In PowerTorque’s ongoing guide to the alt-power conundrum, Erik Gustafson, R&D Engineer, Chart Industries explains some of the basic issues, like, gaseous versus liquid hydrogen storage.
Upstream of the fuel cell is the hydrogen storage system. Most hydrogen FCEVs operating today store hydrogen as a high pressure compressed gas, either 35 MPa (approx. 5,000 psi) or 70 MPa (approx 10,000 psi). Standard nomenclature for compressed hydrogen fuel tanks is derived from these two pressures, namely H35 and H70.
A compressed gas hydrogen storage system consists of a series of tanks tied together in parallel that fill and empty in unison. Typically, these tanks are about 4~8 per cent weight efficient depending upon working pressure. That is, a tank that stores 10kg of hydrogen will itself weigh about 250 to 500 kg. H35 tanks are more weight efficient but hold less hydrogen and are less volume efficient than their higher pressure H70 counterparts.
The hydrogen in gaseous tanks is regulated down in pressure from the storage pressure to a pressure that the FC can accept, typically in the range of 6 to 7 bar. A fuel cell electric truck equipped with a CHSS ‘backpack’ ,behind-the-cab rack, has considerably more range with less weight penalty than a battery electric version of the same truck.
Consequently, an FCET with a CHSS already has significant weight and driving range advantages versus a BEV but can we make it even better?
How about if it was possible to store more hydrogen on board coupled with less weight? This is precisely the reason to consider using liquid hydrogen storage on-board vehicles.
Liquid Hydrogen Storage
Liquid hydrogen is obtained by refrigerating hydrogen gas to -253°C in a process known as cryogenic liquefaction. Once condensed to its liquid form, hydrogen is over 50 per cent denser than its 70 MPa compressed form. At such low temperatures it has to be kept in well-insulated tanks, but because it is stored at relatively low pressure (around 6-7bar (100psi)), the tanks themselves require thinner material and are lighter than their H35 and H70 gaseous counterparts.
A liquid hydrogen storage system can easily achieve weight efficiencies exceeding 15 per cent. Furthermore, truck OEMs and retrofitters can locate the LHSS directly on the frame rails of the vehicle much the same way as diesel tanks are mounted today.
Locating the LH2 tanks on the frame rails saves a large amount of weight that is spent on the tank support structure for a backpack system that many CHSS require. The frame system for compressed hydrogen storage can weigh twice as much as the tanks that it supports. LHSS can also be integrated into a backpack system if even more range is required than a frame-rail mount system.
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