Friday, 20 Oct 2017

Georgia Tech develops novel circuit design that boosts thermoelectric generators for wearables

These thermoelectric generators can produce enough electricity to power small sensors, in the range of microwatts to milliwatts - enough for simple heart rate sensors

8 Oct 2017 | Editor

Researchers at Georgia Institute of Technology have developed thermoelectric generator using flexible conducting polymers and novel circuitry patterns printed on paper. These proof-of-concept wearable thermoelectric generators demonstrate it is possible to harvest energy from body heat to power simple biosensors for measuring heart rate, respiration or other factors.

The researchers said that thermoelectric generators, which convert thermal energy directly into electricity, have been available for decades, but standard designs use inflexible inorganic materials that are too toxic for use in wearable devices.

Power output depends on the temperature differential that can be created between two sides of the generators, which makes depending on body heat challenging. Getting enough thermal energy from a small contact area on the skin increases the challenge, and internal resistance in the device ultimately limits the power output.

Researchers Akanksha Menon and collaborators in the laboratory of Assistant Professor Shannon Yee designed a device with thousands of dots composed of alternating p-type and n-type polymers in a closely-packed layout.

According to the publication the novel patterning converts more heat per unit area due to large packing densities enabled by inkjet printers. By placing the polymer dots closer together, the interconnect length decreases, which in turn lowers the total resistance and results in a higher power output from the device.

The new circuit design also has another benefit: its fractally symmetric design allows the modules to be cut along boundaries between symmetric areas to provide exactly the voltage and power needed for a specific application. That eliminates the need for power converters that add complexity and take power away from the system.

Georgia Tech - Dot pattern for thermoelectric circuitry

Figure: Georgia Tech - Dot pattern for thermoelectric circuitry

So far, the devices have been printed on ordinary paper, but the researchers have begun exploring the use of fabrics. There use of fabric opens up the potential to energy harvesting being integrated into clothing.

With the novel design, the researchers expect to get enough electricity to power small sensors, in the range of microwatts to milliwatts. That would be enough for simple heart rate sensors, but not more complex devices like fitness trackers or smartphones. The generators might also be useful to supplement batteries, allowing devices to operate for longer periods of time.

Among the challenges ahead are protecting the generators from moisture and determining just how close they should be to the skin to transfer thermal energy - while remaining comfortable for wearers.

The researchers use commercially-available p-type materials, and are working with chemists at Georgia Tech to develop better n-type polymers for future generations of devices that can operate with small temperature differentials at room temperatures. Body heat produces differentials as small as five degrees, compared to a hundred degrees for generators used as part of piping and steam lines.

"The attraction of thermoelectric generators is that there is heat all around us."

"If we can harness a little bit of that heat and turn it into electricity inexpensively, there is great value. We are working on how to produce electricity with heat from the body."

"We want to integrate our device into the commercial textiles that people wear every day."

"People would feel comfortable wearing these fabrics, but they would be able to power something with just the heat from their bodies."

Akanksha Menon, Ph.D. student at the Georgia Institute of Technology

"Instead of connecting the polymer dots with a traditional serpentine wiring pattern, we are using wiring patterns based on space filling curves, such as the Hilbert pattern - a continuous space-filling curve."

"The advantage here is that Hilbert patterns allow for surface conformation and self-localization, which provides a more uniform temperature across the device."

Kiarash Gordiz, a co-author who worked on the project while he was a Ph.D. student at Georgia Tech

"One future benefit of this class of polymer material is the potential for a low-cost and abundant thermoelectric material that would have an inherently low thermal conductivity."

"The organic electronics community has made tremendous advances in understanding electronic and optical properties of polymer-based materials. We are building upon that knowledge to understand thermal and thermoelectric transport in these polymers to enable new device functionality."

"Among the other prospects for the materials being developed are localized cooling devices that reverse the process, using electricity to move thermal energy from one side of a device to another."

"Cooling just parts of the body could provide the perception of comfort without the cost of large-space air conditioning."

Assistant Professor Shannon Yee, directs the lab as part of the Woodruff School of Mechanical Engineering

This research was supported by the Air Force Office of Scientific Research (AFOSR) under Award No. FA9550-15-1-0145 and by PepsiCo, Inc.

Interconnect patterns for printed organic thermoelectric devices with large fill factors

Kiarash Gordiz | Akanksha K. Menon | Shannon K. Yeea

Free Published Online: September 2017 | Accepted: September 2017

Journal of Applied Physics 122, 124507 (2017); doi: http://dx.doi.org/10.1063/1.4989589


Organic materials can be printed into thermoelectric (TE) devices for low temperature energy harvesting applications. The output voltage of printed devices is often limited by (i) small temperature differences across the active materials attributed to small leg lengths and (ii) the lower Seebeck coefficient of organic materials compared to their inorganic counterparts. To increase the voltage, a large number of p- and n-type leg pairs is required for organic TEs; this, however, results in an increased interconnect resistance, which then limits the device output power. In this work, we discuss practical concepts to address this problem by positioning TE legs in a hexagonal closed-packed layout. This helps achieve higher fill factors (∼91%) than conventional inorganic devices (∼25%), which ultimately results in higher voltages and power densities due to lower interconnect resistances. In addition, wiring the legs following a Hilbert spacing-filling pattern allows for facile load matching to each application. This is made possible by leveraging the fractal nature of the Hilbert interconnect pattern, which results in identical sub-modules. Using the Hilbert design, sub-modules can better accommodate non-uniform temperature distributions because they naturally self-localize. These device design concepts open new avenues for roll-to-roll printing and custom TE module shapes, thereby enabling organic TE modules for self-powered sensors and wearable electronic applications.

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