Abstract:
A method using controlled forced-convection to couple (or de-couple) a refrigerant-evaporator thermally to a refrigerated compartment, and a method to use heat from a fresh food compartment (or other compartment maintained above 0C) of a two or more temperature refrigeration appliance to defrost a freezer evaporator automatically without using a controller or heater. When thermally coupled to its compartment, an evaporator can provide efficient cooling to the compartment. The ability to de-couple the evaporator from its compartment enables refrigerant flow through the evaporator at significantly different temperatures than its compartment, but without significant heat transfer. This ability enables using a single refrigeration system (with one or no valves) to remove heat (sequentially) at two or more different temperatures from two or more refrigerated compartments.

Description:
This is a non-provisional United States (U.S.) patent application based on two provisional U.S. patent applications including (i) a first provisional U.S. patent application entitled “Method For Increasing Efficiency In Multiple-Temperature Forced-Air Refrigeration Systems” (app. Ser. No. 60/120,469; filed Feb. 17, 1999 and (ii) a second provisional U.S. patent application entitled “Method For Increasing Efficiency In Multiple-Temperature Forced-Air Refrigeration Systems” (app. Ser. No. 60/132,774; filed May 6, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of refrigeration. 
     2. Background of Art Related to the Invention 
     For many decades, domestic refrigerators have included a freezer section and a fresh food section. The fresh food section is maintained at a significantly higher temperature than the freezer section. While the basic laws of thermodynamics provide empirical evidence that it is increasingly more difficult to cool (i.e., remove heat from) an item as its temperature decreases, domestic refrigerators typically have been designed with more consideration focused on cost than thermodynamics. For example, many domestic refrigerators use a one-stage refrigeration system including a single evaporator located in the freezer section. Since the total heat load dissipation is through this single evaporator, this one-stage refrigeration system possesses less than optimal energy efficiency. 
     Recently, in order to increase system efficiency, some refrigerators have been constructed with two separate refrigeration systems; namely, one refrigeration system is responsible for cooling the freezer section while the other refrigeration system is responsible for cooling the fresh food section. Consequently, this dual refrigeration system includes repetitive condensing units, each featuring a compressor and a condenser. This repetition of equipment increases the cost and size of the refrigerator. Also, these repetitive condensing units produce a greater amount of noise. 
     Another example involves yacht refrigerators which have been implemented with refrigeration systems having valves to sequentially, but not simultaneously, connect a single, high-capacity condensing unit to multiple evaporators operating at differing temperatures. The refrigeration system may use thermal energy storage (TES) material to provide stable temperatures during the period between evaporator operations. 
     Preferably, TES material is an aqueous solution such as a salt solution having water and sodium chloride (NaCl). This composition provides high heat storage capacity, emits a large amount of heat isothermally upon changing phase from a liquid to a solid, is-non-toxic and can be produced for a low cost. Unfortunately, this TES material is highly corrosive to most metals, tends to expand when frozen which would damage the thin wall of the heat exchanger and tends to freeze first on the heat exchange surfaces which would hamper further heat transfer. This requires the TES material to be separated from the thin-walled metal tubing of the heat exchanger. One technique of separation involves encapsulating TES material into separate expandable capsules as described in U.S. Pat. No. 5,239,839 by the named inventor. 
     Additionally, the use of TES material adversely affects the efficiency of conventional defrosting cycles. The reason is that conventional defrost methods, if implemented, would require the entire TES material to melt before actual defrosting could begin. 
     U.S. Pat. Nos. 4,712,387 and 4,756,164 by the named inventor describe a heat pipe based method for efficiently transferring heat into and out of TES material and a method for thermally de-coupling the TES material from the cooled space to enable simple and efficient defrosting of the evaporator. These methods fail to provide any suggestion of the multi-stage refrigeration system and/or control protocol used to control this refrigeration system. 
     PCT International Patent Application No. PCT/US97/20151 describes a cost-effective evaporation unit and an energy efficient control protocol to maintain steady temperatures for each section of a refrigeration unit. An additional element of that disclosure is the use and design of a simple sensor for determining the frozen fraction of a TES module in order to control on-and-off cycling of the compressor for temperature stabilization. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a method using controlled forced-convection to couple (or de-couple) a refrigerant-evaporator thermally to a refrigerated compartment. When thermally coupled to its compartment, an evaporator can provide efficient cooling to the compartment. The ability to de-couple the evaporator from its compartment enables refrigerant flow through the evaporator at significantly different temperatures than its compartment, but without significant heat transfer. This ability enables using a single refrigeration system (with no valves) to remove heat (sequentially) at two or more different temperatures from two or more refrigerated compartments. This capability enables achieving the maximum energy efficiency for the heat removal process from a multi-temperature system (e.g., a domestic refrigerator/freezer appliance) with a single refrigeration system. 
     The present invention also comprises a method to use heat from a fresh food compartment (or other compartment maintained above 0 C) of a two or more temperature refrigeration appliance to defrost a freezer evaporator automatically without using a controller or heater. The benefits of this innovation come from use of this innovation in conjunction with one of the frost-free methods also disclosed herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following description of the present invention in which: 
     FIG. 1 presents an illustration of a simple two-temperature implementation of one aspect of the present invention. 
     FIG. 2 presents an illustration of another implementation of one aspect of the present invention. 
     FIG. 3 presents an illustration of a defrost method in accordance with the present invention. 
     FIG. 4 presents an illustration of a defrost method in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The implementation of the present invention illustrated in FIG. 1 may use a thermal energy storage device  20  and  22  in either or both compartments  24  and  26 , respectively. This implementation uses a single compressor  21  and condenser  23 , and uses insulating barriers  28  and  30  to help isolate the evaporators  32  and  34  in each of their compartments from their compartment, except when the fan  36  or  38  is operating. This implementation achieves further isolation (when the fan is off) by having the lower temperature compartment  26  below the warmer temperature compartment  24 . This configuration minimizes natural convection. For example when the fan  36  in the upper (higher temperature) compartment is on, the refrigerant temperature may be significantly higher than the temperature of the lower compartment  26 . In this situation the portion of the refrigerant circuit in the lower compartment  26  will be warmer than the compartment and not establish free convection involving the lower compartment. 
     Another implementation appears in FIG.  2 . In this case, a single compressor  21  and condenser  23  are used, and one or both thermal energy storage devices  20  and  22  may be used. There is only one fan  40  and one air/refrigerant heat exchange surface  42  (expansion device) and the thermal coupling (de-coupling) occurs by opening and closing flow passages  44  and  46  for air flow to either of the compartments  24  and  26 . This method provides the same energy efficiency benefits that have motivated others to add multiple valves in the refrigerant circuit of a multiple temperature system so that multiple evaporators can be operated sequentially and independently. This new approach eliminates the cost, reliability, and energy consumption concerns often associated with multiple refrigerant valves. 
     The capability of thermally disconnecting or connecting an evaporator  42  can be combined with a three-way refrigerant valve  50  as in FIG. 3 below. Here the motivation is to establish an integral connection of the thermal energy storage device ( 20  or  22 ) to the refrigerant in one of the refrigerated compartments  24  or  26 . The desire for a better thermal connection between the thermal energy storage device (TES) and refrigerant or the desire to eliminate a second fan (including its power consumption and dissipation) may make this configuration desirable in some markets. 
     The benefits of this design include: 
     1. Enhanced energy efficiency, by removing heat from each compartment rather than allowing all the heat to flow to the lowest temperature for removal there. 
     2. Enhanced-efficiency de-frost of the freezer section evaporator. During operation of the fan in the higher temperature compartment, the refrigerant flowing through the section of the evaporator in the freezer compartment will heat this evaporator. This heating can offset the required heating to defrost this evaporator. In the configuration of FIG. 2 there is just one surface and it operates at both the low and higher temperature. Defrosting would be performed after operation at the higher temperature. 
     3. Isolation of the air in a refrigerator compartment from a freezer compartment, hence minimizing frost accumulation in the freezer and desiccation of the stored food in the refrigerator section, 
     4. Optional addition of TES to either or both compartments. The forced convection provided by the fan would provide thermal coupling between the evaporator and the TES. The TES could be positioned so that natural convection provides continuous coupling between the TES and the compartment. 
     The capability of thermally disconnecting or connecting an evaporator can be combined with a three-way refrigerant valve  50  as in FIG. 3 below. Here the motivation is to establish an integral connection of the thermal energy storage device to the ( 20  or  22 ) to the refrigerant in one of the refrigerated compartments  24  or  26 . The desire for a better thermal connection between the thermal energy storage device TES and refrigerant or the desire to eliminate a second fan (including its power consumption and dissipation) may make this configuration desirable in some markets. 
     The foregoing is essentially a method to in effect, have another “valve” in the system so that one can achieve most of (or virtually all of) the “two stage” efficiency benefit to having dual refrigeration systems. This approach supports frost-free designs and has many advantages over the use of dual compressors, (as is often the case in European appliances). The fan, when on thermally connects the freezer box to the evaporator and when off, it is disconnected. TES if desired could be placed (widely distributed with very large surface area) throughout the compartment (e.g., compartment walls). 
     A defrost method in accordance with the present invention is illustrated in FIG.  4 . In this Figure, the elements indicated by the numerals 1 through 12 are as follows: 
     1. Refrigerant tube between fresh foods evaporator with TES and no-frost forced convection evaporator in freezer compartment, 
     2. Compressor, 
     3. Condenser, 
     4. Three way valve directs refrigerant flow to be through both evaporators or only through the freezer evaporator, 
     5. Freezer evaporator, 
     6. Refrigerator evaporator (with integral TES) 
     7. Refrigerant tube for singular operation of freezer evaporator and not fresh foods evaporator, 
     8. Thermal barrier between freezer evaporator and freezer compartment 
     9. Freezer TES, heat removal by forced convection from the evaporator 
     10. Fan to thermally couple evaporator in freezer to freezer compartment when operated. 
     11. Expansion device for Freezer evaporator 
     12. Expansion device for refrigerator evaporator 
     During the period when the compressor is not operating, heat transferred from the fresh-foods compartment to the evaporator in the freezer compartment will warm and eventually melt frost build up on its surfaces. Because there are long periods between compressor operation (owing to the extended cooling provided by TES) there is ample time to transfer heat and defrost the freezer evaporator. The desire for a long period of compressor off condition argues strongly for use of a TES module in the freezer section (as described in concept #3 in provisional application “Methods for increasing efficiency in multiple-temperature forced-air refrigeration systems” ). During the compressor off-time the freezer evaporator is thermally disconnected from the thermal mass of the freezer compartment and its contents (as described in Methods for increasing efficiency in multiple-temperature forced-air refrigeration systems). This allows the evaporator to warm up and defrost due to small amounts of heat transferred from the warmer compartment. 
     Heat transfer can be provided simply and economically by the existing tube  1 . In particular, tube  1  can provide heat transfer by either conduction through the tube walls or by a thermal siphon (heat pipe effect) or a combination of the two. Additionally, arranging the air flow (either natural convection or forced) in the fresh foods section, so that it has appropriate heat transfer to the tube  1  in the fresh foods compartment will maintain the appropriate temperature of tube  1  at the point where it passes into the freezer compartment. The trap in tube  1  is an option to block the liquid refrigerant from freely flowing during compressor operation into the fresh food compartment while the three-way-valve directs refrigerant flow to the freezer evaporator through tube  7 . Varying the height and position of the trap will vary the heat transfer provided by the thermal siphon effect. 
     Additional components required (relative to a conventional single evaporator Domestic US refrigerator/freezer): 
     1. TES evaporator in fresh foods section (can be “foamed in” the walls so essentially hidden) 
     2. Three-way valve to control the refrigerant flow and select proper expansion device. 
     The benefits (relative to a conventional single evaporator Domestic U.S. refrigerator/freezer) include: 
     1. Minimum energy used to defrost 
     2. Defrost energy used to cool the refrigerator section 
     3. Eliminate need for heater 
     4. Eliminate need for defrost controller 
     5. Permits a smaller evaporator in the freezer (As a result of having the higher efficiency two-stage system and elimination of the additional heat input traditionally supplied by a defrost heater.) 
     6. Smaller compressor (result of having higher efficiency with the two stage system) 
     7. No requirement for air passages between compartments (simpler design and assembly as well as cleaner internal design) 
     While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention apparent to persons skilled in the art to which the invention pertains, are deemed to lie within the spirit and scope of the invention. Thus, the invention should be measured in terms of the claims which follow.