TET 4190 Power Electronics for Renewable Energy
Project number 1: Electric and hybrid electric vehicles
Group A:
Richard Tokle Schytte Ole Berdiin Olesen
Erik Martinsen
Supervisors: Torbjörn Thiringer, Chalmers tekniska högskola
Ole Jakob Sørdalen, Eltek Tore Marvin Undeland, NTNU
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Summary The purpose of this paper was to look at different vehicle technologies. The electric motor has been compared to conventional propulsion technology in cars, namely the internal combustion engine (ICE). When investigating the technologies, electrical vehicles were divided into three subgroups; battery electrical vehicles (BEV), hybrid electric vehicles (HEV), and plug in hybrid electric vehicles (PHEV). BEV is the most common and simple form of electric vehicles, and is powered purely by electricity. The power is supplied from the battery pack onboard, and even though efficiency is high, the range is far less than for conventional vehicles. HEV utilize an ICE in addition to the electric motor. The battery pack is smaller compared to BEV, but the range is increased because of the internal combustion engine. A PHEV is a HEV with a larger battery pack and can be charged from the main grid. This will greatly increase its electric range. For the PHEV and HEV, the two engines can be connected in basically two ways, either parallel or in series. In parallel, both the ICE and electric motor provide torque to the wheels. In a series configuration the ICE drives a generator both powering the electric motor and charging the battery with excess power. There are benefits with both configurations; the series allows the ICE to run at constant power, increasing efficiency, while parallel avoids the loss due to conversion of mechanical energy into electrical. Power split is a combination of these two configurations. This allows the vehicle to operate as series connected during low speeds, and parallel during high speeds. This configuration is typically regarded as most efficient and versatile. The power split configuration will be best suited to most customers because of its all over high efficiency and extended driving range. Emissions from the use of electric vehicles are closely tied to the local energy production, and especially whether the power is produced through fossil fuels or renewables like sun or hydro. Power electronics in electric vehicles compared to conventional cars mainly include a dc-ac converter, an ECU to control the battery voltage, and possibly a full bridge dc-dc converter.
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Table of contents Summary ............................................................................................................................. 3 1 - Introduction ................................................................................................................... 5 2 - Engine types .................................................................................................................. 6
2.1 - Internal combustion engine .................................................................................... 6 2.2 - Electrical engine ..................................................................................................... 6 3.1 - Battery electric vehicle (BEV) ............................................................................... 7 3.2 - Hybrid electrical vehicle (HEV) ............................................................................. 7 3.3 - Plug-in hybrid electrical vehicle (PHEV) ............................................................... 8
4 - Technological architecture ............................................................................................ 8 5 - Efficiency and emissions ............................................................................................. 10 6 - Motor drives ................................................................................................................ 10
6.1 - Induction motor drives .......................................................................................... 10 6.2 - Synchronous motor drives .................................................................................... 12
7 - Batteries ....................................................................................................................... 12 7.1 - Materials ............................................................................................................... 12 7.2 - Performance .......................................................................................................... 12
8 - The converter ............................................................................................................... 13 9 - Conclusion ................................................................................................................... 15 Appendix ........................................................................................................................... 16
Table of figures Figure 1 – Parallel configuration ........................................................................................ 9 Figure 2 - Serie configuration ............................................................................................. 9 Figure 3 - Power split configuration ................................................................................. 10 Figure 4 - Torque-speed characteristic for induction motor ............................................. 11 Figure 5 - Duty Cycle ....................................................................................................... 13 Figure 6 - The Converter - Fundamental Circuit .............................................................. 14 Figure 7 - Singel phase full-bridge inverter ...................................................................... 16 Figure 8 - Three phase waveforms .................................................................................... 16
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1 -‐ Introduction The electric vehicle is gaining more and more popularity in today's society, especially in the context of growing concerns about the environment and fuel costs. These vehicles might represent a large amount of fast responding power and energy storage potential. The grid operator can improve power flow in the network by using the batteries as buffers. Thereby the vehicles could provide extra power during demand peaks. In this paper different technologies used in electrically driven vehicles will be presented and discussed to find the best solution for a growing market and a shift in the consumers interest in the environment. The switch-mode converter is described. It is used to enable power flow in both directions between the battery and the motor. Suitable materials are developed for the purpose of the batteries used in the different configurations, to secure optimal lifetime of the batteries.
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2 -‐ Engine types
2.1 -‐ Internal combustion engine An internal combustion engine utilizes the expanding power caused by the ignition of pressurized fuel, in order to create propulsion. The process of converting gasoline into propulsion consists of injection, pressurizing, ignition and expansion. The expansion drives the piston, which in turn drives the shaft of the car. ICE is well known technology, with a lot of infrastructure built up around it. It became popular in parts because liquid fuel is easy to handle and contain a lot of energy, giving the car a long driving range and a lot of power. The availability of fossil fuels has traditionally been quite good. The engine produces a lot of noise and releases unwanted gases like carbon dioxide, which can be a health and environmental hazard in cities. ICE has a thermodynamically theoretical maximum efficiency of 37%1, and an average of approximately 20%. The poor efficiency is partly related to the size of the engine, most engines are over dimensioned because it need to produce a large amount of force to accelerate at low speeds.
2.2 -‐ Electrical engine The electrical machine consists of two main parts, the stator and the rotor. In a synchronous machine with permanent magnets, the magnets are placed on a cylindrical rotor, which in turn is placed inside the stator. Coils are placed on the stator, and when three-phase alternating current are applied to the coils, a rotating magnetic field is created. The field from the rotor will align with the stator field and rotate the axle. The permanent magnets produced by rare earth materials are expensive due to poor availability. In an induction machine, the rotor has metal bars instead of magnets. When a rotating magnetic field from the stator affects these bars, a current is induced producing a magnetic field. As long as the fields do not align, a torque is acting upon the rotor. If the rotor field rotates faster than the stator field, the machine will generate power. The efficiency of an electrical machine is a lot higher than for an ICE. For example, a 50 kW induction motor from Siemens has an efficiency of 87,7 %, while a similar 85 kW motor from the same manufacturer has an efficiency of 92,1%2. For the ones typically used in cars, the efficiency is about 90 %. There are no release of harmful gases, and very little noise. The machine itself requires little maintenance since it only has two moving parts.
1 http://courses.washington.edu/me341/oct22v2.htm 2 http://www.industry.usa.siemens.com/drives/us/en/electric-drives/hybrid-drives/Documents/elfa-components-data-sheets.pdf
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3 -‐ Vehicle configurations There are many ways to configure the motors of electrically (full or partially) driven cars3. The torque is supplied to the wheels by an electric motor that is powered by a battery alone, or in combination with an ICE. In this paper battery electric vehicles (BEV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV) are presented.
3.1 -‐ Battery electric vehicle (BEV) Electric vehicles are solely propelled by electric motors. Chemical energy stored in battery packs supplies the motor with power. The battery pack can be recharged during regenerative breaking, and through the main electricity grid at stand still. Since electric vehicles are propelled only through electricity there are no emissions associated with the use of the car. The emissions from the car must be traced back to the production of the car and emissions during production of the electricity, if such exist. It is possible to use the momentum from the vehicle through the tires when driving downhill to reverse the power flow in the motor, acting as a generator, and then charge the batteries. This is done automatically. Limitations in the battery technology have made the driving range for such vehicles relatively low. This is in many ways the greatest challenge for the manufacturers of electric vehicles today. As long as the range is limited the electric vehicle will not be able to compete fully against ordinary combustion vehicles. By driving smart and letting the engine work at maximum efficiency most of the time, it is possible to minimize current drawn from the batteries and increase the range. Accelerating calmly and letting the batteries recharge during braking and downhill driving will contribute to this effect.
3.2 -‐ Hybrid electrical vehicle (HEV) The hybrid vehicle is a compromise between the electrical and the ordinary combustion car. The efficiency and low emissions from the electric vehicle are exploited, as well as the range of cars with combustion engines. It is common to distinguish between mild and full hybrids, where the main difference is how much the electric engine participates in propulsion of the car. A mild hybrid is depending on constant torque from the combustion engine to stay running. The electric motor is only used to help when high amounts of energy are needed. An example of this can be during strong accelerations. In this way the losses from the combustion engine are reduced compared to the engine operating alone. The electric motor is not able to propel the car by itself. However in a full hybrid the electric engine can operate the car alone. The two motors can work either alone or together to maximize energy efficiency. For example the combustion engine can be turned off when driving slowly, hence reducing the emissions an idle combustion engine would produce. Further, the engines will work together when much power is needed. The ICE can charge the batteries when driving at cruising speed, where this engine has relatively good efficiency.
3 Plug-in Hybrid and Battery-Electric Vehicles: State of the research and development and comparative analysis of energy and cost efficiency - Françoise Nemry, Guillaume Leduc, Almudena Muñoz
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3.3 -‐ Plug-‐in hybrid electrical vehicle (PHEV) The IEEE defines a PHEV as any hybrid electric vehicle with a battery storage system of at least 4kWh, which enables recharging from the main grid and from regenerative breaking. The car must have an all-electric range (AER4) of at least ten miles.
The fact that the batteries can be recharged makes this car a better alternative than ordinary hybrids since the electric motor contributes to more efficient transportation. Fuel cost and emissions will be reduced. When the batteries are depleted the car will automatically use the combustion engine for propulsion until the batteries are charged again.
4 -‐ Technological architecture There are different kinds of transmission design in HEVs. It is separated between series (figure 2) and parallel (figure 1) 5. Another configuration is the combined series-parallel, called power split (figure 3) 6. In the parallel configuration the electric motor and the ICE are connected in parallel, separated by a differential gear. This way both engines can work together or separately. This is the technology used in the Insight, Civic, and Accord hybrids from Honda7. When power demand is low the ICE runs the electric motor as a generator recharging the battery. In the parallel configuration the inefficient conversion of mechanical energy to electric energy is eliminated since the ICE propels the wheels directly, making the parallel set up efficient in highway driving. The batteries in this type of configuration may rely only on regenerative breaking because it is possible to use smaller batteries. Parallel hybrids can further be categorized depending on the balance between the two engines. For example can the combustion engine be the dominant one, thus providing most of the power. In such a configuration the hybrid would be categorized as a mild hybrid. The opposite situation is categorized as a full hybrid. In the series configuration it is the electric motor alone that propels the vehicle. The ICE provides torque to a generator supplying the electric motor with power. Excess power is stored in the batteries. During energy intensive loads the electric motor can use both the batteries and the ICE as power sources. There are no mechanical connection between the ICE and the wheels making it possible to run the motor at constant speed independent of the car speed. This way the ICE can be run at as high efficiency as possible, all the time. The vehicle will use energy from the batteries during starts and stops, this reduces the use of the ICE in slow traffic and in cities. This configuration performs best in stop-and-start driving which makes it most appealing for busses and urban use vehicles. The ICE is typically smaller in series configuration because it only has to meet average driving power demand, while the battery storage is more powerful than in the parallel configuration. 4 Plug-in Hybrid and Battery-Electric Vehicles: State of the research and development and comparative analysis of energy and cost efficiency - Françoise Nemry, Guillaume Leduc, Almudena Muñoz 5 http://en.wikipedia.org/wiki/Hybrid_vehicle_drivetrain 6 http://en.wikipedia.org/wiki/Hybrid_vehicle_drivetrain 7 http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html
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The power split architecture is a merge of the series and parallel configuration. The engine can either propel the wheels or be disconnected, in combination with the electric motor. This configuration combines the duality of the engines to make the ICE operate as often as possible close to its optimum efficiency. At low speed the series configuration is applied, while the parallel configuration is used at higher speeds. The duality is regulated considering optimization of loss. This configuration performs better than the series or parallel configuration alone. Toyota Prius and Chevy Volt are two vehicles based upon this concept8. It is important to configure the vehicles architecture related to the use of the vehicle. Series and parallel configuration both has its benefits regarding driving patterns. Series is more efficient in slow traffic and city driving, while parallel has the benefit of a long driving range and becomes more efficient on highway driving. The power split configuration has the benefits of both designs, although the higher asset price, this configuration in combination with possibility of charging the vehicle from the main grid will be the best option for the modern family. This configuration uses the benefits of the electric car in short range driving, and the hybrid configuration in long range driving. The Lexus Hybrid Drive is an example of a power split system with two electric motors. There are electric motors placed in the front and in the back of the car. The ICE is placed in the front. The rear electric motor will only contribute to propulsion when energy demand is high. In this case all three motors will run. The front electric motor is able to power the car by itself and will do so when energy demand is low, allowing the ICE to be disconnected.
Figure 1 – Parallel configuration
Figure 2 - Serie configuration
8 http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=06237548&tag=1
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Figure 3 - Power split configuration
5 -‐ Efficiency and emissions The emissions associated with the electrical vehicle depend on the source of energy production. If the main energy source is renewable, as in Norway, the emissions will be negligible compared to an ICE car. Reductions in pollutions as HC, CO, NOx, SO and particles is dependent of the energy mix. If zero emission sources such as nuclear, hydro, solar or wind power are used, the pollutions will be significant reduced. If an energy mix of coal- and oil-fired plants is used, the production of HC, CO and NOx will be reduced, but the amounts of SO2 and particles would increase9. Electric vehicles have less noise pollution than an ICE, independent of if it is at standstill or in motion. There are different ways to rate the efficiency of an electric vehicle. “Well –to wheel” and “tank –to wheel” are two different measurements of efficiency. The well –to wheel efficiency has far less to do with the vehicle itself, and more to do with the energy production. The efficiency of the vehicle, regarding emissions, could be directly related to the energy mix used in the production of electricity and the losses during transmission. The “tank –to wheel” efficiency of an EV is, according to the European Commission10, three times higher than for ICE vehicles. For instance the EV do not consume energy when stationary while the ICE consume fuel constantly.
6 -‐ Motor drives
6.1 -‐ Induction motor drives In an induction motor11, the synchronous speed is dependent upon the frequency of the voltage applied to the stator coils. Provided that the slip is small, the rotor speed varies linearly with synchronous speed. By keeping the magnetic flux in the air-gap between stator and rotor constant, the torque produced by the motor becomes a linear function of slip frequency. Assuming constant slip, slip frequency is a function of applied frequency. The flux is expressed as the relationship between applied voltage and applied frequency. Meaning that the voltage must be increased linearly with the frequency, if not the produced torque will decrease.
9 http://www.electroauto.com/info/pollmyth.shtml 10 http://ec.europa.eu/transport/urban/vehicles/road/electric_en.htm 11 Chapter 14 – Induction motor drives – Power Electronics by Mohan, Undeland, Robbins
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When the motor is running at high speed, the flux is reduced to increase motor speed even further. The motor will normally operate between points C and E, see figure 412. Power electronics used to control the motor must be able to vary both frequency and amplitude of the applied ac voltage. It should be able to carry a continuous current regardless of frequency, and have bidirectional power flow. A computer and the battery will control the voltage. A full-bridge DC-DC converter may be installed between the battery and the DC/AC converter, in order to achieve a stable DC voltage between the battery and converter. There is a potential for a large current flowing in to the motor, during start-up of induction motors. This follows from the torque needed during start-up being very high, requiring high flux and slip frequency, again resulting in high currents13. A computer, ECU, limits the start-up frequency to avoid this. When the machines rated voltage is reached, and further increase is no longer encouraged, increasing the frequency will result in gains in speed at cost of reduced torque. The motor will then enter a constant power region where it draws maximum power from the battery, until the motors max speed is reached. After this, the motor enters its high-speed region, where torque rapidly decreases as a response to increased frequency and motor speed.
Figure 4 - Torque-speed characteristic for induction motor
12 http://ecmweb.com/motors/understanding-induction-motor-nameplate-information 13 Chapter 14.4.2 Power Electronics by Mohan, Undeland, Robbins
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6.2 -‐ Synchronous motor drives In a synchronous motor14, the rotor field is often supplied by permanent magnets installed on the rotor. It may also be supplied by windings on the rotor that are applied a dc-voltage. The stator is the same as in an induction machine. As for operation, the main difference is that torque is proportional to applied stator current, provided the flux is kept constant which is the case when using permanent magnets. This is the motor commonly used in electrically powered vehicles. The speed depends on applied frequency, since motor speed equals synchronous speed.
7 -‐ Batteries The energy storage capacity determines the distances the vehicle can drive, as well as the space the batteries will occupy. In a BEV the battery capacity varies in the range of 30-50 kWh, in a HEV 1-2 kWh and in PHEVs the capacity needs to be above 4 kWh, according to IEEE15. PHEVs usually have a capacity in the range of 4-30 kWh. In the discussion considering battery capacity, it is of great importance to take into account both high specific energy and energy density. It is crucial to have high capacity of energy storage at the same time as the additional mass of batteries is within acceptable margins.
7.1 -‐ Materials In mass-produced HEVs Nickel Metal Hydride (NiMH) batteries are currently used. NiMH batteries are considered to have reached their technical and economical potential. Lithium-ion batteries offer higher energy density and a lower discharge rate than NiMH. Li-ion batteries are the best option to meet the requirements set by PHEV and BEV. The availability of lithium poses no challenge in this century.16
7.2 -‐ Performance The lifetime of the electric car and the batteries is essential when a customer decides to purchase a car. The battery needs to have a high resistance against degradation over time and must be able to undergo many deep discharge cycles. There are two different modes of driving related to the batteries state of charge (SOC). In charge depletion-operating mode (CD) the energy stored in the battery supplies the motor with power. The SOC curve gradually decreases towards a minimum level of around 20%. The technology related to energy storage in BEV is crucial and the battery must be able to undergo deep charging cycles. In the second driving mode, called charge-sustaining mode (CS), the SOC will have an average value remaining at its initial level. This means that the ICE and the regenerative breaking ability are charging the battery. HEVs operate in CS mode and require therefor only micro cycles, while BEVs require deep charging cycles. PHEVs batteries must be designed to comply with both modes. In figure 4 the duty cycle is illustrated.
14 Chapter 15 – Synchronous Motor Drives – Power Electronics by Mohan, Undeland, Robbins 15 Plug-in Hybrid and Battery-Electric Vehicles: State of the research and development and comparative analysis of energy and cost efficiency - Francoise Nemry, Guillaume Leduc, Almudena Muñoz 16http://www.eenews.net/assets/2011/07/27/document_gw_02.pdf
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Table 1 - BEV, HEV and PHEV Energy and Range17
Figure 5 - Duty Cycle
8 -‐ The converter AC motors have become the most popular choice for electric vehicle propulsion. Since the battery only is capable of supplying DC voltage it is necessary to have a converter acting between the battery and the motor. The most frequently used three-phase inverter circuit is made of three converter legs, one for each phase needed as output to the AC motor. The fundamental circuit is shown in figure 518. Each leg consists of two switches and two diodes. The capacitors are of equal high magnitude and eliminate possible DC components in the output current i0. The switching intervals are determined in a variety of different ways but the idea is that a control signal of equal frequency as the desired output frequency is compared to a triangular waveform. Each phase has its own control signal separated 120° from each other.
17 Plug-in Hybrid and Battery-Electric Vehicles: State of the research and development and comparative analysis of energy and cost efficiency - Francoise Nemry, Guillaume Leduc, Almudena Muñoz 18 Chapter 8-4; Power Electronics – Mohan, Undeland, Robins
BEV HEV PHEV AER (miles) 150-200 0 10-60 Battery Material Li-ion NiMH Li-ion Energy storage (kWh) 30-60 1.3 4-30 Specific energy (Wh/kg) 110-160 46 110-160 Energy density (Wh/l) 400-600 200-350 400-600 Deep charging cycles >2500
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For example, if vcontrol, A > vtri the circuit is switched so that vA = Vd. Repetition of this for every period generates the fundamentally sinusoidal waveform in figure 6 in appendix. This is done for every phase producing a set of three 120° spaced sinusoidal voltages which is what we want as input for the asynchronous motor. When fed with inverted power from the battery the asynchronous machine will act as a motor driving the vehicle forward. However, if the flow of power is reversed the energy needs to be put back into the battery. The converter is therefore often referred to as a switch-mode converter. The per phase waveform generated by rectification are shown in figure 7 in appendix. It is evident that with three phases spaced 120° from each other will produce a fundamentally constant current Io that is supplied to the battery. The switch-mode converter is therefore well suited for the needs of electric vehicles and is perhaps the most fundamental component in electric vehicle design.
Figure 6 - The Converter - Fundamental Circuit
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9 -‐ Conclusion At the moment the electric vehicle is a well-known concept. The increase in knowledge amongst the consumers and the rapid development of EV technology has made both full and partially electric cars a common sight. It is evident that we are now seeing a change in the worldwide car park where cars with ICEs are replaced with HEVs and BEVs. There are big expectations to the further development of this technology in the years to come and it is certain that this vehicle type is a better substitute to ordinary cars than the hybrids of today. Synchronous and induction machines were briefly discussed in context with AC motor drives. The induction motor is preferred for most EV applications because of its ability to be controlled through changing frequency and field current. An AC/DC switch mode converter has been investigated. It uses transistors and diodes to invert DC-current to a set of three phase currents powering the induction motor. Power flow can be reversed through the same device to rectify three-phase AC current to DC when driving downhill and breaking. This power electronic component is probably the most important engine part and is needed for all kinds of electric vehicles to be efficient. Batteries with high energy density and low discharge rates are crucial for the future of all electrical vehicles. The Li-ion batteries have proven to be the best option today. Weight reduction and durability increase of the batteries should be of high concern. The technology that pays best off when considering use and economy is highly dependent on the consumer’s intentions for the car. If the consumers every day driving distance is below the range of a BEV, this might be the preferable option because of the low fuel costs. Longer drives might provoke the need for a hybrid with a backup combustion engine. The PHEV is applicable for both uses and combines the benefits of both types, at the same time as gaining extra weight. There are still many problems that have to be solved before electric vehicles can be considered as a better option than ICE vehicles.
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Appendix
Figure 7 - Singel phase full-bridge inverter
Figure 8 - Three phase waveforms
