PDF《WINDPUMPING》

these are through the creation of either lift or drag force (or through a combination of the two). The difference between drag and lift is illustrated by the difference between using a spinnaker sail, which fills like a parachute and pulls a sailing boat with the wind, and a Bermuda rig, the familiar triangular sail which deflects with wind and allows a sailing boat to travel across the wind or slightly into the wind. Drag forces provide the most obvious means of propulsion, these being the forces felt by a person (or object) exposed to the wind. Lift forces are the most efficient means of propulsion but being more subtle than drag forces are not so well understood. The basic features that characterise lift and drag are: ? drag is in the direction of air flow ? lift is perpendicular to the direction of air flow ? generation of lift always causes a certain amount of drag to be developed ? with a good aerofoil, the lift produced can be more than thirty times greater than the drag ? lift devices are generally more efficient than drag devices Types and characteristics of rotors There are two main families of windmachines: vertical axis machines and horizontal axis machines. These can in turn use either lift or drag forces to harness the wind. The horizontal axis lift device is the type most commonly used. In fact other than a few experimental machines virtually all windmills come under this category. There are several technical parameters that are used to characterise windmill rotors. The tip- speed ratio is defined as the ratio of the speed of the extremities of a windmill rotor to the speed of the free wind. Drag devices always have tip-speed ratios less than one and hence turn slowly, whereas lift devices can have high tip-speed ratios (up to 13:1) and hence turn quickly relative to the wind. The proportion of the power in the wind that the rotor can extract is termed the coefficient of performance (or power coefficient or efficiency;

symbol Cp) and its variation as a function of tip-speed ratio is commonly used to characterise different types of rotor. As mentioned earlier there is an upper limit of Cp = 59.3%, although in practice real wind rotors have maximum Cp values in the range of 25%-45%. Solidity is usually defined as the percentage of the area of the rotor, which contains material rather than air (see Figures

1 &

2 below). High-solidity machines carry a lot of material and have coarse blade angles. They generate much higher starting torque (torque is the twisting or rotary force produced by the rotor) than low-solidity machines but are inherently less efficient than low-solidity machines. The windpump is generally of this type. Low-solidity machines tend to be used for electricity generation. High solidity machines will have a low tip-speed ratio and vice versa. Windpumping Practical Action

4 The choice of rotor is dictated largely by the characteristic of the load and hence of the end use. Some common rotor types and their characteristics are shown in Table

1 below. Type Speed Torque Cp Solidity (%) Use Horizontal axis Multi blade Low High 0.25 - 0.4

50 C

80 Mechanical power Three-bladed aerofoil High Low up to 0.45 Less than

5 Electricity production Vertical axis Panemone Low Medium less than 0.1

50 Mechanical power Darrieus Moderate Very low 0.25 - 0.35 10........