METHOD FOR DETERMINING OPTIMAL COEFFICIENT

OF PERFORMANCE IN A TRANSCRITICAL

VAPOR COMPRESSION SYSTEM

BACKGROUND OF THE INVENTION

The present invention relates generally to a method for optimizing the coefficient of performance of a transcritical vapor compression system by detecting a gas cooler exit temperature and determining an optimal high side pressure of the vapor compression system based solely on the gas cooler exit temperature to optimize the coefficient of performance.

Carbon dioxide is an environmentally friendly refrigerant that is commonly used in transcritical vapor compression systems. Carbon dioxide has a low critical point, and most vapor compression systems utilizing carbon dioxide as the refrigerant run transcritically or partially above the critical point. The pressure of a subcritical fluid is a function of temperature under saturated conditions (when both liquid and vapor are present). However, when the temperature of the fluid is higher than the critical temperature (supercritical), the pressure becomes a function of the density of the fluid and is independent of the heat sink temperature. Therefore, for any set of heat sink conditions, it is possible to operate at many high side pressures. However, a maximum coefficient of performance exists that corresponds to one high side pressure. Therefore, it is important to regulate the high side pressure of the transcritical vapor compression system because the high side pressure has a large effect on the capacity and efficiency of the system.

In one prior vapor compression system, both the temperature and the pressure of the refrigerant at the outlet of the gas cooler is measured. From both these measurements, the optimal high side pressure is determined. The high side pressure is then adjusted to the optimal high side based on both these measurements according to a pre-determined control strategy to optimize the coefficient of performance. The optimal high side pressure is selected to optimize the capacity and efficiency of the vapor compression system for a cooling mode. In another prior vapor compression system, the high side pressure and the low side pressure are

measured and then coupled according to a pre-determined control strategy to optimize the coefficient of performance.

A drawback to prior vapor compression systems is that at least two sensors are needed to determine the optimal high side pressure. In the first example, both a temperature sensor and a pressure sensor are needed to determine that optimal high side pressure. In the second example, two pressure sensors are needed to determine the optimal high side pressure.

There is a need for a method of optimizing the coefficient of performance of a vapor compression system that optimizes the capacity and efficiency during a heating mode, that uses only one sensor and that overcomes the drawbacks and shortcomings of the prior art.

SUMMARY OF THE INVENTION

A transcritical vapor compression system includes a compressor, a gas cooler, an expansion device, and a condenser. Refrigerant circulates through the closed circuit vapor compression system. Preferably, carbon dioxide is employed as the refrigerant. High pressure refrigerant flowing through the gas cooler is cooled by a fluid, such as water, that flows in an opposing direction through a heat sink. The refrigerant exits the gas cooler at a gas cooler exit temperature. In a transcritical vapor compression system, the high side pressure is independent of the operating conditions of the vapor compression system. Therefore, for any set of operating conditions, it is possible to operate the system at a wide range of high side pressures. However, there is an optimal high side pressure which corresponds to an optimal coefficient of performance. The optimal high side pressure is dependent on the gas cooler exit temperature, regardless of the outdoor air temperature. For any gas cooler exit temperature, a single optimal high side pressure optimizes the coefficient of performance of the vapor compression system.

The dependence of the optimal high side pressure as a function of the gas cooler exit temperature is programmed into a control based on values obtained experimentally or obtained through a pre-determined model. A sensor measures the gas cooler exit temperature. Based on the measured gas cooler exit temperature and the information programmed into the control, the optimal high side pressure is

determined. The high side pressure is determined solely on the gas cooler exit temperature. The high side pressure is not sampled. The high side pressure is only changed based on the measured gas cooler exit temperature.

These and other features of the present invention will be best understood from the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 illustrates a schematic diagram of a transcritical vapor compression system of the present invention;

Figure 2 illustrates a graph relating high side pressure to a coefficient of performance in the transcritical vapor compression system for a specific set of operating conditions;

Figure 3 illustrates a graph relating a gas cooler exit temperature to an optimal high side pressure at various outdoor air temperatures; and

Figure 4 illustrates a flow chart of the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 illustrates a schematic diagram of a vapor compression system 20.

The vapor compression system 20 includes a compressor 22, a gas cooler 24, an expansion device 26, and an evaporator 28. Refrigerant circulates though the closed circuit vapor compression system 20. The refrigerant exits the compressor 22 at a high pressure and a high enthalpy and flows through the gas cooler 24 and loses heat, exiting the gas cooler 24 at a low enthalpy and a high pressure. A fluid medium accepts heat from the refrigerant passing through the gas cooler 24. The refrigerant then passes through the expansion device 26 and is expanded to a low pressure. After expansion, the refrigerant flows through the evaporator 28 and rejects heat to a fluid medium. The refrigerant exits the evaporator 28 at a high

enthalpy and a low pressure. The refrigerant then enters the compressor 22, completing the cycle.

Preferably, carbon dioxide is used as the refrigerant. While carbon dioxide is described, other refrigerants may benefit from this invention. Because carbon dioxide has a low critical point, vapor compression systems utilizing carbon dioxide as the refrigerant usually run transcritically.

In a transcritical vapor compression system 20, the high side pressure is independent of the operating conditions (such as the outdoor air temperature) of the vapor compression system 20. Therefore, for any set of operating conditions, it is possible to operate the vapor compression system 20 at many high side pressures. However, for any set of operating conditions, there is an optimal high side pressure which corresponds to an optimal coefficient of performance of the vapor compression system 20.

The coefficient of performance represents the efficiency of the vapor compression system 20. The coefficient of performance equals the total useful heat transferred by the vapor compression system 20 divided by the work put into the vapor compression system 20 by system components, such as fans. The high side pressure influences the coefficient of performance, and it is therefore important to regulate the high side pressure to optimize the coefficient of performance of the vapor compression system 20.

Figure 2 illustrates the relationship between the high side pressure of the vapor compression system 20 and the coefficient of performance at a given set of operating conditions. For the given set of operating conditions, one high side pressure (the optimal high side pressure) corresponds to the optimum coefficient of performance. In the illustrated example, the coefficient of performance varies between approximately 2.7 and 3.1 and reaches a maximum of approximately 3.1 at a high side pressure of approximately 1350 psia.

The optimal high side pressure of the vapor compression system depends strongly on the gas cooler exit temperature. The gas cooler exit temperature is the temperature of the refrigerant exiting the gas cooler 24 and is measured by a sensor 30. Figure 3 illustrates the relationship between the gas cooler exit temperature and the optimum high side pressure at various outdoor air temperatures. At gas cooler

exit temperatures less than 100° F, the optimal high side pressure is independent of the outdoor air temperature. However, at gas cooler exit temperatures greater than 100 ⁰ F, the outdoor air temperature has an effect on the optimal high side pressure. Therefore, the optimal high side pressure is generally only a function of the gas cooler exit temperature.

Figure 4 illustrates a flowchart showing the method of determining the optimal high side pressure of the vapor compression system 20. First, the dependence of the optimal high side pressure as a function of the gas cooler exit temperature (the heat sink temperature) is determined based on the performance of the compressor 22 and the gas cooler 24. The dependence can be obtained either experimentally or through a pre-determined model. The results of the previous testing or the pre-determined model are programmed into a control 32.

A correlation is constructed that relates the gas cooler exit temperature to the optimal high side pressure. This information generates the graph shown in Figure 3. An outdoor air temperature correction factor can also be included in the correlation if needed. This information is also programmed in the control 32.

The gas cooler exit temperature is then detected by the sensor 30. The constructed correlation is then used to relate the gas cooler exit temperature detected by the sensor 30 to determine the optimal high side pressure that optimizes the coefficient of performance. The constructed correlation is based solely on the gas cooler exit temperature and not on the pressure. The sensor 30 detects the gas cooler exit temperature and provides this information to the control 32. Based only on the gas cooler exit temperature detected by the sensor 30, the control 32 uses the correlation to determine the optimal high side pressure based on the data preset into the control 32 and the detected gas cooler exit temperature. This approach is implemented using the linear relationship between the optimal high side pressure and the gas cooler exit temperature, as shown in Figure 3. The optimal high side pressure is determined and selected independent of the outdoor air conditions. The optimal high side pressure of the vapor compression system 20 is determined based solely on measured gas cooler exit temperature detected by the sensor 30. The high side pressure is not sampled when determining the optimal high side pressure.

Therefore, the efficiency and the capacity of the vapor compressor system 20 can be maximized when running in a heating mode.

If the control 32 determines that the gas cooler exit temperature measured by the sensor 30 changes, the control 32 uses the detected gas cooler exit temperature to determine the new optimal high side pressure based on the data programmed unto the control 32. The control 32 then determines the proper expansion device 26 setting and adjusts the expansion device 26 to change the high side pressure to the optimal high side pressure. The high side pressure is adjusted until the gas cooler exit temperature detected by the control is the optimal high side pressure. By determining the optimal high side pressure by measuring the gas cooler exit temperature with the sensor 30 and adjusting the expansion device 26 to maintain the optimal high side pressure, the optimum coefficient of performance can be maintained over a wide range of operating conditions.

If the high side pressure is above the optimal high side pressure, the control 32 sends a signal to the expansion device 26 to open the expansion device 26 and allow more refrigerant to flow through the expansion device 26. This decreases the high side pressure. The high side pressure is adjusted until the gas cooler exit temperature detected by the control 32 is the optimal high side pressure.

Alternately, if the high side pressure is below the optimal high side pressure, the control 32 sends a signal to the expansion device 26 to close the expansion device 26 and allow less refrigerant to flow through the expansion device 26. This increases the high side pressure. The high side pressure is adjusted until the gas cooler exit temperature detected by the control 32 is the optimal high side pressure.

Alternately, the sensor 30 detects the heat sink temperature to determine the optimal high side pressure to maximize the coefficient of performance. This is the temperature of the fluid in the gas cooler 24. The fluid can be water or air.

The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be

practiced otherwise than as specially described. For that reason the following claims should be studied to determine the true scope and content of this invention.