FIELD OF THE INVENTION
[0001] The subject matter disclosed herein relates to defrosting of refrigeration systems,
and in particular to efficient defrosting of HVAC heat pump systems.
BACKGROUND
[0002] Heat pump systems generally build frost on an outdoor heat exchanger coil when
operating in a heating mode. This frost buildup can gradually degrade the heat exchanger and
system performance in the form of heating capacity and efficiency. If the frost is not removed, it
can continue to build up until the heat exchanger coil becomes completely blocked with ice. At
this point, in some heat pump systems, protective devices typically cause the system to shut
down. If the protective devices are not effective, equipment failure may occur.
[0003] For these reasons, it is common practice in most heat pump systems to incorporate
a way to defrost. For example, most heat pump systems switch to operate in a cooling mode for
short periods of time, thereby reversing the flow of refrigerant in the system with the help of a
reversing valve. Also, during this defrost cycle, the outdoor fan, which blows air over the
outdoor heat exchanger coil, is typically stopped. When the heat pump operates in the cooling
mode without the outdoor fan running, the outdoor heat exchanger coil heats up quickly, to melt
the frost.
[0004] Defrosting in this manner may have penalties. For example, running the heat
pump in cooling mode while a conditioned space needs heating capacity may lead to wasted
energy. As such, an associated water loop may be cooled while defrosting, which may decrease
the performance (e.g., integrated heating capacity) of the heat pump, disrupt the stability of the
water loop, and disturb the oil management in the heat pump which may affect reliability.
[0005] Further, regulations may impose minimum efficiency levels (e.g., Seasonal
Coefficient of Performance) for heat pumps at different conditions in order to be certified (e.g.,
CE marking). Such efficiency levels may be difficult to attain for some systems such as fixed
speed heat pump systems. The efficiency levels may be significantly impacted by degradation of
evaporator performance due to the frost buildup on the outdoor coil and standard defrost modes.
[0006] Shah (U.S. Pub. 2007/0180838) describes a method for automatically adjusting
the defrost interval in a heat pump system. The method utilizes measurement of the duration of
the previous defrost cycle or cycles, and adjusts the time interval before initiating the next
defrost cycle so that any frost buildup can be defrosted without unnecessary defrost cycles.
[0007] Said et al. (U.S. Pat. No. 6,334,321) describes a method and system for defrost
control on reversible heat pumps. A control algorithm controls a coil defrosting cycle on a
reversible heat pump by storing values representing performance of a clean coil without frost
buildup, and monitoring those values as they evolve over time. The values are used to create a
"frost factor" whose values varies between 0%, signifying a clean coil, and 100% signifying a
heavily frosted coil. When the frost factor reaches a predetermined value close to 100%, the
refrigerant cycle of the heat pump is reversed to achieve coil defrosting.
BRIEF DESCRIPTION OF THE INVENTION
[0008] In one aspect, a heat pump system is provided. The heat pump system includes a
refrigerant circuit, at least one compressor, an evaporator, and a controller programmed to
defrost the evaporator in a defrost mode, wherein in the defrost mode the controller is
programmed to monitor the evaporator to detect frost creation thereon, and reduce the speed of
the at least one compressor and/or reduce the number of some, but not all operating compressors
of the at least one compressor, if frost creation is detected on the evaporator.
[0009] In addition to one or more of the features described above, or as an alternative,
further embodiments may include wherein in the defrost mode the controller is further
programmed to subsequently monitor a temperature of the evaporator to determine if the
monitored temperature increases and exceeds a predetermined temperature after the compressor
speed reduction and/or the reduced operating compressor numbers; wherein the controller is
programmed to defrost the evaporator in a second defrost mode, wherein in the second defrost
mode the controller is programmed to monitor the evaporator to detect frost creation thereon,
turn off the at least one compressor when frost is detected on the evaporator, and operate a fan to
force ambient air over the evaporator to defrost the evaporator; a heat transfer loop thermally
coupled to the condenser, wherein the heat transfer loop circulates a heat exchange medium to a
building for thermal conditioning thereof; wherein in the second defrost mode the controller is
programmed to perform the steps of turning off the at least one compressor and operating the fan
only if the ambient air temperature of the ambient air forced by the fan is above 0 °C; wherein
the controller is programmed to defrost the evaporator using the defrost mode and the second
defrost mode without utilizing a reverse cycle of the refrigerant circuit; wherein in the defrost
mode the controller is programmed to maintain the at least one compressor at the reduced speed
and/or reduced operating number if the monitored temperature is determined to increase and
exceed the predetermined temperature; wherein the predetermined temperature is 0 °C; and/or
wherein in the defrost mode the controller is programmed to monitor a temperature of the
evaporator to determine if the monitored temperature increases and exceeds a predetermined
temperature after the compressor speed reduction and/or the reduced operating compressor
numbers, and initiate the second defrost mode if the monitored temperature is determined to be
below the predetermined temperature after a predetermined amount of time.
[0010] In another aspect, a method of defrosting a heat exchanger of a refrigerant circuit
having at least one compressor is provided. The method includes monitoring the heat exchanger
to detect frost creation thereon, and operating, if frost is sensed on the heat exchanger, in a
defrosting mode. The defrosting mode includes reducing the speed of the at least one
compressor and/or reducing the number of some, but not all operating compressors of the at least
one compressor, if frost is sensed on the heat exchanger.
[001 1] In addition to one or more of the features described above, or as an
alternative, further embodiments may include wherein the defrost mode further comprises
subsequently monitoring a temperature of the heat exchanger to determine if the monitored
temperature increases and exceeds a predetermined temperature after the compressor speed
reduction and/or the reduced operating compressor numbers; operating in a second defrost mode
if the monitored temperature is determined to be below the predetermined temperature after a
predetermined amount of time, and if frost is sensed on the heat exchanger, wherein the second
defrost mode includes turning off the at least one compressor, and operating a fan to force
ambient air over the heat exchanger to defrost the heat exchanger; wherein the heat exchanger is
an outdoor evaporator and the fan forces outdoor ambient air; wherein the second defrost mode
further comprises turning off the at least one compressor and operating the fan only if the
ambient air temperature of the ambient air forced by the fan is above the freezing temperature of
water; and/or wherein defrosting the heat exchanger with the defrost mode and the second
defrost mode is performed without reversing the cycle of the refrigerant circuit to defrost the heat
exchanger.
[0012] In yet another aspect, a method of defrosting an evaporator of a heat pump system
having a refrigerant circuit and a plurality of compressors is provided. The method includes
monitoring the evaporator to detect frost creation thereon, and operating, if frost is detected on
the evaporator, in a first defrosting mode. The first defrosting mode includes reducing the speed
of at least one compressor of the plurality of compressors and/or reducing the number of some,
but not all operating compressors of the plurality of compressors, if frost is detected on the
evaporator, providing, while defrosting in the first defrosting mode, heating capacity to the heat
pump system with the reduced speed compressors and/or the remaining operating compressors,
and subsequently monitoring a temperature of the evaporator to determine if, during the
defrosting in the first defrosting mode, the monitored temperature increases and exceeds a
predetermined temperature after the compressor speed reduction and/or the reduced operating
compressor numbers. The method includes subsequently operating in a second defrosting mode,
if frost is detected on the evaporator and if the monitored temperature is determined to be below
the predetermined temperature after a predetermined amount of time. The second defrosting
mode includes turning off each compressor of the plurality of compressors, and operating, only
when the outdoor ambient air is above the freezing temperature of water, a fan to force outdoor
ambient air over the evaporator to defrost the evaporator, wherein the evaporator is defrosted
using the first and second defrost modes and without reversing the cycle of the refrigerant circuit
to defrost the heat exchanger.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The subject matter which is regarded as the invention is particularly pointed out
and distinctly claimed in the claims at the conclusion of the specification. The foregoing and
other features, and advantages of the invention are apparent from the following detailed
description taken in conjunction with the accompanying drawings in which:
[0014] FIG. 1 is a schematic illustration of an exemplary heat pump system;
[0015] FIG. 2 illustrates a graph of an exemplary power consumption of the heat pump
system cycling between a normal operating mode and a free defrost mode compared to a
standard defrost mode;
[0016] FIG. 3 illustrates a graph of an exemplary heating capacity of the heat pump
system cycling between the normal operating mode and the free defrost mode compared to the
standard defrost mode;
[0017] FIG. 4 illustrates a graph of an exemplary power consumption of the heat pump
system cycling between the normal operating mode and a positive defrost mode compared to a
standard defrost mode; and
[0018] FIG. 5 illustrates a graph of an exemplary heating capacity of the heat pump
system cycling between the normal operating mode and the positive defrost mode compared to a
standard defrost mode.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Described herein are systems and methods for defrosting a heat pump system.
The heat pump system may be defrosted in a "free defrost" mode, a "positive defrost" mode, or a
combination of the free defrost mode and the positive defrost mode, without operating the heat
pump system in a reverse cycle.
[0020] FIG. 1 illustrates an exemplary heat pump system 10 generally having a
refrigerant circuit 12 for conditioning a fluid circulated in a heat transfer circuit or loop 14. In
some embodiments, heat pump system 10 is an air-to-air or an air-to-water heat pump system.
[0021] Refrigerant circuit 12 generally includes one or more compressors 20, a condenser
22, expansion devices 24, 26, and one or more evaporator 28. Condenser 22 is arranged to
receive high pressure refrigerant in a vapor state from compressor 20 via a discharge line 30.
The refrigerant in condenser 22 is cooled using cooling water, air, or the like, in heat transfer
loop 14, which carries away the heat of condensation. The refrigerant is condensed in condenser
22 and is then supplied to expansion device 24.
[0022] Expansion device 24 (e.g., an expansion valve) is mounted within a conduit line
32 and serves to throttle the liquid refrigerant down to a lower pressure and to regulate the flow
of refrigerant through the system. Due to the expansion process, the temperature and pressure of
the refrigerant is reduced prior to entering evaporator 28.
[0023] In evaporator 28, the refrigerant is brought into heat transfer relationship with a
heat transfer medium such as circulated outdoor ambient air. The refrigerant at the lower
pressure absorbs heat from the heat transfer medium and the refrigerant is subsequently
vaporized. The refrigerant vapor is then drawn from evaporator 28 via compressor inlet line 34
and compressed to begin the cycle over again.
[0024] In the exemplary embodiment, heat pump system 10 includes reversing valves 36
and 38 configured to selectively switch refrigerant circuit 12 between a heating mode and a
cooling mode. As illustrated, reversing valve 36 is a four-way valve and reversing valve 38 is a
three-way valve. System 10 may include one or more controllers 100 programmed to selectively
operate refrigerant circuit 12 reversibly between the cooling mode and the heating mode. As
used herein, the term controller refers to an application specific integrated circuit (ASIC), an
electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or
more software or firmware programs, a combinational logic circuit, and/or other suitable
components that provide the described functionality. However, system 10 may have various
other valving configurations that enables system 10 to function as described herein.
Alternatively, heat pump system 10 may not include reversible valves 36, 38, or a reversing
conduit 46 with expansion device 26.
[0025] Heat transfer loop 14 exchanges thermal energy between condenser 22 and a
serviced space 40 (e.g., a building). Heat transfer loop 14 includes a supply line 42, a return line
44, and a supply fan or pump (not shown) that supplies air/water warmed by condenser 22 to
serviced space 40 where a fan draws air over a coil to warm a space as known in the art. Cooled
return air/water is transferred via return line 44 where it may be directed back to condenser 22.
In typical space heating applications, the heat pump system is dimensioned to provide a building
with sufficient heating capacity in some "design condition," which represents a severe but not
uncommon outdoor air temperature condition.
[0026] During operation of heat pump system 10, frost may accumulate on coils of
evaporator 28. Standard defrost methods include reversing the refrigerant cycle by actuating
reversing valves 36, 38. However, such standard defrost methods may extract thermal energy
from heat transfer loop 14, thereby decreasing the integrated performance of heat pump system
10. In contrast to the standard defrost method, exemplary heat pump system 10 utilizes a "free
defrost" method and/or a "positive defrost" method to defrost evaporator 28.
[0027] Both the "free defrost" and the "positive defrost" methods do not reverse the
refrigerant cycle, and extract the thermal energy necessary to defrost evaporator 28 from the
outdoor air instead of heat transfer loop 14. However, in some embodiments, heat pump system
10 may also utilize a reverse cycle in addition to the "free defrost" and "positive defrost"
methods if frost buildup on evaporator 28 is excessive.
[0028] The "free defrost" method considers the expected cycling (i.e., switching
compressors off) to match a heat demand of space 40, and utilizes outdoor ambient air for
defrosting. As such, system 10 reduces or prevents frost accumulation without having to reverse
the refrigerant cycle. In the free defrost method, evaporator 28 is defrosted when a
predetermined level or amount (e.g., a small layer) of frost accumulation is detected by controller
100, by utilizing thermal energy of outdoor air that is above the freezing point. This is in
contrast to some prior art systems that wait until a significant, thick frost layer is formed. By
activating an outdoor heat exchanger fan(s) 48 and turning off compressor(s) 20, cooling of the
heat transfer loop 14 during the defrost cycle can be reduced or prevented.
[0029] In operation, heat pump system 10 is monitored for frost creation. For example,
one or more sensors 50 may be operatively associated with evaporator 28 to detect the creation
of frost on the coils or other components of evaporator 28. Sensor 50 may be a temperature
sensor that senses the refrigerant temperature and/or the ambient air temperature. However,
system 10 may use any suitable method to detect frost creation on evaporator 28 such as sensing
the refrigerant pressure inside the evaporator, sensing an increase in the differential air-side
pressure drop across the evaporator coil, etc.
[0030] When the ambient air temperature is above the freezing point of water (i.e., > 0
°C at sea-level) and a predetermined level frost is detected on evaporator 28, controller 100
powers off compressor(s) 20 and activates outdoor heat exchanger fan(s) 48 to force ambient air
over evaporator 28. Because the ambient air temperature is above freezing, the air flow will melt
the frost formed on evaporator 28. In the exemplary embodiment, system 10 detects the
beginning of frost creation (i.e., before fully formed frost) so that system 10 is only required to
operate in the free defrost mode for short periods of time to eliminate the small layers of frost.
[0031] Once a predetermined frost reduction condition is met, controller 100 turns
compressor(s) 20 back on and system 10 is operated normally. In the exemplary embodiment,
compressor(s) 20 are turned on and the defrost cycle is terminated when a predetermined
temperature of the refrigerant is reached at an appropriate point in the heat exchanger coil. For
example, sensor 50 may include a coil temperature sensor to detect increased coil temperature
and signal controller 100 to terminate the defrost cycle. Alternately, a pressure sensor or
pressure switch can be used, or the defrost cycles may be run for a fixed duration of time.
However, the free defrost cycle may be terminated when other conditions occur, such as when
the differential air-side pressure drop across the evaporator coil returns below a predetermined
level.
[0032] Accordingly, because compressor(s) 20 are turned off, power consumption of
system 10 is reduced. Further, because system 10 is not operated in a reverse cycle, condenser
22 is not utilized as an evaporator, which would result in unwanted cooling of the fluid circulated
within heat transfer loop 14.
[0033] FIG. 2 illustrates a graph of an exemplary power consumption of heat pump
system 10 cycling between a normal operating mode and the free defrost mode (line 104)
compared to cycling between the normal operating mode and a standard defrost mode (line 102)
where refrigerant circuit 12 is operated in a reverse cycle.
[0034] FIG. 3 illustrates a graph of an exemplary heating capacity of the heat pump
system 10 cycling between the normal operating mode and the free defrost mode (line 106)
compared to cycling between the normal operating mode and the standard defrost mode (line
[0035] The "positive defrost" method reduces or prevents frosting by reducing the
capacity of heat pump system 10 in consideration of the expected reduced heat load requirements
of space 40, and utilizes outdoor ambient air for defrosting. However, although capacity is
reduced, the method still provides some degree of capacity. As such, system 10 reduces the
speed of compressor(s) 20 and/or shuts off some compressors 20, while still providing adequate
heating capacity for heat transfer loop 14.
[0036] In operation, heat pump system 10 is monitored for frost creation. For example,
sensor 50 may be operatively associated with evaporator 28 to detect the creation of frost on the
coils or other components of evaporator 28. Sensor 50 may be a temperature sensor that senses
the refrigerant temperature and/or the ambient air temperature. However, system 10 may use any
suitable method to detect frost creation on evaporator 28, as described herein.
[0037] When the ambient air temperature is above the freezing point of water (i.e., > 0
°C at sea-level) and a predetermined small level of frost is detected on evaporator 28, controller
100 reduces the speed of variable speed compressors 20 and/or reduces the number of operating
compressors 20 (in a multi-compressor system). The coil temperature of evaporator 28 is then
monitored to determine if the refrigerant temperature increases and exceeds 0 °C (or another
predetermined value) after reducing compressor speed and/or the number of operating
compressors.
[0038] If the temperature exceeds, for example, 0 °C, controller 100 maintains the
compressor conditions and the coil temperature is monitored to determine when the refrigerant
temperature is stabilized above 0 °C. In this operation, the resulting warmer evaporator coil may
be enough to melt the small frost layer present while still providing some heating capacity to heat
transfer loop 14. In the exemplary embodiment, compressor(s) 20 are returned to normal
operation (i.e., running at normal speed and/or all compressors turned on) and the defrost cycle is
terminated when a predetermined temperature of the refrigerant is reached at an appropriate
point in the heat exchanger coil. For example, sensor 50 may include a coil temperature sensor
to detect increased coil temperature and signal controller 100 to terminate the defrost cycle.
Alternately, a pressure sensor or pressure switch can be used, or the defrost cycles may be run
for a fixed duration of time. However, the free defrost cycle may be terminated when other
conditions occur, such as when the differential air-side pressure drop across the evaporator coil
returns below a predetermined level.
[0039] If, for a predetermined time, the refrigerant temperature remains below or equal to
0 °C or is decreasing, system 10 may be switched to free defrost mode, and compressors 20 are
turned off and fan 48 is operated to heat the evaporator coil with outdoor ambient air (if above
the freezing point of water).
[0040] FIG. 4 illustrates a graph of an exemplary power consumption of heat pump
system 10 cycling between a normal operating mode and the positive defrost mode (line 110)
compared to cycling between a normal operating mode and a standard defrost mode (line 112)
where refrigerant circuit 12 is operated in a reverse cycle.
[0041] FIG. 5 illustrates a graph of an exemplary heating capacity of the heat pump
system 10 cycling between the normal operating mode and the positive defrost mode (line 114)
compared to cycling between the normal operating mode and the standard defrost mode (line
116).
[0042] System 10 may use various configurations of compressors 20. For example, a
first configuration includes a fixed speed single compressor, a second configuration includes a
variable speed single compressor, a third configuration includes multiple fixed speed
compressors, and a fourth configuration includes fixed and variable speed compressors. System
10 may be operated in the free defrost mode for all four configurations, and system 10 may be
operated in the positive defrost mode for the second, third, and fourth configurations.
[0043] Described herein are systems and methods for defrosting a heat pump system.
The heat pump system may be defrosted in a free defrost mode, a positive defrost mode, or a free
and positive defrost mode, without operating the heat pump system in a reverse cycle. The free
defrost mode includes powering off refrigerant cycle compressors and operating fans to force
ambient air over a frosted evaporator for defrosting. The positive defrost mode includes
reducing the speed of the compressors and/or powering off some of the total of compressors to
raise the refrigerant temperature for evaporator defrosting. The free and positive defrost mode
includes operating the heat pump system in both free defrost mode and positive defrost mode
simultaneously or separately in any order.
[0044] As such, the Coefficient of Performance of the heat pump system may be
significantly increased, with little or no impact on integrated heating capacity, and with little or
no additional hardware costs. In some cases, the integrated heating capacity of the heat pump
system can be enhanced at full load, which improves the cost per delivered heating capacity.
The system increases the Seasonal Coefficient of Performance (e.g., by 15%). In addition to
energy efficiency increase, the described defrost methods may maintain the stability of the
building's air or water loop, increase the reliability of the unit, and reduce laboratory test time.
[0045] While the invention has been described in detail in connection with only a limited
number of embodiments, it should be readily understood that the invention is not limited to such
disclosed embodiments. Rather, the invention can be modified to incorporate any number of
variations, alterations, substitutions or equivalent arrangements not heretofore described, but
which are commensurate with the spirit and scope of the invention. Additionally, while various
embodiments of the invention have been described, it is to be understood that aspects of the
invention may include only some of the described embodiments. Accordingly, the invention is
not to be seen as limited by the foregoing description, but is only limited by the scope of the
appended claims.

CLAIMS:
1. A heat pump system comprising:
a refrigerant circuit;
at least one compressor;
an evaporator; and
a controller programmed to defrost the evaporator in a defrost mode, wherein in
the defrost mode the controller is programmed to:
monitor the evaporator to detect frost creation thereon; and
reduce the speed of the at least one compressor and/or reduce the number
of some, but not all operating compressors of the at least one compressor, if frost
creation is detected on the evaporator.
2. The heat pump system of claim 1, wherein in the defrost mode the
controller is further programmed to subsequently monitor a temperature of the evaporator to
determine if the monitored temperature increases and exceeds a predetermined temperature after
the compressor speed reduction and/or the reduced operating compressor numbers.
3. The heat pump system of claim 1, wherein the controller is programmed to
defrost the evaporator in a second defrost mode, wherein in the second defrost mode the
controller is programmed to:
monitor the evaporator to detect frost creation thereon;
turn off the at least one compressor when frost is detected on the
evaporator; and
operate a fan to force ambient air over the evaporator to defrost the
evaporator.
4. The heat pump system of claim 1, further comprising a heat transfer loop
thermally coupled to a condenser of the refrigerant circuit, wherein the heat transfer loop
circulates a heat exchange medium to a building for thermal conditioning thereof.
5. The heat pump system of claim 3, wherein in the second defrost mode the
controller is programmed to perform the steps of turning off the at least one compressor and
operating the fan only if the ambient air temperature of the ambient air forced by the fan is above
0 °C.
6. The heat pump system of claim 3, wherein the controller is programmed to
defrost the evaporator using the defrost mode and the second defrost mode without utilizing a
reverse cycle of the refrigerant circuit.
7. The heat pump system of claim 1, wherein in the defrost mode the
controller is programmed to maintain the at least one compressor at the reduced speed and/or
reduced operating number if the monitored temperature is determined to increase and exceed the
predetermined temperature.
8. The heat pump system of claim 7, wherein the predetermined temperature
is 0 °C.
9. The heat pump system of claim 3, wherein in the defrost mode the
controller is programmed to:
monitor a temperature of the evaporator to determine if the monitored temperature
increases and exceeds a predetermined temperature after the compressor speed reduction and/or
the reduced operating compressor numbers; and
initiate the second defrost mode if the monitored temperature is determined to be
below the predetermined temperature after a predetermined amount of time.
10. The heat pump system of claim 1, wherein in the defrost mode the
controller is programmed to return the at least one compressor to a normal operation by
increasing the speed of the at least one compressor and/or turning on all compressors of the at
least one compressor when the detected frost is melted by operating in the defrost mode.
11. The heat pump system of claim 3, wherein in the second defrost mode the
controller is programmed to return the at least one compressor to a normal operation by turning
on the at least one compressor when the detected frost is melted by operating in the second
defrost mode.
12. A method of defrosting a heat exchanger of a refrigerant circuit having at
least one compressor, the method comprising:
monitoring the heat exchanger to detect frost creation thereon;
operating, if frost is sensed on the heat exchanger, in a defrosting mode, wherein
the defrosting mode comprises:
reducing the speed of the at least one compressor and/or reducing the number of
some, but not all operating compressors of the at least one compressor, if frost is sensed on the
heat exchanger.
13. The method of claim 12, wherein the defrost mode further comprises
subsequently monitoring a temperature of the heat exchanger to determine if the monitored
temperature increases and exceeds a predetermined temperature after the compressor speed
reduction and/or the reduced operating compressor numbers.
14. The method of claim 13, further comprising operating in a second defrost
mode if the monitored temperature is determined to be below the predetermined temperature after
a predetermined amount of time, and if frost is sensed on the heat exchanger, wherein the second
defrost mode includes:
turning off the at least one compressor; and
operating a fan to force ambient air over the heat exchanger to defrost the heat
exchanger.
15. The method of claim 14, wherein the heat exchanger is an outdoor
evaporator and the fan forces outdoor ambient air.
16. The method of claim 14, wherein the second defrost mode further
comprises turning off the at least one compressor and operating the fan only if the ambient air
temperature of the ambient air forced by the fan is above the freezing temperature of water.
17. The method of claim 14, wherein defrosting the heat exchanger with the
defrost mode and the second defrost mode is performed without reversing the cycle of the
refrigerant circuit to defrost the heat exchanger.
18. A method of defrosting an evaporator of a heat pump system having a
refrigerant circuit and a plurality of compressors, the method comprising:
monitoring the evaporator to detect frost creation thereon;
operating, if frost is detected on the evaporator, in a first defrosting mode,
wherein the first defrosting mode comprises:
reducing the speed of at least one compressor of the plurality of
compressors and/or reducing the number of some, but not all operating compressors of
the plurality of compressors, if frost is detected on the evaporator;
providing, while defrosting in the first defrosting mode, heating capacity
to the heat pump system with the reduced speed compressors and/or the remaining
operating compressors; and
subsequently monitoring a temperature of the evaporator to determine if,
during the defrosting in the first defrosting mode, the monitored temperature increases
and exceeds a predetermined temperature after the compressor speed reduction and/or the
reduced operating compressor numbers;
subsequently operating in a second defrosting mode, if frost is detected on the
evaporator and if the monitored temperature is determined to be below the predetermined
temperature after a predetermined amount of time, wherein the second defrosting mode
comprises:
turning off each compressor of the plurality of compressors; and
operating, only when the outdoor ambient air is above the freezing temperature
of water, a fan to force outdoor ambient air over the evaporator to defrost the evaporator,
wherein the evaporator is defrosted using the first and second defrost modes and without
reversing the cycle of the refrigerant circuit to defrost the heat exchanger.