Nanostorage May Enhance Gas-Fueled Autos’ Efficiency

Scientists are developing new storage systems that may allow automotive engineers to build more cars that can operate more efficiently on gas-based fuels.

Nanogas systems are built out of metal organic frameworks that can store more gas, such as methane, hydrogen or carbon dioxide, than empty cylinders. It may sound like science fiction x –  a vessel that is larger on the inside than the outside – but MOFs are crystal-based solids with nanoscopic pores inside them that have a huge amount of total surface area.

MOFs can accommodate gases that tend to adsorb, or stick, to solid materials. One gram of MOF material (about the size of a pea) can have a surface area of up to 40 tennis courts, according to Tom Robinson, CEO of MOF Technologies based at Queen’s University Belfast in Northern Ireland.

Researchers are considering the potential for MOFs and adsorbed-natural-gastechnology for the automotive industry. "The primary application envisaged for ANG technology is for powering cars," says Ozgur Yazaydin, of the department of chemical engineering at the U.K’s University of Surrey.

There already are cars and buses runningon compressed natural gas: Honda sold 1,500 CNG cars in 2012 in the U.S. and 1,500 in 2011, according to data from Pike Research. The vehicles store natural gas at about 250 bars using a tank made from composite material.

However, Yazaydin points out that for existing CNG vehicles, “this is not a very efficient way of storing natural gas, and the range of these cars is limited. With the discovery of MOF materials we can potentially design new materials to store higher amounts of gas (and) with a longer driving range."

The exact weight of such a vehicle would depend on how efficiently the MOF particles pack in a fuel tank: Yazaydin estimates 22-44 lbs. (10-20 kg) of MOF would be needed to fill a 1.4-cu.-ft. (40-L) fuel tank.

"Because the size of the tank for ANG would be similar to the fuel tanks currently in use, the weight of the MOF placed in the tank is not going to be very different than the weight of the gasoline," he says.

Robinson envisions MOFs being used to store a large amount of natural gas, but notes this "depends on the gas, the operating pressure and the particular MOF. But for methane you can typically achieve approximately five times the storage capacity in the latest MOFs. For acetylene, the increase in storage capacity can be more than 200 times.”

One gas which might not be helped by the new technology, however, is hydrogen.

“I believe the MOF community is moving away from hydrogen storage as an application,” Robinson says. “Hydrogen is just too light, so it's difficult to get the molecules to stick to the material, especially at room temperature."

And even though there is great promise regarding other gases, manufacturing such tanks will demand sophisticated techniques. "The challenge is, how we are going to manufacture tons and tons of MOFs for millions of cars running every day on our roads?" Yazaydin says.

There also are engineering problems.

"To ensure fast response to the fuel demand of the engine – for example, during acceleration or going uphill – the ANG tank will require precise heat management," Yazaydin notes. During delivery of the gas, "the temperaturein the ANG tank will naturally decrease, which in turn slows the delivery rate of the gas."

Another issue of these dense frameworks is their sensitivity to humidity, which could decrease their performance.

“Their long-term stability under different environments and conditions, such as the humidity and impurities present in natural gas, remain to be addressed," Yazaydin says.

The good news is that the key issue of safety in accidents does not appear to present any particular challenges.

Asked if there are safety issues with CNG gas and MOFs, Yazaydin replies: "I don't think so. There are already cars and busesrunning on” CNG. The CNG Honda car in California would not have been on the roads without the approval of the authorities regarding its safety."

But if safety is not an issue, then the embryonic nature of this technology is, because it is fighting competing systems for limited financing. The European Union's European Green Cars Initiative is focusing on longer-life lithium batteries, rather than MOF-enhanced storage space.