Abstrict
Refrigerant is circulated through a vapor compression system including
a compressor, a gas cooler, an expansion device, and an evaporator.
An auxiliary electric heater is activated to further heat the heated
water exiting the gas cooler when the heating capacity of the system
is low. The auxiliary electric heater can be located on the water
line exiting the gas cooler, in a water tank that stores the heated
water, or on the refrigerant line proximate to the compressor discharge.
Claims
What is claimed is:
1. A vapor compression system comprising: a compression device
to compress a refrigerant to a high pressure; a heat rejecting heat
exchanger for cooling said refrigerant; an expansion device for
reducing said refrigerant to a low pressure; a heat accepting heat
exchanger for evaporating said refrigerant; and an auxiliary heater
that selectively heats at least one of the refrigerant and a fluid
that absorbs heat from said refrigerant flowing through said heat
rejecting heat exchanger.
2. The vapor compression system as recited in claim 1 wherein said
heat rejecting heat exchanger includes a fluid outlet, and wherein
said auxiliary heater directly heats a fluid that exits said heat
rejecting heat exchanger through said fluid outlet.
3. The vapor compression system as recited in claim 2 including
a fluid temperature sensor to detect a temperature of said fluid
that exits said fluid outlet and a control that activates said auxiliary
heater when said fluid temperature sensor detects that said temperature
of said fluid is below a threshold value.
4. The vapor compression system as recited in claim 2 including
an ambient temperature sensor to detect an outdoor air temperature
and a control that activates said auxiliary heater when said ambient
temperature sensor detects that said outdoor air temperature is
below a threshold value.
5. The vapor compression system as recited in claim 2 including
an ambient temperature sensor to detect an outdoor air temperature,
a water pump to pump a fluid through said heat rejecting heat exchanger
and a control, and said control increases a speed of said water
pump to lower an exit temperature of said fluid exiting said heat
rejecting heat exchanger and activates said auxiliary heater to
heat said fluid exiting said heat rejecting heat exchanger when
said ambient temperature sensor detects that said outdoor air temperature
is below a threshold value.
6. The vapor compression system as recited in claim 1 including
a water tank and wherein said heat rejecting heat exchanger includes
a fluid outlet in fluid communication with said water tank, and
said auxiliary heater heats a fluid in said water tank.
7. The vapor compression system as recited in claim 6 further including
a fluid temperature sensor to detect a temperature of said fluid
in said water tank.
8. The vapor compression system as recited in claim 7 further including
a control, and said control activates said auxiliary heater when
said fluid temperature sensor detects that said temperature of said
fluid is below a first threshold value and said control deactivates
said auxiliary heater when said fluid temperature sensor detects
that said temperature of said fluid is above a second threshold
value
9. The vapor compression system as recited in claim 1 wherein said
compression device further includes a compressor discharge, and
wherein said auxiliary heater heats said refrigerant that exits
said compressor through said compressor discharge.
10. The vapor compression system as recited in claim 9 including
an ambient temperature sensor that detects a temperature of outdoor
air.
11. The vapor compression system as recited in claim 10 further
including a control, and said control activates said auxiliary heater
when said ambient temperature sensor detects that said temperature
of said outdoor air is below a threshold value.
12. The vapor compression system as recited in claim 11 including
a defrost sensor that detects a defrosting condition of said heat
accepting heat exchanger, and said control activates said auxiliary
heater when said defrost sensor detects said defrosting condition.
13. The vapor compression system as recited in claim 1 wherein
said auxiliary heater is an electric heater.
14. The vapor compression system as recited in claim 1 wherein
said refrigerant is carbon dioxide.
15. A method of increasing heating capacity of a transcritical
vapor compression system having at least one auxiliary heater comprising
the steps of: compressing a refrigerant to a high pressure; rejecting
heat from said refrigerant into a fluid; expanding said refrigerant
to a low pressure; evaporating said refrigerant; and activating
the auxiliary heater to selectively further heat at least one said
fluid and said refrigerant with said auxiliary heater.
16. The method as recited in claim 15 wherein the step of further
heating said fluid includes directly heating said fluid after the
step of rejecting heat.
17. The method as recited in claim 15 wherein the step of further
heating said fluid includes directly heating said refrigerant after
the step of compressing and before the step of rejecting heat.
18. The method as recited in claim 15 further including the step
of detecting a temperature of said fluid, and the step of activating
said auxiliary heater includes activating said auxiliary heater
when said temperature is below a threshold value.
19. The method as recited in claim 15 further including the step
of detecting a temperature of outdoor air, and the step of activating
said auxiliary heater includes activating said auxiliary heater
when said temperature is below a threshold value.
Description BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to a transcritical
vapor compression system including an auxiliary electric heater
that further heats the water that exchanges heat with the refrigerant
in the gas cooler.
[0002] Chlorine containing refrigerants have been phased out in
most of the world due to their ozone destroying potential. Hydrofluoro
carbons (HFCs) have been used as replacement refrigerants, but these
refrigerants still have high global warming potential.
[0003] "Natural" refrigerants, such as carbon dioxide
and propane, have been proposed as replacement fluids. Carbon dioxide
can be used as a refrigerant in automotive air conditioning systems
and other heating and cooling applications. Carbon dioxide has a
low critical point, which causes most air conditioning systems utilizing
carbon dioxide as a refrigerant to run transcritically, or partially
above the critical point, under most conditions.
[0004] A vapor compression system usually operates under a wide
range of operating conditions. When the outdoor air temperature
varies, the temperature of the refrigerant exiting the evaporator
varies. Therefore, the heating capacity of the vapor compression
system in the summer is generally four to five times greater than
the heating capacity of the vapor compression system in the winter,
and the refrigerant mass flow rate of the vapor compression system
in the summer is generally eight to ten times greater than the refrigerant
mass flow rate of the vapor compression system in the winter. Although
the heating capacity of the system changes as the operating conditions
change, the required heating load of the system does not change
as the operating conditions change.
[0005] A vapor compression system must be able to provide enough
heating capacity to meet the load requirements during the winter
when the outdoor air temperature is the lowest. In the prior art,
the vapor compression system is oversized to provide enough heating
capacity in the winter. However, oversizing the vapor compression
system causes the heating capacity to be higher than necessary for
most of the ambient conditions, significantly increasing cost.
[0006] Hence, there is a need in the art for a vapor compression
system that has a high heating capacity and is cost effective. This
invention includes an auxiliary electric heater that further heats
the water that exchanges heat with the refrigerant in the gas cooler.
SUMMARY OF THE INVENTION
[0007] The present invention provides a vapor compression system
that includes an auxiliary electric heater that further heats the
water that exchanges heat with the refrigerant in the gas cooler.
[0008] Refrigerant circulates through a vapor compression system.
In one example, carbon dioxide is used as the refrigerant. As carbon
dioxide has a low critical point, systems utilizing carbon dioxide
as the refrigerant usually run transcritically. The refrigerant
is compressed in a compressor and then cooled in a gas cooler. The
refrigerant rejects heat to water flowing through the gas cooler,
and the water exits the gas cooler in a heated state. The refrigerant
is then expanded to a low pressure in an expansion device. After
expansion, the refrigerant flows through an evaporator and is heated
by outdoor air. The refrigerant then reenters the compressor, completing
the cycle.
[0009] The system further includes an auxiliary electric heater
that further heats the heated water exiting the gas cooler. The
auxiliary electric heater is activated to further heat the water
exiting the gas cooler when the heating capacity of the vapor compression
system does not meet the demand.
[0010] In one example, the auxiliary electric heater is positioned
on the water line exiting the gas cooler. If the water pump is a
single speed water pump, the auxiliary electric heater is activated
when a temperature sensor on the water line exiting the heat sink
outlet or supply detects the temperature of the water exiting the
heat sink outlet or supply is below a threshold value. Alternately,
the auxiliary electric heater is activated when an ambient temperature
sensor detects the temperature of the outdoor air is below a threshold
value.
[0011] The auxiliary electric heater can also be positioned in
a water tank that stores the heated water or on the refrigerant
line proximate to the compressor discharge.
[0012] The auxiliary electric heater can also be located on the
refrigerant line proximate to the compressor discharge. In this
example, the auxiliary electric heater can also decrease the time
of the defrost cycle. When the surface temperature of the evaporator
is below the dew-point temperature of the moist outdoor air, water
droplets condense onto and freeze on the evaporator fins. A defrost
cycle is initiated to defrost the evaporator. When a defrost sensor
detects a condition that necessitates defrosting, the control turns
on the auxiliary electric heater to heat the refrigerant exiting
the compressor discharge and reduce the time of the defrost cycle.
[0013] These and other features of the present invention will be
best understood from the following specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The various features and advantages of the invention will
become apparent to those skilled in the art from the following detailed
description of the currently preferred embodiment. The drawings
that accompany the detailed description can be briefly described
as follows:
[0015] FIG. 1 schematically illustrates a diagram of a first embodiment
of a vapor compression system employing an auxiliary electric heater;
[0016] FIG. 2 schematically illustrates a diagram of a second embodiment
of a vapor compression system employing an auxiliary electric heater;
and
[0017] FIG. 3 schematically illustrates a diagram of a third embodiment
of a vapor compression system employing an auxiliary electric heater.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0018] FIG. 1 illustrates an example vapor compression system 20
that includes a compressor 22, a heat rejecting heat exchanger (a
gas cooler in transcritical cycles) 24, an expansion device 26,
and a heat accepting heat exchanger (an evaporator) 28. Refrigerant
circulates through the closed circuit system 20.
[0019] The refrigerant exits the compressor 22 at a high pressure
and a high enthalpy. The refrigerant then flows through the gas
cooler 24 at a high pressure. A fluid medium 30, such as water or
air, flows through a heat sink 32 of the gas cooler 24 and exchanges
heat with the refrigerant flowing through the gas cooler 24. In
the gas cooler 24, the refrigerant rejects heat into the fluid medium
30, and the refrigerant exits the gas cooler 24 at a low enthalpy
and a high pressure. A water pump 34 pumps the fluid medium through
the heat sink 32. The cooled fluid medium 30 enters the heat sink
32 at the heat sink inlet or return 36 and flows in a direction
opposite to the direction of the flow of the refrigerant. After
exchanging heat with the refrigerant, the heated water 38 exits
the heat sink 30 at the heat sink outlet or supply 40. The heated
water can be stored in a water tank 64. In one example, the water
tank 64 is sized to meet expected peak demand at all times.
[0020] The refrigerant then passes through the expansion valve
26, which expands and reduces the pressure of the refrigerant. The
expansion device 26 can be an electronic expansion valve (EXV) or
other known type of expansion device.
[0021] After expansion, the refrigerant flows through the passages
80 of the evaporator 28 and exits at a high enthalpy and a low pressure.
In the evaporator 28, the refrigerant absorbs heat from the outdoor
air 44, heating the refrigerant. The outdoor air 44 flows through
a heat sink 46 and exchanges heat with the refrigerant passing through
the evaporator 28 in a known manner. The outdoor air 44 enters the
heat sink 46 through the heat sink inlet or return 48 and flows
in a direction opposite to or cross to the direction of flow of
the refrigerant. After exchanging heat with the refrigerant, the
cooled outdoor air 50 exits the heat sink 46 through the heat sink
outlet or supply 52. The temperature difference between the outdoor
air 44 and the refrigerant in the evaporator 28 drives the thermal
energy transfer from the outdoor air 44 to the refrigerant as the
refrigerant flows through the evaporator 28. A fan 54 moves the
outdoor air 44 across the evaporator 28, maintaining the temperature
difference and evaporating the refrigerant. The refrigerant then
reenters the compressor 22, completing the cycle.
[0022] The system 20 transfers heat from the low temperature energy
reservoir (ambient air) to the high temperature energy sink (heated
hot water). The transfer of energy is also achieved with the aid
of electrical energy input at the compressor 22.
[0023] The system 20 can also include an accumulator 56. The accumulator
56 stores excess refrigerant from the system 20 to control the high
pressure of the system 20, and therefore the coefficient of performance.
[0024] In one example, carbon dioxide is used as the refrigerant.
Although carbon dioxide is described, other refrigerants may be
used. Because carbon dioxide has a low critical point, systems utilizing
carbon dioxide as a refrigerant usually run transcritically.
[0025] The heating capacity of a vapor compression system 20 is
defined as the capacity of the system 20 to heat the water 30 that
flows through the gas cooler 24 and accepts heat from the refrigerant
in the gas cooler 24. A vapor compression system 20 usually operates
under a wide range of operating conditions. For example, the temperature
of the outdoor air 44 can vary between -10.degree. F. in the winter
and 120.degree. F. in the summer, which causes the temperature of
the refrigerant exiting the evaporator 28 to vary between approximately
-20.degree. F. and 90.degree. F. Therefore, the heating capacity
of the vapor compression system 20 in the summer is generally four
to five times greater than the heating capacity of the vapor compression
system 20 in the winter, and the refrigerant mass flow rate of the
vapor compression system 20 in the summer is generally eight to
ten times greater than the refrigerant mass flow rate of the vapor
compression system 20 in the winter. Although the heating capacity
of the vapor compression system 20 changes as operating conditions
change, the heating load of the vapor compression system 20 does
not change as operating conditions change.
[0026] FIG. 1 illustrates a first embodiment of the vapor compression
system 20 including a single speed water pump 34. The vapor compression
system 20 includes an auxiliary electric heater 58 that further
heats the heated water 38 exiting the gas cooler 24 to increase
the heating capacity of the vapor compression system 20. The auxiliary
electric heater 58 can be located anywhere on the water line exiting
the gas cooler 24. By employing an auxiliary electric heater 58,
the vapor compression system 20 can be designed smaller to reduce
manufacturing costs. The auxiliary electric heater 58 is activated
to further heat the water exiting the heat sink outlet or supply
40 when the heating capacity of the vapor compression system 20
does not meet the demand.
[0027] In one example, a temperature sensor 60 detects the temperature
of the water exiting the heat sink outlet or supply 40. When the
temperature sensor 60 detects the temperature of the water 38 exiting
the heat sink outlet or supply 40 is below a threshold value, a
control 62 activates the auxiliary electric heater 58 to further
heat the water 38 exiting the gas cooler 24. When the temperature
sensor 60 detects that the temperature of the water 38 exiting the
heat sink outlet or supply 40 is above the threshold value, the
control 62 deactivates the auxiliary electric heater. In one example,
the threshold value is 140.degree. F. However, it is to be understood
that the threshold value can be any desired temperature, and one
skilled in the art who has the benefit of this description would
know what the threshold temperature would be.
[0028] The auxiliary electric heater 58 is only activated when
the system 20 is in operation and when the temperature sensor 60
detects that the temperature of the water 38 exiting the heat sink
outlet or supply 40 is below the threshold value. That is, when
the compressor 22 is inactive, the auxiliary electric heater 58
is inactive.
[0029] In another example, an ambient temperature sensor 82 determines
the temperature of the outdoor air 44. When the ambient temperature
sensor 82 detects the temperature of the outdoor air 44 is below
a threshold value and the compressor 22 is operating, the control
62 activates the auxiliary electric heater 58 to further heat the
water 38 exiting the gas cooler 24. When the ambient temperature
sensor 82 detects that the temperature of the outdoor air 44 is
above the threshold value, the control 62 deactivates the auxiliary
electric heater.
[0030] The auxiliary electric heater 58 is only activated when
the system 20 is in operation and when the ambient temperature sensor
82 detects the temperature of the outdoor air 44 is below the threshold
value. That is, when the compressor 22 is inactive, the auxiliary
electric heater 58 is inactive.
[0031] The vapor compression system 20 can also include a variable
speed water pump 34. The ambient temperature sensor 82 detects the
temperature of the outdoor air 44. When the ambient temperature
sensor 82 detects the temperature of the outdoor air 44 is below
a first threshold value, the control 62 increases the speed of the
water pump 34 to lower the temperature of the water exiting the
heat sink outlet or supply 40 to a value slightly below the desired
customer temperature. The control 62 activates the auxiliary electric
heater 58 to further heat the water 38 exiting the gas cooler 24
to raise the temperature of the water exiting the heat sink outlet
or supply 40 to the desired customer temperature. When the ambient
temperature sensor 82 detects the outdoor air 44 temperature is
above a second threshold value, the control 62 deactivates the auxiliary
electric heater 58.
[0032] For example, if the customer desired temperature is 140.degree.
F., the control 62 increases the speed of the water pump 24 to lower
the temperature of the water exiting the heat sink outlet or supply
40 to 120.degree. F. The control 62 activates the auxiliary electric
heater 58 to further heat the water 38 exiting the gas cooler 24
to raise the temperature of the water exiting the heat sink outlet
or supply 40 to 140.degree. F.
[0033] Although only one auxiliary electric heater 58 is illustrated
and described, it is to be understood that multiple auxiliary electric
heaters 58 can be employed to further heat the water 38 exiting
the gas cooler 24.
[0034] Alternately, as shown in FIG. 2, an auxiliary electric heater
66 is installed in the water tank 64 that stores the heated water
38. The auxiliary electric heater 66 can further heat the water
38 in the water tank 64 with or without starting and operating the
vapor compression system 20. If the vapor compression system 20
cannot be operated due to a component malfunction, the auxiliary
electric heater 66 can temporarily heat the water 38 in the water
tank 64. The auxiliary electric heater 66 also compensates for any
standby heat losses that may occur through the water tank 64 when
the vapor compression system 20 is not operating, reducing the startup
and shutdown times of the compressor 22.
[0035] A temperature sensor 68 in the water tank 64 detects the
temperature of the water in the water tank 64. When the temperature
sensor 68 detects the temperature of the water in the water tank
64 is below a first threshold value, a control 70 activates the
auxiliary electric heater 66 to heat the water in the water tank
64. When the temperature sensor 68 detects that the temperature
of the water in the water tank 64 is above a second threshold value,
a control 70 deactivates the auxiliary electric heater 66.
[0036] Alternately, as shown in FIG. 3, an auxiliary electric heater
72 is installed near the compressor discharge 76 of the compressor
22. The auxiliary electric heater 72 is only activated when the
system 20 is in operation. When the auxiliary electric heater 72
is activated, the refrigerant exiting the compressor 22 is further
heated, increasing the temperature of the refrigerant entering the
gas cooler 24. The heat generated by the auxiliary electric heater
72 is transferred to the water flowing through the gas cooler 24
via the refrigerant flowing through the gas cooler 24, increasing
the amount of heat transferred to the water flowing through the
gas cooler 24.
[0037] An ambient temperature sensor 82 detects the temperature
of the outdoor air 44. When the ambient temperature sensor 82 detects
the temperature of the outdoor air 44 is below a threshold value,
a control 76 activates the auxiliary electric heater 72 to additionally
heat the water 38 exiting the heat sink 32. When the ambient temperature
sensor 82 detects that the outdoor air 44 temperature is above the
threshold value, the control 76 deactivates the auxiliary electric
heater 72 to stop heating the water 38 exiting the gas cooler 24.
In one example, the threshold temperature is 32.degree. F.
[0038] The auxiliary electric heater 72 is only activated when
the system 20 is in operation and when the ambient temperature sensor
82 detects the temperature of the outdoor air 44 is below the threshold
value. That is, when the compressor 22 is inactive, the auxiliary
electric heater 72 is inactive.
[0039] The auxiliary electric heater 72 can also be activated to
decrease the time of the defrost cycle. When the surface temperature
of the evaporator 28 is below the dew-point temperature of the moist
outdoor air, water droplets condense onto the evaporator fins 42.
When the surface temperature of the evaporator 28 is below freezing,
the water droplets can freeze on the evaporator 28. Frost crystals
grow from the frozen droplets and block the passage of air across
the evaporator fins 42. The blockage increases the pressure drop
through the evaporator 28, reducing the airflow through the evaporator
28, degrading heat pump performance, and reducing heating capacity.
[0040] A defrost cycle is initiated to defrost the evaporator 28
when a defrost sensor 78 detects a condition that necessitates defrosting.
In one example, defrosting is needed when frost accumulates on a
coil of the evaporator 28.
[0041] During a defrost cycle, hot refrigerant flows through the
evaporator 28 to melt the frost crystals on the evaporator 28. The
evaporator 28 can be defrosted by converting the compressor 22 power
input into heat that is transferred to the evaporator 28 by the
refrigerant. The evaporator 28 can also be defrosted by deactivating
the water pump 34 in the gas cooler 24. The hot refrigerant from
the compressor 22 flows through the gas cooler 24 without rejecting
heat to the water 30 flowing through the gas cooler 24. The hot
refrigerant is expanded in the expansion device 26 and flows through
the evaporator 28 to defrost the evaporator 28.
[0042] The coefficient of performance of a defrost cycle is always
less than one due to heat losses. Therefore, the refrigerant mass
flow rate and the compressor 22 power draw are always very low,
increasing defrost cycle times and decrease the heating capacity
of the vapor compression system 20.
[0043] The auxiliary electric heater 72 can be operated to reduce
the defrost cycle time. When the frost sensor 78 detects a condition
that necessitates defrosting, the control 76 turns on the auxiliary
electric heater 72 to further heat the refrigerant exiting the compressor
discharge 76. The heated refrigerant flows through the evaporator
28 during the defrost cycle to melt any frost, decreasing the defrost
cycle time. When the defrost sensor 78 detects that defrosting is
no longer necessary, the control 76 turns off the auxiliary electric
heater 72, allowing the system 20 to return to normal operation.
[0044] The auxiliary electric heaters 58, 66 and 72 are activated
at low ambient conditions when the refrigerant mass flow and compressor
22 power draw are low, such as in the winter. Therefore, the total
electric capacity required by the vapor compression system 20 will
not increase. By increasing the heating capacity of the vapor compression
system 20 at low outdoor air temperatures, the system 20 can be
designed smaller, decreasing the manufacturing cost. That is, one
or more of any of the auxiliary electric heaters 58, 66 and 72 can
be employed without any appreciable cost increase for the overall
system.
[0045] It is to be understood that the vapor compression system
20 can include any combination of the auxiliary electric heater
58 that directly heats the hot water 38 exiting the gas cooler 24,
the auxiliary electric heater 66 that heats the water in the water
tank 64 and the auxiliary electric heater 72 that directly heats
the refrigerant exiting the compressor 22 as described above.
[0046] The foregoing description is only exemplary of the principles
of the invention. Many modifications and variations of the present
invention are possible in light of the above teachings. The preferred
embodiments of this invention have been disclosed, however, so that
one of ordinary skill in the art would recognize that certain modifications
would come within the scope of this invention. It is, therefore,
to be understood that within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described.
For that reason the following claims should be studied to determine
the true scope and content of this invention. |