18 It is compared with the power of the free air stream wh
ich flows through the same cross sectional area A, without mechanical power being extracted from it. This power was:
Watt 15
2.2.4.2. Power Coefficient and Tip Speed Ratio
When the wind stream passes the turbine, a part of its kinetic energy is transferred to the rotor and the air leaving the turbine carries the rest away. Actual power produced by a
rotor would thus be decided by the efficiency with which this energy transfer from wind to the rotor takes place. This efficiency is usually termed as the power coefficient Cp. Thus,
the power coefficient of the rotor can be defined as the ratio of actual power developed by the rotor to the theoretical power available in the windMathew 2006. It is expressed as follows:
16
After some re-arrangement, the power coe fficient can be specified directly as a
function ofthevelocity ratio
v
2
v
1
: 17
Figure 2.12 Power coe fficient compared to the flow velocity ratio of
the flow before and after the energy converterHau 2006 perpustakaan.uns.ac.id
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19 The power coe
fficient, the ratio of the extractable mechanical power to the power contained in the air stream, therefore, now only depends on the ratio of the air velocities
before and after the converter. If this inter relationship is plotted graphically naturally, an analytical solution can also be found easily, it can be seen that the power coe
fficient reaches a maximum at a certain velocity ratio Fig.2.7. With
v
2
v
1
= 13, the maximum “ideal power
coe fficient” Cp becomes:
18 This is
called Betz‟s limits where the ideal coefficient power will not over than the ideal number which is 0,593.
Speed is conventionally given as a non-dimensional ratio known as the tip speed ratio λ, which is the ratio of the speed of the rotor tip at radius R in meter, when rotating at ω
radians per seconds, to the speed of the wind
v
.
19
This graphic shows tip speed ratio λ and power coefficient C
p
of various wind turbines.
Figure 2.13 Characteristic of PowerCoefficient and TipSpeed Ratio for some wind turbinesHau 2006
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20
2.2.4.3. Torque
Apart from the rotor power, there are other parameters which are of significance in characterizing rotor performance. The most important of these is the behaviour of the torque.
The rotor torque can present as follows:
20 Where the rotor radius R is the reference parameter. Since the torque can be
calculated by dividing power by the rotational speed,the following simple relationship between power and torque coefficient is obtained:
21
The tip speed ratio can be given by the ratio between the power coefficient and torque coefficient of the rotor.
Figure 2.14 Characteristic of Torque Coefficient and Tip Speed Ratio for Some Wind TurbinesAkwa, Vielmo et al. 2012
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21
CHAPTER III RESEARCH METHOD
3.1. Research Place
The research was conducted at Heat Transfer Laboratory of Mechanical Engineering Department of Engineering Faculty of Sebelas Maret University in Surakarta
3.2. Apparatus
a. Model of Vertical Axis Wind Turbine Savonius
Contrive the wind turbine model as following specifications: -
Turbine diameter : 20 cm
- Axis height
: 18 cm -
Overlap of blade : 1 cm
- Number of blade
: 2 blades -
Material of blade : Aluminum
a. Two-blade Savonius Rotor
b. Savonius Rotor with Guide Vane
Figure 3.1 Models of Vertical Axis Wind Turbine Savonius perpustakaan.uns.ac.id
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