.

Figure 3.1 The energy extracting streamtube of a wind turbine.