Abstract:
A system and method for cooling power electronics using heat sinks, and including a heat pump. The heat pump includes a main refrigerant circuit having a compressor, an indoor heat exchanger, and an outdoor heat exchanger, and a reversing valve. A biflow expansion valve is configured to receive condensed liquid refrigerant and to expand the refrigerant. A cooling circuit in fluid communication with the main refrigerant line includes an expansion device configured to receive a portion of condensed liquid refrigerant from the main refrigerant circuit and to expand the portion of condensed liquid refrigerant. A heat sink is configured to receive the expanded portion of refrigerant from the expansion device. Power electronics are coupled to the heat sink such that the portion of expanded refrigerant from the expansion device passes through the heat sink and cools the power electronics.

Description:
BACKGROUND 
     The present invention relates to a system and method for cooling the power electronics of a variable speed heat pump. 
     High efficiency heat pumps utilizing both a compressor and supply air fan with variable speed drives reduce overall annual energy consumption compared to systems without such drives. These variable speed drives, controlled electronically, include power semiconductors and other electronic components that require cooling, i.e., temperature control, for efficient operation and reliability. 
     SUMMARY 
     In one embodiment, a heat pump includes a main refrigerant circuit. The main refrigeration circuit includes a compressor configured to compress a refrigerant, an indoor heat exchanger, and an outdoor heat exchanger. A biflow expansion valve is configured to receive condensed liquid refrigerant and to expand the refrigerant. A reversing valve is movable between a first position that directs refrigerant from the compressor sequentially to the outdoor heat exchanger, the biflow expansion valve, and the indoor heat exchanger in a cooling mode, and a second position that directs compressed refrigerant from the compressor sequentially to the indoor heat exchanger, the biflow expansion valve, and the outdoor heat exchanger in a heating mode. A cooling circuit in fluid communication with the main refrigerant line includes an expansion device configured to receive a portion of condensed liquid refrigerant from the main refrigerant circuit and to expand the portion of condensed liquid refrigerant. A heat sink is configured to receive the expanded portion of refrigerant from the expansion device. Power electronics are coupled to the heat sink such that the portion of expanded refrigerant from the expansion device passes through the heat sink and cools the power electronics. 
     In another embodiment, a heat pump includes a main refrigerant circuit. The main refrigeration circuit includes a compressor configured to compress a refrigerant, an indoor heat exchanger, and an outdoor heat exchanger. At least one expansion valve is configured to receive condensed liquid refrigerant and to expand the refrigerant. A reversing valve is movable between a first position that directs refrigerant from the compressor sequentially to the outdoor heat exchanger, the at least one expansion valve, and the indoor heat exchanger in a cooling mode, and a second position that directs compressed refrigerant from the compressor sequentially to the indoor heat exchanger, the at least one expansion valve, and the outdoor heat exchanger in a heating mode. A cooling circuit in fluid communication with the main refrigerant line includes a heat sink configured to receive expanded refrigerant. A first orifice check valve is disposed between the heat sink and a first branch point on the main refrigerant circuit between the indoor heat exchanger and the at least one expansion valve. A second orifice check valve is disposed between the heat sink and a second branch point on the main refrigerant circuit between the outdoor heat exchanger and the at least one expansion valve. Each of the first and second orifice check valves is configured to receive a portion of condensed liquid refrigerant from the main refrigerant circuit and to expand the portion of condensed liquid refrigerant. Power electronics are coupled to the heat sink such that the portion of expanded refrigerant from one of the first orifice check valve and the second orifice check valve passes through the heat sink and cools the power electronics. 
     In another embodiment, a method of operating a heat pump includes directing compressed refrigerant from a compressor sequentially to an outdoor heat exchanger to condense the refrigerant, at least one expansion valve to expand the refrigerant, and an indoor heat exchanger to evaporate the refrigerant in a cooling mode. The method also includes directing compressed refrigerant from the compressor sequentially to the indoor heat exchanger to condense the refrigerant, the at least one expansion valve to expand the refrigerant, and the outdoor heat exchanger to evaporate the refrigerant in a heating mode. The method further includes directing a portion of the condensed refrigerant from a point upstream of the at least one expansion valve toward a heat sink coupled to power electronics, expanding the portion of condensed refrigerant with a fixed orifice expansion device, and directing the portion of expanded refrigerant to the heat sink. The method also includes cooling the heat sink and the power electronics with the expanded portion of the refrigerant. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a high efficiency heat pump having a system for cooling variable speed drive power electronics. 
         FIG. 2  is a perspective view of the cooling system of  FIG. 1  located within a heat pump indoor housing. 
         FIG. 3  is another perspective view of the cooling system shown in  FIG. 2 . 
         FIG. 4  is a schematic of a high efficiency heat pump having an alternatively configured system for cooling variable speed drive power electronics. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     Schematically illustrated in  FIG. 1  is a water-source heat pump system  100 . The system  100  includes an indoor heat exchanger  110  and an outdoor heat exchanger  114 . In the illustrated embodiment, the indoor heat exchanger  110  is a refrigerant-to-air heat exchanger and the outdoor heat exchanger  114  is a refrigerant-to-water heat exchanger, but the heat exchangers  110 ,  114  are not so limited. For example, in some constructions the outdoor heat exchanger  114  can be a refrigerant-to-air heat exchanger. A variable speed indoor fan  118  forces air across the indoor heat exchanger  110  and supplies that air to a space  120  in order to temper the environment of the space  120 . The outdoor heat exchanger  114 , which could be, for example, a ground loop or geothermal type of heat exchanger, is in fluid communication with a source of water, which may include a natural source, such as ground water. 
     A compressor  124 , such as a rotary or scroll compressor, discharges gaseous refrigerant to a reversing valve  128 . Refrigerant piping includes suction piping  134 , which connects the suction port of the compressor  124  to the reversing valve  128 , and discharge/return piping  138 , which connects the reversing valve  128  to the indoor and outdoor heat exchangers  110 ,  114 , as is commonly known to those of skill in the art. Referring again to  FIG. 1 , the system  100  includes a bi-flow thermostatic expansion valve (“TXV”)  144  positioned in piping  148  connecting the indoor and outdoor heat exchangers  110 ,  114 . The TXV  144  is controlled through a thermal bulb  150  positioned on the suction line  134  and has a separate bleed line orifice  154  that bypasses a portion of the refrigerant flow, for example, 15%. The bi-flow TXV  144 , which receives condensed liquid refrigerant and expands it to a vapor/liquid phase mixture, permits in-line direction reversal of the system refrigerant flow to accommodate both the heating mode and the cooling mode of the heat pump system  100  with a single expansion valve. The indoor heat exchanger  110 , indoor fan  118 , compressor  124 , reversing valve  128 , and TXV  144  are located within an indoor housing  160 . 
     The reversing valve  128  is movable between a first position that directs refrigerant from the compressor  124  sequentially to the outdoor heat exchanger  114 , the TXV  144 , and the indoor heat exchanger  110  in a cooling mode (arrow  170 ), and a second position that directs refrigerant from the compressor  124  sequentially to the indoor heat exchanger  110 , the TXV  144 , and the outdoor heat exchanger  114  in a heating mode (arrow  180 ). In the space cooling mode of operation  170 , the compressor  124  discharges high temperature/high pressure refrigerant gas to the outdoor heat exchanger  114 . The outdoor heat exchanger  114  condenses the refrigerant through thermal contact with the source of cooling water. The condensed refrigerant flows out of the outdoor heat exchanger  114  to the bi-flow TXV  144 , where it expands to a lower temperature and pressure, and into the indoor heat exchanger  110 , where it vaporizes as heat is transferred from the air directed across the heat exchanger  110  by the fan  118 . In the space heating mode of operation  180 , the direction of refrigerant flow through the system  100  is reversed as are the functions of the indoor and outdoor heat exchangers  110 ,  114 . In this mode, the indoor heat exchanger  110  functions as a refrigerant condenser while the outdoor heat exchanger  114  functions as a refrigerant evaporator. 
     In the heat pump system  100  of the present construction, the employment of variable speed drives, specifically a variable speed compressor  124  and a variable speed indoor fan  118 , results in the need for power electronics components  164  to control compressor and fan speed. Such components  164 , located within the housing  160 , inherently generate large amounts of heat, which must be dissipated to prevent the malfunction of the system  100  and its controls. 
     A cooling circuit  200  includes a cooling line  204  connected at a first end  208  to one side of the TXV  144  at a first branch point on the main refrigerant circuit, and at a second end  212  to the opposite side of the TXV  144  at a second branch point on the main refrigerant circuit. More specifically, the first end  208  of the cooling line  204  corresponds to high pressure condensed refrigerant during the heating mode  180  and low pressure refrigerant in the cooling mode  170 . The second end  212  of the cooling line  204  corresponds to high pressure condensed refrigerant during the cooling mode  170  and low pressure refrigerant in the heating mode  180 , as shown in  FIG. 1 . 
     The cooling line  204  includes a thermal contact portion  220 , illustrated as a serpentine tube, intermediate the first end  208  and the second end  212  and which partially forms a heat sink  224 , to be further described below. A first orifice check valve  234  is disposed inline with a first leg  238  of the cooling line  204  between the first end  208  and the serpentine tube  220 , and a second orifice check valve  242  is disposed inline with a second leg  246  of the cooling line  204  between the second end  212  and the serpentine tube  220 . As shown in  FIG. 1 , each orifice check valve  234 ,  242  includes a fixed or variable orifice/restrictor  250  in parallel with a check valve  254 . Each orifice check valve  234 ,  242  is arranged to meter refrigerant from the high pressure refrigerant side (dependent on system mode) upstream of the bi-flow TXV  144  to the serpentine tube  220  and to permit substantially unrestricted passage of refrigerant from the serpentine tube  220  to the low pressure refrigerant side downstream of the bi-flow TXV  144 . 
     Referring to  FIG. 2 , the cooling circuit  200  is shown located within the housing  160 . Clamped about the serpentine tube  220  of the cooling circuit  200  is a block of material  260 . The block of material  260  is preferably fabricated in two sections  264 ,  268  cooperating to define an internal passage (not shown) into which the serpentine tube  220  can be secured, and is further preferably formed from a heat conducting material such as aluminum. The effect of clamping the halves  264 ,  268  of the material block  260  tightly over the serpentine tube  220  of the cooling line  204  is to create an efficient path for the transfer of heat between the block  260  and the serpentine tube  220 , which together form the heat sink  224 . 
     While the curved ends  270  of the serpentine tube  220  are illustrated as exposed and outside of the block  260 , the block  260  can alternatively be fabricated to define a cooperating serpentine passage such that none of the serpentine tube  220  is exposed. In a further alternative, rather than running through the block of material  260 , the serpentine tube  220  could be interrupted and the block  260  spliced into the cooling line  204  so that system refrigerant flows through and in direct contact with the block  260 . In such a case, the block  260  may be a unitary piece into which a flow passage has been cast, with the interrupted ends of the serpentine tube  220  brazed into the passage orifices of the block  260 . 
     The serpentine tube  220  is not limited to four passes through the block  260  and can have fewer or more than four passes depending on the size of the block  260  and the amount of heat to be absorbed (itself dependent on the power electronics used and the size of the equipment). In other constructions, the tube  220  need not be in serpentine form and other tube shapes, as well as variations in the configuration of the block  260 , are considered to be within the scope of the present invention. For instance, refrigerant might pass through the block unidirectionally and/or in a single pass. 
     Referring again to  FIG. 2 , the block  260  is supported within the housing  160  by fasteners, such as bolts, which pass through the block  260  and a panel  280  of the housing  160 , with the exact location a matter of application preference based on the capacity of the system  100 . For example, the panel  280  of  FIG. 2  may be a rear panel of an externally accessible power electronics box of the housing  160 . Referring to  FIG. 3 , the block  260  is configured to accept the mounting of power electronic modules  290 . The term “power electronic modules” will be used herein to refer to all electronic components mounted on the block  260  through which the speed of the compressor  124  and/or the speed of the indoor fan  118  is/are controlled and varied. These components function with and are connected to power leads (not shown), which direct power to the compressor and fan  124 ,  118 , and it will be appreciated that a large amount of heat is generated within the modules  290 . The modules  290  are attached to the block  260  in a manner that facilitates the transfer of heat to the block  260 . For example, the modules  290  can be attached to a circuit card or board  294  on which various other compressor and/or fan speed control related components are mounted. The reliability and life of the modules  290  is to a significant degree dependent upon precluding such components from operating at high temperatures and/or precluding their exposure to thermal shock. 
     In some applications, a layer of insulation (not shown) is disposed around the outer edge of the block  260  to hinder heat absorption from ambient conditions inside the housing  160  or from other sources other than the modules  290 . 
     In the cooling mode of operation  170 , refrigerant passes from the compressor  124  first to the outdoor heat exchanger  114 , where it condenses, and then to the bi-flow TXV  144 . A portion of the refrigerant upstream of the TXV  144  is redirected through the second end  212  of the cooling line  204 . This portion of refrigerant passes within the second leg  246 , through the second orifice check valve  242  (and specifically through the orifice/restrictor  250  of the second orifice check valve  242 , which expands the refrigerant), and to the serpentine tube  220 . As this low-temperature refrigerant passes through the serpentine tube  220  in thermal contact with the block  260 , the heat generated within the modules  290  used to power and control the compressor  124  passes into the heat sink  224 , which absorbs heat due to the temperature differential between the heat generating modules  290  and the refrigerant being pumped through the serpentine tube  220 . The refrigerant then passes from the tube  220  to the first leg  238 , through the first orifice check valve  234  (and specifically through the open check valve  254  of the first orifice check valve  234 ), and to the first end  208  of the cooling line  204 , where it joins and mixes with the main refrigerant flow in piping  148  downstream of the TXV  144  and upstream of the indoor heat exchanger  110 . 
     In the heating mode  180 , the flow of refrigerant is reversed, with refrigerant passing from the compressor  124  first to the indoor heat exchanger  110  and to the TXV  144 . A portion of refrigerant is redirected through the first end  208  of the cooling line  204  and the orifice/restrictor  250  of the first orifice check valve  234 , through the serpentine tube  220 , past the open check valve  254  of the second orifice check valve  242 , and to the second end  212  of the cooling line  204 . This refrigerant joins and mixes with the main refrigerant flow in piping  148  downstream of the TXV  144  and upstream of the outdoor heat exchanger  114 . 
     The amount of refrigerant redirected to the cooling circuit is a function of the pressure differential across the bi-flow TXV  144  and in normal operation is at or less than approximately 10-15 Ibm of refrigerant per hour in both cooling and heating modes  170 ,  180 . It is to be noted that the faster the speed of the compressor  124  in operation, the greater is the pressure differential across the TXV  144  and therefore the greater the amount of refrigerant redirected through the cooling circuit  200  in a given period of time. The circuit  200  is therefore self-regulating in that when the compressor  124  is running at higher speeds due to increased load a greater quantity of refrigerant is pumped through the cooling circuit  200  and is brought into a heat exchange relationship with the modules  290  generating the heat. 
     Referring to  FIG. 4 , in an alternative construction, a cooling line  304  includes a serpentine tube  320  downstream of both a first end  308  at a first branch point on the main refrigerant circuit and a second end  312  at a second branch point on the main refrigerant circuit, and which partially forms a heat sink  324 . The heat sink  324  includes a block  360 , substantially identical to the block  260  of the heat sink  224 . A first orifice check valve  334  is disposed inline with a first leg  338  of the cooling line  304  between the first end  308  and the serpentine tube  320 , and a second orifice check valve  342  is disposed inline with a second leg  346  of the cooling line  304  between the second end  312  and the serpentine tube  320 . As shown in  FIG. 4 , each orifice check valve  334 ,  342  includes a fixed or variable orifice/restrictor  350  in series with a check valve  354  and is arranged to meter refrigerant from the high pressure refrigerant side upstream of the bi-flow TXV  144  to the serpentine tube  320 . The series arrangement of the orifice/restrictors  350  and respective check valves  354 , together with the orientation of the check valves  354 , inhibits the flow of refrigerant to the low pressure refrigerant side downstream of the bi-flow TXV  144 , i.e., to first end  308  during the cooling mode  170  or to the second end  312  during the heating mode  180 . 
     The first leg  338  and the second leg  346  meet at an intersection  352  to form a third leg  356  extending therefrom. From the third leg  356 , the refrigerant flows to the serpentine tube  320 . As opposed to returning to the low pressure side downstream of the TXV  144 , the refrigerant instead flows out of the serpentine tube  320  and through a fourth leg  358  leading to the compressor suction line  134 . In a variation of the alternative construction, in lieu of the first orifice check valve  334  in the first leg  338  and the second orifice check valve  342  in the second leg  346 , a single orifice restrictor similar to orifice  350  can be positioned in the third leg  356 , with each of the first and second legs  338 ,  346  including only a check valve similar to the check valve  354 . In some constructions, the legs  338 ,  346 ,  356  can form a Y-shape, although other shaped configurations are within the scope of the invention. 
     In the heating mode of operation  180 , refrigerant passes from the compressor  124  first to the indoor heat exchanger  110 , where it condenses, and then to the bi-flow TXV  144 . A portion of the refrigerant upstream of the TXV  144  is redirected through the first end  308  of the cooling line  304 . This portion of refrigerant passes within the first leg  338 , through the first orifice check valve  334 , to the third leg  356 , and to the serpentine tube  320  where it absorbs heat from the block  360  in thermal contact with the power modules  290 . Upon exiting the serpentine tube  320 , the refrigerant is directed through the fourth leg  358  to the compressor suction line  134  upstream of the compressor  124  and mixes with the refrigerant evaporated by the outdoor heat exchanger  114 . 
     In the cooling mode  170 , the flow of refrigerant is reversed, with refrigerant passing from the compressor  124  first to the outdoor heat exchanger  114  and to the TXV  144 , where a portion of refrigerant is redirected through the second end  312  of the cooling line  304  and the orifice/restrictor  350  of the second orifice check valve  342  before proceeding through the serpentine tube  320 , the fourth leg  358 , and to the compressor suction line  134 , substantially as described above. 
     Portions of the present invention are equally applicable to cooling-only air conditioning applications, i.e., in which the flow of refrigerant is at all times from a compressor to an outdoor heat exchanger coil. 
     The heat produced from the power electronics and other speed control components must be efficiently transported away to prevent their failure due to over-heating. If the operating temperatures of critical compressor speed control components can be maintained at less than 185° F., the reliability and life of such components is dramatically enhanced. Testing has determined that under normal operating conditions, the surface temperature of the block  260 ,  360  ranges between about 25° F. and about 90° F. over the complete system  100  operating range, indicating that compressor speed control components are operating at temperatures well below acceptable upper limits. 
     Various features and advantages of the invention are set forth in the following claims.