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.

Description:
[0001]    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. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is important to maintain the temperature of perishable produce held in refrigerated storage spaces at a desired or setpoint temperature. The setpoint temperature is chosen to keep the perishable produce such as meat, vegetables and fruit, at correct temperature to avoid quality degradation. It is known in the art to apply temperature control protocols that selectively control operational states of heaters, cooling devices and internal fans such as evaporator fans of cooling units, coupled to the refrigerated storage space, to maintain a setpoint air temperature inside the refrigerated storage space. The refrigerated storage space may for example comprise a transport volume of a refrigerated container. 
         [0003]    The typical cooling unit or refrigeration unit used in refrigerated storage spaces is based on the so-called vapour compression refrigeration cycle. This cycle comprises at least a compressor, a condenser, an expansion device, an evaporator and a capacity regulating device. The compressor sucks refrigerant vapour from the evaporator and compresses the refrigerant vapour which subsequently flows to the condenser at high pressure. The condenser ejects its heat to a medium outside the refrigerated storage space while condensing the refrigerant vapour. The liquefied refrigerant then flows to the expansion device in which a refrigerant pressure drops. The low pressure refrigerant then flows to the evaporator where the refrigerant evaporates while extracting the required heat from the refrigerated storage space. A capacity regulating device, which may comprise a suction modulating valve, controls the cooling capacity of the cooling unit. Cooling capacity is the amount of heat absorbed by the evaporator per unit of time. A typical characteristic of vapour compression refrigeration cycles is that their energy efficiency reduces in part-load operation, i.e. whenever the compressor continues to be driven or active while the capacity regulating device reduces the cooling capacity. 
         [0004]    EP 2 116 798 A1 discloses a refrigeration system which has an energy saving operation mode performing a first action in which a compressor and an internal fan are driven while the cooling capacity of an evaporator is regulated. In a second action, when the blow-off side air temperature in a cold storage is kept at a set value in the first action, the cooling capacity of the evaporator is increased to lower the blow-off side air temperature to a lower limit temperature of a desired temperature range containing the set value and the compressor and the internal fan are then stopped. In a third action in which, when the blow-off side air temperature after the second action rises to an upper limit temperature of the desired temperature range, the first action is restarted. 
         [0005]    Despite various attempts to lower energy consumption of the cooling units, substantial amounts of energy are still consumed in today&#39;s refrigerated storage spaces. Consequently, there is a continuing need for providing temperature control algorithms and systems with improved energy efficiency to reduce energy costs and reduce CO 2  emissions from cooling or heating the refrigerated storage spaces. 
         [0006]    In one embodiment, the present invention provides an improved energy saving methodology for controlling the respective operational states or modes of heating units, cooling devices and internal fans of existing cooling units of refrigerated storage spaces. The method allows temperature control systems of existing refrigerated storage spaces to benefit from the invention without any need for hardware replacements or modifications. The improved temperature control methodology may advantageously be implemented as embedded control software executed on a microprocessor of a temperature control system associated with the refrigerated storage space to improve energy efficiency of existing heating units, cooling units and internal fans or circulation fans. Consequently, the present invention may conveniently, but not exclusively, be implemented by a software update of existing embedded control software or program code of the temperature control system. 
       SUMMARY OF INVENTION 
       [0007]    A first aspect of the invention relates to a method of controlling air temperature of 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 supply air into the refrigerated storage space to control the 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.       
 
         [0013]    The time average of the supply air temperature preferably deviates with less than +/−0.5° C., or more preferably with less than +/−0.2° C., from the reference temperature under steady-state operation of a temperature control system or algorithm comprising the present methodology of controlling air temperature of the refrigerated storage space. 
         [0014]    The refrigerated storage space may comprise various types of stationary or transportable refrigerated compartments or spaces such as spaces of household freezers and refrigerators, cold storage houses and refrigerated containers. 
         [0015]    In accordance with the present invention, the temperature of the cooled supply air or supply air is adjusted based on the temperature-error integral, which is an integral over the difference between the supply air temperature and the predetermined reference temperature. The use of the supply air temperature-error integral allows wider fluctuation of instantaneous supply air temperature while yet providing accurate control of average supply air temperature over time. In contrast, traditional temperature control algorithms or schemes seek to maintain the instantaneous supply air temperature at setpoint temperature, possibly within upper and lower temperature limits. The wider fluctuations of the instantaneous supply air temperature afforded by the present temperature control methodology or scheme enables exclusively switching operational states of the cooling unit or device between ON and OFF while maintaining accurate control over the time-averaged supply air temperature. The exclusive switching of operational states of the cooling unit between ON and OFF, while maintaining accurate control over the time-averaged supply air temperature such as within the above-mentioned temperature deviations, may be provided by pure integral control (1-control) based adjustment of the supply air temperature based on a current value of an error signal derived from, or constituted by, the temperature-error integral. 
         [0016]    In the present specification, the ON state of the cooling unit or device means it operates at, or close to, its maximum capacity, i.e. about 100% capacity such as at more than 85% of its maximum cooling capacity, even more preferably above 90 or 95% of its maximum cooling capacity. The ON operation at, or close to, the maximum capacity of the cooling unit avoids energy inefficient part-load cooling. The ON state is furthermore preferably placed at a percentage of the maximum cooling capacity where operation is highly efficient. This percentage may vary between specific cooling units but typically lies in the above-mentioned range between 85-100% of the maximum cooling capacity. 
         [0017]    Despite the allowed wider fluctuations of the instantaneous supply air temperature, average supply air temperature is accurately controlled. The temperature of the commodity load, which typically comprises perishable produce, situated within the transport volume of the container is maintained within tight limits or bounds due to thermal inertia of the commodity load despite the wider fluctuations of the instantaneous supply air temperature. 
         [0018]    The reference temperature may comprise, or be set to, an adjusted setpoint temperature or a setpoint temperature of the refrigerated storage space. The adjusted setpoint temperature is preferably derived from the setpoint temperature and possibly one or more additional temperature variables of the temperature control algorithm. 
         [0019]    The present methodology for air temperature control within the transport volume preferably comprises controlling both the cooling unit and a heating unit. Accordingly, in one embodiment, the present methodology comprises a further step of:
       circulating the return air from the refrigerated storage space through the heating unit to provide a flow of heated supply air at the supply air temperature. This embodiment enables the present temperature control methodology to provide a flow of both cooled supply air and heated supply air, depending on the circumstances, to maintain the commodity load at a desired temperature across a wide range of external environmental temperatures outside the refrigerated storage space.       
 
         [0021]    In the present specification, the ON state of the heating unit or device means it operates at, or close to, its maximum capacity, i.e. about 100% capacity such as at more than 85% of its maximum heating capacity, even more preferably above 90 or 95% of its maximum heating capacity. Furthermore, the ON state is preferably placed at a percentage of the maximum heating capacity where operation is highly efficient. This percentage may vary between specific types of heating units. 
         [0022]    The computation of the temperature-error integral of the supply air is preferably performed by the temperature control system operatively coupled to the refrigerated storage space. The temperature control system may reside in a dedicated cooling unit mounted to a wall section of the refrigerated storage space. Alternatively, certain parts of the temperature control system may be situated remotely and coupled to control operation of the cooling unit, heating unit etc through a wired or wireless communications interface. 
         [0023]    The temperature control system may comprise a microprocessor operating according to a set of embedded program instructions or embedded software. The embedded software may be adapted to receive and process supply air temperature data provided by the first temperature sensor and determine appropriate control actions of the cooling unit and/or heating unit to adjust the supply air temperature in a desired direction to reach the desired air temperature inside the refrigerated storage space. The temperature-error integral is preferably computed at regular time intervals such as time intervals smaller than 1 minute, preferably smaller than 10 seconds such as about every second. 
         [0024]    In accordance with a preferred embodiment of the invention, the method of controlling air temperature comprises a step of:
       adjusting the reference temperature as a function of the setpoint temperature and a temperature of the return air measured by a return air temperature sensor or second temperature sensor. Hereafter this adjusted reference temperature or adjusted setpoint temperature is denoted by T set     —     quest . The setpoint temperature is normally identical to a displayed setpoint temperature of the refrigeration unit of the refrigerated storage space. By adjusting the reference temperature as function of the setpoint temperature and the temperature of the return air, the inventors have demonstrated improved quality preservation of the perishable produce by improved produce temperature control. This has been achieved because average produce temperature within the refrigerated transport volume typically lies somewhere in-between the supply air temperature and a few degrees above the return air temperature due to temperature gradients within the transport volume.       
 
         [0026]    In one embodiment, the adjustment of the reference temperature is made such that the average of the supply air temperature and the return air temperature substantially equals, preferably within +/−0.2 degree C., the setpoint temperature unless upper and lower limits for the reference temperature prohibit this. The average produce temperature is thereby maintained close to the setpoint temperature which represents the desired commodity load temperature. 
         [0027]    The method of controlling air temperature according to the invention preferably comprises a step of controlling respective operational states of the heating unit or the cooling unit or both to adjust the supply air temperature. As previously mentioned, the respective operational states of the heating and cooling units are preferably switched between ON and OFF. In one embodiment, the method comprises a step of exclusively switching operational states of the cooling unit between ON and OFF to make the adjustment of the supply air temperature. This embodiment is particularly advantageous since it avoids time periods of inefficient part-load refrigeration, leading to markedly improved energy efficiency. 
         [0028]    In the present specification, the ON state of a heating or cooling unit means the unit in question operates at, or close to, its maximum capacity. In certain embodiments, the cooling unit and/or the heating unit may only possess a fixed number of discrete operational states such as two, three or four etc. In other embodiments, the operational state of the cooling unit and/or the heating unit may be continuously variable between for example 0% (OFF) and the maximum capacity. 
         [0029]    The cooling unit may comprise a compressor coupled to an evaporator with a capacity regulating device regulating the cooling capacity of the cooling unit. In one such embodiment the compressor is coupled to the evaporator via the capacity regulating device, such as a suction valve, mounted on a fluid connection between the compressor and the evaporator. In such an embodiment, the present methodology may comprise a step of:
       controlling a refrigerant flow rate between the evaporator and the compressor by a capacity regulating device to adjust a cooling capacity of the evaporator. In a particularly advantageous embodiment, the operational state of the capacity regulating device is such that the refrigeration capacity is substantially maximized during time periods, preferably all time periods, where the compressor is ON, i.e. operating at, or close to, its maximum capacity.       
 
         [0031]    According to yet another preferred embodiment, the temperature-error integral of the supply air is controlled to stay within upper and lower bounds or limits as defined by first and second integral error thresholds, respectively. The method comprising further steps of:
       comparing the temperature-error integral of the supply air with the integral first integral error threshold and the second integral error threshold,   changing an operational state of at least one of the cooling unit, the heating unit and the internal fans if the temperature-error integral exceeds the first integral error threshold or falls below the second integral error threshold.       
 
         [0034]    The operational state of the cooling unit is controlled such that the cooling unit preferably is activated if the first integral error threshold is exceeded indicating the average supply air temperature over a preceding time period, such as a time period of a current cycle, was too high. Likewise, the heating unit is preferably activated if temperature-error integral falls below the second integral error threshold indicating that average supply air temperature over the past time period was too low. 
         [0035]    Experimental investigations by the present inventors indicate that the first integral error threshold may be set to a value between 50 and 200° C.*minutes (the unit being ° C. times minutes) and the second integral error threshold set to a value between −100 and −10° C.*minutes for typical refrigerated storage spaces. A difference between the first integral error threshold and the second integral error threshold may be set to a value between 20 and 200° C.*minutes to have sufficient, but not too much, bandwidth or distance between cooling and heating operation. 
         [0036]    The methodology of controlling air temperature-error integral to stay between the first and second integral error thresholds may comprise 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 smaller than the second integral error threshold.       
 
         [0039]    The active operational state of the cooling unit is preferably ON at all times and the active operational state of the heating unit preferably ON at all times as well. Each of the first and second minimum time periods may be set to a value between 1 and 10 minutes such as around 5 minutes to avoid unnecessary wear and tear of the compressor and electrical contactors. 
         [0040]    According to another preferred embodiment, the method of controlling air temperature comprises a step of:
       limiting the reference temperature to a lower temperature limit dependent on the setpoint temperature. Optionally, the reference temperature may also be limited to an upper temperature limit. Limiting how low the reference temperature can drop relative to the setpoint temperature is very helpful in avoiding chilling or freezing damage to the commodity load caused by possible prolonged drops of the supply air temperature. For some setpoint temperatures the lower temperature limit is to be substantially equal to the setpoint temperature. For other setpoint temperatures the lower temperature limit is set to a value between 0.1 and 2.0° C. below the setpoint temperature.       
 
         [0042]    In another embodiment, the method of controlling air temperature comprises steps of, during circulation periods, comparing the temperature-error integral to a first heating threshold and maintaining the internal fans at a first preset speed if the temperature-error integral is below the first heating threshold. In the present specification, the term “circulation period” means time periods where the operational state of the heating unit is OFF and the operational state of the cooling unit is OFF. The internal fans may be configured to operate at a number of discrete preset speed settings such as OFF, Low and High with a predefined speed ratio between the Low and High speed settings such as a ratio of 2 or 3 or more. The OFF, Low and High speed settings may for example correspond to air flow rates of 0, 3000 and 6000 m 3  per hour, respectively. The first preset speed may in this situation be either Low or High. By forcing the internal fans to run at the first preset speed if the temperature-error integral lies below the first heating threshold, the temperature control system or algorithm only activates the heating unit if fan energy does not suffice to supply required heat energy. Fan energy is an advantageous heating source due to its double effect contributing with both heating of the supply air and mixing of the air inside the refrigerated storage space. 
         [0043]    In the present specification each of the discussed fan speed settings of the internal fan or fans may be provided by joined operation of all internal fans present in the refrigerated storage space. Different fan speed settings may be achieved by changing the actual speed of one or several individual fan(s) or by turning a certain number of fans ON or OFF. 
         [0044]    This embodiment may comprise 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 is below the second heating threshold; the second preset speed is higher than the first preset speed. In the above described situation with a set of discrete fan speeds, the second preset speed may be High if the first preset speed is Low. In other embodiments, the internal fans speed may be adjustable over a continuous speed range and the second preset speed set to any speed higher than the first preset speed. One advantage of using multiple heating thresholds is a maximization of the ratio air flow divided by energy input because physics dictate that air flow generated by a fan is a linear function of the cube of its power draw.       
 
         [0047]    In another embodiment, the present methodology comprises 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 . This embodiment ensures effective air circulation within the refrigerated storage space during the current circulation period if a previous circulation period was relatively short, i.e. below the circulation time threshold t ct . Such relatively short previous circulation periods may indicate a relatively large net heat load on the refrigerated storage space which makes it important to provide high air circulation to limit or decrease air temperature distribution differences within the refrigerated storage space. In the present specification, the “net heat load” means the sum of heat ingress into the refrigerated storage space and autonomous heat production of the commodity load within the refrigerated storage space.       
 
         [0050]    According to a further refinement of the above-mentioned embodiment, the circulation time threshold t ct  depends on a change of the temperature-error integral during the previous circulation period. 
         [0051]    A number of embodiments of the present invention are advantageously configured to control relative humidity (RH) of the air inside refrigerated storage spaces in addition to controlling the air temperature. In one embodiment the methodology comprises further steps of:
       simultaneously heating and cooling the supply air at a first speed setting of 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 the first fan speed setting is a lower fan speed than the second fan speed setting. The first fan speed setting may be Low and the second fan speed setting High. The lower speed of the first fan speed setting is preferred because it increases a dehumidification capacity of a refrigeration unit controlled by the temperature control system.       
 
         [0055]    In another embodiment, the above embodiment with switching between the first and second fan speed settings may comprise further steps of:
       setting the heating unit to an ON state during circulation periods when a measured relative humidity of the air is higher than a third humidity threshold derived from the setpoint value of the relative humidity. By increasing heat production during circulation periods as outlined above, the duration of the circulation period is shortened by evoking earlier cooling and therefore also earlier dehumidification.       
 
         [0057]    A second aspect of the invention relates to a refrigerated storage space comprising a refrigerated volume for housing a commodity load. A cooling unit is configured to receive return air from the refrigerated volume and generate a flow of cooled supply air at a supply air temperature and an air flow passage is coupled to the refrigerated volume to discharge the supply air therein and control air temperature within the refrigerated volume. A first temperature sensor is adapted to measuring the supply air temperature. A temperature control system is 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. The temperature control system is additionally adapted to 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. 
         [0058]    The return air from the refrigerated volume may be conveyed to the cooling unit or the heating unit through a second air flow passage. The second air flow passage may in certain embodiments be located close to a ceiling portion of the refrigerated storage space. A second or return air temperature sensor may be provided for determining the return air temperature. As previously mentioned, the computation of the temperature-error integral of the supply air is preferably performed by a temperature control algorithm executed by the temperature control system operatively coupled to the refrigerated storage space. The temperature control system may comprise a microprocessor operating according to a set of embedded program instructions or embedded software to execute the temperature control algorithm. Alternatively, the temperature control system may comprise dedicated computation hardware such as programmable logic or hardwired arithmetic and logic circuit blocks configured to execute the required computational steps of the temperature control algorithm. 
         [0059]    As previously mentioned, the refrigerated storage space preferably comprises a heating unit configured to supply heated supply air at the supply air temperature by circulating the return air from the refrigerated storage space. The temperature control system may be adapted to controlling respective operational states of the heating unit or the cooling unit or both to adjust the supply air temperature. 
         [0060]    To maximize cooling energy efficiency of the cooling unit, the temperature control system may be adapted to exclusively switching operational states of the cooling unit between ON and OFF states to make the adjustment of the supply air temperature. This may be achieved by switching operational states of a compressor of the cooling unit between exclusively ON and OFF states so as to avoid energy inefficient part-load operation of the compressor. 
         [0061]    According to an advantageous embodiment, the temperature control system of the refrigerated storage space is adapted to adjust the reference temperature as a function of a setpoint temperature and a temperature of the return air measured by a return air temperature sensor. In this embodiment, the temperature control system is preferably further 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. By controlling the average of the supply air temperature and the return air temperature to the setpoint temperature, improved produce quality preservation can be achieved due to improved produce temperature control. The present inventors have exploited the fact that average produce temperature in the refrigerated transport volume is closer to the mean of the supply and return air temperature than to the supply air temperature. 
         [0062]    The temperature control system of the present refrigerated storage space may naturally be further adapted or refined to comprise any of the previously described functions or features in accordance with the first aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0063]    A preferred embodiment of the invention will be described in more detail in connection with the appended drawings, in which: 
           [0064]      FIG. 1  is a simplified side cross-sectional view of a refrigerated container in accordance with a preferred embodiment of the invention, 
           [0065]      FIG. 2  is a state diagram illustrating respective operational modes of a cooling unit, a heating unit and internal fans as function of a temperature-error integral of supply air discharged into a transport volume of the refrigerated container in accordance with the preferred embodiment of the invention, 
           [0066]      FIGS. 3 and 4  are flow charts illustrating program steps executed by a microprocessor-implemented temperature control algorithm of a temperature control system of the refrigerated container in accordance with a preferred embodiment of the invention, 
           [0067]      FIG. 5  illustrates logic rules applied by the temperature control algorithm for controlling internal fans speed, 
           [0068]      FIG. 6  comprises a series of graphs illustrating experimentally recorded values of various key variables of the temperature control algorithm under high net heat load conditions in accordance with the preferred embodiment of the invention, 
           [0069]      FIG. 7  comprises a series of graphs illustrating the same variables as  FIG. 6  but under low net heat load conditions, 
           [0070]      FIG. 8  comprises a series of graphs illustrating the same variables as  FIG. 6  but under conditions where heating is required; and 
           [0071]      FIG. 9  is a graph illustrating experimentally recorded values of produce temperature versus supply air temperature. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0072]    In  FIG. 1 , the refrigerated container  1  comprises a frontal section having a refrigeration unit  40  housing multiple, usually two or three, variable speed internal fans  10  (depicted schematically as a single fan), heating unit or element  20  and a cooling unit  15 . A load or cargo section  30  of the refrigerated container  1  comprises a commodity load comprising a plurality of stackable transport boxes  35  arranged within a transport volume  45  such as to leave appropriate clearance at a ceiling and a floor structure for air flow passages above and beneath the commodity load. The frontal section comprises a refrigeration unit  40  comprising return air temperature sensor  5  adapted to measure a temperature of a return air flow  50  comprising air that has been circulated through the transport volume  45 . Another temperature sensor  25  is adapted to measuring a supply air temperature of heated or cooled supply air  55  discharged into the transport volume  45  through an air flow passage. A temperature control system (not illustrated) comprises a programmed microprocessor which controls respective operational states of the variable speed internal fans  10 , the heating unit  20  and the cooling unit  15  in accordance with a temperature control algorithm defined by a set of microprocessor program instructions. The temperature control system additionally comprises a user interface, for example a LCD display, where an operator or ship technician can enter or modify certain parameter values of the temperature control algorithm such as a setpoint temperature of the refrigerated container  1 . The operation of the temperature control algorithm is explained in detail below with reference to  FIG. 2  and  FIGS. 5 &amp; 6 . 
         [0073]    The state diagram  200  of  FIG. 2  schematically illustrates how switching between control modes or states of the present temperature control algorithm is performed in a preferred embodiment as function of a temperature-error integral of the supply air discharged into a transport volume of the refrigerated container. Arrow  202  points in direction of increasing values of the temperature-error integral (TEI). 
         [0074]    The state or domain diagram  200  comprises a number of temperature-error integral thresholds or limits in-between individual control states  204 ,  206 ,  208 ,  210  and  212 . A first threshold, TEI_heat_stage — 3_lim, constitutes a lower integral error threshold such that the heating unit is switched ON if a current value of the temperature-error integral falls below this threshold. In the upper portion of the state diagram  200  in-between control states  204  and  206 , a first threshold, TEI_max_cool, constitutes a first or upper integral error threshold such that the cooling unit is switched ON if a current value of the temperature-error integral exceeds this upper threshold. 
         [0075]    Three intermediate states, where the operational states of both the cooling unit and the heating unit are OFF, are located in abutment in-between the upper control state  204 , cooling, and the lowermost control state  212 , heating. These three intermediate states comprise two heating states, control states  208  and  210 , and a circulation state  206 . In the control states  208  and  210 , the heating unit resides in operational state OFF and the internal fans are exploited to both supply heat to the supply air and add circulation to the air inside the transport volume. In the control state  208  the internal fans are set in the Low speed operational state while the internal fans speed is set to High speed in the control state  210  reflecting a requirement for higher heat production due to the decreasing value of the TEI as indicated by the arrow  202 . 
         [0076]    In the circulation control state  206 , the internal fans may be switched between three different operational states having different fan speeds such as Off, Low and High. In this embodiment internal fans speed is switched between High, Low and Off during circulation periods. The speed of the internal fans is maintained at a maximum or High speed during a current circulation period if the previous circulation period was smaller than the circulation time threshold t ct . Otherwise the fan speed is kept at Low speed at the start of the circulation period and subsequently enters a fan speed cycling program during which:
       the fan speed is reduced by one step such as from High to Low speed after maintaining the fan speed for its predetermined maximum duration setting. An additional condition for switching from Low to Off is that the previous circulation fan speed was High.   The fan speed is increased by one step after maintaining the fan speed for its predetermined maximum duration setting. An additional condition for making a change from Low to High speed is that the previous circulation period fan speed was Off.   The fan speed is reduced by one step if the fan speed was increased a preset time period ago such as 10 or 5 minutes, while changes to the return air temperature Tret in that preset time period fell within predetermined upper and lower limits or bounds.   The fan speed is increased by one step if the change of the return air temperature Tret since the start of the current fan speed setting fell outside predetermined upper and lower limits or bounds.       
 
         [0081]    This above set of rules implies that the fan speed setting of the internal fans always changes speed setting one step at a time, so never from Off to High or vice versa. The above-described algorithm or rules for switching between fans speed setting during the circulation period is schematically illustrated by  FIG. 5 . 
         [0082]    In the present embodiment, the setting of the heating thresholds between the individual states  204 ,  206 ,  208 ,  210  and  212  are: 
         [0000]    TEI_max_cool=90° C.*min,
 
TEI_heat_stage — 1_lim=0° C.*min,
 
TEI_heat_stage — 2_lim=−10° C.*min,
 
TEI_heat_stage — 3_lim=−30° C.*min.
 
         [0083]    The variables used in describing the present embodiments of the temperature control algorithm are defined below in Table 1. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Definition of variables of the temperature control algorithm. 
               
             
          
           
               
                 Variable/acronym 
                 Description 
               
               
                   
               
               
                 Tset_quest [° C.] 
                 Reference temperature to which cycle-averaged supply air 
               
               
                   
                 temperature is controlled 
               
               
                 cycle_length [min] 
                 A length of a current cycle 
               
               
                 Cycle 
                 Period of time used in the temperature control algorithm. In usual 
               
               
                   
                 cooling operation it is the time elapsed between two consecutive 
               
               
                   
                 cooling compressor starts. 
               
               
                 Tret_avg [° C.] 
                 Average return air temperature during last completed cycle 
               
               
                 TEI [° C.*min] 
                 Supply air temperature-error integral, i.e. integral over Tsup minus 
               
               
                   
                 Tset_quest. 
               
               
                 Tsup [° C.] 
                 Supply air temperature 
               
               
                 Tret [° C.] 
                 Return air temperature 
               
               
                 Tset [° C.] 
                 Current temperature setpoint 
               
               
                 ΔTset_quest_min 
                 Tset_quest_min = Tset + ΔTset_quest_min is a lower bound on 
               
               
                 [° C.] 
                 Tset_quest. This bound may esp. be hit during high heat load. 
               
               
                 ΔTset_quest_max 
                 Tset_quest_max = Tset + ΔTset_quest_max [° C.] is an upper bound 
               
               
                 [° C.] 
                 on Tset_quest. This bound may esp. be hit during large negative 
               
               
                   
                 heat load, like may occur when Tset = +30° C. in Canadian winters. 
               
               
                   
               
               
                 TEI_max_cool [° C.*min] 
                 See FIG. 2. Cooling switches ON if   
           ∫     t   0     t            (         T   sup          (   τ   )       -     Tset_quest        (   τ   )         )                   d                 τ       &gt;     TEI_max        _cool   .           
 
               
               
                   
               
               
                   
                 It will only switch off again when min. on-time has passed by and 
               
               
                   
               
               
                   
                 
                   
                     
                       
                         
                           
                             ∫ 
                             
                               t 
                               0 
                             
                             t 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   
                                     T 
                                     sup 
                                   
                                    
                                   
                                     ( 
                                     τ 
                                     ) 
                                   
                                 
                                 - 
                                 
                                   Tset_quest 
                                    
                                   
                                     ( 
                                     τ 
                                     ) 
                                   
                                 
                               
                               ) 
                             
                              
                             
                                 
                             
                              
                             d 
                              
                             
                                 
                             
                              
                             τ 
                           
                         
                         ≤ 
                         
                           TEI_max 
                            
                           
                             _cool 
                             . 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 TEI_heat_stage3_lim [° C.*min] 
                 See FIG. 2. Heating switches on when   
           ∫     t   0     t            (         T   sup          (   τ   )       -     Tset_quest        (   τ   )         )                   d                 τ       &lt;     TEI_heat      _stage3        _lim   .           
 
               
               
                   
               
               
                   
                 It will only switch off again when min. on-time has passed by and 
               
               
                   
               
               
                   
                 
                   
                     
                       
                         
                           
                             ∫ 
                             
                               t 
                               0 
                             
                             t 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   
                                     T 
                                     sup 
                                   
                                    
                                   
                                     ( 
                                     τ 
                                     ) 
                                   
                                 
                                 - 
                                 
                                   Tset_quest 
                                    
                                   
                                     ( 
                                     τ 
                                     ) 
                                   
                                 
                               
                               ) 
                             
                              
                             
                                 
                             
                              
                             d 
                              
                             
                                 
                             
                              
                             τ 
                           
                         
                         ≥ 
                         
                           TEI_heat 
                            
                           _stage3 
                            
                           
                             _lim 
                             . 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 TEI_max [° C.*min] 
                 To avoid integral wind-up the value of TEI is limited to the range 
               
               
                   
                 TEI_min ≦ TEI ≦ TEI_max. 
               
               
                 TEI_min [° C.*min] 
                 To avoid integral wind-up the value of TEI is limited to the range 
               
               
                   
                 TEI_min ≦ TEI ≦ TEI_max. 
               
               
                 t ct  [min] 
                 circulation time threshold. 
               
               
                   
               
             
          
         
       
     
         [0084]    In the present embodiment of the invention, the calculated supply air temperature-error integral TEI is bounded to the interval [TEI_min, TEI_max]. TEI(t) [° C.*min] is calculated in time discrete format by: 
         [0000]      TEI_now=( T sup− T set_quest)* ts.  
 
         [0000]      TEI=max(TEI_min,min(TEI_max,TEI+TEI_now)); 
         [0000]    where ts is a sampling interval or time period. 
         [0085]    Experiments have revealed that the sampling time period, ts, preferably should be less than 10 seconds such as about 1 second. 
         [0086]    A starting value for TEI is determined anytime the temperature control algorithm has not been operating. This condition will occur following algorithm power-up. In these cases, the internal fans are first run at High speed for 15 seconds and then an initial value of TEI is calculated using: 
         [0000]      TEI=max(TEI_min,min(TEI_max, 40*( T ret— T set_quest)+30); wherein
 
         [0000]    the anti-integral windup precautions max( . . . , min( . . . , . . . )) avoid the integral from getting excessively large. 
         [0087]    In the present embodiment of the invention the reference temperature is an adjusted setpoint temperature Tset_quest. The setpoint adjustment is made such that an average of the supply air temperature Tsup and a measured return air temperature Tret substantially equals a setpoint temperature Tset. More specifically, in the present temperature control algorithm, a cycle-averaged supply air temperature Tsup is controlled to the adjusted setpoint temperature Tset_quest. 
         [0088]    The cycle is defined as a period of time starting at an end of a previous cycle and ending when one of the following three conditions applies:
       1. Cooling unit switches ON,   2. Heating unit switches OFF,   3. Last cycle ended more than 1 hour ago       
 
         [0092]    During each cycle, the cycle length and the average of the return air temperature are updated at regular time intervals. These values are used to calculate Tset_quest at the start of the next cycle. At the start of the first cycle after power-up Tset_quest is set to Tset. Following this initialization, Tset_quest is calculated at the beginning of each subsequent cycle according to the equations below: 
         [0000]        T set_quest_new=(1−0.2*cycle_length/60)* T set_quest+0.2*cycle_length/60*( T set−( T ret_avg− T set));
 
         [0000]        T set_quest=max( T set+Δ T set_quest_min;min( T set+Δ T set_quest_max; T set_quest_new)).
 
         [0093]    Please refer to the definition of the above variables in Table 1. 
         [0094]    The flowchart extending across  FIGS. 3 and 4  provides a simplified summary of the above-described operation of the present temperature control algorithm or algorithm during each call to the temperature control algorithm. The algorithm starts in step  301  and proceeds to step  303  wherein the value of the temperature-error integral TEI is updated using the current supply air temperature and Tset_quest. In step  303  also the average return air temperature Tret_avg is updated using a current return air temperature. 
         [0095]    In step  305  the algorithm checks whether all respective minimum ON or OFF time periods associated with the heating unit, cooling unit and internal fans speed have expired. Suitable minimum ON and OFF times are generally in the range between 1 and 10 minutes such as about 5 minutes but may be adjusted based on specific characteristics of various components of the refrigeration unit. 
         [0096]    If these minimum ON or OFF time periods have not expired, the algorithm proceeds to step  319  and maintains current operational states of the heating unit, cooling unit and internal fans. Thereafter, the algorithm proceeds to step  401 . On the other hand if all minimum ON or OFF time periods have expired, the algorithm proceeds to step  307  and tests whether the current value of the temperature-error integral TEI exceeds the upper bound or first error threshold TEI_max_cool of the temperature-error integral. If the current value of the temperature-error integral TEI exceeds the upper bound it indicates that the average supply air temperature through the current cycle is getting too high. Therefore, the operational state of the cooling unit is switched to ON in step  321  and the state of the heating unit is set to Off. The internal fans speed is also switched to, or maintained at, High speed setting. The High speed setting during cooling is advantageous for several reasons, one of them being less dehumidification and therefore less weight loss to the commodity load. After step  321 , the algorithm proceeds to step  401 . 
         [0097]    On the other hand if the current value of the temperature-error integral TEI is smaller than the upper bound, the algorithm either switches the operational state of the cooling unit to OFF, or maintains an already existing OFF state, in step  309 . The algorithm proceeds to step  311  and tests whether the current value of the temperature-error integral TEI is smaller than the lower bound or second integral error threshold TEI_heat_stage3_lim of the temperature-error integral. If the current value of the temperature-error integral TEI is smaller than the lower bound it indicates that the average supply air temperature through the current cycle is getting too low. Therefore, the algorithm proceeds to step  323  and switches the operational state of the heating unit to ON. The internal fans speed is also switched to, or maintained at, the High setting so as to add heat and homogenise or minimize temperature variations within the transport volume. After step  323 , the algorithm proceeds to step  401  with the effect described below. 
         [0098]    On the other hand if the current value of the temperature-error integral TEI is larger than the lower bound, the algorithm proceeds to step  313  and sets or switches the operational state of the heating unit to Off and proceeds to step  315 . In step  315  it is evaluated whether the previous circulation period was shorter than the circulation time threshold t ct . If true (Y), the algorithm proceeds to step  325  and sets the internal fans speed to High, after which the algorithm proceeds to step  401 . 
         [0099]    On the other hand if in step  315  it is evaluated that the previous circulation period was not (N) shorter than the circulation time threshold t ct , the algorithm leaves the fan speed to the circulation period&#39;s fan cycling program in step  317 . The algorithm proceeds to step  317  and determines the appropriate internal fans speed or state (i.e. OFF, Low, High) by applying the logic rules governing internal fans state during circulation periods as outlined above in connection with the description of  FIG. 2 . After setting the appropriate internal fans speed, the algorithm proceeds to step  401  (refer to  FIG. 4 ). 
         [0100]    In step  401 , the algorithm checks or determines whether or not the current cycle has been completed. This is done by evaluating the three logic rules or conditions outlined before and determining if one of these conditions applies. The algorithm proceeds to step  405  if the algorithm determines that the cycle has been completed and computes an updated value of the adjusted setpoint temperature Tset_quest based on setpoint temperature Tset and the average return air temperature Tret_avg. 
         [0101]    The value of the average return air temperature Tret_avg is thereafter skipped and preparations are made for calculation of the average return air temperature in the upcoming cycle. The algorithm proceeds to its ending in step  407 . 
         [0102]    If the algorithm in step  401  determines that the current cycle has not been completed, a current value of the adjusted setpoint temperature Tset_quest is maintained in step  403 . After that the algorithm proceeds to its ending in step  407 . Thereafter the algorithm proceeds to await a next call of a control algorithm at step  301 . The next call will typically occur after a certain delay period, i.e. the sampling time interval minus computation time. 
         [0103]      FIG. 6  is series of graphs  601 ,  603  and  605  illustrating experimentally recorded values of selected variables of the above-described temperature control algorithm under high net heat load conditions of the refrigerated container. 
         [0104]    Graph  601  shows temperature values in ° C. on the y-axis for the adjusted setpoint temperature Tset_quest (long dotted line), the return air temperature Tret (short dotted line) and the supply air temperature Tsup (full line). The x-axis unit is time in minutes. 
         [0105]    Graph  603  shows corresponding (to graph  601 ) values of the computed temperature-error integral (TEI) in units of ° C.*min on the y-axis and time in minutes on the y-axis. The full line represents values of the TEI and the horizontal dotted line represents a value of 90° C.*min for a first or upper integral threshold error TEI_max_cool of the temperature-error integral. 
         [0106]    Finally, graph  605  shows corresponding (to graphs  601  and  603 ) operating states of the cooling unit, heating unit and internal fans where the states are indicated on the y-axis. The respective ON states of the cooling unit and heating unit are indicated by the value “1” and OFF states as the value “0”. For the internal fans, the High setting or state (maximum fan speed) is indicated as “2”, the Low setting as “1” and the OFF setting as “0”. 
         [0107]    As illustrated, the supply air temperature on graph  601  varies considerably between 0.3° C. and −3.5° C. while the return air temperature varies considerably less between about between 0.2° C. and −0.2° C. The low variability of the return air temperature is caused by thermal inertia of the produce in the transport volume  45  (of  FIG. 1 ). The adjusted setpoint temperature, Tset_quest (long dotted line), is kept constant at about −1.5° C. 
         [0108]    By comparing the value of the supply air temperature on graph  601  and the ON periods of the cooling unit on graph  605 , the sudden drop or increase of supply air temperature in response to switching the cooling unit between Off and ON states is evident. 
         [0109]    By inspection of the TEI curve on graph  603  and the ON periods of the cooling unit on graph  605 , it is indicated how a TEI value above the 90° C.*min upper bound on the TEI, TEI_max_cool, leads to activation of the cooling unit which stays ON until the flow of cooled supply air has caused the TEI to drop below the upper bound (and the minimum ON time has passed). Consequently, the present temperature control algorithm does not activate the cooling unit based on certain preset limits or bounds on the supply air temperature or return air temperature but instead activates the cooling unit based on limits or constraints placed on the TEI. 
         [0110]    The activation of the cooling unit on graph  605  also shows that the operational state of internal fans remains High (state “2”) for the entire depicted time period while the heating unit remains OFF as expected in view of the high net heat load. The fan speed remains High during this short circulation period because the duration of the circulation period is shorter than the circulation time threshold t ct . 
         [0111]      FIG. 7  is series of graphs  701 ,  703  and  705  illustrating experimentally recorded values of selected variables of the above-described temperature control algorithm under low net heat load conditions of the refrigerated container. The individual curves of these graphs correspond to those of  FIG. 6 . The main difference to the graphs of  FIG. 6  is the length of the circulation periods, i.e. time periods where both the heating unit and the cooling unit are in OFF state, as evidenced by the relative scaling of the time axes and the curves of graphs  605  and  705  which indicate respective operational states of the heating unit, cooling unit and internal fans under the different net head load conditions. During these circulation periods in  FIG. 7 , the internal fans speed starts cycling between Low and Off according to the earlier described rules for controlling the fan speed during circulation periods. Time periods where the operational state of the cooling unit is ON are always accompanied by a High speed setting of the internal fans in the present embodiment of the invention. 
         [0112]      FIG. 8  is series of graphs  801 ,  803  and  805  illustrating experimentally recorded values of selected variables of the above-described temperature control algorithm under environmental conditions where heating is required to maintain the commodity load at or close to the adjusted setpoint temperature. Consequently, the coherent nature of the present temperature control algorithm capable of supplying heated or cooled supply air as required is demonstrated. As indicated by curve  813  on graph  803 , a second or lower integral error threshold is set to a value −10° C.*minutes. When the TEI curve  809  falls below this lower integral error threshold  813 , the heating unit is switched ON as depicted on graph  805  which as expected leads to increasing supply air temperature as evidenced by the supply air temperature curve  807  of graph  801 . As shown on graph  803 , the increasing supply air temperature starting at about t=9 minutes leads to an increasing value of the TEI after a short delay period. The internal fans speed is maintained High during the entire time period while the operational state of the cooling unit is OFF for the entire time period since no cooling is required. 
         [0113]      FIG. 9  is a graph illustrating experimentally recorded values of produce temperature, on curve  901 , versus supply air temperature, depicted on curve  903 , measured at the same position in the transport volume of the refrigerated container where the supply air enters. It is evident that produce temperature is kept within very tight limits of about +/−0.1° C. despite considerable variation of the supply air temperature from about 1.5 to 6.4° C. Thus, demonstrating that the thermal inertia of the produce suffices to annihilate air temperature fluctuations of this frequency. The present temperature control system exploits this thermal inertia to maintain highly accurate control over produce temperature by controlling the temperature-error integral so as to stay within the upper integral error threshold and the lower integral error threshold.