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Pyroelectric energy harvesting using liquid-based switchable thermal interfaces
Sensors and Actuators A 189 (2013) 100–107
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Pyroelectric energy harvesting using liquid-based switchable thermal interfaces Gilhwan Cha, Y. Sungtaek Ju ∗ Mechanical and Aerospace Engineering Department, University of California, Los Angeles, CA 90095-1597, USA
a r t i c l e
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Article history: Received 25 April 2012 Received in revised form 14 September 2012 Accepted 14 September 2012 Available online 24 September 2012 Keywords: Pyroelectric Energy harvesting Thermal interfaces
a b s t r a c t The pyroelectric effect offers an intriguing solid-state approach for harvesting ambient thermal energy to power distributed networks of sensors and actuators that are remotely located or otherwise difficult to access. There have been, however, few device-level demonstrations due to challenges in converting spatial temperature gradients into temporal temperature oscillations necessary for pyroelectric energy harvesting. We demonstrate the feasibility of a device concept that uses liquid-based thermal interfaces for rapid switching of the thermal conductance between a pyroelectric material and a heat source/sink and can thereby deliver high output power density. Using a thin film of a pyroelectric co-polymer together with a macroscale mechanical actuator, we operate pyroelectric thermal energy harvesting cycles at frequencies close to 1 Hz. Film-level power densities as high as 110 mW/cm3 were achieved, limited by slow heat diffusion across a glass substrate. When combined with a laterally interdigitated electrode array and a MEMS actuator, the present design offers an attractive option for compact high-power density thermal energy harvesters. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Harvesting ambient energy offers attractive solutions to powering distributed or portable electronic sensors and actuators with limited accessibility. These include wirelessly networked devices and systems for monitoring the integrity of buildings and other civil structures, patient health, energy usage, and environmental conditions. Solid-state thermal-to-electric conversion devices, most notably those based on the Seebeck effect, have been subjects of intense research and development activities over the past two decades. Practical applications of compact Seebeck devices, however, have been very limited due in part to often-overlooked difficulty in creating large spatial temperature gradients (i.e. the need for large heat sinks) and in part to relatively poor devicelevel conversion efficiency even for those incorporating so-called nanostructured thermoelectric materials [1]. The pyroelectric effect offers an intriguing alternative to the Seebeck effect for directly converting thermal energy into electricity as it potentially delivers superior performance and reduce heat sinking requirements [2]. Pyroelectric energy harvesting is based on the temperature dependence of the spontaneous electric polarization of a certain class of dielectric materials.
∗ Corresponding author. Tel.: +1 310 825 0985; fax: +1 310 206 2302. E-mail address: [email protected] (Y.S. Ju). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.09.019
A number of recent studies reported basic material characterization, modeling, and also circuit-level implementation of pyroelectric energy harvesters [3–15]. Aside from early studies by Olsen and co-workers [6], however, actual device-level designs of thermal-to-electric conversion have been rather lacking. This is in part due to difficulty in converting spatial temperature gradients into temporal temperature oscillations necessary for pyroelectric thermal energy harvesting. The previous device by Olsen and coworkers concept consisted of a stack of pyroelectric materials housed inside a chamber where a piston pump circulated a dielectric fluid in alternating directions between two heat exchangers. These alternating fluid flows subjected the pyroelectric materials to temporal temperature oscillations at frequencies of the order of or below 0.01 Hz. Although this device design may be suitable for large-scale deployments of pyroelectric waste heat harvesting, the bulky construction and more importantly very low operating frequency render it illsuited for realizing compact high power-density thermal energy harvesters. Another potential approach is to integrate some type of solidstate thermal conductance switches into pyroelectric energy harvesters. Previous studies “proposed” such thermal switches based on anisotropic heat conduction in liquid crystal-nanotube composites [16] or near field radiation [17]. These switches, however, offer limited switching performance (large off-state conductance for the liquid crystal switch and small on-state conductance of the near field radiation switch). Furthermore, practical implementation of these switches is expected to be exceedingly
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difficult. In fact even a standalone experimental demonstration of such switching devices has never been reported. A more recent study [18] proposed a pyroelectric energy harvester based on a bimorph cantilever beam that exploits differential thermal expansion to make alternating thermal contacts between a pyroelectric material and heat sink/source. However, neither the effectiveness/reliability of such direct solid–solid contacts nor the actual energy harvesting performance was reported. The strong dependence of the thermal contact conductance on mechanical loading pressure is also a serious challenge for direct solid–solid contacts. Past studies showed that significant loading pressure (>100 kPa) is necessary to achieve thermal contact conductance >104 W/m2 K due to trapped gas layers and/or surface roughness. In fact, we could not perform any meaningful energy harvesting experiments because free-standing pyroelectric materials and/or thin substrates easily fracture before any adequate thermal contact is made with a heat source/sink. We report a new device concept that utilizes a liquid-based switchable thermal interface we recently reported [19,20] to effectively and reliably convert a spatial temperature gradient to rapid temporal temperature oscillations. We present experimental results obtained from its early implementation to demonstrate its feasibility and identify areas for further improvement. Our device concept is readily scalable and can be adapted to other types of MEMS thermal energy harvesters or thermal control devices.
2. Pyroelectric energy harvesting The spontaneous polarization Ps of pyroelectric materials is a function of temperature. When a pyroelectric material experiences a temporal temperature change, it results in a flow of charges, called the pyroelectric current, to or from the surfaces of the material. This is the basis of pyroelectric thermal energy harvesting. For an illustrative purpose, Fig. 1(a) shows experimentally measured isothermal hysteresis curves for a vinylidene fluoride–trifluoroethylene copolymer containing 56 mol% VDF (56/44 P(VDF-TrFE)) at two different temperatures, highlighting the strong temperature dependence of the polarization of the co-polymer. Fig. 1(b) illustrates an actual thermodynamic cycle for thermal energy harvesting one may use between the two temperatures, often referred to as the Ericsson cycle. Pyroelectric polymers are particularly interesting for their low cost and high energy density per mass. The high dielectric strength of many polymers may also allow application of high bias fields, which in turn enables substantial improvement in harvested energy density. The Curie temperature of certain co-polymers of PVDF, around which the pyroelectric effect is expected to be most pronounced, is relatively low (<70 ◦ C), making them attractive for low-grade waste heat harvesting. Fig. 2 illustrates an operation of the Ericsson cycle, which consists of two constant-temperature processes and two constantfield processes. As the temperature of the pyroelectric material is decreased [1 → 2], its polarization and hence surface bound charges increase. If the pyroelectric material is connected to an external circuit, the free charges on its electrodes will be redistributed to compensate for the change in the surface bound charges. Such charge redistribution results in a pyroelectric current flow in the circuit. More charges will accumulate as the external bias field is increased [2 → 3]. Next, as the pyroelectric material is heated [3 → 4], the sign of pyroelectric current is reversed. The cycle is completed by reducing the external bias field back to its initial value [4 → 1]. Electrical work per cycle corresponds to the area enclosed by the process lines 1–2–3–4. Following previous work, we write a differential change in the electric displacement as dD = εdE + pdT, where ε is a dielectric
Fig. 1. (a) Isothermal hysteresis curves for a 56/44 P(VDF-TrFE) co-polymer thin film at two different temperatures, 59 ◦ C and 113 ◦ C and (b) an example of the Ericsson cycle based on the co-polymer film.
permittivity and p is the primary pyroelectric coefficient under constant strain x and electric field E:
p≡
∂D ∂T
= x,E
∂ε ∂T
E+ x,E
∂Ps . ∂T
(1)
The first and second term represent the dielectric and polarization contribution to the pyroelectric coefficient, respectively. Approximating that the ε = (∂D/∂E)T is a function of temperature but is constant with respect to E (that is, D is a linear function of E at a fixed temperature) over the electric field ranges considered [21], we express the magnitude of changes in the electric displacement along each constant-field process by integrating Eq. (1):
D(EH ) = [ε(TC ) − ε(TH )]EH + [PS (TC ) − PS (TH )] D(EL ) = [ε(TC ) − ε(TH )]EL + [PS (TC ) − PS (TH )]
(2) (3)
The area enclosed by the cycle on the D–E diagram is then
w=
EdD =
1 (EH − EL ){D(EL ) + D(EH )} 2
= [PS (TC ) − PS (TH )](EH − EL ) +
[ε(TC ) − ε(TH )] 2 (EH − EC2 ) 2
(4)
The first term on the right-hand side represents the polarization contribution and the second the dielectric contribution. The above formulation is used to calculate the electrical energy that
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Fig. 2. Ericsson thermal-to-electric conversion cycle for a pyroelectric energy harvesting with schematically illustrations of the charge distribution in each state and the device operation.
can be harvested per unit volume of a pyroelectric material over one thermodynamic cycle, which we call simply the energy density. It is well-established that the pyroelectric and related electrocaloric effect are strong functions of mechanical boundary conditions [22]. All our measurements were performed on thin film samples deposited on nominally identical glass substrates to ensure that all the mechanical boundary conditions are consistent. Due to difficulty in making adequate thermal contacts, we could not make any measurements on free standing samples to directly observe the impact of different mechanical boundary conditions.
4. Experimental 4.1. Pyroelectric sample preparation The cross section of test samples is shown in Fig. 4. A 20 nm Ti/200 nm Al film was first deposited and patterned on a 160 mthick glass substrate to form the bottom electrode of width approximately 3 mm. Powders of 56/44 P(VDF-TrFE) copolymer were dissolved in a methyl ethyl ketone (MEK) solvent at a concentration of 12 wt%. The solution was continuously stirred at 60 ◦ C
3. Device design Fig. 3 schematically shows one possible design for a pyroelectric energy harvester that incorporates liquid-based switchable thermal interfaces. An electrode assembly containing a pyroelectric material is located between two substrates, which serve either as a heat source at TH or a heat sink at TC . On the surface of each substrate is an array of circular hydrophilic islands, which are separated from each other by a contiguous hydrophobic coating. In the thermally non-conducting state (off state), the assembly is physically separated from the hot (cold) surface and the liquid exists as discrete droplets on the hot (cold) surface. In the thermally conducting state (on state), the assembly is pressed against the hot (cold) surface to deform the liquid droplets and force them to merge and form a continuous thin liquid layer. These surface-tension driven morphological transformation of the microscale liquid elements is reversible. Our previous work [19,20] showed that, despite the low thermal conductivity of a dielectric liquid, the thermal interface mediated by such a thin (<10 m) liquid layer can achieve a thermal resistance comparable to that of direct solid–solid contacts at loading pressures orders of magnitude smaller. This is possible because the liquid layer helps create a complete contact and eliminate trapped gas layers. Liquid-mediated interfaces are also expected to be immune to fracture, cold-welding, and other failure mechanisms that may limit practical applications of direct solid–solid contacts.
Fig. 3. Pyroelectric energy harvesting module. The electrode assembly containing a pyroelectric material is actuated up and down to make alternating thermal contacts with the heat source (hot side) or source (cold side) via switchable thermal interfaces.
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Fig. 4. Cross section of the electrode assembly containing a pyroelectric co-polymer thin film. A photograph (top view) of an actual sample is shown on the right.
until the powders were completely dissolved. Next, the solution was filtered through a 0.45 m pore size polypropylene filter to remove possible contaminants. A copolymer layer of a thickness of approximately 5 m was obtained by spin-coating the solution at 500 rpm for 60 s on the glass substrate. The sample was subsequently annealed in a vacuum oven at 140–145 ◦ C overnight to evaporate any residual solvent and volatile impurities. A second 20 nm Ti/200 nm Al film was deposited through a shadow mask of opening about 3 mm to serve as the top electrode. Finally, the top and bottom surfaces of the sample were coated with Teflon® . Electrical leads were attached to the aluminum electrodes using a silver epoxy. 4.2. Switchable liquid interface Thermally oxidized silicon substrates were used to create chemical patterns for liquid-based thermal interfaces. Each substrate was spin-coated with a layer of Teflon® and then a photoresist layer. The latter is then lithographically patterned to serve as a mask layer during the O2 plasma etching of Teflon® . The chemically patterned silicon substrates are bonded to metal blocks using thermal grease. We select glycerol as our interface liquid due to its relatively high contact angle (approximately 110◦ ) on Teflon® and a boiling point of 290 ◦ C. 4.3. Pyroelectric energy harvesting characterization setup The experimental setup consists of two sub-systems: (1) electrical sub-system to control bias electric fields and monitor pyroelectric currents and (2) thermal/mechanical sub-system to control heat transfer between the heat sink/source and the pyroelectric material. 4.3.1. Thermal/mechanical sub-system Fig. 5 schematically shows the thermal/mechanical sub-system used in the present study. The glass substrate coated with the copolymer layer is located between two temperature-controlled metal blocks, one serving as the hot surface (heat source) maintained at TH and the other as the cold surface (heat sink) maintained at TC . Currents applied to Peltier devices attached to the metal blocks are dynamically adjusted using two independent PID controllers (Newport model 350B), with readings from thermistors embedded in the blocks as feedback inputs. A precision electromagnetic actuator provides mechanical motions to make alternative thermal contacts in a controlled manner with the hot or cold surface. The total cycle time is the sum of the mechanical actuation time and residence time on either the hot or cold surface. The actuation time is determined primarily by the mechanical characteristics of the linear actuator and the traveling distance. It was fixed to be 1.2 s in all the experiments reported here. The cycle frequency was
varied by changing the residence time on each surface from 0.02 s to 4.5 s. The cycle frequency was set sufficiently high to keep any potential error due to finite leakage currents across the pyroelectric films below 10% of the harvested energy density [23]. 4.3.2. Electronic sub-system An analog output from a data acquisition card (National Instruments) is amplified using a high-voltage amplifier (Trek PZD 700) to apply electric fields in a controlled manner to the sample. A modified Sawyer–Tower circuit is used to measure pyroelectric currents (Fig. 6). The voltage drop across a reference capacitor (5 F), which is connected electrically in series with the sample, is measured using a voltage follower made of a high input impedance operational amplifier (LMC6081) to determine current flows across the sample and hence changes in the electric displacement. The applied voltage was also independently measured by monitoring the voltage across resistor R2 . Application of the high (EH ) or low (EL ) bias fields is synchronized with the z-stage motion (and hence temperature cycling) to implement the Ericsson cycle. 4.4. Experimental procedures 4.4.1. Hysteresis loop measurement Isothermal hysteresis curves of the pyroelectric material were first measured by maintaining the sample in contact with the heat source to achieve isothermal conditions. A triangle waveform with a peak amplitude of 500 kV/cm was applied at frequencies of the order of 15 Hz. The electric displacement D was measured using the modified Sawyer–Tower circuit to construct the D–E hysteresis curves. 4.4.2. Conversion cycle The conversion cycle started at a state defined by the high temperature (TH ) and high electric field (EH ). After the pyroelectric
Fig. 5. Schematic of the experimental setup for characterizing pyroelectric energy harvesting.
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Fig. 6. A modified Sawyer–Tower circuit with a voltage follower to electrically bias a pyroelectric material and measure pyroelectric currents.
film/glass substrate assembly was placed on the hot surface for a finite amount of time, the bias field was switched to the low value (EL ). The voltage changes across R2 and C1 were measured during this switching. The assembly was then moved up to make a thermal contact with the cold surface. The bias voltage was next changed from the low to high value while once again monitoring the voltage changes across R2 and C1 . The sample was returned to the hot surface to complete the cycle. Experiments were performed for several different combinations of the bias fields and hot/cold temperatures. The cold surface temperature was fixed at 40 ◦ C, and the hot side temperature was varied between 60 ◦ C and 120 ◦ C. Large polarization changes are expected when a pyroelectric material experiences the ferroelectric–paraelectric phase transition across its Curie temperature, Tc , which was expected to be approximately 70 ◦ C. In one set of experiments, the low-bias field EL was varied from 50 kV/cm to 400 kV/cm while the high-bias field EH was maintained at 500 kV/cm. In another set of experiments, EH was varied from 200 to 800 kV/cm while EL was maintained at 100 kV/cm. The cycle frequency was also varied to investigate how deleterious parasitic effects, such as finite thermal diffusion time and leakage currents, impact the harvestable power density.
temperatures of 39 ◦ C and 59 ◦ C, which are below the Curie temperature. The D–E loops in Fig. 7 were measured with the peak bias field of 500 kV/cm, above which leakage currents become important. This, however, is below the field necessary to saturate the polarization of the films. 5.2. Harvested energy density at a fixed cycling frequency Fig. 8 shows representative energy conversion cycles for different sets of applied bias fields. The plots of four independent cycles are aligned such that each initial state coincides with one another. Note that the partial hysteresis loop measured between EH and EL does not necessarily follow the full bi-polar hysteresis loops of Fig. 7, which were measured while cycling the bias field between EH and −EH. The displacement does depend on the history of applied fields. Fig. 9 plots the harvested energy and power per cycle per unit volume of the pyroelectric material as a function EL . The high-end bias field EH is fixed at 500 kV/cm. The various lines correspond to data measured at different values of TH . Since the electrical energy output scales as the product of the electric field span E = EH − EL (Eq. (4)) and the spontaneous polarization, one would expect the energy output to increase monotonically with decreasing EL . In
4.4.3. Film temperature measurement Due to a finite thermal diffusion time across the glass substrate and its finite thermal mass, there exist appreciable differences in temperature between the hot/cold surface and the pyroelectric film, especially at high cycling frequencies. To account for these effects, independent measurements were performed using a dummy sample that incorporated a type K thin-film thermocouple in place of a pyroelectric film. We shall call these temperatures as the corrected film temperatures in subsequent discussion. 5. Results and discussion 5.1. Hysteresis loops Fig. 7 shows the displacement D versus applied electric field E characteristics of our 56/44 P(VDF-TrFE) copolymer films measured at different temperatures. The hysteresis loops are reported without any additional correction for leakage currents, which are negligible at our measurement frequencies of approximately 15 Hz. The copolymer films exhibited clear ferroelectricity at
Fig. 7. Isothermal hysteresis loops of the co-polymer films measured at different temperatures.
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Fig. 8. Representative of conversion cycles at f = 0.4 Hz for different values of EH .
actual experiments, however, the energy density peaks at a finite value of EL due to finite hysteresis in the polarization–electric field characteristics of the material. Fig. 10 illustrates several conversion cycles for different sets of bias electric fields. The hysteresis loss, which can be estimated from the areas of the enclosed elliptic loops in the partial hysteresis curves, indeed increases as EL is reduced. The power densities achieved in the present experiments are >100 mJ/cm3 over a nominal temperature difference T (=TH − TC ) of 50 ◦ C between the heat sink and source. A further increase in TH (and hence T) does not lead to correspondingly larger power outputs since the pyroelectric effect becomes less effective as the temperature far exceeds the Curie temperature (approximately 65 ◦ C in the present co-polymer) and the material becomes paraelectric. The full bi-polar hysteresis loops shown in Fig. 7 show that the polarization is reduced rather than increased as the sample cools down from 59 ◦ C to 39 ◦ C under a constant electric field (at least below 500 kV/cm). The conversion cycle then would not produce positive electrical work between these two temperatures. Indeed, when the cycle was operated between these temperatures, the conversion cycle was not viable for EL below 250 kV/cm. However, at higher values of EL (>250 kV/cm), the cycle can still produce positive electrical work output due in part to the dielectric contribution to the work output.
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Fig. 10. D–E diagram of the Ericsson cycle for different sets of bias fields (fixed EH ). The shaded area corresponds to the harvestable energy that excludes the hysteresis loss.
For a fixed value of EL (100 kV/cm) and the fixed hot side and cold side temperatures of 100 and 40 ◦ C, respectively, the energy density initially increases almost linearly with EH , as suggested in Eq. (4), but reaches a peak of approximately 550 mJ/cm3 at electric fields of approximately 750 kV/cm. The energy density subsequently drops with increasing EH due to rapidly increasing leakage currents. Our recent study [23] highlighted the impact of leakage currents on accurate characterization of the intrinsic pyroelectric energy harvesting capability of thin films. By setting the temperature cycling frequency to be sufficiently high, we keep any potential error due to leakage currents (wloss ) relative small fractions of the calculated work output (w). Fig. 11 shows the ratio between the two quantities as a function of the mean film temperature for three different mean bias fields (EH + EL ). The maximum potential error due to leakage currents is estimated to be below 10% for temperatures up to 80 ◦ C.
5.3. Effect of cycle frequency The data reported so far were obtained while keeping the cycle frequency at a constant value. Fig. 12 shows the measured energy and power density as a function of cycle frequency. The temperature of the heat source and sink was maintained at 100 ◦ C and
Fig. 9. Energy density (a) and power density (b) as a function of EL for several different values of TH and fixed values of EH and cycling frequency (∼0.6 Hz).
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material can be circumvented using a recently proposed laterally interdigitated electrode array [24]. The present device concept for pyroelectric energy harvesting can be integrated with a magnetothermal actuator [25] to eliminate the need for external mechanical actuators. The magneto-thermal actuator, also known as the Curie heat engine, converts thermal energy into mechanical oscillations by using the temperature dependence of the magnetization of a ferroelectric material. 6. Summary and conclusion
Fig. 11. Ratio of the leakage “loss” to the output energy density as a function of the mean film temperature.
40 ◦ C, respectively, and the lower and upper end of the bias field at 100 kV/cm and 400 kV/cm, respectively. The energy density, which is once again defined as the energy harvested per unit volume of the pyroelectric film per cycle, first increases with increasing cycling frequency due to the reduced impact of leakage currents during shorter cycle periods. The energy density then decreases with further increase in frequency as the copolymer film undergoes smaller temperature excursions due to the finite thermal mass of and slow heat diffusion through the glass substrate. The power density, which is computed as the product of the energy density and the cycling frequency, exhibits a similar trend, increasing first with increasing frequency and reaching a peak at approximately 0.6 Hz. The measured energy density (per cycle) is compared with model predictions we make from Eq. (4) using the film temperatures independently measured from the thin-film thermocouple (corrected film temperature) and the dielectric permittivity and spontaneous polarization measured as a function of temperatures. The predictions agree reasonably well with the experimental results, capturing well the general trend and the frequency at which the energy density peaks. The negative impact of the slow diffusion across a “thick” substrate and the low thermal diffusivity of a polymeric pyroelectric
We demonstrate the feasibility of a device concept that uses liquid-based thermal interfaces to enable rapid switching of the thermal conductance between a pyroelectric material and a heat source/sink and thereby achieve high output power density. The switchable interfaces utilize reversible morphological transformation of microscale liquid to realize rapidly and reproducibly switchable thermal contacts between a pyroelectric material and a heat sink/source. Using a thin film of a pyroelectric co-polymer (P(VDF-TrFE) co-polymer (56:44 mol%)) together with a macroscale mechanical actuator, we operate pyroelectric thermal energy harvesting cycles at frequencies close to 1 Hz. Film-level power densities as high as 110 mW/cm3 were achieved, which was limited by slow heat diffusion across a glass substrate. When combined with a laterally interdigitated electrode array and a MEMS actuator, the present design offers an attractive option for compact high-power density thermal energy harvesters. The present work did not consider energy input necessary for mechanical actuation or application of bias fields. We do not, in fact, envision that macroscale pyroelectric energy harvesters will provide sufficiently high “net” efficiency to merit their use in large-scale stand-alone energy harvesting applications. We believe instead that our pyroelectric energy harvester concept is bestsuited as part of distributed hybrid energy harvesters for powering wireless sensor networks or related applications. One possible hybrid device configuration combines our pyroelectric energy harvester with a piezoelectric vibrational energy harvester and a so-called Curie heat engine where we exploit the temperature dependence of the magnetization of a ferroelectric material to convert spatial temperature variations into mechanical oscillations [25]. By using these mechanical oscillations to subject pyroelectric materials to thermal cycling, we can eliminate the need for any external actuation power. Advanced energy storage/biasing circuitry is another area that needs further investigation. The net power input to the biasing circuit must be sufficiently smaller than the power harvested by a cycle to justify its practical use. A high-efficiency storage capacitor and associated switching/rectification circuits need to be developed to bias pyroelectric materials during thermodynamic cycles while minimizing circuit loss. Some of the recent studies [26] reported progress in related areas. Acknowledgments
Fig. 12. The experimentally measured energy (per cycle) and power density as a function of cycling frequency.
The present article is based on work sponsored in part by the United States ARPA-E under Award Number (DE-AR0000120) and in part by the United States National Science Foundation under Award Number (CBET-1048726). Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accurate, completeness, or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by
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trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. References [1] C.B. Vining, An inconvenient truth about thermoelectrics, Nature Materials 8 (2) (2009) 83–85. [2] G. Sebald, D. Guyomar, A. Agbossou, On thermoelectric and pyroelectric energy harvesting, Smart Materials and Structures 18 (December (12)) (2009) 125006. [3] J. Xie, X.P. Mane, C.W. Green, K.M. Mossi, K.K. Leang, Performance of thin piezoelectric materials for pyroelectric energy harvesting, Journal of Intelligent Material Systems and Structures 21 (February (3)) (2010) 243–249. [4] G. Sebald, S. Pruvost, D. Guyomar, Energy harvesting based on Ericsson pyroelectric cycles in a relaxor ferroelectric ceramic, Smart Materials and Structures 17 (February (1)) (2008) 015012. [5] R.B. Olsen, D. Evans, Pyroelectric energy conversion: hysteresis loss and temperature sensitivity of a ferroelectric material, Journal of Applied Physics 54 (10) (1983) 5941. [6] R.B. Olsen, D.A. Bruno, J.M. Briscoe, E.W. Jacobs, Pyroelectric conversion cycle of vinylidene fluoride–trifluoroethylene copolymer, Journal of Applied Physics 57 (11) (1985) 5036. [7] P. Mane, J. Xie, K.K. Leang, K. Mossi, Cyclic energy harvesting from pyroelectric materials, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 58 (1) (2011) 10–17. [8] S.B. Lang, S. Muensit, Review of some lesser-known applications of piezoelectric and pyroelectric polymers, Applied Physics A 85 (September (2)) (2006) 125–134. [9] L. Kouchachvili, M. Ikura, Pyroelectric conversion – effects of P(VDF-TrFE) preconditioning on power conversion, Journal of Electrostatics 65 (March (3)) (2007) 182–188. [10] A. Khodayari, S. Pruvost, G. Sebald, D. Guyomar, S. Mohammadi, Nonlinear pyroelectric energy harvesting from relaxor single crystals, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 56 (4) (2009) 693– 699. [11] D. Guyomar, G. Sebald, E. Lefeuvre, A. Khodayari, Toward heat energy harvesting using pyroelectric material, Journal of Intelligent Material Systems and Structures 20 (February (3)) (2009) 265–271.
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Pyroelectric energy harvesting using liquid-based switchable thermal interfaces Gilhwan Cha, Y. Sungtaek Ju ∗ Mechanical and Aerospace Engineering Department, University of California, Los Angeles, CA 90095-1597, USA
a r t i c l e
i n f o
Article history: Received 25 April 2012 Received in revised form 14 September 2012 Accepted 14 September 2012 Available online 24 September 2012 Keywords: Pyroelectric Energy harvesting Thermal interfaces
a b s t r a c t The pyroelectric effect offers an intriguing solid-state approach for harvesting ambient thermal energy to power distributed networks of sensors and actuators that are remotely located or otherwise difficult to access. There have been, however, few device-level demonstrations due to challenges in converting spatial temperature gradients into temporal temperature oscillations necessary for pyroelectric energy harvesting. We demonstrate the feasibility of a device concept that uses liquid-based thermal interfaces for rapid switching of the thermal conductance between a pyroelectric material and a heat source/sink and can thereby deliver high output power density. Using a thin film of a pyroelectric co-polymer together with a macroscale mechanical actuator, we operate pyroelectric thermal energy harvesting cycles at frequencies close to 1 Hz. Film-level power densities as high as 110 mW/cm3 were achieved, limited by slow heat diffusion across a glass substrate. When combined with a laterally interdigitated electrode array and a MEMS actuator, the present design offers an attractive option for compact high-power density thermal energy harvesters. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Harvesting ambient energy offers attractive solutions to powering distributed or portable electronic sensors and actuators with limited accessibility. These include wirelessly networked devices and systems for monitoring the integrity of buildings and other civil structures, patient health, energy usage, and environmental conditions. Solid-state thermal-to-electric conversion devices, most notably those based on the Seebeck effect, have been subjects of intense research and development activities over the past two decades. Practical applications of compact Seebeck devices, however, have been very limited due in part to often-overlooked difficulty in creating large spatial temperature gradients (i.e. the need for large heat sinks) and in part to relatively poor devicelevel conversion efficiency even for those incorporating so-called nanostructured thermoelectric materials [1]. The pyroelectric effect offers an intriguing alternative to the Seebeck effect for directly converting thermal energy into electricity as it potentially delivers superior performance and reduce heat sinking requirements [2]. Pyroelectric energy harvesting is based on the temperature dependence of the spontaneous electric polarization of a certain class of dielectric materials.
∗ Corresponding author. Tel.: +1 310 825 0985; fax: +1 310 206 2302. E-mail address: [email protected] (Y.S. Ju). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.09.019
A number of recent studies reported basic material characterization, modeling, and also circuit-level implementation of pyroelectric energy harvesters [3–15]. Aside from early studies by Olsen and co-workers [6], however, actual device-level designs of thermal-to-electric conversion have been rather lacking. This is in part due to difficulty in converting spatial temperature gradients into temporal temperature oscillations necessary for pyroelectric thermal energy harvesting. The previous device by Olsen and coworkers concept consisted of a stack of pyroelectric materials housed inside a chamber where a piston pump circulated a dielectric fluid in alternating directions between two heat exchangers. These alternating fluid flows subjected the pyroelectric materials to temporal temperature oscillations at frequencies of the order of or below 0.01 Hz. Although this device design may be suitable for large-scale deployments of pyroelectric waste heat harvesting, the bulky construction and more importantly very low operating frequency render it illsuited for realizing compact high power-density thermal energy harvesters. Another potential approach is to integrate some type of solidstate thermal conductance switches into pyroelectric energy harvesters. Previous studies “proposed” such thermal switches based on anisotropic heat conduction in liquid crystal-nanotube composites [16] or near field radiation [17]. These switches, however, offer limited switching performance (large off-state conductance for the liquid crystal switch and small on-state conductance of the near field radiation switch). Furthermore, practical implementation of these switches is expected to be exceedingly
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difficult. In fact even a standalone experimental demonstration of such switching devices has never been reported. A more recent study [18] proposed a pyroelectric energy harvester based on a bimorph cantilever beam that exploits differential thermal expansion to make alternating thermal contacts between a pyroelectric material and heat sink/source. However, neither the effectiveness/reliability of such direct solid–solid contacts nor the actual energy harvesting performance was reported. The strong dependence of the thermal contact conductance on mechanical loading pressure is also a serious challenge for direct solid–solid contacts. Past studies showed that significant loading pressure (>100 kPa) is necessary to achieve thermal contact conductance >104 W/m2 K due to trapped gas layers and/or surface roughness. In fact, we could not perform any meaningful energy harvesting experiments because free-standing pyroelectric materials and/or thin substrates easily fracture before any adequate thermal contact is made with a heat source/sink. We report a new device concept that utilizes a liquid-based switchable thermal interface we recently reported [19,20] to effectively and reliably convert a spatial temperature gradient to rapid temporal temperature oscillations. We present experimental results obtained from its early implementation to demonstrate its feasibility and identify areas for further improvement. Our device concept is readily scalable and can be adapted to other types of MEMS thermal energy harvesters or thermal control devices.
2. Pyroelectric energy harvesting The spontaneous polarization Ps of pyroelectric materials is a function of temperature. When a pyroelectric material experiences a temporal temperature change, it results in a flow of charges, called the pyroelectric current, to or from the surfaces of the material. This is the basis of pyroelectric thermal energy harvesting. For an illustrative purpose, Fig. 1(a) shows experimentally measured isothermal hysteresis curves for a vinylidene fluoride–trifluoroethylene copolymer containing 56 mol% VDF (56/44 P(VDF-TrFE)) at two different temperatures, highlighting the strong temperature dependence of the polarization of the co-polymer. Fig. 1(b) illustrates an actual thermodynamic cycle for thermal energy harvesting one may use between the two temperatures, often referred to as the Ericsson cycle. Pyroelectric polymers are particularly interesting for their low cost and high energy density per mass. The high dielectric strength of many polymers may also allow application of high bias fields, which in turn enables substantial improvement in harvested energy density. The Curie temperature of certain co-polymers of PVDF, around which the pyroelectric effect is expected to be most pronounced, is relatively low (<70 ◦ C), making them attractive for low-grade waste heat harvesting. Fig. 2 illustrates an operation of the Ericsson cycle, which consists of two constant-temperature processes and two constantfield processes. As the temperature of the pyroelectric material is decreased [1 → 2], its polarization and hence surface bound charges increase. If the pyroelectric material is connected to an external circuit, the free charges on its electrodes will be redistributed to compensate for the change in the surface bound charges. Such charge redistribution results in a pyroelectric current flow in the circuit. More charges will accumulate as the external bias field is increased [2 → 3]. Next, as the pyroelectric material is heated [3 → 4], the sign of pyroelectric current is reversed. The cycle is completed by reducing the external bias field back to its initial value [4 → 1]. Electrical work per cycle corresponds to the area enclosed by the process lines 1–2–3–4. Following previous work, we write a differential change in the electric displacement as dD = εdE + pdT, where ε is a dielectric
Fig. 1. (a) Isothermal hysteresis curves for a 56/44 P(VDF-TrFE) co-polymer thin film at two different temperatures, 59 ◦ C and 113 ◦ C and (b) an example of the Ericsson cycle based on the co-polymer film.
permittivity and p is the primary pyroelectric coefficient under constant strain x and electric field E:
p≡
∂D ∂T
= x,E
∂ε ∂T
E+ x,E
∂Ps . ∂T
(1)
The first and second term represent the dielectric and polarization contribution to the pyroelectric coefficient, respectively. Approximating that the ε = (∂D/∂E)T is a function of temperature but is constant with respect to E (that is, D is a linear function of E at a fixed temperature) over the electric field ranges considered [21], we express the magnitude of changes in the electric displacement along each constant-field process by integrating Eq. (1):
D(EH ) = [ε(TC ) − ε(TH )]EH + [PS (TC ) − PS (TH )] D(EL ) = [ε(TC ) − ε(TH )]EL + [PS (TC ) − PS (TH )]
(2) (3)
The area enclosed by the cycle on the D–E diagram is then
w=
EdD =
1 (EH − EL ){D(EL ) + D(EH )} 2
= [PS (TC ) − PS (TH )](EH − EL ) +
[ε(TC ) − ε(TH )] 2 (EH − EC2 ) 2
(4)
The first term on the right-hand side represents the polarization contribution and the second the dielectric contribution. The above formulation is used to calculate the electrical energy that
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Fig. 2. Ericsson thermal-to-electric conversion cycle for a pyroelectric energy harvesting with schematically illustrations of the charge distribution in each state and the device operation.
can be harvested per unit volume of a pyroelectric material over one thermodynamic cycle, which we call simply the energy density. It is well-established that the pyroelectric and related electrocaloric effect are strong functions of mechanical boundary conditions [22]. All our measurements were performed on thin film samples deposited on nominally identical glass substrates to ensure that all the mechanical boundary conditions are consistent. Due to difficulty in making adequate thermal contacts, we could not make any measurements on free standing samples to directly observe the impact of different mechanical boundary conditions.
4. Experimental 4.1. Pyroelectric sample preparation The cross section of test samples is shown in Fig. 4. A 20 nm Ti/200 nm Al film was first deposited and patterned on a 160 mthick glass substrate to form the bottom electrode of width approximately 3 mm. Powders of 56/44 P(VDF-TrFE) copolymer were dissolved in a methyl ethyl ketone (MEK) solvent at a concentration of 12 wt%. The solution was continuously stirred at 60 ◦ C
3. Device design Fig. 3 schematically shows one possible design for a pyroelectric energy harvester that incorporates liquid-based switchable thermal interfaces. An electrode assembly containing a pyroelectric material is located between two substrates, which serve either as a heat source at TH or a heat sink at TC . On the surface of each substrate is an array of circular hydrophilic islands, which are separated from each other by a contiguous hydrophobic coating. In the thermally non-conducting state (off state), the assembly is physically separated from the hot (cold) surface and the liquid exists as discrete droplets on the hot (cold) surface. In the thermally conducting state (on state), the assembly is pressed against the hot (cold) surface to deform the liquid droplets and force them to merge and form a continuous thin liquid layer. These surface-tension driven morphological transformation of the microscale liquid elements is reversible. Our previous work [19,20] showed that, despite the low thermal conductivity of a dielectric liquid, the thermal interface mediated by such a thin (<10 m) liquid layer can achieve a thermal resistance comparable to that of direct solid–solid contacts at loading pressures orders of magnitude smaller. This is possible because the liquid layer helps create a complete contact and eliminate trapped gas layers. Liquid-mediated interfaces are also expected to be immune to fracture, cold-welding, and other failure mechanisms that may limit practical applications of direct solid–solid contacts.
Fig. 3. Pyroelectric energy harvesting module. The electrode assembly containing a pyroelectric material is actuated up and down to make alternating thermal contacts with the heat source (hot side) or source (cold side) via switchable thermal interfaces.
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Fig. 4. Cross section of the electrode assembly containing a pyroelectric co-polymer thin film. A photograph (top view) of an actual sample is shown on the right.
until the powders were completely dissolved. Next, the solution was filtered through a 0.45 m pore size polypropylene filter to remove possible contaminants. A copolymer layer of a thickness of approximately 5 m was obtained by spin-coating the solution at 500 rpm for 60 s on the glass substrate. The sample was subsequently annealed in a vacuum oven at 140–145 ◦ C overnight to evaporate any residual solvent and volatile impurities. A second 20 nm Ti/200 nm Al film was deposited through a shadow mask of opening about 3 mm to serve as the top electrode. Finally, the top and bottom surfaces of the sample were coated with Teflon® . Electrical leads were attached to the aluminum electrodes using a silver epoxy. 4.2. Switchable liquid interface Thermally oxidized silicon substrates were used to create chemical patterns for liquid-based thermal interfaces. Each substrate was spin-coated with a layer of Teflon® and then a photoresist layer. The latter is then lithographically patterned to serve as a mask layer during the O2 plasma etching of Teflon® . The chemically patterned silicon substrates are bonded to metal blocks using thermal grease. We select glycerol as our interface liquid due to its relatively high contact angle (approximately 110◦ ) on Teflon® and a boiling point of 290 ◦ C. 4.3. Pyroelectric energy harvesting characterization setup The experimental setup consists of two sub-systems: (1) electrical sub-system to control bias electric fields and monitor pyroelectric currents and (2) thermal/mechanical sub-system to control heat transfer between the heat sink/source and the pyroelectric material. 4.3.1. Thermal/mechanical sub-system Fig. 5 schematically shows the thermal/mechanical sub-system used in the present study. The glass substrate coated with the copolymer layer is located between two temperature-controlled metal blocks, one serving as the hot surface (heat source) maintained at TH and the other as the cold surface (heat sink) maintained at TC . Currents applied to Peltier devices attached to the metal blocks are dynamically adjusted using two independent PID controllers (Newport model 350B), with readings from thermistors embedded in the blocks as feedback inputs. A precision electromagnetic actuator provides mechanical motions to make alternative thermal contacts in a controlled manner with the hot or cold surface. The total cycle time is the sum of the mechanical actuation time and residence time on either the hot or cold surface. The actuation time is determined primarily by the mechanical characteristics of the linear actuator and the traveling distance. It was fixed to be 1.2 s in all the experiments reported here. The cycle frequency was
varied by changing the residence time on each surface from 0.02 s to 4.5 s. The cycle frequency was set sufficiently high to keep any potential error due to finite leakage currents across the pyroelectric films below 10% of the harvested energy density [23]. 4.3.2. Electronic sub-system An analog output from a data acquisition card (National Instruments) is amplified using a high-voltage amplifier (Trek PZD 700) to apply electric fields in a controlled manner to the sample. A modified Sawyer–Tower circuit is used to measure pyroelectric currents (Fig. 6). The voltage drop across a reference capacitor (5 F), which is connected electrically in series with the sample, is measured using a voltage follower made of a high input impedance operational amplifier (LMC6081) to determine current flows across the sample and hence changes in the electric displacement. The applied voltage was also independently measured by monitoring the voltage across resistor R2 . Application of the high (EH ) or low (EL ) bias fields is synchronized with the z-stage motion (and hence temperature cycling) to implement the Ericsson cycle. 4.4. Experimental procedures 4.4.1. Hysteresis loop measurement Isothermal hysteresis curves of the pyroelectric material were first measured by maintaining the sample in contact with the heat source to achieve isothermal conditions. A triangle waveform with a peak amplitude of 500 kV/cm was applied at frequencies of the order of 15 Hz. The electric displacement D was measured using the modified Sawyer–Tower circuit to construct the D–E hysteresis curves. 4.4.2. Conversion cycle The conversion cycle started at a state defined by the high temperature (TH ) and high electric field (EH ). After the pyroelectric
Fig. 5. Schematic of the experimental setup for characterizing pyroelectric energy harvesting.
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Fig. 6. A modified Sawyer–Tower circuit with a voltage follower to electrically bias a pyroelectric material and measure pyroelectric currents.
film/glass substrate assembly was placed on the hot surface for a finite amount of time, the bias field was switched to the low value (EL ). The voltage changes across R2 and C1 were measured during this switching. The assembly was then moved up to make a thermal contact with the cold surface. The bias voltage was next changed from the low to high value while once again monitoring the voltage changes across R2 and C1 . The sample was returned to the hot surface to complete the cycle. Experiments were performed for several different combinations of the bias fields and hot/cold temperatures. The cold surface temperature was fixed at 40 ◦ C, and the hot side temperature was varied between 60 ◦ C and 120 ◦ C. Large polarization changes are expected when a pyroelectric material experiences the ferroelectric–paraelectric phase transition across its Curie temperature, Tc , which was expected to be approximately 70 ◦ C. In one set of experiments, the low-bias field EL was varied from 50 kV/cm to 400 kV/cm while the high-bias field EH was maintained at 500 kV/cm. In another set of experiments, EH was varied from 200 to 800 kV/cm while EL was maintained at 100 kV/cm. The cycle frequency was also varied to investigate how deleterious parasitic effects, such as finite thermal diffusion time and leakage currents, impact the harvestable power density.
temperatures of 39 ◦ C and 59 ◦ C, which are below the Curie temperature. The D–E loops in Fig. 7 were measured with the peak bias field of 500 kV/cm, above which leakage currents become important. This, however, is below the field necessary to saturate the polarization of the films. 5.2. Harvested energy density at a fixed cycling frequency Fig. 8 shows representative energy conversion cycles for different sets of applied bias fields. The plots of four independent cycles are aligned such that each initial state coincides with one another. Note that the partial hysteresis loop measured between EH and EL does not necessarily follow the full bi-polar hysteresis loops of Fig. 7, which were measured while cycling the bias field between EH and −EH. The displacement does depend on the history of applied fields. Fig. 9 plots the harvested energy and power per cycle per unit volume of the pyroelectric material as a function EL . The high-end bias field EH is fixed at 500 kV/cm. The various lines correspond to data measured at different values of TH . Since the electrical energy output scales as the product of the electric field span E = EH − EL (Eq. (4)) and the spontaneous polarization, one would expect the energy output to increase monotonically with decreasing EL . In
4.4.3. Film temperature measurement Due to a finite thermal diffusion time across the glass substrate and its finite thermal mass, there exist appreciable differences in temperature between the hot/cold surface and the pyroelectric film, especially at high cycling frequencies. To account for these effects, independent measurements were performed using a dummy sample that incorporated a type K thin-film thermocouple in place of a pyroelectric film. We shall call these temperatures as the corrected film temperatures in subsequent discussion. 5. Results and discussion 5.1. Hysteresis loops Fig. 7 shows the displacement D versus applied electric field E characteristics of our 56/44 P(VDF-TrFE) copolymer films measured at different temperatures. The hysteresis loops are reported without any additional correction for leakage currents, which are negligible at our measurement frequencies of approximately 15 Hz. The copolymer films exhibited clear ferroelectricity at
Fig. 7. Isothermal hysteresis loops of the co-polymer films measured at different temperatures.
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Fig. 8. Representative of conversion cycles at f = 0.4 Hz for different values of EH .
actual experiments, however, the energy density peaks at a finite value of EL due to finite hysteresis in the polarization–electric field characteristics of the material. Fig. 10 illustrates several conversion cycles for different sets of bias electric fields. The hysteresis loss, which can be estimated from the areas of the enclosed elliptic loops in the partial hysteresis curves, indeed increases as EL is reduced. The power densities achieved in the present experiments are >100 mJ/cm3 over a nominal temperature difference T (=TH − TC ) of 50 ◦ C between the heat sink and source. A further increase in TH (and hence T) does not lead to correspondingly larger power outputs since the pyroelectric effect becomes less effective as the temperature far exceeds the Curie temperature (approximately 65 ◦ C in the present co-polymer) and the material becomes paraelectric. The full bi-polar hysteresis loops shown in Fig. 7 show that the polarization is reduced rather than increased as the sample cools down from 59 ◦ C to 39 ◦ C under a constant electric field (at least below 500 kV/cm). The conversion cycle then would not produce positive electrical work between these two temperatures. Indeed, when the cycle was operated between these temperatures, the conversion cycle was not viable for EL below 250 kV/cm. However, at higher values of EL (>250 kV/cm), the cycle can still produce positive electrical work output due in part to the dielectric contribution to the work output.
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Fig. 10. D–E diagram of the Ericsson cycle for different sets of bias fields (fixed EH ). The shaded area corresponds to the harvestable energy that excludes the hysteresis loss.
For a fixed value of EL (100 kV/cm) and the fixed hot side and cold side temperatures of 100 and 40 ◦ C, respectively, the energy density initially increases almost linearly with EH , as suggested in Eq. (4), but reaches a peak of approximately 550 mJ/cm3 at electric fields of approximately 750 kV/cm. The energy density subsequently drops with increasing EH due to rapidly increasing leakage currents. Our recent study [23] highlighted the impact of leakage currents on accurate characterization of the intrinsic pyroelectric energy harvesting capability of thin films. By setting the temperature cycling frequency to be sufficiently high, we keep any potential error due to leakage currents (wloss ) relative small fractions of the calculated work output (w). Fig. 11 shows the ratio between the two quantities as a function of the mean film temperature for three different mean bias fields (EH + EL ). The maximum potential error due to leakage currents is estimated to be below 10% for temperatures up to 80 ◦ C.
5.3. Effect of cycle frequency The data reported so far were obtained while keeping the cycle frequency at a constant value. Fig. 12 shows the measured energy and power density as a function of cycle frequency. The temperature of the heat source and sink was maintained at 100 ◦ C and
Fig. 9. Energy density (a) and power density (b) as a function of EL for several different values of TH and fixed values of EH and cycling frequency (∼0.6 Hz).
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material can be circumvented using a recently proposed laterally interdigitated electrode array [24]. The present device concept for pyroelectric energy harvesting can be integrated with a magnetothermal actuator [25] to eliminate the need for external mechanical actuators. The magneto-thermal actuator, also known as the Curie heat engine, converts thermal energy into mechanical oscillations by using the temperature dependence of the magnetization of a ferroelectric material. 6. Summary and conclusion
Fig. 11. Ratio of the leakage “loss” to the output energy density as a function of the mean film temperature.
40 ◦ C, respectively, and the lower and upper end of the bias field at 100 kV/cm and 400 kV/cm, respectively. The energy density, which is once again defined as the energy harvested per unit volume of the pyroelectric film per cycle, first increases with increasing cycling frequency due to the reduced impact of leakage currents during shorter cycle periods. The energy density then decreases with further increase in frequency as the copolymer film undergoes smaller temperature excursions due to the finite thermal mass of and slow heat diffusion through the glass substrate. The power density, which is computed as the product of the energy density and the cycling frequency, exhibits a similar trend, increasing first with increasing frequency and reaching a peak at approximately 0.6 Hz. The measured energy density (per cycle) is compared with model predictions we make from Eq. (4) using the film temperatures independently measured from the thin-film thermocouple (corrected film temperature) and the dielectric permittivity and spontaneous polarization measured as a function of temperatures. The predictions agree reasonably well with the experimental results, capturing well the general trend and the frequency at which the energy density peaks. The negative impact of the slow diffusion across a “thick” substrate and the low thermal diffusivity of a polymeric pyroelectric
We demonstrate the feasibility of a device concept that uses liquid-based thermal interfaces to enable rapid switching of the thermal conductance between a pyroelectric material and a heat source/sink and thereby achieve high output power density. The switchable interfaces utilize reversible morphological transformation of microscale liquid to realize rapidly and reproducibly switchable thermal contacts between a pyroelectric material and a heat sink/source. Using a thin film of a pyroelectric co-polymer (P(VDF-TrFE) co-polymer (56:44 mol%)) together with a macroscale mechanical actuator, we operate pyroelectric thermal energy harvesting cycles at frequencies close to 1 Hz. Film-level power densities as high as 110 mW/cm3 were achieved, which was limited by slow heat diffusion across a glass substrate. When combined with a laterally interdigitated electrode array and a MEMS actuator, the present design offers an attractive option for compact high-power density thermal energy harvesters. The present work did not consider energy input necessary for mechanical actuation or application of bias fields. We do not, in fact, envision that macroscale pyroelectric energy harvesters will provide sufficiently high “net” efficiency to merit their use in large-scale stand-alone energy harvesting applications. We believe instead that our pyroelectric energy harvester concept is bestsuited as part of distributed hybrid energy harvesters for powering wireless sensor networks or related applications. One possible hybrid device configuration combines our pyroelectric energy harvester with a piezoelectric vibrational energy harvester and a so-called Curie heat engine where we exploit the temperature dependence of the magnetization of a ferroelectric material to convert spatial temperature variations into mechanical oscillations [25]. By using these mechanical oscillations to subject pyroelectric materials to thermal cycling, we can eliminate the need for any external actuation power. Advanced energy storage/biasing circuitry is another area that needs further investigation. The net power input to the biasing circuit must be sufficiently smaller than the power harvested by a cycle to justify its practical use. A high-efficiency storage capacitor and associated switching/rectification circuits need to be developed to bias pyroelectric materials during thermodynamic cycles while minimizing circuit loss. Some of the recent studies [26] reported progress in related areas. Acknowledgments
Fig. 12. The experimentally measured energy (per cycle) and power density as a function of cycling frequency.
The present article is based on work sponsored in part by the United States ARPA-E under Award Number (DE-AR0000120) and in part by the United States National Science Foundation under Award Number (CBET-1048726). Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accurate, completeness, or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by
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trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. References [1] C.B. Vining, An inconvenient truth about thermoelectrics, Nature Materials 8 (2) (2009) 83–85. [2] G. Sebald, D. Guyomar, A. Agbossou, On thermoelectric and pyroelectric energy harvesting, Smart Materials and Structures 18 (December (12)) (2009) 125006. [3] J. Xie, X.P. Mane, C.W. Green, K.M. Mossi, K.K. Leang, Performance of thin piezoelectric materials for pyroelectric energy harvesting, Journal of Intelligent Material Systems and Structures 21 (February (3)) (2010) 243–249. [4] G. Sebald, S. Pruvost, D. Guyomar, Energy harvesting based on Ericsson pyroelectric cycles in a relaxor ferroelectric ceramic, Smart Materials and Structures 17 (February (1)) (2008) 015012. [5] R.B. Olsen, D. Evans, Pyroelectric energy conversion: hysteresis loss and temperature sensitivity of a ferroelectric material, Journal of Applied Physics 54 (10) (1983) 5941. [6] R.B. Olsen, D.A. Bruno, J.M. Briscoe, E.W. Jacobs, Pyroelectric conversion cycle of vinylidene fluoride–trifluoroethylene copolymer, Journal of Applied Physics 57 (11) (1985) 5036. [7] P. Mane, J. Xie, K.K. Leang, K. Mossi, Cyclic energy harvesting from pyroelectric materials, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 58 (1) (2011) 10–17. [8] S.B. Lang, S. Muensit, Review of some lesser-known applications of piezoelectric and pyroelectric polymers, Applied Physics A 85 (September (2)) (2006) 125–134. [9] L. Kouchachvili, M. Ikura, Pyroelectric conversion – effects of P(VDF-TrFE) preconditioning on power conversion, Journal of Electrostatics 65 (March (3)) (2007) 182–188. [10] A. Khodayari, S. Pruvost, G. Sebald, D. Guyomar, S. Mohammadi, Nonlinear pyroelectric energy harvesting from relaxor single crystals, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 56 (4) (2009) 693– 699. [11] D. Guyomar, G. Sebald, E. Lefeuvre, A. Khodayari, Toward heat energy harvesting using pyroelectric material, Journal of Intelligent Material Systems and Structures 20 (February (3)) (2009) 265–271.
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