Patent Publication Number: US-2021180838-A1

Title: Electrocaloric heat transfer system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of 62/949,223 filed Dec. 17, 2019, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     A wide variety of technologies exist for cooling applications, including but not limited to evaporative cooling, convective cooling, or solid state cooling such as electrothermic cooling. One of the most prevalent technologies in use for residential and commercial refrigeration and air conditioning is the vapor compression refrigerant heat transfer loop. Vapor compression refrigerant loops effectively provide cooling and refrigeration in a variety of settings, and in some situations can be run in reverse as a heat pump. However, many of the refrigerants can present environmental hazards such as ozone depleting potential (ODP) or global warming potential (GWP), or can be toxic or flammable. Additionally, vapor compression refrigerant loops can be impractical or disadvantageous in environments lacking a ready source of power sufficient to drive the mechanical compressor in the refrigerant loop. Similarly, the weight and power requirements of the compressor can be problematic in various portable cooling applications. Accordingly, there has been interest in developing cooling technologies as alternatives to vapor compression refrigerant loops. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Described herein is a heat transfer system, including an electrocaloric element comprising a nanoparticulate ion scavenger and a co-continuous polymer network of a first polymer phase and a second polymer phase wherein the first polymer phase includes a liquid crystal polymer; a pair of electrodes disposed on opposite surfaces of the electrocaloric element; a first thermal flow path between the electrocaloric element and a heat sink; a second thermal flow path between the electrocaloric element and a heat source; and a controller configured to control electrical current to the electrodes and to selectively direct transfer of heat energy from the electrocaloric element to the heat sink along the first thermal flow path or from the heat source to the electrocaloric element along the second thermal flow path. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the heat source is a conditioned dielectric fluid. The conditioned fluid may be air. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, the nanoparticulate ion scavenger includes freestanding cerium (Ce), platinum (Pt), palladium (Pd), silver (Ag), and gold (Au) nanoparticles. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, the nanoparticulate ion scavenger comprises silica-supported cerium (Ce), platinum (Pt), palladium (Pd), silver (Ag), and gold (Au) nanoparticles. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, the second polymer phase comprises a liquid crystalline mesogenic group. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, the second polymer phase is coated. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, wherein the second polymer phase is a sol-gel. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, the first polymer phase is crosslinked. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, there is an ion barrier layer between the electrocaloric element and the electrodes. The ion barrier layer may have a thickness of 10 nanometers to 10 micrometers. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, first polymer phase comprises a main-chain liquid crystal polymer. 
     Also described herein is a method of using the heat transfer system. The method includes applying an electric field as a voltage differential across the electrocaloric element, thereby causing a decrease in entropy and a release of heat energy by the electrocaloric element; transferring at least a portion of the released heat energy to the heat sink; removing the electric field, thereby causing an increase in entropy and a decrease in heat energy and absorption of heat energy by the electrocaloric element; and transferring heat energy from the heat source to be absorbed by the electrocaloric element. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, at least a portion of the released heat energy to the heat sink is transferred to the heat sink simultaneously with applying the electric field. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, the nanoparticulate ion scavenger comprises freestanding cerium (Ce), platinum (Pt), palladium (Pd), silver (Ag), and gold (Au) nanoparticles. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, the nanoparticulate ion scavenger comprises silica supported cerium (Ce), platinum (Pt), palladium (Pd), silver (Ag), and gold (Au) nanoparticles. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments, an ion barrier layer is located between the electrocaloric element and the electrodes. 
     In addition to one or more of the features described above, or as an alternative, the second polymer phase comprises a liquid crystalline mesogenic group. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic depiction of an exemplary cooling system as described herein; and 
         FIG. 2  is a schematic depiction of another exemplary cooling system as described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the Figures,  FIG. 1  depicts an exemplary cooling system  10 . As shown in  FIG. 1 , a cooling system  10  includes an electrocaloric element  12  having electrodes  14  and  16  on opposite sides of the electrocaloric element  12 . As further described below, the electrocaloric element  12  comprises a co-continuous polymer network of a first polymer phase and a second polymer phase. The first polymer phase includes a liquid crystal polymer, liquid crystal oligomer or a low molecular weight liquid crystal mesogen. The second polymer phase includes a polymer material that forms a support for the first polymer phase. The second polymer phase may also include liquid crystalline mesogenic groups. The assembly of the electrocaloric element  12  and the electrodes  14  and  16  can be prepared by applying an electrode-forming ink (such as metal nanoparticle slurries, metal microparticles slurries, metal/carbon nanowire dispersions, graphene dispersions) or other composition to the electrocaloric element  12 . 
     As used herein, the term “liquid crystal polymer” means a matrix of polymer molecules comprising mesogenic groups that is fluidly deformable by nematic or smectic ordering of the mesogenic groups in response to application of an external field. Liquid crystal polymers (sometime referred to a polymer liquid crystals), comprise polymer molecules that include mesogenic groups. Mesogenic molecular structures are well-known, and are often described as rod-like or disk-like molecular structures having electron density orientations that produce a dipole moment in response to an external field such as an external electric field. Liquid crystal polymers typically comprise numerous mesogenic groups connected by non-mesogenic molecular structures. The non-mesogenic connecting structures and their connection, placement and spacing in the polymer molecule along with mesogenic structures are important in providing the fluid deformable response to the external field. Typically, the connecting structures provide a stiffness that is low enough so that a deformation response is induced by application of the external field, and high enough to provide the characteristics of a polymer when the external field is not applied. 
     In some exemplary embodiments, a liquid crystal polymer can have rod-like mesogenic structures in the polymer backbone separated by non-mesogenic spacer groups having flexibility to allow for re-ordering of the mesogenic groups in response to an external field. Such polymers are also known as main-chain liquid crystal polymers. In some exemplary embodiments, a liquid crystal polymer can have rod-like mesogenic structures attached as side groups attached to the polymer backbone. Such polymers are also known as side-chain liquid crystal polymers. 
     Examples of main-chain liquid crystal polymers include those having the repeating structures shown with C 10  and C 8  polyethylene spacer groups, respectively: 
     
       
         
         
             
             
         
       
     
     Examples of side-chain liquid crystal polymers include those having the repeating structures shown with C 4  and C 10  polyethylene spacer groups, respectively: 
     
       
         
         
             
             
         
       
     
     Of course, the above structures are exemplary. Many other liquid crystal polymers are known, and can be readily utilized by the skilled person. 
     In some embodiments the liquid crystal polymers may be modified with crosslinking. The crosslink density can be adjusted to be low enough so that local molecular flexibility is retained to allow nematic or smectic ordering of the mesogenic groups in response to an external field. However, the crosslink density is set high enough to produce a macro elastic deformation response of the polymer to the external field instead of the Brownian molecular motion that results in a fluid, non-elastic macro response to the external field. The crosslinking reaction can rely on any type of crosslinking mechanism such as including tri- or higher-functional monomer in the monomer reactants during polymerization or by including functional side groups such as hydroxyl attached to the polymer chain, which can be reacted with a crosslinking agent such as a diisocyanate. The functional side groups can be selected to result in a mesogenic group integrated in the crosslink chain, or the mesogenic groups can be attached as side groups on the polymer chain separate from crosslink chains that are non-mesogenic. Many liquid crystal elastomers are known, and can be readily utilized by the skilled person. 
     As mentioned above, the electrocaloric element can comprise a liquid form liquid crystal (first polymer phase) retained in a polymer matrix (second polymer phase). Materials having liquid crystal retained in a polymer matrix are known in the art as polymer dispersed liquid crystal (PDLC), and can be prepared using various techniques including but not limited to polymerization-induced phase separation, thermal-induced phase separation, solvent-induced phase separation, to induce a phase separation resulting in a multiphase matrix having a polymer phase and a liquid crystal phase. The liquid crystal phase can include a molecular liquid crystal, a polymer liquid crystal, or an oligomer liquid crystal. A polymer dispersed liquid crystal (PDLC) a can be prepared using known separation techniques for preparing PDLC in combination with generally known techniques of monomer selection, crosslinking, and processing to prepare a crosslinked polymer network as the PDLC polymer matrix. Exemplary liquid crystal mesogens include n-cyanobiphenyls (n-CB) comprising n=5, 6, 8, 9, 10, 12; 2-methylbutyl 4-(4-decyloxybenzylideneamino)-cinnamate (DOBAMBC); hexyl-oxy benzylideneamino-chloropropyl cinnamate (HOBACPC). 
     Turning again to  FIG. 1 , the electrocaloric element  12  can be in thermal communication with a heat sink  17  through a first thermal flow path  18 . The electrocaloric element  12  can also be in thermal communication with a heat source  20  through a second thermal flow path  22 . A controller  24  is configured to control electrical current to the connected electrodes  14  and  16 . The controller  24  is also configured to control heat flow control devices  26  and  28  to selectively direct the transfer of heat along the first and second heat flow paths  18  and  22 . The type of heat flow path and heat flow control device is not limited and can include, for example, solid state heat thermoelectric switches in thermally conductive contact with the electrocaloric element and the heat source or heat sink, or thermomechanical switches in movable contact to establish thermally conductive contact between the electrocaloric element  12  and the heat source  20  or heat sink  17 . In some exemplary embodiments, described in more detail below, heat transfer between the electrocaloric element  12  and the heat source  20  or heat sink  17  can include convective heat transfer to a flowing fluid in contact with the electrocaloric element  12 . In such cases, the fluid, which should be dielectric to avoid electrochemical interaction with the electrocaloric element  12 , can itself be the heat source (e.g., a conditioned airspace) or the heat sink (e.g., outside air), or the fluid can be a dielectric heat transfer fluid (e.g., an organic compound) flowing between the electrocaloric element  12  and a remote heat source  20  or heat sink  17 . 
     In operation, the system  10  can be operated by the controller  24  applying an electric field as a voltage differential across the electrocaloric element  12  to cause a decrease in entropy and a release of heat energy by the electrocaloric element  12 . The controller  24  activates the heat flow control device  26  to transfer at least a portion of the released heat energy along heat flow path  18  to heat sink  17 . This transfer of heat can occur after the temperature of the electrocaloric element  12  has risen to a threshold temperature. In some embodiments, it may be desirable to avoid excessive temperature increase in order to maintain polymer physical properties of the co-continuous polymer network during this entropy reduction phase, and in some embodiments, heat transfer to the heat sink  17  is begun as soon as the temperature of the electrocaloric element  12  increases to be about equal to the temperature of the heat sink  17 . After application of the electric field for a time to induce a desired release and transfer of heat energy from the electrocaloric element  12  to the heat sink  17 , the electric field is removed. Removal of the electric field causes an increase in entropy and a decrease in heat energy of the electrocaloric element  12  as the first polymer phase returns to its original molecular alignment. This decrease in heat energy manifests as a reduction in temperature of the electrocaloric element  12  to a temperature below that of the heat source  20 . The controller  24  deactivates heat flow control device  26  to terminate transfer of heat energy along heat flow path  18 , and activates heat flow control device  28  to transfer heat energy from the heat source  20  to the colder electrocaloric element  12 . 
     In some embodiments, for example where a heat transfer system is utilized to maintain a temperature in a conditioned space or thermal target, the electric field can be applied to the electrocaloric element  12  to increase its temperature until the temperature of the electrocaloric element reaches a first threshold. After the first temperature threshold, the controller  24  activates heat flow control device  26  to transfer heat from the electrocaloric element  12  to the heat sink  17  until a second temperature threshold is reached. The electric field can continue to be applied during all or a portion of the time period between the first and second temperature thresholds, and is then removed to reduce the temperature of the electrocaloric element  12  until a third temperature threshold is reached. The controller  24  then deactivates heat flow control device  26  to terminate heat flow transfer along heat flow path  18 , and activates heat flow control device  28  to transfer heat from the heat source  20  to the electrocaloric element  12 . The above steps can be optionally repeated until a target temperature of the conditioned space or thermal target (which can be either the heat source or the heat sink) is reached. 
     As mentioned above, in some embodiments, the thermal flow paths can involve a dielectric fluid, either air or other dielectric gas or liquid such as an organic compound heat transfer fluid. In such embodiments, the dielectric fluid can itself be the heat source or heat sink (e.g., outside air), or the dielectric fluid can flow to and transfer heat with a remote heat sink or remote heat source. A technical effect of the use of a dielectric fluid is to facilitate application of electric field to the electrocaloric element with simultaneous transfer of heat from the electrocaloric element to the heat sink. Many prior art systems that rely on conductive heat transfer to or from an electrocaloric element must avoid transferring heat during operation of the electrocaloric element. A cooling system  10 ′ for transferring heat from a heat source (e.g., cooling a conditioned air space) to a heat sink (e.g., an external air space outside of the conditioned air space) is schematically depicted in  FIG. 2 , which has like numbering for like parts as  FIG. 1  that do not require further description below.  FIG. 2  is generally configured and operated like  FIG. 1 , with heat transfer between the electrocaloric element  12  and the heat sink  17  or heat source  20  provided by a dielectric fluid flowing through conduits connecting header spaces  30  and  32  that interface with the electrocaloric element  12  to the heat sink  17  and heat source  20  through control valves  34 ,  36 ,  38 , and  40 . Additionally, the electrocaloric element has flow channels  46  (schematically represented by cross-hatching) for fluid flow through the electrocaloric element  12  between header spaces  30  and  32 . It should be noted that although the fluid flow path through the electrocaloric element is depicted for convenience of illustration as horizontal, it could also be vertical (transverse to the electrocaloric element  12 ) with the channels extending through the electrodes or the electrodes being permeable to the dielectric fluid. 
     The operation of the system  10 ′ is described below with respect to an air conditioning system where the heat source is a conditioned air space and the heat sink is outside air, but it is understood that the system can also be operated in heat pump mode, or with a heat transfer fluid that transfers heat to and from remote heat sources/sinks. In operation, the system  10 ′ can be operated by the controller  24  applying an electric field as a voltage differential across the electrocaloric element  12  to cause a decrease in entropy and a release of heat energy by the electrocaloric element  12 . The controller  24  opens control valves  38  and  40 , closes control valves  34  and  36 , and activates blower  44  to drive airflow from outside air source (heat sink)  17  through the electrocaloric element to transfer at least a portion of the released heat energy from the electrocaloric element  12 . After application of the electric field for a time to induce a desired release and transfer of heat energy from the electrocaloric element  12 , the electric field is removed. Removal of the electric field causes an increase in entropy and a decrease in heat energy of the electrocaloric element  12  as the liquid crystal polymer returns to its original molecular alignment. This decrease in heat energy manifests as a reduction in temperature of the electrocaloric element  12  to a temperature below that of the conditioned space (heat source)  20 . The controller  24  closes control valves  38  and  40  and opens control valves  34  and  36  to direct airflow from blower  44  between the electrocaloric element  12  and the conditioned space. 
     As discussed above, the electrocaloric element includes a co-continuous polymer network. The co-continuous polymer network includes a first polymer phase and a second polymer phase. A co-continuous polymer network refers to a multiphase material in which at least two phases are distributed throughout the matrix without either phase forming discrete and bounded areas. The first polymer phase includes a liquid crystal polymer. The liquid crystal polymer may be cross linked. The second polymer phase may include a film forming polymer, a film forming copolymer, a sol-gel polymer system, or a combination thereof. The second polymer phase may include liquid crystalline mesogenic groups or have polar moieties giving ferroelectric properties. The amount of the first polymer phase may be greater than or equal to 50 weight % based on the total weight of the first polymer phase and the second polymer phase. 
     The film forming polymers, sol-gel polymer systems and various processing methods result in highly porous materials. As used herein the term “highly porous” describes porosities of about 20 volume percent to 90 volume percent. Volume percent porosity is defined as 100 minus the product of 100 and the ratio of the porous film density to the bulk density of the film material in a dense form. The highly porous film may be formed from the film forming polymer using a processing method that results in a highly porous film. Exemplary processing methods include sol-gel methods, solution casting methods, gel processing methods and paste processing methods. 
     Exemplary sol-gel polymer systems include sol-gel precursors comprising metal alkoxides [M(OR) 3 ] and metal chlorides, and other sol-gel precursors which undergo hydrolysis and polycondensation reactions to form a colloid. 
     Exemplary film forming polymers include polytetrafluoroethylene (PTFE), polyvinylidine fluoride (PVDF), poly(ethylene-co-tetrafluoroethylene)(ETFE), poly(tetrafluoroethylene-co-vinylidine fluoride) (PTFE-co-VDF), poly(tetrafluroethylene-co-vinylidine-co-chlorotrifluoroethylene) (PTFE-co-VDF-co-CTFE), poly(trifluoroethylene-co-vinylidine fluoride) (PTrFE-co-VDF), poly(trifluoroethylene-co-vinylidinefluoride-co-chlorotrifluoroethylene) (PTrFE-co-VDF-co-CTFE), siloxane, olefin polymers such as ultra high molecular weight polyethylene (UHMWPE), linear low density polyethylene (LLDPE), polypropylene (PP), and combinations thereof. 
     The highly porous materials may comprise a coating. The coating is chosen to result in a coated material with surface energy such that the wetting of the coated material by the liquid crystal polymer is facilitated. Exemplary coatings include chemical vapor deposition of acrylates, siloxanes, paracyclophanes or any other hydrocarbon monomer compatible with a chemical vapor deposition process. 
     The highly porous materials may also be surface treated using an energetic method to facilitate wetting of the coated film with the liquid crystal polymer. Exemplary surface treatments include exposure to plasma, ozone or heat to alter the surface chemistry of the highly porous material. The highly porous material may also be pre-wetted with an organic solvent to facilitate wetting of the coated material by the liquid crystal polymer or a solution containing the liquid crystal polymer. 
     The electrocaloric element further comprises a nanoparticulate ion scavenger. Nanoparticulate, as used herein, describes a particle having a no single linear dimension greater than 100 nanometers. For example, spherical nanoparticles would not have a diameter greater than 100 nanometers. Exemplary nanoparticulate ion scavengers include freestanding and silica-supported cerium (Ce), platinum (Pt), palladium (Pd), silver (Ag), and gold (Au) nanoparticles. The nanoparticulate ion scavenger addresses issues of ionic purity in the liquid crystal polymer and act to at least partially suppress arcing of the electrocaloric element. 
     In some embodiments, the electrocaloric element includes an ion barrier layer between the electrocaloric element and the electrodes. The ion barrier layer may include india tin oxide, polytetrafluoroethylene, or the like. The ion barrier layer may be applied by chemical vapor deposition, atomic layer deposition, molecular layer deposition, or solution based spin coating. The ion barrier layer may have a thickness of 10 nanometers to 10 micrometers. 
     The systems described herein can be operated in a cooling mode where the heat source is a conditioned space or cooling target. The systems described herein can also be operated in a heat pump mode where the heat sink is a conditioned space or heating target. It should also be noted that the described systems are exemplary in nature and that modifications can of course be made. For example, a single controller  24  is shown in each Figure, but control could be provided by distributed control or smart components such as temperature-sensitive heat transfer control devices. Also, although the systems are depicted with a single electrocaloric element and electrode assembly, it is understood by the skilled person that connected banks or arrays of elements can be used as well. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope 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.