Flipping the Switch on Electric School Buses: Charging Infrastructure: Module 1 (Text Version)
This is a text version of the video for Flipping the Switch on Electric School Buses: Charging Infrastructure: Module 1.
Welcome to Part 4 of the Flipping the Switch on Electric School Buses series, where we will discuss electric school bus charging infrastructure. The Flipping the Switch series contains a number of parts on electric school bus technical assistance. This is currently where we are at in the series. In the upcoming parts we'll discuss charging infrastructure, infrastructure planning and solutions, vehicle in-use performance, training for both drivers and technicians, and then, finally, cost factors. Part 4 of the Flipping the Switch series, "Electric School Bus Charging Infrastructure," consists of two modules. Module 1, which is the module we're talking about today, provides information on how to determine charging needs and how to select a charger. Module 2 will discuss charging infrastructure installation considerations. Now let's dive into Module 1 of "Electric School Bus Charging Infrastructure," Determining Charging Needs and Selecting a Charger.
When transitioning to an electric school bus fleet, consideration should be made for both vehicle acquisitions and infrastructure installations. At the same time, a fleet is deciding which electric buses will serve their vehicle emission needs, discussions should be also surrounding the installation of the necessary charging infrastructure. This includes discussions with the local utility, as discussed in Part 2; planning the best locations for chargers and developing an installation design; performing site construction and charger installation; and then, finally, testing and commissioning the equipment prior to receiving your battery electric bus. In order to understand whether electricity is a good energy source for your fleet, please consult the Alternative Fuels Data Center's Electricity page to determine the benefits of electrifying transportation, the various vehicle options and charging infrastructure, as well as procurement guidance and possible incentives. The AFDC also provides detailed information on charging infrastructure and how to choose the appropriate charger for your fleet.
This includes the procurement of equipment, as well as cost considerations such as charger features, or installation challenges and site requirements. As with any fleet electrification process, the decision on which BEV will suit your fleet's needs is an important consideration. The seating capacity and estimated range must meet or exceed the vehicle's mission requirements. The range of each vehicle is a factor of the operational efficiency and battery capacity. The operational efficiency, when presented in miles per kilowatt-hour, can help determine the energy consumed in a given day of travel, with a higher number representing more efficient operations. These daily energy needs are helpful when selecting the best chargers, which must be capable of recharging the vehicle during a typical dwell period, such as eight hours overnight. Additionally, the battery must be capable of holding the daily energy needs of the vehicle, unless mid-day charging is being considered, in which case the recharge time from the charger is an important calculation. This chart summarizes these metrics for several of the battery electric bus options available on the market as of 2022, and for more details, please refer back to Part 3 in this series.
After determining the optimal battery electric bus, a fleet must decide which charging technology is best for their application. The most important feature to consider when selecting a charger is the charging power, and how rapidly that power can provide the energy your bus will need. Many light-duty EVs can take advantage of a simple 120-volt receptacle and portable level one charger to recharge their battery after most typical days of travel. However, the increased energy demands of larger vehicles make this charging rate too slow for most electric bus applications. This is in contrast to the higher power level of most DC fast charging options that provide 50 kilowatts or more of charging power to the vehicle. While the capability of these chargers may seem tempting, most bus applications would not require this level of power, and installation costs would be much higher compared to level one or two charging – which will be discussed in Part A. The sweet spot for most BEVs is the upper end of level two charging, capable of supplying up to 19 kilowatts of charging power. These units will recharge the vehicle in 1/10 the time a level one will, while the installation costs will be much lower than for DC fast chargers.
In summary, the main considerations to keep in mind when choosing a charger for your electric fleet are: planning for charging installations and BEV acquisition simultaneously to ensure the BEV meets your fleet needs, and the charger can supply the energy your new vehicle will require. Determine the energy needs for your new BEV based on operational efficiency and energy storage capacity to determine when and how often the vehicle must charge. And finally, choose the charging technology that can supply the necessary power while avoiding the most costly installation options.
Now let's talk a little bit more about how to select that best charging technology. When selecting a charger, it is important to understand both the vehicle energy needs and power capabilities of a charger. The energy a bus will require is dependent on how far it must travel and its operational efficiency. This will determine how much energy must be recharged at the end of the day and is typically presented in kilowatt hours. The power capabilities of a charger describe how rapidly the unit is able to supply energy to a vehicle. The more power a charger provides results in a shorter time to recharge a vehicle. Power is typically presented in kilowatts, which describes how many kilowatt-hours of energy can be provided in one hour. For example, a 10-kilowatt charger will supply a vehicle with approximately 10 kilowatt-hours in about one hour.
The charging needs for a BEV are defined by the daily vehicle energy needs and dwell period. Two key metrics are needed in order to estimate the daily energy needs, including the daily vehicle miles traveled and operational efficiency. A typical bus can travel approximately 0.67 miles on a kilowatt hour of energy or, put another way, consumes about 1.5 kilowatt-hours of energy for every mile traveled. The efficiency of an electric bus may be represented with both of these units. However, it is important to remember that more efficient buses will have higher miles per kilowatt hour and relatively lower kilowatt-hours per mile. If a typical bus travels a daily route length of 100 miles, it would require approximately 150 kilowatt-hours to travel that distance. This daily energy need must be recouped during the vehicle's daily dwell period, such as when it's parked overnight. It's important for BEVs to have a consistent parking location to ensure they always have access to their charging station. When selecting a charger, there are two common options: AC charging and DC charging. Although all battery electric bus batteries will store electricity in the form of DC, or direct current, power, the electric grid transmits and provides electricity in the form of AC, or alternating current, power. Therefore, a critical process of charging a BEV is the transformation of AC power into DC power. With AC charging technology such as AC level one or AC level two, this is performed by the vehicle's on-board charger, as indicated on the left side of the image. AC charging provides electricity directly from the grid to the vehicle and is primarily intended to monitor the charge session to ensure power is safely provided to the vehicle. As a result, AC charging technology is a lower-cost charging option, but it's generally limited in how much power can be provided. Alternately, DC chargers transform AC power into DC power in the charging unit, and provide electricity directly to the battery, as indicated on the right side of the image. This enables DC fast chargers to provide more power to the vehicle, but they are also more expensive to purchase and install due to the higher unit costs and equipment upgrades that are typically required.
Selecting a charger comes down to understanding when a vehicle is able to charge, and how quickly it must receive the energy it needs. Most bus schedules have a long overnight dwell period that is great for recharging the vehicle's batteries. However, some buses may also be able to charge in the middle of the day to support longer routes or evening activities. The middle of the day and early morning hours may also provide a great opportunity to mitigate demand charges by avoiding adding to the building peak load. However, the ability to shift this charging load throughout the day is dependent on the recharge time of the vehicle relative to the dwell period. For example, it would take 7.8 hours for a 19.2 kilowatt AC level two charger to provide a battery electric bus with about 150 kilowatt-hours of energy.
In conclusion, when determining which charger is needed for an electric bus fleet, it is important to understand both energy requirements and power needs. The daily energy needs are determined by the vehicle and how it is driven, such as vehicle miles traveled and operational efficiency, while the power considerations are determined by the charging duration for a given technology in comparison to the vehicle's daily dwell period.
Thank you for listening. That concludes Module 1 of Part 4 of Flipping the Switch series, "Electric School Bus Charging Infrastructure." To complete the modules in Part 4 of this series, continue on to listen to Module 2, where we will discuss charging infrastructure installation considerations.
You can find all the content for the Flipping the Switch on Electric School Buses series, including each part of the series and associated modules, as well as handouts with a summary of information and links to all the resources mentioned today, on the Alternative Fuels Data Center's Electric School Bus page.