THE ANALYSIS AND EXPERIMENTAL INVESTIGATION OF HEAT PUMP SYSTEM USING THERMOBANK AND COS EJECTOR CYCLE

Transcription

1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00385 THE ANALYSIS AND EXPERIMENTAL INVESTIGATION OF HEAT PUMP SYSTEM USING THERMOBANK AND COS EJECTOR CYCLE Cuong Le Ngoc 1, Kwang-Il Choi 2, Jong Taek Oh 2* 1 Graduate school, Chonnam National University, 50 Daehak-ro, Yeosu, Chonnam , Republic of Korea. 2 Department of Refrigeration and Air Conditioning Engineering, Chonnam National University, 50 Daehak-ro, Yeosu, Chonnam , Republic of Korea. Presenting Author: cuonglnbk@gmail.com * Corresponding Author: ohjt@chonnam.ac.kr ABSTRACT This paper shows the results of an experimental investigation in which the heat pump system performance of the low pressure refrigerants R404A was compared between a standard cycle and condenser outlet split ejector cycle (COS). The special features of this study are to use the thermobank and an ejector in heat pump system. The thermobank stored heat from superheat vapor of refrigeration cycle and its energy is used for heating room and defrosting process. The ejector is an expansion device capable of work recovery. Therefore, the thermobank and ejector can help improving coefficient of performance (COP). When compared to standard cycle, the heat pump system using thermobank and COS ejector cycle got maximum COP improvements of 24.5%. This heat pump system can be used to keep preservation of agricultural products in cold storage warehouse together with floor panel heating for room in winter. KEYWORDS: Heat pump system, Thermobank, COP, COS(Condenser outlet split ejector cycle), Energy saving 1. INTRODUCTION This study is to develop a high efficiency heat pump system. The special feature of the system is the heat storage equipment (thermobank) and ejector installed in the refrigeration system. The ejector principle has been known for a long time ago. Since ejector is capable of generating low pressure and then lifting pressure, it can be relegated the refrigeration purposes to applications where waste heat is easily available from sources such as automobiles, industrial processes and solar, etc. The ejector refrigeration system provides a promising way to the emergency and be of great potential for wide application due to its electrical energy saving, simplicity in construction, installation and maintenance. However, there are currently no commercially ejector refrigeration systems due to the low COP thermal, whereas high COP mechanical. In the expansion device, throttling loss is one of the thermodynamic losses in the conventional vapor compression refrigeration cycle. The loss can be greatly reduced by the isentropic throttling process instead of the isenthalpic process. In order to recover the potential kinetic energy in the expansion process, various possible methods of the expansion process have been proposed. Recently, the application of liquid-gas ejector has been considered as one of the most efficient methods to reduce the throttling loss in compression refrigeration cycle. In this study, the heat pump system, as is shown in Fig.1: a two-phase ejector is used to recover the kinetic energy loss during the expansion process. The motive flow from the high pressure side of the system is expanded in the motive nozzle, meanwhile the static pressure converts into kinetic energy during the expansion process, and some of the liquid turn into gas due to the pressure drop. Since the pressure at the nozzle outlet is lower than evaporation pressure, gas from the evaporator is sucked into the suction nozzle of the ejector, then motive flow (primary flow) and suction flow (secondary flow) are mixed in the mixture 1

2 section. The velocity of high speed two-phase flow slows down with the increase of the static pressure in the diffuser section, after that the mixture is separated in a separator, the gas is sucked into the compressor and the liquid is sucked into the evaporator. The ejector geometry is shown in Fig.2. Because of the pressure recovery in the ejector, the suction pressure is higher than that of the conventional system, so the compression ratio of the compressor can be reduced. Therefore the compressor energy consumption can be decreased. It eventually improves the system performance in terms of COP and cooling capacity. There are a lot of difficulties in the twophase ejector research due to the complex flow, however, more and more researchers focus on this field due to the potential of energy recovery in the refrigerating system. 2. ANALYSIS In Fig.1 shows ejector working principle. A typical ejector consists of a motive nozzle, a suction chamber, a mixing section, and a diffuser. The working principle of the ejector is based on converting internal energy and pressure related flow work contained in the motive fluid stream into kinetic energy. Energy balance equations in the primary and secondary nozzle sections in Fig. 2. Mass entrainment ration is definite by equation: Mass equations in term of entrainment ratio: (4) (5) Mass, momentum and energy equations in the mixing section: (6) The energy balance in the diffuser section: The overall energy balance in the ejector: ( ) ( ) (8) (1) (2) (3) (7) (9) (10) 3. EXPERIMENTAL APPARATUS The Fig.1 shows the schematic and working principle of heat pump system with using COS ejector cycle and thermobank. In the compressor, low temperature and low pressure refrigerant (1) is pressurized into high temperature and high pressure refrigerant (2). Oil will be separated and come back to compressor, the refrigerant enters a thermobank where heat from the refrigerant is removed and water in thermobank is warmed up. So the refrigerant is precooled before going to condenser (3). The refrigerant goes into condenser and it is condensed into liquid (4). The liquid refrigerant is continuously cooled into subcooling state (5) and stored in receiver before splitting into two parts. One part, the liquid refrigerant (5) is throttled by an thermo expansion valve (10) where a small liquid refrigerant portion is evaporated to vapor and bigger liquid refrigerant portion is continuously reduced pressure to evaporating pressure before entering the evaporator 1. Second part enters ejector and expanded in the motive nozzle and combines with the vapor refrigerant after evaporator 1 (11). In ejector where takes place heat transfer, mixing, reduced pressure and separate processes (6,7,8,9). After ejector, the refrigerant is liquid and vapor state (9). The refrigerant (9) enters evaporator 2. In the evaporator the liquid refrigerant at low pressure and temperature, passes through the evaporator and absorbs heat from the cold storage. The liquid refrigerant boils and 2

3 evaporates when it is heated. The vapor from evaporator 1 goes into the suction gate of ejector (11). All the vapor refrigerant from evaporator 2 comes back to suction line of the compressor (1) for continuing its cycle again. Water is used in the thermobank which stores and transfers the heat. Hot water in thermobank is cyclically pumped to heating panel. Heating panel is used for warming up the house. The heat storage is also used for defrosting process. During the defrosting process, the hot refrigerant from compressor goes through the evaporator for evaporator coils defrosting process. The refrigerant ejects its heat and condenses into liquid. It flows back into the heat storage equipment (thermobank). The liquid refrigerant is re-evaporated by the heat storage of thermobank before entering the compressor and circulates again. So that, the system does not need an accumulator. Fig. 1 The apparatus of heat pump system using COS ejector and thermobank. Fig. 2 The ejector geometry. 4. DISCUSSION The experimental investigation measures the system power input and system COP. Sensor: T-thermocouple, full scale (±0.1 0 ). The Coriolis mass flow rate, full scale 0.075kg/min 5kg/min, accuracy of ±0.5%. Static pressure transducer, full scale 0-3.5Mpa, accuracy of ±0.25%. And dimensions of nozzle throat are : 0.8, 1, 1.2, 1.5, 2mm. Diameter of mixing chamber is 6mm and diffuser chamber is 16mm, dimension between motive nozzle and mixing chamber is 15mm. From fig.3, we can find that as the nozzle throat diameter increases, the mass flow rate of the refrigerant increases. Because mass flow rate is directly proportional to flow section area. When the nozzle throat diameter increases, the mass entrainment ration decreases. Because the nozzle throat diameter increases then velocity out let of motive nozzle decreases, the momentum of the flow decreases. So it could make enough suction pressure at secondary flow of ejector. Therefore, the mass entrainment ration decreases when the nozzle throat diameter increases. Fig.4 shows the effect of nozzle throat dimension and temperature in cold storage to COP. When temperature in cold storage decreases, COP decreases because cooling capacity decreases. COP gets maximum value at 1 mm of motive nozzle diameter. The effect of nozzle throat dimension to cooling capacity and COP is depicted in Fig.5. The temperature in cold storage was set at zero degrees Celsius. When nozzle diameter changes from 0.8 mm to 2 mm, the variation tendency of system refrigerating capacity. When nozzle diameter is 1 mm, system cooling capacity is 2500 W, and the system can achieve the maximum capacity and COP. After that as the nozzle diameter increases, cooling capacity decreases gradually because the mass entrainment ration decreases. 3

4 Fig. 3 The effect of nozzle throat dimension on mass flow rate and mass entrainment ration. Fig. 4 The effect of nozzle throat dimension and temperature in cold storage to COP. Fig. 5 The effect of nozzle throat dimension to cooling capacity. Fig. 6 The effect of entrainment ration and temperature in cold storage to COP. The Fig.6 shows the effect of mass entrainment ration and temperature in cold storage to COP, when mass entrainment ration increases, COP increases, the test condition with 1mm of motive nozzle diameter. Fig. 7 The comparison of COP. 4

5 Fig.7 shows the effect of temperature in cold storage to COP. The COP increases when the temperature in cold storage increases. Thermobank can help improving cooling capacity but power input is still constant compare with standard cycle. When the thermobank and the COS ejector cycle are installed in system, ejector can decrease in compressor displacement relative to a standard vapor compressor cycle and the COP increases about 24.5% in comparison with standard cycle. 5. CONCLUSIONS The heat pump system using thermobank and condenser outlet split ejector cycle has been experimental investigation. The ejector is used in system which the purpose increases in coefficient of performance (COP) and decrease in compressor displacement relative to a standard vapor compressor cycle. The mass flow rate of the refrigerant increases with increasing the nozzle throat diameter. The COP decreases when the temperature in cold storage decreases. This paper also study the effect of nozzle throat dimension to COP. When compared to standard cycle, the heat pump system using thermobank and COS ejector cycle got maximum COP improvements of 24.5%. This heat pump system can be used to keep preservation of agricultural products in cold storage warehouse together with floor panel heating for room in winter. ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by Ministry of Education, Science and Technology (NRF-2016R1D1A1A ). NOMENCLATURE h specific of enthalpy ( J/Kg) a area (m 2 ) v fluid velocity (m/s) p f pressure drop (pa) U mass entrainment ration fluid density (kg/m 3 ) REFERENCE [1] Neal Lawrence, Stefan Elbel Experimental investigation of a two-phase ejector cycle suitable for use with low-pressure refrigerants R134a and R1234yf. Proceeding of international journal of refrigeration [2] Boeis A.M, K.O. Homan, J.H.Davidson and Wei Liu A Variable effectiveness model for indirect thermal storage devices. Proceedings of ASME summer heat transfer conference HT research 25: [3] Yazdani, M., Alahyari, A. A., & Radcliff, T. D. (2012) Numerical modeling of two-phase supersonic ejectors for work-recovery applications. International Journal Heat Mass Transfer, 55, [4] Takeuchi, H., Kume, Y., Oshitani, H., & Ogata. G (2002). Ejector cycle system. U.S. Patent 6,438,993 B2. [5] Banasiak, K, & Hafner, A. (2011). 1D Computational model of a two-phase R744 ejector for expansion work recovery. International Journal of Thermal Sciences, 50, [6] F. P., & DeWitt, D. P. (1990). Fundamental of heat transfer and mass transfer 3 rd edition, John Wiley, New York, [7] Smolka J, Bulinski Z, Fic A, Nowak AJ, Banasiak K, Hafner A, A computational model of a transcritical R744 ejector based on a homogeneous real fluid approach. Applied Mathematical Modelling (2012). [8] Incropera, F. P., & DeWitt, D. P. (1990). Fundamental of heat transfer and mass transfer 3 rd edition, John Wiley, New York, [9] Zhu., Yinhai, Cai., Wenjian, Wen., Changyun, Li, Yanzhong, 2009.Numerical investigation of geometry parameters for design of high performance ejectors. Appl. Therm. Eng. 29, [10] Eames, Ian W., A new prescription for the design of supersonic jet-pumps: the constant rate of momentum change method. Appl. Therm. Eng. 22,