WIND TURBINE PHONE CHARGER

Abstract

The wind turbine phone charger is designed to deliver a maximum of 15W of power to a battery pack, capable of charging various portable electronic devices via USB ports. The turbine comprises a two-level Savonius structure with offset blades to capture wind from any direction. These blades are linked to a shaft that turns a gearbox, enhancing the speed for the generator’s operation. A permanent magnet DC machine serves as the generator, producing an unregulated voltage. This voltage undergoes processing via power electronics and a USB-C interface, generating a 5V output for the battery pack. A microcontroller oversees the generator’s output, calculating wind speed based on this data. Both values—the generator output and wind speed—are showcased on an LCD display. Additionally, the microcontroller supplies 5V to several chips within the power electronics system. While each component functions properly independently, integrating all these parts poses a challenge.

1. Introduction

The use of renewable energy has been steadily increasing over the years, with solar and wind farms generating hundreds of megawatts daily. This trend extends to smaller scales, where residences employ solar panels, and even external phone batteries can be linked to solar panels. However, solar power is reliant on sunlight, and battery packs have limited storage capacity. Little exploration has been done on utilizing small-scale wind power, yet there remains high demand for portable device power supplies in the foreseeable future. The wind turbine phone charger seizes wind energy to provide power when solar and batteries may fall short, demonstrating the potential for expanding small-scale wind power utilization.

The project comprises three main components: the turbine, power electronics, and microcontroller, with further subsections delineated in Figure 1. The turbine block, inclusive of the rotor and generator, transforms wind power ranging between 5-12 m/s into electric power within a voltage range of 0 to 18V. We consistently employed a specific wind turbine type but made some modifications as needed. Subsequently, the power electronics block receives power from the turbine block. Comprised of a DC/DC converter and control unit, its primary role is to regulate any voltage supplied by the generator to output a stable 5V. The design and construction of the power electronics underwent several adjustments. Initially, it encompassed an AC/DC rectifier and a DC/DC converter, assuming the generator would output AC power.

The microcontroller interfaces with all other blocks, monitoring the generator’s voltage and calculating wind speed, both displayed on a 2×16 LCD screen. Moreover, it draws power from the battery and allocates it to low-power 5V sections of the control circuit. The overall structure of the microcontroller block remained largely unchanged from its initial design. Finally, the auxiliary blocks involve the battery and the phone. While the battery is indispensable, it can be interchanged, as can the phone; neither were specifically designed for this project. In summary, the turbine captures wind energy and delivers 2.5-15W at 5V to a connected battery.

2. Design

Turbine

The turbine serves as the initial stage in the powertrain, transforming mechanical power generated by the wind into electrical power. Comprising two primary components—namely, the rotor and the generator—the former includes rotor blades and rotor bases. The generator, interconnected via a shaft and gearbox, is linked to the rotor.

Outlined in detail in Sections 2.1.1, 2.1.2, and 2.1.3 below are specific design modifications we undertook. An additional alteration, unmentioned in the following sections, pertains to the adjustment of the bottom housing unit. Originally envisioned to be smaller and lighter, the need for stability necessitated sufficient space for the shaft. Furthermore, accommodating the gearbox required extra space, achieved through a belt system, resulting in the need for adequate housing space.

Savonius Rotor

The origin of the Savonius design stemmed from the necessity to develop a wind turbine that was compact, portable, and provided stability. Initial research focused on contrasting “Horizontal Axis Wind Turbines” (HAWT) and “Vertical Axis Wind Turbines” (VAWT). Table 1 highlights the essential distinctions between HAWTs and VAWTs.

Table 1 HAWT vs VAWT Comparison [1]

Horizontal Axis Wind Turbine (HAWT)	Vertical Axis Wind Turbine (VAWT)
Must face wind direction to operate	Omni-directional
Requires Yaw mechanism	Does not require Yaw mechanism
Equipment is at a height	Equipment is at ground level
Self-starting	Generally non-self-starting
More efficient at converting wind energy into electricity	Less efficient compared to HAWTs

The VAWT is notably preferable, particularly for our specific usage scenario. Typically, efficiency holds utmost importance in extensive wind farms generating multiple megawatts of power through turbines. However, in our case, the requirement is different as charging a mobile phone demands less than 3 watts of power.

Not mandating a yaw mechanism and positioning the equipment, such as the generator, at ground level proves advantageous for small-scale applications by ensuring portability and minimal user intervention. Although Vertical Axis Wind Turbines (VAWTs) typically lack self-starting capabilities, certain designs deviate from this norm. The imperative need for omnidirectional capability, self-starting functionality, and compactness led to the development of the Savonius design, contrasting with potential stability issues faced by designs like Darrieus due to centrifugal forces exerted on the rotor tips.

In Figure 3, a bird’s eye view of the Savonius design is depicted. The two semi-circular structures represent the buckets, formed by splitting an 8-inch PVC Schedule 40 pipe. For a comprehensive portrayal of the completed Savonius turbine, please refer to Appendix B. Additionally, Figure 2 offers a visual representation of each individual component.

Each floor undergoes a 90-degree shift concerning the other, meaning the top floor experiences a 90-degree phase shift in relation to the bottom floor. This arrangement enables the Savonius turbine to exhibit omnidirectional capabilities and self-starting functionality. This trait is crucial for a VAWT (Vertical Axis Wind Turbine) as it eliminates the need for external assistance to initiate rotation, thus preserving overall efficiency.

Subsequently, these PVC pipes are positioned on the base components of the rotor. The lower base is designated to accommodate various equipment, including the generator, gearbox, power electronics, and the microcontroller unit. Assembling these components together results in the creation of the turbine.

Figure 3 Bird’s Eye View – Savonius Design (Two Bucket)

Below in Figure 4, the correlation between the wind speed accessible and the maximum power derived from the wind is depicted. This correlation embodies the adapted Betz’s Equation, considering that the Betz’s coefficient represents an ideal scenario, whereas real-world implementations deviate from this ideal. Equation 1.1 represents the unaltered Betz’s equation, while Equation 1.2 signifies its modified form.

Equation 1.1

16
𝑃 =
27

𝜌
( ) 𝐴𝑣3
2

[𝑊]

The following calculations are made assuming the following parameters:

Equation 1.2
𝜌 3

𝑃 = 0.18 ∗ (
2

) 𝐴𝑣

[𝑊]

where 𝐴 = ℎ × 𝑟

Air density, ρ = 1.2 kg/m3

Rotor height, h = 0.5 m

Rotor effective radius, R = 0.4 m (this does not reflect the radius of each specific bucket, it is the
effective swept area radius denoted by R as shown in Figure 3.)

As indicated earlier, the primary factor influencing power output is the wind speed. Given that power increases exponentially in relation to wind speed, higher wind speeds will result in greater wattage.

Moreover, the computations mentioned above are based on a conservative power coefficient of 0.18. Different estimates exist for the specific power coefficient applied, leading to variations in our results based on the actual coefficient utilized [2][3][5].

Generator

Receiving its source power from the propeller, the generator functions to convert the mechanical movement of the blades into electrical energy. Currently, the generator in use is a 36W DC permanent magnet motor capable of operating as a DC generator. Employing a suitable gearing arrangement, the slow rotation of the rotor is transformed into high-speed rotation that the motor can effectively utilize. Following discussions with Professor Haran, it was advised that a permanent magnet motor could be conveniently employed as a generator. Considering factors such as relative availability, size, and cost, this specific motor was selected. With a rating of 36W, this generator aligns with our high-level requirement of achieving 2.5W.

Gearbox

Another pivotal element during the design phase involved the integration of gears. Given the disparity between the operational RPMs of the turbine and the generator, employing gears becomes crucial to mitigate inefficiencies. Table 2 showcases the assortment of gears necessary across different wind speeds. The calculation of the gear ratio was executed utilizing the following approach (considering the generator’s rated speed at 2500 RPM).

Power Electronics

DC/DC Converter

The generator produces unregulated DC, which needs to undergo regulation via a converter to attain an appropriate output voltage for the battery bank. Initially, the DC passes through an input filter. Subsequently, the converter operates as either a buck or a boost, depending on whether the output voltage needs to be decreased or increased. It is crucial for the converter to ensure minimal ripple to ensure compatibility with commercial devices.

Determining the appropriate topology was the first step. As previously mentioned, the input voltage ranges between 1-18V, while our desired output is 5V. Hence, a topology capable of both buck and boost functionalities was necessary. Initially, a Ćuk topology was chosen due to its simplicity, as depicted in Figure 5. However, the Ćuk’s output is inverted. Initially, attempts were made to work around this inversion, but the control circuit was incompatible with the inverted output. Consequently, a SEPIC topology was selected instead, shown in Figure 6. Notably, this topology involves an additional capacitor and a change in the position of the diode. Alternatively, incorporating an isolating transformer could enhance safety and broaden the range of usable input voltages. However, implementing an isolating transformer might pose challenges, and for our project, the safety aspect is considered unnecessary as the battery pack is equipped with robust protections, and the generator is designed to handle twice the power we intend to maximize.

To confirm that an isolating transformer wasn’t necessary for expanding the voltage range, we utilized a formula to calculate the required duty ratios. It’s essential to note that this duty ratio calculation factors in V(FWD), representing the voltage drop across the diode. Given that high-current-rated components are used for power transmission, the diode’s voltage drop holds more significance compared to low-power applications and must be accounted for.

Once the circuit topology is chosen, determining part values becomes imperative. A fundamental requirement for the converter circuit is to maintain a low ripple voltage. The converter circuit’s output is directly linked to a commercially available battery. Preserving the integrity of the battery is crucial, presuming it was designed to handle outputs from well-established products. Keeping the output voltage ripple at a minimum is essential, with research indicating it should remain below 100 mV peak-to-peak.

The observed ripple at the output is primarily influenced by the output capacitor, particularly the one nearest to the load as depicted in Figure 6. The capacitance value of this component is calculated using the following equation. Additionally, if the capacitor possessed a noteworthy equivalent series resistance (ESR), it could significantly impact the output ripple. Hence, our focus turned to tantalum capacitors due to their favorable size and lower ESR.

Microcontroller

We opted for the ATMEGA32-16PU microcontroller due to its relatively low power consumption characteristics. This 8-bit controller boasts 2 Kilobytes of SRAM, 32 general-purpose I/O lines, and 32×8 general-purpose working registers. Its I/O functionality includes a master/slave SPI and various other features crucial for communicating with the LCD. The multitude of input/output lines proves beneficial as we’ll be gathering data from the generator and providing 5V to control circuit components. Besides providing essential hardware for our project, the ATmega32 microprocessor operates at a clock speed of 1MHz with 3V and consumes merely 1.1mA, aligning perfectly with our generator’s low power requirements. Furthermore, leveraging the fact that the Arduino Uno also employs the ATMEGA32-16PU, we can utilize the Arduino IDE to program the microcontroller for our specific needs.

3. Design Verification