Utility Capacity and Demand Charges
For utilities to prepare to service a new electric bus base, the first question which will come up is how much peak power is required? The peak power is essential for the utility to correctly size the cables, transformers and other hardware. It is a major driver of the distribution costs and is paid for by monthly ‘demand charges’ (or sometimes excess service fees if it goes unused). Getting an informed estimate of the power demand for each base is an important aspect of preparing an agency wide battery electric bus plan.
Utilities think long term
Electricity utilities routinely connect new customers and new loads, but unexpected large new loads may not fit in their existing infrastructure, causing project delays and potentially costs. Utilities build out transmission cables, substations, generation and feeders to support their long-term estimates of needs. A 5-10-year horizon for planning is typical, with the costs amortized over as much as 40 years. An electric bus base is a large new load, often comparable to thousands of homes. Sometimes projects are fortunate, and the required power is available with minimal upgrades by the utility. In other cases the required power isn’t available without substantial time consuming upgrades. ChargeSim can help evaluate those scenarios and try to find charger configurations and strategies which operate within the available power. As with any design choice, a design margin should be added to accommodate changes or oversights in planning. The sooner you can provide a good estimate of your demand, the sooner the utility can start including that in their projections and plans.
What inputs do you need to use ChargeSim for power projections?
There are two basic inputs to an initial power projection:
- Energy Consumption (kWh)
With those inputs the required energy, and at what time of day power is required, can be simulated. From there, you can start exploring different charge management strategies to reduce the peak vehicle requirement.
The current schedule is often a good starting point for analysis, but there are a few factors to consider if it will be representative of a future schedule.
- Usually small changes, e.g. a specific bus leaving a few minutes earlier or later will have minimal impact on garage power requirements.
- The number of buses is usually set by the size of the parking. It may shrink slightly if more space is needed for charging infrastructure depending on how the base is designed.
- The mix of peak/off-peak/overnight services will impact charging plans. Peak service is usually the easiest to accommodate since the buses are driving fewer hours per day, leaving more time for charging including a large break mid-day. Off-peak service will have more mileage and service hours, requiring more energy and allowing less time on base, thus requiring higher power. Overnight service will often need blocks to be broken up and buses swapped to allow charging. All day service usually has the longest mileage and lowest time on base, therefor requiring opportunity charging in some cases.
- The average speed of the blocks is an important consideration, since a bus driving faster will use more energy than a slower one. A base supporting a BRT line or more suburban services may require more energy per bus than one supporting slower urban routes.
- New projects or strategic changes will impact the schedule. The opening of a new subway line could mean buses are allocated to more daytime, low speed local services as the subway takes over from long range and express bus services. Adding dedicated bus lanes or new more suburban routes can result in higher average speeds, and thus higher energy consumption.
In the case of a new base, a preliminary schedule will need to be prepared to start creating estimates. In preparing a projection, consider what the agencies strategic plans are, and if changes in the mix of service types and speeds is expected.
A key change often needed to go from a traditional diesel or CNG schedule to an electric schedule is to split long blocks due to the finite range of battery electric buses. Diesel buses usually carry more than enough fuel for a full day of driving, so scheduling a 23 hour, 300 mile block where the bus repeats a route all day (swapping drivers on-route) is feasible and reduces deadhead (also called non-revenue) mileage. With electric buses that is only possible with opportunity chargers recharging the bus throughout the day, which may not make sense on all routes. Splitting the blocks into shorter segments allows more time on base for individual buses to charge but results in more deadhead trips. Depending when the blocks are split and the deadhead distance required, it could be possible that the same service schedule will require more electric buses to operate. The additional vehicle needs can be mitigated by splitting blocks during off peak times. Scheduling considering the limited range for electric buses and on-route charging is a complex problem requiring scheduling software which considers electric bus constraints. For a first power approximation, long blocks can be manually split, and deadhead distance and time added; however, this further increases the uncertainty in the estimate.
Energy Consumption (kWh)
The energy consumption (kWh) for each block is another key input, it is usually calculated by taking the specific energy consumption (kWh/mi) and multiplying the distance for each block. Remember the size of the infrastructure needs to be designed for the worst-case day, not the average. Some contributors to energy consumption:
- Variation in driver behaviour will be averaged over the fleet.
- Temperature will impact the whole fleet simultaneously and needs to be considered in the worst case. Note that within a day there will be further variation with early mornings colder than late afternoons. The peak specific energy consumption will be lower if supplementary (usually diesel) heaters are used.
- Different sizes (for example articulated 60’ v. 40’ buses) will have different specific energy consumption and should be calculated separately.
Specific Energy Consumption in kWh/mi is a practical simplification to calculating the energy required for each block considering its unique drive time, average speed, topography and passenger loads. Some loads on the bus, like heating, will vary with time and number of stops (since opening doors loose heat) as opposed to distance. This could also be modeled as a separate term, but for preliminary projection selecting a high estimate combining all of them into one kWh/mi term and adding a safety factor can be sufficient.
The schedule is usually uploaded in excel format to ChargeSim, which then prepares summaries for review. The energy consumption (kWh) is specified for each block in the upload, usually just by multiplying the block distance by the specific energy consumption for the vehicle type. Other methods to estimate per-block energy consumption can be used if more detailed models are available. Usually existing scheduling reports can just be copied and pasted into the format for ChargeSim in just a few minutes. The ChargeSim team can also assist with importing from GTFS or other formats directly.
Using these inputs, and a quick generic charger configuration you can setup in ChargeSim, you can simulate what the charging power profile will be. By adjusting the utility limit setting, you can explore different potential utility limits to find the power level where the charging requirements are efficiently met.
ChargeSim training courses run through examples in detail, and help familiarize you with the simulation tool, and how to fully understand the outputs.