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The ABSOLUTE Hybrid: Combining a Solid Oxide Fuel Cell with a ZEBRA Battery

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.




What kind of well-to-wheels efficiency would we be talking about?


IMHO, PEM fuel cells are a legacy of GM's effort to build a fuel cell car without a battery buffer. The early designs in the latest round of enthusiasm (about 7 years ago) were large (80-90kw) fully throttlable fuel cells. In a design like that dynamic response and power density are key, hence the use of PEM. At the time I recall an ex-Ballard engineer saying "I don't know why they're not trying SOFC with a large battery". The reason, at the time, was that batteries were expensive, hence they should be eliminated, not mention Ballard's patent focus was PEM. But experience has shown dynamic PEM FC's are much more complex and expensive than batteries. Even GM has returned to battery dominant "buffering" in the "Shanghai Volt" concept. Let's hope this means researchers and automakers have finally found a "sweet spot" in the long laundry list of tradeoffs for advanced hybrids.


From the summary:

"For a light delivery van operating with 6 h drive time per day, a fuel cell system model predicted a gasoline equivalent fuel economy of 25.1 km L−1, almost twice that of a gasoline fuelled ICE vehicle of the same size, and CO2 emissions of 71.6 g km−1, well below any new technology target set so far."

That's 59 MPG. The delivery van model weighs 2712 lbs, 40% regen capture, 0.4 Cd, 2 sq meter frontal area.


Wouldn't it be great if the net power to separate H2 was more greatly positive and you didn't need such large heavy tanks to store it; thus the reason that H2 isn't practical unless these problems are solved. Some advances have been made; however, mass production H2 fuel usage is still in the distance future, if ever. At this time H2 is only good for those of us who dream about a perfect fuel and enjoy reading engineering exercises.

We have an immediate problem to solve for the welfare and health of not only our country, but the world: How do we minimize the usage of fossil fuels for automobiles! I see only one path to solving this problem. Move to electric transportation as soon as possible. PHEVs are possible right now and will eventually lead to the mass use of BEVs. H2 powered cars are not practical. And, the use of even more complicated diesel ICEs burning biodiesel still doesn't solve this problem. Biodiesel fuel still pollutes the air with harmful emissions and creates GHGs.

Stan Peterson


That is a pretty small delivery van. The vehicle weight approximates a mini-compact car. Still 59 mpg is impressive but I don't see how this displaces much hydrocarbon fuel.

The battery certinly can be pluged in and charged, but the fuel cell operates always on, and just keeping it cooking at 7-800 degreess Centigrade, even when shut down for the night will alter the total system mileage quite a bit, when that is accounted for. Ditto fro cooking the abttery to keep it at temperature.


You are quite correct. While possible I think there are other better energy shifting mechanisms like PHEVs and BEVs based on other battery and prime mover methods that may offer a more practical vehicle.


"How do we minimize the usage of fossil fuels for automobiles!" (?)

The answer is: we don't. It's currently not politically possible. The good news is we are rapidly exhausting the easy supply. The bad news is that many of the alternatives are even more polluting.

"Biodiesel fuel...creates GHGs."

Not really. It just releases those that were taken up initially while the feedstock was growing.


Lad: Despite the fact that intuitively a PHEV is a lot closer to the current reality than BEVs, I'm not fully convinced that PHEVs are all that much easier than BEVs to produce. Unless you're talking about a parallel hybrid, the battery needs to be pretty big just to provide the power that you need. By the time the battery gets that big it will contain a considerable amount of energy. On top of that the BEV doesn't require an engine at all. That's a lot of moving parts. Of course an ICE engine is a thoroughly understood beast that is already produced cheaply.


Back on topic: I think this is a clever idea that would make the most sense for fleet vehicles that don't have to sit around for any length of time.


From a well-to-wheels point of view, there are a couple of cool advantages with SOFC.

1. They can burn "wet" ethanol. Wet ethanol has a higher percentage of water, is less distilled, and hence requires less energy to create and can be sold much more cheaply. For that matter, SOFC's can run on a wide range of fuels, which completely gets you away from the bad math of hydrogen creation, transportation, and storage.

2. The total system efficiency can be pretty good, because you can repurpose the heat for additional power generation (maybe even a closed loop microturbine generator) like cogeneration.

3. There are some pretty incredible insulating materials now to minimize heat loss.

4. Running a very small SOFC continuously is a bit like having a 3-cylinder ICE generator running at most-optimum RPM to charge a series PHEV. It avoids the inefficient parts of the power curve.

Neil, I agree this is not for every kind of drive cycle. It sucks for Sunday drivers, taxicabs, or even people who drive daily, but not very far.

There are other challenges with SOFC. Even the tube variety are still made of brittle ceramic material that may have durability issues. The cost and power density are still, um, prohibitive. However, since a lot of the techniques are similar to microchip challenges, power densities should continue to go up while costs/KwH go down.

It sounds like their approach is to have a small fuel cell to save on costs and volume.

At scale, the Imperial College crew seems to envision a 3KwH ITSOFC, cranking out ostensibly 72 KwH/day for use in a 450 ah battery. That sounds like 1.5 hours/day of drive time in a small car to me. Does that check with anybody else's numbers?


I am all for fuel cells, but I question the usefulness of this scheme.  If the fuel cell has to operate for 24 hours to allow the vehicle to operate for 6, why not just make the battery 33% larger and eliminate the fuel cell?

Sure, the FC does a better job of converting fuel to work than a combustion engine.  I'm sure it would do even better running 24/7 as a grid-connected base-load powerplant.  The next step could be converting waste biomass to charcoal plus fuel gas for the FC.  The vehicle could charge overnight, the FC runs other loads during the day, the waste heat does the charcoal conversion and provides ancillary heat, and the charcoal goes for whatever purpose is desired.


I really don't think SOFC are the way to go for cars, maybe for submarines and small powerplants but there just to big and expensive for cars. Why not Alkaline Fuel Cells: They are much cheaper to make then PEM or SOFC because they don't need expensive catalysts or fancy ceramics and their more efficient and have lower temp operation. AFCs CO2 poisoning has also been solved via CO2 repelling membranes, CO2 absorbents in the electrolyte ("change electrolytes every 3000 miles"), or using alkaline fuels such as mostly reformed ammonia or zinc paste.

Brent Pieczynski

Legalize hemp as a coffee substitute and tax it, taxes can fund research into how to use the by-products. Nations with people to feed can become fuel producers, food development and agricultural-byproducts often exist together, which allows alternative-energy source creation. Does anyperson know about research done by Ford about using a hemp product which is lighter than steel, for the vehicle body? Henry Ford was correct, hemp is good for at least one thing.


the automotive supplier DELPHI did some studies about SOFC APU's.

With 1-5KW SOFC we will get close to 100mpg ...


Hemp is good for a lot of things, but I don't think coffe is one of those. Rope, carbon fiber panels, cooking oil, hemp seed food products, biodeisel fuel, etc are all good uses of hemp and should be encouraged. But let's not get carried away seeing hemp as a cure all to our problems. Were you thinking of the coca plant instead?

Industrial Hemp is not smokeable (its THC content is way to low) it’s a great energy crop for making ethanol, biodiesel and plastics (or just use the hemp fibers outright), its only disadvantage is that it looks just like the hemp meant for getting high. Munchy-fun-time Hemp is not as good of an energy crop as industrial hemp because of poor fiber and oil concentrations. Coca on the other hand: if why got everyone on that they could just run to work and back everyday no cars needed!

Roger Pham

Quite an ingenious idea for a commercial vehicle with a predictable daily routine. The synergy between IT-SOFC and ZEBRA battery is that they both require high temperatures to operate, thus the waste heat from the IT-SOFC is used to maintain operating temperature without having to waste battery power to generate heat.
"Why not just enlarge the battery 33% and dispense with the FC?" For the above reason: waste heat from the SOFC can be put to good use. Remember that ZEBRA battery is much cheaper and has other advantages above current Li-ion batteries, while having equivalent energy and power density. The reason that ZEBRA battery is not widely used because it requires high temperature, thus wasting heat energy at start up or when not in used. The high cost of SOFC is overcome by using a very small-size unit running 24/7.

"The efficiency of 59 mpg" for a 2700-lb van is good, though not impressive in comparison with that of the Prius II, weighing 2900 lbs and rated at 55 mpg combined cycle.

Wait a minute: I remember that the Prius II is rated at 104 gm of CO2/km emission. This van is rated at 72 gm of CO2/km emission. Dividing 55 mpg by 72 and multiplying by 104 should give an equivalent mpg of 79 mpg, or rounded off to 80mpg! Now that's real impressive!

Henry Gibson

An engine that is only used for charging a battery need only to have one piston and can operate at either the most efficient speed or the most powerful speed as needed. This speed can be very high even more than 20,000 rpm. Single piston engines are more efficient per horsepower, and electromagnetic dampers can eliminate most vibration. Rotary valves will allow very high speed operation. This puts a lot of power in a small package that costs much less than any possible fuel cell. The series hybrid could even use a microturbine with air bearings and not a drop of oil or antifreeze like Capstone and run at 150,000 rpm. fifteen HP would be enough, but engines are more efficient and could be hand lifted out of a vehicle like OPOC.

Now with the TH!NK, Calcars can add a huge model airplane engine to it to get a plug in hybrid with perhaps a ten HP engine. This is similar to the generating trailer for the TZERO.

Just one more thing. With the battery giving instant action, a fully automated charcoal burner could be started in the period of five or ten minutes to produce CO for an engine and start the engine generator, and if the car were in open space when it stopped the burner could continue until the battery was fully charged, or the system could be progammed to use electricity until there was a need for the generator. The automated burner could also be made to use any liquid fuel to make CO or just burn the liquid directly in the engine.

Researchers at a Hawaian university know how to make charcoal from any plant material. Why bother with cellulostic ethanol; run cars on processed grass clippings.

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