2.8.215 Variable capacitor energy harvesting based on polymer dielectric and composite electrode Robert Hahn 1*, Yuja Yang 1, Uwe Maaß 1, Leopold Georgi 2, Jörg Bauer 1, and K.- D. Lang 2 1 Fraunhofer IZM, Gustav-Meyer-Allee 25, 13355 Berlin, Germany 2 Technische Universität Berlin, TiB4/2-1, Gustav-Meyer-Allee 25, 13355 Berlin, Germany August 215 Agenda Introduction Fraunhofer IZM and MATFLEXEND project Capacitive harvesting overview Equivalent circuit analysis FEM simulation of composite electrode Experimental results Fraunhofer Gesellschaft 2 1
2.8.215 Micro Energy at Fraunhofer IZM, Berlin Micro fuel cell Fuel cell stacks Micro DC-DC converter coils Micro batteries Fraunhofer Gesellschaft 4 2
2.8.215 Human based mechanical energy harvesting Many challenges: Low frequency, random motion Low and medium forces, mechanical adaption Devices need to be tunable or broadband Power output limited by device size Conversion effectiveness needs to be as high as possible Many devices have been shown: Fraunhofer Gesellschaft 5 Electrostatic harvester comparison Dielectric elastomers: mechanical force stretches the dielectric, increases capacitor area and reduces dielectric thickness mechanical fixtures are required high voltage, high efficiency relative permittivity: ca. 3 Electrostatic MEMS interdigitated electrodes a) area overlap b) gap closing mostly MEMS relative perittivity: ca. 1 MATFLEXEND capacity is changed by change of top electrode area relative perittivity: ca. 3 3 flexible and low cost Fraunhofer Gesellschaft 6 3
2.8.215 Electrostatic harvester comparison Piezo, electrete and triboelectric: True emf, electric charges are being generated Dielectric elastomers, Electrostatic MEMS and MATFLEXEND: These are variable capacitors which have to be charged and discharged at the right time of the conversion cycle Fraunhofer Gesellschaft 7 Capacitive harvesting with variable electrode area Fraunhofer Gesellschaft 8 4
2.8.215 Capacitive Harvesting Charge constrained with variable electrode area Voltage constrained E = 1 2 V max 2 C min 1 C min C max E = 1 2 V max 2 C max C min 2 C min C max Fraunhofer Gesellschaft 9 How much energy can be harvested? E = 1 2 V max 2 C max C min 2 C min C max therefore in most cases this circuit is used: high power but the conversion cycle circuit requires sophisticated circuit C max = 1 nf C min = V = 4 V de =.8 mws f=.5 Hz P =.4 mw The voltage of C store is increased stepwise from V 1 to V 2 = 2V 1 1 steps at f=.5 Hz: P=,2 mw Fraunhofer Gesellschaft 1 5
2.8.215 Electrode material: two versions liquid metals and conducting elastomers Liquid metal electrode Conducting elastomer electrode Nano-doped polymer dielectric PolyHIPE droplet-separator spring structure Fraunhofer Gesellschaft 11 FP7 MATFLEXEND: Project Summary Matflexend investigates New materials which enable capacitive-mechanical energy harvesting based on high-k dielectric composites and electrically conducting elastomers as variable capacitor electrodes The technology will be demonstrated in applications like wearable electronics and smart cards including novel near-hermetic packaging. Fraunhofer Gesellschaft 12 6
2.8.215 FP7 MATFLEXEND: Applications A M T END Fraunhofer Gesellschaft 13 Equivalent circuit model: Parasitics R s serial resistance, mainly ohmic resistance of the conductive elastomer electrode R p parallel resistance, leakage current through the dielectric MATFLEXEND variable capacitor Fraunhofer Gesellschaft 14 7
2.8.215 Voltage and current waveforms Fraunhofer Gesellschaft 15 Design example of circuit analysis Fraunhofer Gesellschaft 16 8
2.8.215 Case 1 Fraunhofer Gesellschaft 17 Case 2 Fraunhofer Gesellschaft 18 9
2.8.215 Measurements Capacity of composite electrode on dielectric as function of mechanical pressure Current flow during cycling of capacitor as function of speed Current of measurement circuit and voltage at output capacitor Fraunhofer Gesellschaft 19 Elastomer electrode configuration 3D shape (molded) planar, rough planar, smooth material strain generates restoring force thicker device, lower power density thin device, high powerdensity, stackable lower specific capacity (rough electrode additional spacer/spring elements required Fraunhofer Gesellschaft 2 1
capacity [pf] area [mm²] capacity [pf] normal force [N] 2.8.215 Dielectrics for proof-of-concept testing Different commercially available dielectrics were tested on isolation resistance, capacity and leakage current PE, PP, PTFE foils Low leakage currents, good linearity, cheap, good availability of very thin foils, but low permittivity Thin-film dielectrics (Ta 2 O 5, Al 2 O 3 ) Very thin, good permittivity, medium leakage current, but very high costs Fraunhofer Gesellschaft 21 Capacity change with 3D shaped electrodes Spherical electrode 7.5 mm BaTiO 3 -dielectric 5 4 normal force 5 4 3 2 capacity 3 2 45 4 45 4 1 1 35 3 25 capacity area 35 3 25 5 1 15 2 2 2 15 1 5 1 2 3 4 5 normal force [N] 15 1 5 Simultaneous measurement of force, capacity and electrode area Fraunhofer Gesellschaft 22 11
Charge [nas] Capacity [nf/cm²] Capacity [nf/cm²] Charge [nas] 2.8.215 Capacity of composite electrode as function of mechanical pressure: flat electrodes 25 metal electrode 15 metal electrode 2 15 smooth rough 1 cm² 1 cm².25 cm².25 cm² 1 cm² 1 cm² 1 smooth rough 1 cm² 1 cm².25 cm².25 cm² 1 cm² 1 cm² 1 5 5 1 2 3 4 5 Top Electr. Dielectric Bottom El Force [N] Elastomer Ta 2 O 5 Silicon 1 2 3 4 5 Top Electr. Dielectric Bottom El. Force [N/cm²] Elastomer Al 2 O 3 Silicon Top Electr. Dielectric Bottom El. Elastomer Mylar Cu Capacity with composite electrode is 3... 5 times smaller than metal el. Fraunhofer Gesellschaft 23 Current measurement: slow cycling V1 = 2 V 6 6 4 4 2 2-5 5 1 15-2,5, 2,5 Top Electr. Elastomer Dielectric Mylar 1 Bottom El. Cu Top Electr. Elastomer Dielectric Al2O3 Bottom El. Si Charge loss at slow cycle speed Q max ca. 5 nas Fraunhofer Gesellschaft 24 24 12
Charge [nas] current [na] current [na] current [na] Charge [nas] Charge [nas] 2.8.215 Current measurement: slow compression fast release 15 6 V1 = 2 V 1 5 4-5 2-1 5 1 15 5 1 15 Top Electr. Dielectric Bottom El. Elastomer Mylar 4 Cu Fraunhofer Gesellschaft 25 Current measurement (V M ) with harvester circuit 5 25 4 2 (fast release) V1 = 1 V 3 15 2 1 1 5 3 25 -,2,,2,4,6,8 8 6 -,4 -,2,,2,4,6 here electrode slowly detaches from dielectric Top Electr. Dielectric Bottom El. Elastomer Mylar 4 Cu 2 15 1 5 4 2 V1 = 25, 5, 1, 2 V Source V1 Icharge D1 Rser V2 = 5, 1, D2 2, 4 V V2 VM Iharvest RM -,5,,5 -,1 -,5, Charvest Fraunhofer Gesellschaft 26 13
Voltage [V] 2.8.215 Charging the output capacitor Voltage at C store 22 2 18 16 4.5 N/cm² 4.5 N/cm² 7.1 N/cm² V 14 12 1 5 1 15 2 25 3 35 Top Electr. Dielectric Bottom El. Elastomer Mylar 13 Cu C min = 1 pf/cm 2 C max = 4 pf/cm 2 C store = 3.3 nf frequency range.25 2 Hz tested minor charge reduction at 2 Hz degradation during long term test observed Fraunhofer Gesellschaft 27 Conclusions Capacitive energy harvesting principle with elastomer electrode proven. An equivalent circuit model was used to investigate the influnce of the leakage currents (R p ) and the bulk resistivity of the elastomer electrode (R s ). Losses in R s are only a function of rise and falling times of the capacitor changes. For typical low frequency operation (.1 1 Hz) R p should be above 1 G per square centimeter. Charges between 25 and 7 nas per cm 2 have been transferred per cycle at 1 V/2 V with state of the art dielectric and conducting elastomer materials. Fraunhofer Gesellschaft 28 14
2.8.215 Required material properties Dielectric: High-k values (2 ), specific capacity > 1 nf/cm 2 Low leakage current of thin films (R p > G cm 2 ) Polymers capable for roll-to-roll manufacture or screen print Dielectric losses are not critical since only low frequency operation (.1 1 Hz) Elastomer electrode: Low modulus mechanically flexible material Medium to low electrical resistance (Rs < 5k cm 2 ) High density/homogenity of conducting particles at the surface (distances between conducting phase < 1 µm) Roll-to-roll coating or screen print Fraunhofer Gesellschaft 29 Thank you for your attention Contact: Robert Hahn Fraunhofer Institute for Reliability and Micro Integration Gustav-Meyer-Allee 25 13355 Berlin, Germany robert.hahn@izm.fraunhofer.de +49 3 4643 611 This project is funded by the European Union under Contract: A M T END Fraunhofer Gesellschaft 3 15