Researchers in China have developed a new compressed air energy storage system that uses flooded roads in abandoned coal mines to store compressed air and heat for nighttime power generation. Simulations show that the design can achieve a thermal efficiency of 71.5%, stable performance and higher energy density at greater depths, with minimal long-term effects of air leakage.
An international group of researchers led by scientists from China’s Chongqing University has developed and thermodynamically assessed a new compressed air energy storage (CAES) system that uses the roads of abandoned coal mines for underground storage.
The system is called Flooded Coal Mine Roadway Pressed Air Energy Storage (FM-CAES) and collects energy during the day and releases it at night. When electricity supply is plentiful during the day, excess energy from wind and solar power powers a multi-phase process that compresses air. This compressed air is then stored in abandoned coal mine tunnels. At the same time, the heat produced during compression is captured by intercoolers and transferred to a thermal storage tank. At night, the stored air is reheated and expanded through a turbine to produce electricity. The retained thermal energy is used to heat the air during this exhaust process, while the thermal storage medium cools back to ambient temperature.
The system consists of three phases: in the charging phase, the cooled compressed air enters the air storage track via the air injection channel. As air enters, the water level in the roadway gradually drops and the displaced water flows to other deeper paths. In the air storage phase, the high-pressure compressed air is maintained at a constant pressure by the combined action of the geological formations and the hydrostatic pressure of the surrounding water. In the final blow-out phase, the air outlet duct is opened, allowing the compressed air to escape. The hydrostatic pressure of the water provides nearly constant air pressure, minimizing fluctuations during drainage.
The FM-CAES system also uses water pressure to maintain constant air pressure, in an effort to improve efficiency and reduce leakage. It operates with charging and discharging periods of 8 hours each, with compressed air storage and water flooding lasting 4 hours. In the COMSOL 6.0 software simulation, the ambient temperature was 292.65 K, the air mass flow rate was 26.91 kg/s, and the air flow rate was 6.2 MPa. The nominal discharge power was 10 MW. Roads had a radius of 2.1 m, with a revetment thickness of 0.3 m.
Image: Chongqing University, Energy, CC BY 4.0
Through the simulations, the scientists found that the thermal efficiency of the FM-CAES system can reach 71.5%, with an energy storage density (ESD) of 29.72 MJ/m3. They also found that the system’s efficiency remained stable and was “largely” unaffected by air mass flow. “Increasing the water pressure within the roadway from 4 MPa to 12 MPa resulted in an increase in storage time by 38.2% and a reduction in air flow rate by 27.6%, accompanied by a slight improvement in system efficiency,” they pointed out.
The analysis also showed that effective air trapping was achieved with low permeability around rock and cladding. A critical permeability threshold of 1 × 10⁻¹⁷ m² was established for the surrounding rock, below which the variation in air leakage was minimal. For concrete coverings, permeability of less than 1 × 10⁻¹⁹ m² effectively stabilized system efficiency. Deeper roads showed higher average air leakage rate (AALR) and ESD, although after a 30-year simulation period, system efficiency was consistent across depths. This indicates that the impact of leakage rates on long-term efficiency is approaching saturation.
“The ESD increased more rapidly with depth, improving from 4.3 MJ/m3 to 6.7 MJ/m3 for each additional 100 meters of burial depth,” the academics concluded. “The exergy destruction of the system occurred mainly during the compression and expansion phases, resulting in exergy losses of 9% and 10.5%, respectively. Other phases suffered exergy losses of less than 4.3%.”
The system was presented in “Research on compressed air energy storage in abandoned flooded coal mine: thermodynamic analysis and applicability study”, published in Energy. Researchers from China’s Chongqing University, Sweden’s Luleå University of Technology and the French National Center for Scientific Research participated in the study.
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