Mitsubishi Hitachi Power System (MHPS) has been striving to combine is solid oxide fuel cell, called MEGAMIE, with a micro…
Each MEGAMIE unit uses a cell stack – a cylindrical substrate tube designed to trigger reactions for power generation. Cell stacks are made entirely of ceramics and take about a year of development.
The pressurizing system MHPS uses in MEGAMIE combines the delicate ceramics with a gas turbine that must withstand extreme temperature and pressure conditions. These different components had conflicting properties but they had to be integrated into a single complex system. “Many industry-leading players and research institutes have tried to commercialize similar fuel cells,” Kobayashi noted, “but combining these technologies proved to be too difficult.”
Two-stage power gen process
Power generation takes place in two stages within the MEGAMIE system: within the solid oxide fuel cell (SOFC) itself and within the micro gas turbine. Heat is removed from the high-temperature turbine exhaust gases to produce steam or hot water.
Versatile in fuel use, MEGAMIE can run on multiple types of fuel gases – from city gas and LPG in local infrastructure to methane gas from sludge, food waste and agricultural waste. The system can also leverage pressurized gas, as in conventional power systems which use gas turbines.
"Pressurized gas produces more power,” Kobayashi explained. “When you look at the shape of the cell, you notice it needs to be sealed only at two locations at both ends of the cylinder. That is sufficient to shield the fuel flowing inside the cell from the air outside. With fewer sealing locations, the cell could be more readily combined with gas turbines.”
Cost barriers and quality challenges
Cost is one of the barriers to greater market penetration. The first commercial 250 kW class MEGAMIE started operation in 2019 at the large Marunouchi Building, but going forward, “MEGAMIE must be offered at a much more reasonable price to increase adoption across the globe,” MHPS noted. To that end, preparations for mass production of the cell stack have been underway with NGK Spark Plug, a top ceramic manufacturer.
Increasing production yield and ensuring quality control throughout the supply chain were other big challenges. The “balance of plant” (BOP) was of particular concern, and for MEGAMIE, this term applies to micro gas turbines, heat exchangers, piping, valves, and electrical components.
Kobayashi needed to ascertain that the suppliers of BOP components would be willing to provide the parts in good condition even as the fuel cell had yet to go to market. “To ensure the quality of all raw materials, you have to deploy your people to the manufacturers’ factories,” he said. “Project members also kept talking to the partners. They negotiated costing of the BOP components and made improvements to boost the performance of the SOFC.”
Another issue is how to ensure safe and efficient operations. Polymer electrolyte fuel cells used for automobiles work within a relatively low-temperature range of 60-100°C; thus, start/stop functions would not pose major difficulties. However, SOFCs work in temperatures as high as 900°C, and take much longer to start or stop.