September 2006   Vol. XXI   No. 9   ISSN 1080-8019
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Linde Develops Ionic Compressor for More Efficient Compressed Hydrogen Storage

VIENNA – Large-volume storage of compressed hydrogen gas is likely to become simpler, cheaper, more efficient and less maintenance-intensive with the development of a so-called ionic compressor at an r&d center operated here by Linde Gas, the industrial gas company.

It doesn’t look all that different, but it’s likely to improve future compressed hydrogen storage: Linde r&d project manager Markus Mayer proudly demonstrates the company’s first ionic compressor in its compressed natural gas version, shown here refueling a CNG car at Vienna gas utility WienEnergie.

So far three beta-phase versions of the device have been undergoing testing for natural gas applications at WienEnergie, a Vienna gas utility, and another one for hydrogen fuel delivery - at carmaker BMW in Germany, Robert Adler, an expert in thermodynamics and head of the development team at Linde’s Applications Technology Center here, told H&FCL in a telephone interview last month. The company expects to sell another 14 so-called medium pressure compressors rated at 450 bar (6,500 psi) to WienEnergie which uses them mostly to refuel some 100 taxis in Vienna running on natural gas, according to Adler.

Another bigger hydrogen compressor rated at 900 bar (13,000 psi) has been sold to Infraserv Hoechst, an infrastructure services division of chemical manufacturer Hoechst that, among other areas, handles high-pressure hydrogen applications for the Frankfurt, Germany-based company.

Storage is Big Issue in Future H2 Economy

Storage of hydrogen in any form - compressed gas, liquid, hydrides, nanotubes, microspheres - is a big issue. The U.S. Energy Department has said that unresolved or unsatisfactory onboard hydrogen storage problems are a major stumbling block in road transportation in the commercialization of hydrogen cars and trucks.

In conventional piston and screw compressors, the process of squeezing the gas to higher pressures somewhere between 200 and 1,000 bar (2,900-14,500 psi), is inevitably accompanied by a rise in temperature. That heat has to be siphoned off, usually with a heat exchanger.

An ionic compressor does that work at constant temperature - so-called isothermal compression - something that minimizes energy waste because just about all of the expended energy goes into actual compression, and not out the window as excess heat. Linde says isothermal compression has been the target and dream of engineers for a century and a half or so.

Fewer Moving Parts, Less Maintenance

In an article in the January 2006 issue of the company’s “Linde Technology” magazine, Adler explained conventional piston compressors have many moving parts. They must be exquisitely machined to assure tight fit to prevent gas from escaping and must have good lubrication to prevent wear. Excellent sealing is essential to prevent lubricants from contaminating the hydrogen gas, a potential death knell for hydrogen-fueled PEM fuel cells, for example, which are easily poisoned by contaminants such as CO2 in the gas.

The upshot is that lubrication is a no-no in compressors for natural gas and hydrogen filling station applications, Adler, explained in the article. This is a problem for machinery in a future hydrogen economy that will have to handle up to 1,500 cu. m. of hydrogen per day and run for 8,000 hours or so - about 500 days - without maintenance.

Enter the ionic compressor. Essentially, the new device replaces the conventional metal piston of a conventional compressor with a specially designed, nearly incompressible “ionic liquid” that does the actual compression work.

Ionic liquids are described in the article as organic salts with melting points below 100 deg. Celsius. Unlike ordinary molecular liquids, ionic liquids consist entirely of particles with negative and positive electric charges which gives them new and unusual properties that vary widely depending on the combination of anions and cations. Most salts have high melting points, but those of ionic liquids can be drastically lowered through the judicious selection of cations and anions.

Environmentally Friendly “Designer Liquids”

They are not volatile or combustible, they have no measurable vapor pressure and they can hold very high concentrations of a wide range of materials in solution: from fats, oils and pharmaceuticals to metals, polymers and even minerals. The very large number of possible anion/cation combinations permits the preparation of an almost limitless varieties of “designer liquids” for many needs.

Put to work in the ionic compressor, the advantages are manifold. The absence of vapor pressure means that molecules do not evaporate from the liquid, which in turn means the ionic liquid does not mix with the atmosphere, making them environmentally benign as well. These mixtures are not flammable or electrically conducive, they act to prevent corrosion, and they have good lubricating qualities.

Adler says he and his development team’s initial beta compressors work at very slow speeds - about 3 rpm equivalent - for testing purposes. It makes it easier to examine and test the liquids’ long-term stability and their ability to lubricate.

The ones under construction now are so-called medium-speed machines operating at about 70 rpm - better, 70 piston up-and-down movements/minute - reducing the unit’s volume by factor of 25 for the same output.

Linde’s Vienna engineers have been working on compressor designs for hydrogen stations in a collaboration with DaimlerChrysler for about five years: Adler says about half of the 70-plus existing conventional hydrogen stations worldwide have come from his labs and shops.

Development Started 3 Years Ago

Work on the ionic concept began about three years ago. Because the ionic liquid does not mix with the gas, researchers were able to do away with seals and bearings, potentially a major cost savings: “In contrast to a conventional compressor with some 500 moving parts, we now need only eight,” the article quoted Adler as saying. He adds the ionic liquid design improves the service life of a compressor by about a factor of ten compared to standard technology, and energy costs are reduced by as much as 20%.

Maintenance needs are greatly reduced: The only place where conventional parts are put to work is in the pump that shifts the liquid back and forth between two cylinders to move the liquid column up and down; while uncompressed gas is being drawn into one cylinder, the gas in the other cylinder is compressed by the communicating cylinder.

Heat exchangers, normally located on the outside of the cylinder are done away with as well. With a special design, which for competitive reasons Linde won’t discuss, the heat is removed in the cylinder itself where it is generated. The result: almost 100% of the energy going into the process is being utilized, with little energy wasted as reject heat.

Cost Comparable to Conventional Units

As to cost, Adler says they have to be roughly comparable in price to existing conventional compressors, which in the case of 300 cu. m./hour natural gas compressors is around Euro 200,000-250,000 ($156,000-195,500). Given the fact that these early versions are largely hand-made, “we don’t exactly earn big money with them,” he says. On the other hand, Linde can’t ask a customer like WienEnergie to subsidize the new devices, he adds.

Series production may start some time next year, he adds, at a location which at this stage is a closely held secret - if in fact the place has been picked as yet.

In the near term, Adler sees market potential in the natural gas market, such as fueling stations for forklift truck fleets. He says countries such as Thailand, Iran and the United Arab Emirates have plans for “hundreds” of these stations, with hydrogen stations likely to follow later. Beyond that, he sees applications such as the charging of airbags, production of ethylene and in diecasting processes where conventional compressor types have long dominated.

And another advantage: “It makes very little noise,” says Adler.

Source/Contact: Linde-Gas Austria (media) Linde Technology magazine, January 2006;; Linde media, Stefan Metz, +49/611/770-487,