EP4607112A1

EP4607112A1 - Method, control unit and heat pump unit

Info

Publication number: EP4607112A1
Application number: EP24158602.3A
Authority: EP
Inventors: Marie FLUHR; Mathieu GUERPILLON; Baptiste BLAINEAU; Axel BUKRABA; Billy SENG; Alain MAKHOUR; Florian ANTOINE
Original assignee: BDR Thermea Group BV
Current assignee: BDR Thermea Group BV
Priority date: 2024-02-20
Filing date: 2024-02-20
Publication date: 2025-08-27

Abstract

The present invention relates to a method for timing defrosting of a heat pump unit for at least heating a refrigerant based on a defrost parameter DP. The heat pump unit comprises an evaporator for heat exchange between the refrigerant characterized by an evaporating temperature T_evap and ambient air having an air temperature T_air, a temperature sensor for measurement of the air temperature T_air, and a temperature sensor for measurement of the evaporating temperature T_evap. The method includes measuring/determining the evaporating temperature T_evap and the air temperature T_air; determining whether the air temperature T_air is higher than the evaporating temperature T_evap, i.e. whether T_air > T_evap; incrementing the defrost parameter DP to reflect the temperature difference Δ = T_air - T_evap, if T_air > T_evap, e.g. adding to the defrost parameter DP the temperature difference Δ = T_air - T_evap; and optionally starting the defrosting, if the (incremented) defrost parameter DP exceeds the predetermined defrost parameter threshold A.

Description

Technical field

The invention concerns a method, and control unit and heat pump unit having such method or control unit.

Background

Heat pump units using a coil battery heat exchanger, i.e. a gas/refrigerant heat exchanger, as an evaporator are known. Such heat pump unit may be an air source heat pump for at least heating a fluid, such as an air/water heat pump, or an air source heat pump water heater, or an air destination heat pump for at least cooling, such as an air conditioner, such as an air/air heat pump.
There is a risk of icing and frosting of the refrigerant and/or the evaporator, when refrigerant in the evaporator (battery) may be less or about 0°C and/or when the air flowing through the evaporator may be less or about 0°C. (The freezing temperature may e.g. "deviate" from exact 0°C due to pressurization and/or salinity for example.)
When a heat pump unit with an air-cooled heat exchanger is used on the cold side (as outdoor unit), there is a risk that it will freeze over. This means that a layer of ice may form on top of the heat exchanger fins, preventing from efficient heat exchange between the air and the refrigerant. This phenomenon occurs when the air and/or the refrigerant temperature is below the freezing temperature (threshold value), for example 0°C. Other conditions, such as air humidity, air pressure, salinity, may affect the speed and conditions of frosting. When a frost situation is identified, a defrost measure is set up to eliminate frost from the evaporator.
Several defrosting measures are known, such as natural defrosting by ventilation, cycle inversion, electric heating of the heat exchanger, use of hot gas, etc. Whatever the defrosting measure is, it is important to define the point in time at which the defrosting shall be launched. Optimizing defrost start-up is expedient for maintaining good energy performance and system reliability, for the following reasons: If defrost is started too early, there may be a significant loss of energy and a risk of premature ageing of the components. Energy loss is associated with the consumption of useful energy for defrosting purposes, instead of being transferred to the refrigerant/medium to be heated. This results in a reduction of the energy performance coefficient. If defrosting is initiated too late, there is also a risk of a drop in system efficiency, notably due to a drop in heat exchange performance when the evaporator is frosted to a significant amount. Such defrosting launch may also lead to discomfort for the user, as defrosting will take significantly longer than optimal defrosting, which corresponds to a point in time during which there is no heat transfer by the refrigerant being heated. This may result in a shortage of domestic hot water or a drop in the temperature of the area to be heated, for example. At the same time, late defrosting may lead to breakage of heat pump circuit components.
Thus, there is a need for defrosting at an appropriate point in time.

Summary of the invention

The present invention relates to a (control) method for timing defrosting of a heat pump unit for heating and/or cooling fluid (at least for heating or only for heating), in particular for domestic hot water and/or heating/cooling of a building, based on (i.e. in consideration of) a defrost parameter DP. The heat pump unit comprises an evaporator for heat exchange between (ambient) air having an air temperature T_air, and the refrigerant characterized by an evaporating temperature T_evap, a sensor for (direct or indirect) measurement of the air temperature T_air, and a sensor for (direct or indirect) measurement of the evaporating temperature T_evap. The method includes measuring/determining the evaporating temperature T_evap and the air temperature T_air; determining the defrost parameter DP based on an integral of a function of the air temperature T_air and of the evaporating temperature T_evap over time, namely DP (t) = ∫f(T_air,T_evap)dt, and determining the timing for defrosting based on the value of the defrost parameter DP (wherein further parameters are not excluded). Generally, dt denotes the time difference between two successive measurements or iterations of the parameter DP.
The idea underlying the invention is that an appropriate defrosting time corresponds to a time at which the energetic benefit of the defrosting is particularly relevant (compared to the energetic loss during defrosting). Defrosting may mean the desire to remove ice from the evaporator, i.e. de-icing. Specifically, the inventors found that the two main criteria are (1) the temperature of the refrigerant in the evaporator (the evaporating temperature T_evap) by direct or indirect measurement (e.g. on the walls of the heat exchanger tubes or the tubes in the vicinity, measured as a temperature with a temperature determination device or as physical property related to temperature, such as pressure) and (2) the temperature of the air entering/crossing the heat exchanger (air temperature T_air). Especially, it has been found that the quantity of the difference between these temperatures over the time is always related to an appropriate (preferably optimal) defrosting time for a same air temperature T_air. In particular, including the temperature difference between the evaporating temperature and the temperature reflects the "impact" of the frosting, in particular compared to approaches not considering these temperatures, e.g. a temperature difference (e.g. merely counting the number of iterations/occasions at which the air temperature T_air is above the evaporating temperature T_evap). That means that with (e.g. only) these two parameters together, it is possible to determine a sufficiently precise value for the optimal defrosting point in time. For example, it is considered that other potential parameters impacting the optimal defrost time launch, such as humidity of air, are already reflected in this approach and, thus, already included, e.g. correlated with the measurement of the temperature difference between the temperature of the air entering the evaporator (T_air) and the temperature of the refrigerant at the evaporator (T_evap). In some embodiments, only T_air and T_evap may be measured to determine the timing of defrosting. Put differently, the function may depend only on Tair and Tevap.
The corresponding quantity/parameter reflecting the temperature of the air entering the evaporator and the temperature of the refrigerant at the evaporator is herein referred to as "defrost parameter" DP (which may be seen as a "frost unit"). It may be measured in Kelvin·sec (or any temperature unit and time unit such as Kelvin·min; Kelvin hour; Fahrenheit day etc.). On this basis, an appropriate timing for defrosting can be determined.
The defrost parameter DP is the integral/sum of a function f depending on the air temperature T_air and on the evaporating temperature T_evap over time, namely DP (t) = ∫f(T_air,T_evap) dt (wherein further parameters/dependencies are not excluded). The defrost parameter DP may represent a counter. The function f may be an affine function or a polynomial function.
In some embodiments, the function may depend on the difference between the air temperature and the evaporating temperature, i.e. DP (t) = ∫f(T_air - T_evap) dt, wherein f is an affine function or a polynomial function, but preferably an affine function.
Optionally, the function may relate to the difference between these temperatures and the evolution of the quantity over time corresponds to the mathematical sum or integration of the difference over time and may be described as DP (t) = ∫ T_air - T_evap dt. The defrost parameter DP may, in this case, be seen as the cumulative difference between the air temperature T_air and the evaporating temperature T_evap over time.
As explained above, it has been found that the defrost parameter is quite similar whatever the conditions of the test (setting temperature of the destination fluid, humidity etc..) are, and is only/primarily dependent on the air temperature and that, based on this defrost parameter, the determination of the optimal defrosting launch may be achieved. The defrost parameter may be regarded as related to the energy loss.
The defrost parameter may depend on the power capacity of the heat pump, i.e. the compressor or its operating mode. For example, it has been found that an appropriate value of the defrost parameter (corresponding to the threshold A) to launch the defrosting is in the range [20 000; 200 000] Kelvin · seconds for a heat pump having a heating capacity between 6 and 16 kW at -10°C Air source /50°C water destination. This may apply to an air temperature range from -20°C to 45°C and a water temperature range from 15°C to 70°C. This range may depend on the specific heat pump unit and may be determined based on experimental data. By comparing the "defrost parameter" value during the operation of the heat pump unit to this threshold identified as a particularly appropriate value of the defrost parameter, it is possible to control the defrost cycle of a heat pump unit to launch it at an appropriate point in time.
Specifically, for a heat pump unit having a heating capacity between 6 and 16 kW, preferably between 6 and 8 kW, measured at -10°C Air source /55°C Water destination, the defrost threshold A is between 40 000 and 200 000 K · s, preferably between 40 000 K·s and 60 000 K.s, even preferably between 45 000 K·s and 55 000K·s, when measured at 2°C Air / 35°C Water.
In this context, it is noted that the absolute temperature difference may not necessarily be referred to, but that a value indicative or reflective of the temperature difference may be sufficient. E.g. the temperatures T_evap and T_air as measured may be normalized prior to be compared to each other.
The evaporating temperature T_evap refers to the thermodynamic equilibrium temperature and, hence, the saturation temperature of the refrigerant (or indirectly the refrigerant temperature during phase change).
The temperature of the refrigerant at the evaporator may relate to the temperature of the refrigerant as such (e.g. measured inside the refrigerant piping) or to the temperature of the evaporator (e.g. at an outer surface of the refrigerant piping or a similar entity or a piping directly connected to the evaporator). The temperature of the air may relate to the temperature of the ambient air, e.g. of the air entering/crossing the evaporator or its vicinity. For example, one could measure the temperature of the air in a neighbored area, with the assumption that the temperature of the air which will cross the heat exchanger will be close to the one measured. The invention may be implemented when the ambient air is in indoor and/or based on a measurement of air inside the duct of the heat pump unit. If the heat pump unit is an outdoor unit, ambient air is the air located outside. For example, for an air heat pump heat exchanger, the air temperature can be measured inside the duct of the heat pump unit and/or outside the duct, in the vicinity of the heat pump.
Ambient air may be understood as air in the vicinity of the evaporator. Thus, ambient air can designate the air of the installation site (room, basement,) for an indoor heat pump unit, as well as the outside for an outdoor heat pump unit.
The invention enables more accurate and simpler control of an appropriate start of the defrosting. The invention is simpler because it may be based on just two physical property measurements, namely the temperature of the ambient air and of the refrigerant/evaporator, or associated indirect measurements, such as pressure measurements (e.g. a pressure sensor and the saturation pressure is converted in a saturation temperature). It has been found that the calculation method behind the invention allows phenomena associated with other physical parameters to be taken into consideration, to which icing is correlated without these parameters being measured.
Defrosting may mean that the evaporator is defrosted by means of inversion of the heat pump cycle. Other defrosting measures are also conceivable.
The defrost parameter may integrate only the sum of events that will actually generate or contribute to icing. For example, special conditions may reduce icing and automatically lead to partial defrosting without a specific command having been issued. It may therefore be appropriate to subtract a quantity of the defrost parameter obtained during the cycles from the quantity calculated above. Specific examples are as follows.
Optionally, the method includes, further optionally determining whether the air temperature T_air is higher than the evaporating temperature T_evap, i.e. whether T_air > T_evap, optionally if the compressor is working; obtaining the defrost parameter DP in particular by integrating the function over time, optionally if T_air >T_evap, optionally incrementing the defrost parameter DP to reflect the temperature difference Δ = T_air - T_evap, if T_air > T_evap, according to as DP (t) = ∫ T_air - T_evap dt, e.g. adding to the defrost parameter DP the temperature difference Δ = T_air - T_evap; and optionally starting the defrosting, if the (incremented) defrost parameter DP reaches, e.g. exceeds, the predetermined defrost parameter threshold A. The parameter A may be a fixed value or may be a variable dependent on the heat pump unit operating conditions, in particular T_air. The threshold value A is a parameter value which may be fixed or dependent on the air temperature T_air. It may be determined based on a table depending on T_air or being a function of T_air. Preferably, only T_air and T_evap are used to control the defrost launch process.
Optionally, the evaporating temperature, the air temperature and the defrost parameter DP are periodically and repeatedly determined, e.g. every second or every minute. This supports accurate determination and control.
Embodiments described in connection with the drawings and as preferred embodiments in the following are based on the approach that the defrost parameter has an initial value of zero and increases during integration over time, in particular until a threshold is exceeded. However, also other approaches may be used in other embodiments of the invention, such as DP having a high initial value and decreasing over time, in particular until a threshold of zero is reached. A further approach in an embodiment of the invention may be to relate DP to a rate and indicate percentages, such as a change from 100% to 0% of DP or from 0% to 100% of DP. Accordingly, in addition to the calculation defined in the claims, further calculations may be carried out in embodiments of the invention, such as relating DP to a reference value.
Optionally, the method includes determining whether the air temperature T_air reaches an air temperature threshold B1, if the compressor is not working, and decrementing, i.e. reducing, the defrost parameter DP to reflect the temperature difference between the air and the air temperature threshold, if T_air > B1, e.g. by subtracting from the defrost parameter DP the temperature difference Δ = Y(T_air - B2). The parameter Y is a weighting factor. More generally, this may mean that it is determined whether the air temperature T_air reaches an air temperature threshold B1, if the compressor is not working, and the defrost parameter DP is obtained in particular by subtracting the integral of the function over time while T_air > B1, from the preceding value of DP with Y = ]0 ; 1].
When the compressor is off, the quantity corresponding to natural defrosting may be subtracted when the outside air has a positive temperature (above 0°C or above a threshold temperature allowing for effective defrosting, e.g. above +2°C), as this would contribute to (natural) de-icing/warming the refrigerant/evaporator. The modification of the defrost parameter therefore corresponds to the integral function, optionally of the temperature difference between the air temperature and the threshold value B1. The threshold value B1 is a parameter value which may be fixed or dependent on the air temperature Tair. It may be determined with a table depending of Tair or be a function of Tair. Alternatively, or cumulatively, a weighting factor may be used to reduce the impact of this correction. A weighting Y factor may be used, with a value 0<Y≤1, in particular between 0.5 and 1. Hence, the cumulative temperature difference between the air and a threshold value B2, when the compressor is not running, may be deducted from the defrost parameter DP. The associated decrementation may be DP(t)= DP(t-dt) - Y(T_air-B2)dt, wherein Y and B2 may be a function of the air temperature. It is noted that B1 as used in (Tair>B1) and B2 as used in Y(Tair-B2)dt may differ, i.e. optionally B1≠B2. In particular, B1 and B2 may differ, if Y is below 1 and acts as a weighting factor. For example, it can be relevant to ensure that Tair>2°C (B1=2°C), but to count as Y(Tair-0°C) (B2=0°C), with 0 < Y < 1. Alternatively, B1 can be equal to B2.
Optionally, known and/or natural defrosting measures may be combined with the defrosting of the present invention. For example, the operational state of the fan may support natural de-icing. The fan may increase the air flow along and through the evaporator, thus making de-icing more efficient. Specifically, the operation of the fan may be decided based on value of the defrost parameter DP. More specifically, the fan may be switched off or on for natural de-icing depending the value of DP: If DP is high, the fan is switched on. If DP is not yet high, the fan is not switched on, as ventilation may be less useful. Thresholds with respect to the switching of the fan are possible. If the temperature is above a first threshold temperature, the fan is switched on. If the temperature is above a second threshold temperature, the fan is switched off. Alternatively or additionally, Y value may depend on the state of the fan.
Optionally, the method may include determining whether the evaporating temperature T_evap reaches a predetermined evaporating temperature threshold C1, if the compressor is working, and decrementing the defrost parameter DP to reflect the temperature difference Δ = Z(T_evap - C2), e.g. by subtracting from the defrost parameter DP the temperature difference Δ = T_evap - C2. When the compressor is working, a defrosting is produced when the temperature of refrigerant in the battery (and the temperature of air entering the battery) is positive or above the threshold C1 (e.g. +2°C). In this case, the temperature of the refrigerant may be seen as too hot, so that it de-ices itself and the evaporator. The amount of the defrost parameter corresponding to this (automatic) defrosting corresponds to the integral of the temperature difference between the refrigerant and the threshold value C1. Hence, the cumulative temperature difference between the refrigerant temperature and the threshold value C2, when the compressor is running and the refrigerant temperature is above the threshold C1, may be deducted from the defrost parameter DP. Thus, an amount of the defrost parameter is compensated. The threshold value C1 is a parameter value which may be fixed or dependent of the air temperature Tair or the refrigerant temperature Tevap. It may be determined with a table depending of Tair or being a function of Tair. Alternatively, or cumulatively, a weighting factor may be used to reduce the impact of this correction. The weighting parameter Z may be 0<Z≤1, in particular between 0.5 and 1. The associated decrementation is DP(t)=DP(t-dt)- Z(T_evap -C2)dt, wherein Z and C2 may be a function of the air temperature and/or the refrigerant temperature. More generally, the defrost parameter DP is determined in particular by subtracting the integral of the function over time while T_evap > C1. It is noted that C1 as used in (Tevap>C1) and C2 as used in Z(Tevap -C2)dt may differ, i.e. optionally C1≠C2. In particular, C1 and C2 may differ if Z is below 1 and acts as a weighting factor.. Alternatively, C1 can be equal to C2.
Optionally, the method includes determining whether the defrost parameter DP reaches a second predetermined defrost parameter threshold D, which is lower than the defrost parameter threshold A (or is, more generally, reached before the defrost parameter threshold A has been reached), if the defrost parameter DP is below (or, more generally, has been reached) the predetermined defrost threshold A, and determining whether the defrost parameter DP differs from an earlier defrost parameter DP(t-n·dt) by at least a predetermined defrost parameter difference threshold E, if DP(t) > D, and starting the defrosting, if DP(t) - DP(t-n.dt) > E, and if DP(t)>D. The preceding defrost parameter DP(t-n·dt) may be a defrost parameter at an earlier time t-n.dt, wherein n may be an integer n=1, 2,3, .... It may, but does not have to be the immediately preceding point in time, namely DP(t-dt), n=1.
The "behavior" of the defrost parameter DP over the time (which corresponds to the mathematical derivative of the defrost parameter) is also relevant to manage the identification of the optimal defrost launch. For example, the more the evolution of the defrost parameter DP over time, the more it means that the threshold value A to launch the optimal defrost time will be achieved quickly, i.e. in the nearest future and that de-icing is inevitable. Thus, it can be specified that the defrosting can be started a bit before the threshold value A, for example to compensate for a certain amount of system inertia when the defrosting is triggered. Knowing the evolution of this parameter DP can therefore help to avoid this inertia, by launching preliminary steps (by referring to the threshold E and a previous parameter value DP(t-n·dt)) to ensure that the defrosting is actually executed at the optimum moment, and not just its command. Alternatively, or cumulatively, a weighting of this factor can be used to reduce the impact of this correction.
Optionally, the method includes re-setting the defrost parameter DP after the defrosting. Further optionally, re-setting of the defrost parameter DP includes setting the defrost parameter to its initial value DP_initial, e.g. DP=0, in particular if the defrosting was successful. Alternatively or additionally, re-setting of the defrost parameter DP may optionally include setting, optionally decrementing, the defrost parameter DP, in particular to between 50% to 90% of the predetermined defrost parameter threshold A, preferably 80%, in particular if the defrosting was unsuccessful. At the end of defrosting, the defrost parameter-count DP may be reset to zero. This is equivalent to considering that there are no more traces of ice on the evaporator, that the heat pump unit has been reset and that the next icing will occur after the same amount of frost as the first launch of the defrosting. However, defrost parameter DP may be only partially reset, for example between 50 and 90% of the maximum threshold value. This may result in a future defrosting starting earlier than if the defrost parameter were to be re-set to zero. An associated evolution may be DP=X·A, X being a parameter between 0 and 1. The parameter X may be a fixed value or dependent on other parameters related to the status of the previous defrosting (duration, success, temperature and/or pressure measurements, etc.) This partial reset may be carried out in particular when parameters measured in the heat pump unit suggest that defrosting has not been carried out completely/optimally. For example, this may be considered if pressure or temperature measured values in the system after defrost are not in the usual range, or evolving not as usual, or premature stopping of defrosting due to a power cut occurred.
The invention is also directed to a control unit for controlling start of defrosting of a heat pump unit for heating and/or cooling applications based on a defrost parameter DP, the heat pump unit comprising an evaporator for heat exchange between air having an air temperature T_air and refrigerant of the heat pump unit characterized by an evaporating temperature T_evap, a sensor for determination of the air temperature T_air, and a sensor for determination of the refrigerant temperature inside the evaporator T_evap, wherein the control unit is configured to determine the evaporating temperature T_evap and the air temperature T_air, determine the defrost parameter DP based on an integral over time of a function depending on the air temperature T_air and on the evaporating temperature T_evap,, and determine the timing for defrosting based on the value of the defrost parameter DP. The dependent claims also relate to the control unit.
The invention is further directed to a computer-implemented method for controlling start of defrosting of a heat pump unit for heating and/or cooling applications based on a defrost parameter DP, the heat pump unit comprising an evaporator for heat exchange between air having an air temperature T_air and refrigerant of the heat pump unit characterized by an evaporating temperature T_evap, a sensor for determination of the air temperature T_air, and a sensor for determination of the refrigerant temperature inside the evaporator T_evap, wherein the method includes determining the evaporating temperature T_evap and the air temperature T_air, determining the defrost parameter DP based on an integral over time of a function depending on the air temperature T_air and on the evaporating temperature T_evap, and determining the timing for defrosting based on the value of the defrost parameter DP. The dependent claims also relate to the computer-implemented method.
The invention is also directed to a heat pump unit comprising the control unit and/or implementing the control method of the invention. Specifically, the heat pump unit of the invention comprises a compressor for compressing and, thus, heating the refrigerant, a condenser for heat exchange with a fluid for at least heating the fluid, an expansion valve for expanding and cooling the refrigerant, and the evaporator for heat exchange between the refrigerant having an evaporating temperature T_evap and ambient air having an air temperature T_air. The fluid to be heated may be water for domestic consumption and/or primary water to be transferred to emitters to heat a building.
The invention is also directed to a computer program comprising instructions to cause the heat pump unit of the invention to execute the steps of the method of the invention, and to a computer-readable medium having stored thereon the computer program of the invention.
The invention, in general, refers to any kind of heat pump unit having a air/refrigerant heat exchanger used as an evaporator, e.g. a reversible water/air heat pump - and/or air source-heat pump unit for at least a heating application. The heat pump unit may be for heating, optionally also cooling, of buildings and/or of domestic water. The heat pump units may be indoor or outdoor units. A heat pump unit of the heat pump system may be a split unit or a monobloc unit. The heat pump system may be configured to heat and/or cool air or water (of a closed loop or of an open loop, such as domestic hot water).
A way to check whether the invention is implemented/is used to start the defrosting of a heat pump, may be to elaborate an experimental plan in a climate chamber including measuring the time period before defrost launch, Tair and Tevap of a target heat pump unit at different humidity level, compressor frequency or Tair, and identify if a pattern is present between defrost launch, Tair and Tevap.
For instance, in this case, measurements of Tair may be made at 7°C; -2°C ; - 7°C; and -15°C, the frequency of compressor may be taken from 40% to 100% of nominal frequency, and the humidity levels may be made between 20 and 80%.
In this case, a pattern may be identified using only defrost time. This may mean that the method for timing defrosting according to the invention has been used. Also alternative or additional ways to check realization of the invention are possible.
Detailed embodiments and further advantages and features related to the present invention are described in the following, wherein these examples shall not be regarded as limiting the invention.

Brief description of the drawings

Fig. 1: shows a diagram reflecting the defrost parameter of the present invention and temperature measurements.
Fig. 2: shows control steps according to an embodiment of the present invention.
Fig. 3: shows control steps according to an embodiment of the present invention.
Fig. 4: shows control steps according to an embodiment of the present invention.
Fig. 5: shows control steps according to an embodiment of the present invention.
Fig. 6: shows control steps according to an embodiment of the present invention.

Detailed description

Figure 1 schematically shows measurement results for the evaporating temperature T_evap and for ambient air having an air temperature T_air, over time, wherein the horizontal line is the 0°C reference. Also, the sum of the difference of these values over time, i.e. the integral DP (t) = ∫ T_air - T_evap dt, is indicated as the hatched area between the graph reflecting the air temperature T_air and the graph reflecting the evaporating temperature T_evap at the time t. This indicates the defrost parameter DP, in particular the defrost parameter DP(t) at a specific time t. Accordingly, in the embodiment reflected in the detailed description, the function f in DP (t) = ∫f(T_air,T_evap )dt is f = T_air - T_evap.
In the embodiment shown in the figures, the timing of defrosting of a heat pump unit is based on this defrost parameter DP and a predetermined defrost parameter threshold A, the latter being determined based on previous measurements, in particular with similar heat pump units. E.g. the defrost parameter threshold A may be between 20 000 and 200 000 K · s.
The heat pump unit comprises an evaporator for heat exchange between the refrigerant characterized by an evaporating temperature T_evap and ambient air having an air temperature T_air, a temperature sensor for direct measurement of the air temperature T_air, and a temperature sensor for direct measurement of the evaporating temperature T_evap. Alternatively, the temperatures may be determined indirectly, e.g. via pressure sensors. The evaporating temperature T_evap and the air temperature T_air are, hence, directly or indirectly, measured.
Figure 2 shows a diagram reflecting steps 1 to 5 of the control of the invention. In step 1, it is determined if the compressor is working. If so, it is determined in step 2 whether the air temperature Tair is higher than the evaporating temperature Tevap. If Tair > Tevap, the defrost parameter DP is incremented, i.e. increased, to reflect the temperature difference Δ = Tair - Tevap, meaning DP(t)=DP(t-dt)+(Tair-Tevap)dt, in Step 3. That means, DP(t) > DP(t-dt). If it is determined in Step 4 that DP(t) > A, i.e. that the defrost parameter DP exceeds the predetermined defrost parameter threshold A, defrosting is started in Step 5.
Figure 3 shows a diagram reflecting in particular Steps 1a and 1b of the control of the invention. In Step 1a, it is determined whether the air temperature T_air exceeds an air temperature threshold B1, after it has been determined in Step 1 that the compressor is not working. The air temperature threshold B1 may be a slightly positive temperature, e.g. +2°C. In Step 1b, the defrost parameter DP is decremented, i.e. reduced, to reflect the temperature difference between the air and the air temperature threshold, meaning DP(t)=DP(t-dt) - Y(Tair-B2)dt, in Step 3. That means, DP(t) < DP(t-dt).
Figure 4 shows a diagram reflecting in particular specific Steps 2a and 2b of the control of the invention. In Step 2a, it is determined whether the evaporating temperature T_evap exceeds a predetermined evaporating temperature threshold C1, if it has been found in Step 1 that the compressor is working.
If T_evap > C1, in Step 2b, the defrost parameter DP is decremented, i.e. reduced, to reflect the temperature difference Δ = T_evap - C2, meaning DP=DP(t-dt)- Z(T_evap -C2)dt, in step 3. That means, DP(t) < DP(t-dt). As an example, C2 may be 0° or 2°C. The Z parameter between may be between 0.5 and 0.95 and used to limit the impact of the defrost compared to frosting. Alternatively, for example, Z=1 and C2 equals C1
Figure 5 shows a diagram reflecting in particular Steps 4a and 4b of the control of the invention. Subsequent to Step 4, it is determined in Step 4a whether the defrost parameter DP reaches a second predetermined defrost parameter threshold D, which is lower than the defrost parameter threshold A, if it is has been found in Step 4 that the defrost parameter DP is below the (first) predetermined defrost threshold A. This means that A > DP(t) > D. As an example, D may be between 75% and 99.9%, in particular about 95% of A. In Step 4b, it is then determined whether the defrost parameter DP exceeds a preceding defrost parameter DP by at least a predetermined defrost parameter difference threshold E, e.g. between successive measurements if DP - DP(t-dt) > E. The defrost parameter difference threshold E may strongly depend on dt and may be determine so that, in the case of constant speed of icing, the next value or the next five values for DP will not exceed the value A.
Figure 6 shows a diagram reflecting in particular specific Steps 6a and 6b of the control of the invention. After the defrosting in Step 5, the defrost status is checked in Step 6 and the defrost parameter DP is reset after the defrosting in Steps 6a or 6b. In Step 6a, the re-setting of the defrost parameter DP includes setting the defrost parameter to e.g. zero, DP=0, in particular if the defrosting was successful. In Step 6b, the re-setting of the defrost parameter DP includes decrementing the defrost parameter DP, in particular to between 50% to 90% of the predetermined defrost parameter threshold A, preferably 80%, in particular if the defrosting was unsuccessful.

Abbreviations

DP: defrost parameter
Tair (Tair): air temperature
Tevap (Tevap): refrigerant temperature in evaporator (evaporating temperature)
A: (first) defrost parameter threshold
B1, B2: air temperature threshold
C1, C2: evaporating temperature threshold
D: (second) defrost parameter threshold
E: defrost parameter difference threshold
X, Y, Z: weighting parameters

Claims

Method for timing defrosting of a heat pump unit for heating and/or cooling applications, based on a defrost parameter DP,
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, a sensor for determination of the air temperature T_air, and a sensor for determination of the evaporating temperature T_evap, wherein the method includes

determining the evaporating temperature T_evap and the air temperature T_air,

determining the defrost parameter DP based on an integral over time of a function depending on the air temperature T_air and on the evaporating temperature T_evap, and

determining the timing for defrosting based on the value of the defrost parameter DP.
Method of claim 1, wherein the method additionally includes consideration of a predetermined defrost parameter threshold A and
determining the defrost parameter DP based on integration of the function overtime, optionally incrementing DP to reflect the temperature difference Δ = T_air - T_evap, and

starting defrosting, if the defrost parameter DP reaches the predetermined defrost parameter threshold A.
Method of any of the preceding claims, wherein the method includes
determining whether the air temperature T_air reaches an air temperature threshold B1, determining the defrost parameter DP based on a subtraction of the integral of the function over time while T_air > B1, optionally decrementing DP to reflect the temperature difference Δ = T_air - B2, while T_air > B1, wherein B1 and B2 are air temperature thresholds.
Method of any of the preceding claims, wherein the method includes
determining whether the evaporating temperature T_evap reaches a predetermined evaporating temperature threshold C1 and

determining the defrost parameter DP based on a subtraction of the integral of the function over time while T_evap > C1, optionally decrementing the defrost parameter DP to reflect the temperature difference Δ = T_evap - C2 over time, while T_evap > C1, wherein C1 and C2 are evaporating temperature thresholds.
Method of any of the preceding claims, wherein the method includes
if the defrost parameter DP has reached the predetermined defrost threshold A, determining whether the defrost parameter DP reaches a second predetermined defrost parameter threshold D, reachable before the defrost parameter threshold A,

determining whether the defrost parameter DP(t) differs from an earlier defrost parameter DP(t-n·dt) by at least a predetermined defrost parameter difference threshold E, if DP(t) > D(t-n.dt), wherein n is an integer above zero, and

starting defrosting, if DP(t) - DP(t-n·dt) > E and if DP(t)>D.
Method of any of the preceding claims, wherein the method includes re-setting the defrost parameter DP after defrosting,
optionally to its initial value DP_initial, further optionally to zero, in particular if the defrosting was successful, and/or

optionally to between 50% to 90%, preferably 80%, of the predetermined defrost parameter threshold A, in particular if the defrosting was unsuccessful.
Control unit for controlling the timing of defrosting of a heat pump unit for heating and/or cooling applications, based on a defrost parameter DP,
the heat pump unit comprising an evaporator for heat exchange between air having an air temperature T_air and refrigerant of the heat pump having an evaporating temperature T_evap, a sensor for determination of the air temperature T_air, and a sensor for determination of the evaporating temperature T_evap, wherein the control unit is configured for

determining the evaporating temperature T_evap and the air temperature T_air,

determining the defrost parameter DP based on an integral over time of a function depending on the air temperature T_air and on the evaporating temperature T_evap, and

determining the timing for defrosting based on the value of the defrost parameter DP.
Control unit of claim 7, wherein the control unit is additionally configured to include a predetermined defrost parameter threshold A and configured for
determining the defrost parameter DP based on integration of the function over time, if T_air >T_evap , optionally incrementing DP to reflect the temperature difference Δ = T_air - T_evap, and

determining the timing for defrosting, if the defrost parameter DP reaches the predetermined defrost parameter threshold A.
Control unit of claim 7 or 8, wherein the control unit is configured for
determining whether the air temperature T_air reaches an air temperature threshold B1, determining the defrost parameter DP based on a subtraction of the integral of the function over time while T_air > B1, optionally decrementing DP to reflect the temperature difference Δ = T_air - B2, while T_air > B1, wherein B1 and B2 are air temperature thresholds.
Control unit of any of the preceding claims 7 to 9, wherein the control unit is configured for
determining whether the evaporating temperature T_evap reaches a predetermined evaporating temperature threshold C1, and

determining the defrost parameter DP based on a subtraction of the integral of the function over time while T_evap > C1, optionally decrementing the defrost parameter DP to reflect the temperature difference Δ = T_evap - C2 over time, if T_evap > C1, wherein C1 and C2 are evaporating temperature thresholds.
Control unit of any of the preceding claims 7 to 10, wherein the control unit is configured for
if the defrost parameter DP has reached the predetermined defrost threshold A, determining whether the defrost parameter DP reaches a second predetermined defrost parameter threshold D, reachable before the defrost parameter threshold A,

determining whether the defrost parameter DP(t) differs from an earlier defrost parameter DP(t-n·dt) by at least a predetermined defrost parameter difference threshold E, if DP(t) > D(t-n·dt), wherein n is an integer above zero, and

determining the timing for defrosting, if DP(t) - DP(t-n.dt) > E and if DP(t)>D.
Control unit of any of the preceding claims 7 to 11, wherein the control unit is configured for re-setting the defrost parameter DP after defrosting,
optionally to its initial value DP_initial, further optionally to zero, in particular if the defrosting was successful, and/or

optionally to between 50% to 90%, preferably 80%, of the predetermined defrost parameter threshold A, in particular if the defrosting was unsuccessful.
Heat pump unit having a sensor for determining the ambient air temperature T_air and 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 any of the preceding claims 7 to 12 and/or adapted to execute the steps of the method of any of the preceding claims 1 to 6.
Heat pump unit according to claim 13 having a heating capacity between 6 and 16 kW, preferably between 6 and a kW, measured at -10°C Air source /55°C Water destination, in which the defrost threshold A is between 40 000 and 200 000 K · s, preferably between 40 000 K · s and 60 000 K · s, more preferably between 45 000 K · s and 55 000K · s, when measured at 2°C Air/ 35°C Water.
A computer program comprising instructions to cause the heat pump unit of claim 13 or 14 to execute the steps of the method of any of the preceding claims 1 to 6.