EP4790333A1

EP4790333A1 - Method, control unit and heat pump unit

Info

Publication number: EP4790333A1
Application number: EP25156005.8A
Authority: EP
Inventors: Fabrice DURRHEIMER; Leandro MOLON; Gilles WITTMANN
Original assignee: BDR Thermea Group BV
Current assignee: BDR Thermea Group BV
Priority date: 2025-02-05
Filing date: 2025-02-05
Publication date: 2026-08-12

Abstract

The invention relates to a method for determining a timing for a defrosting operation for defrosting of a heat pump unit for heating and/or cooling applications. The heat pump unit comprises an evaporator for heat exchange between air having an air temperature T_air and a refrigerant having an evaporating temperature T_evap, wherein a pinch P reflects a temperature difference ΔT between the air temperature T_air and the evaporating temperature T_evap. The method includes (1) obtaining a pinch difference ΔP indicative of a difference of the pinch at first t1 and second t2 points in time, and (2) determining the timing for defrosting including: (2a) comparing the pinch difference ΔP with a first pinch difference threshold ΔP_max, and (2b) if the comparison indicates that the pinch difference ΔP at least reaches the first pinch difference threshold ΔP_max, shifting the first pinch difference threshold ΔP_max further away from a pinch difference reference ΔP_ref relative to which the pinch difference ΔP is indicated; and (2c) using the shifted first pinch difference threshold as first pinch difference threshold ΔP_max, and (3) continuing the determination of the timing for defrosting until a defrosting threshold DF is reached.

Description

The invention relates to a method for determining a timing for a defrosting operation for defrosting of a heat pump unit for heating and/or cooling applications, a control unit for controlling the timing of defrosting of a heat pump unit for heating and/or cooling applications, a heat pump unit and a computer program product comprising instructions to cause the heat pump unit to execute the corresponding steps.
Icing of a heat pump unit is an undesirable but natural phenomenon in particular at colder outside air temperatures, when a heat exchanger is used to extract calories from the air and transfers them to the refrigerant circulating in the exchanger. When the heat exchanger, more specifically the evaporating refrigerant, becomes too cold, i.e. after the refrigerant has cooled considerably, and the air conditions cause this phenomenon (e.g. depending on the air temperature, humidity and air flow rate), frost may form on the evaporator. Frosting may lead to a layer of ice on the evaporator. Specifically, the moisture in the air may freeze upon contact with the evaporator fins. This frost may significantly reduce the performance of the exchanger, as the heat exchange between the air and the exchanger will be reduced/obstructed by the layer of ice.
When this phenomenon occurs, it may be necessary to defrost the evaporator. A defrosting operation may include heating up the evaporator by circulating a warmer refrigerant and/or allowing air to circulate to melt the ice.
Timing to launch the defrosting is of interest. Defrosting too early may reduce the heat pump's operating time and/or performance because actions designed to heat the evaporator are taken instead of heating the actual (destination) fluid, e.g hot water. Defrosting too late may reduce performance because the heat pump system may operate with significantly degraded performance as long as the evaporator is covered (at least partly) in ice.
To avoid inappropriate defrosting actions and improve determination of de-icing points in time, it is desired to determine/estimate when the evaporator is starting or continuing to ice up so that the appropriate action can be taken at an appropriate timing.
Thus, the object of the invention may be seen as how to improve determination of an appropriate point in time for defrosting operations for defrosting of a heat pump unit.
The object is solved by a method for determining a timing for a defrosting operation for defrosting of a heat pump unit for heating and/or cooling applications, according to claim 1. Specifically, the heat pump unit comprises an evaporator for heat exchange between air having an air temperature T_air and a refrigerant having an evaporating temperature T_evap, wherein a pinch P reflects a temperature difference ΔT between the air temperature T_air and the evaporating temperature T_evap. The method includes (1) obtaining a pinch difference ΔP indicative of a difference of the pinch at first t1 and second t2 points in time, and (2) determining the timing for defrosting including: (2a) comparing the pinch difference ΔP with a first pinch difference threshold ΔP_max, and (2b) if the comparison indicates that the pinch difference ΔP at least reaches the first pinch difference threshold ΔP_max, shifting the first pinch difference threshold ΔP_max further away from a pinch difference reference ΔP_ref relative to which the pinch difference ΔP is indicated, and (2c) using the shifted first pinch difference threshold as the first pinch difference threshold ΔP_max. Then, (3) the determination of the timing for defrosting is continued, until a defrosting threshold DF is reached. In particular, the pinch difference ΔP may be indicative of a temperature difference increasing from the first t1 to the second t2 point in time.
The invention may be regarded as including evaluating the evolution (for example, the first derivative) of the temperature difference between the air temperature and the evaporation temperature (i.e. of the pinch) over time. As such, the pinch difference reflects the first derivative of pinch. The change in this difference, i.e. the derivation of the pinch, may make it possible to identify whether frost is setting in the air exchanger. As a result of the comparison according to (2a), the first pinch difference threshold ΔP_max is shifted further away from the pinch difference reference ΔP_ref relative to which the pinch difference ΔP is indicated in step (2b), if the pinch difference ΔP at least reaches the first pinch difference threshold ΔP_max. For a subsequent iteration (2c), the shifted first pinch difference threshold replaces the first pinch difference threshold ΔP_max. For determination of the timing for defrosting, the determination is continued, in particular steps (1) and (2a) to (2c) are repeated, until the defrosting threshold DF is reached. The defrosting threshold DF indicates that the criteria for defrosting are met. In other words, the defrosting threshold DF indicates (sufficient/substantive) frosting of the evaporator.
At least reaching the threshold may mean that the threshold is met exactly or crossed and went beyond ("violated"), e.g. exceeded.
As such, it may be possible to adjust, update, adapt or shift, the pinch difference threshold ΔP_max, based on the (current, e.g. measured) pinch difference ΔP.
ΔP > ΔP_max may indicate frosting. The frost identification may be seen as leading to shift the updated pinch difference threshold further away from the current pinch difference threshold, if frost is detected. In case of repeated iterations of the method, this may mean a shift further away from the initial pinch difference threshold.
Once the pinch difference approaches the pinch difference threshold, the pinch difference threshold is shifted/moved away in the approaching direction. For example, increasing the pinch difference threshold may mean shifting it further in the direction of the increasing pinch difference, in particular it further in the direction of the increasing pinch difference relative to the pinch difference reference.
The first pinch difference threshold may be an upper/maximum pinch difference threshold.
The pinch difference reference ΔP_ref may be 0. The pinch difference reference ΔP_ref may be seen as a common reference point for the (current) pinch difference and the pinch difference threshold.
According to the invention, it is possible to adapt the pinch difference threshold ΔP_max by shifting it "in the direction" of the current pinch difference. This may allow to adjust the sensitivity of the pinch difference threshold ΔP_max according to the current pinch difference. Specifically, if frost is being detected, the sensitivity may be increased by shifting the pinch difference threshold accordingly. For example, the pinch difference threshold ΔP_max may be increased relative to pinch difference reference. More specifically, the sensitivity of pinch difference comparison is adjusted (improved), so that for subsequent increasing pinch differences, the first pinch difference threshold is larger. For example, this may allow to more appropriately distinguish between differently strongly increasing pinch differences. Based on the more sensitive comparison with the pinch difference threshold, the determination of the timing for defrosting may be improved, in particular may be more accurate.
The outcome of the determination of the (current) pinch difference with the pinch difference threshold is used for determining the timing for defrosting. Based on the shifted first pinch difference threshold ΔP_max, continuing the determination of the timing for defrosting until the defrosting threshold DF is reached. For example, if in two successive measurements ΔP ≥ ΔP_max or alternatively ΔP > ΔP_max, this may be seen as reaching the defrosting threshold DF, which is decisive for determining a need for defrosting. Alternatively, an (optional) counter may be used to count the number of results ΔP ≥ ΔP_max or ΔP > ΔP_max, wherein based on the value of the counter, more specifically based on a comparison of the value of counter with a counter threshold, the determination for defrosting is made. Here, the counter may function as defrosting threshold DF. Using such counter may delay the timing of the start of the defrosting until a certain amount of frost has been build up.
Specifically, based on the present invention, it may be possible to obtain more reliable detection of (relevant) frosting, e.g. one may distinguish icing from other possible errors/events (such as tap water consumption), which may lead to greater system accuracy, and/or reduce the number of sensors and data required, and therefore reduce the risk of data being incorrect, which may avoid misdiagnosis, and/or to adjust the defrost start-up with appropriate delay, such as via a counter and more or more threshold values, and/or to allow the defrost start delay to be adjusted if (natural, auxiliary) defrosting occurs, e.g. by decreasing a counter.
The present invention is particularly suited for air-source thermodynamic water heaters, i.e. means for producing domestic hot water using calories from the air to heat the water. However, the present invention is not limited in this regard and may be used e.g. for air source heat pumps with a wider range of applications, such as: air/air heat pump (= air conditioner), air/water heat pump (the water being the primary water in a heating circuit and not domestic hot water), etc. As such, the heat pump unit may be an air source heat pump unit.
Optionally, the heat pump unit includes a fixed speed compressor. But variable speed compressors are not excluded for a method of the present invention.
The subject matter underlying the present invention may (alternatively or supplementarily) be summarized as follows: A temperature difference ΔT indicates a difference between the air temperature T_air and the evaporating temperature T_evap and increases from a first point in time to a second point in time, wherein the pinch difference ΔP is based on the temperature difference ΔT = ΔT2 - ΔT1, wherein ΔT1 indicates a difference between the air temperature Tair and the evaporating temperature T_evap at the first point in time, and ΔT2 indicates a difference between the air temperature Tair and the evaporating temperature T_evap at the second point in time, the pinch difference ΔP indicating the pinch difference relative to a pinch difference reference ΔP_ref. On this basis, the pinch difference ΔP may be determined (e.g. calculated). It is evident that the pinch difference may be expressed as ΔP = P2-P1, wherein P2 is the pinch at the second point in time and P1 is the pinch at the first point in time. In Fig. 1, the second point in time is indicated as t and by means of the corresponding pinch difference ΔP(t), and the first point in time is indicated as t-1 and the by means of the corresponding pinch difference ΔP(t-1). The second point in time may be later than the first point in time.
At least two points in time need to be considered to obtain a pinch difference between these two points in time. But more than two points in time are possible. For example, first to n-th points in time may be considered.
For the present invention, the (first and second, nth) points in time may refer to (first and second, nth) time intervals, respectively. For example, instead of a measurement at a single point in time, a plurality of measurements over a time span may be averaged so as to indicate an average value for the time span/interval. Hence, the pinch difference may be measured at first and second points in time (including averaging over time intervals).
The subject matter underlying the present invention may further (alternatively or supplementarily) be summarized as follows: Obtaining a first pinch value indicative of the temperature difference between the air temperature T_air and the evaporating temperature T_evap for a first time interval; obtaining a second pinch indicative of the temperature difference between the air temperature T_air and the evaporating temperature T_evap for a second time interval; and obtaining at least one pinch difference ΔP between the first pinch and the second pinch. The temperature difference ΔT may be determined as follows: ΔT =x*T_air-y*T_evap, wherein x and y are predetermined parameters, e.g. x=1 and/or y=1.
The pinch difference may be measured directly or may be calculated based on the measured evaporating temperature (and optionally the measured air temperature). The air temperature may be obtained otherwise, e.g. provided by third parties. Sensors may be used for determining directly or indirectly the temperature (e.g. via tube, via pressure).
Timing may relate to an appropriate time/need of start of the (next) defrosting operation. However, this determination represents, in the first place, a suggestion for initiating defrost operation. Hence, whether defrosting is actually performed at the point in time as determined, is not implicated. There may be reasons for not running/launching the defrosting operating at the point in time as determined.
Timing may be an absolute timing in the sense of a start timing specified as/equivalent to a day, hour, minute, second (e.g. 7 Oct 2024, 16:16:20) or a relative timing, such as a time interval/period/span, in a minute, or immediately, in particular as relative to an specific moment in time, relative to an specific hour, specific minutes, specific second or relative to "now".
In a specific application of the present invention, e.g. in the case of a thermodynamic water heater, it may be possible to help to distinguish between behavior associated with evaporator icing, i.e. frosting, and behavior associated with other occurrences, such as water consumption.
Optionally, the shift of the first pinch difference threshold ΔP_max depends on the pinch difference ΔP, according to claim 2. This may allow for a stepwise adaption of threshold based on the size/amount of the (current, measured) pinch difference. This may reflect an appropriate step width when shifting the first pinch difference threshold ΔP_max. Further optionally, the shift of the first pinch difference threshold ΔP_max is proportional to the pinch difference ΔP. A specific example of a linear function f(a) reflecting the dependence of the shifted first pinch difference threshold ΔP_max on the pinch difference ΔP may be: shifted first pinch difference threshold ${ΔP}_{\max} is f (α) = α * {ΔP}_{\max} + (1 - α) ΔP .$
Optionally, determining the timing for defrosting further includes a counter c indicating frosting of the evaporator, wherein the determination of the timing for defrosting is based on a comparison of the counter with a counter threshold c_th, per claim 3. The counter is, as indicated above, optional for the method of the present disclosure. The counter may be seen as counting and hence reflecting the status of frosting of the evaporator. The counter counts the number of results ΔP ≥ ΔP_max or ΔP > ΔP_max wherein based on the value of the counter, more specifically based on a comparison of the value of counter with the counter threshold, the determination for defrosting is made. The counter may represent a means to delay the determination of the timing for defrosting until a specific frosting has been reached. For example, depending on the counter threshold c_th, the timing for the defrosting operation does not start when freezing occurs for the first time, but when freezing has been ongoing for a specific period. Determination of the timing for defrosting may directly or at least indirectly be based on the counter, e.g. by means of a counter threshold c_th. For example, c = c_th may trigger the determination of the timing for defrosting.
Optionally, the counter c is changed according to a first mathematical operation if the comparison indicates that the pinch difference ΔP at least reaches the first pinch difference threshold ΔP_max, according to claim 4. For example, this may mean that the counter is incremented, i.e. increases, if ΔP ≥ ΔP_max or, alternatively, if ΔP > ΔP_max. A result of this incrementation may be that only a "relevant" increase of the pinch difference (i.e. a sufficiently large pinch difference) is counted, i.e. reflected by the counter. If ΔP < ΔP_max, this pinch difference may not be reflected by a change of the counter c. This may represent an efficient way for counting.
Optionally, the pinch difference ΔP is compared to a second pinch difference threshold ΔP_min, according to claim 5. This may allow for more specific, detailed method for determining the timing for defrosting. Further optionally, the second pinch difference threshold ΔP_min is closer to the pinch difference reference ΔP_ref than the first pinch difference threshold ΔP_max. The second pinch difference threshold ΔP_min may be a smaller than than the first pinch difference threshold ΔP_max relative to the pinch difference reference ΔP_ref. The second pinch difference threshold ΔP_min may be a lower pinch difference threshold than the first pinch difference threshold ΔP_max. relative to the pinch difference reference ΔP_ref. The second pinch difference threshold ΔP_min may be minimum pinch difference threshold. The second pinch difference threshold ΔP_min may be closer to the pinch difference reference ΔP_ref than the first pinch difference threshold ΔP_max relative to the pinch difference reference ΔP_ref. In particular, the second pinch difference threshold ΔP_min may inbetween the first pinch difference threshold ΔP_max and the pinch difference reference ΔP_ref. This may mean that the first pinch difference threshold is further away from the pinch difference reference than the second pinch difference threshold is away from the pinch difference reference. This may further support improved timing for the defrosting.
Optionally, for determining the timing for defrosting, the method includes, if the comparison indicates that the pinch difference ΔP does not at least reach the second pinch difference threshold ΔP_min, shifting the first pinch difference threshold ΔP_max towards the pinch difference reference ΔP_ref, according to claim 6. This may mean that, if ΔP < ΔP_min, the first pinch difference threshold ΔP_max is decreased. For example, the second pinch difference threshold ΔP_min may be and remain below the first pinch difference threshold ΔP_max. This may more or less lead to a return to the initial value ΔP_max, _in for ΔP_max. Put differently, this may mean that (current) ΔP_max is set to a lower value. For example, a linear function h(γ) may determine the shift, e.g. wherein ΔP_max is proportional to ΔP_max, _ini. Exemplarily, the shifted first pinch difference threshold ${ΔP}_{\max} is h (γ) = γ * {ΔP}_{\max, ini} + (1 - γ) {ΔP}_{\max} .$
This may lead to a moderate or no increase in the pinch difference and that is not counted because it does not reflect a sufficiently large/excessive increase in the pinch difference, and, hence, frosting. The re-adjusting of the first pinch difference threshold closer to its initial value may mean that the sensitivity is (automatically) increased.
Optionally, the counter c changes according to a second mathematical operation opposite to the first mathematical operation, if the comparison indicates that the pinch difference ΔP does not at least reach the second pinch difference threshold ΔP_min, according to claim 7. If the counter is incremented, according to the first mathematical operation, the counter is decremented according to the second mathematical operation, as incrementing (addition) is opposite to decrementing (subtraction). In other words, it is counted negatively. Hence, an opposite mathematical operation may mean "+" instead of "-", and vice versa. This change of the counter may be carried out if ΔP ≤ ΔP_min or, alternatively, ΔP < ΔP_min. This may be considered as reflecting in the counter that a natural de-icing, i.e. de-frosting occurs. The counter may not be negative, i.e. go below zero.
Optionally, if the comparison indicates that the pinch difference ΔP is between the first pinch difference threshold ΔP_max and the second pinch difference threshold ΔP_min, the first pinch difference threshold ΔP_max is shifted towards the pinch difference reference ΔP_ref, according to claim 8. The shift may be according to a linear function g(β), i.e. proportional to ΔP_max. Exemplarily, the shift may be related to the initial first pinch difference threshold ΔP_max. _ini according to the shifted first pinch difference threshold ${ΔP}_{\max} being g (β) = β * {ΔP}_{\max, ini} + (1 - β) {ΔP}_{\max} .$
This reflects that a moderate increase or no increase in the pinch difference if not of relevance because it does not reflect an excessive increase of the pinch difference, and, hence, no frosting. The shift may lead to a decrease of the first pinch difference threshold ΔP_max. It may serve to re-adjust the first pinch difference threshold ΔP_max so that the sensitivity is increased again.
Optionally, the counter c is not changed if the comparison indicates that the pinch difference ΔP is between the first pinch difference threshold ΔP_max and the second pinch difference threshold ΔP_min, according to claim 9. This may mean that minor changes in the pinch difference are not considered (counted), as they do not indicate excessive frosting. This may mean that the counter is kept constant, if ΔP_min < ΔP < ΔP_max.
Optionally, the time difference between the first and second points in time is at least 60 s, optionally at least 300 s, more optionally less than 600 s, according to claim 10. This may represent an appropriate precision while reasonably reducing measurement efforts. The points in time may reflect time spans/periods which are reflected in a measurement indicative of a point in time, as explained above. In embodiments where more than two points are used, such where first to n-th points in time are considered, this may be understood as that the time difference between successive points in time of the first to n-th points is at least 60 s, optionally at least 300 s, more optionally less than 600 s.
Optionally, the method includes re-setting the counter c after starting the defrosting operation, further optionally after completion of the defrosting operation, according to claim 11. For example, the counter may be re-set to zero. Hereby, a further execution of the method for determining a timing for a next defrosting operation may start with a newly initialized which may improve the performance of the method.
The invention is also directed to a method including carrying out the defrosting, namely the method of claim 12, including defrosting a heat pump, including timing the defrosting operation according to the invention, wherein the method further includes starting the defrosting operation, in accordance with the determined timing for defrosting. For example, based on the counter (if c=c_th, but not necessarily based on the counter), the defrosting operation is actually launched. Defrosting may start once the timing for defrosting has been determined, optionally once the counter threshold c_th has been reached. Alternatively, a point in time/ a time period for the start of defrosting, is set, optionally once the counter threshold has been reached.
The present invention is also directed to a control unit for controlling the timing of defrosting of a heat pump unit for heating and/or cooling applications, wherein the control unit is configured to carry out the steps of the method of the invention, according to claim 13. Hence, the present invention is also directed to the following: A control unit for determining a timing for a defrosting operation for defrosting of a heat pump unit for heating and/or cooling applications, the heat pump unit comprising an evaporator for heat exchange between air having an air temperature T_air and a refrigerant having an evaporating temperature T_evap, wherein a pinch P reflects a temperature difference ΔT between the air temperature T_air and the evaporating temperature T_evap, wherein the control unit is configured to: obtain a pinch difference ΔP indicative of a difference of the pinch at first t1 and second t2 points in time and indicative of the temperature difference increasing from the first t1 to the second t2 point in time, and determine the timing for defrosting including: comparing the pinch difference ΔP with a first pinch difference threshold ΔP_max, and if the comparison indicates that the pinch difference ΔP at least reaches the first pinch difference threshold ΔP_max, shifting the first pinch difference threshold ΔP_max further away from a pinch difference reference ΔP_ref relative to which the pinch difference ΔP is indicated. Optionally, the shift of the first pinch difference threshold ΔP_max depends on the pinch difference ΔP, optionally the shift of the first pinch difference threshold ΔP_max being proportional to the pinch difference ΔP. Optionally, determining the timing for defrosting further includes: a counter c as indicator for frosting of the evaporator, wherein the determination of the timing for defrosting is based on a comparison of the counter c with a counter threshold c_th. Optionally, the counter c is changed according to a first mathematical operation if the comparison indicates that the pinch difference ΔP at least reaches the first pinch difference threshold ΔP_max. Optionally, the pinch difference ΔP is compared to a second pinch difference threshold ΔP_min, optionally the second pinch difference threshold ΔP_min being closer to the pinch difference reference ΔP_ref than the first pinch difference threshold ΔP_max. Optionally, the control unit is configured for determining the timing for defrosting, if the comparison indicates that the pinch difference ΔP does not at least reach the second pinch difference threshold ΔP_min, including shifting the first pinch difference threshold ΔP_max towards the pinch difference reference ΔP_ref. Optionally, the counter c changes according to a second mathematical operation opposite to the first mathematical operation, if the comparison indicates that the pinch difference ΔP does not at least reach the second pinch difference threshold ΔP_min. Optionally, the control unit is configured for determining the timing for defrosting, if the comparison indicates that the pinch difference ΔP is between the first pinch difference threshold ΔP_max and the second pinch difference threshold ΔP_min, including shifting the first pinch difference threshold ΔP_max towards the pinch difference reference ΔP_ref. Optionally, the counter c is not changed if the comparison indicates that the pinch difference ΔP is between the first pinch difference threshold ΔP_max and the second pinch difference threshold ΔP_min. Optionally, the time difference between the first and second points in time is at least 60 s, optionally at least 300 s, more optionally less than 600 s. Optionally, the control unit is configured to re-set the counter c after starting the defrosting operation, optionally after completion of the defrosting operation. Optionally, the control unit is configured to control defrosting a heat pump, including timing the defrosting operation according to the invention, and including starting the defrosting operation, in accordance with the determined timing for defrosting.
The invention is also directed to a heat pump unit, having or being arranged to cooperate with a sensor for determining the temperature T_evap of refrigerant in the evaporator and a control unit, according to claim 14. The control unit is the control unit of the present invention and/or configured to execute the steps of the method of the present invention. The sensor for determining the temperature T_evap may support precise and temperature determination of the temperature of the refrigerant in the evaporator, which may result in an improved defrosting performance of the heat pump unit.
Optionally, the heat pump unit has or is arranged to cooperate with a sensor for determining the ambient air temperature T_air. This may support precise and up-to-date air temperature determination, which may result in an improved defrosting performance of the heat pump unit.
The invention is also directed to a computer program product comprising instructions to cause the heat pump unit of the present invention to execute the steps of the method of the present invention, when the program is executed by a computer or by the control unit of the heat pump unit., according to claim 15. The invention is also directed to a computer readable data carrier having stored thereon the computer program product of the present invention. The invention is also directed to a data carrier signal carrying the computer program product of the present invention.
The invention is also, in an independent form, directed to the aspect of claim 6. More specifically, the invention is directed to the following: A method for determining a timing for a defrosting operation for defrosting of a heat pump unit for heating and/or cooling applications, wherein the heat pump unit comprises an evaporator for heat exchange between air having an air temperature T_air and a refrigerant having an evaporating temperature T_evap, wherein a pinch P reflects a temperature difference ΔT between the air temperature T_air and the evaporating temperature T_evap. The method further includes, if the comparison indicates that the pinch difference ΔP does not at least reach a second pinch difference threshold ΔP_min, shifting a first pinch difference threshold ΔP_max towards a pinch difference reference ΔP_ref, relative to which the pinch difference ΔP is indicated. Based on the shifted first pinch difference threshold ΔP_max, the determination of the timing for defrosting is continued until a defrosting threshold DF is reached The second pinch difference threshold ΔP_min and the first pinch difference threshold ΔP_max are also indicated relative to the pinch difference reference ΔP_ref, and the second pinch difference threshold ΔP_min may be closer to the pinch difference reference ΔP_ref than the first pinch difference threshold ΔP_max is relative to pinch difference reference ΔP_ref. The remaining aspects e.g. as described in connection with the claims, are compatible and combinable with this independent aspect.
In the figures, the subject-matter of the invention is schematically shown, wherein identical or similarly acting elements are usually provided with the same reference signs.

Figure 1: shows a schematic diagram representing the pinch as a function of time.
Figure 2: shows a schematic diagram of the pinch as a function of time for different occurrences.
Figure 3: shows a flow chart representing a method of an embodiment of the invention.
Figure 4(a), (b) and (c): show schematic diagrams representing a freezing occurrence.
Figure 5(a), (b) and (c): show schematic diagrams representing a tapping occurrence.

Figure 1 shows a diagram reflecting the pinch P (representing the difference between the air temperature T_air and the evaporating temperature T_evap) as a function of time. The pinch P reflects/indicates a temperature difference ΔT between the air temperature T_air and the evaporating temperature T_evap. Here, the pinch P is calculated based on discrete temperature values per point in time, e.g. for t-2, t-1 and t. However, as indicated above, temperature values may alternatively relate to a time interval and may e.g. be averaged, so that the temperature values per point in time t-2, t-1 and t etc. are average values.
Specifically, the pinch P increases over time. This implicates that the first derivative of P over time is positive. Further, Fig. 1 reflects that the difference of the pinch increases over time, as ΔP_i(t-1) < ΔP_i (t), wherein ΔP_i (t-1) = P_t-1 - P_t-2 and ΔP_i (t) = P_t - P_t-1. The subscript i indicates that the values relate to an iteration or cycle i. Thus, the slope of the pinch difference ΔP over time implicates that the second derivative of P over time is positive.
Figure 2 shows a diagram reflecting the pinch P, when different circumstances occur in the heat pump system. In a first constellation, the evaporator of the heat pump is frosted, i.e. ice is formed on the evaporator. This is reflected in an increase of the pinch P as schematically indicated with the continuous line and having a convex shape. In a second constellation, the evaporator of the heat pump is not being frosted, but exemplarily tap water is consumed from the liquid (destination) circuit. This is reflected in an increase of the pinch P as schematically indicated with the dashed line and having a concave shape. Hence, the increase of the pinch differs depending on the reason for the increased pinch.
In a preferred application of the present invention, ice forming on the evaporator and, hence, of frosting of the evaporator, is primarily handled. This may mean that the application may concern determination of the timing for defrosting based on a characteristic of the occurrence of frosting at the evaporator. For such handling, optionally, a characteristic of the occurrence of frosting at the evaporator may be distinguished from of another occurrence in the system. More specifically, the application may support appropriate determination of timing for defrosting while other effects or impacts may occur such as hot water tapping or sharp change of air temperature. This is explained later in connection with Fig. 4 and 5.
The basis for focussing the determination for the timing for defrosting on characteristics relating to icing of the evaporator is provided by means of the following method, which is reflected in Fig. 3, supported by Fig. 1, and explained in the following. Here, per iteration i, the pinch difference ΔPi which is referred to as ΔP - which may also be referred to in this application as without explicit reference to iteration i as ΔP for easier readability, is shown.
A timing for a defrosting operation for defrosting of a heat pump unit for heating and/or cooling applications is determined. The heat pump unit comprises an evaporator for heat exchange between air having an air temperature T_air and a refrigerant having an evaporating temperature T_evap. A pinch difference ΔP indicative of a difference of the pinch at first t1 (or: t-1, as shown in Fig. 1) and second t2 (or: t, as shown in Fig. 1) points in time and indicative of the temperature difference increasing from the first t1 to the second t2 point in time is obtained. In other words, the temperature difference between the air temperature T_air and the evaporation (evaporating) temperature T_evap is determined, leading to the pinch P (or Pi, if referred to per iteration): $Pi = T_{air} - T_{evap}$
Then, the evolution/change of the Pi between different measurements at first (t-1) and second (t) points in time is determined: $Δ Pi = {Pi}_{t} - {Pi}_{t - 1}$
A measurement between t and t-1 may be realized with a delay of between 1 and 10 min.
In an alternative notation, this may be expressed as follows: The pinch P is monitored and averaged for n successive time intervals, each interval having a length dt, e.g., expressed in seconds: $Pi (n) = \frac{1}{dt} \sum_{j = n - dt}^{n} (T_{air} (j) - T_{evap} (j))$
The difference between two consecutive pinch values at n and n-dt is expressed as pinch difference: $Δ Pi (n) = Pi (n) - Pi (n - dt)$
For determination of the timing for defrosting, at least steps Step2 and Step2c are carried out in at least one iteration i. These steps comprise comparing, in an iteration i, the pinch difference ΔP with a first pinch difference threshold ΔP_max, and if the comparison indicates that the pinch difference ΔP at least reaches (or alternatively exceeds) the first pinch difference threshold ΔP_max, the first pinch difference threshold ΔP_max is shifted further away from a pinch difference reference ΔP_ref relative to which the pinch difference ΔP is indicated. In step Step2c, in this example, ΔP_max is shifted to a value of f(a)= a * ΔP_max + (1- α) ΔP. For the subsequent iteration i+1, the shifted ΔP_max is used. This is repeated until the defrost threshold DF is reached. Hence, at the end of a number of iterations i, the timing of defrosting is determined, when the defrost threshold DF is reached.
If ΔP > ΔP_max, this indicates that the behavior of the evaporator is characteristic of a frosting. ΔP_max may be a predetermined value, such as a value determined by a test lab, and which indicates from which value of ΔP (=from which level of growth of the pinch curve) the evaporator is icing / frost.
Hence, method includes, if an iteration does not directly lead to the determination of the timing for defrosting, the shifting of the first pinch difference threshold ΔP_max, namely to f(a).
The remaining steps in Fig. 3 are not essential. In particular, a method only including steps Step2 and Step2c is feasible. Specifically, this may mean that it is irrelevant for steps Step2 and Step2c in isolation whether ΔP is smaller or larger than ΔP_min, per step Step1. Also, the determination of the timing is understood to follow in another iteration of the algorithm schematically shown in Fig. 3 than in the iteration in which step Step2c is actually carried out.
In an embodiment, the shift of the first pinch difference threshold ΔP_max depends on the pinch difference ΔP, namely to the effect that the shift of the first pinch difference threshold ΔP_max is proportional to the pinch difference ΔP. Specifically, this is carried out in each of steps Step1b, Step2c and Step3 of Fig. 3.
As the objective is often not to launch the defrosting action as soon as the frost is started, a counter c may be used which may have the effect to delay the timing of the defrosting until a certain amount of frost has been build up. In the embodiment of Fig. 3, the counter c is used as indicator for frosting of the evaporator, wherein the determination of the timing for defrosting is based on a counter threshold c_th, which may be a maximum value the counter c may have. The counter c is applied in steps Step1a and Step2a in Fig. 3.
The counter c is changed according to a first mathematical operation if the comparison indicates that the pinch difference ΔP at least reaches the first pinch difference threshold ΔP_max. The counter is changed in step Step2a by means of incrementing the counter c. Specifically, each time the comparison indicates ΔP > ΔP_max, the counter c is incremented.
In step Step1, the pinch difference ΔP is compared to a second pinch difference threshold ΔP_min, which is closer to the pinch difference reference ΔP_ref than the first pinch difference threshold ΔP_max. If the comparison indicates that the pinch difference ΔP does not at least reach the second pinch difference threshold ΔP_min, the first pinch difference threshold ΔP_max is shifted towards the pinch difference reference ΔP_ref. This is reflected in step Step1b of Fig. 3. In this example, ΔP_max is shifted according to h(y)= γ * ΔP_max, _ini + (1- γ) ΔP_max.. With reference to step Step1a, if ΔP < ΔP_min, it indicates that the heat pump system is de-icing (due to external, natural conditions, such as a temperature increase). If so, c = max (0; c-1), which means that the counter c decreases by 1, without going below 0. This may be seen as delaying the timing of the start of defrosting.
In this case, the counter c is changed according to a second mathematical operation opposite to the first mathematical operation, as the comparison indicates that the pinch difference ΔP does not at least reach the second pinch difference threshold ΔP_min, see step Step1a in Fig. 3.
If the comparison indicates that the pinch difference ΔP is between the first pinch difference threshold ΔP_max and the second pinch difference threshold ΔP_min, the first pinch difference threshold ΔP_max is shifted towards the pinch difference reference ΔP_ref. This is reflected in step Step3 of Fig. 3. In this example, ΔP_max is shif to the shifted first pinch difference threshold ΔP_max being g(β)= β * ΔP_max, _ini + (1- β) ΔP_max. Fig. 3 further reflects no counter change in this case.
Fig. 3 further shows at the end of the number of iterations, in a final iteration i, the determination for defrosting in step Step4. Here, the defrost threshold DF is met (DF=1) and the counter c has reached the counter threshold c_th in step Step2b. This may lead to triggering the defrost mode. When c reaches the threshold value c_th (dependent on the heat pump system by itself but also dependent on e.g. the time between two successive measurements), the defrosting action may be launched.
In the method for defrosting a heat pump, the defrosting operation is started, in accordance with the determined timing for defrosting. E.g. after a time interval has lapsed after the determination for defrosting was made, or at a specific point in time.
As an example, the following equations may be used for determining the shift of ΔP_max, as indicated above: $f (α) = α * {ΔP}_{\max} + (1 - α) ΔP$ $g (β) = β * {ΔP}_{\max, ini} + (1 - β) {ΔP}_{\max}$ $h (γ) = γ * {ΔP}_{\max, ini} + (1 - γ) {ΔP}_{\max}$
The functions f, g,h may be predetermined, so as to obtain a suitable shift of ΔP_max, i.e. of the determination of the (next) pinch difference ΔP reaching the (next) ΔP_max.
A suitable time difference between the first and second points in time (e.g. measurement time points) is about 60 s, optionally at least 300 s, more optionally less than 600 s. Hence, the algorithm reflected in Fig. 3 is run/repeated every 60, or 300 or 600 s.
Fig. 3 does not show a final step of re-setting the counter c after starting/completing the defrosting operation. However, this step may be carried out in Step 4 or in or during the step referred to as "Defrost mode" at the end of the diagram shown in Fig. 3.
The invention is also directed to a control unit for controlling the timing of defrosting of a heat pump unit for heating and/or cooling applications, wherein the control unit is configured to carry out the steps of the method described above, in particular of the steps shown in Fig. 3.
The invention is also directed to a heat pump unit (not shown), having a sensor (not shown) for determining the temperature T_evap of refrigerant in the evaporator and the control unit, wherein the control unit is the control unit of the present invention and/or configured to execute the steps of the method as described above. The sensor is for determining the ambient air temperature T_air.
The invention is also directed to a computer program product comprising instructions to cause the heat pump unit to execute the steps of the method of the present invention, when the program is executed by a computer.
A specific embodiment of the present invention is now described with reference to Fig. 4 and 5. Fig. 4 shows the first constellation of freezing, i.e. frosting of the evaporator. Here, the pinch P exhibits a convex shape, i.e. it increases with a gradually increasing slope, see Fig. 4(a). Fig. 4(b) shows the measurements for ΔP at time intervals 1 to 8. ΔP starts increasing after time interval 1. The first pinch difference threshold ΔP_max is increased over time, as of time interval 2, e.g. according to step Step2c. At time intervals 4/5, ΔP reaches ΔP_max, which means that, according to step Step2a, the counter is incremented. This is reflected in Fig. 4(c) which shows that the counter counts as of time interval 4.
Fig. 5 shows the second constellation of non-frosting of the evaporator, such as tap water consumption. In the specific example, tap water is consumed between time intervals 1 and 7. Here, the pinch P exhibits a concave shape, i.e. it has a decreasing slope, see Fig. 5(a). A typical function exhibiting such behaviour is the square root or logarithmic function. Fig. 5(b) shows the measurements for ΔP at time intervals 1 to 8. ΔP starts increasing strongly after time interval 1. The first pinch difference threshold ΔP_max is increased and then decreased over time, e.g. according to step Step2c. At time intervals 1/2, ΔP reaches ΔP_max, which means that, according to step Step2a, the counter is incremented. At time intervals 4/5, ΔP is lower than ΔP_max, which means that, according to step Step1a, the counter is not changed. This is reflected in Fig. 5(c) which shows that the counter counts only between time intervals 1 and 4. Hence, once a "transitory effect" of the pinch difference dampens (as hot water tapping is stopped), the sensitivity converges again to its initial value so that the original sensitivity is recovered without decrementing the counter if ΔP_min < ΔP < ΔP_max or the counter c may even decrease if the pinch difference is below ΔP_max.
Hence, while progressive icing of the evaporator may result in a convex curve (like P(x)=x² or exponential function), water consumption and associated production may result in a concave curve (like P(x)=√(x) or ln(x)).
A reason may be that, if the evaporator temperature decreases due to tap water consumption, the evaporator temperature begins to rise again, which represents a difference between the first and second constellations as to the practical handling of the two occurrences. For example, the counter c is incremented so as to reflect the situation of freezing appropriately and determine defrosting timing appropriately. The counter c is not, or at least less strongly, incremented if the change of pinch difference ΔP does not result from freezing (but results from tap water consumption, leading to drop in water temperature and, hence, in a drop in the evaporator temperature). Also, an increase of first pinch difference threshold ΔP_max helps to achieve the situation that the lowering pinch difference ΔP due to tap water consumption is not being counted, i.e. that the timing for the de-frosting is appropriately adjusted.
The present invention also proposes, in one embodiment, a control configured to determine different system states by means of e.g. two sensors, at least on the basis of a determination of the air temperature T_air and the evaporating temperature T_evap. The evaporating temperature T_evap may reflect the temperature of the air entering the evaporator, i.e. the temperature of the source of the heating medium (e.g. a thermodynamic water heater). This temperature is measured by a temperature sensor positioned in the air flow, before the air passes through the exchanger and is cooled during the heat exchange. For example, this sensor is positioned at the air duct inlet, inside the heat pump unit. Alternatively, it may be external to the heat pump unit, for example in an air duct or in the environment from which the air is drawn. Further alternatively, the temperature may come from a sensor or data that does not belong to the heat pump unit but is communicated to it, such as meteorological data from a local authority, sensor from the home automation system on the installation site, etc.
The temperature of the refrigerant at the evaporator is the temperature of the refrigerant at the phase change from liquid to gas. This temperature is the evaporating temperature under the conditions of use. It is often measured "indirectly" by means of a temperature sensor positioned on a tube in the exchanger or at the exchanger inlet or outlet. The tube may be made of a highly thermally conductive material such as copper, which makes it possible to obtain the temperature of the refrigerant without direct contact between the refrigerant and the tube. Alternatively, the pressure of the refrigerant in the circuit may be used to determine the evaporating temperature T_evap.
A specific advantage achievable by the present invention may be that, instead of evaluating the values of the pinch difference, or even more broadly of a function based on this difference (such as a polynomial, use of constant etc.), the evolution of the pinch difference is considered. Indeed, it has been recognized by the inventors that the shape of the curve of a function of the development of pinch difference over time differs according to the state of the system. More generally, the derivative of a function of the pinch difference in temperatures surprisingly reveals different product behaviours, in particular when comparing frosting to nominal operation or when comparing frosting to hot water tapping.

List

P, Pit, Pit-1, Pit-2: pinch
ΔP, ΔPi(t), ΔPi(t-1): pinch difference
ΔPmax: first/maximum pinch difference threshold
ΔPmin: second/minimum pinch difference threshold
ΔPref: pinch difference reference
ΔPmax, ini: initial first/maximum pinch difference threshold
t1, t2, t-2, t-1, t: points in time
DF: defrost threshold
c: counter
cth: counter threshold
f, g, h: functions to determine ΔP_max
α, β, γ: parameters
Tair: air temperature
Tevap: evaporating temperature
i: iteration

Claims

Method for determining a timing for a defrosting operation for defrosting of a heat pump unit for heating and/or cooling applications,
the heat pump unit comprising an evaporator for heat exchange between air having an air temperature (T_air) and a refrigerant having an evaporating temperature (T_evap), wherein a pinch (P) reflects a temperature difference (ΔT) between the air temperature (T_air) and the evaporating temperature (T_evap), wherein the method includes:
obtaining a pinch difference (ΔP) indicative of a difference of the pinch at first (t1; t-1) and second (t2; t) points in time,

determining the timing for defrosting including the steps of:
comparing the pinch difference (ΔP) with a first pinch difference threshold (ΔP_max), and

if the comparison indicates that the pinch difference (ΔP) at least reaches the first pinch difference threshold (ΔP_max), shifting the first pinch difference threshold (ΔP_max) further away from a pinch difference reference (ΔP_ref) relative to which the pinch difference (ΔP) is indicated, and

using the shifted first pinch difference threshold as the first pinch difference threshold (ΔP_max), and

continuing the determination of the timing for defrosting until a defrosting threshold (DF) is reached.
Method of claim 1, wherein the shift of the first pinch difference threshold (ΔP_max) depends on the pinch difference (ΔP), optionally the shift of the first pinch difference threshold (ΔP_max) being proportional to the pinch difference (ΔP).
Method of claim 1 or 2, wherein determining the timing for defrosting further includes:
a counter (c) for indicating the frosting of the evaporator, wherein the determination of the timing for defrosting is based on a comparison of the counter with a counter threshold (c_th).
Method of claim 3, wherein the counter (c) is changed according to a first mathematical operation if the comparison indicates that the pinch difference (ΔP) at least reaches the first pinch difference threshold (ΔP_max).
Method of any of the preceding claims, wherein the pinch difference (ΔP) is compared to a second pinch difference threshold (ΔP_min), optionally the second pinch difference threshold (ΔP_min) being closer to the pinch difference reference (ΔP_ref) than the first pinch difference threshold (ΔP_max).
Method of claim 5, including, for determining the timing for defrosting,
if the comparison indicates that the pinch difference (ΔP) does not at least reach the second pinch difference threshold (ΔP_min), shifting the first pinch difference threshold (ΔP_max) towards the pinch difference reference (ΔP_ref).
Method of any of the preceding claims, if dependent on claims 3, 4 and 5, wherein the counter (c) changes according to a second mathematical operation opposite to the first mathematical operation, if the comparison indicates that the pinch difference (ΔP) does not at least reach the second pinch difference threshold (ΔP_min).
Method of any of the preceding claims, if dependent on claim 5, including, for determining the timing for defrosting,
if the comparison indicates that the pinch difference (ΔP) is between the first pinch difference threshold (ΔP_max) and the second pinch difference threshold (ΔP_min), shifting the first pinch difference threshold (ΔP_max) towards the pinch difference reference (ΔP_ref).
Method of any of the preceding claims, if dependent on claims 3 and 5, wherein the counter (c) is not changed if the comparison indicates that the pinch difference (ΔP) is between the first pinch difference threshold (ΔP_max) and the second pinch difference threshold (ΔP_min).
Method of any of the preceding claims, wherein the time difference between the first and second points in time is at least 60 s, optionally at least 300 s, more optionally less than 600 s.
Method of any of the preceding claims, wherein the method includes re-setting the counter (c) after starting the defrosting operation, optionally after completion of the defrosting operation.
Method for defrosting a heat pump, including timing the defrosting operation according to any of the preceding claims, wherein the method further includes
starting the defrosting operation, in accordance with the determined timing for defrosting.
Control unit for controlling the timing of defrosting of a heat pump unit for heating and/or cooling applications, wherein the control unit is configured to carry out the steps of any of the preceding claims.
Heat pump unit, having a sensor for determining the temperature T_evap of refrigerant in the evaporator and a control unit, wherein the control unit is the control unit of claim 13 and/or configured to execute the steps of the method of any of the preceding claims 1 to 12, optionally the heat pump unit having a sensor for determining the ambient air temperature T_air.
A computer program product comprising instructions to cause the heat pump unit of claim 14 to execute the steps of the method of any of the preceding claims 1 to 12, when the program is executed by a computer or by the control unit of the heat pump unit.