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High entropy alloys (HEAs) are defined as a multi-element alloy including more than 4 elements with near equi-atomic percentage. In general, the configurational entropy of the HEAs is known to be sufficient to stabilize a single solid solution, such as body-centered cubic (BCC), face-centered cubic (FCC) and hexagonal-closed pack (HCP). Compared to BCC single-phase alloys, FCC single-phase alloys draw extensive attention because they are advantageous in manufacturing and processing. FCC-based HEAs show excellent ductility but limited strength, so many research on improving strength has been conducted. Outstanding mechanical properties with a balance of strength and ductility are rarely achieved in single-phase FCC-based HEAs. This is why most alloys for structural applications exhibit a multi-phase microstructure. In this study, we aimed to develop multi-phase FCC-based HEA with superior mechanical properties than single-phase CoCrFeMnNi HEA, via Co substitution in CoCrFeMnNi HEA by Cu, which has a high mixing enthalpy. It was found that the CuCrFeMnNi HEA is composed of two FCC phases and one BCC phase. The CuCrFeMnNi HEA was cold-rolled, and subsequently aged at 500, 700, 900oC for 1 hour. As the annealing temperature increased, the volume fraction of the FCC phase (FCC1 + FCC2) increased and the residual stress was gradually relieved by recrystallization. Furthermore, small amount of sigma phase was formed at 900oC. The effect of the microstructural evolution on the mechanical properties, such as hardness and tensile properties at room temperature, will be discussed.

The design of photoactive materials and interface engineering between organic/inorganic layers play a critical role in achieving enhanced performance in energy-harvesting devices. Two-dimensional transitional dichalcogenides (TMDs) with excellent optical and electronic properties are promising candidates in this regard. In this study, we demonstrate the fabrication of size-controlled MoS2 quantum dots (QDs) and present fundamental studies of their optical properties and their application as a hole-transport layer (HTL) in organic solar cells (OSCs). Optical and structural analyses reveal that the as-prepared MoS2 QDs show a fluorescence mechanism with respect to the quantum confinement effect and intrinsic/extrinsic states. Moreover, when incorporated into a photovoltaic device, the MoS2 QDs exhibit a significantly enhanced performance (5/10-nanometer QDs: 8.30%/7.80% for PTB7 and 10.40%/10.17% for PTB7-Th, respectively) compared to those of the reference device (7.24% for PTB7 and 9.49% for PTB7-Th). We confirm that the MoS2 QDs clearly offer enhanced transport characteristics ascribed to higher hole-mobility and smoother root mean square (Rq) as a hole-extraction material. This approach can enable significant advances and facilitate a new avenue for realizing high-performance optoelectronic devices.

Stem cell therapy is one of the novel and prospective fields. The ability of stem cells to differentiate into different lineages makes them attractive candidates for several therapies. It is essential to understand the cell fate, distribution, and function of transplanted cells in the local microenvironment before their applications. Therefore, it is necessary to develop an accurate and reliable labeling method of stem cells for imaging techniques to track their translocation after transplantation. The graphitic quantum dots (GQDs) are selected among various stem cell labeling and tracking strategies which have high photoluminescence ability, photostability, relatively low cytotoxicity, tunable surface functional groups, and delivering capacity. Since GQDs interact easily with the cell and interfere with cell behavior through surface functional groups, an appropriate surface modification needs to be considered to get close to the ideal labeling nanoprobes. In this study, polyethylene glycol (PEG) is used to improve biocompatibility while simultaneously maintaining the photoluminescent potentials of GQDs. The biochemically inert PEG successfully covered the surface of GQDs. The PEG-GQDs composites show adequate bioimaging capabilities when internalized into neural stem/progenitor cells (NSPCs). Furthermore, the bio-inertness of the PEG-GQDs is confirmed. Herein, we introduce the PEG-GQDs as a valuable tool for stem cell labeling and tracking for biomedical therapies in the field of neural regeneration.

The charge excitation and decay pathways of two-dimensional heteroatomic quantum dots (QDs) are affected by the quantum confinement effect, bandgap structure and strong exciton binding energy. Recently, semiconducting transition metal dichalcogenides (TMDs) have been intensively studied; however, the charge dynamics of metallic phase QDs (mQDs) of TMDs remain relatively unknown. Herein, we investigate the photophysical properties of TMD-mQDs of two sizes, where the TMD-mQDs show different charge excitation and decay pathways that are mainly ascribed to the defect states and valence band splitting, resulting in a large Stokes shift and two excitation bands for maximum photoluminescence (PL). Interestingly, the dominant excitation band redshifts as the size increases, and the time-resolved PL peak redshifts at an excitation wavelength of 266 nm in the smaller QDs. Additionally, the lifetime is shortened in the larger QDs. From the structural and theoretical analysis, we discuss that the charge decay pathway in the smaller QDs is predominantly affected by edge oxidation, whereas the vacancies play an important role in the larger QDs. Metallic phase transition metal dichalcogenides quantum dots show different pathways of optical charge excitation and decay according to the size and sort of defects, resulting into the large Stoke shift, two bands for charge excitation, and TRPL peak shift. This result is mainly ascribed to the valance band splitting and the emerging defect states originated from atomic vacancy of basal plane and edge oxidation.

Highly dispersive molybdenum disulfide nanoflakes (MoS2 NFs), without any phase transition during the exfoliation process, are desirable for full utilization of their semiconductor properties in practical applications. Here, we demonstrate an innovate approach for fabricating MoS2 NFs by using hydrazine-assisted ball milling via the synergetic effect of chemical intercalation and mechanical exfoliation. The NFs obtained have a lateral size of 600&ndash, 800 nm, a thickness less than 3 nm, and high crystallinity in the 2H semiconducting phase. They form a stable dispersion in various solvents, which will be helpful for many applications, due to the oxygen functional group. To investigate production of a two-dimensional (2D) photodetector, 2D semiconducting MoS2, MoS2&ndash, p-Si vertical devices were fabricated, and their optical properties were characterized. The photodiode exhibited consistent responses with excellent photo-switching characteristics with wavelengths of 850, 530, and 400 nm.

Two-dimensional hexagonal boron nitride (h-BN) has excellent and useful mechanical and thermal properties, and can be used as a novel filler to enhance the thermal conductivity of polymer composites. We prepared exfoliated h-BN nanoplatelets (BNNP) with hydroxyl functional groups using a hydroxide-assisted ball milling process, and demonstrated the enhanced thermal conductivity of its epoxy nanocomposites. The prepared BNNP are highly soluble and retain their in-plane structure. The dielectric constant and dielectric loss of the nanocomposites increase with the addition of BNNP owing to interfacial polarization by the large surface area of BNNP and the ionic and electronic relaxation polarizations by the surface functional groups on the BNNP. Notably, the thermal conductivity of the epoxy nanocomposites with 10% mass fraction of BNNP was 0.57 W/m·K, which is 2.85-times higher than that of neat epoxy. Finally, compared with neat epoxy, BNNP nanocomposite exhibits excellent heat dissipation capability in flat chip packaging. This enhanced performance is ascribed to the high quality and dispersion of the BNNP and their strong interfacial bonding with the epoxy matrix, which is produced by the hydroxyl functional groups on the BNNP. The overall results suggest that BNNP nanocomposites have strong potential for application as electronic packaging materials.

One important strategy for developing high performance smart materials is to utilize the magnetic properties of elastomers. Here we prepared magnetic elastomer nanocomposites using the latex compounding method followed by in situ methods. Our strategy exploited the synergetic effect of carbon nanofibers (CNFs) and ferric oxide (Fe2O3), which, when combined, can improve the dispersion within the elastomer matrix by introducing hybrid nanomaterial networks. When Fe2O3 decorated CNFs (CNF–Fe2O3) were embedded in an SBR matrix, they produced a remarkable improvement in the composite material's mechanical and thermal properties. This was attributed to the efficient dispersion of the CNF–Fe2O3, and the enhanced interfacial interaction between the filler particles and the elastomer matrix. Furthermore, the magnetic properties of the elastomer nanocomposites were modulated by the addition of the prepared CNF–Fe2O3 hybrids. The synergistic reinforcement of SBR achieved by the incorporation of CNF–Fe2O3 hybrids can support the development of high performance magnetic elastomers for applications in electronic appliances, magnetoresistive sensors, actuators, and automotive parts.

With the development of “green tires” in the tire industry, the conventional carbon black filler that is used in tread formulations is being replaced with silica. Generally, this requires the addition of a processing aid, containing zinc ion, which acts as a lubricant and dispersing agent. However, because zinc is a heavy metal, zinc-free processing aids (ZFAs) are required to satisfy worldwide environmental concerns. We present herein a series of catalytically synthesized ZFAs and evaluate the effects of replacing zinc ion-containing processing aids (ZCAs) on a silica tread formulation. Interestingly, replacing ZCA with ZFA in a two parts per hundred rubber (phr) by weight formulation improved both its tensile strength and elongation by as much as 31% and 20%, respectively. ZFA-rubber formulations also exhibited a twofold enhancement in fatigue properties over those of ZCA-rubber formulations. Furthermore, pneumatic tires were fabricated from our ZFA-rubber formulation and compared against tires containing ZCAs. The ZFA-rubber composite exhibited improved dry and wet braking and rolling resistance due to enhanced dispersion of silica in the rubber matrix. These results show that rubber composites prepared with ZFAs may be promising in tire engineering applications.

Evaporation of tellurium (Te) occurs during most of the fabrication methods which leads to a strong deviation on thermoelectric properties. Due to the existence of volatile tellurium has been a great challenge to determine the accurate chemical compositions in the Bi0.5Sb1.5Te3 compound for optimizing the carrier concentration. Herein, we systematically verify the reliability and understand the correlations between chemical composition and thermoelectric properties at different portions of large (25 mm), thick (30 mm) samples. The results reveal that spark plasma sintered samples didn’t show any significant evaporation of Te. However, the chemical composition slightly changed, resulting in the final thermoelectric figure of merit at different portions of thick samples varied typically within a 10% error. Moreover, this work supports that the selection of the powder fabrication and compaction method plays a crucial role on the final thermoelectric properties of the material, as the combining of gas atomization and spark plasma sintering can prohibit the evaporation of tellurium and any possible inhomogeneous stress field or temperature distribution in large-sized graphite die to SPS.

Homogeneous and stable dispersion of nanoparticles in polymer matrix remains a significant challenge. Here, we propose a simple and effective route for synthesizing functionalized TiO2 nanoparticles through oleic acid. The Oleic-TiO2 nanoparticles homogeneously dispersed in SBR matrix show remarkable improvement of mechanical properties such as modulus, tensile strength, and fatigue properties, even at low loadings, which are ascribed to the enhanced interfacial bonding with the polymer matrix. Moreover, the incorporation of the Oleic-TiO2 into SBR matrix significantly prevents photo and thermal degradation. Therefore, we confirm that our approach can suggest fascinating potentials to be scalable and commercial in polymer engineering. POLYM. COMPOS., 38:E174–E180, 2017. © 2016 Society of Plastics Engineers

The photoluminescence of graphene quantum dots (GQDs) is rigorously investigated due to their potential applications. However, GQDs from graphene oxide are inherently embedded with non-negligible defects and oxygen grafted onto the edge and basal plane, which induce a change of innate electronic structure in the GQDs. Thus, graphene oxide based GQDs can misrepresent the characteristic properties of primitive GQDs. Here we report the size and pH dependent photophysical properties of GQDs that minimize the content of oxygen and defects. From auger electron spectroscopy, the oxygen content of the GQDs with two different lateral sizes (∼2 nm and ∼18 nm) was probed and found to be ∼5% and ∼8%, respectively. Two common photoluminescence (PL) peaks were observed at 436 nm (the intrinsic bandgap) and 487 nm (the extrinsic bandgap) for both GQDs. The characteristic PL properties in extrinsic and intrinsic bandgaps examined by optical spectroscopic methods show that the emission peak was red-shifted and that the peak width was widened as the size increased. Moreover, the PL lifetime and intensity were not only reversibly changed by pH but also depended on the excitation wavelength. This is in line with our previous report and is ascribed to the size variation of sp2 subdomains and edge functionalization.

Hexagonal boron nitride nanoplatelets (BNNPs) can serve as two-dimensional (2D) fillers for elastomer nanocomposites due to their excellent and intriguing mechanical and thermal properties. Homogeneous and stable dispersion of BNNPs in elastomer is key to successful composite applications. Herein we propose a facile and easily industrialized approach for incorporating fully exfoliated BNNPs with hydroxyl functional groups into elastomer using the latex compounding method. The prepared BNNPs are highly dispersed in various solvents and their size of a few microns preserves their in-plane structure. The environmentally friendly, cost-effective, scalable preparation method proposed shows great potential for advancing the performance of elastomer nanocomposites. Specifically, the incorporation of BNNPs into the elastomer matrix remarkably improved the elastic modulus and tensile strength, even at low loadings; this was ascribed to the enhanced interfacial bonding of the BNNPs with the elastomer matrix. Moreover, well-dispersed BNNPs within the elastomer provided outstanding thermal conductivity and gas permeability.

Feasibility tests for the hydraulic fracturing were conducted in order to secure additional saline groundwater for irrigating to the land-based aquaculture farm. Two boreholes were placed to the aquaculture farm A and B, respectively. A hydraulic fracturing using single packer was applied to major fracture zones within two boreholes. To identify effects of hydraulic fracturing on securing additional saline groundwater, some selective methods including well logging methods, pumping tests, and groundwater quality analysis were commonly applied to the boreholes before and after the hydraulic fracturing. Enlarging/creating fracture zones, increasing water contents in bedrock near boreholes, and increasing transmissivity were observed after the hydraulic fracturing. Even though the hydraulic fracturing could be an alternative to secure additional saline groundwater to the land-based aquaculture farm, salinity of the groundwater did not meet optimal thresholds for each fingerling in two farms: Fresh submarine groundwater discharge flowed the more into borehole of the farm A that resulted in decreasing a salinity value. Increased saline groundwater quantity in the borehole of the farm B rarely affect to the salinity. Although salinity problem of groundwater limited its direct use for the farms, the mixing with seawater could be effectively used for the fingerlings during the early stage. A horizontal radial collector well placed in the alluvial layer could be an alternative for the farms as well.

The development of renewable energy technologies that can replace fossil fuels is environmentally important; however, such technologies must be economically feasible. Economic analyses are important for assessing new projects such as geothermal heating-cooling systems, given their large initial costs. This study analyzed the economics and carbon dioxide emissions of: a SCW (standing column well), a vertical closed loop boiler, a gas boiler, and an oil boiler. Life cycle cost analysis showed that the SCW geothermal heating-cooling system had the highest economic feasibility, as it had the highest cost saving and also the lowest carbon dioxide emissions. Overall, it appears that geothermal systems can save money when applied to large-scale controlled agriculture complexes and reclaimed land.

This study was conducted to provide the preliminary data on preparation for policy decisions regarding the groundwater supply scheme for N. Korea vulnerable to drought. Agricultural activities oriented to upland field due to the mountainous terrain more than 79% as well as the diversity of rainfall distribution over the country make more vulnerable to structural problems in the drought in N. Korea. Therefore, in anticipation of the expansion of exchange policies in agricultural sector, the available amount of groundwater needed for sustainable water resources supply was estimated for each administrative district after analyzing alluvium and hydrogeology distribution in N. Korea. Overall, the available amount of groundwater was estimated to 22.3 billion m3 (0.18 million m3/km2). The available amount of groundwater per unit area in Ryanggangdo and Gaesungsi was appeared very high in each of 0.56 and 0.39 million m3/km2, respectively, and it would be interpreted that two districts have relatively wide area of volcanic rocks and alluvium with highly permeable characteristics, respectively. Finally, to maximize the utilization of this study result, the available amount of groundwater distribution map was developed on the basis of the 1 × 1 km grid network over the entire N. Korea.

Purpose: Treatment of patients with para-aortic lymph node metastasis from colorectal cancer is controversial. The goal of this study was to investigate the technical feasibility of laparoscopic intrarenal para-aortic lymph node dissection in patients with colorectal cancer and clinically suspected para-aortic lymph node dissection. Methods: The inclusion criteria for the laparoscopic approach were patients with infrarenal para-aortic lymph node metastasis from colorectal cancer. Patients who had any other distant metastatic lesion or metachronous para-aortic lymph node metastasis were excluded from this study. Perioperative outcomes and survival outcomes were analyzed. Results: Between November 2004 and October 2013, 40 patients underwent laparoscopic para-aortic lymph node dissection. The mean operating time was 192.3 ± 68.8 minutes (range, 100–400 minutes) and the mean estimated blood loss was 65.6 ± 52.6 mL (range, 20–210 mL). No patient required open conversion. The postoperative complication rate was 15.0%. Sixteen patients (40.0%) had pathologically positive lymph nodes. In patients with metastatic para-aortic lymph nodes, the 3-year overall survival rate and disease-free survival rate were 65.7% and 40.2%, respectively. Conclusion: The results of our study suggest that a laparoscopic approach for patients with colorectal cancer with metastatic para-aortic lymph nodes can be a reasonable option for selected patients.

Decoupling the Seebeck coefficient, electrical and thermal conductivity has been a great challenge in efforts to obtain thermoelectric materials with a high figure of merit (ZT) for power generation and cooling applications. In this work, p-type Bi0.5Sb1.5Te3 samples were fabricated by combining gas atomization and spark plasma sintering. Heat-treatment and subsequent consolidation during SPS stimulated the recrystallization of grains and reduced texture, contributing to reduced carrier scattering. This enhanced carrier mobility by 29%. The recrystallization also induces a hierarchy of fine and coarse crystal grains with random orientations. This mixed microstructure contributed to the simultaneous enhancement of the Seebeck coefficient and electrical conductivity, while reducing lattice thermal conductivity, due to effective phonon scattering by the fine grains and Sb2O3 nanoparticles. The significant improvement in power factor and relatively low κL resulted in a peak ZT of 1.1 at 350 K for the 470 °C treated sample. This was about 47% higher than the initial sample (ZT = 0.75 at 350 K). The average ZT for all the heat treated samples reached over unity in the range of 300–500 K. This work suggests that the spark plasma heat treatment and consolidation processes enable the recrystallization, allowing the thermoelectric properties to be decoupled, enhancing ZT.

We experimentally demonstrate the simultaneous enhancement of Raman and photoluminescence (PL) of core-shell hybrid nanoparticles consisting of Ag (core) and polydiacetylene (PDA, shell) through the assistance of localized surface plasmon (LSP) effect for the effective biosensor. Core-shell nanoparticles (NPs) are fabricated in deionized water through a sequential process of reprecipitation and self-assembly. The Raman signal of PDA on core-shell NPs is enhanced more than 100 times. Also, highly enhanced photoluminescence is observed on Ag/PDA hybrid NPs after coupling of the complementary t-DNA with p-DNA which are immobilized on PDA shell. This indicates that the core Ag affects the Raman and PL of PDA through the LSP resonance, which can be caused by the energy and/or charge transfer caused by the LSP coupling and the strong electromagnetic field near Ag NP surface. Only electrons present on the surface interact with the PDA shell, not involving the electrically neutral part of the electrons inside the Ag NP. Furthermore, this work shows that as prepared Ag/PDA NPs functionalized by probe DNA can sense the target DNA with an attomolar concentration (100 attomole).

Graphene nanosheets have been among the most promising candidates for a high-performance anode material to replace graphite in lithium ion batteries (LIBs). Studies in this area have mainly focused on nanostructured electrodes synthesized by graphene oxide (GO) or reduced graphene oxide (rGO) and surface modifications by a chemical treatment. Herein, we propose a cost-effective and reliable route for generating a defect-free, nanoporous graphene nanostructure (df-GNS) through the sequential insertion of pyridine into a potassium graphite intercalation compound (K-GIC). The as-prepared df-GNS preserves the intrinsic property of graphene without any crystal damage, leading to micro-/nano-porosity (microporosity: ~10–50 µm, nanoporosity: ~2–20 nm) with a significantly large specific surface area. The electrochemical performance of the df-GNS as an anode electrode was assessed and showed a notably enhanced capacity, rate capability, and cycle stability, without fading in capacity or decaying. This is because of the optimal porosity, with perfect preservation of the graphene crystal, allowing faster ion access and a high amount of electron pathways onto the electrode. Therefore, our work will be very helpful for the development of anode and cathode electrodes with higher energy and power performance requirements.

The grid-based water balance of watershed scale was assessed in the mountainous area of Pyosun catchment in Jeju Island after analyzing precipitation, evapotranspiration, and runoff from January 2008 to December 2013. The existing results of direct runoff, evapotranspirtion, and groundwater recharge comparing to precipitation were presented 22.0%, 25.6%, and 52.4%, respectively, in Pyosun catchment. However, this study indicated each component shows 14.5%, 24.2%, and 61.0%, respectively, in the mountainous area of Pyosun catchment. Therefore, groundwater recharge rate in the mountainous area appears higher than 10% comparing to the overall catchment. It would be analyzed that the amount of direct runoff is relatively small. Moreover, this difference could be generated because of the spatial discontinuities in the process of estimating the total amount of precipitation in the mountainous area. Therefore, the grid-based spatial analysis to maximize the spatial continuity would be useful for providing a more reasonable result when the total amount of water resources are evaluated in mountainous areas in the future.

To estimate water balance of Pyosun watershed in Jeju Island, a three-dimensional finite difference model MODFLOW was applied. Moreover, the accuracy of groundwater flow modeling was evaluated through the comparison of the recharge rate by flow modeling and the existing one from water balance model. The modeling result under the steady-state condition indicates that groundwater flow direction was from Mt. Halla to the South Sea and groundwater gradient was gradually lowered depending on the elevation. Annual recharge rate by the groundwater flow modeling in Pyosun watershed was calculated to 236 million m 3 /year and it was found to be very low as compared to the recharge rate 238 million m 3 /year by the existing water balance model. Therefore, groundwater flow modeling turned out to be useful to estimate the recharge rate in Pyosun watershed and it would be available to make groundwater management policy for watershed in the future. Key words : Water balance, Pyosun watershed, MODFLOW, Recharge rate, Groundwater modeling

Nano-fillers have provided a big advantage for enhancing the performance of rubber composites through leading the synergy effects in the physical and chemical properties. However, despite various approaches having been explored, the process to make a homogenous and stable dispersion of nano-filler in the rubber matrix remains a major challenge in this field. Herein, we propose a simple and effective route for synthesizing nanocomposites of rubber with homogenous and stable dispersed low defect graphene flakes (l-GFs), which are prepared using l-GFs/SBR composites via aqueous-phase mixing of exfoliated l-GFs with SBR latex. The l-GFs embedded SBR matrix shows a remarkable improvement in the modulus and tensile strength even at the low loading rate, which is ascribed to the efficient dispersion of the l-GFs enhancing interfacial interaction with the rubber matrix. The integration of l-GFs into the SBR matrix significantly improves the thermal and electrical conductivities, as well as the gas barrier property of the rubber composites. This method is water-mediated, green and scalable, showing great potential for the production of various l-GFs-based rubber composites at an industrial level.

최근 지열에너지 활용 증가 추세와 더불어 지하수 부존량이 풍부한 지역에서 수직개방형의 효율이 더 높다는 연구결과가 제시됨에도 불구하고, 국내 지열냉난방시스템의 열교환 방식은 수직밀폐형 방식이 주를 이루고 있다. 따라서, 본 연구에서는 지하수 부존량이 풍부한 지역에서 수직개방형 방식의 효율성을 검증하기 위하여, 수리지질 및 열적 특성을 반영한 최적의 지열에너지 활용방식을 수치 모의하였다. 1차 모의 결과, 지하수 부존량이 풍부한 지역에서 지열냉난방시스템을 활용하는 경우 수직밀폐형보다 지하수를 직접 이용하는 수직개방형이 더욱 효과적인 것으로 분석되었으며, 수직개방형 중에서는 SCW (standing column well)형 보다는 주입정과 추출정이 분리된 복수관정형에서 지반과 열교환을 통해 얻을 수 있는 주입수와 추출수의 온도차 (Δ)가 커 더욱 효율적인 것으로 나타났다. 2차 모의에서는 수직개방형 지열에너지 공급방식인 단일관정형, SCW형, 복수관정형을 대상으로 열 이송,이격거리 및 유량, 지하수 수리경사 등을 고려한 최적의 활용 방식을 검토하였으며, 이를 바탕으로 지열냉난방시스템의 지중 열교환 방식 선정 시 활용할 수 있는 흐름도를 제시하였다. 본 연구결과 제시된 다양한 선정 기준을 기초로 실제 선정 시에는 전체 지열냉난방 시스템에 대한 COP (coefficient of performance) 계산 및 세부적인 타당성 검토가 필요할 것으로 판단된다.주요어: 지열에너지 , 지열냉난방시스템 , 열교환 방식, 수직개방형 , 지하수While the vertical open type of heat exchanger is more effective in areas of abundant groundwater, and is becoming morewidely used, the heat exchanger most commonly used in geothermal heating and cooling systems in Korea is the verticalclosed loop type. In this study, we performed numerical simulations of the optimal utilization of geothermal energy based onthe hydrogeological and thermal properties to evaluate the efficiency of the vertical open type in areas of abundant ground-water supply. The first simulation indicated that the vertical open type using groundwater directly is more efficient than thevertical closed loop type in areas of abundant groundwater. Furthermore, a doublet system with separated injection and extrac-tion wells was more efficient because the temperature difference (Δ) between the injection and extraction water generated byheat exchange with the ground is large. In the second simulation, we performed additional numerical simulations of the opti-mal utilization of geothermal energy that incorporated heat transfer, distance, flow rate, and groundwater hydraulic gradienttargeting a single well, SCW (standing column well), and doublet. We present a flow diagram that can be used to select theoptimal type of heat exchanger based on these simulation results. The results of this study indicate that it is necessary to exam-ine the adequacy of the geothermal energy utilization system based on the hydrogeological and thermal properties of the areaconcerned, and also on a review of the COP (coefficient of performance) of the geothermal heating and cooling system.Key words: geothermal energy, geothermal heating and cooling system, type of heat exchanger, vertical open type, groundwater*Corresponding author: jeong@anu.ac.krⓒ 2014, The Korean Society of Engineering Geology

Rural groundwater monitoring network has been managed by Korea Rural Community Corporation (KRC) since 1998. The network consists of two kinds of subnetworks; rural groundwater management network (RGMN) and seawater intrusion monitoring network (SIMN). RGMN has been operated to promote a sound and sustainable development of rural groundwater within the concerned area for groundwater quality and quantity. SIMN has been operated to protect the crops against hazards by the saline water in coastal areas in which the shortage of irrigation water become a main problem for agriculture. Currently, a total of 283 monitoring wells has been installed; 147 wells in 79 municipalities for RGMN and 136 wells in 52 ones for SIMN, respectively. Two subnetworks commonly monitor three hydrophysical properties (groundwater level, temperature, and electric conductivity) every hour. Monitored data are automatically transferred to the management center located in KRC. Data are opened to the public throughout website named to be the Rural Groundwater Net (www.groundwater.or.kr). Annual reports involving well logging and hydrochemical data of RGMN and SIMN have been published and distributed to the rural water management office of each municipalities. In addition, anyone who concerns about RGMN an SIMN can freely download these reports throughout the Rural Groundwater Net as well.