Pressure Cylinders

The need for lighter gas storage has led to the development of cylinders made of lightweight composite rather than steel. Conventional carbon-wrapped aluminum cylinders can store hydrogen at pressures of

Hydrogen Pressure Tanks

FIGURE 5.1 Concept of "conformable" pressure tanks giving up to 50% better space filling than conventional cylinders. (Redrawn from Thiokol, 2001.)

Hydrogen Pressure Tanks

FIGURE 5.1 Concept of "conformable" pressure tanks giving up to 50% better space filling than conventional cylinders. (Redrawn from Thiokol, 2001.)

up to 55 MPa (550 bar/8000 psi), although national legislation and codes of practice may limit the allowable pressure to a value well below this. In most countries, gas cylinders are typically filled up to a maximum of 24.8 or 30 MPa (248 bar/3600 psi and 300 bar/4350 psi, respectively). At the higher pressure, a modern composite tank reaches a hydrogen mass fraction of approximately 3%, i.e., only 3% of the weight of the full cylinder consists of hydrogen.

In a further development, so-called "conformable" tanks have been produced in order to give better spacefilling than packed cylinders — see Fig. 5.1. Thiokol Propulsion has developed a tank based on this concept that weighs 29 kg when full. It holds 1.5 kg of hydrogen, giving it a 5.2 wt% storage density (Golde, 1998). Hydrogen storage may also require the use of a polymer barrier to reduce gas permeability.

Figure 5.5 later in this chapter compares the different direct and indirect (chemical — see Section 5.4) storage methods.

Pressure storage of hydrogen has been applied in a range of prototype buses developed by Ballard Power Systems, by DaimlerChrysler, and others. In June 1995, Ballard introduced its second bus, a full-size, 40-foot prototype zero emission vehicle (ZEV) powered by a 275-horsepower (205-kW) fuel cell engine. At a range of 400 kilometers (250 miles), this bus meets the operating performance of a diesel transit bus. It runs on compressed hydrogen at a delivery pressure of 30 psig (207 kPa). The hydrogen is stored in rooftop tanks at 3600 psig (24.8 MPa), which is the standard for compressed natural gas.

DaimlerChrysler's 1997 NEBUS roof system consists of seven 150-liter cylinders at a pressure of 300 bar (30.0 MPa). These supply the fuel cell with approximately 45,000 liters of hydrogen. Depending on application profile, the NEBUS in this configuration has an operating range of up to 250 km.

General Motors' current compressed hydrogen gas storage systems typically hold 2.1 kg of hydrogen in a 140-liter/65-kg tank at 350 bar that is good for 170 km (106 miles). The target here is a 230-liter/ 110-kg tank that would hold 7 kg of hydrogen at 700 bar, giving the same range as liquid hydrogen (see Section 5.2.2), 700 km (438 miles) (Hydrogen & Fuel Cell Letter, 2001). See also Section 10.2.1.

In 2001, Californian Quantum Technologies WorldWide demonstrated a composite hydrogen pressure storage tank with a nominal operating pressure of almost 700 bar (10,000 psi), giving an 80% capacity increase over tanks operating at 350 bar. The new tank underwent a hydrostatic burst test during which it failed under 1620 bar (23,500 psi). This test was done along the lines given in the regulations drafted by the European Integrated Hydrogen Project (EIHP). The tank has an in-tank regulator that provides a gas supply under no more than 10 bar (150 psi).

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