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1 HybridComponentsLaboratory,CEALITEN,Grenoble,France 2 Sofileta,25petiteruedelaplaine,Bourgoinjallieu,France 3 METIS,27BdStMichel,Bourgoinjallieu,France Heading for a largely improved interface between individuals and electronics this paper presents an enabling technology for the energy harvesting using body heat. Since the user produces thermalenergyasbodyheat(80to700Watt),apartofthisenergycanbeconvertedintoelectricalpower viaathermoelectricfabric.Thus,alowcostmethodtomanufacturealargeareaofthermoelectricfabric with industrial machine has been developed. After a thermal and electrical modeling of the fabric structure, specific characterizations have been achieved. 4mW/m2 could be produced by thermoelectric fabricwhenatemperaturedifferencebetweenthetwosidesofthefabricisequaltotendegrees. Thermoelectricity,Fabric,Humanheatenergy Inthisreport,weproposeanalternativemethodto compensatethelowthermoelectricefficiencybya large area of thermo elements. It consists in Recently, “Smart clothes” are an effort to make manufacturing a thermoelectric fabric self electronicdevicesagenuinepartsourdailylifeby powered by body heat energy. In this paper embedding entire systems into clothing and thermoelectric performances of fabric panel have accessories. Power is perhaps the most limiting been examined with a specific measurement factor in mobile technology. It restricts the system. autonomy, weight and size of portable devices. Batteriesmustberechargedandtheaccesstothe electricalnetworkisnotalwaysaccessible.Thus, newapproachesemergetoharvestbodyheatfrom thermoelectric devices. Thermoelectric devices show advantages compared to batteries, they are robust, consist of environmentally friendly In this study, ordinary thermocouple wire productsandprovideanunlimitedlifetime.Thus, materials are employed, namely, chromel and thermoelectricity presents an interesting constantan wires 50 m in diameter. They are alternativetobatteries. manufactured at the same time with the textile This concept has been explored by Seiko with during the industrial process. Flexibility of the thermoelectricwristwatches[1].Inourknowledge textile is perfectly preserved to guarantees the it is the unique example of commercial user’scomfortandfabriccolorationisdefinedby applicationofthermogeneratorfromhumanbody. the textile structure. In this example, textile Moreover, the energy consumption of the Seiko structure was made with polyester varns. Two watches is 1W. In this way, Infineon evaluated pictures of yellow and white textile panel are shown in Fig.1. Copper wires have been used in newmaterialswithamicromachined arraysuited the ends to allow electric connection. The details forcomfortableintegrationintoclothing[2] of the textile structure are described by a patent Today IMEC demonstrates that thermoelectricity depositedbySOFILETAandCEA[4]. is a good candidate to develop wireless pulse oximeterfullypoweredbyawatchstyleTEG[3]. 311 thesestructures.Thedifferencesbetweenstructure A and B are fabrics thickness and thermocouple density. These two characteristics depend on manufacturingmethod. According to the fabric structure and thermal conductivities of chromel, constantan and polyester,thefabricthermalconductivityhasbeen estimatedto0.19W/mK. Thermocouplesarearrangedinpairsonbothsides ofthefabric.Thus,apatternofthermocouplecells may be produced by repetition. Several thermocouple cells are connected in parallel to form various colons and connected in series to form various lines. Thermogenerator fabric formedbythermocoupleshasbeenrepresentedby an electric circuit where each resistance corresponds to the resistance of chromel wires, Rchromel, (or resistance of constantan wires, Rconstantan) between two knots alternated on the fabricsides.Totalresistanceofthefabricdevice, notedR,dependsofCandL,whereCandLare thenumbersofcolonsandlinesrespectively(See Fig2). ( tan tan ) (1) 2 Fig. 2 shows a schematic thermal circuit representing the thermogenerator and its environment. Three thermal components are very important: the thermal resistance of the thermoelectric fabric, the thermal resistance between the body and the fabric and the thermal resistanceoftheair.Inafirstapproximation,only the thermal conductivity of the fabric will be estimated. In this study, prototypes of thermoelectric textile weremanufacturedbyarranged75colonsand50 lines, and two structures, noted A and B, have beenanalyzed.Table1givesthespecificationsof Fig.2:Schematicthermalcircuitrepresentingthe generatoranditsenvironment Fabrics have been rolled up around a cylinder heatedbyJouleeffect(Fig.3).Thetemperatureof the internal side of the textile varies from 0 to 50°C, controlled by a thermocouple. The outside of the textile has been left with the ambient air and convection phenomenon could be simulated with ventilation. Thus, information of thermal resistanceofairwillbeinferred. Betweenthetwoendsofthetextile,resistanceand voltage have been measured. A system of acquisitionallowsthelayoutofseebeckvoltageas functionoftemperaturedifferenceexploited. 312 Structure A B sample L C A1 A2 A3 A4 B1 B2 50 75 Thickness (mm) 2 1.1 Area (cm2) 650 645 617 624 245 268 R() 1.3 1.1 Table 2 provides reproducibility measurement from four identical fabrics. These results have shownthatourmanufacturingprocessisreliable. Measurements of thermal power density are collectedbyflowmeteronhotsideofthetextile. Accordingwithequations3and4,theslopeofthe linear fitting (U=f( F )) was given by the ratio LSe/λ. It is an indirect method to obtain the thermalconductivityofthetextile. F L $ (2)and $ (3) where, Ф is thermal power density (W/m2), λ is thermal conductivity (W/mK) and e is fabric thickness (m), Tjunction is the temperature differencebetweenthetwosidesofthetextile,U, the terminal voltage (V), L, the numbers of lines and S, seebeck coefficient of chromel/constantan couples. Fig. 4 shows the evolution of output voltage measured in the open circuit state versus thermal power density that flow through the thermoelectric fabric, for sample A1. The potentialincreasesdirectlywiththeheatflow,due to an increase of the temperature difference exploited by the thermoelectric fabric. For example,avoltageof4.2mVwasreachedwhena heatflowandtemperaturedifferenceexploitedby the thermoelectric fabric were 100W/m2 and 1.2, respectively. The thermal conductivity given by experimental measurement is 0.16 ± 0.02 W/mK (Table 2). This value is in good agreement with our previous estimated value (i.e. 0.19 W/mK). Human body produces around 100W/m2 of heat energy for a sitting person working in office [5]. Thus, for this value, thermoelectric performances for structures A and B have been measured and reportedinTable3. N° Ф (W/m2) λ (W/m.K) U (mV) R () 100 0,14 0.15 0,17 0,19 0,17 4.2 3.9 3,9 3,25 3,9 4,3 4.5 4,6 4,4 4,5 A1 A2 A3 A4 Useful power (W/m2) 16 13.5 13 10 13,5 Area (cm2) U(mV) R() A B 635 257 3.8 2.1 4.5 2.2 Useful power (W/m2) 12.6 19 An estimation of convection effect has been simulated with our characterization method (ventilation off or on, with open or closed system). For a constant temperature of the fabric hotside(around37°C),dependenceonconvection effect has been estimated. Three conditions have 313 been simulated and lead to the performances for various surrounding conditions of the wearable fabric (without, indoor and outdoor convection). Results are reported in Table 4. The convection effect increases the heat flow and consequently the useful power. Variation of surrounding conditions could also increase the useful power from16to116W/m2. In addition, 4mW/m2 could be produced by thermoelectric fabric when a temperature difference between the two sides of the fabric is equaltotendegrees. Without convection Indoor convection Outdoor convection Heat Useful U R Flow power (mV) () (W/m2) (W/m2) 83 1.8 2 16 Transfert coefficient (W/m2K) 4 130 2.8 2 41 6 210 4.4 1.7 116 10 4 Outdoor V o la tg e [m V] 3 Indoor 2 1 0 13:26:24 13:29:17 13:32:10 13:35:02 time (h:min:s) 13:37:55 13:40:48 13:43:41 Finally, these results have been confronted with outdoorcondition.Sampleof250cm2inareawith BstructuretextilehasbeenintegratedintoaTee shirt for outdoor use. Voltage generated by this TeeShirt was measured and reported on Fig 5. When the person have no physical activity in room at 21°C, 0.9 mV has been measured, whereaswhenthepersonisoutdoor(at19°C),the increaseofvoltagereachedto3mV.Accordingto our previous results, voltage increase depends on convection conditions and external temperature. 314 Nevertheless lower voltages than our previous results (Table 4) were observed due to poor thermal contact between human body and tee shirt. In this report, a thermoelectric fabric scavenging energy from wasted human body heat was proposed for the first time. The manufacture of thisdeviceisverylowcostandtotallyindustrial. Thermoelectric performances were evaluated undercontrolledandrealconditions.Itwasfound thattheoutputpowerofthefabricsvarieswithits structure and the environmental conditions. Despiteoflowpowerdensitymeasurement,large area of thermoelectric fabric could be employed and compensate it. Nevertheless, specific application using low power and low voltage devices could use this new approach of energy harvestingfrombodyheat. This study has offered a basic evaluation of the characteristics of a thermoelectric fabric, but optimization of the structure, electrical contact resistance and thermal contact resistance should beimproved. Thanks to Metis partner for his financial participationinthisproject. [1] M. Kishi, et al., MicroThermoelectric Modules and their application to Wrist watchesasanEnergySource, pp301307,1999 [2] M. Strasser, R. Aigner, M. Franosch, G. Wachutka, Miniaturized Thermoelectric Generators Based on PolySi and PolySiGe Surface Micromachining, ,Vol.1,pp2629,2001 [3] V. Leonov,R.J.M.Vullers,PulseOximeter Fully Powered by Human Body Heat Tom Torfs, in , vol.80,Issue6,pp12301238,2007 [4] M.Plissonnier,C.Salvi,T.Lanier,Patent06 03292(2006) [5] T. Starner, Umanpowered wearable computing, ,Vol.35,pp 618629,1996
