Distributing hydrogen

2008-11-30

in Daily updates

Among a number of other strong points, this WWF report (PDF) on “The end of the oil age” highlights some of the problems with hydrogen as a fuel, particularly for vehicles. One major issue raised is the difficulty of transporting the stuff:

Despite having a high specific energy (i.e. energy content per unit mass) of 142 MJ/kg, the physical density of hydrogen is just 84 g/m^3, which means that one kilogramme of the gas occupies around 12 m^3 at normal temperature and pressure (NTP). By comparison, one kilogramme of natural gas displaces 1.4 m^3 and packs a specific energy of 54 MJ/kg. This means the volumetric energy density of hydrogen is only one-third that of natural gas, making the cost of a hydrogen pipeline around six times higher than a natural gas pipeline of equivalent energy capacity. The IEA projects that worldwide investment required to develop a hydrogen pipeline network might be in the order of US$ 2.5 trillion, while noting that the energy required transporting hydrogen via pipeline is on average 4.6 times higher per unit of energy than for natural gas. This equates to an efficiency loss of ten percent over a distance of 1,200 km; the same energy would move natural gas 5,000 km.

As an alternative to pipeline distribution, like natural gas, hydrogen may be either compressed to around 200 atm or chilled close to absolute zero for transportation via truck or ship. Both processes are energy intensive, resulting in additional efficiency losses in the hydrogen supply chain, and super-cooling requires venting that can further deplete the stored fuel. According to one study, it takes 22 tube trailers at 200 atm or 4.5 liquid hydrogen tankers to carry the energy contained in a single gasoline tanker of the same gross weight.

Comparing the hydrogen distribution efficiencies with our electron pathway, we know that electricity grid transmission and distribution (T&D) losses of around 6-8% are typical in OECD countries. Whether carried by pipeline, tanker or ship, it is therefore inconceivable that centrally-produced hydrogen will ever match the efficiency of the electricity grid. Only if it is synthesised at or close to the point of use would hydrogen avoid significant energy losses associated with distribution. Even then, mindful that our guiding principle is the exclusive use of energy from sustainable renewable resources, hydrogen produced in localised facilities would still need to be compressed for storage and/ or delivery directly to the vehicle, which would of course incur further energy losses.

As I have said several times before, hydrogen is a low quality fuel, difficult to handle with low energy per volume. Even with electrolysis at 80% efficiency, the report concludes that hydrogen vehicles would have an overall efficiency of 28%, when production, transport, and the operation of fuel cells are taken into account. Given that vehicles using hydrogen fuel cells would actually be using electricity to drive their motors anyhow, it seems more sensible to focus our efforts on battery technology, which the report concludes to be 23% more efficient, with a lot less new infrastructure to build.

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{ 5 comments… read them below or add one }

Tristan November 30, 2008 at 5:09 pm

Compressed air also seems like an interesting choice of fuel (conceiving of fuel not as a source of energy but a manner of storing energy).

http://www.popularmechanics.com/automotive/new_cars/4251491.html

. November 30, 2008 at 10:31 pm
Milan December 19, 2009 at 6:19 am
. April 23, 2010 at 4:34 pm

“From the beginning, the cloud hanging over the whole hydrogen enterprise has not been the power source as such, but the intractable difficulty of distributing and storing the stuff. It is not hard to see why. Hydrogen atoms are the smallest and lightest in the universe. The next heaviest element in the periodic table, the inert gas helium, is used for detecting cracks in pressure vessels and the like. Even though helium atoms are four times chunkier than hydrogen atoms, they are still small enough to find all the weak spots as they worm their way through the crystalline structure of solid steel several centimetres thick. If hydrogen were used as a crack detector (it is not because of the fire hazard), it would escape four times faster.

Devising a fuel tank to constrain hydrogen has always been a challenge. To have a useful range of 480km (300 miles) or so, an electric car using a fuel cell instead of a battery pack would require around 9kg (20 pounds) of hydrogen. Storing hydrogen as a gas or liquid in a vessel containing “reversible” crystalline metal hydrides is one way to carry it around. Another is to use high-tech pressure vessels made of carbon-fibre. Some researchers are working on sponges made of carbon nanotubes that soak up hydrogen. Whichever technology is chosen, a vessel for storing hydrogen on-board a car costs hundreds of times more than a conventional petrol tank.”

. April 23, 2010 at 4:37 pm

“Meanwhile, transporting hydrogen from its production facility has presented other difficulties. Natural-gas pipelines cannot be used because hydrogen makes the steel tubing brittle and attacks the welds. Special production processes are needed to make pipes for carrying hydrogen. For that reason, few exist. The alternative is to liquefy the hydrogen at great expense and transport it in road tankers refrigerated with liquid nitrogen. Either way, the hydrogen fuel finishes up costing way too much. And all this assumes that hydrogen can be made cheaply and without producing large amounts of carbon emissions. So far, it can’t.”

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