• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.8
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• pp.694-701
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• 2007

The applications of a transcritical $CO_2$ cycle into water heaters show advantages over conventional systems in the respect of power consumption and heating efficiency because the $CO_2$ cycle has a high compressor discharge temperature. Besides, the heating performance of the transcritical $CO_2$ cycle can be improved by optimizing operating conditions. In this study, the heating performance of a variable speed $CO_2$ heat pump was measured and analyzed by varying refrigerant charge amount, EEV opening, compressor frequency and outdoor temperature. As a result, the optimum normalized charge for heating was 0.226. The COPs at the compressor frequencies of 40, 50 and 60 Hz were 2.94, 2.75 and 2.25, respectively. The heating performance of the $CO_2$ cycle with charge amount was more sensitive than the cooling performance. Moreover, the heating performance was improved significantly by optimizing of compressor frequency and EEV opening.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.4
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• pp.208-215
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• 2013

An experimental study of an ORC (Organic Rankine Cycle) system has been performed for small-scale applications in the range of a few kW for low-grade-recovery heat sources. The ORC system was equipped with a scroll expander. Experimental tests were carried out using this system, and showed good performance and reliability for the small-scale system. The effects of various operating conditions were selected as the main parameters for the performance of ORC system, such as the expander speeds and mass flow rates of R-134a for expander inlet temperatures ranging from $100^{\circ}C$ to $190^{\circ}C$, as well as the thermal power, thermal efficiency, expansion efficiency, and volumetric efficiency.

• Journal of Advanced Marine Engineering and Technology
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• v.9 no.4
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• pp.317-327
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• 1985

In the case of temperature control of air conditioning and refrigerating system, it is possible to operate the system continuously by controlling the cooling capacity of refrigerating machines. But on-off control system has been adopted for refrigerating system which has more large capacity than that required to remove the generated heat. In this on-off control system, it can be considered that there exists some optimum condition for the refrigerating capacity, operating cycle, running hour, and the temperature difference between thermostat setting value and real one. In this paper, an equation was derived to express the temperature variation of the refrigerated object (Nybrine) and later two evaluating functions were derived. One is for the temperature difference and another is for operating cycle and running hour. The weighted sum of these two functions is defined as the criterion function for the evaluation of the control performance of the system, and then the optimum running condition is investigated in the sense of minimizing the criterion function. Experiments showed that the heat balance equation derived for the temperature variation and the estimation of the time constant of the refrigerated system are appropriate. By conclusion, if a proper weighting factor D is selected, the optimum conditions exist for the refrigerating capacity, running hour, and operating cycle in the on-off temperature control of the refrigerating system.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.21 no.11
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• pp.621-628
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• 2009

The objectives of this study are to analyze the performance of a river water-source heat pump and to carry out economic assessment for the heat pump. The COP of the river water-source heat pump was 3-21% higher than that of the air-source heat pump because river water provides stable operating temperature compared with air temperature throughout the year. The economic analysis was carried out by comparing the initial and operating cost of the river water-source heat pump with those of the conventional air-source heat pump. The ratio of the life cycle operating cost to the life cycle cost increased with the increase of building capacity. The payback period was found to be less than 3.5 years when the capacity of the river water-source heat pump was larger than 10 RT.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.7
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• pp.505-512
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• 2011

Recently, many researchers have analyzed the performance of the transcritical $CO_2$ refrigeration cycle in order to identify opportunities to improve the system energy efficiency. The reduction of the expansion process losses is one of the key issues to improve the efficiency of the transcritical $CO_2$ refrigeration cycle. In this study, the analytical study on the performance characteristics of $CO_2$ cycle with an ejector carried out with a variation of outdoor temperature, gascooler inlet air velocity, evaporator inlet air velocity, and evaporator inlet air temperature. As a result, the system performance could be improved over 85% by using an ejector for various operating condition because of the reduction of compressor work. Moreover, the cooling capacity increased about 18% for variable outdoor condition. Therefore, the high performance of an ejector system could be maintained for wide operating conditions and system reliability could be improved compared to that of a basic system.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.22 no.10
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• pp.657-664
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• 2010

The theoretical analysis of performance characteristics in a $CO_2$ cycle with the heat exchanger type was carried out. The size and performance of the fin-tube and microchannel heat exchanger were compared with operating conditions. As a result, the performance of the fin-tube gascooler and evaporator were more sensitive to the variation of operating condition compared to that of the microchannel gascooler and evaporator. Beside, the sizes of microchannel gascooler and evaporator could be decreased by 73% and 76%, respectively, compared to those of the fin-tube type gascooler and evaporator with the similar capacity. The COP and reliability of the $CO_2$ system can be increased by using a microchannel heat exchanger.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.45 no.4
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• pp.553-557
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• 1996

Heat cycle aging of insulating materials in large generator stator winding has been investigated using both on-line and off-line test methods. On this study, principally, off-line test against actual generator in service was carried out to acquire information about polarization index(PI) and dissipation factor, dissipation factor tip-up, maximum partial discharge for the purpose of remnant breakdown voltage and life assessment. It was found from the tests that both dissipation factor and maximum partial discharge decreased with the increase of operating hours and starting numbers. It was found from off-line tests that the remnant breakdown voltage had a strong relationship with both dissipation factor and maximum partial discharge the remnant breakdown voltage as a results of both operating hours and starting number and the nondestructive tests were proposed as parameters which can predict the remnant lifetime of insulating materials in large generator stator windings. (author). 8 refs., 8 figs., 2 tabs.

• JSTS:Journal of Semiconductor Technology and Science
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• v.13 no.2
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• pp.152-156
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• 2013

This paper presents a fast-lock dual-loop successive approximation register-controlled duty-cycle corrector (SARDCC) circuit using a mixed (binary+sequential) search algorithm. A wider duty-cycle correction range, higher operating frequency, and higher duty-cycle correction accuracy have been achieved by utilizing the dual-loop architecture and the binary search SAR that achieves the fast duty-cycle correcting property. By transforming the binary search SAR into a sequential search counter after the first DCC lock-in, the proposed dual-loop SARDCC keeps the closed-loop characteristic and tracks variations in process, voltage, and temperature (PVT). The measured duty cycle error is less than ${\pm}0.86%$ for a wide input duty-cycle range of 15-85 % over a wide frequency range of 0.5-2.0 GHz. The proposed dual-loop SARDCC is fabricated in a 0.18-${\mu}m$, 1.8-V CMOS process and occupies an active area of $0.075mm^2$.

• 제어로봇시스템학회:학술대회논문집
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• 1997.10a
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• pp.1520-1523
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• 1997

A rigorous dynamic simulation was performed in binary gas mixture H$_{2}$/CO (70:30 vol.%) to determinate start-up operating conditions of PSA(Pressure Swing Adsorption) processes. The rigorous dynamic model for the PSA process contains an Ergun equation for expressing the pressure drop in a bed, and valve equations to compute the boundary pressure change of the bed. As the result of the continuous dynamic simulation of 100 operating cyles in various initial conditions, the unsteady-state appeared in the early period and the cyclic steady-state came out about 20th cycle in feed condition and vaccum condition, and 30th cycle in pure H$_{2}$ condition. As time goes by valve equations made change the pressure at each end of the bed in ressurization, countercurrunt-depressurization and pressure equalization steps. The H$_{2}$ purity and the recovery is 99.99% and 86.73% respectively, which is slightly higher than the experimental data. Main contributiion of this study includes supplying fundamental technologies of handling combined variables PSA processes by developing rigorous models.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.2
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• pp.279-287
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• 1996

A cycle analysis is performed to investigate how the non-ideal gas behavior of helium reduces the heating capacity of VM heat pumps. Since the operating pressures of VM heat pumps are as high as 1 to 20 MPa, the compressibility factor of helium becomes clearly greater than 1 and the non-ideal behavior always represents a thermal loss in heating. To calculate the amount of the losses, an adiabatic cycle analysis is performed with the real properties of helium and the net enthaply flows through the two regenerators are numerically obtained. It is shown that the non-ideal gas losses could be as much as 8% in the heating capacity when the operating pressures are greater than 10MPa. The effects of the operating temperatures and the dead volumes on the loss are presented.