• Tunnel and Underground Space
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• v.23 no.5
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• pp.442-453
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• 2013

A large-scale high-temperature thermal energy storage(TES) was numerically modeled and the heat loss through storage tank walls was analyzed using a commercial code, FLAC3D. The operations of rock cavern type and above-ground type thermal energy storages with identical operating condition were simulated for a period of five consecutive years, in which it was assumed that the dominant heat transfer mechanism would be conduction in massive rock for the former and convection in the atmosphere for the latter. The variation of storage temperature resulting from periodic charging and discharging of thermal energy was considered in each simulation, and the effect of insulation thickness on the characteristics of heat loss was also examined. A comparison of the simulation results of different storage models presented that the heat loss rate of above-ground type TES was maintained constant over the operation period, while that of rock cavern type TES decreased rapidly in the early operation stage and tended to converge towards a certain value. The decrease in heat loss rate of rock cavern type TES can be attributed to the reduction in heat flux through storage tank walls followed by increase in surrounding rock mass temperature. The amount of cumulative heat loss from rock cavern type TES over a period of five-year operation was 72.7% of that from above-ground type TES. The heat loss rate of rock cavern type obtained in long-period operation showed less sensitive variations to insulation thickness than that of above-ground type TES.

• Tunnel and Underground Space
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• v.24 no.4
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• pp.297-307
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• 2014

Cavern Thermal Energy Storage system stores thermal energy in caverns to recover industrial waste heat or avoid the sporadic characteristics of renewable-energy resources, and its advantages include high injection-and-extraction powers and the flexibility in selecting a storage medium. In the present study, the structural stability of rock mass pillar between these silo-type storage caverns was assessed using a coupled thermal-mechanical model in $FLAC^{3D}$. The results of numerical simulations showed that thermal stresses due to long-term storage depended on pillar width and had significant effect on the pillar stability. A sensitivity analysis of main factors indicated that the influence on the pillar stability increased in the order cavern depth < pillar width < in situ condition. It was suggested that two identical caverns should be separated by at least one diameter of the cavern and small-diameter shaft neighboring the cavern should be separated by more than half of the cavern diameter. Meanwhile, when the line of centers of two caverns was parallel to the direction of maximum horizontal principal stress, the shielding effect of the caverns could minimize an adverse effect caused by a large horizontal stress.

• Tunnel and Underground Space
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• v.23 no.3
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• pp.241-259
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• 2013

A packed bed thermal energy storage (TES) consisting of solid storage medium of rock or concrete through which the heat transfer fluid is circulated is considered as an attractive alternative for high temperature sensible heat storage, because of the economical viability and chemical stability of storage medium and the simplicity of operation. This study introduces the technologies of packed bed thermal energy storage, and presents a numerical model to analyze the thermal energy balance and the performance efficiency of the storage system. In this model, one dimensional transient heat transfer problem in the storage tank is solved using finite difference method, and temperature distribution in a storage tank and thermal energy loss from the tank wall can be calculated during the repeated thermal charging and discharging modes. In this study, a high temperature thermal energy storage connected with AA-CAES (advanced adiabatic compressed air energy storage) was modeled and analyzed for the temperature and the energy balance in the storage tank. Rock cavern type TES and above-ground type TES were both simulated and their results were compared in terms of the discharging efficiency and heat loss ratio.

• Economic and Environmental Geology
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• v.43 no.2
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• pp.197-205
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• 2010

Recently abrupt climate changes have been occurred in global and regional scales and $CO_2$ reduction technologies became an important solution for global warming. As a method of the solution shallow underground thermal energy storage (UTES) has been applied as a reliable technology in most countries developing renewable energy. The geothermal energy system using thermal source of soil, rock, and ground water in aquifer or cavern located in shallow ground is designed based on the concept of thermal energy recovery and storage. UTES technology of Korea is in early stage and consistent researches are demanded to develop environmental friendly, economical and efficient UTES systems. Aquifers in Korea are suitable for various type of ground water source heat pump system. However due to poor understanding and regulations on various UTES high efficient geothermal systems have not been developed. Therefore simple closed U-tube type geothermal heat pump systems account for more than 90% of the total geothermal system installation in Korea. To prevent becoming wide-spread of inefficient systems, UTES systems considering to the hydrogeothemal properties of the ground should be developed and installed. Also international collaboration is necessary, and continuous UTES researches can improve the efficiency of shallow geothermal systems.