Abstract:
A refrigeration system for merchandisers, drug cabinets and similar enclosures ( 10 ) that continues to provide temperature control during periods when external power is not necessarily available. The system typically has a compressor/condenser subsystem ( 11 ) that is powered by mains electricity ( 16 ) and a second subsystem ( 12 ) that includes an insulated eutectic tank. The compressor/condensor ( 11 ) cools the tank ( 12 ) using external electrical power, when available, while the tank cools the enclosure ( 10 ) without requiring external power. A refrigerant loop ( 14 ) between the second subsystem ( 12 ) and the enclosure ( 10 ) operates by way of convection and/or gravity and a simple controller ( 15 ).

Description:
FIELD OF THE INVENTION 
   This invention relates to refrigeration systems for enclosed spaces and in particular but not only to a eutectic system that continues to provide temperature control over an enclosed space during periods when external power is not necessarily available. 
   BACKGROUND TO THE INVENTION 
   Many refrigeration systems are required to provide cooling without necessarily having access to a continuous supply of electricity. In some cases electrical power is not available for large parts of the day in remote areas or for mobile systems. In others, the systems are required to avoid consumption of power during peak periods. Conventional eutectic systems have been developed to operate under these circumstances, but do not provide adequate temperature control for many purposes. Solar power systems with storage batteries have been developed but are relatively expensive and cannot guarantee that electricity will be available. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a refrigeration system using a eutectic subsystem with temperature control that can operate without external power for useful periods of time, or at least to provide an alternative to existing systems. 
   In one aspect the invention is a refrigeration system for an enclosure, including: a first cooling subsystem that is powered by an external source, a second cooling subsystem that is not necessarily powered by an external source, a first thermal pathway by which the first cooling subsystem, when powered, cools the second cooling subsystem, a second thermal pathway by which the second cooling subsystem cools the enclosure, and a controller in the second thermal pathway that operates to maintain the enclosure at a predetermined temperature. 
   Preferably the second thermal pathway is a refrigerant loop that conveys heat from the enclosure to the second cooling subsystem by convection. Refrigerant in the loop circulates by evaporation from a relatively low location to a relatively high location in the enclosure, followed by condensation and descent under gravity within the second subsystem. Preferably the controller includes a valve that regulates the flow of refrigerant around the loop without need of power from an external source. 
   Preferably the first cooling subsystem includes a compressor/condenser arrangement that is powered by mains electricity and the second cooling subsystem includes an insulated eutectic tank. In one embodiment the first thermal pathway includes a refrigerant loop between the first cooling subsystem and the second cooling subsystem, separate from the second thermal pathway. In another embodiment the first and second pathways are combined, so that the first cooling subsystem, when powered, cools both the second cooling subsystem and the enclosure. 
   In another aspect the invention is a method of cooling an enclosure, including: operating a powered cooling system to extract heat from a non-powered cooling system, and cooling the enclosure by convective transfer of heat from the enclosure to the non-powered cooling system. 
   Preferably the method further includes ceasing operation of the powered cooling system during periods when power is not available, and continuing to cool the enclosure during such periods by convective transfer of heat from the enclosure to the non-powered cooling system. Transfer of heat from the enclosure to the non-powered system is controlled to maintain the enclosure at a predetermined temperature 
   The enclosure may be a merchandiser, a cold storage room, a cabinet for medical supplies, a transportable container or an air conditioned room, for example. 

   
     LIST OF FIGURES 
     Preferred embodiments of the invention will be described with respect to the accompanying drawings, of which: 
       FIGS. 1A and 2B  schematically show alternative refrigeration systems, 
       FIGS. 2A and 2B  schematically show the alternative systems in more detail, 
       FIG. 3  shows a solenoid device that may be used as a valve in either system, 
       FIG. 4  shows a heat exchanger that may be used in either system, and 
       FIGS. 5   a ,  5   b ,  5   c  are views of a merchandiser with a refrigeration system. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to the drawings it will be appreciated that the invention may be implemented in a range of different ways for a range of different purposes. The systems described here are given by way of example only. It will also be appreciated that many components of these systems are of a conventional nature and need not be described in detail. 
     FIGS. 1A and 1B  show alternative refrigeration systems, each arranged to cool an enclosure  10 . Each system includes a first cooling subsystem  11 , typically compressor/condenser equipment that is electrically powered from an external source  16  such as mains electricity, and a second cooling subsystem  12 , typically a eutectic device that is generally without an external power supply. A first thermal pathway  13 , typically a refrigerant loop, links the first and second cooling subsystems, while a second thermal pathway  14 , also typically a refrigerant loop, links the second cooling subsystem with the enclosure. A temperature detector  17  in the second cooling system determines when operation of the first cooling system is required, while a temperature detector  18  in the enclosure determines when operation of the second cooling system is required. 
   In  FIG. 1A  the thermal pathways are separate and the first cooling subsystem  11  acts to cool the second cooling subsystem  12  which in turn cools the enclosure. In  FIG. 1B  the pathways are partially combined so that the first cooling subsystem cools both the second subsystem and the enclosure. In both cases, movement of refrigerant along the first pathway is generally driven by electrical power supplied to the first cooling system, while movement of refrigerant along the second pathway is generally driven by gravity and/or convection without need of external electrical power. A controller  15  such as a solenoid valve is provided in the second thermal pathway to control movement of the refrigerant in response to the detector  18  and thereby control the temperature of the enclosure. Various alternative arrangements of the subsystems and pathways are possible. 
     FIG. 2A  shows the refrigeration system of  FIG. 1A  in more detail. The first cooling system  11  includes a compressor  20 , a condenser  21 , a float  22 , a heat exchanger  23  and a capillary brake  24 . The second cooling system  12  includes an insulated tank  28  containing a eutectic solution or other material, such as brine or ethylene glycol. The enclosure  10  is a refrigeration cabinet in this example. A refrigerant loop including an accumulator  25  forms the first thermal pathway  13  between the cooling systems, and might be considered as part of the first cooling system. A refrigerant loop forms the second thermal pathway  14  between the second cooling system and the enclosure, and includes one or more evaporators  26  and  27  in the enclosure. The second loop might be considered as part of the second cooling system. 
   The compressor system  11  in  FIG. 2A  is able to cool the system  12  when power is available from source  16 . Refrigerant in the loop  13  enters the compressor  20  as a relatively cool low pressure gas and is delivered to the condenser  21  as a relatively warm high pressure gas. The condenser dissipates heat from the gas into the atmosphere and produces a warm liquid within the loop. The float  22  and brake  24  are control devices that regulate the flow of liquid along loop  13  from the condenser to the eutectic tank, particularly when the system is started and the tank is relatively warm. The liquid is cooled by expansion through these devices. Once in the tank  28  the liquid refrigerant in loop  13  absorbs heat from the eutectic material by evaporating and then returning to the compressor through the heat exchanger as a gas. The accumulator  25  is a trap that prevents any unevaporated liquid refrigerant from reaching the compressor. 
   The eutectic system  12  in  FIG. 2A  cools the enclosure  10  without necessarily using power from an external source or being in direct contact with the enclosure. Refrigerant loop  14  is arranged so that the refrigerant circulates in response to the effects of gravity and convection with the overall rate of flow determined by the controller  15 . Refrigerant cools and descends within tank  28  and passes as a liquid from the tank into the enclosure. The refrigerant enters at a relatively low point in the enclosure and depending on the temperature of the enclosure, is either pushed up toward the roof evaporator  27  or begins to evaporate initially in the base evaporator  26 . The liquid thereby absorbs heat from the enclosure and returns to the tank  28  as a gas from a relatively high point in the enclosure. 
     FIG. 2B  shows a refrigeration system in which the thermal pathways are combined, as an alternative to the system in  FIG. 2A . The compressor subsystem  11  cools either the eutectic subsystem  12  alone, or both the eutectic subsystem and the enclosure, depending on the status of controller  15 . The system of  FIG. 2B  cools the enclosure more quickly under a heavy load but the combined pathways require a common refrigerant and are more difficult to repair in the event of a leak. On the other hand, the system of  FIG. 2B  allows use of different refrigerants that may be selected for performance of the particular loop. 
     FIG. 3  shows a solenoid valve  15  in more detail. The valve is operated by a microprocessor (not shown) that monitors the temperature detectors  17  and  18  and draws power from a battery (not shown). A pair of coils  30  are pulsed to open and close the seat  31  of the valve when required by the microprocessor. The valve is normally held in a closed position by a spring  32  and requires no power in that position. Similarly the valve may be held open by a magnet  33  without additional power. An appropriate coil is pulsed to change the open or closed status of the seat requiring minimal power for a short period of time. Other valve systems that operate from temperature differentials and do not require battery power might also be used. 
     FIG. 4  shows a heat exchanger  23  of  FIGS. 2A and 2B  in more detail. Warm liquid refrigerant passing from the condenser  21  through the high side float  22  reaches the heat exchanger as a cool liquid with some vapour. Relatively cold vapour from the eutectic tank also passes through the heat exchanger when moving back to the compressor  20 . The cold vapour from the tank sub-cools the liquid and vapour from the float to form a cool liquid without vapour moving towards the capillary brake  24 . The level of heat exchange between the inflowing and outflowing liquids and vapours is determined to enhance the efficiency of the compressor. 
     FIGS. 5   a ,  5   b  and  5   c  are sectional views of a merchandiser that incorporates a refrigeration system as shown in  FIG. 2A  or  2 B. The merchandiser includes a cabinet  50  with front doors  51 , shelves  52  for products such as food or drink, and may be mounted on wheels  53 . Refrigerant from a eutectic tank  12  located in the rear of the cabinet flows through the base evaporator  26  upwards to the roof evaporator  27 , as indicated, and then returns to the tank. Valve  15  between the eutectic tank and the roof evaporator controls the flow of refrigerant. An optional fan  54  in the roof of the cabinet drives air flow downwards through the roof evaporator to the base evaporator, as indicated. The fan is powered by mains electricity and is generally not operated when power is not available. 
   As shown in  FIG. 5   c , the compressor  20  is located in an upper part of the rear of the cabinet in this example. The condenser  21  is located on one side at the rear of the cabinet and may have a fan  55  to assist dispersal of heat when power is available. A relatively small compressor can be used because the effect of sudden or heavy loads in the cabinet, such as opening of the front doors and stocking of the shelves, is buffered by heat absorption in the eutectic tank. Operation of the compressor can also be optimised for predetermined time periods with a reduced number of start events.