Defying skeptics who said reformers were not feasible in micro fuel cells, UltraCell developed a portable fuel cell power source that runs off reformed methanol, achieving higher performance than comparable direct methanol fuel cell (DMFC) systems. At the heart of this fuel cell system is a proprietary “micro reformer” that converts a highly concentrated methanol solution into fuel-cell-ready hydrogen. The micro reformer has been combined with a fuel cell stack, electronics, and other elements to create a prototype fuel cell system for the U.S. Army’s Communications-Electronics Research, Development and Engineering Center (CERDEC). This organization recently awarded UltraCell a contract to deliver a 25-W pocket-sized fuel cell power system to assist U.S. forces on extended missions.
Powering a micro fuel cell off of reformed methanol has several advantages versus the direct methanol approach. Because the reformer delivers hydrogen to the fuel cell stack, the fuel cell stack achieves higher power density and generates a higher output voltage on each cell in the fuel cell stack. (The open circuit output of the stack is 11 V, which is then stepped up to either 12 V or 24 V by a dc-dc converter within the fuel cell unit.)
According to Norm Allen, cofounder and chief operating officer of UltraCell, running the fuel cell off hydrogen doubles the power density-per-square centimeter on the fuel cell’s membrane assembly versus the power density in a DMFC membrane. As a result, the hydrogen-fed fuel cell requires only about half the number of cells in the fuel cell stack, saving space and material cost.
While providing the high performance associated with hydrogen, the reformer also maintains the convenience associated with using methanol fuel cartridges, which are an attraction of DMFCs. However, the reformer runs off of a more highly concentrated form of methanol than a DMFC. Allen notes that the fuel used in UltraCell’s fuel cell system is 67% methanol versus just 3% to 5% methanol for the typical DMFC.
In addition to higher power density, another benefit of using reformed methanol versus the DMFC approach is that the fuel cell produces very little residual water. Consequently, UltraCell’s fuel cell system requires no mechanism for handling waste water that would be a byproduct of a DMFC.
According to UltraCell, it is the first company to integrate a reformer within a micro fuel cell.
Although it won’t reveal the techniques used to create this micro reformer, Ultracell says it had to overcome daunting engineering challenges in thermal management and packaging. Specifically, the fuel cell system must allow for very high internal temperatures, while keeping the case temperature of the system within safe levels. The micro reformer operates at 270°C, while the fuel cell stack operates between 150°C and 180°C.
Through proper insulation and air cooling, UltraCell maintains these high internal temperatures while keeping the external case temperature on the fuel cell system low. In the current, alpha-stage fuel cell prototypes, the case temperature is around 50ۥC, said Allen. However, going forward, it’s expected that this case temperature will be further reduced as commercial applications such as laptops require a case temperature closer to 45°C.
Ultracell’s fuel cell technology is not considered a direct battery replacement, but rather a runtime extender. Its benefits are exemplified by the XX25, the 25-W prototype fuel cell systems UltraCell developed for the U.S. Army. Weighing just 40 oz, the power unit is about the size of a paperback novel. Without the methanol fuel cartridge, the unit measures 91 mm × 210 mm × 35 mm and has a volume of 730 ml. A single methanol cartridge adds another 200 ml to the unit.
Because fuel cell runtime depends on the number of fuel cell cartridges, a comparison with battery packs must be made on the basis of required runtime. In military applications, 72 hr represents a typical runtime figure. For that runtime, the XX25 achieves a gravimetric energy density of 550 wh/kg with a similar value for volumetric energy density in wh/l. Those values are significantly higher than the energy density values associated with primary and secondary battery packs. Note that the proper comparison is with battery packs rather than individual battery cells.
Ultracell states that, based on a 72-hr mission at 20 W, the Army expects the XX25 to have up to a 3-to-1 advantage over lithium primary batteries, such as the BA-5590 currently in use, and up to a 4-to-1 advantage over currently available military rechargeable batteries. Longer missions at higher power levels will show greater improvements.
Note the XX25 itself uses a small internal battery for startup since the fuel cell has a warm-up period. Typically, that warm up takes between 15 min and 20 min. However, the company expects to reduce that warm up to about 10 min. But because of this warm-up period, a battery is still required in the target application, which is why UltraCell’s fuel cell system is positioned as a runtime extender. The company plans to field test several hundred units of the XX25 in military applications next spring with production likely to follow in 2007.
This power source also is being developed for commercial use as the UltraCell25, which will be available for professional, industrial and mobile computing applications in 2006. Laptop computers represent a key target for the commercial product, and UltraCell is seeking OEM partners to develop the technology for laptop applications. In such applications, development issues will revolve around the requirements of “hybridization”—properly using the external fuel cell power source in combination with the laptop’s battery. Although a laptop may have peak power requirements of 60 W to 70 W for short periods, it is not cost effective to size the fuel cell to deliver that power. However, by intelligently managing power from the ac adapter, the fuel cell and the battery, it’s possible to size the fuel cell unit appropriately (cost effectively) and still achieve the desired extension of laptop runtime.