Japanese auto giant Toyota has developed a system to recycle “boil-off” gas for liquid hydrogen-fuelled vehicles. While liquid H2 has a higher energy density by volume than compressed gas, which would reduce refuelling costs, the molecules must also be kept at temperatures below minus-253°C, or close to absolute zero.
However, there is an inherent problem with storing liquid hydrogen — ambient temperatures cause H2 at the edges of storage tanks to warm up, turning it into a gas (known as “boil-off”), which must be vented to prevent a build-up of pressure that could cause the tank to explode.
Currently, hydrogen boil-off is released straight into the atmosphere, which presents a problem both for project economics (since expensive volumes of H2 are lost) and environmental impact (since hydrogen acts as an indirect greenhouse gase).
As such, Toyota has developed a new system for liquid-hydrogen-fuelled vehicles that uses the boil-off gas to generate energy on the move, which it has trialled in its GR Corolla H2 Concept, a car which has a hydrogen-based internal combustion engine.
In this system, when boil-off gas builds up in the storage tank, it is vented into “a self-pressuriser”, which increases pressure by between two and four times without using external electricity. The pressurised gas is then directed into the engine to be burned as a fuel.
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Surplus boil-off gas that is not re-pressurised is fed into a small fuel-cell stack to generate electricity, which can then be used to power the liquid hydrogen pump for the motor and increase overall energy efficiency.
The liquid hydrogen-fueled GR Corolla H2 Concept car. Photo: Toyota
Source: Hydrogen Insight
