Machines and Power Electronics

The work on electrical machines and power electronics has concentrated on the design of novel machines for power take off systems in renewable energy converters, such as direct drive wave, wind and tidal current systems. Power electronic converters are being developed for interfacing these renewable energy systems to the grid and for control to optimise performance. In addition there is also continuing work on electronic power supplies, including switched mode power supplies, resonant mode and uninterruptible power supplies. Summaries of current research projects are given below.

Core Research Areas:

• Novel Generator Designs for Renewable Power Generation
• Thermal and Mechanical Analysis for Electrical Machines
• Power Conversion and Control for Renewable Energy Converters

• Electromechanical and Hydrodynamic Modelling of the Snapper Device
• Structural Optimisation of Direct Drive Permanent Magnet Generators
• Direct-drive permanent magnet generators for the next generation offshore wind turbines
• 50kW Linear Generator for Direct Drive Wave Energy Converters
• Integration of C-GEN to Wave and Tidal Devices
• Reliability of Generators in the Marine Environment
• Lightweight Low Speed Rotary Electrical Generators for Direct Drive Wind Systems
• Machine Design Optimisation using Genetic Algorithms
• Lightweight Low Speed Linear and Rotary Electrical Generators for Direct Drive Wave, Wind and Tidal Current Systems
• Structural Design Tools for Low Speed High Torque Machines used in Direct Drive Renewable Energy Converters
• Feasibility Analysis of High Temperature Superconducting Generators for Renewable Energy

Richard Crozier (EU FP7)
Supervisors: Dr Markus Mueller, Dr John Chick

Snapper is a novel linear machine design combining magnetic coupling with a spring element in order to increase the relative velocities experienced between the generator parts in comparison to a low velocity, high force input received from the prime mover. The object of this research is to create a combined electromechanical and hydrodynamic model of the Snapper device in combination with a heaving buoy and to use this model to create a design for prototype development. This work is being performed in collaboration with Helen Bailey and is an EU FP7 project.

Structural Optimisation of Direct Drive Permanent Magnet Generators

Aristeidis Zavvos (Hopewell)
Supervisors: Dr Markus Mueller, Dr John Chick

In terms of drive train technologies, modern wind turbines can be split into two categories: Those with gearboxes between the turbine blades and high speed generator and those with a low speed generator directly-driven by the turbine blades. Direct drive generators with permanent magnets are highly efficient since they omit the cost of the gearbox and the maintenance cost related to them. Furthermore, there is no need for external excitation, so the excitation losses are eliminated; allowing smaller pole pitches to be used. On the downside, the high torque requirements lead to structures with great diameters that are expensive and difficult to build, transport and install. 60-70% of the mass of a direct drive generator is due to the structural mass required to support the active electromagnetic part of the generator. The structure is required to overcome the magnetic attraction forces within the machine and also the wind induced loadings. In this project optimisation techniques to reduce structural mass will be investigated. Structural optimisation will be applied to a number of promising direct drive generator technologies. This work will provide an in depth study and comparison of the various generator topologies, taking into account the structural and active mass of such machines.

Direct-drive permanent magnet generators for the next generation offshore wind turbines

Dr Alasdair McDonald (EU FP6)
Supervisors: Dr Markus Mueller

UpWind is a European project funded under the EU's Sixth Framework Programme (FP6). The project looks towards the wind power of tomorrow, more precisely towards the design of very large wind turbines, both onshore and offshore. Edinburgh's work was part of the Transmission and Conversion work package. This work included the structural modelling, design and optimisation of very large direct-drive permanent magnet generators and was carried out with partners at Aalborg and Delft Universities. Axial-flux and radial-flux generators were studied and optimised.

50kW Linear Generator for Direct Drive Wave Energy Converters

Dr Neil Hodgins (Carbon Trust)
Supervisors: Dr Markus Mueller, Dr John Chick

This is a prototype for a series of lightweight linear generators designed specifically for marine renewables. A 2m core of Permanent Magnets are driven past a 4m long section of coils generating 50kW at peak load. The generator has been tested at TUV NEL in East Kilbride with a hydraulic power pack driving the test rig. The generator was built at Fountain Design and the project was funded by the Carbon Trust. [more...]

Integration of C-GEN to Wave and Tidal Devices

Ozan Keysan (npower Juice)
Supervisor: Dr Markus Mueller

C-GEN is a new topology of direct-drive permanent magnet generator being developed at Edinburgh University. The main benefits are reduced overall system mass and ease of manufacturing, due to the use of an air-cored winding, but with a modular PM rotor consisting of C-core modules. This project, funded by Npower Juice, aims to investigate the feasibility of C-GEN to two tidal and two wave energy converters. An analytical design tool is developed to find out the most optimum solution for each device. Analytical design tool combines electromagnetic, structural and thermal design aspects for the C-GEN. Also, FEA simulations are performed for each device to validate the analytical tool. [more...]

Reliability of Generators in the Marine Environment

Isaac Portugal (CONACYT, EPSRC UK-China Project)
Supervisors: Dr Markus Mueller, Dr John Chick

Wave energy is still on its infancy; most of devices are still on research, development or demonstration phase. A key challenge that the industry has yet to overcome is the reliability of the devices. High-energy density places in the sea are typically located some kilometres away from the shores where the harsh weather conditions could limit the maintenance accessibility. More over, the nature of the marine resources (i.e. turbulence and wave spectrum) could create high-load patterns that could jeopardise the reliability economic feasibility of the devices. This project has the objective of studying and improving the reliability of the marine energy converters, with special focus on electrical generators, to increase the possibility of commercial deployment in a not-too-distant future.

Lightweight Low Speed Rotary Electrical Generators for Direct Drive Wind Systems

Dr Alasdair McDonald, Dr Jonathan Shek (Scottish Enterprise - PRP)
Supervisor: Dr Markus Mueller

In direct drive wind turbines the electrical generator is directly coupled to the slow moving prime mover, which rotates. It has been convention to use iron-cored machines, that is a machine have iron on both the stationary and moving parts, but the magnetic field between the two results in a large undesirable magnetic attraction force, which requires a significant mechanical structure to maintain the physical airgap between the two. Removal of the iron from one component eliminates the magnetic attraction force completely. Such machines are termed "air-cored". In this project the main objective is to demonstrate a novel form of air-cored permanent magnet machine integrated in a commercially available small-scale wind turbine. A prototype machine will be designed and built to demonstrate the concept: a 15kW, 150rpm rotary machine. The prototype will initially be tested at the University of Edinburgh and subsequently transported to a wind turbine test site for field testing. This project is funded by Scottish Enterprise PRP grant.

Machine Design Optimisation using Genetic Algorithms

Richard Crozier (EPSRC SuperGen Marine)
Supervisors: Dr Markus Mueller, Dr John Chick

The aim of this research is to develop models for several linear machine topologies for use in wave energy applications, and to use these models to optimise each machine, taking into account electrical, thermal and mechanical characteristics. The optimised designs will then be used to perform an economic comparison of the main machine types.

Lightweight Low Speed Linear and Rotary Electrical Generators for Direct Drive Wave, Wind and Tidal Current Systems

Dr Alasdair McDonald, Dr Kenneth Ochije (Scottish Enterprise - Proof of Concept)
Supervisor: Dr Markus Mueller

In direct drive wave, wind and marine current energy converters the electrical generator is directly coupled to the slow moving prime mover, which either rotates (wind and marine current) or reciprocates (wave). In the latter case linear generators have been developed. It has been convention to use iron-cored machines, that is a machine have iron on both the stationary and moving parts, but the magnetic field between the two results in a large undesirable magnetic attraction force, which requires a significant mechanical structure to maintain the physical airgap between the two. Removal of the iron from one component eliminates the magnetic attraction force completely. Such machines are termed "air-cored". In this project the main objective is to investigate a novel form of air-cored permanent magnet machine which makes effective use of the magnetic circuit, but without the magnetic attraction forces present in iron-cored machines. Prototype machines will be design and built to demonstrate the concept: a 20kW, 100rpm rotary machine and 3kW linear machine. Both prototypes will be tested at the University of Edinburgh. This project is funded by Scottish Enterprise under the Proof of Concept Fund.

Structural Design Tools for Low Speed High Torque Machines used in Direct Drive Renewable Energy Converters

Alasdair McDonald (EPSRC)
Supervisors: Dr Markus Mueller, Dr Ewen Macpherson

Direct drive systems offer better reliability compared to gearbox drive trains. However, because of the low speed the generators have a large outside diameter and are very heavy. There have been a number of design studies comparing different machine topologies, but in these studies only the active mass is included and the structural mass (or so-called inactive mass) required to maintain the airgap is excluded. In this project we are developing analytical tools for estimating the inactive mass, which will then be used in a more comprehensive comparison of electrical machine topologies for direct drive power take off systems.

Feasibility Analysis of High Temperature Superconducting Generators for Renewable Energy

Ozan Keysan
Supervisors: Dr Markus Mueller, Dr Ewen Macpherson

There has been a remarkable trend towards higher power for offshore wind turbines. This trend is limited by the excessive mass of drive-train for turbines greater than 5 MW. More over, the installation cost of the turbine is directly related to the mass of the drive-train. One promising candidate to reduce the overall mass and volume of large wind turbine power take-off systems is High-Temperature Superconducting Generators (HTSG). There have been designed some commercial HTS machines but there is not a feasible topology for offshore wind turbines. The aim of this project is to develop a HTSG topology that can answer the needs of the industry and penetrate into market.In order to compete with the available options, the suitable generator should have a high-power density should have simple cryocooler system, and should be reliable. We expect this research to provide valuable information and foundation for future research projects in this area.

• Thermal Analysis of Electrical Generators for Renewable Energy Converters
• Bearings for Linear Machines
• Thermal Modelling of Synchronous Machines
• High Speed Electrical Power Conversion for Oscillating Water Columns

Eddie Chong (in collaboration with Motor Design Ltd.)
Supervisors: Dr John Chick, Dr Markus Mueller

Innovations in design have resulted in an increase in the power density of electrical generators, making these a much more attractive proposition for many renewable energy applications. However, the power density is usually limited by maximum operating temperature of the machine. In order to optimize the generator design, the electrical design has to be coupled to the thermal design, with taking into account the internal and external air-flow of the machine. The major deliverable from this project will be an air-flow modeling tool for incorporation into electromagnetic-thermal machine design tools to allow full optimization taking into account internal and external airflows and the operating environment of the generator. The models developed will be generic so that they can be applied to any type of electrical machine, but will be demonstrated on the following machines: 20kW 4-pole synchronous machine, 20kW 4-pole induction machine, and 20kW direct drive PM generator.

Bearings for Linear Machines

Sarah Caraher (EPSRC SuperGen Marine)
Supervisors: Dr Markus Mueller, Dr John Chick

The purpose of the work is to identify a fail to safe, low maintenance, low wear bearing that will maintain alignment between the moving and stationary components of a direct drive linear generator, primarily the permanent magnet linear generator, and then lend this knowledge to other types of novel direct drive linear generators.The bearings are an integral part of the design as their failure could render the whole generator useless. Due to the nature of the device, a submerged offshore wave energy converter, maintenance is also a huge problem. The priority outcome of this reseach will be to design a linear bearing that; can survive the harsh operating environment of the ocean for a period of years and hence does not require regular maintenance, will provide a mechanism to prevent catastrophic failure of the generator if it fails and operate fully flooded. Types of bearings investigated include the fluid film variety, such as water fed hydrostatic and hydrodynamic bearings, then more focus was put on the use of novel polymer materials to act as a water lubricated plain bearings. The bearings selected will be compared by their lifespan and power requirement. Testing of these novel bearing materials that are suitable for operation in wet and dry conditions was undertaken in a vertically aligned application specific test facility designed and built as part of this project.

Improved Lumped Parameter Thermal Modelling of Synchronous Generators

Carlos Mejuto (EPSRC Industrial CASE Award with Cummins Power Generation)
Supervisors: Dr Markus Mueller, Dr Ewen Macpherson

Within the existing available mix of numerical and analytical thermal analysis options, lumped parameter thermal modelling is selected as the operational backbone to develop an improved novel synchronous generator thermal modelling package. The objective is for the creation of a user friendly quick feedback tool, which can serve as a means to make quick machine design thermal calculations and answer customer queries quickly and reliably. Furthermore, thermally improved generator designs will allow for inevitable operational losses to be channelled away from the machine more efficiently. As a result, machine component temperatures will be reduced, allowing lower generator thermal ratings. The end result will be smaller, longer lasting, more efficient generators, with the ability to be adapted with greater ease to particular applications. With the contribution of selected numerical analysis techniques, mainly finite element analysis for the distribution of iron losses, the MySolver thermal modelling package is developed and presented. It is this combination of numerical and analytical tools that improves synchronous generator thermal modelling accuracy, but ultimately it is the lumped parameter nature of the thermal models developed that makes MySolver succeed as a reliable quick feedback electrical machine thermal design tool, validated using experimental results for a wide range of operating conditions.

High Speed Electrical Power Conversion for Oscillating Water Columns

Neil Hodgins (EPSRC Industrial CASE Award with Wavegen)
Supervisors: Dr Markus Mueller, Dr John Chick

It is widely accepted that 10% of the UK electricity needs could be met from marine energy, principally off the NW coast of Scotland, and thus make a significant contribution to the reduction in CO₂ emissions. The Oscillating Water Column (OWC) used in the LIMPET device, operated by Wavegen, has proved the feasibility of extracting energy from waves, and can be applied both on and offshore. Wavegen have identified a clear potential in making use of existing and new-build breakwater structures to extract wavepower using low powered OWCs. In their initial testing of a prototype device on Islay they have realised that the electrical generation system has to be different to the one used in the LIMPET main power outlet. In order to obtain optimum efficiency from the directly coupled air turbine it is necessary to operate over a wide speed range so the applied damping generated by the unit maintains its matching with the energy in the collector. The entire system from wave to wire will be simulated in Simulink and the results correlated to the current set up. The simulations will be used to compare the technologies and develop control strategies for the 20kW device. Based upon the simulation results a prototype system will be built and tested in the laboratory at Edinburgh, which will allow demonstration of the system and verification of the simulation results. If possible the system will be installed and tested on the 20kW device in LIMPET.

• Control for Component Failure Prevention in Wave Energy Converters
• Performance Optimisation for Hybrid Renewable Energy Systems
• Electrical Power Conversion in Arrays of Linear Direct Drive Wave Energy Converters
• Control of Linear Generators for Wave Energy Converters in Irregular Seas
• Power Converters for Low Power Hybrid Renewable Energy Systems
• Phase and Amplitude Control of Wave Energy Converters using Linear Electrical Generators

Dr Jonathan Shek (EPSRC SuperGen Marine)
Supervisor: Dr Markus Mueller

For offshore marine energy converters, maintenance is regarded as a key issue which directly affects the overall performance of a device or a group of devices. It is well-known that component failure results in significant downtime and poses the risk of damage to other components. Hence minimising failure occurances is crucial for effective maintenance. An initial study suggests that the time-to-failure of certain components within a marine energy conversion system can be extended by controlling the device to operate under different conditions. Previous work has shown that device control is possible through intelligent control of the power conversion stage. This project looks at methodologies for component failure prevention and aims to develop control algorithms that will extend component time-to-failure. The project will also analyse the effect that controlling for failure prevention has on the energy conversion system has a whole in order to optimise the overall performance.

Increasing the efficiency of an Hybrid Diesel Wind System

Juan-Pablo Echenique (CONICYT, BECAS CHILE)
Supervisors: Dr Markus Mueller, Dr Ewen Macpherson

Hybrid power generation systems are an attractive solution for the electrification of isolated communities, since they integrate different energy sources, achieving a cost reduction of the energy supplied. However, many of the existing solutions are capable of providing energy to develop very basic activities. In this sense, an isolated community will be faced with a considerable barrier to achieve higher levels of development. A hybrid system based on a diesel generator and a renewable energy source (e.g. sun, wind, hydro, etc) is one of the most common solutions. The challenges of such systems are the ability to integrate intermittent energy sources, providing quality energy to a community where the energy demand is variable. An interesting problem to solve here is the minimisation of the system costs (investment and operating costs), maintaining a certain level of quality and availability for electricity consumers. This problem can be analyzed from different viewpoints: finding an optimal size of the electrical devices (generators, support banks, etc) or choosing an optimal control strategy in order to manage the power flows. This work aims to increase the efficiency of an experimental hybrid system based on: a variable speed diesel and wind generator, and a bank of ultra capacitors, minimizing the energy losses due to: a) Intermittency in energy resources (wind) and loads, and b) Operation of the power converters associated to each device (generators and support banks). The work will enable the design of an optimal strategy for power management, increasing the autonomy of the overall system.

Electrical Power Conversion in Arrays of Linear Direct Drive Wave Energy Converters

Zaki Annuar
Supervisors: Dr Ewen Macpherson, Dr Markus Mueller

There have been various proposals for interconnecting wave energy converters (WECs) in future offshore wave energy farms. Currently there is no agreed method for power conversion within marine renewable arrays. The project addresses issues regarding the power collection within arrays of linear direct drive WECs into power “bundles” for transfer to shore. This conversion requires careful power handling (control) as the power generated from each WEC in a farm varies due to the different wave spectrum experienced by them. This objective will be achieved through the development of system models, which will include: devices; electrical power take off and associated power converters; modes of power transfer to shore; and converters used in grid connection.

Control of Linear Generators for Wave Energy Converters in Irregular Seas

Bin Li (EPSRC SuperGen Marine)
Supervisors: Dr Ewen Macpherson, Dr Markus Mueller

Systems where the driving and driven elements are connected or coupled directly without any mechanical interface between them are known as direct drive systems. This project focuses on maximum power extraction from irregular waves using a linear generator directly connected with WEC. In order to make the Wave energy converter move in optimum phase relative to the waves, new methods to control the power take off force are developed. Device displacement needs to be considered as well to limit the excursion.

Power Converters for Low Power Hybrid Renewable Energy Systems

Paul Stott (EPSRC)
Supervisors: Dr Markus Mueller, Dr Ewen Macpherson

There is increasing interest in domestic renewable power systems, such as domestic CHP, PV, and roof mounted wind turbines. Such systems could be used to offset the power supplied by the grid, but could equally be used to generate power back into the grid. However, any power conversion or power management system would need to comply with Grid Codes laid down by the Distribution Network Operator. The system needs to be intelligent, so that it can optimise the power flows from (or to) the grid and from the small-scale energy converters. Power converters must be able to generate voltage and frequency within the current regulations for use by existing domestic and industrial loads.In this project we will focus on the development of power converters and controllers for low power wind systems connected to domestic or industrial units. In addition fully variable speed hybrid wind-diesel systems will be included in the study.

Phase and Amplitude Control of Wave Energy Converters using Linear Electrical Generators

Jonathan Shek (EPSRC)
Supervisors: Dr Ewen Macpherson, Dr Markus Mueller

This project aims to develop control strategies for optimising the energy absorbed from a wave energy device, such as heaving buoy, using a directly coupled linear electrical generator. The energy captured by a wave energy device is a function of the frequency of the incoming waves and their amplitude. Maximum energy is captured when the incoming wave frequency matches the resonant frequency of the device, but this condition can never be guaranteed. Waves are random, and hence some form of control is required to optimise the energy captured. Phase control ensures the wave excitation force and the velocity are in phase, which is always the case at resonance. This is achieved at off resonant frequencies by applying an additional spring force by mechanical means. However, in a previous project it has been shown by simulation that a directly coupled linear electrical generator can be used to provide this additional spring force. The aim of this project is to develop control strategies for direct phase and amplitude control using a linear electrical generator. An electromechanical test rig for emulating wave energy devices has been installed in the machines lab at Edinburgh so that the system can be demonstrated.

Group Leader

Dr Markus Mueller