Proceedings of the Winter KGCOE Multi-Disciplinary Engineering Design Conference Page 7

Project Number: 05300

Investigation and design of A sELF-sUSTAINED ENERGY Mini-Scale Energy Generation System

Peter C. Gravelle / Borce Goreveski
Nicholas Ieva / Dr. Sergey E. Lyshevski

Department of Electrical Engineering, Rochester Institute of Technology, Rochester, NY 14623

Proceedings of the Winter KGCOE Multi-Disciplinary Engineering Design Conference Page 7

Abstract

In this project, we design and examine a self-sustained mini-scale submersible, permanent-magnet, axial-topology generation system. Due to performance requirements, cost and time constraints, the originally envisioned generation system was re-designed. As a result, a high-performance cost-efficient mini-scale generator was designed, built, tested, and documented. The proposed self-sustained generation system utilizes and uniquely integrates microelectronics and permanent-magnet technologies. The output is a stabilized 3.3 DC voltage at any angular velocity above a minimum speed. The system fits within a small volume depending on the power requirements. The electronic design utilizes commercial converter topologies. Also, this paper discusses possible improvements on the design to improve performance and enhance functionality.

introduction

Energy harvesting is a rapidly-developing sector in engineering due to the environmental, social, and economic impact of non-renewable fossil fuel sources. According to the Energy Information Administration, Americans use 350 million BTU capita each year, and approximately 85 % of this energy is derived from non-renewable fossil fuel sources. Rapidly-developing nations, such as China and India, have a demand for energy that is much greater than that of the United States.

Due to the increasing costs of energy, it is important to design self-sustainable and renewable energy sources utilizing new technologies and advancements in electronics, energy conversion, and electric machinery. It is also imperative to increase the efficiency of systems. Currently, most energy systems are non-renewable. Utilizing renewable natural resources to generate energy is the ideal solution. Conserving energy is important due to cost and environmental issues. Geothermal, solar, water, and wind power systems reduce the dependence on the power grid as well as reduce the amount of resources consumed. Thus, energy-harvesting is on the forefront of this conservationist engineering pursuit.

In addition to the above applications, self-sustained renewable energy sources have many other applications. Our power system is aimed to power sensors and actuators that can be installed on the surface of underwater and surface vehicles such as ships and submersible vehicles. These vessels require a thick and impenetrable hull with minimum drag and optimized stream flow. Though it is virtually impossible to supply power to actuators and sensors from within, vehicles should be able to easily perceive conditions outside and ensure control. For these marine applications, it is feasible to install a small turbine on the outer hull. This turbine harnesses the energy from the water flowing around the vessel to power sensors and actuators. A current DARPA project [1] challenges engineers to design mini-scale generator systems. This particular project places emphasis on mounting the generators on sharks. In this application, the issues of self-containment, uncertainty, and environment are critical and must be taken into consideration.

Mini-scale generators can be attached to the bodies of aircraft and cars in order to power a variety of sensors, actuators, electronics, and communication systems. This will reduce the load on the main battery or energy generation system. Regenerative braking concepts could be applied through these turbines and would be used to recapture energy for air and water vehicles, instead of just cars.

We have developed a small-scale energy harvesting system that can be utilized with many different types of sensors. This system harvests the energy of passing water to rotate a turbine shown in Figure 1. The turbine has a multi-pole magnet attached to it. On the stationary member, the axial coils induce an AC voltage due to the changes of the magnetic field in the water-filled airgap. This AC voltage is supplied to a two-stage circuit that first rectifies it, and then boosts the voltage to 5 VDC. The first-stage booster operates this way as the induced voltage is above 2.7 V and less than 5 V. When the rectified input is greater than 5 VDC, the output of the first stage will be greater than 5 VDC, but this is satisfactory due to the second converter stage. When the output of the first stage booster exceeds 4.3 V, a step-down converter is used to regulate the output voltage to 3.3 V. Thus, the output voltage is stabilized at 3.3V when the angular velocity is above a minimum speed.

Nomenclature

AC – Alternating Current

BTU – British Thermal Unit

CAM – Computer Aided Machining

CMOS – Complementary Metal Oxide Semiconductor

DARPA – Defense Advance Research Project Agency

DC – Direct Current

IC – Integrated Circuit

MOSFET – Metal-Oxide Semiconductor Field Effect Transistor

NdFeB – Neodymium Iron Boron

PVC – Polyvinyl Chloride

SmCo – Samarium Cobalt

System Architecture

Turbine. The system utilizes a wheel with a diameter of 82.3 mm (3.24 in). The wheel’s curved blades are modeled after the Pelton Turbine, noted both for its efficiency and stability. However, true Pelton Turbines are difficult to fabricate because they require two cups per vane. This difficulty is exacerbated on the small scale this project targets. Therefore, rather than formed into a full cup, the vanes were curved. This curvature does not significantly affect the efficiency of the system. In addition, it is significantly cheaper and simpler to manufacture. To increase stability, the shaft of the turbine was lengthened to penetrate deep into the case. This idea has been extensively used in classical windmill design.

Figure 1: Drawing of the Turbine

Magnet and Windings. We designed a permanent-magnet axial-topology generation system with axial windings as documented in Figures 2 and 3. The magnet is axially magnetized and the windings are wound and placed below the magnets in the stationary casing. The windings’ width is equal to the width of the magnets. Due to the cost and time constants, an off-the-shelf ring permanent magnet and windings were used. The SmCo magnet has an outer diameter of the 36 mm (0.65 in) and an inner diameter of 16.5 mm (0.65 in). The thickness is 4 mm (0.16 in). The ring-shaped permanent-magnet has six axially-oriented poles. Correspondingly, there are six axial windings. The windings and magnet together create a three-phase alternating current permanent-magnet synchronous axial-topology generator. This design leads to the best-performing generator.

Figure 2: 6-Pole Permanent Magnetic Strip on a Shaft

Figure 3: Three-Phase Axial Windings

AC to DC Converter. A three-phase full-rectifier with filtering capacitor was used to convert the generated AC voltage to DC voltage. The rectifier is illustrated in Figures 4 and 5. The rectifier is built using six diodes. These diodes are arranged to provide the same polarity of output voltage and current for the six possible polarities of input power. In this way, the circuit also protects its load components from a polarity reversal. A capacitor was added between the positive and negative terminals of the DC output to filter the voltage ripple by presenting a low impedance to the high-frequency components of the ripple voltage. The diodes have a very small forward voltage drop, can withstand high currents, and have a high switching speed.

Figure 4: Schematic of the Rectifier

Figure 5: Photograph of the Rectifier on the Evaluation Board

Boost Converter. The boost converter allows the system to ensure its target voltage at lower angular velocities. The converter was designed to ensure DC 5 V output voltage with an input of at least 2.7 V. The main issue with this solution is that if the input voltage exceeds 5 V, the output voltage is equal to the input voltage minus a diode’s voltage drop. The boost converter works in two cycles. During the first cycle, a MOSFET is turned on, storing energy in an inductor. Correspondingly, the output capacitor supplies current to the load. The diode is reverse-biased to prevent backflow of the higher voltage to the input. During the second cycle, the MOSFET turns off, and the inductor offloads its stored energy to the output capacitor and load. The converter can be found in Figures 6 and 7.

Figure 6: Schematic of the Boost Converter

Figure 7: Photograph of the Boost Converter on the Evaluation Board

Flyback Converter. The flyback converter was designed to step-down the output voltage of the boost converter to the desired stabilized DC voltage of 3.3V. The converter utilizes a transformer to step-down the voltage, to isolate the input from the output, and as an inductor. When the power transistor switches on, current in the primary stores energy in the transformer core, and produces a polarity that turns off the output diode. When the transistor switches off, the voltage polarity reverses and flies back, passing current through the output diode to the output capacitor and load. The output contains a low-amplitude (±0.2 V) saw-tooth ripple. This ripple produced by the transformer’s switching is eliminated by adding a 0.9 F supercapacitor in parallel with the load. The output voltage is now a constant 3.3 V. The designed power harvesting circuitry mimics a forward converter.

Figure 8: Schematic of the Flyback Converter

By changing the values of the four feedback resistors (two for each circuit) Rfb1 and RBfb2 (on Figure 6 and 8 those resistors denoted as R1 and R2), it is possible to set the output voltage to any arbitrary value. The boost converter and flyback converter use almost the same parts, thus reducing the overall production cost.

Figure 9: Photograph of the Flyback Converter

Case. The team designed a case to be made from Acetal plastic due to its ruggedness and lack of expansion in water. Due to cost and time constraints, this case was not manufactured., Therefore, alternatives were sought and implemented. A waterproof case made of PVC was purchased and then machined for the team’s specific purpose. A hole was drilled though the center of the case’s cover, a bearing was fitted into the hole, and a partially hollow shaft was placed inside to seal the hole up. This was done so that the turbine can rest on top of the bearing. The wheel must be kept as frictionless as possible, so a single steel ball was dropped into the shaft to act as a needle bearing on the turbine’s shaft.

Figure 10: Three-Dimensional View of the Ideal Case

Figure 11: Transparent Diagram of the Ideal Case with Shaft

Energy Storage. Many energy-harvesting systems use batteries as a storage media. Due to the small volume of the application, most batteries are impractical. Batteries are also non-ideal because of the rapid charge-discharge cycles inherent in energy-harvesting. Most rechargeable batteries have a limited life-span. The life span would be shortened significantly under the predicted operating conditions. In addition, batteries require complex circuitry, such as overcharge detection. Finally, many are highly reactive with water (especially lithium-ion batteries). Using supercapacitor technology, it is possible to avoid these problems. Supercapacitors have a small form factor, and have a capacity in the range of farads, rather than microfarads in case conventional capacitors. Supercapacitors can charge and discharge infinitely many times. In addition, supercapacitors are very robust and have favorable characteristics in the various temperature ranges seen by submersible systems. Supercapacitors provide a very high power density. Also, circuitry is not required for overcharge detection as the supercapacitor will stop accepting electrons when fully charged. It is impossible to overcharge a supercapacitor with current. The capacitor this project is using has a capacity of 0.9 F and is 42 x 17 x 4 mm (1.73 x 0.67 x 0.16 in) in size.

Objective

The objective of this project is to design, test and characterize a self-sustained energy-harvesting system. This system should be

1. submersible,

2. produce a required DC output voltage for any speeds above a certain critical angular velocity,

3. optimized by utilizing a permanent-magnet axial topology generator.

The key constraint for the system is cost while ensuring maximum achievable performance. Other notable goals are the maximization of power density and the minimization of volume.

Future Direction

The cost and time constraints required the team make compromises and seek innovative solutions to overcome the challenges in order to accomplish the overall objectives. This section will detail the areas the team sees for improvement. These should be seen as guidelines should this project be attempted by other team, or, as directions for further research.

CMOS Technology. A power harvesting circuitry design utilizing CMOS technology to ensure voltage control can be envisioned. That is, instead of using the boost and flyback converters, an integrated circuit solution can be sought. Utilizing this microelectronics-centered circuit, it is feasible to ensure higher efficiency than was achieved in the prototype because a CMOS-based circuit will utilize less power. The CMOS power harvesting circuitry is a modification of a system developed at Johns Hopkins University [2].

Figure 12: Circuit Diagram of the Regulator Circuit

Controller. The regulator circuit (Figure 12) ensures a stabilized 3.3 VDC output voltage. The circuit uses a transconductance amplifier to control the gate of a large PMOS transistor. The large transistor leads to a wide gate voltage. The output voltage is regulated through negative feedback.

Figure 13: Circuit Diagram of the Voltage Reference

Voltage Reference. The voltage reference (Figure 13) uses a current bias circuit that allows a supply independent reference to be generated using only standard CMOS devices. The circuit produces an 800 mV reference voltage with a dependence of less than 1 % on the supply.