Studies on the performance characteristics and system optimization of cascade heat pump

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2014. 2. 김민수.In this study, the studies of cascade heat pump performance characteristics and optimization for heating and hot water supply in extremely cold region were conducted. In general, conventional heat pump water heater system shows a limited performance at low ambient temperature whereas the heating demand increases. In order to increase the water discharge temperature, multi-stage cycle was suggested and among the multi-stage cycle, cascade cycle shows the best performance at high condensing temperature condition. In order to increase the cascade performance at low ambient temperature, R410A refrigerant which is suitable for low temperature application was adopted in bottoming cycle. The R134a refrigerant which has a higher critical temperature was adopted in topping cycle to increase the water discharge temperature. In order to increase the cascade system efficiency, several attempts were conducted. The determination of optimal refrigerant charge amount was studied by numerical simulation and experiment. The optimized coefficient of performance was obtained at the optimal charge amount condition and its corresponding degree of subcool at each cycle was suggested. The intermediate temperature which determines the pressure ratio of each cycle was also optimized by numerical anaysis based on the reverse-Carnot model. The verification of optimized numerical intermediate temperature was conducted by experiment and numerical intermediate temperature well predicted the experimental optimal intermediate temperature. The performance characteristics of cascade heat pump with water temperature lift at condenser were conducted by experiment. Despite of several advantages of heat pump than conventional boiler, slower thermal response is the weakness of heat pump water heater system. In order to increase the water discharge temperature, mass flow rate of water should be reduced. In case of decreasing water mass flow rate, the performance characteristics of cascade heat pump were obtained. The fast response of heat pump system is the key issue for hot water supply. The optimal control logic which is suitable for cascade heat pump was designed and the verification of control logic was conducted. The optimized PI controller based on the genetic algorithm showed enhanced performance than conventional PI tuning method.1. Introduction 1 1.1 Background of the study 1 1.2 Literature survey 11 1.3 Objectives and scopes 19 2. Determination of the refrigerant charge on cascade system 21 2.1 Introduction 21 2.2 Single cycle charge optimization 23 2.2.1 System description and experimental apparatus for single cycle 23 2.2.2 Test conditions, data reduction and uncertainty of measurements 24 2.2.3 Test results and discussion 30 2.2.4 Simulation results for single cycle and discussion 37 2.3 Cascade cycle charge optimization 49 2.3.1 System description and experimental apparatus for cascade cycle 49 2.3.2 Test conditions, data reduction and uncertainty of measurements 50 2.3.3 Test results and discussion 51 2.3.4 Simulation results for cascade cycle and discussion 64 2.4 Conclusion 69 3. Optimal intermediate temperature on cascade system 71 3.1 Introduction 71 3.2 System description and experimental apparatus 72 3.2.1 System description 72 3.2.2 Experimental apparatus and test procedure 74 3.2.3 Test conditions, data reduction and uncertainty of measurements 78 3.3 Numerical analysis of optimum intermediate temperature 80 3.4 Experimental analysis of optimum intermediate temperature 85 3.4.1 Characteristics of cascade system 85 3.4.2 Experimental results for heating capacity change 90 3.4.3 Experimental results for water inlet temperature change 93 3.4.4 Experimental results for ambient temperature change 96 3.4.5 Validation of numerical analysis 98 3.5 Conclusion 101 4. Performance characteristics of cascade heat pump with regard to water temperature lift 102 4.1 Introduction 102 4.2 Performance of cascade heat pump performance with water temperature lift 103 4.2.1 System description and experimental apparatus 103 4.2.2 Test conditions, data reduction and uncertainty of measurements 104 4.2.3 Test results and discussion 111 4.3 Effect of water temperature lift on optimaum intermediate temperature 124 4.3.1 Optimum intermediate temperature and test conditions 124 4.3.2 Test results and discussion 128 4.4 Effect of water temperature lift on transient performance 134 4.4.1 Transient heat pump performance and test conditions 134 4.4.2 Test results and discussion 135 4.5 Conclusion 140 5. Optimized control logic of cascade heat pump 142 5.1 Introduction 142 5.2 System identification 144 5.2.1 Input signal for the system identification 145 5.2.2 System model determination 153 5.3 Optimization of controller with genetic algorithm 157 5.4 Performance of optimized PI controller 163 5.4.1 Transient performance of cascade heat pump 163 5.4.2 Ziegler-Nichols PI controller 168 5.4.3 Gengitc algorithm PI controller 169 5.5 Conclusions 173 6. Concluding remarks 175 References 177 Abstract (in Korean) 185Docto