A public-private partnership created a plan for U.S. school districts on the lowest cost to charge electric school buses.
The blueprint developed by the Center for Transportation and the Environment (CTE), environmental consultant NV5, and electric charging company The Mobility House was based on a study of California’s Stockton Unified School District (SUSD), funded by a $199,903 grant from the California Energy Commission focused on how the district could charge 100 percent of its electric school bus (ESB) fleet in current operations.
As a result of the study, the project team recommends school districts install Level 2 (AC) chargers unless there is a need for Direct Current (DC) fast charging and utilize a Charge Energy Management (CEM) system to limit charging to the most cost-effective times.
The team also found under California’s transition from Net Energy Metering 2.0 to Net Billing Tariff (NEM3.0), which went into effect on April 15 to increase the value of battery storage through solar power generation installed on homes and buildings, that solar photovoltaic, or PV, systems installed without CEM do not provide positive 25-year net present value savings for school bus charging when most of the charging will occur overnight.
Added costs associated with Vehicle-to-Grid (V2G) technology diminish returns and make it difficult to justify the potential cost benefits at this time, the team concludes adding that V2G use should continue to be researched as policies, tariffs and technology changes to make bi-directional charging more favorable.
While the study was framed by California rate structures — particularly electric vehicle time-of-use tariffs and incentivizing the Net Energy Metering 3.0 tariff for onsite solar generation to influence how district fleet managers orient their charging strategy — the results are considered broadly applicable to school districts planning for infrastructure to support ESBs.
The rapid transition to ESBs dovetails with other significant electric grid operations changes such as future V2G programs as well as solar production incentives with California’s anticipated NEM 3.0 transition in California.
Scheduling ESB charging to prioritize solar PV self-consumption will be key to making solar installations under NEM3.0 financially sound, the team notes. In considering AC and DC charging, the blueprint added that AC charging is sufficient for the charging scenarios evaluated for Stockton Unified, as higher-power DC charging is unnecessary. It added that AC hardware, installation, and ongoing costs are lower compared to DC charging.
The research team notes CEM usage can result in significant savings by limiting charging spikes and avoiding high-demand charges. In a 100–percent, AC-charging scenario, CEM is modeled to save the district more than $160,000 annually, or roughly 40 percent of energy costs from charging.
CEM will be critical in coordinating charging timing with time-of-use tariffs. Energy production from solar PV coordinating charging with solar production makes PV a viable option under NEM3.0. In the study, without CEM the majority of PV production is sent back to the grid at a low value and unmanaged charging occurs during peak Time-of-Use rates.
Regarding onsite distributed energy resources of PV and PV plus a battery energy storage system, or BESS, PV under NEM3.0 is only justified on a cost-savings basis if paired with CEM to optimize ESB charging to periods when PV generation is available, therefore reducing exported PV generation, the paper finds.
Charging the ESBs with a PV system sized to offset 90 percent of annual consumption results in lower costs to Stockton compared to a baseline without PV, where ESBs charge primarily overnight with a CEM.
The team also found using BESS to store and use PV generation was not cost-effective due to the current high cost.
Using CEM to shift ESB charging and manage demand was found to be a more cost-effective solution for Stockton.
For school districts able to secure a NEM2.0 solar PV system, solar PV sized to 90 percent offset is cost-effective with or without CEM, the team notes.
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“Supply chain is a bit of a challenge on chargers, but it can usually be mitigated because charging infrastructure projects generally take a while — a few months at least — to plan so you can plan around the charger lead times if you know them,” said Sam Hill-Cristol, who handles business development for The Mobility House. “Every once in a while, there will be a project with a quick turnaround. Lead time will limit what chargers we can sell.”
Hill-Cristol noted a more constraining supply chain issue is electrical infrastructure components like transformers and switchgears.
“Some big components, either on the customer side of the meter or the utility side if there is a service upgrade, can take over a year,” he added. “That is a huge issue. This is where we see the right software playing a big role.
“The biggest challenge for school bus electrification in the coming years is site-specific power upgrades on the distribution system that take the utilities a long time and make it hard for districts to plan. Inexpensive ways of overcoming that gap, such as the right charge management software, are key to making sure the grid can handle all of the new electrical load.”