EP2551612A2

EP2551612A2 - Supercritical-cycle heat pump

Info

Publication number: EP2551612A2
Application number: EP12177749A
Authority: EP
Inventors: Youhei Hotta; Yoshiyuki Kimata; Hajime Sato; Toshiyuki Goto
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2011-07-29
Filing date: 2012-07-25
Publication date: 2013-01-30
Anticipated expiration: 2032-07-25
Also published as: EP2551612A3; JP2013029269A; JP5798830B2; EP2551612B1

Abstract

A supercritical-cycle heat pump (1) that employs CO₂ refrigerant as a working medium that is provided with an injection circuit (11) for injecting intermediate-pressure refrigerant gas being separated by a gas-liquid separator 6 into a sealed housing (14), having intermediate pressure in the interior thereof, in a multistage compressor (2), a controller (25) is provided, which determines the viscosity of the POE oil (17) on the basis of the solubility of the POE oil (17) in the CO₂ refrigerant and temperature of the POE oil (17), the solubility of the POE oil (17) in the CO₂ refrigerant being determined by pressure and temperature of intermediate-pressure refrigerant and which controls the pressure of the intermediate-pressure refrigerant by controlling a first electronic expansion valve (5) so that the pressure thereof is controlled within a preset usage limit range in order to keep the viscosity of the POE oil (17) above the certain viscosity zone.

Description

Technical Field

The present invention relates to a supercritical-cycle (CO₂-cycle) heat pump employing CO₂ refrigerant.

{Background Art}

With supercritical-cycle heat pumps employing CO₂ refrigerant, which are employed in air conditioners, hot-water supply units, and so forth, it is known that polyalkylene-glycol-based oil (PAG oil), polyol-ester-based oil (POE oil), or mixed oil thereof is employed as a compressor lubricant (refrigerator oil) (for example, see Patent Literatures 1 and 2). In addition, there are known units that employ, as compressors, multistage compressors that are provided with lower-stage compression mechanisms and higher-stage compression mechanisms; that discharge intermediate-pressure refrigerant gas compressed by the lower-stage compression mechanisms into sealed housings; and that take this refrigerant gas into the higher-stage compression mechanisms to perform two-stage compression to high pressure (for example, see Patent Literatures 3 and 4).
Furthermore, among CO₂-cycle heat pumps employing multistage compressors such as those described in Patent Literatures 3 and 4, there are ones proposed by, for example, Patent Literatures 5 and 6, etc. wherein gas-liquid separators are installed in refrigerant circuits between radiators and evaporators, with first electronic expansion valves and second electronic expansion valves provided upstream and downstream thereof, and the multistage compressors are provided with injection circuits, with which the intermediate-pressure refrigerant gas separated by the gas-liquid separators is injected into the sealed housings, having an intermediate-pressure atmosphere in the interior thereof.

Citation List

Patent Literature

{PTL 1} Japanese Unexamined Patent Application, Publication No. 2002-174462
{PTL 2} Japanese Unexamined Patent Application, Publication No. 2008-185290
{PTL 3} Japanese Unexamined Patent Application, Publication No. 2001-271776
{PTL 4} Japanese Unexamined Patent Application, Publication No. 2005-257240
{PTL 5} Japanese Unexamined Patent Application, Publication No. 2008-163894
{PTL 6} Japanese Unexamined Patent Application, Publication No. 2008-190377

Summary of Intention

Technical Problem

Relatively speaking, the use of polyalkylene-glycol-based oil (PAG oil) is more common in compressors which employ CO₂ refrigerant. However, because PAG oil has low compatibility with CO₂ refrigerant, it tends to be separated from the refrigerant in a low-temperature range, and thus, there is a problem in that oil return to the compressor from the system side tends to deteriorate. In particular, as indicated in Patent Literatures 5 and 6, there are concerns that, in compressors provided with the injection circuits, the oil will be separated in gas-liquid separators, causing the oil return from the injection circuits to also deteriorate in addition to that from the low-pressure side refrigerant circuits, which affects the lubrication performance of the compressors, and so on.
On the other hand, the polyol-ester-based oil (POE oil) has high compatibility with the refrigerant, which makes the occurrence of problems described above unlikely; however, there are concerns that the refrigerant will increase the dilution ratio, decrease the oil viscosity, and so on. Although these concerns are reduced in compressors employing multistage compressors with sealed housings, having intermediate pressure in the interior thereof, because of the temperature and pressure conditions in the intermediate-pressure housings, as compared with compressors with high-pressure housings or low-pressure housings, it is necessary to somehow restrict the effects in question because they are considered to affect the lubrication performance.
The present invention has been conceived in light of the above-described circumstances, and an object thereof is to provide a supercritical-cycle heat pump with which the capacity to return oil from the system side where an injection circuit is provided can be enhanced by employing POE oil as lubricant for a compressor thereof, and concerns with regard to an increase in the dilution ratio and a decrease in the oil viscosity when the POE oil is employed can be eliminated. Solution to Problem
In order to solve the above-described problems, a supercritical-cycle heat pump of the present invention employs the following solutions.
Specifically, a supercritical-cycle heat pump according to the present invention is a supercritical-cycle heat pump that employs CO₂ refrigerant as a working medium including a multistage compressor that is provided with a lower-stage compression mechanism and a higher-stage compression mechanism, that discharges intermediate-pressure refrigerant gas compressed by the lower-stage compression mechanism into a sealed housing, and that takes this refrigerant gas into the higher-stage compression mechanism to perform compression to high pressure; a radiator; a first electronic expansion valve; a gas-liquid separator; a second electronic expansion valve; an evaporator; an injection circuit for injecting intermediate-pressure refrigerant gas separated at the gas-liquid separator into the sealed housing of the multistage compressor; and a controller that controls pressure of the intermediate-pressure refrigerant by controlling the first electronic expansion valve so that the pressure of the intermediate-pressure is controlled within a preset usage limit range in order to keep viscosity of oil which is employed as lubricant for the multistage compressor above a certain viscosity zone, the controller determining the viscosity of the oil on the basis of solubility of the oil in the CO₂ refrigerant and temperature of the oil, the solubility of the oil in the CO₂ refrigerant being determined by pressure temperature of the intermediate-pressure refrigerant, wherein a refrigerating cycle is formed in which the multistage compressor, the radiator, the first electronic expansion valve, the gas-liquid separator, the second electronic expansion valve, and the evaporator are connected in this order, and wherein polyol-ester based oil or mixed oil of polyol-ester based oil is employed as the lubricant for the multistage compressor.
With the present invention, in the supercritical-cycle heat pump that is provided with the injection circuit for injecting the intermediate-pressure refrigerant gas which is separated by the gas-liquid separator into the sealed housing, having intermediate pressure in the interior thereof, in the multistage compressor and that employs CO₂ refrigerant as a working medium, because the polyol-ester-based oil or the mixed oil thereof is employed as the lubricant for the multistage compressor and also because the controller is provided, which controls the pressure of the intermediate-pressure refrigerant via the first electronic expansion valve so that the pressure thereof is controlled within the preset usage limit range in order to keep the viscosity of the oil above the certain viscosity zone, the capacity to return oil to the multistage compressor from the system side can be enhanced by employing the polyol-ester-based oil having high compatibility with the CO₂ refrigerant or the mixed oil (POE oil) thereof, and it is also possible to prevent an increase in the dilution ratio of the oil and a decrease in the oil viscosity, which are affected by the pressure are temperature of the refrigerant, by keeping the viscosity of the oil above the certain viscosity zone by controlling the pressure of the intermediate-pressure refrigerant within the preset usage limit range. Therefore, it is possible to eliminate a decrease in the lubrication performance due to insufficient lubricant in the multistage compressor, an increase in the dilution ratio of the oil, a decrease in the viscosity thereof, and so forth, which makes it possible to ensure sufficient reliability.
Furthermore, with a first aspect of the supercritical-cycle heat pump of the present invention, in the above-described supercritical-cycle heat pump, further includes an intermediate pressure sensor for detecting the pressure of the intermediate-pressure refrigerant to be injected from the injection circuit to the sealed housing of the multistage compressor; an intermediate temperature sensor for detecting the temperature of the intermediate-pressure refrigerant to be injected from the injection circuit to the sealed housing of the multistage compressor; an intake pressure sensor for detecting the pressure of intake refrigerant for the multistage compressor; an intake temperature sensor for detecting the temperature of the intake refrigerant for the multistage compressor, wherein the controller controls refrigerant superheating temperatures of the intermediate-pressure refrigerant and the intake refrigerant to respective target superheating temperatures with the first electronic expansion valve and the second electronic expansion valve.
With the present invention, because the controller is configured to control refrigerant superheating temperatures of the intermediate-pressure refrigerant and the intake refrigerant to respective target superheating temperatures with the first electronic expansion valve and the second electronic expansion valve, by controlling the existing first electronic expansion valve and second electronic expansion valve, provided upstream and downstream of the gas-liquid separator connected to the injection circuit, the viscosity of the POE oil can be kept above the certain viscosity zone by merely changing the software for the controller without additionally providing new devices, which makes it possible to prevent an increase in the dilution ratio of the oil and a decrease in the oil viscosity. Therefore, it is possible to achieve improved lubrication performance in a simple manner by employing the POE oil, which has high compatibility, while avoiding an increase in complexity of the hardware configuration.
Furthermore, with the supercritical-cycle heat pump of the present invention, in any of the above-described supercritical-cycle heat pumps, the controller changes usage limit ranges in accordance with the viscosity of the oil in the sealed housing in a lower-stage usage limit range and a higher-stage usage limit range, which are preset based on the relationship between low pressure and intermediate pressure and that between intermediate pressure and high pressure, thus enabling operation in portions of limited pressure ranges even when the viscosity of the oil does not reach a first rated value so long as a second rated value, which is lower than the first rated value, is reached.
Furthermore, with the present invention, the controller changes, in accordance with the viscosity of the oil in the sealed housing, the usage limit ranges in the lower-stage usage limit range and the higher-stage usage limit range, which are preset based on the relationship between the low pressure and intermediate pressure on the lower-stage side and the relationship between the intermediate pressure and high pressure on the higher-stage side, thus enabling operation in portions of the limited pressure ranges, even when the viscosity of the oil does not reach the first rated value, so long as the second rated value, which is lower than the first rated value, is reached; therefore, even when the viscosity of the POE oil in the sealed housing does not reach the rated value, operation is possible in the limited pressure ranges, which are portions of the above-described usage limit ranges (for example, the pressure ranges below the limit lines L shown in Fig. 2) so long as the second rated value is reached. Therefore, it is possible to reliably prevent a decrease in the lubrication performance due to a decrease in the viscosity of the POE oil caused by an increase in the oil temperature, and it is also possible to ensure a sufficient degree of freedom for the operation on the system side.

{Advantageous Effects of Invention}

With the present invention, because the capacity to return oil to a multistage compressor from a system side can be enhanced by employing polyol-ester-based oil having high compatibility with CO₂ refrigerant or mixed oil (POE oil) thereof, and also because an increase in the dilution ratio of oil and a decrease in the oil viscosity, which are affected by the pressure and temperature of the refrigerant, can be prevented by keeping the viscosity of the oil above the certain viscosity zone by controlling the pressure of intermediate-pressure refrigerant within a preset usage limit range, it is possible to eliminate a decrease in the lubrication performance caused by a lack of lubricant in the multistage compressor, an increase in the dilution ratio of the oil, a decrease in the viscosity thereof, and so forth, which makes it possible to ensure sufficient reliability. Brief Description of Drawings

Fig. 1 is a diagram of a refrigerating cycle of a supercritical-cycle heat pump according to an embodiment of the present invention.
Fig. 2 is a map showing a usage limit range in terms of pressure of POE oil filled in a multistage compressor of the supercritical-cycle heat pump shown in Fig. 1.
Fig. 3 is a diagram showing a pressure-solubility characteristic between CO₂ refrigerant and POE oil, with temperature as a parameter.
Fig. 4 is a diagram showing a temperature-viscosity characteristic between the CO₂ refrigerant and the POE oil, with solubility as a parameter.

{Description of Embodiment}

An embodiment of the present invention will be described below with reference to Figs. 1 to 4.
Fig. 1 shows a diagram of a refrigerating cycle of a supercritical-cycle heat pump employing CO₂ refrigerant according to an embodiment of the present invention.
A supercritical-cycle heat pump (CO₂ cycle heat pump) 1 is provided with a multistage compressor 2, and a closed-cycle refrigerant circuit (refrigerating cycle) 10 is formed by sequentially connecting the multistage compressor 2, an oil separator 3, a radiator 4, a first electronic expansion valve 5, a gas-liquid separator 6, a second electronic expansion valve 7, and an evaporator 8 in this order via refrigerant pipes 9.
Furthermore, the above-described refrigerant circuit (refrigerating cycle) 10 is provided with an injection circuit 11 for injecting intermediate-pressure refrigerant gas separated by the gas-liquid separator 6 into a sealed housing 14, having intermediate pressure in the interior thereof, in the multistage compressor 2, and is also provided with an oil-return circuit 13 that returns lubricant separated from the refrigerant gas at the oil separator 3 to an intake refrigerant pipe 9A in the multistage compressor 2 after performing heat exchange thereof with the intermediate-pressure refrigerant gas via a heat exchanger 12 provided in the injection circuit 11.
The multistage compressor 2 has an electric motor (not shown) built into a single sealed housing 14 and is also provided with two compression mechanisms, that is, a lower-stage compression mechanism 15 and a higher-stage compression mechanism 16, that are driven by the electric motor. The multistage compressor 2 is configured such that the lower-stage compression mechanism 15 takes in low-pressure refrigerant gas evaporated by the evaporator 8, compresses it to intermediate pressure, and discharges it into the sealed housing 14; and the higher-stage compression mechanism 16 takes in the intermediate-pressure refrigerant gas, performs tow-stage compression to high pressure, and discharges the high-pressure refrigerant gas to the oil separator 3 connected to the multistage compressor 2. Note that a single type or mixed types of compression mechanisms among the rotary type, scroll type, and various other types may be employed as the lower-stage compression mechanism 15 and the higher-stage compression mechanism 16.
A certain amount of lubricant (refrigerator oil) 17, which lubricates sliding portions in the lower-stage compression mechanism 15 and the higher-stage compression mechanism 16, is filled at a bottom portion of the sealed housing 14, having intermediate pressure in the interior thereof, in the multistage compressor 2, and it is force fed to the sliding portions via an oil-supply pump. In this embodiment, polyol-ester-based oil (POE oil), which has high compatibility with CO₂ refrigerant, or mixed oil thereof (hereinafter, simply referred to as POE oil) is employed as the lubricant 17.
The oil separator 3 separates the lubricant 17 contained in the CO₂ refrigerant discharged from the multistage compressor 2 and returns it to the intake refrigerant pipe 9A in the multistage compressor 2 via the oil-return circuit 13. The radiator 4 performs heat exchange between high-temperature, high-pressure refrigerant gas and a cooling medium, thus causing the refrigerant gas to release heat to reach a supercritical state or a condensed liquefied state, and thereby causes the refrigerant to flow out toward the first electronic expansion valve 5. The first electronic expansion valve 5 depressurizes the high-pressure refrigerant to intermediate pressure and supplies it to the gas-liquid separator 6. The first electronic expansion valve 5 measures the pressure and temperature of the intermediate-pressure refrigerant and controls superheating temperature thereof to a target value so that the performance and capacity of the heat pump 1 are maximized and the viscosity of the POE oil employed as the lubricant 17 is kept above a certain viscosity zone, as described later.
The gas-liquid separator 6 performs gas-liquid separation of gas-liquid two-phase CO₂ refrigerant which has been depressurized to the intermediate pressure, injects gaseous refrigerant into the sealed housing 14 of the multistage compressor 2 by making it pass through the injection circuit 11 from the gas-liquid separator 6, and also causes liquid refrigerant to flow out toward the second electronic expansion valve 7. The second electronic expansion valve 7 depressurizes the intermediate-pressure liquid refrigerant, supplies it to the evaporator 8 as low-pressure, low-temperature gas-liquid two-phase refrigerant, measures the pressure and temperature of the low-pressure refrigerant gas to be taken into the multistage compressor 2, and controls the refrigerant superheating temperature at an outlet of the evaporator 8 to a target value.
The evaporator 8 performs heat exchange between the gas-liquid two-phase refrigerant from the second electronic expansion valve 7 and a medium to be cooled, and evaporates the gas-liquid two-phase refrigerant by absorbing heat from the medium to be cooled, thereby causing the multistage compressor 2 to take it in as low-temperature, low-pressure gaseous refrigerant. The above-described components form the supercritical-cycle heat pump 1 provided with the injection circuit 11 for injecting the intermediate-pressure refrigerant gas from the gas-liquid separator 6 into the sealed housing 14, having an intermediate-pressure atmosphere in the interior thereof, in the multistage compressor 2.
In the refrigerant circuit (refrigerating cycle) 10 of the supercritical-cycle heat pump 1 described above, a discharge pipe from the multistage compressor 2 is provided with a discharge pressure sensor 18 and a temperature sensor 19 that detect the pressure and temperature of the discharged refrigerant gas; the intake refrigerant pipe 9A in the multistage compressor 2 is provided with an intake pressure sensor 20 and a temperature sensor 21 that detect the pressure and temperature of the intake refrigerant gas; and, additionally, the injection circuit 11 is provided with an intermediate pressure sensor 22 and a temperature sensor 23 that detect the pressure and temperature of the intermediate-pressure refrigerant. In addition, the sealed housing 14 in the multistage compressor 2 is provided, at the bottom portion thereof, with an oil temperature sensor 24 that detects the oil temperature of the lubricant 17.
Detected values from the discharge pressure sensor 18 and the temperature sensor 19 are used for high-pressure protection, discharge-temperature control, discharge-superheating temperature control, or the like, and the intake pressure sensor 20 and the temperature sensor 21 are employed for low-pressure protection and intake-superheating temperature control by the second electronic expansion valve 7. Furthermore, the detected values from the intermediate pressure sensor 22, the temperature sensor 23, and the oil temperature sensor 24 are used for the following control for keeping the viscosity of the POE oil, employed as the lubricant 17, in the certain viscosity zone.
The viscosity of the POE oil 17 is controlled in the following way via a controller 25.
The viscosity of the POE oil 17 depends on its solubility in the CO₂ refrigerant, which is determined by the pressure and temperature of the CO₂ refrigerant. The solubility of the POE oil 17 in the CO₂ refrigerant has the characteristic that the solubility increases with an increase in pressure if the temperature is the same, and, in addition, the solubility increases with a decrease in temperature if the pressure is the same, as is clear from a pressure-solubility characteristic diagram shown in Fig. 3, with temperature as a parameter; for example, when the pressure is 5.4 MPa, the solubility is 20 wt% if the temperature is 60 °C.
On the other hand, the viscosity of the POE oil 17 when dissolved in the CO₂ refrigerant has the characteristic that the viscosity decreases with an increase in the solubility if temperature of the POE oil 17 is the same and, in addition, the viscosity decreases with an increase in temperature of the POE oil 17 if the solubility is the same, as is clear from a temperature-viscosity characteristic diagram shown in Fig. 4, with solubility as a parameter; for example, when the solubility is 20 wt%, as described above, the viscosity is 5.0 mPa•s if temperature is 40 °C. In this way, the viscosity of the POE oil 17 filled in the sealed housing 14, having intermediate pressure in the interior thereof, in the multistage compressor 2 can be ascertained by measuring the pressure and temperature of the intermediate-pressure refrigerant, and this fact indicates that the viscosity of the POE oil 17 can he controlled by controlling the pressure and temperature of the intermediate-pressure refrigerant.
Therefore, the pressure and temperature of the intermediate-pressure refrigerant are controlled by the controller 25 via the first electronic expansion valve 5 so that the viscosity of the POE oil 17 is kept above the certain viscosity zone so as to prevent an increase in the dilution ratio when the refrigerant dissolves in the POE oil 17, which has a high compatibility with the CO₂ refrigerant, as well as a resultant decrease in the viscosity of the oil. Specifically, the pressure and temperature of the intermediate-pressure refrigerant are measured by the intermediate pressure sensor 22 and the temperature sensor 23, the solubility of the POE oil in the CO₂ refrigerant under that pressure and temperature is determined by using Fig. 3, and the viscosity of the POE oil 17 is determined from that solubility and temperature of the POE oil 17 by using Fig. 4. Then, in order to keep that viscosity above the certain viscosity zone, the pressure and temperature of the intermediate-pressure refrigerant are controlled by the controller 25 via the first electronic expansion valve 5 so that the pressure thereof is controlled within a preset usage limit range, and thereby, the viscosity of the POE oil 17 is set above the certain viscosity zone.
In addition, the controller 25 detects the pressure and temperature of the intermediate-pressure refrigerant to be injected into the sealed housing 14 from the injection circuit 11 and those of the intake refrigerant for the multistage compressor 2 with the intermediate pressure sensor 22, the temperature sensor 23, the intake pressure sensor 20, and the temperature sensor 21 respectively; controls refrigerant superheating temperatures of the intermediate-pressure refrigerant and the intake refrigerant to respective target superheating temperatures (for example, intermediate-pressure saturation temperature + α deg. and intake-pressure saturation temperature + α deg.) with the first electronic expansion valve and the second electronic expansion valve; and also controls the pressure and temperature of the intermediate-pressure refrigerant via the first electronic expansion valve 5 so that the pressure thereof is controlled within the preset usage limit range, in order to keep the viscosity of the POE oil 17 above the certain viscosity zone.
Furthermore, in this embodiment, because the multistage compressor 2 of the supercritical-cycle heat pump 1 is a two-stage compressor, as pressure ranges in which it can be operated, a lower-stage usage limit range and a higher-stage usage limit range are preset on the basis of the relationship between the low pressure and intermediate pressure on the lower-stage side and the relationship between intermediate pressure and high pressure on the higher-stage side in consideration of the above-described points, as shown in Fig. 2. Then, for example, when a rated value (first threshold) for the viscosity of the POE oil 17 is 5.0 mPa•s, the supercritical-cycle heat pump 1 (multistage compressor 2) can be operated in the entire regions of the above-described usage limit ranges, so long as the viscosity of the POE oil 17 is at or above this rated value.
In addition, in this embodiment, operation is possible in portions of the regions even if the viscosity of the POE oil 17 in the sealed housing 14 does not reach the rated value (first threshold) described above. Specifically, so long as the viscosity of the POE oil 17 in the sealed housing 14 reaches a second rated value (second threshold), for example, 3.0 mPa•s, which is lower than the above-described rated value 5.0 mPa•s (for example, when the viscosity is 4.0 mPa•s), operation is possible only in the pressure ranges below limit lines L in the above-described usage limit ranges. This is a result of a decreased required viscosity caused by a decrease in the load on a bearing in the ranges below the imit lines L. Because of this it is possible to reliably prevent a decrease in the lubrication performance due to a decrease in the viscosity of the POE oil 17 which is caused by an increase in the oil temperature, and it is also possible to ensure a sufficient degree of freedom for the operation of the supercritical-cycle heat pump 1.
With the above-described configuration, this embodiment affords the following operational advantages.
In the above-described supercritical-cycle heat pump 1, the CO₂ refrigerant compressed to the intermediate pressure at the lower-stage compression mechanism 15 of the multistage compressor 2 is discharged into the sealed housing 14 and is taken into the higher-stage compression mechanism 16 together with the intermediate-pressure refrigerant gas injected into the sealed housing 14 from the injection circuit 11. This refrigerant undergoes two-stage compression to high pressure at the higher-stage compression mechanism 16, is discharged toward the refrigerant circuit (refrigerating cycle) 10, and is introduced into the radiator 4 after the lubricant 17 in the refrigerant is separated at the oil separator 3.
The refrigerant introduced into the radiator 4 reaches a supercritical state or a condensed liquefied state by releasing heat to the cooling medium, is depressurized to intermediate pressure by the first electronic expansion valve 5, thereby reaching the gas-liquid separator 6 in the gas-liquid two-phase state, and is separated therein into the intermediate-pressure liquid refrigerant and the intermediate-pressure gaseous refrigerant. The separated intermediate-pressure gaseous refrigerant is injected into the sealed housing 14 of the multistage compressor 2 via the injection circuit 11, as described above. On the other hand, the intermediate-pressure liquid refrigerant is depressurized again by the second electronic expansion valve 7, thereby being supplied to the evaporator 8 in the form of low-temperature, low-pressure gas-liquid two-phase refrigerant.
while circulating in the evaporator 8, the low-pressure, low-temperature, gas-liquid two-phase refrigerant that has flowed into the evaporator 8 undergoes heat exchange with the medium to be cooled and is evaporated by absorbing heat from the medium to be cooled. This low-pressure refrigerant gas merges with the oil from the oil-return circuit 13 via the intake refrigerant pipe 9A and is taken into the lower-stage compression mechanism 15 of the multistage compressor 2 to be recompressed. While repeating the above cycle, the heat released at the radiator 4 can be utilized for space heating, heating, supplying hot water, and so forth, and, on the other hand, the heat-absorption effect at the evaporator 8 can be utilized for cooling air or cooling.
During this process, in the case in which the heat released from the radiator 4 is utilized for space heating, heating, supplying hot water, and so forth, because the injected intermediate-pressure refrigerant is added to the refrigerant flowing in the radiator 4, the circulated volume of the refrigerant is increased, and the capacities for space heating, heating, or supplying hot water can be enhanced by a corresponding amount. In addition, in the case in which the heat absorption at the evaporator 8 is utilized for space cooling, cooling, and so forth, because enthalpy is increased and the heat energy of the refrigerant evaporated at the evaporator 8 is increased, the capacities for space cooling or cooling can be enhanced by a corresponding amount.
In addition, in this embodiment, the POE oil, which has high compatibility with the CO₂ refrigerant, is employed as the lubricant (refrigerator oil) 17 filled in the sealed housing 14 of the multistage compressor 2. Because of this, even with the supercritical-cycle heat pump 1 provided with the injection circuit 11 with a gas-liquid separation system including the gas-liquid separator 6, there is no risk of creating situations such as insufficient lubricant in the multistage compressor 2 due to deterioration of the oil-return capacity, resulting from the oil being separated at the gas-liquid separator 6, and so on, and it is possible to reliably ensure sufficient lubrication performance in the multistage compressor 2, which makes it possible to enhance the reliability thereof.
Furthermore, because the POE oil 17 has high compatibility with the CO₂ refrigerant, there are concerns that the refrigerant will increase the dilution ratio and decrease the viscosity; however, the controller 25 is provided, which determines the viscosity of the POE oil 17 from the solubility and the temperature of the POE oil 17 on the basis of the solubility of the POE oil 17 in the CO₂ refrigerant under the pressure and the temperature of the intermediate-pressure refrigerant, controls the pressure and the temperature of the intermediate-pressure refrigerant via the first electronic expansion valve 5 so that the pressure thereof is controlled within the preset usage limit range in order to keep the viscosity of the POE oil 17 above the certain viscosity zone, and, by doing so, the viscosity of the POE oil 17 is kept above the certain viscosity zone by controlling the pressure of the intermediate-pressure refrigerant within the preset usage limit range, which thereby prevents an increase in the dilution ratio of the POE oil 17 and a decrease in the viscosity thereof, which are affected by the pressure and the temperature of the refrigerant. Accordingly, it is possible to eliminate a decrease in the lubrication performance due to an increase in the dilution ratio of the POE oil 17, a decrease in the viscosity thereof, and so forth in the multistage compressor 2, which makes it possible to ensure sufficient reliability.
In addition, the above-described controller 25 is configured so as to detect the pressure and the temperature of the intermediate-pressure refrigerant gas injected into the sealed housing 14 of the multistage compressor 2 via the injection circuit 11 and those of refrigerant gas that is taken into the multistage compressor 2; to control respective refrigerant superheating temperatures to the target superheating temperatures (for example, intermediate-pressure saturation temperature + α deg. and intake-pressure saturation temperature + α deg.) with the first electronic expansion valve 5 and the second electronic expansion valve 7; and to also control the pressure and the temperature of the intermediate-pressure refrigerant via the first electronic expansion valve 5 so that the pressure thereof is controlled within the preset usage limit range in order to keep the viscosity of the POE oil 17 above the certain viscosity zone.
Because of this, by controlling the existing first electronic expansion valve 5 and second electronic expansion valve 7, provided upstream and downstream of the gas-liquid separator 6 connected to the injection circuit 11, the viscosity of the POE oil 17 can be kept above the certain viscosity zone by merely changing software for the controller 25 without additionally providing new devices, which makes it possible to prevent an increase in the dilution ratio of the POE oil 17 and a decrease in the viscosity thereof. Therefore, it is possible to achieve improvements in the lubrication performance in a simple manner by employing the POE oil 17, which has high compatibility, while avoiding an increase in complexity of the hardware configuration.
Furthermore, in this embodiment, the multistage compressor 2 is a two-stage compressor, and, as shown in Fig. 2, the usage limit ranges are changed in accordance with the viscosity of the POE oil 17 in the sealed housing 14 in the lower-stage usage limit range and the higher-stage usage limit range, which are preset on the basis of the relationship between the low pressure and intermediate pressure on the lower-stage side and the relationship between the intermediate pressure and high pressure on the higher-stage side, thus enabling the operation in portions of limited pressure ranges even when the viscosity of the POE oil 17 does not reach the rated value (first threshold), so long as it reaches the second rated value (second threshold), which is lower than the rated value. Because of this, even when the viscosity of the POE oil 17 in the sealed housing 14 does not reach the rated value, operation is possible in the limited pressure ranges, which are portions of the usage limit ranges (for example, the pressure ranges below the limit lines L shown in Fig. 2) so long as the second rated value is reached. Therefore, it is possible to reliably prevent a decrease in the lubrication performance due to a decrease in the viscosity of the POE oil 17 which is caused by an increase in the oil temperature, and it is also possible to ensure a sufficient degree of freedom for the operation of the supercritical-cycle heat pump 1.
Note that the present invention is not limited to the invention according to the above-described embodiment, and appropriate alterations are possible within a range that does not depart from the spirit thereof. For example, although the above-described embodiment has been described in terms of an example in which the oil separator 3, the heat exchanger 12, and the oil-return circuit 13 are provided, they are not essential and they may be omitted. In addition, in the multistage compressor 2, the electric motor, the lower-stage compression mechanism 15, and the higher-stage compression mechanism 16, which are built into the sealed housing 14, may be arranged in any way.
Furthermore, the supercritical-cycle heat pump 1 according to the present invention is applicable to a wide range of usages without limitation to air conditioners, hot-water supply units, and so forth; and it is, of course, applicable to a unit in which a four-way switching valve is provided between the discharge side and the intake side of the multistage compressor 2, which makes it possible to switch the refrigerant circuit (refrigerating cycle) 10 between a heating cycle and a cooling cycle.

Reference Signs List

1: supercritical-cycle heat pump
2: multistage compressor
4: radiator
5: first electronic expansion valve
6: gas-liquid separator
7: second electronic expansion value
8: evaporator
10: refrigerant circuit (refrigerating cycle)
11: injection circuit
14: sealed housing
15: lower-stage compression mechanism
16: higher-stage compression mechanism
17: lubricant (POE oil)
18: discharge pressure sensor
19, 21, 23,: temperature sensor
20: intake pressure sensor
22: intermediate pressure sensor
24: oil temperature sensor
25: controller
L: limit line

Claims

A supercritical-cycle heat pump that employs CO₂ refrigerant as a working medium comprising;
a multistage compressor (2) that is provided with a lower-stage compression mechanism (15) and a higher-stage compression mechanism (16), that discharges intermediate-pressure refrigerant gas compressed by the lower-stage compression mechanism (15) into a sealed housing, and that takes this refrigerant gas into the higher-stage compression mechanism (16) to perform compression to high pressure; characterized in that it comprises
a radiator (4);
a first electronic expansion valve (5);
a gas-liquid separator (6);
a second electronic expansion valve (7);
an evaporator (8);
an injection circuit (11) for injecting intermediate-pressure refrigerant gas separated at the gas-liquid separator (6) into the sealed housing of the multistage compressor (2); and
a controller (25) that controls pressure of the intermediate-pressure refrigerant by controlling the first electronic expansion valve (5) so that the pressure of the intermediate-pressure is controlled within a preset usage limit range in order to keep viscosity of oil which is employed as lubricant for the multistage compressor (2) above a certain viscosity zone, the controller (25) determining the viscosity of the oil on the basis of solubility of the oil in the CO₂ refrigerant and temperature of the oil, the solubility of the oil in the CO₂ refrigerant being determined by pressure and temperature of the intermediate-pressure refrigerant,
wherein a refrigerating cycle (10) is formed in which the multistage compressor (2), the radiator (4), the first electronic expansion valve (5), the gas-liquid separator (6), the second electronic expansion valve (7), and the evaporator (8) are connected in this order, and
wherein polyol-ester based oil or mixed oil of polyol-ester based oil is employed as the lubricant for the multistage compressor (2).
A supercritical-cycle heat pump according to Claim 1, further comprising:
an intermediate pressure sensor (22) for detecting the pressure of the intermediate-pressure refrigerant to be injected from the injection circuit (11) to the sealed housing of the multistage compressor (2);

an intermediate temperature sensor (23) for detecting the temperature of the intermediate-pressure refrigerant to be injected from the injection circuit (11) to the sealed housing of the multistage compressor (2);

an intake pressure sensor (20) for detecting the pressure of intake refrigerant for the multistage compressor (2);

an intake temperature sensor (21) for detecting the temperature of the intake refrigerant for the multistage compressor (2),

wherein the controller (25) controls refrigerant superheating temperatures of the intermediate-pressure refrigerant and the intake refrigerant to respective target superheating temperatures with the first electronic expansion valve (5) and the second electronic expansion valve (7).
A supercritical-cycle heat pump according to Claim 1 or 2, wherein the controller (25) changes usage limit ranges in accordance with the viscosity of the oil in the sealed housing within a lower-stage usage limit range and a higher-stage usage limit range, which are preset based on the relationship between low pressure and intermediate pressure and that between intermediate pressure and high pressure, thus enabling operation in portions of limited pressure ranges even when the viscosity of the oil does not reach a first rated value so long as a second rated value, which is lower than the first rated value, is reached.