• Energy Research

This paper is focused on the dynamic modelling of the polycrystalline silicon wafer-based photovoltaic cells under various operational and fault conditions. The models are drawn from the impedance changes observed using electrochemical impedance spectroscopy. In this paper, tests were carried out at different voltage bias levels under illumination, dark, uniform partial shading, and cell-mismatch conditions. The cell model parameters are extracted from the obtained Nyquist plots and fitted to appropriate equivalent circuits. Results obtained show that, the variations of the AC model parameters can be used for in-line characterization and real-time condition monitoring of the photovoltaic cells/arrays under the different operational and fault conditions.

Abstract This paper presents fault detection techniques in an attempt to discriminate between two common faults in axial-flux permanent magnet wind generators namely; static eccentricities and interturn short circuit faults. A combination of condition monitoring systems is proposed to achieve the discrimination; with detection techniques derived from current and vibration signatures for both faults using signal processing techniques. Parametric spectral estimation technique is applied as an alternative to the Fourier transform to process the fault signatures. This effectively overcame the problem of non-stationarity of signals due to variable wind speed as high frequency resolution was attained with less than three seconds of measurement data. The results will advance the development of standards and technical guidelines in the condition monitoring of permanent magnet synchronous generators in wind turbines.

This paper presents a laboratory-based implementation of a variable-speed Permanent Magnet (PM) Wind Energy Conversion System (WECS). The theory behind such a system is presented, after which simulated and experimental results are presented. The behavior of a wind turbine is emulated through the use of a torque-controlled DC-motor coupled to the PMSG. A full back-to-back power converter is employed to rectify and subsequently invert the produced power and supply it to the grid through a LCL-type grid-filter. The control of the PMSG is realized through the appropriate control of the machine-side power converter while the grid-side converter maintains the DC-link voltage while transferring the produced power to the grid.

Developers and operators are interested in improving the reliability and reducing the associated costs of wind power plants (WPPs) because of the continuous increase in the power capacity of wind energy conversion systems (WECSs) and the increasing development of WPPs. The electrical subsystem of the WPP experiences the highest failure rate and constitutes a significant proportion of its total cost. Reliability of the WECS can be increased and its cost reduced by eliminating the wind turbine transformer from the electrical subsystem. This study gives a techno-economic evaluation of a five-level nested neutral point clamped (NNPC) converter topology for transformer-less connection of high- power WECSs. The approach entailed the calculation of reliability of five-level NNPC converter topology deployed in the grid-side of a WECSs. This method presents a mathematical formula for deriving the reliability of a five-level NNPC converter topology by using the reliability block diagram and reliability estimation-based models in the military handbook (MIL-HDBK-217F). The cost analysis model shows that the total cost of the five-level diode clamped converter topology was higher than the five-level NNPC converter topology. The study could be extended by carrying out accurate modelling of the mission profile of the presented converter by using multi-domain simulation technique.

This paper presents the thermal and structural analysis of an electromechanical battery energy storage system designed to enhance rural electrification in sub-Saharan Africa. The system consists of a flywheel rotor, an electrical machine, bearings and a containment structure. The flywheel rotor was constructed from E-glass fiber, the machine from imported NdFeB magnets and commercial energy efficient bearings. With the exception of the power electronics and magnets, local materials were used for the manufacture of the flywheel system. The system was designed to operate between 8,000 rpm to 25,000 rpm with a rated storage capacity of 300Wh. Numerical stress analysis was performed during the design stage to ensure that the maximum tensile strength is not exceeded. A lumped parameter thermal model was used to estimate the temperature distribution to ensure safe operating conditions of the flywheel system and environment. The results of both analyses are presented.

A thermal model of an electromechanical flywheel with a brushless DC machine is presented. Thermal analysis is particularly important during vacuum operation of the flywheel. Accurate prediction of temperature facilitates material selection, structural integrity and good performance. In addition, the choice of vacuum level, bearings, conductor sizes and system overload capability can be attributed to a good thermal model. A thermal model was developed and simulated on MATLAB Simulink. The simulation results were compared with experimental results and a good correlation was observed.

This paper presents the design and analysis of an electromechanical flywheel energy storage system to enhance rural electrification in sub-Saharan Africa. The system consists of a flywheel rotor, an electrical machine, control system, bearings, and a containment structure. With the exception of the power electronics and magnets, local materials were used for the manufacture of the flywheel system. The flywheel rotor is made from glass fiber-epoxy composite, designed using novel shape profiles and utilizes a stress based solution by introducing a central hole for shaft inclusion. The system was accelerated to 6000 r/min storing up to 227 kJ. Numerical stress analyses were performed during the design stage to ensure that the maximum tensile strength is not exceeded. A lumped parameter thermal model is used to estimate the temperature distribution to ensure safe operating conditions of the flywheel system and environment. A life cycle cost analysis performed found that by integrating a flywheel system into a Solar Home System implies a cost savings of 35% per kilowatthour when compared with lead-acid batteries.

This paper focuses on using the fuel cell's characteristics for developing a power electronic interface. To gain the specifications for the DC/DC converter, a 1kW HTPEMFC semi-empirical model was developed to better understand the electrical behaviour of the fuel cell. The DC/DC converter is a crucial component of the power electronic interface, for stationary applications, as it regulates the unstable voltage produced by the fuel cell. Since the focus of this research is towards the residential market, the requirements of the converter is also to obtain a high efficiency while being cost effective and flexible. A two-stage DC/DC boost converter was selected as the most feasible choice. In this paper, a semi-empirical 1kW HTPEM fuel cell model is firstly developed to attain the specifications for the DC/DC converter interface. This is followed by the design of a two-stage DC/DC boost converter, for the fuel cell system. The initial design is improved through an iterative process, all of which are documented and presented in this paper. Finally the performance of the converter is quantified.