Abstract:
The present invention provides a heat pump that includes: a compressor, a switching valve, an indoor heat exchanger, a first expansion valve, and an outdoor heat exchanger, wherein the outdoor heat exchanger includes: a primary heat exchanging unit connected with the first expansion valve through a refrigerant pipe and through which outdoor air exchanges heat; a secondary heat exchanging unit connected with the switching valve through a refrigerant pipe, disposed behind the primary heat exchanging unit in the flow direction of outdoor air, and through which the outdoor air, which has exchanged heat through the primary heat exchanging unit, exchanges heat; a connection pipe that connects the primary heat exchanging unit and the secondary heat exchanging unit with each other; and a second expansion valve that is disposed in the connection pipe. Therefore, it is possible to improve both cooling performance and heating performance with minimum parts and a simple structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the Korean Patent Application No. 10-2013-0126750 filed on Oct. 23, 2013, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a heat pump, and more particularly, to a heat pump including an outdoor heat exchanger having a plurality of heat exchanging units. 
         [0004]    2. Description of the Conventional Art 
         [0005]    In general, a heat pump is a heating/cooling device that transfers heat from a low-temperature heat sink to a high-temperature destination or from a high-temperature heat sink to a low-temperature destination, using evaporating or condensing heat of a refrigerant. 
         [0006]    Heat pumps include a compressor, a switching valve, an outdoor heat exchanger, an expansion device, and an indoor heat exchanger. 
         [0007]    In cooling operation of a heat pump, a refrigerant can flow sequentially through the compressor, the switching valve, the outdoor heat exchanger, the expansion device, the indoor heat exchanger, the switching valve, and the compressor. 
         [0008]    In heating operation of a heat pump, a refrigerant can flow sequentially through the compressor, the switching valve, the indoor heat exchanger, the expansion device, the outdoor heat exchanger, the switching valve, and the compressor. 
         [0009]    When a heat pump operates for heating at low outdoor temperature, the outdoor heat exchanger may frost due to low-temperature outdoor air. 
         [0010]    A heat pump may be equipped with a separate defrosting heater for heating the outdoor heat exchanger, and in this case, when the outdoor heat exchanger frosts too much, it is possible to defrost the outdoor heat exchanger by heating it with the defrosting heater. 
         [0011]    Further, when the outdoor heat exchanger frosts too much in the heating operation of a heat pump, it is possible to defrost the outdoor heat exchanger by performing defrosting operation in which a refrigerant of the heat pump flows as in the cooling operation. 
         [0012]    Frequent defrosting operation may deteriorate the heating performance of heat pumps, so it is preferable to use the defrosting operation as little as possible. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention has been made in an effort to provide a heat pump that can minimize or prevent frosting in heating operation with a simple structure and minimum parts. 
         [0014]    According to an aspect of the present invention, there is provided a heat pump including a compressor, a switching valve, an indoor heat exchanger, a first expansion valve, and an outdoor heat exchanger, in which a refrigerant flows sequentially through the compressor, the switching valve, the outdoor heat exchanger, the first expansion valve, the indoor heat exchanger, the switching valve, and the compressor in cooling, and the refrigerator flows sequentially through the compressor, the switching valve, the indoor heat exchanger, the first expansion valve, the outdoor heat exchanger, the switching valve, and the compressor in heating. The outdoor heat exchanger includes: a primary heat exchanging unit that is connected with the first expansion valve through a refrigerant pipe and through which outdoor air exchanges heat; a secondary heat exchanging unit that is connected with the switching valve through a refrigerant pipe, disposed behind the primary heat exchanging unit in the flow direction of outdoor air, and through which the outdoor air, which has exchanged heat through the primary heat exchanging unit, exchanges heat; a heat exchanging unit connection pipe that connects the primary heat exchanging unit and the secondary heat exchanging unit with each other; and a second expansion valve that is disposed in the heat exchanging unit connection pipe. 
         [0015]    The primary heat exchanging unit and the secondary heat exchanging unit may be integrally combined. 
         [0016]    The primary heat exchanging unit may include a primary microchannel heat exchanger having a pair of headers connected by a microchannel, and the secondary heat exchanging unit may include at least one secondary microchannel heat exchanger having a pair of headers connected by a microchannel and disposed behind the primary microchannel heat exchanger in the flow direction of outdoor air. 
         [0017]    The secondary microchannel heat exchanger may include: a first secondary microchannel heat exchanger disposed behind the primary microchannel heat exchanger in the flow direction of outdoor; and a second secondary microchannel heat exchanger disposed behind the first secondary microchannel heat exchanger in the flow direction of outdoor air. 
         [0018]    The headers of the primary microchannel heat exchanger and the headers of the first secondary microchannel heat exchanger may be coupled to each other, and the headers of the first secondary microchannel heat exchanger and the headers of the second secondary microchannel heat exchanger may be coupled to each other. 
         [0019]    The first secondary microchannel heat exchanger may be connected with the primary microchannel heat exchanger through the heat exchanging unit connection pipe. 
         [0020]    The second secondary microchannel heat exchanger may be connected with the first secondary microchannel heat exchanger through a microchannel heat exchanger connection pipe. 
         [0021]    The second secondary microchannel heat exchanger may be connected with the switching valve through a refrigerant pipe. 
         [0022]    The primary heat exchanging unit may include a primary tube, and the secondary heat exchanging unit may include at least one secondary tube disposed behind the primary tube in the flow direction of outdoor air. 
         [0023]    The outdoor heat exchanger may further include common fins being in contact with the primary tube and the secondary tube. 
         [0024]    The secondary tube may include: a first secondary tube disposed behind the primary tube in the flow direction of outdoor air; and a second secondary tube disposed behind the first secondary tube in the flow direction of outdoor air. 
         [0025]    The first secondary tube may be connected with the primary tube through the heat exchanging unit connection pipe. 
         [0026]    The second secondary tube may be connected with the first secondary tube through a tube connection pipe. 
         [0027]    The second secondary tube may be connected with the switching valve through a refrigerant pipe. 
         [0028]    In a heating operation at an outdoor temperature over a predetermined temperature, the first expansion valve may be controlled to open at a degree for expanding a refrigerant and the second expansion valve may fully open. 
         [0029]    In a heating operation at an outdoor temperature less than a predetermined temperature, the first expansion valve may fully open and the second expansion valve may be controlled to open at a degree for expanding a refrigerant. 
         [0030]    The heat exchanging unit connection pipe may include: a primary heat exchanging unit connection pipe that connects the primary heat exchanging unit with the second expansion valve with each other; and a secondary heat exchanging unit connection pipe that connects the secondary heat exchanging unit and the second expansion valve. 
         [0031]    The secondary heat exchanging unit may have a larger heat exchange area for heat exchange with outdoor air than the primary heat exchanging unit. 
         [0032]    The secondary heat exchanging unit may have a longer pipe through which a refrigerant passes than the primary heat exchanging unit. 
         [0033]    The secondary heat exchanging unit may be larger in number than the primary heat exchanging unit. 
         [0034]    According to the heat pump of the present invention which the configuration described above, it is possible to delay or prevent frosting in a heating operation and improve cooling performance and heating performance with minimum parts and a simple structure in which the second expansion valve is disposed between the primary heat exchanging unit and the secondary heat exchanging unit of the outdoor heat exchanger. 
         [0035]    Further, since the outdoor air increases in temperature and decrease in relative humidity through the primary heat exchanging unit and then flows to the secondary heat exchanging unit, it is possible to delay or prevent frosting in a large area of the secondary heat exchanging unit and to achieve excellent effect on frosting delay. 
         [0036]    Further, since the primary heat exchanging unit and the secondary heat exchanging unit are integrated, it is easy to assemble and service the outdoor heat exchanger. 
         [0037]    Further, when a separate supercooling heat exchanger is additionally provided for the outdoor heat exchanger, it is possible to reduce the volume occupied by the outdoor heat exchanger and to achieve a compact outdoor heat exchanger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  is a view showing refrigerant flow in a cooling mode of a heat pump according to an embodiment of the present invention; 
           [0039]      FIG. 2  is a view showing refrigerant flow in a first heating mode of the heat pump according to an embodiment of the present invention; 
           [0040]      FIG. 3  is a view showing refrigerant flow in a second heating mode of the heat pump according to an embodiment of the present invention; 
           [0041]      FIG. 4  is a perspective view showing the outdoor heat exchanger shown in  FIGS. 1 to 3  through which outdoor air exchanges heat; and 
           [0042]      FIG. 5  is a perspective view showing an outdoor heat exchanger of a heat pump according to another embodiment of the present invention through which outdoor air exchanges heat. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    Hereinafter, embodiments of a heat pump according to the present invention are described with reference to the accompanying drawings. 
         [0044]      FIG. 1  is a view showing refrigerant flow in a cooling mode of a heat pump according to an embodiment of the present invention,  FIG. 2  is a view showing refrigerant flow in a first heating mode of the heat pump according to an embodiment of the present invention,  FIG. 3  is a view showing refrigerant flow in a second heating mode of the heat pump according to an embodiment of the present invention, and  FIG. 4  is a perspective view showing heat exchange between an outdoor heat exchanger shown in  FIGS. 1 to 3  and the outdoor air AO. 
         [0045]    A heat pump according to the present embodiment may include a compressor  1 , a switching valve  4 , an indoor heat exchanger  8 , a first expansion valve  10 , and an outdoor heat exchanger  12 . 
         [0046]    The heat pump can operate in a cooling mode. In the cooling mode of the heat pump, a refrigerant can flow sequentially through the compressor  1 , the switching valve  4 , the outdoor heat exchanger  12 , the first expansion valve  10 , the indoor heat exchanger  8 , the switching valve  4 , and the compressor  1 . 
         [0047]    The heat pump can operate in a heating mode. In the heating mode of the heat pump, the refrigerant can flow sequentially through the compressor  1 , the switching valve  4 , the indoor heat exchanger  8 , the first expansion valve  10 , the outdoor heat exchanger  12 , the switching valve  4 , and the compressor  1 . 
         [0048]    A compressor inflow pipe  2  through which the refrigerant that has passed through the switching valve  4  enters the compressor  1  may be connected to the compressor  1 . A compressor exit pipe  3  through which the refrigerant that has been compressed by the compressor  1  is discharged to the switching valve  4  may be connected to the compressor  1 . 
         [0049]    The switching valve  4  may be connected with the outdoor heat exchanger  12  through a refrigerant pipe. The refrigerant pipe connecting the switching valve  4  and the outdoor heat exchanger  12  to each other may be a switching valve-outdoor heat exchanger connection pipe  5 . The switching valve  4  may be connected with the indoor heat exchanger  8  through a refrigerant pipe. The refrigerant pipe connecting the switching valve  4  and the indoor heat exchanger  8  to each other may be a switching valve-indoor heat exchanger connection pipe  6 . 
         [0050]    The heat pump may further include an indoor fan  7  that allows indoor air AI to exchange heat with the indoor heat exchanger  8 . 
         [0051]    The indoor heat exchanger  8  may be a heat exchanger that allows indoor air AI to exchange heat with the refrigerant. The indoor heat exchanger  8  may be connected with the first expansion valve  10  through a refrigerant pipe. The refrigerant pipe connecting the indoor heat exchanger  8  with the first expansion valve  10  may be an indoor heat exchanger-first expansion valve connection pipe  9 . 
         [0052]    The heat pump may be used as a separation type air conditioner, in which the indoor fan  7  and the indoor heat exchanger  8  may be disposed in an indoor unit I and the indoor fan  7  can suck indoor air AI into the indoor unit I and then supply it back to the indoor through the indoor heat exchanger  8 . 
         [0053]    The first expansion valve  10  may be connected with the outdoor heat exchanger  12  through a refrigerant pipe. The refrigerant pipe connecting the first expansion valve  10  with the outdoor heat exchanger  12  may be a first expansion valve-outdoor heat exchanger connection pipe  11 . The first expansion valve  10  may be controlled to open at the degree for expanding the refrigerant in the cooling mode. The first expansion valve  10  may be controlled to open at the degree for expanding the refrigerant or fully open in the heating mode. 
         [0054]    The outdoor heat exchanger  12  may include a pipe through which the refrigerant passes. The pipe of the outdoor heat exchanger  12  may include a heat exchange pipe through which the refrigerant exchanges heat with the outdoor air AO and an expansion pipe for expanding the refrigerant. 
         [0055]    The outdoor heat exchanger  12  may include a plurality of heat exchange pipes spaced from each other in the flow direction of the refrigerant. In the outdoor heat exchanger  12 , an expansion pipe may be disposed between the heat exchange pipes. 
         [0056]    When the refrigerant passes through the outdoor heat exchanger  12 , it can pass through the expansion pipe after passing through a first one of the heat exchange pipes and then pass through a second one of the heat exchange pipes. In the cooling mode of the heat pump, the refrigerant can pass through the expansion pipe after passing through a first one of the heat exchange pipes and then pass through a second one of the heat exchange pipes. In the heating mode of the heat pump, opposite to the cooling mode, the refrigerant can pass through the expansion valve after passing through the second one of the heat exchange pipes and then pass through the first one of the heat exchange pipes. 
         [0057]    In the outdoor heat exchanger  12 , the refrigerant may pass through the expansion pipe without expanding and all of the heat exchange pipes may function as evaporation pipes for evaporating the refrigerant. In the outdoor heat exchanger  12 , the refrigerant may pass through the expansion pipe without expanding and all of the heat exchange pipes may function as condensation pipes for condensing the refrigerant. In the outdoor heat exchanger  12 , refrigerant can expand through the expansion pipe, some of the heat exchange pipes can function as condensation pipes for condensing the refrigerant, and the other of the heat exchange pipes can function as evaporation pipes for evaporating the refrigerant. 
         [0058]    The outdoor heat exchanger  12  may include a plurality of heat exchanging units  20  and  30 . The heat exchanging units  20  and  30  may be sequentially arranged in the flow direction of outdoor air AO. The outdoor heat exchanger  12  may include a primary heat exchanging unit  20  through which outdoor air AO exchanges heat. The primary heat exchanging unit  20  may be connected with the first expansion valve  10  through the first expansion valve-outdoor heat exchanger connection pipe  11 . The outdoor heat exchanger  12  may include a secondary heat exchanging unit  30  that is disposed behind the primary heat exchanging unit  20  in the flow direction of the outdoor air AO and allows the outdoor air AO, which has exchanged heat through the primary heat exchanging unit  20 , to exchange heat. The secondary heat exchanging unit  30  may be connected with the switching valve  4  through the switching valve-outdoor heat exchanger connection pipe  5 . The outdoor heat exchanger  12  may include a heat exchanging unit connection pipe  40  that connects the heat exchanging units  20  and  30  with each other. The heat exchanging unit connection pipe  40  may connect the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  with each other. The outdoor heat exchanger  12  may include a second expansion valve  50  disposed in the heat exchanging unit connection pipe  40 . 
         [0059]    The heat pump may further include an outdoor fan  14  that allows outdoor air AO to exchange heat with the outdoor heat exchanger  12 . The outdoor fan  14  can blow the outdoor air AO into the outdoor heat exchanger  12  regardless of the cooling mode and the heating mode. The outdoor fan  14  can blow the outdoor air AO through the primary heat exchanging unit  20  and then the secondary heat exchanging unit  30 . The outdoor air AO can exchange heat primarily through the primary heat exchanging unit  20  and secondarily through the secondary heat exchanging unit  30 , regardless of the cooling mode and the heating mode. 
         [0060]    The heat pump may be a separation type air conditioner, in which the compressor  1 , the switching valve  4 , the first expansion valve  10 , the outdoor heat exchanger  12 , and the outdoor fan  14  may be disposed in the outdoor unit O, and the outdoor fan  14  may suck the outdoor air AO into the outdoor unit O and then discharge it back to the outdoor through the outdoor heat exchanger  12 . 
         [0061]    The first expansion valve-outdoor heat exchanger connection pipe  11  that is the refrigerant pipe connecting the first expansion valve  10  and the outdoor heat exchanger  12  with each other may be the refrigerant pipe connecting the primary heat exchanging unit  20  and the first expansion valve  10  with each other. 
         [0062]    The switching valve-outdoor heat exchanger connection pipe  5  that is the refrigerant pipe connecting the switching valve  4  and the outdoor heat exchanger  12  with each other may be the refrigerant pipe connecting the secondary heat exchanging unit  30  and the switching valve  4  with each other. 
         [0063]    In the cooling mode of the heat pump, the refrigerant flowing in the switching valve-outdoor heat exchanger connection pipe  5  from the switching valve  4 , as shown in  FIG. 1 , can flow to the secondary heat exchanging unit  30 , sequentially pass through the secondary heat exchanging unit  30 , the second expansion valve  50 , and the primary heat exchanging unit  20 , and then flow in the first expansion valve-outdoor heat exchanger connection pipe  11 . 
         [0064]    In the heating mode of the heat pump, the refrigerant flowing in the first expansion valve-outdoor heat exchanger connection pipe  11  from the first expansion valve  10 , as shown in  FIGS. 2 and 3 , can flow to the primary heat exchanging unit  20 , sequentially pass through the primary heat exchanging unit  20 , the second expansion valve  50 , and the secondary heat exchanging unit  30 , and then flow to the switching valve-outdoor heat exchanger connection pipe  5 . 
         [0065]    The heat pump can operate in the cooling mode in which the refrigerant expands through the first expansion valve  10  but it does not expand through the second expansion valve  50 . In the cooling mode of the heat pump, the first expansion valve  10  may be controlled to open at the degree for expanding the refrigerant and the second expansion valve  50  may be controlled to fully open. 
         [0066]    The heat pump can operate in a first heating mode in which the refrigerant expands through the first expansion valve  10  but it does not expand through the second expansion valve  50 . In the first heating mode of the heat pump, the first expansion valve  10  may be controlled to open at the degree for expanding the refrigerant and the second expansion valve  50  may be controlled to fully open. 
         [0067]    The heat pump can operate in a second heating mode in which the refrigerant does not expand through the first expansion valve  10  but it expands through the second expansion valve  50 . In the second heating mode of the heat pump, the first expansion valve  10  may be controlled to fully open and the second expansion valve  50  may be controlled to open at the degree for expanding the refrigerant. 
         [0068]    In the cooling mode of the heat pump, the secondary heat exchanging unit  30  may function as a condenser that condenses a refrigerant and the primary heat exchanging unit  20  may function as a supercooling heat exchanger that supercools the refrigerant condensed through the secondary heat exchanging unit  30 . 
         [0069]    In the cooling mode of the heat pump, the refrigerant compressed by the compressor  1  may flow sequentially through the secondary heat exchanging unit  30 , the second expansion valve  50 , and the primary heat exchanging unit  20 . The refrigerant flowing in the secondary heat exchanging unit  30  may condense by exchanging heat with the outdoor air AO that has passed through the primary heat exchanging unit  20 . The refrigerant condensed through the secondary heat exchanging unit  30  may flow into the primary heat exchanging unit  20  after passing through the second expansion valve  50 . The refrigerant flowing in the primary heat exchanging unit  20  may exchange heat with the outdoor air AO and may be supercooled by the outdoor air AO through the primary heat exchanging unit  20 . 
         [0070]    In the first heating mode of the heat pump, the primary heat exchanging unit  20  may function as an evaporator through which the refrigerant that has expanded through the first expansion valve  10  evaporates before evaporating through the secondary heat exchanging unit  30 . In the first heating mode of the heat pump, the secondary heat exchanging unit  30  may function as an evaporator through which the refrigerant that has passed through the second expansion valve  50  after evaporating through the primary heat exchanging unit  20  evaporates. That is, in the first heating mode of the heat pump, the refrigerant condensed through the indoor heat exchanger  8  may flow into the primary heat exchanging unit  20  after expanding through the first expansion valve  10  and then sequentially flow through the primary heat exchanging unit  20 , the second expansion valve  50 , and the secondary heat exchanging unit  30 , and the refrigerant expanding through the first expansion valve  10  may additionally evaporate through the secondary heat exchanging unit  30  after evaporating first through the primary heat exchanging unit  20 . 
         [0071]    In the second heating mode of the heat pump, the primary heat exchanging unit  20  may function as a supercooling heat exchanger that supercools the refrigerant that has passed through the first expansion valve  10  after being condensed through the indoor heat exchanger  8 . In the second heating mode of the heat pump, the secondary heat exchanging unit  30  may function as an evaporator through which the refrigerant that has expanded through the second expansion valve  50  evaporates by exchanging heat with the outdoor air AO. In the second heating mode of the heat pump, the refrigerant that has condensed through the indoor heat exchanger  8  may flow into the primary heat exchanging unit  20  without expanding through the first expansion valve  10  and then flow sequentially through the primary heat exchanging unit  20 , the second expansion valve  50 , and the secondary heat exchanging unit  30 . The refrigerant that has condensed through the indoor heat exchanger  8  may be supercooled through the primary heat exchanging unit  20  by exchanging heat with the outdoor air AO. The refrigerant that has supercool through the primary heat exchanging unit  20  may flow into the secondary heat exchanging unit  30  after expanding through the second expansion valve  50  and then evaporate through the secondary heat exchanging unit  30 . 
         [0072]    The secondary heat exchanging unit  30  may have a larger heat exchange area for heat exchange with the outdoor air AO than the primary heat exchanging unit  20 . The secondary heat exchanging unit  30  may be thicker than the primary heat exchanging unit  20 . The secondary heat exchanging unit  30  may have a longer pipe through which a refrigerant passes than the primary heat exchanging unit  20 . The secondary heat exchanging unit  30  may be larger in number than the primary heat exchanging unit  20 . 
         [0073]    The heat exchanging unit connection pipe  40  may include a primary heat exchanging unit connection pipe  42  connecting the primary heat exchanging unit  20  and the second expansion valve  50  with each other. The heat exchanging unit connection pipe  40  may include a secondary heat exchanging unit connection pipe  44  connecting the secondary heat exchanging unit  30  and the second expansion valve  50  with each other. 
         [0074]    The primary heat exchanging unit connection pipe  42  may be formed by a first refrigerant tube between the primary heat exchanging unit  20  and the second expansion valve  50 . The secondary heat exchanging unit connection pipe  44  may be formed by a second refrigerant tube between the secondary heat exchanging unit  30  and the second expansion valve  50 . 
         [0075]    The refrigerant may, as shown in  FIG. 1 , pass sequentially through the secondary heat exchanging unit  30 , the secondary heat exchanging unit connection pipe  44 , the second expansion valve  50 , the primary heat exchanging unit connection pipe  42 , and the primary heat exchanging unit  20 . The refrigerant may, as shown in  FIGS. 2 and 3 , pass sequentially through the primary heat exchanging unit  20 , the primary heat exchanging unit connection pipe  42 , the second expansion valve  50 , the secondary heat exchanging unit connection pipe  44 , and the secondary heat exchanging unit  30 . 
         [0076]    The second expansion valve  50  may be connected to the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  through the heat exchanging unit connection pipe  40  and may be integrated with the primary heat exchanging unit  20  and the secondary heat exchanging unit  30 . 
         [0077]    The primary heat exchanging unit  20  and the secondary heat exchanging unit  30  may be integrally combined. 
         [0078]    In the outdoor heat exchanger  12 , the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  both may be microchannel heat exchangers, the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  both may be fin-tube heat exchangers, and any one of the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  may be a microchannel heat exchanger and the other one may be a fin-tube heat exchanger. 
         [0079]    Hereafter, an example that the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  are both microchannel heat exchangers is described with reference to  FIG. 4 . 
         [0080]    The primary heat exchanging unit  20 , as shown in  FIG. 4 , may include a primary microchannel heat exchanger  20 A with a pair of headers  21  and  22  connected by a microchannel  23 . The microchannel may have a channel through which a refrigerant passes and may be a primary heating member allowing a refrigerant passing through the channels to exchange heat with outdoor air AO. The microchannel  23  may have a plurality of channels and a refrigerant can exchange heat with outdoor air AO through the channels of the microchannel  23 . The primary heat exchanging unit  20  may include a plurality of microchannels  23 . The microchannels  23  may connect the pair of headers  21  and  22 . The primary heat exchanging unit  20  may further include fins  24  that promote heat transfer of the microchannels  23 . The fins  24  may be disposed between the microchannels  23 . One 21 of the pair of headers of the primary microchannel heat exchanger  20 A may be connected with the first expansion valve-outdoor heat exchanger connection pipe  11 . 
         [0081]    The secondary heat exchanging unit  30  may include one or more secondary microchannel heat exchangers  30 A and  30 B that have a pair of headers  31  and  32  connected by a microchannel (not shown) and are disposed behind the primary microchannel heat exchanger in the flow direction of outdoor air AO. 
         [0082]    The secondary microchannel heat exchangers  30 A and  30 B may have the same structure as that of the primary microchannel heat exchanger  20 A and may include a plurality of microchannels. A plurality of microchannels (not shown) may connect the pair of headers  31  and  32  and the secondary microchannel heat exchangers  30 A and  30 B may include fins (not shown) that promote heat transfer of the microchannels connecting the pair of headers  31  and  32 . 
         [0083]    The secondary microchannel heat exchangers  30 A and  30 B may include a first secondary microchannel heat exchanger  30 A disposed behind the primary microchannel heat exchanger  20 A in the flow direction of outdoor air AO and a second secondary microchannel heat exchanger  30 B disposed behind the first secondary microchannel heat exchanger  30 A in the flow direction of outdoor air AO. 
         [0084]    The first secondary microchannel heat exchanger  30 A may be connected with the primary microchannel heat exchanger  20 A by the heat exchanging unit connection pipe  40 . The first secondary microchannel heat exchanger  30 A may be connected with the heat exchanging unit connection pipe  40  at one of the pair of headers  31  and  32 . The headers of the first secondary microchannel heat exchanger  30 A may be coupled to the headers of the primary microchannel heat exchanger  20 A by welding or an adhesive. The headers of the first secondary microchannel heat exchanger  30 A may be coupled to the headers of the second secondary microchannel heat exchanger  30 B by welding or an adhesive. 
         [0085]    The second secondary microchannel heat exchanger  30 B may be connected with the first secondary microchannel heat exchanger  30 A by a microchannel heat exchanger connection pipe  34 . The second secondary microchannel heat exchanger  30 B may be connected with the heat exchanging unit connection pipe  40  at one of the pair of headers  31  and  32 . The second secondary microchannel heat exchanger  30 B may be connected with the switching valve-outdoor heat exchanger connection pipe  5  at one of the pair of headers. 
         [0086]    In the cooling mode, the refrigerant in the switching valve-outdoor heat exchanger connection pipe  5  can pass sequentially through the second secondary microchannel heat exchanger  30 B, the microchannel heat exchanger connection pipe  34 , the first secondary microchannel heat exchanger  30 A, the heat exchanging unit connection pipe  40 , and the primary microchannel heat exchanger  20 A and then flow to the first expansion valve-outdoor heat exchanger connection pipe  11 . 
         [0087]    In the first heating mode and the second heating mode, the refrigerant in the first expansion valve-outdoor heat exchanger connection pipe  11  can pass sequentially through the primary microchannel heat exchanger  20 A, heat exchanging unit connection pipe  40 , the first secondary microchannel heat exchanger  30 A, the microchannel heat exchanger connection pipe  34 , and the second secondary microchannel heat exchanger  30 B and then flow to the switching valve-outdoor heat exchanger connection pipe  5 . 
         [0088]    The heat pump may include an outdoor temperature sensor that senses the outdoor temperature and one of the first heating mode and the second heating mode may be performed in accordance with the outdoor temperature sensed by the outdoor temperature sensor in the heating operation. 
         [0089]    In the cooling operation of the heat pump, the cooling mode may be performed regardless of the outdoor temperature, in which the first expansion valve  10  may be controlled to open at the degree for expanding the refrigerant and the second expansion valve  50  may fully open. 
         [0090]    When the heat pump operates for heating at an outdoor temperature over a predetermined temperature, the first heating mode may be performed, in which the first expansion valve  10  may be controlled to open at the degree for expanding the refrigerant and the second expansion valve  50  may fully open. A predetermined temperature may be a temperature above 0° C. predetermined by a user to avoid or minimize frosting on the outdoor heat exchanger, i.e. 0.5° C. or 1° C. 
         [0091]    When the heat pump operates for heating at an outdoor temperature over a predetermined temperature, there is a small possibility that the outdoor heat exchanger  12  frosts, in which both of the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  can function as an evaporator. 
         [0092]    When the heat pump operates for heating at an outdoor temperature less than a predetermined temperature, the second heating mode may be performed, in which the first expansion valve  10  may fully open and the second expansion valve  50  may be controlled to open at the degree for expanding the refrigerant. In a heating operation at an outdoor temperature less than a predetermined temperature, there is a large possibility that the outdoor heat exchanger  12  frosts, in which when the high-temperature refrigerant that has condensed through the indoor heat exchanger  8  flows into the primary heat exchanging unit  20 , it is possible to delay or prevent frosting of the primary heat exchanging unit  20 . On the other hand, the outdoor air AO decreasing in humidity and increasing in temperature by exchanging heat through the primary heat exchanging unit  20  can delay or prevent frosting of the secondary heat exchanging unit  30  by flowing into it. A predetermined temperature may be a temperature above 0° C. predetermined by a user to avoid or minimize frosting on the outdoor heat exchanger, i.e. 0.5° C. or 1° C. 
         [0093]    The operation of the heat pump according to the present invention is described hereafter. 
         [0094]    First, in the cooling mode, the compressor  1  may be in operation, the indoor fan  7  and the outdoor fan  14  may be in operation, the first expansion valve  10  may be open at the degree of expanding the refrigerant, and the second expansion valve  50  may be fully open. 
         [0095]    When the indoor fan  7  operates, the indoor air AI can be supplied to the interior through the indoor heat exchanger  8 , and when the outdoor fan  14  operates, the outdoor air AO can pass through the primary heat exchanging unit  20  and then the secondary heat exchanging unit  30 . 
         [0096]    In the cooling mode, as shown in  FIG. 1 , the refrigerant is compressed through the compressor  1  and then guided to the switching valve-outdoor heat exchanger connection pipe  5  by the switching valve  4 . The refrigerant passing through the switching valve-outdoor heat exchanger connection pipe  5  flows to the secondary heat exchanging unit  30  and condenses through the secondary heat exchanging unit  30  by exchanging heat with the outdoor air AO. The refrigerant that has condensed through the secondary heat exchanging unit  30  flows to the primary heat exchanging unit  20  after passing through the second expansion valve  50 . The refrigerant flowing in the primary heat exchanging unit  20  supercools through the primary heat exchanging unit  20  by exchanging heat with the outdoor air AO. The refrigerant that has passed through the primary heat exchanging unit  20  flows into the first expansion valve-outdoor heat exchanger connection pipe  11 , expands through the first expansion valve  10 , and then exchanges heat with the indoor air AI through the indoor heat exchanger  8 . The refrigerant passing through the indoor heat exchanger  8  is evaporated by the indoor air AI and then guided into the compressor  1  by the switching valve  4 . 
         [0097]    In the first heating mode, the compressor  1  may be in operation, the indoor fan  7  and the outdoor fan  14  may be in operation, the first expansion valve  10  may be open at the degree of expanding the refrigerant, and the second expansion valve  50  may be fully open. 
         [0098]    When the indoor fan  7  operates, the indoor air AI can be supplied to the interior through the indoor heat exchanger  8 , and when the outdoor fan  14  operates, the outdoor air AO can pass through the primary heat exchanging unit  20  and then the secondary heat exchanging unit  30 . 
         [0099]    The refrigerant is compressed by the compressor  1  and then guided to the indoor heat exchanger  8  by the switching valve  4 . The refrigerant in the indoor heat exchanger  8  is condensed through the indoor heat exchanger  8  by the indoor air AI. The refrigerant that has condensed through the indoor heat exchanger  8  expands through the first expansion valve  10  and then flows to the first expansion valve-outdoor heat exchanger connection pipe  11 . The refrigerant in the first expansion valve-outdoor heat exchanger connection pipe  11  flows to the primary heat exchanging unit  20  and evaporates through the primary heat exchanging unit  20  by exchanging heat with the outdoor air AO. The refrigerant that has evaporated through the primary heat exchanging unit  20  flows to the secondary heat exchanging unit  30  after passing through the second expansion valve  50 . The refrigerant flowing in the secondary heat exchanging unit  30  evaporates through the secondary heat exchanging unit  30  by exchanging heat with the outdoor air AO. The refrigerant that has passed through the secondary heat exchanging unit  30  flows to the switching valve-outdoor heat exchanger connection pipe  5  and is then guided into the compressor  1  by the switching valve  4 . The refrigerant is evaporated by the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  while sequentially passing through the primary heat exchanging unit  20  and the secondary heat exchanging unit  30  and the entire outdoor heat exchanger  12  may function as an evaporator. 
         [0100]    In the second heating mode, the compressor  1  may be in operation, the indoor fan  7  and the outdoor fan  14  may be in operation, the first expansion valve  10  may be fully open, and the second expansion valve  50  may be open at the degree of expanding the refrigerant. 
         [0101]    When the indoor fan  7  operates, the indoor air AI can be supplied to the interior through the indoor heat exchanger  8 , and when the outdoor fan  14  operates, the outdoor air AO can pass through the primary heat exchanging unit  20  and then the secondary heat exchanging unit  30 . 
         [0102]    The refrigerant is compressed by the compressor  1  and then guided to the indoor heat exchanger  8  by the switching valve  4 . The refrigerant in the indoor heat exchanger  8  is condensed through the indoor heat exchanger  8  by the indoor air AI. The refrigerant that has condensed through the indoor heat exchanger  8  passes through the first expansion valve  10  and then flows to the first expansion valve-outdoor heat exchanger connection pipe  11 . The refrigerant in the first expansion valve-outdoor heat exchanger connection pipe  11  flows to the primary heat exchanging unit  20  and exchanges heat with the outdoor air AO through the primary heat exchanging unit  20 . The refrigerant passing through the primary heat exchanging unit  20  does not expand through the first expansion valve  10  after condensing through the indoor heat exchanger  8 . Further, The refrigerant passing through the primary heat exchanging unit  20  passes through the primary heat exchanging unit  20 , at a higher temperature than in the first heating mode and it not only increases the temperature of the outdoor air AO passing through the primary heat exchanging unit  20 , but decreases the relative humidity of the outdoor air AO passing through the primary heat exchanging unit  20 . For example, when the outdoor air AO flowing to the primary heat exchanging unit  20  has a temperature of 2° C. and relative humidity of 84%, it may increase in temperature to 3.5° C. and decrease in relative humidity to 75% while passing through the primary heat exchanging unit  20 , by the refrigerant passing through the primary heat exchanging unit  20 . The refrigerant passing through the primary heat exchanging unit  20  may supercool by exchanging heat with the outdoor air AO and the supercooling refrigerant expands through the second expansion valve  50 . The refrigerant that has expanded through the second expansion valve  50  flows to the secondary heat exchanging unit  30 . The refrigerant flowing in the secondary heat exchanging unit  30  evaporates through the secondary heat exchanging unit  30  by exchanging heat with the outdoor air AO that has increased in temperature and decreased in relative humidity through the primary heat exchanging unit  20 . When the refrigerant passes through the secondary heat exchanging unit  30 , it exchanges heat with the outdoor air AO that has increased in temperature and decreased in relative humidity through the primary heat exchanging unit  20  and frosting of the secondary heat exchanging unit  30  can be delayed or prevented. The refrigerant that has evaporated through the secondary heat exchanging unit  30  flows to the switching valve-outdoor heat exchanger connection pipe  5  and is then guided into the compressor  1  by the switching valve  4 . 
         [0103]      FIG. 5  is a perspective view showing an outdoor heat exchanger of a heat pump according to another embodiment of the present invention through which outdoor air AO exchanges heat. 
         [0104]    In an outdoor heat exchanger  12  according to the present embodiment, a primary heat exchanging unit  20  may include a primary tube  26  and a secondary heat exchanging unit  30  may include one or more secondary tubes  36  and  37  disposed behind the primary tube  26  in the flow direction of outdoor air AO. Other configuration and operation of the outdoor heat exchanger  12  are the same as or similar to those of the previous embodiment of the present invention, so the same reference numerals are used and the detailed description is not provided. 
         [0105]    The primary tube  26  and the secondary tubes  36  and  37  may each include a plurality of straight tubes elongated perpendicular to the flow direction of the outdoor air AO and may each include a plurality of return bands each connecting two straight tubes. The primary tube  26  and the secondary tubes  36  and  37  may be generally arranged in a zigzag shape. 
         [0106]    The outdoor heat exchanger  12  may further include common fins  27  being in contact with the primary tube  26  and the secondary tubes  36  and  37 . A plurality of common fins  27  may be coupled to the primary tube  26  and the secondary tubes  36  and  37 . The outdoor heat exchanger  12  may further include a pair of end plates  28  and  29 . The common fins  27  may be disposed between the pair of end plates  28  and  29 . The common fins  27 , in a second heating mode, can receive the heat from the refrigerant passing through the primary tube  26  and transfer it to the secondary tubes  36  and  37  by means of conduction. The straight tubes of the primary tubes  26  and the secondary tubes  36  and  37  may be arranged through the common fins  27 , and holes through which the primary tube  26  is disposed and holes through which the secondary tubes  36  and  37  are disposed may be formed in the common fins  27 . 
         [0107]    The secondary tubes  36  and  37  may include a first secondary tube  36  disposed behind the primary tube  26  in the flow direction of outdoor air AO and a second secondary tube  37  disposed behind the first secondary tube  36  in the flow direction of outdoor air AO. 
         [0108]    When the outdoor heat exchanger  12  includes the primary tube  26 , the first secondary tube  36 , and the second secondary tube  37 , holes through which the straight tubes of the primary tube  26  is disposed and holes through which the straight tubes of the first secondary tube  36  are disposed, and holes through which the straight tubes of the second secondary tube  37  are disposed may be formed in the common fins  27 . 
         [0109]    A first expansion valve-outdoor heat exchanger connection pipe  11  may be connected to the primary tube  26 . The first expansion valve-outdoor heat exchanger connection pipe  11  may be connected to one of the straight tubes of the primary tube  26 . 
         [0110]    The first secondary tube  36  may be connected with the primary tube  26  through a heat exchanging unit connection pipe  40  and a second expansion valve  50  may be disposed in the heat exchanging unit connection pipe  40 . The heat exchanging unit connection pipe  40  may include a primary tube connection tube that connects the primary tube  26  and the second expansion valve  50  with each other and a secondary tube connection tube that connects the second expansion valve  50  and the first primary tube  36  with each other. The primary tube connection tube may be the primary heat exchanging unit connection pipe  42  of the previous embodiment and the secondary tube connection tube may be the secondary heat exchanging unit connection pipe  44  of the previous embodiment. 
         [0111]    The second secondary tube  37  may be connected with the first secondary tube  36  through a tube connection pipe  38 . The tube connection pipe  38  may be a straight tube connection tube that connects one of the straight tubes of the first secondary tube  36  and one of the straight tubes of the second secondary tube  37  with each other. The straight tube connection tube that is the tube connection pipe  38  may be the same as the return bands of the primary tube  26 , the second secondary tube  37 , and the first secondary tube  36 . 
         [0112]    The second secondary tube  37  is connected with a switching valve  4  through a refrigerant pipe, that is, a switching valve-outdoor heat exchanger connection pipe  5  may be connected to the second secondary tube  37 . The switching valve-outdoor heat exchanger connection pipe  5  may be connected to one of the straight tubes of the second secondary tube  37 . 
         [0113]    According to the outdoor heat exchanger of the present embodiment, in a cooling mode, the refrigerant in the switching valve-outdoor heat exchanger connection pipe  5  can pass sequentially through the second secondary tube  37 , the tube connection pipe  38 , the first secondary tube  36 , the heat exchanging unit connection pipe  40 , and the primary tube  26  and then flow to the first expansion valve-outdoor heat exchanger connection pipe  11 . 
         [0114]    According to the outdoor heat exchanger  12  of the present embodiment, in a first heating mode and a second heating mode, the refrigerant in the first expansion valve-outdoor heat exchanger connection pipe  11  can pass sequentially through the primary tube  26 , the heat exchanging unit connection pipe  40 , the first secondary tube  36 , the tube connection pipe  38 , and the second secondary tube  37 , and then flow to the switching valve-outdoor heat exchanger connection pipe  5 . 
         [0115]    It should be understood that the present invention is not limited to the embodiments and may be implemented in various ways within the scope of the present invention.