Energy Storage Systems for EV Fleet Charging: A Case Study On Stanford University's Bus Depot

Electric vehicle (EV) fleets charged by solar energy can help reduce the carbon footprint of the transportation sector, which accounts for 28% of US greenhouse gas emissions (US EPA). Coupling solar and energy storage enables charging stations to operate with flexible schedules without increasing grid demand and significantly reduces the associated emissions.

Case Study: Stanford University Electric Bus Fleet

An interesting research paper was recently published by a group of researchers at Stanford University looking at optimizing the operations of electric bus fleets, on-site solar arrays, and battery energy storage systems to minimize costs and emissions. The case study was done on Stanford University's shuttle depot, and sheds light on how battery storage can play a critical role in EV fleet charging.

In 2013, Stanford’s diesel bus system was emitting 2,800 tons of CO2 annually, with a traveled distance of 1.1 million miles per year. However, Stanford purchased its first three all-electric buses in 2014, and the university was operating a fleet of 38 all-electric buses by 2020.

Stanford completed the transition to 100% renewable energy in March 2022, using a 5-MW solar carport on-campus and a 117-MW solar farm off-campus. Additionally, the solar carport is equipped with EV charging stations for small vehicles and Stanford’s electric bus fleet.

However, transitioning to 100% renewable energy doesn't mean zero-emission electricity. As shown in the chart below, solar generation is only available from around 7:00 to 18:00, and largely concentrated around noon. However, EV charging stations often operate outside of this schedule — relying on grid electricity.

Nighttime EV charging with grid electricity has both financial and environmental impacts. As shown in the graph above, grid emissions are significantly higher at night. This happens because, in the absence of solar generation, California must increase its electricity output from gas-fired power stations (NPR). Grid electricity is also expensive since California has some of the highest electricity rates in the US. As of September 2024, commercial customers were paying an average rate of 29.13 cents per kWh in the Golden State (US EIA).

Using optimized charging schedules and battery dispatch for its electric bus fleet, Stanford University can achieve the following benefits over a 10-year period:

• Direct electricity savings of $2.3 million, consisting of $0.9 million in energy charges and $1.4 million in demand charges. This estimate assumes that electricity rates continue to increase at 5% annually.
• Estimated transportation resiliency savings of $1.4 million, offering a total financial benefit of $3.7 million for Stanford University.
• A 98% reduction of bus fleet emissions, from a baseline of 3,160 tCO2 during 10 years to an estimated 70 tCO2.

ConclusionEnergy storage systems allow the sustainable operation of EV fleets, addressing both financial and environmental challenges. As demonstrated by Stanford University’s electric bus fleet, battery systems can improve the operational efficiency of solar-powered charging stations while achieving significant cost savings and lowering emissions.