Thermoelectric generators may lack the visible appeal of solar panels, but their demonstrated usefulness is spurring more research into lower-cost fabrication techniques.
Thermoelectric generators (TEGs) use heat—or more accurately, temperature differences—and the well-known Seebeck effect to generate electricity. Their applications range from energy harvesting of available heat and especially “wasted” heat in industrial and other situations, to being the heat-to-electrical energy converter using radioactive-based powers sources for spacecraft in radioisotope thermal generators (RTGs).
TEG-based RTGs use the heat of natural decay of plutonium-238. They have been used in nearly every space mission since 1961 (see References) as well as for remote Earth-based applications. They don’t get a lot of attention compared to highly visible, clean-looking, often dazzling solar panels in space, but the reality is that solar panels alone can’t provide adequate power themselves, even for many orbiting or close-to-Earth missions. The option of electrochemical batteries is a non-starter as they don’t function in the intense cold of space, which is at about 2.7 K if there is no solar-heating effect.
TEGs, as with most energy-harvesting transducers and arrangements seem like a good idea in principle since you are getting something worthwhile for almost nothing. In practice, however, they have several drawbacks: they are relatively hard to manufacture (especially in bulk), and they are inefficient (around 10%). That efficiency figure, while low, is often acceptable when the heat would otherwise be wasted, or there is no other viable choice.
We usually associate the Seebeck effect with bimetallic-junction thermocouples and temperature measurement rather than energy capture. In fact, many heat-recovery TEG devices use highly doped semiconductors made from bismuth telluride (Bi2Te3), lead telluride (PbTe), calcium manganese oxide (Ca2Mn3O8), as well as other materials, depending on application and temperature.
The other problem with TEGs is that they are hard to manufacture in quantity and difficult to produce inexpensively. These shortcomings are also incentives for researchers to see what enhancements or improvements can be made in their materials and production processes, as two very different projects clearly demonstrate.
A team led by researchers at the University of Notre Dame (Indiana) addressed the problem that TEGs generally lack a high-throughput processing method, and so they developed a much-faster way to create high-performance devices. They used machine-learning techniques to optimize sintering the thermoelectric materials rapidly while maintaining their high thermoelectric properties, Figure 1.
Figure 1 The researchers use a three-stage interactive process, with (i) laser-driven sintering followed by (ii) assessment of thermoelectric properties and then (iii) Bayesian optimization, leading back to (i). Source: University of Notre Dame
The novel process uses intense pulsed light to sinter thermoelectric material in less than a second, while conventional sintering in thermal ovens can take hours. The team sped up this method of turning nanoparticle inks into flexible devices by using machine learning to determine the optimum conditions for the ultrafast but complex sintering process.
They integrated high-throughput experimentation and Bayesian optimization (BO) to accelerate the discovery of the optimum sintering conditions of silver–selenide TE films using an ultrafast intense pulsed light (flash) sintering technique. Due to the nature of the high-dimensional optimization problem of flash sintering processes, a Gaussian process regression (GPR) machine learning model was used to rapidly recommend the optimum flash sintering variables based on Bayesian expected improvement, Figure 2.
Figure 2 The feature-feature correlation matrix of the top features guides the improvement process. Source: University of Notre Dame
They produced a flexible TE film with an ultrahigh-power factor of 2205 μW/m–K2 and with a zT of 1.1 at 300 K; zT is a dimensionless figure of merit, where zT = S2ρ−1κ−1T, and it is calculated from the Seebeck coefficient (S), electrical resistivity (ρ), and thermal conductivity (κ). The sintering time was less than one second, which is several orders of magnitude shorter than that of conventional thermal-sintering techniques.
The films also showed excellent flexibility with 92% retention of the power factor (PF) after one-thousand bending cycles with a 5-mm bending radius, Figure 3. In addition, a wearable thermoelectric generator based on the flash-sintered films generates a very competitive power density of 0.5 mW/cm2 at a temperature difference of 10 K.
Figure 3 The flexibility test of the flash-sintered films under different bending angles demonstrated the film’s resilience and longevity. Source: University of Notre Dame
They believe that ultrafast flash sintering assisted by machine learning will make it possible to produce high-performance devices much faster and at far lower cost than possible at present. The work is described in detail in their 12-page paper “Machine learning-assisted ultrafast flash sintering of high-performance and flexible silver–selenide thermoelectric devices” published in the journal Energy & Environmental Science; there is also a posted 17-page Supplementary Information file which provides additional insight and information.
A team at the prestigious Karlsruhe Institute of Technology (KIT) (Germany) has developed a way to produce TEGs using printable thermoelectric polymers and composite materials using a low-cost, fully screen-printed flexible design. Using a unique two-step “origami-style” folding technique, they produced a mechanically stable 3D cuboidal device from a 2D layout printed on a thin flexible substrate with thermoelectric inks based on PEDOT [poly(3,4-ethylene dioxythiophene)] nanowires and a TiS2: Hexylamine-complex material, Figure 4.
Figure 4 Details of the fabrication and folding technique. [Yellow: n-type material, blue: p-type material, gray: substrate material. Arrows indicate the current flow through the device resulting from an applied temperature difference (hot side: red, cold side: cyan). Dashed arrows indicate folding procedures.] a) 2D layout of four thermocouples on a substrate with an extra strip of unprinted substrate. b) Origami folded TEG with four thermocouples with inlaid substrate material for electrical insulation of the thermocouples. Source: Karlsruhe Institute of Technology
The device’s architecture resulted in a high thermocouple density of 190 units per cm² by using the thin substrate as electrical insulation between the thermoelectric elements, yielding a high-power output of 47.8 µW/cm² from a 30 K temperature difference. The device properties are adjustable via the print layout, and the thermal impedance of the TEGs can be tuned over several orders of magnitudes, thus enabling matching of the thermal impedance to any heat source, Figure 5.
Figure 5 a) A 2D print layout for an origami TEG with 254 p-legs (blue) and 253 n-legs (yellow) (green: overlapping area) arranged in a checkerboard pattern of 13 columns by 39 rows. b) Screen printed TEGs with TiS2 as n-type material and PEDOT as p-type material with extended contact fields of PEDOT deposited by calligraphy. c) First folding step stacking all columns plus one extra strip of substrate. d) Fully folded thermoelectric ribbon. e) Thermoelectric ribbon creased at the fold lines. f) Fully folded thermoelectric generator fixed with a Kapton ribbon. Source: Karlsruhe Institute of Technology
They tested the units under various conditions, Figure 6. The output power at the maximum power point (MPP) was high enough to supply low-power electronic circuits. The output power increased with ΔΤ² reaching 243 µW for ΔT = 60 K. Even for ΔT = 30 K, this device generated PMPP = 63.4 µW and an open-circuit voltage Voc = 534 mV, corresponding to a power density of 47.8 µW/cm2 while the internal resistance is 1124 Ω.
Figure 6 a) A histogram of the internal electrical resistance of the devices unfolded after printing (light) and after the origami folding (dark). b) TEG characterization setup with two copper blocks as thermal contacts. c) Open-circuit voltage versus applied temperature difference for TEG #6. d) I–V characteristics and output power versus voltage for different applied-temperature differences at TEG #6. e) Output power versus electrical load for different applied-temperature differences at TEG #6. f) Histogram of the maximal output power and the open-circuit voltages of all produced TEGs at ∆T = 30 K. Source: Karlsruhe Institute of Technology
As a practical test and demonstration of the usefulness, they built a self-powered weather station measuring ambient temperature, humidity, and pressure using off-the-shelf components including a Bosch BME280 sensor and a Texas Instruments power-management IC, all reporting via a BLE (Bluetooth Low Energy) interface.
Full details on their process, the deep analysis of the material-science physics behind it, and their test arrangements and results are in the eight-page paper “Fully printed origami thermoelectric generators for energy-harvesting” published in Nature; there’s also a 13-page Supplementary Information file which has further analysis as well as full weather-station construction detail, plus a 30-second video of the first stage of the production process.
Have you ever used a TEG, other than a basic bimetallic thermocouple, for energy harvesting or power capture? Did it work out technically or were there unexpected issues which made it an inadequate choice for “free” power?
This article was originally published on EDN.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.