Barcelona Filling Station |
The volumetric energy density of hydrogen gas under ambient conditions
is much lower than that of gasoline or diesel (cf. section 1.2). Hydrogen
is therefore compressed in order to reduce the size of the filling
station storage, to keep space requirements onboard the vehicle at
a reasonable level, and to ensure enough range for daily bus operation.
This is not entirely new as it also applies to natural gas, but the
volumetric energy density of hydrogen compared to methane – the
most important constituent of natural gas – is more than three
times lower.
One solution for compensating this disadvantage is to move to higher
onboard gas pressures, from 200 bar (standard technology for mobile
applications so far, both hydrogen and natural gas) to 350 bar, and
most likely 700 bar in the future. HyFLEET:CUTE is the first major
trial which follows this 350 bar concept, requiring a technology step
for the refuelling infrastructure. The main components of a filling
station for compressed gaseous hydrogen (CGH2) storage and dispensing
are compressor (one or more, cf. below), storage vessels and dispenser
with filling nozzle.
Liquid hydrogen (LH2) performs about as well as natural gas at 200
bar regarding volumetric energy density, even when considering the
volume for the insulation of the cryogenic tank. Liquid hydrogen storage
can be employed both at stations and in vehicles. One of the HyFLEET:CUTE
cities, London, will demonstrate external supply of LH2 and its storage
on site at the station. Liquid onboard storage is not realised in HyFLEET:CUTE
as buses have sufficient room on the roof to accommodate enough 350
bar pressure vessels to enable the desired range. The main components
for a filling station for CGH2 dispensing with LH2 storage are cryogenic
vessel, cryogenic pump for pressurising the liquid, vaporiser and dispenser.
Other equipment at both types of station is, for example, hydrogen
sensors and other safety equipment, depending on local or country-specific
standards (e.g. flame detectors, sprinkler installations etc.).
General Requirements
Key requirements for the HyFLEET:CUTE hydrogen filling stations were:
- A turn-key solution from only one supplier per site (including
on-site hydrogen generation, if applicable)
- Compact, modular units and components that can easily be integrated
into existing facilities, namely a bus depot, not interfering with
day-to-day business there
- Pre-assembled, skid-mounted delivery of the plant
- Small footprint
- A full-service and maintenance contract with short response times
from the turn-key supplier
- Automatic operation and 24 hours surveillance possible (both by
supplier and operator)
- Simple handling of the refuelling process
- Refuelling time per bus not more than 30 minutes
- Refuelling of the 3 buses feasible without or with only a short
interval
- Hydrogen quality not affected along the chain from on-site production
or trailer feed-in, respectively, to the refuelling nozzle
- In case of on-site generation, the possibility to produce at part
load during periods of reduced demand
Details varied from site to site and deviated partly from the above
list. For example, the hydrogen storage size may have been limited
to a certain value by the approving authority due to the vicinity of
other specific installations in the depot or due to nearby residential
houses. In case of a small storage, the interval between two bus fillings
may be several hours, until, for example, the on-site unit has produced
enough gas to refuel another vehicle.
Compression and Storage Concepts
Overflow Filling The rated pressure of the station storage is higher
than the one of the vehicle tank after refuelling. Refuelling is simply
achieved by gas overflow from the station into the vehicle vessels
and pressure levelling between the two. This is optimised by dividing
the storage into several banks that are consecutively connected to
the vehicles tank where only the last bank has to be charged with a
pressure above the final vehicle tank level. A compressor will only
be needed to recharge the storage of the station but is not involved
in the refuelling.
Booster Filling
The station storage has a rated pressure below that of the vehicle
tank, so pressure downstream the station vessels must be sufficiently
enhanced in order to fully charge the vehicle. This requires a “booster” compressor
with a rated inlet pressure high above ambient conditions which will
be working during refuelling. A second compressor may be required
to recharge the storage of the station, depending on the characteristics
of hydrogen supply.
These were only the principle solutions and had numerous variants.
For example, a two-step system may be realised with step one using
a pressure differential and in step two the filling is completed by
means of a booster (denoted as “overflow + booster” in
the above table). And for the case of compressor failure, by-passes
should enable at least a partial vehicle tank filling.
In the case of liquid hydrogen storage and gaseous refuelling, the
liquid can be pressurised upstream the vaporiser using a cryogenic
pump. No compressor for the gas phase will be required and refuelling
is achieved by overflow filling. |
