Defrost for Cascade Heat Pump System

The application claims priority from U.S. Provisional Application No. 63/661,309, filed Jun. 18, 2024, and entitled “DEFROST FOR CASCADE HEAT PUMP SYSTEM,” the disclosure of which is incorporated herein by reference in its entirety.

The present disclosure relates generally to improved defrost processes and design for a cascade heat pump system.

Cascade systems have been utilized in select applications to transport heat, often across a significant temperature gradient. In general, these systems include two (or more) complete refrigerant circuits where refrigerant in each circuit is circulated via a compressor and heat is transferred between the circuits via heat exchangers.

Given this arrangement, cascade systems offer various additional complexity over single-loop systems, particularly when attempting to defrost one or more of the coils associated with a heat exchanger. For example, in a single circuit heat pump the typical mode of defrost is to reverse the direction of refrigerant flow, and therefore reverse the direction of heat flow. The side effect of this method is that now the heat sink becomes the heat source which negatively impacts the operation of the heat pump that is attempting to heat a target, e.g., a working fluid. In a cascade system the defrost mode is more complex because there are two refrigerant circuits that need to be managed.

This issue can be particularly challenging for a high-temperature cascade heat pump. In some applications the heat source may be subject to low temperature conditions and ice may form at that heat exchanger exposed to these conditions even though the heat sink may be attempting to achieve high temperatures. For example, the heat source may be an outdoor ambient environment, and ice may form on the coils exposed to this environment. Presently, no existing solutions appropriately address this problem, and the present disclosure provides an improved system and method for addressing this ice formation via a defrosting process.

The present disclosure thus includes, without limitation, the following example embodiments.

Some example implementations provide 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 includes 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.

Some example implementations provide a cascade system including 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. The cascade system includes 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 These and other features, aspects, and advantages of the disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below.

The disclosure includes any combination of two, three, four, or more of the above-noted embodiments as well as combinations of any two, three, four, or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined in a specific embodiment description herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosed disclosure, in any of its various aspects and embodiments, should be viewed as intended to be combinable unless the context clearly dictates otherwise.

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.

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.

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.

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.

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,shows an example depiction of a cascade heat pump, andshows an example depiction of a cascade heat pump with additional valves for use in the defrosting process disclosed herein.

To first walk through, the cascade heat pumpincludes a first stageand a second stage. In the depicted example, the first stageis the low, temperature stage, which is thermally coupled to the heat source/sinkfor the system, and the second stageis the high, temperature stage, which rejections/absorbs heat into a working fluid. Each of these stages includes a compressor (,) and a metering device (,), e.g., a thermal expansion valve, an electronic expansion valve, a capillary tube, orifice, etc. The first stagefurther includes a source-side heat exchanger, and the second stagealso includes a usage-side heat exchanger. The first and second stages (and) are also thermally coupled at an interstage heat exchanger. The first and second stages also includes devices (and) for moving a fluid across the source-side heat exchangerand the usage-side heat exchanger, respectively. These devices may facility heat transfer between the various fluids at the respective heat exchangers. For example, an outdoor air fanmay be used to transport air from the heat source/sink over the source-side heat exchanger. Similarly, a pumpmay be used to circulate a working fluid over the source-side heat exchanger. Other configurations may also be utilized.

The cascade heat pumpmay also include various sensors to control the operation of one or more components. For example, the depicted example, includes a temperature sensorlocated on a coil of the source-side heat exchanger. 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.

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 exchangermay 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 exchangermay 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 exchangeris then transported through the cascade heat pumpto provide heat at a much higher temperature at the usage-side heat exchanger, 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.

Further, in the depicted example in heating mode, the first stageabsorbs heat at the source-side heat exchangerand transfers that heat to the second stageat the interstage heat exchanger. To walk through the first stage, the compressordischarges high temperature refrigerant to the interstage heat exchanger. At that heat exchanger the refrigerant in the first stagemay be condensed and reject heat into the refrigerant in the second stage. The refrigerant in the first stagedischarged from the interstage heat exchangermay be in a predominately liquid form, and that refrigerant from the interstage heat exchangermay be circulated to the first stage metering device. The metering devicemay reduce the pressure of the refrigerant in the first stageprior to entering the source-side heat exchanger. By lowering the pressure at the metering device, the refrigerant may flash and evaporate at the source-side heat exchanger, allowing it to absorb heat from the heat source/sink, 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 exchangermay be routed to the source-side heat exchangervia a conduit. It is understood that additional conduits may be utilized and arranged in different manners. Again, in some examples, the cascade heat pumpmay be tasked with absorbing heat from an outdoor environment at cold (and even extremely cold) temperatures, and thus, the first stagemay 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/sinkat these low temperature conditions.

In the depicted example, the first stagefurther includes a fanthat moves air from the heat source/sinkto help facilitate the thermal exchange at the source-side heat exchanger. In some examples, the fandirects outdoor ambient air over the heat exchangerto allow heat transfer to occur via convection, increasing the heat transfer between the heat source/sinkand the refrigerant in the first stageat the source-side heat exchanger. 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. Still other examples may be used, and in some examples, no mechanism is needed to transport fluid associated with the heat sink/sourceto practice the examples disclosed herein.

In the depicted example, the heat absorbed in the first stageat the source-side heat exchangeris transferred to the refrigerant in the second stageat the interstage heat exchanger. That heat is transported via the second stageto the usage-side heat exchangerand then into a working fluid. As discussed above, the cascade heat pumpmay be used for multiple different applications, and various working fluidsmay be utilized. For example, as shown in the depicted example, the cascade heat pump may include a pumpto 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. Pumpmay 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 stageat heat exchanger. 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 exchangerand circulated to conditioned spaces via a circulation fan (not shown). Still other applications and fluids may be utilized in accordance with the disclosure herein.

To walk through the second stage, the compressordischarges high temperature refrigerant to the source-side heat exchanger. At that heat exchanger, the refrigerant in the second stagemay be condensed and reject heat into the working fluid. The refrigerant in the second stagedischarged from the usage-side heat exchangermay be in a liquid form, and that refrigerant may continue to circulate to the second stage metering device. The metering devicemay reduce the pressure of the refrigerant in the second stageprior to entering the interstage heat exchanger. Similar to the first stage, by lowering the pressure at the metering device, the refrigerant in the second stagemay flash and evaporate at the interstage heat exchanger, allowing it to absorb heat from the refrigerant in the first stage. Similar to the first stage, 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 pumpmay be tasked with transporting heat across a significant temperature gradient, and thus, the second stagemay be designed for that purpose, e.g., the selected refrigerant, heat exchanger size/type, etc.

shows an example depiction of a cascade heat pumpthat includes the same or similar components discussed above in connection with cascade heat pumpshown in; however, cascade heat pumpalso includes additional components that allows this system to efficiently implement a defrost process. For example, cascade heat pumpincludes a switch-over-valve (SOV value)in the low temperature, first stage. This switch-over-valvemay 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-valveincludes a first position and second position. In these examples, the first position may circulate the refrigerant in the first circuitin a heating mode, e.g., the same or similar to the process described above in connection with, and the second position of the switch-over-valvemay circulate the refrigerant in the first circuitin a defrost mode (or cooling mode) as shown and described in connection with. In the depicted example, the cascade heat pumpalso includes a bypass linein the high temperature, second stage. This bypass linemay allow the second stageto supply heat to the first stagewithout reversing the flow of refrigerant. Each of these additional components are discussed in more detail below.

In the first stageof cascade heat pump, the switch-over-valveallows the first stage to reverse the flow of the refrigerant and correspondingly the flow of heat. For example, the first stagemay operate in accordance with a heating mode as described above where heat is absorbed at the source-side heat exchanger. The switch-over-valvemay reverse the flow of refrigerant in the first stage, allowing the first stageto discharge heat at the source-side heat exchanger, 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 exchangerto facilitate melting the ice. For example, in the depicted example, the fanmay circulate outdoor air over the coils of the source-side heat exchangerduring 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.

To walk through the refrigerant flow in this configuration, e.g., a defrost mode, the first stage compressordischarges high temperature refrigerant to the switch-over-valve. In the depicted configuration, e.g., defrost mode, the switch-over-valveis in a second position and routes the high temperature refrigerant to the source-side heat exchanger. In this mode, the source-side heat exchangerrejects heat from the refrigerant into the heat source/sink, which in defrost mode is designed to melt any ice formed on the coils of that heat exchanger. 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 stagevia a conduit. In some examples, this refrigerant bypasses (not shown) the first stage metering deviceand/or pass through metering devicewithout any significant change in pressure, e.g., the refrigerant is not depressurized by a metering device. The refrigerant then circulates through the interstage heat exchangerwhere it absorbs heat from the refrigerant in the second stage. In some examples an additional metering device (not shown) is utilized to reduce the pressure of the refrigerant in the first stageprior to entering the interstage heat exchangerto allow the refrigerant in the first stage to receive heat from the refrigerant in the second stage; however, in some examples, the temperature of the refrigerant in the second stageis 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 exchangeracts as an evaporator for the refrigerant in the first stage, causing the refrigerant to evaporate prior to returning to the suction side of the compressorvia the switch-over-valve.

In the second stageof the cascade heat pumpthe bypass lineis used during a defrost mode to supply heat to the first stagein defrost mode. For example, the bypass linemay be used to allow the second stageto transfer heat to the first stageduring defrost mode without the need to reverse the refrigerant flow in the second stageand/or having any undesirable impact on the usage-side heat exchangerand/or the working fluid, e.g., no heat is absorbed from the usage-side heat exchangerand/or the working fluidduring the defrost mode.

In the depicted example, the bypass linecouples to the main refrigerant circuit of the second stagebetween the discharge port of the compressorand the usage-side heat exchanger. In this example, the bypass circuitroutes the refrigerant to the interstage heat exchanger, bypassing the usage-side heat exchangerand the second stage metering device.

In the depicted example, the bypass linefurther includes a 3-way valvecoupled to the main circuit at the location where the bypass lineroutes 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, or through the bypass line. 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.

In some examples, multiple valves are used to control the flow into the bypass lineand/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, potentially stopping or limiting the flow of refrigerant in the main circuit to the usage-side heat exchanger. 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 compressoroperates appropriately in this defrost mode with the varying changing conditions.

In some examples, during defrost mode there is no need to circulate the working fluidover the usage-side heat exchanger. As stated above, in some examples, one benefit to the bypass lineis that the refrigerant bypasses the usage-side heat exchanger, and thus, by shutting off the pumpassociated with the working fluid the system reduces energy consumption. Shutting off that pumpalso avoids (or at least limits) any undesirable heat transfer between the working fluidand the refrigerant in the second stageduring defrost mode. In other examples, the pumpmay 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 (,), to assist in defrost, or for another purpose.

To illustration these components and the disclosed defrost process, further reference is made to cascade heat pumpdepicted in. 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. This ice formation may be detected using a temperature sensor (e.g.,) based on a temperature of the coil of the source-side heat exchanger, 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.

Once the cascade heat pumpenters a defrost mode, the first stagereverses the flow of refrigerant via the switch-over-valve. As discussed above, this reverses the flow of heat at the source-side heat exchanger, to initiate melting any ice formed on that coil. The defrost process also engages the bypass linein the second stagevia the 3-way valve(or other flow control methos), which was also discussed above, allows the second stageto transfer heat to the first stagevia the interstage heat exchanger.

To further illustrate an example, defrost mode in the cascade heat pump, an example of the refrigerant flows and the heat transfers in both the first stageand the second stageare described. In some examples, in defrost mode, the first stage compressordischarges refrigerant at an elevated temperature to the source-side heat exchangervia the switch-over-valve. The refrigerant discharged from the compressor rejects heat to melt ice formed on the coils of the source-side heat exchanger. 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 stageto the interstage heat exchangervia a conduit. At the interstage heat exchanger, the refrigerant in the first stage absorbs heat from the second stage, 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 exchangerwithout the need for a metering device to reduce the pressure of the refrigerant within the first stage prior to entering the interstage heat exchanger. 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 exchangervia 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 stageis discharged from the interstage heat exchangerafter absorbing heat from the refrigerant in the second stage. In some examples, the refrigerant in the first stagemay evaporate at the interstage heat exchanger, and the refrigerant in the first stageis then routed back to the suction side of the first stage compressorvia the switch-over-valve. At the first stage compressor, the refrigerant is then again compressed, increasing the temperature (and pressure) of that refrigerant to repeat this cycle during the defrost mode.

Turning to the second stage, during the defrost mode in this example, the second stage compressordischarges high temperature refrigerant which is routed to the bypass circuitvia the 3-way valve(or other flow control process). The bypass circuit routes this high temperature refrigerant directly to the interstage heat exchanger, bypassing the usage-side heat exchangerand/or the second stage expansion valve. At the interstage heat exchangerthis high temperature refrigerant in the second stagerejects heat into the refrigerant circulating within the first stage. In some examples, the refrigerant in the second stagemay be at an elevated temperature such that the refrigerant discharged from the interstage heat exchangerin the second stageonly changes temperature, but not phases, e.g., it remains a gas despite rejecting heat at the interstage heat exchanger. That refrigerant, e.g., the refrigerant in the second stagedischarged from the interstage heat exchanger, may then be routed back to the second stage compressorwhere it is compressed, and discharged from the compressor at an elevated temperature (and pressure) to repeat this cycle.shows that a metering devicemay be installed on the bypass circuitbefore the interstage heat exchanger. In some examples, the metering devicemay reduce the pressure of the refrigerant before the refrigerant enters the interstage heat exchanger. This metering devicemay be used to balance the refrigerant circuit and/or refrigerant flows, by reducing the pressure associated that refrigerant discharged from the compressor. Further, in some examples, the refrigerant leaving the metering devicemay be a gas and the refrigerant discharged from the interstage heat exchangerin the second stagemay also remain a gas despite being going through the metering deviceand rejecting heat at the interstage heat exchanger.

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.

In some examples, this defrost cycle may continue until any ice built up on the source-side heat exchangeris removed. For example, again, a sensor may be located proximate the source-side heat exchanger (e.g., sensor), and that sensor may be used to monitor the temperature associated with the coil at the source-side heat exchanger. 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.

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.depicts a processfor defrosting the cascade system in response to an indication of ice formation on a first stage heat exchanger during the heating mode.depicts a processfor maintaining the refrigerant discharged from the first stage heat exchanger is in predominately a liquid form during the defrost mode. The processshown indepicts 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,depicts a processfor operating the cascade system in a heating mode.

The processofbegins with operating the cascade system of some implementation of the disclosed disclosure in a heating mode, at step. The cascade system may be the systemofthat includes the 3-way valve, the bypass circuitin the second stage, e.g., circuit, and the switch-over valvein the first stage, e.g., circuit, 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, when it is detected that the ice is formed on the heat exchanger of the first stage, the processinitiates 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 sensorof.

During the defrost mode, stepexecutes by reversing the refrigerant flow in the first stage. This may be executed by a switch-over valve, e.g., the switch-over-valve. 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.

Further, stepexecutes by diverting the refrigerant flow in the second stage of the cascade system, potentially via the 3-way valve, 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 exchangerwhich 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 circuiton 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, 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.

The above stepsandmay 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, processwill terminate the defrost mode, as shown at stepand the cascade system resume to operate the heating mode at step.

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.

Processofshows 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, and by evaporating the refrigerant discharged from the heat exchanger of the first stage at the heat exchanger of the interstage, at step. The stepmay 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 inand thus, are omitted herein for simplicity.

Processofshows 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, stepexecutes by discharging the refrigerant from the second stage compressor, e.g., the compressor. Next, stepexecutes by diverting the refrigerant to the bypass line via a bypass valve, e.g., 3-way valve. As shown in, as an example, the bypass line routes this high temperature refrigerant directly to the interstage heat exchanger, without going through the usage-side heat exchangerand/or the second stage expansion valve. At the interstage heat exchangerthis high temperature refrigerant in the second stagerejects heat into the refrigerant circulating within the first stage. Next, stepexecutes by routing the refrigerant through the heat exchanger of the interstage. After that, stepexecutes 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.

depicts a processfor 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.

During a heating mode (step), stepexecutes by the source-side heat exchangerabsorbing 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 exchangerabsorbs heat from the outdoor environment at outdoor ambient environment temperatures higher than 32° F. At step, 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, stepexecutes 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.

illustrates an apparatus, 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 (,) 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 processorconnected to a memory.

The processoris 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.

The processormay be configured to execute computer programs such as computer-readable program code, which may be stored onboard the processor or otherwise stored in the memory. 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.

The memoryis generally any piece of computer hardware capable of storing information such as, for example, data, computer-readable program codeor 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, causes the apparatusto perform various operations as described herein, some of which may in turn cause the components discussed herein to perform various operations.

In addition to the memory, the processormay also be connected to one or more peripherals such as a network adapter, one or more input/output (I/O) devices or the like. The network adapter is a hardware component configured to connect the apparatusto 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.