Rising air injection rates boost compressed air energy storage efficiency

A research team from China’s Hunan University of Technology has analyzed energy storage rates in compressed air energy storage (CAES) caverns under different air injection processes.

The research aimed to determine how different air injection strategies affect the thermodynamic behavior and energy storage performance of CAES caverns, while accounting for heat transfer during air transport through the wellbore and heat exchange between the cavern and surrounding rock. The researchers sought to identify operating and structural parameters that could improve energy storage rates and provide guidance for the design and operation of underground CAES systems.

The researchers developed a numerical model of a wellbore-cavern CAES system that accounts for turbulent flow, the thermophysical properties of real air, gas-solid conjugate heat transfer, and heat conduction in the surrounding rock.

“Compared with conventional lumped thermodynamic models, the proposed model can describe the spatially non-uniform evolution of the temperature field, pressure field, and wall heat loss inside the cavern,” the group said. “Compared with computational fluid dynamics (CFD) models in which the injection and discharge boundaries are set directly at the cavern inlet, this study further considers the thermodynamic state variations of air during wellbore transport and their coupling with the cavern flow and heat transfer processes.”

The scientists built a numerical model that solved equations for the conservation of air mass, momentum and energy. It used a standard k–ε turbulence model and calculated heat transfer through the wellbore and cavern walls into the surrounding rock.

They validated the model against operating data from Germany’s Huntorf CAES plant, representing its cavern as a simplified cylinder with a radius of 20 meters and a height of 112.2 meters. The maximum differences between the simulated and measured values were 2.66 K for temperature and 0.30 MPa for pressure, which the researchers described as “acceptable.”

For subsequent simulations, the researchers modeled a spherical cavern with a radius of 40 meters and a volume of 268,000 m3, connected to a 30-meter wellbore.

They compared four air injection patterns: a flow rate increasing linearly from 10 kg/s to 150 kg/s; a rate of 110 kg/s for five hours followed by 30 kg/s for three hours; intermittent injection with pauses and progressively higher flow rates; and a constant rate of 80 kg/s. Each pattern injected the same total mass of air over eight hours.

In separate simulations, the researchers compared wellbore diameters of 0.5 meters, 1.0 meters, 1.5 meters and 2.0 meters. They also tested surrounding-rock thermal conductivities corresponding to basalt at 2.17 W/(m·K), granite at 2.90 W/(m·K), and quartzite at 5.53 W/(m·K).

“Among the different air injection strategies, the linearly increasing mass flow rate mode increases the average temperature by 1 K, the average pressure by 0.13 MPa, the work done on the gas by approximately 12.7%, and the wall heat loss to the surrounding rock by approximately 11% compared with the constant mass flow rate injection mode,” the researchers said. “The energy storage rate increases from 85.5% under the constant injection mode to 87.5% under the linearly increasing injection mode.”

The researchers also found that wellbore diameter is an important structural parameter affecting heat loss during air transport. Increasing the diameter from 0.5 meters to 2.0 meters reduced heat loss through the wellbore wall from 8.5 × 10^8 J to 2.5 × 10^8 J, while increasing the average air temperature in the cavern by about 1 K.

“When the surrounding rock changes from basalt with a thermal conductivity of 2.17 W/(m·K) to quartzite with a thermal conductivity of 5.53 W/(m·K), the conductive heat loss increases by approximately 7%,” the team said. “At the end of air injection, the adiabatic model overestimates the average temperature and pressure by 1.7 K and 0.05 MPa, respectively. At the end of air discharge, the corresponding overestimations are 2.2 K and 0.06 MPa, respectively.”

The researchers presented their findings in “Analysis of energy storage rate in compressed air energy storage caverns under different air injection processes,” published in Case Studies in Thermal Engineering.

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