DE102015010593B4 - Operating procedure for a refrigeration system and associated refrigeration system - Google Patents

Abstract

Method for setting an optimum operating point of a refrigeration system (1), comprising the following steps:
a) checking whether a required temperature is reached at an evaporator (12);
b) if the required temperature is not reached, increasing the power of a compressor (2) to the power limit;
c) iteratively adjusting the expansion valve (11) until the refrigeration system (1) is operated at an optimum operating point at which the required temperature at the evaporator (12) is reached.

Description

The invention relates to a method for setting an optimal operating point of a refrigeration system.

Cooling systems are currently being developed for motor vehicles that use R744 (CO ₂ ) as the refrigerant. Such a refrigeration system comprises a compressor, a gas cooler, an internal heat exchanger, an adjustable expansion valve, an evaporator and a battery (collecting container). Depending on the refrigerant used, the gas cooler is also referred to as a capacitor. In the case of supercritical process control, cooling of the refrigerant takes place in the gas cooler, in the case of subcritical process control, the refrigerant condenses therein, analogously to the known condenser.

Currently, different expansion organs are used. These include Fixed Orifice Tubes (FOT), possibly with a bypass. It is an expansion valve with a fixed opening cross-section. Alternatively, thermal expansion devices or electronic expansion devices are also common. The regulation of the refrigeration system is carried out by appropriate control of the expansion device, for this purpose, the values to be set on the basis of a characteristic curve, a map or by a model calculated in parallel.

If such a refrigeration system is operated with refrigerant R744, there are two parameters to be set, namely a high pressure as a function of the outlet temperature of the refrigerant at the gas cooler for the supercritical region, which is preferably measured after the gas cooler, and a temperature or the pressure and temperature detectable subcooling of the refrigerant for the subcritical region, which is preferably also measured after the gas cooler.

However, the control of such refrigeration systems is difficult, especially when energy-efficient operation is required. The maps for the two parameters to be set high pressure and subcooling are consuming to determine. They are subject to a certain degree of inaccuracy and can change with the system structure of the refrigeration system. When changing a single component of the refrigeration system then also the maps must be determined again. In addition, measurement inaccuracies in pressure and temperature sensors play a major role, which can result in further efficiency losses. Finally, the possible accuracy of setting an electronic expansion device also influences optimal operation. Usually, in such refrigeration systems, a value for the optimum high pressure is determined on the basis of the temperature measured after the gas cooler. The high pressure is thus a function of the temperature, by varying the opening cross-section of an electronic expansion element, the optimum high pressure is set. In unfavorable cases, measurement errors of the sensors for temperature and pressure as well as inaccuracies in the adjustment of the opening cross-section can add up. Therefore, even small deviations can still have a major impact on the energy efficiency.

From the publication US 5,735,134 For example, a method for optimizing a working point in gas compression cycles is known. Environmental conditions, such as indoor and outdoor temperature, and heat load are evaluated to determine operating parameters for optimum energy efficiency operation. In addition, the system monitors system properties in real time and links them back as part of the determination of the operating state.

The publication US 2009/0241566 A1 discloses an apparatus for controlling an expansion valve. In this case, an expansion valve of a cooling device is controlled, which cools a closed area, for which purpose a parameter of the closed area is detected and the evaporation valve is controlled in dependence of an evaporator pressure determined as a function of the parameter.

An air conditioner with a closed cooling circuit is from the document US 2005/0198980 A1 known. By a controller, a compressor and an expansion valve of the refrigeration cycle are controlled to control the input pressure of the expansion valve. The control is such that the inlet temperature of the expansion valve is controlled to a desired value.

A method for controlling a performance of a gas compression cycle is disclosed in the document WO 03/019085 A1 known. Depending on the power to be delivered, the speed of a compressor is controlled. A flow control device is controlled as a function of the temperature and / or the pressure of the coolant, wherein the temperature or the pressure are measured on the high pressure side.

The invention is therefore based on the object to provide a method for setting an optimal operation of a refrigeration system, on the one hand ensures the required supercooling and on the other hand, an energy efficient operation is possible.

To solve this problem, a method with the method steps of claim 1 is provided.

The method according to the invention comprises the following steps: checking whether a required temperature is reached at an evaporator; if the required temperature is not reached, increase the power of the compressor to the power limit; iteratively adjusting the expansion valve until the refrigeration system is operated at an optimum operating point at which the required temperature is reached at the evaporator.

Deviating from conventional control methods, the control is carried out according to the invention by means of relative changes in the measured values. Decisive are tendencies of the measured values. In this way it can be iteratively determined how a changed adjustment of the opening cross section of the electronic expansion element and / or a variation of the power of the compressor has an effect. On the one hand, it is noted that the setpoint value of the subcooling is achieved, on the other hand, when the setpoint value is reached, it is attempted to reduce the drive power of the compressor.

According to the invention, an optimum operating point is set by targeted activation of the electronic expansion element with simultaneous monitoring of the power consumption and power output of the refrigeration system. The determination of the optimal operating point is not primarily based on the absolute measured values, but on the basis of relative changes in the measured values. This has the advantage that statistical measurement errors of the individual sensors have no relevance. For the control, it is not necessary that the performance of the refrigeration system is measured directly, but rather the power can be determined via indirect measured variables or maps. Essential for the regulation is the tendency of a change.

In the method according to the invention, it can be provided that the refrigerant temperature (subcooling) is measured after the gas cooler in order to monitor the function of the control, but also to set a first operating point of the system, then iteratively approached the ideal operating point of the system and is set. In this case, based on the measured refrigerant temperature, an optimal high-pressure value can be calculated and compared with the current operating point.

A development of the method according to the invention provides that when the required temperature is reached at the evaporator, the expansion valve is iteratively adjusted until the refrigeration system is operated at an optimum operating point at which the power consumed by the compressor is minimal. In this part of the scheme, the performance of the evaporator is optimized. By adjusting the expansion valve, an optimum operating point is set, in which the required amount of subcooling or the optimum high pressure is reached exactly.

In a further embodiment of the method according to the invention, it can be provided that the generated high pressure is reduced in order to minimize the power consumption of the compressor. This control process also takes place iteratively, whereby, of course, it is monitored whether the required cooling capacity or the required setpoint of the air temperature at the evaporator is still reached. In the case where, after adjusting the expansion valve, the temperature at the evaporator is higher than the required temperature (set point / set point input), the process continues with increasing the capacity of the compressor. The method according to the invention thus comprises two different sections. On the one hand, a performance optimization takes place on the evaporator until a required blow-off temperature is reached, on the other hand, after reaching the required setpoint, an efficiency optimization is carried out in order to produce the provision of the refrigerating capacity in an energy-efficient manner.

According to a further embodiment of the method, it can be provided that the recorded power is determined in a driven by an electric drive compressor as a product of voltage and current and in a mechanically driven drive based on a map. Preference is given to a compressor with electric drive, since its performance can be determined very easily.

A particularly preferred variant of the method according to the invention provides that it is carried out either during a cooling operation or during a heating operation. The cooling operation is used for cooling and dehumidifying the supply air to the interior of a motor vehicle, in the heating mode, the refrigeration system is used as a heat pump to generate heat. The heat pump function can be used for example in electric vehicles or hybrid vehicles in which no waste heat of a cooling circuit of an internal combustion engine is available.

The inventive method is particularly suitable for refrigeration systems that are operated with carbon dioxide (CO ₂ ) as a refrigerant. In principle, however, all other refrigerants come into question, for example HFC (such as R134a or R1234yf), which are operated with an electric or mechanical compressor, wherein, as already described, Performance data of the drive unit must be available or measurable and at the same time have at least one externally controllable by the control unit expansion device, which is the basic requirement for adjustability of the optimal system operating point. However, the respective control parameters may vary in this case or resemble those of R744. Other target values can be optimal high pressure, optimal subcooling (after condenser / gas cooler / internal heat exchanger) and overheating (after evaporator / internal heat exchanger). The concept of adjustability of the optimal operating point can be realized with different or any refrigerants, that is it is not limited to R744 (CO ₂ ). However, with other refrigerants operating parameters of the process must be adjusted or adjusted accordingly.

In the method according to the invention, it may be provided that when starting the refrigeration system, a start value for the expansion valve is selected, which is either a fixed value or is selected based on a characteristic curve or on the basis of a characteristic diagram.

Starting from this starting value, a first operating point for the system is determined and set in a next step, that is, the starting high pressure or the starting supercooling. This takes place analogously by means of a characteristic relationship, etc.

In addition, the invention relates to a refrigeration system, with a compressor, a gas cooler, an inner heat exchanger, an adjustable expansion valve, an evaporator and a battery as a reservoir.

The refrigeration system according to the invention is characterized in that it has a control device which is designed to carry out the method described.

An internal heat exchanger is not necessarily required for a refrigeration system.

The refrigeration system according to the invention can be operated either in a refrigeration cycle for generating cold or in a heating circuit as a heat pump for generating heat.

The invention will be explained below with reference to an embodiment with reference to the drawing. The drawing is a schematic representation and shows the essential components of a refrigeration system according to the invention.

The in the 1 shown refrigeration system 1 includes a compressor 2 which is driven electrically or mechanically. The compressor 2 Downstream are a pressure sensor 3 and a temperature sensor 4 , which measure the pressure and temperature of the refrigerant. The pressure sensor 3 and the temperature sensor 4 are with a control device 5 connected, the measured variables are supplied as input variables.

The refrigeration system 1 is filled with refrigerant. In the illustrated embodiment, this is R744. After passing the compressor 2 the refrigerant, which has a subcritical or supercritical pressure depending on the load applied to the system, reaches a gas cooler 6 , The gas cooler 6 is often referred to as a capacitor, wherein a condensation takes place in the subcritical operating condition. At the gas cooler 6 there is a temperature sensor 7 that measures the temperature of the ambient air. The gas cooler 6 are another pressure sensor 8th and another temperature sensor 9 downstream, via appropriate lines with the control device 5 are connected. When passing the gas cooler 6 downstream internal heat exchanger 10 there is a further cooling of the refrigerant. After the high pressure side section of the internal heat exchanger 10 the refrigerant reaches an expansion element, which in the illustrated embodiment as an electronically controllable expansion valve 11 is trained. Passing the expansion valve 11 an isenthalpe expansion takes place into the wet steam area. The liquid phase of the refrigerant is then in the evaporator 12 evaporates, whereby the desired cooling effect of the evaporator flowing through the air flow occurs. A temperature sensor 13 , the evaporator 12 is arranged, the temperature of the air after the evaporator measures 12 ,

A the evaporator 12 downstream battery 14 serves as storage for the refrigerant, then takes the refrigerant when passing the low pressure side portion of the internal heat exchanger 10 Heat up, overheats and gets back to the compressor 2 , whereby the refrigerant circuit is closed.

It is understood that the individual components of the refrigeration system 1 are connected to each other via pipes or hose lines, the pressure sensors and temperature sensors mentioned are via corresponding electrical lines with the control device 5 connected. In addition, the control device 5 with the expansion valve 11 connected and can control the opening cross-section. Next to it is the control device 5 connected to the compressor to ensure its control or to communicate with this.

The procedure for setting an optimal operating point of the refrigeration system 1 is explained below.

After the start, in a first step it is checked whether a required temperature has been reached. This temperature is also called a "set point". It is a setpoint for the air temperature after the evaporator 12 , At this point, the desired cooling effect occurs, that is, a stream of air is cooled and fed by means of a blower to the interior of a motor vehicle. The first step is with the temperature sensor 13 checked whether the required temperature is reached, that is, whether the air temperature after the evaporator 12 is equal to or less than the required temperature. If this condition is met, the method continues with a first branch of the method, if this condition is not met, the method continues with a second branch of the method.

A second step is achieved if the evaporator temperature to be set, ie the required cooling capacity, is not reached. It is then checked if the compressor 2 is operated at its power limit. The power can be increased by increasing the speed, an increase in speed is accompanied by an increase in electrical power consumption. If the compressor 2 is not operated at its power limit, the power is increased in a third step, then the first step is branched.

Otherwise, if the compressor 2 is already operated at its power limit, the current operating parameters are stored in a fourth step. These include at least the means of the temperature sensor 13 measured temperature of the air after the evaporator and that of the compressor 2 recorded performance.

Subsequently, the cooling performance at the evaporator is tried 12 at constant power of the compressor 2 to increase. In addition, in a fifth step, the expansion valve 11 selectively controlled, then the resulting change in cooling capacity is recorded with respect to previous values and compared. The actually set minimum air temperature after the evaporator 12 is a measure of the cooling capacity.

In a sixth step it is checked whether after the activation of the expansion valve 11 in the fifth step has reduced the temperature difference between the actual temperature of the air after the evaporator and the set point. If this is not the case, the system branches back to the first step, so that the procedure is run through again.

Otherwise, if the targeted control of the expansion valve 11 has led to the desired reduction is tested in a seventh step, if the required temperature (set point) is already reached. If this is not the case, the process branches to the second step from there, so that the second branch of the process is run through again in order to optimize the cooling capacity.

Otherwise, if in the seventh step it has been determined that the setpoint for the air temperature after the evaporator 12 is reached, is branched to the first branch of the process to the operation of the refrigeration system 1 energy efficient. For this, the operating parameters are stored in an eighth step. The system data includes the current air temperature after the evaporator 12 , which by means of the temperature sensor 13 is measured. Further, the operating parameters include those of the compressor 2 recorded performance.

Subsequently, in a ninth step, the expansion valve 11 targeted, ie open or closed. By varying the opening cross-section of the expansion valve 11 it changes in the refrigeration system 1 ruling high pressure. Opening the expansion valve 11 reduces the high pressure, the closing of the expansion valve 11 has its increase. At the same time, the power consumption of the compressor 2 monitored and compared with previous values. If the power changes "positively", ie in the sense of reducing the drive power of the compressor 2 , so will the expansion valve 11 further controlled in the same direction of actuation. In an operating condition, the expansion valve 11 it can be closed in another operating state. If a "negative" change in the absorbed power in the sense of an increase in the drive power of the compressor 2 is detected, the "direction" of the control is reversed, that is, when the expansion valve 11 has been closed, it will now open and vice versa. In detail, it is checked in a tenth step, if the setpoint for the air temperature after the evaporator 12 , which is set point, reached. If so, it is checked in an eleventh step if the performance of the compressor 2 by controlling the expansion valve 11 was reduced. If so, the procedure is repeated from the eighth step. Otherwise, if the control of the expansion valve 11 has not led to the performance of the compressor has decreased, the direction of the drive is reversed and the procedure is also run again from the eighth step.

If the test in the tenth step has revealed that the setpoint for the air temperature after the evaporator 12 (set point) has not been reached, is branched from there to the second step in the second branch of the method.

Because the compressor 2 has an electric drive motor, a change in the drive power can be easily detected. Based on the drive motor, the applied voltage and the current consumption can be determined and the control device 5 be supplied. The product of current and voltage corresponds to the absorbed electrical power for operation of the electric compressor 2 at the current operating point. The first branch of the method is used, with unchanged, that is, constant performance of the evaporator 12 the electrical power consumption of the compressor 2 minimize, thereby reducing the overall efficiency of the refrigeration system 1 is optimized.

In the case of a mechanically driven compressor, it is possible instead to resort to a characteristic diagram in which the required drive torque is specified as a function of the rotational speed of the engine. About the respective transmission ratio and the pressure ratio, a calculation of the power can be done.

For all types of compressors, the pressures of the low pressure side and the high pressure side can be detected by integrated pressure sensors. If no such sensor is present on the low-pressure side, an indirect pressure determination can take place on the basis of the measured value of the air outlet temperature downstream of the evaporator and a conversion to a corresponding evaporation pressure, whereby the low pressure sought can be obtained.

In the 1 shown refrigeration system can also be operated as a heat pump after modifications in the system design. On the feedback of the power consumption of the compressor can also be detected and adjusted an optimal operating point with the lowest drive power.

Thanks to the iterative procedure, the process always finds the optimum operating point, regardless of components installed or used, or different system designs. It is not absolutely necessary to determine elaborate characteristics or maps, but a recourse to approximate formulas for characteristics is possible, which are known from experiments or the literature, for a temporary operation of the refrigeration system, but especially for the tarnish and possibly for a transient Operation are sufficiently accurate. In the described refrigeration system, statistical measurement errors of the pressure sensors and the temperature sensors have less of an effect on the efficiency of the system. The performance of the refrigeration system is increased, so that even at higher temperatures, the required cooling capacity can be provided. Delivered messages from the compressors that are used can be used and used directly to increase system efficiency.

Claims (10)

Method for setting an optimum operating point of a refrigeration system (1), comprising the following steps: a) checking whether a required temperature is reached at an evaporator (12); b) if the required temperature is not reached, increasing the power of a compressor (2) to the power limit; c) iteratively adjusting the expansion valve (11) until the refrigeration system (1) is operated at an optimum operating point at which the required temperature at the evaporator (12) is reached. Method according to Claim 1 characterized in that, when the required temperature at the evaporator (12) is reached, the expansion valve (11) is iteratively adjusted until the refrigeration system (1) is operated at an optimum operating point at which the pressure from the compressor (2) recorded power is minimal. Method according to Claim 2 characterized in that for minimizing the power consumption of the compressor (2) the generated high pressure is increased or reduced in particular for supercritical plant operation, wherein to minimize the power consumption of the compressor (2) the generated subcooling to the gas cooler (6) or condenser in particular for a uncritical system operation is increased or reduced, and / or that to minimize the power consumption of the compressor (2), the generated superheat after evaporator (12) is increased or reduced. Method according to Claim 2 or 3 , characterized in that, if after the setting of the expansion valve (11), the temperature at the evaporator (12) is higher than the required temperature, the method with the step b) of Claim 1 will continue. Method according to one of the preceding claims, characterized in that the power absorbed in a driven by an electric drive compressor (2) is determined as a product of voltage and current and in a mechanically driven drive based on a map or that the power consumption as a direct output value of electrically driven compressor (2) is provided. Method according to one of the preceding claims, characterized in that it is carried out either during a cooling operation or during a heating operation. Method according to one of the preceding claims, characterized in that carbon dioxide (CO ₂ ) is used as the refrigerant. Method according to one of the preceding claims, characterized in that when starting the refrigeration system (1), a starting value for the expansion valve (11) is selected, which is either a fixed value or is selected based on a characteristic or based on a map. Refrigeration system (1), with a compressor (2), a gas cooler (6), an internal heat exchanger (10), an adjustable expansion valve (11), an evaporator (12) and a battery (14), characterized in that the refrigeration system (1) a control device (5), which for carrying out the method according to one of Claims 1 to 8th is trained. Refrigeration system after Claim 9 , characterized in that it is operable in a heating circuit as a heat pump.

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