Modelling and Simulation of Hybrid Electric Trains Powered by Hydrogen Fuel Cells and Batteries for Routes in the Highlands of Scotland : Preliminary Results

This report builds on an earlier review for the Scottish Association for Public Transport on the potential of batteries, hydrogen fuel cells and other short-term energy storage systems for railway and tramway applications. It outlines the development of a train performance model and associated computer simulation software for a design of hybrid multiple unit, powered by a combination of hydrogen fuel cells and batteries. Assumptions underlying the model are discussed in detail. The chosen mode of operation involves steady state conditions for the fuel cells, with the batteries being used to provide additional stored energy for use on gradients and when the train is accelerating. The simulation techniques involve a mix of conventional “forward” simulation and an approach based on an “inverse” simulation method. Simulation results presented are for a case study involving a short section of route chosen to be typical of sections of many rural routes in Scotland, such as the West Highland lines and routes north and west of Inverness or to and from Stranraer. Data relating to the performance of Class 156 diesel multiple units currently used on non-electrified railway lines in Scotland have provided a point of reference in assessing the performance of the hybrid multiple units. Although other studies of hybrid rail vehicles involving hydrogen fuel cell and battery combinations have been published, those have involved routes that are shorter, with more intermediate stations and no prolonged gradients. Conclusions are presented in terms of fuel cell and battery power levels and battery storage capacity required for operation on the type of route being considered. The most important conclusion is that a preliminary specification for a hybrid two-coach unit could involve two 200 kW traction motors, fuel-cells providing a maximum power output of 350 kW and a battery pack giving a maximum power output of 250 kW and 75 kWh of electrical energy storage capacity. Using standard components that are available commencially, approximate calculations suggest that a design based around these power ratings could be implemented within a target weight of 90 tonnes for a two-coach unit. However, it is thought that the limitations of the UK loading gauge could present difficulties in terms of the space required and implementation might only be possible at the cost of some passenger space. Suggestions are made in the report for further simulation work involving a three-coach configuration and for the addition of a pantograph and associated electrical equipment to allow power to be drawn from 25 kV overhead wiring when the unit is operating on electrified routes. Another important recommendation for further work involves development of a detailed route model for a typical line, including exact information about gradients, curvature and local speed restrictions. Assessment of possible journey-time reductions is also important and preliminary results are presented, for the specification given above, using inverse simulation methods. Potential journey-time reductions over a complete route or specific sections could be investigated in future work. Issues of weight could also be linked to performance within the simulation software and advice could be provided to the user when space or weight constraints are violated. The report includes discussion of possible benefits of developing more detailed, physics-based, sub-models of elements such as fuel cells, batteries, traction motors and power electronic components which could be used to replace the much simpler sub-models used in the existing simulation model. This might allow use of well-established and validated sub-models and would extend the range of issues that could be addressed through simulation and allow more accurate assessment of losses in batteries, power electronic components and traction motors over the full range of operating conditions. This could also be of value for checking underlying assumptions within the model and for the development of control and energy management strategies. The report recommends the use of both forward and inverse methods of simulation for applications of this kind as these two approaches, taken together, can provide additional insight that is not obtained so readily from the use of conventional forward simulation methods alone

Powering Future Transport in Scotland: A Review for the Scottish Association for Public Transport

This report discusses energy costs and emissions associated with transport in Scotland and reviews options for future power sources for different modes of public transport. Transport provides a major contribution to greenhouse gas and other harmful emissions worldwide and efforts to reduce these are important for all forms of public transport, as well as for private cars and for the movement of freight. The effects of transport policy decisions are recognised, increasingly, as being very important for the electricity supply industry at national and local levels, largely because of the growth in the numbers of electric and hybrid road vehicles. Moving from oil to low carbon energy for transport raises important issues for electrical power generation and distribution systems in addition to challenges already being faced by the electrical power industry as the proportion of generating capacity involving renewables increases. The report starts by considering current energy costs and emissions for different forms of passenger transport and then outlines some current developments in areas such as internal combustion engine technology, battery storage systems and hydrogen fuel cells. Systems involving short-term energy storage and recovery of energy that would otherwise be dissipated as heat during braking are also discussed. Such systems generally involve the use of super-capacitors, flywheels or hydraulic devices. References are provided to the sources of data used in the analysis carried out for this review and, also, to sources of information about relevant developments in science and engineering. For all the new developments mentioned, there is a brief review of some transport applications in the United Kingdom and elsewhere. The possible impact of autonomous vehicles on future car ownership is still not known and the effects of this technology on public transport remain uncertain. As well as discussing autonomous road vehicles, the report makes brief mention of the potential of autonomous systems and increased automation for rail transport and for tramway operations. The benefits of further conventional railway electrification are reviewed in terms of energy usage, costs and emissions and the advantages of a more integrated approach to the provision of public transport in Scotland are emphasised. The value of using mathematical modelling and simulation methods to explore options in transport systems developments and planning is discussed, and the importance of testing simulation models in ways that are appropriate for the intended application is emphasised. This review presents the first results from a continuing study which was started in 2018 and is intended to provide information that should be relevant for those involved in decision-making in Scotland at the time of publication. The quantitative information contained within it clearly needs to be updated on a regular basis. The review concludes with recommendations for the Scottish Association for Public Transport about possible priorities for its efforts to increase public awareness about transport issues and is intended to be the first of a series of publications on transport and energy issues in the Scottish context. The references form an important part of the report and provide a potentially important bibliography which must be augmented and updated regularly

Methods of system identification, parameter estimation and optimisation applied to problems of modelling and control in engineering and physiology

Mathematical and computer-based models provide the foundation of most methods of engineering design. They are recognised as being especially important in the development of integrated dynamic systems, such as “control-configured” aircraft or in complex robotics applications. These models usually involve combinations of linear or nonlinear ordinary differential equations or difference equations, partial differential equations and algebraic equations. In some cases models may be based on differential algebraic equations. Dynamic models are also important in many other fields of research, including physiology where the highly integrated nature of biological control systems is starting to be more fully understood. Although many models may be developed using physical, chemical, or biological principles in the initial stages, the use of experimentation is important for checking the significance of underlying assumptions or simplifications and also for estimating appropriate sets of parameters. This experimental approach to modelling is also of central importance in establishing the suitability, or otherwise, of a given model for an intended application – the so-called “model validation” problem. System identification, which is the broad term used to describe the processes of experimental modelling, is generally considered to be a mature field and classical methods of identification involve linear discrete-time models within a stochastic framework. The aspects of the research described in this thesis that relate to applications of identification, parameter estimation and optimisation techniques for model development and model validation mainly involve nonlinear continuous time models Experimentally-based models of this kind have been used very successfully in the course of the research described in this thesis very in two areas of physiological research and in a number of different engineering applications. In terms of optimisation problems, the design, experimental tuning and performance evaluation of nonlinear control systems has much in common with the use of optimisation techniques within the model development process and it is therefore helpful to consider these two areas together. The work described in the thesis is strongly applications oriented. Many similarities have been found in applying modelling and control techniques to problems arising in fields that appear very different. For example, the areas of neurophysiology, respiratory gas exchange processes, electro-optic sensor systems, helicopter flight-control, hydro-electric power generation and surface ship or underwater vehicles appear to have little in common. However, closer examination shows that they have many similarities in terms of the types of problem that are presented, both in modelling and in system design. In addition to nonlinear behaviour; most models of these systems involve significant uncertainties or require important simplifications if the model is to be used in a real-time application such as automatic control. One recurring theme, that is important both in the modelling work described and for control applications, is the additional insight that can be gained through the dual use of time-domain and frequency-domain information. One example of this is the importance of coherence information in establishing the existence of linear or nonlinear relationships between variables and this has proved to be valuable in the experimental investigation of neuromuscular systems and in the identification of helicopter models from flight test data. Frequency-domain techniques have also proved useful for the reduction of high-order multi-input multi-output models. Another important theme that has appeared both within the modelling applications and in research on nonlinear control system design methods, relates to the problems of optimisation in cases where the associated response surface has many local optima. Finding the global optimum in practical applications presents major difficulties and much emphasis has been placed on evolutionary methods of optimisation (both genetic algorithms and genetic programming) in providing usable methods for optimisation in design and in complex nonlinear modelling applications that do not involve real-time problems. Another topic, considered both in the context of system modelling and control, is parameter sensitivity analysis and it has been found that insight gained from sensitivity information can be of value not only in the development of system models (e.g. through investigation of model robustness and the design of appropriate test inputs), but also in feedback system design and in controller tuning. A technique has been developed based on sensitivity analysis for the semi-automatic tuning of cascade and feedback controllers for multi-input multi-output feedback control systems. This tuning technique has been applied successfully to several problems. Inverse systems also receive significant attention in the thesis. These systems have provided a basis for theoretical research in the control systems field over the past two decades and some significant applications have been reported, despite the inherent difficulties in the mathematical methods needed for the nonlinear case. Inverse simulation methods, developed initially by others for use in handling-qualities studies for fixed-wing aircraft and helicopters, are shown in the thesis to provide some important potential benefits in control applications compared with classical methods of inversion. New developments in terms of methodology are presented in terms of a novel sensitivity based approach to inverse simulation that has advantages in terms of numerical accuracy and a new search-based optimisation technique based on the Nelder-Mead algorithm that can handle inverse simulation problems involving hard nonlinearities. Engineering applications of inverse simulation are presented, some of which involve helicopter flight control applications while others are concerned with feed-forward controllers for ship steering systems. The methods of search-based optimisation show some important advantages over conventional gradient-based methods, especially in cases where saturation and other nonlinearities are significant. The final discussion section takes the form of a critical evaluation of results obtained using the chosen methods of system identification, parameter estimation and optimisation for the modelling and control applications considered. Areas of success are highlighted and situations are identified where currently available techniques have important limitations. The benefits of an inter-disciplinary and applications-oriented approach to problems of modelling and control are also discussed and the value in terms of cross-fertilisation of ideas resulting from involvement in a wide range of applications is emphasised. Areas for further research are discussed

Development of a multi-rate simulation model of an unmanned underwater vehicle for real-time applications

This paper describes work involving the further development and refinement of a mathematical model of an unmanned underwater vehicle (UUV), together with the development of an associated real-time multi-rate simulation that includes both high-speed power electronic subsystems and slower components. The chosen vehicle uses a battery as its energy source and this feeds an a.c. motor drive through a d.c. to a.c. converter. The drive powers the vessel which is modelled as a six-degree of freedom vehicle with control surfaces. Tests carried out indicate that a careful choice of frame rates can increase the speed of solution by factors of several hundred over solution times when the shortest frame rate is used throughout. These multi-rate solutions were executed faster than real time on a typical laptop even when using 3-D graphical output for visualisation of vehicle motion. The conclusions of the paper are that the modelling and simulation of the UUV has provided a useful test-bed for ideas on multi-rate simulation and has demonstrated that multi-rate real-time simulation is feasible and useful for an application of this kind that includes very fast power electronic subsystems and relatively slow systems such as the vehicle and battery