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Organic Thermoelectric Generators

Final Report Summary - OTEGS (Organic Thermoelectric Generators)

In response to the thread of environmental and ecological degradation along with projected fossil fuel depletion the active search for efficient renewable energy conversion technologies has been attempted in various research areas including the field of thermoelectrics. Despite the availability of considerable amounts of waste and natural heat stored in warm fluids (<250 °C) a lack of environmentally friendly materials with high natural abundance, low manufacturing cost and high thermoelectric efficiency impedes the widespread use of thermoelectric generators for energy harvesting on a large scale. Today’s thermoelectric materials of choice for low temperature energy conversion are bismuth antimony telluride alloys. The constituting atomic elements have a low natural abundance and therefore expensive. Additionally, low natural abundance goes hand in hand with some level of toxicity for the environment. Thereby, before our discovery it was unthinkable to enable a widespread use of thermoelectric installations for solar and waste heat recovery.
Our findings provide an amazing opportunity. We have demonstrated that widely available (constituting elements of high natural abundance (e.g. C, S, O, N)), although less efficient organic thermoelectric materials, such as conducting polymers, emerge as a new class of thermoelectric materials. The chemical synthesis of such materials can be readily scale-up for mass production. Those conducting polymers are not toxic, they are even biocompatible. In TEGs, half of the cost is from the material and the other half is from the manufacturing process. Beside the low material cost, organic thermoelectric material can be processed, patterned at relatively low-cost via low-temperature and/or solution-based manufacturing processes. Finally, large-area OTEGs could be made flexible, adaptable to various shapes in new re-designed heat exchangers or integrated in flexible solar cells. Thermoelectric generators (TEGs) are semiconducting electronic devices that transform a heat flow into electricity. There is no mechanical part that move and worn out. My research is related to the heat-to-charge conversion, i.e. the thermoelectric conversion in materials. The material conversion efficiency is given by the so-called thermoelectric figure-of-merit ZT (T is the temperature). We demonstrated that organic conducting polymers is a new class of thermoelectric materials for low temperature applications [20-250C]. We showed a first strategy to increase their thermoelectric properties by tuning the oxidation level of the polymer. We reached a ZT= 0.25 at room temperature; while the best inorganic thermoelectric materials bismuth telluride alloys reach ZT=1.1. We also fabricated the first organic thermoelectric module as proof of concept. Several research groups all around the world are now actively working in this new field of research. We explain that those conducting polymers can be semimetals. We demonstrate yet another strategy to increase their thermoelectric performance: decreasing the molecular disorder. This is the first demonstration of a semi-metallic polymer with obvious advantages for thermoelectric applications. The potential impact of these new materials is provided by the theoretical limit for the efficiency of an ideal thermoelectric module; which is about 2% [4%] for ZT=0.3 [0.6] @RT and a temperature gradient of 100C. Although much work is needed to truly exploit the potential of organic thermoelectric materials into efficient thermoelectric generators, our findings might become part of the multiple solutions to the energy and environmental issues in our society by collecting 2-4% of the huge amount of thermal energy stored in warm fluids and solar radiation.