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A waste heat utilization device includes a refrigeration cycle having a compressor, a Rankine cycle using a condenser in common with the refrigeration cycle, and a control unit which controls operation of the refrigeration cycle and the Rankine cycle. The control unit performs a continuation control

A waste heat utilization device includes a refrigeration cycle having a compressor, a Rankine cycle using a condenser in common with the refrigeration cycle, and a control unit which controls operation of the refrigeration cycle and the Rankine cycle. The control unit performs a continuation control of the Rankine cycle, in which the Rankine cycle is continuously operated regardless of an operation state of the compressor, when a load of the refrigeration cycle is lower than a predetermined load. In contrast, the control unit performs an intermittent control of the Rankine cycle, in which the Rankine cycle is intermittently operated in accordance with the operation state of the compressor such that the mechanical energy recovered by the expansion unit is larger than a driving energy of the compressor.

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What is claimed is: 1. A waste heat utilization device comprising: a refrigeration cycle having a compressor which draws a low-pressure refrigerant flowing from a side of an evaporator for cooling air, compresses the drawn refrigerant to have a high temperature and high pressure, and discharges the

What is claimed is: 1. A waste heat utilization device comprising: a refrigeration cycle having a compressor which draws a low-pressure refrigerant flowing from a side of an evaporator for cooling air, compresses the drawn refrigerant to have a high temperature and high pressure, and discharges the compressed refrigerant toward a condenser in which refrigerant is cooled and condensed; a Rankine cycle using the condenser in common with the refrigeration cycle, the Rankine cycle including a pump for pumping and sending refrigerant, and an expansion unit which expands refrigerant sent by the pump and heated by waste heat from a heat generating unit so as to recover a mechanical energy; and a control unit which controls operation of the refrigeration cycle and the Rankine cycle, wherein: the control unit performs a continuation control of the Rankine cycle, in which the Rankine cycle is continuously operated regardless of an operation state of the compressor, when a load of the refrigeration cycle is lower than a predetermined load; and the control unit performs an intermittent control of the Rankine cycle, in which the Rankine cycle is intermittently operated in accordance with the operation state of the compressor when the load of the refrigeration cycle is higher than the predetermined load, wherein, in the intermittent control, a quantity of mechanical energy recovered by the expansion unit is larger than a quantity of driving energy required by the compressor for the operation of the Rankine cycle. 2. The waste heat utilization device according to claim 1, wherein: the control unit performs an intermittent operation of the compressor, in which the compressor is intermittently operated when performing the intermittent control of the Rankine cycle; and the control unit performs the intermittent control of the Rankine cycle to be reversed in an operation state with the intermittent operation of the compressor. 3. The waste heat utilization device according to claim 2, wherein: the compressor is a fixed displacement compressor that is driven by a driving source and has a predetermined discharge capacity per rotation; and the control unit performs the intermittent operation of the compressor by switching a clutch connected with the driving source or turning on or off the driving source. 4. The waste heat utilization device according to claim 2, wherein: the compressor is a variable displacement compressor with an adjustable discharge capacity per rotation; and the control unit performs the intermittent operation of the compressor by switching the discharge capacity between a first capacity and a second capacity larger than the first capacity. 5. The waste heat utilization device according to claim 1, wherein: the Rankine cycle further includes a switching unit for opening and closing a refrigerant path provided therein; and the control unit controls the switching unit to close the refrigerant path when the operation of the Rankine cycle is stopped during the intermittent control of the Rankine cycle. 6. The waste heat utilization device according to claim 5, wherein: the control unit stops the pump after the switching unit closes the refrigerant path when the operation of the Rankine cycle is stopped during the intermittent control of the Rankine cycle. 7. The waste heat utilization device according to claim 5, wherein: the switching unit is located at a refrigerant inlet side of the expansion unit. 8. The waste heat utilization device according to claim 1, wherein: the control unit determines the load of the refrigeration cycle by using an outside air temperature. 9. The waste heat utilization device according to claim 1, wherein: the control unit determines the load of the refrigeration cycle by combining information for controlling the heat generating unit and information for controlling the refrigeration cycle. 10. The waste heat utilization device according to claim 9, wherein: the heat generating unit is an internal combustion engine for a vehicle; the information for controlling the heat generating unit is at least one of a vehicle speed, a rotation speed of the internal combustion engine, an outside air temperature, a coolant temperature of the internal combustion engine. 11. The waste heat utilization device according to claim 9, wherein: the information for controlling the refrigeration cycle is at least one of an interior temperature of a compartment to be air-conditioned, a target air temperature of air to be blown into the compartment, an air temperature of air cooled by the evaporator, and a refrigerant pressure in the refrigeration cycle. 12. A control method of a waste heat utilization device that compnses: a refrigeration cycle including a compressor which draws a low-pressure refrigerant flowing from a side of an evaporator in which refrigerant is evaporated, compresses the drawn refrigerant to have a high temperature and high pressure, and discharges the compressed refrigerant toward a condenser in which refrigerant is cooled and condensed; and a Rankine cycle which uses the condenser in common with the refrigeration cycle, and includes a pump for pumping and sending refrigerant, and an expansion unit which expands refrigerant sent by the pump and heated by waste heat from a heat generating unit so as to recover a mechanical energy, the control method comprising: performing a continuation control of the Rankine cycle, in which the Rankine cycle is continuously operated regardless of an operation state of the compressor, when a load of the refrigeration cycle is lower than a predetermined load; and performing an intermittent control of the Rankine cycle, in which the Rankine cycle is intermittently operated in accordance with the operation state of the compressor when the load of the refrigeration cycle is higher than the predetermined load, wherein, in the intermittent control, a quantity of mechanical energy recovered by the expansion unit is larger than a quantity of driving energy required by the compressor for the operation of the Rankine cycle. 13. The control method of the waste heat utilization device according to claim 12, further comprising: performing an intermittent operation of the compressor, in which the compressor is intermittently operated when the intermittent control of the Rankine cycle is performed; and controlling the intermittent control of the Rankine cycle to be reversed in an operation state with the intermittent operation of the compressor. 14. The control method of the waste heat utilization device according to claim 13, wherein: the compressor is a fixed displacement compressor that is driven by a driving source and has a predetermined discharge capacity per rotation; and the intermittent operation of the compressor is performed by switching a clutch connected with the driving source or turning on or off the driving source. 15. The control method of the waste heat utilization device according to claim 13, wherein: the compressor is a variable displacement compressor with an adjustable discharge capacity per rotation; and the intermittent operation of the compressor is performed by switching the discharge capacity between a first capacity and a second capacity larger than the first capacity. 16. The control method of the waste heat utilization device according to claim 12, wherein the Rankine cycle further includes a switching unit for opening and closing a refrigerant path provided therein, the control method further comprising controlling the switching unit to close the refrigerant path when the operation of the Rankine cycle is stopped during the intermittent control of the Rankine cycle. 17. The control method of the waste heat utilization device according to claim 16, further comprising stopping the pump after the switching unit closes the refrigerant path when the operation of the Rankine cycle is stopped during the intermittent control of the Rankine cycle. 18. The control method of the waste heat utilization device according to claim 16, wherein: the switching unit is located at a refrigerant inlet side of the expansion unit. 19. The control method of the waste heat utilization device according to claim 12, further comprising determining the load of the refrigeration cycle by using an outside air temperature. 20. The control method of the waste heat utilization device according to claim 12, further comprising determining the load of the refrigeration cycle by combining information for controlling the heat generating unit and information for controlling the refrigeration cycle. 21. The control method of the waste heat utilization device according to claim 20, wherein: the heat generating unit is an internal combustion engine for a vehicle; the information for controlling the heat generating unit is at least one of a vehicle speed, a rotation speed of the internal combustion engine, an outside air temperature, a coolant temperature of the internal combustion engine. 22. The control method of the waste heat utilization device according to claim 20, wherein: the information for controlling the refrigeration cycle is at least one of an interior temperature of a compartment to be air-conditioned, a target air temperature of air to be blown into the compartment, an air temperature of air cooled by the evaporator, and a refrigerant pressure in the refrigeration cycle. 23. The control method of the waste heat utilization device according to claim 12, further comprising determining the load of the refrigeration cycle by using information for controlling the refrigeration cycle, wherein the information includes at least one of an interior temperature of a compartment to be air-conditioned, an outside air temperature, a target air temperature of air to be blown into the compartment, an air temperature of air cooled by the evaporator, and a refrigerant pressure in the refrigeration cycle. 24. The waste heat utilization device according to claim 1, wherein the control unit causes the refrigeration cycle and the Rankine cycle to perform a single operation of the refrigeration cycle, a single operation of the Rankine cycle, and a combination operation of the refrigeration cycle and the Rankine cycle; and the control unit performs the intermittent control of the Rankine cycle in the combination operation of the refrigeration cycle and the Rankine cycle. 25. The control method of the waste heat utilization device according to claim 12, wherein the control unit causes the refrigeration cycle and the Rankine cycle to perform a single operation of the refrigeration cycle, a single operation of the Rankine cycle, and a combination operation of the refrigeration cycle and the Rankine cycle; and the control unit performs the intermittent control of the Rankine cycle in the combination operation of the refrigeration cycle and the Rankine cycle. 26. A waste heat utilization device comprising: a refrigeration cycle having a compressor which draws a low-pressure refrigerant flowing from a side of an evaporator for cooling air, compresses the drawn refrigerant to have a high temperature and high pressure, and discharges the compressed refrigerant toward a condenser in which refrigerant is cooled and condensed; a Rankine cycle using the condenser in common with the refrigeration cycle, the Rankine cycle including a pump for pumping and sending refrigerant, and an expansion unit which expands refrigerant sent by the pump and heated by waste heat from a heat generating unit so as to recover a mechanical energy; and a means for controlling operation of the refrigeration cycle and the Rankine cycle to perform a continuation control of the Rankine cycle, in which the Rankine cycle is continuously operated regardless of an operation state of the compressor when a load of the refrigeration cycle is lower than a predetermined load, and to perform an intermittent control of the Rankine cycle, in which the Rankine cycle is intermittently operated in accordance with the operation state of the compressor when the load of the refrigeration cycle is higher than the predetermined load, and in the intermittent control, a quantity of mechanical energy recovered by the expansion unit is larger than a quantity of driving energy required by the compressor for the operation of the Rankine cycle.