H2Tech - Q4 2022 - 31

REFUELING STATIONS
Billing accuracy for H2
refueling stations
H2 is recognized as playing a crucial
) emissions. From transportation
is already expected
role in reducing global carbon dioxide
(CO2
to heating homes, H2
to play a significant part in replacing fossil
fuels in net-zero policies in the UK
and around the world. There are plans for
partial or full replacement of natural gas
with H2
in natural gas grids, as well as ambitious
targets to enhance the production
of fuel cell vehicles and the development
of H2
will form the infrastructure of a future
H2
one party (a seller) to another (a buyer).
Fuel cell electric vehicles (FCEVs) and
battery electric vehicles (BEV) are considered
the most promising candidates
for the future of transportation. FCEVs
are EVs that use H2
as fuel. H2
reacts with
oxygen in a reverse electrolysis in their
fuel cells to generate the required electricity.
This process is free of carbon emissions,
with the only byproduct of the reaction
being water. FCEVs offer significant
advantages, especially for larger vehicles
such as buses and heavy goods vehicles.
The H2
tank of an FCEV (small or large)
can be filled in a few minutes vs. hours to
charge a BEV. However, increasing the
use of FCEVs requires the development
of relevant infrastructure such as H2
refueling
stations, and technologies such as
accurate H2
flowmeters and regulations.
All these aspects are in their early stages
of development but growing at a fast pace.
H2
is sold based on mass (in kilograms)
in H2 vehicle refueling stations. However,
accurate billing needs accurate metering
of H2
, which is a challenge. Liquid fuels
such as petrol (gasoline) and diesel must
be measured to 0.5% accuracy in the refueling
stations based on the recommenFIG.
1. View of a H2
refueling station.
H2Tech | Q4 2022 31
at
refueling stations (FIG. 1). These
network. Accurate metering of H2
different points of this network is crucial,
especially when H2
is transferred from
vehicle
M. SADRI, TÜV SÜD National Engineering Laboratory, Glasgow, Scotland, UK
dations of the International Organization
of Legal Metrology (OIML) (Accuracy
Class 0.5 in the document OIML R117).
The required accuracy for the measuring
system of gaseous fuels such as compressed
natural gas is 1.5% (Class 1.5 in
OIML R139). However, OIML R139
separates H2
from all other types of gaseous
fuels and recommends Class 2 and
Class 4 (2% and 4% accuracy of the measuring
system, respectively) for its measurements.
It is expected that many counties
will enforce Class 2 of OIML R139 in
the coming years.
There are several factors that make H2
metering challenging at H2
refueling stations.
H2 has a very high gravimetric energy
density of 140 MJ/kg. This means
that it stores a lot of energy relative to
its weight, much more than natural gas
(53.6 MJ/kg), diesel (45.6 MJ/kg) and
lithium-ion batteries (< 5 MJ/kg). In
volumetric terms, H2
is the least dense
of any gas and takes up more space than
both natural gas and diesel.
carrier, H2
To improve its efficiency as an energy
is compressed to pressures
as high as 700 bar in H2
compressed state, H2
vehicles. In this
occupies about the
vehicles
same space as a battery, for much less
weight. Another advantage to H2
is the fast refueling time. However, when
H2
is rapidly compressed to 700 bar, a lot
of heat is generated. To stay within safe
operating limits, the quickest fuelling
protocols pre-cool the gas to -40°C.
Hydrogen refueling stations are therefore
required to operate across a wide
range of pressures (up to 875 bar) and
temperatures (-40°C-60°C). This is very
challenging from a measurement perspective,
since the accuracy of most flowmeter
technologies is adversely affected by
extreme pressure and temperature conditions,
as well as the transient flow encountered
for vehicle filling.
Coriolis meters have dominated the
market of H2
dispensers. They have several
advantages, but the most important
one might be their capability of mea
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