The use of stored electrical energy as the sole source of electricity has not been substantially developed in the transportation sector, except for those vehicles with dedicated routes, such as trams, trolleybuses, subways, the use of fixed electrical installations with catenary and telescopic booms trains, and trains that take electricity from the ground. Electric road vehicles or other functional vehicles are generally used only for duty (for example, conveying tasks in confined areas such as airports and factories), but their economic and environmental advantages are disproportionate to the cost of consumption due to the small scope of application. In the development of electrified transportation, only encouraging policies that can greatly improve battery performance can lead people to change their ideas.
The adoption of full electrification in the transportation sector has always been the goal of electrical experts, and the benefits are:
1) Use clean energy without generating waste and pollution.
2) The same system can realize both power output and control.
3) Braking energy recovery.
Achieving pure electric drive remains a long-term goal, and the main obstacle is electric energy storage, which is incomparable with hydrocarbon-based fuels. Roughly calculated, IL gasoline can provide 40MJ of energy, and the fuel tank can store 10MW of power. The energy density of gasoline is so high that no other energy storage system can match it, as shown in Figure 1.
Figure 1 Volume energy density of typical fuels and comparison with batteries
At the same time, it is difficult to imagine what a cable with a load capacity of 10MW would look like. This necessitates a substantial increase in the transmission voltage level to reduce the cross-sectional area of the cable, which in turn creates safety concerns. When the voltage is 500V, the current will reach 20000A, even if the energy storage system can absorb such a large current, the cross-sectional area of the copper wire reaches 2000mm², and the current density is 10A/mm² (this is already a very high value) , the wire diameter will be up to 50mm. Therefore, the copper required per meter of wire is about 13kg. Not only that, but the losses on the wires are far greater than the gasoline pump.
Compared with the internal combustion engine, the electric motor has a higher conversion efficiency, which makes up for the shortage of energy storage and transmission to a certain extent. We'll review in-depth the constraints associated with using electricity to drive a car. The first electric car is very old, called the "Electrobat," and was built in Philadelphia, USA, in 1894. Peugeot's "Jamais Contente" electric car in 1899 traveled more than 100km/h. Due to the rapid development of internal combustion engines and bottlenecks in battery energy storage technology, the development of electric vehicles has come to an abrupt end. Its production ceased completely in 1918, and restarted after a long time. At present, the various types of electric vehicles that are about to go on the market or have already come on the market mostly revolve around the internal combustion engine, which has begun to be replaced by the electric motor.
At first, DC motors with brushes were widely used, which controlled the speed of the motor by changing the armature voltage. This type of motor is very suitable for electric traction (this type of motor is still widely used in electric locomotives, subways and trams from the past to the present). The low-speed torque of the DC motor is high and allows direct drive, eliminating the need for a gearbox.
There are also quite a few using AC motors, which are more reliable, require less maintenance, and can convert more power. The AC motor is controlled by an inverter connected to a DC bus. The most reliable motors are asynchronous motors (no brushes required), in which a three-phase full-bridge inverter controls the voltage and frequency on the motor's stator, thereby keeping the motor's magnetic flux constant. Vector control technology is ideal for controlling asynchronous motors and efficiently controlling the speed and torque of the motor. Due to losses in the motor rotor, the overall efficiency of an asynchronous motor in generating torque is lower. From this point of view, a synchronous motor is the better solution. In addition, synchronous motors also allow the inverter to operate at a more optimal operating point if the magnetic field can be changed.
Although the motor is very suitable for high-speed occasions, the direct drive system can only reach 400r/min at most. Variable reluctance motors are a good solution. The first implementation was proposed by the Jarret brothers, but other structures may be more reasonable. These schemes usually consist of a multi-tooth rotor and a stator with multi-phase windings that pass through the variable frequency voltage, and the motor can also be equipped with permanent magnets to improve performance. Although this type of motor can easily achieve variable frequency speed regulation, it has two main disadvantages: heavy weight and vibration.
The easiest way is to use a self-synchronizing motor, which has the same external characteristics as a brushed self-excited DC motor. For brushless motors, the inverter is a simple current commutator. The synchronous motor has permanent magnets to ensure excitation; the inverse structure of the stator inside and the rotor outside allows us to mount the motor directly on the outer edge of the wheel. This technology is used by the Canadian company TM4 on the 18.5kW motor. The French company Alstom used the asynchronous motor on the wheel drive of the bus and tested it on a mobile workbench (EC-CE workbench) equipped with a 30kW motor. The technology will result in a new type of car structure that saves space inside the car. But it has three disadvantages: ① There is no mechanical connection between the motors, so the corresponding operating point must be set for the inverter that controls the motor to achieve mechanical differential action (electronic differential); ② The position of the inverter and its controller It must be planned reasonably; ③The motor connected to the wheel increases the sprung mass and affects the driving effect of the vehicle. However, if the technology is used on all four wheels of the vehicle, it is easy to install ABS, anti-skid, recuperation and four-wheel drive.
The classic solution is to connect an electric motor directly to the mechanical differential, while functions such as ABS are achieved through conventional techniques. But in any case, the most important thing is to make the driver feel like driving a traditional car, especially in terms of motor braking by controlling the power electronics.
There are four battery technologies. It can be used in electric vehicles, namely lead-acid batteries, nickel-hydrogen batteries, lithium batteries (lithium ion or lithium polymer batteries) and nickel chloride batteries.
The weight of an electric vehicle is the primary issue to consider when designing, and the battery accounts for a large part of the weight of the vehicle. Although the reference data given in some literatures are not completely consistent, the weight of the battery in the whole vehicle is still obvious. In order to continuously drive 100km autonomously, an electric vehicle needs 30kW.h of electricity, 850kg if it uses a lead-acid battery, and 270kg if it uses a lithium-ion battery. At the same time, the placement of these batteries is also problematic, and it is not optimal to place them in a cube. Lithium or NiMH batteries are structurally flexible and can be easily installed in cars. Therefore, it is necessary to design for the needs of electric vehicles from the beginning, a trend that can be fully reflected in the so-called second-generation electric vehicles.
Battery technology currently faces many difficulties:
(1) Energy density:
The energy density of lead-acid batteries is 40W·h/kg, while new batteries such as lithium batteries or Jin-hydrogen batteries can reach 220W·h/kg and 100W·h/kg, respectively.
(2) Number of charge and discharge cycles:
Refers to the number of charge and discharge cycles that a battery can withstand without affecting its capacity. Unlike batteries used in combustion engine vehicles, batteries in electric vehicles have to be deeply discharged, which can seriously affect their lifespan. Among high-energy batteries, lead-acid batteries have a very low cycle life of 180 cycles, and lithium and nickel-metal hydride batteries are slightly better, but only 1,000 cycles. Although high-power batteries can achieve 1,000, 20,000 and 25,000 cycles, respectively, they are not suitable for electric vehicles.
(3) Self-discharge rate:
For nickel-metal hydride batteries, the charge continues to decrease due to the diffusion of hydrogen into the brocade electrode, causing self-discharge.
Lithium batteries are very expensive compared to lead-acid batteries, and NiMH batteries are slightly cheaper than Lithium batteries.
(5) Low temperature characteristics:
The performance of lead-acid batteries will be reduced in low temperature environments, and this characteristic is similar to other batteries, especially lithium batteries. And this problem is very critical, because electric vehicles should be able to start normally at -20 ℃.
Lead-acid batteries can be recycled by the manufacturer's commitment, and if other types of batteries are to be used in large quantities in electric vehicles, the relevant manufacturers should also form recycling methods that can be industrialized.
3) The efficiency of the battery
Due to the existence of various losses, the energy stored in the battery cannot be fully utilized. Measured in Faraday efficiency, newer batteries such as nickel-metal hydride or lithium batteries have efficiencies close to 1, while lead-acid batteries are a little less. Its efficiency is related to the Joule effect of the battery and the losses of the power electronic converter in the battery charger. In order to make full use of the capacity of the electric vehicle battery, the overall efficiency of the battery energy storage system (usually 70% to 90%) is a very important parameter.
4) Voltage class
There is no uniform standard for the voltage level used in electric vehicles, and it is completely customized by each manufacturer according to the requirements of the optimized design, mainly considering the constraints of the vehicle's mobility, weight and volume. The voltage level of the DC bus is usually set at 300V, but the original design was only about 100V. Regardless of the design, the batteries need to be connected in series and used in groups, so the battery group must be fully charged frequently to ensure the voltage balance between the individual battery cells.
5) Battery characteristics
Generally speaking, the main characteristics of batteries include energy density and power density, application fields, number of charge and discharge cycles, high and low temperature characteristics, self-discharge rate, aging, life, and price.
For an electric vehicle, the battery is its only source of energy. In a conventional car, the gasoline or diesel gauge gives a very clear indication of the remaining fuel so that the onboard ECU can calculate how far the car can go, and the same should be true for electric cars. Therefore, the state of charge (SOC) of the battery must be monitored to correctly calculate the distance the car can continue to travel. It is not easy to measure the state of charge of lead-acid batteries, and corresponding measurement technologies have emerged for Jin-hydrogen batteries and lithium batteries. If the state of charge of the battery at a certain time is determined, the increase or decrease of the battery charge can be calculated by integrating the current over time. Theoretical values for battery energy density are not readily available unless the generator draws very little current over a long period of time. Once its current density is non-negligible and the system is emitting power, the potential difference shrinks due to the polarization effect of the battery and the impedance of the electrodes to their active components, which creates a certain voltage drop. Therefore, only the information under no-load condition can accurately judge the state of charge of the battery, and the battery must stand for a period of time so that the charge can fully participate in the reaction. At the same time, it must also be clear that the state of charge of the battery is related to its temperature.
One performance metric for electric vehicles is depth of discharge (DOD), which describes the rate of discharge that is allowed without permanent loss of battery capacity. If used for electric traction, it is best to choose a battery with a higher depth of discharge (such as 80%).
Here is another relatively subjective performance indicator - state of health (SOH), which considers the battery's progressive damage, rechargeable capacity, internal resistance, and voltage and current during self-discharge. Therefore, for electric vehicles, the state of health of the battery is as important as the state of charge.
6) Auxiliary functions of electric vehicles
In addition to the need for electricity for traction, electric vehicles are also necessary to power some auxiliary functions. We have already seen the electrification of auxiliary functions on conventional petrol or diesel cars, and in electric vehicles, all auxiliary functions except heating and air conditioning can be implemented in the same way. There is no longer a large heat source on electric vehicles, and even if the inverter or motor needs to be cooled, the temperature rise level of its heat generation is far less than that of the radiator of a traditional engine.
Satisfying heating requirements with small burners burning hydrocarbon fuels is unimaginable on an electric vehicle, and it would be a shame to dissipate so much electricity in electrical resistance. Corresponding solutions include: electrothermal cooling (Peltier effect) or magnetocaloric cooling, but the latter is still under investigation. The auxiliary functions of electric vehicles are one of the main parts of energy consumption, and improving their efficiency can increase the mileage of the car.
Not only does the cockpit need to be cooled, but also parts of the electrical drive system, such as circuit boards, motors, and batteries or supercapacitors. Their operating temperature range is narrow and generally below 60°C. Other types of batteries must be maintained between 60 and 80°C to maintain optimal conditions (eg, Li-polymer batteries).
7) Charging the battery
Charging the battery is an important issue. The charger can be external or integrated in the car to charge the battery through the traditional AC grid. Charging the battery by electromagnetic induction rather than physical connection has always been imagined, but even at high operating frequencies, the efficiency of the system cannot reach a satisfactory level, because the two elements that rely on electromagnetic coupling for energy exchange Devices are susceptible to various interferences.
Here is a charging example of a 98A.h lead-acid battery, and its discharge limit is given by the manufacturer for reference only.
(1) The amount of electricity that can be released at a 5h discharge rate is 98A·h.
(2) The discharge time at 200A is 14min.
(3) The normal charging current is 19.6A.
(4) The fast charging current is 39.6A.
(5) The transient peak charging current is 100A.
(6) The peak value of discharge current within 1min is 450A.
Other types of batteries should also report similar data.
As you can imagine, fast charging is generally used to charge cars outside of the garage. Moreover, the power of the external charger must be large, which requires more up-front investment and a mature business operation model, as well as standardized interfaces. An average garage will have much less power, as only the car battery needs to be charged in the normal way. Regardless of the situation, a safe and standard quick connection is required. In addition, the charger should be able to adapt to different types of batteries, and it seems that integrating the charger in the car is the only feasible solution, but this will add a lot of weight to the car.
It is also possible to replace a standardized battery, that is, to replace a depleted battery with a fully charged battery. But this requires specialized swap facilities and ample battery reserves. Currently, this method is only available for fleets that are dispatched.
8) Range extender
In order to make up for the loss of battery life in electric vehicles, a small generator can be installed on the vehicle to charge the battery when the state of charge is too low. The internal combustion engine runs at a constant speed to maintain a high efficiency. This solution will not affect the normal operation of the electric vehicle, but there is a fact that cannot be ignored, that is, the DC bus voltage will increase due to the change in the direction of the battery current. The generator provides part of the power to the propulsion engine, and the mode is very similar to that of a hybrid. Such electric vehicles, which are charged from the grid, are often referred to as "plug-in hybrids," as shown in Figure 2.
Figure 2 Schematic diagram of an electric vehicle with a range extender
Fuel cells are commonly referred to as proton exchange membrane fuel cells (PEFCs) and can also be used as range extenders. The AUXIPAC planned by the French company Axane, and the concept bus (H2O car) that Peugeot has already demonstrated, use this technology.
9) Typical second generation electric vehicle
CleanNova and BlueCar are two electric vehicles that are about to be put on the market in France. The car plans to use lithium-ion or lithium-polymer batteries to obtain much higher energy density and shorter charging time than lead-acid batteries, and the battery is not easy to age. The characteristics of the two cars are summarized as follows:
CleanNova is manufactured by SVE (Dassault-Heuliez). It has an electric motor with integrated differential provided by TM4, which can work at different voltage levels; uses 16~30kW·h lithium-ion batteries; charging time is 8h (16A), 4h (32A) or 30min ( 150A); use a heat pump for heating. When configured with a range extender, an additional small generator can be decoupled from the propulsion motor and meshed with the traction motor, acting as an electric motor and outputting torque.
BlueCar is manufactured by Batscap in France. It is equipped with a motor with a rated speed of 10000r/min and a power of 30kW; using a 27kW.h lithium metal polymer battery, the charging time is 6h, and the expected life is 10 years or 150000km ; The voltage range of the DC bus is 243~375V; the cruising range is 200~250km; the total battery weight is 200kg; the maximum speed is 125km/h.
In addition, according to relevant information, Chevrolet Motor Company announced that an electric vehicle it manufactured has a cruising range of 60km and uses a 180kg lithium-ion battery.
We can also focus on recreational vehicles like those used on golf courses, or other four-wheeled electric vehicles that are lighter or heavier, but with special restrictions on travel speeds. This type of car can use 12 48V, 240A batteries, and its cruising range can reach 100km. In order to reduce the cost of battery replacement, lead-acid batteries are often used.
10) Energy management and modeling
Energy management in electric vehicles, like hybrids, must be considered from the very beginning of vehicle design. The design is mainly based on the model of the car (mechanical model, road tire contact model), the elements on the traction chain (motor, power electronics, battery), but also the necessary auxiliary functions or comfort functions. The energy is managed by an on-board ECU, which takes into account the battery state of charge and the driver's commands. We can even imagine an intelligent management system that takes into account the actual journey situation (start-point and distance given by GPS).
2. Heavy goods vehicles and buses
There are also examples of electric buses. GEPEBUS (a branch of French Gruau and Ponticelli) has launched two electric buses with 22 and 25 seats. The Eu-ropolis series buses of IRISBUS are equipped with a 140kW motor and sodium nickel chloride battery (Zebra) battery, with a cruising range of 120km.
3. Two-wheeled vehicles
We have already discussed four-wheeled vehicles, but the essence of the problem is the same for two-wheeled vehicles such as electric motorcycles or power-assisted bicycles. Electric motorcycles are far less popular in Europe than they are in Asia, and a French manufacturer has stopped production due to a lack of market demand, but also because the design of the product is more suitable for motorcycles with internal combustion engines than electric motorcycles. The space in the car is so small that the battery that can be carried is very limited, so the power supply voltage level is only 36V, and the power of the propulsion motor is only 2kW.
Electric motorcycles can be redesigned using in-wheel motor technology, while energy storage that combines NiMH batteries and supercapacitors is well suited for braking energy recovery. Space constraints in the car make integrating the charger a problem, and pulling the battery stack out for charging is a more reliable method, but this requires a stable and secure electrical connection system. The average capacity of batteries used in electric motorcycles is 20A·h.
All power-assist bicycles use electric and existing motorized control technology: the engine is placed on the front or rear hub, providing power to the pedals and friction on the tires. Even if there is the possibility of human riding, the cruising range of a power-assisted bicycle is directly related to the energy stored in the battery. The power of the power-assisted motor is 150 ~ 250W, and a well-trained cyclist can make it run at 250W for 1 hour. above. If a 36V, 7.5A·h NiMH battery is used, the motor can be directly driven without raising the voltage level. Moped batteries are usually removable for easy charging.
We can describe recreational two-wheeled vehicles in the same way, such as electric scooters or, more specifically, Segway electric bicycles.
4. Guided vehicles (trains, subways, trams, trolleybuses)
For guided vehicles such as trains, high-speed trains, trams or trolleybuses, the drive does not have any impact on the energy storage. The driving of railway locomotives is done by pantographs and telescopic booms on the railway, and some countries still use the trolley wire on the third track (the power supply track) to provide energy for the locomotive. The power supply situation of the subway is similar to that of the railway. The power supply scheme of the trams running around the city is more diverse, and they generally need to be equipped with a guided power supply network.
For hot tourist attractions, people are studying the power supply scheme of rail locomotives without using pantographs. The method of taking electricity from the ground has begun to be applied, but there are also studies focusing on the method of on-board energy storage. There are currently two energy storage technologies in favor, namely inertial flywheel energy storage and supercapacitors. Although it is possible to set up charging stations for flywheel energy storage at each parking site, the problem is how to transfer a large amount of energy in a short period of time. For example, a flywheel energy storage system on a tram can provide 3kW.h of energy and 300kW of power. An older application like an energy storage trolley can carry a 3.3kW.h flywheel energy storage and capacitor bank at the same time.
Energy storage, whether in the form of mechanical energy, electrical energy or chemical energy, can realize the recovery of braking energy, thereby reducing the energy consumed by the vehicle during driving, and can accelerate the locomotive when it starts again. It can achieve 15% energy saving, and because the operating characteristics are fully grasped, it is also more convenient in management.
The stored electrical energy is necessary to perform auxiliary functions when the conventional power supply system fails (such as a catenary break). In the event of a sudden stop, the trains, subways or trams must have electrical power for their internal signals, and lighting and air conditioners must be ensured as far as possible. These emergency battery packs are generally installed in passenger compartments, and trains also use 72V lead-acid batteries for energy storage.
Finally, we imagine ways to deliver more energy, such as fuel cell generators. If a polymer electrolyte fuel cell (PEFC) is used, storage and recovery equipment or a hydrocarbon reformer is also required; or an auxiliary power unit (APU) with a solid oxide fuel cell (SOFC). For the latter, hydrocarbon fuels such as gasoline or petroleum are the source of energy.
5. Maritime traffic - yachts
In order to improve passenger comfort (reduce noise and pollution), it is necessary to replace the internal combustion engine of boats traveling on canals or lakes with electric motors. The yacht does not require much starting and the speed of the boat is low to allow ample sightseeing time for the passengers on board, so the electric motor does not need a lot of power, and the range depends on the number of batteries the yacht can carry. There is enough space in the cabin to place the battery, and the charging can be completed when the yacht is docked at night, but the ventilation of the cabin must be ensured during charging. Such yachts can be seen on the Canal Saint-Martin in France or the banks of the Doubs in Switzerland.
Other electrical applications include small pleasure boats, or driving a small auxiliary motor, or powering navigational instruments.