Hydrogen Centralized or Decentralized Distribution

If hydrogen is produced from gasoline or diesel fuels on board vehicles via reformers, then the existing distribution network could be used with essentially no modifications. If the reformer technology can be made economically viable, it would however, not solve our dependence on decreasing oil resources and would still produce considerably increased amounts of CO2. In the long term, only reformers using methanol offer a perspective of sustainability.

In case hydrogen gas itself were to become the energy source of the future, it would need to be easily available anywhere at an affordable price, and its distribution should be safe and user-friendly - similar to today's hydrocarbon-based fuels. This implies the creation of a totally new infrastructure specifically designed for the transportation, storage and distribution of hydrogen. Hydrogen could also be produced directly in local fueling stations by natural gas reforming or via water electrolysis by electricity. A decentralized hydrogen production would not require a nationwide delivery system involving trucks or pipelines, but it would be very expensive and energy-consuming. Only some 60 hydrogen-producing and refueling stations are currently operating worldwide in Germany, the United States, Japan, and other countries, providing hydrogen for a small number of automobiles and buses. California, in its Hydrogen Highway program, is planning to construct 200 such fuel stations within a decade. Locally distributed hydrogen generation may be the preferred option as long as the number of hydrogen-powered vehicles and the demand for hydrogen remain low. However, even so there are drawbacks: hydrogen being generated in limited amounts has a high cost, and on-site production by reforming of natural gas emits CO2 which, considering the scattered locations and small scale, cannot be economically captured or recycled with present-day technology. Electrolysis is only emission-free if the electricity used comes from renewable or atomic energy sources. Iceland for example, which produces electricity almost exclusively from rich geothermal and hydro resources, has the ambition to become the world's first hydrogen-based economy. In most countries, however, if electricity is taken from the existing grid, a significant part of it will have been generated by fossil fuel-burning power plants, thereby eliminating any possible benefits expected from the use of hydrogen. The local manufacture of leak-prone and explosive hydrogen in populated areas would also raise serious safety issues.

The other solution would be to produce hydrogen in large quantities in centralized plants and then to transport it to the local stations by road, rail, or pipelines. In such plants, the production costs would be lower, the efficiency higher, and CO2 capture and sequestration (or recycling) easier to implement. Furthermore, hydrogen generation is not limited only to methane reforming or water electrolysis, but can also involve other technologies such as coal and biomass gasification or thermal splitting of water in high-temperature nuclear reactors. However, such means of hydrogen production require the establishment of an extensive storage and delivery system in order to service customers. Currently, the options taken in consideration for delivering hydrogen are by road transport using trailers containing hydrogen in its liquid or high-pressure form, and hydrogen gas pipelines.

The transportation by truck of hydrogen as a cryogenic liquid is today commonly used when delivering hydrogen to industries with limited needs, and where on-site generation would be uneconomical. It is by this means, for example, that the hydrogen required as propellant for Space Shuttle launches is transported from Louisiana to the NASA launch pads in Florida. Whereas the density of liquid hydrogen is about ten-fold lower than that of gasoline or diesel oil, it still has the advantage of being relatively compact compared to compressed hydrogen. Commercially available trailer-trucks can transport some 3500 kg hydrogen in liquid form, and this is energetically equivalent to about 13 000 L of gasoline [97]. Because the cryogenic trailers used must be double hulled and vacuum-insulated to avoid excessive blow-off, they are expensive. It has been suggested that transportation of hydrogen in its liquid form from centralized production centers to local distribution stations could play an important role in the initial transition phase to a hydrogen infrastructure. On the large scale necessary however, this solution is not suitable. As mentioned earlier, up to 40% of the energy content of the shipped hydrogen is consumed by its liquefaction, making the process far too expensive from both energetic and economic viewpoints. Hydrogen compression requires less energy than liquefaction. The steel cylinders and other on-board equipment needed for the safe handling of highly pressurized hydrogen is both heavy and expensive. Moreover, with road transport being weight-limited, and considering the extremely low density of hydrogen, this means that the current tube trailers used to transport hydrogen under high pressure (200 atm) can each deliver only about 300 kg of hydrogen. Even taking into account any expected future technical advances, a 40 000-kg truck would enable the delivery of only 400 kg of hydrogen, or about 1% of its dead weight [96]. In comparison, a similar truck could deliver some 26 tonnes of gasoline, containing more than twenty times more energy than the compressed hydrogen truck. So, instead of one driver and truck, more than twenty would be needed to deliver the equivalent amount of energy as gasoline; this in turn would generate higher costs and increase traffic congestion. The introduction of lightweight materials for high-pressure hydrogen storage, such as those currently under development for use in motor cars, could potentially be utilized, but the transportation capacity is expected to remain modest. Besides economic and energetic considerations, a substantial increase in the transportation of highly flammable and explosive hydrogen both in liquid and compressed form by road would imply considerable safety issues and risks.

The most commonly used system for the transmission of hydrogen in large quantities for the chemical and petrochemical industries is by pipelines. To date, worldwide, these have a combined length of only about 2500 km, of which 1500 km are located in Europe and 900 km in the United States [98]. Transport by pipeline allows a direct connection to be made between main hydrogen producers and users. Hydrogen pipelines requiring the use of special steels or metals, seals and pumps, and they are also expensive to build, maintain and operate. In contrast, there are many hundreds of thousand kilometers of existing pipelines for natural gas, oil and other hydrocarbon products around the world, though these are not suited for hydrogen transport. Most of the metal pipelines, when exposed to hydrogen, would allow it to diffuse through and themselves become brittle over time. Due to the small size of hydrogen, leakage - especially during transportation over long distances - is likely to occur and should therefore be carefully controlled in order to minimize significant losses and explosion hazards. The cost of hydrogen transport is also at least about 50% higher than that for natural gas and for the same volume [99], while hydrogen contains three times less energy than natural gas. Pipeline shipment and dispensing of hydrogen is estimated to cost some $1 kg-1 with current technology (ca. $0.7 kg-1 expected by future improvements). It is thus, much more expensive than the $0.19 per gallon currently paid to ship and distribute gasoline [91]. Nonetheless, the transport of hydrogen by pipelines may be the best solution to date, though the installation of a large hydrogen pipeline infrastructure would be highly capital-intensive. Indeed, it would only be an option for the long term, when the numbers of cars and other fuel cell-driven devices running on hydrogen would be sufficient to support the vast investments needed.

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