Ever more mechanical and hydraulic components in vehicles or aircraft are being replaced by highly controllable electrical systems, in order to make lighter and more efficient systems. Where multiple electrical energy supplies are combined in the same system (e.g. generator, fuel cells, battery), careful energy management is required in order to maximise system efficiency.
Hybrid and all-electric vehicles
The abundance of mid to large passenger carrying and urban delivery vehicles means that improvements in their energy usage will significantly impact the carbon economy. Importantly, they are often modular and can accommodate technology retrofits, and have a dynamic duty cycle that would most benefit from advances in power electronics, electric machines, energy storage media, advanced power sources and intelligent energy management. One ongoing research project investigates components of an advanced battery/supercapacitor energy store and cell management system which are located in the Bristol Energy Management Laboratories and which will be interfaced to a modern high-efficiency diesel engine located at the University of Bath, using mathematical models representing the electrical machine, power converter and transmission. The outcome will be demonstration of the fuel economy benefits of a mild hybrid taking into account the non-linear properties of electrochemical energy storage, and an engine and its fuel system.
Aircraft electrical systems
Adoption of more-electric technologies will enable major changes to how aircraft are designed and flown whilst realising significant improvements in fuel burn. The size/weight/reliability of electric motors and power electronic converters are now at a stage where, for example, an electrically driven rotor is emerging as a serious prospect.
An example research project in the Electrical Energy Management Group is the investigation of a fault-tolerant electric tail rotor system. The project will assess how typical fault modes impact aircraft flightworthiness. A fully rated electric tail rotor motor drive located in the Group’s laboratory will be integrated with an aircraft simulator operated by a trained pilot at the sponsoring aircraft company’s site. Initially the real-time performance of the hardware will be assessed over a flight mission. Typical single point faults will then be emulated in order to demonstrate fault management and the effect of the resultant reduced capability on the aircraft and pilot’s correcting actions.
Wind turbine blades and aircraft wings would benefit from intelligent structural monitoring and the controlled morphing of their shape. The Group has embedded functional materials into composite materials and is researching ways of using these new high-tech materials to increase the performance, efficiency and life of rotor blades.
The research into electric actuator technology encompasses all elements of the system including the actuator, power electronic driving circuitry and control strategies. For this we have developed automated test facilities for accurately characterising electric actuators, containing:
- high bandwidth, high power (3.3 kVA) linear amplifiers capable of driving reactive loads;
- force, displacement, acceleration and temperature sensors;
- computer-controlled signal generation and measurement equipment.
Once an actuator has been fully characterised, we develop power electronic systems optimised to the specific actuator, minimising the volume and weight of the circuitry and maximising efficiency.
New lightweight, multifunctional structures
The ACARE goals of a 70% reduction in CO2 from aircraft by 2050 can only be realised by embracing the system level, energy efficiency benefits of electric propulsion. This disruptive change in aircraft design and operation will only come about if the power per weight and volume of electrical machines can be reduced substantially. The EU JTI Clean Program Technology Roadmap identifies the need for electrical machine with a specific output of >30 kW/kg by 2035; a x6 – x10 improvement on the state-of-art (3-5 kW/kg). High efficiency is also an essential requirement, if weight savings are not to be overshadowed by the additional fuel load needed to supply the power conversion losses. It will not be possible to deliver this through incremental changes and radical new approaches to electrical machine manufacture and design are needed.
The University of Bristol is addressing the challenges of light weighting electrical machines through multi-discipline research, joining together internationally recognised expertise in composite materials, and electrical machine loss and thermal analysis, with advanced composite and additive manufacture capabilities and state-of test and measurement facilities.
With directional electrical conductivity, directional magnetic permeability, directional dynamic force reaction and good heat extraction. Exploratory research has demonstrated high-strength containment materials with tailored permeability for electrical machine rotors and high-volume loadings of conductors. 3D printing has been employed to form optimal conductor cross sections for loss reduction and integrated cooling channels.
For more information, please contact Phil Mellor.