Thermoelectric Generators from Waste Heat to Electricity

A large amount of electrical energy consumed is released as waste heat from industrial and residential areas leading to global warming. As per a recent estimation, about 60 TWh is lost as waste heat from industries annually. Recovering just 2% of this waste heat can reduce about 0.5 Trillion grams of CO₂ emission into the environment [1]. This process of recovering the heat can increase system efficiency and lower power consumption. The conventional technique of converting the heat into electricity is used by the Rankine cycle heat engines, where the system maintenance and the size stands as a major problem for practical applications [2]. Solid-state semiconductor devices called Thermoelectric Generators (TEG) are explored in recent times for their excellent heat to electricity conversion efficiency [3]. TEGs works on the principle of seebeck effect, an applied temperature difference across the device induces the development of a voltage called seebeck voltage. TEGs doesn’t have any moving part like heat engines/steam turbines, and they are emission-free, a maintenance-free system with a life span over 25 years.

Figure 1. TEG device architecture and performance [4].

As we know, all machines from jet engines to microprocessors, generate heat, and in manufacturing processes ranging from steel to food production. So, TEGs are employed in a wide range of applications like waste heat harvesting from automobile engines, power source for wireless sensors, power source for deep space exploration and also as wearable electronics.

Automotive/internal combustion applications

Heavy vehicle’s internal combustion engines release the heat between 400-500^oC through the flue gas exhaust. This arises the concern of environment organization to reduce the emission and reuse the heat for air conditioning in the vehicles. In this aspect, a European company has introduced a product called VIPER2. This extra module, when attached to the exhaust pipe, can generate electric power to run the air conditioners, lighting systems inside the vehicle. Many leading automobile manufacturers like Land rover, BMW, Ford and Jaguar have implemented this system to increase the fuel efficiency.

Aerospace applications

As we move away from the sun, the spacecraft loses the solar radiation, and using photovoltaic modules for power generation is impossible. In this aspect, TEGs plays a major role in deep space exploration projects. NASA’s space probes Voyager 1 and Voyager 2 was launched in 1977 and it still operates by the power generated from the radioisotope TEGs. Recently, commercial and military aircraft uses sensors and sensor networks powered by thermoelectric generators to monitor the aircraft skin for damage that can cause stresses and structural weakness.

Wearable and bioimplant applications

Figure 2. Wearable Thermoelectric Generators [5].

A healthy human body always maintains a temperature of 37^oC irrespective of the environment. This temperature generated from the human body is used to generate power through TEGs. In the future, wearable electrics will play an inevitable role in our day-to-day life for monitoring our physical activities, health conditions and social networking. Using Li-ion batteries in gadgets has become more dangerous due to their explosions. In this aspect, TEGs replace the role of batteries by powering the gadget through our body heat. Wearable and flexible thermoelectric generators are being widely studied for these applications [5,6].

Unlike Photovoltic technology, owing to the fabrication difficulties, there are very few commercial manufacturers for the TEGs namely Marlow, Ferrotec and TE technology. To employ TEGs in more practical applications, the device needs to be shape conformable over the heat surface and have low device fabrication cost. The brittle nature of the thermoelectric materials hinders implementation in heavy industries. Researchers are working on different strategies to overcome these disadvantages through material processing techniques like spark plasma sintering, selective laser sintering and additive manufacturing.

In conclusion, TEGs are the future of renewable energy and wearable electronic technology. Because they are easily scalable for microwatt to megawatt power generation projects, moreover, they are emission-free and compact to be integrated with any heat-emitting systems. TEGs are easily disposable in the environment comparing to the toxic battery wastes. The use of TE technology to recover the waste heat released from thermal power plants can save over 5% of its power generation cost. To achieve the goal of zero carbon emission 2050, various governments around the globe have introduced their policies to use TEGs.

References

R. Freer and A. V Powell, “Realising the potential of thermoelectric technology: a Roadmap,” J. Mater. Chem. C, 8, 441 (2020).
L. E. Bell, “Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems,” Science, 321, 1457 (2008).
S. Twaha et al., “A comprehensive review of thermoelectric technology: Materials, applications, modelling and performance improvement,” Renew. Sustain. Energy Rev., 65, 698 (2016).
T. Sugahara et al., “Fabrication with Semiconductor Packaging Technologies and Characterization of a Large-Scale Flexible Thermoelectric Module,” Adv. Mater. Technol., 4, 1 (2019).
V. Karthikeyan et al., “Wearable and flexible thin film thermoelectric module for multi-scale energy harvesting,” J. Power Sources, 455, 227983 (2020).
Y. Du et al., “Flexible thermoelectric materials and devices,” Appl. Mater. Today, 12, 366 (2018).

Blog Written by

Dr. K. Vaithinathan

Department of Materials science and engineering

City University of Hong Kong, Hong Kong

Editors

Dr. A. S. Ganeshraja

Dr. K. Rajkumar

Dr. S. Chandrasekar

Reviewers

Dr. Y. Sasikumar

Dr. S. Thirumurugan

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