Energy-efficient and reliable urban rail transit: A new framework incorporating underground energy storage systems

As the scale of urban rail transit system (URTS) gradually expands, issues related to energy consumption, economic benefits, carbon emissions (CEs), and power supply resilience are becoming prominent. First, urban rail transit systems (URTSs) operate with a high volume of passengers and frequent schedules, leading to substantial energy consumption. Moreover, the heavy reliance of URTSs on electricity exacerbates the challenge of CEs. The current energy structure of power grids is still dominated by fossil fuels, resulting in a significant carbon footprint during the electricity generation process. Additionally, the quasi-public goods nature of URTSs and rising energy costs further exacerbate their operating deficits. Furthermore, disruptions to the power supply due to failures or unforeseen events could have significant impacts on the safety of URT operations.

The research mentioned in this paper summarizes and analyzes the innovation explorations in promoting the energy conservation, emission reduction, and resilience enhancement for URTSs. However, there is still a lack of systematic and holistic approaches to energy conservation measures. The application potential of green energy remains to be tapped. And there is insufficient flexibility to adjust resource allocation for improving power supply resilience.

We proposed a novel framework of urban rail transit incorporating underground energy storage systems (UESS), exploring the potential of different forms of energy in URTS and forming a new low-carbon, efficient, safe, and reliable energy use mode. URTSs traverse residential, commercial, and industrial zones, featuring both underground and elevated lines. Expertise in underground construction has addressed the security challenges of integrating UESS. Elevated rooftops have facilitated increased penetration of renewable energy. The UESS serves as a versatile and dependable regulation hub in URTS, attributed to its swift response rate, elevated energy density, and robust underground space protection capabilities. The PV generation system and RBE recovery system collectively contribute to the provisioning of renewable energy for the electrical infrastructure within stations and trains.

The comprehensive benefits of utilizing UESS in URTS mainly focuses on economic efficiency, carbon emission reduction, and resilience enhancement. The most concerned economic benefits are achieved through improving URTS load characteristics. The UESS-URTS addresses price mechanisms and renewable energy impacts, drastically cutting energy costs. Meanwhile, low-carbon benefits arise from reducing fossil fuel consumption and shifting loads during high carbon intensity periods. Moreover, resilience enhancement is reflected in minimizing power outage losses and providing energy support in extreme conditions, leveraging underground space for reliable backup power and utilizing stored energy for operational continuity. The longitudinal-depth protection framework enhances the usability of UESS in extreme conditions, providing emergency power for vehicle traction, underground station lighting, and tunnel ventilation, thereby mitigating the consequences of secondary disasters. The case study demonstrated the effectiveness of proposed framework applied in the Nanjing Metro Line 3. The NPV achieves 54.802 million CNY and 71.642 million t-CO₂eq CE are reduced.

The study is published on 03 June 2025, in iEnergy.

About iEnergy

iEnergy publishes original research on exploring all aspects of power and energy, including any kind of technologies and applications from power generation, transmission, distribution, to conversion, utilization, and storage. It provides a platform for delivering cutting-edge advancements of sciences and technologies for the future-generation power and energy systems.  It has been indexed by ESCI, Ei Compendex, Scopus (CiteScoreTracker 2025 9.0), Inspec, CAS, and DOAJ.

iEnergy is a quarterly journal launched on March 2022. It has published 4 volumes (13 issues). Authors come from 21 countries, including China, the United States, Australia, etc., and world’s top universities and research institutes, including University of Nebraska Lincoln, Columbia University, Imperial College of Science and Technology, Tsinghua University, etc. 12 published articles are written by academicians from various countries. The published papers have also attracted an overwhelming response and have been cited by 179 journals, including top journals in the field of power and energy like Nature Materials, Advanced Materials, Advanced Functional Materials, Advanced Energy Materials, etc., from 45 countries.