A CO.sub.2 refrigeration system includes a CO.sub.2 refrigeration subsystem that provides cooling for a refrigeration load using carbon dioxide (CO.sub.2) as a refrigerant. The CO.sub.2 refrigeration system further includes a direct CO.sub.2 heat exchange subsystem that uses the CO.sub.2 refrigerant from the CO.sub.2 refrigeration subsystem to provide heating or cooling for a building zone. The direct CO.sub.2 heat exchange subsystem includes a heat exchanger that exchanges heat directly between the CO.sub.2 refrigerant and an airflow provided to the building zone.

Provided is a heat-pump system including a plurality of compressors, wherein the plurality of compressors includes a first compressor and a second compressor that compress refrigerant, an oil separator provided on a discharge side of the plurality of compressors to separate oil mixed with refrigerant compressed by the plurality of compressors, an oil separation pipe extended from the oil separator to allow the plurality of compressors to recover oil, and a compressor side oil balance pipe extended from the second compressor to allow the first compressor to recover oil stored in the second compressor.

Provided is a heat-pump system including a plurality of compressors, wherein the plurality of compressors includes a first compressor and a second compressor that compress refrigerant, an oil separator provided on a discharge side of the plurality of compressors to separate oil mixed with refrigerant compressed by the plurality of compressors, an oil separation pipe extended from the oil separator to allow the plurality of compressors to recover oil, and a compressor side oil balance pipe extended from the second compressor to allow the first compressor to recover oil stored in the second compressor.

In some embodiments, an air-cooled ammonia refrigeration system comprises: a plurality of air-cooled condensers, each having a heat exchanger and at least one axial fan and having a first operating state capable of condensing vaporous ammonia to form liquid ammonia; an evaporator coupled to the air-cooled condenser; a subcooler positioned between the air-cooled condenser and the evaporator; a compressor coupled to the evaporator; an oil cooler coupled to the compressor; and a plurality of valves coupled to the plurality of air-cooled condensers and having a first configuration corresponding to the first operating state of the plurality of air-cooled condensers, and a second configuration corresponding to a second operating state of one or more of the plurality of air-cooled condensers such that the one or more of the plurality of air-cooled condensers functions as an evaporator capable of evaporating liquid ammonia to form vaporous ammonia.

Cold storage unit apparatuses designed for high energy efficiency are provided, which may include: a refrigeration system; a box enclosing an interior space, the box formed by a plurality of insulated sides; an entry into the interior space including a plurality of openable barriers facilitating preventing entry of heat or moisture into the interior space; and a power system connected to the refrigeration system, the power system facilitating independent operation of the refrigeration system when not connected to a power grid, and further facilitating a net energy use of zero from a power grid when connected to a power grid. Cold storage units may further include systems for using the refrigerant of the refrigeration system to defrost one or more components of the cold storage unit, and for using the refrigerant to facilitate maintaining a desired internal temperature of the interior space of the cold storage unit.

Cold storage unit apparatuses designed for high energy efficiency are provided, which may include: a refrigeration system; a box enclosing an interior space, the box formed by a plurality of insulated sides; an entry into the interior space including a plurality of openable barriers facilitating preventing entry of heat or moisture into the interior space; and a power system connected to the refrigeration system, the power system facilitating independent operation of the refrigeration system when not connected to a power grid, and further facilitating a net energy use of zero from a power grid when connected to a power grid. Cold storage units may further include systems for using the refrigerant of the refrigeration system to defrost one or more components of the cold storage unit, and for using the refrigerant to facilitate maintaining a desired internal temperature of the interior space of the cold storage unit.

A refrigeration cycle apparatus includes refrigerant circuits each configured to circulate a refrigerant of the same composition. The refrigerant circuit is provided with a radiator configured to condense the refrigerant to transfer heat to external fluid, and the refrigerant circuit is provided with a radiator configured to transfer heat to the external fluid while allowing the refrigerant to be maintained in a supercritical state. The radiator is arranged upstream of the radiator in a direction of a flow of the external fluid. A capacity of a refrigerant flow channel of the radiator is smaller than a capacity of a refrigerant flow channel of the radiator.

A refrigeration cycle apparatus includes refrigerant circuits each configured to circulate a refrigerant of the same composition. The refrigerant circuit is provided with a radiator configured to condense the refrigerant to transfer heat to external fluid, and the refrigerant circuit is provided with a radiator configured to transfer heat to the external fluid while allowing the refrigerant to be maintained in a supercritical state. The radiator is arranged upstream of the radiator in a direction of a flow of the external fluid. A capacity of a refrigerant flow channel of the radiator is smaller than a capacity of a refrigerant flow channel of the radiator.

A refrigeration system including a first compressor and a second compressor, operating in tandem, wherein the first compressor includes a compression capacity different than the compression capacity of the second compressor. A method for operating a refrigeration system including a first compressor and a second compressor operating in tandem; wherein the first compressor comprises a first compression capacity and the second compressor comprises a second compression capacity larger than the first compression capacity the method comprising the steps of: determining a cooling demand; operating the first compressor in a first stage of cooling; operating the second compressor in a second stage of cooling; and operating the first compressor and the second compressor in a third stage of cooling.

A refrigeration system including a first compressor and a second compressor, operating in tandem, wherein the first compressor includes a compression capacity different than the compression capacity of the second compressor. A method for operating a refrigeration system including a first compressor and a second compressor operating in tandem; wherein the first compressor comprises a first compression capacity and the second compressor comprises a second compression capacity larger than the first compression capacity the method comprising the steps of: determining a cooling demand; operating the first compressor in a first stage of cooling; operating the second compressor in a second stage of cooling; and operating the first compressor and the second compressor in a third stage of cooling.