INTEGRATION OF DISPENSER-PRINTED ULTRALOW VOLTAGE THERMOELECTRIC AND ENERGY STORAGE DEVICES

Zuoqian Wang1, Deepa Madan1, Alic Chen1, Rei-cheng Juang2, Michael Nill1, 3, James W Evans3,

Paul K Wright1 1Department of Mechanical Engineering, University of California, Berkeley

2Green Energy & Environment Research Laboratories, Industrial Technology Research Institute 3Department of Material Science and Engineering, University of California, Berkeley

*Presenting Author: zqwang@me.berkeley.edu Abstract: This work reports on an integrated energy harvesting prototype consisting of dispenser-printed thermoelectric energy harvesting and electrochemical energy storage devices. The use of a commercially available DC-to-DC converter is explored to step-up the mV output voltage of the printed thermoelectric device to several volts for charging printable zinc-based micro-batteries. Two separate parallel-connected thermoelectric devices were designed and fabricated, one on flexible polyimide substrates with evaporated gold contacts and another on commercially available flexible printed circuit boards. The prototype presented in this work demonstrated the feasibility of deploying printed energy harvesting systems in practical applications. Keywords: integration, printed energy harvesting, energy storage, ultralow voltage and power source INTRODUCTION Research related to self-powered wireless sensor systems has been an emerging area in recent years for its wide range of potential applications [1]. Perpetual power solutions are typically composed of three essential components: an energy harvester, power regulation circuitry and energy storage devices [2, 3]. A custom dispenser printer has previously been developed for fabricating both planar thermoelectric generators (TEG) [4-5] for thermal energy harvesting and electrochemical micro-batteries [6-8] for energy storage. Dispenser printing is a cost effective, additive-process manufacturing method to precisely deposit and pattern layered components onto various substrates. The printing methods are also scalable towards mass manufacturing, including traditional screen and flexographic printing techniques. In previous work, a 50-couple thermoelectric device printed on a flexible polyimide substrate was capable of producing 10.5µW at 171.6mV for a 20K temperature difference at matched load resistance [5]. Printed zinc-manganese dixoide microbatteries with gel polymer electrolytes achieved an average of 1 mAh/cm2 and 1.2 mWh/cm2 for discharge rates between C/2 – C/7 [8]. Although we have individually demonstrated the performance of printed thermal energy harvesting and energy storage devices, practical applications require integrated DC-to-DC conversion. While low currents output (µA~mA) from TEGs are sufficient for slowly charging microbatteries, a voltage output higher than the battery open circuit voltage is always necessary (1.5~5V depending on the battery types). This presents difficulties for small-scale thermoelectric generators at low temperature differences since device output voltages typically fall within the mV range. Thus, we explore the use of a

commercially available voltage step-up DC-to-DC converter (Linear Tech LTC3108) to charge a printed microbattery using ultralow power and energy harvested from a printed thermoelectric device. Fig. 1 and 2 show a schematic and image of the components in an integrated circuit.

Fig. 1: A schematic of printable low voltage thermoelectric energy harvesting and energy storage devices integration. THERMOELECTRIC DEVICE DESIGN Since the DC-to-DC converter is designed as a 12Ω load resistance, the printed thermoelectric generator was redesigned to reduce device resistance (typically 2-5kΩ) to match the load resistance. The reduction in device resistance was achieved by decreasing the number of thermoelectric couples in series and instead, placing them in parallel. However, this results in a trade-off between voltage output and device resistance (Fig. 3). Fig. 4 demonstrates the optimization between voltage output, number of couples in series and temperature difference for a printed TEG consisting of 50 couples. To achieve a 30mV output (the minimum input required by the

converter), a 50 couple device consisting of 10 parallel sets of 5 couples in series requires a temperature difference of 40K. The optimized device power output (ΔT=20K) is 22.5mW (power requirement of data transmitting mode is typically 5/10mW).

Fig. 2: Proof of concept integrated devices with a LED output.

Fig. 3: Theoretical calculations show the tradeoff between internal resistance and open circuit voltage.

Fig. 4: An optimization of internal connection of thermoelectric devices for highest voltage and power. EXPERIMENTAL Thermoelectric Devices Fabrication

Details of the thermoelectric materials and prototype fabrication can be found in [6]. Two different substrates were used for the printed thermoelectric device prototypes: (1) polyimide substrates with evaporated gold contacts and (2) custom flexible printed circuit boards (Flex PCB) by

PCB Universe, Inc. Evaporated Polyimide Substrate The polyimide substrates were first prepared by shadow-mask evaporation of gold metal contacts onto a 63.5µm thick flexible polyimide well taped on a glass substrate. Next, the p-type and n-type elements were dispenser printed onto the substrate to form lines spanning across the metal contacts on both sides. The printed devices were then cured in a vacuum oven at 250°C for six hours. Finally, the devices were connected using silver epoxy to two copper wires to form electrical leads. Fig. 5 shows a picture of two printed parallel thermoelectric devices on a flexible polyimide substrate with evaporated gold contacts. On each device, 7 parallel sets of 5 couples in series consisting of elements that were 5mm long was fabricated and characterized to test performance.

Fig. 5: Printed two sets of thermoelectric devices in parallel on flexible polyimide substrate, with one set connected to copper wire. Flexible Printed Circuit Board Considering the thermal inefficiencies and high costs related to the evaporated substrate, a flexible double layer PCB substrate was alternatively designed, The PCB consisted of metal traces connecting the parallel elements to the back of the substrate. The elements in front were connected to the back through vias in the through-thickness direction. The illustration and a photo with both of the front and back view of the actual device are shown in Fig. 6. On each substrate, there are 5 parallel sets of 10 couples in series consisting of elements that were 3 mm long. The design was chosen to increase voltage generation and reduce device resistance. Sets of the devices were conveniently connected in parallel using common copper wires through the vias as shown on Fig. 2. Integration and Testing Both prototypes were connected in parallel and stacked together to reduce the overall resistance to 10 Ohms. Devices were placed on a hot plate and connected to the voltage step-up converter. Thermal compound was used to improve interfacial thermal conductivity between the devices and the heat source. A small fan provided convective flow to increase the

Fig. 6: Illustration and actual printed devices on Flex PCB substrate. temperature gradients. Both the input and output voltages and currents were recorded. A 1 cm2 Zinc based micro-battery was printed on the free space of the conversion circuit and connected to the output terminal [8]. When no thermal energy is available, the microbattery discharges and provides energy for the sensing and transmitting applications. The prototype presented in this work uses a LED light for demonstration purposes.

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Fig. 7: Comparison of devices resistance from theoretical calculations and experimental results. RESULTS & DISCUSSION Devices On Evaporated Polyimide Substrates The first thermoelectric prototype fabricated resulted in a higher-than-expected internal device resistance. Each set of 5 couple in series had a device resistance of approximately 630Ω. Thus, 56 sets of 5 couples in series were placed in parallel to reduce the device resistance to 12Ω. Next, the device was tested on a hot plate to achieve a 79mV open circuit voltage. The closed circuit input voltage, however, only reached a maximum value of 22.5mV. This limits the maximum measurable circuit output voltage to 3.27V. Table 1 summarizes the experimental results. Devices On Flex PCB’s The Flex PCB devices showed reasonable internal

resistances as compared to theoretical calculations. From Fig. 7, a resistance of 8.8Ω was achieved when 10 devices were connected in parallel. As expected, a higher open circuit voltage than the first set of devices was achieved from this set of devices for its higher number of couples in-series (10 couples). Temperature differences between 30-60K provided TEG open circuit voltages of up to 200mV, allowing for all four preset step-up voltage outputs to be obtained. However, as the testing temperatures increased, the internal resistance of the device also increased. This limited the device power and current output. Table 2 summarizes the experimental results. Fig. 8 shows the current and circuit efficiency as a function of input voltage for a 1M Ohm external load (similar to that of a printed zinc microbattery). The maximum efficiency of the circuit was 12.4% when the voltage was 36 mV.

Fig. 8: Output current and efficiency vs input voltage.

Fig. 9: A printed multilayer microbatteries charging time analysis based on an capacity of 1 mAh/cm2 for each layer and a circuit efficiency of 12.4%. Micro-batteries Charging Time Analysis The 2-5V stepped-up output voltage from the printed TEG’s is sufficient for charging the printed electrochemical energy storage devices. Battery charging time is a function of the current output from the converter and the efficiency of the conversion. Fig. 9 presents a multilayer microbattery charging time analysis based on the obtained voltage step-up circuit efficiency and an optimized input power of 22.5mW. As the capacity of the battery increases, the charging time increases proportionally. For a small 1cm2 battery, the charging time will only range between a

few minutes. However, for large surface area batteries and multi-layered systems, the charging time can exceed 10 hours. Future work will be focused on characterizing the performance of the printed battery charged from the printed TEG described in this work. CONCLUSIONS An integrated dispenser-printed energy harvesting system was investigated in this work. The system consisted of printed thermoelectric energy generators, a commercial voltage step-up converter and printed electrochemical energy storage devices. The TEG input voltages between 20-36mV were successfully stepped up to 2.35-5V at currents between 1.28-5µA. These values are sufficient for slow charging of printed electrochemical batteries. Future work will focus on improving the current output and characterization of the charge-discharge behavior of printed zinc micro-batteries in the integrated system. ACKNOWLEDGEMENTS

The authors thank the California Energy Commission for supporting this research under contract 500-01-43. We would also like to thank Christine Ho, Brian Mahlstedt, Michael Seidal, Christopher Sherman, Jay Keist and Peter Minor for their contributions.

REFERENCES [1] James E P et al. 2004 An investigation of self-

powered systems for condition monitoring applications Sens. and Actuators A 110 171-176

[2] Koeneman P B, Busch-Vishniac I J and Wood K L 1997 Feasibility of micro power supplies for MEMS J. Microelectromech. Syst. 6 355–62

[3] Harb JN, LaFollette RM, and Howell LL 2002 Microbatteries for self-sustained hybrid micropower supplies J. Power Sources 104 46–51

[4] Madan D, Chen A, Wright P K, Evans J W 2010 Dispenser printed composite thermoelectric thick films for thermoelectric generator applications J. Applied Physics 109 034904 (6pp).

[5] Chen A, Madan D, Wright P K, Evans J W 2011 Dispenser-printed planar thick film thermoelectric generators J. Micromech. Microeng. 21, 389698 (8pp). In press.

[6] Ho, C.C., D.A. Steingart, J.W. Evans, and P.K. Wright 2008 Tailoring Electrochemical Capacitor Energy Storage Using Direct Write Dispenser Printing ECS Transactions, Vol. 16 (1), 35-47.

[7] Ho, C.C., Murata, K., Steingart, D.A., Evans, J.W., and Wright, P.K 2009 A super ink jet printed zinc-silver 3D microbattery J. Micromech. Microeng. 19, 094013 5.

[8] Ho, C.C., J.W. Evans, and P.K. Wright 2010 Direct write dispenser printing of a zinc microbattery with an ionic liquid gel electrolyte J. Micromech. Microeng. 20, 104009 (9pp).

Table 1: Measured circuit performance for thermoelectric devices on evaporated polyimide substrate, consisting of 56 parallel sets of 5 couple in series.

Open Circuit Voltage

VOC (mV)

Input Voltage Vin (mV)

Input Current Iin (mA)

Circuit Output Set Voltage

Vset (V)

Output Voltage2 Vout (V)

Output Current2 Iout (µA)

2.35 2.34 12.4 3.3 3.27 12.3 4.1 – 1 – 1 79 22.5 2.9

5 – 1 – 1 1. Input voltage was insufficient for measurable circuit output; 2. No external load applied.

Table 2: Measured circuit performance for printed thermoelectric devices on Flex PCB, consisting of 50 parallel sets of 10 couple in series.

Open Circuit Voltage

VOC (mV)

Input Voltage Vin (mV)

Input Current Iin (mA)

Circuit Output Set Voltage

Vset (V)

Output Voltage1 Vout (V)

Output Current1 Iout (µA)

110 21.5 2.9 2.35 1.175 1.28 115 22.2 3.17 2.35 1.224 1.38 115 23 3.3 2.35 2.05 2.2 118 23.9 3.4 2.35 1.43 1.6 120 25 3.5 2.35 2.35 2.5 130 26 4.5 3.3 2.78 3.13 150 27.8 4.7 3.3 3.3 3.6 180 31 5.2 4.1 4.1 4.5 200 36 5.6 5 5 5

1. Applied external load: 1 M Ohm.