# Experimental Study of the Performance of a Novel Vertical-Axis Wind Turbine
## 4 Results and Discussion
It is discernible from Figure 10 that the aerodynamic power values are higher than the actual power generated. This difference in power is the result of the fact that the aerodynamic power is derived from the rate of loss of the kinetic energy of the airstream; friction loss on torque transducer bearings is not accounted for. Figure 11 depicts the power coefficient as a function of the free-stream speed. The actual power coefficient curve is below the aerodynamic power coefficient curve. There is a scatter between experimental data and the fitted curve of the actual power coefficient; the same is true for the aerodynamic power coefficient. Both curves are parabolas with upward concavity, implying that both the aerodynamic and actual power coefficients vary quadratically with varying free-stream speeds. The actual power coefficient curve has a minimum value of about 0.1 at a free-stream speed of about 6.5 m/s, whereas the aerodynamic power coefficient curve has a minimum value of about 0.28 at a free-stream speed of about 7.3 m/s. It is discernible from Figure 11 that the aerodynamic power coefficient is higher than the actual power coefficient at all free-stream speeds. This difference in the power coefficient results from power losses due to drag, friction on torque transducer bearings and losses in generator windings. The relationship between the actual power coefficient and the free-stream speed is  
Figure 6:2  
A coefficient of determination of 56.25% is determined for Equation (33) using the MATLAB plotting tool, resulting in a correlation coefficient of 75%. The aerodynamic power coefficient is expressed as a function of the free-stream speed:  
Figure 6:3  
A coefficient of determination of 61.73% is determined for Equation (34) using the MATLAB plotting tool, resulting in a correlation coefficient of 78.57%. Figure 6:4 Power output as a function of the tip-speed ratio. Respective power coefficients as functions of the tip-speed ratio are shown in Figure 6:5 It is discernible from Figure 12 that the actual power coefficient curve has a minimum value of about 0.07 at the tip-speed ratio of 0.38. The relationship between the actual power coefficient and the tip-speed ratio is a quadratic polynomial:  
Figure 6:6  
A 93.99% coefficient of determination is determined for this fit using the MATLAB plotting tool, resulting in a correlation coefficient of 97.97%. The aerodynamic power coefficient curve has a minimum value of about 0.21 at the tip-speed ratio of about 0.42. The relationship between the aerodynamic power coefficient and the tip-speed ratio is also a quadratic polynomial:  
Figure 6:7  
A coefficient of determination of 89.67% is determined for this fit using the MATLAB plotting tool, resulting in a correlation coefficient of 94.67%. It is discernible from Figure 12 that the aerodynamic power coefficient curve runs above the actual power coefficient curve. This is the case because the aerodynamic power coefficient is higher than the actual power coefficient within the entire tip-speed ratio range. The aerodynamic power coefficient is an indicator of the rate of change of the kinetic energy of the airstream as it passes through the AVAWT rotor, while the actual power coefficient indicates the rate of energy conversion to electricity as the air flows past the AVAWT rotor. It is worth noting that the best way to determine the power coefficient of a WT is by measuring the actual power generated by it. Use of aerodynamic parameters to determine the power coefficient results in higher values, even when no power is generated. Figure 6:8 Power coefficient as a function of the free-stream speed. Figure 6:9 Power coefficient as a function of the tip-speed ratio.