U.S. patent application number 09/837107 was filed with the patent office on 2001-12-06 for heat-pump water heater.
Invention is credited to Kobayakawa, Tomoaki, Kuroki, Jyouji, Kusakari, Kazutoshi, Noro, Shinya, Saikawa, Michiyuki, Sakakibara, Hisayoshi.
Application Number | 20010048031 09/837107 |
Document ID | / |
Family ID | 26590413 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010048031 |
Kind Code |
A1 |
Noro, Shinya ; et
al. |
December 6, 2001 |
Heat-pump water heater
Abstract
In a heat-pump water heater, an ECU sets a target temperature
difference between water flowing into a water heat exchanger and
refrigerant flowing out from the water heat exchanger, and controls
a valve opening degree of an expansion valve so that the target
temperature difference is obtained. When a refrigerant temperature
discharged from a compressor is higher than a predetermined value,
the target temperature difference is increased until the
refrigerant temperature discharged from the compressor becomes
lower than the predetermined value. Further, when water-heating
capacity is decreased due to the increase of the target temperature
difference, a rotation speed of the compressor is increased.
Inventors: |
Noro, Shinya; (Kariya-city,
JP) ; Sakakibara, Hisayoshi; (Nishio-city, JP)
; Kuroki, Jyouji; (Kariya-city, JP) ; Kobayakawa,
Tomoaki; (Tokyo, JP) ; Kusakari, Kazutoshi;
(Urawa-city, JP) ; Saikawa, Michiyuki;
(Zushi-city, JP) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, PLC
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
26590413 |
Appl. No.: |
09/837107 |
Filed: |
April 18, 2001 |
Current U.S.
Class: |
237/2B ;
237/19 |
Current CPC
Class: |
F25B 49/02 20130101;
F25B 2700/21152 20130101; Y02B 30/70 20130101; F25B 2700/21161
20130101; F25B 30/02 20130101; F25B 41/34 20210101; F25B 2600/021
20130101; F25B 2339/047 20130101; F25B 9/008 20130101; F25B
2309/061 20130101; F25B 2700/2116 20130101; F25B 2341/063 20130101;
F24H 4/04 20130101; F25B 2600/17 20130101 |
Class at
Publication: |
237/2.00B ;
237/19 |
International
Class: |
F24H 001/22; F24D
003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 19, 2000 |
JP |
2000-118394 |
Oct 11, 2000 |
JP |
2000-311142 |
Claims
What is claimed is:
1. A heat pump fluid heater for heating a fluid using a heat pump
cycle as a heating source, comprising: a compressor which
compresses sucked refrigerant to have a pressure equal to or higher
than a critical pressure of refrigerant, and discharges compressed
refrigerant; a heat exchanger which is disposed to perform a heat
exchange between refrigerant from the compressor and the fluid, in
such a manner that a flow direction of refrigerant is opposite to a
flow direction of the fluid in the heat exchanger; and a control
unit for controlling operation of the heat pump cycle, wherein: the
control unit controls a high-pressure side refrigerant pressure
from the compressor and before being decompressed in the heat pump
cycle, so that a temperature difference between the fluid flowing
into the heat exchanger and refrigerant discharged from the heat
exchanger becomes a target temperature difference; the control unit
has a detection member for detecting one of a refrigerant
temperature and a physical amount relative to the refrigerant
temperature discharged from the compressor; and the control unit
changes the target temperature difference to be increased when a
detection value of the detection member is more than a
predetermined value.
2. The heat-pump fluid heater according to claim 1, wherein the
control unit increases the target temperature difference until the
detection value of the detection member is less than the
predetermined value.
3. The heat-pump fluid heater according to claim 1, wherein:
wherein the control unit sets the target temperature difference
larger as a low-pressure side refrigerant pressure after being
decompressed in the heat pump cycle becomes lower, when the
low-pressure side refrigerant pressure in the heat pump cycle is
lower than a predetermined pressure.
4. The heat-pump fluid heater according to claim 1, wherein: the
detection member detects at least one of a refrigerant pressure and
a refrigerant temperature sucked into the compressor; and a
detection value of the detection member is used as the physical
amount relative to the high-pressure side refrigerant pressure.
5. The heat-pump fluid heater according to claim 1, wherein: the
detection member detects a refrigerant pressure discharged from the
compressor; and the refrigerant pressure detected by the detection
member is used as the physical amount.
6. The heat-pump fluid heater according to claim 1, wherein: the
control unit determines whether or not a load of the compressor is
excessive; and the control unit changes the target temperature
difference to be increased to a value, when it is determined that
the load of the compressor is excessive.
7. The heat-pump fluid heater according to claim 6, wherein: the
control unit detects an electrical current applied to the
compressor, and determines that the load of the compressor is
excessive when the electrical current applied to the compressor is
larger than a predetermined value.
8. The heat-pump fluid heater according to claim 6, further
comprising an inverter circuit for driving the compressor, the
inverter circuit having a protection circuit which restricts an
output current for protecting the inverter circuit, wherein the
control unit detects an output restriction due to the protection
circuit, and determines that the load of the compressor is
excessive when the output restriction due to the protection circuit
is detected.
9. The heat-pump fluid heater according to claim 6, wherein the
control unit determines whether the load of the compressor is
excessive based on at least one condition of a target fluid
temperature to be heated, an outside air temperature and a rotation
speed of the compressor.
10. The heat-pump fluid heater according to claim 6, wherein when
the control unit determines that the load of the compressor is
excessive, the control unit changes any one of a target fluid
temperature to be heated and the target temperature difference, in
accordance with the load of the compressor.
11. The heat-pump fluid heater according to claim 1, further
comprising an expansion valve for decompressing refrigerant, which
is disposed to electrically adjust a valve opening degree, wherein
the control unit adjusts the valve opening degree of the expansion
valve based on the target temperature difference to control the
high-pressure side refrigerant pressure.
12. The heat-pump fluid heater according to claim 11, wherein the
control unit increases a rotation speed of the compressor to obtain
a target heating capacity of the fluid, when the target temperature
difference is changed to be increased.
13. The heat-pump fluid heater according to claim 1, wherein the
fluid to be heated is water in a hot water supply system.
14. A heat pump fluid heater for heating a fluid using a heat pump
cycle as a heating source, comprising: a compressor which
compresses sucked refrigerant to have a pressure equal to or higher
than a critical pressure of refrigerant, and discharges compressed
refrigerant; a heat exchanger which is disposed to perform a heat
exchange between refrigerant from the compressor and the fluid, in
such a manner that a flow direction of refrigerant is opposite to a
flow direction of the fluid in the heat exchanger; and a control
unit for controlling operation of the heat pump cycle, wherein: the
control unit controls a high-pressure side refrigerant pressure
from the compressor and before being decompressed in the heat pump
cycle, so that a temperature difference between the fluid flowing
into the heat exchanger and refrigerant discharged from the heat
exchanger becomes a target temperature difference; the control unit
determines whether or not a load of the compressor is excessive;
and the control unit changes the target temperature difference to
be increased to a value, when it is determined that the load of the
compressor is excessive.
15. The heat-pump fluid heater according to claim 14, wherein: the
control unit detects an electrical current applied to the
compressor, and determines that the load of the compressor is
excessive when the electrical current applied to the compressor is
larger than a predetermined value.
16. The heat-pump fluid heater according to claim 14, further
comprising an inverter circuit for driving the compressor, the
inverter circuit having a protection circuit which restricts an
output current for protecting the inverter circuit, wherein the
control unit detects an output restriction due to the protection
circuit, and determines that the load of the compressor is
excessive when the output restriction due to the protection circuit
is determined.
17. The heat-pump fluid heater according to claim 14, wherein the
control unit determines whether the load of the compressor is
excessive based on at least one condition of a target fluid
temperature to be heated, an outside air temperature and a rotation
speed of the compressor.
18. The heat-pump fluid heater according to claim 14, wherein when
the control unit determines that the load of the compressor is
excessive, the control unit changes stepwise any one of a target
fluid temperature to be heated and the target temperature
difference in accordance with the load of the compressor.
19. The heat-pump fluid heater according to claim 14, wherein when
the control unit determines that the load of the compressor is
excessive, the control unit changes continuously any one of a
target fluid temperature to be heated and the target temperature
difference in accordance with the load of the compressor.
20. A heat pump fluid heater for heating a fluid using a heat pump
cycle as a heating source, comprising: a compressor which
compresses sucked refrigerant to have a pressure equal to or higher
than a critical pressure of refrigerant, and discharges compressed
refrigerant; a heat exchanger which is disposed to perform a heat
exchange between refrigerant from the compressor and the fluid, in
such a manner that a flow direction of refrigerant is opposite to a
flow direction of the fluid in the heat exchanger; and a control
unit for controlling operation of the heat pump cycle, wherein: the
control unit has a temperature detection sensor for detecting a
refrigerant temperature discharged from the compressor; the control
unit controls a high-pressure side refrigerant pressure from the
compressor and before being decompressed in the heat pump cycle, so
that a temperature difference between the fluid flowing into the
heat exchanger and refrigerant discharged from the heat exchanger
becomes a target temperature difference when the refrigerant
temperature detected by the temperature sensor is lower than a
predetermined temperature; and when the refrigerant temperature
detected by the temperature sensor is higher than the predetermined
temperature, the control unit controls the high-pressure side
refrigerant pressure of the heat pump cycle so that the refrigerant
temperature detected by the temperature sensor becomes lower than
the predetermined temperature.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is related to Japanese Patent Applications
No. 2000-118394 filed on Apr. 19, 2000, and No. 2000-311142 filed
on Oct. 11, 2000, the contents of which are hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a heat-pump water heater
that heats water using a super-critical (transcritical) heat pump
cycle as a heating source.
[0004] 2. Description of Related Art
[0005] In a conventional heat-pump water heater, low-temperature
water is heat-exchanged with high-temperature refrigerant in a
water heat exchanger, and high-temperature water heated in the
water heat exchanger is stored in a water tank to be supplied to a
user after being temperature-adjusted. In the heat-pump water
heater, a target temperature difference .DELTA.T between water
flowing into the water heat exchanger and refrigerant discharged
from the water heat exchanger is set, and high-pressure side
refrigerant pressure of the heat pump cycle is controlled based on
the target temperature difference .DELTA.T for increasing a cycle
efficiency of the heat pump cycle. Generally, the high-pressure
side refrigerant pressure is controlled by adjusting a valve
opening degree of the expansion valve.
[0006] However, when the high-pressure side refrigerant pressure is
controlled based on the target temperature difference .DELTA.T when
the heat-pump water heater is used under a low temperature, a
low-pressure side refrigerant pressure (e.g., evaporation pressure)
of the heat pump cycle is decreased, and temperature of refrigerant
discharged from a compressor may exceed a normal operation
temperature area of the compressor.
[0007] On the other hand, when the high-pressure side refrigerant
pressure of the heat pump cycle is increased due to an outside air
increase, a water temperature increase, a rotation speed increase
of the compressor or a deterioration of operation performance of
the water heat exchanger, load of the compressor increases, and a
normal operation of the heat pump cycle may be affected. In this
case, when the rotation speed of the compressor is decreased for
preventing the overload of the compressor, it is difficult to
obtain a necessary heating capacity in the water heater only by
controlling the valve opening degree of the expansion valve.
SUMMARY OF THE INVENTION
[0008] In view of the foregoing problems, it is an object of the
present invention to provide a heat-pump fluid heater for heating a
fluid (e.g., water) using a heat pump cycle as a heating source, in
which a refrigerant temperature discharged from a compressor can be
controlled in an operation temperature area even when the heat pump
cycle is used under a low temperature.
[0009] It is an another object of the present invention to provide
a heat-pump fluid heater which prevents a problem of a heat pump
cycle due to a load increase of a compressor, while obtaining a
desired water-heating capacity in a water supply system.
[0010] According to the present invention, in a heat pump fluid
heater for heating a fluid (e.g., water) using a heat pump cycle as
a heating source, a control unit for controlling operation of the
heat pump cycle controls a high-pressure side refrigerant pressure
from the compressor and before being decompressed in the heat pump
cycle, so that a temperature difference between the fluid flowing
into a heat exchanger and refrigerant discharged from the heat
exchanger becomes a set target temperature difference. Further, the
control unit has a detection member for detecting one of a
refrigerant temperature and a physical amount relative to the
refrigerant temperature discharged from the compressor, and the
control unit changes the target temperature difference to be
increased when a detection value of the detection member is more
than a predetermined value. When the target temperature difference
is changed and becomes larger, a heat-exchanging efficiency of the
heat exchanger is decreased, and a heat-exchanging amount in the
heat exchanger is reduced. That is, in this case, because the
refrigerant pressure discharged from the compressor is controlled
to be decreased, the refrigerant temperature discharged from the
compressor is decreased. Accordingly, even when the heat pump cycle
is used under a low temperature condition, the refrigerant
temperature discharged from the compressor can be controlled in an
operation temperature area.
[0011] Preferably, the control unit sets the target temperature
difference larger as a low-pressure side refrigerant pressure after
being decompressed in the heat pump cycle becomes lower, when the
low-pressure side refrigerant pressure in the heat pump cycle is
lower than a predetermined pressure. When the low-pressure side
refrigerant pressure (e.g., evaporation pressure) of the heat pump
cycle is decreased due to a decrease of outside air temperature,
for example, load of the compressor is increased and refrigerant
temperature discharged from the compressor is increased.
Accordingly, by setting the target temperature difference larger as
the low-pressure side refrigerant pressure becomes lower, it can
effectively restrict the refrigerant temperature discharged from
the compressor from being increased.
[0012] On the other hand, the control unit determines whether or
not a load of the compressor is excessive, and the control unit
changes the target temperature difference to be increased to a
value when it is determined that the load of the compressor is
excessive. In this case, when the target temperature difference is
made larger, the compressor continuously operates with a relatively
lower high-pressure. Accordingly, is can prevent a problem of a
heat pump cycle due to an increased load of the compressor.
Further, when the fluid is water in a hot water supply system, a
desired water-heating capacity can be obtained.
[0013] According to the present invention, the control unit has a
temperature detection sensor for detecting a refrigerant
temperature discharged from the compressor, and the control unit
controls a high-pressure side refrigerant pressure from the
compressor and before being decompressed in the heat pump cycle, so
that a temperature difference between the fluid flowing into the
heat exchanger and refrigerant discharged from the heat exchanger
becomes a target temperature difference when the refrigerant
temperature detected by the temperature sensor is lower than a
predetermined temperature. On the other hand, when the refrigerant
temperature detected by the temperature sensor is higher than the
predetermined temperature, the control unit controls the
high-pressure side refrigerant pressure of the heat pump cycle so
that the refrigerant temperature detected by the temperature sensor
becomes lower than the predetermined temperature. Accordingly, the
refrigerant temperature discharged from the compressor can be
directly controlled without changing the target temperature
difference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Additional objects and advantages of the present invention
will be more readily apparent from the following detailed
description of preferred embodiments when taken together with the
accompanying drawings, in which:
[0015] FIG. 1 is a schematic diagram of a heat-pump water heater
according to a first preferred embodiment of the present
invention;
[0016] FIG. 2 is a graph (T-H diagram) showing a relationship
between temperature and enthalpy in a heat pump cycle using carbon
dioxide as refrigerant, according to the first embodiment;
[0017] FIG. 3 is a flow diagram showing a control process of an
electronic control unit (ECU) according to the first
embodiment;
[0018] FIG. 4 is a flow diagram showing a control process of the
ECU according to a second preferred embodiment of the present
invention;
[0019] FIG. 5 is a characteristic view showing a relationship
between an evaporation temperature Ts of refrigerant and a target
temperature difference .DELTA.T, according to the second
embodiment;
[0020] FIG. 6 is a flow diagram showing a control process of the
ECU according to a third preferred embodiment of the present
invention;
[0021] FIGS. 7A and 7B are graphs (T-H diagrams), respectively,
each showing a relationship between temperature and enthalpy in a
heat pump cycle using carbon dioxide as refrigerant, according to
the third embodiment;
[0022] FIG. 8 is a graph showing a relationship between a driving
current and a load of a compressor according to a modification of
the present invention; and
[0023] FIG. 9 is a graph showing a stepwise change of the target
temperature difference .DELTA.T according to an another
modification of the present invention.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0024] Preferred embodiments of the present invention will be
described hereinafter with reference to the accompanying
drawings.
[0025] A first preferred embodiment of the present invention will
be now described with reference to FIGS. 1-3. As shown in FIG. 1, a
heat-pump water heater 1 is a hot water supply system, in which
heated hot water is stored in a tank 2 and is supplied to a user
after being temperature-adjusted. The heat-pump water heater 1
includes the tank 2, an electrical pump 3 forcibly circulating
water in a water cycle, and a super-critical (trans-critical) heat
pump cycle 4.
[0026] The tank 2 is made of a metal having a corrosion resistance,
such as a stainless steel, and has a heat insulating structure so
that high-temperature hot water can be stored for a long time. Hot
water stored in the tank 2 can be supplied to a kitchen, a bath or
the like, and can be used as a heating source for a floor heater or
a room heater or the like.
[0027] The electrical pump 3, the tank 2 and a water heat exchanger
7 of the heater pump cycle 4 are connected by a water pipe 5 to
form the water cycle. Therefore, water circulates between the tank
2 and the water heat exchanger 7, and water circulating amount in
the water cycle can be adjusted in accordance with a rotation speed
of a motor disposed in the electrical pump 3.
[0028] The super-critical heat pump cycle 4 uses carbon dioxide
having a low-critical pressure as refrigerant, for example, so that
a high-pressure side refrigerant pressure becomes equal to or
greater than the critical pressure of carbon dioxide. As shown in
FIG. 1, the heater pump cycle 4 includes a compressor 6, the water
heat exchanger 7, an expansion valve 8, an air heat exchanger 9 and
an accumulator 10.
[0029] The compressor 6 includes an electrical motor 6a which is
driven by an inverter circuit 16. The compressor 6 compresses
sucked gas refrigerant by the rotation of the electrical motor 6a,
so that refrigerant discharged from the compressor 6 has the
pressure equal to or greater than the critical pressure of
refrigerant. The water heat exchanger 7 is disposed to perform a
heat exchange between high-pressure gas refrigerant discharged from
the compressor 6 and water pumped from the electrical pump 3. In
the water heat exchanger 7, a flow direction of refrigerant is set
opposite to a flow direction of water.
[0030] The expansion valve 8 is constructed so that a valve opening
degree can be electrically adjusted. The expansion valve 8 is
disposed at a downstream side of the water heat exchanger 7 in a
refrigerant flow direction, and decompresses refrigerant cooled in
the water heat exchanger 7 in accordance with a valve opening
degree. A fan 11 for blowing air (i.e., outside air) toward the air
heat exchanger 9 is disposed so that refrigerant decompressed in
the expansion valve 8 is heat-exchanged with air in the air heat
exchanger 9. Therefore, refrigerant is evaporated in the air heat
exchanger 9 by absorbing heat from air.
[0031] Refrigerant from the air heat exchanger 9 flows into the
accumulator 10 and is separated into gas refrigerant and liquid
refrigerant in the accumulator 10. Only separated gas refrigerant
in the accumulator 10 is sucked into the compressor 6, and surplus
refrigerant in the heat pump cycle 4 is stored in the accumulator
10.
[0032] The heat-pump water heater 1 has an electrical control unit
(hereinafter, referred to as ECU) 15, and plural sensors 12-14.
Specifically, the plural sensors 12-14 are a first refrigerant
temperature sensor 12 for detecting a temperature Td of refrigerant
discharged from the compressor 6, a water temperature sensor 13 for
detecting temperature Tw of water flowing into the water heat
exchanger 7, and the second refrigerant temperature sensor 14 for
detecting temperature Tr of refrigerant flowing out from the water
heat exchanger 7. Detection signals from the sensors 12-14 are
input into the ECU 15, and the ECU 15 controls operation of the
heat pump cycle 4.
[0033] The ECU 15 controls a high-pressure side refrigerant
pressure in the heat pump cycle 4 based on a temperature difference
between water flowing into the water heat exchanger 7 and
refrigerant flowing out from the water heat exchanger 7, so that
the heat pump cycle 4 can be operated with a high efficiency. That
is, a target temperature difference .DELTA.T between water flowing
into the water heat exchanger 7 and refrigerant flowing out from
the water heat exchanger 7 is set as an index of the cycle
efficiency, and the valve opening degree of the expansion valve 8
is electrically controlled so that the target temperature
difference .DELTA.T is obtained.
[0034] Next, the control process of the ECU 15 according to the
first embodiment will be now described with reference to FIG. 3.
First, at step S10, the high-pressure side refrigerant pressure of
the heat pump cycle 4 is controlled by controlling the valve
opening degree of the expansion valve 8, so that a set target
temperature difference .DELTA.T (e.g., 10.degree. C.) is obtained.
Next, at step S20, the refrigerant temperature Td discharged from
the compressor 6 is detected by the first refrigerant temperature
sensor 12.
[0035] At step S30, it is determined whether or not the refrigerant
temperature Td discharged from the compressor 6 is equal to or
higher than a predetermined value Tdp. In the first embodiment, the
predetermined value Tdp is set based on a permissible upper limit
temperature of the compressor 6. When it is determined that the
refrigerant temperature Td discharged from the compressor 6 is
equal to or higher than the predetermined value Tdp at step S30,
the target temperature difference .DELTA.T is increased at step
S40. Thereafter, the control routine returns to step S10.
Accordingly, the target temperature difference .DELTA.T is
gradually increased until the refrigerant temperature Td discharged
from the compressor 6 becomes smaller than the predetermined value
Tdp. On the other hand, when it is determined that the refrigerant
temperature Td discharged from the compressor 6 is lower than the
predetermined value Tdp at step S30, it is determined whether or
not a water heating capacity reaches a target water heating
capacity at step S50. For example, the water heating capacity can
be determined based on a heat quantity of hot water that is heated
by refrigerant in the water heat exchanger 7 and is stored in the
tank 2. Here, the heat quantity of hot water is calculated in
accordance with a hot water temperature and a hot water flow
amount. Specifically, when the heat quantity transmitted into water
for a predetermined time is equal to or larger than a predetermined
value, it is determined that the target water heating capacity is
obtained.
[0036] When it is determined that the water heating capacity
reaches the target water heating capacity at step S50, the control
routine is finished. On the other hand, when it is determined that
the water heating capacity does not reach the target water heating
capacity at step S50, the rotation speed of the motor 6a of the
compressor 6 is increased for obtaining the target water heating
capacity. Thereafter, the control routine moves to step S10.
[0037] According to the first embodiment of the present invention,
when the refrigerant temperature Td discharged from the compressor
6 is higher than the predetermined value Tdp, the target
temperature difference .DELTA.T is changed to be increased, and
therefore, the opening degree of the expansion valve 8 becomes
larger. FIG. 2 shows both states of the heat pump cycle 4, before
and after the valve opening degree of the expansion valve 8 becomes
larger. In FIG. 2, Q' indicates a heat radiating capacity of the
water heat exchanger 7 before the valve opening degree of the
expansion valve 8 becomes larger, Q indicates the heat radiating
capacity of the water heat exchanger 7 after the valve opening
degree of the expansion valve 8 becomes larger, L' indicates a
compression operation amount (i.e., consumed power) before the
valve opening degree of the expansion valve 8 becomes larger, and L
indicates the compression operation amount after the valve opening
degree of the expansion valve 8 becomes larger. Before the valve
opening degree of the expansion valve 8 becomes larger, the target
temperature difference .DELTA.T' is in a permissle range, but the
refrigerant temperature Td' discharged from the compressor 6 is
higher than the predetermined value Tdp. This cycle state is
readily caused when the outside air temperature becomes lower and
the low-pressure side refrigerant pressure of the heat pump cycle 4
becomes lower.
[0038] After the valve opening degree of the expansion valve 8
becomes larger, because the high-pressure side refrigerant pressure
of the heat pump cycle 4 decreases, the compression operation
amount of the compressor 6 is decreased (L'.fwdarw.L), and the heat
radiating amount of the water heat exchanger 7 is decreased
(Q'.fwdarw.Q). As a result, the refrigerant temperature Td
discharged from the compressor 6 decreases. Until the refrigerant
temperature Td discharged from the compressor 6 is decreased to the
operation temperature area of the compressor 6, the target
temperature difference .DELTA.T is changed to be increased.
According to the first embodiment, because the refrigerant
temperature Td discharged from the compressor 6 can be decreased to
be in the operation temperature area, a problem affected to the
compressor 6 can be prevented.
[0039] In the above-described first embodiment, instead of the
refrigerant temperature Td detected by the first refrigerant
temperature sensor 12, a physical amount relative to the
refrigerant temperature Td, such as an evaporation pressure, an
evaporation temperature and a refrigerant pressure discharged from
the compressor 6, may be used. Further, when the refrigerant
temperature Td discharged from the compressor 6 is higher than the
predetermined temperature Tdp, the valve opening degree of the
expansion valve 8 can be directly controlled so that the
refrigerant temperature Td becomes lower than the target
temperature Tdp, without changing the target temperature difference
.DELTA.T or without firstly setting the target temperature
difference .DELTA.T.
[0040] A second preferred embodiment of the present invention will
be now described with reference to FIGS. 4 and 5. In the second
embodiment, the target temperature difference .DELTA.T is set based
on a low-pressure side refrigerant temperature (e.g., evaporation
temperature TS of refrigerant). In the second embodiment, the other
parts are similar to those of the above-described first
embodiment.
[0041] FIG. 4 is a flow diagram showing a control process of the
ECU 15 according to the second embodiment. First, at step S110, the
valve opening degree of the expansion valve 8 is controlled so that
a set target temperature difference .DELTA.T can be obtained. Next,
an evaporation temperature Ts of refrigerant is detected at step
S120, and it is determined whether or not the evaporation
temperature Ts is equal to or lower than a predetermined
temperature Ts1 (i.e., protection control start temperature) at
step S130. When the evaporation temperature Ts of refrigerant is
equal to or lower than the predetermined temperature Ts1 at step
S130, the target temperature difference .DELTA.T is determined
based on the evaporation temperature Ts of refrigerant in
accordance with the graph of FIG. 5. In FIG. 5, Tp indicates a
protection control start point. On the other hand, when the
evaporation temperature Ts of refrigerant is higher than the
predetermined temperature Ts1 at step S130, the control routine
moves to step S170.
[0042] After the target temperature difference .DELTA.T is
determined at step S140, an actual temperature difference .DELTA.T0
is detected at step S150, and the set target temperature difference
.DELTA.T is compared with the actual temperature difference
.DELTA.T0 at step S160. That is, at step S160, it is determined
whether or not the set target temperature difference .DELTA.T is
agreement with the actual temperature difference .DELTA.T0. When it
is determined that the set target temperature difference .DELTA.T
is agreement with the actual temperature difference .DELTA.T0, the
control routine moves to step S170. On the other hand, when it is
determined that the set target temperature difference .DELTA.T is
not agreement with the actual temperature difference .DELTA.T0, the
control routine moves to step S110.
[0043] At step S170, it is determined whether or not a water
heating capacity reaches a target water heating capacity. When it
is determined that the water heating capacity reaches the target
water heating capacity, the control routine is finished. On the
other hand, when it is determined that the water heating capacity
does not reach the target water heating capacity, the rotation
speed of the motor 6a of the compressor 6 is increased at step S180
for obtaining the target water heating capacity. Thereafter, the
control routine moves to step S10.
[0044] According to the second embodiment of the present invention,
when the refrigerant evaporation temperature Ts is lower than the
predetermined temperature Ts1, the target temperature difference
.DELTA.T is set to be larger than a general control based on the
refrigerant evaporation temperature Ts. Therefore, the opening
degree of the expansion valve 8 becomes larger, the refrigerant
pressure discharged from the compressor 6 becomes lower, and
refrigerant temperature Td discharged from the compressor 6 can be
decreased to the operation temperature area. As a result, it can
prevent a problem affected to the compressor 6 in the heat pump
cycle 4. In the second embodiment, when the refrigerant temperature
Td discharged from the compressor 6 becomes lower due to a decrease
of water temperature, the predetermined temperature Ts1 (protection
control start temperature) may be set at a low value.
[0045] A third preferred embodiment of the present invention will
be now described with reference to FIGS. 6, 7A and 7B. In the third
embodiment, it is determined whether or not the load applied to the
compressor 6 is excessive (i.e., larger than an upper limit value),
and the target temperature difference .DELTA.T is set larger when
the load of the compressor 6 is excessive. In the third embodiment,
for determining the load of the compressor 6, an operation state of
a protection circuit (not shown), which restricts output current
for protecting the inverter circuit 16, is detected. When the
output current is restricted by the protection circuit, it is
determined that the load of the compressor 6 is larger than the
upper limit value. That is, in this case, it is determined that the
load of the compressor 6 is excessive.
[0046] FIG. 6 is a flow diagram showing a control process of the
ECU 15 according to the third embodiment. First, at Step S210, the
high-pressure side refrigerant pressure of the heat pump cycle 4 is
controlled by controlling the valve opening degree of the expansion
valve 8, so that a set target temperature difference .DELTA.T can
be obtained. Next, at step S220, it is determined whether or not a
current restriction due to the inverter circuit 16 is performed in
the compressor 6. When the current restriction is performed at step
S220, the target temperature difference .DELTA.T is changed to
become larger (e.g., 15.degree. C.) at step S230, and thereafter,
the control routine moves to step S210.
[0047] On the other hand, when the current restriction is not
performed at step S220, it is determined whether or not a water
heating capacity reaches a target water heating capacity at step
S240. For example, the water heating capacity can be determined
based on a heat quantity of hot water that is heated by refrigerant
in the water heat exchanger 7 and is stored in the tank 2. Here,
the heat quantity of hot water is calculated in accordance with a
hot water temperature and a hot water flow amount. Specifically,
when the heat quantity transmitted into water for a predetermined
time is equal to or larger than a predetermined value, it is
determined that the target water heating capacity is obtained.
[0048] When it is determined that the water heating capacity
reaches the target water heating capacity, the control routine is
finished. On the other hand, when it is determined that the water
heating capacity does not reach the target water heating capacity,
the rotation speed of the motor 6a of the compressor 6 is increased
at step S250 for obtaining the target water heating capacity.
Thereafter, the control routine moves to step S210.
[0049] According to the third embodiment, in a normal operation of
the heat pump cycle 4, as shown in FIG. 7A, the high-pressure side
refrigerant pressure is controlled so that the set target
temperature difference .DELTA.T (e.g., 10.degree. C.) can be
obtained, and a suitable heat-exchanging state of the water heat
exchanger 7 can be obtained. On the other hand, when the load of
the compressor 6 becomes excessive due to some reason, the target
temperature difference .DELTA.T (e.g., 10.degree. C.) is changed to
be increased by a value (e.g., 5.degree. C.) as compared with the
normal operation state, as shown in FIG. 7B. Even in this case, the
heat pump cycle operates with a high-pressure side refrigerant
pressure lower than that in the normal operation state.
[0050] In the third embodiment, even when the current restriction
due to the inverter circuit 16 is performed, the current
restriction can be canceled by increasing the target temperature
difference .DELTA.T, and it can prevent the refrigerant flow amount
from being decreased due to a reduce of the rotation speed of the
compressor 6. As a result, a necessary water heating capacity can
be obtained in the heat-pump water heater 1 without throttling the
valve opening degree of the expansion valve 8 more than a necessary
degree.
[0051] Although the present invention has been fully described in
connection with the preferred embodiments thereof with reference to
the accompanying drawings, it is to be noted that various changes
and modifications will become apparent to those skilled in the
art.
[0052] For example, In the above-described embodiments, the present
invention is typically applied to the heat-pump water heater 1 for
heating water. However, the present invention may be applied to a
heat-pump fluid heater for heating a fluid using the heat pump
cycle 4 as a heating source.
[0053] In the above-described first and second embodiments, the
valve opening degree of the expansion valve 8 is controlled so that
the set target temperature difference .DELTA.T can be obtained.
However, a water discharge amount of the electrical pump 3 may be
controlled, so that the flow amount of water flowing into the water
heat exchanger 7 is changed and the target temperature difference
.DELTA.T is obtained.
[0054] In the above-described third embodiment, the excessive load
of the compressor 6 is determined based on the current restriction
due to the inverter circuit 16. However, electrical current applied
to the motor 6a of the compressor 6 from the inverter circuit 16 is
detected, and the load of the compressor 6 may be determined based
on the applied electrical current. For example, as shown in FIG. 8,
when electrical current applied to the motor 6a is equal to or
larger than a determination value, it is determined that the load
of the compressor 6 is equal to or larger than a set upper limit
value, and the target temperature difference .DELTA.T is changed to
be larger.
[0055] Further, the excessive load of the compressor 6 may be
determined based on at least one physical amount relative to the
load of the compressor, such as a target heating temperature of
water, an outside air temperature and a rotation speed of the
compressor 6.
[0056] Further, in the third embodiment, when the target
temperature difference .DELTA.T is changed, the target temperature
difference may be stepwise changed or may be gradually continuously
changed. For example, as shown in FIG. 9, the target temperature
difference .DELTA.T can be changed stepwise based on a combination
of the outside air temperature and a target water temperature to be
heated. In this case, a determination range of the target
temperature difference .DELTA.T may be changed in accordance with
the rotation speed of the compressor 6. That is, as the rotation
speed of the compressor 6 is higher, the target temperature
difference .DELTA.T is corrected to become larger.
[0057] Such changes and modifications are to be understood as being
within the scope of the present invention as defined by the
appended claims.
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