Hybrid storage of compressed air and thermal energy in post-mining infrastructure

Scientists in Poland have developed compressed air energy storage technology using thermal energy storage systems (TES) embedded in abandoned mine shafts. The system operates without an external heat source and uses an air compressor, a compressed air storage tank with a built-in thermal energy storage system, and an air expander.

A team of scientists from the Silesian University of Technology in Poland has developed compressed air energy storage technology (CAES), which uses thermal energy storage systems (TES) built into abandoned mines to efficiently repurpose compressed air tanks. “Our storage concept aims to reuse and protect underground infrastructure after mining, which is often irreparably damaged after mine closure,” said Lukasz Bartela, correspondent for pv magazine, author of the study.

The Group believes that mining areas have the potential to provide low cost energy infrastructure. “The mines are often located near power plants and/or distribution stations,” the researchers said. “This makes it possible to use the existing grid connection infrastructure. In addition, proximity to highly developed industrial areas reduces power transmission losses. There is no need to build above-ground storage tanks for thermal power plants, which saves the limited available space.”

The system operates without the use of an external heat source and uses an air compressor, a compressed air storage tank with built-in thermal energy storage and an air expander. System elements can be single-stage or two-stage.

In the proposed configuration of the system, the TPP tank is built-in and connected to the concentrator. This helps to reduce heat loss even as the heat leaves the storage material and passes through the air in the shaft tank. The TPP system adapts to the geometry of the mine, reducing the heat transfer field, which has a positive effect on the energy efficiency of the heat storage process.

“The biggest advantage of placing the TES system in the volume of a pressure tank is the possibility of using a thin-walled shell structure that houses the material for storing thermal energy,” explained Bartela. “This can significantly reduce the cost of the CHP system.”

To divide the tank into sections, steel cylinders with a perforated bottom are used, which makes it easy to install and periodically inspect the layer of heat-absorbing material. “Communication between sections will be possible with the help of ladders, which are also part of the TES vertical positioning system,” the scientists noted.

During the charging phase, electricity is used to drive the compressor. Hybrid underground systems supply hot compressed air to the tank through an inlet piping with built-in shut-off valves. The air then passes through the TES system, heating the stored material.

During the unloading phase, air passes through the TES system, removing heat from the accumulated material. The hot air then enters an expander, which drives a generator to generate electricity. “It is advantageous to fill the TES device with a sufficient volume of suitable thermal storage material to absorb heat, which is important for high cooling of the stored air,” the group stressed. “By limiting the temperature of the air in the storage that is in contact with the underground tank shell, heat loss will be reduced, which will increase the energy storage efficiency of the CAES system.”

They calculated a compressed air storage capacity of 60,000 cubic meters and a maximum pressure of 5 megapascals (MPa). Based on this, they calculated that the facility would have an energy storage capacity of 140 MWh with a round trip efficiency of about 70 percent and a storage tank with an energy efficiency of 95 percent.

They also explain that in special cases, the air pressure can be maintained at a level of up to 8 MPa, depending on the specific characteristics of the mine. “In this case, the energy capacity of the system can exceed 200 MWh,” emphasizes Bartela. “From an economic point of view, it would be most beneficial to use ordinary rocks in the TES system, such as granite or basalt. However, research into alternative synthetic materials is currently being carried out at the Silesian University of Technology.”

The group presented the concept of energy storage in a study recently published in the Energy Storage Journal evaluating the energy potential of an adiabatic post-harvest compressed air energy storage system based on a new thermal energy storage system. “Currently, we are only working on optimizing the design of the heat storage tank, which can reduce the cost of the CHP modules,” concludes Bartela.

According to the group, Poland has 139 active coal mines and 34 active copper and rock salt mines. There are currently plans to decommission 39, about half of which are used for pumping water. The deepest mine has a depth of over 1300 meters.

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