When it comes to fighting climate change, electric buses are a triple threat: they encourage energy-efficient levels of urban population density; take dozens of polluting vehicles off the street; and don't release tailpipe emissions.
The popularity of this approach presents its own challenges, however: cities can deploy electric buses faster than their power grids can keep up with the increased demand.
For University of Utah engineering professor Xiaoyue Cathy Liu , this challenge is an opportunity—not just to solve the immediate problem of grid stability, but to radically rethink how public transportation systems are integrated into other parts of civic infrastructure.
"Integrating onsite solar power generation and energy storage at bus depots introduces a brand new renewable energy production and management mode," Liu said, "transforming a public transport depot into an energy hub that produces more electricity than it consumes."
A professor in Price College of Engineering's Department of Civil & Environmental Engineering, Liu recently published a study in the journal Nature Energy that analyzes the potential of this approach using data from Beijing's fleet of electric buses. The international collaboration includes researchers from China's Beihang University, Sweden's Chalmers University of Technology and Germany's Fraunhofer Institute for Systems and Innovation Research ISI.
Beijing's 27,000 buses form the largest public transportation system in the world. More than 90% of those in service as of 2022 are low- or no-emission vehicles. These battery-powered buses recharge through a network of more than 700 bus depots spread across 6,500 square miles, a substantial piece of physical infrastructure that runs in parallel with the region's electrical grid. And given the power demands of the vehicles they serve, these depots put a heavy load on that grid, raising the potential for localized brownouts or other disruptions.
Using advanced data science scene techniques, Liu and her colleagues are exploring whether locally generated solar power would be sufficient to counterbalance this demand. Critically, they are also studying the complicated economic factors that would determine this approach's feasibility.
"More than meeting demand, our simulations show that these depots could net out to be energy producers, further stabilizing the grid," Liu said.
The study is based on a computer model of the Beijing bus network, replete with real-world data on air temperature and solar irradiance at each depot, recorded over the course of 2020. Combined with the rooftop surface area of each depot, the researchers were able to predict the electric output of solar panels that could be installed there.
Adding to the complexity of this model is the degree of variation between depots, in terms of both supply and demand. With more buses to charge, busier depots can make the most of a day's sunshine, while more remote depots would need to store or redistribute their excess electricity lest it go to waste.
"We found energy storage to be the most expensive factor in the model, so smarter and strategic charging schedules would need to be implemented," Liu said. "That responsiveness is critical, as variable energy pricing schemes have such a large impact on the overall economics."
The researchers aim to further generalize their model, providing a pathway for other countries to estimate the return-on-investment of similarly transforming bus depots and other pieces of civic infrastructure into energy hubs.
