H2Tech - Q1 2021 - 30

INFRASTRUCTURE AND DISTRIBUTION

Tackling flow measurement challenges
for hydrogen fuels
D. ANDERSON, TÜV SÜD National Engineering Laboratory, Glasgow, Scotland, UK

Hydrogen is recognized as playing a
crucial role in global net-zero carbon targets, through its potential use in vehicles
and in domestic heating. This is because
H2 contains no carbon, so when used in
a fuel cell or combustion engine its only
product is water vapor. However, due to
the low abundance of elemental H2 in the
earth's atmosphere, H2 must be first produced before it can be used as a fuel. This
means that it is referred to as an energy
carrier and, unlike hydrocarbons, is not
an energy source.
When discussing the use of H2 in a lowcarbon economy, green H2 is commonly
referred to; this is H2 generated from the
electrolysis of water, using renewable
energy such as solar, wind or tidal. Generation of H2 from renewable sources provides a buffer that allows excess energy to
be stored in periods of peak generation.
However, the most common method
for H2 production is either through steam
methane reforming (SMR) or autothermal reforming (ATR) of hydrocarbons,
such as natural gas, primarily due to the
lower cost relative to electrolysis. This
process produces carbon dioxide as a byproduct; therefore, CO2-reducing technologies, such as carbon capture, utilization and storage (CCUS), are required to
avoid CO2 being released to atmosphere.
When a carbon-reduction technology
such as CCUS is used alongside SMR or
ATR, the H2 produced is commonly referred to as blue hydrogen.
Flow measurement challenges for
hydrogen vehicles. The use of H2 as

an alternative to refined oil products and
natural gas for transport is receiving much
attention. Like battery-powered electric
vehicles (BEVs), the use of H2 fuel cell
electric vehicles (FCEVs) will reduce local air pollution due to the absence of tail-

30 Q1 2021 | H2-Tech.com

pipe emissions. Provided that either green
or blue H2 is used, overall CO2 emissions
will also be reduced. For BEVs, which use
electricity from the electricity grid, the
overall CO2 emissions will be reduced
if the method of electricity generation
emits less CO2 per charged vehicle than
those which use hydrocarbons as a fuel.
Globally, BEVs are significantly larger
in number than FCEVs, at present. The
capital costs associated with building an
H2 refueling station mean that they are
less common than the relatively low-cost
BEV charging points. However, FCEVs
do have several advantages, such as a larger range of 400 km and above, compared
to a range of around 250 km for BEVs. In
addition, FCEVs can be refueled in a few
minutes, whereas BEVs can take several
hours to recharge their batteries.
One aspect that is commonly overlooked for FCEVs is the ability to effectively trade H2. For BEVs, this is relatively
simple since the necessary standards are
already in place to measure electricity usage. For the FCEV industry, the challenge
of measuring H2 dispensed into the tank is
not as simple, particularly when customers using these refueling stations will expect a similar level of measurement error
to what is presently achieved in gasoline
and diesel refueling-i.e. -1 % to +0.5 %.
The accuracy requirements for dispensers at H2 refueling stations are set
out in the international recommendation
OIML R139. A maximum permissible
error (MPE) of ±1.5 % is stipulated for
the flowmeter and ±2% for the complete
measuring system at initial verification,
which is challenging due to the operating
conditions at H2 refueling stations. These
are specified in the worldwide accepted
standard SAE J2601. A schematic diagram
of an H2 refueling station is shown in FIG. 1.
The flowmeters used at refueling sta-

tions are Coriolis mass flowmeters, with
the exact location of these dependent on
the design of the refueling station. Two
common locations are upstream of the
precooler near the compressor, and in the
dispenser unit downstream of the precooler; both locations are shown in FIG. 1. Coriolis mass flowmeters perform best when
calibrated at temperatures and pressures
close to field operating conditions, and
ideally using the same fluid. However, vehicles are filled at up to 700 bar, and the
temperature within the refueling station
can reach up to 60°C from precooling at
-40°C. At present, no independent flow
calibration laboratories can operate with
H2 at these conditions to enable the calibration of Coriolis mass flowmeters.
Some sources of flow measurement
error are unrelated to the accuracy of the
flowmeter. For example, for safety reasons,
the dispenser hose must be vented after
use. Since the hose is located downstream
of the flowmeter, this represents a quantity
of H2 that has been measured but not delivered to the customer. A correction must be
applied for the vented H2; otherwise, it is
an additional source of measurement error.
In addition, once the dispenser hose
is vented, a large volume of pressurized
H2 may still remain in the piping further
upstream, particularly if the flowmeter is
installed upstream of the precooler. This
" dead volume " contains gas that has been
measured by the flowmeter but not delivered to the receiving vehicle. If the current
user fills their vehicle to a higher pressure than the previous user, there will be
a positive error and the user will be overcharged, and vice versa. Once again, corrections must be applied to ensure that the
user has been billed correctly.
To assess the magnitude of error at the
dispenser, several field test standards have
been developed, with the U.S. National

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