Transportation accounted for 26% of global energy consumption in 2020 and contributed 20% of global carbon dioxide (CO2) emissions in 2021. The sector is undergoing rapid electrification to achieve net-zero goals worldwide, with China leading the charge. However, this transition could strain electricity grids, particularly with high-power charging modes such as those required for battery electric buses (BEBs). What challenges will electricity grids face if BEBs replace all diesel and natural gas buses in a city? How can public transport (PT) operators mitigate these impacts through strategic and tactical measures? This research stems from the authors’ extensive work in bus fleet electrification, aiming to answer these critical questions.
Now, I am pleased to share the story behind the paper titled “Transforming public transport depots into profitable energy hubs.” The inspiration for our research emerged from the growing focus on integrating transportation with renewable energy systems. We were interested in the energy island and self-sufficiency in the beginning. Therefore, we introduce solar photovoltaic (PV) and battery energy storage at bus depots (charging hubs). Leveraging extensive bus operational and meteorological data from Beijing, housed at ACME Lab (http://www.trevorma.com/), the concept took shape and became the cornerstone of the first author’s Ph.D. dissertation in 2021. To turn this vision into a publication, the corresponding author reached out to co-authors, sharing the ideas and datasets to foster a global collaboration with their expertise from energy and transportation research in 2023. We began our journey by diving into the available datasets. Through several rounds of discussion, we developed our research framework. This process was enriched by the expertise of Sonia Yeh in energy economics and energy system modeling, as well as Patrick Plötz’s deep knowledge of transportation electrification. For example, Sonia and Patrick suggested that we look at the grid as a whole, consider economics, and find the most economically optimal solutions for the bus depot operators. After receiving the reviewers’ comments from the first round, we decided to increase the real-world relevance of our study. We reached out to the largest BEB charging service provider in Beijing to understand the details of charging regulations and strategies from the perspective of a public transport operator. This collaboration allowed us to align our research with real operational conditions, transitioning our work to practical, applicable solutions.
Our framework first simulates a baseline scenario in which BEBs replace all buses of other fuel types in Beijing. The complete bus fleet electrification is simulated from bus GPS trajectory data, vehicle type data, grid electricity price schemes, and bus depot data. These four real-world datasets are used to simulate BEB energy consumption, optimize BEB battery capacities, and optimize BEB charging schedules under perfect foresight. To transform bus depots into energy hubs, we estimate solar PV generation based on bus depot data, air temperature data, and solar irradiance data. Combined with three scenarios related to subsidy policies for solar PV, we maximize the economic profits for solar PV and energy storage by optimizing the installed capacity of solar PV, energy storage capacity, bus charging schedules, solar PV use, and energy storage use. This work uses Beijing’s entire public bus transport network to illustrate how the different datasets are used to quantify the technical, economic, and environmental benefits of transforming PT depots into energy hubs. This work outlines the daily BEB charging load curves at each bus depot under a 100% electrified bus system scenario. The works simulates daily BEB charging schedules, solar PV usage, and energy storage usage, offering minute-by-minute resolution and accounting for seasonal variations.
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Figure 1. Data-driven framework for transforming PT depots into energy hubs.
We show that solar PV reduces the grid’s net charging load by 23% during electricity generation periods and lowers the net charging peak load by 8.6%. Integrating energy storage amplifies these reductions to 28% and 37.4%, respectively. While unsubsidized solar PV yields profit 64% above costs, adding battery storage cuts profits to 31% despite offering grid benefits. The sensitivity analysis demonstrates the impact of energy storage cost and grid electricity pricing on the net profit of integrating solar PV with energy storage at bus depots. As energy storage technology continues to evolve, the economic benefits of solar PV and energy storage are expected to increase with reductions in energy storage costs. Additionally, this analysis indicates that the economic advantages of combining solar PV with energy storage are stronger in scenarios with a substantial difference between peak and off-peak grid electricity prices. Negative marginal abatement gains for CO2 emissions underscore the economic sustainability. Our findings provide a model for cities worldwide to accelerate their commitments toward sustainable transport and energy systems.
One of the angles we had not explored is the “willingness to pay” from the grid operators’ perspective to reduce the load from large local users like bus depots without adding more grid capacity. Another point is to extend the current study to multimodal transportation systems. In 2023, the number of electric light-duty vehicles on the roads globally surpassed 40 million, and this upward trend is set to continue. Imagine a scenario where over 100,000 electric light-duty vehicles are charging simultaneously in a single city. How can we plan the charging infrastructure to meet the diverse demands of buses, trucks, and light-duty vehicles? How can new technologies—such as advanced charging systems, innovative batteries, vehicle-to-grid integration, hydrogen energy, and solar PV energy—be harnessed to improve transportation and energy system interaction and resilience? This decentralized approach examined in our paper have multiple benefits that we are yet to explore. However, it is crucial to understand these advantages within the context of multimodal transportation systems. These questions are vital as we consider human acceptance and behavior, urban development, economic performance, and the challenges of climate change. We propose an open topic to address these concerns, acknowledging that significant work is needed in areas such as charging infrastructure planning and management, human behavior analysis, integrated transportation and energy system modeling, mobility equity, and energy vulnerability. By focusing on these areas, we can pave the way for a more sustainable and resilient future towards transportation and energy.