Method and system for temperature control in refrigerated storage spaces

Abstract

The present invention relates to a method of controlling air temperature within a refrigerated storage space based on a temperature-error integral of supply air discharged into the storage space. Another aspect of the invention relates to a refrigerated storage space comprising a corresponding temperature control system.

1 . A method of controlling air temperature within a refrigerated storage space, the method comprising steps of:
 circulating return air from the refrigerated storage space through a cooling unit to provide a flow of cooled supply air at a supply air temperature,   discharging the flow of cooled supply air into the refrigerated storage space to control air temperature inside the refrigerated storage space,   measuring the supply air temperature by a first temperature sensor,   computing a temperature-error integral of the supply air based on a difference over time between the supply air temperature and a reference temperature,   adjusting the supply air temperature based on the temperature-error integral such that a time average of the supply air temperature substantially equals the reference temperature.   
     
     
         2 . A method of controlling air temperature according to  claim 1 , wherein the reference temperature comprises an adjusted setpoint temperature or a setpoint temperature. 
     
     
         3 . A method of controlling air temperature according to  claim 1 , comprising a further step of:
 circulating the return air from the refrigerated storage space through a heating unit to provide a flow of heated supply air at the supply air temperature.   
     
     
         4 . A method of controlling air temperature according to  claim 1 , comprising a step of:
 controlling respective operational states of the heating unit or the cooling unit or both to adjust the supply air temperature.   
     
     
         5 . A method of controlling air temperature according to  claim 1 , comprising a step of:
 switching operational states of the cooling unit between exclusively ON and OFF states to make the adjustment of the supply air temperature.   
     
     
         6 . A method of controlling air temperature according to  claim 5 , comprising a further step of:
 regulating the cooling capacity of the cooling unit by a capacity regulating device coupled to vapour compression refrigeration cycle, containing a compressor and an evaporator.   
     
     
         7 . A method of controlling air temperature according to  claim 6 , wherein the cooling capacity of the cooling unit is regulated by a step of:
 controlling a refrigerant flow rate between the evaporator and the compressor by a suction valve.   
     
     
         8 . A method of controlling air temperature according to  claim 1 , comprising further steps of:
 comparing the temperature-error integral of the supply air with a first integral error threshold and a second integral error threshold,   changing an operational state of at least one of the cooling unit, the heating unit and internal fans if the temperature-error integral exceeds the first integral error threshold or falls below the second integral error threshold.   
     
     
         9 . A method of controlling air temperature according to  claim 8 , wherein a difference between the first integral error threshold and the second integral error threshold is set to a value between 20° C.*minutes and 200° C.*minutes. 
     
     
         10 . A method of controlling air temperature according to  claim 8 , comprising further steps of:
 setting the operational state of the cooling unit to active for at least a first minimum time period if the temperature-error integral exceeds the first integral error threshold,   setting the operational state of the heating unit to active for at least a second minimum time period if the temperature-error integral is below the second integral error threshold.   
     
     
         11 . A method of controlling air temperature according to  claim 8 , wherein:
 the first integral error threshold is set to a value between 50 and 200° C.*minutes; and   the second integral error threshold is set to a value between −100 and −10° C.*minutes.   
     
     
         12 . A method of controlling air temperature according to  claim 1 , comprising a step of:
 adjusting the reference temperature as a function of a setpoint temperature and a temperature of the return air measured by a return air temperature sensor.   
     
     
         13 . A method of controlling air temperature according to  claim 12 , wherein the adjustment of the reference temperature is made such that the average of the supply air temperature and the return air temperature substantially equals the setpoint temperature. 
     
     
         14 . A method of controlling air temperature according to  claim 12 , comprising a step of:
 limiting the reference temperature to a lower temperature limit dependent on the setpoint temperature, and optionally a further step of:   limiting the reference temperature to an upper temperature limit.   
     
     
         15 . A method of controlling air temperature according to  claim 8 , comprising steps of, during circulation periods,
 comparing the temperature-error integral to a first heating threshold,   maintain internal fans at a first preset speed if the temperature-error integral is below the first heating threshold.   
     
     
         16 . A method of controlling air temperature according to  claim 15 , comprising a further step of:
 comparing the temperature-error integral to a second heating threshold smaller than the first heating threshold,   maintain the internal fans at a second preset speed if the temperature-error integral falls below the second heating threshold;   wherein the a second preset speed is higher than the first preset speed.   
     
     
         17 . A method of controlling air temperature according to  claim 5 , comprising further steps of:
 comparing a duration of a previous circulation period with a circulation time threshold t ct ,   maintain the speed of the internal fans at a maximum speed during a current circulation period if the previous circulation period was smaller than the circulation time threshold t ct .   
     
     
         18 . A method of controlling air temperature according to  claim 17 , wherein the circulation time threshold t ct  depends on a change of the temperature-error integral during the previous circulation period. 
     
     
         19 . A method of controlling air temperature according to  claim 1 , wherein the temperature-error integral is computed at regular time intervals such as time intervals smaller than 1 minute, preferably smaller than 10 seconds such as about every second. 
     
     
         20 . A method of controlling air temperature and relative humidity according to  claim 1 , comprising further steps of:
 simultaneous heating and cooling the supply air at a first speed setting of the internal fans when a measured relative humidity (RH) of the air inside the transport volume is higher than a first humidity threshold derived from the setpoint value of the relative humidity (RH set ),   setting a second fan speed of the internal fans during circulation periods when a measured relative humidity (RH) of the air is higher than a second humidity threshold derived from the setpoint value of the relative humidity;   wherein fan speed in the first fan speed is lower than fan speed at the second fan speed setting.   
     
     
         21 . A method of controlling air temperature and relative humidity according to  claim 20 , comprising further steps:
 setting the operational state of the heating unit to ON during circulation periods when a measured relative humidity (RH) of the air is higher than a third humidity threshold derived from the setpoint value of the relative humidity.   
     
     
         22 . A refrigerated storage space comprising:
 a refrigerated volume for housing a commodity load,   a cooling unit configured to receive return air from the refrigerated volume and generate a flow of cooled supply air at a supply air temperature,   an air flow passage coupled to the refrigerated volume to discharge the supply air therein and control air temperature within the refrigerated volume,   a first temperature sensor adapted to measuring the supply air temperature,   a temperature control system adapted to:
 computing a temperature-error integral of the supply air based on a difference over time between the supply air temperature and a reference temperature, 
 adjusting the supply air temperature based on the temperature-error integral such that a time average of the supply air temperature is substantially equal to the reference temperature. 
   
     
     
         23 . A refrigerated storage space according to  claim 22 , wherein the reference temperature comprises an adjusted setpoint temperature or a setpoint temperature. 
     
     
         24 . A refrigerated storage space according to  claim 22 , comprising a heating unit configured to supply a flow of heated supply air at the supply air temperature from the return air from the refrigerated storage space. 
     
     
         25 . A refrigerated storage space according to  claim 24 , wherein the temperature control system is adapted to:
 controlling respective operational states of the heating unit or the cooling unit or both to adjust the supply air temperature.   
     
     
         26 . A refrigerated storage space according to  claim 25 , wherein the temperature control system is adapted to:
 switching operational states of the cooling unit between exclusively ON and OFF states to make the adjustment of the supply air temperature.   
     
     
         27 . A refrigerated storage space according to  claim 22 , wherein the temperature control system is adapted to:
 adjusting the reference temperature as a function of a setpoint temperature and a temperature of the return air measured by a return air temperature sensor.   
     
     
         28 . A refrigerated storage space according to  claim 27 , wherein the temperature control system is adapted to adjust the reference temperature such that the average of the supply air temperature and the return air temperature substantially equals the setpoint temperature. 
     
     
         29 . A refrigerated storage space according to  claim 22 , comprising a refrigerated container wherein the refrigerated volume comprises a transport volume of the refrigerated container.