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Electrification is vital to decarbonizing the transportation sector, which includes public transit fleets. Medium- and heavy-duty vehicles—including trucks and buses—are the second largest source of transportation-related greenhouse gas emissions, and their zero-emission vehicle sales are expected to reach 30% by 2030 and 100% by 2040. Although adoption of electric buses is increasing, they comprised only 2% of the U.S. transit bus fleet in 2021. Fleets are committed to retiring fossil-fuel-powered buses for electric buses, including New York City’s Metropolitan Transportation Authority (MTA), which is aiming to make all 5,800 of its buses zero emission by 2040.
As adoption increases, so does demand for charging. Technology improvements like direct-current fast charging reduce charging times and increase adoption. Current scenarios project an estimated 80,000 battery-electric buses in operation by 2050, meaning charging infrastructure must keep pace and depot operations must adapt.
The added infrastructure demand from increased adoption, especially for the large batteries that power medium- and heavy-duty vehicles, requires a deeper understanding of electric grid impacts and creates a need to evaluate opportunities to increase cost-effectiveness and energy-effectiveness. Greater cost efficiency and energy efficiency are critical for deploying zero-emission transportation, but high-power charging challenges must be addressed, including:
Larger and more variable charging loads that yield higher utility costs.
Higher and more variable utility price structures, which often include demand charges and time-of-use components—different rates charged at different times.
Costs for upgrading existing transmission and distribution infrastructure.
Increased reliance on the electrical grid for electric vehicle (EV) and electric bus charging.
Increased congestion due to charging on main roads.
Transit fleets are complex operations with many moving parts, adding a layer of challenges for fleet operators looking to transition to electric fleets.
“Greater electricity demand and associated higher costs are the price of admission with high-power charging,” said Roberto Vercellino, mobility engineer at the National Renewable Energy Laboratory (NREL). “But behind-the-meter resources are an available, effective means of tackling those challenges without impacting operation.”
Behind-the-Meter Storage and Distributed Energy Resources: Addressing EV Charging Challenges
Distributed energy resources—small generation and storage units located near sites of electricity use, like rooftop solar, EVs, and battery storage systems—are key to the future grid, expanding energy generation opportunities. Behind-the-meter (BTM) energy storage resources are distributed energy resources that can create a cost-effective, reliable, resilient, and sustainable power system.
Pairing EV and battery-electric bus fast charging infrastructure with BTM energy storage and generation resources can provide a solution to many of the challenges presented here. BTM resources can help minimize the demands vehicle electrification can place on the electrical grid while optimizing cost efficiency and energy efficiency of EV charging systems.
BTM battery storage is being leveraged at commercial, industrial, and residential levels, as it proves effective in assisting EV fast charging, particularly for fleet vehicles. On-site photovoltaic generation adds further benefits, producing clean and cheap electricity that can be stored and allowing customers to sell electricity back to the grid via net metering. BTM resources can lessen the load impact on the electrical grid, reducing or deferring potential distribution or transmission upgrades. On-site energy storage also enhances an EV charging station’s resilience during service interruptions.
“If we’re going to decarbonize the transportation sector, including transit bus and other fleets, we need to make sure that electrification is as cost efficient and energy efficient as possible,” said NREL mobility researcher, Gustavo Campos. “We’re seeing BTM resources deployed effectively in this way throughout the country, so we have a great opportunity to demonstrate its value to transportation decision makers.”
MTA and BTM Storage: A Case Study
On an average weekday, 5,800 New York MTA buses transport more than 2.1 million riders. MTA has committed to transitioning its entire bus fleet to zero-emission vehicles and battery-electric buses by 2040. Pilot testing has revealed range limitations and the need for expanded investment in charging infrastructure. BTM storage presents a solution for MTA and other organizations looking to electrify their transportation fleets. Researchers studied modeling data from MTA’s fleet of electric buses and the potential for BTM storage.
“The Joint Office of Energy and Transportation [Joint Office], in support of the Federal Transit Administration, is providing free technical assistance to Low- or No-Emission Grant Program applicants. Through this avenue MTA reached out and asked us about assessing the impacts of BTM resources on bus electrification and on the grid,” said NREL mobility project manager Ryan Frasier. “Our modeling and simulation capabilities can help MTA plan for fleet electrification, and later be replicated by other agencies.”
Modeling electric bus energy consumption, researchers simulated bus routes for an entire year based on real schedules. Using EVI-EDGES, a modeling and analysis tool from NREL, they utilized high-performance computing and optimization techniques to produce high-fidelity simulations of BTM storage and generation integrated with MTA bus fleet operation.
“The Joint Office is ready to support transit agencies, school districts, and other public agency fleet managers with detailed technical assistance to ensure a smooth transition to an electric future,” said Jeff Peel, Joint Office deployment manager. “This analysis for MTA showed the potential to save over $2 million in operations annually—money the MTA can reinvest into more and better service for passengers.”
MTA’s Kingsbridge bus depot was selected as the initial location for this study after MTA requested technical assistance from the Joint Office through the Low-No Emission Transit Vehicle concierge service. Here, researchers evaluated the benefits of BTM resources under different combinations of utility rates, bus routes, charging schedules, and charging station configurations to account for uncertainty in the future. Averaged across scenarios, appropriately sized and controlled BTM resources showed more than 35% in annual utility cost savings. Subtracting the investment for the storage and photovoltaics, these savings translated into a 19% reduction, equivalent to $15 million—or $2.08 million annual operations savings—in the total cost over the project’s lifetime of 20 years. These results were shown to be highly robust and not particularly sensitive to input assumptions like technology costs, equipment lifetime, and discount rate.
“Modeling tools can be used to optimize the design of BTM resources, reducing energy and cost concerns,” Campos said of NREL’s work. “We can showcase the benefits of electrification, behind-the-meter resources, and high-power charging all before any technology is deployed.”
“Overcoming the challenges presented by fast charging is important to EV charging electrification,” Vercellino said. “As we showed in the MTA case, BTM resources can address those energy challenges while providing significant cost savings.”
Courtesy of the Joint Office of Energy and Transportation.
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