Andrew Cruden

Energy storage in electrochemical capacitors: designing functional materials to improve performance

Electrochemical capacitors, also known as supercapacitors, are becoming increasingly important components in energy storage, although their widespread use has not been attained due to a high cost/performance ratio. Fundamental research is contributing to lowered costs through the engineering of new materials. Currently the most viable materials used in electrochemical capacitors are biomass-derived and polymer-derived activated carbons, although other carbon materials are useful research tools. Metal oxides could result in a step change for electrochemical capacitor technology and is an exciting area of research. The selection of an appropriate electrolyte and electrode structure is fundamental in determining device performance. Although there are still many uncertainties in understanding the underlying mechanisms involved in electrochemical capacitors, genuine progress continues to be made. It is argued that a large, collaborative international research programme is necessary to fully develop the potential of electrochemical capacitors.

Modeling the Benefits of Vehicle-to-Grid Technology to a Power System

Electric vehicle (EV) numbers are expected to significantly increase in the coming years reflecting their potential to reduce air pollutants and greenhouse gas emissions. Charging such vehicles will impose additional demands on the electricity network but given the pattern of vehicle usage, the possibility exists to discharge the stored energy back to the grid when required, for example when lower than expected wind generation is available. Such vehicle-to-grid operation could see vehicle owners supplying the grid if they are rewarded for providing such services. This paper describes a model of an electric vehicle storage system integrated with a standardized power system (the IEEE 30-node power system model). A decision-making strategy is established for the deployment of the battery energy stored, taking account of the state of charge, time of day, electricity prices and vehicle charging requirements. Applying empirical data, the benefits to the network in terms of load balancing and the energy and cost savings available to the vehicle owner are analyzed. The results show that for the case under study, the EVs have only a minor impact on the network in terms of distribution system losses and voltage regulation but more importantly the vehicle owners costs are roughly halved.

Optimizing for Efficiency or Battery Life in a Battery/Supercapacitor Electric Vehicle

A novel energy control strategy for a battery/supercapacitor vehicle, which is designed to be tunable to achieve different goals, is described. Two possible goals for adding a pack of supercapacitors are examined for a test vehicle using lead-acid batteries: 1) improving the vehicles efficiency and range and 2) reducing the peak currents in the battery pack to increase battery life. The benefits of hybridization are compared with those achievable by increasing the size of the battery pack by a comparable mass to the supercapacitors. The availability of energy from regenerative braking and the characteristics of the supercapacitors are considered as impact factors. Supercapacitors were found to be effective at reducing peak battery currents; however, the benefits to range extension were found to be limited. A battery life extension of at least 50% is necessary to make supercapacitors cost effective for the test vehicle at current prices.

Three-Phase, Three-Wire, Five-Level Cascaded Shunt Active Filter for Power Conditioning, Using Two Different Space Vector Modulation Techniques

The three-phase, three-wire, five-level cascaded Inverter is used as a medium-voltage shunt active power filter. The capacitor voltage-control technique used as a harmonic current extraction method for the two-level inverter is extended to the five-level shunt active power filter, with a technique proposed for balancing capacitor voltages. Predictive current control based on the supply current (not the active filter current) is employed. Two different space vector modulation (SVM) techniques viz., phase-shifted SVM and hybrid SVM, are used for multilevel inverter pulsewidth-modulation generation. The proposed five-level shunt active power filter is validated by simulation and practically for both modulation techniques. The proposed technique results in the same software and hardware requirements for any m-level inverter.

A Variable Speed Magnetic Gear Box Using Contra-Rotating Input Shafts

This paper presents analytical, simulation, and practical analyses of a variable speed magnetic gearbox (MG) that uses contra-rotating (CR) input shafts. The CR turbine comprises two sets of rotors one behind the other: one rotor rotates in a clockwise direction while the other rotor rotates in an anticlockwise direction. Our magnetic gear magnetically combines the two CR input shafts into a single variable speed output shaft, and consists of three rotating parts: two contra-rotating low-speed input shafts and a single high-speed output shaft. Theoretical analysis is presented to show how to control the speed of the output shaft via control of the speeds of the two input shafts. This analysis is then supported by simulation and experimentation. The paper discusses the MG torque transmission capability by presenting pull-out torque test results and efficiency information when being driven through the different load torque shafts. The effective ratio of the MG is explained in terms of its torque curve result. A prototype gearbox was built, having been designed with an electromagnetic finite-element software package, and evaluated using a practical test rig.

Optical crystal based devices for current and voltage measurement

This paper describes novel designs for magneto-optic and electro-optic crystal sensors providing potentially separate and combined current and voltage measurement in power systems. The results of testing programmes are presented and details of the work associated with field trials given. >

Temperature compensation for optical current sensors

An analysis of an optical current transducer (OCT) considering in particular the influence of temperature induced variations in the Verdet constant is presented. The analysis is supported by an experimental evaluation of a prototype OCT over a range of operating temperatures and current values and concludes with a laboratory demonstration of a temperature compensation scheme that improves the measurement precision to better than 0.7%.

An Improved Lead–Acid Battery Pack Model for Use in Power Simulations of Electric Vehicles

A new model for a lead-acid battery pack is proposed for use in power simulations of electric vehicles. A linear approximation using a constant voltage drop has been used to model the charge-transfer resistance of the battery pack, and an exponential voltage-recovery equation has been used to model the transient capacitance effects following a period of discharge. The new model is easy to implement with simple calculations and easily acquired parameters, combining speed of implementation with accuracy. The new model was found to have a peak error of 3.1% in drive cycle tests, thus comparing favorably to existing models of similar complexity. An initial assessment of the models suitability for use with a lithium-ion battery pack was also performed, finding a peak error of 5%.

Grid connected inverter suitable for economic residential fuel cell operation

Due to environmental concerns and limited hydrocarbon fuel reserves, R&D effort is being put into alternative energy sources like geothermal, photovoltaic, wave and wind, as well as fuel cells which use hydrogen to generate electricity without increasing pollution. Fuel cells for a residential application, with combined heat and power (CHP), may offer a viable energy source when both thermal and electrical outputs are associated with energy storage. The addition of grid connection improves reliability and offers the option to sell surplus energy when economically justified. A fuel cell has a variable, low voltage dc output requiring a power electronic interface to any energy storage and to ac loads. A simple inverter topology can be adopted based on minimum component count. This paper presents the design, simulation and experimental verification of an economic arrangement utilising an intermediate voltage storage battery and a bidirectional dc/ac interface suitable for domestic ac loads and grid synchronisation. The design was modelled in Matlab/Simulink. An economic model of the fuel cell/battery/grid system, operating in micro-CHP mode, was developed to assess the prospects for commercial exploitation and to characterise the relationship between domestic load profile, fuel cell rating and battery capacity to optimize life time cost. This model assumes natural gas for the fuel cell, as hydrogen infrastructure is not yet available. The economic model results and their interpretation will be the subject of a future publication

Electronic differential with sliding mode controller for a direct wheel drive electric vehicle

Traction drives used in electric vehicles can be divided into two categories, (i) single drive systems, and (ii) multi-drive systems. With multi-drive systems the motor controllers must additionally be configured to provide an electronic differential effect i.e. they must also perform a similar function as their mechanical differential counterpart. Thus the electronic differential must take account of the speed difference between the two wheels when cornering. This paper presents a design for an electronic differential utilising a sliding mode controller employing a 4-switch 3-phase inverter. This type of inverter is particularly suitable for this application as the supply batteries can be easily split into two separate battery strings. The system is evaluated on a test vehicle in which the rear wheels are directly driven by permanent magnet brushless motors. Results indicate that this arrangement can be successfully implemented into an electric vehicle drive train.