UC Berkeley

Thermoelectric generators can potentially be used to generate electricity from this low–grade waste heat and play an important role in powering the condition monitoring sensors. This work presents a novel method to synthesize thermoelectric materials to print scalable thermoelectric generator (TEG) devices in a cost effective way. The main focus of this work is performance advancements of dispenser printed composite thermoelectric materials and devices. Thermoelectric device design for condition monitoring applications, novel composite thermoelectric materials and cost effective and scalable manufacturing methods are foundational aspects of this work.

WSNs are a promising technology for ubiquitous, active monitoring in residential, industrial and medical applications. A current bottleneck for widespread adoption of WSNs is the power supplies. While the power demands can be somewhat alleviated through novel electronics, any primary battery will have a finite lifetime. This can pose a major problem if the network is large or the nodes are located in difficult to reach areas. Battery replacement is thus undesirable, costly, and inefficient for large–scale deployments of WSNs. Thermal energy is an attractive option to power WSNs due to the availability of low–grade ambient waste heat sources.

TEGs provide solid–state energy by converting temperature differences into usable electricity. These solid–state TEGs have great appeal due to their silent nature, have no moving parts and are CO₂ emission free. In order to be used for powering the condition monitoring WSNs, the TEG should be able to provide certain average power at desired voltage levels. Based on heat transfer TEG design, high voltage output requires large number of couples packed in a small area in addition to high Seebeck coefficient and high temperature difference across the device.

The performance of TEGs devices depends on both material properties and device geometry The efficiency of TEG is governed by the dimensionless figure of merit, ZT, which depends on material properties. It has been challenging to increase ZT beyond 1 for commercial thermoelectric materials like Bi₂Te₃ as n–type and Bi_0.5Sb_1.5Te₃ as p–type, since the thermoelectric parameters of ZT are generally interdependent. There are challenges on the device design side as well. The electrical resistance and the temperature difference across the device depend on the element length of the device. Electrical resistance increases with increase in element length resulting in lower power output. Temperature difference across the device increases with increase in element length resulting in higher power output. Therefore, a trade–off occurs between device element length and power output, which ultimately depend on the TEG application. Therefore, an application specific optimized device length is required to maximize power output.

Devices, utilizing waste heat to generate power, should be low cost in order to be competitive. Traditional pick and place methods to manufacture TEG devices are labor, materials and energy intensive. Whereas, the micro–fabrication technology involves expensive and complicated processes like lithography and thin-film deposition and is limited to micro–scale regime. These methods have limited cost-effective scalability for manufacturing of application–specific TEGs. The limitations of the commonly used manufacturing technologies provide an opportunity for additive manufacturing methods such as direct-write printing. Printing utilizes additive processing steps, thus reducing materials waste and cost per unit area. It is an automated process that can be used to print high–aspect–ratio devices with minimum labor.

Printing of high–aspect–ratio TEG devices requires thermoelectric materials that are readily synthesized, air stable, and solution–process able to create patterns on large areas. In this regard, polymer thermoelectric composites are very attractive, as they require relatively simple manufacturing processes. While the ZT of inorganic composite materials may be lower than that of conventional materials, the reduction of manufacturing costs associated with printing is significant. As a result, the cost of energy generated from TEGs is improved through printed manufacturing. In this work, we utilize a custom developed dispenser printer to print high–aspect–ratio planar single–element TEGs. Off the shelf, Bi₂Te₃ and Sb₂Te₃ were chosen as the starting thermoelectric material because of high ZT values at room temperature. The maximum ZT at room temperature for an n–type Bi₂Te₃–epoxy composite and p–type Sb₂Te₃–epoxy composite cured at 250°C was 0.16 and 0.18. Mechanical alloy and additives helped to improve the ZT for n and p–type thermoelectric composite materials to 0.2 and 0.18 respectively. Single element n–type planar prototype device was printed on a custom designed flexible polyimide substrate to form a TEG. The device produced 25μW at 0.23mA and 109mV for 20K temperature difference. These results indicate an areal power density of 130μW/cm², which is quite close to ideal power density 135μW/cm². Similarly p–type planar prototype device produced 20.5μW power at 0.15mA and 130mV for 20K temperature difference and areal power density of 150μW/cm².

We have explored Bi as n–type thermoelectric composite material as it is easily and cheaply available and its toxicity is less as compared to commonly used thermoelectric materials like Bi₂Te₃. We present a unique way of fabricating circular TEGs using dispenser–printing methods, which can be easily mounted on hot surface or wrapped around pipe carrying hot fluid to generate electricity to power condition monitoring sensors. In order to realize practical thermoelectric devices, both p–type and n–type elements connected in series are essential to achieve reasonable efficiency. Therefore, Bi–epoxy has been used as n–type composite thermoelectric material and MA Bi_0.5Sb_1.5Te₃ with 8wt% extra Te epoxy composites as p–type thermoelectric material to print circular TEGs. The maximum power output of 130μW at 70K temperature difference is achieved for a 10–couple device that resulted in measured power density (1230μW/cm²).

The TEG device design was designed based on n and p–type composite thermoelectric film properties and optimized thermo–element leg length obtained was 3.5mm. The Flexible PCB consisting of nickel and gold plated copper traces on a flexible polyimide substrate was built by Rigiflex Technology Inc. Thick gold plated nickel and copper metal contacts resulted in reduced electrical contact resistance between metal contacts and printed TE elements. Flexible polyimide has low thermal conductivity that helps to maintain temperature difference across the device, electrical insulation helps to separate the gold contacts and high temperature tolerance make curing feasible for printed elements at high temperature. The 50–couple prototype device produced a power output of 33μW at 0.75mA and 43mV for a temperature difference of 20K resulting in a device areal power density of 280μW/cm². TEGs were mounted on hot surface pipes and without controlling the cold side temperature; power measurements with variable load resistance were done. At 100°C hot surface temperature, 33μW of maximum power has been obtained. Power generated by these TEG devices is sufficient to charge batteries used in condition monitoring WSNs on the hot surfaces of equipment. While the efficiency of printed thermoelectric composites is not as high as that of some state–of–the–art materials with high figure of merit, the results are encouraging. The ease of processing and device fabrication with printed materials provides deployment advantages over such materials.