Flipping the Switch on Electric School Buses: Vehicle In Use Performance: Module 3 (Text Version)
This is a text version of the video for Flipping the Switch on Electric School Buses: Vehicle In Use Performance: Module 3.
Lauren: Welcome to Part 6 of the Flipping the Switch on Electric School Buses series, where we will discuss electric school bus vehicle in-use performance. If you've been following along to Part 6 of the Flipping the Switch series, Electric School Bus Vehicle In-Use Performance, you'll already know that it consists of three modules. Module 1 provided information on how to track electric school bus performance and why it is important to do so. Module 2 discussed electric school bus energy use and fuel economy, and finally Module 3, which I'll be discussing today, will provide information on electric school bus range and reliability.
Now let's get started with Module 3 of Electric School Bus Vehicle In-Use Performance, Range and Reliability. I'll start with range. The range capability of a battery electric bus is based on efficiency, and efficiency is affected by many factors. We have noted three primary factors from electric transit bus studies that are applicable to electric school buses.
First we have the duty cycle of the bus, or how the bus is operated, such as the route demands, distance, speed, terrain, number of stops, stuff like that. The driver style is also a key factor where aggressive driving behaviors will increase the energy use demands, which reduces the efficiency and therefore decreases the overall range capability of the bus. And finally, the surrounding environment because that influences the heating, ventilation, and air conditioning demands which impacts the energy consumption of the bus. So let's dive into each of these factors a little bit more.
First we'll start with a quick review of the efficiency of a diesel bus and compare it to a battery electric bus. So this lovely whale tail looking image is an example of a flow diagram of energy in a diesel bus. The key thing to notice here is that the diesel engine experiences a decent amount of energy loss due to different aspects of energy conversion and this impacts the engine's efficiency. Also note that the diesel engine buses' energy flow is an open-ended system, so there aren't any energy losses like from the engine or braking that are being recaptured and used in this flow.
The key takeaway is to understand that there is a lot of energy going in, and that's kind of the bigger blue part that looks like the whale tail, and then it reduces down to an open-ended system. But here we have the same type of diagram for the energy flow in a battery electric bus. Notice that the losses in the top part of the diagram are lower because electric motors are very efficient. The battery electric bus is also a closed loop system where the energy loss from braking is recaptured and the energy is redistributed back to the battery.
There are still some system losses similar to a diesel engine bus due to the overall architecture of the bus such as auxiliary loads, the drag, and rolling resistance. And here you can also see that this is where the duty cycle largely influences the efficiency because different aspects are impacted by the way the bus is operated. So for example demanding terrain which may require higher speeds, or even fluctuating speeds, or has steeper grades will dictate the energy demands and therefore some energy losses within the system. So your drag, your rolling, your braking, all of those factors are going to be impacted here.
However, some of those impacts can be influenced by driver style such as aggressive acceleration or deceleration habits. So as the driver navigates through those terrain characteristics and therefore the duty cycle demands it will change based off of the behaviors of the driver. Also as I previously mentioned the auxiliary load is one of the greatest impacts to the battery electric bus, and this is generally due to the HVAC demand. So this brings me to one of the primary factors influencing the range and that is HVAC.
Here is an example of how HVAC demands impacted battery electric bus efficiency and therefore the overall range capability for the transit electric bus, and this is applicable to an electric bus. So although propulsion tends to always be the greatest demand and consume the most energy in a battery electric bus the auxiliary loads is the next greatest impact. As you can see by the green triangles on the graph the HVAC system consumed more energy than the power steering and air compressor components. And as the HVAC demand increases the estimated range which is that solid black line decreases.
So HVAC demands are largely ambient temperature dependent. Right? So if it's hot outside you're going to run the AC and if it's cold outside you're going to crank that heater on. And therefore the efficiency of the battery electric bus drops for both heating and cooling energy demands, although according to our transit studies heating tends to have a larger impact or require more energy. But transit agencies in colder climates have also reported HVAC taking as much as 50% of the total power to heat the interior. So key takeaway, that HVAC is greatly going to impact your efficiency, which therefore impacts the range of the bus.
Now let's talk about reliability a bit. So this is a metric called the mean distance between failure, and it is measured in miles, and we utilize it as a measure of reliability. Basically a well-maintained bus should have fewer failures on the road. Right? And maintenance technicians that keep up to date with all preventative maintenance will often catch and then be able to repair issues before they cause a road failure. And that road failure during service causes many disruptions that tend to impact multiple other factors and can therefore increase the total cost of ownership.
So this measure of reliability is dependent on how long a bus can go between failures. And you want that to be a longer time. Right. So you want a greater mean distance between failures, and in order to get that you just can make sure that maintenance technicians keep up to date with all preventative maintenance, and then that you conduct repairs as needed.
Availability – and this is a percentage – is another key performance indicator that helps assess reliability. So availability is calculated by the number of days buses can operate and is divided by the number of planned service days, and then we multiply that by 100, because again it's a percent of availability. So the point here is that the availability of fleets or the bus in a fleet is not defined just by the total number of buses in the fleet. Right?
It doesn't do your feet much good if you have 100 buses but only 2 of them tend to be available. Right? So this is just again ensuring or helping to track and ensure that the buses are properly maintained and available for use in order to be deemed reliable. So the availability of a bus is something to track to ensure the reliability of that technology meets your fleet's needs.
So the range capability of a battery electric bus as I mentioned is impacted by efficiency, which is dependent on many factors. The driver's style where aggressive driving behaviors can also increase the energy demands, reducing the efficiency and therefore decreasing your overall range capability. We also talked about auxiliary loads, specifically the HVAC demands, which are influenced by the ambient temperatures, and then the duty cycle demands of the bus such as route demands of distance, speed, terrain, and number of stops.
And then tracking the mean distance between failures will help ensure that a bus is being properly maintained to avoid downtime and track your reliability of the bus. It's also important to observe the availability of a bus to determine if the technology isn't meeting the needs of your fleet and causing too much downtime or maintenance cost, which then indicates excessive down times, then helps to troubleshoot problems. So again we have the efficiency and the reliability factors.
And thank you very much for listening. That concludes Module 3 of the Flipping the Switch series, Electric School Bus Vehicle In-Use Performance.
Abby: Now that you've completed Part 6 of the Flipping the Switch series Vehicle In-Use Performance, you'll want to see what's coming next. In the following parts we'll discuss training for both drivers and technicians, and finally cost factors.
Lauren: And as a reminder, you can find all the content for the Flipping the Switch on Electric School Bus series including each part of the series and associated modules, as well as handouts with the summary of information and links to all the resources mentioned today on the Alternative Fuels Data Center's Electric School Bus page.