2018 Update: Annex VIII: Co-operative program on the development of thermoelectric materials for waste heat recovery in transportation industries

International Round-robin Studies of Thermoelectric Materials and Devices
Annex VIII is addressing one of the most important issues facing the thermoelectric community: reliability of transport data. The round-robin results showed significant problems in two of the four measurements that determine the figure of merit (ZT). The test procedures developed by Annex VIII is critical to the transition from laboratory materials development to module and device production. It has significant impact on the commercialization and scale-up and manufacturing of thermoelectric devices.

First international Round-robin study:
The first round-robin on bismuth telluride was conducted in 2010 all the participating labs completed testing two sets of specimens (n-type and p-type). The results showed significant problem in thermal conductivity measurements (thermal diffusivity and specific heat). The largest measurement error came from heat capacity measurements. The Annex round-robin results were well received by the thermoelectric community. It was considered a timely and very important contribution.

Second international Round-robin study:
After the discussion on the first round-robin results, the second round-robin testing began in August of 2010 and was completed by August 2011. Two sets of p-type materials were measured at 7 labs in 4 countries. The results of round-robin 1 were given to all the labs as reference. The issues identified in round-robin 1 were discussed by participating labs. The same test instruments used for round-robin 1 were used for all the transport properties in round-robin two. All the labs completed tests on all specimens. In some cases, thermal and electrical properties testing were carried out at different labs within the same country.

Third international Round-robin study:
The 3rd International round-robin study was carried out from 2013 to 2015 focusing on high temperature (300-800 K) thermoelectric materials for vehicle waste heat recovery. N-type half-Heusler material made by GMZ Energy was used. Transport properties were tested by 13 labs in 6 countries. Following the test procedures developed by the annex, the results show significant improvement in uncertainties of the properties. Due to the error propagation in calculating ZT, scatter among various labs can be expected just above +15%.

Fourth international Round-robin study:
While the materials round-robins were coming to an end, annex VIII gather a small international team and conducted a survey of thermoelectric device efficiency testing. The results were published in 2015. From 2015 to 2017, the 4th international round-robin on thermoelectric devices was underway. Bismuth telluride modules from the Marlow Industries were selected for efficiency testing from 300 – 425 K. Test results showed large scatter in efficiency values mainly due to the uncertainties of determining heat flow though the devices.

Overview of Scope
Dr. Shengqiang Bai of Shanghai Institute of Ceramics, China took over this Annex by unanimous vote of the Executive Committee as the international round robin tests are drawing to a close. The scope will expand to include thermal management of electric vehicles (EV). The objective of the Annex now is to conduct cooperative research and development on energy efficiency improvement by new materials and techniques in thermal storage, transfer, and conversion, which will lead the exploration of the potential applications in marine, EV and HEV.

Activities/Accomplishments
As an environmentally friendly energy conversion technique, thermoelectric power generation devices based on the Seebeck effect have long been regarded as one of the best solutions to improve efficiency of energy consumption by turning waste heat directly into electric power. The most attractive features include all solid-state devices with no moving parts, no emissions and easy adaption to generate energy from any heat source. Some build-up demo system of exhaust heat recovery based on thermoelectric materials and realized a FE improvement of 1-3% with the output power of about 500 Watts. For the lack of international testing standard of thermoelectric materials and devices, the reported data from different affiants has a huge deviation. Focusing on the goal of testing standard, Annex VIII carried out a series of international round robin testing for both thermoelectric materials and modules. After systemic analysis, Annex VIII proposed the testing standard and published in the scientific paper, which was recognized by the thermoelectric society. In 2018-2019, the 5th international round robin test of thermoelectric module was in processing. Two half-Heusler modules have been selected. Fraunhofer Institute in Freiburg, Germany and SICCAS have exchanged modules and completed testing. The modules have been sent to ORNL. Due to the COVID-19, the round-robin test has been temporarily paused.

With the technical aids of Artificial Intelligence (AI) and Internet of Things (IoT), the accelerated approaching of automobile electrification makes the traditional automobile industry facing the unprecedented challenges. According to this transformation, the initial technical route focusing on the internal combustion engine (ICE) need re-evaluated and adapted to meet the future demands. Almost all the passenger cars in the future will be transformed into electric vehicles. Meanwhile, there is still some areas need ICEs, such as marine and extra long-range trucks. Energy efficiency improvement and emission reduction based on the new materials/techniques will be the important technical target of ICEs in the next decades.

Thermal management is one of the critical technical elements in electric vehicles (EV) or hybrid electric vehicles (HEV). For example, Battery Thermal Management (BTM) not only affects the energy efficiency but also the safety. The air-condition in EV, especially the Heating Ventilation Air Conditioning (HVAC) is a high energy consumption part, which critically reduced the driving range. In recent years, some attempts have been carried out in the BTM and HVAC system in EVs by introducing new materials or techniques (e.g. phase change materials) to reduce energy consumption.
With the goal of energy saving and emission reduction, Annex VIII will extend the research scope to developing new materials and techniques in thermal storage, transfer, conversion and utilize in the potential applications in marine, EV and HEV in the next years.