WO2015165937A1

WO2015165937A1 - A method for estimating and/or controlling a temperature of foodstuff stored in a refrigerated cavity

Info

Publication number: WO2015165937A1
Application number: PCT/EP2015/059292
Authority: WO
Inventors: Rasmus PEDERSEN; John SCHWENSEN; Torben Green; Benjamin BIEGEL; Jakob Stoustrup
Original assignee: Danfoss A/S
Priority date: 2014-05-01
Filing date: 2015-04-29
Publication date: 2015-11-05

Abstract

A method for estimating and/or controlling a temperature of foodstuff stored in a refrigerated cavity of a refrigeration system is disclosed. A time constant for the temperature of the foodstuff is established, said time constant being determined based on a time interval between a relatively large change of opening degree of an expansion valve and until variance of the opening degree of the expansion valve decreases to a relatively low variance. The time constant is applied to a low pass filter, said low pass filter providing a relation between the temperature of the foodstuff and the air temperature, and the low pass filter is applied to the monitored air temperature, thereby obtaining an estimate for the temperature of the foodstuff stored in the refrigerated cavity.

Description

A METHOD FOR ESTIMATING AND/OR CONTROLLING A TEMPERATURE OF FOODSTUFF STORED IN A REFRIGERATED CAVITY

FIELD OF THE INVENTION

The present invention relates to a method for estimating and/or controlling a temperature of foodstuff stored in a refrigerated cavity, such as a cooling or freezing compartment, a display case or a refrigerated storage room, e.g., of the kind used in a supermarket

BACKGROUND OF THE INVENTION

When goods, such as foodstuff, are stored in a refrigerated cavity, such as a display case in a supermarket, it is desirable to control the temperature inside the refrigerated cavity in such a manner that the goods are stored at a temperature which is within an acceptable

temperature range. To this end the air temperature inside the refrigerated cavity is normally measured, and a refrigeration system is controlled in such a manner that the air temperature is maintained within an acceptable temperature range. However, the air temperature inside the refrigerated cavity is not necessarily the same as the temperature of the stored goods. Therefore, the temperature of the stored goods may very well be maintained within an acceptable temperature range, even if the air temperature inside the refrigerated cavity is allowed to exceed the boundaries of the acceptable temperature range for a short period of time. This may, e.g ., be used if it is desired to use the stored goods for energy storage, or if it is desired to implement cooling down of new goods being added to the refrigerated cavity in an energy efficient manner.

It is therefore desirable to operate the refrigeration system on the basis of the temperature of the stored goods, e.g . , a surface temperature of the stored goods, rather than on the basis of the air temperature inside the refrigerated cavity. However, it is difficult, and may even be impossible, to measure the temperature of the stored goods. EP 1 762 801 Al discloses a method for controlling the temperature inside a cavity of a cooling appliance provided with a temperature sensor inside the cavity and with actuator means, such as a compressor, a damper and/or a fan, for adjusting the cooling capacity of the appliance. The food temperature is estimated on the basis of the value from the temperature sensor and on a predetermined function of the status of the actuator means. The predetermined function includes a parameter which depends on the appliance and on the considered load condition, an average value of which must be found during the design phase of the appliance. Accordingly, the method disclosed in EP 1 762 801 Al is not readily applicable to any random system or appliance, but must be adapted to a specific system or appliance by determining a parameter, which is appropriate for that specific system or appliance.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for controlling a temperature in a refrigerated cavity, which ensures that goods, such as foodstuff, stored in the refrigerated cavity are maintained at a temperature within an acceptable temperature range, without the need to acquire knowledge regarding appliance or expected load conditions during a design phase.

It is a further object of embodiments of the invention to provide a method for controlling a temperature in a refrigerated cavity, which can readily be applied to any random system or appliance.

It is an even further object of embodiments of the invention to provide a method for estimating a temperature of goods, such as foodstuff, stored in a refrigerated cavity, without the need to acquire knowledge regarding appliance or expected load conditions during a design phase.

The invention provides a method for estimating and/or controlling a temperature of foodstuff stored in a refrigerated cavity of a refrigeration system, the refrigeration system comprising a compressor, a condenser, an expansion valve and an evaporator, arranged in a refrigerant path, the evaporator being arranged in thermal contact with the refrigerated cavity, and the expansion valve being provided in the refrigerant path upstream of the evaporator, the method comprising the steps of: monitoring a temperature of air inside the refrigerated cavity,

- monitoring an opening degree of the expansion valve,

- establishing a time constant for the temperature of the foodstuff, said time constant being determined based on a time interval between a relatively large change of opening degree of the expansion valve and until variance of the opening degree of the expansion valve decreases to a relatively low variance, and

- applying the time constant to a low pass filter, said low pass filter providing a relation between the temperature of the foodstuff and the air temperature, and applying the low pass filter to the monitored air temperature, thereby obtaining an estimate for the temperature of the foodstuff stored in the refrigerated cavity.

The method according to the invention is for estimating and/or controlling a temperature of foodstuff stored in a refrigerated cavity of a refrigeration system. Thus, the method of the invention may be used for estimating such a temperature, for controlling such a temperature, or for estimating such a temperature as well as controlling it.

In the present context the term "refrigeration system' should be interpreted to mean a system which is capable of providing cooling for a refrigerated cavity. The refrigeration system comprises a compressor, a condenser, an expansion valve and an evaporator arranged in a refrigerant path. The expansion valve is provided in the refrigerant path upstream of the evaporator, and thereby the supply of refrigerant to the evaporator is controlled by means of the expansion valve. Refrigerant flowing in the refrigerant path is alternatingly compressed by means of the compressor and expanded by means of the expansion valve. Heat exchange takes place in the condenser and the evaporator in such a manner that heat is rejected from the refrigerant flowing through the condenser, due to the refrigerant condensing, and heat is absorbed by the refrigerant flowing through the evaporator, due to the refrigerant evaporating.

The evaporator is arranged in thermal contact with the refrigerated cavity. Thereby cooling is provided to the refrigerated cavity by means of the evaporator. The method comprises monitoring a temperature of air inside the refrigerated cavity and monitoring an opening degree of the expansion valve. The temperature of air inside the refrigerated cavity may, e.g., be monitored by means of a temperature sensor arranged inside the refrigerated cavity.

The method further comprises establishing a time constant for the temperature of the foodstuff stored in the refrigerated cavity. The time constant is determined based on a time interval between a relatively large change of opening degree of the expansion valve and until variance of the opening degree of the expansion valve decreases to a relatively low variance. When a thermal equilibrium exists between the foodstuff stored in the refrigerated cavity and the air inside the refrigerated cavity, it is normally not necessary to perform significant changes to the supply of refrigerant to the evaporator. Therefore, under such circumstances, the variance of the opening degree of the expansion valve is normally at a low level. On the other hand, when changes occur which affects the thermal equilibrium between the stored foodstuff and the air inside the refrigerated cavity, or changes in the refrigeration load of the system, larger changes in the opening degree of the expansion valve may be required in order to re-establish the thermal equilibrium as quickly as possible. Thus, the time constant reflects, or is based on, how quickly the system responds to changes in the refrigeration load, e.g., in terms of how quickly the system is capable of restoring a thermal equilibrium following a change in refrigeration load. The time constant is applied to a low pass filter. The low pass filter provides a relation between the temperature of the foodstuff and the air temperature. Finally, the low pass filter is applied to the monitored air temperature, thereby obtaining an estimate for the

temperature of the foodstuff stored in the refrigerated cavity.

Accordingly, the temperature of the foodstuff is estimated on the basis of the monitored air temperature and the monitored opening degree of the expansion valve. This is an advantage, because the air temperature and the opening degree of the expansion valve are easy to monitor, and they are already monitored in existing refrigeration system for other purposes. This allows the temperature of the foodstuff to be estimated without the need for additional sensors. Furthermore, since the time constant applied to the low pass filter reflects the ability of the system, including the stored foodstuff, to react to changes in the refrigeration load, a reliable estimate for the temperature of the foodstuff is obtained. This allows the refrigeration system to be operated in a manner, which allows the air temperature to exceed the acceptable temperature limits, as long as it is ensured that the temperature of the foodstuff does not exceed the acceptable temperature limits. Thereby the refrigeration system may be operated in a more energy efficient manner, and the foodstuff may be used for energy storage.

In summary, the foodstuff temperature is estimated by applying a low pass filter to the monitored air temperature. The low pass filter is defined by a time constant, which is established on the basis of the monitored behaviour of the opening degree of the expansion valve. Since the dynamics of the opening degree closely follows the dynamics of the system, including thermal dynamics of the foodstuff, the time constant determined in this manner is also representative for the thermal dynamics of the foodstuff. Accordingly, the method can readily be applied to any system or appliance, irrespective of various system parameters or expected load conditions. The 'model' which correlates the monitored air temperature and the estimated foodstuff temperature, i.e. the low pass filter, is simply adapted to reflect conditions related to the system or appliance in question by determining the relevant time constant as described above. Thereby it is not necessary to determine any system specific parameters during the design phase of the system or appliance. According to one embodiment, the invention provides a method for estimating and/or controlling a temperature of foodstuff stored in a refrigerated cavity of a refrigeration system, the refrigeration system comprising a compressor, a condenser, an expansion valve and an evaporator, arranged in a refrigerant path, the evaporator being arranged in thermal contact with the refrigerated cavity, and the expansion valve being provided in the refrigerant path upstream of the evaporator, the method comprising the steps of: monitoring a temperature of air inside the refrigerated cavity,

- monitoring an opening degree of the expansion valve, establishing a time constant for the temperature of the foodstuff, said time constant reflecting the ability of the system to react to changes in refrigeration load, and

- applying the time constant to a low pass filter, said low pass filter providing a relation between the temperature of the foodstuff and the air temperature, and applying the low pass filter to the monitored air temperature, thereby obtaining an estimate for the temperature of the foodstuff stored in the refrigerated cavity. The method may further comprise the step of:

- controlling a temperature in the refrigerated cavity by controlling the opening degree of the expansion valve, based on the monitored air temperature and the monitored opening degree of the expansion valve, the applied low pass filter, as well as the established time constant. According to this embodiment, a temperature in the refrigerated cavity is controlled on the basis of the process described above. The controlled temperature may, e.g ., be the temperature of air inside the refrigerated cavity, or the temperature of the foodstuff stored in the refrigerated cavity.

Thus, the method may further comprise the step of controlling the temperature of the foodstuff stored in the refrigerated cavity, based on the monitored air temperature and the monitored opening degree of the expansion valve, and the step of controlling the

temperature of the foodstuff stored in the refrigerated cavity may be performed on the basis of the estimated temperature of the foodstuff stored in the refrigerated cavity. According to this embodiment, the temperature of the foodstuff is directly controlled. Thereby it can efficiently be ensured that the temperature of the foodstuff does not exceed the acceptable temperature limits, even if the air temperature inside the refrigerated cavity is allowed to exceed the acceptable temperature limits.

The step of establishing a time constant may be repeated after a time period has elapsed, thereby establishing a new time constant, and the step of applying a low pass filter to the monitored air temperature may subsequently be performed using the new time constant.

According to this embodiment the time constant, which is applied to the low pas filter, is determined in a dynamical manner, in the sense that a new time constant is established at a later point in time. Thereby the applied time constant will always reflect the current operating conditions, and therefore a very accurate estimate for the temperature of the foodstuff stored in the refrigerated cavity is obtained.

According to one embodiment the time period may be based on a selected magnitude of change in opening degree of the expansion valve. For instance, a new time constant may be established when a change in the opening degree of the expansion valve exceeds a certain threshold value. Such significant changes in the opening degree of the expansion valve may, e.g ., be occasioned by an abrupt change in the refrigeration load. Thus, according to this embodiment, a new time constant is established when there has been a significant change in the refrigeration load, and it must therefore be expected that the operating conditions have changed .

As an alternative, the time period may simply be a fixed time period. In this case a new time constant is established each time the fixed time period has elapsed .

The new time constant may be established based on the following formula :

T(i+ 1) = a_Tnew(i) + ( l -a)T(i) where τ(ί) is the previous time constant, T_ne„(i) is a time constant obtained by fitting a model to monitored data regarding the opening degree of the expansion valve, a is a tuning parameter reflecting an accuracy of the model, and τ(ϊ+1) is the new time constant to be applied to the low pass filter.

According to this embodiment, the time constant for the low pass filter is established in a dynamical and iterative manner, based on the time constant which was applied to the low pass filter the last time a time constant was established (τ(ί)), as well as based on a time constant which is obtained by means of a model (T_new(i)). The model may advantageously be a model which defines a relation between air temperature inside the refrigerated cavity and the opening degree of the expansion valve. For instance, T_ne„(i) may be obtained by applying measured data relating to the air temperature inside the refrigerated cavity and to the opening degree of the expansion valve to a suitable low pass filter. a is a tuning parameter which reflects an accuracy of the model, which is used for obtaining TnewCO- Thus, a provides a 'confidence level' for the model, or a measure for how reliable the model is. For instance, if a = l the new time constant, τ(ί+ 1), is equal to the model based time constant, T_new(i). In this case it is believed that the model is very reliable. On the other hand, in the case that a = 0 the new time constant, τ(ί+ 1), is equal to the previous time constant, τ(ϊ). In this case it is believed that the model is not very reliable, and the time constant provided by means of the model is therefore not taken into account at all.

When 0<α < 1, the new time constant, τ(ι^'+1), is a linear combination of the model based time constant, T_new(i), and the previous time constant, τ(ί), and the value of a determines how much weight is put on the model based time constant, T_ne„(i).

The new time constant may, e.g ., be established in the following manner. Correlated data relating to opening degree of the expansion valve and air temperature inside the refrigerated cavity are obtained and stored . The data is applied to a model, e.g ., in the form of a low pass filter, e.g., with a time constant corresponding to the previously established time constant, T(i). Based on this, the model based time constant, T_new(i), is established .

The tuning parameter, a, is then identified. The hypothesis is that a time constant relating to the opening degree of the expansion valve depends on a time constant related to the dynamics of the foodstuff stored in the refrigerated cavity, a should reflect how the applied model reflects the actual relationship between the currently prevailing air temperature and the opening degree.

Finally, the new time constant, τ(ϊ+1), is calculated, using the formula above. Since the new time constant, τ(ϊ+ 1), is established as a linear combination of the model based time constant, T_new(i), and the previous time constant, τ(ί), it is ensured that a model reflecting the present operating conditions is taken into account, on the one hand, while holding on to historical conditions, on the other hand . Thereby it is ensured that no sudden changes in the applied time constant occur, and robustness is added to the method. The step of estimating the temperature of the foodstuff may comprise applying a lower constraint on the time constant, said lower constraint having a value defined from foodstuff having a relatively low thermal mass compared to a thermal mass of other foodstuff stored in the same refrigerated cavity. According to this embodiment, the time constant is not allowed to be lower than a value which is dictated by the foodstuff having the lowest thermal mass. Sometimes various kinds of foodstuff are stored in the same refrigerated cavity. The thermal mass, and thereby the ability of the foodstuff to maintain a temperature when the ambient temperature changes, may vary from one kind of foodstuff to another. Therefore, when the air temperature inside a refrigerated cavity changes, this may have an insignificant impact on the temperature of some of the foodstuff items stored in the refrigerated cavity, i.e. the foodstuff items having a relatively high thermal mass, while it may have a significant impact on other foodstuff items stored in the refrigerated cavity, i .e. the foodstuff items having a relatively low thermal mass. Therefore, in order to ensure that none of the foodstuff items stored in the refrigerated cavity is allowed to reach a temperature which exceeds the acceptable temperature limits, a lower constraint is applied on the time constant.

The foodstuff having a relatively low thermal mass may be regarded as foodstuff items having a thermal mass which is lower than an average thermal mass, or lower than a representative thermal mass, of the foodstuff item stored in the refrigerated cavity. Or the foodstuff having a relatively low thermal mass may be regarded as the foodstuff with the fastest known dynamics.

Alternatively or additionally, the step of estimating the temperature of the foodstuff may com rise applying an upper constraint on the time constant, said upper constraint having a value defined from foodstuff having a relatively high thermal mass compared to a thermal mass of other foodstuff stored in the same refrigerated cavity. According to this embodiment, the time constant is not allowed to be higher than a value which is dictated by the foodstuff having the highest thermal mass.

The foodstuff having a relatively high thermal mass may be regarded as foodstuff items having a thermal mass which is higher than an average thermal mass, or higher than a representative thermal mass, of the foodstuff items stored in the refrigerated cavity. Or the foodstuff having a relatively high thermal mass may be regarded as the foodstuff with the slowest known dynamics.

The step of establishing the time constant may be performed at moments of time, where the relatively large change of opening degree of the expansion valve takes place due to changes of one or more operating parameters of the refrigeration system, said changes of one or more operating parameters being changes dependent on other changes of operation of the refrigeration system than change of time constant. 15 059292

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According to this embodiment, a new time constant is established each time changes in the ambient conditions or operating conditions occur, and the changes in ambient conditions or operating conditions require large changes in the opening degree of the expansion device in order to reestablish a thermal equilibrium in the refrigerated cavity. Such changes in the ambient conditions may, e.g ., include changes in the refrigeration load .

For instance, the changes of operating parameters may take place due to defrost of the refrigeration system being initiated. When defrost is initiated, the expansion valve is closed in order to remove ice which has formed on the evaporator. This constitutes a significant change in the opening degree of the expansion valve. Furthermore, the air temperature inside the refrigerated volume increases.

As an alternative, the changes of operating parameters may take place due to defrost of the refrigeration system being terminated. When defrost is terminated, the expansion valve is opened in order to restart the cooling of the refrigerated cavity. Often the expansion valve is opened fully, i.e. the highest possible opening degree is selected, in order to quickly reduce the air temperature inside the refrigerated cavity, which was increased during defrost. Thus, this also constitutes a significant change in the opening degree of the expansion valve.

Preferably, the changes of operating parameters may be occasioned by an entire defrost cycle, including initiation and termination of defrost, as described above.

As another alternative, the changes of operating parameters may take place due to opening of a door or hatch of the refrigerated cavity, said opening of the door or hatch resulting in air from outside entering the cavity, said air having a temperature which exceeds the air temperature inside the refrigerated cavity, thereby resulting in an increase of the

temperature inside the refrigerated cavity, and thereby in a sudden increased demand for cooling of the air in the refrigerated cavity. This sudden increased demand for cooling of the air in the refrigerated cavity constitutes an increased refrigeration load . In order to meet the increased refrigeration load, the supply of refrigerant to the evaporator must be increased, and therefore the opening degree of the expansion valve is increased by a large amount.

As yet another alternative, the changes of operating parameters may take place due to an amount of foodstuff being supplied to the refrigerated cavity, said foodstuff having a temperature which is higher than the air temperature inside the refrigerated cavity, said supply of the amount of foodstuff thereby resulting in a sudden increase of air temperature inside the refrigerated cavity, and thereby in a sudden increased demand for cooling of the air in the refrigerated cavity. Similarly to the situation described above, the sudden increased demand for cooling of the air in the refrigerated cavity constitutes an increased refrigeration load, which results in the opening degree of the expansion valve being increased by a large amount.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a block diagram of a system for estimating a temperature of foodstuff using a method according to an embodiment of the invention,

Fig. 2 is a graph illustrating correlation between opening degree and food temperature following a step in reference for the air temperature,

Fig. 3 is a graph illustrating a fit of foodstuff temperature estimation using a first order filter, and

Figs. 4a and 4b are graphs illustrating iterative update of a time constant of a low pass filter used for estimating temperature of foodstuff according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram of a system 1 for estimating a temperature of foodstuff using a method according to an embodiment of the invention. Foodstuff is arranged in a refrigerated cavity in the form of a display case 2, e.g., of the kind used in a supermarket. A temperature of air, Tair, inside the refrigerated cavity 2 is monitored and supplied to a unit 3, which calculates a control error, Err= T_air, _ref-T_air. The control error is supplied to an air temperature controller 4, which controls an opening degree, OD, of an expansion valve in accordance with the signal received from the unit 3. The opening degree of the expansion valve determines the supply of refrigerant to an evaporator arranged in thermal contact with the refrigerated cavity 2, and thereby the temperature of air inside the refrigerated cavity 2 is controlled in order to obtain an air temperature which is equal to the reference air temperature, Τ_{3|Γ( ref}.

The air temperature controller 4 supplies the opening degree and a control state to an estimator unit 5. The control state provides information regarding when a defrost cycle is initiated or terminated. Finally, the reference air temperature, T_air, ret, is supplied to the estimator unit 5. Based on the received information, the estimator unit 5 establishes or calculates a time constant, τ. The calculated time constant, τ, represents a time interval elapsing from a point in time where a relatively large change of the opening degree of the expansion valve takes place, until variance of the opening degree of the expansion valve decreases to a relatively low variance. As described above, the time constant is thereby representative for how quickly the refrigeration system reacts to significant changes in ambient conditions or operating conditions, such as significant changes in the refrigeration load,

The time constant, τ, is applied to a low pass filter 6. The low pass filter 6 provides a relation between the temperature of the foodstuff stored in the refrigerated cavity 2 and the air temperature, T_a,_r, inside the refrigerated cavity 2. Accordingly, based on the monitored air temperature, T_air, the low pass filter 6 is capable of estimating the temperature of the foodstuff stored in the refrigerated cavity 2,

Thus, the monitored air temperature, T_alr, is applied to the low pass filter 6, and based on the monitored air temperature, T_air, and using the time constant, τ, calculated on the basis of the monitored opening degree of the expansion valve, the low pass filter 6 provides an estimate for the temperature, T_f00d, of the foodstuff stored in the refrigerated cavity 2.

A new time constant, τ, may be established after a time period has been allowed to lapse. Thereby it is ensured that the time constant, τ, applied to the low pass filter 6, and used for estimating the temperature of the foodstuff, T_f00d, always reflects the currently prevailing operating conditions. Fig. 2 is a graph illustrating correlation between opening degree and food temperature following a step in reference for the air temperature. In the graph, opening degree (OD) of an expansion valve and various temperatures (T) are plotted as a function of time (t).

At time t, a reference temperature 7 of air temperature inside a refrigerated cavity is abruptly decreased. In response thereto the opening degree 8 of the expansion valve is increased significantly and abruptly. This results in a significant and abrupt increase in the supply of refrigerant to the evaporator, in order to increase the cooling effect of the evaporator and drive the air temperature inside the refrigerated cavity down to reach the new reference temperature 7.

The air temperature 9 of air inside the refrigerated cavity is quickly reduced and reaches the reference air temperature 7. When the air temperature 9 has reached a steady state, the opening degree 8 is slowly decreased. The foodstuff temperature 10, i.e. the temperature of foodstuff stored inside the refrigerated cavity, slowly decreases during this, until it reaches the air temperature 9 at time t₂. Similarly, at time t₃ the reference temperature 7 is abruptly increased, and the opening degree 8 is consequently decreased in order to decrease the refrigerant supply to the evaporator, and allowing the air temperature 9 inside the refrigerated cavity to increase. It can be seen that the air temperature 9 quickly reaches the steady state, and the opening degree 8 is slowly increased again. The foodstuff temperature 10 slowly increases.

It is clear from the graph of Fig. 2 that the dynamics of the foodstuff temperature 10 closely follows the dynamics of the opening degree 8 of the expansion valve. In particular, the time period elapsing from an abrupt and significant change in the opening degree 8 until the opening degree 8 is once again substantially constant, is substantially equal to the time period elapsing from the abrupt and significant change in the opening degree 8 until the foodstuff temperature 10 reaches a steady state, i .e. the foodstuff temperature 10 reaches the air temperature 9. This corresponds to the time period from t- to t₂.

This suggests that the foodstuff temperature 10 can be estimated by a simple first order low pass filter applied to the air temperature 9. Fig . 3 is a graph illustrating a fit of foodstuff temperature estimation using a first order filter. Opening degree 11 is monitored. Furthermore, air temperature inside a refrigerated cavity and a foodstuff temperature in the form of a surface temperature 12 of foodstuff stored in the refrigerated cavity are measured . It is normally not possible to measure the surface temperature of the foodstuff in a display case of a supermarket. However, the data underlying the graph of Fig. 3 is obtained from a test facility where it is possible to measure the surface temperature 12 of the foodstuff, in order to be able to test the ability of the model to predict the temperature of the foodstuff.

The data shown in Fig. 3 represents variations in opening degree and temperatures following defrost. An optimal time constant, τ, is found on the basis of the monitored opening degree data, as the best fit to the opening degree data. The derived time constant, τ, is applied to a low pass filter. Using the low pass filter, an estimated foodstuff temperature 14 is obtained.

It can be seen from Fig . 3 that the estimated foodstuff temperature 14 provides a relatively good estimate for the actual surface temperature 12 of the foodstuff. Figs. 4a and 4b are graphs illustrating iterative update of a time constant of a low pass filter used for estimating temperature of foodstuff according to an embodiment of the invention . Foodstuff temperature, in the form of a surface temperature 12 of foodstuff stored inside a refrigerated cavity, and estimated foodstuff temperature 14 are shown.

The time constant applied to the low pass filter which is used for estimating the foodstuff temperature 14 is updated iteratively each time an event occurs which is expected to significantly change the operating conditions. These events show up as spikes in the surface temperature 12. The time constant is updated using the following formula : τ(ϊ+1) = aTnew(i) + ( l-a)T(i) where τ(ί) is the previous time constant, T_new(i) is a time constant obtained by fitting a model to monitored data regarding the opening degree of the expansion valve, a is a tuning parameter reflecting an accuracy of the estimate, and τ(ϊ+1) is the new time constant to be applied to the low pass filter. Thereby the new time constant is calculated with due consideration to the applied model and the behaviour of the opening degree of the expansion valve, and with due consideration to the previously calculated time constant.

The iterative process is started by providing an initial guess for the foodstuff temperature. In Fig. 4a as well as in Fig. 4b the initial guess is that the foodstuff temperature is equal to the air temperature. Furthermore, an initial guess for the time constant, τ, is provided.

In Fig. 4a the initial guess for the time constant, τ, is close to the correct value of τ. This has the consequence that the estimated foodstuff temperature 14 closely follows the actual foodstuff temperature 12, already at the first iteration. However, the agreement between the estimated foodstuff temperature 14 and the actual foodstuff temperature 12 improves for each iteration, i.e. each time a significant event occurs, and new opening degree data is obtained.

In Fig. 4b the initial guess for the time constant, τ, is poor. This has the consequence that, during the first iteration, there is a significant disagreement between the estimated foodstuff temperature 14 and the actual foodstuff temperature 12. However, agreement between the estimated foodstuff temperature 14 and the actual foodstuff temperature 12 is quickly improved, and after a few iteration the agreement is substantially as good as it was in the case illustrated in Fig. 4a, where the initial guess for the time constant, τ, was good. This illustrates that the iterative process described above ensures that a correct time constant, τ, is quickly obtained, even if the initial guess is poor.

Claims

1. A method for estimating and/or controlling a temperature of foodstuff stored in a refrigerated cavity of a refrigeration system, the refrigeration system comprising a compressor, a condenser, an expansion valve and an evaporator, arranged in a refrigerant path, the evaporator being arranged in thermal contact with the refrigerated cavity, and the expansion valve being provided in the refrigerant path upstream of the evaporator, the method comprising the steps of:

- monitoring a temperature of air inside the refrigerated cavity,

- monitoring an opening degree of the expansion valve, establishing a time constant for the temperature of the foodstuff, said time constant being determined based on a time interval between a relatively large change of opening degree of the expansion valve and until variance of the opening degree of the expansion valve decreases to a relatively low variance, and applying the time constant to a low pass filter, said low pass filter providing a relation between the temperature of the foodstuff and the air temperature, and applying the low pass filter to the monitored air temperature, thereby obtaining an estimate for the temperature of the foodstuff stored in the refrigerated cavity.

2. A method according to claim 1, further comprising the step of:

- controlling a temperature in the refrigerated cavity by controlling the opening degree of the expansion valve, based on the monitored air temperature and the monitored opening degree of the expansion valve, the applied low pass filter, as well as the established time constant.

3. A method according to claim 2, further comprising the step of controlling the temperature of the foodstuff stored in the refrigerated cavity, based on the monitored air temperature and the monitored opening degree of the expansion valve, and wherein the step of controlling the temperature of the foodstuff stored in the refrigerated cavity is performed on the basis of the estimated temperature of the foodstuff stored in the refrigerated cavity.

4. A method according to any of the preceding claims, wherein the step of establishing a time constant is repeated after a time period has elapsed, thereby establishing a new time constant, and wherein the step of applying a low pass filter to the monitored air temperature is subsequently performed using the new time constant.

5. A method according to claim 4, wherein the time period is based on a selected magnitude of change in opening degree of the expansion valve,

6. A method according to claim 4 or 5, wherein the new time constant is established based on the following formula ; τ(ί+1) = crr_ne„(i) + (1-α)τ (i) where τ(ί) is the previous time constant, T_new(i) is a time constant obtained by fitting a model to monitored data regarding the opening degree of the expansion valve, a is a tuning parameter reflecting an accuracy of the model, and τ(ί + 1) is the new time constant to be applied to the low pass filter.

7. A method according to any of the preceding claims, wherein the step of estimating the temperature of the foodstuff comprises applying a lower constraint on the time constant, said lower constraint having a value defined from foodstuff having a relatively low thermal mass compared to a thermal mass of other foodstuff stored in the same refrigerated cavity.

8. A method according to any of the preceding claims, wherein the step of estimating the temperature of the foodstuff comprises applying an upper constraint on the time constant, said upper constraint having a value defined from foodstuff having a relatively high thermal mass compared to a thermal mass of other foodstuff stored in the same refrigerated cavity.

9. A method according to any of the preceding claims, wherein the step of establishing the time constant is performed at moments of time, where the relatively large change of opening degree of the expansion valve takes place due to changes of one or more operating parameters of the refrigeration system, said changes of one or more operating parameters being changes dependent on other changes of operation of the refrigeration system than change of time constant.

10. A method according to claim 9, where the changes of operating parameters take place due to defrost of the refrigeration system being initiated.

11. A method according to claim 9, where the changes of operating parameters take place due to defrost of the refrigeration system being terminated .

12. A method according to claim 9, where the changes of operating parameters take place due to opening of a door or hatch of the refrigerated cavity, said opening of the door or hatch resulting in air from outside entering the cavity, said air having a temperature which exceeds the air temperature inside the refrigerated cavity, thereby resulting in an increase of the temperature inside the refrigerated cavity, and thereby in a sudden increased demand for cooling of the air in the refrigerated cavity.

13. A method according to claim 9, where the changes of operating parameters take place due to an amount of foodstuff being supplied to the refrigerated cavity, said foodstuff having a temperature which is higher than the air temperature inside the refrigerated cavity, said supply of the amount of foodstuff thereby resulting in a sudden increase of air temperature inside the refrigerated cavity, and thereby in a sudden increased demand for cooling of the air in the refrigerated cavity.