Method for determination of the coefficient of performanace of a refrigerating machine

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.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to co-pending German Patent Application Ser. No. 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

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.

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.

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.

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

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.

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.

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.

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.

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.

In the method in accordance with the invention in accordance with claim4, 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.

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.

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.

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.

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.

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.

DETAILED DESCRIPTION

A first embodiment of a refrigeration machine in accordance with the invention is shown inFIG. 1. The refrigeration machine includes a closed circuit10which has a refrigerant and in which an evaporator12, a compressor14, a condenser16and an expansion valve18are arranged.

For the determination of the refrigerant temperature, a temperature sensor28is arranged in the region of the inlet of the compressor14, a temperature sensor30is arranged in the region of the outlet of the condenser16and a temperature sensor32is arranged in the region of the outlet of the expansion valve18. The temperature sensors28,30,32are connected to an evaluation unit26which can be integrated in a control of the refrigeration machine.

The refrigeration machine is described here in its function as a heat pump.FIG. 2shows 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 liquid20and saturated gas22are drawn.

The point E inFIG. 2designates the state of the refrigerant after the expansion through the expansion valve18. An evaporation (E-A) and overheating (A-B) of the refrigerant takes place in the evaporator12.

The compressor14provides 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 evaporator12up to approximately +90° C. by the compressor14.

A condensing (C-D) of the refrigerant takes place in the condenser16, 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 valve18(D-E), with it cooling down to approximately 0° C., for example.

In the following, the temperature of the gaseous refrigerant at the inlet of the compressor14is designated as T1; the temperature of the liquid refrigerant at the outlet of the condenser16as T2; the temperature of the expanded refrigerant at the outlet of the expansion valve18as T3; and the temperature of the gaseous refrigerant at the outlet of the compressor14as T4.

The evaporation pressure, i.e. that is the pressure of the gaseous refrigerant at the outlet of the evaporator12is designated as P1and the condensing pressure, i.e. that is the pressure of the liquid refrigerant at the outlet of the condenser16as P2.

First the enthalpy H1is determined at the outlet of the condenser16, the enthalpy H2at the inlet of the compressor14and the enthalpy H3at the outlet of the compressor14to determine the coefficient of performance of the refrigeration machine.

In this respect, the enthalpy H1is a function of the refrigerant temperature T2at the outlet of the condenser, the enthalpy H2is a function of the refrigerant temperature11at the inlet of the compressor14and of the refrigerant pressure P1at the outlet of the evaporator12; and the enthalpy H3is a function of the refrigerant temperature T4at the outlet of the compressor14and of the refrigerant pressure P2at the outlet of the condenser16:
H1=f(T2)  (1)
H2=f(P1,T1)  (2)
H3=f(P2,T4)  (3)

In the embodiment shown inFIG. 1, the determination of the temperatures T1, T2, T3takes place by measurement using the temperature sensors28,30and32respectively. The temperature values T1, T2, T3detected by the temperature sensors28,30,32are communicated to the evaluation unit26.

Using the pressure equation of the refrigerant used, the evaluation unit26calculates the pressure P2from the received value for the temperature T2at the outlet of the condenser16and the pressure P1from the temperature value T3at the outlet of the expansion valve18. The generally known Clausius-Clapeyron equation can be used, for example, as the pressure equation.

With knowledge of the temperatures T1and T2and of the pressure P1, the enthalpies H1and H2can now be determined by equations (1) and (2).

The enthalpy H3is calculated from the compressor model since the temperature T4is not known.

It is assumed for this purpose that approximately 95% of the electrical power taken up by the compressor14is induced into the refrigeration circuit. The electrical power Qe1taken up by the compressor14is in this respect not determined by an electricity meter, but is rather calculated by a model describing the thermodynamic properties of the compressor14, e.g. a 10-coefficient model.

Not only the electrical power taken up by the compressor14can be calculated using this model, but also the refrigerating capacity Q0of the compressor14, the electrical current I taken up by the compressor14and the mass flow m° of the refrigerant flowing through the compressor14.

In this respect, the values calculated only apply to the documented operating point of the compressor14either at a constant overheating or at a constant suction gas temperature, i.e. at a constant temperature T1of 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 T1.

The electrical power Qe1taken up by the compressor14is divided by the mass flow m° to determine the enthalpy difference H3-H2.
Qe1/m°=H3−H2  (4)

Since the enthalpy H2is known from equation (2), the enthalpy H3can be calculated easily from the enthalpy difference H3−H2.

For control, the refrigerant temperature T4at the compressor outlet is calculated from the point of intersection of the line of enthalpy H3with the line of the pressure P2in the log p-h diagram ofFIG. 2.

Subsequently, the heat output Qh of the refrigeration machine is calculated from the difference of the calculated enthalpies H3and H1in accordance with the equation
Qh=m°*(H3−H1)  (5).

The electrical power Qe1taken up by the compressor14was already determined using the compressor model and is preoperational to the difference of the enthalpies H3and H2in accordance with equation (4).

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 Qe1still has to be formed:
COP=Qh/Qe1=(H3−H1)/(H3−H2)  (6).

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 Qe1can 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.

A second embodiment of a refrigeration machine in accordance with the invention is shown inFIG. 3which differs from the embodiment described above in that a fourth temperature sensor34connected to the evaluation unit26is arranged in the region of the compressor14to determine the refrigerant temperature T4at the compressor outlet. In this embodiment, the refrigerant temperature T4at the compressor outlet therefore does not need to be estimated using a compressor model, but is rather measured directly.

In accordance with the first embodiment, while using the pressure equation of the refrigerant used, the evaluation unit26calculates the pressure P2from the received value for the temperature T2at the outlet of the condenser16and the pressure P1from the temperature T3at the outlet of the expansion valve18. Subsequently, in accordance with equations (1) to (3), the enthalpies H1, H2and H3are determined from the measured temperatures T1, T2, T4and from the calculated pressures P1, P2and the coefficient of performance is determined from these in accordance with equation (6).

A third embodiment of a refrigeration machine in accordance with the invention is shown inFIG. 4which differs from the first embodiment described with reference toFIG. 1in that, instead of the third temperature sensor32, a pressure sensor36is arranged in the region of the outlet of the evaporator12to measure the pressure P1of the refrigerant there. The pressure sensor36is connected to the evaluation unit26to communicate the measured refrigerant pressure P1to it.

In this embodiment, the pressure P1therefore does not need to be calculated from the refrigerant temperature T3at the outlet of the expansion valve18, but is rather measured directly. Only the pressure P2has to be calculated using the pressure equation of the refrigerant used from the temperature T2at the outlet of the condenser16and the refrigerant temperature T4at the compressor outlet has to be calculated, as explained with reference toFIG. 1, using a compressor model so that the enthalpies H1, H2and H3can 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.

A fourth embodiment of a refrigeration machine in accordance with the invention is shown inFIG. 5which differs from the third embodiment shown inFIG. 4in that a fourth temperature sensor34connected to the evaluation unit26is arranged in the region of the outlet of the compressor14to determine the refrigerant temperature T4at the compressor outlet. Unlike in the third embodiment, the refrigerant temperature T4at 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 inFIG. 2. As in the embodiments described above, the pressure P2is also calculated from the refrigerant temperature T2at the outlet of the condenser16here.

Subsequently, the enthalpies H1, H2and H3are calculated in accordance with equations (1) to (3) from the measured temperatures T1, T2, T4and the measured pressure P1as well as the calculated pressure P2, and the coefficient of performance is determined therefrom in accordance with equation (6).

A fifth embodiment of a refrigeration machine in accordance with the invention is shown inFIG. 6which differs from the third embodiment shown inFIG. 4in that a second pressure sensor38connected to the evaluation unit26is arranged in the region of the outlet of the condenser16to determine the refrigerant pressure P2at the condenser outlet.

Unlike in the third embodiment, the pressure P2therefore does not have to be calculated using the pressure equation of the refrigerant used from the temperature T2at the outlet of the condenser16in this embodiment, but it is rather measured directly. Only the refrigerant temperature T4at the compressor outlet is calculated using a compressor model in this embodiment as described with reference toFIG. 1.

Subsequently, in accordance with equations (1) to (3), the enthalpies H1, H2and H3are calculated from the measured temperatures T1, T2and the measured pressures P1, P2and from the calculated temperature T4and the coefficient of performance is determined therefrom in accordance with equation (6).

A sixth embodiment of a refrigeration machine in accordance with the invention is shown inFIG. 7which differs from the fifth embodiment shown inFIG. 6in that a third temperature sensor34connected to the evaluation unit26is arranged in the region of the outlet of the compressor14to determine the refrigerant temperature T4at the compressor outlet. Unlike in the fifth embodiment, the refrigerant temperature T4at the compressor outlet therefore does not need to be estimated using a compressor model in this embodiment, but is rather measured directly.

Subsequently, in accordance with equations (1) to (3), the enthalpies H1, H2and H3are calculated from the measured temperatures T1, T2and T4and the measured pressures P1, P2and the coefficient of performance is determined therefrom in accordance with equation (6).