*2.3. Experimental Stand*

A small test stand was built for the purpose of testing the wind turbine model.

Figure 7 shows a schematic diagram of the stand. The test stand includes a virtual measurement and control device implemented on a PC class computer and a physical model of a miniature wind tunnel. The virtual measurement and control device was created in the LabView programming environment. Figure 8 shows the view of the front panel. The device can set the output value of the fan power, the value of the PWM duty cycle for the DC/DC converter, and the pitch value of the wind turbine blade. In addition, the instrument displays and records the following measured values: wind speed *v*, turbine speed *n*, generator output voltage *UG*, output voltage of the DC/DC converter *UO*, and load current *IO*. The power delivered to the load *PO* is also determined.

**Figure 7.** Schematic diagram of the stand for testing the wind microturbine model.

**Figure 8.** Virtual measuring and control device—front panel.

The model of the wind tunnel made, shown in Figure 9a,b, consists of a tunnel pipe (1) fixed in a frame (2) which also acts as a honeycomb (air stream straightener). Attached to the frame is a brushless BLDC fan motor (3) with a 203 mm (8-inch) diameter propeller and 101 mm (4-inch) pitch, and a DC brush motor (4) with permanent magne<sup>t</sup> excitation acting as a DC generator. The generator is powered by a propeller that functions as a wind turbine. At the entrance to the tunnel there is an anemometer sensor (5) measuring the speed of the wind. The BLDC motor of the fan is powered by a three-phase inverter (6). The research model has a controller (7) based on a 32-bit STM32F1 microcontroller which controls the operation of the fan, DC/DC converter, and the servomechanism (12) of the turbine propeller pitch (11). The controller also measures wind speed *v*, turbine rotational speed *ω*, generator output voltage *UG*, converter output voltage *UO*, and load current *IO* (8). The controller, via the USB serial interface (9), communicates with the master computer on which the virtual measurement and control device program is running. The model is powered by a power supply (10) with a voltage of 12 V. Due to the small size of the model, some of its elements could be produced using a 3D printer.

**Figure 9.** (**a**) View of the research model; (**b**) View of the wind tunnel.

#### **3. Results and Discussion**

The virtual measurement and control device has the ability to automatically record measured values for ten set fan power values and one hundred set values of the PWM wave fill factor for the transistor in the DC/DC converter (buck converter). Because of this solution, a matrix of 1000 values is obtained for each measured value without the tedious involvement of the person conducting the measurements and data recording. Moreover, the measuring and control device has the possibility of automatic adjustment of the blade setting angle in accordance with the control function described by the expression dependent on the measured instantaneous rotational speed of the turbine *β = f(n)*. Ultimately, the practical solution of the microturbine provides for a possibly simple and cheap mechanical solution using the centrifugal force of the rotating mass, which would change the pitch of the blades. The optimal control of the power plant is based on achieving the maximum power for a given wind speed value. Power control, in this system solution of the power plant, is carried out by changing the duty cycle of the PWM signal that controls the operation of the switch in a simple DC/DC converter, which has a direct impact on the value of the converter output voltage. In the conducted research, measurements were carried out for a load with a non-linear current–voltage characteristic, which is a lithium-polymer battery cell. The electromotive force EMF of the cell used, depending on the state of charge, was in the range (3.3 ÷ 4.3) V and its internal resistance was determined as *Rw* ≈ 0.25 Ω.

#### *3.1. Wind Turbine with a Classic Aerofoil Profile Blade*

In order to compare the results of the research on the influence of the adjustment of the pitch angle of the blades on the efficiency of the wind turbine operation, a propeller with a blade with a classic aerodynamic profile and a constant pitch angle was used in the experiment. Examples of power generated by a wind turbine for a resistive load as a function of the duty cycle of the PWM signal (control quantity) for different wind speeds are shown in Figure 10.

**Figure 10.** The power generated by a wind turbine for a linear load as a function of the control value—the duty cycle of the PWM signal, for different wind speeds.

It is worth noting that for a linear load such as a resistor, the optimal operation of the power plant for higher wind values is achieved for a narrow range of the PWM duty cycle (0.50 ÷ 0.65). The limitation of the power of the power plant in this case is mainly due to the relatively high resistance value of the generator winding. For lower wind speeds, the power limitation results mainly from the low value of mechanical power achieved by the wind turbine (propeller), which is transferred to the generator shaft. Examples of power generated by a wind turbine loaded with a Li-Po cell as a function of the duty cycle of the PWM signal (control quantity) for various wind speeds are shown in Figure 11. It is worth noting that for a non-linear load such as a Li-Po cell, the optimal control of the power plant operation requires a wider range of PWM duty cycle values within the range (0.4 ÷ 0.85). It is also necessary to take into account changes in the EMF voltage of the cell depending on the degree of its charge.

**Figure 11.** The power generated by a wind turbine for a non-linear load, as a function of the control value—duty cycle of the PWM signal, for different wind speeds.

Figure 12 presents the obtained characteristics of the maximum power of the power plant depending on the wind speed. It can be seen that at higher wind speeds, the slope of the characteristic no longer increases. The cause may be a significant increase in the thrust force *Fr*, resulting from the increase in the rotational speed of the turbine—the frontal resistance of the airfoil. In the conducted experiment, the turbine achieves a high tip speed ratio *λ* >10: the turbine blades are positioned almost perpendicular to the wind direction. In addition, taking into account the static friction of bearings and brushes and the cogging torque resulting from changes in the reluctance of the generator's magnetic circuit, the start-up of such a turbine is very difficult and takes place only at a relatively high wind speed of 4 m/s, compared to the wind speed of 1.5 m/s at which it stops.

**Figure 12.** The maximum power of a wind turbine.

Preliminary results of the measurements indicate that the direct connection of the battery to the generator, often used in microturbines, whose charging characteristics are strongly non-linear and varies depending on the battery charge level, may be the cause of poorer efficiency of micro power plants. The solution to this problem may be the use of a possibly simple DC/DC converter, which could linearize the load characteristics of the wind microturbine.

Tests were carried out of a micro wind power plant model, controlled by a simple HCS algorithm presented in Figure 6. Figure 13 shows selected results of the experiment in the form of graphs: microturbine wind power, PWM duty factor, and turbine rotation speed. The results were obtained at a constant wind speedv=5 m/s.

**Figure 13.** Graphs: microturbine power, duty factor PWM, and turbine rotation speed of a microturbine model controlled by HCS algorithm.

The results of the experiment indicate the possibility of using a simple HCS algorithm to control the operation of a wind microturbine only for a stable value of wind speed. In the case of rapid and greater decreases in wind speed, the presented simple HCS algorithm no longer works optimally, even leading to the wind turbine stopping and the need to restart it. Some improvement in operation can be achieved in this case by using the measured turbine rotational speed in the control algorithm.

#### *3.2. Wind Turbine with Adjustable Pitch*

For the purposes of the research on the impact of the adjustment of the blade angle on the operating efficiency range of a wind turbine, a propeller with a straight profile and a zero twist angle of the blade was used in the experiment—Figure 9. Figure 14 shows the power characteristics obtained in the tests carried out on a wind turbine with a constant pitch *β* = 15◦ as a function of the value of the PWM signal duty factor (control variable) for various wind speeds.

**Figure 14.** The power generated by a wind turbine with a constant pitch, as a function of the control value—duty cycle of the PWM signal, for different wind speeds.

On the other hand, Figure 15 shows the power characteristics obtained in the tests carried out on a wind turbine with adjustable pitch depending on the rotation speed *n* [rpm] of the turbine *β*(*n*) *=* 30◦ − *n*/150, as a function of the duty cycle of the PWM signal for various wind speeds. It is worth noting that for a non-linear load such as a Li-Po cell, optimal operation of the power plant requires a wider range of PWM control variable values in the range (0.45 ÷ 0.95). It is also necessary to take into account changes in the EMF voltage of the cell depending on the degree of its charge.

**Figure 15.** The power generated by a wind turbine with adjustable pitch, as a function of the duty cycle, for different wind speeds.

Figure 16 compares the obtained two characteristics of the maximum turbine power achieved as a function of wind speed for the case of fixed (*β = const*) and adjustable angle of the pitch (*β* = *f(n)*) of turbine blades. It can be seen that for the case of the adjustable angle of the pitch, the efficiency of the power plant is higher. In the experiment, the turbine achieves a high value of tip speed ratio *λ* ≈ 10; the blades of the turbine are positioned almost perpendicular to the direction of the wind. In addition, taking into account the static friction of bearings and brushes and the cogging torque resulting from changes in the reluctance of the generator's magnetic circuit, the start-up of such a turbine is very difficult and takes place only at a relatively high wind speed of 4 m/s, compared to the wind speed of 1.5 m/s at which it stops. Adjusting the pitch angle of the blades can greatly facilitate the start-up of a wind turbine and allow it to operate at lower wind speeds. In addition, through the appropriate selection of the dependence *β = f(n)*, it is also possible to limit the maximum rotational speed of the turbine at which *β(nmax) = 0.*

**Figure 16.** The maximum power of a wind microturbine for the case of constant and adjustable pitch.

The results of the measurements indicate that the direct connection of the battery to the generator, often used in wind microturbines, whose charging characteristics is strongly non-linear and changes depending on the battery charge level, may be the cause of poorer performance of the microturbine. The solution to this problem may be the use of a possibly simple converter that could linearize the load characteristics of the microturbine.

Tests were carried out of a wind microturbine model, controlled by an HCS algorithm, presented in Figure 6. Figure 17 shows selected results of the experiment in the form of graphs: power of the wind farm, value of the PWM duty factor, and the rotation speed of the turbine. The results were obtained at constant wind speed *v* = 6.5 m/s.

**Figure 17.** Graphs: microturbine power, duty factor PWM, and turbine rotation speed of a microturbine model controlled by HCS algorithm for adjustable pitch angle of the turbine blade.

The obtained results of the experiment show the possibility of using a simple HCS algorithm to control the operation of a wind microturbine only for a stable value of wind speed. In the case of rapid and greater decreases in wind speed, the presented simple HCS algorithm no longer works optimally, leading to a significant decrease in the turbine's rotational speed and its re-acceleration. A slight improvement in operation can be achieved in this case by using a simple mechanism for adjusting pitch angle of the blade *β = f(n),* which prevents sudden stalling and facilitates the acceleration of the turbine. Some improvement can also be obtained by introducing additional actions to the control algorithm resulting from additional measurement of the instantaneous rotational speed of the turbine.
