• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3407-3412
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• 2007

The performance characteristics of the polymer electrolyte fuel cells (PEFCs) were investigated under various humidification conditions at steady-state and transient conditions. The PEFC studied in this study was characterized by I-V curves in potentiostatic mode. The I-V curves representing steady-state performance were obtained from OCV to 0.25V, and the dynamic performance responses were obtained at some points of voltages. The anodic external humidification was applied and the humidity was controlled from 20% to 100%. The effects of relative humidity of hydrogen were measured with the dry air at the cathode. At high voltage region, the performance at high temperature was higher, but at low voltage region, low temperature condition showed the higher performance. The dynamic responses were observed at the instant when the voltage of the PEFC was changed. It was observed that the performance reached steady-state earlier with the increase of temperature.

• 한국수소및신에너지학회논문집
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• 제29권2호
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• pp.148-154
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• 2018

This paper presents a comparative analysis of thermodynamic performance of ammonia-water Rankine cycles with and without regeneration and Kalina cycle for recovery of low-temperature heat source. Special attention is paid to the effect of system parameters such as ammonia mass fraction and turbine inlet pressure on the characteristics of the system. Results show that maximum net power can be obtained in the regenerative Rankine cycle for high turbine inlet pressures. However, Kalina cycle shows better net power and thermal efficiency for low turbine inlet pressures, and the optimum ammonia mass fractions of Kalina cycle are lower than Rankine cycles.

• 전력전자학회논문지
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• 제26권6호
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• pp.421-428
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• 2021

Lithium-ion secondary batteries exhibit advantageous characteristics such as high voltage, high energy density, and long life, allowing them to be widely used in both military and daily life. However, the lithium-ion secondary battery does have its limitation; for example, the output power and capacity are readily decreased due to the increased internal impedance during discharging at a lower temperature (-32℃, military requirement). Also, during charging at a lower temperature, lithium dendrite growth is accelerated at the anode, thereby decreasing the battery capacity and life as well. This paper describes a study that involves increasing the internal temperature of lithium-ion secondary battery by energy circulation operation in a low-temperature environment. The energy circulation operation allows the lithium-ion secondary battery to alternately charge and discharge, while the internal resistance of lithium-ion battery acts as a heating element to raise its own temperature. Therefore, the energy circulation operation method and device were newly designed based on the electrochemical impedance spectroscopy of the lithium-ion secondary battery to mediate the battery performance at a lower temperature. Through the energy circulation operation of lithium ion secondary battery, as a result of the heat generated from internal resistance in an extremely low-temperature environment, the temperature of the lithium-ion secondary battery increased by more than 20℃ within 10 minutes and showed a 75% discharging capacity compared with that at room temperature.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.266-271
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• 2001

In this study, the performance of the simultaneous heating & cooling water making system using R134a was investigated by simulation. The most important effect upon heating COP was intermediate pressure depending on input water temperature. With the input water temperature of $10^{\circ}C\;and\;20^{\circ}C$, optimum intermediate pressure were 923 and 1040kPa, respectively. At that optimum intermediate pressure, the maximum heating COP of the system operated between $0^{\circ}C$ evaporating temperature and $70^{\circ}C$ condensing temperature were 4.15 and 3.83. With installation of the subcoolers in high or low pressure section, the system COP was increased by reducing the refrigerant mass flow rate. Under the optimum pressure and $10^{\circ}C$ input water temperature, it was found that heating COP was maximized when the low-subcooler and high-subcooler capacity rate were taken by 14% and 13%, respectively.

• 한국수소및신에너지학회논문집
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• 제24권1호
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• pp.91-97
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• 2013

Recently a novel cycle named organic flash cycle (OFC) has been proposed which has improved potential for power generation from low-temperature heat sources. This study carries out thermodynamic performance analysis of OFC using various working fluids for recovery of low-grade heat sources in the form of sensible energy. Special attention is focused on the optimum flash temperature at which the exergy efficiency has the maximum value. Under the optimal conditions with respect to the flash temperature, the thermodynamic performances of important system variables including mass flow ratio, separation ratio, heat addition, specific volume flow rate at turbine exit, and exergy efficiency are thoroughly investigated. Results show that the exergy efficiency has a peak value with respect to the flash temperature and the optimum working fluid which shows the best exergy efficiency varies with the operating conditions.

• 한국유체기계학회 논문집
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• 제18권1호
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• pp.29-36
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• 2015

Recently, Stirling engines are emerging as a key device for power conversion of renewable energy or waste energy. This study develops a LTDSE(Low Temperature Differential Stirling Engine) using a water spray for higher heat transfer and performs load performance tests for various flow rates and temperatures of hot water spray for variable engine loads emulated by a mechanical friction device. Internal temperature and pressure, working frequency and inlet and outlet temperature of the supply water are measured. As a result, the increases in flow rate and temperature of hot water respectively enhance the power output, efficiency and the working frequency, while the increasing engine load leads to decreases in working frequency but increases in the pressure amplitude. Eventually, it is revealed there exists a maximum shaft power of the test engine.

• 한국수소및신에너지학회논문집
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• 제23권4호
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• pp.382-389
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• 2012

Power generation cycle using ammonia-water mixture as working fluid has attracted much attention because of its ability to efficiently convert low-temperature heat source into useful work. If an ammonia-water power cycle is combined with a power cycle using liquefied natural gas (LNG), the conversion efficiency could be further improved owing to the cold energy of LNG at $-162^{\circ}C$. In this work parametric study is carried out on the thermodynamic performance of a power cycle consisted of an ammonia-water Rankine cycle as an upper cycle and a LNG cycle as a bottom cycle. As a driving energy the combined cycle utilizes a low-temperature heat source in the form of sensible heat. The effects on the system performance of the system parameters such as ammonia concentration ($x_b$), turbine 1 inlet pressure ($P_{H_1}$) and temperature ($T_{H_1}$), and condenser outlet temperature ($T_{L_1}$) are extensively investigated. Calculation results show that thermal efficiency increases with the increase of $P_{H_1}$, $T_{H_1}$ and the decrease of $T_{L_1}$, while its dependence on $x_b$ has a downward convex shape. The changes of net work generation with respect to $P_{H_1}$, $T_{H_1}$, $T_{L_1}$, and $x_b$ are roughly linear.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2009년도 9th International Meeting on Information Display
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• pp.951-952
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• 2009

The long way toward new silicon technology, processed at very low temperature on any substrate, is described. The technology is based on CMIS (Complementary Metal Insulator Semiconductor) structure that shown its efficiency with known CMOS electronics. Present performance of this new technology is discussed through electrical parameters and reliability of transistors.

• 대한기계학회논문집B
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• 제40권9호
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• pp.589-596
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• 2016

최근 들어 작동유체가 액체 상태를 유지하며 흡열 과정이 일어나는 증기동력사이클인 유기플래시 사이클이 제안되었다. 본 연구에서는 재생을 이용한 수정 유기플래시 사이클을 제안하고 현열 형태의 저온 열원을 사용하는 시스템의 열역학적 성능 해석을 수행하였으며 작동유체나 플래시 온도가 순생산 동력이나 열효율 등 시스템의 성능에 미치는 영향을 체계적으로 분석하고 논의하였다. 해석 결과는 시스템의 순생산동력은 플래시 온도에 대해 최대값을 갖지만 열효율은 플래시 온도에 따라 상승함을 보여준다. 재생 사이클은 기존의 유기플래시 사이클에 비해 시스템 열효율이 높고 저온 열원의 동력 변환에 있어 성능 개선을 위한 잠재성을 보여준다.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2009년도 춘계학술대회 논문집
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• pp.569-572
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• 2009

In this study, HFC ORCs (Organic Rankine Cycles) are investigated for a low-temperature geothermal power generation by a simulation method. A steady-state simulation model is developed to analyze and optimize cycle's performance. The model contains a turbine, a pump, an expansion valve and heat exchangers. The turbine and pump are modelled by an isentropic efficiency. Simulations were carried out for the given heat source and sink inlet temperatures, and given flow rate that is based on the typical power plant thermal-capacitance-rate ratio. 3 HFC fluids are considered as a candidate for a working fluid of low-temperature ORCs. In this study, all optimized HFC ORCs are shown to yield almost the same performance in terms of power for a low-temperature heat source of about $100^{\circ}C$.