EP4589214A1

EP4589214A1 - Two-stage cascade refrigeration cycle device, and two-stage cascade refrigeration cycle device control method

Info

Publication number: EP4589214A1
Application number: EP23865489.1A
Authority: EP
Inventors: Akinobu ODA; Yoshinari MAEMA; Shohei NAKATA; Kazuki KANEI
Original assignee: Fujitsu General Ltd
Current assignee: Fujitsu General Ltd
Priority date: 2022-09-13
Filing date: 2023-09-11
Publication date: 2025-07-23
Also published as: AU2023341653A1; JP7459907B2; CN119866422A; JP2024040636A; WO2024058136A1

Abstract

In a case where a refrigerating cycle circuit which employs a multistage refrigerating circuit and which includes a heat accumulator is employed, the present invention can secure a sufficient amount of a refrigerant in the refrigerating cycle circuit during the operation restrain a decrease in heating capacity, and maintain heat accumulation capacity without spoiling comfortability. A cascade refrigeration cycle system includes: a high-order side refrigerant circuit (1) through which a high-order side refrigerant circulates; a low-order side refrigerant circuit (2) through which a low-order side refrigerant circulates; a high-order side heat accumulation circuit (3) including a heat accumulation heat exchanger (H) provided in parallel with a cascade heat exchanger (C) through which the high-order side refrigerant circulates; a low-order side heat accumulation circuit (4) including the heat accumulation heat exchanger (H) through which the low-order side refrigerant circulates; a refrigerant temperature detecting unit (5) configured to measure or calculate a saturation temperature of the low-order side refrigerant discharged from a low-order side compressor (21); a heat accumulation temperature sensor (HS) configured to measure the temperature of a heat accumulation material; and a control unit (6). At a time of performing a heating operation, the control unit (6) acquires information on the saturation temperature and information on the temperature of the heat accumulation material and controls the low-order side compressor (21) such that a temperature difference between the saturation temperature and the temperature of the heat accumulation material becomes a first predetermined value.

Description

Technical Field

Embodiments of the present invention relate to a cascade refrigeration cycle system and a cascade refrigeration cycle system control method.

Background Art

In recent years, for reduction in CO2 emissions, it has been considered to replace a conventional combustion-type water heating air-conditioner with a heat pump, for example. A heat pump is generally constituted by an indoor unit installed in a room and an outdoor unit installed outdoors, and the heat pump adjusts the temperature or the humidity in the room by circulating a refrigerant such as water through a refrigerating circuit formed by coupling the indoor unit to the outdoor unit.
It is considered that the replacement of the combustion-type water heating air-conditioner with the heat pump faces a problem "to supply water at high temperature" and "to secure comfortability in a defrosting operation." That is, in terms of the former problem, the temperature of water to be supplied from the heat pump tends to be low in comparison with the combustion-type water heating air-conditioner. Accordingly, in order to achieve heating capacity at an equivalent level to the combustion-type water heating air-conditioner, the heat pump requires a configuration to increase the temperature of water to be supplied.
As a method to secure a higher temperature of water to be supplied in a case where the heat pump is used, a heat pump system in which a low-stage cycle and a high-stage cycle are connected via a cascade condenser is proposed as described in PTL 1, for example.
The cascade condenser works as a condenser in the low-stage cycle and works as an evaporator in the high-stage cycle. By combining refrigerating cycles at multiple stages as such, COP can be improved in comparison with only a single-stage refrigerating cycle, and therefore, hot water at high temperature can be more efficiently supplied.
In the meantime, in terms of the defrosting operation as the latter problem, generally, at the time when a heating operation is performed in an air conditioner, a low-temperature refrigerant flows through an outdoor heat exchanger. In a case where the refrigerant absorbs heat from external air, it is necessary for the refrigerant to be colder than the external air. However, for example, in a case where the outdoor temperature is below the freezing point, when the temperature of the refrigerant becomes lower than the dew point temperature of the external air, frost is attached to the outdoor heat exchanger, and it becomes hard to perform heat-exchange with the external air. Accordingly, in a case where the heating operation is performed, it is necessary to perform the defrosting operation regularly.
The defrosting operation is a necessary operation to operate the air conditioner as such. However, while the defrosting operation is performed, the heating operation stops. The defrosting operation itself is an unnecessary operation for the combustion-type water heating air-conditioner, and besides, the heating operation stops while the defrosting operation is performed as such. This results in a gradual decrease in temperature in the room, which may spoil comfortability.
In view of this, PTL 2 discloses an invention in which the heating operation is continued even during the defrosting operation to prevent a decrease in temperature in the room which decrease is caused due to the heating operation stopping temporarily, thereby making it possible to maintain comfortability.
In the invention disclosed in PTL 2, a heat accumulator is placed in a refrigerating circuit separately from an indoor heat exchanger. In the defrosting operation, heat accumulated in the heat accumulator is used to avoid the heating operation from stopping.

Citation List

Patent Literature

PTL 1: JP 2013-113534 A
PTL 2: WO 2015/128980 A1

Summary of Invention

Technical Problem

From PTL 1 and PTL 2, it is considered that a refrigerating cycle circuit which employs refrigerating circuits at multiple stages and which includes a heat accumulator can be used, for example. With such a circuit, it is possible to supply warmer air to the room by supplying high-temperature water, and it is not necessary to stop the heating operation even in a case where the defrosting operation is performed. Accordingly, it is considered that an air conditioner that does not spoil comfortability can be provided.
However, in a case where such a refrigerating circuit which employs refrigerating circuits at multiple stages and which includes a heat accumulator is used, the following problem may occur. That is, in a case of a multistage refrigerating circuit as described in PTL 1, generally, an operation is performed such that a difference in pressure on the high-stage side and a difference in pressure on the low-stage side become appropriate to improve COP.
In a case where such a refrigerating circuit includes a heat accumulator as disclosed in PTL 2, a refrigerant having the same temperature, that is, a high-temperature refrigerant is supplied to a cascade condenser and the heat accumulator. In the heat accumulator, a gas-phase refrigerant flowing therein from a compressor exchanges heat and condenses, but as the temperature of the heat accumulator becomes lower than the temperature of the refrigerant, the amount of a liquid-phase refrigerant staying in the heat accumulator increases.
The gas-phase refrigerant condenses in the heat accumulator and is turned into a liquid-phase refrigerant. Part of the liquid-phase refrigerant flows into an expansion valve, but the other part of the refrigerant remains in the heat accumulator. As the temperature of the heat accumulator becomes low relative to the temperature of the refrigerant, the refrigerant is cooled down more excessively in the heat accumulator, so that the amount of the refrigerant remaining in the heat accumulator increases. When such a state occurs, the amount of the refrigerant circulating through the refrigerating circuit gradually decreases, and the amount of the refrigerant to be sent to the cascade condenser decreases. Because of this, heat exchange capacity between the high-stage cycle and the low-stage cycle in the cascade condenser decreases, and as a result, the performance of the heat pump decreases.
An object of the present invention is to provide a cascade refrigeration cycle system and a cascade refrigeration cycle system control method each of which secures a sufficient amount of a refrigerant in a refrigerating cycle circuit during the operation, restrains a decrease in heating capacity, and does not spoil comfortability even in a case where the refrigerating cycle circuit employs a multistage refrigerating circuit and includes a heat accumulator to supply high-temperature water and to maintain a heating operation during a defrosting operation in the heat accumulator.

Solution to Problem

A cascade refrigeration cycle system according to one aspect of the present invention includes: a high-order side refrigerant circuit configured such that a high-order side compressor, a high-order side heat exchanger, a first high-order side pressure reduction mechanism, and a cascade heat exchanger are sequentially connected via refrigerant pipes to circulate a high-order side refrigerant through the high-order side refrigerant circuit; a low-order side refrigerant circuit configured such that a low-order side compressor, the cascade heat exchanger, a first low-order side pressure reduction mechanism, and a low-order side heat exchanger are sequentially connected via refrigerant pipes to circulate a low-order side refrigerant through the low-order side refrigerant circuit; a high-order side heat accumulation circuit configured such that the high-order side compressor, the high-order side heat exchanger, a second high-order side pressure reduction mechanism, and a heat accumulation heat exchanger provided in parallel with the cascade heat exchanger and including a heat accumulation material are sequentially connected via refrigerant pipes to circulate the high-order side refrigerant through the high-order side heat accumulation circuit; a low-order side heat accumulation circuit configured such that the low-order side compressor, the heat accumulation heat exchanger, a second low-order side pressure reduction mechanism, and the low-order side heat exchanger are sequentially connected via refrigerant pipes to circulate the low-order side refrigerant through the low-order side heat accumulation circuit; a refrigerant temperature detecting unit configured to measure or calculate a saturation temperature of the low-order side refrigerant discharged from the low-order side compressor; a heat accumulation temperature sensor configured to measure a temperature of the heat accumulation material provided in the heat accumulation heat exchanger; and a control unit configured to control the high-order side compressor, the low-order side compressor, the first high-order side pressure reduction mechanism, the second high-order side pressure reduction mechanism, the first low-order side pressure reduction mechanism, and the second low-order side pressure reduction mechanism. The high-order side refrigerant and the low-order side refrigerant performs heat-exchange in the cascade heat exchanger or the heat accumulation heat exchanger. At a time of performing a heating operation, the control unit acquires the saturation temperature and the temperature of the heat accumulation material and controls the low-order side compressor such that a temperature difference between the saturation temperature and the temperature of the heat accumulation material becomes a first predetermined value.
A cascade refrigeration cycle system control method according to one aspect of the present invention is a control method for controlling a cascade refrigeration cycle system including: a high-order side refrigerant circuit configured such that a high-order side compressor, a high-order side heat exchanger, a first high-order side pressure reduction mechanism, and a cascade heat exchanger are sequentially connected via refrigerant pipes to circulate a high-order side refrigerant through the high-order side refrigerant circuit; a low-order side refrigerant circuit configured such that a low-order side compressor, the cascade heat exchanger, a first low-order side pressure reduction mechanism, and a low-order side heat exchanger are sequentially connected via refrigerant pipes to circulate a low-order side refrigerant through the low-order side refrigerant circuit; a high-order side heat accumulation circuit configured such that the high-order side compressor, the high-order side heat exchanger, a second high-order side pressure reduction mechanism, and a heat accumulation heat exchanger provided in parallel with the cascade heat exchanger and including a heat accumulation material are sequentially connected via refrigerant pipes to circulate the high-order side refrigerant through the high-order side heat accumulation circuit; a low-order side heat accumulation circuit configured such that the low-order side compressor, the heat accumulation heat exchanger, a second low-order side pressure reduction mechanism, and the low-order side heat exchanger are sequentially connected via refrigerant pipes to circulate the low-order side refrigerant through the low-order side heat accumulation circuit; a refrigerant temperature detecting unit configured to measure or calculate a saturation temperature of the low-order side refrigerant discharged from the low-order side compressor; a heat accumulation temperature sensor configured to measure a temperature of the heat accumulation material provided in the heat accumulation heat exchanger; and a control unit configured to control the high-order side compressor, the low-order side compressor, the first high-order side pressure reduction mechanism, the second high-order side pressure reduction mechanism, the first low-order side pressure reduction mechanism, and the second low-order side pressure reduction mechanism. The control method includes: the control unit acquiring the saturation temperature and the temperature of the heat accumulation material at a time of performing a heating operation; and controlling the low-order side compressor such that a temperature difference between the saturation temperature and the temperature of the heat accumulation material becomes a first predetermined value.

Advantageous Effects of Invention

With the present invention, even in a case where a refrigerating cycle circuit which employs a multistage refrigerating circuit and which includes a heat accumulator is employed to supply high-temperature water and to maintain a heating operation during a defrosting operation in the heat accumulator, it is possible to secure a sufficient amount of a refrigerant in the refrigerating cycle circuit during the operation and to restrain a decrease in heating capacity.

Brief Description of Drawings

FIG. 1 is a refrigerant circuit diagram of a cascade refrigeration cycle system according to an embodiment of the present invention;
FIG. 2 is an explanatory drawing illustrating the concept of a control method of a heating and heat-accumulation operation by a control unit in the cascade refrigeration cycle system according to the embodiment of the present invention;
FIG. 3 is a refrigerant circuit diagram illustrating the flow of a refrigerant at the time when the cascade refrigeration cycle system according to the embodiment of the present invention performs the heating and heat-accumulation operation;
FIG. 4 is a refrigerant circuit diagram illustrating the flow of the refrigerant at the time when the cascade refrigeration cycle system according to the embodiment of the present invention performs a defrosting operation;
FIG. 5 is a flowchart illustrating a control flow at the time when a heating and defrosting operation is performed in the cascade refrigeration cycle system according to the embodiment of the present invention;
FIG. 6 is a flowchart illustrating a control flow at the time when the heating and heat-accumulation operation is performed in the cascade refrigeration cycle system according to the embodiment of the present invention; and
FIG. 7 is a flowchart illustrating a control flow at the time when the defrosting operation is performed in the cascade refrigeration cycle system according to the embodiment of the present invention.

Description of Embodiments

A structure of a cascade refrigeration cycle system S according to an embodiment of the present invention will be described with reference to FIG. 1. FIG. 1 is a refrigerant circuit diagram of the cascade refrigeration cycle system S according to the embodiment of the present invention. The cascade refrigeration cycle system S is used for a cooling operation in a case where a high-order side heat exchanger 12 (described later) is used as an evaporator. The cascade refrigeration cycle system S is a refrigeration cycle device that can be used for an operation to make hot water or a heating operation in a case where the high-order side heat exchanger 12 is used as a condenser.
Note that, hereinafter, an operation to make hot water and an operation to provide warm air to an indoor space may be collectively referred to as a "heating operation." The embodiment of the present invention deals with a case where the cascade refrigeration cycle system S is used for the heating operation, a heat accumulation operation to accumulate heat in a heat accumulation heat exchanger H (described later), and a defrosting operation on a low-order side heat exchanger 23.
The cascade refrigeration cycle system S according to the embodiment of the present invention as illustrated in FIG. 1 includes a binary refrigeration cycle in which respective refrigerant circuits are provided on a high-order side and on a low-order side. A high-order side refrigerant circuit 1 is provided on the high-order side includes, and a low-order side refrigerant circuit 2 is provided on the low-order side.
The high-order side refrigerant circuit 1 is configured such that a high-order side compressor 11, a high-order side heat exchanger 12, a first high-order side pressure reduction mechanism 13, and a cascade heat exchanger C are sequentially connected via refrigerant pipes P to circulate a high-order side refrigerant through the high-order side refrigerant circuit 1. The high-order side heat exchanger 12 is disposed inside an indoor unit installed in the indoor space and generates hot water by performing heat-exchange between the high-order side refrigerant and water circulating through a water supply path (not illustrated). Alternatively, the high-order side heat exchanger 12 supplies, to the indoor space, air warmed by heat-exchange performed between the high-order side refrigerant and air flowing into the indoor unit. The indoor unit also includes a room temperature sensor configured to measure the room temperature of the indoor space.
In the meantime, the low-order side refrigerant circuit 2 is configured such that a low-order side compressor 21, the cascade heat exchanger C, a first low-order side pressure reduction mechanism 22, and a low-order side heat exchanger 23 are sequentially connected via refrigerant pipes P to circulate a low-order side refrigerant through the low-order side refrigerant circuit 2. The low-order side heat exchanger 23 performs heat-exchange between the low-order side refrigerant and external air.
Here, the cascade heat exchanger C is a heat exchanger configured to perform heat-exchange between the high-order side refrigerant and the low-order side refrigerant. For example, in a case where the cascade refrigeration cycle system S performs the heating operation, the cascade heat exchanger C works as an evaporator for the high-order side refrigerant.
The cascade refrigeration cycle system S according to the embodiment of the present invention includes a high-order side heat accumulation circuit 3 and a low-order side heat accumulation circuit 4 for the high-order side and the low-order side, respectively.
The high-order side heat accumulation circuit 3 is configured such that the high-order side compressor 11, the high-order side heat exchanger 12, a second high-order side pressure reduction mechanism 31, and the heat accumulation heat exchanger H are sequentially connected via refrigerant pipes P to circulate the high-order side refrigerant through the high-order side heat accumulation circuit 3. In the meantime, the low-order side heat accumulation circuit 4 is configured such that the low-order side compressor 21, the heat accumulation heat exchanger H, a second low-order side pressure reduction mechanism 41, and the low-order side heat exchanger 23 are sequentially connected via refrigerant pipes P to circulate the low-order side refrigerant through the low-order side heat accumulation circuit 4.
The heat accumulation heat exchanger H is a heat exchanger configured to perform heat-exchange between a heat accumulation material filled in the heat accumulation heat exchanger H and the high-order side refrigerant or the low-order side refrigerant. The heat accumulation heat exchanger H includes a heat accumulation tank. The heat accumulation tank includes, for example, a high-order side heat exchanger including a flow path through which the high-order side refrigerant flows, the flow path being formed in a meandering manner on the same surface, and a low-order side heat exchanger including a flow path through which the low-order side refrigerant flows, the flow path being formed in a meandering manner on the same surface, and the high-order side heat exchanger and the low-order heat exchanger are disposed at positions where their respective surfaces face each other. The high-order side refrigerant flows in and flows out from the high-order side heat exchanger, and the low-order side refrigerant flows in and flows out from the low-order side heat exchanger, so that the heat accumulation material exchanges heat with the high-order side refrigerant or the low-order side refrigerant.
As such, the high-order side heat exchanger and the low-order side heat exchanger are disposed in the heat accumulation tank of the heat accumulation heat exchanger H, and the heat accumulation material is filled in the heat accumulation heat exchanger H to surround the high-order side heat exchanger and the low-order side heat exchanger. Heat supplied to the heat accumulation heat exchanger H is accumulated in the heat accumulation material, and the heat thus accumulated is used for the heating operation on the high-order side and is used for the defrosting operation on the low-order side, as will be described later. In addition, a heat accumulation temperature sensor HS (described later) is provided at a position where the heat accumulation temperature sensor HS can measure the temperature of the heat accumulation material.
The heat accumulation heat exchanger H is disposed in parallel with the cascade heat exchanger C as illustrated in FIG. 1, so that the high-order side heat accumulation circuit 3 and the low-order side heat accumulation circuit 4 are formed.
In the high-order side refrigerant circuit 1, a high-order side four-way valve 14 is connected to a discharge side of the high-order side compressor 11, and the high-order side four-way valve 14 is configured to switch between a state where the high-order side refrigerant discharged from the high-order side compressor 11 flows toward the high-order side heat exchanger 12 side and a state where the high-order side refrigerant flows toward the cascade heat exchanger C side.
Similarly, in the low-order side refrigerant circuit 2, a low-order side four-way valve 24 is connected to a discharge side of the low-order side compressor 21, and the low-order side four-way valve 24 is configured to switch between a state where the low-order side refrigerant discharged from the low-order side compressor 21 flows toward the cascade heat exchanger C side and a state where the low-order side refrigerant flows toward the low-order side heat exchanger 23 side.
The cascade refrigeration cycle system S includes a refrigerant temperature detecting unit 5 configured to measure or calculate a saturation temperature of the low-order side refrigerant circulating through the low-order side refrigerant circuit 2 or the low-order side heat accumulation circuit 4 and discharged from the low-order side compressor 21. The low-order side refrigerant circuit 2 includes a pressure sensor 51 configured to measure a discharge pressure of the low-order side refrigerant discharged out from the low-order side compressor 21, and a refrigerant temperature sensor 52 configured to measure the temperature of the low-order side refrigerant.
The refrigerant temperature detecting unit 5 calculates the saturation temperature of the low-order side refrigerant by use of a pressure value measured by the pressure sensor 51. The saturation temperature, that is, the temperature of saturated steam is determined depending on a refrigerant type and can be calculated from a high pressure that is a detection value from the pressure sensor 51. A program to calculate the saturation temperature of the low-order side refrigerant may be stored in the refrigerant temperature detecting unit 5 or may be stored in a storage unit (not illustrated) provided separately. Information on the saturation temperature of the low-order side refrigerant which saturation temperature is calculated by the refrigerant temperature detecting unit 5 is transmitted to a control unit 6 (described later).
The heat accumulation heat exchanger H includes the heat accumulation temperature sensor HS as described above, and the heat accumulation temperature sensor HS measures the temperature of the heat accumulation material provided in the heat accumulation heat exchanger H. In other words, the heat accumulation temperature sensor HS is a sensor configured to measure the amount of heat accumulated in the heat accumulation heat exchanger H. Information on the temperature of the heat accumulation material which temperature is measured by the heat accumulation temperature sensor HS is transmitted to the control unit 6.
The control unit 6 controls respective openings of the high-order side compressor 11, the first high-order side pressure reduction mechanism 13, the second high-order side pressure reduction mechanism 31, the low-order side compressor 21, the first low-order side pressure reduction mechanism 22, and the second low-order side pressure reduction mechanism 41.
Particularly, when the control unit 6 controls the opening of each pressure reduction mechanism, the flow rate of the high-order side refrigerant or the low-order side refrigerant flowing through a corresponding one of the high-order side refrigerant circuit 1, the low-order side refrigerant circuit 2, the high-order side heat accumulation circuit 3, and the low-order side heat accumulation circuit 4 can be adjusted.
At the time of a heating and heat-accumulation operation to perform the heating operation and the heat accumulation operation at the same time, the control unit 6 controls the low-order side compressor 21 by use of information on the saturation temperature calculated by the refrigerant temperature detecting unit 5 and information on the temperature of the heat accumulation material which temperature is measured by the heat accumulation temperature sensor HS. The control unit 6 also performs a control at the time when the defrosting operation on the low-order side heat exchanger 23 is performed.
In view of this, next will be described further details of the controls performed by the control unit 6. FIG. 2 is an explanatory drawing illustrating the concept of a control method performed by the control unit 6 right after the start of the heating and heat-accumulation operation, in the cascade refrigeration cycle system S according to the embodiment of the present invention. Note that the heating and heat-accumulation operation is an operation to perform a heating operation with the high-order side heat exchanger 12 being used as a condenser and to accumulate heat in the heat accumulation heat exchanger H by releasing heat from a refrigerant to the heat accumulation material.
In the explanatory drawing illustrated in FIG. 2, the horizontal axis indicates time (t), and the vertical axis indicates temperature (T). First of all, a dotted line drawn laterally denotes a preset temperature in the room. That is, the dotted line denotes a preset temperature, and with reference to the temperature (T) in the vertical axis, the dotted line is constant. Accordingly, the dotted line is drawn linearly to be parallel with the horizontal axis.
Curves indicative of a plurality of temperatures are also drawn from left to right along the passage of time, and a curve indicated by a solid line among those curves denotes a room temperature. The measurement of the room temperature is performed by the room temperature sensor provided for the indoor unit, and a measurement result is transmitted to the control unit 6 as described above.
The cascade refrigeration cycle system S is controlled by the control unit 6 such that the room temperature becomes the preset temperature or is maintained at the preset temperature. Accordingly, the room temperature drawn in the solid line first exhibits a movement to increase rapidly. This is because the cascade refrigeration cycle system S is controlled such that the room temperature reaches the preset temperature quickly.
When the room temperature reaches the preset temperature once, heating in the room is performed such that the room temperature is maintained at the preset temperature after that. Accordingly, as illustrated in FIG. 2, the room temperature becomes lower or higher than the preset temperature around a temperature near the preset temperature such that the difference between the room temperature and the preset temperature gradually becomes small.
Then, a broken line starting from generally the same temperature as an initial temperature of the room temperature indicated by the solid line denotes the temperature of the heat accumulation material provided in the heat accumulation heat exchanger H. The temperature of the heat accumulation material is a temperature measured by the heat accumulation temperature sensor HS as described above.
In FIG. 2, the temperature of the heat accumulation material is illustrated in a curve exhibiting that the temperature gradually increases, exceeds the preset temperature at a time point at about half of the horizontal axis indicating time (t) and finally reaches a temperature higher than the room temperature or the preset temperature. Note that the temperature change of the heat accumulation material in FIG. 2 is just an example and can vary depending on the type of the heat accumulation material or various situations where the heat accumulation heat exchanger H is placed.
In the meantime, a temperature which has a starting point slightly higher than that of the broken line indicative of the temperature of the heat accumulation material and which exhibits changes generally similar to an increase in the temperature of the heat accumulation material is a first saturation temperature of the low-order side refrigerant, and the first saturation temperature is a value calculated from the high pressure as a detection value from the pressure sensor 51. The first saturation temperature is illustrated in an alternate long and short dash line curve in FIG. 2. The first saturation temperature of the low-order side refrigerant becomes equivalent to the temperature of the heat accumulation material in the end. This is to prevent the temperature of the heat accumulation material from further exchanging heat after the temperature of the heat accumulation material reaches a setting value.
Here, there are two linear broken lines intersecting with the dotted line indicative of the preset temperature and parallel to the vertical axis. Out of these broken lines, one linear broken line closer to the vertical axis indicative of temperature (T) extends downward from a base point that is a point where the dotted line indicative of the preset temperature first intersects with the solid line indicative of the room temperature, and the one linear broken line intersects with the horizontal axis indicative of time (t). A time at which the one linear broken line intersects with the horizontal axis indicative of time (t) is hereinafter referred to as "time a."
That is, a period (time) from the start of the heating operation by the cascade refrigeration cycle system S to the time a indicates a time taken until the room temperature reaches the preset temperature. After the time a, the heating and heat-accumulation operation of the cascade refrigeration cycle system S is continued so that the room temperature can be maintained at the preset temperature, as described above.
At the other linear broken line farther from the vertical axis indicative of the temperature (T), the temperature of the low-order side refrigerant is equivalent to the temperature of the heat accumulation material. That the temperature of the heat accumulation material is equivalent to the temperature of the low-order side refrigerant indicates that the temperature of the heat accumulation material reaches a predetermined setting value of the heat accumulation material, and heat is sufficiently accumulated in the heat accumulation heat exchanger H. The setting value is set appropriately depending on the heat accumulation material of the heat accumulation heat exchanger H. Accordingly, after a time (hereinafter referred to as "time b" appropriately) at a point where the other linear broken line intersects with the time (t), the temperature of the low-order side refrigerant and the temperature of the heat accumulation material are maintained at the same temperature, so that further heat accumulation is restrained (hereinafter referred to as a heat accumulation operation stop).
Finally, an alternate long and two short dashes line starting from a point where the one linear broken line intersects with the first saturation temperature of the low-order side refrigerant which first saturation temperature is illustrated in the alternate long and short dash line curve indicates a second saturation temperature of the low-order side refrigerant. The second saturation temperature of the low-order side refrigerant exhibits a temperature further higher than the first saturation temperature of the low-order side refrigerant, but the second saturation temperature becomes lower than the temperature of the heat accumulation material at the time b when the heat accumulation operation is stopped, similarly to the first saturation temperature of the low-order side refrigerant.
Note that the first saturation temperature and the second saturation temperature of the low-order side refrigerant denote condensing temperatures of the low-order side refrigerant. The reason why two types of saturation temperatures of the low-order side refrigerant are illustrated in FIG. 2 is as follows.
As described above, in the cascade refrigeration cycle system S according to the embodiment of the present invention, the cascade heat exchanger C and the heat accumulation heat exchanger H are connected in parallel. Accordingly, the low-order side refrigerant discharged out from the low-order side compressor 21 is split as illustrated in FIG. 1 and flows into the cascade heat exchanger C and the heat accumulation heat exchanger H, separately.
For example, in a case where heat is not accumulated so much in the heat accumulation heat exchanger H, and the temperature of the heat accumulation material in the heat accumulation heat exchanger H is not high, when a high-temperature low-order side refrigerant similar to the low-order side refrigerant flowing into the cascade heat exchanger C from the low-order side compressor 21 flows into the heat accumulation heat exchanger H, a gas-phase low-order side refrigerant flowing into the heat accumulation heat exchanger H exchanges heat with the heat accumulation material in the heat accumulation heat exchanger H and condenses. Part of the condensed liquid-phase reduction side refrigerant flows into the second low-order side pressure reduction mechanism 41 on the downstream side, but the other part of the reduction side refrigerant remains in the heat accumulation heat exchanger H. As the temperature of the heat accumulation material is lower than the temperature of the reduction side refrigerant, the reduction side refrigerant is cooled down more excessively in the heat accumulation heat exchanger H, so that the amount of the refrigerant remaining in the heat accumulation heat exchanger H increases.
In such a state, as described above, a large amount of the liquid-phase low-order side refrigerant is distributed in the heat accumulation heat exchanger H, and the amount of the low-order side refrigerant to circulate decreases relatively. The decrease in the amount of the low-order side refrigerant to circulate leads to a decrease in the amount of heat exchange with the high-order side refrigerant in the cascade heat exchanger C, and the heating capacity of the cascade refrigeration cycle system S may decrease.
The reason why a large amount of the liquid-phase low-order side refrigerant is distributed in the heat accumulation heat exchanger H is because a difference between the saturation temperature of the low-order side refrigerant flowing into the heat accumulation heat exchanger H and the temperature of the heat accumulation agent constituting the heat accumulation heat exchanger H is large. A supercooling degree of the low-order side refrigerant becomes larger as the temperature difference is larger. Since a liquid single-phase region in a path of the low-order side refrigerant in the heat accumulation heat exchanger H becomes larger as the supercooling degree is larger, the amount of a liquid-phase refrigerant to accumulate increases.
In view of this, the control unit 6 controls a rotation speed of the low-order side compressor 21 so that the difference between the saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material does not become large. More specifically, the saturation temperature of the low-order side refrigerant is calculated by the refrigerant temperature detecting unit 5 by use of a value of the pressure of the low-order side refrigerant discharged from the low-order side compressor 21 which pressure is measured by the pressure sensor 51, and a temperature of the low-order side refrigerant which temperature is measured by the refrigerant temperature sensor 52.
That is, measurement results from the pressure sensor 51 and the refrigerant temperature sensor 52 are input into the refrigerant temperature detecting unit 5. Then, information on the saturation temperature calculated by the refrigerant temperature detecting unit 5 by use of the input measurement results is transmitted to the control unit 6. In the meantime, the temperature of the heat accumulation material is measured by the heat accumulation temperature sensor HS and input into the control unit 6, as described above.
Hereby, the control unit 6 can acquire the information on the saturation temperature and the information of the temperature of the heat accumulation material. In consideration of the operation condition of the cascade refrigeration cycle system S, the control unit 6 controls the heating and heat-accumulation operation based on the difference between the saturation temperature and the temperature of the heat accumulation material.
In view of this, a control in a situation where the operation of the cascade refrigeration cycle system S is just started will be described first. In this case, it is necessary to warm the indoor space. That is, in the heating and heat-accumulation operation, it is necessary to perform the heating operation in priority to the heat accumulation operation. At this time, the second low-order side pressure reduction mechanism 41 is controlled to an opening at which the flow rate of a refrigerant passing through the second low-order side pressure reduction mechanism 41 is smaller than the flow rate of a refrigerant passing through the first low-order side pressure reduction mechanism 22. More specifically, the second low-order side pressure reduction mechanism 41 is controlled to a slightly open state.
On this account, the control unit 6 controls the heating and heat-accumulation operation such that the difference between the first saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material becomes a "first predetermined value T1" determined in advance. The first predetermined value T1 is determined in advance based on an appropriate value as a temperature difference between them which value is obtained by a test or the like, for example, and is a value from 2°C to 3°C, for example.
The control unit 6 controls the rotation speed of the low-order side compressor 21 such that the determined value as the first predetermined value T1 is maintained. That is, the control unit 6 performs the control such that the first saturation temperature of the low-order side refrigerant does not become too high as compared to the temperature of the heat accumulation material. In other words, the control unit 6 performs the control such that the rotation speed of the low-order side compressor 21 does not increase, so as to bring the first saturation temperature of the low-order side refrigerant close to the temperature of the heat accumulation material in the heat accumulation heat exchanger H.
When such a control is performed, it is possible to reduce such a possibility that the difference between the first saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material increases due to a sudden increase in the first saturation temperature, and the low-order side refrigerant discharged from the low-order side compressor 21 condenses and is turned into a liquid-phase low-order side refrigerant at the time when the low-order side refrigerant flows into the heat accumulation heat exchanger H.
Note that the saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material, which are necessary for the control of the control unit 6, are input into the control unit 6 from the refrigerant temperature detecting unit 5 and the heat accumulation temperature sensor HS as needed. Alternatively, information on the saturation temperature of the low-order side refrigerant and information on the temperature of the heat accumulation material may be transmitted to the control unit 6 from the refrigerant temperature detecting unit 5 and the heat accumulation temperature sensor HS at given timings.
As described above, after the start of the heating and heat-accumulation operation to the time a, the control unit 6 controls the cascade refrigeration cycle system S to preferentially perform the heating operation out of the heating operation and the heat accumulation operation. That is, in the low-order side refrigerant circuit 2, the control unit 6 controls respective openings of the first low-order side pressure reduction mechanism 22 and the second low-order side pressure reduction mechanism 41 such that a more amount of the refrigerant flows into the cascade heat exchanger C than the heat accumulation heat exchanger H. By immediately warming the indoor space as such, it is possible to provide comfortability.
When the room temperature reaches the preset temperature, a heating operation to maintain the room temperature at the preset temperature is demanded after that, in terms of the relationship between the room temperature and the preset temperature. That is, from the time a to the time b, a load in the cascade refrigeration cycle system S during the heating operation becomes smaller than a load from the start of the heating and heat-accumulation operation to the time a, so that surplus energy of the heating capacity of the cascade refrigeration cycle system S can be applied to the heat accumulation operation. At this time, in the low-order side refrigerant circuit 2, the control unit 6 controls respective openings of the first low-order side pressure reduction mechanism 22 and the second low-order side pressure reduction mechanism 41 so that a more amount of the refrigerant flows into the heat accumulation heat exchanger H than the cascade heat exchanger C.
In view of this, the control unit 6 controls the cascade refrigeration cycle system S to perform heat accumulation in priority to heating during a period (time) from the time a to the time b. However, this does not mean that the heating operation is not performed during this period, and the heat accumulation operation is also performed while the heating operation is performed so as not to spoil comfortability.
When the control unit 6 grasps that the room temperature reaches the preset temperature, the control unit 6 continues the heating operation and sets a second predetermined value T2 so as to preferentially perform the heat accumulation operation. This is a target value of the difference between the second saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material, and the second predetermined value T2 is determined in advance to be a value equal to or more than the first predetermined value T1.
In FIG. 2, the second predetermined value T2 is a value indicated by a two-way arrow between the alternate long and two short dashes line curve indicative of the second saturation temperature of the low-order side refrigerant and the broken line curve indicative of the temperature of the heat accumulation material.
When the second predetermined value T2 determined to be T1 ≤ T2 is set, the rotation speed of the low-order side compressor 21 is increased more than before to increase the saturation temperature of the low-order side refrigerant, thereby making it possible to accumulate more heat in the heat accumulation heat exchanger H. That is, the heat accumulation capacity of the heat accumulation heat exchanger H is increased such that the heat accumulation operation is performed in priority to the heating operation. In this sense, it may be said that the first predetermined value T1 and the second predetermined value T2 are set to adjust the heat accumulation capacity.
As described above, the control unit 6 acquires information on the temperature of the heat accumulation material from the heat accumulation temperature sensor HS. In a case where the temperature of the heat accumulation material becomes higher than the value set in advance, that is, in a case where the control unit 6 determines that a sufficient amount of heat is accumulated in the heat accumulation material, the heat accumulation operation is stopped. The heat accumulation operation is stopped in such a manner that the opening of the second low-order side pressure reduction mechanism 41 is controlled to be in a slightly open state to restrict the amount of the low-order side refrigerant flowing into the heat accumulation heat exchanger H, and the temperature of the low-order side refrigerant and the temperature of the heat accumulation material are maintained at the same temperature.
In FIG. 2, the temperature of the heat accumulation material exhibits a maximum value at the time b, and its value is larger than the saturation temperature of the low-order side refrigerant. Accordingly, the time indicated by the time b corresponds to a heat accumulation stop time.
Note that, in the heating and heat-accumulation operation, the control unit 6 controls the low-order side compressor 21 so that the first predetermined value T1 and the second predetermined value T2 are set and maintained, but each pressure reduction mechanism is controlled as follows.
That is, in the heating and heat-accumulation operation, the control unit 6 performs a control to open the first high-order side pressure reduction mechanism 13, the first low-order side pressure reduction mechanism 22, and the second low-order side pressure reduction mechanism 41 and to close the second high-order side pressure reduction mechanism 31. The second high-order side pressure reduction mechanism 31 is controlled to be closed to prevent heat accumulated in the heat accumulation heat exchanger H from being used during the heating and heat-accumulation operation.
FIG. 3 is a refrigerant circuit diagram illustrating the flow of the refrigerant at the time when the cascade refrigeration cycle system S according to the embodiment of the present invention performs the heating and heat-accumulation operation. As described above, the control unit 6 controls the second high-order side pressure reduction mechanism 31 to be closed.
On this account, in the circuit diagram, no high-order side refrigerant flows through a refrigerant pipe P in which one end is connected to the second high-order side pressure reduction mechanism 31 and the other end is connected between the cascade heat exchanger C and the high-order side four-way valve 14 across the heat accumulation heat exchanger H, and the refrigerant pipe P is illustrated in a broken line. Accordingly, the high-order side refrigerant does not circulate through the high-order side heat accumulation circuit 3. Note that a refrigerant pipe P through which the low-order side refrigerant or the high-order side refrigerant flows is illustrated in a solid line.
On the high-order side, the control unit 6 controls the first high-order side pressure reduction mechanism 13 to be open. Accordingly, at the time when the heating and heat-accumulation operation is performed, the high-order side refrigerant discharged from the high-order side compressor 11 in the high-order side refrigerant circuit 1 flows from the high-order side four-way valve 14 into the high-order side heat exchanger 12, flows through the first high-order side pressure reduction mechanism 13 and the cascade heat exchanger C, and is then sucked into the high-order side compressor 11, as indicated by arrows in FIG. 3.
In the meantime, the low-order side refrigerant discharged from the low-order side compressor 21 in the low-order side refrigerant circuit 2 on the low-order side flows from the low-order side four-way valve 24 into the cascade heat exchanger C, flows through the first low-order side pressure reduction mechanism 22 and the low-order side heat exchanger 23, and is then sucked into the low-order side compressor 21.
At this time, in the cascade heat exchanger C, the low-order side refrigerant and the high-order side refrigerant exchange heat with each other. The cascade heat exchanger C functions as an evaporator for the high-order side refrigerant, so that the high-order side refrigerant exchanges heat with air or water at the high-order side heat exchanger 12 in the high-order side refrigerant circuit 1. Hereby, warm air or hot water is supplied to the indoor space.
The control unit 6 also controls the first low-order side pressure reduction mechanism 22 and the second low-order side pressure reduction mechanism 41 to be open. That is, the low-order side refrigerant discharged from the low-order side compressor also flows through the heat accumulation heat exchanger H provided in parallel with the cascade heat exchanger C, as well as the cascade heat exchanger C.
As described above, the low-order side heat accumulation circuit 4 is configured such that one end is connected between the low-order side four-way valve 24 and the cascade heat exchanger C and the other end is connected between the first low-order side pressure reduction mechanism 22 and the low-order side heat exchanger 23.
At the time when the heating and heat-accumulation operation is performed in the cascade refrigeration cycle system S, the low-order side refrigerant discharged from the low-order side compressor 21 in the low-order side refrigerant circuit 2 flows out from the low-order side four-way valve 24 and flows into the heat accumulation heat exchanger H through the low-order side heat accumulation circuit 4, as indicated by arrows in FIG. 3 . Then, the low-order side refrigerant passes through the second low-order side pressure reduction mechanism 41, flows through the low-order side heat exchanger 23, and is then sucked into the low-order side compressor 21.
As described above, at start of the heating operation, the control unit 6 sets the first predetermined value T1 as the difference between the first saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material and controls the rotation speed of the low-order side compressor 21 such that the first predetermined value T1 is maintained. Accordingly, the low-order side refrigerant flows into the heat accumulation heat exchanger H in a state where the difference between the first saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material does not increase.
This can reduce a region in the heat accumulation heat exchanger H which region is filled with the liquid-phase low-order side refrigerant. Accordingly, it is possible to restrain a lot of liquid-phase low-order side refrigerant from being distributed in the heat accumulation heat exchanger H. The low-order side refrigerant discharged from the heat accumulation heat exchanger H passes through the second low-order side pressure reduction mechanism 41 and the low-order side heat exchanger 23 and is then sucked into the low-order side compressor 21.
In the meantime, one end of the high-order side heat accumulation circuit 3 is connected between the high-order side heat exchanger 12 and the first high-order side pressure reduction mechanism 13, and the other end thereof is connected between the cascade heat exchanger C and the high-order side four-way valve 14, but the control unit 6 controls the second high-order side pressure reduction mechanism 31 to be in a closed state, as described above.
Accordingly, the high-order side refrigerant does not flow through the high-order side heat accumulation circuit 3 and does not exchange heat with the low-order side refrigerant in the heat accumulation heat exchanger H. Besides, at the time when the high-order side refrigerant exchanges heat with air or water in the high-order side heat exchanger 12, heat accumulated in the heat accumulation heat exchanger H is not used.
The control unit 6 performs a control to open the first low-order side pressure reduction mechanism 22, the second low-order side pressure reduction mechanism 41, and the first high-order side pressure reduction mechanism 13 and to close the second high-order side pressure reduction mechanism 43. When the control unit 6 performs this control, the cascade refrigeration cycle system S can perform the heating operation and also perform the heat accumulation operation on the heat accumulation heat exchanger H.
That is, the cascade heat exchanger C functions as an evaporator for the high-order side refrigerant, so that the low-order side refrigerant and the high-order side refrigerant exchange heat via the cascade heat exchanger C. In the meantime, the low-order side refrigerant flows into the heat accumulation heat exchanger H, but no high-order side refrigerant flows into the heat accumulation heat exchanger H. Accordingly, heat accumulated in the heat accumulation heat exchanger H is not used for the heating operation, and heat of the low-order side refrigerant is accumulated in the heat accumulation material of the heat accumulation heat exchanger H.
This is the control method of the control unit 6 in the heating and heat-accumulation operation. Refrigerant circuits indicative of respective flows of the high-order side refrigerant and the low-order side refrigerant are as illustrated in FIG. 3. In view of this, next will be described a case where the defrosting operation is performed in the cascade refrigeration cycle system S.
As described above, in a case where the heating operation is performed, it is necessary to perform the defrosting operation regularly. In a case where the defrosting operation is performed, the low-order side four-way valve 24 is switched from a heating operation state to a cooling operation state to supply a warm low-order side refrigerant discharged from the low-order side compressor 21 to the low-order side heat exchanger 23, and therefore, the heating operation stops during the defrosting operation. Accordingly, warm air and hot water are not supplied to the indoor space during the defrosting operation, which may spoil comfortability.
In view of this, the cascade refrigeration cycle system S according to the embodiment of the present invention performs the defrosting operation without stopping the heating operation, so that comfortability is secured and the heating capacity of the cascade refrigeration cycle system S is maintained.
FIG. 4 is a refrigerant circuit diagram illustrating the flow of the refrigerant at the time when the cascade refrigeration cycle system S according to the embodiment of the present invention performs the defrosting operation. As described above, it is necessary to supply a warm low-order side refrigerant to the low-order side heat exchanger 23 at the time when the defrosting operation is performed, and therefore, the low-order side four-way valve 24 is switched to supply the low-order side refrigerant to the low-order side heat exchanger 23 instead of the cascade heat exchanger C and the heat accumulation heat exchanger H.
That is, the low-order side refrigerant discharged from the low-order side compressor 21 first flows into the low-order side heat exchanger 23 via the low-order side four-way valve 24. Frost attached to the low-order side heat exchanger 23 is melted by the low-order side refrigerant thus flowing into the low-order side heat exchanger 23.
At the time when the defrosting operation is performed, the control unit 6 performs a control to bring the first low-order side pressure reduction mechanism 22 into a closed state and to bring the second low-order side pressure reduction mechanism 41 into an open state. By controlling the first low-order side pressure reduction mechanism 22 and the second low-order side pressure reduction mechanism 41 as such, the low-order side refrigerant discharged from the low-order side heat exchanger 23 does not flow into the low-order side refrigerant circuit 2 but flows into the low-order side heat accumulation circuit 4.
Accordingly, the low-order side refrigerant discharged from the low-order side heat exchanger 23 passes through the second low-order side pressure reduction mechanism 41 and the heat accumulation heat exchanger H and is then sucked into the low-order side compressor 21. Accordingly, in the defrosting operation, the low-order side refrigerant does not flow into the cascade heat exchanger C.
In the meantime, on the high-order side, the low-order side refrigerant does not flow into the cascade heat exchanger C, as described above. In the first place, the low-order side refrigerant has already undergone heat-exchange in the low-order side heat exchanger 23 to defrost the low-order side heat exchanger 23, and therefore, the low-order side refrigerant and the high-order side refrigerant cannot exchange heat in the cascade heat exchanger C.
In view of this, the control unit 6 performs a control to bring the first high-order side pressure reduction mechanism 13 into a closed state. However, in a state where heat-exchange cannot be performed in the cascade heat exchanger C, warm air cannot be supplied to the indoor space, so that the room temperature gradually decreases, which may spoil comfortability.
In order to avoid such a situation, the cascade refrigeration cycle system S according to the embodiment of the present invention performs a control to continue the heating operation by use of heat accumulated in the heat accumulation heat exchanger H. That is, the control unit 6 controls the second high-order side pressure reduction mechanism 31 to be brought into an open state and guides the high-order side refrigerant discharged from the high-order side heat exchanger 12 to the high-order side heat accumulation circuit 3.
At the time when the high-order side refrigerant is passed through the heat accumulation heat exchanger H, the high-order side refrigerant absorbs heat accumulated in the heat accumulation heat exchanger H and flows into the high-order side heat exchanger 12 via the high-order side compressor 11 again, so that warm air can be supplied to the indoor space.
As described above, the control unit 6 performs a control to bring the first high-order side pressure reduction mechanism 13 and the first low-order side pressure reduction mechanism 22 into a closed state so as not to circulate the high-order side refrigerant and the low-order side refrigerant through the high-order side refrigerant circuit 1 and the low-order side refrigerant circuit 2, as indicated by broken lines in FIG. 4. When the control unit 6 performs such a control, the low-order side heat exchanger 23 is defrosted, and the heating operation is continued by use of heat accumulated in the heat accumulation heat exchanger H.
In addition, the control unit 6 also controls the first low-order side pressure reduction mechanism 22 and the first high-order side pressure reduction mechanism 13 to be closed, so that the low-order side refrigerant and the high-order side refrigerant do not flow into the cascade heat exchanger C and do not exchange heat with each other. Accordingly, the low-order side refrigerant used for the defrosting of the low-order side heat exchanger 23 does not flow into the cascade heat exchanger C, thereby making it possible to prevent the cold on the low-order side from being transmitted to the high-order side.

[Operation]

Next will be described a control flow of the cascade refrigeration cycle system S by the control unit 6 in the heating and heat-accumulation operation and the defrosting operation, with reference to FIGS. 5 to 7. FIGS. 5 and 6 are flowcharts each illustrating a control flow at the time when the heating and heat-accumulation operation is performed in the cascade refrigeration cycle system S according to the embodiment of the present invention. FIG. 7 is a flowchart illustrating a control flow at the time when the defrosting operation is performed in the cascade refrigeration cycle system S according to the embodiment of the present invention.
First described is the flow of the heating and heat-accumulation operation. As illustrated in FIG. 5, when the heating and heat-accumulation operation is started in the cascade refrigeration cycle system S (ST1), the control unit 6 adjusts the opening of a pressure reduction mechanism to a predetermined initial opening so that the low-order side refrigerant discharged from the low-order side compressor 21 flows into the cascade heat exchanger C and the heat accumulation heat exchanger H (ST2).
Herein, a collective term "pressure reduction mechanism" is used, but the pressure reduction mechanism indicates, for example, the first low-order side pressure reduction mechanism 22 and the second low-order side pressure reduction mechanism 41 illustrated in FIG. 3, for example. The initial opening is set to secure reliability at the time of the start.
The first low-order side pressure reduction mechanism 22 is set to an opening which can prevent the discharge temperature from increasing excessively and which can avoid a liquid back flow to the low-order side compressor 21. The second low-order side pressure reduction mechanism 41 is set to an opening which can avoid liquid accumulation in the heat accumulation heat exchanger H and which can prevent an excessive flow rate of the refrigerant flowing into the heat accumulation heat exchanger H.
In the meantime, on the high-order side, the control unit 6 adjusts the opening of the first high-order side pressure reduction mechanism 13 to a predetermined initial opening (ST3). In the meantime, the control unit 6 adjusts the opening of the second high-order side pressure reduction mechanism 31 to a closed state (ST4). The initial opening is set to secure reliability at the time of the start. The first high-order side pressure reduction mechanism 13 is set to an opening which can prevent the discharge temperature from increasing excessively and which can avoid a liquid back flow to the high-order side compressor 11.
Due to the control by the control unit 6, the high-order side refrigerant circulates only through the high-order side refrigerant circuit 1 and does not flow into the high-order side heat accumulation circuit 3. Accordingly, heat accumulated in the heat accumulation heat exchanger H is not used for the heating operation, thereby making it possible to preferentially perform the heating operation with the heat accumulation operation being also performed and to warm the indoor space.
Then, the refrigerant temperature detecting unit 5 calculates the saturation temperature of the low-order side refrigerant by use of measurement results from the pressure sensor 51 and the refrigerant temperature sensor 52. The temperature of the heat accumulation material is measured by the heat accumulation temperature sensor HS provided for the heat accumulation heat exchanger H. Information on the calculated saturation temperature and information on the temperature of the heat accumulation material are input into the control unit 6 (ST5).
The control unit 6 acquires the first predetermined value T1, for example, from a storage unit (not illustrated) (ST6). The control unit 6 controls the rotation speed of the low-order side compressor 21 such that the difference between the saturation temperature of the low-order side refrigerant and the temperature of the heat accumulation material is maintained at the first predetermined value T1 thus acquired (ST7) .
Note that the order of the control of the control unit 6 until step ST7 from the start of the heating and heat-accumulation operation in step ST1 is described in the aforementioned order for illustrative purposes. However, the order is not limited to the aforementioned order, and the order is changeable. Further, controls on respective pressure reduction mechanisms are also performable at the same time.
While the heating and heat-accumulation operation is performed, the room temperature is measured by the room temperature sensor in the indoor unit at given periods (ST8) . This is because it is necessary to check whether or not the room temperature reaches the preset temperature or is maintained at the preset temperature.
The control unit 6 compares the room temperature measured by the room temperature sensor with the preset temperature and determines whether or not the room temperature has reached the preset temperature (ST9). In a case where the room temperature has not reached the preset temperature yet (NO in ST9), the process returns to step ST7 again, so that the heating and heat-accumulation operation is continued, and the measurement of the room temperature is also performed. Then, a comparison between a newly measured room temperature and the preset temperature is performed.
In the meantime, in a case where the room temperature has reached the preset temperature (YES in ST9), the control unit 6 acquires information on the calculated saturation temperature and information of the temperature of the heat accumulation material again (ST10 in FIG. 6). Then, the control unit 6 acquires, from the storage unit, for example, the second predetermined value T2 (for example, 5°C) determined in advance based on a temperature difference between a saturation temperature and the temperature of the heat accumulation material which temperature difference is obtained by a test or the like (ST11). Note that the second predetermined value T2 is set such that the relationship between the first predetermined value T1 and the second predetermined value T2 satisfies T1 ≤ T2.
The control unit 6 controls the low-order side compressor 21 so that the second predetermined value T2 is maintained, and performs the heat accumulation operation (ST12). The control unit 6 acquires information on the temperature of the heat accumulation material from the heat accumulation temperature sensor HS (ST13).
Then, it is determined whether or not the acquired temperature of the heat accumulation material is equal to or more than a setting value determined in advance (ST14). The setting value can be set appropriately depending on the heat accumulation material of the heat accumulation heat exchanger H.
In a case where the temperature of the heat accumulation material has not reached the setting value yet (NO in ST14), the process returns to step ST12, and the heat accumulation operation is continued. In the meantime, in a case where the temperature of the heat accumulation material has reached the setting value (YES in ST14), the heat accumulation operation is stopped (ST15). The heat accumulation operation is stopped in such a manner that the opening of the second low-order side pressure reduction mechanism 41 is controlled to be in a slightly open state to restrict the amount of the low-order side refrigerant flowing into the heat accumulation heat exchanger H, and the temperature of the low-order side refrigerant and the temperature of the heat accumulation material are maintained at the same temperature. Here, the heating and heat-accumulation operation is ended.
Next will be described a control flow of the defrosting operation with reference to FIG. 7. As described above, the defrosting operation is performed regularly while the heating operation is performed. For example, the heating operation is switched to the defrosting operation in a case where an outside temperature is equal to or less than 5°C and the heating operation is continued for three hours or a case where the temperature of the low-order side heat exchanger 23 becomes -15°C or less. However, the timing when the heating operation is switched to the defrosting operation can be set as appropriate.
When the defrosting operation is started (ST31), the control unit 6 adjusts the opening of the first low-order side pressure reduction mechanism 22 to be closed so that the low-order side refrigerant flowing out from the low-order side heat exchanger 23 does not flow into the cascade heat exchanger C (ST32). Further, the control unit 6 adjusts the opening of the first high-order side pressure reduction mechanism 13 to be closed so that the high-order side refrigerant does not flow from the high-order side heat exchanger 12 into the cascade heat exchanger C (ST33).
Further, the control unit 6 adjusts the openings of the second low-order side pressure reduction mechanism 41 and the second high-order side pressure reduction mechanism 31 to be in an open state so that the low-order side refrigerant and the high-order side refrigerant both flow into the heat accumulation heat exchanger H (ST34, ST35). Note that, as described above, the controls on the pressure reduction mechanisms by the control unit 6 may be performed in any order as described above or may be performed at the same time.
The control unit 6 performs the defrosting operation and determines whether or not defrosting of the low-order side heat exchanger 23 is completed, that is, whether or not the control unit 6 is to end the defrosting operation (ST36). For example, in a case where the defrosting operation time has passed a predetermined time, or in a case where the temperature of the low-order side heat exchanger 23 has reached a predetermined temperature, the control unit 6 determines that defrosting is completed. In a case where the control unit 6 determines that defrosting of the low-order side heat exchanger 23 has not been completed yet (NO in ST36), the control unit 6 continues the defrosting operation.
In the meantime, in a case where the control unit 6 determines that defrosting of the low-order side heat exchanger 23 is completed (YES in ST36), the control unit 6 stops the defrosting operation (ST37).
With the cascade refrigeration cycle system including the refrigerant circuits and the cascade refrigeration cycle system control method as described above, even in a case where a refrigerating cycle circuit which employs a multistage refrigerating circuit and which includes a heat accumulator is employed, it is possible to secure a sufficient amount of a refrigerant in the refrigerating cycle circuit during the operation, to restrain a decrease in heating capacity, and to maintain heat accumulation capacity without spoiling comfortability.
Note that the invention described herein is not limited to the above embodiment as it is and is just an example of the present invention. The invention can be embodied by modifying an element without departing from the gist of the invention when the invention is carried out, and the above embodiment can be variously modified or altered. Further, various inventions can be achieved by combining a plurality of elements disclosed in the above embodiment appropriately.
For example, some elements may be deleted from all elements described in the embodiment, for example. Furthermore, elements described in different embodiments may be combined appropriately, and embodiments with the changes or improvements can be also included in the present invention. The embodiments and their modifications are included in the scope and the gist of the invention and are also included in the invention described in the claims and in its equivalent range.

Reference Signs List

1: high-order side refrigerant circuit
2: reduction side refrigerant circuit
3: high-order side heat accumulation circuit
4: reduction side heat accumulation circuit
5: refrigerant temperature detecting unit
11: high-order side compressor
12: high-order side heat exchanger
13: first high-order side pressure reduction mechanism
14: high-order side four-way valve
21: reduction side compressor
22: first low-order side pressure reduction mechanism
23: reduction side heat exchanger
24: reduction side four-way valve
31: second high-order side pressure reduction mechanism
41: second low-order side pressure reduction mechanism
51: pressure sensor
52: refrigerant temperature sensor
C: cascade heat exchanger
H: heat accumulation heat exchanger
HS: heat accumulation temperature sensor
P: refrigerant pipe
S: cascade refrigeration cycle system

Claims

A cascade refrigeration cycle system comprising:
a high-order side refrigerant circuit configured such that a high-order side compressor, a high-order side heat exchanger, a first high-order side pressure reduction mechanism, and a cascade heat exchanger are sequentially connected via refrigerant pipes to circulate a high-order side refrigerant through the high-order side refrigerant circuit;

a low-order side refrigerant circuit configured such that a low-order side compressor, the cascade heat exchanger, a first low-order side pressure reduction mechanism, and a low-order side heat exchanger are sequentially connected via refrigerant pipes to circulate a low-order side refrigerant through the low-order side refrigerant circuit;

a high-order side heat accumulation circuit configured such that the high-order side compressor, the high-order side heat exchanger, a second high-order side pressure reduction mechanism, and a heat accumulation heat exchanger provided in parallel with the cascade heat exchanger are sequentially connected via refrigerant pipes to circulate the high-order side refrigerant through the high-order side heat accumulation circuit;

a low-order side heat accumulation circuit configured such that the low-order side compressor, the heat accumulation heat exchanger, a second low-order side pressure reduction mechanism, and the low-order side heat exchanger are sequentially connected via refrigerant pipes to circulate the low-order side refrigerant through the low-order side heat accumulation circuit;

a refrigerant temperature detecting unit configured to measure or calculate a saturation temperature of the low-order side refrigerant discharged from the low-order side compressor;

a heat accumulation temperature sensor configured to measure a temperature of a heat accumulation material provided in the heat accumulation heat exchanger; and

a control unit configured to control respective openings of the high-order side compressor, the low-order side compressor, the first high-order side pressure reduction mechanism, the second high-order side pressure reduction mechanism, the first low-order side pressure reduction mechanism, and the second low-order side pressure reduction mechanism, wherein:
the high-order side refrigerant and the low-order side refrigerant perform heat-exchange in the cascade heat exchanger or the heat accumulation heat exchanger; and

at a time of performing a heating operation, the control unit acquires information on the saturation temperature and information on the temperature of the heat accumulation material and controls the low-order side compressor such that a temperature difference between the saturation temperature and the temperature of the heat accumulation material becomes a first predetermined value.
The cascade refrigeration cycle system according to claim 1, further comprising:
an indoor unit provided in an indoor space and including the high-order side heat exchanger and a room temperature sensor configured to measure a room temperature of the indoor space, wherein, in a case where the room temperature reaches a preset temperature set in the indoor unit, the control unit controls the low-order side compressor such that the temperature difference between the saturation temperature and the temperature of the heat accumulation material becomes the second predetermined value.
The cascade refrigeration cycle system according to claim 2, wherein the control unit sets the second predetermined value to be equal to or more than the first predetermined value.
The cascade refrigeration cycle system according to claim 3, wherein:
the control unit acquires information on the temperature of the heat accumulation material; and

in a case where the temperature of the heat accumulation material reaches a predetermined temperature, the control unit stops heat accumulation in the heat accumulation heat exchanger.
The cascade refrigeration cycle system according to any one of claims 1 to 4, wherein, during the heating operation using the first predetermined value or the second predetermined value, the control unit performs a control to open the first high-order side pressure reduction mechanism, the first low-order side pressure reduction mechanism, and the second low-order side pressure reduction mechanism and to close the second high-order side pressure reduction mechanism.
The cascade refrigeration cycle system according to claim 1, wherein the control unit performs a control to close the first low-order side pressure reduction mechanism and the first high-order side pressure reduction mechanism at a time of performing a defrosting operation on the low-order side heat exchanger.
A control method for controlling a cascade refrigeration cycle system including
a high-order side refrigerant circuit configured such that a high-order side compressor, a high-order side heat exchanger, a first high-order side pressure reduction mechanism, and a cascade heat exchanger are sequentially connected via refrigerant pipes to circulate a high-order side refrigerant through the high-order side refrigerant circuit,

a low-order side refrigerant circuit configured such that a low-order side compressor, the cascade heat exchanger, a first low-order side pressure reduction mechanism, and a low-order side heat exchanger are sequentially connected via refrigerant pipes to circulate a low-order side refrigerant through the low-order side refrigerant circuit,

a high-order side heat accumulation circuit configured such that the high-order side compressor, the high-order side heat exchanger, a second high-order side pressure reduction mechanism, and a heat accumulation heat exchanger provided in parallel with the cascade heat exchanger are sequentially connected via refrigerant pipes to circulate the high-order side refrigerant through the high-order side heat accumulation circuit,

a low-order side heat accumulation circuit configured such that the low-order side compressor, the low-order side heat exchanger, a second low-order side pressure reduction mechanism, the heat accumulation heat exchanger, and the low-order side compressor are sequentially connected via refrigerant pipes to circulate the low-order side refrigerant through the low-order side heat accumulation circuit,

a refrigerant temperature detecting unit configured to measure or calculate a saturation temperature of the low-order side refrigerant discharged from the low-order side compressor,

a heat accumulation temperature sensor configured to measure a temperature of a heat accumulation material provided in the heat accumulation heat exchanger, and
a control unit configured to control respective openings of the high-order side compressor, the low-order side compressor, the first high-order side pressure reduction mechanism, the second high-order side pressure reduction mechanism, the first low-order side pressure reduction mechanism, and the second low-order side pressure reduction mechanism,

the control method comprising:
the control unit acquiring information on the saturation temperature and information on the temperature of the heat accumulation material at a time of performing a heating operation; and

controlling the low-order side compressor such that a temperature difference between the saturation temperature and the temperature of the heat accumulation material becomes a first predetermined value.
The control method according to claim 7, further comprising:
determining whether or not a room temperature of an indoor space reaches a preset temperature set in an indoor unit which is provided in the indoor space and which includes the high-order side heat exchanger and a room temperature sensor configured to measure the room temperature of the indoor space;

acquiring the information on the saturation temperature and the information on the temperature of the heat accumulation material in a case where the room temperature is determined to reach the preset temperature; and

controlling the low-order side compressor such that the temperature difference between the saturation temperature and the temperature of the heat accumulation material becomes a second predetermined value.
The control method according to claim 8, wherein the control unit sets the second predetermined value to be equal to or more than the first predetermined value.
The control method according to claim 8, further comprising:
acquiring the information on the temperature of the heat accumulation material;

determining whether or not the temperature of the heat accumulation material reaches a predetermined temperature; and

in a case where the temperature of the heat accumulation material is determined to reach the predetermined temperature, stopping heat accumulation in the heat accumulation heat exchanger.
The control method according to any one of claims 7 to 10, further comprising:
opening the first high-order side pressure reduction mechanism, the first low-order side pressure reduction mechanism, and the second low-order side pressure reduction mechanism during the heating operation using the first predetermined value or the second predetermined value; and

closing the second high-order side pressure reduction mechanism.
The control method according to claim 7, further comprising:
closing the first low-order side pressure reduction mechanism and the first high-order side pressure reduction mechanism at a time of performing a defrosting operation on the low-order side heat exchanger.