A new method for preventing air-source heat pumps and refrigerators from frosting

Transcription

1 12 th IEA Heat Pump Conference 2017 A new method for preventing air-source heat pumps and refrigerators from frosting Central Research Institute of Electric Power Industry L. Zhang, T. Fujinawa, K. Hashimoto*, M. Saikawa May, 2017 ROTTEROAM

2 ubackground Contents uintroduction of the proposed frost-free heat pump and simulation results uintroduction of the proposed frost-free refrigerator and simulation results uconfirming experimentally whether frost-free could be realized uconclusions 2

3 Background 3

4 Background frosting problem of heat pump u Frost formation on the outdoor heat exchanger (evaporator) is one of the major problems for air source heat pump in winter. Decrease heat transfer and obstruct air flow humid air frost fin tube refrigerant Outdoor heat exchanger(evaporator)of heat pump Heating operation Defrosting operation Problems: Reduced energy efficiency Heating shutdown 4

5 Background frosting problem of refrigerator u Frost formation is a common phenomenon observed in refrigerators, including household refrigerator-freezers, refrigerated display cabinets, refrigerated warehouses and so on. <Frost layer on the surface of the evaporator of a refrigerated display cabinet> Frost Defrosting heater As shown in this photo, although the fin pitch of the heat exchanger is over 10mm, the air flow is obstructed by the frost layer. Cooling operation Problems: Reduced energy efficiency Cooling shutdown Overflowing of defrosted water from tray Defrosting operation 5

6 uconcept of frost-free How to realize frost-free Humid air desiccant Dried air Frost-free refrigerant Heat exchanger (evaporator etc.) u Energy is needed to regenerate the desiccant fin tube Exhausted air The humid air is dehumidified via desiccant before it enters evaporator. Therefore, frostfree could be achieve. heating Regeneration air 6

7 Introduction of the proposed frost- free heat pump and simulation results 7

8 The proposed frost-free heat pump water heater system Desiccant-coated heat exchanger (DCHE): desiccant is coated on the surface of the heat exchanger DCHE refrigerant Evaporator V1 compressor Condenser or gas-cooler Hot water D1 V2 D3 Air pass D2 V1, V2-expansion valve D1, D2, D3- damper The system is composed of compressor, condenser or gas-cooler, DCHE, evaporator, two expansion valves (V1-V2) and three air dampers (D1-D3). With respect to the moisture moving between the desiccant and the air, the operation process of the proposed system can be classified into two modes: adsorption mode (AD mode) and desorption mode(de mode). 8

9 uadsorption mode (AD mode) DCHE D1 open OA P throttled V2 Evaporator AA frost -free D3 close D2 open EA Condenser DCHE Evaporator Enthalpy throttled V1 compressor (1) P-h diagram of refrigerant (AD) Condenser or gas-cooler Hot water OA- outside air AA- adsorption air EA- exhausted air During AD mode, refrigerant is throttled by V1 and V2 sequentially. So DCHE works as another evaporator. Hot water is produced at the condenser. Dampers D1 and D2 are opened and D3 is closed. Outside air (OA) is passed through DCHE, in which it is dehumidified. The dry air leaving from DCHE exits heat to refrigerant at the evaporator, and exhausted as exhausted air (EA). Because the dew point of the dry air (AA) is lower than the evaporation temperature of evaporator, frost-free can be realized. 9

10 udesorption mode(de mode) DCHE D1 close P V2 Evaporator DA throttled D3 open RA water D2 close Q DCHE =Q eva DCHE Evaporator open V1 compressor Q Con =W com Condenser Enthalpy (2) P-h diagram of refrigerant (DE) Condenser or gas-cooler DA- desorption air RA- recycled air Hot water During DE mode, V1 is opened and V2 is throttled. So DCHE works as another condenser and the condensation heat is used to regenerate the desiccant. Dampers D1, D2 are closed and D3 is opened. So the air recycles between the DCHE and evaporator. As a result, the condensation heat of DCHE is completely recovered at the evaporator. Therefore, the electrical consumption of the compressor is equal to the heat output to the water. Heating shutdown caused by conventional defrosting methods is solved by applying the air-recycling method of the system. 10

11 usimulation results The main parameters of the calculation: Air temperature (T oa ) : -7-5ºC, relative humidity (RH oa ): 60-80%; Inlet and outlet water temperatures: 5, 65ºC; The adiabatic efficiency of the compressor (η com ): 0.75 (assumed); Refrigerant: CO2 COP: the ratio of the heat output to water to the electrical consumption of the compressor 5.0 COP The proposed system RH oa =70% RH oa =80% The COP of the proposed system is 5-20 % higher than that of the hot gas defrosting system T oa (ºC) Be calculated by assuming that COP will be reduced by 20% [1] compared to a base system [2], which operates at non-frosting conditions. Fig.1 Comparison of COP between the proposed system and a conventional hot gas defrosting system [1]: Byun JS, et al. 2008, Frost retardation of an air-source heat pump by the hot gas bypass method, Int. J. Refrig. 31: [2]: Neksa P, et al. 1998, CO 2 heat pump water heater: characteristics, system design and experimental results, Int. J. Refrig. 21 (3):

12 Introduction of the proposed frost-free refrigerator and simulation results The method of using a desiccant to prevent the heat pump from frosting can also be applied to refrigeration systems, such as household refrigerator-freezers, refrigerated display cabinets, refrigerated warehouses and so on. 12

13 The proposed frost-free household refrigerator-freezer Freezer Compartment Refrigerator Compartment Air cooling HE capillary Compressor Air-cooled HE gas-liquid separator Expansion valve DCHE (desiccant-coated HE) The system comprises a compressor, an air-cooled HE, an expansion valve, a DCHE, a gas-liquid separator, a capillary and an air cooling HE. With respect to the available cooling load temperature and the moisture removing of the desiccant, the system can be classified into two modes: u Freezing-Refrigeration-ADsorption mode (F-R-AD mode) u Refrigeration-DEsorption (R-DE mode). 13

14 ufreezing-refrigeration-adsorption mode (F-R-AD mode) SA(F) Freezer Compartment FA SA(R) Refrigerator Compartment RA Evaporator DA DA MA DCHE (Adsorption) Condenser EA OA 3 throttled The expansion valve (3) is throttled, so the DCHE works as another evaporator. The air (MA) mixed by the freezer air (FA) and refrigerator air (RA) is dehumidified at DCHE due to the adsorption process of the desiccant. The dry air (DA) exiting DCHE is split into two air streams: SA(R) is supplied to the refrigerator compartment, the rest (SA(F)) is further cooled at the evaporator and supplied to the freezer compartment. Because the dew point of the dry air (DA) is lower than the evaporation temperature of evaporator, frost-free can be realized. 14

15 urefrigeration-desorption (R-DE mode) Freezer Compartment SA(R) Refrigerator Compartment RA Evaporator EA OA DCHE (Desorption) 3 opened The expansion valve (3) is opened, so the DCHE (4) works as a condenser The DCHE is heated by the condensation heat of the refrigerant and is regenerated by discharging moisture to the outside air (OA). The refrigerator air (RA) is cooled at the evaporator and returned to the refrigerator compartment. Note that the evaporation temperature of the evaporator should be operated to keep it higher than the dew point of RA, therefore frost-free operation is also feasible in R-DE mode. 15

16 usimulation results The main parameters of the calculation: The air flow rates of the refrigerator (R.) and freezer (F.) compartments are 0.1 and 0.3 m³/min. The air temperature and relative humidity in R. and F. compartments are (4ºC, 50%) and (-18ºC, 50%). The adiabatic efficiency of the compressor (η com ) is assumed to be 0.7. Refrigerant: Isobutane (R600a) MJ/day MJ 40% (F. compartment) 60% (R. compartment) overall cooling load 4.08MJ (1.13kWh) electricity consumption The total cooling load including F. and R. compartments, the electric power consumption of the compressor is 7.42MJ and 4.08MJ for one day. So the average COP, the ratio of the cooling load to electric power consumption, is calculated to be Fig.2 The overall cooling load and electric power consumed by the compressor for one day 16

17 Confirming experimentally whether frost-free could be realized The results of the theoretical analysis show that the two proposed systems can prevent evaporators from frosting and have high energy efficiency. In the following, we want to confirm experimentally whether a frost-free state could be realized and whether the DCHE could be effectively regenerated under the condensation temperature of heat pumps and refrigerators. 17

18 uexperimental apparatus DCHE AQSOA DCHE Polymer DCHE DPout DCHE DPin Process air (Aluminum fin and copper tube) (All aluminum, plate tube with corrugated fins) Cooling brine Hot brine Objectives of this experiment: Ø Whether can frost-free be realized? Ø Regeneration temperature of DCHE AQSOA: a kind of desiccant belonging to the zeolite family, developed by Mitsubishi Chemical Corporation). Polymer: a kind of sorption material, developed by Japan Exlan. Parameters DCHE (AQSOA) DCHE (polymer) Height (m) Length (m) Width (m) Fin (m) Mass of desiccant(kg)

19 uexperimental results Ø Whether can frost-free be realized? The evaluation criteria of the frost-free operation of the proposed heat pump and refrigerator are given as, Experimental result of AQSOA- DCHE for frost-free heat pump application DP #$% < T ()* DP out : is the measured dew point of the DCHE outlet air T eva : is the evaporation temperatures, assumed to be -5ºC and -28ºC for the proposed frost-free heat pump and refrigerator, individually. The evaluation criteria were satisfied and the operation time of frost-free was about 20 minutes. 19

20 Experimental result of polymer DCHE for frost-free refrigerator application The polymer sorbent is capable of adsorbing moisture from the air of - 18ºC and the operation time satisfying the evaluation criteria of the frost-free is about 30 minutes. Ø Whether can the DCHE be regenerated under the condensation temperature level? The above experimental results of AQSOA and polymer DCHEs were obtained after the DCHEs were regenerated by passing 55ºCand 43ºC hot brines, separately. 55ºC: the same temperature levels as the condensation temperature of the heat pump 43ºC: the same temperature levels as the condensation temperature of the refrigerator 20

21 Conclusions The frost-free air-source heat pump and frost-free household refrigerator-freezer were proposed in our study. Theoretical study shows that the two proposed systems have high energy efficiency. Experimental results showed that the frost-free operation of the proposed systems could be realized when the two kinds of desiccants: AQSOA and polymer, were applied. Furthermore, it was verified that AQSOA and polymer could be regenerated under the condensation temperatures of heat pumps and refrigerators. 21

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