Compressed air energy storage enhanced by gravity

Scientists in China have simulated an advanced adiabatic compressed air energy storage, to which they added an elastic airbag with a heavy load situated above it. The energy, exergy, and economic analysis of the system showed that, due to the constant weight of the heavy load, the airbag’s pressure level remains unaltered during operation.

A research group from China’s Northeast Electric Power University has proposed a novel advanced adiabatic compressed air energy storage (AA-CAES) system.

The proposed system utilizes a heavy load, an elastic airbag, and an abandoned vertical mine shaft, transforming the AA-CAES into a gravity-assisted isobaric system, where the pressure remains constant.

“By designing a novel isobaric air storage reservoir, the system achieves isobaric operation,” the researchers explained. “An abandoned vertical mine shaft is used as the air storage reservoir to maximize the land use. In addition, upon completion of the discharging phase, the air that has been stored can be entirely expelled, improving the energy storage density.”

The system was simulated using MATLAB software for energy, exergy, and economic analysis. It utilizes excess power from PV, wind, or grid power to drive a compressor. That, in turn, turns the ambient atmosphere into high-pressure air, which is then stored in an air storage reservoir (ASR). The process involves five stages of sequential compression to reduce energy consumption, accompanied by five intercoolers that capture the heat generated during compression. The ASR includes an abandoned vertical mineshaft, a heavy load, and an elastic airbag.

The elastic airbag was placed at the bottom of the mineshaft, while the heavy load was installed above the airbag. During the charging phase, the valve V1 opens and the valve V2 closes, with the high-pressure air entering the airbag, which increases the volume of the airbag and the height of the airbag. However, due to the constant weight of the heavy load and the constant contact area between the heavy load and airbag, the airbag’s pressure level remains unaltered during the charging operation, achieving an isobaric air charging process.

“Upon completion of the charging procedure and reaching the intended peak volume, the airbag is fully charged, symbolizing the fulfillment of its designed maximum capacity. During the discharging phase, the valve V2 opens and the valve V1 closes. With the action of gravitational potential energy of the heavy load, the air is squeezed out from the airbag. By the same principle mentioned above, an isobaric air discharging process can be achieved. The airbag is fully discharged when its volume reaches the designed minimum value and the discharging process is over,” the scientists explained.

The simulations assumed an ambient temperature of 25 C and an ambient pressure of 0.1 MPa. The density of the heavy load was set at 7,870 kg/m3, with the compressor consuming 5,880.82 kW. Prices for the system were based on the chemical engineering plant cost index (CEPCI): price of electricity for the discharging period was $0.18/ kWh, price of heavy load was $0.1/ kg, price of hot water $0.018/ kWh, and the price of electricity for the charging period was $0.04/ kWh. The system was assumed to operate 350 days per year with a service life of 25 years.

The analysis showed that the energy efficiency (ENE) was 87.1%, the exergy efficiency was 70.07%, the air energy storage density was 2.68 kWh/m³, and the occupied space energy storage density was calculated to be 2.29 kWh/m³. The turbine and compressor experience the largest exergy losses, accounting for 35.21% and 30.98% of the total exergy losses of the system, respectively; while the intercooler and interheater have lower exergy efficiency with the lowest 63.54% and 50.60%, respectively.

Additionally, the economic analysis results revealed a levelized cost of energy (LCOE) of $0.0804/kWh. The net present value (NPV) was calculated to be $1.6 million, with an internal rate of return (IRR) of 17.93% and a dynamic payback period (DPP) of 8.36 years. The results have appeared in “3E analysis and multi-objective optimization of a novel isobaric compressed air energy storage system with a gravity-enhanced air storage reservoir,” published in Case Studies in Thermal Engineering.