Abstract:
The present invention relates to a method for the determination of the coefficient of performance of a refrigeration machine, in particular of a heat pump, which includes a closed circuit which has a refrigerant and in which an evaporator, a compressor, a condenser and an expansion valve are arranged. In the method, at least three temperatures of the refrigerant are determined using temperature sensors arranged in the circuit. Alternatively, at least two temperatures and at least one pressure of the refrigerant is determined using sensors arranged in the circuit. Enthalpies of the circuit are calculated from the determined refrigerant temperatures and refrigerant pressures and the heat output and the taken up electrical power of the refrigeration machine are calculated therefrom to determine the coefficient of performance of the refrigeration machine from the quotient of the calculated heat output and the calculated taken up electrical power. The invention also relates to a refrigeration machine for the carrying out of such a method.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to co-pending German Patent Application Serial Number 10 2008 061 631.1, filed Dec. 11, 2008, the entirety of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a method for the determination of the coefficient of performance of a refrigeration machine, in particular of a heat pump, which includes a closed circuit which has a refrigerant and in which an evaporator, a compressor, a condenser and an expansion valve are arranged. 
         [0003]    The quotient from the heat output of the refrigeration machine and the taken up electrical power of the refrigeration machine is called the coefficient of performance (COP) of a refrigeration machine. Conventionally, the electrical power take-up of the refrigeration machine is detected via an electricity meter, whereas the heat output of the refrigeration machine is determined by a temperature measurement and a volume flow measurement on the water side of the refrigerant circuit, i.e. that is behind the condenser. 
         [0004]    A method is also known in which the temperatures and the pressures of the refrigerant are detected using two pressure sensors and three temperature sensors at different points of the circuit and are used for the calculation of the coefficient of performance. The electrical power take-up of the refrigeration machine is also detected by means of an electricity meter. The heat output of the refrigeration machine can then be calculated by multiplying the coefficient of performance by the taken up electrical power. 
         [0005]    It proves to be problematic with the known methods or refrigeration machines that both the electricity meter and the pressure sensors represent a not unsubstantial cost factor. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In a method in accordance with the invention, at least three temperatures of the refrigerant are determined for the determination of the coefficient of performance of a refrigeration machine, in particular of a heat pump, which includes a closed circuit which has a refrigerant and in which an evaporator, a compressor, a condenser and an expansion valve are arranged, using at least three temperature sensors which are arranged in the circuit. Enthalpies and pressures of the circuit are calculated from the determined refrigerant temperatures and both the heat output and the taken up electrical power of the refrigeration machine are calculated from differences of the calculated enthalpies. The coefficient of performance is finally determined from the quotient of the calculated heat output and the calculated taken up electrical power. 
         [0007]    In a method in accordance with the invention, the coefficient of performance of the refrigeration machine is in other words determined only with reference to temperature values which are delivered by three temperature sensors arranged in the refrigerant circuit, with a specific knowledge of the thermodynamic properties of the system, in particular of the refrigerant and of the compressor, being required. A minimum of information on the refrigerant circuit which is required to be able to determine the coefficient of performance of the refrigeration machine is determined by the measurement of the refrigerant temperatures at three different points of the refrigerant circuit. 
         [0008]    A use of additional sensors, e.g. of further temperature sensors or pressure sensors, which are typically approximately ten times more expensive than temperature sensors, is thus generally not required. The use of a costly electricity meter can in particular be dispensed with. The use in accordance with the invention of a minimal number of temperature sensors therefore makes it possible to determine the coefficient of performance of a refrigeration machine with a minimal cost effort. 
         [0009]    In accordance with an advantageous embodiment of the method, a first temperature is measured in the region of the inlet of the compressor, a second temperature is measured in the region of the outlet of the condenser and a third temperature is measured in the region of the outlet of the expansion valve. The refrigerant temperatures measured at these points of the refrigerant circuit are generally sufficient to determine the enthalpies of the circuit and ultimately to determine the coefficient of performance of the refrigeration machine from them. 
         [0010]    Alternatively, a fourth temperature can additionally be determined by means of a fourth temperature sensor and can be used for the determination of the coefficient of performance, with the fourth temperature preferably being determined in the region of the outlet of the compressor. By the measurement of the refrigerant temperature at the compressor outlet, this temperature no longer has to be calculated by a compressor model, but it can rather be determined exactly. The coefficient of performance can be determined more simply, faster and more precisely in this manner. 
         [0011]    In the method in accordance with the invention in accordance with claim  4 , at least two temperatures and one pressure of the refrigerant are determined for the determination of the coefficient of performance of a refrigeration machine using at least two temperature sensors and at least one pressure sensor which are arranged in the refrigerant circuit. Enthalpies of the circuit are calculated from the determined refrigerant temperatures and the determined refrigerant pressure and the heat output and the taken up electrical power of the refrigeration machine are calculated from differences between the enthalpies. The coefficient of performance of the refrigeration machine is then determined from the quotient of the calculated heat output and the calculated taken up electrical power. 
         [0012]    In this variant of the method in accordance with the invention, the coefficient of performance of the refrigeration machine can also be determined using a minimal number of sensors and in particular without an electricity meter and thus particularly cost-effectively. In this case, the determination of the coefficient of performance takes place only with reference to the measured values delivered by the two temperature sensors and by the one pressure sensor, with specific knowledge of the system, in particular of the thermodynamic properties of the refrigerant and of the compressor, also having to be required here. 
         [0013]    In accordance with an advantageous embodiment of the method, a first temperature is measured in the region of the inlet of the compressor, a second temperature is measured in the region of the outlet of the condenser and a first pressure is measured in the region of the outlet of the evaporator. 
         [0014]    In addition, a third temperature can be determined and can be used for the determination of the coefficient of performance, with the third temperature preferably being determined in the region of the outlet of the compressor. Due to the additional measurement of a third temperature, it is possible to replace calculations which are required on the use of only three sensors for the determination of the enthalpies, in particular for the determination of the coolant temperature at the compressor outlet, by an actual measurement, whereby the determination of the coefficient of performance of the refrigeration machine can take place more simply, faster and with a higher precision. 
         [0015]    Alternatively or additionally, a second pressure can be determined and can be used for the determination of the coefficient of performance, with the second pressure preferably being determined in the region of the outlet of the condenser. The measurement of the second pressure also contributes to a faster and more precise determination of the coefficient of performance in that the calculation of the pressure value required without the direct measurement can be dispensed with. 
         [0016]    Further subject matters of the invention are moreover the refrigeration machines disclosed herein. The methods in accordance with the invention can be carried out particularly easily and the above advantages can be achieved correspondingly using these refrigeration machines. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will be described in the following purely by way of example with reference to advantageous embodiments and to the enclosed drawings. There are shown: 
           [0018]      FIG. 1  a schematic representation of a first embodiment of a refrigeration machine in accordance with the invention; 
           [0019]      FIG. 2  a log p-h diagram of the refrigerant of the refrigeration machine of  FIG. 1  and the associated cycle; 
           [0020]      FIG. 3  a schematic representation of a second embodiment of a refrigeration machine in accordance with the invention; 
           [0021]      FIG. 4  a schematic representation of a third embodiment of a refrigeration machine in accordance with the invention; 
           [0022]      FIG. 5  a schematic representation of a fourth embodiment of a refrigeration machine in accordance with the invention; 
           [0023]      FIG. 6  a schematic representation of a fifth embodiment of a refrigeration machine in accordance with the invention; and 
           [0024]      FIG. 7  a schematic representation of a sixth embodiment of a refrigeration machine in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    A first embodiment of a refrigeration machine in accordance with the invention is shown in  FIG. 1 . The refrigeration machine includes a closed circuit  10  which has a refrigerant and in which an evaporator  12 , a compressor  14 , a condenser  16  and an expansion valve  18  are arranged. 
         [0026]    For the determination of the refrigerant temperature, a temperature sensor  28  is arranged in the region of the inlet of the compressor  14 , a temperature sensor  30  is arranged in the region of the outlet of the condenser  16  and a temperature sensor  32  is arranged in the region of the outlet of the expansion valve  18 . The temperature sensors  28 ,  30 ,  32  are connected to an evaluation unit  26  which can be integrated in a control of the refrigeration machine. 
         [0027]    The refrigeration machine is described here in its function as a heat pump.  FIG. 2  shows for this purpose a log p-h diagram of the refrigerant used in the refrigeration machine, with the pressure p of the refrigerant being entered logarithmically as the function of the enthalpy H. In addition, the limits of saturated liquid  20  and saturated gas  22  are drawn. 
         [0028]    The point E in  FIG. 2  designates the state of the refrigerant after the expansion through the expansion valve  18 . An evaporation (E-A) and overheating (A-B) of the refrigerant takes place in the evaporator  12 . 
         [0029]    The compressor  14  provides a compression (B-C) of the refrigerant which is accompanied by a corresponding temperature increase. The temperature of the refrigerant can be increased, for example, from approximately +10° C. at the outlet of the evaporator  12  up to approximately +90° C. by the compressor  14 . 
         [0030]    A condensing (C-D) of the refrigerant takes place in the condenser  16 , with the condensation temperature being able to amount, for example, to +50° C. The now liquid refrigerant which is only 50° C. warm is subsequently expanded by the expansion valve  18  (D-E), with it cooling down to approximately 0° C., for example. 
         [0031]    In the following, the temperature of the gaseous refrigerant at the inlet of the compressor  14  is designated as T 1 ; the temperature of the liquid refrigerant at the outlet of the condenser  16  as T 2 ; the temperature of the expanded refrigerant at the outlet of the expansion valve  18  as T 3 ; and the temperature of the gaseous refrigerant at the outlet of the compressor  14  as T 4 . 
         [0032]    The evaporation pressure, i.e. that is the pressure of the gaseous refrigerant at the outlet of the evaporator  12  is designated as P 1  and the condensing pressure, i.e. that is the pressure of the liquid refrigerant at the outlet of the condenser  16  as P 2 . 
         [0033]    First the enthalpy H 1  is determined at the outlet of the condenser  16 , the enthalpy H 2  at the inlet of the compressor  14  and the enthalpy H 3  at the outlet of the compressor  14  to determine the coefficient of performance of the refrigeration machine. 
         [0034]    In this respect, the enthalpy H 1  is a function of the refrigerant temperature T 2  at the outlet of the condenser, the enthalpy H 2  is a function of the refrigerant temperature  11  at the inlet of the compressor  14  and of the refrigerant pressure P 1  at the outlet of the evaporator  12 ; and the enthalpy H 3  is a function of the refrigerant temperature T 4  at the outlet of the compressor  14  and of the refrigerant pressure P 2  at the outlet of the condenser  16 : 
         [0000]        H 1= f ( T 2)  (1) 
         [0000]        H 2= f ( P 1, T 1)  (2) 
         [0000]        H 3= f ( P 2, T 4)  (3) 
         [0035]    In the embodiment shown in  FIG. 1 , the determination of the temperatures T 1 , T 2 , T 3  takes place by measurement using the temperature sensors  28 ,  30  and  32  respectively. The temperature values T 1 , T 2 , T 3  detected by the temperature sensors  28 ,  30 ,  32  are communicated to the evaluation unit  26 . 
         [0036]    Using the pressure equation of the refrigerant used, the evaluation unit  26  calculates the pressure P 2  from the received value for the temperature T 2  at the outlet of the condenser  16  and the pressure P 1  from the temperature value T 3  at the outlet of the expansion valve  18 . The generally known Clausius-Clapeyron equation can be used, for example, as the pressure equation. 
         [0037]    With knowledge of the temperatures T 1  and T 2  and of the pressure P 1 , the enthalpies H 1  and H 2  can now be determined by equations (1) and (2). 
         [0038]    The enthalpy H 3  is calculated from the compressor model since the temperature T 4  is not known. 
         [0039]    It is assumed for this purpose that approximately 95% of the electrical power taken up by the compressor  14  is induced into the refrigeration circuit. The electrical power Qe 1  taken up by the compressor  14  is in this respect not determined by an electricity meter, but is rather calculated by a model describing the thermodynamic properties of the compressor  14 , e.g. a 10-coefficient model. 
         [0040]    Not only the electrical power taken up by the compressor  14  can be calculated using this model, but also the refrigerating capacity Q 0  of the compressor  14 , the electrical current I taken up by the compressor  14  and the mass flow m° of the refrigerant flowing through the compressor  14 . 
         [0041]    In this respect, the values calculated only apply to the documented operating point of the compressor  14  either at a constant overheating or at a constant suction gas temperature, i.e. at a constant temperature T 1  of the refrigerant at the compressor inlet. To calculate the values of the real operating point, the values have to be corrected in dependence on the real compressor inlet temperature T 1 . 
         [0042]    The electrical power Qe 1  taken up by the compressor  14  is divided by the mass flow m° to determine the enthalpy difference H 3 -H 2 . 
         [0000]      Qe1/ m°=H 3− H 2  (4) 
         [0043]    Since the enthalpy H 2  is known from equation (2), the enthalpy H 3  can be calculated easily from the enthalpy difference H 3 −H 2 . 
         [0044]    For control, the refrigerant temperature T 4  at the compressor outlet is calculated from the point of intersection of the line of enthalpy H 3  with the line of the pressure P 2  in the log p-h diagram of  FIG. 2 . 
         [0045]    Subsequently, the heat output Qh of the refrigeration machine is calculated from the difference of the calculated enthalpies H 3  and H 1  in accordance with the equation 
         [0000]      Qh=m°*( H 3− H 1)  (5). 
         [0046]    The electrical power Qe 1  taken up by the compressor  14  was already determined using the compressor model and is preoperational to the difference of the enthalpies H 3  and H 2  in accordance with equation (4). 
         [0047]    To determine the coefficient of performance COP or the efficiency of the refrigeration machine, subsequently only the quotient of the heat output Qh and of the electrical power Qe 1  still has to be formed: 
         [0000]      COP=Qh/Qe1=( H 3− H 1)/( H 3− H 2)  (6). 
         [0048]    In addition, the annual performance index of the refrigeration machine can be determined by an integration of the coefficient of performance over time. Accordingly, the heat output Qh and the electrical power Qe 1  can be integrated over time to indicate the heating energy and the taken up electrical energy. The power take-up of additional devices such as pumps, electronics, etc. can in this respect be taken into the calculation through suitable parameters. 
         [0049]    A second embodiment of a refrigeration machine in accordance with the invention is shown in  FIG. 3  which differs from the embodiment described above in that a fourth temperature sensor  34  connected to the evaluation unit  26  is arranged in the region of the compressor  14  to determine the refrigerant temperature T 4  at the compressor outlet. In this embodiment, the refrigerant temperature T 4  at the compressor outlet therefore does not need to be estimated using a compressor model, but is rather measured directly. 
         [0050]    In accordance with the first embodiment, while using the pressure equation of the refrigerant used, the evaluation unit  26  calculates the pressure P 2  from the received value for the temperature T 2  at the outlet of the condenser  16  and the pressure P 1  from the temperature T 3  at the outlet of the expansion valve  18 . Subsequently, in accordance with equations (1) to (3), the enthalpies H 1 , H 2  and H 3  are determined from the measured temperatures T 1 , T 2 , T 4  and from the calculated pressures P 1 , P 2  and the coefficient of performance is determined from these in accordance with equation (6). 
         [0051]    A third embodiment of a refrigeration machine in accordance with the invention is shown in  FIG. 4  which differs from the first embodiment described with reference to  FIG. 1  in that, instead of the third temperature sensor  32 , a pressure sensor  36  is arranged in the region of the outlet of the evaporator  12  to measure the pressure P 1  of the refrigerant there. The pressure sensor  36  is connected to the evaluation unit  26  to communicate the measured refrigerant pressure P 1  to it. 
         [0052]    In this embodiment, the pressure P 1  therefore does not need to be calculated from the refrigerant temperature T 3  at the outlet of the expansion valve  18 , but is rather measured directly. Only the pressure P 2  has to be calculated using the pressure equation of the refrigerant used from the temperature T 2  at the outlet of the condenser  16  and the refrigerant temperature T 4  at the compressor outlet has to be calculated, as explained with reference to  FIG. 1 , using a compressor model so that the enthalpies H 1 , H 2  and H 3  can be determined in accordance with equations (1) to (3) and, in accordance with equation (6), the coefficient of performance of the refrigeration machine can be determined from them. 
         [0053]    A fourth embodiment of a refrigeration machine in accordance with the invention is shown in  FIG. 5  which differs from the third embodiment shown in  FIG. 4  in that a fourth temperature sensor  34  connected to the evaluation unit  26  is arranged in the region of the outlet of the compressor  14  to determine the refrigerant temperature T 4  at the compressor outlet. Unlike in the third embodiment, the refrigerant temperature T 4  at the compressor outlet therefore does not have to be calculated using a compressor model in this embodiment, but is rather measured directly in a similar manner to the second embodiment shown in  FIG. 2 . As in the embodiments described above, the pressure P 2  is also calculated from the refrigerant temperature T 2  at the outlet of the condenser  16  here. 
         [0054]    Subsequently, the enthalpies H 1 , H 2  and H 3  are calculated in accordance with equations (1) to (3) from the measured temperatures T 1 , T 2 , T 4  and the measured pressure P 1  as well as the calculated pressure P 2 , and the coefficient of performance is determined therefrom in accordance with equation (6). 
         [0055]    A fifth embodiment of a refrigeration machine in accordance with the invention is shown in  FIG. 6  which differs from the third embodiment shown in  FIG. 4  in that a second pressure sensor  38  connected to the evaluation unit  26  is arranged in the region of the outlet of the condenser  16  to determine the refrigerant pressure P 2  at the condenser outlet. 
         [0056]    Unlike in the third embodiment, the pressure P 2  therefore does not have to be calculated using the pressure equation of the refrigerant used from the temperature T 2  at the outlet of the condenser  16  in this embodiment, but it is rather measured directly. Only the refrigerant temperature T 4  at the compressor outlet is calculated using a compressor model in this embodiment as described with reference to  FIG. 1 . 
         [0057]    Subsequently, in accordance with equations (1) to (3), the enthalpies H 1 , H 2  and H 3  are calculated from the measured temperatures T 1 , T 2  and the measured pressures P 1 , P 2  and from the calculated temperature T 4  and the coefficient of performance is determined therefrom in accordance with equation (6). 
         [0058]    A sixth embodiment of a refrigeration machine in accordance with the invention is shown in  FIG. 7  which differs from the fifth embodiment shown in  FIG. 6  in that a third temperature sensor  34  connected to the evaluation unit  26  is arranged in the region of the outlet of the compressor  14  to determine the refrigerant temperature T 4  at the compressor outlet. Unlike in the fifth embodiment, the refrigerant temperature T 4  at the compressor outlet therefore does not need to be estimated using a compressor model in this embodiment, but is rather measured directly. 
         [0059]    Subsequently, in accordance with equations (1) to (3), the enthalpies H 1 , H 2  and H 3  are calculated from the measured temperatures T 1 , T 2  and T 4  and the measured pressures P 1 , P 2  and the coefficient of performance is determined therefrom in accordance with equation (6).