Electric Vehicles

 

Electric Vehicles

 

Introduction

Electric vehicles have been on the horizon of the transport world for many years. In certain areas of transportation they have already become the standard, e.g., electric trains, trams and increasingly certain types of buses. Although the technology for electric vehicles and the potential for its development have existed since the late 1800’s, both eventually took a backseat to the increasing dominance of fossil fueled propulsion systems, which became cheaper and allowed for travelling longer distances. However, with current advances in technology and concerns over greenhouse gas emissions and energy sustainability, the electric vehicle is seen as one of several solutions for the future of transportation due to its having the potential to be clean as well as that it can obtain energy from a variety of resources.

Types of Electric Vehicles

Electric vehicles include cars, trucks, motorcycles, boats, trains, trams, buses, airplanes and spacecraft, or virtually any form of vehicle. Defining what is an electric vehicle involves considering the many motor and energy storage configurations that are possible. Vehicles that run on electricity include:

  • Battery powered electric vehicles, e.g., electric cars
  • Gasoline powered electric vehicles, e.g., hybrid vehicles
  • Fuel cell powered vehicles, e.g., hydrogen vehicles
  • Overhead electric line powered vehicles, e.g., electric trains

Battery Powered Electric Vehicles

Electric vehicles that run from a battery source can include cars, trucks, boats, buses and planes. The basic configuration involves a vehicle with an electric motor that derives its power from onboard batteries. The batteries are charged from the electric grid and can be replaceable or not. Currently batteries are one of the main issues with electric vehicle development because of their high costs, large mass and the more limited range they allow for. This has had the effect of positioning electric vehicles as a means for short distance and city driving, but as battery technologies improve and a greater recharging infrastructure is created, the range issue (called “range anxiety”) will become less important.

Hybrid Vehicles

Hybrid vehicles derive electric power from both batteries and combustion engines, where the combustion engine can be used to either generate electricity for the batteries, or as a traditional combustion engine. There are three standard configurations for a hybrid vehicle:

  • Parallel hybrid
    • Has both an electric motor and combustion engine that can simultaneously transmit power to the wheels through the transmission. In current production models the combustion engine generally provides the majority of force. A parallel hybrid can never transmit 100% electrical power to the wheels and must always rely on the combustion engine.
  • Series hybrid
    • Has both an electric motor and combustion engine where the electric motor is always used to transmit power to the wheels and the combustion engine is always used to charge the batteries
  • Power-split hybrid
    • Has the properties of both the parallel and series hybrids, and can take advantage of performance differences, where the electric motor performs better at slower speeds and the combustion engine at higher speeds

Fuel Cell Powered Vehicles

Fuel cell powered vehicles are actually not classed as electric vehicles, but rather as fuel cell vehicles or hydrogen vehicles, but that they run on electricity allows them to be compared with (battery powered) electric vehicles.

Fuel cell vehicles use an electric motor that is driven by electricity created in the fuel cell (see Fuel Cells article this web site). A fuel cell is similar to a battery, in that it contains an anode, cathode and electrolyte, the difference being that whereas a battery has a limited amount of fuel to convert, a fuel cell allows for fuel external to the system to be constantly fed in, allowing for greater range. Fuels for fuel cells are mainly hydrogen, but depending on the fuel cell type can also be butane, methanol and biogas.

Overhead Electric Line Powered Motors

Electric vehicles that derive power from overhead electric lines, e.g., trains and trams, is a well established industry and won’t be treated in this article. For further information see the below links:

http://en.wikipedia.org/wiki/Electric_locomotive

http://en.wikipedia.org/wiki/Electric_multiple_unit

Motors and Batteries

Motors (AC vs. DC)

Unlike a combustion engine that derives its power from burning fossil fuels, an electric motor derives its power from electricity, and because there are two types of electricity, i.e., AC and DC, both types of motors exist. Regarding which type of system is better depends on the many advantages and disadvantages that each motor type has compared to the other and the type of vehicle it will be used for.

DC motors (brushless DC motor) for vehicles require powerful magnets, which involves using expensive rare earth elements, they also provide lower variable performance than AC motors. On the other hand they generate less heat so waste less energy. Currently DC motors are used mainly in hybrid vehicles, where power is shared between a combustion engine and an electric motor.

AC motors (induction motor) don’t require magnets, because a magnetic field is generated once electricity is sent through the system. However, because batteries supply DC electricity, they do require a DC to AC inverter (see Electric Motors and Generators article, this website). AC motors are able to adjust voltage to load, which means they are more efficient at handling variable load requirements. This ultimately becomes advantageous particularly when higher performance is required, and means that for pure electric vehicles, AC motors are ultimately a better option.

Batteries

Batteries present one of the largest hurdles for electric vehicle development and fall under the greater category of energy storage. Fossil fuels are an efficient means of energy storage, and developing a system that can compete with this involves significant advances in technology. The principle battery technology for both combustion engine and electric vehicles has been lead acid rechargeable batteries due to the maturity of the technology. However, with recent developments in the electronics sector, mainly for computers and mobile telephones, new battery technologies have come to the forefront, namely lithium ion batteries, which improve over lead acid batteries in many areas including power and weight. However the maturity of lithium ion battery technology is moderate and there remain issues including a lower number of cycles, a shorter lifespan, toxicity and the risk of combustion.

Due to the enormous demand for efficient energy storage systems, development in this area is ongoing. Promising technologies include lithium ion, lithium-ion polymer and zinc-air batteries, as well as advances in ultracapacitors that use carbon nanotube technology and create the “ultrabattery” when combined with a battery.

Charging and Energy Recovery

An ongoing issue with electric vehicle adoption is “range anxiety”, where lower range performance at higher costs has a prohibitive effect on consumers used to combustion engine standards. Resolving this requires improvements in batteries and charging options. Charging options include:

  • Conductive (direct) coupling
  • Inductive coupling
  • Charging time
  • Battery swapping
  • Charging stations
  • Regenerative braking
  • Online Electric Vehicle charging

Conductive (direct) Coupling

This is the traditional means of accessing electricity through a cable and connection plug. In the case of electric vehicles, it involves a high power electric cable from a public or private recharging station that can plug into a weatherproof socket in the vehicle.

Inductive Coupling

This is a means of battery charging where there is no physical contact between the source of electricity and the vehicle. Inductive coupling involves transferring electricity from one wire to another by means of electromagnetic induction (Faraday’s law of induction), where a voltage is created when there is movement between a conductor and a magnetic field that are not in physical contact. For electric vehicles a variety of setups exist including one where the electricity source is on the floor of a garage and the receiver is on the bottom of the car. With inductive charging there is no risk of electrical hazard as there is with conductive charging.

Charging Time

The charging time for electric vehicle batteries is an issue due to the comparative quickness of refueling combustion engine vehicles. Resolving this depends on technological options and usage behavior. Recharging an electric vehicle can take between 30 minutes to 26 hours [1] depending on the type of battery, vehicle configuration and the available voltage levels from the grid (higher voltage means quicker charging). Beyond technological restraints, the charging time influences usage, e.g., daily commutes vs. long trips. For a daily commute, charging options exist at home for overnight charging or at recharging points in town for recharging during working hours, but for long trips the recharging infrastructure is less predictable, particularly when it might take several hours before a recharge is complete.

Battery Swapping

An alternative to recharging batteries is to swap them at an exchange station, which saves recharging time. However, this would require more standardization of electric vehicle batteries, something that will prove difficult at this early stage of the industry’s development.

Charging Stations

Charging stations for electric vehicles operate similarly to petrol stations as supply networks. Options include converting existing petrol stations to include electric charging services, as well as installing recharging points throughout urban areas. Because of the ubiquity of the electric grid, providing electricity is much simpler than providing petrol and means that recharging points can be almost anywhere that the electric grid is, and even where it isn’t in the case of autonomous renewable energy setups. Because charging times are faster with higher voltages, charging standards exist to regulate charging outlets, known as Level 1, 2, and 3 charging. Home charging outlets are usually level 1 (120 volts) or 2 (208 – 240 volts), and public outlets are level 2 or 3 (300 – 600 volts).

Regenerative Braking

Regenerative braking is a system that allows energy to be recovered through braking, where the recovered energy is sent as electricity back to the source of electrical supply, which can be on-board batteries or, in the case of electric trains, the grid. Depending on driving conditions regenerative braking can recover up to 20% of the energy used in electric vehicles. It is highest in city driving where more braking takes place.

Online Electric Vehicle Charging

The future of electric vehicles holds many dynamic solutions, and one of them is Online Electric Vehicle charging. This is a system being developed by The Korea Advanced Institute of Science and Technology that involves using an inductive charging system where an electric power strip under a road constantly powers/charges an electric vehicle moving along it, i.e., an invisible electric rail system for vehicles.

Efficiency

Analyzing vehicle energy efficiency is very complex due to the number of factors involved in putting a vehicle on the road. When strictly analyzing overall energy consumption efficiency certain analytical definitions are used. These are:

  • Well-to-Station
    • Includes all energy consumption involved in getting energy from a raw material state to a supply station
  • Station-to-Wheel
    • Includes all energy consumption involved in getting energy from a supply station to a turning the wheels state
  • Well-to-Tank
    • Includes all energy consumption involved in getting energy from a raw material state to an onboard vehicle state
  • Tank-to-Wheel
    • Includes all energy consumption in getting energy from an onboard vehicle state to a turning the wheels state
    • Is the same as Miles Per Gallon Equivalent (MPGE)
      • MPGE is a measure that allows for comparing standard combustion engine vehicle energy efficiency with that of alternative fuel vehicles
  • Well-to-Wheel
    • The complete energy consumption cycle of getting energy from its raw material state to a turning the wheels state

Applying some of these measures to different vehicle types yields a variety of results that include:

  • A 2003 study [2] found that efficiencies varied depending on which measure was used when comparing vehicle types:
Well-to-Tank Tank-to-Wheel Well-to-Wheel
More efficient Combustion Engine Fuel Cell Hybrid
Hybrid Electric Fuel Cell
Electric Hybrid Electric
Less efficient Fuel Cell Combustion Engine Combustion Engine
    • Considering the supporting infrastructure that exists for conventional vehicles, this breakdown makes sense. However, were a supporting infrastructure for electric and fuel cell vehicles to be further developed, then these two technologies would be the most efficient for all measures.
    • Electric vehicles convert 75 – 80% of the battery energy to power the wheels, whereas combustion engines convert only 20% of the petrol energy to power the wheels (Tank-to-Wheel) [3]
  • A 2010 study [4] for the Tesla Roadster (electric vehicle) found that:
Well-to-Station Well-to-Wheel
More efficient Combustion Engine Electric
Hybrid Hybrid
Fuel cell Combustion Engine
Less efficient Electric Fuel Cell

The details for these measures will continue to change. However, it is now generally accepted that electric vehicles, even in the conventional energy infrastructure, are the most energy efficient overall [5] [6].

Operation

An electric vehicle is much simpler that a combustion engine and has about 5 moving parts compared to the combustion engine’s many moving parts. In basic operation an accelerator pedal is connected to a controller that varies the amount of electricity delivered from the batteries to the electric motor. The greater the amount of electricity delivered, the faster the motor goes.

Electric vehicles don’t require gear or transmission systems because torque is a function of current and means that a responsive and smooth acceleration can be achieved at any speed.

There are several motor configurations for an electric vehicle. Power can be delivered to the wheels as in a traditional combustion engine where the motor delivers rotational power through an axle, or motors can be placed on one or more  wheels, thereby eliminating the axle. The advantages of combining motors with wheels include that power can be more effectively recovered through braking and shock absorption, there are less motor parts, there is a lower center of gravity and traction is improved.

Advantages and Disadvantages

Advantages of electric vehicles include:

  • Reduction of CO2 emissions
  • Simpler motor technology
  • Better energy efficiency
  • Cheaper to drive

Disadvantages of electric vehicles include:

  • Still rely on energy that is generated from fossil fuels
  • Shorter range
  • Battery technologies need more development
  • Long recharging times
  • Less cabin heating available in winter

References

  1. Battle of the Batteries: Comparing Electric Car Range, Charge Times, gigaom.com
  2. Gauging Efficiency, Well to Wheel, css.engineering.uiowa.edu
  3. Electric Vehicles, U.S. Environmental Protection Agency, www.fueleconomy.gov
  4. The 21st Century Electric Car, www.stanford.edu
  5. Electric Car: Energy Efficiency, Wikipedia
  6. Fuel Cell Vehicle: Efficiency, Wikipedia

Links

http://en.wikipedia.org/wiki/Electric_vehicle

http://en.wikipedia.org/wiki/Battery_electric_vehicle

http://en.wikipedia.org/wiki/Electric_car

http://www.howstuffworks.com/electric-car.htm

http://www.carinsurance.org/alternative-fuel

Comments are closed.