• 설비공학논문집
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• 제24권11호
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• pp.792-798
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• 2012

A general concept on the definition of the second law efficiencies of thermodynamic cycles is introduced. The efficiency is defined to be proportional to the entropy generation divided by the maximum possible entropy generation. This way of definition of the cycle efficiency is clear and concise and, moreover, follows faithfully the concept of the second law of thermodynamics. This definition is applied to heat engine, refrigerator and heat pump. The second law efficiencies of heat engine and refrigeration cycles are derived, which are the same as the existing ones, respectively. The second law efficiency of heat pump, however, finds to be different from the existing one. Discussion is given about the difference and its cause.

• International Journal of Automotive Technology
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• 제2권1호
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• pp.1-7
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• 2001

This paper presents further investigation into the effect of over-expansion cycle in a spark-ignition engine. On the basis of the results obtained in previous studies, several combinations of late-closing (LC) of intake valve and expansion ratio were tested using a single-cylinder production engine. A large volume of intake capacity was inserted into the intake manifold to simulate multi-cylinder engines. With the large capacity volume, LC can decrease the pumping loss and then increase the mechanical efficiency. Increasing the expansion ratio from 11 to 23.9 with LC application can produce about 13% improvement of thermal efficiency which was suggested to be caused by the increased cycle efficiency. The decrease of compression ratio from 11 to 5.5 gives little effect on the thermal efficiency if the expansion ratio could be kept constant. Thus, the expansion ratio is revealed to be a determining factor for cycle efficiency, while compression ratio is no more important, which suggests the usefulness of controlling the intake charge with intake valve closure timing. These were successfully explained by simple thermodynamic calculation and thus the mechanism could be verified by the estimation.

• 한국수소및신에너지학회논문집
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• 제22권2호
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• pp.223-231
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• 2011

The thermal efficiency of energy-to-power conversion becomes uneconomically low when the temperature of heat source drops below $370^{\circ}C$. ORC (Organic Rankine Cycle) has attracted much attention in last few years due to its potential in reducing consumption of fossil fuels and relaxing environmental problems, and its favorable characteristics to exploit low-temperature heat sources. In this work thermodynamic performance of ORC using nine working fluids is comparatively assessed. Special attention is paid to the effect of system parameters such as turbine inlet temperature and pressure on the characteristics of the system such as volumetric flow rate and quality at turbine exit, latent heat, net work as well as thermal efficiency. Results show that in selection of working fluid it is required to consider various criteria of performance characteristics as well as the thermal efficiency. Results also show that the system efficiencies become same irrespective of kind of working fluid when the temperature of heat source decreases to low range.

• 대한기계학회논문집
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• 제16권1호
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• pp.77-86
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• 1992

본 연구에서는 사이클은 터빈 출구로 부터 배출되는 폐열을 최대한 회수하여 얻은 증기를 연소기내에 분사시킴으로써 부가적인 압축기 및 비출력의 상향을 기할수 있다.아울러 폐열이용 암모니아 흡수기 냉동기를 구동하여 압축기 입구 온도를 낮 춤에 의해 열효율 및 비출력의 증대는 물론 대기온도 변화에 따른 기관 성능의 변동을 감소시킬 수 있다.

• 한국수소및신에너지학회논문집
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• 제31권2호
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• pp.234-241
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• 2020

This paper presents a thermodynamic performance analysis of a combined cycle consisting of regenerative organic Rankine cycle (ORC) and liquefied natural gas (LNG) Rankine cycle to recover low-grade heat source and the cold energy of LNG. The mathematical models are developed and the system performances are analyzed in the aspect of thermodynamics. The effects of the turbine inlet pressure and the working fluid on the system performance such as the mass flow rates, heat transfers at heat exchangers, power productions at turbines, and thermal efficiency are systematically investigated. The results show that the thermodynamic performance of ORC such as net power production and thermal efficiency can be significantly improved by the regenerative ORC and the LNG cold energy.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2005년도 창립60주년 기념 추계 학술대회
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• pp.101.1-101.1
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• 2005

• 대한기계학회논문집B
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• 제36권9호
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• pp.937-944
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• 2012

선박의 주 추진용 디젤엔진에서 배출되는 배기가스의 폐열을 회수하는 발전시스템에 대하여, 작동유체로서 물이 적용된 3 변 사이클에 대한 열역학적 특성을 이론적으로 조사하였다. 그 결과로, 하나의 열원이 주어지면, 에너지 및 엑서지 효율은 터빈입구에서 작동유체에 대한 압력 및 온도의 특정한 조건에 의하여 최대화될 수 있었다. 그러한 조건에 대하여 응축온도의 증가에 따라, 터빈의 체적 팽창비를 적절하게 감소시킬 수 있었는데, 열원의 엑서지 손실률 및 응축기에서 엑서지 파괴율이 크게 증가되었다. 따라서, 상부 사이클에서 버려지는 엑서지를 회수하기 위하여, 저온 열원에 적합한 유기랭킨사이클을 하부 사이클로 적용하는 복합 사이클이 유용할 수 있다.

• 설비공학논문집
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• 제5권1호
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• pp.73-83
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• 1993

Thermodynamic properties of alternatives for R12 and R22 were estimated and performances of refrigerating cycle using these refrigerants were compared. In this study, we adopt R134a, R22/R142b, R22/R152a, R22/R152a/R124 as alternatives for R12 and R32/R134a for R22. Thermodynamic properties of these refrigerants were estimated using modified CSD equation of state. Cycle simulations of the refrigerating system considering heat source were carried out in order to compare the performance of the system. R134a shows relatively lower COP than R12 but very similar VCR. R22/R142b(50/50 mass fraction), R22/R152a(10/90), R22/R152a/R124(30/25/45) are good for the substitutes of R12 and R32/R134a(30/70) is appropriate for that of R22 in view of COP and VCR.

• 설비공학논문집
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• 제19권7호
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• pp.485-490
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• 2007

The LNG BOG re-liquefaction system for LNG carriers was designed based on the Claude refrigeration cycle and the thermodynamic analysis was carried out in order to find the design point of the three heat exchangers constituting the system. The thermodynamic analysis revealed that the system state could be defined by the three cold endpoint temperatures of the three-pass heat exchanger. Hence the iso-lines of the specific liquefaction work, taken as the performance indicator, were presented in terms of those three temperatures and discussed. The system was found most economical when those three temperatures approached a single temperature of $-140^{\circ}C$ and thus this system state could be taken as the design point for the heat exchangers.

• 설비공학논문집
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• 제23권10호
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• pp.662-669
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• 2011

A numerical study was performed for thermodynamic efficiencies of a basic ORC (Organic Rankine Cycle), ORC with a FLH (Feed Liquid Heater), and ORC with a regenerator. The efficiencies of the basic ORC were higher in the order of R113, R123, R245ca, and R245fa for its working fluids. It was confirmed that an optimal FLH pressure existed to maximize efficiency of the ORC with a FLH. A correlation was developed to predict the optimal FLH pressure as a function of evaporation and condensation pressure and its average absolute deviation was 0.505%. The efficiency enhancement of the basic ORC with a FLH was higher than that with a regenerator. It was presented that the basic ORC efficiency could be improved more than 10% by a FLH with $30^{\circ}C$ condensation and over $110^{\circ}C$ evaporation temperatures.