Method for defrosting a heat pump and heat pump comprising a defrost system

Abstract

Method for defrosting an air source heat pump (2) comprising a plurality of outdoor evaporators (6, 6', 6", 6‴) each comprising a number of coils (20, 20', 20", 20‴) each comprising fins (8, 8', 8", 8‴) and pipes (30, 30', 30", 30‴). The heat pump (2) comprises a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils (20, 20', 20", 20‴). The method comprising: a) by using at least one temperature sensor (12) measuring at least one temperature (T1, T2, T3) of at least one of the coils (20, 20', 20", 20‴) of at least one of the outdoor evaporators (6, 6', 6", 6‴); b) activating the defrost system and hereby performing several defrost processes each having a duration (Tdefrost) separated by a pause (Tpause) without defrosting, when the at least one temperature (T1, T2, T3) is below a predefined temperature value, d) by using at least one temperature sensor (16, 16', 16") measuring at least one outdoor temperature (Tout 1, Tout 2, Tout 3), The method comprises the following steps: determine a maximum allowable pause time (Tpause, max) in dependency of the at least one outdoor temperature (Tout 1, Tout 2, Tout 3).

Description

Field of invention

[0001] The present invention relates to a method for defrosting an air source heat pump comprising a plurality of outdoor evaporators each comprising a number of coils each comprising fins and pipes. The present invention also relates to an air source heat pump comprising a plurality of outdoor evaporators each comprising a number of coils, said heat pump comprising a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils.Prior art

[0002] Different defrost control strategies have been used in the air source heat pumps with the time control method being the most common one. These control strategies include time control, pressure difference control, and temperature control.

[0003] Defrosting is an energy demanding process. Accordingly, one would like to wait as long as possible to initiate defrosting process. The performance, however, decreases as the frost is created and moreover, it is difficult to defrost if one waits too long time.

[0004] The prior art methods for controlling defrost of heat pumps detect a fin temperature and applies it for determining when to start and stop the defrost procedure. These systems are, however, based on a single or two control methods. Accordingly, the prior art solutions have a poor performance.

[0005] Thus, there is a need for a method and an air source heat pump which enables a higher performance, and which reduces or even eliminates the above mentioned disadvantages of the prior art.Summary of the invention

[0006] The object of the present invention can be achieved by a method as defined in claim 1 and by an air source heat pump as defined in claim 10. Preferred embodiments are defined in the dependent subclaims, explained in the following description and illustrated in the accompanying drawings.

[0007] The method according to the invention is a method for defrosting an air source heat pump comprising a plurality of outdoor evaporators each comprising a number of fans and a number of coils each comprising fins and pipes, wherein the heat pump comprises a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils, the method comprising: a) by using at least one (coil) temperature sensor measuring at least one (coil) temperature of at least one of the coils of at least one of the outdoor evaporators; b) activating the defrost system and hereby performing several defrost processes each having a duration (T defrost ) separated by a pause (T pause ) without defrosting, when the at least one (coil) temperature is below a predefined temperature value, d) by using at least one (ambient) temperature sensor measuring at least one outdoor (ambient) temperature (T out 1 , T out 2 , T out 3 ), wherein the method comprises the following steps: determine a maximum allowable pause time (Tpause, max) in dependency of the at least one outdoor (ambient) temperature (Tout 1, Tout 2, Tout 3).

[0008] Hereby, it is possible to provide a method that enables a higher performance, and which reduces or even eliminates the above mentioned disadvantages of the prior art.

[0009] The method according to the invention is a method for defrosting an air source heat pump. The heat pump comprises a plurality of outdoor evaporators each comprising a number of fans and a number of coils each comprising fins and pipes.

[0010] The heat pump comprises a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils.

[0011] The method comprising: a) by using at least one temperature sensor measuring at least one temperature of at least one of the coils of at least one of the outdoor evaporators; b) activating the defrost system and hereby performing several defrost processes each having a duration (T defrost ) separated by a pause (T pause ) without defrosting, when the at least one temperature is below a predefined temperature value, d) by using at least one temperature sensor measuring at least one outdoor temperature (T out 1 , T out 2 , T out 3 ).

[0012] The method comprises the following steps: determine a maximum allowable pause time (T pause , max ) in dependency of the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ).

[0013] By determining a maximum allowable pause time (T pause , max ) in dependency of the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ), it is possible to provide an adaptive method that automatically adapts to the the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ).

[0014] The term "determine" in the expression: "determine a maximum allowable pause time (T pause , max ) in dependency of the at least one outdoor temperature (T out 1 , T out 2 , T out 3 )" may by way of example refer to: 1) a calculation by using one or more predefined formulas or; 2) application of a lookup table comprising predefined values; 3) application of a predefined algorithm.

[0015] In an embodiment, the maximum allowable pause time (T pause , max ) is in the range 10-300 min.

[0016] In an embodiment, the maximum allowable pause time (T pause , max ) is in the range 60-200 min.

[0017] In an embodiment, the maximum allowable pause time (T pause , max ) is in the range 100-150 min.

[0018] In an embodiment, the temperature is a surface temperature.

[0019] In an embodiment, the predefined temperature value is 0°C.

[0020] In an embodiment, the predefined temperature value is 0.5°C.

[0021] In an embodiment, the predefined temperature value is 0.9°C.

[0022] In an embodiment, the defrosting medium is water containing glycol.

[0023] In an embodiment, the defrosting medium is a high temperature refrigerant.

[0024] In an embodiment, the defrosting medium is a medium temperature refrigerant.

[0025] In an embodiment, the at least one temperature sensor is arranged to detect a temperature at a surface of at least one of the coils.

[0026] In an embodiment, at least one temperature is an average temperature calculated on the basis of two or more temperatures detected by at least two temperature sensors.

[0027] The pause (T pause ) without defrosting is the elapsed time since the end of the last defrosting process for evaporator in question.

[0028] In an embodiment, the method comprising: by using at least one sensor detecting a pressure difference (ΔP) across an evaporator and activating the defrost system when the pressure difference (ΔP) across the evaporator is above a predefined pressure difference level (ΔP max ).

[0029] Hereby, it is possible to apply the pressure difference (ΔP) across the evaporator to activate the defrost system.

[0030] In an embodiment, the method comprising: by using at least one humidity sensor detecting the relative humidity (H) of the ambient air, wherein the method comprising: activating the defrost system in dependency of the relative humidity (H).

[0031] In an embodiment, the method comprising: determining a minimum allowable pause time (T pause min safety ), ensuring that the defrost system is only activated when the pause (T pause ) has exceeded the minimum allowable pause time (T pause min safety ).

[0032] In an embodiment, the method comprising: determining a maximum allowable pause time (T pause max safety ), determining the time / pause (T pause ) since the last defrost process and activating the defrost system when the pause (T pause ) has exceeded the maximum allowable pause time (T pause , max ).

[0033] In an embodiment, the method comprising: detecting a temperature indicative of the temperature of the defrosting medium, wherein the duration (T defrost ) of a defrost process is determined in dependency of: a) the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ) and b) the temperature indicative of the temperature of the defrosting medium.

[0034] In an embodiment, the method comprising: detecting a temperature of the defrosting medium, wherein the duration (T defrost ) of a defrost process is determined in dependency of: a) the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ) and b) the temperature of the defrosting medium.

[0035] In an embodiment, the method comprising: detecting a temperature of a liquid in a return water line (38) wherein the duration (T defrost ) of a defrost process is determined in dependency of: a) the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ) and b) the temperature of the liquid in a return water line (38).

[0036] In an embodiment, the method defrosts all evaporators sequentially one at a time.

[0037] In an embodiment, no break is provided in between.

[0038] In an embodiment, the method defrosts a fraction of the evaporators sequentially one at a time. Hereby, the remaining fraction of the evaporators can be operated.

[0039] In an embodiment, no break is provided in between.

[0040] In an embodiment, a pause is provided between the defrost of the last evaporator and the next process (the subsequent defrost of the first evaporator).

[0041] In an embodiment, no pause is provided between the defrost of the last evaporator and the next process (the subsequent defrost of the first evaporator).

[0042] In an embodiment, the predefined pressure difference level (ΔP max ) corresponds to 20-50 % blockage ratio.

[0043] In an embodiment, the "blockage ratio" is defined as the ratio between the frost thickness and half of the fin spacing (the distance between adjacent fins).

[0044] In an embodiment, the predefined pressure difference level (ΔP max ) corresponds to 30-45 % blockage ratio.

[0045] In an embodiment, the predefined pressure difference level (ΔP max ) corresponds to 40 % blockage ratio.

[0046] In an embodiment, the defrosting medium is recircled water-containing liquid.

[0047] In an embodiment, the defrosting medium comprises glycol.

[0048] In an embodiment, the defrosting medium is cooled refrigerant.

[0049] In an embodiment, the defrosting medium is heated refrigerant.

[0050] In an embodiment, the the heat pump comprises an air inlet and an air outlet, wherein the method comprises detecting a temperature of the air inlet and a temperature of the air outlet, wherein: a) when the difference the temperature of the air inlet and temperature of the air outlet during a defrost process exceeds a predefined level, operation of the fans of the heat pump is initiated and maintained for a predefined time period.

[0051] This is done to keep the heat in the casing by running the fans upwards for a user defined period. When the time period has exceeded a new measurement of one of the sensors is needed to start the period.

[0052] In an embodiment, the method comprising: determining a maximum allowable pause time T pause, max safety , ensuring that the defrost system is activated when the pause (T pause ) has exceeded the maximum allowable pause time T pause, max safety .

[0053] In an embodiment, the maximum allowable pause time T pause, max safety is set by the user, wherein the maximum allowable pause time T pause, max safety is overruling the determined T pause, max .

[0054] In an embodiment, the T pause max savety is in the range 15-600 min.

[0055] In an embodiment, the T pause max savety is in the range 90-400 min.

[0056] In an embodiment, the T pause max savety is in the range 200-600 min.

[0057] T pause max savety may depend on the number of evaporators og the given system.

[0058] In an embodiment, the method comprising: defining a minimum allowable pause time T pause, min safety , ensuring that the defrost system is not activated when before the pause (T pause ) has exceeded the required minimum allowable pause time T pause, min safety .

[0059] In an embodiment, the T pause min safety is in the range 10-100 min.

[0060] In an embodiment, the T pause min safety is in the range 10-75 min.

[0061] In an embodiment, the T pause min safety is in the range 10-25 min.

[0062] T pause min savety may depend on the number of evaporators og the given system.

[0063] The heat pump according to the invention is an air source heat pump comprising a plurality of outdoor evaporators each comprising a number of fans and a number of coils, said heat pump comprising a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils, wherein the heat pump comprises at least one temperature sensor arranged and configured to measure at least one temperature (T 1 , T 2 , T 3 ) of at least one of the coils of at least one of the outdoor evaporators, wherein the defrost system is configured to perform several defrost processes each having a duration (T defrost ) separated by a pause (T pause ) without defrosting when the at least one temperature (T 1 , T 2 , T 3 ) is below a predefined temperature value, the defrost system comprising: at least one temperature sensor arranged and configured to measure at least one ambient (outdoor) temperature (T out 1 , T out 2 , T out 3 ), wherein the defrost system is configured to: determining a maximum allowable pause time (T pause , max ) in dependency of: a) the at least one ambient (outdoor) temperature (T out 1 , T out 2 , T out 3 ).

[0064] Hereby, it is possible to provide a provide a heat pump that enables a higher performance, and which reduces or even eliminates the above mentioned disadvantages of the prior art.

[0065] The heat pump is an air source heat pump comprising a plurality of outdoor evaporators. Each outdoor evaporator comprises a number of coils.

[0066] The heat pump comprises a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils.

[0067] The heat pump comprises at least one temperature sensor arranged and configured to measure at least one temperature (T 1 , T 2 , T 3 ) of at least one of the coils of at least one of the outdoor evaporators.

[0068] In an embodiment, an average temperature is calculated on the basis of temperature measurements made by several temperature sensors.

[0069] The defrost system is configured to perform several defrost processes each having a duration (T defrost ) separated by a pause (T pause ) without defrosting when the at least one temperature (T 1 , T 2 , T 3 ) is below a predefined temperature value.

[0070] The defrost system comprises: at least one temperature sensor arranged and configured to measure at least one ambient (outdoor) temperature (T out 1 , T out 2 , T out 3 ).

[0071] The defrost system is configured to: determine a maximum allowable pause time (T pause , max ) in dependency of: a) the at least one ambient (outdoor) temperature (T out 1 , T out 2 , T out 3 ).

[0072] The term "determine" may by way of example refer to: 1) a calculation by using one or more predefined formulas or; 2) application of a lookup table comprising predefined values; 3) application of a predefined algorithm.

[0073] By determining a maximum allowable pause time (T pause , max ) in dependency of the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ), it is possible to provide an adaptive defrost system i that automatically adapts to the the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ).

[0074] In an embodiment, the defrost system comprises: at least one sensor arranged and configured to detect a pressure difference (ΔP) across an evaporator, wherein the defrost system is configured to activating the defrost system when the pressure difference (ΔP) across the evaporator is above a predefined pressure difference level (ΔPmax).

[0075] Hereby, the defrost system can take into consideration the pressure difference (ΔP) across the evaporator.

[0076] In an embodiment, the defrost system comprises: at least one humidity sensor arranged and configured to detect the relative humidity (H) of the ambient air, wherein the defrost system is configured to: determine a maximum allowable pause time (T pause , max ) in dependency of the relative humidity (H).

[0077] Hereby, the defrost system can take into consideration the relative humidity (H) of the ambient air.

[0078] The term "determine" may by way of example refer to: 1) a calculation by using one or more predefined formulas or; 2) application of a lookup table comprising predefined values; 3) application of a predefined algorithm.

[0079] In an embodiment, the air source heat pump is configured to ensure that the defrost system is only activated when the pause (T pause ) has exceeded the minimum allowable pause time (T pause min safety ).

[0080] Hereby, it is possible ensure that pause (T pause ) is not less than the minimum allowable pause time (T pause , min ).

[0081] In an embodiment, the air source heat pump is configured to: determine the time since the last defrost process and activate the defrost system when the pause (T pause ) has exceeded a predefined maximum allowable pause time (T pause , max ).

[0082] In an embodiment, the air source heat pump is configured to: detect a temperature of the defrosting medium, wherein the duration (T defrost ) of a defrost process is determined in dependency of: a) the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ) and b) the temperature of the defrosting medium.

[0083] In an embodiment, the air source heat pump is configured to defrost a fraction of the evaporators at a time only.

[0084] In an embodiment, the air source heat pump is configured to defrost all evaporators sequentially one at a time. In an embodiment, no break is provided in between.

[0085] In an embodiment, the air source heat pump is configured to defrost a fraction of the evaporators sequentially one at a time.

[0086] In an embodiment, a pause is provided between the defrost of the last evaporator and the next process (the subsequent defrost of the first evaporator).

[0087] In an embodiment, no pause is provided between the defrost of the last evaporator and the next process (the subsequent defrost of the first evaporator).

[0088] In an embodiment, the predefined pressure difference level (ΔP max ) corresponds to 20-50 % blockage ratio.

[0089] In an embodiment, the heat pump comprises an air inlet and an air outlet, wherein the heat pump is configured to detect a temperature of the air inlet and a temperature of the air outlet, wherein the heat pump is configured to ensure that: a) when the difference the temperature of the air inlet and temperature of the air outlet during a defrost process exceeds a predefined level, operation of the fans is initiated and maintained for a predefined time period.

[0090] In an embodiment, the least one temperature sensor is arranged and configured to measure at least one surface temperature of at least one of the coils of at least one of the outdoor evaporators.

[0091] In an embodiment, the predefined pressure difference level (ΔP max ) corresponds to 35-40 % blockage ratio.

[0092] In an embodiment, the at least one temperature sensor is arranged to detect a temperature at a surface of at least one of the coils.

[0093] In an embodiment, the at least one temperature is an average temperature calculated on the basis of two or more temperatures detected by at least two temperature sensors.Description of the Drawings

[0094] The invention will become more fully understood from the detailed description given herein below. The accompanying drawings are given by way of illustration only, and thus, they are not limitative of the present invention. In the accompanying drawings: Fig. 1shows a schematic view of a heat pump according to the invention; Fig. 2shows a flowchart of a method according to the invention; Fig. 3Ashows a schematic cross-sectional view of the fins of coil of a heat pump according to the invention; Fig. 3Bshows another schematic cross-sectional view of the fins of the coil shown in Fig. 3A; Fig. 4shows a table illustrating the defrosting processes of a method according to the invention; Fig. 5shows a heat pump according to the invention comprising an indoor unit installed in a building and Fig. 6shows a flowchart of a method according to the invention. Detailed description of the invention

[0095] Referring now in detail to the drawings for the purpose of illustrating preferred embodiments of the present invention, a heat pump 2 of the present invention is illustrated in Fig. 1.

[0096] Fig. 1 illustrates a schematic view of a heat pump 2 according to the invention. The heat pump 2 comprises an indoor unit 29 comprising compressors or a pump depending on the application (not shown). The indoor unit 29 is connected to several outdoor coils 20, 20', 20", 20‴ by using forward lines 48', 50' and return lines 48, 50. The dotted lines are used in the defrost circuit. The solid lines are used in the refrigerant circuit.

[0097] Each of the outdoor coils 20, 20', 20", 20‴ comprises: an evaporator 6, 6', 6", 6‴; a fan 42; a pipe 30, 30', 30", 30‴ for conducting the refrigerant through the coil 20, 20', 20", 20‴ and a pipe 31, 31', 31", 31‴ for conducting a defrosting medium through the coil 20, 20', 20", 20'".

[0098] The refrigerant flows from the indoor unit 29 to the forward lines 50' to the first coil 20 through a first refrigerant pipe 30, to the second coil 20' through a second refrigerant pipe 30', to the third coil 20 through a third refrigerant pipe 30" and to the fourth coil 20‴ through a fourth refrigerant pipe 30'".

[0099] The refrigerant is returned to the indoor unit 29 from the coils 20, 20', 20", 20‴ through the return line 50.

[0100] The indoor unit 29 supplies a defrosting medium to the outdoor coils 20, 20', 20", 20‴ by using a forward line 48 that is connected toto the first coil 20 through a first defrosting pipe 31, to the second coil 20' through a second defrosting pipe 31', to the third coil 20 through a third defrosting pipe 31" and to the fourth coil 20‴ through a fourth defrosting pipe 31'".

[0101] The defrosting medium is returned to the indoor unit 29 from the coils 20, 20', 20", 20‴ through the return line 48.

[0102] In an embodiment, the indoor unit 29 supplies hot water to a plate heat exchanger (not shown) via a supply water line 36. The the indoor unit 29 receives water from the heat exchanger via a return water line 38.

[0103] A differential pressure sensor 14 is arranged and configured to detect the pressure difference ΔP across the first evaporator 6. Hereby, the differential pressure sensor 14 is configured to detect when the pressure difference ΔP across the first evaporator 6 is above a predefined pressure difference level ΔP max .

[0104] A first temperature sensor 16 is arranged to detect an ambient (outdoor) temperature T out 1 . The ambient (outdoor) temperature T out 1 is typically measured at the air-side inlet of the first evaporator 6. In an embodiment, additional outdoor temperatures are detected. In an embodiment, the ambient temperature T out 1 is the temperature of the inlet (an inlet temperature). The temperature sensors applied for measuring the ambient (outdoor) temperature T out 1 or other ambient temperatures are typically arranged outside the coils 20, 20', 20", 20‴.

[0105] In an embodiment, a first ambient (outdoor) temperature T out 1 is measured at an air-side inlet of the first evaporator 6 by using a first temperature sensor 16, wherein a second ambient (outdoor) temperature T out 2 is measured at an air-side inlet of the first evaporator 6 by using a second temperature sensor 16'.

[0106] In an embodiment, a first ambient (outdoor) temperature T out 1 is measured outside the first evaporator 6 by using a first temperature sensor 16, wherein a second ambient (outdoor) temperature T out 2 is measured outside the first evaporator 6 by using a second temperature sensor 16', wherein a third ambient (outdoor) temperature T out 3 is measured outside one of the remaining evaporators (e.g. the second evaporator 6') by using a third temperature sensor 16". In general, the ambient (outdoor) temperatures may be measured by using temperature sensors arranged outside, typically at or in close proximity to an air-side inlet.

[0107] In an embodiment, an outlet temperature T outlet is measured at the outlet of the first evaporator 6 by using a temperature sensor 46.

[0108] In an embodiment, a temperature of the liquid in the return water line 38 is measured by using a temperature sensor 32.

[0109] In an embodiment, a temperature of the fins of the first evaporator 6 is measured by using a temperature sensor 12.

[0110] In an embodiment, heat pump 2 comprises a humidity sensor 40 arranged and configured for detecting the relative humidity of the ambient air. In an embodiment, the humidity sensor 40 is outside one of the coils 20, 20', 20", 20"'. In an embodiment, the humidity sensor 40 is at the air-side inlet of an evaporator. In general, the relative humidity of the ambient air may be measured by using one or more humidity sensors arranged outside, typically at or in close proximity to an air-side inlet.

[0111] In an embodiment, heat pump 2 comprises a control unit 10 arranged and configured for carrying out the method according to the invention. In an embodiment, the control unit 10 arranged and configured for receiving data from the sensors of the heat pump 2. The data may be transmitted to the control unit 10 via wires or via wireless connections. In an embodiment, the control unit 10 is electrically connected to the indoor unit 29. The indoor unit 29 is connected to an energy source (e.g. the mains) which is not shown in Fig. 1.

[0112] The heat pump 2 comprises an air inlet S3 and an air outlet S4.

[0113] In an embodiment, the heat pump 2 is configured to detect a temperature of the air inlet S3 and a temperature of the air outlet S4. The temperatures may be detected by using temperature sensors.

[0114] In an embodiment, the heat pump 2 is configured to ensure that: a) when the difference the temperature of the air inlet S3 and temperature of the air outlet S4 during a defrost process exceeds a predefined level, operation of the fans is initiated and maintained for a predefined time period.

[0115] Fig. 2 illustrates a flowchart of a method according to the invention. The first step is to start the heat pump 2.

[0116] In the next step it is evaluated if the temperature of the fins T Fin is below a predefined temperature level. In an embodiment, the predefined temperature level is 0°C.

[0117] If the temperature of the fins T Fin is above the predefined temperature level (e.g. 0°C), no defrosting is initiated.

[0118] If the temperature of the fins T Fin is below the predefined temperature level (e.g. 0°C), it is evaluated if: a) the pressure difference ΔP across the first evaporator 6 is above a predefined pressure difference level ΔP max or b) the pause T pause has exceeded the maximum allowable pause time T pause, max .

[0119] If either of these two conditions a), b) is meet, the defrosting is initiated and the defrosting is maintained for a time period T defrost .

[0120] In an embodiment, it is ensured that the time period T defrost exceeds a predefined minimum defrosting time period T defrost, min . When the time period T defrost exceeds the predefined minimum defrosting time period T defrost, min , the defrosting of the evaporator can be stopped as long as the fin temperature is above the predefined threshold. If T defrost exceeds T defrost max safety , the defrost process is stopped.

[0121] In an embodiment, the minimum defrosting time period T defrost, min is determined (e.g. calculated) on the basis of at least one detected air temperature and / or a return temperature of the defrosting media.

[0122] In an embodiment, the maximum allowable pause time T pause, max is determined (e.g. calculated) on the basis of at least one detected air temperature and / or a relative humidity measurement of ambient air.

[0123] Even though it is not shown, the fin temperature is detected after "start defrosting". Moreover, when the time exceeds the T defrost,min , the fin temperate will be checked and if the fin temperature is above a predefined threshold, or the T defrost exceeds T defrost max safety , the defrost process will be stopped.

[0124] Fig. 3A illustrates a schematic cross-sectional view of the fins 8, 8', 8", 8‴ of coil 20 of a heat pump according to the invention. An air flow 24 enters the air gap 22 between adjacent fins 8, 8', 8", 8". Since the fin temperature is above 0°C, no ice is present on the fins 8, 8', 8", 8". Accordingly, the distance between adjacent fins (fin spacing) D 2 is maximum and thus the pressure difference (ΔP) across the evaporator of the coil 20 is low and not above a predefined pressure difference level ΔP max .

[0125] Fig. 3B illustrates another schematic cross-sectional view of the fins 8, 8', 8", 8‴ of the coil 20 shown in Fig. 3A. The fin temperature is below 0°C and the fins 8, 8', 8", 8" are covered by a layer of ice 26.

[0126] Accordingly, the distance between adjacent fins (fin spacing) D 1 is lower than in Fig. 3A. Accordingly, the pressure difference (ΔP) across the evaporator of the coil 20 has increased.

[0127] Fig. 4 illustrates a table illustrating the defrosting processes of a method according to the invention. No pause is provided between the defrost of each evaporator and the next process (the subsequent defrost of the first evaporator).

[0128] At time t 0 , defrost of evaporator 1 is initiated. At time t 1 evaporator 1 has been defrosted.

[0129] At time t 1 , defrost of evaporator 2 is initiated. At time t 2 evaporator 2 has been defrosted.

[0130] At time t 2 , defrost of evaporator 3 is initiated. At time t 3 evaporator 3 has been defrosted.

[0131] At time t 3 , defrost of evaporator 4 is initiated. At time t 4 evaporator 4 has been defrosted.

[0132] A pause is provided between time t 4 and t 5 .

[0133] At time t 5 , defrost of evaporator 1 is initiated. At time t 6 evaporator 1 has been defrosted.

[0134] At time t 6 , defrost of evaporator 2 is initiated. At time t 7 evaporator 2 has been defrosted.

[0135] At time t 7 , defrost of evaporator 3 is initiated. At time t 8 evaporator 3 has been defrosted.

[0136] At time t 8 , defrost of evaporator 4 is initiated. At time t 9 evaporator 4 has been defrosted. A break is conducted from time t 9 to t 10 .

[0137] Fig. 5 illustrates a heat pump 2 according to the invention comprising an indoor unit 29 installed in a building 44. The heat pump 2 comprises an outdoor coil 20, in which a fan 42 is arranged and configured to suck an air flow 24 into the coil 20 through air gaps between adjacent fins 8

[0138] Fig. 5 illustrates an air to air heat pump 2. A heat exchanger 120 is arranged and configured to heat water inside the building 44. In an embodiment the water is used for district heating. The heat exchanger 120 is arranged and configured to heat the room in which the heat exchanger 120 is placed.

[0139] The heat pump 2 comprises an indoor unit 29 arranged and configured to receive a refrigerant through a return line 52 that connects the outlet of the outdoor coil 20 to the indoor unit 29. The indoor unit 29provides pressurised refrigerant outdoor coil 20 via the forward line 54.

[0140] The indoor unit 29 is in fluid communication with the heat exchanger 120 via the forward line 56 and via the return line 58.

[0141] The heat pump 2 comprises a defrosting circuit that delivers a defrosting medium from the indoor unit 29 to the outdoor coil 20 through the forward defrosting line 62. The defrosting medium is returned to the indoor unit 29 via the return defrosting line 64. In an embodiment, the defrosting medium is refrigerant received by the compressor via the forward line 56.

[0142] Fig. 6 illustrates a flowchart of a method according to the invention. The first step is to start the heat pump.

[0143] In the next step it is evaluated if T pause (the elapsed time since the end of the last defrosting process for evaporator in question) has exceeded a predefined level, T pause min safety . If T pause has not yet exceeded the predefined level T pause min safety , a waiting step is conducted until T pause exceeds the predefined level, so the following condition is meet: T pause > T pause min safety .

[0144] In the next steps it is evaluated if: a) the pressure difference ΔP across the evaporator is above a predefined pressure difference level ΔP max and b) If T pause (the elapsed time since the end of the last defrosting process for evaporator in question) has exceeded a determined maximum allowable time T pause, max .

[0145] If either condition a) or condition b) is meet, it is evaluated if the coil temperature exceeds a predefined threshold 1. If the coil temperature exceeds the predefined threshold 1, the defrost process is initiated. If not, the previous steps (after Start) are repeated.

[0146] If the pressure difference ΔP across the evaporator is not above the predefined pressure difference level ΔP max and if T pause (the elapsed time since the end of the last defrosting process for evaporator in question) has not exceeded a predefined maximum allowable level T pause, max , it is evaluated if the pressure difference ΔP exceeds the predefined pressure difference level ΔP max and if T pause has exceeded a the predefined maximum allowable level T pause, max .

[0147] During the defrost process the defrost time T defrost is initially set to zero and the defrost process continues until the defrost time T defrost exceeds a predefined minimum requires defrost time level, T defrost, min .

[0148] In the next step is evaluated if: c) the defrost time T defrost has exceeded a predefined maximum allowable defrost time T defrost max safety and d) the coil temperature has exceeded a predefined threshold 2.

[0149] If either of the conditions c) or d) are meet, the defrost process (of evaporation 1) is stopped.

[0150] If the defrost time T defrost has not exceeded the predefined maximum allowable defrost time T defrost, max the defrost process continues.

[0151] Similarly, if the coil temperature has not yet exceeded the predefined threshold 2 the defrost process continues.

[0152] When the defrost process of evaporator 1 has ended the defrost process of evaporator 2 is initiated. This process continues until all evaporators have been defrosted.

[0153] In an embodiment, the threshold 1 is 0.5 °C or 0°C.List of reference numerals

[0154] 2Heat pump 6, 6', 6", 6‴Evaporator 8, 8', 8" 8"Fin 10Control unit 12Temperature sensor 14Pressure differential sensor 16Temperature sensor 20, 20', 20", 20‴Coil 22Air gap 24Air flow 26Ice 28Compressor 29Indoor unit 30, 30', 30", 30‴Pipe ΔPPressure difference T out 1 , T out 2 , T out 3 Ambient temperature T outlet Outlet temperature T 1 , T 2 , T 3 Temperature T pause Pause (duration) T pause, max Maximum allowable pause time ΔP min Predefined pressure difference level T defrost Duration HRelative humidity of the ambient air S3Air inlet S4Air outlet D 1 , D 2 Distance between adjacent fins (fin spacing) 32Temperature sensor 36Supply water line 38Return water line 40Humidity sensor 42Fan 44Building 46Temperature sensor 48Forward line 48'Return line 50Forward line 50'Return line 52Return line 54Forward line 56Forward line 58Return line 60Valve 62Defrosting line 64Defrosting line 120Heat exchanger

Claims

1. Method for defrosting an air source heat pump (2) comprising a plurality of outdoor evaporators (6, 6', 6", 6‴) each comprising a number of fans (42) and a number of coils (20, 20', 20", 20‴) each comprising fins (8, 8', 8", 8‴) and pipes (30, 30', 30", 30‴), wherein the heat pump (2) comprises a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils (20, 20', 20", 20‴), the method comprising: a) by using at least one temperature sensor (12) measuring at least one temperature (T1, T2, T3) of at least one of the coils (20, 20', 20", 20‴) of at least one of the outdoor evaporators (6, 6', 6", 6‴); b) activating the defrost system and hereby performing several defrost processes each having a duration (Tdefrost) separated by a pause (Tpause) without defrosting, when the at least one temperature (T1, T2, T3) is below a predefined temperature value, d) by using at least one (ambient) temperature sensor (16, 16', 16") measuring at least one ambient (outdoor) temperature (Tout 1, Tout 2, Tout 3), characterised in that the method comprises the following steps: determine a maximum allowable pause time (Tpause, max) in dependency of the at least one ambient (outdoor) temperature (Tout 1, Tout 2, Tout 3).

2. A method according to claim 1, wherein the method comprising: - by using at least one sensor (14, 14', 14") detecting a pressure difference (ΔP) across an evaporator (6, 6', 6", 6‴) and - activating the defrost system when the pressure difference (ΔP) across the evaporator (6, 6', 6", 6‴) is above a predefined pressure difference level (ΔPmax).

3. A method according to claim 1 or 2, wherein the method comprising: - by using at least one humidity sensor detecting the relative humidity (H) of the ambient air, wherein the method comprising: - determining a maximum allowable pause time (Tpause, max) in dependency of the relative humidity (H)4. A method according to one of the preceding claims, wherein the method comprising: - defining a minimum allowable pause time (Tpause, min safety), - ensuring that the defrost system is only activated when the pause (Tpause) has exceeded the minimum allowable pause time (Tpause, min safety).

5. A method according to one of the preceding claims, wherein the method comprising: - determining a maximum allowable pause time (Tpause, max), - determining the time / pause (Tpause) since the last defrost process and - activating the defrost system when the pause (Tpause) has exceeded the maximum allowable pause time (Tpause, max).

6. A method according to one of the preceding claims, wherein the method comprising: - detecting a temperature of a liquid in a return water line (38) or a temperature the defrosting medium, wherein the duration (Tdefrost) of a defrost process is determined in dependency of: a) the at least one outdoor temperature (Tout 1, Tout 2, Tout 3) and b) the temperature of the liquid in a return water line (38) or the temperature of the defrosting medium.

7. A method according to one of the preceding claims, wherein the method comprising: defrosting a fraction of the evaporators (6, 6', 6", 6‴) at a time only.

8. A method according to one of the preceding claims, wherein the predefined pressure difference level (ΔPmax) corresponds to 20-50 % of the blockage ratio.

9. A method according to one of the preceding claims, wherein the the heat pump (2) comprises an air inlet (S3) and an air outlet (S4), wherein the method comprises detecting a temperature of the air inlet (S3) and a temperature of the air outlet (S4), wherein: a) when the difference the temperature of the air inlet (S3) and temperature of the air outlet (S4) during a defrost process exceeds a predefined level, operation of the fans is initiated and maintained for a predefined time period.

10. Air source heat pump (2) comprising a plurality of outdoor evaporators (6, 6', 6", 6‴) each comprising a number of fans (42) and a number of coils (20, 20', 20", 20‴), said heat pump (2) comprising a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils (20, 20', 20", 20‴), wherein the heat pump (2) comprises at least one temperature sensor (12) arranged and configured to measure at least one temperature (T1, T2, T3) of at least one of the coils (20, 20', 20", 20‴) of at least one of the outdoor evaporators (6, 6', 6", 6‴), wherein the defrost system is configured to perform several defrost processes each having a duration (Tdefrost) separated by a pause (Tpause) without defrosting when the at least one temperature (T1, T2, T3) is below a predefined temperature value, the defrost system comprising: - at least one temperature sensor (16, 16', 16") arranged and configured to measure at least one outdoor temperature (Tout 1, Tout 2, Tout 3), characterised in that the defrost system is configured to: - determining a maximum allowable pause time (Tpause, max) in dependency of: a) the at least one outdoor temperature (Tout 1, Tout 2, Tout 3).

11. Air source heat pump (2) according to clam 10, wherein the defrost system comprises: - at least one sensor (14, 14', 14") arranged and configured to detect a pressure difference (ΔP) across an evaporator (6, 6', 6", 6‴), wherein the defrost system is configured to activating the defrost system (4) when the pressure difference (ΔP) across the evaporator (6, 6', 6", 6‴) is above a predefined pressure difference level (ΔPmax).

12. Air source heat pump (2) according to claim 10 or 11, wherein the defrost system (4) comprises: - at least one humidity sensor arranged and configured to detect the relative humidity (H) of the ambient air, wherein the defrost system is configured to: - determine a maximum allowable pause time (Tpause, max) in dependency of the relative humidity (H).

13. Air source heat pump (2) according to claim 10-12, wherein the air source heat pump (2) is configured to ensure that the defrost system is only activated when the pause (Tpause) has exceeded the minimum allowable pause time (Tpause, min safety).

14. Air source heat pump (2) according to claim 10-13, wherein the air source heat pump (2) is configured to: - determine the time / pause (Tpause) since the last defrost process and - activate the defrost system when the pause (Tpause) has exceeded a predefined maximum allowable pause time (Tpause, max).

15. Air source heat pump (2) according to claim 10-14, wherein the air source heat pump (2) is configured to: - detect a temperature of the defrosting medium, wherein the duration (Tdefrost) of a defrost process is determined in dependency of: a) the at least one ambient (outdoor) temperature (Tout 1, Tout 2, Tout 3) and b) the temperature of the defrosting medium.

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