Patent Publication Number: US-2010122540-A1

Title: Thermoelectric cooler for economized refrigerant cycle performance boost

Description:
BACKGROUND OF THE INVENTION 
     This application relates to a refrigerant system having an economizer circuit, wherein a thermoelectric cooler provides additional cooling and a performance boost to assist the conventional economizer circuit. 
     Refrigerant compressors circulate a refrigerant through a refrigerant system to condition a secondary fluid. Typically, in a basic refrigerant cycle, a compressor compresses a refrigerant and delivers it to a heat rejection heat exchanger. 
     Refrigerant from the heat rejection heat exchanger passes through an expansion device, in which its pressure and temperature are reduced. Downstream of the expansion device, the refrigerant passes through a heat accepting heat exchanger, and then back to the compressor. As known, the heat accepting heat exchanger is typically an evaporator. The heat rejecting heat exchanger is a condenser, in subcritical applications, and a gas cooler, in transcritical applications. 
     One option in a refrigerant system design to enhance performance is the use of an economizer, or so-called vapor injection function. When an economizer function is activated, a portion of refrigerant is tapped from a main refrigerant stream downstream of the heat rejection heat exchanger. In one configuration, this tapped refrigerant is passed through an auxiliary expansion device, to be expanded to an intermediate pressure and temperature, and then this partially expanded tapped refrigerant passes in heat exchange relationship with a main refrigerant flow in an economizer heat exchanger. In this manner, the main refrigerant is cooled such that it will have a greater thermodynamic potential when it reaches the heat accepting heat exchanger. The tapped refrigerant, typically in a superheated thermodynamic state, is returned to an intermediate compression point in the compressor downstream of the economizer heat exchanger. As known, there are other arrangements involving economizer heat exchangers as well as flash tanks that provide similar functionality. One other option which has been recently proposed for incorporation into refrigerant systems is the use of a thermoelectric cooler. The thermoelectric cooler essentially takes advantage of specific thermoelectric properties of dissimilar semiconductor materials and is based on two phenomena—the Peltier effect and Seebeck effect concurrently taking place during operation of the thermoelectric device. The Peltier effect is associated to the release or absorption of a finite heat flux at the junction of two electrical conductors, made from different materials and kept at constant temperature, at the presence of electric current. Similarly, the Seebeck effect is related to the same arrangement, where the two junctions are maintained at different temperatures, which would create a finite potential difference, and an electromotive force that would drive an electric current in the closed-loop electric circuit. The Peltier and Seebeck effects are presented simultaneously in the thermoelectric cooler that is preferably made from the materials that have dissimilar absolute thermoelectric powers. The finite electric current passing through the two junctions triggers two heat transfer interactions with two cold and hot reservoirs kept at different temperatures. For steady thermoelectric cooler operation, heat fluxes associated with the two junctions should have opposite signs. If the external system maintains potential difference and drives electric current against this difference, the two junction system becomes a thermoelectric cooling device. 
     A typical thermoelectric cooler consists of an array of P-type and N-type semiconductor elements that act as the two dissimilar conductors. The P-type material has an insufficient number of electrons and the N-type material has extra electrons. These electrons in the N-type material and so-called “holes” in the P-type material, in addition to carrying an electric current, become a transport media to move the heat from the cold junction to the hot junction. The heat transport rate depends on the current passing through the circuit and the number of moving electron-hole couples. As an electric current is passed through one or more pairs of P-N elements, there is a decrease in temperature at the cold junction resulting in the absorption of heat from the object to be cooled. The heat is carried through the thermoelectric cooler by electron transport and released at the hot junction as the electrons move from a high to a low energy state. Although the thermoelectric devices are inherently irreversible, since heat and electric current must flow through the circuit during their operation, they do not have moving parts that makes them extremely reliable. Further, the refrigerant of the conventional vapor compression system is replaced by electrons carrying energy from a cold junction to a hot junction, created by two conductors with dissimilar absolute thermoelectric powers and connected electrically in series and thermally in parallel. To date, the use of thermoelectric coolers has only been proposed to be positioned downstream of a heat rejection heat exchanger in a conventional refrigerant cycle. Such thermoelectric coolers have never been proposed for providing an additional performance boost to an economizer cycle. Performance enhancement of economized refrigerant systems becomes especially crucial in a view of a limited capability of the economizer cycle in the air conditioning application range and continuously raising efficiency standards. Furthermore, alternate refrigerants, such as carbon dioxide operating in a transcritical regime, require extra means, in addition to the economizer function, to achieve the performance levels comparable to the performance levels of refrigerant systems charged with conventional refrigerants. 
     SUMMARY OF THE INVENTION 
     In disclosed embodiments of this invention, refrigerant systems are provided with economizer circuits. A thermoelectric cooler is operable to provide an additional performance boost to the economized refrigerant system by providing extra cooling either to an economized refrigerant flow or directly to a main refrigerant flow or both. In either case, a thermoelectric cooler can be positioned upstream or downstream of an economizer, in relation to a respective refrigerant flow. In all disclosed refrigerant system configurations, the thermoelectric cooler allows for additional cooling of the main refrigerant flow and its temperature reduction upstream of the main expansion device. Therefore, the thermodynamic cooling potential of the refrigerant flow entering an evaporator and the overall performance of the refrigerant system are increased. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a first schematic. 
         FIG. 1B  shows a second schematic. 
         FIG. 2A  shows a third embodiment. 
         FIG. 2B  shows a fourth schematic. 
         FIG. 3A  shows a fifth schematic. 
         FIG. 3B  shows a sixth schematic. 
         FIG. 4A  is a P-h diagram for  FIGS. 1A ,  1 B,  2 A and  2 B. 
         FIG. 4B  is a P-h diagram for  FIGS. 3A and 3B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An economized refrigerant system  20  is illustrated in  FIG. 1A . As known, a compressor  22  compresses a refrigerant and delivers it downstream to a heat rejection heat exchanger  24 . As also known, the heat rejection heat exchanger  24  is a gas cooler for a transcritical cycle and a condenser for a subcritical cycle. From the heat rejection heat exchanger  24 , the refrigerant passes through an economizer heat exchanger  26 . In this embodiment, a main refrigerant flow passes through the economizer heat exchanger  26 , and a tap refrigerant line  30  taps a portion of the refrigerant from a main refrigerant line  28  downstream of the economizer heat exchanger. The tap line  30  passes through an economizer expansion device  32 , and then once again through the economizer heat exchanger  26 . In the expansion device  32 , the tapped refrigerant is expanded to an intermediate pressure and temperature, and therefore can cool the refrigerant in the main refrigerant line  28  during heat transfer interaction in the economizer heat exchanger  26 . The refrigerant from the tap refrigerant line  30  passes through an injection refrigerant line  34  back to an intermediate compression point at the compressor  22 . 
     Refrigerant in the main refrigerant line  28  downstream of the economizer heat exchanger  26  passes through a main expansion device  40 , and then through a heat accepting heat exchanger (evaporator)  42 . From the heat accepting heat exchanger  42 , the refrigerant passes back to the compressor  22 . 
     The economized refrigerant system described above is generally known in the art. The present invention enhances performance of this known economized refrigerant system by including a thermoelectric cooler  38  downstream of the economizer heat exchanger  26 . Thermoelectric cooler  38  provides further cooling to the refrigerant in the main refrigerant line  28  by providing a thermal contact between a cold junction of the thermoelectric cooler  38  and a refrigerant in the main refrigerant line  28 . Moreover, the thermoelectric cooler  38  also cools the refrigerant which is tapped into the tap refrigerant line  30 , and thus provides even greater thermodynamic cooling potential for the tapped refrigerant and a higher heat transfer rate between the main refrigerant in the main refrigerant line  28  and tapped refrigerant in the tap refrigerant line  30 , in the economizer heat exchanger  26 . The thermoelectric cooler  38  may be of any type or configuration known in the art. For instance, the hot junction of the thermoelectric cooler  38  may be cooled by an air stream. In embodiments, the components may be positioned such that a single air moving device moves air over both the gas cooler  24  and the thermoelectric cooler  38  to reject heat from both components. Alternatively, separate air moving devices can be utilized. Obviously, other heat rejection means from the hot junction of the thermoelectric cooler  38  are also feasible. The attachment of the cold junction of the thermoelectric cooler  38  to the main refrigerant line to provide sufficient thermal contact can be, for instance, by a mechanical contact, brazing, soldering, welding, gluing, or any other means. 
     Analogously to the  FIG. 1A  embodiment  20 , the tap refrigerant line  30  for the economizer circuit can be positioned upstream of the thermoelectric cooler  38 . Similar benefits can be achieved in this configuration as well. 
       FIG. 1B  shows an embodiment  50 , which is similar to the embodiment  20  depicted in  FIG. 1A , other than in the location of the thermoelectric cooler  52 . Here, the thermoelectric cooler  52  is positioned intermediate the economizer expansion device  32 , and the economizer heat exchanger  26 . In this case, the thermoelectric cooler can be made more compact, since it has to cool only a portion of refrigerant tapped into the tap refrigerant line  30 . This embodiment provides higher temperature difference between a refrigerant in the main refrigerant line  28  and a refrigerant in the tap refrigerant line  30 , leading to a higher heat transfer rate in the economizer heat exchanger  26 . Obviously, the thermoelectric cooler can also be positioned between the tap point  33  and the economizer expansion device  32 . 
       FIG. 2A  shows yet another embodiment  60 . In the embodiment  60 , a portion of refrigerant is tapped into the refrigerant line  30  upstream of the economizer heat exchanger  26 , and passed through a thermoelectric cooler  62  prior to reaching the economizer expansion device  32 . Again, refrigerant in the tap refrigerant line  30  will be colder, and thus will be able to cool the refrigerant in the main refrigerant line  28  to an even greater extent, in the economizer heat exchanger  26 . 
       FIG. 2B  shows an embodiment  70  that is similar to the embodiment  60  of  FIG. 2A , other than locating the thermoelectric cooler  72  downstream of the economizer expansion device  32 . 
       FIG. 3A  shows still another embodiment  80  where the economizer heat exchanger  26  of previous embodiments is replaced by a flash tank  44 . Economized systems with a flash tank are known in the art. The flash tank separates liquid and vapor refrigerant phases, with the liquid phase flowing through the main circuit and the vapor phase delivered to an intermediate compression point in the compressor  22 . A thermoelectric cooler  46  is placed between the economizer expansion device  32  and the flash tank  44  to provide extra liquid content in the refrigerant mixture flowing into the flash tank  44  and an additional performance boost to the refrigerant system  80 . 
       FIG. 3B  shows another embodiment  90 , which is similar to the embodiment  80  of  FIG. 3A , with the exception that a thermoelectric cooler  48  is positioned between the flash tank  44  and the main expansion device  40 . In this case, rather than increasing the liquid content in the two-phase mixture flowing into the flash tank  44 , the thermoelectric cooler  48  further cools the liquid exiting the flash tank  44 , thus enhancing performance of the refrigerant system  90 . 
     The benefits of providing a thermoelectric cooler at the locations as shown in this application can be seen from the P-h diagrams of  FIGS. 4A and 4B . In general, the P-h diagram depicted in  FIG. 4A  shows additional capacity provided by a thermoelectric cooler for the refrigerant systems having an economizer circuit containing an economizer heat exchanger, as shown in  FIGS. 1A ,  1 B,  2 A and  2 B. Similarly,  FIG. 4B  shows benefits provided by a thermoelectric cooler for the refrigerant systems including a flash tank, as exhibited in  FIG. 3B . 
     The economized refrigerant systems incorporating thermoelectric coolers disclosed in this invention can be used in both conventional subcritical applications, as shown in  FIG. 4A , and transcritical applications, as exhibited in  FIG. 4B . Since transcritical applications, such as those employing carbon dioxide as a refrigerant, are inherently less efficient, the thermoelectric cooler would be the most advantageous in those applications. Also, since the augmentation provided by an economizer cycle for air conditioning applications is limited by a reduced pressure ratio, the thermoelectric cooler integration would provide additional benefits for air conditioning applications, especially in a view of continuously raising efficiency standards and diminishing returns of standard performance enhancement methods. 
     Furthermore, the thermoelectric cooler can provide additional flexibility in unloading economized refrigerant systems. Turning the thermoelectric device on will supply additional capacity to compensate for thermal load demand in the conditioned space. On the other hand, switching the thermoelectric device off will allow for unloading of the refrigerant system when only part-load capacity is required to meet the space demand. 
     It should be pointed out that many different compressor types could be used in this invention. For example, scroll, screw, rotary, or reciprocating compressors can be employed. The refrigerant systems that utilize this invention may have various options and enhancement features, such as, for instance, tandem components, reheat circuits, intercooler heat exchangers, etc., and can be used in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigeration truck-trailer units, and supermarket refrigeration systems. 
     While embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.