Methods for Defrosting a Heat Pump and Heat Pumps Comprising a Defrost System

Abstract

Methods are disclosed for defrosting an air source heat pump having a plurality of outdoor evaporators each comprising a number of coils each comprising fins and pipes. The heat pump comprises a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils. The method includes: a) 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 (Tdefrost) separated by a pause (Tpause) without defrosting, when the at least one temperature is below a predefined temperature value; c) using at least one temperature sensor to measure at least one outdoor temperature; and d) determining a maximum allowable pause time (Tpause, max) depending on the at least one outdoor temperature.

What is claimed is: 
     
         1 . A method for defrosting an air source heat pump having a plurality of outdoor evaporators each comprising a number of fans and a number of coils each comprising fins and pipes, the heat pump further comprising a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils, the method comprising:
 a) using at least one temperature sensor 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;   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 (T 1 , T 2 , T 3 ) is below a predefined temperature value;   c) using at least one ambient temperature sensor to measure at least one ambient outdoor temperature (T out 1 , T out 2 , T out 3 ); and   d) determining a maximum allowable pause time (T pause, max ) depending on the at least one ambient outdoor temperature (T out 1 , T out 2 , T out 3 ).   
     
     
         2 . The method according to  claim 1 , further comprising:
 using at least one sensor to detect 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 ).   
     
     
         3 . The method according to  claim 2 , further comprising:
 using at least one humidity sensor to detect a relative humidity (H) of ambient air; and   determining a maximum allowable pause time (T pause, max ) depending on the relative humidity (H).   
     
     
         4 . The method according to  claim 2 , wherein the predefined pressure difference level (ΔP max ) corresponds to 20-50% of a blockage ratio. 
     
     
         5 . The method according to  claim 1 , further comprising:
 using at least one humidity sensor to detect a relative humidity (H) of ambient air; and   determining a maximum allowable pause time (T pause, max ) depending on the relative humidity (H).   
     
     
         6 . The method according to  claim 1 , further comprising:
 defining a minimum allowable pause time (T pause, min safety ); and   ensuring that the defrost system is only activated when a pause (T pause ) has exceeded the minimum allowable pause time (T pause, min safety ).   
     
     
         7 . The method according to  claim 1 , further comprising:
 determining a maximum allowable pause time (T pause, max );   determining a pause (T pause ) since a last defrost process; and   activating the defrost system when the pause (T pause ) has exceeded the maximum allowable pause time (T pause, max ).   
     
     
         8 . The method according to  claim 1 , further comprising:
 detecting a temperature of a liquid in a return water line, wherein the duration (T defrost ) of a defrost process is determined depending on:   a) the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ); and   b) the temperature of the liquid in the return water line.   
     
     
         9 . The method according to  claim 1 , further comprising:
 detecting a temperature of a defrosting medium, wherein the duration (T defrost ) of a defrost process is determined depending on:   a) the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ); and   b) the temperature of the defrosting medium.   
     
     
         10 . The method according to  claim 1 , further comprising:
 defrosting only a fraction of the evaporators at a time.   
     
     
         11 . The method according to  claim 1 , wherein the heat pump comprises an air inlet and an air outlet, the method further comprising:
 detecting a temperature of the air inlet and a temperature of the air outlet; and   when a difference between the temperature of the air inlet and the temperature of the air outlet during a defrost process exceeds a predefined level, initiating and maintaining operation of the fans for a predefined time period.   
     
     
         12 . An air source heat pump comprising:
 a plurality of outdoor evaporators each comprising a number of fans and a number of coils;   a defrost system that applies a defrosting medium to provide thermal energy to defrost the coils;   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; and   at least one temperature sensor arranged and configured to measure at least one outdoor temperature (T out 1 , T out 2 , T out 3 );   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; and 
 determine a maximum allowable pause time (T pause, max ) depending on the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ). 
   
     
     
         13 . The air source heat pump according to  claim 12 , wherein the defrost system further comprises:
 at least one sensor arranged and configured to detect a pressure difference (ΔP) across an evaporator;   wherein the defrost system is configured to activate the defrost system when the pressure difference (ΔP) across the evaporator is above a predefined pressure difference level (ΔP max ).   
     
     
         14 . The air source heat pump according to  claim 13 , wherein the defrost system comprises:
 at least one humidity sensor arranged and configured to detect a relative humidity (H) of the ambient air;   wherein the defrost system is configured to determine a maximum allowable pause time (T pause, max ) depending on the relative humidity (H).   
     
     
         15 . The air source heat pump according to  claim 12 , wherein the defrost system comprises:
 at least one humidity sensor arranged and configured to detect a relative humidity (H) of the ambient air;   wherein the defrost system is configured to determine a maximum allowable pause time (T pause, max ) depending on the relative humidity (H).   
     
     
         16 . The air source heat pump according to  claim 12 , wherein the air source heat pump is configured to ensure that the defrost system is only activated when a pause (T pause ) has exceeded a minimum allowable pause time (T pause, min safety ). 
     
     
         17 . The air source heat pump according to  claim 12 , wherein the air source heat pump is configured to:
 determine a pause (T pause ) since a last defrost process; and   activate the defrost system when the pause (T pause ) has exceeded a predefined maximum allowable pause time (T pause, max ).   
     
     
         18 . The air source heat pump according to  claim 12 , wherein the air source heat pump is configured to:
 detect a temperature of a defrosting medium, wherein the duration (T defrost ) of a defrost process is determined depending on:   a) the at least one outdoor temperature (T out 1 , T out 2 , T out 3 ); and   b) the temperature of the defrosting medium.   
     
     
         19 . The air source heat pump according to  claim 12 , wherein the heat pump further comprises an air inlet and an air outlet, and 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 when a difference between the temperature of the air inlet and the 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. 
     
     
         20 . The air source heat pump according to  claim 12 , wherein the at least one temperature sensor is arranged to detect a temperature at a surface of at least one of the coils.