image: Various types of thermoelectric power generator modules, including multistage ones.
Credit: Korea Electrotechnology Research Institute
Drs. SuDong Park, Byungki Ryu, and Jaywan Chung of the Korea Electrotechnology Research Institute (KERI) developed a new thermoelectric efficiency formalism and a high-efficiency multistage thermoelectric power generator module. This innovation can boost nuclear battery performance, crucial for space probes, and has attracted attention from the German Aerospace Research Institute.
A Radioisotope Thermoelectric Generator (RTG), known as a thermoelectric-based nuclear battery, is a dependable power source that has been used in space probes, rovers, and other remote operations. In an RTG, radioisotopes like plutonium-238 and americium-241 decay within a sealed vessel, producing substantial heat—typically ranging from 400-700 degrees Celsius. The RTG captures this heat and directly converts the thermal energy to electrical energy in the cold environment of space.
The core components of RTG technology are the "Radioisotope Heat Unit (RHU)", which harnesses radioactive isotopes as a heating element, and the "thermoelectric power generator module" that converts this heat into electricity. While the development of the RHU is constrained by international restrictions, South Korea's thermoelectric module fabrication technology is considered to be globally competitive.
In RTGs, thermoelectric power modules are designed with a layered arrangement of thermoelectric materials, transitioning from the hot to the cold sides, each optimized for peak performance within specific temperature ranges. This multistage design is crucial given the inherent temperature dependence of thermoelectric material efficiency. Strategically positioning the top-performing materials based on temperature distribution is essential. KERI's landmark accomplishment is their world-class design, synthesis, and analysis of this highly effective layered thermoelectric module.
Initially, the research team identified the shortcomings and constraints of the 'dimensionless thermoelectric figure of merit (ZT)', a traditional metric conventionally used in academia to evaluate thermoelectric performance. They then successfully formulated a new thermoelectric efficiency formalism and equations that allow for precise efficiency predictions. Leveraging this formalism and the thermoelectric data held by KERI, they can predict the performance of thermoelectric power generator modules across more than 100 million potential thermoelectric semiconductor stack combinations. By utilizing the thermoelectric device design program, pykeri, this design and search process has been expedited by several hundred times compared to previous methods. This innovation marks a substantial leap forward from earlier approaches that depended on single-stage thermoelectric materials and the traditional metric.
The KERI research team successfully fabricated multistage thermoelectric modules, achieving an efficiency that surpasses traditional single-stage modules by over 3% when the hot side exceeds 500 degrees Celsius.
Additionally, their innovative fabrication method permits these modules to be comprised of two to four layers, all fitting compactly within a height of just a few millimeters. This advancement not only ensures heightened efficiency but also offers superior compactness and a lightweight design compared to previous methods. Such an internationally competitive milestone stands out prominently in the space auxiliary power market—particularly for small satellites and exploration rovers—garnering significant attention in the civilian commercial sector.
SuDong Park of KERI remarked, "We are the first institute in Korea to conduct thermoelectric power generation research and have a long history and abundant source technology and practical data." He further added, "This achievement is the culmination of convergence research that incorporates mathematics and physics into materials science."
“The module technology developed at KERI is excellent when compared internationally” said Pawel Ziolkowski, Deputy Head of a group of Thermoelectric Functional Materials and Systems, at the German Aerospace Center, adding that "The achieved level of technological maturity provides the best conditions for the development of new RTG-based energy systems for space exploration. This makes a significant contribution to an expanding scope of human space exploration."
The research team believes that this achievement has applications not only in the aerospace and defense sectors that utilize nuclear energy but also in various industries such as industrial waste heat recovery, cooling of communication equipment and optical devices and temperature control of electric vehicle batteries, and plans to strengthen cooperation with related organizations and companies.
Meanwhile, KERI is a government-funded research institute under the National Research Council of Science & Technology of the Ministry of Science and ICT.