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In a heat-pump water heater with a super-critical refrigerant cycle, a valve open degree of a decompression valve is controlled to control a pressure of high-pressure side refrigerant so that a temperature difference between refrigerant flowing out from the water-refrigerant heat exchanger and water

In a heat-pump water heater with a super-critical refrigerant cycle, a valve open degree of a decompression valve is controlled to control a pressure of high-pressure side refrigerant so that a temperature difference between refrigerant flowing out from the water-refrigerant heat exchanger and water flowing into a water-refrigerant heat exchanger is set in a predetermined temperature range. Thus, the pressure of high-pressure side refrigerant in the super-critical refrigerant cycle can be controlled, thereby suitably adjusting heat-exchange performance of an internal heat exchanger, and restricting the temperature of refrigerant discharged from the refrigerant compressor from being uselessly increased.

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What is claimed is: 1. A super-critical refrigerant cycle system comprising: a refrigerant compressor for compressing refrigerant to a pressure equal to or higher than critical pressure of the refrigerant; a heating heat exchanger having a first section through which the refrigerant flows and a se

What is claimed is: 1. A super-critical refrigerant cycle system comprising: a refrigerant compressor for compressing refrigerant to a pressure equal to or higher than critical pressure of the refrigerant; a heating heat exchanger having a first section through which the refrigerant flows and a separate second section through which a fluid other than the refrigerant flows, the heating heat exchanger heating the fluid by performing heat-exchange between the fluid and the refrigerant discharged from the refrigerant compressor; a refrigerant evaporator for evaporating refrigerant; an internal heat exchanger for performing heat-exchange between refrigerant flowing out from the heating heat exchanger and refrigerant flowing toward the refrigerant compressor after leaving the refrigerant evaporator; a decompression valve for decompressing refrigerant coming directly from the internal heat exchanger, and for supplying the decompressed refrigerant to the refrigerant evaporator; and a controller that controls a valve open degree of the decompression valve to control a pressure of high-pressure side refrigerant after leaving the heating heat exchanger and before being decompressed to continuously set a state where a difference between a refrigerant outlet temperature of the first section of the heating heat exchanger and a fluid inlet temperature of the second section of the heating heat exchanger approaches a predetermined temperature value. 2. The super-critical refrigerant cycle system according to claim 1, further comprising: a fluid-temperature detection device for detecting the fluid inlet temperature at an inlet side of the fluid in the heating heat exchanger; an outlet refrigerant-temperature detection device for detecting the refrigerant outlet temperature at an outlet side of refrigerant in the heating heat exchanger. 3. The super-critical refrigerant cycle system according to claim 1, wherein the internal heat exchanger includes a first refrigerant heat-exchanging part disposed between an outlet of the heating heat exchanger and the decompression valve, and a second refrigerant heat-exchanging part disposed between an outlet of the refrigerant evaporator and a suction port of the refrigerant compressor, the system further comprising: a first refrigerant-temperature detection device for detecting an inlet temperature of refrigerant flowing into the second refrigerant heat-exchanging part of the internal heat exchanger; and a second refrigerant-temperature detection device for detecting an outlet temperature of refrigerant flowing out from the second refrigerant heat-exchanging part of the internal heat exchanger; and the controller controls the valve open degree of the decompression valve such that a deference between the outlet temperature of refrigerant and the inlet temperature of refrigerant in the second refrigerant heat-exchanging part is set smaller than a predetermined temperature. 4. The super-critical refrigerant cycle system according to claim 1, further comprising a discharge refrigerant-temperature detection device for detecting a discharge temperature of refrigerant discharged from the refrigerant compressor, wherein the controller controls the valve open degree of the decompression valve such that the discharge temperature of refrigerant becomes lower than a predetermined temperature. 5. The super-critical refrigerant cycle system according to claim 1, further comprising an accumulator including a storage chamber for temporarily storing refrigerant flowing from the refrigerant evaporator, and an outlet pipe inserted into the accumulator for mainly supplying gas refrigerant from the storage chamber to the refrigerant compressor through the internal heat exchanger, wherein: the outlet pipe has an opening at its top end in the storage chamber, from which gas refrigerant is introduced from the storage chamber into the outlet pipe, an oil return hole at its lower portion in the storage chamber, for introducing an oil in the refrigerant from the storage chamber into the outlet pipe, and a liquid-refrigerant return hole at its upper portion upper than the oil return hole in the storage chamber, for introducing liquid refrigerant from the storage chamber into the outlet pipe. 6. The super-critical refrigerant cycle system according to claim 5, wherein: the liquid-refrigerant return hole is provided at a position which becomes equal to or lower than a liquid refrigerant surface in the storage chamber when the temperature of the fluid flowing into the heating heat exchanger is low, and which becomes higher than the liquid-refrigerant surface in the storage chamber when the temperature of the fluid flowing into the heating heat exchanger is high. 7. The super-critical refrigerant cycle system according to claim 5, wherein: the refrigerant evaporator is disposed to evaporate refrigerant by absorbing heat from air; and the liquid-refrigerant return hole is provided at a position which becomes equal to or lower than a liquid refrigerant surface in the storage chamber when the temperature of air flowing to the refrigerant evaporator is low, and which becomes higher than the liquid-refrigerant surface in the storage chamber when the temperature of air flowing to the refrigerant evaporator is high. 8. The super-critical refrigerant cycle system according to claim 5, wherein an open area of the liquid-refrigerant return hole is set smaller than that of the opening at the top end of the outlet pipe. 9. The super-critical refrigerant cycle system according to claim 5, wherein: the oil is a lubrication oil used for the refrigerant compressor, that is undissolvable with liquid refrigerant in the storage chamber; and the oil has a density larger than that of the liquid refrigerant. 10. The super-critical refrigerant cycle system according to claim 1, wherein: the heating heat exchanger is disposed to heat water to be supplied by using the fluid as a heating source. 11. The super-critical refrigerant cycle according to claim 1, wherein: the fluid is water to be supplied; and the heating heat exchanger is disposed to perform heat exchange between the water and the refrigerant discharged from the compressor to heat the water to be supplied. 12. A water heater for heating water to be supplied, comprising: a refrigerant compressor for compressing refrigerant to a pressure equal to or higher than critical pressure of the refrigerant; a heating heat exchanger having a first section through which the refrigerant flows and a separate second section through which the water flows, the heating heat exchanger heating the water to a predetermined temperature by performing heat-exchange between the water and the refrigerant discharged from the refrigerant compressor; a refrigerant evaporator for evaporating refrigerant by absorbing heat from air; an internal heat exchanger for performing heat-exchange between refrigerant flowing out from the heating heat exchanger and refrigerant flowing toward the refrigerant compressor from the refrigerant evaporator; a decompression valve for decompressing refrigerant from the internal heat exchanger, and for supplying the decompressed refrigerant to the refrigerant evaporator; a water-temperature detection device for detecting a water inlet temperature before being heat-exchanged in the heating heat exchanger; an outlet refrigerant-temperature detection device for detecting a refrigerant outlet temperature after being heat-exchanged in the heating heat exchanger; and a controller that controls a valve open degree of the decompression valve to continuously set a state where a difference between a refrigerant outlet temperature of the first section of the heating heat exchanger and a water inlet temperature of the second section of the heating heat exchanger approaches a predetermined temperature value. 13. The super-critical refrigerant cycle according to claim 1, wherein heating heat exchanger has a fluid passage through which the fluid flows, and a refrigerant passage through which the refrigerant flows in a flow direction opposite to that of the fluid in the fluid passage. 14. The water heater according to claim 12, wherein heating heat exchanger has a water passage through which water flows, and a refrigerant passage through which the refrigerant flows in a flow direction opposite to that of water in the water passage. 15. The super-critical refrigerant cycle system according to claim 1, wherein the predetermined temperature value is changed in accordance with an outside air temperature. 16. The super-critical refrigerant cycle system according to claim 1, wherein the predetermined temperature value is set in a range of 5-15째 C. 17. The super-critical refrigerant cycle system according to claim 1, wherein the fluid is water to be supplied to a water tank, and the heating heat exchanger heats the water flowing from the water tank. 18. The super-critical refrigerant cycle system according to claim 1, wherein the internal heat exchanger has a first portion and a separate second portion, the first portion being disposed between the first section of the heating heat exchanger and the decompression valve, the second portion being disposed between the evaporator and the compressor. 19. The super-critical refrigerant cycle system according to claim 12, wherein the internal heat exchanger has a first portion and a separate second portion, the first portion being disposed between the first section of the heating heat exchanger and the decompression valve, the second portion being disposed between the evaporator and the compressor.