Refuelling Station Technology


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.

 

Madrid Dispenser

Hydrogen Storage Banks at the Hamburg Station

The Two Options for Gaseous Hydrogen Refuelling

 

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