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Table 5.8. Safety-related properties of hydrogen and other fuels (with use of Zittel and Wurster, 1996).

Table 5.8. Safety-related properties of hydrogen and other fuels (with use of Zittel and Wurster, 1996).

Compressed storage in gaseous form. The low volume density of hydrogen at ambient pressure (Table 5.5) makes compression necessary for energy storage applications. Commercial hydrogen containers presently use pressures of 20-30x106 Pa, with corresponding energy densities of 1900-2700x10" J m-3, which is still less than 10% of that of oil. Research is undertaken for increasing pressures to about 70 MPa, using high-strength composite materials such as Kevlar fibres. Inside liners of carbon fibres (earlier glass/aluminium) are required to reduce permeability. Compression energy requirements influence storage cycle efficiencies and involve long transfer times. The work required for isothermal compression from pressure p1 to p2 is of the form

W = A T log (P2/P1), where A is the hydrogen gas constant 4124 J K-1 kg-1 times an empirical, pressure-dependent correction (Zittel and Wurster, 1996). To achieve the compression, a motor rated at Bm must be used, where m is the maximum power throughput and B, depending on engine efficiency, is around 2.

Liquid hydrogen stores. Because the liquefaction temperature of hydrogen is 20 K (—253°C), the infrastructure and liquefaction energy requirements are substantial (containers and transfer pipes must be super—insulated). On the other hand, transfer times are low (currently 3 minutes to charge a passenger car). The energy density is still 4—5 times lower than for conventional fuels (see Table 5.5). The liquefaction process requires very clean hydrogen, as well as several cycles of compression, liquid nitrogen cooling, and expansion.

Metal hydride storage. Hydrogen diffused into appropriate metal alloys can achieve storage at volume densities over two times that of liquid hydrogen. However, the mass storage densities are still less than 10% of those of conventional fuels (Table 5.5), making this concept doubtful for mobile applications, despite the positive aspects of near loss-free storage at ambient pressures (0—6 MPa) and transfer effected by adding or withdrawing modest amounts of heat (plus high safety in operation), according to

where the hydride may be body-centred cubic lattice structures with about 6x1028 atoms per m3 (such as LaNi5H6, FeTiH2). The currently highest density achieved are for metal alloys absorbing two hydrogen atoms per metal atom (Toyota, 1996). The lattice absorption cycle also performs a cleaning of the gas, because impurities in the hydrogen gas are too large to enter the lattice.

Methanol storage. One current prototype hydrogen—fuelled vehicle uses methanol as storage, even if the desired form is hydrogen (because the car uses a hydrogen fuel cell to generate electricity for its electric motor; Daimler-Chrysler-Ballard, 1998). This is due to the simplicity of methanol storage and filling infrastructure. In the long run, transformation of hydrogen to methanol and back seems too inefficient, and it is likely that the methanol concept will be combined with methanol fuel cells (cf. section 4.7), while hydrogen—fuelled vehicles must find simpler storage alternatives.

Graphite nanofibre stores. Current development of nanofibres has suggested wide engineering possibilities, regarding both electric and structural adaptation, including the storage of foreign atoms inside "balls" or "tubes" of large carbon structures (Zhang et al., 1998). Indications are that hydrogen may be stored in nanotubes in quantities exceeding that of metal hydrides, and at a lower weight penalty, but no designs exists yet (Service, 1998).

Regeneration of power from hydrogen

Retrieval of energy from stored hydrogen may be by conventional low-efficiency combustion in Otto engines or gas turbines, or it may be through fuel cells at a considerably higher efficiency, as described in section 4.7 and in Serensen (2004).

Batteries

Batteries may be described as fuel cells where the fuels are stored inside the cell rather than outside it. Historically, batteries were the first controlled source of electricity, with important designs being developed in the early 19th century by Galvani, Volta and Daniell, before Grove's discovery of the fuel cell and Plante's construction of the lead-acid battery. Today, available batteries use a wide range of electrode materials and electrolytes, but despite considerable development efforts aimed at the electric utility sector, battery storage is still in practice restricted to small-scale use (consumer electronics, motor cars etc.).

Type Elec- Energy Energy Power-densities Cycle Operating tro- effici- density Peak Sustained life tempera-lyte ency (%) (Whkg-1) (W kg-1)(W kg-1) (cycles) tures (°C)

Type Elec- Energy Energy Power-densities Cycle Operating tro- effici- density Peak Sustained life tempera-lyte ency (%) (Whkg-1) (W kg-1)(W kg-1) (cycles) tures (°C)

Commercial:

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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