Metal hydride hydrogen storage tank for fuel cell utility vehicles

The use of fuel cells (FC) in heavy duty utility vehicles, including material handling units / forklifts or underground mining vehicles, has a number of advantages over similar battery-driven vehicles including: constant power during the entire shift, and shorter refuelling time as compared to the time to recharge the battery. Most of vehicular FC power systems demonstrated so far have utilised compressed H₂ stored in gas cylinders at pressures up to 350 bar. This solution, however, results in too light weight of the FC power modules for the utility vehicles which require additional ballast for a proper counterbalancing to provide vehicle stability. In the underground applications, the use of pressurised hydrogen (> 20 bar) is not acceptable at all for the safety reasons. A promising alternative is the application of metal hydrides (MH) for the on-board hydrogen storage [1]. The “low-temperature” intermetallic hydrides with hydrogen storage capacities below 2 wt% can provide compact H2 storage simultaneously serving as ballast. Thus, their low weight capacity, which is usually considered as a major disadvantage to their use in vehicular H₂ storage applications, is an advantage for the heavy duty utility vehicles [2]. Here, we present new engineering solutions [3, 4] of a MH hydrogen storage tank for FC utility vehicles which combines compactness, adjustable high weight, as well as good dynamics of hydrogen charge / discharge. The tank is an assembly of several MH cassettes. Each cassette comprises several MH containers made of stainless steel tube with embedded (pressed-in) perforated copper fins and filled with a powder of a composite MH material which contains AB2- and AB5-type hydride forming alloys and expanded natural graphite. H₂ input / output pipelines are ended by gas filters inside the MH containers and connected to a common gas manifold from the opposite side. The assembly of the MH containers staggered together with heating / cooling tubes is encased in molten lead followed by the solidification of the latter. During lead encasing, the inner space of the MH containers is evacuated providing initial activation of the MH material. After cooling down, the MH cassette is filled with pressurised H₂ for the initial H₂ charge which starts immediately and completes in about 1.5 hours. One MH cassette comprising of five 51.3x800 mm MH containers (each filled with ~3 kg of the MH material) has hydrogen storage capacity about 2.5 Nm³ H₂. When heated with a running water to T=40– 50 °C (typical coolant temperature during the operation of a PEMFC stack), the cassette can release more than 60% of this maximum amount at the H₂ flow rate of 25 NL/min that corresponds to 1 hour long full load operation of 2.5 kWe stack at 50% efficiency. Furthermore, at the heating temperature about 40 °C and H2 output flow rate of 15 NL/min (equivalent to the stack power of 1.38 kWe at the same efficiency) the H₂ release remains stable during >2 hours providing utilisation of ~80% of the stored H₂.