EP4660555A1

EP4660555A1 - Heat pump device

Info

Publication number: EP4660555A1
Application number: EP24750393.1A
Authority: EP
Inventors: Kentaro SAGAWA; Ryo Takaoka
Original assignee: Fujitsu General Ltd
Current assignee: Fujitsu General Ltd
Priority date: 2023-02-02
Filing date: 2024-02-01
Publication date: 2025-12-10
Also published as: WO2024162444A1; JP7485111B1; JP2024110060A

Abstract

A heat pump device according to an embodiment of the present invention includes: a compressor, a use-side heat exchanger that exchanges heat between water and a refrigerant, an electronic expansion valve, a heat source-side heat exchanger, and control means for controlling a rotation speed of the compressor and an opening degree of the electronic expansion valve. The control means controls the electronic expansion valve on the basis of predetermined upper-limit discharge temperature and lower-limit discharge temperature, sets the upper-limit discharge temperature in accordance with the magnitude of the load of the compressor, reduces the opening degree of the electronic expansion valve such that the discharge temperature is equal to or larger than the lower-limit discharge temperature when the discharge temperature is less than the lower-limit discharge temperature, increases the opening degree of the electronic expansion valve such that the discharge temperature is less than the upper-limit discharge temperature when the discharge temperature is equal to or larger than the upper-limit discharge temperature, and controls the opening degree of the electronic expansion valve such that the subcooling is a target subcooling when the discharge temperature is equal to or larger than the lower-limit discharge temperature and less than the upper-limit discharge temperature.

Description

Technical Field

The present invention relates to a heat pump device such as a hot-water heater of a heat pump type.

Background Art

Air conditioners in which a refrigerant exchanges heat with air, and hot-water heaters in which a refrigerant exchanges heat with water are known as heat pump devices. The hot-water heater includes a refrigerant circuit including a compressor, a use-side heat exchanger, an electronic expansion valve, and a heat source-side heat exchanger, and causes a high-temperature gas refrigerant compressed by the compressor to exchange heat with water in the use-side heat exchanger to heat the water by the heat dissipated by the refrigerant (see, for example Patent Literature 1).
The hot-water heater can include a use-side heat exchanger smaller in size than an air conditioner. This is because water has a higher heat transfer coefficient than air, and the heat transfer area between the refrigerant and water can be made smaller. A heat pump device used as a hot-water heater has a narrow subcooling range in which the efficiency of operation is high because of the small-sized use-side heat exchanger. For that reason, if the subcooling range is not adjusted appropriately, the efficiency of operation may decrease.
In this regard, in the device described in Patent Literature 1, a target subcooling is obtained from the condensation pressure and the rotation speed of the compressor, and the opening degree of the electronic expansion valve is adjusted such that the subcooling in the refrigerant circuit is the target subcooling. This enables optimal adjustment of the subcooling and thus an efficient operation.

Citation List

Patent Literature

Patent Literature 1: Japanese Patent Application Laid-open No. 2011-69570

Disclosure of Invention

Technical Problem

However, if the electronic expansion valve is adjusted in accordance with the subcooling, the state of the suctioned refrigerant and a discharge temperature are left to chance. Accordingly, the discharge temperature may be too high or too low due to installation conditions or variations in the amount of charged refrigerant, which may reduce the reliability of the compressor. Further, the discharge temperature may tend to be low or high depending on the type of refrigerant.
In view of the circumstances as described above, it is an object of the present invention to provide a heat pump device that is capable of an efficient operation while ensuring reliability of a compressor.

Solution to Problem

A heat pump device according to an embodiment of the present invention includes:

a refrigerant circuit in which a compressor, a use-side heat exchanger that exchanges heat between water and a refrigerant, an electronic expansion valve, and a heat source-side heat exchanger are connected to each other via piping;
discharge temperature detection means for detecting a discharge temperature, the discharge temperature being a temperature of the refrigerant discharged from the compressor;
subcooling detection means for detecting a subcooling of the refrigerant that flows out of the use-side heat exchanger;
load detection means for detecting a magnitude of a load of the compressor; and
control means for controlling a rotation speed of the compressor and an opening degree of the electronic expansion valve.

The control means

controls the electronic expansion valve on the basis of a predetermined upper-limit discharge temperature and a predetermined lower-limit discharge temperature,
sets the upper-limit discharge temperature in accordance with the magnitude of the load of the compressor,
reduces the opening degree of the electronic expansion valve such that the discharge temperature is equal to or larger than the lower-limit discharge temperature when the discharge temperature is less than the lower-limit discharge temperature,
increases the opening degree of the electronic expansion valve such that the discharge temperature is less than the upper-limit discharge temperature when the discharge temperature is equal to or larger than the upper-limit discharge temperature, and
controls the opening degree of the electronic expansion valve such that the subcooling is a target subcooling when the discharge temperature is equal to or larger than the lower-limit discharge temperature and less than the upper-limit discharge temperature.

The load detection means may include a discharge pressure detection section that detects a discharge pressure, the discharge pressure being a pressure of the refrigerant discharged from the compressor, and a suction pressure detection section that detects a suction pressure, the suction pressure being a pressure of the refrigerant suctioned into the compressor.
The control means may set the upper-limit discharge temperature on the basis of a virtual discharge temperature that is a discharge temperature when the refrigerant suctioned into the compressor is assumed as saturated vapor, the virtual discharge temperature being calculated on the basis of at least the discharge pressure and the suction pressure.
The lower-limit discharge temperature may be set in advance on the basis of a predetermined minimum superheat determined for the compressor.
The control means may set the upper-limit discharge temperature such that the upper-limit discharge temperature is higher than the lower-limit discharge temperature over an entire load region of the compressor.
Further, the control means may set the upper-limit discharge temperature by adding a correction value to the virtual discharge temperature.
Further, the control means may increase the correction value as the load of the compressor becomes lower.
For example, the heat pump device may further include a storage section that stores a plurality of temperature values set in advance in accordance with the magnitude of the load of the compressor, and the control means may select the correction value from the plurality of temperature values on the basis of a current magnitude of the load of the compressor.
The control means may determine that the load of the compressor becomes larger as a differential pressure that is a difference between the discharge pressure and the suction pressure becomes larger.
The refrigerant that is sealed in the refrigerant circuit is typically a refrigerant in which a specific heat ratio in saturated vapor at 10°C is less than 1.25.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a heat pump device that is capable of an efficient operation while ensuring reliability of a compressor.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a refrigerant circuit diagram of a heat pump device according to an embodiment of the present invention.
[Fig. 2] Fig. 2 is a diagram showing the relationship between a subcooling and a COP when an outdoor air temperature is 7°C by comparing a hot-water heater (black circles in the figure) and an air conditioner (black squares in the figure).
[Fig. 3] Fig. 3 is a block diagram showing a configuration of a control device in the heat pump device.
[Fig. 4] Fig. 4 is a diagram showing an example of a target subcooling table.
[Fig. 5] Fig. 5 is a conceptual diagram for describing a method of extracting a target subcooling.
[Fig. 6] Fig. 6 is a conceptual diagram for describing a method of determining a correction value of an upper-limit discharge temperature.
[Fig. 7] Fig. 7 is a diagram showing an example of an upper-limit discharge temperature and a lower-limit discharge temperature under two conditions different in refrigerant and operating state.
[Fig. 8] Fig. 8 is a flowchart showing an example of a processing procedure executed in the control device. Mode(s) for Carrying Out the Invention

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[Basic Configuration of Heat Pump Device]

Fig. 1 is a refrigerant circuit diagram of a heat pump device 100 according to an embodiment of the present invention. The heat pump device 100 of this embodiment is a heat-pump underfloor heating device (hot-water heater).
The heat pump device 100 includes a refrigerant circuit 10 including a compressor 11, a four-way valve 12, a use-side heat exchanger 13, an electronic expansion valve 14, and a heat source-side heat exchanger 15 serving as an outdoor heat exchanger, which are sequentially connected to each other by piping, and a control device 20 serving as control means.
The type of refrigerant sealed in the refrigerant circuit 10 is not particularly limited, and for example, hydrocarbon-based refrigerants and fluorocarbon refrigerants, which are natural refrigerants, can be employed. In particular, in this embodiment, as will be described later, a refrigerant that may cause inversion between a lower limit temperature of a discharge temperature that is the temperature of the refrigerant discharged from the compressor 11 (hereinafter, also referred to as lower-limit discharge temperature) and an upper limit temperature of the discharge temperature (hereinafter, also referred to as upper-limit discharge temperature), or more specifically, a refrigerant whose specific heat ratio γ in saturated vapor at 10°C (ratio of molar specific heat at constant pressure Cp to molar specific heat at constant volume Cv: Cp/Cv)) is less than 1.25, is used. As this type of refrigerant, for example, R290 (specific heat ratio γ: 1.24), R1234yf (specific heat ratio γ: 1.17), and R454C (specific heat ratio γ: 1.24) are included.
The compressor 11 is a variable-capacity compressor that compresses a low-temperature, lowpressure refrigerant and discharges a high-temperature, high-pressure refrigerant. The high-temperature, high-pressure refrigerant discharged from the compressor 11 is supplied to a port a of the four-way valve 12 via a pipe 91.
The four-way valve 12 is a flow path switching valve including the port a, a port b, a port c, and a port d. The four-way valve 12 is switched to one of a first state in which the port a and the port b communicate with each other and the port c and the port d communicate with each other, which is shown by the solid lines in Fig. 1, and a second state in which the port a and the port d communicate with each other and the port b and the port c communicate with each other, which is shown by the broken lines in Fig. 1, on the basis of a command from the control device 20. The four-way valve 12 is switched to the first state when a heating operation is performed, and is switched to the second state when a defrosting operation of the heat source-side heat exchanger 15 is performed. If the four-way valve 12 is switched to the first state, the refrigerant circulates through the refrigerant circuit 10 in the direction shown by the arrows in Fig. 1.
The port b of the four-way valve 12 is connected to the use-side heat exchanger 13 via a pipe 92. The port c of the four-way valve 12 is connected to a suction port of the compressor 11 via a pipe 95. The port d of the four-way valve 12 is connected to the heat source-side heat exchanger 15 via a pipe 94.
The use-side heat exchanger 13 is a radiator (condenser) that exchanges heat between the high-temperature, high-pressure refrigerant discharged from the compressor 11 and the water passing through an underfloor heating panel 18. The use-side heat exchanger 13 can adopt various types of heat exchangers capable of exchanging heat between water and the refrigerant, such as a plate heat exchanger, a doubletube heat exchanger, and a multi-tube heat exchanger. The refrigerant that has exchanged heat with water in the use-side heat exchanger 13 is supplied in sequence to the electronic expansion valve 14 and the heat source-side heat exchanger 15 through a pipe 93.
The heat pump device 100 further includes a hot water circuit 96 that is formed by sequentially connecting the use-side heat exchanger 13, the underfloor heating panel 18, and a hot water pump 19. In the hot water circuit 96, the water (hot water) that has exchanged heat with the refrigerant in the use-side heat exchanger 13 circulates. The hot water circulates in the hot water circuit 96 in the direction shown by the arrows in Fig. 1. The hot water circuit 96 includes a meandering passage 96a provided in the underfloor heating panel 18, and heats the underfloor heating panel 18 by the hot water flowing through the meandering passage 96a.
The electronic expansion valve 14 is a pressure reducer for reducing the pressure of the refrigerant flowing out of the use-side heat exchanger 13 via the pipe 93. The opening degree of the electronic expansion valve 14 is controlled on the basis of a command from the control device 20.
The heat source-side heat exchanger 15 is an evaporator that exchanges heat between the refrigerant flowing out of the electronic expansion valve 14 and outside air. The type of the heat source-side heat exchanger 15 is not particularly limited, and for example, various types of heat exchangers that can exchange heat between air and a refrigerant, such as a parallel flow heat exchanger, a fin tube heat exchanger, and a plate fin heat exchanger, can be adopted. A fan for blowing air (not shown) may be disposed near the heat source-side heat exchanger 15.
In the heat source-side heat exchanger 15, the refrigerant that has exchanged heat with the outside air is returned to the compressor 11 via the pipe 94, the four-way valve 12 in the first state, and the pipe 95. Note that the pipe 95 may be provided with an accumulator (not shown) for separating a liquidphase refrigerant from the refrigerant returned to the compressor 11 via the pipe 95.
Note that the pipe 91 is provided with a discharge temperature sensor 31 serving as discharge temperature detection means for detecting a discharge temperature, the discharge temperature being the temperature of the refrigerant discharged from the compressor 11, and a discharge pressure sensor 32 serving as a discharge pressure detection section that detects a discharge pressure, the discharge pressure being the pressure of the refrigerant discharged from the compressor 11. Further, the pipe 93 located between the use-side heat exchanger 13 and the electronic expansion valve 14 is provided with a refrigerant temperature sensor 33 that detects the temperature of the refrigerant flowing out from the use-side heat exchanger 13. Furthermore, the pipe 95 is provided with a suction pressure sensor 34 serving as a suction pressure detection section that detects a suction pressure that is the pressure of the refrigerant suctioned into the compressor 11. In addition, a hot water temperature sensor 35 that detects the hot water temperature is provided on the outlet side of the use-side heat exchanger 13 in the hot water circulation path 96.
The control device 20 is a computer that includes a CPU, a memory, and the like and comprehensively controls the operation of the heat pump device 100. More specifically, the control device 20 outputs and controls signals to indicate the rotation speed of the compressor 11, the opening degree of the electronic expansion valve 14, and the like on the basis of the detection values of the discharge temperature sensor 31, the discharge pressure sensor 32, the refrigerant temperature sensor 33, the suction pressure sensor 34, the hot water temperature sensor 35, and the like.
For example, the control device 20 rotates the compressor 11 such that the current hot water temperature detected by the hot water temperature sensor 34, that is, the temperature of the water heated by the use-side heat exchanger 13, is a target temperature set in advance (target hot water temperature).
Further, the control device 20 calculates a subcooling (degree of supercooling) of the refrigerant flowing out of the use-side heat exchanger 13 on the basis of the detection value of the discharge pressure sensor 32 and the detection value of the refrigerant temperature sensor 33, and controls the opening degree of the electronic expansion valve 14 such that the calculated result has a target value. The discharge pressure sensor 32 and the refrigerant temperature sensor 33 correspond to subcooling detection means for detecting the subcooling of the refrigerant.
In general, the hot-water heater can include a use-side heat exchanger smaller in size than the air conditioner. This is because water has a higher heat transfer coefficient than air, and a heat transfer area between the refrigerant and water can be made smaller. A heat pump device used as a hot-water heater has a narrow subcooling range in which the efficiency of operation is high because of the small-sized use-side heat exchanger. In other words, in the relationship between a subcooling and a coefficient of performance (COP), which represents the efficiency of operation, the changes in subcooling have a greater impact on the COP in the hot-water heater than in the air conditioner, and if the subcooling is not strictly controlled to keep the subcooling within a narrow range, the COP deteriorates, resulting in an inefficient operation.
For example, Fig. 2 shows the relationship between the subcooling and the COP when an outdoor air temperature is 7°C by comparing a hot-water heater (black circles in the figure) and an air conditioner (black squares in the figure). As shown in Fig. 2, it can be found that the COP of the hot-water heater is significantly lower than that of the air conditioner when the subcooling is 5°C or higher, and the change in subcooling has a significant impact on the COP. Water has a higher heat transfer coefficient than air, but in a subcooling region, the heat transfer coefficient on the refrigerant side significantly decreases, and there is no longer a large difference in the heat passage coefficient (since the heat passage coefficient is the reciprocal of the sum of the thermal resistance, the point of high thermal resistance becomes dominant). Thus, the percentage of the two-phase region in the entire heat exchanger when the subcooling is increased decreases more rapidly in the hot-water heater. When the area of the two-phase region with a higher heat passage coefficient rapidly decreases, the discharge pressure increases, and the COP drops sharply as the subcooling increases. In such a manner, if the subcooling is not adjusted to be maintained in an appropriate range in the hot-water heater, the efficiency of operation will drop sharply.
In this regard, there is known subcooling control in which a target subcooling is obtained from the condensation pressure and the rotation speed of the compressor, and the opening degree of the electronic expansion valve is controlled such that the subcooling of the refrigerant circuit is the target subcooling (see, for example, Patent Literature 1). The subcooling control is to subtract the target subcooling from the current subcooling, and when the subtraction result is positive, the opening degree of the electronic expansion valve is increased to correspond to the value of the subtraction result, and when the subtraction result is negative, the opening degree of the electronic expansion valve is reduced to correspond to the value of the subtraction result. This enables optimal adjustment of the subcooling, resulting in a more efficient operation.
However, if the electronic expansion valve is adjusted in accordance with the subcooling, unlike the case where the opening degree of the electronic expansion valve is controlled on the basis of the target discharge temperature, the state of the refrigerant suctioned into the compressor and the discharge temperature are left to chance. Accordingly, the discharge temperature may be too high or too low due to the length of the pipe 92 or pipe 93 or variations in the amount of charged refrigerant, which may reduce the reliability of the compressor.
For example, a lower-limit discharge temperature and an upper-limit discharge temperature are set for the discharge temperature. The lower-limit discharge temperature is set in advance on the basis of a predetermined minimum superheat determined for the compressor 11. The predetermined minimum superheat is the minimum superheat required to ensure the lubricity of the compressor and is, for example, the minimum discharge SH determined as the specifications of the compressor 11. Therefore, if the subcooling control is executed when the discharge temperature is less than the lower-limit discharge temperature, there is a risk that the degree of wetness of the refrigerant suctioned into the compressor may become excessively large. On the other hand, the upper-limit discharge temperature is set so as to protect the compressor. Therefore, if the subcooling control is executed when the discharge temperature is equal to or larger than the upper-limit discharge temperature, the discharge temperature may rise excessively.
Further, since the lower-limit discharge temperature and the upper-limit discharge temperature differ depending on the type of refrigerant, the operating state of the compressor, and the like, the discharge temperature may tend to be lower or higher depending on the type of refrigerant used.
To solve such problems, the control device 20 controls the opening degree of the electronic expansion valve 14 on the basis of the upper-limit discharge temperature and the lower-limit discharge temperature (also referred to as predetermined temperature range) set in advance depending on the type of refrigerant. The control device 20 sets the upper-limit discharge temperature in accordance with the magnitude of the load of the compressor 11. For example, if the discharge temperature is out of the predetermined temperature range, the opening degree of the electronic expansion valve 14 is controlled such that the discharge temperature falls within the predetermined temperature range. If the discharge temperature falls within the predetermined temperature range, the opening degree of the electronic expansion valve 14 is controlled such that the subcooling has a target value. Specifically, the control device 20 reduces the opening degree of the electronic expansion valve 14 such that the discharge temperature is equal to or larger than the lower-limit discharge temperature when the discharge temperature is less than the lower-limit discharge temperature; increases the opening degree of the electronic expansion valve 14 such that the discharge temperature is less than upper-limit discharge temperature when the discharge temperature is equal to or larger than the upper-limit discharge temperature; and controls the opening degree of the electronic expansion valve 14 such that the subcooling is the target subcooling when the discharge temperature is equal to or larger than the lower-limit discharge temperature and less than the upper-limit discharge temperature.

[Details of Control Device]

Hereinafter, the details of the control device 20 will be described. Fig. 3 is a block diagram showing a configuration of the control device 20.
As shown in Fig. 3, the control device 20 includes a target subcooling extraction section 21, a load detection section 22, an upper-limit discharge temperature setting 23, a discharge temperature determination section 24, and a storage section 25. The target subcooling extraction section 21, the load detection section 22, the upper-limit discharge temperature setting section 23, and the discharge temperature determination section 24 are functional blocks of the CPU of the control device 20, and the storage section 25 is a semiconductor memory or another storage device.

(Target Subcooling Extraction Section)

The target subcooling extraction section 21 extracts the target subcooling on the basis of the rotation speed indicated to the compressor 11 by the control device 20 and the detection value of the discharge pressure sensor 32. The storage section 25 stores a target subcooling table obtained in advance on the basis of the condensation pressure state of the refrigerant in the use-side heat exchanger 13 and the rotation speed of the compressor 11. The target subcooling extraction section 21 extracts the target subcooling from the target subcooling table stored in the storage section 25 on the basis of the rotation speed indicated to the compressor 11 by the control device 20 and the discharge pressure sensor 32. The control device 20 controls the opening degree of the electronic expansion valve 14 such that the current subcooling calculated on the basis of the detection values of the discharge pressure sensor 32 and the refrigerant temperature sensor 33 is the target subcooling. The current subcooling is calculated by subtracting the detection value of the refrigerant temperature sensor 33 from the value obtained by converting the detection value of the discharge pressure sensor 32 to a saturation temperature.
Fig. 4 shows an example of the target subcooling table. Here, the target subcooling table shows items, on the left side in order from top to bottom, "condensation pressure state", "condensation pressure threshold (MPaG), and "rotation speed (rps (rotation per second))".
The "condensation pressure state" distinguishes whether the condensation pressure is rising or dropping. In practice, when the detection value of the discharge pressure sensor 32, which is acquired by the control device 20 at a predetermined cycle, changes from below to above with respect to the previous detection value, it is determined to be "rising," and when the detection value changes from above to below with respect to the previous detection value, it is determined to be "dropping".
The "rotation speed" is divided into three zones (70 rps or more, 40 rps or more and less than 70 rps, and less than 40 rps). The "condensation pressure threshold" is divided into three zones as shown in Fig. 5, and a target subcooling is set for each zone. The condensation pressure thresholds that divide those three zones have hysteresis corresponding to the rise/drop of the condensation pressure in order to reduce hunting in control.
For example, when the rotation speed of the compressor 11 is 40 rps or more and less than 70 rps and when the pressure tends to rise, the condensation pressure threshold is divided into zones of less than 3.0 MPaG, 3.0 MPaG or more and less than 3.6 MPaG, and 3.6 MPaG or more, and the target subcooling is specified as 10°C, 8°C, and 6°C, respectively, in order from the zone with the lower pressure. On the contrary, when the pressure tends to drop, the condensation pressure threshold is divided into zones of less than 2.8 MPaG, 2.8 MPaG or more and less than 3.4 MPaG, and 3.4 MPaG or more, and the target subcooling is specified as 10°C, 8°C, and 6°C, respectively, in order from the zone with the lower pressure. Since the target subcooling is determined to be the subcooling with the highest COP for each condensation pressure and each rotation speed of the compressor 11, even if the condensation pressure changes, the target subcooling can be switched accordingly, which makes it possible to provide strict subcooling control that can maintain the high COP even if the condensation pressure changes.
Further, when the rotation speed of the compressor 11 is less than 40 rps, the target subcooling is specified as 6°C, 5°C, and 4°C in the order from the zone with the lower condensation pressure. When the rotation speed of the compressor 11 is 70 rps or more, the target subcooling is specified as 12°C, 10°C, and 7°C in the order from the zone with the lower condensation pressure.
Note that the values of the condensation pressure threshold, the rotation speed, and the target subcooling in Fig. 4 are merely examples and can be discretionally changed in accordance with the type of refrigerant, installation conditions, operating conditions, and the like.

(Load Detection Section)

The load detection section 22 determines the magnitude of the load of the compressor 11. In this embodiment, the load detection section 22 calculates the load of the compressor 11 on the basis of the differential pressure between the discharge pressure (high pressure) and the suction pressure (low pressure) and determines to which one of the plurality of zones set in advance the calculated magnitude of the load belongs. The discharge pressure sensor 32 and the suction pressure sensor 34 correspond to load detection means for detecting the magnitude of the load of the compressor 11.
Fig. 6 is a conceptual diagram for describing the plurality of regions described above, showing a method of determining a correction value of the upper-limit discharge temperature selected in accordance with the magnitude of the load of the compressor 11. Here, three zones are provided as shown in Fig. 6, and a correction value of the upper-limit discharge temperature is set for each zone. The differential pressures that divide those three zones have hysteresis corresponding to the rise/drop of the differential pressure in order to reduce hunting in the control.
The correction value of the upper-limit discharge temperature is set to be a higher temperature value as the magnitude of the load of the compressor 11 (differential pressure) becomes smaller. For example, during a rising tendency of the differential pressure, the pressure threshold is divided into zones of less than 1.5 MPa, 1.5 MPa or more and less than 2.3 MPa, and 2.3 MPa or more, and the correction value of the upper-limit discharge temperature is specified as 30°C, 15°C, and 0°C, respectively, in order from the zone with the lower pressure. On the contrary, during a dropping tendency of the differential pressure, the pressure threshold is divided into zones of less than 1.3 MPa, 1.3 MPa or more and less than 2.1 MPa, and 2.1 MPa or more, and the correction value of the upper-limit discharge temperature is specified as 30°C, 15°C, and 0°C, respectively, in order from the zone with the lower pressure. Note that the correction value of the upper-limit discharge temperature or the pressure threshold can be discretionally set in accordance with the type of refrigerant and are determined by performing experiments and the like in advance to be stored in the storage section 25.
In such a manner, the storage section 25 stores a plurality of temperature values set in advance (30°C, 15°C, and 0°C in the example in Fig. 6) in accordance with the magnitude of the load of the compressor 11, and the load detection section 22 selects a correction value of the upper-limit discharge temperature from the plurality of temperature values on the basis of the current magnitude of the load of the compressor 11. Note that the temperature value (correction value of the upper-limit discharge temperature) is 0°C when the differential pressure is equal to or larger than a predetermined threshold (2.3 MPa (during rise of the differential pressure) or 2.1 MPa (during drop of the differential pressure)), and thus the upper-limit discharge temperature is not corrected when the differential pressure is equal to or larger than the threshold, and a virtual discharge temperature stored in the storage section 25 is treated as the upper-limit discharge temperature.

(Upper-Limit Discharge Temperature Setting Section)

The upper-limit discharge temperature setting section 23 sets the upper-limit discharge temperature in accordance with the magnitude of the load of the compressor 11. The upper-limit discharge temperature is set by adding the correction value of the upper-limit discharge temperature (see Fig. 6), which is obtained in accordance with the magnitude of the load of the compressor 11, to the virtual discharge temperature stored in the storage section 25.
The virtual discharge temperature is the discharge temperature when the refrigerant suctioned into the compressor 11 is assumed as saturated vapor (dryness is 1), calculated on the basis of the discharge pressure and the suction pressure, and means the discharge temperature when adjusted to an ideal refrigeration cycle. The parameters for calculating the virtual discharge temperature may include the discharge pressure and the suction pressure as well as the rotation speed of the compressor 11.
Here, as the refrigerant used has a smaller specific heat ratio γ, the virtual discharge temperature becomes lower. Therefore, for example, a refrigerant with a small specific heat ratio γ, such as R290, may cause inversion between the upper-limit discharge temperature and the lower-limit discharge temperature (upper-limit discharge temperature < lower-limit discharge temperature) depending on the operating state (the magnitude of the load) of the compressor 11. In such a case, the above inversion makes it impossible to stably perform desired subcooling control. In this embodiment, the correction value of the upper-limit discharge temperature, which is determined in accordance with the magnitude of the load of the compressor 11, is added to the virtual discharge temperature to prevent the inversion between the upper-limit discharge temperature and the lower-limit discharge temperature, thereby achieving appropriate subcooling control and maintaining a high COP.
Fig. 7 shows an example of the upper-limit discharge temperature and the lower-limit discharge temperature when R290 is used as the refrigerant, compared with a refrigerant (R32 (specific heat ratio γ in saturated vapor at 10°C: 1.53)) in which the inversion between the upper-limit discharge temperature and the lower-limit discharge temperature is relatively less likely to occur. For example, in the operating state (state 1) where the evaporation temperature is 0°C and the condensation temperature is 37°C, if the refrigerant is R32 (minimum superheat: 15°C), the lower-limit discharge temperature is 52°C, and the upper-limit discharge temperature is 73.5°C, with a temperature difference between them being over 20°C. Meanwhile, if the refrigerant is R290 (minimum superheat: 10°C), in the same operating state (state 1), the lower-limit discharge temperature is 47°C, and the upper-limit discharge temperature is 46.7°C, where the upper-limit discharge temperature and the lower-limit discharge temperature are inverted.
Further, in the operating state (state 2) where the evaporation temperature is 2°C and the condensation temperature is 57°C, if the refrigerant is R32, the lower-limit discharge temperature is 72°C, and the upper-limit discharge temperature is 110.8°C, with a temperature difference between them being approximately 40°C. If the refrigerant is R290, in the same operating state (state 2), the lower-limit discharge temperature is 67°C, and the upper-limit discharge temperature is 69.7°C, where the upper-limit discharge temperature is close to the lower-limit discharge temperature, which makes it difficult to maintain the temperature difference between them.
If the load of the compressor 11 is large, neither the inversion between the upper-limit discharge temperature and the lower-limit discharge temperature nor the proximity of the upper-limit discharge temperature and the lower-limit discharge temperature occurs, but such inversion and proximity occur in the operating state where the load of the compressor 11 is relatively small as in the state 1 and the state 2 shown in Fig. 7. In this regard, in this embodiment, as the load of the compressor 11 becomes smaller, a larger temperature correction value (in this example, 30°C) is added to the virtual discharge temperature to increase the upper-limit discharge temperature, thereby preventing the inversion between the upper-limit discharge temperature and the lower-limit discharge temperature.
Further, setting a plurality of correction values of the upper-limit discharge temperature in accordance with the magnitude of the load of the compressor 11 makes it possible to achieve subcooling control suitable for the operating state of the compressor 11, which changes from time to time.
The storage section 25 stores the target subcooling table (Fig. 4), the correction values of the upper-limit discharge temperature (Fig. 6), and the virtual discharge temperature, as well as the lower-limit discharge temperature. The upper-limit discharge temperature setting section 23 sets the upper-limit discharge temperature such that the upper-limit discharge temperature is higher than the lower-limit discharge temperature over the entire load region of the compressor 11.

(Discharge Temperature Determination Section)

The discharge temperature determination section 24 compares the temperature of the refrigerant discharged from the compressor 11 (discharge temperature) with the lower-limit discharge temperature and upper-limit discharge temperature stored in the storage section 25, and determines whether or not the discharge temperature is equal to or larger than the lower-limit discharge temperature and less than the upper-limit discharge temperature.
When the discharge temperature is less than the lower-limit discharge temperature, the control device 20 reduces the opening degree of the electronic expansion valve 14 such that the discharge temperature is equal to or larger than the lower-limit discharge temperature. Further, when the discharge temperature is equal to or larger than the upper-limit discharge temperature, the control device 20 increases the opening degree of the electronic expansion valve 14 such that the discharge temperature is less than the upper-limit discharge temperature. Furthermore, when the discharge temperature is equal to or larger than the lower-limit discharge temperature and less than the upper-limit discharge temperature, the control device 20 controls the opening degree of the electronic expansion valve 14 such that the subcooling is the target subcooling.

[Operation of Heat Pump Device]

Subsequently, a specific control procedure executed in the control device 20 will be described together with the operation of the heat pump device 100. Fig. 8 is a flowchart showing an example of a processing procedure executed in the control device 20.
First, the control device 20 starts the operation of the hot water pump 19 to circulate water (hot water) through the hot water circuit 96 and switches the four-way valve 12 to the first state shown in Fig. 1. The control device 20 then operates the heat pump device 100 by determining the rotation speed of the compressor 11 for activation such that the detection value of the hot water temperature sensor 35 reaches a target hot water temperature set in advance (ST101).
When a predetermined time has elapsed until stabilization of the refrigerant circulation in the refrigerant circuit 10, the control device 20 acquires the pressure of the refrigerant discharged from the compressor 11 (discharge pressure) and the pressure of the refrigerant suctioned into the compressor (suction pressure) (ST102). The discharge pressure is acquired from the discharge pressure sensor 32, and the suction pressure is acquired from the suction pressure sensor 34.
The control device 20 (load detection section 22) calculates the magnitude of the load (differential pressure) of the compressor 11 on the basis of the acquired discharge pressure and suction pressure, and determines whether or not it is equal to or larger than a predetermined threshold set in advance (ST103).
The predetermined threshold is the pressure value (differential pressure between the discharge pressure and the suction pressure) for determining whether or not a predetermined correction value (Fig. 6) should be added to the upper-limit discharge temperature, which is 2.3 MPa (during rise of the differential pressure) or 2.1 MPa (during drop of the differential pressure) in the example of Fig. 6. When the differential pressure is less than the above predetermined threshold (No in ST103), the control device 20 (upper-limit discharge temperature setting section 23) corrects the upper-limit discharge temperature (ST104).
The upper-limit discharge temperature is corrected by adding the correction value of the upper-limit discharge temperature to a virtual discharge temperature. A temperature value set in advance in a stepwise manner in accordance with the magnitude of the differential pressure between the discharge pressure and the suction pressure as shown in Fig. 6 is selected for the correction value of the upper-limit discharge temperature. Thus, as the magnitude of the load of the compressor 11 (differential pressure) becomes smaller, a higher upper-limit discharge temperature is set for the virtual discharge temperature.
Note that if the magnitude of the load of the compressor 11 is equal to or larger than a predetermined threshold (Yes in ST103), the virtual discharge temperature stored in advance in the storage section 25 is still used as the upper-limit discharge temperature without correcting the upper-limit discharge temperature.
Subsequently, the control device 20 (discharge temperature determination section 24) determines whether or not the discharge temperature is less than the lower-limit discharge temperature (ST105). The discharge temperature is acquired from the discharge temperature sensor 31. If the discharge temperature is less than the lower-limit discharge temperature (Yes in ST105), the control device 20 reduces the opening degree of the electronic expansion valve 14 such that the discharge temperature is equal to or larger than the lower-limit discharge temperature (ST106), and the processing is returned to ST102. If the discharge temperature is equal to or larger than the lower-limit discharge temperature (No in ST105), the processing moves to ST107 without controlling the opening degree of the electronic expansion valve 14.
Subsequently, the control device 20 determines whether or not the discharge temperature is equal to or larger than the upper-limit discharge temperature (ST107). If the discharge temperature is equal to or larger than the upper-limit discharge temperature (Yes in ST107), the control device 20 increases the opening degree of the electronic expansion valve 14 such that the discharge temperature is less than the upper-limit discharge temperature (ST108), and the processing is returned to ST102. If the discharge temperature is less than the upper-limit discharge temperature (No in ST107), the processing moves to ST109 without controlling the opening degree of the electronic expansion valve 14.
Through ST105 to 108, the opening degree of the electronic expansion valve 14 is controlled such that the discharge temperature is equal to or larger than the lower-limit discharge temperature and less than the upper-limit discharge temperature. In this state, the control device 20 executes the subcooling control (ST109). Note that the subcooling control in ST109 is the above-mentioned control based on the target subcooling and the current subcooling.
In other words, the control device 20 (target subcooling extraction section 21) extracts the target subcooling from the target subcooling table (Fig. 4) stored in the storage section 25 on the basis of the rotation speed of the compressor 11 and the condensation pressure of the refrigerant in the use-side heat exchanger 13. The condensation pressure can be calculated, for example, on the basis of the detection value of the discharge pressure sensor 32.
Subsequently, the control device 20 calculates the current subcooling on the basis of the detection values of the discharge pressure sensor 32 and the refrigerant temperature sensor 33, compares the calculated subcooling with the target subcooling extracted from the target subcooling table (Fig. 4), and adjusts the opening degree of the electronic expansion valve 14 on the basis of this difference. In other words, the control device 20 subtracts the target subcooling from the current subcooling, and when the subtraction result is positive, the control device 20 controls the opening degree of the electronic expansion valve 14 to be increased to correspond to the value of the subtraction result, and when the subtraction result is negative, the control device 20 controls the opening degree of the electronic expansion valve 14 to be reduced to correspond to the value of the subtraction result. By such control performed in this manner, the current subcooling is controlled to have a target subcooling value constantly, resulting in maintaining the COP at a high level.
The control device 20 repeatedly executes the processing of ST102 to 109 in a predetermined cycle, so that the subcooling control is executed in the state in which the discharge temperature is maintained to be equal to or larger than a predetermined lower-limit discharge temperature and less than a predetermined upper-limit discharge temperature.
According to this embodiment, the discharge temperature can be maintained to be equal to or larger than the lower-limit discharge temperature, so that it is possible to avoid suctioning an excessively wet refrigerant vapor into the compressor 11. Further, since the upper-limit discharge temperature is set in accordance with the magnitude of the load of the compressor 11, and the discharge temperature is maintained to be less than the upper-limit discharge temperature, it is possible to avoid an excessive increase in the discharge temperature. This allows the desired subcooling control to be performed stably, thus ensuring the reliability of the compressor 11 and maintaining the COP of the heat pump device 100 at a high level.
Further, according to this embodiment, since the upper-limit discharge temperature is set on the basis of the virtual discharge temperature, which is calculated on the basis of at least the discharge pressure and the suction pressure, the upper-limit discharge temperature can be determined with reference to the discharge temperature when adjusted to an ideal refrigeration cycle. This makes it possible to set the discharge temperature to an appropriate upper limit value corresponding to the magnitude of the load of the compressor 11.
Further, according to this embodiment, since the upper-limit discharge temperature is set to be larger than the lower-limit discharge temperature over the entire load region of the compressor 11, even if a refrigerant that easily causes inversion between the upper-limit discharge temperature and the lower-limit discharge temperature is used, the target subcooling control can be stably executed without causing a control failure due to that inversion.
Further, according to this embodiment, since the upper-limit discharge temperature is set by adding the correction value to the virtual discharge temperature, the upper-limit discharge temperature can be appropriately set even if the magnitude of the load of the compressor 11 changes from time to time.
Further, according to this embodiment, since the correction value becomes larger as the load of the compressor 11 becomes lower, the upper-limit discharge temperature can be appropriately set in accordance with the load of the compressor 11.
Further, according to this embodiment, since the correction value described above is selected from a plurality of temperature values on the basis of the current magnitude of the load of the compressor 11, it is possible to simplify the control and to reduce system construction costs as compared to the case where the correction value is calculated by complicated calculation processing.
Furthermore, according to this embodiment, it is determined that, as the differential pressure that is a difference between the discharge pressure and the suction pressure becomes larger, the load of the compressor 11 becomes larger. Thus, an existing sensor can be used to determine the magnitude of the load of the compressor 11 without separately using parameters required to determine the load of the compressor 11.
Hereinabove, the embodiment of the present invention has been described, but the present invention is not limited to the embodiment described above and can be variously modified as a matter of course.
For example, in the embodiment described above, the differential pressure between the discharge pressure and the suction pressure has been used to determine the magnitude of the load of the compressor 11, but the present invention is not limited thereto. For example, the magnitude of the load of the compressor 11 may be determined with reference to only the discharge pressure or discharge temperature. Furthermore, the magnitude of the load of the compressor 11 may be determined with reference to the rotation speed of the compressor 11, the current value of the motor that drives the compressor 11, or the like.
Further, in the embodiment described above, the upper-limit discharge temperature is changed in accordance with the magnitude of the load of the compressor 11 so as to avoid a control failure caused by the inversion between the upper-limit discharge temperature and the lower-limit discharge temperature, but the present invention is not limited thereto. For example, in the low load state of the compressor 11 where the above inversion may occur, the upper-limit discharge temperature may be ignored to cease to perform the processing of ST107 and ST108 of Fig. 8 or may be set a lower limit value for the upper-limit discharge temperature to obtain an upper-limit discharge temperature that does not cause the above inversion even in such a low load state.
Further, in the embodiment described above, the discharge pressure sensor 32 for detecting the discharge pressure is installed on the pipe 91 between the discharge port of the compressor 11 and the four-way valve 12, but the location of the discharge pressure sensor 32 to be installed is not particularly limited as long as it is between the discharge port of the compressor 11 and the electronic expansion valve 14. Further, a heat exchange temperature sensor may also be installed on the use-side heat exchanger 13 to calculate a discharge pressure on the basis of its detection value (condensation temperature).
Similarly, the suction pressure sensor 34 for detecting the suction pressure is installed on the pipe 95 between the four-way valve 12 and the suction port of the compressor 11, but the location of the suction pressure sensor 34 to be installed is not particularly limited as long as it is between the electronic expansion valve 14 and the suction port of the compressor 11. Further, a heat exchange temperature sensor may also be installed on the heat source-side heat exchanger 15 to calculate a suction pressure on the basis of its detection value (evaporation temperature).
Further, in the embodiment described above, the hot-water heater (underfloor heating device) has been exemplified as the heat pump device 100, but the present invention is not limited thereto and is applicable to a heat-pump hot-water supply device or the like.

Reference Signs List

10: refrigerant circuit
11: compressor
12: four-way valve
13: use-side heat exchanger
14: electronic expansion valve
15: heat source-side heat exchanger
20: control device
21: target subcooling extraction section
22: load detection section
23: upper-limit discharge temperature setting section
24: discharge temperature determination section
25: storage section
31: discharge temperature sensor
32: discharge pressure sensor
33: refrigerant temperature sensor
34: suction pressure sensor
100: heat pump device

Claims

A heat pump device, comprising:
a refrigerant circuit in which a compressor, a use-side heat exchanger that exchanges heat between water and a refrigerant, an electronic expansion valve, and a heat source-side heat exchanger are connected to each other via piping;

discharge temperature detection means for detecting a discharge temperature, the discharge temperature being a temperature of the refrigerant discharged from the compressor;

subcooling detection means for detecting a subcooling of the refrigerant that flows out of the use-side heat exchanger;

load detection means for detecting a magnitude of a load of the compressor; and

control means for controlling a rotation speed of the compressor and an opening degree of the electronic expansion valve, wherein

the control means
controls the electronic expansion valve on a basis of a predetermined upper-limit discharge temperature and a predetermined lower-limit discharge temperature,

sets the upper-limit discharge temperature in accordance with the magnitude of the load of the compressor,

reduces the opening degree of the electronic expansion valve such that the discharge temperature is equal to or larger than the lower-limit discharge temperature when the discharge temperature is less than the lower-limit discharge temperature,

increases the opening degree of the electronic expansion valve such that the discharge temperature is less than the upper-limit discharge temperature when the discharge temperature is equal to or larger than the upper-limit discharge temperature, and

controls the opening degree of the electronic expansion valve such that the subcooling is a target subcooling when the discharge temperature is equal to or larger than the lower-limit discharge temperature and less than the upper-limit discharge temperature.
The heat pump device according to claim 1, wherein
the load detection means includes
a discharge pressure detection section that detects a discharge pressure, the discharge pressure being a pressure of the refrigerant discharged from the compressor, and

a suction pressure detection section that detects a suction pressure, the suction pressure being a pressure of the refrigerant suctioned into the compressor, and

the control means sets the upper-limit discharge temperature on a basis of a virtual discharge temperature that is a discharge temperature when the refrigerant suctioned into the compressor is assumed as saturated vapor, the virtual discharge temperature being calculated on a basis of at least the discharge pressure and the suction pressure.
The heat pump device according to claim 2, wherein
the lower-limit discharge temperature is set in advance on a basis of a predetermined minimum superheat determined for the compressor, and

the control means sets the upper-limit discharge temperature such that the upper-limit discharge temperature is higher than the lower-limit discharge temperature over an entire load region of the compressor.
The heat pump device according to claim 3, wherein
the control means sets the upper-limit discharge temperature by adding a correction value to the virtual discharge temperature.
The heat pump device according to claim 4, wherein
the control means increases the correction value as the load of the compressor becomes lower.
The heat pump device according to claim 5, further comprising
a storage section that stores a plurality of temperature values set in advance in accordance with the magnitude of the load of the compressor, wherein

the control means selects the correction value from the plurality of temperature values on a basis of a current magnitude of the load of the compressor.
The heat pump device according to claim 1, wherein
the load detection means includes
a discharge pressure detection section that detects a discharge pressure, the discharge pressure being a pressure of the refrigerant discharged from the compressor, and

a suction pressure detection section that detects a suction pressure, the suction pressure being a pressure of the refrigerant suctioned into the compressor, and

the control means determines that the load of the compressor becomes larger as a differential pressure that is a difference between the discharge pressure and the suction pressure becomes larger.
The heat pump device according to any one of claims 1 to 6, wherein
the refrigerant that is sealed in the refrigerant circuit is a refrigerant in which a specific heat ratio in saturated vapor at 10°C is less than 1.25.