|The IT-SOFC/Zebra powertrain for the ABSOLUTE hybrid. Click to enlarge.|
An engineering team at Imperial College London has developed and bench-tested a series-hybrid powertrain that combines an intermediate-temperature solid oxide fuel cell (IT-SOFC) with a ZEBRA sodium nickel-chloride battery.
Dubbed the ABSOLUTE Hybrid (Advanced Battery Solid Oxide Linked Unit to maximize Efficiency), the prototype power system pairs a 300 watt SOFC stack from Versa Power Systems with a 45 Ah sodium nickel-chloride battery. This type of relatively low-weight, high-performance ZEBRA battery (manufactured by Beta Research & Development Ltd.) has been used in experimental electric vehicles for the past decade.
The ABSOLUTE powertrain combination exploits thermal synergy between the two technologies to overcome known limitations of each.
Solid Oxide Fuel Cells operate at higher temperature than PEM fuel cells, and have greater fuel flexibility—the heat produced by the SOFC can service a fuel reformer. They have lower power density than PEMFCs, however, and poor thermal and redox cycling properties—they don’t like start/stop cycling. They also have long start-up times from cold.
An intermediate-temperature SOFC operates at a range lower than that of high-temperature SOFCs—700°-800°C in the case of the ABSOLUTE unit. The IT-SOFCs offer faster start-up, better thermal and redox transient performance, simpler system requirements and lower cost. Hybridization with a battery to accommodate the maximum and dynamic load can further offset some of the issues with high-temperature SOFCs.
ZEBRA batteries have an established track record in transportation applications. Gassing is not an issue, and they don’t self-discharge; capacity is independent of discharge rate; they have a high charge/discharge efficiency; and they have a high specific energy (around 120 Wh/kg for a complete system). They also offer an excellent safety record; have been demonstrated in more than 200 electric and hybrid electric vehicles worldwide; and are highly durable, with a long calendar life.
However, they require a high operating temperature of around 270°-350° C—the high temperature is required to ensure adequate ionic conductivity of the β-alumina electrolyte. As a result, the battery must be kept hot when not in use to be ready for operation. Battery energy is used to sustain temperature.
In the ABSOLUTE design, the battery provides maximum power and satisfies load transients. The fuel cell operates in a predominantly always-on mode, during drive and non-drive time (so avoiding excessive redox and thermal cycling during stop/start operations).
The SOFC supplies constant power to the battery during drive and non-drive time, thereby reducing exposure of the fuel processor to load transients. The battery is thus not reliant on a grid charge, although it can certainly plug-in. The intention of the design, says the Imperial College team, is to minimize the size of the IT-SOFC and DC/DC converter.
The SOFC works with a fuel processor, which uses heat generated by the SOFC as well as from the battery. In turn, the fuel processor can supply heat to the battery.
The Imperial strove for charge neutrality over 24 hours in the system design—i.e., full discharge and recharge over a 24 hour driving/non-driving cycle. The bench-top system is about 1/10th the size of an actual vehicle system.
Researchers modeled out different drive cycles for different sized vehicles, with different fuels—compressed hydrogen, CNG and LPG.
So far, they have found that LPG and CNG deliver substantial range and fuel economy, although compressed hydrogen is not suitable because of range limitations due resulting from storage requirements. Vehicle modelling implies that the ABSOLUTE hybrid will suit small to medium size vehicles run up to 10 hours per day.
Design and Control Options for an Intermediate Temperature Solid Oxide Fuel Cell / ZEBRA Battery Hybrid System for Vehicle Applications (Project ABSOLUTE) (presentation)
P. Aguiara, D.J.L. Brett and N.P. Brandona; “Feasibility study and techno-economic analysis of an SOFC/battery hybrid system for vehicle applications” Journal of Power Sources (In press) doi:10.1016/j.jpowsour.2006.12.049