HK1103789B - Water heater system having a heat pump with an expansion valve and water pump and method controlling thereof - Google Patents

Description

Water heater system of heat pump with expansion valve and water pump and control method thereof

Technical Field

The present invention relates to vapor compression systems, and more particularly, to a method of controlling a warm-up process of a vapor compression system.

Background

Vapor compression systems are often used in heat pumps, for example, to heat and cool air, water, or other fluids. The simplest compression systems operate in a subcritical state, where the refrigerant in the vapor compression system is in a mixed liquid-vapor state. However, to provide additional degrees of freedom in the control of the compression system, the user may choose to use a supercritical compression system that allows the refrigerant to reach a supercritical vapor state.

If the supercritical vapor compression system is used as a heat pump in a heat pump water heater, the water heater must undergo a warm-up process at startup to bring the heat pump to a steady state where the components in the heat pump are all at their target states. During the warm-up process, various over-fire conditions can occur within the water heater, causing the water heater to shut down in an attempt to protect the water heater. Moreover, the signals from the expansion valve and the water pump can be aligned to undesirably reduce the operating efficiency of the water heater. Heat pumps employing supercritical vapor compression systems can be particularly prone to shutdowns due to improper startup with their extra degree of freedom.

It would be desirable to provide a method of stabilizing a heat pump in a water heater without causing various over-fire conditions or incorrect system sequencing that results in reduced energy efficiency.

Disclosure of Invention

The present invention relates to a method of controlling the start-up operation of a heat pump water heater system that prevents inadvertent shutdowns and/or low operating efficiencies. In one embodiment, the method includes selecting to open the expansion valve to near a desired steady state value at startup to ensure high system capacity as early as possible, setting the water pump signal to a high level to maximize cycle efficiency, and performing closed loop control of the expansion valve and the water pump to gradually increase the pressure of the system in a controlled manner by comparing the actual pressure to the desired pressure. Once the water heater assembly reaches a steady state operating condition, the closed loop control can continue to maintain the steady state, if desired.

By providing closed loop control of the system components during start-up, the present invention ensures that the system components reach their steady state without various over-fire conditions or loss of efficiency. The above approach is effective even if the system uses a supercritical vapor compression system as a heat pump that provides an additional degree of freedom that typically results in system instability.

In particular, the present invention proposes a method of controlling a water heater system of a heat pump having an expansion valve and a water pump, comprising:

providing a heat pump having a compressor, at least two heat exchangers and an expansion valve and circulating a refrigerant through said heat pump;

providing a water circuit, water driven by a water pump through at least one of said two heat exchangers being heated by said refrigerant;

starting the water heater; monitoring a change in refrigerant during a start-up and monitoring a characteristic of water passing through at least one of the two heat exchangers;

controlling the expansion valve based on the monitored change in refrigerant, while controlling the water pump based on the monitored characteristic of water; and

after the start step, the water pump signal is set to a high level.

Furthermore, the present invention also proposes a water heater system comprising:

a heat pump having an expansion valve, a water circuit with a water pump, and a pressure sensor;

a controller operatively connected to the expansion valve, the water pump, and the pressure sensor, wherein the controller controls at least one of the expansion valve and the water pump based on the pressure detected by the pressure sensor; and

and a controller setting the water pump signal to a high level.

Drawings

FIG. 1 is a representative diagram of a vapor compression system used in an embodiment of the present invention;

FIG. 2 is a graph illustrating an example of the relationship between system pressure and enthalpy;

FIG. 3 is a representative diagram of a heat pump water heater controlled according to an embodiment of the method of the present invention;

FIG. 4 is a flow chart depicting a method of one embodiment of the present invention; and

FIG. 5 is a graph illustrating an example of system pressure over time during system startup and warm-up.

Detailed Description

Fig. 1 is a typical diagram of a typical vapor compression system in which the method of the present invention may be used. Vapor compression systems are often used in heat pumps, for example, to heat and cool air, water, or other fluids. As shown in fig. 1, the compression system 100 includes a compressor 102 that compresses gaseous refrigerant within a conduit to a high pressure, which thereby heats the vapor. The steam then flows through a first heat exchanger 106 where the heat in the steam is released for heating a fluid, such as air or water. As the heat of the compressed vapor is absorbed by the fluid, the vapor cools. The cooled vapor is sent to an expansion valve 108 to regulate the amount of expansion of the vapor. As the steam expands, it cools significantly, and as the steam flows through the second heat exchanger 110, it is used to cool another fluid. The cycle continues as the vapor returns to the compressor 102. Thus, the compression system 100 may heat the fluid flowing through the first heat exchanger 106 and cool the fluid flowing through the second heat exchanger 110.

For purposes of illustration only, figure 2 graphically illustrates one example of a relationship between pressure and enthalpy in a vapor compression system. The figure shows a liquid-vapor dome 112 defined by the boundary formed by a particular pressure versus enthalpy relationship. If the compression system is operating at a level below the dome 112, as is the case with a subcritical compression system, the refrigerant in the compression system maintains a liquid/vapor mixture state. For a simple subcritical vapor compression system, the entire compression cycle operates at a range of pressures and enthalpies below the liquid-vapor dome 112. Thus, the pressure and temperature are coupled together and thus affect each other.

To provide additional degrees of freedom, the compression system 100 may be designed as a supercritical vapor compression system that allows the pressure and enthalpy to move above the dome 112 and bring the refrigerant in the compression system 100 to a supercritical vapor state. The pressure and temperature in the compression system 100 no longer interact to provide greater operating flexibility within the compression system 100, and this allows the system to often reach higher operating temperatures than subcritical systems.

As mentioned above, the supercritical vapor compression system may be used as a heat pump 150 in a heat pump water heater 152, the water heater 152 being depicted in the exemplary form in fig. 3. The water heater 152 has a water pump 154 that circulates water through the water heater 152 and a water tank 156. An evaporator fan (not shown) within heat exchanger 106 extracts heat from the air and directs it to heat exchanger 110 so that heat exchanger 110 can more easily absorb heat from the air. The controller 160 controls the operation of the water heater 152 components and may include a processor 162 that provides closed loop control of the overall heat pump 150, for example, by monitoring the pressure of the overall water heater system through the pressure sensor 155 and the operating conditions of the compressor 102, expansion valve 108, and water pump 154.

Temperature sensors 164 may be provided at various points in the system, such as: at a hot water outlet 166, a cold water inlet 168, and/or an ambient environment 170. The temperature sensor 164 is coupled to the controller 160 to provide more data for controlling the operation of the system. For example, the processor 162 in the controller 160 may use the temperature sensors 164 of the hot water outlet 166 and the cold water inlet 168 to determine whether to change the volume of water pumped by the water pump 154, while the temperature sensor 164 in the ambient environment 170 tells the controller 160 how much energy in the air is available to heat the water for the heat exchanger 106.

To ensure that the water heater 152 quickly reaches its operating state, the water heater 152 undergoes a warm-up process at startup to place the heat pump 150 in a steady state where the expansion valve 108, water pump 154, and heat pump 150 all reach their target states. As mentioned above, heat pumps employing supercritical vapor compression systems are particularly sensitive to shutdowns caused by improper startup due to the additional degrees of freedom. For example, during the warm-up process, if various over-fire conditions (e.g., over-temperature and/or over-pressure in any of the water heater components) occur at a time, all components within the heat pump 150 may be undesirably shut down in an attempt to protect the entire water heater 152. Moreover, the signals from the expansion valve 108 and the water pump 154 may be arranged to undesirably operate the water heater 152 at a low coefficient of performance (COP) operating vapor compression cycle.

To avoid these problems, the method of the present invention is directed to controlling the startup and warm-up processes so that the water heater uses a supercritical vapor compression system in the heat pump. FIG. 4 is a flow chart illustrating a method according to an embodiment of the present invention. Generally, this method imposes relatively tight control on the heat pump components to ensure that they quickly reach their steady state operation without encountering various over-fire conditions or low COP values.

To do so, the controller 160 first selects a value to open the expansion valve to near the desired steady state (block 200). The desired steady state value for a given environmental condition (e.g., ambient air temperature, water temperature, etc.), for example, may be obtained empirically or stored in a table, which may be referenced by the controller 160.

Next, the controller 160 activates the compressor 102, the heat pump 150, and the evaporator fan (block 202), and sets the water pump signal to a high level, thus avoiding inefficient cycle operation of the heat pump 150 (block 204). More particularly, a high pump signal ensures that a large amount of water is pumped through the water heater 152 early in the warm-up cycle, ensuring that the system extracts as much energy as possible from the ambient air to maximize cycle efficiency.

The controller 160 then performs closed loop control of the expansion valve 108 so the controller 160 can alter the expansion valve opening based on the desired pressure and the measured pressure (block 206). Fig. 5 is an explanatory graph depicting a desired warm-up operation in relation to the pressure detected by the pressure sensor 155. As shown in fig. 5, during the warm-up time 256, after startup 250, the pressure of the heat pump 150 is desirably ramped up to maintain the pressure within the heat pump 150 stable, even though the supercritical system allows the heat pump to operate with an additional degree of freedom. The closed loop in the system allows the controller 160 to continuously compare the pressure sensed by the pressure sensor 155 to the desired system pressure 254 at a given time and, if necessary, adjust the expansion valve 108 to match the increase in the actual system pressure 252 to the increase in the desired system pressure curve 254. This continuous monitoring and adjustment prevents the pressure of the water heater 152 from experiencing a fire condition and reaching a level that would cause the system to shut down.

The controller 160 also performs closed loop control of the water pump 154 to allow the water pump 154 to be controlled based on operating conditions before reaching its steady state (block 208). Controlling the water pump 154 to maintain the hot water outlet 166 at a given water temperature; for example, if the temperature sensor 164 at the hot water outlet 166 indicates that the delivered water is too hot, the water pump 154 may pump more water through the system 100 to reduce the water temperature. Similarly, if the temperature sensor 164 at the cold water inlet 168 is cooler than desired, the water pump 154 will pump less water to allow more time for the water to absorb more energy as it flows through the water heater 152.

Closed loop control of the expansion valve 108 and water pump 154 continues until the pressure sensor 155 detects that the system has reached the desired steady state operating pressure 258 (block 210). At this point, the controller 160 may continue closed loop control of the expansion valve 108 and water pump 154 to allow the system to continue normal steady state operation 258 even if changes, such as temperature and/or pressure changes, occur.

It will be appreciated that many variations of the embodiments of the invention described herein may be used in the practice of the invention. The following defines the scope of the invention and, therefore, covers methods and apparatus within the scope of these equivalents.

Claims (14)

1. A method of controlling a water heater system of a heat pump having an expansion valve and a water pump, comprising:

providing a heat pump having a compressor, at least two heat exchangers and an expansion valve and circulating a refrigerant through said heat pump;

providing a water circuit, water driven by a water pump through at least one of said two heat exchangers being heated by said refrigerant;

starting the water heater;

monitoring a change in refrigerant during a start-up and monitoring a characteristic of water passing through at least one of the two heat exchangers; controlling the expansion valve based on the monitored change in refrigerant, while controlling the water pump based on the monitored characteristic of water; and

after the start step, the water pump signal is set to a high level.

2. The method of claim 1, wherein the controlling step comprises closed loop controlling both the expansion valve and the water pump.

3. The method of claim 2, wherein a starting position of the expansion valve is selected to be around a desired steady state value, and the closed loop then controls the expansion valve from the starting position.

4. The method of claim 1, wherein the controlling step comprises closed loop controlling the expansion valve by:

comparing the system pressure to an ideal system pressure; and

the expansion valve is adjusted to bring the system pressure into agreement with the desired system pressure.

5. The method of claim 4, wherein during startup, the desired system pressure increases linearly with time.

6. The method of claim 4, wherein the system pressure allows the refrigerant in the heat pump to reach a supercritical vapor state.

7. The method of claim 1, further comprising the step of continuing to monitor and control after the system reaches a steady state.

8. The method of claim 1, wherein the controlling step comprises closed loop controlling the water pump.

9. The method of claim 8, wherein the water pump is also closed loop controlled based on at least one of a hot water outlet temperature and a cold water inlet temperature.

10. The method of claim 1, further comprising measuring an ambient air temperature, wherein the controlling step is also performed based on the ambient air temperature.

11. A water heater system, comprising:

a heat pump includes:

an expansion valve is arranged in the air-conditioning system,

a water circuit with a water pump, and

a pressure sensor;

a controller operatively connected to the expansion valve, the water pump, and the pressure sensor, wherein the controller controls at least one of the expansion valve and the water pump based on the pressure detected by the pressure sensor; and

the controller sets the water pump signal to a high level.

12. The water heater system of claim 11, further comprising:

a water tank having a hot water outlet and a cold water inlet; and

at least one temperature sensor connected to at least one of the hot water outlet and the cold water inlet, wherein the controller controls the water pump based on a temperature detected by the at least one temperature sensor.

13. The water heater system as recited in claim 11 wherein the heat pump is a supercritical compression system.

14. The water heater system of claim 11, further comprising at least one temperature sensor for measuring ambient air temperature, wherein the controller controls at least one of the expansion valve and the water pump based on the ambient air temperature.