a new method for preventing air-source heat pumps and ... ·...
TRANSCRIPT
Central Research Institute of Electric Power Industry
A new method for preventing air-source heat pumps and refrigerators from frosting
12th IEA Heat Pump Conference 2017
15-18 May, 2017 ROTTEROAM
L. Zhang, T. Fujinawa, K. Hashimoto*, M. Saikawa
ContentsuBackgrounduIntroduction of the proposed frost-free heat pump
and simulation resultsuIntroduction of the proposed frost-free refrigerator
and simulation resultsuConfirming experimentally whether frost-free could
be realizeduConclusions
2
Background
3
Background −frosting problem of heat pumpu Frost formation on the outdoor heat exchanger (evaporator) is one of the major
problems for air source heat pump in winter.
Outdoor heat exchanger（evaporator）of heat pump
refrigeranttubefin
frosthumid air
H2O H2O
H2O H2OH2O H2O
H2OH2O H2O
Decrease heat transfer and obstruct air flow
Defrosting operation
Heating operation
Problems:• Reduced energy efficiency • Heating shutdown
4
Background −frosting problem of refrigerator
5
＜Frost layer on the surface of the evaporator of a refrigerated display cabinet＞
Defrosting heater
Frost
As shown in thisphoto, althoughthe fin pitch of theheat exchanger isover 10mm, the airflow is obstructedby the frost layer.
u Frost formation is a common phenomenon observed in refrigerators, includinghousehold refrigerator-freezers, refrigerated display cabinets, refrigeratedwarehouses and so on.
Defrosting operation
Cooling operation
Problems:• Reduced energy efficiency • Cooling shutdown• Overflowing of defrosted
water from tray
How to realize frost-freeuConcept of frost-free
Heat exchanger（evaporator etc.)
refrigerant
tubefin
Frost-freeHumid air
H2O H2OH2O H2OH2O H2O
H2OH2O H2O
desiccant
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Dried air
Regeneration airExhausted air
heating
H2O
H2O
H2O
H2O
H2O
H2OH2O
H2OH2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
uEnergy is needed to regenerate the desiccant
The humid air is dehumidified via desiccant before it enters evaporator . Therefore, frost-free could be achieve.
6
Introduction of the proposed frost- free heat pump and simulation results
7
The proposed frost-free heat pump water heater system
compressor
V1
V2
Hot water
D1 D2D3
DCHE Evaporator
Air pass V1, V2-expansion valveD1, D2, D3- damper
Condenser or gas-cooler
refrigerant
8
• 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 operationprocess of the proposed system can be classified into two modes: adsorption mode(AD mode) and desorption mode(DE mode).
Desiccant-coated heatexchanger (DCHE):desiccant is coated onthe surface of the heatexchanger
uAdsorption mode (AD mode)
Enthalpy
P
DCHEEvaporator
Condenser
compressor
V1
V2
Hot water
D1 D2D3
DCHE Evaporator
Condenser or gas-cooler
throttled
throttled
open openclose
OA- outside airAA- adsorption airEA- exhausted airOA EA
AA
(1) P-h diagram of refrigerant (AD)
frost -free
9
• During AD mode, refrigerant isthrottled by V1 and V2 sequentially.So DCHE works as anotherevaporator. Hot water is produced atthe condenser.
• Dampers D1 and D2 are opened andD3 is closed. Outside air (OA) ispassed through DCHE, in which it isdehumidified. The dry air leaving fromDCHE exits heat to refrigerant at theevaporator, and exhausted asexhausted air (EA).
• Because the dew point of the dry air(AA) is lower than the evaporationtemperature of evaporator, frost-freecan be realized.
10
uDesorption mode(DE mode)
Enthalpy
P
Evaporator
CondenserDCHE
QDCHE=Qeva QCon=Wcom
(2) P-h diagram of refrigerant (DE)
compressor
V1
V2
Hot water
D1 D2D3
DCHE Evaporator
Condenser or gas-cooleropen
throttled
close closeopen
DA- desorption airRA- recycled air
RA
DA
water
• During DE mode, V1 is opened and V2 isthrottled. So DCHE works as anothercondenser and the condensation heat isused to regenerate the desiccant .
• Dampers D1, D2 are closed and D3 isopened. So the air recycles between theDCHE and evaporator. As a result, thecondensation heat of DCHE iscompletely recovered at the evaporator.Therefore, the electrical consumption ofthe compressor is equal to the heatoutput to the water.
• Heating shutdown caused byconventional defrosting methods issolved by applying the air-recyclingmethod of the system.
11
uSimulation results
2.0
3.0
4.0
5.0
-8 -6 -4 -2 0 2 4 6
COP
Toa (ºC)
The proposed system
RHoa=80%
RHoa=70%
The main parameters of the calculation:• Air temperature (Toa) : -7 - 5ºC, relative humidity (RHoa): 60 - 80%; • Inlet and outlet water temperatures: 5, 65ºC;• The adiabatic efficiency of the compressor (ηcom): 0.75 (assumed); Refrigerant: CO2• COP: the ratio of the heat output to water to the electrical consumption of the compressor
Be calculated by assuming that COP will bereduced by 20% [1] compared to a base system[2], which operates at non-frosting conditions.
The COP of the proposedsystem is 5-20 % higherthan that of the hot gasdefrosting 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: 328-334.[2]: Neksa P, et al. 1998, CO2 –heat pump water heater: characteristics, system design and experimental results, Int. J. Refrig. 21 (3): 172-179.
Fig.1 Comparison of COP between the proposed system and a conventional hot gas defrosting system
Introduction of the proposed frost-free refrigerator and simulation results
12
The method of using a desiccant to prevent the heat pump from frosting can also be applied torefrigeration systems, such as household refrigerator-freezers, refrigerated display cabinets,refrigerated warehouses and so on.
The proposed frost-free household refrigerator-freezer
13
Freezer Compartment
Refrigerator Compartment
Air-cooled HE
Compressor
Expansion valve
gas-liquid separator
Air cooling HE
capillary
DCHE (desiccant-coated HE)
• The system comprises a compressor,an air-cooled HE, an expansion valve,a DCHE, a gas-liquid separator, acapillary and an air cooling HE.
• With respect to the available coolingload temperature and the moistureremoving of the desiccant, the systemcan be classified into two modes:u Freezing-Refrigeration-ADsorption
mode (F-R-AD mode) u Refrigeration-DEsorption (R-DE
mode).
14
uFreezing-Refrigeration-ADsorption mode (F-R-AD mode)
• The expansion valve (3) is throttled, so theDCHE works as another evaporator.
• The air (MA) mixed by the freezer air (FA)and refrigerator air (RA) is dehumidified atDCHE due to the adsorption process of thedesiccant.
• The dry air (DA) exiting DCHE is split intotwo air streams: SA(R) is supplied to therefrigerator compartment, the rest (SA(F)) isfurther cooled at the evaporator andsupplied to the freezer compartment.
• Because the dew point of the dry air (DA) islower than the evaporation temperature ofevaporator, frost-free can be realized.
Freezer Compartment
Refrigerator Compartment
OA
EA
RA
FA
MA
SA(R)
SA(F)
DA
DA
3throttled
DCHE(Adsorption)
Evaporator
Condenser
15
Freezer Compartment
Refrigerator Compartment
OA
EA
RA
SA(R)
3
Evaporator
opened
DCHE(Desorption)
uRefrigeration-DEsorption (R-DE mode)
• The expansion valve (3) is opened, so theDCHE (4) works as a condenser
• The DCHE is heated by the condensationheat of the refrigerant and is regenerated bydischarging moisture to the outside air (OA).
• The refrigerator air (RA) is cooled at theevaporator and returned to the refrigeratorcompartment.
• Note that the evaporation temperature of theevaporator should be operated to keep ithigher than the dew point of RA, thereforefrost-free operation is also feasible in R-DEmode.
16
uSimulation results
0
2
4
6
8
overall cooling load electricityconsumption
MJ/d
ay
4.08MJ(1.13kWh)
60%(R. compartment)
40%(F. compartment)
7.42MJ • The total cooling load including F. and R.compartments, the electric powerconsumption of the compressor is 7.42MJand 4.08MJ for one day.
• So the average COP , the ratio of thecooling load to electric power consumption,is calculated to be 1.82.
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 (ηcom) is assumed to be 0.7. • Refrigerant: Isobutane (R600a)
Fig.2 The overall cooling load and electric power consumed by the compressor for one day
Confirming experimentally whether frost-free could be realized
The results of the theoretical analysis show that the two proposed systems can preventevaporators from frosting and have high energy efficiency. In the following, we want to confirmexperimentally whether a frost-free state could be realized and whether the DCHE could beeffectively regenerated under the condensation temperature of heat pumps and refrigerators.
17
18
uExperimental apparatus
Parameters DCHE (AQSOA) DCHE (polymer)Height (m) 0.242 0.2Length (m) 0.3 0.5Width (m) 0.0508 0.016Fin (m) 1.8 × 10-3 1.2 × 10-3
Mass of desiccant(kg) 0.756 0.0878
AQSOA DCHE Polymer DCHE
DCHE
AQSOA: a kind of desiccant belonging to the zeolitefamily, developed by Mitsubishi Chemical Corporation).Polymer: a kind of sorption material, developed by JapanExlan.
(Aluminum fin and copper tube)
(All aluminum, plate tube with corrugated fins)Process air
DCHE
Cooling brine
Hot brine
DPinDPout
Objectives of this experiment:Ø Whether can frost-free be realized?Ø Regeneration temperature of DCHE
19
uExperimental resultsØ Whether can frost-free be realized?
• The evaluation criteria of the frost-free operation of the proposed heat pump and refrigerator are given as,
𝐷𝑃#$% < 𝑇()*
• Experimental result of AQSOA-DCHE for frost-free heat pumpapplication
The evaluation criteria were satisfiedand the operation time of frost-freewas about 20 minutes.
DPout: is the measured dew point of theDCHE outlet airTeva: is the evaporation temperatures,assumed to be -5ºC and -28ºC for theproposed frost-free heat pump andrefrigerator, individually.
20
• Experimental result of polymer DCHE for frost-free refrigerator application
The polymer sorbent is capable ofadsorbing moisture from the air of -18ºC and the operation timesatisfying the evaluation criteria ofthe 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
Conclusions
21
• 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.
