Analyzing the Potential of Energy Storage on Electrified Transit Systems

Transcription

1 Analyzing the Potential of Energy Storage on Electrified Transit Systems M. Chymera, A.C. Renfrew, M. Barnes University of Manchester, School of Electrical and Electronic Engineering, Manchester, M60 1QD, UK Abstract The paper assesses the potential implementation of energy storage system on a tram on the Blackpool tramway. Energy flow is analyzed for drive cycles measured on the tramway and it is demonstrates that using energy storage could result in the energy consumption of a tram being reduced by 30.6%. 1. Introduction Environmental and sustainability concerns, significant rises in energy prices and an unpredictable future result in a requirement to reduce energy consumption. A significant proportion of energy in electrified mass transit systems is dissipated through braking. Regenerative braking allows this energy to be reused by other vehicles in the system. Difficulties have prevented wide spread implementation of regenerative braking on electrified mass transit systems. Recent developments in energy storage devices, particularly batteries, supercapacitors and flywheels, have renewed the interest in recovering energy from braking. Energy storage provides two advantages for electrified transit systems, reducing the overall energy consumption, and therefore operating costs, and reducing peak loading. The nature of electrified transit systems means that current peaks are large. This means the system infrastructure needs to be designed to meet the peak requirements. Energy storage enables peak shaving, and hence infrastructure costs can be reduced, or the capacity of an existing infrastructure can be increased to cope with increased service demand. In transit systems energy storage can potentially be placed on board vehicles or at the track side. Using on board energy storage allows a vehicle to capture regenerated energy from braking, and reuse the energy during acceleration. This paper assesses the use of energy storage on the Blackpool tramway, determining the sizing and the potential energy saving. The analysis looks at the implementation on an existing tram using over the current drive cycle. 2. Energy Storage Devices Significant developments in energy storage devices have recently been made, particularly for electric vehicle, power system and aerospace applications. For use in railways systems in conjunction with regenerative braking, an energy storage device with large power density would be required. Supercapacitors, flywheels, and SMES are potentially suitable for railway applications. Supercapacitors consist of two solid electrodes in a liquid electrolyte. An ion permeable separator is used to electrically insulate the electrodes, but allowing ions of the electrolyte to pass through. Supercapacitors store charge at the interface of the solid electrodes and the electrolyte, forming a double layer [1]: a capacitance is formed by the two monolayers. The distance between the charge layers is only a few atomic diameters, hence a capacitance much greater than that achieved by conventional capacitors is possible. A double layer is formed at each electrode.

2 Flywheels store energy in the form of rotating inertia. Using magnetic bearings and containing the flywheel in a vacuum has reduced losses, enabling flywheels to store energy for longer periods more efficiently [2]. Superconducting Magnetic Energy Storage (SMES) devices store energy in a magnetic field [3]. By applying a DC current to a coil, a magnetic field is created, storing magnetic energy. When the DC potential is removed, the energy is released. By using low loss superconducting coils, high amounts of energy can be stored in the magnetic field. SMES are used to improve power quality in distribution networks [4]. Supercapacitors provide an ideal energy storage form for tractive applications. Currently supercapacitors are more readily available than other energy storage technologies, and as a result would be ideal for a tram on the Blackpool tramway. 3. Application of Energy Storage to the Blackpool Tramway An ideal energy flow diagram for an electrified vehicle is shown in Figure 1. The diagram is idealized, in the energy transfer processes, climbing and accelerating, energy is dissipated through loss mechanisms, driveline losses, and kinetic energy is dissipated through frictional forces. Electrical energy transmitted to the system is also dissipated through electrical transmission losses. The diagram shows that energy is dissipated through braking. Acceleration KINETIC Braking ELECTRICAL Climbing (decelerating) Acceleration (on decline) Climbing POTENTIAL Braking Figure 1 Idealised energy flow diagram for an electric vehicle The principle of using energy storage is to recapture kinetic energy when a vehicle slows down or stops, and to transfer it into a form of stored energy. The objective of using energy storage is to add an additional form of energy to the vehicle, electrostatic energy in the case of a supercapacitor, and hence allowing energy, currently dissipated in braking to be reused. Figure 2 shows a modified energy flow diagram. The diagram shows energy from braking being reused.

3 ELECTRICAL Acceleration Climbing KINETIC Climbing (decelerating) Acceleration (on decline) POTENTIAL Regenerative Braking Acceleration Climbing Regenerative Braking STORAGE Figure 2 Idealised Energy Flow with Energy Storage Energy storage can be applied to any electrified transit system, and could potentially result in significant energy savings. The magnitude of the energy savings is dependent on the system. The energy savings achieved depends on a number of factors, the stop spacing, the gradient profile, the location and the vehicles. Drive cycles on systems with stops close together will include more braking, and therefore the potential benefit of capturing regenerative braking is greater. Gradients have an impact on the effectiveness of energy storage, some systems with large gradients require braking to be applied to maintain constant speed downhill, providing a potential for energy to be regenerated. Other systems are designed so that stations are at a higher level than the track, and hence there is an uphill gradient coming into stations and a downhill gradient leaving stations. [5] The effect of these gradients reduces the amount of braking required, and hence would reduce energy available for energy storage. In such systems, energy storage is already employed; kinetic energy converted into potential energy as the vehicles slow down and on departure from the stations the potential energy is used to accelerate the vehicles. The geography of the tram system has an important impact on the amount of energy dissipated through braking. Street running systems are likely to require vehicles to stop and start for traffic and pedestrian interfaces than segregated systems. The potential benefits of energy storage on systems consisting of on-road and pedestrian sections could potentially yield greater energy savings by employing energy storage. This study focuses on the Blackpool tramway. The Blackpool tramway contains street running and segregated sections. Stops are frequently spaced on the Blackpool tramway, with a total of 61 stops along the 18km route, meaning that the average distance between stops is 300m. The short stop separation means that the amount of energy that could be saved by reusing braking energy is likely to be significant. There are some small gradients on the system. The fleet of trams at Blackpool is aged, many of the traction drives employing DC drives with resistor control and some newer trams with chopper controls. Recently a prototype tram has been built by Tram Power Ltd, the City Class tram. This tram has been tested on the Blackpool tramway. The City Class tram is a modern tram containing an inverter driven induction motor. The City Class tram will be considered in this study.

4 4 Energy Storage Device Selection and Energy Management The energy available from braking is dependent on the rate of braking and the speed when braking commences. By simulating the driveline at different braking rates, the energy captured by the energy storage device can be calculated. Figure 3 shows the energy available for the 22 tonne tram at different braking rates and from different speeds m/s/s m/s/s Energy Captured (MJ) m/s/s 0.8m/s/s 0.6m/s/s 0.4m/s/s 0.2m/s/s Increasing Braking Rate Speed when braking starts (m/s) Figure 3 Energy available from braking at different braking rates and starting speeds. Examination of the speed profile measured on the Blackpool tramway, Figure 6 reveals that tram speeds at Blackpool are relatively slow, and speeds infrequently exceed 10m/s (36km/h). When sizing the energy storage device it is important to select a device which can capture most of the available braking energy most of the time, whilst avoiding over sizing the device. Select an oversized device which is not fully utilized adds additional costs and mass to the vehicle. The graph, Figure 3 shows that the energy available when braking commences at 10m/s does not exceed 1.05MJ even at the highest braking rates. Selecting an energy storage device that can store approximately 1.05MJ would provide sufficient storage capacity to store all the braking energy from a tram braking from 10m/s. The Maxwell HTM Power Supercapictor [6] is a 1.02MJ supercapacitor, and hence is an ideal choice. To control the energy flow between the supercapacitor and the inverter DC bus a DC-DC converter is required. The sizing of the converter is important to determine how much of the braking power can be captured. As with the selection of the storage device, over sizing the converter adds additional mass and cost to the vehicle.

5 The instantaneous braking power can be determined by calculating the change in kinetic energy. This can be determined from the speed profile. To analyze the energy that can be captured on the Blackpool tramway a measured drive cycle can be used, Figure 6. When decelerating, some of vehicle s kinetic energy is dissipated through frictional forces. Some of the remaining braking energy is not transmitted to the energy storage device, losses occur in the mechanical driveline, the motor and the power electronics. Further losses occur in the energy storage device. The energy can be determined by modeling the drive line (see section 5) to determine the power available for the supercapacitor. By applying limits to the established power profile, the converter rating, and subtracting the converter and supercapacitor power losses, the supercapacitor charging power can be determined, this is integrated to find the total energy stored. Figure 4 shows the relationship between converter size and braking energy captured over the drive cycle. Results of the analysis show that the amount of energy captured from regenerative braking initially rises rapidly as the converter size is increased the amount of energy then proceeds to level off as the converter size is increased further Energy Captured ( Converter Rating (kw ) Figure 4 Percentage of energy captured for converter sizing Selecting a point on the curve where it starts to flatten off allows a converter to be chosen, which captures a sufficient amount of the energy available. A converter rating of 50kW is selected for this application, allowing 3.1MJ of braking energy to be captured. An energy management system is used to determine how much of the tractive power is taken from the supercapacitor. During braking, as much of the braking energy as possible should be stored in the supercapacitor. The storage power is limited by the converter power, and the energy storage device must not be allowed to exceed its maximum voltage level. Braking energy not stored by the energy storage device is dissipated in resistor banks. During motoring, the traction power can be taken from the energy storage device or the traction supply. To ensure that the supercapacitor has enough remaining capacity to absorb braking energy, it is beneficial to use energy from the storage device first. The power from the energy storage is limited by the converter rating, 50kW there is also a limit of the device. The internal

6 resistance, and component resistances, the converter and transmission line resistances limit how much power can be transferred. The supercapacitor should not be allowed to fall below half the working voltage, and hence the power taken from the supercapacitors is limited to prevent the voltage falling too low. 5 Traction System Modelling Modeling is essential for the assessment of the benefits of adding energy storage to an electrified transit system. The purpose of the modeling is to establish how much energy is dissipated in braking, and therefore determining if there is any potential benefit of installing energy storage to a system. The model is then modified to include energy storage. Figure 5 shows a flow diagram to represent the model. SPEED ACCELERATION GRADIENT DYNAMIC MODEL TORQUE ANGULAR VELOCITY DATA FLOW SIMULATION DATA FLOW MECHANICAL TRANSMISSION MODEL TORQUE ANGULAR VELOCITY INDUCTION MOTOR VOLTAGE FREQUENCY CURRENT INVERTER POWER TRACTION SUPPLY SYSTEM POWER MANAGEMENT POWER BRAKING RESISTOR OR STORAGE Figure 5 Model Flow Diagram To model a tram over a specified drive cycle requires an inverse dynamic modeling technique to be employed. An inverse dynamic technique takes a required output of a system and determines the required inputs. Each stage is modeled, the dynamics, mechanical transmission, induction motor, inverter, and supply, each stage determines the required output of the previous stage, as shown in the flow diagram, Figure 5. Consideration of energy flow is crucial; understanding where energy is dissipated in order to establish the magnitude of the return energy flows, and calculate how much energy can be stored and reused. Each part of the electrified transit system is modeled to consider energy flows, outputting the energy dissipated in each section. The model is constructed based on the energy flows described. Firstly considering the kinetic energy, modeling the frictional loss, using the Davis equation [7] and determining the energy required to accelerated the vehicle. The potential energy change is determined by considering

7 the gradient. The total kinetic and potential energy requirements determine the power requirement for the driveline. The driveline consists of a power electronic drive, a 3-phase inverter, and induction motor and mechanical transmission. For the city class tram this arrangement is duplicated at both ends of the vehicle. These components are modeled in reverse, hence the power requirement of the mechanical power requirement is added to the power loss of the mechanical transmission, which is added to the power loss of the induction motor which in turn is added to the power loss of the power electronic drive to determine the total tractive power required. Adding this to the auxiliary load of the vehicle determines the total power requirement of the vehicle. The electrical energy is transmitted from the local distribution network through a traction substation, consisting of a transformer and rectifier and conductor system. The electrical losses are also determined in the model, by considering the conductive losses and the substation losses. The model is used to simulate the vehicle with and without energy storage over the same drive cycle. This allows a comparison of performance to be made. 6 Drive Cycle To establish the potential benefits of energy storage, the vehicle should be compared over a realistic drive cycle. Speed measurements made on the Blackpool tramway allow this to be done. The drive cycle Figure 6 taken from GPS date on the Blackpool Tramway is over a 9.2km journey from Fleetwood Ferry to North Pier Speed (m/s) Time (s) Figure 6 Speed profile measured on the Blackpool tramway

8 7 Traction System Performance Analysis of the 22 tonne City Class tram over the drive cycle gives the following consumption levels. With a total consumption level of 1.1kWh/km, corresponding to consumption levels measured during testing [8]. Table 1 Energy Consumption of vehicle components Electrical Energy dissipated (kwh/km) Frictional Forces Gears Motor Power electronics Auxiliary Electrical supply Braking Resistors The analysis shows that 55% of the energy is dissipated in braking. This energy could potentially be used to charge an energy storage device. The energy storage device will also contain losses, and hence the braking energy calculated does not represent the potential energy saving. 8 System Performance with Energy Storage A supercapacitor model was added to the model. The supercapacitor selected was a 390V supercapacitor, the Maxwell HTM Power Supercapictor [6] with a capacitance of 17F. A 50kW converter was used with the supercapacitor. Table 2 Energy dissipation with Energy Storage Electrical Energy dissipated (kwh/km) Frictional Forces Gears Motor Power electronics Auxiliary Electrical supply Energy Storage System Losses Braking Resistors A total of 0.763kWh/km is consumed on a vehicle with energy storage over the drive cycle. This corresponds to a 0.337kWh/km reduction in energy consumption, a 30.6% saving in energy over the drive cycle.

9 Speed (m/s) Time (s) Figure 7 Portion of the drive cycle Power (kw) Supply Power -40 Energy Storage Power -60 Time (s) Figure 8 Supply and Supercapacitor Power Figure 8 shows the supercapacitor power profile and the supply power profile over a short portion of the drive cycle, corresponding to Figure 7. The selected portion shows the immediate use of the stored energy when the tram begins to accelerate and the additional use of power from the supply when the converter power limit is reached. The use of power from the energy storage device stops when the stored energy is depleted.

10 9 Discussion The analysis demonstrates that significant energy saving can be achieved by installing a supercapacitor on board a tram on the Blackpool tramway. The analysis is conducted using drive cycles measured on a vehicle on the Blackpool tramway. The way trams are driven depends on how the drivers are trained. At Blackpool some energy is saved by employing coasting, braking is then used to control the deceleration of a vehicle. The driving techniques could be modified to limit the levels of braking to maximise energy captured for the storage device, only exceeding this level of braking when necessary. Slight alterations to the deceleration could yield further energy savings, without any significant impact on performance. 10 Conclusions The results show that a 0.337kWh/km energy saving could potentially be made by installing an on board energy storage device to the City Class Tram on the Blackpool Tramway. In service a tram at Blackpool could complete up to 280km a day, hence a daily energy saving of 95kWh could be achieved with the implementation of energy storage on the vehicle. A significant saving is achievable, which warrants further investigation and cost analysis to determine the financial benefits, if any. As the price of energy storage continues to decrease and energy prices increase the potential of energy storage increases. The analysis shows that the amount of energy that could be saved is significant. Acknowledgements The research has been conducted as part of an Engineering Doctorate research project carried out at the University of Manchester. The research is funded by the UK Engineering Physical Science Research Council (EPSRC) and HILTech Developments Ltd. It is working on aspects of new modeling software, PowerQ (all rights reserved). The authors are grateful to Blackpool Transportation, particularly their chief electrical engineer, Mr Peter Brown for permission to take speed measurements on their system. The authors a thankful of Professor Lewis Lesley and Tram Power Limited for data provided on the City Class Tram used in this study. References [1] M. Endo, T. Takeda, Y. J. Kim, K. Koshiba, and K. Ishii, "High power electric double layer capacitor (EDLC's); from operating principles to pore size control in advanced active carbons," Carbon Science, vol. 1, pp , [2] J. G. Bitterly, "Flywheel technology: past, present, and 21st century projections," Aerospace and Electronic Systems Magazine, IEEE, vol. 13, pp , [3] C.-S. Hsu and W.-J. Lee, "Superconducting magnetic energy storage for power system applications," Industry Applications, IEEE Transactions on, vol. 29, pp , [4] R. Schottler and R. G. Coney, "Commercial application experiences with SMES," Power Engineering Journal [see also Power Engineer], vol. 13, pp , [5] "Central London railway," Engineering, vol. 65, pp , [6] Maxwell Technologies, "HTM Power Series 390v Datasheet." [7] W. J. Davis, Jr., "The tractive resistance of electric locomotives and cars," General Electric Review, vol. 29, pp , [8] L. Lesley, A. Winstanley, A. C. Renfrew, M. Barnes, and M. Chymera, "Power Consumption in a New LRV," presented at 9th International Conference and Exhibition on Railway Engineering, London, 2007.