PhD defence by Seyedmohammad Mortazavinatanzi on The Manufacturing Processes of Flexible Thermoelectric Generators
11.03.2021 kl. 12.00 - 15.00
Seyedmohammad Mortazavinatanzi, Department of Energy Technology, will defend the thesis "The Manufacturing Processes of Flexible Thermoelectric Generators"
The Manufacturing Processes of Flexible Thermoelectric Generators
Professor Lasse Rosendahl
Associate Professor Alireza Rezaniakolaei
Associate Professor Torsten Berning
Associate Professor Kaiyuan Lu, Dept. of Energy Technology, Aalborg University (Chairman)
Professor Andrea Reale, University of Rome Tor Vergata
Professor Ngo Van Nong, Nagoya University
It has been widely accepted among the scientific community that the rising greenhouse gas level is the main contributor to global warming. Greenhouse gases reduction by 20% compared to 1990 or by 30% (if all the countries involved are committed to playing the roles equally) as well as enhancing the share of the renewable source of energy to 20% in the overall energy consumption are the major objectives of the Paris climate agreement in 2015. The main contributors to the increasing trend of greenhouse gases are power plants and fossil fuel consumption. Nevertheless, a significant portion of the generated energy is released in the surrounding environment in waste heat, both in the industry, residential and transportation sectors. The amount of waste heat also significantly large for the transportation sector (60% energy loss). Generally speaking, only 20% of the global energy consumption is converted to effective work, while the rest is lost in the form of waste heat.
Thermoelectric devices are solid-state technology that can convert waste heat into useful electricity. They generate a voltage potential upon applying a temperature difference across them. Commercial thermoelectric devices typically consist of arrays of semiconductor materials sandwiched between the two parallel electrical insulative sheets. Their solid-state nature introduces them as reliable solutions for waste heat recovery applications compared to other techniques such as the organic Rankine cycle (ORC) based waste heat recovery. Besides, thermoelectric devices can be utilized for direct electrical energy generation by imposing them on a renewable source of heat energy, such as solar radiation. In this case, the device is called a solar thermoelectric generator (STEG), which can be considered a sustainable solution for solar energy conversion.
Despite the mentioned advantages, the low efficiency of the thermoelectric materials, high material cost, and lack of a scalable manufacturing method have been the main barriers to the widespread utilization of thermoelectric devices. There have been many attempts to develop efficient and low-cost thermoelectric materials. until now, bismuth telluride has shown the best thermoelectric energy conversion efficiency at room temperatures (4% to 7%). This relatively low performance is not sufficient for current commercial thermoelectric material to be adopted for large scale industrial waste heat recovery. It is also worth mentioning that the most efficient thermoelectric materials contain rare earth elements that can lead to a semiconductor market disruption in large-scale utilization. Consequently, any effort in material development research should be in the direction of finding new materials that are earth-abundant to solve the material scarcity problem.
On the other hand, finding inexpensive and efficient thermoelectric material does not tackle integrating these materials into a final applicable device. In other words, the lack of scalable manufacturing methods for high-throughput fabrication of thermoelectric devices leads to the high cost of thermoelectric systems and limitations in the areas of applications. In this Ph.D. thesis, one of the main goals is to develop a new manufacturing concept for industrial-scale automated manufacturing of thermoelectric devices. The high-throughput fabrication results in lowering the total thermoelectric system cost and availability for large volume applications. The commercial thermoelectric devices are also fabricated in small sizes and non-flexible structure (rigid). Besides, most of the waste heat is emitted from the large area and none flat (curved) surface into the surrounding environment.
Consequently, introducing fabrication methods to bring flexibility to thermoelectric devices has been the focus of many researchers' attempts. These works can be categorized into two different directions: flexible thermoelectric materials development and fabrication of flexible devices through rigid thermoelectric materials. The first category mostly has studied the polymer-based organic thermoelectric materials, which can be applied in printable inks or pastes. Regardless of proving high mechanical flexibility, these groups of materials suffer from low thermoelectric efficiency. In the second direction, the same bulk thermoelectric materials applied in commercial devices have been used in a flexible structure to provide the required level of flexibility. As the main part of this Ph.D. thesis, the same strategy (category two) was chosen to develop a manufacturing platform for automated high-throughput fabrication of flexible thermoelectric devices. The proposed concept is similar to a method called Flexible Hybrid Electronics (FHE), which enables the combination of rigid silicon electronics and printed electronics to provide a flexible printed electronics product. Nano-silver (Ag) bonding also was investigated to bond the bulk thermoelectric materials on top of a flexible substrate. This relatively novel bonding technique, which is increasingly used as the die-attach material in power electronics, shows promising performances compared to the conventional bonding materials for thermoelectric fabrication like low-temperature solder pastes.