**4. Design of Slow Wind Speed Wind Turbines**

As previously stated, the problem of concern here is that existing commercial turbines are generally designed for wind speeds greatly in excess of typical wind speeds for major por‐ tions of the planet. Rather than simply exclude wind power from the potential energy sce‐ nario for these regions, we would like to design a small wind turbine especially for low wind speed regions [9]. Most of South East Asia (SEA) lies in a region of relatively low wind speeds. Wind speed data form a test site in Malaysia is shown in figure 5. Wind power prob‐ ability is derived by multiplying the wind speed probability by the cube of the wind speed. The highest wind power probability is at approximately 3 m/s. At this wind speed commer‐ cial turbines will produce very little power.

**Figure 5.** Wind and normalized wind power probability from a low wind speed test site

In order to improve power extraction the wind turbine requires a fundamental redesign. Equation 1 provides us the first indication of how to proceed. For a given wind speed we are left with modifying the turbine area, and optimizing the coefficient of performance. Control over the ambient air density is beyond the scope of this text. Lengthening the blades will increase the cross sectional area of the turbine, increasing the power of the turbine. This will, however, also increase the load on the turbine and tend to result in a slower rate of rotation. Electrical power production from a generator is proportional to the square of the rotational speed, so it may be advantageous to adjust the pitch angle in order to maximize the TRS, and thus increase the generator speed. For low wind speeds both the turbine hub and gener‐ ator will need re-optimization for the larger blades required to achieve a reasonable level of power production.

#### **4.1. Overall Turbine Design**

namic stresses. The centrifugal forces are proportional to the blade weight, blade length and square of the turbine speed, and limit the maximum speed of the turbine. Assuming similar materials and blade design, in order to achieve the same level of stress a larger, heavier, blade will have to spin at a lower speed than a smaller blade. This maximum speed of tur‐ bine operation becomes one of the limiting factors in the wind turbine, requiring either an extremely robust design or an active speed control system. Stall control systems are mechan‐ ically simple to implement, and thus common on small turbine systems. As the wind speed increases above the rated speed, a large electrical load, generally a high power resistor bank, is applied to the output of the generator. This increases the torque load on the turbine, slow‐ ing it. As the TSR is reduced, the angle of attack is raised above the optimum, and lift drops off as the blade begins to stall. This subsequently reduces the turbine's torque, slowing it further. This technique has been shown effective at preventing over speed in small turbines.

As previously stated, the problem of concern here is that existing commercial turbines are generally designed for wind speeds greatly in excess of typical wind speeds for major por‐ tions of the planet. Rather than simply exclude wind power from the potential energy sce‐ nario for these regions, we would like to design a small wind turbine especially for low wind speed regions [9]. Most of South East Asia (SEA) lies in a region of relatively low wind speeds. Wind speed data form a test site in Malaysia is shown in figure 5. Wind power prob‐ ability is derived by multiplying the wind speed probability by the cube of the wind speed. The highest wind power probability is at approximately 3 m/s. At this wind speed commer‐

**4. Design of Slow Wind Speed Wind Turbines**

**Figure 5.** Wind and normalized wind power probability from a low wind speed test site

In order to improve power extraction the wind turbine requires a fundamental redesign. Equation 1 provides us the first indication of how to proceed. For a given wind speed we are left with modifying the turbine area, and optimizing the coefficient of performance. Control over the ambient air density is beyond the scope of this text. Lengthening the blades will

cial turbines will produce very little power.

272 Advances in Wind Power

As a starting point for the design we will chose a system capable of providing power for a model rural dwelling typical of the remote regions of SEA. Such dwellings generally use re‐ chargeable automotive lead acid batteries to power electrical lights, radios and televisions. These batteries are transported weekly to a diesel powered generator station for recharging. Transporting the batteries weekly is a significant burden to the rural residents which can be alleviated with the use of a wind power system. With improved access to electrical power the electrical power consumption will probably increase significantly. Additional power is likely to go to improved lighting, and additional appliances such as fans and even refrigera‐ tors. The actual power required will vary widely, but we will assume here that the typical house will consume approximately 1kWh per day.

The wind power system should have sufficient storage capacity for at lease one week with no power generation, thus we require at least 7kWh of electrical storage. As in most small off grid electrical power applications power will be stored in 12V automotive batteries. To minimize power transmission losses we will chose the highest system voltage considered safe for such applications. An operating voltage of 48V can be achieved with 4 batteries in series, and the constraint of 7kWh of energy storage then translates into a battery capacity of about 150Ah, similar to common truck batteries.

With good turbine sighting on a hill top, peak power probabilities of around 5m/s are possi‐ ble in some costal regions of SEA. From long term measurements we can determine that the wind may achieve this target speed about 20% of the time, or 4.8 hours per day. Assuming that the turbine needs to generate about 1/3 more than the daily required power to compen‐ sate for loses in the system, we'll require about 1.3kWh per day of electrical power produc‐ tion. At 4.8 hours of power production per day the system will have to produce approximately 270W in the 5m/s wind. Assuming a generator efficiency of 80% and a Cp of 0.29, from equation 1 we can determine the area of the turbine to be 15.9m2 , yielding a blade length of approximately 2.25m. If we accept a conventional TSR of around 8, the turbine will be spinning at 170 rpm. Based on some initial measurements it was determined that a con‐ ventional generator design would require a much higher rotational speed to achieve the de‐ sired power output, so we will target twice this speed, or 340rpm. The operational TSR will be optimized via turbine blade pitch adjustment during field testing of the system, but we will target a TSR of 16, twice the conventional ratio. The operating current of the generator at this point will be approximately 5.8A.

Leveraging off of existing small turbine designs [1] the generator is to be a 3-phase, axial flux synchronous permanent magnet generator. We have selected a 12 pole design with 25 x 50mm, 11mm thick nickel plated NdFeB magnets. The generator is based around an auto‐ motive wheel bearing and disk brake, thereby defining the rotor diameters. The initial speci‐ fications for the turbine are listed in table 2.


**Table 2.** Wind turbine and generator initial specifications
