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Trends in Electric Propulsion

By Kaushik Rajashekara, Fellow IEEE, Department of Electrical Engineering, The University of Texas at Dallas, Richardson, TX

Electric vehicles (EV) have been around since late 1800’s. However, in the past, EV development activities were discontinued because of low cost of gasoline and advancement of internal combustion engines. In the last decade, Electric vehicles (EV) and Plug-in Hybrid Electric Vehicles (PHEV) are gaining increasing interest in North America and in other countries due to rising fuel prices, concern for the environment and the sustainability of fossil fuel based transportation.

The critical subsystem required in an electric vehicle is the propulsion system, which provides the tractive force to propel the vehicle. This propulsion system consists of an energy storage system, the power converter, the propulsion motor and associated controllers as shown in Figure 1.
electric-propulsion-1

Figure 1: Typical propulsion System components of a EV Power-train
Click Image to Enlarge

Electric motor(s) converts the energy supplied by the battery into mechanical energy to provide traction power to the wheels. Today, interior permanent magnet (IPM) synchronous motor widely used in automotive propulsion system because of its high efficiency, high torque, high power density and relative ease of field weakening operation. Toyota Prius, Ford Escape, Chevy Volt are some of the vehicles that use IPM machine. However, there are rising concerns about the availability of rare-earth based magnets and their increasing costs. A number of companies and researchers are working on the development of motors that do not use permanent magnets, but achieve the same performance as IPM motors. These include Induction, Switched Reluctance, Synchronous Reluctance, and PM-assist Synchronous Reluctance Motors. In the near future, the interior PM motor will likely continue to dominate the market.

In the area of power electronics, presently Insulated Gate Bipolar Transistor (IGBT) devices are being used in almost all commercially available EVs, HEVs, and PHEVs. The IGBTs will continue to be the technology of choice until the silicon carbide (SiC) and gallium nitride (GaN) based devices are commercially available at a cost similar to that of silicon IGBTs. Various properties of silicon carbide such as; wider band gap, larger critical electric field, and higher thermal conductivity enables the SiC devices to operate at higher temperatures and higher voltages offering higher power density and higher current density than the pure Si devices. Gallium devices are projected to have significantly higher performance over silicon-based devices, and much better performance than SiC devices, due to their excellent material properties such as high electron mobility, high breakdown field, and high electron velocity. GaN-based power electronics also feature both low on-resistance and fast switching, leading to substantial reduction in both conduction and switching losses. Achieving the highest power density in a compact package (considering the thermal aspects and reliability) is critical for successful deployment of power electronics systems in electric and hybrid vehicles.
Table I: Characteristics of commonly used batteries in EVs Click Image to Enlarge

Table I: Characteristics of commonly used batteries in EVs
Click Image to Enlarge

Lithium-based energy storage technologies (ie. lithium-ion batteries) are leading the way to meet the storage requirements of EV/HEVs. In the past, Lead-acid batteries and Nickel-Metal-Hydride (NiMH) batteries were popular in EV/HEVs. The Tesla Roadster was the one of first production automobile to use lithium-ion battery cells to travel more than 200 miles per charge. Presently, the Nissan Leaf (BEV) and the GM Chevy Volt (PHEV) also use lithium-ion based batteries [1-2]. Typical values of energy, power and cycle life for lead acid, NiMH and lithium-ion batteries are shown in Table I and Figure 2. Lithium-ion presents a higher-density and more efficient way to power modern hybrids and EVs. The future of EV batteries could be based on lithium-air technology. The energy density of lithium-air batteries theoretically is equivalent to the energy density of gasoline. This is because it has an “air cathode” made of a porous materials that draw in oxygen from the surrounding air. When the lithium combines with the oxygen, it forms lithium oxide and releases energy. Since the oxygen doesn’t need to be stored in the battery, the cathode is much lighter than that of a lithium-ion battery, which gives lithium-air batteries their higher energy density. Toyota Motor Corp and BMW have announced a joint research program on lithium-air battery that will be expected to be more energy-dense than the lithium-ion batteries of today [3]. This technology is also being studied by other researchers, including IBM, working to develop a lithium-air battery that would enable electric vehicles a range of 500 miles on a single charge [4]. Researchers have demonstrated coin-sized, rechargeable lithium-air batteries with a current density of 600 mAh/g (much higher than lithium-ion batteries at 100 to 150 mAh/g). However, lithium-air batteries are still experiencing challenges, such as limited charge/discharge cycles and a relative slow charging process.
Figure 2: Power (acceleration) and energy (range) by battery type Click Image to Enlarge

Figure 2: Power (acceleration) and energy (range) by battery type
Click Image to Enlarge

The U.S. government’s current rules for the Corporate Average Fuel Economy (or CAFÉ) standards mandates an average of about 29 miles per gallon gradually increasing to 35.5 mpg by 2016 and 54.5 miles per gallon by 2025. In order to meet these standards, automakers will gradually switch from the current pure internal combustion engine based vehicles to various forms of plug-in-hybrid and battery-electric vehicles. Once battery technology and costs are achieved to provide about 300 miles per charge, the electric vehicles will be more prevalent than the PHEVs. Another significant change to look for is the advancement of the clean diesel, diesel hybrids, diesel engine based plug-in hybrids, and liquid natural gas based vehicles. Clean diesel based hybrids may make it possible for automakers to stretch towards the 100 mpg mark in coming years. Although fuel cell technology had shown a great promise, the full fuel cell vehicle continues to remain only as demonstration vehicles. Issues related to cost of manufacturing, robustness of the technology, hydrogen production, and the hydrogen distribution infrastructure will limit adoption. However, with the advancement of Polymer Electrolyte Membrane (PEM) and Solid oxide Fuel Cell (SOFC) technologies, the fuel cells could be used as range extenders in place of internal combustion engine-generators in series hybrid vehicles. These plug-in, fuel-cell hybrid vehicles (PFCV) consisting of a smaller fuel cell and a larger battery (battery dominant) could be a future direction for automobiles.

Continuing worldwide R&D by industry, academia, and government will advance the propulsion technologies further, making EVs more viable with longer range, higher performance, and lower cost. All electric vehicles will be joined by PHEVs and PFCVs to serve broader segment of the transportation market. The acceptance these vehicles will be judged based on their performance, reliability, lifespan, and cost.
Source electricvehicle.ieee.org

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