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.

What is claimed is: 
     
         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.