Heat pump defrost control

Abstract

A water heating system includes a compressor unit fluidly coupled to a first heat exchanger and a controller operatively coupled to the compressor unit. The controller is configured to operate the compressor unit in a heating mode to circulate refrigerant between the compressor unit and the first heat exchanger in a first direction, determine a loss of superheat in the first heat exchanger while the compressor unit is operating in the heating mode, and initiate a defrosting sequence in response to determining the loss of superheat in the first heat exchanger.

What is claimed is: 
     
         1 . A water heating system, comprising:
 a compressor unit fluidly coupled to a first heat exchanger; and   a controller operatively coupled to the compressor unit, the controller configured to:
 operate the compressor unit in a heating mode to circulate refrigerant between the compressor unit and the first heat exchanger in a first direction, 
 determine a loss of superheat in the first heat exchanger while the compressor unit is operating in the heating mode, and 
 initiate a defrosting sequence in response to determining the loss of superheat in the first heat exchanger. 
   
     
     
         2 . The water heating system of  claim 1 , wherein the controller is configured to:
 monitor a temperature of refrigerant exiting the first heat exchanger during the defrosting sequence; and   initiate a termination sequence in response to the temperature of the refrigerant exiting the first heat exchanger exceeding a threshold.   
     
     
         3 . The water heating system of  claim 1 , wherein the controller is configured to operate the compressor unit in a defrosting mode during the defrosting sequence. 
     
     
         4 . The water heating system of  claim 3 , wherein the compressor unit is configured to circulate refrigerant between the compressor unit and the first heat exchanger in a second direction in the defrosting mode, wherein the second direction is opposite the first direction. 
     
     
         5 . The water heating system of  claim 4 , wherein:
 the first heat exchanger functions as an evaporator when the compressor unit is operating in the heating mode; and   the first heat exchanger functions as a condenser when the compressor unit is operating in the defrosting mode.   
     
     
         6 . The water heating system of  claim 1 , wherein the controller is configured to determine the loss of superheat in the first heat exchanger based on a comparison of (i) an inlet refrigerant temperature of the first heat exchanger and (ii) an outlet refrigerant temperature of the first heat exchanger. 
     
     
         7 . The water heating system of  claim 6 , wherein the controller is configured to determine the loss of superheat in the first heat exchanger in response to the outlet refrigerant temperature of the first heat exchanger not exceeding the inlet temperature of the first heat exchanger over a predetermined time duration. 
     
     
         8 . The water heating system of  claim 1 , further comprising:
 a water pump fluidly coupled to a second heat exchanger and operatively coupled to the controller;   wherein the controller is configured to operate the water pump to provide a first water flow rate through the second heat exchanger during the defrosting sequence, wherein the first water flow rate is lower than a second water flow rate associated with normal heating operations.   
     
     
         9 . The water heating system of  claim 8 , wherein:
 the compressor unit is fluidly coupled to the second heat exchanger; and   the controller is configured to initiate a preheating sequence by:
 stopping the water pump to substantially stop water circulation through the second heat exchanger, and 
 operating the compressor unit in the heating mode to circulate refrigerant through the first heat exchanger and the second heat exchanger. 
   
     
     
         10 . The water heating system of  claim 9 , wherein the controller is configured to:
 monitor a characteristic of the refrigerant during the preheating sequence; and   terminate the preheating sequence in response to the characteristic of the refrigerant exceeding a limit.   
     
     
         11 . The water heating system of  claim 10 , wherein the characteristic of the refrigerant is a temperature of refrigerant between the second heat exchanger and the first heat exchanger and the limit is a temperature limit. 
     
     
         12 . The water heating system of  claim 10 , wherein the characteristic of the refrigerant is a pressure of refrigerant exiting a compressor of the compressor unit and the pressure is a pressure limit. 
     
     
         13 . The water heating system of  claim 10 , wherein the characteristic of the refrigerant is a temperature of refrigerant entering a compressor of the compressor unit and the limit is a temperature limit. 
     
     
         14 . The water heating system of  claim 9 , wherein the controller is configured to initiate the preheating sequence in response to determining the loss of superheat in the first heat exchanger. 
     
     
         15 . The water heating system of  claim 2 , further comprising:
 a fan operatively coupled to the controller, the fan configured to provide airflow to the first heat exchanger;   wherein the controller is configured to initiate the termination sequence by:
 operating the fan to provide airflow to the first heat exchanger, and 
 configuring the compressor unit to operate in the heating mode. 
   
     
     
         16 . A method for controlling a heat pump water heater, comprising:
 operating a compressor unit in a heating mode to circulate refrigerant between the compressor unit and a first heat exchanger in a first direction;   determining a loss of superheat in the first heat exchanger while the compressor unit is operating in the heating mode; and   initiating a defrosting sequence in response to determining the loss of superheat in the first heat exchanger.   
     
     
         17 . The method of  claim 16 , further comprising:
 monitoring a temperature of refrigerant exiting the first heat exchanger during the defrosting sequence; and   initiating a termination sequence in response to the temperature of the refrigerant exiting the first heat exchanger exceeding a threshold.   
     
     
         18 . The method of  claim 16 , wherein the compressor unit operates in a defrosting mode during the defrosting sequence. 
     
     
         19 . The method of  claim 18 , wherein the compressor unit is configured to circulate refrigerant between the compressor unit and the first heat exchanger in a second direction in the defrosting mode, wherein the second direction is opposite the first direction. 
     
     
         20 . The method of  claim 19 , wherein:
 the first heat exchanger functions as an evaporator when the compressor unit is operating in the heating mode; and   the first heat exchanger functions as a condenser when the compressor unit is operating in the defrosting mode.   
     
     
         21 . The method of  claim 16 , wherein determining the loss of superheat in the first heat exchanger includes a comparison of (i) an inlet refrigerant temperature of the first heat exchanger and (ii) an outlet refrigerant temperature of the first heat exchanger. 
     
     
         22 . The method of  claim 21 , wherein determining the loss of superheat in the first heat exchanger includes a determination that the outlet refrigerant temperature of the first heat exchanger has not exceeded the inlet temperature of the first heat exchanger for a predetermined period of time. 
     
     
         23 . The method of  claim 16 , further comprising operating a water pump to provide a first water flow rate through a second heat exchanger during the defrosting sequence, wherein the first water flow rate is lower than a second water flow rate associated with normal heating operations. 
     
     
         24 . The method of  claim 23 , further comprising initiating a preheating sequence by:
 stopping the water pump to substantially stop water circulation through the second heat exchanger; and   operating the compressor unit in a heating mode to circulate refrigerant through the first heat exchanger and the second heat exchanger.   
     
     
         25 . The method of  claim 24 , further comprising:
 monitoring a characteristic of the refrigerant during the preheating sequence; and   terminating the preheating sequence in response to the characteristic of the refrigerant exceeding a limit.   
     
     
         26 . The method of  claim 25 , wherein the characteristic of the refrigerant is a temperature of refrigerant between the second heat exchanger and the first heat exchanger and the limit is a temperature limit. 
     
     
         27 . The method of  claim 25 , wherein the characteristic of the refrigerant is a pressure of refrigerant exiting a compressor of the compressor unit and the pressure is a pressure limit. 
     
     
         28 . The method of  claim 25 , wherein the characteristic of the refrigerant is a temperature of refrigerant entering a compressor of the compressor unit and the limit is a temperature limit. 
     
     
         29 . The method of  claim 24 , further comprising initiating the preheating sequence in response to the loss of superheat in the first heat exchanger. 
     
     
         30 . The method of  claim 17 , further comprising initiating the termination sequence by:
 operating a fan to provide airflow to the first heat exchanger; and   configuring the compressor unit to operate in the heating mode.   
     
     
         31 . The water heating system of  claim 8 , wherein the controller is further configured to operate the water pump to provide a third water flow rate through the second heat exchanger during the defrosting sequence, wherein the third water flow rate is greater than the first flow rate and lower than the second water flow rate. 
     
     
         32 . The water heating system of  claim 31 , wherein the controller is further configured to progressively increase a water flow rate of the water pump from the first water flow rate to the third water flow rate at a predetermined time rate of change. 
     
     
         33 . The water heating system of  claim 32 , wherein the controller is further configured to:
 monitor a characteristic of the refrigerant while progressively increasing the water flow rate of the water pump;   compare the characteristic of the refrigerant to a threshold; and   terminate the defrosting sequence in response to determining that the characteristic of the refrigerant is below the threshold.   
     
     
         34 . The water heating system of  claim 33 , wherein the characteristic of the refrigerant is a temperature of refrigerant between the first heat exchanger and the second heat exchanger and the threshold is a temperature limit. 
     
     
         35 . The method of  claim 23 , further comprising operating the water pump to provide a third water flow rate through the second heat exchanger during the defrosting sequence, wherein the third water flow rate is greater than the first flow rate and lower than the second water flow rate. 
     
     
         36 . The method of  claim 35 , further comprising progressively increasing a water flow rate of the water pump from the first water flow rate to the third water flow rate at a predetermined time rate of change. 
     
     
         37 . The method of  claim 36 , further comprising:
 monitoring a characteristic of the refrigerant while progressively increasing the water flow rate of the water pump;   comparing the characteristic of the refrigerant to a threshold; and   terminating the defrosting sequence in response to determining that the characteristic of the refrigerant is below the threshold.   
     
     
         38 . The method of  claim 37 , wherein the characteristic of the refrigerant is a temperature of refrigerant between the first heat exchanger and the second heat exchanger and the threshold is a temperature limit.