Goran Mandic

Indoor Power Harvesting Using Photovoltaic Cells for Low-Power Applications

Utilization of low power indoor devices such as remote sensors, supervisory and alarm systems, distributed controls, and data transfer system are on steady rise. Due to remote and distributed nature of these systems, it is attractive to avoid using electrical wiring to supply power to them. Primary batteries have been used for this application for many years but they require regular maintenance at usually hard to access places. This paper provides a complete analysis of a PV harvesting system for indoor low power applications. The characteristics of a target load, photovoltaic (PV) cell and power conditioning circuit are discussed. Different choices of energy storage are also explained. Implementation and test results of the system are presented that highlights the practical issues and limitations of the system.

Active Torque Control for Gearbox Load Reduction in a Variable-Speed Wind Turbine

With the advent of power electronics, the size, weight, and cost of power converters have been drastically reduced while efficiency is improved. The use of variable-speed wind power generators has seen considerable growth. The use of gearboxes in the wind turbines allows for smaller size, lower weight, and higher speed generators. However, gearboxes have shown to be one of the least reliable components of the wind turbines. In this paper, we propose a method that can extend the life and reliability of wind turbine gearboxes by reducing the mechanical stress on gearbox components. Reduction of mechanical stress is achieved by the generator torque control that minimizes resonant torsional vibrations within a drivetrain caused by variations in wind velocity. A detailed model for the drivetrain of a 750-kW wind turbine, including a gearbox is presented. Experimental results are used to calculate the parameters of the gearbox. A controller is designed to adjust the generator torque at the end of the drivetrain to remove the unwanted and damaging torque variations from the drivetrain. Simulation results verify the effectiveness of the proposed method.

Utilizing energy storage with PV for residential and commercial use

This paper addresses the intermittency of PV energy by supporting it with an energy storage element in order to increase the penetration of renewable energy. A PV grid-tied system composed of photovoltaic module, uni-directional DC to DC converter, bi-directional DC to AC converter and storage battery is proposed. The efficiency of two different system operation options is evaluated and the operation option with the higher efficiency is simulated and practically implemented. The model developed and implemented in this paper can be extended, with little modifications, to wind and other renewable energy sources that are intermittent. The combined system will be modeled to assess the technology available and to set the base for a higher power rating system implementation.

Mechanical stress reduction in variable speed wind turbine drivetrains

The use of variable-speed wind power generation has seen considerable growth. The use of gearboxes in wind turbines allows for smaller size, lower weight, and higher speed generators. In this paper, we attempt to describe methods to extend the life and reliability of wind turbine drivetrains by reducing the mechanical stress. We will represent a two-mass model for the drivetrain of a 750-kW wind turbine. This model is verified by using simulation and experimental results. A controller is designed to adjust the generator torque at the end of the drivetrain to remove the unwanted and damaging torque variations from the drivetrain. The ultimate goal of this study is to present a detailed model of the drivetrain, including the gearbox and ways to mitigate the stress on the gearbox.

Modeling and simulation of a wind turbine system with ultracapacitors for short-term power smoothing

This paper describes the modeling and simulation of a Permanent Magnet Synchronous Generator (PMSG) based Wind Energy Conversion System (WCES). The model includes all components from wind to the grid. This model is used to study various control algorithms as well as integration of ultracapacitor energy storage aimed at reduction of output power variations. Ultracapacitors are placed on the DC bus of the double conversion system to avoid extra power conversion stage.

Lithium-Ion Capacitor Energy Storage Integrated With Variable Speed Wind Turbines for Power Smoothing

Utilization of wind energy in modern power systems creates many technical and economical challenges that need to be addressed for successful large scale wind energy integration. Variations in wind velocity result in variations of output power produced by wind turbines. Variable power output becomes a challenge as the share of wind energy in power systems increases. Large power variations cause voltage and frequency deviations from nominal values that may lead to activation of protective relay equipment, which may result in disconnection of the wind turbines from the grid. Particularly community wind power systems, where only one or few wind turbines supply loads through a weak grid such as distribution network, are sensitive to supply disturbances. Energy storage integrated with wind turbines can address this challenge. In this paper, Li-ion capacitors are investigated as a potential solution for filtering power variations at the scale of tens of seconds. A novel topology and control technique has been introduced to integrate capacitors and power conversion circuitry. Modeling and scaled-down experimental results are provided to verify the theoretical analyses.

Wind power smoothing using rotor inertia aimed at reducing grid susceptibility

Due to wind speed variations, the output power of wind turbines fluctuates. This power fluctuation makes the wind power undispatchable. Furthermore, it can cause frequency deviations and power outage particularly when wind power penetration is significant. Energy storage devices such as batteries, ultracapacitors, super inductors, and flywheels can be utilized in a hybrid system to solve this problem. These methods, although, are effective but impose a significant additional cost to the system. This paper presents a novel control method to mitigate the power fluctuations using the rotor inertia as an energy storage component. Therefore, the additional energy storage system is not required. The proposed method is also modified to obtain better energy capturing efficiency. The method is analyzed using mathematical and physical characteristics of the system. The efficiency of the algorithm is evaluated based on obtained equations. The energy extracting capability using this method is comparable with other methods such as Maximum Power Extraction (MPE) algorithm. Simulation results for various cases are performed on a permanent magnet synchronous generator that verify theoretical analysis.

Testing and modeling of lithium-ion ultracapacitors

The main problem of renewable energy sources is their intermittency. Integration of energy storage devices with renewable energy systems can offer solution to this problem simply by storing excess produced energy for utilization when needed later on. Energy storage devices are also essential for other applications such as automotive. Accurate knowledge of the electrical attributes and expected response of an energy storage device is of high importance when it is utilized in any power system in order to attain proper charging/discharging without inflicting damages. In this paper, a relatively new type of energy storage device, Li-ion ultracapacitor, is thoroughly studied. This type of ultracapacitors has high energy density, high power density, high efficiency, and long cycle life. Testing, analysis, and modeling of this energy storage device are presented in detail. A series of DC and AC tests has been conducted to develop an electrical model, which has been verified for accuracy by simulation and comparison with test results.

Development of an Electrical Model for Lithium-Ion Ultracapacitors

Accurate knowledge of the electrical characteristics of an energy storage device is of high importance when utilized in any power system to achieve proper charging and discharging modes without inflicting any damage to its structure. In this paper, a Li-ion ultracapacitor, a hybrid type of energy storage, is thoroughly studied. This type of ultracapacitors has high energy density, high power density, high efficiency, long cycle life, and superior performance under high temperatures. Testing, analysis, and modeling of this energy storage device are presented in details. A battery-based model and a capacitor-based model are proposed. A series of dc and ac tests has been conducted at various temperatures to develop an electrical model spanning, the entire domain of temperatures under which the ultracapacitor can normally and safely operate. These models have been verified for model accuracy via simulation and comparison with the test results for both a single ultracapacitor cell and a 360 V ultracapacitor module built in the laboratory. Both coulombic and energy efficiency have also been investigated at different temperatures and current ratings.