HK1106819B - Method of controlling a carbon dioxide heat pump water heating system - Google Patents

Description

Control method of carbon dioxide heat pump water heating system

Technical Field

The present invention relates generally to a method of operating a heat pump water heating system, and more particularly to a method of detecting and diagnosing operating conditions of a heat pump water heating system.

Background

Chlorine-containing refrigerants have been removed from service due to environmental concerns. Many alternatives have been proposed to replace chlorine-containing refrigerants, such as carbon dioxide. Carbon dioxide has a low critical point which allows most air conditioning system sections utilizing carbon dioxide to operate above the critical point or, in most cases, transcritical. In saturated conditions (both liquid and vapor present), the fluid pressure below any critical point is a function of temperature. However, when the temperature of the fluid is above the critical temperature, the pressure becomes a function of the density of the fluid.

Transcritical refrigeration systems employ a refrigerant that is compressed at high pressure and temperature in a compressor. As the refrigerant enters the gas cooler, heat is removed from the refrigerant and transferred to a fluid medium, such as water. In the heat pump water heater, water heated in the gas cooler is used to heat water in the hot water tank. The refrigerant flows from the gas cooler to the expansion valve. The expansion valve regulates the flow of refrigerant between a high pressure and a low pressure. Control of the refrigerant by the expansion valve controls the flow and efficiency of the refrigerant circuit. The refrigerant flows from the expansion valve to the evaporator.

In the evaporator, the low-pressure refrigerant receives heat from the air and becomes superheated. Superheated refrigerant from the evaporator flows into the compressor to repeat the cycle.

The system is controlled to vary the flow of refrigerant and water depending on the current operating conditions. Degradation of system devices can adversely affect system performance and operating costs. Furthermore, in some cases the change in system performance is not noticeable and therefore not noticeable. Operating costs are greatly reduced by operating the system under preferred conditions. In addition, the reduction of system down time also greatly reduces operating costs.

Accordingly, it is desirable to develop a method of detecting system defects and diagnosing system problems to reduce system downtime and increase operating efficiency.

Disclosure of Invention

The present invention is a method of detecting and diagnosing operating conditions of a heat pump water heating system by monitoring operating changes and their response to system inputs.

The method for detecting the operating condition of the heat pump comprises the following steps:

a) compressing a refrigerant with a compressor;

b) cooling the refrigerant by heat exchange with a fluid medium in a heat exchanger;

c) expanding the refrigerant to a low pressure in an expansion device;

d) evaporating the refrigerant in an evaporator;

e) monitoring a condition of the expansion device and determining an expected refrigerant pressure between a compressor and a heat exchanger based on a given input to the expansion device;

f) monitoring an actual refrigerant pressure between the compressor and the heat exchanger that varies in response to operation of the expansion device;

g) determining a pressure differential between the monitored refrigerant pressure and an expected refrigerant pressure that is expected to be generated in response to a given input to the expansion device;

h) comparing the pressure differential to a desired range; and

i) indicating a fault in response to the pressure differential exceeding the desired range.

The heat pump water heating system includes a transcritical vapor compression circuit. The vapor compression circuit includes a compressor, a gas cooler, and an evaporator. The gas cooler transfers heat to the water circuit, which in turn heats the water in the hot water tank. The water temperature is adjusted by varying the water flow through the gas cooler. Slower water flow provides more heat absorption resulting in higher water temperature. The increase in water flow reduces heat absorption, resulting in a drop in water temperature.

The controller controls the heat pump water heating system to provide and maintain a desired water temperature in the water tank. To achieve optimum operation, sensors throughout the system are constantly monitored and parameters adjusted. The system detects and diagnoses system problems by monitoring and comparing actual measured conditions to predicted conditions based on system inputs. Detection and diagnosis of problems reduces system maintenance and downtime, thereby improving system efficiency.

Accordingly, the method of detecting and diagnosing system operating conditions of the present invention may reduce system downtime and improve operating efficiency.

Drawings

The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:

FIG. 1 is a schematic diagram of a carbon dioxide heat pump water heater.

Detailed Description

Referring to fig. 1, a heat pump system 10 is schematically illustrated and includes a refrigerant compressor 14, the refrigerant compressor 14 driving refrigerant through a vapor compression circuit 12. Preferably, the refrigerant used in the present system is carbon dioxide. Because carbon dioxide has a low critical point, vapor compression circuits employing carbon dioxide refrigerant typically operate transcritically. Although the use of carbon dioxide is preferred, it is within the scope of the present invention to use other refrigerants known to those skilled in the art. The vapor compression circuit 12 includes a compressor 14, a heat exchanger 16, an expansion valve 20, and an evaporator 18. The evaporator 18 includes a fan 30, the fan 30 being selectively activated to blow air across the evaporator 18.

The water circuit 13 is in thermal contact with the vapour compression circuit 12 at a heat exchanger 16. The pump 34 drives water through the water circuit 13. In the heat exchanger 16, the water flowing through the water path 13 absorbs heat released from the refrigerant. The water in the waterway 13 then transfers heat to the water in the feed tank 38.

The vapor compression circuit 12 operates by alternately compressing and expanding refrigerant to absorb heat and transfer it to water in a water tank 13. The refrigerant leaving the compressor 14 is at a high temperature and pressure. This high temperature, high pressure refrigerant flows through the heat exchanger 16. In the heat exchanger 16, the refrigerant releases heat to the water circuit 13. The refrigerant exiting the heat exchanger 16 then passes to an expansion valve 20. The expansion valve 20 controls the flow of refrigerant from a high pressure to a low pressure. Preferably, the expansion valve 20 is variable to allow for refrigerant flow changes to change operating conditions. The expansion valve 20 may have any construction known to those skilled in the art.

System efficiency is affected by many different parameters and environmental conditions. For example, loss of refrigerant due to leakage or evaporation may reduce the amount of heat absorbed and released. The method of the present invention detects and diagnoses system operating conditions of a heat pump water heating system by monitoring system parameters and comparing actual measured parameters to predicted parameters based on current system conditions and inputs.

The present method monitors the amount of refrigerant in the system 10 to detect a decrease in refrigerant below a desired value. The amount or charge of refrigerant is monitored by measuring the refrigerant pressure and temperature between the evaporator 18 and the compressor 14. A temperature sensor 28 and a pressure sensor 26 are disposed in the vapor compression circuit 12 between the compressor 14 and the evaporator 18. Although the pressure and temperature sensors 26, 28 are disposed between the evaporator 18 and the compressor, it will be appreciated by those skilled in the art having the benefit of this disclosure that the refrigerant temperature and pressure may be monitored at other locations in the vapor compression circuit 12.

If the refrigerant is in a saturated state, the pressure and temperature of the refrigerant are directly related. Thus, measurement and monitoring of the refrigerant pressure in the saturated state provides insight into the refrigerant temperature. However, when the refrigerant is not in a saturated state, this relationship no longer holds and a direct temperature measurement is required.

In some cases, the saturation temperature corresponding to the refrigerant pressure is very different from the actual temperature of the refrigerant. This condition is known in the art as an overheat condition. A superheat condition occurs when the actual temperature is greater than the saturation temperature for a given refrigerant pressure. A superheat condition is evidence of a loss of refrigerant in the system.

The system compares the actual temperature provided by temperature sensor 28 with a predicted temperature corresponding to the pressure of the refrigerant provided by pressure sensor 26. The predicted temperature is calculated as a function of ambient conditions (typically air and water temperatures), for example, empirically determined by using a look-up table. The environmental conditions must be sensed by appropriate sensors. A difference between the actual temperature and the predicted temperature outside of a predetermined range indicates a loss of refrigerant. In response to the detected low refrigerant condition, the controller 46 activates a prompt 47 to alert of the problem. In addition, the controller 46 may also shut down the system 10 to prompt for maintenance.

The temperature sensor 28 and the pressure sensor 26 between the compressor 14 and the evaporator 18 are also used to determine if there is a failure of the fan 30. If the fan 30 is operating properly, heat will be absorbed from the air in the evaporator 18 in a predictable manner. The refrigerant temperature should respond in a predictable manner to the actuation of the fan 30 and the corresponding airflow over the evaporator 18.

If the difference between the predicted refrigerant temperature and the actual measured temperature monitored by the temperature sensor 28 is greater than the desired value, a problem with the fan 30 is indicated. If the refrigerant temperature and pressure match, but do not represent a predicted level for a given operating condition of the fan 30, a problem with the fan 30 is indicated. Based on the appearance of a failure of the fan 30, the controller 46 will provide a prompt to warn and indicate that maintenance is to be performed on the source of the problem.

Another example of a condition monitored by the system 10 includes monitoring of the expansion valve 20. The expansion valve 20 operates to vary the flow of refrigerant through the vapor compression circuit 12. If the expansion valve 20 is malfunctioning, the refrigerant flow will not react as expected. Poor operation of the expansion valve 20 can cause the difference between the high and low pressures in the vapor compression circuit 12 to go outside of desired ranges. Furthermore, the ideal range is determined empirically and is a function of environmental conditions. A pressure sensor 22 disposed between the compressor 14 and the heat exchanger 16 monitors the refrigerant pressure. The refrigerant pressure between the compressor 14 and the heat exchanger 16 should be in accordance with the setting of the expansion valve 20.

If the difference between the expected pressure and the actual pressure between the compressor 14 and the heat exchanger 16 at a given input to the expansion valve 20 is outside of a desired range, this may indicate that a problem with the expansion valve 20 may occur. Actuation of the expansion valve 20 causes a desired refrigerant pressure between the compressor 14 and the heat exchanger 16. A fault is indicated in response to a difference between the expected and actual refrigerant pressures exceeding a desired range. In response to an indication of an expansion valve failure, the controller 46 initiates a prompt to alert and indicate attention to the failure.

Another condition monitored by the system is water pump speed. The water pump 34 regulates the flow of water through the waterway 13 to maintain the temperature of the water in the water tank 38. Failure of the water pump 34 or degradation of the heat exchanger 16 may reduce the efficiency of the system 10. The temperature sensor 32 monitors the temperature of the water in the water circuit 13. The speed of the water pump 34 coincides with the predicted increase in water temperature. The predicted water temperature at a given water pump speed is compared to the actual temperature value measured by the temperature sensor 32. A speed sensor 36 monitors pump speed. Sensor 36 provides pump speed information for predicting a desired water temperature range. The sensor 36 may be of any type known to those skilled in the art. If the difference between the actual and expected values of the water temperature is greater than a predetermined range, a fault is detected and either the system is shut down or a fault condition is indicated. As described above, the predetermined range depends on the environmental conditions.

There are several possibilities for causing a difference between the actual and predicted water temperatures. One possible reason is that the pump 34 may not be rotating at a sufficient speed for a given input of the pump 34. The pump 34 is preferably driven by an electric motor, as is well known. The current supplied to the motor governs the speed of the pump 34. The current supplied to the motor can be measured to indicate the desired pump speed which can be compared to the actual pump speed measured by speed sensor 36. In addition, the current drawn by the motor is associated with a given pump speed. The pump speed measured by the speed sensor 36 is correlated to the predicted water temperature. The difference between the predicted and actual water temperatures causes the controller 46 to indicate a fault in the system 10.

Another cause of the difference between the predicted water temperature and the actual water temperature is calcium build-up on the heat exchanger 16. Condensation in the heat exchanger 16 may result in calcium build-up, degrading heat transfer between the vapor compression circuit 12 and the water circuit 13. Calcium degrades heat transfer, causing the actual water temperature to not change as expected in response to changes in pump speed. Further, in this case, the controller 46 will activate an alarm to prompt for maintenance of the system 10.

The heat pump hot water heating system of the invention detects and diagnoses the operation condition to improve the reliability, detects the system deterioration condition, reduces the system maintenance and improves the efficiency of the whole system.

The foregoing description is exemplary and not just a specific illustration. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. It is therefore to be understood that within the scope and content of this invention is to be determined by the following claims.

Claims (11)

1. A method of detecting an operating condition of a heat pump, comprising the steps of:

a) compressing a refrigerant with a compressor device;

b) cooling the refrigerant by heat exchange with a fluid medium in a heat exchanger;

c) expanding the refrigerant to a low pressure in an expansion device;

d) evaporating the refrigerant in an evaporator;

e) monitoring a condition of the expansion device and determining an expected refrigerant pressure between a compressor and a heat exchanger based on a given input to the expansion device;

f) monitoring an actual refrigerant pressure between the compressor and the heat exchanger that varies in response to operation of the expansion device;

g) determining a pressure differential between the monitored refrigerant pressure and an expected refrigerant pressure that is expected to be generated in response to a given input to the expansion device;

h) comparing the pressure differential to a desired range; and

i) indicating a fault in response to the pressure differential exceeding the desired range.

2. The method of claim 1, wherein the refrigerant is carbon dioxide.

3. The method of claim 1, wherein the heat pump is in heat exchange with a water heater.

4. The method as set forth in claim 1, wherein in said step g), a fault condition is determined in response to operation of said expansion device not being accompanied by a corresponding change in refrigerant pressure monitored between the compressor and the heat exchanger.

5. The method of claim 1, wherein the second pressure is monitored between an evaporator and a compressor, and the temperature of the refrigerant is monitored between the compressor and the evaporator.

6. The method of claim 5, wherein a loss of refrigerant is determined in response to a predicted refrigerant temperature based on the second pressure not corresponding to an actual monitored refrigerant temperature.

7. The method of claim 5, wherein the evaporator includes a fan for blowing air across the evaporator, the fan determined to be malfunctioning in response to the actual refrigerant temperature differing from the expected refrigerant temperature.

8. The method of claim 1, wherein the fluid medium is water, the method comprising a second temperature sensor disposed in the water circuit to measure a temperature of the water entering the heat exchanger.

9. The method of claim 8, wherein a water pump failure is detected in response to the water temperature being less than a predicted water temperature.

10. The method of claim 8, comprising monitoring a sensor of pump speed and determining that the heat exchanger has calcified in response to a predetermined difference between a predicted water temperature based on the pump speed and an actual water temperature.

11. The method of claim 1, wherein a loss of refrigerant is determined in response to detecting a superheat condition, wherein the superheat condition is a difference between a predicted refrigerant temperature corresponding to the monitored refrigerant pressure and an actual refrigerant temperature.