순차적 카르노 사이클의 열역학적 특성과 열펌프 시스템에서의 응용에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2015. 8. 김민수.Since the fossil fuels, which meet most of the energy requirements in the world, are not environmental friendly and its present stocks are finite, various renewable energy sources have been introduced as a promising option to fill increasing demand of worldwide energy. Although some typical renewable energy sources, such as solar photovoltaic energy, wind energy, and hydrogen energy of fuel cells, are directly converted into electricity, a lot of renewable sources, such as solar thermal energy, industrial waste heat, geothermal energy, and ocean thermal energy, still exist in the form of heat. These thermal energy sources categorized as low grade heat source because they have low temperature and relatively small heat capacity, requires some thermodynamic cycles to generate useful power. Generally, Carnot cycle, has been used in the evaluation of the thermal system as a standard. However, considering that the characteristics of renewable heat sources are different from those of conventional sources, it may not be the best choice to use just a single Carnot cycle for analyzing renewable thermal energy system as it deals with the heat source of infinite heat capacity. Therefore, the sequential Carnot cycle has been suggested, where a number of individual Carnot cycles are arranged in parallel. In contrast to the original Carnot cycle, the sequential cycle considers the temperature change in heat sources which occurs at the heat transfer process between the cycle and heat sources. Moreover, in case of a sequential Carnot cycle, the heat transfer rate between the cycle and heat sources is calculated by taking account of temperature difference between them. In this study, the thermodynamic characteristics of sequential Carnot cycles, such as the efficiency and power output of the system in various conditions, are investigated. Using basic model for the sequential system, which is advanced from the initial one suggested earlier, the performance of sequential Carnot cycles is calculated in an analytical way. For the analysis, only fundamental relations of thermodynamics and heat transfer are utilized, instead of complex numerical techniques. The effect of major variables, which are the number of individual Carnot cycles in the system, the inventory of heat exchangers used and the final temperature of heat sources, on the system performance is researched and some optimization processes are also conducted based on those results. During the analysis, some symbolic expressions are obtained, which can be utilized effectively in actual situations. Also, to bridge the gap between the practical thermal system and the sequential Carnot cycle, more detailed sequential system models are proposed. Different from the previous one, this model adopts the heat sink with finite heat capacity and has some internal irreversibilities in contrast with Carnot cycles. How these features influences the system performance is discovered by analytical equations which are derived from the process similar to that used in the basic model. By the way, throughout the theoretical research, the interesting phenomenon is obeserved, which is that the efficiency of the sequential system increases along with the number of individual Carnot cycles in the system. Paying attention to the possibility that this result can also be applied to actual thermodynamic systems, the numerical simulation using thermodynamic properties of real working fluids is carried out targeting organic Rankine cycles (ORC) and heat pumps. As a result, it is found that the concept of sequential cycles has advantages in actual cases. Finally, the experimental setup for a sequential heat pump, which has two refrigeration cycles in one system, is prepared to validate the result of numerical calculation. The experiment is conducted with not only a sequential heat pump, but also a simple conventional heat pump, which have the same heat capacity. After comparing coefficients of performance (COP) of both systems, it can be found that the sequential heat pump has higher efficiency than the simple heat pump. It can be expected that the result of this research, such as simple expressions from sequential Carnot cycles, is widely used in various situations related with renewable thermal energy sources or a lot of researches about sequential thermal cycles will be initiated by this study.Abstract i Contents v List of Figures viii List of Tables xiii Nomenclature xiv Chapter 1. Introduction 1 1.1 Background of the study 1 1.2 Literature survey 6 1.3 Objectives and scopes 11 Chapter 2. Thermodynamic analysis of basic sequential Carnot cycles 14 2.1 Introduction 14 2.2 System description 16 2.3 Analytical modeling of the cycle 19 2.4 Parametric study for the performance of the cycle 22 2.5 Analytical modeling for exergy analysis 30 2.6 Result of exergy analysis 36 2.7 Summary 42 Chapter 3. Advanced analysis and optimization study on sequential Carnot cycles 44 3.1 Introduction 44 3.2 Sequential Carnot cycle with finite heat sink 46 3.2.1 Analytical modeling of the cycle 46 3.2.2 Performance of advanced sequential Carnot cycles 50 3.3 Sequential Carnot cycle with internal irreversibility 60 3.3.1 Analytical modeling of the cycle 60 3.3.2 Performance of sequential cycles with internal irreversibility 65 3.4 Optimization of sequential systems 72 3.4.1 The optimal distribution of heat exchanger inventory 72 3.4.2 The optimal TF in sequential systems 85 3.4.3 The optimal NTUH/NTUL in sequential systems 93 3.5 Summary 101 Chapter 4. Practical research for sequential heat pump. 103 4.1 Introduction 103 4.2 Numerical simulation of the sequential system 105 4.2.1 Simulation model of the system 105 4.2.2 The results numerical simulation 111 4.3 The experimental study on the sequential heat pump 121 4.3.1 Experimental methodology 121 4.3.2 Experimental result 129 4.4 Summary 142 Chapter 5. Concluding remarks 143 References 146 Abstract (in Korean) 152Docto

R134a와 R410A를 사용한 캐스케이드 냉동 시스템의 최적 중간온도에 대한 연구

학위논문 (석사)-- 서울대학교 대학원 : 기계항공공학부, 2012. 2. 김민수.The eco-friendly heat pump system which brings high efficiency has gained a lot of attention and becomes one of the major facilities in refrigeration and air-conditioning. In case of producing hot water, however, replacement of existing hot water supply systems which use fossil fuel as a heat source with heat pump water heaters has not been progressed effectively because providing hot water with heat pump systems has some disadvantages. Major weaknesses of heat pump water heaters are that their heat capacity is deteriorated in cold climate and their maximum water temperature has some limitations by refrigerants used and systems. To overcome those drawbacks, a cascade cycle using R134a and R410A is adopted in a heat pump water heater system. A cascade cycle in a refrigeration system has two different refrigerants as working fluids in separated loops: a high-temperature cycle, and a low-temperature cycle. By operating two divided loops in one system, the problems mentioned earlier can be handled properly. While the cascade system can solve the problems of existing heat pump water heaters, it raises some control issues since the number of devices composing the cycle increases in a cascade system. One of the most important and unique control parameters in a cascade refrigeration cycle is the intermediate temperature which is an evaporating temperature of a high-temperature cycle. In this study, the optimal intermediate temperature for the maximum COP was investigated in an analytical and an experimental way. In an analytical study, a new equation which can calculate the optimal intermediate temperature was suggested. This formula was derived by fundamental thermodynamic laws without additional numerical data. Some assumptions were employed to model a complex system, such as an isothermal heat transfer process. In addition, regression analysis was also applied in the equation to reflect the thermodynamic characteristic of refrigerants. Optimal values from this equation were compared with an existing correlation and its tendency according to other conditions was studied. To prove the validity of an analytical result, experiments were carried out. Experiments were conducted under a steady-state condition with an intermediate temperature changed. Major operating conditions were also adjusted to identify their effects and experimental optimum value of intermediate temperature and corresponding COP were determined in each case. Finally, the experimental results were compared with an analytical result to secure the validity of an analytic solution.친환경적이며 고효율로 작동하는 히트펌프 시스템은 많은 관심을 받아왔으며 이제는 냉동 공조분야의 주요설비로 자리잡았다. 고온의 물을 만들어 내는 경우에 있어서는 여전히 히트펌프가 가지는 몇몇 단점들로 인하여 기존의 화석연료이용 급탕기를 효과적으로 대체하고 있지 못하는 상황이다. 주요 단점으로는 히트펌프가 영하의 온도와 같은 추운 날씨에 충분한 열량을 확보하기 어렵다는 점과 냉매 및 열 교환의 한계로 인하여 온수의 최대 온도를 높이 올리기 어렵다는 점을 들 수 있다. 이러한 약점을 극복하기 위하여 히트펌프 급탕시스템에 R134a와 R410A를 사용하는 캐스케이드 사이클이 도입되었다. 캐스케이드 사이클은 고온부와 저온부로 분리된 두개의 루프에 서로 다른 냉매를 작동유체로 하는 시스템이다. 하나의 시스템을 두 개의 루프로 분리시켜 작동함으로써 앞서 언급한 단점들을 줄일 수 있게 된다. 그럼에도 불구하고 캐스케이드 시스템은 기존의 시스템에 비해 두 배의 장치가 설치되기 때문에 이로 인하여 제어 문제들이 몇몇 생겨난다. 그 중 가장 대표적이며 캐스케이드 사이클에서만 특수하게 나타나는 주제가 바로 중간 온도를 제어하는 것으로, 이때의 중간온도는 고온부 사이클의 증발온도를 의미한다. 본 연구를 통하여 시스템의 성능계수를 최대화 할 수 있는 최적 중간온도를 분석적 및 실험적 방법을 통하여 알아보고자 한다. 분석적 연구부분에서는 최적 중간온도를 찾기위하여 사용할 수 있는 새로운 방정식을 고안하였다. 이 식은 부수적인 수치 데이터 없이 기본적인 열역학적 법칙들을 사용하여 유도되었다. 복잡한 시스템을 파악하기 위하여 등온 열교환 과정과 같은 몇몇 가정이 도입되었으며 냉매의 물성치에 따른 영향을 반영하기 위하여 관련 상관식을 제작, 도입하였다. 식을 통하여 얻은 최적값은 기존에 존재하는 관련 상관식의 결과와 비교를 하였으며 조건의 조작을 통해서 어떻게 값이 변화하는지 또한 살펴보았다. 분석적 연구를 통해 얻은 결과의 타당성을 증명하기 위하여 실험을 수행하였다. 실험은 국제 표준 조건을 위주로 중간온도를 변화시켜 가며 실행하였다. 또한 주요 작동변수들의 영향을 파악하기 위해 조금씩 변화시켜가며 실험을 수행하였다. 각 경우마다 최적 중간온도의 값과 그에 대응하는 성능계수를 구하였다. 실험 결과는 앞서 유도한 식의 결과와 비교되었으며 변화시킨 작동변수들의 영향도 함께 파악되었다.Maste