DEFROST FOR CASCADE HEAT PUMP SYSTEM

Abstract

A method and apparatus for defrosting a cascade heat pump. The cascade heat pump may include a first stage compressor circulating refrigerant in a first stage refrigerant circuit, a second stage compressor circulating refrigerant in a second stage refrigerant circuit, and an interstage heat exchanger thermally coupling the first stage and second stage. The defrost process for the cascade heat pump may include initiating a defrost mode in response to an indication of ice formation on a first stage heat exchanger, reversing a flow of refrigerant in the first stage during the defrost mode, and diverting a flow of refrigerant in the second stage through a bypass line during the defrost mode, wherein the bypass line directs the flow of refrigerant to the interstage heat exchanger and to bypass a second stage heat exchanger.

Claims

1. A method of defrosting a cascade system, the cascade system including a first stage compressor configured to circulate refrigerant in a first stage refrigerant circuit, and a second stage compressor configured to circulate refrigerant in a second stage refrigerant circuit, wherein the first stage and second stage are thermally coupled at an interstage heat exchanger, the method comprising: operating the cascade system in a heating mode; initiating a defrost mode in response to an indication of ice formation on a first stage heat exchanger; reversing a flow of refrigerant in the first stage during the defrost mode; and diverting a flow of refrigerant in the second stage through a bypass line during the defrost mode, wherein the bypass line directs the flow of refrigerant to the interstage heat exchanger and to bypass a second stage heat exchanger.

2. The method of claim 1, wherein the first stage heat exchanger is a source-side heat exchanger configured to exchange thermal energy between an outdoor ambient environment and the refrigerant in the first stage, and the second stage heat exchanger is a usage-side heat exchanger configured to exchanger thermal energy between a working fluid and the refrigerant in the second stage, wherein operating the cascade system in the heat mode further includes: absorbing heat from the outdoor ambient environment at the source-side heat exchanger; transferring heat from the refrigerant in the first stage to the refrigerant in the second stage at the interstage heat exchanger; rejecting heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger.

3. The method of claim 2, wherein absorbing heat from the outdoor ambient environment at the source-side heat exchanger during heating mode further includes absorbing heat at outdoor ambient environment temperatures of 32 F. or lower; and wherein rejecting heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger during heating mode further includes rejecting heat from the refrigerant in the second stage to discharge the working fluid at temperatures of 150 F. or higher.

4. The method of claim 2, wherein the cascade system further includes an outdoor fan configured to move outdoor air through the source-side heat exchanger to facilitate the exchange of thermal energy between the outdoor ambient environment and the refrigerant in the first stage, and a pump configured to move the working fluid through the usage-side heat exchanger to facilitate the exchange of thermal energy between the working fluid and the refrigerant in the first stage, and the method further comprises: operating the outdoor fan during the defrost mode; and shutting off the pump during the defrost mode.

5. The method of claim 1, wherein diverting the flow of refrigerant in the second stage further includes directing the flow of refrigerant through the interstage heat exchanger in the same direction during defrost mode as the refrigerant in the second stage flows through the interstage heat exchanger during heating mode.

6. The method of claim 1, wherein the bypass line further directs the flow of refrigerant to bypass a second stage metering device.

7. The method of claim 1, further comprises: routing the refrigerant discharged from the first stage heat exchanger to the interstage stage heat exchanger at substantially the same pressure during the defrost mode.

8. The method of claim 7, wherein the refrigerant discharged from the first stage heat exchanger is in predominately a liquid form during the defrost mode, and the method further comprises: evaporating the refrigerant discharged from the first stage heat exchanger at the interstage heat exchanger during the defrost mode.

9. The method of claim 7, further comprising: maintaining the refrigerant in the second stage refrigerant circuit in predominately a gas form through a full cycle of the second stage refrigerant circuit during the defrost mode, the full cycle including: discharging the refrigerant from the second stage compressor, diverting the refrigerant to the bypass line via the bypass valve, routing the refrigerant through the interstage heat exchanger, and returning the refrigerant to the second stage compressor.

10. The method of claim 1, further comprising: terminating the defrost mode in response to an indication that ice formation on the first stage heat exchanger has been removed; and resuming operation of the heating mode in response to terminating the defrost mode.

11. A cascade system comprising: a first stage refrigerant circuit including a first stage compressor configured to circulate refrigerant in the first stage refrigerant circuit, a first stage heat exchanger configured to exchange thermal energy between the refrigerant in the first stage and an ambient environment, and a switch-over-valve configured to reverse the flow of refrigerant from the first stage compressor to the first stage heat exchanger, wherein reversing the flow of refrigerant reverses the exchange of thermal energy between the refrigerant in the first stage and the ambient environment; a second stage refrigerant circuit including a second stage compressor configured to circulate refrigerant in the second stage refrigerant circuit, a second stage heat exchanger configured to exchange thermal energy between the refrigerant in the second stage and a working fluid, and a bypass valve configured to selectively divert a flow of refrigerant in the second stage through a bypass line, wherein the bypass valve includes at least a first position and a second position, the first position allows the refrigerant in the second circuit to flow through the bypass valve to the second stage heat exchanger and bypass the bypass line, and the second position allows the refrigerant in the second circuit to flow through the bypass line and bypass the second stage heat exchanger; an interstage heat exchanger configured to thermally couple the refrigerant in the first stage and the refrigerant in the second stage; and a control circuit configured to: operate the cascade system in a heating mode; initiate a defrost mode in response to an indication of ice formation on the first stage heat exchanger; adjust the position of the switch-over-valve to reverse the flow of refrigerant in the first stage during the defrost mode; and adjust the position of the bypass valve to the second position during the defrost mode to divert the flow of refrigerant in the second stage through the bypass line, wherein the bypass line directs the flow of refrigerant to the interstage heat exchanger and to bypass the second stage heat exchanger.

12. The cascade system of claim 11, wherein the first stage heat exchanger is a source-side heat exchanger configured to exchange thermal energy between an outdoor ambient environment and the refrigerant in the first stage, wherein the second stage heat exchanger is a usage-side heat exchanger configured to exchanger thermal energy between the working fluid and the refrigerant in the second stage, wherein the control circuitry configured to operate the cascade system in heating mode further includes control circuitry configured to: position the switch-over-valve in a heating mode position, wherein the heating mode position directs refrigerant discharged from the first stage compressor to the interstage heat exchanger prior to flowing through the first stage heat exchanger and allow heat from the outdoor ambient environment to be absorbed at the source-side heat exchanger; position the bypass valve in a first position to direct refrigerant discharged from the second stage compressor to the usage-side heat exchanger and allow heat from the refrigerant in the second stage to be rejected into the working fluid.

13. The cascade system of claim 12, wherein absorbing heat from the outdoor ambient environment at the source-side heat exchange during heating mode further includes absorbing heat at outdoor ambient environment temperatures of 32 F. or lower; and wherein rejecting heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger during heating mode further includes rejecting heat from the refrigerant in the second stage to elevate the discharge temperature of the working fluid to 150 F. or higher.

14. The cascade system of claim 12, further comprising: an outdoor fan configured to move outdoor air through the source-side heat exchanger to facilitate the exchange of thermal energy between the outdoor ambient environment and the refrigerant in the first stage; and a pump configured to move the working fluid through the usage-side heat exchanger to facilitate the exchange of thermal energy between the working fluid and the refrigerant in the first stage, wherein the control circuitry is further configured to: operate the outdoor fan during the defrost mode; and shut off the pump during the defrost mode.

15. The cascade system of claim 11, wherein the bypass line directs the flow of refrigerant in the second stage through the interstage heat exchanger in the same direction during defrost mode as the refrigerant in the second stage flows through the interstage heat exchanger during heating mode.

16. The cascade system of claim 11, wherein the first stage heat exchanger and the interstage heat exchanger are coupled via a conduit, the conduit including a metering device and configured to route refrigerant in a predominately liquid form between the first stage heat exchanger and the interstage heat exchanger, wherein, during the heating mode, the conduit is configured to route the refrigerant in predominately liquid form from the interstage heat exchanger to the first stage heat exchanger, and the metering device is configured to depressurize the refrigerant prior to entering the first stage heat exchanger; and wherein, during the defrost mode, the conduit is configured to route the refrigerant in predominately liquid form from the first stage heat exchanger to interstage heat exchanger at substantially the same pressure.

17. The cascade system of claim 16, wherein the interstate stage heat exchanger is configured to evaporate the refrigerant discharged from the source-side heat exchanger during the defrost mode.

18. The cascade system of claim 16, wherein, in defrost mode, the refrigerant circulated in the second stage refrigerant circuit remains in predominately a gas form while circulating through a full cycle of the second stage refrigerant circuit, the full cycle including the refrigerant being discharged from the second stage compressor, diverting to the bypass line via the bypass valve, routing through the interstage heat exchanger, and returning to a suction side of the second stage compressor.

19. The cascade system of claim 11, further comprising: a temperature sensor configured to monitor a temperature of the first stage heat exchanger, wherein the control circuitry configured to initiate the defrost mode is configured to initiate the defrost mode based on temperature measurements provided by the temperature sensor.

20. The cascade system of claim 19, wherein the control circuitry is further configured to: terminate the defrost mode in response to an indication from the temperature sensor that ice formation on the first stage heat exchanger has been removed; and resume operation of the heating mode in response to terminating the defrost mode.

Description

BRIEF DESCRIPTION OF THE FIGURE(S)

[0010] Reference will now be made to the accompanying figures, which are not necessarily drawn to scale, and wherein:

[0011] FIG. 1 illustrates a diagram of an example cascade heat pump, according to some example implementations;

[0012] FIG. 2 illustrates a diagram of an example cascade heat pump with additional components, according to some example implementations;

[0013] FIGS. 3A, 3B, 3C, and 3D illustrate flowcharts of processes for defrosting a cascade system, according to some example implementations.

[0014] FIG. 4 illustrates control circuitry according to some example implementations.

DESCRIPTION

[0015] Some implementations of the present disclosure will now be described more fully hereinafter with reference to the accompanying figures, in which some, but not all implementations of the disclosure are shown. Indeed, various implementations of the disclosure may be embodied in many different forms and should not be construed as limited to the implementations set forth herein; rather, these example implementations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like reference numerals refer to like elements throughout.

[0016] The disclosure herein describes various systems and methods for operating a cascade refrigeration system in a defrost mode. These disclosed examples are particularly applicable to cascade heat pumps that are used to provide heating for a given application, e.g., conditioning of a particular space. In these systems the heat source is typically an ambient environment, e.g., the outside ambient environment, and the cascade heat pump system may be used to transport this heat across a significant temperature gradient to provide the heating for the given application.

[0017] An issue that arises in these systems is accounting for ice formation on the heat exchanger exposed to the ambient conditions. That heat exchanger may be subject to highly variable conditions, particularly if it is located outside and required to provide heating during cold climate conditions. However, traditional defrost processes for single-circuit systems are often insufficient for cascade heat pumps. This is because cascade systems are not simply balancing loads between a single heat source/sink circuit arrangement, but instead these cascade systems are balancing loads across multiple different circuits. Thus, to reverse the flow of heat across each of these circuits may be difficult, particularly when accounting for the significant temperature gradient between the different circuits. Another option is to focus only on the circuit associated with the heat exchanger exposed to the cold conditions; however, that may result in load imbalances at that stage, potentially damaging the compressor or other components.

[0018] Accordingly, the present disclosure presents an improved process for defrosting a heat-source heat exchanger in a cascade heat pump. This process reverses the first, low temperature stage of the cascade system, which reverses the flow of heat at that stage. This allows heat to be rejected (as opposed to absorbed) at the heat exchanger exposed to the cold conditions, which allows the heat exchanger to melt ice formed on its coils. However, as opposed to reversing the second, high temperature stage, the process utilizes a bypass line on the second, high temperature stage. This bypass line bypasses the usage-side heat exchanger and allows that second stage to deliver heat to the first, low temperature stage during defrost mode without actually reversing the system. In addition, this bypass line in the second stage prevents that stage from pulling heat from the heat sink/source, e.g., the working fluid, and primarily passes along the heat of compression associated with the second stage. This results in a significant benefit for high temperature heat pumps because it also prevents excessively high suction temperatures on the second stage compressor, which could result from reversing the second stage in these systems. Thus, the disclosed process allows for a more efficient defrost process along with a simpler overall system design.

[0019] The below walks through an example system and method for the disclosed defrost process. These examples are directed to an application that uses a cascade heat pump to deliver elevated heating to a working fluid, e.g., air, water, etc., however it is understood that these examples may be utilized on other systems and designs in accordance with the disclosure herein. To further illustrate these examples, FIG. 1 shows an example depiction of a cascade heat pump, and FIG. 2 shows an example depiction of a cascade heat pump with additional valves for use in the defrosting process disclosed herein.

[0020] To first walk through FIG. 1, the cascade heat pump 100 includes a first stage 102 and a second stage 104. In the depicted example, the first stage 102 is the low, temperature stage, which is thermally coupled to the heat source/sink 106 for the system, and the second stage 104 is the high, temperature stage, which rejections/absorbs heat into a working fluid 108. Each of these stages includes a compressor (110, 112) and a metering device (114, 116), e.g., a thermal expansion valve, an electronic expansion valve, a capillary tube, orifice, etc. The first stage 102 further includes a source-side heat exchanger 118, and the second stage 104 also includes a usage-side heat exchanger 120. The first and second stages (102 and 104) are also thermally coupled at an interstage heat exchanger 122. The first and second stages also includes devices (124 and 126) for moving a fluid across the source-side heat exchanger 118 and the usage-side heat exchanger 120, respectively. These devices may facility heat transfer between the various fluids at the respective heat exchangers. For example, an outdoor air fan 124 may be used to transport air from the heat source/sink over the source-side heat exchanger 118. Similarly, a pump 126 may be used to circulate a working fluid over the source-side heat exchanger 120. Other configurations may also be utilized.

[0021] The cascade heat pump 100 may also include various sensors to control the operation of one or more components. For example, the depicted example, includes a temperature sensor 128 located on a coil of the source-side heat exchanger 118. That temperature sensor may monitor the temperature at that location and may provide an indication of whether ice has formed on that coil and/or ice has been removed from that coil. It is understood that additional sensors, different sensor types, and/or different sensor location may be utilized for this purpose, e.g., pressure sensors, sensors located proximate the suction side of the first stage compressor, etc. Further, in some examples, no sensors are utilized. Still other examples and techniques may be used.

[0022] To walk through an example, the cascade heat pump may be designed to transport heat from an outdoor ambient environment to provide an elevated temperature at a usage-side heat exchanger. In these examples, the source-side heat exchanger 118 may be designed to absorb heat from the outdoor ambient environment. Because, in this example the system is designed for heating (or may be operating in a heating mode), the temperature of the outdoor ambient environment can vary considerably and can be exposed to extreme temperatures. For example, in some locations/climates, the source-side heat exchanger 118 may be asked to absorb heat from a sub-freezing outdoor environment, e.g., 32 F. or lower, and potentially as low as 0 F. or lower. The heat absorbed at this source-side heat exchanger 118 is then transported through the cascade heat pump 100 to provide heat at a much higher temperature at the usage-side heat exchanger 120, potentially over 200 F. or higher. This high temperature of refrigerant at the usage-side heat exchanger allows for elevated discharge working fluid temperatures, e.g., 150 F. or higher, and potentially over 180 F. This elevated temperature can have several advantages, for example, it can allow for a greater thermal capacity to be stored in the same volume or flow rate of a working fluid.

[0023] Further, in the depicted example in heating mode, the first stage 102 absorbs heat at the source-side heat exchanger 118 and transfers that heat to the second stage 104 at the interstage heat exchanger 122. To walk through the first stage 102, the compressor 110 discharges high temperature refrigerant to the interstage heat exchanger 122. At that heat exchanger the refrigerant in the first stage 102 may be condensed and reject heat into the refrigerant in the second stage 104. The refrigerant in the first stage 102 discharged from the interstage heat exchanger 122 may be in a predominately liquid form, and that refrigerant from the interstage heat exchanger 122 may be circulated to the first stage metering device 114. The metering device 114 may reduce the pressure of the refrigerant in the first stage 102 prior to entering the source-side heat exchanger 118. By lowering the pressure at the metering device 114, the refrigerant may flash and evaporate at the source-side heat exchanger 118, allowing it to absorb heat from the heat source/sink 106, which in this example is the outdoor ambient environment. The refrigerant in the first stage may be routed via conduits coupling the various components. For example, the refrigerant from the interstage heat exchanger 122 may be routed to the source-side heat exchanger 118 via a conduit. It is understood that additional conduits may be utilized and arranged in different manners. Again, in some examples, the cascade heat pump 100 may be tasked with absorbing heat from an outdoor environment at cold (and even extremely cold) temperatures, and thus, the first stage 102 may be designed for that purpose, e.g., the selected refrigerant, heat exchanger size/type, etc., may be selected to allow heat to be absorbed at the heat source/sink 106 at these low temperature conditions.

[0024] In the depicted example, the first stage 102 further includes a fan 124 that moves air from the heat source/sink 106 to help facilitate the thermal exchange at the source-side heat exchanger 118. In some examples, the fan 124 directs outdoor ambient air over the heat exchanger 118 to allow heat transfer to occur via convection, increasing the heat transfer between the heat source/sink 106 and the refrigerant in the first stage 102 at the source-side heat exchanger 118. As will be understood, other heat/sources and sinks may be utilized. For example, water may be circulated as the heat source/sink mechanism, in which case a pump may be used to circulate that fluid and facilitate the heat transfer at the first stage heat exchanger 118. Still other examples may be used, and in some examples, no mechanism is needed to transport fluid associated with the heat sink/source 106 to practice the examples disclosed herein.

[0025] In the depicted example, the heat absorbed in the first stage 102 at the source-side heat exchanger 118 is transferred to the refrigerant in the second stage 104 at the interstage heat exchanger 122. That heat is transported via the second stage 104 to the usage-side heat exchanger 120 and then into a working fluid 108. As discussed above, the cascade heat pump 100 may be used for multiple different applications, and various working fluids 108 may be utilized. For example, as shown in the depicted example, the cascade heat pump may include a pump 126 to circulate the working fluid, which may be used as part of a hydronic system. In these systems the working fluid may be water, and the heated water may be circulated to air handler units (not shown) to supply a heating source for those units. Pump 126 may circulate the water to the air handler units and may also assist in the convection heat exchange that may occur between the working fluid and the refrigerant in the second stage 104 at heat exchanger 120. In other examples, the cascade heat pump may be part of a forced air system, and the working fluid may be circulation air that is heated by the usage-side heat exchanger 120 and circulated to conditioned spaces via a circulation fan (not shown). Still other applications and fluids may be utilized in accordance with the disclosure herein.

[0026] To walk through the second stage 104, the compressor 112 discharges high temperature refrigerant to the source-side heat exchanger 120. At that heat exchanger 120, the refrigerant in the second stage 104 may be condensed and reject heat into the working fluid 108. The refrigerant in the second stage 104 discharged from the usage-side heat exchanger 120 may be in a liquid form, and that refrigerant may continue to circulate to the second stage metering device 116. The metering device 116 may reduce the pressure of the refrigerant in the second stage 104 prior to entering the interstage heat exchanger 122. Similar to the first stage 102, by lowering the pressure at the metering device 116, the refrigerant in the second stage 104 may flash and evaporate at the interstage heat exchanger 122, allowing it to absorb heat from the refrigerant in the first stage 102. Similar to the first stage 102, the refrigerant in the second stage may also be routed between the various components via conduit(s) and these conduits may be arranged in various ways as will be understood by a person of ordinary skill in the art. Again, in some examples, the cascade heat pump 100 may be tasked with transporting heat across a significant temperature gradient, and thus, the second stage 104 may be designed for that purpose, e.g., the selected refrigerant, heat exchanger size/type, etc.

[0027] FIG. 2 shows an example depiction of a cascade heat pump 200 that includes the same or similar components discussed above in connection with cascade heat pump 100 shown in FIG. 1; however, cascade heat pump 200 also includes additional components that allows this system to efficiently implement a defrost process. For example, cascade heat pump 200 includes a switch-over-valve (SOV value) 202 in the low temperature, first stage 102. This switch-over-valve 202 may be a 4-way valve that allows the first stage circuit to switch between a heat mode and a cooling mode, or in several examples disclosed herein, a defrost mode. In some examples, the switch-over-valve 202 includes a first position and second position. In these examples, the first position may circulate the refrigerant in the first circuit 102 in a heating mode, e.g., the same or similar to the process described above in connection with FIG. 1, and the second position of the switch-over-valve 202 may circulate the refrigerant in the first circuit 102 in a defrost mode (or cooling mode) as shown and described in connection with FIG. 2. In the depicted example, the cascade heat pump 200 also includes a bypass line 204 in the high temperature, second stage 104. This bypass line 204 may allow the second stage 104 to supply heat to the first stage 102 without reversing the flow of refrigerant. Each of these additional components are discussed in more detail below.

[0028] In the first stage 102 of cascade heat pump 200, the switch-over-valve 202 allows the first stage to reverse the flow of the refrigerant and correspondingly the flow of heat. For example, the first stage 102 may operate in accordance with a heating mode as described above where heat is absorbed at the source-side heat exchanger 118. The switch-over-valve 202 may reverse the flow of refrigerant in the first stage 102, allowing the first stage 102 to discharge heat at the source-side heat exchanger 118, melting any ice formed on the coils of that heat exchanger. In some examples, fluid associated with the heat source/sink is moved across the coils of the source-side heat exchanger 118 to facilitate melting the ice. For example, in the depicted example, the fan 124 may circulate outdoor air over the coils of the source-side heat exchanger 118 during defrost mode to assist in the defrost process. In other examples, the fan may be turned off to limit any impact the heat source/sink may have on the ice formation/melting process, e.g., in certain cold conditions circulating outdoor air over ice while the system is melting the ice may be counterproductive.

[0029] To walk through the refrigerant flow in this configuration, e.g., a defrost mode, the first stage compressor 110 discharges high temperature refrigerant to the switch-over-valve 202. In the depicted configuration, e.g., defrost mode, the switch-over-valve 202 is in a second position and routes the high temperature refrigerant to the source-side heat exchanger 118. In this mode, the source-side heat exchanger 118 rejects heat from the refrigerant into the heat source/sink 106, which in defrost mode is designed to melt any ice formed on the coils of that heat exchanger 118. In this example, the refrigerant may condense at the source-side heat exchanger, which may cause this heat exchanger to act as a condenser. In this example, the refrigerant discharged from source-side heat exchanger, which may be a liquid, continues to circulate through the first stage 102 via a conduit. In some examples, this refrigerant bypasses (not shown) the first stage metering device 114 and/or pass through metering device 114 without any significant change in pressure, e.g., the refrigerant is not depressurized by a metering device. The refrigerant then circulates through the interstage heat exchanger 122 where it absorbs heat from the refrigerant in the second stage 104. In some examples an additional metering device (not shown) is utilized to reduce the pressure of the refrigerant in the first stage 102 prior to entering the interstage heat exchanger 122 to allow the refrigerant in the first stage to receive heat from the refrigerant in the second stage 104; however, in some examples, the temperature of the refrigerant in the second stage 104 is sufficiently elevated above the temperature of the refrigerant in the first stage at the interstage heat exchanger during the defrost mode that an additional metering device in the first stage is not necessary. Further, in some examples, the interstage heat exchanger 122 acts as an evaporator for the refrigerant in the first stage 102, causing the refrigerant to evaporate prior to returning to the suction side of the compressor 110 via the switch-over-valve 202.

[0030] In the second stage 104 of the cascade heat pump 200 the bypass line 204 is used during a defrost mode to supply heat to the first stage 102 in defrost mode. For example, the bypass line 204 may be used to allow the second stage 104 to transfer heat to the first stage 102 during defrost mode without the need to reverse the refrigerant flow in the second stage 104 and/or having any undesirable impact on the usage-side heat exchanger 120 and/or the working fluid 108, e.g., no heat is absorbed from the usage-side heat exchanger 120 and/or the working fluid 108 during the defrost mode.

[0031] In the depicted example, the bypass line 204 couples to the main refrigerant circuit of the second stage 104 between the discharge port of the compressor 112 and the usage-side heat exchanger 120. In this example, the bypass circuit 204 routes the refrigerant to the interstage heat exchanger 122, bypassing the usage-side heat exchanger 120 and the second stage metering device 116.

[0032] In the depicted example, the bypass line 204 further includes a 3-way valve 206 coupled to the main circuit at the location where the bypass line 204 routes refrigerant from the main circuit to bypass the various components. In some examples, the 3-way valve is designed to route all refrigerant either through the main circuit, e.g., to the usage-side heat exchanger 120, or through the bypass line 204. In other examples, the 3-way valve splits the flow between the main circuit and the bypass line, allowing a portion of the refrigerant to flow in both directions.

[0033] In some examples, multiple valves are used to control the flow into the bypass line 204 and/or allocate the refrigerant between the main circuit and the bypass circuit. For example, each line may have a modulating valve (not shown) and these modulating valves may selectively control the refrigerant follow into the bypass line 204, potentially stopping or limiting the flow of refrigerant in the main circuit to the usage-side heat exchanger 120. In some examples, the control may be via solenoid valves which either allow or block the flow of refrigerant via the bypass circuit and the main circuit. In these examples, a subsequent valve downstream of the bypass circuit, potentially a metering device, may assist in controlling the flow into the bypass circuit. In some examples, the various valves associated with selectively diverting the refrigerant flow between the main circuit and the bypass circuit are used to ensure an appropriate pressure ratio is achieved within by the refrigerant in these various circuits. In some examples, the flow/pressure control may be directed to ensuring the compressor 112 operates appropriately in this defrost mode with the varying changing conditions.

[0034] In some examples, during defrost mode there is no need to circulate the working fluid 108 over the usage-side heat exchanger 120. As stated above, in some examples, one benefit to the bypass line 204 is that the refrigerant bypasses the usage-side heat exchanger 120, and thus, by shutting off the pump 124 associated with the working fluid the system reduces energy consumption. Shutting off that pump 126 also avoids (or at least limits) any undesirable heat transfer between the working fluid 108 and the refrigerant in the second stage 104 during defrost mode. In other examples, the pump 124 may be activated (potentially at reduced power) to allow for some heat to be transferred between these fluids. This heat transfer may be used to account for any imbalance in load or temperature occurring in the first and/or second stages (102, 104), to assist in defrost, or for another purpose.

[0035] To illustration these components and the disclosed defrost process, further reference is made to cascade heat pump 200 depicted in FIG. 2. In this example, the defrost process may be initiated through any process. For example, ice formation may be detected on the coil of the source-side heat exchanger 118. This ice formation may be detected using a temperature sensor (e.g., 128) based on a temperature of the coil of the source-side heat exchanger 118, by detecting a reduced capacity and/or thermal transfer at that coil, a larger than normal differential between the refrigerant temperature and the ambient temperature, and/or through another process.

[0036] Once the cascade heat pump 200 enters a defrost mode, the first stage 102 reverses the flow of refrigerant via the switch-over-valve 202. As discussed above, this reverses the flow of heat at the source-side heat exchanger 118, to initiate melting any ice formed on that coil. The defrost process also engages the bypass line 204 in the second stage 104 via the 3-way valve 206 (or other flow control methos), which was also discussed above, allows the second stage 104 to transfer heat to the first stage 102 via the interstage heat exchanger 122.

[0037] To further illustrate an example, defrost mode in the cascade heat pump 200, an example of the refrigerant flows and the heat transfers in both the first stage 102 and the second stage 104 are described. In some examples, in defrost mode, the first stage compressor 110 discharges refrigerant at an elevated temperature to the source-side heat exchanger 118 via the switch-over-valve 202. The refrigerant discharged from the compressor rejects heat to melt ice formed on the coils of the source-side heat exchanger 118. The refrigerant in the first stage may condense as part of this process, but regardless, the refrigerant discharged from the source-side heat exchanger is routed via the first stage 102 to the interstage heat exchanger 122 via a conduit. At the interstage heat exchanger 122, the refrigerant in the first stage absorbs heat from the second stage 104, and in some examples, the refrigerant in the first stage evaporates as a result of this heat transfer. As discussed above, in some examples, the temperature of the refrigerant in the second stage may be sufficiently high to evaporate the refrigerant in the first stage at the interstage heat exchanger 122 without the need for a metering device to reduce the pressure of the refrigerant within the first stage prior to entering the interstage heat exchanger 122. In these examples, the refrigerant discharged from the source-side heat exchanger may be in a predominately liquid form and may be routed to the interstage heat exchanger 122 via a conduit without any substantial change in pressure, e.g., the pressure of the predominately liquid refrigerant from the source-side heat exchanger may be substantially the same as the predominately liquid refrigerant entering the interstage heat exchanger. Regardless, in the defrost mode, the refrigerant in the first stage 102 is discharged from the interstage heat exchanger 122 after absorbing heat from the refrigerant in the second stage 104. In some examples, the refrigerant in the first stage 102 may evaporate at the interstage heat exchanger 112, and the refrigerant in the first stage 102 is then routed back to the suction side of the first stage compressor 110 via the switch-over-valve 202. At the first stage compressor 110, the refrigerant is then again compressed, increasing the temperature (and pressure) of that refrigerant to repeat this cycle during the defrost mode.

[0038] Turning to the second stage 104, during the defrost mode in this example, the second stage compressor 112 discharges high temperature refrigerant which is routed to the bypass circuit 204 via the 3-way valve 206 (or other flow control process). The bypass circuit routes this high temperature refrigerant directly to the interstage heat exchanger 122, bypassing the usage-side heat exchanger 120 and/or the second stage expansion valve 116. At the interstage heat exchanger 122 this high temperature refrigerant in the second stage 104 rejects heat into the refrigerant circulating within the first stage 102. In some examples, the refrigerant in the second stage 104 may be at an elevated temperature such that the refrigerant discharged from the interstage heat exchanger 122 in the second stage 104 only changes temperature, but not phases, e.g., it remains a gas despite rejecting heat at the interstage heat exchanger 122. That refrigerant, e.g., the refrigerant in the second stage 104 discharged from the interstage heat exchanger 122, may then be routed back to the second stage compressor 112 where it is compressed, and discharged from the compressor at an elevated temperature (and pressure) to repeat this cycle. FIG. 2 shows that a metering device 130 may be installed on the bypass circuit 204 before the interstage heat exchanger 122. In some examples, the metering device 130 may reduce the pressure of the refrigerant before the refrigerant enters the interstage heat exchanger 122. This metering device 130 may be used to balance the refrigerant circuit and/or refrigerant flows, by reducing the pressure associated that refrigerant discharged from the compressor 112. Further, in some examples, the refrigerant leaving the metering device 130 may be a gas and the refrigerant discharged from the interstage heat exchanger 122 in the second stage 104 may also remain a gas despite being going through the metering device 130 and rejecting heat at the interstage heat exchanger 122.

[0039] In some examples, as discussed above, the refrigerant in the second stage refrigerant circuit remains in predominately a gas from through the full cycle of the second stage circuit during the defrost mode. For example, as discussed above, the refrigerant discharged from the second stage compressor may be a high temperature gas. That refrigerant may be diverted via the 3-way valve, e.g., the bypass valve, to the bypass line in this high temperature gas form. The bypass line may route the refrigerant in the second stage from the bypass line, and potentially a metering device within the bypass line, to the interstage heat exchanger. The refrigerant may reduce pressure and temperature while flowing through these components, but it may remain in a predominately gas form. The refrigerant in the second stage discharged from the interstage heat exchanger, again in predominately gas form, may then be returned to the second stage compressor to repeat the cycle.

[0040] In some examples, this defrost cycle may continue until any ice built up on the source-side heat exchanger 118 is removed. For example, again, a sensor may be located proximate the source-side heat exchanger (e.g., sensor 128), and that sensor may be used to monitor the temperature associated with the coil at the source-side heat exchanger 118. The defrost process may determine all ice has melted from the coil based on the temperatures received from that sensor indicating that the ice has melted, e.g., the temperature of the coil may be sufficiently high to indicate that the ice has melted, the temperature change of the coil may indicate ice has melted, the temperature difference between the refrigerant and the ambient temperature, etc. To walk through one example, the coil temperature may be substantially constant when ice is formed on the coil, e.g., the temperature may be associated with the temperature of the ice. While the ice is melting the phase change associated with the ice may keep the monitored temperature generally constant; however, once the ice melts the temperature may change rapidly. This change in the rate of temperature change may indicate that the ice has melted. Other methods may also be used to determine the ice has melted, e.g., a timer, etc., and the system should transition out of the defrost mode to another mode, e.g., heating mode.

[0041] FIGS. 3A-3D illustrate flowcharts of processes for defrosting a cascade system according to the implementations of the disclosure. In general, the defrosting mode is performed during a heating mode. FIG. 3A depicts a process 300 for defrosting the cascade system in response to an indication of ice formation on a first stage heat exchanger during the heating mode. FIG. 3B depicts a process 330 for maintaining the refrigerant discharged from the first stage heat exchanger is in predominately a liquid form during the defrost mode. The process 340 shown in FIG. 3C depicts a flowchart for maintaining the refrigerant in the second stage refrigerant circuit in predominately a gas form through a full cycle of the second stage refrigerant circuit during the defrost mode. Moreover, FIG. 3D depicts a process 350 for operating the cascade system in a heating mode.

[0042] The process 300 of FIG. 3A begins with operating the cascade system of some implementation of the disclosed disclosure in a heating mode, at step 302. The cascade system may be the system 200 of FIG. 2 that includes the 3-way valve 206, the bypass circuit 204 in the second stage 104, e.g., circuit 2, and the switch-over valve 202 in the first stage 101, e.g., circuit 1, for instance. The first stage heat exchanger is a source-side heat exchanger configured to exchange thermal energy between an outdoor ambient environment and the refrigerant in the first stage, and the second stage heat exchanger is a usage-side heat exchanger configured to exchanger thermal energy between a working fluid and the refrigerant in the second stage. At step 304, when it is detected that the ice is formed on the heat exchanger of the first stage, the process 300 initiates the defrost mode. As described above, the indication of the ice formation may be done by detecting the coil temperature of the heat exchanger using a temperature sensor, e.g., the temperature sensor 128 of FIG. 2.

[0043] During the defrost mode, step 306 executes by reversing the refrigerant flow in the first stage. This may be executed by a switch-over valve, e.g., the switch-over-valve 202. In some examples, the switch-over valve includes a first position and second position. The first position may circulate the refrigerant in the first circuit of the first stage in a heating mode and the second position may circulate the refrigerant in the first circuit of the first stage in a defrost mode (or cooling mode) as shown and described in connection with FIG. 2.

[0044] Further, step 308 executes by diverting the refrigerant flow in the second stage of the cascade system, potentially via the 3-way valve 206, so that the second stage can supply heat to the first stage without reversing the flow of refrigerant. The bypass line directs the flow of refrigerant to the interstage heat exchanger and to bypass a second stage heat exchanger. By reversing the flow of heat at the first stage, the heat is rejected (as opposed to absorbed) at the heat exchanger of the first stage, e.g., the heat exchanger 118 which is exposed to the cold conditions, which allows the heat exchanger to melt ice formed on its coils. As opposed to reversing the second, high temperature stage, the process utilizes a bypass line, e.g., bypass circuit 204 on the second, high temperature stage. This bypass line bypasses the usage-side heat exchanger and allows that second stage to deliver heat to the first, low temperature stage during the defrost mode without actually reversing the system. As mentioned previously, this bypass line in the second stage may also prevents that stage from pulling heat from the heat sink, e.g., the working fluid 108, and only passes along the heat of compression associated with the second stage. This results in a significant benefit for high temperature heat pumps because it also prevents excessively high suction temperatures on the second stage compressor, which could result from reversing the second stage in these systems.

[0045] The above steps 306 and 308 may be repeated a few times until the ice formation on the heat exchanger of the first stage is removed. Again, the ice formation being removed may be indicated by the temperature of the heat exchanger of the first stage detected by the temperature sensor. Once the ice formation is removed, process 300 will terminate the defrost mode, as shown at step 310 and the cascade system resume to operate the heating mode at step 312.

[0046] In the example implementations of the disclosed disclosure, during the defrost mode, the refrigerant discharged from the heat exchanger of the first stage may be in predominately a liquid form. Further, during the defrost mode, the refrigerant in the second stage refrigerant circuit may be in predominately a gas form through a full cycle of the second refrigerant circuit. The processes for maintaining the refrigerant in predominately liquid form in the first stage and in predominately gas form in the second stage will be described in FIGS. 3B and 3C.

[0047] Process 330 of FIG. 3B shows that the refrigerant discharged from the heat exchanger of the first stage is maintained in predominately a liquid form by routing the refrigerant discharged from the heat exchanger of the first stage to the heat exchanger of the interstage at substantially the same pressure during the defrost mode, at step 332, and by evaporating the refrigerant discharged from the heat exchanger of the first stage at the heat exchanger of the interstage, at step 334. The step 332 may be executed by directing the refrigerant bypassing the metering device or passing the metering device without reducing the pressure of the refrigerant. Details of these steps have been described in FIG. 2 and thus, are omitted herein for simplicity.

[0048] Process 340 of FIG. 3C shows the refrigerant in the second stage refrigerant circuit in predominately a gas form through a full cycle of the second stage refrigerant circuit during the defrost mode. As shown, step 342 executes by discharging the refrigerant from the second stage compressor, e.g., the compressor 112. Next, step 344 executes by diverting the refrigerant to the bypass line via a bypass valve, e.g., 3-way valve 206. As shown in FIG. 2, as an example, the bypass line routes this high temperature refrigerant directly to the interstage heat exchanger 122, without going through the usage-side heat exchanger 120 and/or the second stage expansion valve 116. At the interstage heat exchanger 122 this high temperature refrigerant in the second stage 104 rejects heat into the refrigerant circulating within the first stage 102. Next, step 346 executes by routing the refrigerant through the heat exchanger of the interstage. After that, step 348 executes by routing the refrigerant to the second stage compressor. At the second stage compressor, the refrigerant is compressed and discharged from the compressor at an elevated temperature (and pressure) to repeat this cycle.

[0049] FIG. 3D depicts a process 350 for operating the cascade system in the heating mode. As previously discussed, the first stage heat exchanger is a source-side heat exchanger configured to exchange thermal energy between an outdoor ambient environment and the refrigerant in the first stage, and the second stage heat exchanger is a usage-side heat exchanger configured to exchanger thermal energy between a working fluid and the refrigerant in the second stage.

[0050] During a heating mode (step 352), step 354 executes by the source-side heat exchanger 118 absorbing heat from the outdoor ambient environment. One example of absorbing the heat from the outdoor ambient environment at the source-side heat exchanger during the heating mode includes absorbing heat at outdoor ambient environment temperatures of 32 F. or lower. In some examples, the source-side heat exchanger 118 absorbs heat from the outdoor environment at outdoor ambient environment temperatures higher than 32 F. At step 356, the first stage may transfer the heat from the refrigerant in the first stage to the refrigerant in the second stage at the interstage heat exchanger. That heat may be transported via the second stage to the usage-side heat exchanger and then into a working fluid. Further, at the interstage heat exchanger, step 358 executes by rejecting the heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger. One example of rejecting heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger during heating mode is to reject heat from the refrigerant in the second stage to discharge the working fluid at temperatures of 150 F. or higher. In some examples, the working fluid is discharged at temperatures lower than 150 F.

[0051] FIG. 4 illustrates an apparatus 400, e.g., control circuitry, according to some example implementations of the present disclosure. Generally, an apparatus of exemplary implementations of the present disclosure may comprise, include or be embodied in one or more fixed or portable electronic devices. Examples of suitable electronic devices include any of the controllers, processors, or other electrical devices discussed herein, and to the extent not already discussed: smartphone, tablet computer, laptop computer, desktop computer, workstation computer, server computer, PLC, circuit board or the like. In some examples, these electronic devices may be associated with the cascade heat pumps (100, 200) discussed above, e.g., one or more controllers (not shown) for those heat pumps. The apparatus may include one or more of each of a number of components such as, for example, a processor 402 connected to a memory 404.

[0052] The processor 402 is generally any piece of computer hardware capable of processing information such as, for example, data, computer programs and/or other suitable electronic information. The processor includes one or more electronic circuits some of which may be packaged as an integrated circuit or multiple interconnected integrated circuits (an integrated circuit at times more commonly referred to as a chip). The processor may be a number of processors, a multi-core processor or some other type of processor, depending on the particular implementation.

[0053] The processor 402 may be configured to execute computer programs such as computer-readable program code 406, which may be stored onboard the processor or otherwise stored in the memory 404. In some examples, the processor may be embodied as or otherwise include one or more ASICs, FPGAs or the like. Thus, although the processor may be capable of executing a computer program to perform one or more functions, the processor of various examples may be capable of performing one or more functions without the aid of a computer program.

[0054] The memory 404 is generally any piece of computer hardware capable of storing information such as, for example, data, computer-readable program code 406 or other computer programs, and/or other suitable information either on a temporary basis and/or a permanent basis. The memory may include volatile memory such as random access memory (RAM), and/or non-volatile memory such as a hard drive, flash memory or the like. In various instances, the memory may be referred to as a computer-readable storage medium, which is a non-transitory device capable of storing information. In some examples, then, the computer-readable storage medium is non-transitory and has computer-readable program code stored therein that, in response to execution by the processor 402, causes the apparatus 400 to perform various operations as described herein, some of which may in turn cause the components discussed herein to perform various operations.

[0055] In addition to the memory 404, the processor 402 may also be connected to one or more peripherals such as a network adapter 408, one or more input/output (I/O) devices or the like. The network adapter is a hardware component configured to connect the apparatus 400 to one or more networks to enable the apparatus to transmit and/or receive information via the one or more networks. This may include transmission and/or reception of information via one or more networks through a wired or wireless connection using Wi-Fi, Bluetooth, BACnet, LonTalk, Modbus, ZigBee, Zwave, or the like, or other suitable wired or wireless communication protocols.

[0056] The I/O devices may include one or more input devices 410 capable of receiving data or instructions for the apparatus 400, and/or one or more output devices 412 capable of providing an output from the apparatus. Examples of suitable input devices include a keyboard, keypad or the like, and examples of suitable output devices include a display device such as a one or more light-emitting diodes (LEDs), a LED display, a liquid crystal display (LCD), or the like.

[0057] As explained above and reiterated below, the present disclosure includes, without limitation, the following example implementations. [0058] Clause 1: A method of defrosting a cascade system, the cascade system including a first stage compressor configured to circulate refrigerant in a first stage refrigerant circuit, and a second stage compressor configured to circulate refrigerant in a second stage refrigerant circuit, wherein the first stage and second stage are thermally coupled at an interstage heat exchanger, the method comprising: operating the cascade system in a heating mode; initiating a defrost mode in response to an indication of ice formation on a first stage heat exchanger; reversing a flow of refrigerant in the first stage during the defrost mode; and diverting a flow of refrigerant in the second stage through a bypass line during the defrost mode, wherein the bypass line directs the flow of refrigerant to the interstage heat exchanger and to bypass a second stage heat exchanger. [0059] Clause 2: The method of any of the clauses, wherein the first stage heat exchanger is a source-side heat exchanger configured to exchange thermal energy between an outdoor ambient environment and the refrigerant in the first stage, and the second stage heat exchanger is a usage-side heat exchanger configured to exchanger thermal energy between a working fluid and the refrigerant in the second stage, wherein operating the cascade system in the heat mode further includes: absorbing heat from the outdoor ambient environment at the source-side heat exchanger; transferring heat from the refrigerant in the first stage to the refrigerant in the second stage at the interstage heat exchanger; rejecting heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger. [0060] Clause 3: The method of any of the clauses, wherein absorbing heat from the outdoor ambient environment at the source-side heat exchanger during heating mode further includes absorbing heat at outdoor ambient environment temperatures of 32 F. or lower; and wherein rejecting heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger during heating mode further includes rejecting heat from the refrigerant in the second stage to discharge the working fluid at temperatures of 150 F. or higher. [0061] Clause 4: The method of any of the clauses, wherein the cascade system further includes an outdoor fan configured to move outdoor air through the source-side heat exchanger to facilitate the exchange of thermal energy between the outdoor ambient environment and the refrigerant in the first stage, and a pump configured to move the working fluid through the usage-side heat exchanger to facilitate the exchange of thermal energy between the working fluid and the refrigerant in the first stage, and the method further comprises: operating the outdoor fan during the defrost mode; and shutting off the pump during the defrost mode. [0062] Clause 5: The method of any of the clauses, wherein diverting the flow of refrigerant in the second stage further includes directing the flow of refrigerant through the interstage heat exchanger in the same direction during defrost mode as the refrigerant in the second stage flows through the interstage heat exchanger during heating mode. [0063] Clause 6: The method of any of the clauses, wherein the bypass line further directs the flow of refrigerant to bypass a second stage metering device. [0064] Clause 7: The method of any of the clauses, further comprises: routing the refrigerant discharged from the first stage heat exchanger to the interstage stage heat exchanger at substantially the same pressure during the defrost mode. [0065] Clause 8: The method of any of the clauses, wherein the refrigerant discharged from the first stage heat exchanger is in predominately a liquid form during the defrost mode, and the method further comprises: evaporating the refrigerant discharged from the first stage heat exchanger at the interstage heat exchanger during the defrost mode. [0066] Clause 9: The method of any of the clauses, further comprising: maintaining the refrigerant in the second stage refrigerant circuit in predominately a gas form through a full cycle of the second stage refrigerant circuit during the defrost mode, the full cycle including: discharging the refrigerant from the second stage compressor, diverting the refrigerant to the bypass line via the bypass valve, routing the refrigerant through the interstage heat exchanger, and returning the refrigerant to the second stage compressor. [0067] Clause 10: The method of any of the clauses, further comprising: terminating the defrost mode in response to an indication that ice formation on the first stage heat exchanger has been removed; and resuming operation of the heating mode in response to terminating the defrost mode. [0068] Clause 11: A cascade system comprising: a first stage refrigerant circuit including a first stage compressor configured to circulate refrigerant in the first stage refrigerant circuit, a first stage heat exchanger configured to exchange thermal energy between the refrigerant in the first stage and an ambient environment, and a switch-over-valve configured to reverse the flow of refrigerant from the first stage compressor to the first stage heat exchanger, wherein reversing the flow of refrigerant reverses the exchange of thermal energy between the refrigerant in the first stage and the ambient environment; a second stage refrigerant circuit including a second stage compressor configured to circulate refrigerant in the second stage refrigerant circuit, a second stage heat exchanger configured to exchange thermal energy between the refrigerant in the second stage and a working fluid, and a bypass valve configured to selectively divert a flow of refrigerant in the second stage through a bypass line, wherein the bypass valve includes at least a first position and a second position, the first position allows the refrigerant in the second circuit to flow through the bypass valve to the second stage heat exchanger and bypass the bypass line, and the second position allows the refrigerant in the second circuit to flow through the bypass line and bypass the second stage heat exchanger; an interstage heat exchanger configured to thermally couple the refrigerant in the first stage and the refrigerant in the second stage; and a control circuit configured to: operate the cascade system in a heating mode; initiate a defrost mode in response to an indication of ice formation on the first stage heat exchanger; adjust the position of the switch-over-valve to reverse the flow of refrigerant in the first stage during the defrost mode; and adjust the position of the bypass valve to the second position during the defrost mode to divert the flow of refrigerant in the second stage through the bypass line, wherein the bypass line directs the flow of refrigerant to the interstage heat exchanger and to bypass the second stage heat exchanger. [0069] Clause 12: The cascade system of any of the clauses, wherein the first stage heat exchanger is a source-side heat exchanger configured to exchange thermal energy between an outdoor ambient environment and the refrigerant in the first stage, wherein the second stage heat exchanger is a usage-side heat exchanger configured to exchanger thermal energy between the working fluid and the refrigerant in the second stage, wherein the control circuitry configured to operate the cascade system in heating mode further includes control circuitry configured to: position the switch-over-valve in a heating mode position, wherein the heating mode position directs refrigerant discharged from the first stage compressor to the interstage heat exchanger prior to flowing through the first stage heat exchanger and allow heat from the outdoor ambient environment to be absorbed at the source-side heat exchanger; position the bypass valve in a first position to direct refrigerant discharged from the second stage compressor to the usage-side heat exchanger and allow heat from the refrigerant in the second stage to be rejected into the working fluid. [0070] Clause 13: The cascade system of any of the clauses, wherein absorbing heat from the outdoor ambient environment at the source-side heat exchange during heating mode further includes absorbing heat at outdoor ambient environment temperatures of 32 F. or lower; and wherein rejecting heat from the refrigerant in the second stage to the working fluid at the usage-side heat exchanger during heating mode further includes rejecting heat from the refrigerant in the second stage to elevate the discharge temperature of the working fluid to 150 F. or higher. [0071] Clause 14: The cascade system of any of the clauses, further comprising: an outdoor fan configured to move outdoor air through the source-side heat exchanger to facilitate the exchange of thermal energy between the outdoor ambient environment and the refrigerant in the first stage; and a pump configured to move the working fluid through the usage-side heat exchanger to facilitate the exchange of thermal energy between the working fluid and the refrigerant in the first stage, wherein the control circuitry is further configured to: operate the outdoor fan during the defrost mode; and shut off the pump during the defrost mode. [0072] Clause 15: The cascade system of any of the clauses, wherein the bypass line directs the flow of refrigerant in the second stage through the interstage heat exchanger in the same direction during defrost mode as the refrigerant in the second stage flows through the interstage heat exchanger during heating mode. [0073] Clause 16: The cascade system of any of the clauses, wherein the bypass line further directs the flow of refrigerant in the second stage to bypass a second stage metering device. [0074] Clause 17: The cascade system of any of the clauses, wherein the first stage heat exchanger and the interstage heat exchanger are coupled via a conduit, the conduit including a metering device and configured to route refrigerant in a predominately liquid form between the first stage heat exchanger and the interstage heat exchanger, wherein, during the heating mode, the conduit is configured to route the refrigerant in predominately liquid form from the interstage heat exchanger to the first stage heat exchanger, and the metering device is configured to depressurize the refrigerant prior to entering the first stage heat exchanger; and wherein, during the defrost mode, the conduit is configured to route the refrigerant in predominately liquid form from the first stage heat exchanger to interstage heat exchanger at substantially the same pressure. [0075] Clause 18: The cascade system of any of the clauses, wherein the interstate stage heat exchanger is configured to evaporate the refrigerant discharged from the source-side heat exchanger during the defrost mode. [0076] Clause 19: The cascade system of any of the clauses, wherein, in defrost mode, the refrigerant circulated in the second stage refrigerant circuit remains in predominately a gas form while circulating through a full cycle of the second stage refrigerant circuit, the full cycle including the refrigerant being discharged from the second stage compressor, diverting to the bypass line via the bypass valve, routing through the interstage heat exchanger, and returning to a suction side of the second stage compressor. [0077] Clause 20: The cascade system of any of the clauses, further comprising: a temperature sensor configured to monitor a temperature of the first stage heat exchanger, wherein the control circuitry configured to initiate the defrost mode is configured to initiate the defrost mode based on temperature measurements provided by the temperature sensor. [0078] Clause 21: The cascade system of any of the clauses, wherein the control circuitry is further configured to: terminate the defrost mode in response to an indication from the temperature sensor that ice formation on the first stage heat exchanger has been removed; and resume operation of the heating mode in response to terminating the defrost mode.

[0079] Many modifications and other implementations of the disclosure set forth herein will come to mind to one skilled in the art to which the disclosure pertains having the benefit of the teachings presented in the foregoing description and the associated figures. Therefore, it is to be understood that the disclosure is not to be limited to the specific implementations disclosed and that modifications and other implementations are intended to be included within the scope of the description provided herein. Moreover, although the foregoing description and the associated figures describe example implementations in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative implementations without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0080] Unless specified otherwise or clear from context, references to first, second or the like should not be construed to imply a particular order. Also, while reference may be made herein to quantitative measures, values, geometric relationships or the like, unless otherwise stated, any one or more if not all of these may be absolute or approximate to account for acceptable variations that may occur, such as those due to engineering tolerances or the like.

[0081] As used herein, unless specified otherwise or clear from context, the or of a set of operands is the inclusive or and thereby true if and only if one or more of the operands is true, as opposed to the exclusive or which is false when all of the operands are true. Thus, for example, [A] or [B] is true if [A] is true, or if [B] is true, or if both [A] and [B] are true. Further, the articles a and an mean one or more, unless specified otherwise or clear from context to be directed to a singular form.