A method for detecting performance degradation of a vapour compression system

Abstract

A method for detecting performance degradation of a vapour compression system (1) is disclosed. A performance baseline is provided, the performance baseline specifying corresponding values of mean duration of OFF periods of the compressor (2) and mean running compressor capacity. While operating the vapour compression system (1), durations of OFF periods of the compressor (2) and running compressor capacity (18) are measured. The measured durations of OFF periods of the compressor (2) and running compressor capacity (18) are compared to the performance baseline, and it is determined whether or not performance degradation of the vapour compression system (1) is occurring, based on the comparison.

Description

[0001] A METHOD FOR DETECTING PERFORMANCE DEGRADATION OF A VAPOUR COMPRESSION SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for detecting performance degradation of a vapour compression system, such as a refrigeration system, a heat pump or an air condition system. The method according to the invention provides accurate and reliable detection of performance degradation in an easy and fast manner.

[0004] BACKGROUND OF THE INVENTION

[0005] Vapour compression systems, such as refrigeration systems, heat pumps or air condition systems, normally comprise a compressor unit with one or more compressors, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path. Refrigerant flowing in the refrigerant path is compressed by means of the one or more compressors before being supplied to the heat rejecting heat exchanger where heat exchange takes place with the ambient or a secondary fluid, in such a manner that heat is rejected from the refrigerant. Refrigerant leaving the heat rejecting heat exchanger is passed through the expansion device where it undergoes expansion before being supplied to the evaporator. In the evaporator, the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place between the refrigerant and the ambient or a secondary fluid, in such a manner that heat is absorbed by the refrigerant. Finally, the refrigerant leaving the evaporator is once again supplied to the compressor unit. Thus, the refrigerant flowing in the refrigerant path is alternatingly compressed and expanded, while heat exchange takes place at the heat rejecting heat exchanger and the evaporator, respectively, thus providing heating or cooling.

[0006] It is desirable that a vapour compression system operates efficiently, appropriately and with good performance. For instance, it is desirable that the energy consumption of the vapour compression system is minimal and that the cooling or heating provided via the heat rejecting heat exchanger or the evaporator meets certain requirements, such as maintaining a certain temperature. However, the performance of the vapour compression system may degrade, e.g. because one or more of the components of the vapour compression system is malfunctioning or degrading, or because the vapour compression system is operated in a non-optimal manner. It is desirable to be able to detect when the performance of the vapour compression system is degraded, because this will allow for measures to be taken in order to restore the performance of the vapour compression system. In order to detect performance degradation, it may be considered relevant to observe the power consumption of the compressor unit, since an increased power consumption may be an indication that the vapour compression system may be operating in a non-optimal manner and / or that the performance of the vapour compression system is degrading. However, the power consumption of the compressor unit is affected by a number of various factors or circumstances, such as ambient temperature, variations in cooling load, etc. It may therefore be difficult to determine what has caused a detected increase in the power consumption of the compressor unit, and it may therefore not be possible to accurately detect performance degradation of the vapour compression system solely by observing the power consumption of the compressor unit.

[0007] DESCRIPTION OF THE INVENTION

[0008] It is an object of embodiments of the invention to provide a method for detecting performance degradation of a vapour compression system, in which performance degradation can be detected accurately and in an easy manner.

[0009] The invention provides a method for detecting performance degradation of a vapour compression system, the vapour compression system comprising a compressor, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path, the method comprising the steps of: providing a performance baseline, the performance baseline specifying corresponding values of mean duration of OFF periods of the compressor and mean running compressor capacity,

[0010] - while operating the vapour compression system, measuring durations of OFF periods of the compressor and running compressor capacity,

[0011] - comparing the measured durations of OFF periods of the compressor and running compressor capacity to the performance baseline, and

[0012] - determining whether or not performance degradation of the vapour compression system is occurring, based on the comparison.

[0013] Thus, the invention provides a method for detecting performance degradation of a vapour compression system. As described above, the vapour compression system comprises a compressor, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path. Accordingly, refrigerant flowing in the refrigerant path is alternating ly compressed by the compressor and expanded in the expansion device, while heat exchange takes place in the heat rejecting heat exchanger and the evaporator, respectively.

[0014] In the present context the term 'performance degradation' should be interpreted to mean a state in which the vapour compression system operates in a non-optimal manner, under the given operating conditions, such as ambient temperature, cooling / heating load, etc. Thus, when the vapour compression system experiences performance degradation it performs worse than expected. This could, e.g., include increased energy consumption and / or that certain cooling or heating requirements, such as maintaining a certain temperature in a refrigerated or heated volume, are not met.

[0015] Performance degradation may, e.g., be caused by malfunction or degradation of one or more components of the vapour compression system, e.g. the compressor, the evaporator, the heat rejecting heat exchanger, one or more fans driving a secondary fluid flow across the evaporator or the heat rejecting heat exchanger, or any other relevant component of the vapour compression system.

[0016] In the case that the vapour compression system is a refrigeration system, the evaporator is arranged in thermal contact with a refrigerated volume, and the heat exchange taking place in the evaporator provides cooling of the refrigerated volume. For instance, the refrigerated volume may be a display case or a cooling or freezing compartment, configured to hold goods that need to be stored at a specified temperature or within a specified temperature interval. Accordingly, the vapour compression system is operated in such a manner that the temperature inside the refrigerated volume is maintained within the specified temperature interval. This could include monitoring the temperature inside the refrigerated volume, and in the case that the temperature increases to a level which exceeds an upper threshold value, e.g. referred to as a cut-in temperature, starting the compressor so as to cause refrigerant to flow in the refrigerant path and provide cooling to the refrigerated volume via the evaporator. This will cause the temperature in the refrigerated volume to decrease, and when the temperature reaches a lower threshold value, e.g. referred to as a cut-out temperature, the compressor is stopped, thus stopping the refrigerant flow and causing the temperature in the refrigerated volume to increase again.

[0017] The vapour compression system is preferably of a kind in which only one evaporator is connected to the compressor, thus allowing for the refrigerant flow through the evaporator, and thus the temperature inside the refrigerated volume, to be controlled by starting and stopping the compressor as described above. Such a vapour compression system is sometimes referred to as a 'one-to-one' vapour compression system. In the method according to the invention, a performance baseline is initially provided. In the present context the term 'performance baseline' should be interpreted to mean a set of parameters or a state of the vapour compression system which can be expected to occur when the vapour compression system is operating in an optimal, non-degraded manner.

[0018] The performance baseline may be generated by the vapour compression system, either during operation or initially, such as before the vapour compression system is installed at the operating site. This will be described in further detail below. As an alternative, the performance baseline may be generated without participation of the vapour compression system, e.g. in the form of a standard performance baseline which may be applicable to multiple vapour compression systems.

[0019] In any event, the performance baseline specifies corresponding values of mean durations of OFF periods of the compressor and mean running compressor capacity. In the present context the term 'OFF periods of the compressor' should be interpreted to mean the periods where the compressor has been stopped while the temperature inside the refrigerated volume increases from the cut-out temperature to the cut-in temperature. The mean duration of the OFF period reflects how fast the temperature in the refrigerated volume increases when no cooling is provided via the evaporator.

[0020] In the present context the term 'mean running compressor capacity' should be interpreted to mean a mean capacity at which the compressor is running during a time period which includes several ON / OFF periods. For instance, in the case that the compressor is a variable speed compressor, the mean running compressor capacity may reflect the mean speed of the compressor during its ON periods and over several cycles between the cut-out temperature and the cut-in temperature. In the case that the compressor is a fixed speed compressor, the mean running compressor capacity may reflect the duty cycle of the compressor during its ON periods and over several cycles between the cut-out temperature and the cut-in temperature. In any event, the mean running compressor capacity reflects the work that needs to be performed by the compressor in order to provide the required compression and refrigerant flow to the evaporator to ensure that the cooling demand is met.

[0021] Thus, the performance baseline provides a set of corresponding values reflecting how fast the temperature in the refrigerated volume increases when no cooling is provided via the evaporator, in the form of the mean duration of OFF periods, and the work that needs to be performed by the compressor, in the form of the mean running compressor capacity, as expected when the vapour compression system is operating in an optimal, non-degraded manner. In the present context the term 'corresponding values' should be interpreted to mean values which occur simultaneously or within the same time period, i.e. temporally coinciding, and thus under the same operating conditions, such as ambient conditions, and state of the vapour compression system. Thus, the performance baseline may be regarded as specifying a mean running compressor capacity which is expected when a given mean duration of OFF periods of the compressor is occurring. In other words, the corresponding values of mean duration of OFF periods and mean compressor capacity defined by the performance baseline represents sets of such corresponding values, under certain not- specified operating conditions, but given that the vapour compression system operates in an optimal manner under these operating conditions.

[0022] Next, while operating the vapour compression system, durations of OFF periods of the compressor and running compressor capacity are measured, and the measured durations of OFF periods and running compressor capacity are compared to the performance baseline. The measured durations of OFF periods of the compressor and running compressor capacity are also corresponding, i.e. temporally coinciding, values in the sense of the definition provided above with regard to the performance baseline, since they are measured within the same time period, and thus reflect operating conditions, such as ambient conditions, and state of the vapour compression system during that time period.

[0023] Thus, the present state of the vapour compression system, in the form of the corresponding values of the duration of the OFF periods and the running compressor capacity, is directly compared to the values expected under optimal, non-degraded operation of the vapour compression system.

[0024] As described above, the values being measured and compared to the performance baseline reflect, on the one hand, how fast the temperature in the refrigerated volume increases when no cooling is provided via the evaporator and, on the other hand, the work that needs to be performed by the compressor in order to meet the cooling demand. The duration of the OFF periods provides an indication of the ambient temperature as well as of the changes in cooling load. For instance, when the ambient temperature is high, the temperature inside the refrigerated volume will increase faster than when the ambient temperature is low, thus resulting in shorter duration of the OFF periods. Similarly, if the cooling load increases, e.g. because new goods have been added to the refrigerated volume and / or a door or a cover has been opened for a certain period of time or multiple times, this will also result in shorter duration of the OFF periods.

[0025] Accordingly, by comparing values of duration of OFF periods as well as running compressor capacity, obtained during the same time period, to the values defined by the performance baseline, it is ensured that the impact on the operation of the compressor, notably the power consumption of the compressor, which is due to various ambient factors or circumstances, is reflected in the comparison. Thus, it is possible to distinguish to what extent a possible increase in power consumption of the compressor is due to performance degradation of the vapour compression system, and to what extent it is due to other factors or circumstances, such as an increase in ambient temperature and / or adding of new goods to the refrigerated volume.

[0026] Finally, it is determined whether or not performance degradation of the vapour compression system is occurring, based on the comparison. For instance, it may be determined that a performance degradation is occurring if there is a deviation between the set of measured values and the set of values specified by the performance baseline. In particular, performance degradation is likely occurring if the measured running compressor capacity is higher than a mean running compressor capacity specified by a performance baseline which further specifies a mean duration of OFF periods of the compressor which is essentially equal to the measured duration of OFF periods of the compressor.

[0027] Since the method according to the invention allows for distinguishing between increased power consumption caused by ambient factors or circumstances and by performance degradation, for the reasons set forth above, performance degradation can be detected accurately, reliably, and in an easy manner.

[0028] The performance baseline may further specify an upper threshold running compressor capacity corresponding to the mean duration of OFF periods of the compressor, and the step of comparing may comprise comparing the measured running compressor capacity to the upper threshold running compressor capacity.

[0029] According to this embodiment, in addition to specifying mean running compressor capacity, the performance baseline also specifies an upper threshold running compressor capacity, corresponding to the mean duration of OFF periods of the compressor which is also specified by the performance baseline. Thus, in this case, the performance baseline species an upper limit which the running compressor capacity should not be allowed to exceed when the duration of OFF periods of the compressor is at a specified level. When performing the comparing step, the measured running compressor capacity is compared to the upper threshold running compressor capacity, thus revealing whether or not the measured running compressor capacity is approaching, or even above, the upper threshold running compressor capacity.

[0030] The upper threshold running compressor capacity may, e.g., be selected such that a running compressor capacity above the upper threshold running compressor capacity is a clear sign of performance degradation of the vapour compression system. Thus, the step of determining whether or not performance degradation of the vapour compression system is occurring may comprise determining that a performance degradation of the vapour compression system is occurring if the measured running compressor capacity exceeds the upper threshold running compressor capacity.

[0031] The step of providing a performance baseline may comprise the steps of:

[0032] - while operating the vapour compression system for a predetermined period of time, measuring duration of OFF periods of the compressor and running compressor capacity, thus obtaining temporally coinciding measurements of durations of OFF periods of the compressor and running compressor capacity,

[0033] - calculating a mean duration of the OFF periods of the compressor, associated with the predetermined period of time, based on the measured durations of OFF periods of the compressor,

[0034] - calculating a mean running compressor capacity, associated with the predetermined period of time, based on the measured running compressor capacity, and

[0035] - deriving the performance baseline as a combination of the calculated mean duration of the OFF periods of the compressor and the calculated mean running compressor capacity.

[0036] According to this embodiment, the performance baseline is generated by means of the vapour compression system, and it is, thus, customized to the vapour compression system, and possibly also to the ambient conditions prevailing at the site where the vapour compression system is positioned. More particularly, the performance baseline is generated in the following manner.

[0037] The vapour compression system is operated for a predetermined period of time. The predefined period of time is preferably a relatively long period of time, covering several cycles of the temperature inside the refrigerated volume reaching the cut-in temperature and the cut-out temperature, and thus covering several ON / OFF cycles of the compressor. The predetermined period of time could, e.g., be at least one hour, such as several hours, such as at least 24 hours, or even several days.

[0038] While operating the vapour compression system, duration of OFF periods of the compressor and running compressor capacity are measured. Thereby temporally coinciding, and thus corresponding, measurements of durations of OFF periods and running compressor capacity are obtained, during the predetermined period of time.

[0039] Next, a mean duration of the OFF periods of the compressor, associated with the predetermined period of time, is calculated, based on the measured durations of OFF periods of the compressor. Accordingly, a representative value for the duration of OFF periods of the compressor, and thus for how fast the temperature in the refrigerated volume increases when no cooling is provided via the evaporator, under the operating conditions prevailing during the predetermined period of time, is obtained.

[0040] Similarly, a mean running compressor capacity, associated with the predetermined period of time, is calculated, based on the measured running compressor capacity. Accordingly, a representative value for the running compressor capacity, and thus for the work that needs to be performed by the compressor in order to meet the cooling demand, under the operating conditions prevailing during the predetermined period of time, is obtained.

[0041] Accordingly, a set of representative values of duration of OFF periods of the compressor and running compressor capacity is obtained, associated with the predetermined period of time, and thus temporally coinciding and reflecting the same operating conditions. Since these representative values are in the form of mean values calculated from measurements obtained during a relatively long period of time, it is ensured that they reflect the operation of the vapour compression system under stable operating conditions, and that any short term variations or impact, e.g. caused by opening a door or a cover to the refrigerated volume or adding new goods to the refrigerated volume, are essentially disregarded. Thus, the obtained set of mean values may be regarded as representing the true performance of the vapour compression system.

[0042] Accordingly, the performance baseline is derived as a combination of the calculated mean duration of the OFF periods of the compressor and the calculated mean running compressor. In particular, if the measurements of the durations of the OFF periods and the running compressor capacity were obtained under conditions where it is known that the performance of the vapour compression system is not degraded, the performance baseline derived in this manner may be considered a suitable representation of non-degraded operation of the vapour compression system, and thus a suitable performance baseline.

[0043] The method may further comprise the step of calculating a variation of the measured durations of the OFF periods of the compressor, and the step of calculating a variation of the measured running compressor capacity, and the calculated variation of the measured durations of the OFF periods of the compressor and the calculated variation of the measured running compressor capacity may further form part of the derived performance baseline.

[0044] According to this embodiment, in addition to calculating the mean values of the duration of the OFF periods of the compressor and the running compressor capacity, the respective variations of these two parameters are also calculated. Furthermore, the calculated variations are also included in the performance baseline. Thus, if one or both of the measured values varies significantly during the predetermined period of time, this will be reflected in the derived performance baseline, in the form of a large variance. When subsequently comparing measured values to the performance baseline, a certain deviation from the mean values specified by the performance baseline may be allowed without concluding that performance degradation is occurring. On the other hand, if the calculated variances specified by the performance baseline are small, such deviations may not be allowed, and it may instead be concluded that performance degradation is occurring.

[0045] The calculated variations may, e.g., be in the form of calculated standard deviations. In this case the respective variations may be calculated as: where S is the standard deviation, n is the number of measurements, Xi are measurement values, and x is the mean of the measurement values.

[0046] The method may further comprise the step of calculating an upper threshold running compressor capacity, based on the calculated mean running compressor capacity and the calculated variation of the measured running compressor capacity, and the calculated upper threshold running compressor capacity may further form part of the derived performance baseline. For instance, the upper threshold running compressor capacity may be the sum of the mean running compressor capacity and the variation of the running compressor capacity. In this case the upper threshold running compressor capacity represents an uppermost boundary of a range within which the running compressor capacity can be expected if performance degradation is not occurring.

[0047] According to this embodiment, when comparing the measured durations of OFF periods of the compressor and the measured running compressor capacity to the performance baseline, this may include comparing the measured running compressor capacity to the upper threshold running compressor capacity, having been calculated as described above. This has already been described in detail above. The method may further comprise the step of identifying outliers with regard to measured durations of OFF periods of the compressor and / or measured running compressor capacity, and the identified outliers may be omitted when calculating the mean duration of OFF periods of the compressor and when calculating the mean running compressor capacity.

[0048] According to this embodiment, outliers in the measurements performed during the predetermined period of time are identified and discarded before the mean values of the duration of the OFF periods of the compressor and the running compressor capacity are calculated. Such outliers may be regarded as non-representative for the general operation the vapour compression system under stable operating conditions, and they may likely be due to abnormal or unusual circumstances. Thus, by omitting such outliers from the calculation of the mean values it is ensured that the resulting performance baseline truly reflects non-degraded performance behaviour of the vapour compression system under stable operating conditions, thus resulting in an appropriate and suitable performance baseline.

[0049] The deriving of the performance baseline as described above may be performed after the vapour compression system has been installed at the site where it is supposed to operate, possibly while the vapour compression system is in fact operating under real operating conditions. In this case the performance baseline may be derived at any time after installing the vapour compression system, e.g. immediately after installation and / or after the vapour compression system has been operating for a while, possibly repeatedly. Deriving the performance baseline after installation of the vapour compression system ensures that the real operating conditions of the vapour compression system are taken into account in the performance baseline.

[0050] As an alternative, the performance baseline may be derived while the vapour compression system is arranged at a manufacturing site, a storage facility, a test site or similar. In this case the real operating conditions of the vapour compression system will not be taken into account. However, the performance baseline may, in this case, be derived under well-defined and controllable conditions.

[0051] As an alternative to deriving the performance baseline in the manner described above, the performance baseline may be provided in any other manner, e.g. in the form of a generic or standard performance baseline which may apply to several vapour compression systems of similar type.

[0052] The method may further comprise the step of, in the case that the step of comparing reveals that the measured durations of OFF periods of the compressor deviates by at least a predefined amount from the mean duration of OFF periods of the compressor specified by the performance baseline, obtaining a new performance baseline and subsequently applying the new performance baseline during the step of comparing.

[0053] In the case that the measured durations of OFF periods of the compressor deviates significantly from the mean duration of OFF periods of the compressor specified by the performance baseline, this is an indication that the temperature in the refrigerated volume increases faster or slower than what is expected under the operating conditions underlying the performance baseline. This could, e.g., be due to a change in the average ambient temperature, e.g. due to seasonal changes. Thus, when this occurs, it is an indication that the performance baseline no longer corresponds to the operating condition which the vapour compression system is experiencing. Therefore a new performance baseline, which appropriately reflects the new operating conditions, e.g. including the expected ambient temperatures, is obtained, and the new performance baseline is subsequently applied. Thus, it is ensured that the performance baseline applied during the comparing step keeps reflecting the actual operating conditions experienced by the vapour compression system. The new performance baseline may, e.g., be obtained by deriving the performance baseline in the manner described above.

[0054] The method may further comprise the step of estimating a refrigeration load on the vapour compression system, based on the measured durations of OFF periods of the compressor. As described above, the durations of the OFF periods of the compressor represent how fast the temperature in the refrigerated volume increases when no cooling is provided via the evaporator. It can be expected that a high refrigeration load will result in a fast increase of the temperature, and thus in short durations of the OFF periods of the compressor. Similarly, a low refrigeration load can be expected to result in long durations of the OFF periods of the compressor. Thus, information regarding the refrigeration load on the vapour compression system can be derived from the durations of the OFF periods of the compressor.

[0055] The method may further comprise the step of deriving a performance index of the vapour compression system based on the comparison. In the present context the term 'performance index' should be interpreted to mean a measure for the performance of the vapour compression system. For instance, the performance index may be in the form of a percentage or a number between 0 and 1, where 100% or 1 indicates no performance degradation and 0% or 0 indicates full performance degradation. According to this embodiment, it may be determined that performance degradation is occurring if the performance index is below a specified level. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The invention will now be described in further detail with reference to the accompanying drawings in which

[0057] Fig. 1 is a diagrammatic view of a vapour compression system adapted for performing a method according to an embodiment of the invention,

[0058] Fig. 2 is a flow diagram illustrating a method according to an embodiment of the invention,

[0059] Fig. 3 shows graphs illustrating temperature inside a refrigerated volume and running compressor capacity of a vapour compression system as a function of time,

[0060] Fig. 4 illustrates deriving of a performance baseline as part of a method according to an embodiment of the invention,

[0061] Fig. 5 illustrates the performance baseline derived in Fig. 4, and

[0062] Fig. 6 illustrates detection of performance degradation of a vapour compression system in accordance with a method according to an embodiment of the invention.

[0063] DETAILED DESCRIPTION OF THE DRAWINGS

[0064] Fig. 1 is a diagrammatic view of a vapour compression system 1 according to an embodiment of the invention. The vapour compression system 1 comprises a compressor 2, a heat rejecting heat exchanger 3, and expansion device 4 and an evaporator 5 arranged in a refrigerant path. Refrigerant flowing in the refrigerant path is compressed by the compressor 2 before being supplied to the heat rejecting heat exchanger 3. In the heat rejecting heat exchanger 3, heat exchange takes place between the refrigerant and a secondary fluid flow across the heat rejecting heat exchanger 3, in such a manner that heat is rejected from the refrigerant. The secondary fluid flow is driven by means of a fan 6.

[0065] The refrigerant leaving the heat rejecting heat exchanger 3 is supplied to the expansion device 4, where it undergoes expansion before being supplied to the evaporator 5. The refrigerant being supplied to the evaporator 5 is, thus, a mixture of liquid and gaseous refrigerant. In the evaporator 5, the liquid part of the refrigerant is at least partly evaporated while heat exchange takes place with the ambient or a secondary fluid flow across the evaporator 5, in such a manner that heat is absorbed by the refrigerant. Finally, the refrigerant is once again supplied to the compressor 2.

[0066] The evaporator 5 is arranged in thermal contact with a refrigerated volume 7, and the heat exchange taking place in the evaporator 5, thus, provides cooling to the refrigerated volume 7. In order to maintain the temperature inside the refrigerated volume 7 within a specified temperature range defined by a lower boundary, in the form of a cut-out temperature, and an upper boundary, in the form of a cut-in temperature, the temperature inside the refrigerated volume 7 is monitored. When the temperature exceeds the cut-in temperature, the compressor 2 is switched on, so as to drive the refrigerant flow in the refrigerant path and provide a supply of refrigerant to the evaporator 5, via the expansion device 4. Thus, cooling is provided to the refrigerated volume 7, and the temperature inside the refrigerated volume decreases.

[0067] When the temperature inside the refrigerated volume reaches the cut-out temperature, the compressor 2 is switched off, thus stopping the supply of refrigerant to the evaporator 5, and the temperature inside the refrigerated volume 7 starts increasing again. Thus, the temperature inside the refrigerated volume 7 cycles between the cut-out temperature and the cut-in temperature, due to the sequential switching on and off of the compressor 2.

[0068] During operation of the vapour compression system 1, the durations of the OFF periods of the compressor 2, i.e. the durations of the periods from when the compressor 2 is switched off because the temperature inside the refrigerated volume 7 has reached the cut-out temperature until the compressor 2 is switched on again because the temperature inside the refrigerated volume 7 has reached the cut-in temperature, are measured. Furthermore, the running capacity of the compressor 2 is measured. The measured durations of the OFF periods of the compressor 2 and running capacity of the compressor 2 are compared to a performance baseline specifying corresponding or correlated values of mean duration of OFF periods of the compressor 2 and mean running compressor capacity under conditions where the performance of the vapour compression system 1 is not degraded. Based on this comparison, it is determined whether or not performance degradation of the vapour compression system 1 is occurring. This will be described in further detail below with reference to Figs. 2-6.

[0069] Fig. 2 is a flow chart illustrating a method according to an embodiment of the invention. At step 8 a unit, i.e. a vapour compression system, is activated. At step 9, data is collected by measuring temporally coinciding values of duration of OFF periods of the compressor and running compressor capacity, while the vapour compression system operates. This continues for a predetermined period of time of approximately one day.

[0070] At step 10, the mean duration of the OFF periods of the compressor as well as the variation of the duration of the OFF periods of the compressor, in the form of the standard deviation, are calculated based on the measured data. In addition, the mean running compressor capacity is calculated based on the measured data. A variation, e.g. the standard deviation, of the running compressor capacity may further be calculated.

[0071] At step 11 it is investigated whether or not the calculated mean duration of OFF periods of the compressor obtained at step 10 agrees with previously obtained mean durations of OFF periods of the compressor. More particularly, it is investigated whether or not the newly calculated mean duration of OFF periods of the compressor is within a range defined by the mean value ± the standard deviation of the durations of OFF periods of the compressor obtained for any previous measurement period. If this is the case, then it is concluded that the currently occurring ambient conditions correspond to the ambient conditions occurring during the previous measurement periods, and that the previously obtained mean durations of OFF periods of the compressor and mean running compressor capacity form a suitable performance baseline for the newly obtained measurements.

[0072] Thus, when this is the case, the process is forwarded to step 12, where the performance of the vapour compression system is evaluated, based on a comparison between the mean running compressor capacity obtained at step 10 and an upper threshold running compressor capacity corresponding to the mean durations of OFF periods of the compressor obtained during the previous measurement periods. For instance, if the mean running compressor capacity exceeds the upper threshold running compressor capacity, it may be determined that performance degradation of the vapour compression system is occurring, whereas it may be determined that performance degradation is not occurring, if the upper threshold running compressor capacity is not exceeded. Subsequently, the process is returned to step 9 to start a new measurement period.

[0073] In the case that step 11 reveals that the calculated mean duration of OFF periods of the compressor differs from mean durations of OFF periods of the compressor obtained during previous measurement periods by more than a standard deviation, it may be concluded that the ambient conditions have changed since the previous data was obtained. The process is then forwarded to step 13. At step 13, it is investigated whether the data collected at step 9 is aligned with data collected for the purpose of training and generating a performance baseline. This allows for identification of outliers with regard to measured durations of OFF periods of the compressor and / or measured running compressor capacity, so that such outliers may be omitted when deriving the performance baseline. This will be described in further detail below with reference to Fig. 4.

[0074] If step 13 reveals that the newly obtained data is not aligned with the previously obtained data, the process is returned to step 9 in order to obtain further data, i.e. the newly obtained data is discarded. In the case that step 13 reveals that the newly obtained data is aligned with the previously obtained data, then the process is forwarded to step 14, where the data obtained at step 9 and the calculations performed at step 10 are added for training a new region, i.e. for obtaining a new performance baseline related to the currently occurring ambient conditions.

[0075] At step 15 it is investigated whether or not sufficient data is available for deriving a suitable performance baseline related to the currently occurring ambient conditions. If this is not the case, the process is returned to step 9 in order to obtain further data.

[0076] In the case that step 15 reveals that sufficient data is available, the process is forwarded to step 16, where a performance baseline is derived. This includes calculating, based on the available data, a mean duration of OFF periods of the compressor, a standard deviation of the duration of OFF periods of the compressor, and a mean running compressor capacity. Finally, the process is returned to step 9.

[0077] Fig. 3 shows two graphs. The upper graph illustrates temperature 17 inside a refrigerated volume of a vapour compression system as a function of time. The lower graph illustrates running compressor capacity 18 of a compressor of the vapour compression system as a function of time.

[0078] It can be seen that the temperature 17 inside the refrigerated volume is maintained within a temperature range delimited by a lower boundary, in the form of a cut-out temperature 19, and an upper boundary, in the form of a cut-in temperature 20, with small undershoots / overshoots. This is obtained in the following manner.

[0079] When the temperature 17 inside the refrigerated volume reaches the cut-in temperature 20, the compressor is started so as to drive a refrigerant flow and ensure a refrigerant supply to an evaporator arranged in thermal contact with the refrigerated volume, thus providing cooling to the refrigerated volume. It can be seen from the lower graph of Fig. 3 that the running compressor capacity 18 is switched from OFF to ON when the temperature 17 inside the refrigerated volume reaches the cut-in temperature 20. As a consequence of the cooling provided to the refrigerated volume via the evaporator, the temperature 17 inside the refrigerated volume decreases until it reaches the cut-out temperature 19. When this happens, the compressor is stopped, i.e. the running compressor capacity 18 is switched from ON to OFF, thus discontinuing the refrigerant supply to the evaporator and causing the temperature 17 inside the refrigerated volume to increase again, until it, once again, reaches the cut-in temperature 20.

[0080] The time periods where the compressor is switched off, i.e. the time periods elapsing from the temperature 17 inside the refrigerated volume reaches the cut-out temperature 19 and the running compressor capacity 18 is switched from ON to OFF, and until the temperature 17 inside the refrigerated volume reaches the cut-in temperature 20 and the running compressor capacity 18 is switched from OFF to ON, may be referred to as OFF periods of the compressor. The duration of the OFF periods of the compressor reflects how fast the temperature 17 inside the refrigerated volume increases when no cooling is provided via the evaporator. Ambient conditions, such as ambient temperature have an impact on this. Furthermore, if goods have been added to the refrigerated volume and / or a door or a cover of the refrigerated volume has been open for an extended time, this will also impact the duration of the OFF periods of the compressor.

[0081] From the ON / OFF cycles of the running compressor capacity 18, a mean running compressor capacity can be derived. This reflects the work required by the compressor in order to meet the cooling demand of the vapour compression system. If the work required by the compressor, and thus the power consumption of the compressor, increases, this may be an indication that performance degradation of the vapour compression system is occurring. However, ambient conditions, notably ambient temperature, and increases in cooling load arising from new goods added to the refrigerated volume and / or an open door or cover, may also have an impact on the power consumption of the compressor.

[0082] In order to allow for accurately determining whether or not performance degradation of the vapour compression system is occurring, the durations of the OFF periods of the compressor and the running compressor capacity 18 are measured, e.g. by deriving them from the graphs of Fig. 3. The durations of the OFF periods of the compressor and the running compressor capacity 18 are then compared to a performance baseline specifying corresponding values of mean duration of OFF periods of the compressor and mean running compressor capacity, under conditions without performance degradation. Since the duration of the OFF periods of the compressor as well as the running compressor capacity 18 are compared to the performance baseline, it is ensured that the impact on the running compressor capacity 18 caused by performance degradation is distinguishable from the impact caused by ambient conditions and changes in cooling load. Fig. 4 illustrates deriving of a performance baseline as part of a method according to an embodiment of the invention. Data is collected during nine separate measurement time periods, each of approximately one day's duration. The data collected includes durations of OFF periods of the compressor and running compressor capacity. For each measurement period, a mean duration of OFF periods of the compressor, a standard deviation for the duration of OFF periods of the compressor, a mean running compressor capacity and a standard deviation of the running compressor capacity are calculated. These values are shown in the table of Fig. 4.

[0083] The values obtained for the respective measurement periods are compared among each other in order to establish whether or not they are sufficiently aligned to be combined into a single performance baseline. This reveals that the mean running compressor capacity obtained during measurement period 3 is significantly higher than the mean running compressor capacity obtained during the other measurement periods, and there is further no overlap between the standard deviation ranges. Therefore measurement period 3 is considered an outlier.

[0084] Furthermore, the mean duration of OFF periods of the compressor obtained during measurement period 6 is significantly higher than the mean duration of OFF periods of the compressor obtained during the other measurement periods, and with no overlap between the standard deviation ranges. Therefore measurement period 6 is also considered and outlier.

[0085] The outlier measurement periods (measurement periods 3 and 6) are omitted and based on the data of the remaining measurement periods, a performance baseline denoted 'Training Result' is derived.

[0086] Fig. 5 illustrates the performance baseline shown in Fig. 4. This performance baseline is indicated as being related to 'Learned Region #1', reflecting that it relates to a certain set of ambient conditions. The row related to 'Learned Region #2' is empty, but could be populated in a similar manner at a later point in time where the ambient conditions have changed, e.g. due to a change in season.

[0087] Fig. 6 illustrates detection of performance degradation of a vapour compression system in accordance with a method according to an embodiment of the invention. For five separate measurement time periods, i.e. measurement periods 10-14, a mean duration of OFF periods of the compressor and a mean running compressor capacity are obtained and compared to the performance baseline of Fig. 5. For measurement periods 10, 11 and 13, the mean duration of OFF periods of the compressor as well as the mean running compressor capacity are within the standard deviation ranges of the duration of OFF periods of the compressor and mean running compressor capacity, respectively, specified by the performance baseline. Accordingly, for these measurement periods it is concluded that no performance degradation is occurring, and that a performance index is therefore 100%.

[0088] For measurement period 12, the mean duration of OFF periods of the compressor deviates from the mean duration of OFF periods of the compressor specified by the performance baseline by more than the standard deviation. It is therefore concluded that the ambient conditions occurring during this measurement period are not comparable to the ambient conditions related to the performance baseline, and that the performance baseline is therefore not suitable for this measurement period. Accordingly, a performance index is not derived for measurement period 12, i.e. it can not be established whether or not performance degradation is occurring.

[0089] For measurement period 14, the mean duration of OFF periods of the compressor is within the standard deviation range of the duration of OFF periods of the compressor specified by the performance baseline, and the performance baseline is therefore considered suitable for this measurement period. However, the mean running compressor capacity is significantly higher than the mean running compressor capacity specified by the performance baseline. It is therefore concluded that performance degradation of the vapour compression system is occu rring.

[0090] Moreover, the performance index for measurement period 14 is calculated as: where CompCap24h is the mean running compressor capacity during measurement period 14 and CompCapref is the mean running compressor capacity specified by the performance baseline. Accordingly, the calculated performance index indicates that the performance of the vapour compression system is 61% of optimal performance. This may be regarded as sufficient performance degradation to trigger actions for restoring the performance of the vapour compression system.

Claims

CLAIMS1. A method for detecting performance degradation of a vapour compression system (1), the vapour compression system (1) comprising a compressor (2), a heat rejecting heat exchanger (3), an expansion device (4) and an evaporator (5) arranged in a refrigerant path, the method comprising the steps of: providing a performance baseline, the performance baseline specifying corresponding values of mean duration of OFF periods of the compressor (2) and mean running compressor capacity,- while operating the vapour compression system (1), measuring durations of OFF periods of the compressor (2) and running compressor capacity (18),- comparing the measured durations of OFF periods of the compressor (2) and running compressor capacity (18) to the performance baseline, and- determining whether or not performance degradation of the vapour compression system (1) is occurring, based on the comparison.

2. A method according to claim 1, wherein the performance baseline further specifies an upper threshold running compressor capacity corresponding to the mean duration of OFF periods of the compressor (2), and wherein the step of comparing comprises comparing the measured running compressor capacity (18) to the upper threshold running compressor capacity.

3. A method according to claim 2, wherein the step of determining whether or not performance degradation of the vapour compression system (1) is occurring comprises determining that a performance degradation of the vapour compression system (1) is occurring if the measured running compressor capacity (18) exceeds the upper threshold running compressor capacity.

4. A method according to any of the preceding claims, wherein the step of providing a performance baseline comprises the steps of: while operating the vapour compression system (1) for a predetermined period of time, measuring duration of OFF periods of the compressor (2) and runningcompressor capacity (18), thus obtaining temporally coinciding measurements of durations of OFF periods of the compressor and running compressor capacity (18),- calculating a mean duration of the OFF periods of the compressor (2), associated with the predetermined period of time, based on the measured durations of OFF periods of the compressor (2),- calculating a mean running compressor capacity, associated with the predetermined period of time, based on the measured running compressor capacity (18), and- deriving the performance baseline as a combination of the calculated mean duration of the OFF periods of the compressor and the calculated mean running compressor capacity.

5. A method according to claim 4, further comprising the step of calculating a variation of the measured durations of the OFF periods of the compressor (2), and the step of calculating a variation of the measured running compressor capacity (18), and wherein the calculated variation of the measured durations of the OFF periods of the compressor (2) and the calculated variation of the measured running compressor capacity (18) further form part of the derived performance baseline.

6. A method according to claim 5, further comprising the step of calculating an upper threshold running compressor capacity, based on the calculated mean running compressor capacity and the calculated variation of the measured running compressor capacity (18), and wherein the calculated upper threshold running compressor capacity further forms part of the derived performance baseline.

7. A method according to any of claims 4-6, further comprising the step of identifying outliers with regard to measured durations of OFF periods of the compressor (2) and / or measured running compressor capacity (18), and wherein the identified outliers are omitted when calculating the mean duration of OFF periods of the compressor (2) and when calculating the mean running compressor capacity.

8. A method according to any of the preceding claims, further comprising the step of, in the case that the step of comparing reveals that the measured durations of OFF periods of the compressor (2) deviates by at least a predefined amount from the mean duration of OFF periods of the compressor (2) specified by the performance baseline, obtaining a new performance baseline and subsequently applying the new performance baseline during the step of comparing.

9. A method according to any of the preceding claims, further comprising the step of estimating a refrigeration load on the vapour compression system (1), based on the measured durations of OFF periods of the compressor (2).

10. A method according to any of the preceding claims, further comprising the step of deriving a performance index of the vapour compression system (1) based on the comparison.

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