Printed, metallic thermoelectric generators integrated with pipe insulation for powering wireless sensors
Introduction
The Internet of Things (IoT) has the capability to transform industrial manufacturing. Pervasive IoT monitoring of vibrations, stresses, corrosion, and thermal fluctuations will provide critical data for better maintenance prediction and scheduling. However, the labor costs associated with battery replacement in IoT devices is impractical, necessitating cost effective, continuous power sources to support this form of ubiquitous sensing. Here, we seek to use abundant waste heat streams available in most industrial settings as the source for this power.
More than 50% of the energy consumption in the United States results in waste heat [1]. Low-grade waste heat (less than 230 °C) is particularly underutilized because typical recovery methods (such as a heat exchanger) are less economically viable at low temperatures. However, this lower quality heat is available in such large quantities (950 trillion kJ per year) that the work potential of this stream is 35% higher than medium grade (230–650 °C) and 110% higher than high grade (>650 °C) sources [2]. Consequently, modestly efficient, low-grade heat recovery may be the ideal solution to powering low-power industrial IoT sensors.
Currently, the most viable route to harnessing this wasted heat is through the use of thermoelectric generators (TEGs). In recent years, the theoretical limits of these generators have been continually pushed by advances in nanotechnology and have made new application areas, such as wireless sensing, more viable [3], [4], [5], [6]. The sheer number of sensors and, consequently, power sources that would be required for these networks has driven a specific focus on creating simple and scalable manufacturing processes for TEGs. Large area printing of TEGs has quickly emerged as a promising path forward [7], [8]. The current benchmarks for high efficiency thermoelectric conversion rely on toxic and relatively rare metals such as bismuth (Bi), lead (Pb), tellurium (Te), and antimony (Sb). While these have been used widely to produce printed modules [9], [10], [11] cost and environmental concerns remain as barriers to large scale implementation in industry. When considering the cost (economically and environmentally) of a potential thermoelectric, the base material cost and the cost of production are the main driving forces. Printed thermoelectrics are capable of simultaneously addressing both concerns due to the wide variety of deposition techniques (screen printing, inkjet printing, molding, lithography, vacuum deposition) and inorganic/organic material combinations possible (carbon nanotubes, semiconducting polymers, metal powders) [12].
A significant amount of prior work has been carried out on the development of high aspect ratio and high power density flexible printed TEGs for use in wearable applications [13], [14], [15] and in industrial facilities [16], [17], [18]. Lu et al. used silk fabric as a substrate for solution based bismuth telluride (BiTe) TEG elements. The final device, composed of 12 thermocouples, exhibited 12 nW of power output at temperature differences possible on the body (5–35 K) [15]. Furthermore, Suarez et al. have shown the capabilities of non-solution based bismuth and telluride elements in a flexible matrix [19]. The wearable devices created showed no increase in resistance after 1000 bending cycles to a radius of 5 mm, thus demonstrating the potential durability of flexible devices. Integration of a printed, flexible BiTe TEG with a voltage boost converter has also been demonstrated for use in wearable applications. Veri et al. have shown that a 140 thermocouple device can generate enough voltage to power up a D.C. to D.C. boost converter, similar to that reported later in this work [20].
As wearable devices have addressed increasing interest in health monitoring the same has been true for devices designed with industry system monitoring in mind. Chen et al. have demonstrated combining a curved steam pipe attachment and a commercial TEG to power a wireless sensor [18] while Jovanovic et al. have designed a flexible bismuth telluride device that would meet the power requirements of a sensing network [21]. More recently, Madan et al. applied a printed, flexible BiTe TEG to an insulated heat pipe. A high power density of 2.8 W m−2 was reported while utilizing forced convection to maintain a temperature gradient [22]. The concept demonstrated in that work shows that the common piping found in many industrial facilities has great potential for further development. It is important to note that, for both the wearable and industrial spaces, the vast majority of the literature work relies on bismuth, antimony, and tellurium due to high efficiencies. Capabilities of other, less toxic thermoelectric elements has been investigated to a lesser extent and utilizing a flexible, printed TEG to power an industrial wireless sensing network has not yet been demonstrated.
In this work, we introduce several new accomplishments: (1) screen printing of a 420 junction TEG module based solely upon relatively inexpensive and non-toxic materials, and (2) utilizing this TEG to power a wireless sensing system in a realistic industrial scenario. Inspiration for a novel device matrix was drawn from standard steam pipe insulation found in facilities worldwide. Single planar modules were embedded in insulation radially, thus creating a harvesting device that takes advantage of waste heat in all directions. A visualization of this setup can be seen in Fig. 1. One six inch (15 cm) section of insulation is demonstrated to power a microcontroller, temperature probe, and wireless transmission relay for 10 min of continuous data transmission at 30 s intervals after a 4 h charging period.
Section snippets
Device fabrication and evaluation
The TEGs in this work were printed from commercial silver (Ag) ink and in-house formulated nickel (Ni) pastes [23]. Silver paste was DuPont 5064 H (discontinued, now 5065). Nickel paste was prepared from 1–5 µm nickel flakes (Atlantic Equipment Engineers, Upper Saddle River, NJ.) The flakes were suspended in ethylene glycol with polyvinylpyrrolidone (PVP, molecular weight 40,000 g/mol) as the binding agent. The total solids content of the final paste was 82% with a PVP fraction of 2%. The silver
Single module improvement and performance
Besides connecting multiple TEG modules into a monolithic device, we also demonstrate improvement upon our previously reported single TEG module design [23] in two important ways: (1) the electrical resistances between the two printed TE legs are better matched, and (2) each module is encapsulated to improve long term durability of our final TEG system.
Power output of a TEG is driven by module resistance. High resistances decrease power output and mismatched leg resistances can further lower
Conclusions
This paper has realized a radial thermoelectric generator integrated with heat pipe insulation that is capable of powering a sensor circuit with wireless transmission capabilities. Twelve modules comprising a total of 420 thermoelectric junctions were embedded in steam pipe insulation and connected in an optimal configuration to power a D.C. to D.C. boost converter. This boost converter was used to charge a pair of storage capacitors that then turned on a Bluetooth low energy enabled
Acknowledgements
This work was supported by a grant from North Carolina State University, Raleigh, NC, USA through the 2015 Chancellor Innovation Fund. We thank Dr. Philip Bradford and Brian Wells (NC State University) for help in sintering the printed devices and Toby Tung (NC State University) with assistance on scanning electron microscope imaging. K. Ankireddy, M. D. Losego and J. S. Jur have small equity interests in and serve as scientific advisors for Thermo-Flex Technologies, Inc., a company that may
Glossary
- P-type element
- Thermoelectric material where electron holes are the dominant charge carrier
- N-type element
- Thermoelectric material where electrons are the dominant charge carrier
- Junction
- Pair of thermoelectric elements, one p-type and one n-type
- Seebeck coefficient
- Magnitude of potential difference created per unit of temperature difference (typically with units of mV K−1 or µV K−1 per junction)
- TEG
- Thermoelectric generator
- Junction
- Connection of one p-type and one n-type element
- Module
- Single 35 junction TEG
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