A Guide to Green Energy Adoption for Transit Agencies Chapter 5: Charger Modeling and Charge Management
- Date: February 13, 2022
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Zero-emission bus (ZEB) deployments differ not only in the operational characteristics of the buses themselves, but also in the fueling infrastructure required. For battery electric bus (BEB) operators, a number of considerations arise when determining how to develop and deploy charging infrastructure for their buses:
- Charger power
- Number of chargers
- Number of charging dispensers per charger
- Physical space constraints
- Expected charging periods
- Utility rate structures
This process attempts to find the right balance between ensuring resilience is built into the infrastructure design (i.e. having one charger go offline will not prevent service requirements being met) and minimizing cost by not overbuilding infrastructure.
Charger Modeling
Charger modeling is one method for agencies to better understand how fueling ZEBs will impact their operational costs. It also has the potential to inform infrastructure project development.
For BEBs, charger modeling begins with an agency’s electric utility. Early engagement with the electric utility allows an agency to ask questions about how their current electricity rate is structured and how that structure will impact the cost of charging BEBs.
Overview of Electric Bills
Electric utilities generally design their rates to account for two different measures – power, which is typically measured in kilowatts (kW), and energy, which is measured in kilowatt-hours (kWh). In addition to charges for power and energy, utility bills will generally include a fixed monthly cost and may also include assorted other charges and/or taxes.
Table 4 shows three different scenarios in which a charger could consume 300 kWh of energy. Power, or the rate that energy is consumed or moved, varies across the three scenarios. This impacts the amount of time needed by the charger to consumed 300 kWh. Actual charge times may be longer due to charge limitations. An engineering analysis is required to determine accurate charge times.
Table 4: Examples of Charging Power and Relationship to Time and Energy
| Charger Power (kW) | x Time Charging (hrs) | = Energy (kWh) |
| 50 | 6 | 300 |
| 150 | 2 | |
| 450 | 0.67 |
Demand Charges
A demand charge is calculated based on the highest amount of power needed by the customer during a specific window of time, typically 15 or 30 minutes. Demand charges allow a utility to cover the cost of building enough energy generation and distribution assets to meet the highest level of demand at any given point across its service territory. While calculating a demand charge is typically straightforward [highest average power (kW) over a specific period of time multiplied by the rate ($/kW)], utilities have different ways they can quantify peak demand.
Some utilities forgo demand charges altogether. Demand charges may be broken up into short- term (hourly) and/or long-term (seasonal) periods, or may be based on an overall annual peak demand.
The following examples illustrate how peak demand is calculated based on different usage Scenarios. Each scenario assumes a 15‐minute demand window, and the calculations below do not take into account efficiency losses, which may result in higher demand charges.
Example 1:
Six 125 kW depot chargers are installed on the same meter that services a transit facility for overnight bus charging. The facility utilizes an average of 200
kW during the day and 50 kW overnight. Under these conditions, the previous peak demand was during the day. Without any charge management strategies, the new peak demand would now occur overnight, from simultaneously charging six buses on the 125 kW chargers. Assuming the buses require more than 15 minutes to charge, the chargers, alone, would create the following power demand:
6 chargers × 125 kW × (15 minutes of charging / 15-minute demand window) = 750 kW
When you add that to your existing average overnight facility demand (50 kW), your new peak demand now occurs overnight:
50 kW (existing overnight demand) + 750 kW (overnight charging demand) = 800 kW
In this example, the chargers created 750 kW of additional demand. However, the previous peak demand, occurring during the day, was 200 kW. The new overnight peak demand was only 800 kW, an increase of 600 kW instead of the entire 750 kW the chargers created. Because the charging occurred during previously off‐peak hours, a portion of the charger demand was offset by the daytime facility usage.
Example 2:
A separately metered 450 kW fast charger is installed. The highest demand possible for that charger would be approximately 450 kW, which would occur if a bus took 15 minutes to charge:
1 charger × 450 kW × (15 minutes of charging / 15-minute demand window) = 450 kW
However, many buses fully charge in less than 15 minutes. If a bus takes 10 minutes to charge, the peak demand would be approximately:
1 charger × 450 kW × (10 minutes of charging / 15-minute demand window) = 300 kW
For separately metered fast chargers, the peak demand will occur during the longest charge event for each month.
Energy Charges
Energy charges are typically calculated by adding up the total amount of energy use (kWh) over a billing period and multiplying that amount of energy by a pre-determined rate ($/kWh). Utilities may charge the same dollar amount per kWh of energy used no matter when it is consumed.
However, utilities may also place different values on a kWh of energy consumption depending on the time of day or season, or tier rates based on the overall amount of energy consumed.
Fixed and Other Costs
Utilities may charge a monthly service fee to connect to the electric grid, which typically makes up a small fraction of a monthly bill. Other bill charges may include local taxes, renewable energy production, energy efficiency, and the method and source of energy production.
Electricity Rate Modeling
An agency can begin to estimate expected charging costs once they have developed an understanding of their electricity rate and how it is structured. Developing an electricity rate model can allow an agency to explore the potential range of outcomes for the cost of charging their BEBs. An electricity rate model should incorporate the following elements.
- Estimated bus mileage and energy efficiency, including seasonal variations
- Estimated or actual charger power, including an estimate of efficiency losses
- Estimate the expected time of day charging will take place
- Estimate the maximum overall peak demand (kW) and energy consumption (kWh)
- Include additional power and energy loads if chargers are on the same utility meter as other facilities
- Include multiple scenarios to estimate a range of outcomes
While an agency will be unable to capture each and every scenario, a utility rate model can provide an expectation of the range out outcomes possible based on vehicle utilization and charging profiles.
Charge Management
Charge management is the process by which a vehicle operator attempts to limit the power needed to charge vehicles. This optimization can be achieved through operational processes or software management solutions. From an operational perspective, there are two main strategies that can be used to limit power needed to charge vehicles:
- Reduce the number of vehicles charging simultaneously
- E.g. A fleet with 10 BEBs charges the first five buses simultaneously, followed by the remaining five buses, cutting the agency’s demand for kilowatts by half.
- Reduce the amount of power used to charge vehicles
- E.g. An agency has an eight-hour window to recharge buses that would only require four hours to recharge using their charger’s full power output. The agency reduces the power output of the chargers to charge the buses over the 8 hours and cuts their demand for kilowatts by half.
It is important that an agency has a clear understanding of the service needs of the BEBs when assessing whether a charge management strategy is viable. Charge management may be difficult when deploying buses that return from service simultaneously and have a short window in which to charge before their next pullout time. However, for agencies that have more flexibility in how they deploy BEBs, there are ways to meet service requirements while minimizing power needs and charging costs.
Software solutions do exist from both charging manufacturers and third-party companies to automatically manage charging. These solutions vary in level of automation but have the potential to provide savings both in charging costs and staff time required to oversee a charge management solution. Software-based charge management is still in the early phases of rollout and utilization in the United States, although fleets in other countries have more experience with these solutions.
There are also training-based solutions that can provide immediate benefits and minimize charging costs. It is recommended that training be provided to all relevant staff including operators, maintenance, and management personnel. Proper training can decrease the likelihood that a bus will be charged during a time that would cause the peak demand to increase beyond the agency’s goal or that buses are left plugged in well beyond when they reach 100% state-of-charge, which can lead to unnecessary additional costs.
Charge modeling and management represent opportunities both pre-and-post-deployment for agencies to develop a better understanding of the impact of BEB operations on charging costs. Charging costs are informed by both the power required by the chargers and overall energy consumption of the BEBs. Early partnerships and conversations with an agency’s utility can lead to a better understanding of how charging will impact electricity bills. These conversations may even lead to the utility agreeing to revisit an agency’s electricity rates and offer new rates that better serve both the customer and utility. And while the number of options for charge management may be limited for some agencies based on service requirements, there are still opportunities to ensure that charging is done in the most efficient and cost-effective way possible.