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The thermal-hydrological-mechanical (T-H-M) behavior of rock mass surrounding a large-scale high-temperature cavern thermal energy storage (CTES) at a shallow depth has been investigated, and the effects of hydrological conditions such as water table and rock permeability on the behavior have been examined. The liquid saturation of ground water around a storage cavern may have a small impact on the overall heat transfer and mechanical behavior of surrounding rock mass for a relatively low rock permeability of . In terms of the distributions of temperature, stress and displacement of the surrounding rock mass, the results expected from the simulation with the cavern below the water table were almost identical to that obtained from the simulation with the cavern in the unsaturated zone. The heat transfer in the rock mass with reasonable permeability was dominated by the conduction. In the simulation with rock permeability of , however, the convective heat transfer by ground-water was dominant, accompanying the upward heat flow to near-ground surface. The temperature and pressure around a storage cavern showed different distributions according to the rock permeability, as a result of the complex coupled processes such as the heat transfer by multi-phase flow and the evaporation of ground-water.

In this technical report we summarize the observational data on thermal and mechanical properties of rocks reported by over 70 best published papers. The experimental results reported individually are integrated and presented in tables and figures here, which will provide fundamental data to fairly determine and evaluate the rock properties at the initial design stage of underground structures exposed to high temperature environment.Key words Thermal and mechanical properties of rock, High temperature초 록 본 기술보고서에서는 현재까지 발표된 국내외 70여 편의 주요 연구들을 토대로 고온 조건하 암석의 열적･역학적 물성에 대한 실험 데이터를 수집하여 요약 ･제시하였다. 대부분 특정 암석에 대해 일회성으로 보고되었던 선행연구들의 실험 결과를 보다 체계적으로 통합하여 제시함으로써 향후 열적 환경에 건설되는 암반구조물의 타당성 조사단계 및 초기 설계단계에서 이들의 활용성을 제고하고자 하였다 . 핵심어 암석의 열적･역학적 물성, 고온 1. 서 론 최근 지하공간을 활용한 에너지저장기술 (압축공기저장, LNG 저장, 열에너지저장)과 지열에너지 개발에 대한 관심이 증대되면서 다양한 온도 조건에서 암반의 열적 ･역학적 특성 변화와 이에 따른 지하구조물의 안정성 및 안정성에 대한 연구가 증가하고 있다. 또한, 방사성 폐기물의 안전한 격리를 위해 심지층 처분이 유력하게 고려되어 온 만큼 암석 및 암반의 열적 ･역학적 특성을 규명하는 것은 지난 수십 년간 중요한 연구 과제로 인식되어 왔다 .고온 조건에서 암석의 열적 ･역학적 특성에 대한 연구는 습곡, 지열, 마그마 관입, 지구조적 작용 (plate tectonics) 등 지각 활동에 대한 이해 (Heuze, 1983)와 고준위 방사성폐기물 처분장의 장기 안전성 평가를 목적으로 수행되어 왔다. 특히 방사성 폐기물의 심지층처분을 고려하는 미국, 스웨덴, 핀란드, 스위스, 일본 등과 같은 선진국을 중심으로, 최고 1200℃ 조건에서 열전도도, 비열, 열팽창계수, 강도, 변형 특성 등을 측정한 실험 연구가 다수 보고된 바 있다 (Robertson, 1988, Robertson과 Hemingway, 1995, Lima와 Paraguassu, 2004). 이러한 연구는 실험실에서뿐만 아니라 대규모 현장시험을 통해서도 이루어지고 있다 . 스웨덴 Aspo Hard Rock Laboratory에서는 암반의 열적･역학적 파괴 거동 예측을 위하여 대규모 필라 안정성 현장 시험 (APSE, Aspo Pillar Stability Experiment)을 실시한 바 있고 (Andersson, 2007, Andersson과 Martin, 2009), 미국 유카 산의 Exploratory Studies Facility(ESF)에서는 고준위 폐기물처분장에서 예상되는 열-수리-역학-화학적 복합거동의 이해를 돕기 위해 대규모의 Drift Scale Test(DST)를 수행한 바 있다. 이 현장시험에서는 처분공에서 발생하는 방사성 붕괴열뿐만 아니라 인접한 처분터널에서 발생하는 열의 영향까지도 고려되고 있다 (Rutqvist와 Tsang, 2002).

Multiple thermal energy storage (TES) caverns can be used for storing thermal energy on a large scale and for a high-aspect-ratio heat storage design to provide good thermal performance. It may also be necessary to consider the use of multiple caverns with a reduced length when a single, long tunnel-shaped cavern is not suitable for connection to aboveground heat production and injection equipments. When using multiple TES caverns, the separation distance between the caverns is one of the significant factors that should be considered in the design of storage space, and the optimal separation distance should be determined based on a quantitative stability criterion. In this paper, we described a numerical approach for determining the optimal separation distance between multiple caverns for large-scale TES utilization. For reliable stability evaluation of multiple caverns, we employed a probabilistic method which can quantitatively take into account the uncertainty of input parameters by probability distributions, unlike conventional deterministic approaches. The present approach was applied to the design of a conceptual TES model to store hot water for district heating. The probabilistic stability results of this application demonstrated that the approach in our work can be effectively used as a decision-making tool to determine the optimal separation distance between multiple caverns. In addition, the probabilistic results were compared to those obtained through a deterministic analysis, and the comparison results suggested that care should taken in selecting the acceptable level of stability when using deterministic approaches.

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.

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.

Developing efficient and reliable energy storage system is as important as exploring new energy resources. Energy storage system can balance the periodic and quantitative mismatch between energy supply and energy demand and increase the energy efficiency. Industrial waster heat and renewable energy such as solar energy can be stored by the thermal energy storage (TES) system at high and low temperatures. TES system using underground rock carven is considered as an attractive alternative for large-scale storage, because of low thermal conductivity and chemical safety of surrounding rock mass. In this report, the development of available thermal energy storage methods and the characteristics of storage media were introduced. Based on some successful applications of cavern storage and high-temperature storage reported in the literature, the applicabilities and practicabilities of storage media and technologies for large-scale cavern thermal energy storage (CTES) were reviewed.

Roughness of rock joint has generally been characterized based upon geometrical aspects of a two-dimensional surface profile. The appropriate description of joint roughness, however, should consider the features of roughness mobilization at contact areas under normal and shear loads. In this study, direct shear tests were conducted on the replicas of tensile fractured gneiss joints and the influence of the shear direction on the shear behavior and effective roughness was examined. In this procedure, a joint surface was represented as a group of triangular planes, and the steepness of each plane was characterized using the concepts of the active and inactive micro-slope angles. The contact areas at peak strength which were estimated by a numerical method showed that the locations of the contact areas were mainly dependent on the distribution of the micro-slope angle and the shear behavior of joint was dominated by only the fractions with active micro-slope angles. Therefore, a three-dimensional coefficient for the quantification of rock joint roughness is proposed based on the distribution of active micro-slope angle: active roughness coefficient, Cr. Comparison of the active roughness coefficient and the peak shear strength obtained from the experiment suggests that the active roughness coefficient is the effective parameter to quantify the surface roughness and estimate the shear behavior of rock joint.