Flipping the Switch on Electric School Buses: Vehicle Requirements: Module 2 (Text Version)
This is a text version of the video for Flipping the Switch on Electric School Buses: Vehicle Requirements: Module 2.
Lauren: Welcome to Part 3 of the series Flipping the Switch on Electric School Buses where we will discuss electric school bus vehicle requirements.
If you've been following along to Part 3 of the Flipping the Switch series, Electric School Bus Vehicle Requirements, you'll already know that it consists of three modules. Module 1 provided information on currently available electric school bus vehicle models and vehicle cost factors. Module 2, which I'll be covering today, discusses how to analyze your vehicle routes to determine if electric is a good option for your fleet, your range requirements, and your efficiencies. And finally, Module 3 where we will provide information on incentives, battery life, and other considerations like battery recycling.
Now, let's step into Module 2 Vehicle Requirements: Route Analysis, Range, and Efficiency Considerations. When initially considering battery electric bus for your application, you should conduct route analysis to ensure that the battery electric bus will meet your route requirements and ensure that it is a proper technology to support the fleet's needs. So this normally involves just understanding the route profiles and characteristics. So how is this vehicle operated? And there are multiple factors when taking this into consideration. It's not just about length and maybe your longest route but your terrain will also impact your efficiency as well as number of stops and maybe temperature. So in most cases, battery electric buses tend to be a good candidate for school bus fleets because their routes are predictable and consistent. They have proper time to charge during dwell periods or times that the bus isn't in use, and they usually benefit from regenerative braking.
So here's just an example of understanding where your truck is driving and how it is being operated from route analysis. We then take the information from the route analysis and understand the vehicle's duty cycle or the total way that the vehicle tends to operate when it's in use. When it is in use. So the log data from route analysis is normally fed into a total model for consideration where you're going to log the baseline bus data and convert the energy used from that duty cycle to an equivalent battery energy. You then can determine what battery capacity is needed to meet those route requirements. So using that vehicle operation data, which you tend to get from a data logger, you can analyze and understand the basic behaviors of that route such as fuel use, fuel efficiency, performance, cost, battery life, etcetera. And then you can use those factors and convert them to predict the efficiencies of other fuels for comparison. So that's a long-winded way of saying that once you do the route analysis and understand the way the vehicle is operated you can convert that into an energy consumption requirement and understand how different technologies might perform under those energy demands in order to compare what technology might best fit your needs.
So in order to do that route analysis, you're going to need that data logger. Some OEMs may actually offer their own route analysis or data logging services and there are some additional third parties such as telematics providers who can provide the equipment to log the data as well as conduct the analysis of the duty cycle and maybe even determine the total energy consumption needs based off of those logged data. So in addition to that, there's also options for more advanced methods of analysis such as a model that NREL provides called FASTSim where we take the data from a data logger and input it into a duty cycle comparison where we consider different metrics, normally again fuel efficiency, performance, cost of the vehicle, battery life of the electric powertrain and compare them across those different powertrain options in order to understand which technology might best suit your needs based off of the way your vehicle is operated.
But the basis of that is all just getting an understanding of those energy requirements and comparing them, right? So an example is presented here where we've estimated the fuel economy of an electric powertrain and we've compared that against the baseline bus, which is diesel the time, and that was used just from a basic fuel economy comparison. So this truck on this route used this much fuel. That much fuel would be an energy equivalent of this and therefore, this is the amount of energy consumed. Now, this energy consumption is then considered when determining the battery size and the range requirements of a battery electric bus for the same duty cycle. So it's not just about the range requirements but also other efficiency considerations that is going to kind of marry with the fuel economy to determine if that battery electric bus is indeed the proper technology to meet your fleet's needs.
Let's go into an example where we can better explain how the range requirements and the efficiency requirements will impact the battery size in order to meet your fleet's needs. In general, the main focus when initially looking at the bus route for evaluation is to understand the energy demands in order to ensure that the battery electric bus will be able to meet your range requirements as well as your energy use requirements. So battery size does not only depend on range requirements to meet that maximum distance. But efficiency is a large factor and that is dependent on the energy used.
In this example, from a transit bus study, we assumed a route demand of 100 miles per day and battery characteristics such as 80% charging capacity remaining at the end of its life. We took these two factors as well as two different cases for energy consumption and determined what that battery size requirement is. And you can see that the determination for battery size varies greatly mainly because of the energy used.
So we have a worst-case scenario where we have a higher energy consumption rate, in this case it's 2.3 kWh per mile, which means that it was a more demanding route and had lower fuel economy. The overall efficiency of this battery electric bus in this scenario was lower because it consumed more energy. However, the lower energy use scenario, which probably had an optimal route where you had flat terrain, nice temperature, and you experienced higher fuel efficiency resulted in a lower battery size requirement because the energy use requirement was not as high, either. So it was more efficient and had less energy consumption rate requirements. Both of these scenarios we did consider range buffers for little peace of mind and then you can see how that range buffer will also affect your battery size. So based off of the route distance requirement assumption, again, we took an 80 percent charging capacity at end of remaining life. This is actually been improved in most of bus technologies today. Two different scenarios for the energy consumption rate and a range buffer all to determine the estimated battery size in kilowatt hours that we would require to support this route.
So something else to note that the energy consumption rate in kilowatt hours per mile is used here to depict the different energy consumptions in the high energy use and the low energy use cases. We normally use kilowatt hours per mile for medium- and heavy-duty applications because kilowatt hours tend to be a larger number than what is seen in light-duty applications where we might just use a watt hour per mile. However, it's also common to utilize or convert to miles per kilowatt hour for a fuel equivalent type of comparison in order to understand the efficiency of the distance per energy. So something to keep in mind as we are looking at that energy consumption rate or the efficiency of distance per energy and how your units might change.
Key takeaways from all of this, though, are to understand the way that your vehicle is operated and what those resulting energy requirements are. In order to do so, you can use data loggers, either in house then utilizing analysis to understand your energy requirements based off of that vehicle operation or OEMs may be willing to help, as well as separate third parties who provide telematics services as well. Then you can determine the battery size based off of the route requirement, range requirements mainly, and the greatest energy requirements.
So that energy requirement is also impacted by other factors which determine the overall battery electric bus efficiency. So the efficiency and range together are affected by many factors. It's not just maximum distance required. The duty cycle of the bus, again as we just described, how the bus is operated, what's the demands of distance, the speed, the terrain, your number of stops; in addition to the driver's style along that route, the payload, and the surrounding environment of that route are all key factors that should be taken into consideration when implementing electric school buses because they will impact the efficiency of the battery electric bus. So now we have the vehicle route requirements. We have the energy consumption rates and the efficiency all to consider together.
Here's an example of how a specific system, in this case it was HVAC, which is your heating, ventilation, and air conditioning system, how the demands of that impacted the battery electric bus efficiency even though the fuel economy of the battery electric bus was better compared to the baseline technology. So even though along a specific route in optimal conditions, the battery electric bus will have better fuel economy, these additional load requirements such as HVAC impacted the efficiency and brought down, if you will, impacted, the total efficiency of the battery electric bus in that application.
So you can see here in the pie chart, that the HVAC system consumed almost a quarter of the energy of the transit bus from this study. The HVAC demands are largely dependent on the ambient temperature. So if it's hot outside, you are going to run the AC. If it is cold outside, you're probably going to run your heater. And the efficiency drops for both heating and cooling. However, in our transit studies, we have seen that heating tends to have a larger impact or require more energy of that bus. Although transit agencies in colder climates have reported that HVAC takes as much as 50% of the total power to heat their interior. So the main take away is that additional systems, such as HVAC, can impact the total efficiency of the battery electric bus even though the fuel economy is better than the baseline technology.
So that battery electric bus efficiency and range are affected by multiple factors. The HVAC system like I just mentioned, which is impacted or mainly defined by the surrounding environment. You also have the payload, the driver's style, and the duty cycle of that bus. So what we've seen from transit is that HVAC use is the most important factor in determining efficiency and range. And meeting requirements for longer-range routes might be a challenge if you have high demands of your HVAC system. So this is a time where you might want to consider mid-day plug-in or alternating buses between morning and afternoon. It's not an end-all kind of scenario if you have high HVAC requirements, it's just something to take into consideration because your efficiency will be impacted.
There are other factors impacting your efficiency such as driver training. As an example, relating to the HVAC system, your driver could use the HVAC system in a more efficient manner by setting it to a set temperature and leaving it there; not using as low of an air conditioning set point in the hotter temperatures in order to compensate or reduce the demand of your energy consumption from the HVAC system. Right? So just examples on how your efficiency might be impacted and another reminder that efficiency is a key impact to your battery electric bus technology meeting your fleet's needs.
So thank you for listening. That concludes Module 2 of Part 3 of the Flipping the Switch series, Route Analysis, Range, and Efficiency Considerations. To complete the modules in Part 3 of the series, continue on to listen to Module 3 where we will provide information on incentives, battery life, and other considerations.
And remember that you can find all the content for the series Flipping the Switch on Electric School Buses, including parts 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.