Abstract:
An improved air-source heat pump for residential and commercial use, employing two closed refrigerant systems having different refrigerants in cascade relationship to each other to address efficiency and space concerns, with the first closed refrigerant system partitionable into a first sub-system and a second sub-system, with the first sub-system working in conjunction with the second closed refrigerant system in heating mode and the second sub-system working independently in cooling mode.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to environmental climate control devices, namely, air-source heat pumps. Specifically, the invention relates to an improvement on cascading air-source heat pumps for residential and commercial use, where two closed refrigerant circuits are used with two different refrigerant fluids, in order to address efficiency and space concerns. Heating occurs with two circuits working in cascade fashion, with a heat output to an external heating system, such as a furnace or boiler or directly to a hot water or hot air line. Cooling occurs with the first closed circuit operating in reverse, using a separate input, such as an air plenum or an external cooling system, but using the same compressor and evaporator/condenser. 
       BACKGROUND 
       [0002]    Air-source heat pumps are well known in the art. See, e.g., U.S. Pat. No. 6,615,602 (Wilkinson), which describes a typical air-source heat pump in detail. Air-source heat pumps incorporate a combination of compressors and condensers in a closed-loop system to draw heat energy from the outside environment for use in heating interior spaces. They can also be used in reverse to provide for air conditioning of interior spaces. Air-source heat pumps rely on well-known principles of thermodynamics to extract energy from a given volume of air. 
         [0003]    As fossil fuels such as oil and natural gas continue to become scarcer, increased emphasis will be placed upon the use of electricity to provide space heating of homes and commercial buildings. Concerns are especially pertinent in cold weather areas such as New England. However, heat pump systems operating in cold weather environments often experience problems with efficiency. It is important to attain the highest level of efficiency as possible, as heat pumps that are more efficient save energy and money. A major problem that most heat pump systems experience is that, as the ambient outside temperature falls, the heating capacity of the heat pump system decreases drastically. Yet it is during these times of low temperatures that heating needs increase. In order to meet these needs, systems have utilized supplemental electrical resistance type heating, cascade technology, and boosters to increase their heating capacity so as to operate in low temperature environments. 
         [0004]    A typical air-source heat pump is arranged with a “high side” and a “low side” configuration, wherein the system refrigerant is at a relatively high pressure and high temperature on the high side and is at a relatively low pressure and low temperature on the low side. Relatively low pressure/low temperature gaseous refrigerant from the low side is introduced into a compressor, which compresses the refrigerant into a high pressure/high temperature gas (compressing a given volume of gas into a smaller volume of gas causes its pressure and temperature to increase). The compressed high pressure/high temperature gas is then forced through a condenser which is in contact with the interior space to be heated; the gas gives up some of its energy in the condenser, thus providing heat to the interior space, and the refrigerant becomes liquefied. The liquid refrigerant is then forced through an expansion device which vaporizes the liquid into a low pressure/low temperature gas. Once the refrigerant has been vaporized into a low pressure/low temperature gas, it is passed through an evaporator which is in contact with the outside air. Heat energy is absorbed from the outside air by the refrigerant, which is then introduced to the compressor, repeating the cycle. The portion of the heat pump system between the compressor and the expansion device is the system high side, and the portion of the system from the evaporator back to the compressor is the system low side. 
         [0005]    The foregoing is a simplified explanation of the mechanics of how an air-source heat pump works. However, it is sufficient to illustrate a phenomenon of thermodynamics which renders the typical air-source heat pump inefficient in cold climates. The maximum energy that can be extracted from a given volume of air by an air-source heat pump is its heating capacity. The heating capacity of an air-source heat pump changes with the temperature of the air from which energy is extracted. As the temperature of the outside air decreases, the expansion device pressurizes less of the refrigerant, resulting in the refrigerant having a lower density (and pressure) for a given volume to achieve a lower boiling point (since the boiling point of the refrigerant must be lower than the temperature of the ambient outside air). As the mass density of the refrigerant decreases eventually the flow of refrigerant will be below the operating capacity of the heat pump. Because air-source heat pumps are designed to handle a specific volume of flow, lowering the amount available lowers the overall heating capacity of the heat pump, because the system high side requires the refrigerant to be of a certain minimum pressure; when the refrigerant pressure is diminished due to decreasing outside air temperatures the compressor must raise the pressure of the refrigerant a greater degree. When the outside temperature becomes cold enough the corresponding pressure differential between the system low side and the system high side becomes too great for the compressor to overcome. To compensate, either compressors with far greater maximum capacity must be used, at great expense and inefficiency, or alternative heat sources must be available when the outside temperature falls too low. Neither of these solutions is practical and thus the use of typical air-source heat pumps is very limited in colder climates, where the need for heat is greatest during those winter months when the outside air is coldest and the resulting heating capacity is lowest. 
         [0006]    A solution to the lack of efficiency of cold climate air-source heat pumps was demonstrated by Gustafsson, involving the use of cascade-connected heat pumps. See, e.g., U.S. Pat. No. 3,984,050 (Gustafsson). Cascading heat pumps are well known in the art. Gustafsson describes a heat pump system capable of extracting heat from relatively low temperature ambient air (−10° C.) to produce hot water (up to 80° C.). Air-source heat pump systems set up in a cascade fashion use the condenser unit of one heat pump arranged in a heat-exchanging relationship with the evaporator of the other heat pump. This “piggy-back” relationship increases the system&#39;s efficiency, i. e., the ratio between the output energy and the input energy. However, while the Gustafsson device and similar systems are able to reach temperatures high enough to produce hot water, they do not also provide air conditioning. 
         [0007]    A cascading heat pump system capable of operating over a wide range of source temperature and of providing supplemental comfort zone air conditioning was disclosed in U.S. Pat. No. 4,391,104 (Wendschlag). The Wendschlag heat pump system uses a first refrigerant fluid and a second refrigerant fluid with separate compression cycle loops passing in heat transfer relationship through a tri-fluid heat exchanger. Having separate circuits allows several different types of refrigerant to be used, which is beneficial because different types of refrigerant are effective under different conditions. This setup also allows the system to operate in several different modes, incorporating a method for selectively heating water by extracting heat from relatively cold outdoor ambient air in cascade, and for heating or cooling air supplied to a comfort zone in non-cascade fashion. Although the Wendschlag system, and others like it, work efficiently in cold weather, they are not effective when it comes to providing air conditioning during periods of warm temperatures. The air conditioning provided by Wendschlag is only supplemental and occurs only as a by-product of heating water. This prevents Wendschalg and similar systems from being efficient sources of comfort zone temperature conditioning, and necessitate that a separate air conditioning system be used in order to provide efficient and sufficient cold air during warm weather months. 
         [0008]    One solution to the problem of cold climate heat pumps which also efficiently provide air conditioning was demonstrated in U.S. Pat. No. 4,149,389 (Hayes, et al.). Hayes, et al., uses either a cascading or non-cascading mode to send hot air into the conditioned space. The non-cascade heating mode also operates in reverse in order to provide non-supplemental air conditioning to the conditioned space. However, the Hayes, et al., device is not set up to run an external heating system and a separate external cooling system. Rather, the output heat energy is exhausted through an air plenum into the ambient interior air, which also serves as the location for the provision of air conditioning. The Hayes, et al., system is therefore impractical for application with existing heating systems or where more efficient heating systems are desired. 
         [0009]    It is therefore an object of the invention to provide an improved air-source heat pump which operates efficiently in cold climates. 
         [0010]    It is a further object of the invention to provide an improved air-source heat pump which incorporates the efficiencies of a cascading dual refrigerant circuit heating system. 
         [0011]    It is yet a further object of the invention to provide an improved air-source heat pump which incorporates the efficiencies of a cascading heating system with a non-cascading cooling system. 
         [0012]    It is yet a further object of the invention to provide an improved air-source heat pump which incorporates different refrigerants to increase the operational temperature range of the device. 
         [0013]    It is yet a further object of the invention to provide an improved air-source heat pump which efficiently integrates with a furnace or a boiler. 
         [0014]    It is yet a further object of the invention to provide an improved air-source heat pump which efficiently integrates directly with a hot water line or a hot air line. 
         [0015]    It is yet a further object of the invention to provide an improved air-source heat pump which efficiently integrates directly with a cold water line or a cold air line. 
         [0016]    It is yet a further object of the invention to provide an improved air-source heat pump wherein the components of a cascading dual refrigerant circuit are simplified resulting in the overall efficiency of the system being maximized and the time and effort for installation being minimized. 
         [0017]    Other objects of this invention will be apparent to those skilled in the art from the description and claims which follow. 
       SUMMARY 
       [0018]    The present invention discloses an improved air-source heat pump heater/air conditioner, using two separate closed loop refrigerant circuits which interact with each other through a first heat exchanger and which further interact with an external heating system through a second heat exchanger. In heating mode, the two refrigerant circuits work together in cascade relation, with heat output to the external heating system, such as a hot air furnace or a hot water boiler or directly with a hot water line or a hot air line. In cooling mode, the first closed refrigerant circuit operation is reversed and the second refrigerant circuit is nonoperational. 
         [0019]    Each of the two closed refrigerant circuits uses a different type of refrigerant fluid, with the first refrigerant circuit using a relatively high pressure, low temperature refrigerant such as R22 or R410A, and the second refrigerant circuit using a relatively low pressure, high temperature refrigerant such as R134A or R236. The use of different refrigerants allows the device to work in cold climates while also producing sufficient heat to boil water. 
         [0020]    In heating mode, a first compressor circulates the first refrigerant fluid through a portion of the first refrigerant circuit, partitioned from the remainder of the circuit by appropriate valving. The vaporized first refrigerant fluid passes through an evaporator/condenser exposed to the outside environment, whereby heat energy is absorbed by the first refrigerant fluid. The vaporized first refrigerant fluid is then compressed by the first compressor into a high/pressure/high temperature gas and passed through a first heat exchanger, where heat energy contained in the first refrigerant circuit is transferred to the second refrigerant circuit. In giving up some of its heat energy in the first heat exchanger, the first refrigerant fluid becomes liquefied. The liquid first refrigerant is then passed through an expansion device which revaporizes the liquid into a low pressure/low temperature gas, which is then directed to the evaporator/condenser to repeat the cycle. 
         [0021]    The use of a relatively high pressure, low temperature refrigerant allows heat energy to be extracted from the outside environment at low temperatures. The use of an unloadable compressor further expands the operational temperature range of the compressor, allowing it to efficiently operate at ever lower temperatures. However, due to practical constraints of the compressor, the ultimate amount of heat energy absorbed by the first refrigerant when the outside ambient temperature is low is insufficient to adequately provide sufficient heat to the external heating system. Therefore, the second refrigerant circuit is employed to augment the available heat. 
         [0022]    The second refrigerant circuit utilizes a second compressor to circulate the second refrigerant fluid. The vaporized second refrigerant fluid passes through the first heat exchanger to absorb heat energy from the first refrigerant circuit. The vaporized second refrigerant fluid is then compressed by the second compressor into a high pressure/high temperature gas and passed through a second heat exchanger, where heat energy contained in the second refrigerant circuit is transferred to the external heating system, resulting in the second refrigerant fluid liquefying. The second refrigerant is then passed through an expansion device to revaporize it into a low pressure/low temperature gas, which is then directed to the first heat exchanger to repeat the cycle. 
         [0023]    In cooling mode, the first compressor circulates the first refrigerant fluid through the first refrigerant circuit in the opposite direction from heating mode. A different portion of the first refrigerant circuit, which extends to the space to be cooled and which is not involved in the heating cycle, is made accessible through appropriate valving. The vaporized first refrigerant fluid passes through an evaporator located in the space to be cooled, extracting heat energy from the inside environment. The vaporized first refrigerant fluid is then compressed by the first compressor and passed through the evaporator/condenser, where heat energy contained in the first refrigerant circuit is exhausted to the outside environment and the first refrigerant fluid liquefies. The first refrigerant is then passed through an expansion device to revaporize and then is directed to the evaporator to repeat the cycle. During cooling mode the first heat exchanger is bypassed by appropriate valving. 
         [0024]    The cooling mode may also be used in conjunction with the heating mode to perform a defrost cycle of the coils of the evaporator/condenser. During heating mode, especially when the outside ambient temperature is low, the evaporator/condenser coils tend to ice up, preventing efficient extraction of heat energy. During cooling mode heat energy is exhausted to the outside environment through the evaporator/condenser coils. By periodically switching from heating mode to cooling mode, the evaporator/condenser coils can be efficiently defrosted. Other means of performing a defrost cycle may also be used, such as heating the coils with external heat sources. 
         [0025]    Because the first refrigerant circuit is partitioned into a first sub-circuit to be used for heating and a second sub-circuit to be use for cooling, the same compressor and evaporator/condenser can be used for both functions, improving the overall efficiency of the device. And because the output from the heating mode is separated from the space to be cooled in cooling mode, the device can be easily retrofitted to existing hot air furnace systems or hot water boiler systems as well as added to heating systems without furnaces or boilers but rather employing direct hot water or hot air lines for rapid heating response, while also providing air conditioning functionality. 
         [0026]    Other features and advantages of the invention are described below. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0027]      FIG. 1  is a schematic depiction of the heat pump of the present invention. 
           [0028]      FIG. 2A  is a schematic depiction of the heat pump of the present invention with the first refrigerant circuit emphasized. 
           [0029]      FIG. 2B  is a schematic depiction of the heat pump of the present invention with the second refrigerant circuit emphasized. 
           [0030]      FIG. 3A  is a schematic depiction of the heat pump of the present invention with the first sub-circuit of the first refrigerant circuit emphasized. Directional arrows are included along the flow conduits of the first and second refrigerant circuits to indicate the direction of flow of refrigerant fluid during heating mode. 
           [0031]      FIG. 3B  is a schematic depiction of the heat pump of the present invention with the second sub-circuit of the first refrigerant circuit emphasized. Directional arrows are included along the flow conduit of the first refrigerant circuit to indicate the direction of flow of refrigerant fluid during cooling mode. An alternative embodiment of the external cooling system to which the heat pump is connected is also shown. 
           [0032]      FIG. 4  is a schematic depiction of the heat pump of the present invention with one embodiment of the defrost means emphasized. Directional arrows are included along the flow conduit of the second sub-circuit of the first refrigerant circuit to indicate the direction of flow of refrigerant fluid during defrost mode. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    The present invention discloses an improved heat pump  1  comprising a first closed refrigerant circuit  100  containing a first refrigerant fluid, see  FIG. 2A , and a second closed refrigerant circuit  200  containing a second refrigerant fluid, see  FIG. 2B , the first and second refrigerant circuits  100 , 200  being in cascade relationship with each other, wherein the improved heat pump  1  is operable both in a heating mode and in a cooling mode. The cascade relationship is achieved by the improved heat pump  1  having a first heat exchanger  300  interposed between and in connection with the first refrigerant circuit  100  and the second refrigerant circuit  200  and a second heat exchanger  400  interposed between and in connection with the second refrigerant circuit  200  and an external heating system  700 . See  FIG. 1 . 
         [0034]    The first refrigerant fluid is selected from the group of known refrigerant fluids having a relatively high pressure at a given temperature and able to efficiently extract heat energy from air at relatively low temperatures, while the second refrigerant fluid is selected from the group of known refrigerant fluids having a relatively lower pressure at a given temperature than that of the first refrigerant fluid and able to efficiently achieve higher temperatures than the first refrigerant fluid. The combined use of the first and second refrigerant fluids expands the operating range of the improved heat pump  1  to permit efficient operation at very low environmental temperatures while achieving high heat output. The first refrigerant fluid may be the well-known refrigerant R22 or R410A. The second refrigerant fluid may be the well-known refrigerant R134A or R236. Use of other refrigerants having the characteristics of the first and second refrigerant fluids described herein are also contemplated by the present invention. 
         [0035]    The first refrigerant circuit  100  is comprised of a first sub-circuit  102  and a second sub-circuit  104 , with the first sub-circuit  102  of the first refrigerant circuit  100  operable in heating mode and the second sub-circuit  104  of the first refrigerant circuit  100  operable in cooling mode. See  FIGS. 3A ,  3 B. When the improved heat pump  1  is operable in heating mode the first refrigerant fluid circulates through the first sub-circuit  102  of the first refrigerant circuit  100  in a first direction, see  FIG. 3A , and when the improved heat pump  1  is operable in cooling mode the first refrigerant fluid circulates through the second sub-circuit  104  of the first refrigerant circuit  100  in a second direction, see  FIG. 3B . The first sub-circuit  102  is comprised of a portion of the first refrigerant circuit  100  and the second sub-circuit  104  is comprised of a different portion of the first refrigerant circuit  100 , whereby at least some sections of the portions of the first refrigerant circuit  100  do not overlap with each other. The use of different sub-circuits within a single refrigerant circuit for the different modes of operation of the improved heat pump  1  increases its efficiency and ability to be retrofitted into existing construction, as well as used in new construction. 
         [0036]    The first refrigerant circuit  100  of the improved heat pump  1  also comprises a first flow conduit  110 . See  FIG. 2A . The first flow conduit  110  is a closed loop suitably adapted to contain the first refrigerant fluid, such that the first refrigerant fluid may flow in a continuous cycle through the first flow conduit  110 . The first flow conduit  110  may therefore be constructed of any manner of suitable tubing or piping, whether rigid or flexible. It may be comprised of any suitable material, such as copper or PEX (cross-linked polyethylene). 
         [0037]    The first refrigerant circuit  100  comprises a first compressor  120 , integrated with the first flow conduit  110 . See  FIG. 2A . The first compressor  120  is suitably adapted to compress the first refrigerant fluid, which is introduced to the first compressor  120  as a relatively low pressure/low temperature gas and which is compressed by the first compressor  120  into a high pressure/high temperature gas. The first compressor  120  ideally is unloadable. In one embodiment the first compressor  120  is oversized, and may run at fifty percent or one hundred percent capacity. The capacity of the first compressor  120  is determined in relation to the temperature of the outside environment; where the outside environment temperature is extremely low, the available heat energy in the air is less and the first compressor  120  operates at full capacity. When the temperature of the outside environment is moderate there is more heat energy in the air and the first compressor  120  operates at partial capacity. Other partial capacities are also contemplated. In yet other embodiments, the improved heat pump  1  utilizes one or more staged auxiliary compressors to operate in parallel with the first compressor  120 . The number of auxiliary compressors operating at any given time will depend on the temperature of the outside environment, with more compressors operating the colder the temperature of the outside environment. 
         [0038]    The first refrigerant circuit  100  also includes a first expansion device  130 , an evaporator/condenser  140 , a second expansion device  132 , a second evaporator  142 , and a reversing valve  170 . The first sub-circuit  102  of the first refrigerant circuit  100  comprises a first portion of the first flow conduit  110 , the first compressor  120 , the reversing valve  170 , the first expansion device  130 , and the evaporator/condenser  140 . See  FIG. 3A . The first compressor  120 , the reversing valve  170 , the first expansion device  130 , and the evaporator/condenser  140  are in respective serial fluid communication with one another in the first sub-circuit  102 , permitting the first refrigerant fluid to circulate in a closed loop in the first direction within the first sub-circuit  102 . The second sub-circuit  104  of the first refrigerant circuit  100  comprises a second portion of the first flow conduit  110 , the first compressor  120 , the reversing valve  170 , the evaporator/condenser  140 , the second expansion device  132 , and the second evaporator  142 . See  FIG. 3B . The first compressor  120 , the reversing valve  170 , the evaporator/condenser  140 , the second expansion device  132 , and the second evaporator  142  are in respective serial fluid communication with one another in the second sub-circuit  104 , permitting the first refrigerant fluid to circulate in a closed loop in the second direction within said second sub-circuit  104 . At least some part of the second portion of the first flow conduit  110  is not coterminous with at least some part of the first portion of the first flow conduit  110 . The use of several of the same elements in both the first sub-circuit  102  and the second sub-circuit  104  reduces redundancy and increases the cost-effectiveness of the improved heat pump  1 . In some embodiments of the present invention the first refrigerant circuit  100  may also include one or more refrigerant expansion tanks  180 , with at least one refrigerant expansion tank  180  located within the first sub-circuit  102  and within the second sub-circuit  104 . The refrigerant expansion tank  180  compensates for the various volumes of refrigerant fluid needed during the different modes of operation. For example, during cooling mode, the evaporator/condenser  140  will hold more first refrigerant fluid than will be held by the first heat exchanger  300  during heating mode. When the mode of operation switches from heating to cooling, the first refrigerant circuit  100  needs more first refrigerant fluid to operate properly. The differential amount of first refrigerant fluid is stored in the refrigerant expansion tank  180 . 
         [0039]    The second refrigerant circuit  200  of the improved heat pump  1  comprises a second flow conduit  210 . See  FIG. 2B . The second flow conduit  210  is a closed loop suitably adapted to contain the second refrigerant fluid, such that the second refrigerant fluid may flow in a continuous cycle through the second flow conduit  210 . The second flow conduit  210  may be of the same configuration and be constructed of the same materials as the first flow conduit  110 . The second refrigerant circuit  200  also comprises a second compressor  220 , integrated with the second flow conduit  210 . The second compressor  220  is suitably adapted to compress the second refrigerant fluid, which is introduced to the second compressor  220  as a relatively low pressure/low temperature gas and which is compressed by the second compressor  220  into a high pressure/high temperature gas. Unlike the first compressor  120 , the second compressor  220  need not be unloadable and ideally is a simple, single capacity compressor. The second refrigerant circuit  200  also includes a third expansion device  234 , whereby the second compressor  220  and the third expansion device  234  are in respective serial fluid communication with one another. The second compressor  220  circulates the second refrigerant fluid in a closed loop within the second flow conduit  210  of the second refrigerant circuit  200 . The second refrigerant circuit  200  is operable in heating mode only. 
         [0040]    The first refrigerant circuit  100  and the second refrigerant circuit  200  are in cascade relation with each other through the first heat exchanger  300 . The first heat exchanger  300  is interposed between and in connection with the first refrigerant circuit  100  and the second refrigerant circuit  200 . The first heat exchanger  300  contains a first section  107  of the first refrigerant circuit  100  and a first section  207  of the second refrigerant circuit  200  in close proximity to each other within the first heat exchanger  300 , such that heat energy carried by the first refrigerant fluid circulating within the first refrigerant circuit  100  is capable of being transferred to the second refrigerant fluid circulating within the second refrigerant circuit  200 . See  FIG. 2A . The first section  107  of the first refrigerant circuit  100  may be adjacent to and in contact with the first section  207  of the second refrigerant circuit  200 , or there may be a conductive element interposed between them to assist in heat transfer. The first section  107  of the first refrigerant circuit  100  and the first section  207  of the second refrigerant circuit  200  may be configured as straight piping, or coils, or any other configuration that allows for efficient heat transfer. The first heat exchanger  300  may utilize an insulated housing that contains the first section  107  of the first refrigerant circuit  100  and the first section  207  of the second refrigerant circuit  200 . Other configurations of the first heat exchanger  300  are also contemplated, as long as the first refrigerant fluid and the second refrigerant fluid remain physically separated from each other within the first heat exchanger  300 . 
         [0041]    The second refrigerant circuit  200  and the external heating system  700  are in cascade relation with each other through the second heat exchanger  400 . The second heat exchanger  400  is interposed between and in connection with the second refrigerant circuit  200  and the external heating system  700 . The second heat exchanger  400  contains a second section  209  of the second refrigerant circuit  200  and a first section  707  of the external heating system  700  in close proximity to each other within the second heat exchanger  400 , such that heat energy carried by the second refrigerant fluid circulating within the second refrigerant circuit  200  is capable of being transferred to either the water or air contained within the external heating system  700 . See  FIG. 2B . The portion of the external heating system  700  which comprises the first section  707  of the external heating system  700  is the heating system interface  710 . The heating system interface  710  connects to the remainder of the external heating system  700 , which may be a standard hot water boiler, hot air furnace, direct forced hot water system, direct forced hot air system, or any other conventional heating system. The heating system interface  710  may be a standard component of the external heating system  700  or a new component integrated with the external heating system  700  and specially designed to provide an interface with the improved heat pump  1 . The configuration of the second section  209  of the second refrigerant circuit  200  and the first section  707  of the external heating system  700  within the second heat exchanger  400  is analogous to the configuration of the first section  107  of the first refrigerant circuit  100  and the first section  207  of the second refrigerant circuit  200  within the first heat exchanger  300 . The second heat exchanger  400  may utilize an insulated housing that contains the second section  209  of the second refrigerant circuit  200  and the first section  707  of the external heating system  700 . Other configurations of the second heat exchanger  400  are also contemplated, as long as the second refrigerant fluid and the heat transfer medium of the external heating system  700  remain physically separated from each other within the second heat exchanger  400 . 
         [0042]    The reversing valve  170  of the first refrigerant circuit  100  is in fluid communication with and interposed between the first flow conduit  110  and the first compressor  120 . See  FIG. 1 . The reversing valve  170  is capable of being movably positioned between a first position and a second position to control the direction of flow of the first refrigerant fluid through the first flow conduit  110 . The first refrigerant fluid flows through the first flow conduit  110  in the first direction when the reversing valve  170  is in the first position and the first refrigerant fluid flows through the first flow conduit  110  in the second direction when the reversing valve  170  is in the second position. The position of the reversing valve  170  may controlled by one or more controllers  530 . See  FIG. 3B . The controller  530  may be a logic controller, such as an integrated circuit incorporated within a printed circuit board. 
         [0043]    In addition to the reversing valve  170 , other valving may be present within the first refrigerant circuit  100  to direct the flow of the first refrigerant fluid in either the first direction or the second direction. In one embodiment the improved heat pump  1  comprises a first check valve  150 , a second check valve  152 , and a third check valve  160 , with all three check valves  150 , 152 , 160  located within the first flow conduit  110 . See  FIG. 1 . Each check valve  150 , 152 , 160  permits the flow of the first refrigerant fluid through it in a single direction, with the first and second check valves  150 , 152  permitting the flow of the first refrigerant fluid in the first direction only and the third check valve  160  permitting the flow of the first refrigerant fluid in the second direction only. In this embodiment each expansion device  130 , 132 , 234  may be a thermal expansion valve, an electronic expansion valve, capillary tubing, orifice tubing, or a mechanical expansion valve with bypass. Each expansion device  130 , 132 , 234  may also comprise a solenoid valve for controlling flow if such capability is not otherwise integrated within its functionality. The first expansion device  130  has an open state and a closed state, each state controlled either by integrated functionality or governed by the solenoid valve, whereby when the first expansion device  130  is in the open state the flow of the first refrigerant fluid through it is permitted in the first direction, and when the first expansion device  130  is in the closed state the flow of the first refrigerant fluid bypasses expansion. The second expansion device  132  also has an open state and a closed state, similarly controlled, whereby when the second expansion device  132  in the open state the flow of the first refrigerant fluid through it is permitted in the second direction, and when the second expansion device  132  is in the closed state the flow of the first refrigerant fluid through it is prevented. When the reversing valve  170  is in the first position, the first expansion device  130  is in the open state, and the second expansion device  132  is in the closed state the first sub-circuit  102  of the first refrigerant circuit  100  is operable and the first refrigerant fluid circulates in a closed loop in the first direction within the first sub-circuit  102 . When the reversing valve  170  is in the second position, the first expansion device  130  is in the closed state, and the second expansion device  132  is in the open state the second sub-circuit  104  of the first refrigerant circuit  100  is operable and the first refrigerant fluid circulates in a closed loop in the second direction within the second sub-circuit  104 . The states of the first expansion device  130  and the second expansion device  132  may controlled by one or more controllers  530 , which may be logic controllers. See  FIG. 3B . 
         [0044]    The operation of the first compressor  120  and the second compressor  220  may be controlled by one or more controllers  510 , 520 . See  FIG. 3A . The controllers  510 , 520  may be logic controllers. In the preferred embodiment where the first compressor  120  is unloadable, the improved heat pump  1  also comprises a sensor  540  for sensing environmental temperatures proximate to the evaporator/condenser  140 . The sensor  540  may be an electronic temperature sensing device. The sensor  540  is in communication with the controller  510  and the controller  510  is responsive to input from the sensor  540 . When the controller  510  receives input from the sensor  540  indicating the environmental temperature proximate to the evaporator/condenser  140  has reached a first predetermined level, the controller  510  operates the first compressor  120  at a first capacity. When the controller  510  receives input from the sensor  540  indicating the environmental temperature proximate to the evaporator/condenser  140  has reached a second predetermined level, the controller  510  operates the first compressor  120  at a second capacity. As an example, the first predetermined level may be 40 degrees Fahrenheit; when the outside temperature is sensed to fall below this temperature, the first compressor  120  operates at 100% capacity. When the outside temperature is sensed to rise above the second predetermined level (ideally somewhat higher than the first predetermined level, to minimize cycling), the first compressor  120  operates at 50% capacity. 
         [0045]    A known downside to using air-source heat pumps in cold climates is the potential for moisture to freeze onto the evaporator. As ice and frost accumulate onto the evaporator&#39;s coils, the transfer of heat becomes less efficient, degrading the entire system&#39;s performance. The improved heat pump  1  of the present invention comprises a defrosting means  600  sufficient to eliminate ice buildup from the evaporator/condenser  140 . See  FIG. 4 . 
         [0046]    One embodiment of the defrosting means  600  of the improved heat pump  1  periodically reverses the flow of the first refrigerant fluid within the first refrigerant circuit  100  (as is done when the improved heat pump  1  is used in cooling mode), thereby sending heated refrigerant fluid through the evaporator/condenser  140 , which then discharges rather than absorbs heat energy, thus defrosting the coils. After allowing the coils to defrost, the flow of the first refrigerant fluid is reversed to permit the first refrigerant fluid to absorb heat energy from the outside air. 
         [0047]    In one embodiment of this defrosting means  600 , the flow of the first refrigerant fluid through the first refrigerant circuit  100  is controlled by one or more controllers  610 , which may be logic controllers, which are suitably adapted to control the states of the first and second expansion devices  130 , 132 , the position of the reversing valve  170 , and operation of the first compressor  120 . The defrosting means  600  utilizes a sensor  620  for sensing temperatures of the first refrigerant fluid proximate to the evaporator/condenser  140  and a timer  630 , both of which are in communication with at least one of the one or more controllers  610 . When the one or more controllers  610  receive input from the sensor  620  indicating the temperature of the first refrigerant fluid proximate to the evaporator/condenser  140  has reached a predetermined level, the controllers  610  activate the timer  630  for a predetermined period of time, set the states of the first and second expansion devices  130 , 132 , set the position the reversing valve  170  to place the improved heat pump  1  in cooling mode, and operate the first compressor  120  such that the first refrigerant fluid flows through the second sub-circuit  104  of the first refrigerant circuit  100  in the second direction until the predetermined period of time as measured by the timer  630  elapses. Thereafter, the controllers  610  reverse the respective states of the first and second expansion devices  130 , 132  and the position of the reversing valve  170  to place the improved heat pump  1  in heating mode and operate the first compressor  120  such that the first refrigerant fluid flows through the first sub-circuit  102  of the first refrigerant circuit  100  in the first direction. 
         [0048]    In another embodiment of this defrosting means  600 , rather than operating for a predetermined period of time governed by a timer, the defrosting means  600  initiates the defrost cycle when the sensor  620  determines the temperature of the first refrigerant fluid proximate to the evaporator/condenser  140  has fallen to a first predetermined level and terminates the defrost cycle when the sensor  620  determines the temperature of the first refrigerant fluid proximate to the evaporator/condenser  140  has risen to a second predetermined level. The second predetermined level is ideally somewhat higher than the first predetermined level to minimize cycling. 
         [0049]    An alternative embodiment of the defrosting means  600  utilizes the application of heat from an auxiliary heat source, such as an electric heater or a natural gas burner, for a predetermined period of time to defrost the coils. 
         [0050]    In one embodiment of the present invention, the improved heat pump  1  may further comprise a third heat exchanger  900 . See  FIG. 3B . The third heat exchanger  900  is interposed between and in connection with the second sub-circuit  104  of the first refrigerant circuit  100  and an external cooling system  800 . The third heat exchanger  900  contains a portion of the second sub-circuit  104  and a portion of the external cooling system  800  in close proximity to each other within the third heat exchanger  900 , such that heat energy carried by the external cooling system  800  is capable of being transferred to the first refrigerant fluid circulating within the second sub-circuit  104  of the first refrigerant circuit  100 . The portion of the external cooling system  800  which is contained within the third heat exchanger  900  is the cooling system interface  810 . The cooling system interface  810  connects to the remainder of the external cooling system  800 , which may be a standard air plenum system, a direct expansion cooling coil, a direct cold water line, a direct cold air line, or any other conventional cooling system. The cooling system interface  810  may be a standard component of the external cooling system  800  or a new component integrated with the external cooling system  800  and specially designed to provide an interface with the improved heat pump  1 . In another embodiment the third heat exchanger  900  contains the second evaporator  142  and a portion of the external cooling system  800  in close proximity to each other within the third heat exchanger  900 . The third heat exchanger  900  may utilize an insulated housing that contains the portion of the second sub-circuit  104  and the portion of the external cooling system  800 . Other configurations of the third heat exchanger  900  are also contemplated, as long as the first refrigerant fluid and the heat transfer medium of the external cooling system  800  remain physically separated from each other within the third heat exchanger  900 . 
         [0051]    Modifications and variations can be made to the disclosed embodiments of the invention without departing from the subject or spirit of the invention as defined in the following claims.