Heat pump water heater with compressor modulation to extend low ambient operation

Abstract

A heat pump water heater system includes a compressor and an evaporator. The system further includes a heat exchanger having a refrigerant inlet that receives heated refrigerant from the compressor whereby water flowing through the heat exchanger is heated. A controller increases a speed of the compressor if a pressure of refrigerant upstream of the compressor is reduced. Increasing the speed of the compressor permits operation of the water heater system at a lower ambient temperature. A defrost cycle may be initiated if increasing compressor speed does not sufficiently maintain inlet pressure.

What is claimed is: 
     
         1 . A heat pump water heater system, comprising:
 a compressor having a compressor motor that is electrically powered, a compressor inlet, and a compressor outlet, the compressor defining a minimum allowable operating pressure at the compressor inlet;   an evaporator having an evaporator inlet and an evaporator outlet, wherein the evaporator outlet is fluidly connected to the compressor inlet;   an evaporator fan that is configured to cause air to flow through the evaporator when the evaporator fan is actuated;   a heat exchanger having an inlet that is fluidly connected to the compressor outlet, and an outlet that is fluidly connected to the evaporator inlet, whereby water flowing through the heat exchanger is heated by refrigerant flowing through the heat exchanger;   a pressure sensor that is configured to detect a pressure of refrigerant upstream of the compressor inlet to provide a measured inlet refrigerant pressure;   a defrost valve system that is configured to cause heated refrigerant from the compressor outlet to flow through the evaporator during a defrost cycle; and   a controller that is configured to: 1) increase a speed of the compressor motor if a measured inlet pressure drops to a threshold pressure that is greater than the minimum allowable operating pressure of the compressor whereby a mass flow rate of the compressor is increased to permit operation of the heat pump water heater system at a lower ambient temperature, and: 2) actuate the defrost valve system if a measured inlet pressure reaches a defrost pressure that is below the threshold pressure despite the increased speed of the compressor motor.   
     
     
         2 . The heat pump water heater system of  claim 1 , including:
 the defrost pressure is greater than the minimum allowable operating pressure of the compressor.   
     
     
         3 . The heat pump water heater system of  claim 1 , wherein:
 the defrost pressure is equal to the minimum allowable operating pressure of the compressor.   
     
     
         4 . The heat pump water heater system of  claim 1 , wherein:
 the controller comprises a Variable-Frequency Drive (VFD) that is configured to provide: 1) a first operating mode in which the controller drives the compressor motor at a first frequency; and: 2) a second operating mode in which the controller drives the compressor motor at a second frequency that is greater than the first frequency; and   the controller is configured to switch from the first operating mode to the second operating mode if the measured inlet pressure drops below the threshold pressure while in the first operating mode.   
     
     
         5 . The heat pump water heater system of  claim 4 , wherein:
 the controller is configured to actuate the defrost valve system if the measured inlet pressure is at or below the defrost pressure while the controller is in the second operating mode.   
     
     
         6 . The heat pump water heater system of  claim 4 , wherein:
 the controller is configured to default to the first operating mode whereby the controller only switches to the second operating mode if predefined operating conditions are detected.   
     
     
         7 . The heat pump water heater system of  claim 6 , wherein:
 the predefined operating conditions comprise at least one operating parameter selected from the group consisting of: 1) a measured inlet pressure at or below the threshold pressure; 2) an ambient temperature at or below predefined temperature; and: 3) a heat demand from a hot water tank system that cannot be met when the VFD is driving the compressor motor at the first frequency.   
     
     
         8 . The heat pump water heater system of  claim 4 , wherein:
 the controller is configured to continuously and/or repeatedly increase a frequency of electrical power to the compressor motor to maintain the measured inlet pressure at or above the threshold pressure.   
     
     
         9 . The heat pump water heater system of  claim 8 , wherein:
 the controller is configured to limit increases in frequency of the electrical power to the compressor motor to a predefined maximum frequency.   
     
     
         10 . The heat pump water heater system of  claim 1 , wherein:
 the controller is configured to optimize Co-efficient-of-Performance (COP) of the heat pump water heater system, wherein the COP is the heat output of the system divided by input power to the system, and wherein the controller is configured to optimize COP by controlling a speed of the compressor and/or a speed of the evaporator fan to minimize input power and maximize heat output based on a plurality of operating parameters.   
     
     
         11 . The heat pump water heater system of  claim 10 , wherein:
 the operating parameters comprise ambient temperature and temperature of water entering and exiting the heat exchanger;   the controller is configured to control: 1) the speed of the compressor, and/or: 2) the speed of the evaporator fan, based at least in part, on ambient temperature, whereby the speeds of the compressor and/or the evaporator fan are inversely related to ambient temperature;   the controller is configured to determine heat output utilizing a difference between the temperatures of water entering and exiting the heat exchanger, and determine COP by dividing heat output by input power to the system.   
     
     
         12 . A heat pump water heater system, comprising:
 a compressor having a compressor motor that is electrically powered, and defining a minimum allowable operating pressure at the compressor inlet;   an evaporator;   a heat exchanger; and   a controller that is configured to: 1) increase electrical power to the compressor motor to maintain a pressure of refrigerant at an inlet of the compressor above the minimum allowable pressure, and: 2) implement a defrost cycle causing heated refrigerant to flow through the evaporator if increasing electrical power to the compressor motor is insufficient to maintain mass flow through the compressor according to predefined criteria.   
     
     
         13 . The heat pump water heater system of  claim 12 , wherein:
 the controller is configured to implement a defrost cycle if a pressure of refrigerant at an inlet of the compressor falls below a predefined pressure despite increases in electrical power to the compressor motor.   
     
     
         14 . The heat pump water heater system of  claim 13 , wherein:
 the controller is configured to increase electrical power to the compressor motor if a pressure of refrigerant at an inlet of the compressor is below a threshold pressure.   
     
     
         15 . The heat pump water heater system of  claim 14 , wherein:
 the threshold pressure is greater than the minimum allowable pressure of the compressor.   
     
     
         16 . The heat pump water heater system of  claim 14 , wherein:
 the controller comprises a Variable-Frequency Drive (VFD) that is configured to provide: 1) a first operating mode in which the controller drives the compressor motor at a first frequency; and: 2) a second operating mode in which the controller drives the compressor motor at a second frequency that is greater than the first frequency; and   the controller is configured to switch from the first operating mode to the second operating mode if the measured inlet pressure drops below the threshold pressure while in the first operating mode.   
     
     
         17 . The heat pump water heater system of  claim 16 , including:
 a defrost valve system that is configured to cause heated refrigerant from the compressor outlet to flow through the evaporator during a defrost cycle; and wherein:   the controller is configured to actuate the defrost valve system if the measured inlet pressure is at or below the defrost pressure while the controller is in the second operating mode.   
     
     
         18 . The heat pump water heater system of  claim 16 , wherein:
 the controller is configured to default to the first operating mode whereby the controller only switches to the second operating mode if predefined operating conditions are detected.   
     
     
         19 . The heat pump water heater system of  claim 18 , wherein:
 the predefined operating conditions comprise at least one operating parameter selected from the group consisting of: 1) a measured inlet pressure at or below the threshold pressure; 2) an ambient temperature at or below a predefined temperature; and: 3) a heat demand from a hot water tank system that cannot be met when the VFD is driving the compressor motor at the first frequency.   
     
     
         20 . A method of controlling a heat pump water heater having a compressor that supplies heated refrigerant to a heat exchanger to heat water, and an evaporator that is exposed to ambient air, the method comprising:
 empirically determining a relationship between operating variables including: 1) compressor speeds and 2) evaporator fan speeds that optimize a Co-efficient-of-Performance (COP) based on input parameters, wherein the COP comprises heat output of the heat pump system divided by input power to the heat pump system, and wherein the input parameter comprises: 1) temperature of water at an inlet and/or an outlet of the heat exchanger; 2) ambient air temperature; and 3) temperature and/or pressure of refrigerant at an inlet and/or an outlet of the compressor; and   controlling compressor speed and/or fan speed utilizing the relationship to maximize COP.