Heat pump water heater and method

Abstract

A heat pump water heater and method is disclosed. The heat pump water heater includes a stratified water tank configured to hold water of different temperatures therein and a heat exchange unit. The heat exchange unit includes a pump configured to remove water from a bottom of the stratified tank, a heat pump having a condenser configured to receive the water from the pump and supply heat to the water flowing through the condenser such that the condenser heats the water to a pre-determined temperature, and a resistance heater configured to receive the heated water from the condenser. The resistance heater supplies additional heat to the water if the pre-determined temperature of the water exiting the condenser does not equal or exceed a pre-determined set-point. The heated water from the resistance heater is supplied to a top of the stratified tank for use by a residence or utility.

We claim:  
     
         1 . A heat pump water heater configured to provide grid related services such as Demand Response, comprising:
 (a) a stratified water tank configured to hold water of different temperatures therein; and   (b) a heat exchange unit, comprising:
 (i) a pump configured to remove water from a bottom of the stratified tank; 
 (ii) a heat pump having a condenser configured to receive the water from the pump and supply heat to the water flowing through the condenser such that the condenser heats the water to a pre-determined temperature; and 
 (iii) a resistance heater configured to receive the heated water from the condenser and supply additional heat to the water if the pre-determined temperature of the water exiting the condenser does not equal or exceed a pre-determined set-point, wherein the heated water from the resistance heater is supplied to a top of the stratified tank for use by a residence or utility. 
   
     
     
         2 . The heat pump water heater according to  claim 1 , wherein the heat pump further includes:
 (a) an evaporator configured to provide heat to a low-pressure, low-temperature refrigerant flowing therethrough;   (b) a compressor configured to receive the refrigerant from the evaporator and compress the refrigerant into a high-temperature, high-pressure refrigerant, wherein the high-temperature, high-pressure refrigerant is supplied to the condenser for heating of the water flowing therethrough; and   (c) an expansion valve configured to receive the refrigerant from the condenser in a high-pressure, moderate temperature state and expand it into a low-pressure, low-temperature state for use in the evaporator.   
     
     
         3 . The heat pump water heater according to  claim 2 , wherein the compressor is of a variable capacity design. 
     
     
         4 . The heat pump water heater according to  claim 1 , wherein the resistance heater is configured to provide elevated temperature storage in advance of Demand Response events or for load shifting. 
     
     
         5 . The heat pump water heater according to  claim 1 , wherein the resistance heater is configured to provide additional heating capacity as needed for fast recovery and to boost capacity due to low ambient temperatures. 
     
     
         6 . The heat pump water heater according to  claim 1 , wherein the resistance heater provides the additional heat to the water so that the heat pump can operate at maximum efficiency. 
     
     
         7 . A method of providing hot water to a residence or utility, comprising the steps of:
 (a) providing a heat pump water heater, having:
 (i) a stratified water tank; and 
 (ii) a heat exchange unit having a heat pump and an in-line resistance heater; 
   (b) pulling water from a bottom of the stratified water tank and supplying it to the heat pump;   (c) using the heat pump to heat the water to a higher temperature;   (d) using the in-line resistance heater to provide additional heat to the water exiting the heat pump; and   (e) storing the heated water in a top of the stratified water tank for use by a residence or utility.