The most common means to warm liquid or cold gaseous hydrogen is the use of ambient air vaporizers using ambient atmospheric. Other means can be through the use of electric heat or process waste heat.  

Gaseous hydrogen can be stored forever as long as the system integrity is maintained. However, liquid hydrogen is “use it or lose it” and will boil from system heat leak and build pressure unless it is used or vented. This is not usually an issue for continuous use or low-pressure applications which can use hydrogen gas pressure directly from the tank.  

For intermittent or high-pressure applications such as vehicle fueling, this can be more challenging.  If a gas compressor is used and if demand is regular enough, often the compressor can recover the boil-off gas. This is more difficult for a pump, but again, if usage is sufficient, then the liquid being removed from the tank can minimize or eliminate the need to vent.