External Hydrogen Supply

Introduction

Hydrogen from a central production plant could in principle be delivered to the HyFLEET:CUTE filling stations via pipeline. In Europe, however, only ca. 1,000 km of hydrogen pipelines exist and none of them runs near one of the HyFLEET:CUTE facilities. So external supply, both on a regular basis and as a back-up source, has to take place via road transport. Hydrogen quality is certified by the suppliers for each delivery. London, Luxembourg and Porto receive all their hydrogen fuel from central sources, Madrid part of the demand. They first all bought compressed gaseous hydrogen (CGH2). London moved to liquid hydrogen supply in May 2005.

 

Liquid Supply

A truck can carry up to about 3.3 tonnes of liquid hydrogen (LH2), equivalent to about 36,700 Nm3. This way of supply has the advantage that one delivery to the local station storage can last for more than 20 days with three buses served there. It is preferable for long distances between production site and consumer, common in the USA.

A drawback of liquid supply is that, due to the very low temperatures, all storage vessels have to be very well insulated. Small amounts of hydrogen can also be lost if the station is not being used for refuelling for prolonged periods as hydrogen can start to boil and has to be vented in order to stay below the maximum pressure of the vessel. This is not however a problem if vehicles are refuelling regularly.

Another disadvantage is the high energy demand for liquefying hydrogen. It amounts to about one third of the energy contained within the liquefied hydrogen (1 Nm3, containing 3.54 kWh, requires more than 1 kWh). Given the comparably short distances from central production sites to hydrogen customers, gaseous delivery is dominant in Europe. Only three facilities for liquefaction exist today.

Gaseous Supply

The standard pressure for road transport of compressed gaseous hydrogen (CGH2) is 200 bar, maximum pressure currently being 300 bar. A trailer can deliver between 300 and 600 kg CGH2. One delivery will thus only last for a very limited span of time. Unless two trailers are parked on site, the schedule for exchanging them will be tight and has to work on a strict justin- time basis to guarantee fuel supply for the buses.

Compared to liquefaction, the energy demand for compression is significantly less (depending on input and output pressure). Gaseous hydrogen, once filled into a pressure vessel, will remain there without losses.

In addition to HyFLEET:CUTE cities that rely on external supply entirely, the majority of the sites with on-site generation have the opportunity to use hydrogen from central sources on a backup basis whenever required, like during maintenance. Other cities were guaranteed a very high availability of the hydrogen production unit from their turn-key supplier and therefore made no arrangements for back-up supply.

External supply of hydrogen saves the investment in a local production facility but it does not necessarily reduce footprint. To the contrary, in case of CGH2, space for at least two trailers must be made available plus room for parking manoeuvres. Some transport operators expected disturbances in their bus depot because of hydrogen trailer traffic and thus opted for an on-site production solution. For this reason or for the lack of space, a few of them even excluded back-up supply as their technology supplier guaranteed sufficient availability of their on-site production unit (cf. above).

Trailer for Liquid Hydrogen Supply
Trailer for Gaseous Supply in Luxembourg

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