Thermal Performance of Electrocaloric Refrigeration using Thermal Switches of Fluid Motion and Changing Contact Conductance

Transcription

1 Americn Journl of Physics nd Applictions 2016; 4(5): doi:.11648/j.jp ISSN: (Print); ISSN: (Online) Therml Performnce of Electrocloric Refrigertion using Therml Switches of Fluid Motion nd Chnging Contct Conductnce Shigeki Hirsw *, Tsuyoshi Kwnmi, Ktsuki Shiri Deprtment of Mechnicl Engineering, Kobe University, Kobe, Jpn Emil ddress: (S. Hirsw) * Corresponding uthor To cite this rticle: Shigeki Hirsw, Tsuyoshi Kwnmi, Ktsuki Shiri. Therml Performnce of Electrocloric Refrigertion using Therml Switches of Fluid Motion nd Chnging Contct Conductnce. Americn Journl of Physics nd Applictions. Vol. 4, No. 5, 2016, pp doi:.11648/j.jp Received: August 23, 2016; Accepted: September 7, 2016; Published: September 19, 2016 Abstrct: Therml performnce of electrocloric refrigertion system composed with thin electrocloric mteril nd therml switches ws numericlly clculted. Two types of therml switches were studied: therml switch of fluid motion nd therml switch by chnging the contct therml conductnce. The following results were obtined. For the therml switch of fluid motion with the frequency Hz, the thicknesses of the electrocloric mteril 200 µm nd the wter flow chnnel 0 µm, the verge het trnsfer efficiency ws 11%. For the therml switch by chnging contct therml conductnce with the frequency 00 Hz, the thicknesses of the electrocloric mteril 20 µm nd the het storge mteril 20 µm, the verge het trnsfer efficiency ws 6% nd the verge het flux trnsferred to the cold side of the system ws 7 x 4 W/m 2. Keywords: Electrocloric Refrigertion, Therml Switch, Fluid, Contct, Therml Resistnce, Het Trnsfer, Efficiency 1. Introduction Recently new cooling technology using the electrocloric effect in thin films of n electrocloric mteril hs ttrcted interest becuse of high energy efficiency. Figure 1 shows bsic model of the electrocloric refrigertion system. A thin electrocloric mteril ws coupled with two therml switches working t high temperture nd working t low temperture. The electrocloric mteril lterntively generted het energy nd cold energy by chnging electric field. Here, cold energy mens negtive het energy. The generted het energy ws trnsferred to the hot side of the system, nd the cold energy ws trnsferred to the cold side seprtely by the opertion of therml switches. The coupling of multi-lyers of thin films of electrocloric mteril with therml switches cn work s the efficient refrigertor. It is esy to enhnce the cooling rte of the electrocloric refrigertor by incresing the frequency of the chnge of the electric field. Het energy Chnging electric field Cold energy working t high temp. Electrocloric mteril working t low temp. Figure 1. Model of electrocloric refrigertion system. Rdebugh et l. [1] reported the fesibility of electrocloric refrigertion for the 4 15 K temperture rnge. They studied two types of therml switches of multiple-lef contct switch nd mgneto therml switch. Epstein nd Mlloy [2] reported thin therml-switching device using liquid crystls. Ji nd Ju [3] reported experimentl results of therml performnce of n electrocloric refrigertion system using mechnicl therml switch t frequency of 0.1 Hz. Sto [4] reported the therml switching behvior of het pipe. Guo et l. [5] designed n electrocloric refrigertion system with therml switch of fluid motion. They reported tht cooling power density ws 3 4 W/m 2 nd percent of Crnot COP (coefficient of performnce) ws 31% t temperture

2 135 Shigeki Hirsw et l.: Therml Performnce of Electrocloric Refrigertion using Therml Switches of Fluid Motion nd Chnging Contct Conductnce spn of 15 K nd frequency of 15 Hz. Ozbolt et l. [6] reviewed the technologies for the electrocloric refrigertion. Hirsw et l. [7 9] reported the therml switching behvior of flt het pipe, chnging the contct therml conductnce nd fluid motion. They reported tht the system using fluid motion therml switch incresed the verge het trnsfer efficiency to 67 % t frequency of 5 Hz for constnt het genertion. They lso reported tht pulse het genertion ws preferble to increse therml performnce [9]. For prcticl use of electrocloric refrigertion to cool electronic devices, more studies re still needed to understnd the therml performnce t high frequency nd the design guideline to increse performnce of the electrocloric refrigertion. In this work, the therml performnce of electrocloric refrigertion system with two types of therml switches ws numericlly clculted for the pulse het genertion. They were therml switch of fluid motion nd therml switch by chnging the contct therml conductnce. Conditions to increse performnce of the electrocloric refrigertion were clrified. 2. Clcultion Method for Therml Switch of Fluid Motion A model of the electrocloric refrigertion system with therml switch of fluid motion ws shown in Figure 2 [9]. Both sides of flt thin film of the electrocloric mteril contct the hot wter nd cold wter s they flow in thin-flow chnnels. The working digrm of the electrocloric mteril nd fluid motion ws shown in Figure 3. The electrocloric mteril genertes het energy nd cold energy lterntely by pulse het genertion with time intervl t e. The het generting period of the pulse het genertion ws ssumed to be (t e / ). In this system, the fluid motion works s the therml switch. When the electrocloric mteril genertes het energy, wter flows in the upper chnnel nd ir is present in the lower chnnel. The wter in the upper chnnel trnsfers the het energy to the hot side of the system. When the electrocloric mteril genertes cold energy, wter flows in the lower chnnel nd ir is present in the upper chnnel. The wter in the lower chnnel trnsfers the cold energy to the cold side of the system. Therefore, the het energy nd the cold energy trnsfer to the hot side nd the cold side of the system seprtely. Wter Wter Het energy Cold energy Electrocloric mteril (length: x e, thickness: b e ) Hot wter Upper chnnel (thickness: b w ) Cold wter Lower chnnel (thickness: b w ) Figure 2. Model of electrocloric refrigertion system with therml switch of fluid motion. Electrocloric mteril 0 t e / Generte het energy Wter flow t e 2t e Generte cold energy Wter flow to cold side Time Figure 3. Working digrm of electrocloric mteril nd fluid motion. The high-frequency behvior of the cooling system is importnt to enhnce the cooling rte of the refrigertor using the electrocloric effect. However, the mteril hs therml time-constnt with respect to chnge temperture ccording to the size. The order of time-constnt t c (s) for therml conduction in film is estimted by t c (ρ c / λ) b 2 [9]. Here, ρ is the density (5800 kg/m 3 for BTiO 3 electrocloric mteril [] nd 00 kg/m 3 for wter [11]), c is the specific het (430 J/kg K for electrocloric mteril nd 4200 J/(kg K) for wter), λ is the therml conductivity (2.6 W/m K for electrocloric mteril nd 0.62 W/(m K) for wter), nd b is the thickness of the film (m). The frequency of the chnging electric field F (Hz) should be determined within the order of the therml time-constnt t c to keep good efficiency, nd it is estimted by F 1 / (2 t c ). The verge het flux q g (W/m 2 ) of the generted het energy in the electrocloric mteril film ws estimted by q g (Q g b e F). Here, b e is the thickness of the electrocloric mteril film, Q g is the generted het per unit volume for ech chnge in the electric field, nd F is the frequency of chnging the electric field. The representtive vlues of the het chnge per unit volume for ech chnge in the electric field were reported to be Q g = 6 7 J/m 3 for chnge in the electric field of V/m [12]. Figure 4 shows the clcultion results of the mximum thicknesses of the electrocloric mteril film b e, wter flow chnnel b w within the limit of the therml time-constnt to chnge temperture, nd the verge het flux q g of the generted het energy in the electrocloric mteril film. b(µm) Upper chnnel 00 Lower chnnel 0 b w b e Figure 4. Mximum thicknesses of electrocloric mteril, wter flow chnnel, het storge mteril, nd verge het flux of generted het energy. 3t e Wter flow to hot side F(Hz) b f q g 1.0E E E+04 q g (W/m 2 )

3 Americn Journl of Physics nd Applictions 2016; 4(5): When the therml switch ws mde by fluid motion in thin-flow chnnel, the order of pressure drop P fluid of the fluid cused by the inerti nd viscosity ws estimted by P fluid {ρ (x e / t c ) 2 + µ x e 2 / (t c b w 2 )} [9]. Here, ρ is the density, µ is the viscosity, t c is the moving time, b w is the thickness of flow chnnel, nd x e is the length of flow chnnel. Figure 5 shows the order of the pressure drop P fluid for the wter. In the clcultion the thickness of flow chnnel b w ws obtined from Figure 4, nd the length of the flow chnnel ws ssumed to be x e = 0.1 m. In prcticl condition the pressure drop P fluid should be less thn the order of 5 P. Then, the mximum frequency becme on the order of F = 20 Hz from Figure 5. In this work, the cse of the frequency F = Hz nd the time intervl of pulse het genertion t e = {1 / (2 F)} = 0.05 s were studied for therml switch of fluid motion. The mximum thicknesses of the electrocloric mteril film nd wter flow chnnel becme on the order of b e = 200 µm nd b w = 0 µm for F = Hz from Figure 4. In this work, the cse of b e = 200 µm nd b w = 0 µm were studied. The generted het flux in the electrocloric mteril film in generting period of (t e / ) is q e = {Q g b e / (t e / )} = W/m 2 for the generted het per unit volume for ech chnge in the electric field Q g = 6 7 J/m 3. The verge het flux of the generted het energy of the electrocloric mteril film for the whole time period becme on the order of q g = W/m 2. Also, the mximum temperture chnge of the electrocloric mteril without ny het trnsfer is T = {Q g / (ρ c)} = 24 K. P fluid (P) 7 1.E+07 1.E E E E+03 1.E+02 P fluid P move 1.E+04 1.E+03 1.E+02 1.E F(Hz) E+00 Figure 5. Pressure drop of wter flow nd pressure to move het storge mteril film. The energy equtions of the wter flow nd the electrocloric mteril film were similr to our previous report [9]. In this work, the wter flow velocity ws ssumed to be u = 8 m/s, which mens tht the flow time to pss the width of the electrocloric mteril x e = 0.1 m is s nd it is less thn the time intervl of het genertion t e. Inlet temperture of the hot wter ws ssumed to be 5 K, inlet temperture of the cold wter ws ssumed to be -5 K, nd temperture difference between the hot wter nd cold wter ws bout (T h P move (P) - T c ) = K for prcticl use of electrocloric refrigertion. In this work tempertures were shown with the difference from the verge temperture of the electrocloric refrigertion system. Het trnsfer efficiency η ws defined s the rtio between the verge het trnsfer rte to the cold side of system q c-ve nd the generted cold energy in the electrocloric mteril q g. The equtions were numericlly clculted using the finite difference method [11]. The number of clcultion meshes ws to the flow direction, nd 5 to the thickness direction (they were T h, T ek-h, T e, T ek-c, nd T c ). The clcultion time step ws 1/ of the het generting time t e. 3. Clcultion Results of Therml Performnce for Therml Switch of Fluid Motion Figure 6 shows the clcultion results of the temperture chnges of the hot wter T h t the exit side, the cold wter T c, nd the electrocloric mteril T e. The temperture of the electrocloric mteril film T e incresed when it generted het energy, nd it decresed when it generted cold energy. The temperture of hot wter t the exit side T h ws higher thn the inlet temperture 5 K nd the temperture of cold wter t the exit side cold wter T c ws lower thn the inlet temperture -5 K. The verge het trnsfer efficiency ws η = 11%, nd the verge het flux trnsferred to the cold side of the system ws q c-ve = W/m 2. The verge het trnsfer efficiency ws less thn 50% becuse some het ws consumed by the temperture chnge of the electrocloric mteril. If the therml switch of fluid motion ws the cse of chnging only the wter flow velocity without ir, the verge het trnsfer efficiency ws less thn zero. It is becuse there is the temperture difference of (T h - T c ) = K between the hot wter nd the cold wter. If the cold wter styed in the flow chnnel during the genertion of het energy in the electrocloric mteril film, the cold wter ws heted to decrese the efficiency. Therefore the system of chnging ir nd wter flow ws indispensble for the therml switch of fluid motion in prctice. T (K) T e T h T c t (s) Figure 6. Temperture chnge for therml switch of fluid motion.

4 137 Shigeki Hirsw et l.: Therml Performnce of Electrocloric Refrigertion using Therml Switches of Fluid Motion nd Chnging Contct Conductnce 4. Clcultion Method for Therml Switch by Chnging Contct Therml Conductnce Figure 7 shows the clcultion model for the electrocloric refrigertion system with therml switches by chnging the contct therml conductnce. The electrocloric mteril film nd het storge mteril films re sndwiched together. There re four therml switches mong the electrocloric mteril film, het storge mteril films, hot side, nd cold side of the system. The opertion of the therml switches is to chnge the contct therml conductnce. When the distnce between both wlls of the therml switch is less thn 1 µm, the therml conductnce is high nd het flows through the therml switch. When the distnce between both wlls is lrger thn 50 µm, the therml conductnce is low nd the therml switch is n insultor. Chnge of the distnce between both wlls cn be chieved by movement of the het storge mteril films driven by chnging the pressure in the spce to vcuum or tmospheric pressure [1, 8]. The movement of the het storge mterils is smll (in the order of 50 µm), nd the working response is very high. Also thin therml switches using liquid crystls, mgneto therml switches, nd MEMS devices were studied [1, 2]. () When generting het energy. (b) When generting cold energy. working t high temp. Moving up Electrocloric mteril working t low temp. Het storge mteril Figure 7. Model of electrocloric refrigertion system with therml switch by chnging contct therml conductnce. The working digrm of het genertion in electrocloric mteril nd therml conductnce of therml switches is similr to Figure 3. The electrocloric mteril genertes het energy nd cold energy lterntely by pulse het genertion with time intervl t e nd frequency F. After the genertion of het energy in the electrocloric mteril, the therml switch working t high temperture hs high therml conductnce, nd the het energy trnsfers to the upper het storge mteril. At tht time, the therml switch working t low temperture is the insultors. On the other hnd, fter the genertion of cold energy in the electrocloric mteril, the therml switch working t low temperture hs high therml conductnce, nd the cold energy trnsfers to the lower het storge mteril. At tht time, the therml switch working t high temperture is insultors. The het energy in the upper het storge mteril trnsfers to the hot side of the system during genertion of cold energy in the electrocloric mteril becuse of the opertion of the therml switch between the upper het storge mteril nd the hot side. The cold energy in the lower het storge mteril trnsfers to the cold side of the system during genertion of het energy in the electrocloric mteril. Therefore, the het energy trnsfers to the hot side of the system, nd the cold energy trnsfers to the cold side seprtely. In this work, the het trnsfer phenomen of working fluids t the hot side nd the cold side of the system were not clculted, nd the tempertures T h, T c of the hot side nd the cold side of the system were ssumed to be constnt. The het storge mteril is n luminum film (density ρ = 2700 kg/m 3, specific het c = 900 J/kg K, nd therml conductivity λ = 220 W/m K). The mximum thickness of het storge mteril b f ws clculted with the equtions of time-constnt nd shown in Figure 4. The mximum thickness within the limit of the therml time-constnt to chnge the temperture of the het storge mteril for the frequency F = 00 Hz ws in the order of b f = 200 µm nd the mximum thickness of the electrocloric mteril film ws b e = 20 µm. When the opertion of the therml switch is driven by the movement of the het storge mteril film cused by chnging pressure, the required pressure P move to move distnce b in time t c is estimted with Eq. (1). It is the inerti force of the het storge mteril film with thickness b f nd densityρ [11]. P move ρ b f (b / t c 2 ) (1) The order of the pressure P move clculted with Eq. (1) is shown in Figure 5. In the clcultion the thickness of het storge mteril film b f ws obtined from Figure 4, nd the move distnce ws ssumed to be b = 50 µm. The pressure P move should be less thn 5 P, which is the difference between vcuum nd tmospheric pressure, nd the mximum frequency ws greter thn F = 4 Hz from Figure 5. In this work, the following conditions were studied: the frequency F = 00 Hz, the time intervl to generte pulse het genertion ws t e = s, the thicknesses of electrocloric mteril b e = 20 µm, the luminum het storge mteril b f = 20 µm, the generted het per unit volume for ech chnge in the electric field Q g = 6 7 J/m 3, the generted het flux in the electrocloric mteril in generting period ws q e = W/m 2, the verge generted het flux in electrocloric mteril ws q g = W/m 2, the chnging therml conductnce of the therml switch ws C = 5 4 W/m K nd 500W/m K (which corresponded to the distnce between both wlls of the therml switch b = 0.5 µm nd 50 µm), nd temperture difference between the hot side nd the cold side of the system ws (T h - T c ) = K. The energy eqution of the electrocloric mteril film is s follows. 2 T e T q e f2 + q f1 ρ ece = λe + Q 2 e t z (2) be Here, T e is the temperture of electrocloric mteril film, Q e is the generted het per unit volume due to the electrocloric effect (W/m 3 ), b e is the thickness of the electrocloric mteril, nd q f2 nd q f1 (W/m 2 ) re the het fluxes trnsferred to the upper nd lower het storge mteril, respectively.

5 Americn Journl of Physics nd Applictions 2016; 4(5): The energy eqution of the het storge mteril film is s follows. The temperture distribution in the het storge mteril ws neglected becuse of lrge therml conductivity. T q + q fi fi f( i 1) ρ fcf = (3) t bf Here, T fi is the temperture of the i-th het storge mteril, b f is the thickness, nd q fi nd q f(i-1) re the inlet het fluxes trnsferred to the het storge mteril. The therml conductnce of therml switch C (W/m 2 K) ws ssumed to be only therml conduction in the ir between both wlls of the therml switch. The het flux through therml switch q fi (W/m 2 ) from the electrocloric mteril to the het storge mteril is s follows q fi ( T T ) = C (4) e fi C = λ / b (5) Here, b is the distnce between both wlls of the therml switch (0.5 µm or 50 µm), nd λ is the therml conductivity of ir (0.026 W/m K). These equtions were numericlly clculted using the implicit finite difference method. The number of clcultion meshes ws 7 in the thickness direction z (they were T h, T f2, T ek-h, T e, T ek-c, T f1, nd T c ). 5. Clcultion Results of Therml Performnce for Therml Switch by Chnging Contct Therml Conductnce Figure 8 shows the clcultion results of the temperture chnges of the electrocloric mteril film T e, the het storge mteril films T f1, T f2, the hot side T h, nd the cold side T c of the electrocloric refrigertion system with therml switch by chnging contct therml conductnce. The temperture of electrocloric mteril film T e ws higher thn the upper het storge mteril film T f2 nd het energy trnsferred to the upper het storge mteril film when they generted het energy. And the temperture of electrocloric mteril film T e ws lower thn the lower het storge mteril film T f1 nd cold energy trnsferred to the lower het storge mteril films when they generted cold energy. Figure 9 shows the chnges of the generted het flux in ech electrocloric mteril q e (= ±2.4 7 W/m 2 ), het flux trnsferred to the hot side q h nd the cold side q c of system. The cold het trnsferred to the cold side of the system during genertion of het energy in the electrocloric mteril becuse of the opertion of the therml switch between the lower het storge mteril nd the cold side. The verge het trnsfer efficiency ws η = 6%, nd the verge het flux trnsferred to the cold side of the system ws q c-ve = 7 4 W/m 2. The verge het trnsfer efficiency for the system with the therml switch by chnging contct therml conductnce ws lower thn tht for the therml switch of fluid motion, but the verge het flux trnsferred to the cold side of the system ws higher becuse of the high frequency. T (K) T f2 T f1 T h T c T e Figure 8. Temperture chnge for therml switch by chnging contct therml conductnce. q(w/m 2 ) 3.E+05 2.E E+05 1.E E+04 0.E E+04-1.E E+05-2.E q c q e Figure 9. Chnge of het flux for therml switch by chnging contct therml conductnce. 6. Conclusions q h t (s) -3.E t(s) The therml performnce of electrocloric refrigertion with two types of therml switches ws numericlly clculted: therml switch of fluid motion nd therml switch by chnging the contct therml conductnce. The following results were obtined. (1) The design guidelines of the electrocloric refrigertion system were clrified. For the therml switch of fluid motion, the mximum frequency ws on the order of 20 Hz. The system of chnging ir nd wter flow ws indispensble for the therml switch of fluid motion in prctice. For the therml switch by chnging contct therml conductnce, the mximum frequency ws greter thn 4 Hz. (2) For the therml switch of fluid motion with the frequency Hz, the thicknesses of the electrocloric mteril 200 µm, the flow chnnel 0 µm, generted het per unit volume for ech chnge in the electric field 4 7 J/m 3, nd the temperture difference

6 139 Shigeki Hirsw et l.: Therml Performnce of Electrocloric Refrigertion using Therml Switches of Fluid Motion nd Chnging Contct Conductnce between the hot wter nd cold wter K, the verge het trnsfer efficiency ws 11% nd the verge het flux trnsferred to the cold side of the system ws W/m 2. (3) For the therml switch by chnging contct therml conductnce with the frequency 00 Hz, the thicknesses of the electrocloric mteril 20 µm nd the het storge mteril 20 µm, the chnging therml conductnce of the therml switch is 5 4 W/m K nd 500W/m K, nd temperture difference between the hot side nd the cold side K, the verge het trnsfer efficiency ws 6% nd the verge het flux trnsferred to the cold side of the system ws 7 4 W/m 2. Acknowledgments Authors thnk to Mr. Mkoto Endo, Mr. Noki Fujimoto, Mr. Yusuke Stke, nd Mr. Ktsuhis Hyshi of Kobe University for helping nlysis nd experiment. Nomenclture b thickness of film, m C therml conductnce, W/m K c specific het, J/kg K F frequency, Hz Q g generted het for ech chnge in electric field, J/m 3 q het fluxes, W/m 2 q g verge het flux of generted het energy, W/m 2 T temperture, K t time, s t c time-constnt or moving time, s t e time intervl of het genertion, s x e length of flow chnnel, m z coordinte to thickness, m P pressure, P η het trnsfer efficiency λ therml conductivity, W/m K ρ density, kg/m 3 µ viscosity, P s Subscripts ir c cold side e electrocloric mteril film ek surfce of electrocloric mteril film f het storge mteril film h hot side References [1] R. Rdebugh, W. N. Lwless, J. D. Siegwrth, A. J. Morrow, "Fesibility of Electrocloric Refrigertion for the 4-15 K Temperture Rnge, Cryogenics," vol. 19, 1979, pp [2] R. I. Epstein, K. J. Mlloy, "Electrocloric Devices Bsed on Thin-Film Het Switches," Journl of Applied Physics, vol. 6, , [3] Y. Ji, Y. S. Ju, "A Solid-Stte Refrigertor Bsed on the Electrocloric Effect," Applied Physics Letters, vol. 0, , [4] W. Sto, "A Study of New Cooling Device Bsed on the Electrocloric Effect," Proc. of the ASME 2013 Interntionl Mechnicl Engineering Congress, Sn Diego, IMECE , [5] D. Guo, J. B. Go, Y. J. Yu, S. Snthnm, A. Slippey, G. K. Fedder, A. J. H. McGughey, S.C. Yo, "Design nd Modeling of Fluid-Bsed Micro-Scle Electrocloric Refrigertion System," Interntionl Journl of Het nd Mss Trnsfer, vol. 72, 2014, pp [6] M. Ozbolt, A. Kitnovski, J. Tusek, A. Poredos, "Electrocloric Refrigertion: Thermodynmics, Stte of the Art nd Future Perspectives," Interntionl Journl of Refrigertion, vol. 40, 2014, pp [7] S. Hirsw, T. Nkmu, T. Kwnmi, K. Shiri, "Study on Unstedy Therml-Switching Function of Flt Het Pipe," Proc. of the ASME 2015 Interntionl Mechnicl Engineering Congress & Exposition, 2015, Houston, IMECE , [8] S. Hirsw, S. Fujimoto, T. Nkmu, T. Kwnmi, K. Shiri, "Experiment on Het Trnsfer Control by Chnging Contct Therml Resistnce," Proc. of the JSME 2014 Mechnicl Engineering Congress, Tokyo, G061, 2014 (in Jpnese). [9] S. Hirsw, T. Kwnmi, K. Shiri, "Efficient Cooling System using Electrocloric Effect," Journl of Electronics Cooling nd Therml Control, vol. 6, 2016, pp [] Y. He, "Het Cpcity, Therml Conductivity, nd Therml Expnsion of Brium Titnte-Bsed Cermics," Thermochimic Act, vol. 419, 2004, pp [11] J. P. Holmn, Het Trnsfer, 9th Ed., McGrw-Hill Interntionl Book Co., Boston, [12] G. Zhng, Q. Li, H. Gu, S. Jing, K. Hn, M. R. Gdinski, M. A. Hque, Q. Zhng, Q. Wng, "Ferroelectric Polymer Nnocomposites for Room-Temperture Electrocloric Refrigertion," Advnce Mteril, vol. 27, 2015, pp