G. Fultz, Emerson, Boulder, Colorado
When most people think of zero-emission vehicles (ZEVs), the
concept suggests an electric motor drive supplied by a rechargeable battery.
This can apply to automobiles, trucks, buses, industrial vehicles and railroad
locomotives. However, another category of ZEVs does not use batteries, but instead
generates power using a fuel cell as a battery alternative. These fuel cell
electric vehicles (FCEVs) can cover the full range of vehicles, with the
possible exception of the smallest, such as a bicycle (FIG. 1).
While there are several commercialized fuel cell
technologies, the one most suited to mobile applications is the proton exchange
membrane (PEM), thanks to its high-energy density, scalability, relatively low
operating temperature and general simplicity. These units use pure hydrogen (H2)
as fuel, combining it with atmospheric oxygen to produce an electric current to
drive a motor, with only water vapor as a byproduct. The downside of PEM
designs is its use of a precious metal catalyst. However, if a unit is
scrapped, the catalyst is easily recoverable for recycling.
Regardless of what technologies are eventually adopted for
widespread use, H2 fueling stations and precise methods for
measuring the amount of fuel transferred to vehicles will be required. Newer
flowmeter technologies address this issue with innovative designs, which have
been proven in use over the past few years.
Fuel cells vs. batteries for ZEVs. This creates a
question: how extensively could fuel cells replace batteries for
transportation? One of the major concerns about the practicality of converting
to ZEVs on a global scale is the availability of batteries. Supplies of lithium
and other rare metals are constrained, and practical battery recycling
technologies are still being developed.
This leaves fuel cells as an effective alternative, boasting
a set of advantages over batteries. For example, when comparing an FCEV to the
same vehicle with batteries, it can be refueled much faster, and it will retain
the fuel indefinitely, unlike batteries, which lose power steadily, even when
not in use.
While FCEVs as consumer cars and SUVs are rare, H2
fuel cells are widely used for industrial material-handling vehicles, such as
forklifts. This approach is practical since it is unnecessary to depend on
widely distributed external H2 refueling infrastructure. The plant
or warehouse receives compressed H2 via a truck from the local
like other industrial gases—and
can place refueling stations in the facility wherever needed. For general
transportation, H2 refueling stations have a main storage tank that
can be supplied via truck, just as with gasoline and diesel (FIG. 2).
When used in this manner, H2 fuel cells have
several advantages over batteries:
Pressure to maximize
fuel capacity. The idea of powering a vehicle fueled by compressed gas is
nothing new. Propane works well and has a high capacity since it remains liquid
even when the pressure is below 14 bar (200 psi). Compressed natural gas is
also used, with pressures typically between 200 bar–250 bar. Higher pressures
have been adopted as the standard to realize the highest practical H2
energy density. For large trucks and commercial vehicles (with some flexibility
for tank size), 350 bar is the standard, but 700 bar use is growing. For cars
and other small vehicles, 700 bar is the standard.
These are very high pressures, and all equipment must be
designed to handle potential safety issues. Moreover, this is complicated
because people using the equipment may not be trained. An average person coming
into a service station wanting to fill a car with H2 must make the
hose connection properly and without assistance, so the equipment must be
highly durable, and user-friendly in many critical ways.
Fueling vehicles with
H2. Like any fueling stop, filling a car or truck with H2
is an exercise in custody transfer. The same concepts apply as when buying
gasoline or diesel: a flowmeter must provide a totalized volume to calculate
the applicable price. Since money changes hands based on the measurement, accuracy
is paramount. For large-scale custody transfer applications, international
metrology standards call for a maximum allowable shift in the accuracy of ±0.167% compared to an international reference standard, even with
disturbed flow conditions present.
Unlike liquid fuels, H2 must be sold by mass
since a volume measurement requires correction for pressure and temperature. A
flowmeter that measures mass natively instead of volume is thus far simpler to
implement, and it avoids complexity with additional measurements and
The most widely used technology for mass flow measurement is
Coriolis, known for its high accuracy and wide turn-down range. It is a
preferred method for custody transfer measurements on a large scale for
hydrocarbons, chemicals and gases. However, applying it, or any flowmeter
technology, to H2 is challenging due to the high pressures involved,
as few off-the-shelf models can handle 700 bar daily.
Fortunately, the line sizes are small for refueling station
service since the flowrates do not need to be very high. Typically, 1/8-in and
1/6-in [diameter nominal (DN)2 and DN3] units are sufficient. This makes
construction for high-pressure much easier.
Designing flowmeters for H2. Handling H2
presents various unique challenges. Its small single-atom size makes it more difficult
to find leaks than any other gas, particularly at such high pressures. Sealing
strategies capable of controlling much larger molecules (e.g., methane) may not
be sufficient with H2. For example, to avoid product loss, some
joints that were previously sealed with O-rings must be welded. The embrittling
effect that H2 has on some common grades of stainless steel must
also be considered, but this is well recognized, so more appropriate alloys are
The author’s company developed a flowmeter familya
that has found successful adoption for H2 fueling applications (FIG. 3). All models
in this group are rated far higher than 350 bar, and most exceed 900 bar due to
the specific alloy XM-19 (austenitic stainless steel) used, chosen for its high
tensile strength and resistance to H2 embrittlement.
This flowmeter series is widely used in Europe and has been adopted
by various companies for their H2 vehicle fueling stations (FIG. 4). These
stations can dispense H2 at either 350 bar or 700 bar to accommodate
both commercial and consumer customers.
Since these flowmeters are being used in a custody transfer
application where money and fuel are changing hands, they fall under relevant
trade regulations, including the International Organization of Legal Metrology
(OIML) standard OIML R 139-1, Compressed Gaseous Fuel Measuring System for
Vehicles and Netherlands Measurement Institute (Nmi) certification.
The Nmi certification is especially important as the metrology
organization has reported that many H2 dispensers have not yet been
tested for metrological accuracy. As a result, measurements can vary
significantly, negatively affecting station owners and FCEV drivers.1
Flowmeters in custody transfer applications require regular calibrations,
but the technology and accuracy requirements determine the interval between
those tasks. The author’s company’s flowmetera has meter
verification software. This is not self-calibration, but it is instead a series
of internal diagnostics able to determine if anything is happening to cause a
loss of accuracy. For example, it can detect corrosion or erosion of internal
The software can generate a time-stamped report, useful for
inspectors and maintenance teams. Many agencies recognize these verification
results as a valid work practice. When indications using this method show no
signs of problems, calibration intervals can often be extended, reducing
required maintenance and downtime.
The growth of H2.
As more countries push for fewer internal combustion-powered vehicles, ZEV options
grow in importance. Battery-powered EVs have taken the limelight for much of
the public, but FCEVs will continue to grow in numbers, driven by constraints
on battery supply chains and the clear advantages of fuel cells.
As shown by a recent incident that occurred in July 2023 while
refueling a H2 bus in California, H2 refueling can be
dangerous if not handled correctly by using proper procedures and components.2
One of the more critical components is the device used to measure H2
flow, as this demanding application requires careful evaluation and selection,
including consultation with knowledgeable suppliers.
The growing availability of H2 (green and blue)
will result in wider refueling infrastructure, making adopting FCEVs easier and
more attractive. The right flowmeters will ensure the new infrastructure is
accurate, efficient and robust. H2T
a Emerson’s Micro Motion high-pressure Coriolis
GENNY FULTZ is a marketing product manager for
Emerson, responsible for Micro Motion products. Before joining Emerson, she
worked in various marketing and manufacturing roles for process industry
companies. Fultz earned BS and MBA degrees from Purdue University.