Abstract:
Heat pump cycle provided with a fluidic loop connecting two heat exchangers. The fluidic loop is filled with an electro-caloric liquid as a heat transfer medium. Applying electric filed in one of the heat exchangers the temperature of the electro-caloric liquid is changed.

Description:
FIELD OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates generally to the field of electrocaloric materials and, more particularly, to a heat pump system that uses liquid-phase electrocaloric materials. 
       BACKGROUND OF THE INVENTION 
       [0002]    Typical heating, ventilation, and air conditioning functionality (“HVAC”) is provided by vapor compression, or reverse Rankine, cycles. These devices use two-phase fluorinated refrigerants which are under high pressure and exhibit significant global warming potential when they inevitably leak into the atmosphere. Also, the compression process cannot be efficiently scaled to small sizes restricting energy savings achievable through distributed heat pumping. Finally, such compressors tend to be noisy. A scalable, quiet, and environmentally friendly alternative is desired. 
         [0003]    Materials that exhibit adiabatic temperature change when subject to mechanical strain, magnetic fields, or electrical fields have been used to create heat pump cycles. For example, field-active materials can include electrocaloric and magnetocaloric materials. Electrocaloric materials exhibit large entropy changes when an electric field is applied to their structure. A basic heat pump cycle that implements an electrocaloric material is shown in  FIG. 1 . At state  1 , a material is at steady temperature and is subject to a steady field applied directly to the material. An increase in the applied field strength increases material temperature at state  2 . Heat is rejected to a hot ambient bringing the material temperature down near the hot ambient value in state  3 . This is best accomplished through direct contact of the ambient air and the active material. Reduction of the field strength reduces material temperature at state  4 . The cycle is then completed by absorbing heat from a cold ambient, again preferably through direct contact, causing the material temperature to rise back to the temperature value at state  1 . This cycle may approximate ideal Carnot, Brayton, or Ericsson cycles depending on the timing of field actuation in relation to heat rejection. 
         [0004]    The adiabatic temperature lift available with known elcctrocaloric or magnetocaloric materials is typically lower than the lift required for most commercial heat pump applications such as environmental control. One well-known means of increasing temperature lift (at the expense of capacity) is thermal regeneration. A typical regenerative heat exchanger depends on thermal storage and reciprocating fluid motion to develop an axial temperature gradient and thus multiply temperature lift. Regenerative heat exchangers are common in cycles that use fluid compression rather than field-active materials to provide heat pumping. For example, Stirling cycle coolers, and thermoacoustic coolers that apply a modified Stirling cycle, use regenerative heat exchangers as common practice. In these regenerative heat exchangers, the work for heat pumping comes from compression/expansion of the fluid within the regenerator and the solid material of the regenerator provides the heat capacity for regeneration. Also, in a thermoacoustic or other pressure-based regenerative cooling cycle, it is necessary to use a heat exchanger to separate the pressurized working fluid from the ambient air resulting in a significant loss in performance. Regenerative heat exchanger use has also been reported in field-active magnetocaloric cooler prototypes. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    In accordance with an embodiment, a heat pump cycle includes providing a fluidic loop between two heat exchangers in fluidic communication with each other; energizing at least a first heat exchanger of the two heat exchangers to generate an electric field in the first heat exchanger, advecting a field-active liquid through the fluidic loop; changing an entropy of the field-active liquid in response to advecting into the electric field of the at least first heat exchanger; and exchanging heat between the field-active liquid and the two heat exchangers in response to the changing of the entropy of the field-active liquid. 
         [0006]    In accordance with another embodiment a regenerative field-active heat pump cycle for heat transport having a regenerator and secondary heat exchanger elements includes energizing the regenerator and a first heat exchanger of the secondary heat exchanger elements to apply an intermittent electric field; changing an entropy of the field-active liquid resident in the regenerator and a first heat exchanger of the secondary heat exchanger elements in response to the electric field; advecting the field-active liquid from the regenerator into the first heat exchanger of the secondary heat exchanger elements while maintaining the electric field; transferring heat from the first heat exchanger to a hot ambient temperature in response to advecting the hot energized field-active liquid into the heat exchanger; releasing the field in the regenerator and a first heat exchanger of the secondary heat exchanger elements; changing an entropy of the field-active liquid resident in the regenerator and a first heat exchanger of the secondary heat exchanger elements in response to releasing the electric field; advecting the cold field-active liquid from the regenerator into the second heat exchanger of the secondary heat exchanger elements while maintaining the electric field; and transferring heat from the second heat exchanger to a cold ambient temperature in response to advecting the cold do-energized field-active liquid into the heat exchanger. 
         [0007]    Technical function of the one or more claims described above provides heat transfer through a field-active liquid that heats or cools upon application of a field, and heat transfer occurs in a heat exchanger with the associated hot or cool environment until the liquid comes into near-equilibrium with the environs while remaining in the field. 
         [0008]    Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0009]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which like elements are numbered alike in the several FIGURES: 
           [0010]      FIG. 1  is a diagram of a field-activated heat pump cycle in accordance with the prior art; 
           [0011]      FIG. 2  is an exemplary system diagram for a heat pump cycle that utilizes a field-active liquid in accordance with an embodiment of the invention; 
           [0012]      FIG. 3A  is a general perspective view of an exemplary heat exchanger that has multiple flow tubes and electrodes in accordance with an embodiment of the invention; 
           [0013]      FIG. 3B  is a side elevation view of an exemplary heat exchanger of  FIG. 3A  that has multiple flow tubes and electrodes in accordance with an embodiment of the invention; 
           [0014]      FIG. 4  is a front elevation view of an exemplary regenerator in accordance with an embodiment of the invention; 
           [0015]      FIG. 5  illustrates an exemplary hybridized regenerator system for use in accordance with embodiments of the invention; and 
           [0016]      FIGS. 6A-6C  illustrates a cascade regenerator system that integrates multiple electrocaloric loops in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Embodiments of the invention described below include using liquid-based electrocaloric materials as the working fluids for heat pumping in heating, ventilation, and air conditioning (“HVAC”) and refrigeration systems, as well as in hybrid systems containing field-active liquid and solid materials. In embodiments, the field-active liquid is circulated through at least two heat exchanger elements, wherein a heat transfer process occurs in the presence of an electric field in one and in the absence of field in the other. The field causes the field-active liquid to either heat or cool (depending on the specific liquid composition), and heat transfer occurs in the heat exchanger with the associated hot or cool environment until the liquid comes into near-equilibrium with the environs while remaining in the field. As the liquid leaves the field it cools or heats (respectively) and the fluid enters a de-energized heat exchanger to once again transfer heat to the cool/hot environment. 
         [0018]    Referring to  FIG. 2 , a basic system  200  for a heat pump cycle is illustrated in accordance with an embodiment of the invention. System  200  includes a plurality of heat exchangers  202  and  204  that are in fluidic communication with each other through a flow tube or passage  206 . Heat exchanger  202  includes electrodes  212  and  214  in order to generate an electric field in the heat exchanger. Flow tube  206  contains a field-active liquid material that is circulated between heat exchangers  202  and  204  and through the flow tube  206  continuously. In an embodiment, flow tube  206  includes insulation  210  in order to prevent heat exchange between the field-active liquid material and an external environment. A pump  208  creates the pressure to advect or pump the field-active liquid material through the flow tube  206  and the heat exchangers  202  and  204 . In some non-limiting examples, pump  206  can be a mechanical pump, an electrostatic electric field pump, an electrophoretic electric field pump, or the like. Also, the field-active liquid material exhibits temperature change when subject to the electrical field in heat exchanger  202  and can be an liquid electrocaloric material. Non-limiting examples of liquid electrocaloric materials can include liquid crystals, ionic liquids, or other similar liquids that can exhibit a temperature change in an electric field. It is to be appreciated that the field-active liquid material serves as the working fluid for the heat pumping cycle as well as enabling heat exchange between heat exchangers  202  and  204  and an external environment  216 . 
         [0019]    Field-active materials including liquid crystals respond to an applied electric field, creating internal order/disorder; and therefore are capable of storing or releasing energy in the form of caloric heat and electrical capacitive energy. The field-active material can alter its order parameter with the applied electric field. As the order parameter is directly related to the system entropy and free energy, cooling and heating are consequences of electric field release or application, or of advection of the field-active material through a localized continuous electric field. 
         [0020]    In an exemplary operation for system  200 , the field-active liquid material is circulated through heat exchanger elements  202  and  204 , wherein an electric field is applied or not applied during a heat transfer process. Field-active liquid material is pumped into heat exchanger  202  where an electric field is applied. The electric field causes the field-active liquid material to transfer heat to the associated hot environment  218  (e.g., outdoors in cooling mode or indoors in heating mode) until the field-active liquid material comes into near-equilibrium with the environs while remaining in the electric field. As the field-active liquid material leaves the electric field it cools and the field-active liquid material enters a de-energized heat exchanger  204  to absorb heat from cold environment  216  (e.g., indoors in cooling mode or outdoors in heating mode). This cycle is repeated continuously. It is to be appreciated that, for maximizing performance of system  200 , the field-active liquid material is energized in the same location that heat exchange occurs as any interruption of electric field will return the field-active liquid material to its original temperature. So, a heat exchanger integrated with electrodes that can apply the required uniform field can be used, for example, as heat exchanger  202 . 
         [0021]      FIG. 3A  illustrates an exemplary heat exchanger that can be used with system  200  of  FIG. 2  to provide an effective cooling device. Preferably, in embodiments, a multiple channel liquid-air heat exchanger with a counter flow configuration or a cross-counter flow configuration can be used, but other configurations of heat exchangers can also be used in accordance with embodiments of the invention. In other embodiments, liquid-gas heat exchangers or liquid-liquid heat exchangers in a counter flow or cross-counter flow configuration can also be used. An exemplary counter flow heat exchanger  300  is illustrated in  FIG. 3A . Heat exchanger  300  is a tube-fin structure heat exchanger and includes a plurality of electrically conductive channels that serve as tubes or conduits  302  for a secondary heat exchange fluid. In one embodiment, this fluid is a liquid such as water or oil. In another embodiment, this fluid is air. Each fluid-containing tube  302  is separated by an insulating material  306  such that each tube  302  and its associated fins, if any, can be energized independently. The space  304  between any two tubes  302  contains field-active liquid material wherein a field can be applied to this liquid by applying a potential to the surrounding conductive tubes  302  without applying any field to the secondary heat transfer fluid. Each tube-fin structure of heat exchanger  300  serves as an electrode and will be energized with potential of opposing polarity. The liquid heat exchanger  300  can be made out of metal tubes but other materials could also be used given the low pressure of the process. Polymer or ceramic-walled heat exchangers with deposited electrodes can also be used. As shown in  FIG. 3B , positive electrodes  310   a - 310   c  and negative electrodes  312   a - 312   c  are placed with opposing polarity to create an electrical field in the flowing field-active liquid material, but are placed with similar polarity surrounding the secondary fluid to avoid any electrical discharge through this fluid. In embodiments, the walls of the heat exchanger could be made of a solid field-active ceramic or polymer such as PZT ceramic or PVDF polymer. One set of electrodes can now energize both active liquid and active solid material simultaneously, increasing the specific capacity of the overall device. The heat exchanger  300  serves a function of a heat transport fluid, enabling a continually flowing pumped loop with continuously applied electric fields. 
         [0022]    It is to be appreciated that performance of the field-active liquid material can be increased by utilizing a mixture of dielectric constituents, both liquid and solid, to improve entropy change and/or extend operating temperature range. For example, particles of an electrocaloric ceramic with large pyroelectric effect can be mixed into an active electrocaloric liquid crystal with lower performance to create a slurry, gaining the performance advantage of the solid material while retaining the system flexibility advantage of using a liquid. In addition to the features of a slurry of an electrocaloric ceramic with an active electrocaloric liquid, other embodiments can include an inactive liquid dielectric material that is added to a solid elcctrocaloric material for the purpose of creating a flowable mixture. As an additional example, two or more different liquid crystals with different active temperature ranges may be mixed to broaden the temperature response of the liquid mixture in the system. As an additional example, additives may be used to lower input requirements for entropy change, such as nanoparticles to lower required field strength. Also, solid-state pumping technology such as electrophoretic pumping could be used to create an entirely solid-state cooling device. 
         [0023]      FIG. 4  illustrates an exemplary variation of a system  400  that uses a regenerative heat exchanger to achieve higher temperature lift than that enabled by the physical properties of the field-active liquid material. System  400  includes a regenerative heat exchanger  402  (or regenerator  402 ) that includes a regenerative matrix made from a solid material that stores heat and acts as an electrode, imposing an electric field on the field-active liquid. A field active liquid reciprocates back-and-forth between bracketed respective hot and cold heat exchangers  404  and  406  and through the regenerative heat exchanger  402  in synchronization with the applied electric field to develop a temperature gradient in the regenerator and thus increase the temperature difference between heat exchangers  404  and  406 . The field-active liquid material can be translated back and forth through the regenerator  402  by an imposed pressure field generated by a mechanical or electrostatic pump or linear actuator. 
         [0024]    Heat exchangers  404  and  406  can include electrodes to apply an electric field to the field-active liquid material. Unlike any other regenerative cycle, the reciprocating field-active liquid is best maintained under constant field, either on or off, when the liquid is reciprocated from regenerator  402  toward either heat exchangers  404  and  406 . When the regenerator is energized and the liquid is translated toward one heat exchanger, that heat exchanger will also be energized. This requires integration of the three heat exchangers  402 ,  404 , and  406  and specific spatial-temporal synchronization of the applied field. 
         [0025]    In operation, application of the field through intimate contact to the field-active liquid in regenerator  402  may increase the material entropy (e.g., temperature). Advecting the now hot field-active liquid into the hot heat exchanger  404  while also maintaining the field in the heat exchanger  404  causes it to reject heat to the hot ambient  408 . Once the heat exchanger  404  cools to the hot ambient  408  temperature, the field in the regenerator  402  is released causing the field-active liquid to cool. The field in hot heat exchanger  404  is also de-energized causing the field-active material inside to cool. Advecting the now cooled field-active material from the hot exchanger  404  toward the cold heat exchanger  406  causes the field-active material to absorb heat from the cold ambient  410  and complete the cycle. The performance of the system  400  may depend on timing and synchronization of the applied field and flow, and that such timing may change with thermal properties of the material, the load, and the temperature lift desired, so careful control of this process may be needed to achieve satisfactory performance. 
         [0026]    The regenerator matrix can be made with field-active materials to create a hybrid liquid-solid matrix, increasing the heat pumping capacity and power density. In one embodiment the regenerator matrix  402  is made from electrically insulating electroactive ceramic or polymer with electrodes on each side and the field-active liquid between the layers. Energizing these electrodes activate both liquid and solid field-active material simultaneously for increased capacity. In another embodiment the regenerator matrix  402  can be made from active solid magnetocaloric materials, elastocaloric materials, or optocaloric materials. Electric field applied to activate the electroactive liquid material is synchronized with a separately applied magnetic, strain, or light field, respectively, to the solid matrix to produce additional capacity. In another embodiment, heat exchangers  404  and  406  can also be made from solid field-active material and energized with the field-active regenerator matrix and field-active liquid to further increase capacity. 
         [0027]      FIG. 5  illustrates an exemplary hybridized system  500  for use in accordance with embodiments of the invention. System  500  illustrates two repeating elements  502  and  504  of a multi-channel regenerative heat exchanger that utilizes combinations of liquid and solid electrocaloric materials as well as materials sensitive to other fields such as magnetic, strain, pressure or radiation fields. In an example for regenerator element  502 , a solid matrix  503  of the regenerator  502  can be made of electrocaloric material such as ferroelectric ceramics or polymers. This material provides thermal storage needed for regeneration as well as providing support for the electrodes  506  and  508 . A pair of electrodes  506  and  508  can energize the electrocaloric solid. A pair of electrodes  508  and  510  can energize the electrocaloric liquid  512  flowing between a pair of solid matrices in regenerator elements  502  and  504 . These electrodes, e.g., electrodes  506 ,  508 , and  510  can simultaneously energize both the field-active liquid (e.g., electrocaloric liquid) and the field-active solid material of the regenerator matrix, in effect offsetting the parasitic thermal dilutive effect of the regenerator material and thus increasing the specific capacity of the device. In another embodiment, the electrode pairs  506 / 508  and  508 / 510  can be energized in sequence to provide additional temperature lift. 
         [0028]    In order to use the principle of offsetting parasitic loss of the regenerator matrix, a solid material can be used which exhibits entropy change in fields other than electric for the regenerator matrix. Use of a magnetocaloric material or material that changes entropy when exposed to strain, pressure, or radiation (including light) as the regenerator matrix and electrode support, combined with the imposition of the respective field synchronized with the electric field imposed on the liquid electrocaloric material, can also increase specific capacity of the device. Similarly, an electrocaloric solid material could be superposed with an optically energized liquid material. 
         [0029]    Using a field-active liquid material serves a function of a heat transport fluid, enabling a continually flowing pumped loop with continuously applied electric fields as described in the embodiments described above in  FIGS. 2-3 . However, using regeneration to multiply temperature lift as described in the embodiments described above in  FIGS. 4-5  requires a less efficient reciprocating fluid motion as well as potentially inefficient temporal variation of the electric field. To achieve high temperature lift with continuous fluid flow and electric field, and thus improved efficiency, a cascaded cycle concept with appropriately integrated heat exchangers is used as is described in  FIG. 6 . 
         [0030]      FIGS. 6A-6C  illustrate an exemplary regenerator system  600  that integrates multiple electrocaloric loops through coupling heat transfer in accordance with an embodiment of the invention. System  600  integrates many individual electrocaloric loops through coupling heat transfers using an electrocaloric liquid crystal but, in embodiments, other field-active liquid materials may also be utilized. As seen in  FIG. 6A , a first electrocaloric loop is illustrated where a cold secondary fluid or ambient is connected through a heat exchanger  604  with the de-energized end  608  of an electrocaloric loop  602  which is driven by a liquid pump  606 . In some non-limiting examples, pump  606  can be a mechanical pump, an electrostatic electric field pump, an electrophoretic electric field pump, or the like. The electrocaloric loop  602  will continually transport heat from the low ambient to a higher temperature. The energized (or hot) end  610  of the loop  602  is in a heat exchange relationship with another heat exchange element  612  to the cold end of another independent loop  614  to pump heat to an even higher temperature as illustrated in  FIG. 6B . Similarly, as illustrated in  FIG. 6C , an energized hot end  616  of loop  614  is in a heat exchange relationship through another heat exchange element  618  to the cold end of another independent loop  620  to pump heat to an even higher temperature. This process continues with another connection between low ambient to a higher temperature through an electrocaloric loop until adequate temperature lift is achieved and then the hot end of the last loop is connected to the hot secondary fluid or ambient through heat exchanger  622 . 
         [0031]    As shown in  FIGS. 6A-6C , combination of electrocaloric loops  602 ,  614 , and  620  is enabled by stacking layers of loops and heat exchangers and then using headers to connect the channels together such that many parallel loops can be driven by one pump, which is similar to brazed or welded plate-fin, minichannel, or compact heat exchanger fabrication known in the industry. In embodiments, a multichannel pump such as a peristaltic pump or other modular pomp could be used to drive flow through multiple cascade elements using a single motor and speed control. The system  600  allows heat pumping while maintaining continuous active fluid and secondary fluid flows combined with steadily applied electric fields to avoid any wasteful reversal of flow or current. Again, many physical embodiments may provide the same functionality of bringing active primary and secondary fluids together at the appropriate temperatures for heat transfer resulting in additional lift. 
         [0032]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.