Electric Vehicles

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Abstract: As a means of utilizing grid electricity for transportation, an electric vehicle is more efficient than other options such as electrofuels. Light-duty vehicles represent a technically feasible application for battery electric powertrains—driving ranges tend to be short and in many cases, batteries can be recharged overnight at home. Charging standards have been developed that define the voltage, current and connection requirements for charging electric vehicles. Electrification of heavy-duty vehicles is more challenging because of the wider range of vehicle weight ratings, travel distances and various commercial considerations. Alternative electric vehicle concepts have been proposed such as pantograph trucks.

Introduction

Electrically powered vehicles are attractive because of the general perception that they emit extremely low or nearly no emissions. While they emit no tailpipe emissions, criteria pollutants and GHGs can be emitted by the power plants needed to charge their batteries. According to the EPA, an electric vehicle in the USA results in a net upstream CO₂ emission equivalent to about 120 g/mile (2010 data). In addition, electric vehicles also produce non-exhaust emissions that originate from their brake and tire wear.

As a means of utilizing electricity from the grid for transportation, an electric vehicle is more efficient than other options such as electrofuels (e-fuels). For a typical light-duty electric vehicle in 2018, 60 to 65% of electric energy supplied to the vehicle is transferred to the wheels, Figure 1. This can increase to about 80% if it is supplemented by regenerative braking [5237].

Charging Technologies

Overview

One critical aspect of electric vehicles is that the battery must be recharged. The frequency can range from several times a day to once every few days or longer depending on how the vehicle is operated.

It is interesting to compare the recharging time of a battery electric vehicle to a typical gasoline fueled vehicle. Retail gasoline pumps dispensing 10 US gal/min (38 L/min) supply about 21 MW of fuel energy to the vehicle’s fuel tank. For a typical mid-size vehicle, this translates to about 250 km/min of driving range supplied to the vehicle during refueling. Depending on the charging station type, electric vehicle battery chargers supply about 3-120 kW to the vehicle’s battery. This translates to about 0.2-10 km/min of range supplied to the battery during charging.

At a minimum, charging the battery of an EV from the grid requires that the AC power from the grid be converted to DC and that the DC voltage be controlled to be compatible with the vehicle’s battery. This is typically accomplished with a battery charger (or simply ‘charger’) that consists of an AC to DC rectifier followed by a DC-to-DC converter. The battery charger can also include components to correct power factor. EVs are typically equipped with on-board chargers but because of space and weight limits, the power output of these can be limited. Faster charging typically requires an off-board charger.

The power rating of the battery charger has a direct impact on the time it takes to charge the vehicles battery and thus the convenience of operating an electric vehicle. For example, for an EV with a 30 kWh battery, charging with a typical North American 15 A/120 V outlet would take 20 h. With an outlet similar to that used for a clothes dryer it would take about 3.3 h. With a 50 kW DC fast charger it would take about 35 min and with a 400 kW charger (Extreme Fast Charging, XFC) it would be similar to the time it takes to refuel a gasoline vehicle.

While conductive charging in which a cable is used to charge the vehicle’s battery is most common, wireless charging offers the potential to charge the vehicle without the need to connect a cable and can take the form of capacitive power transfer (CPT) or inductive power transfer (IPT) [5154]. Inductive charging is the more commonly discussed wireless option and can be applied over a range of gap distances and power levels. Capacitive charging applies to kilowatt-power-levels but requires a small gap. Wireless charging technology that can deliver up to 1.5 MW/m2 is under development [5238].

Both conductive and wireless charging can be applied when the vehicle is stationary or moving. Stationary charging could be used not only when the vehicle is parked but also would enable opportunistic charging that could be used when the vehicle is stopped for a short period of time such as at a stop light. Opportunistic stationary charging is facilitated with wireless technology. Dynamic charging could be used to charge the battery when the vehicle is moving along a dedicated charging lane. Dynamic conductive charging requires a physical connection to the vehicle such as with a pantograph while dynamic wireless charging would likely use technology buried in the roadbed.

Battery exchange or swap can address the challenge of long charge duration—battery swapping can take as little as 90s—but a lack of standardization complicates this and makes it unfeasible for the general vehicle population [5154]. It is primarily suitable for applications such as captive fleets operating a number of similar vehicles that all use the same battery.