Development of Small Power Sources Based on a Micro-SOFC System Operated on Liquid Fuels for Mobile Electric Devices
Authors: Takahara Shinpei , Kato Koki , Iguchi Fumitada , Shimizu Makoto , Yugami Hiroo 
Published: February 2020

In this decade, the progress of computer technology have been enable mobile electric devices to be ubiquitous information tools, and recently, virtual space will corrode real space via such mobile electric deices. As a result, these mobile electric devices such as tablets, smartphones, and laptop computers require higher power and longer usable power sources than current Li-ion secondary batteries. However, the scientific and technical margins for further improvement of Li-ion batteries are being diminished, and new secondary batteries have been being eagerly developed. Separating power sources to be chemical energy resources and energy converters, i.e. fuel cells, is one solution from another aspect. Generally, fuel cells can convert the half of chemical energy of liquid fuels, which show significantly high energy densities than secondary batteries, to electricity. In addition, because energy conversion occurs on the two-dimensional surface of fuel cells, the reactive cell surfaces required for rated power output could be fold in limited space. Fuel cells developed in accordance with the concept by using fabrication technics of micro electro mechanical systems (MEMS) are called micro-FCs, and studied for two decades. Among them, micro-SOFCs can success high power density over 1 Wcm-2 at the temperature range of 400-500 oC with significantly thin cells (<1mm) are considered to be promising candidates for alternative power sources. However, case studies as whole power sources are limited, especially for small ones although high operating temperature requires high-level thermal management.

In this study, we focused on the two thermal requirements and one electrical performance: heat-self-sustainability, thermal consistency and an energy density, and evaluated applicability of energy conversion systems based on micro-SOFCs for mobile electric devices.

At first, heat balance during operation was thermodynamically calculated in the assumed operating temperature range of 200 oC to 500 oC, and excess heat, which could be lost from the system without temperature drop, was calculated. The value was smaller than 2W for 4W class micro-SOFCs, and decreased as operating temperature decreased. Then, package, which contain a micro-SOFC, was designed to satisfy thermal requirements: heat loss from the package surface should be lower that excess heat, and surface temperature should be harm-less for other electronics. Numerical calculation reveals that two requirements were satisfied by using high performance vacuum insulators at the operating temperature range of 200 oC to 400 oC with small package volume (< 20 cm3). However, at 500 oC, it was difficult to heat-self sustainability only by thermal insulation and the assist of heat recycle was suggested to be mandatory. Energy density calculated from the volume of a package and fuel increased as total power increased because the volume of high energy density liquid fuels was only increased. The system showed higher energy density than that of Li-ion batteries at the capacity larger than 20Wh, which values were used for smartphones and tables. These results confirmed applicability of energy conversion systems based on micro-SOFCs for small electric devices.

In accordance with the design, prototypes of the system, which consisted of vacuum insulator and fuel tank for heat exchange, was made as a demonstrator. System volume of the system could be reduced to 12.3 cm3. These prototypes revealed that the designed package could reduce heat loss from the package surface as calculated. Other test results such as internal reforming and power generation tests will be presented in the conference.