JEC COMPOSITES MAGAZINE - Issue #95 - Special JEC Europe - March 2015 - 20

thermoplastics

mance and are lighter still, but
also more costly. Type IV tanks
are the newest, lightest, and most
expensive. Extended exposure
to hydrogen can cause embrittlement of certain metals and metal
alloys, reducing tank durability.
Hybrid (mixed) material tanks
(Types II-IV) are prone to fatigue
issues at the multi-material
interface, and also are difficult to
separate for end-of-life recycling.
All four types are heavier and
bulkier than is desirable for passenger cars. Seeing opportunity
in such challenges, a multi-year,
multi-partner consortium based
in the U.K. has been working on
alternatives to current storage
cylinders in the hopes of finding a
more suitable technology.

Evaluating thermoplastic
alternatives
Between 2010 and 2014, the DuraStor consortium for low-cost,
durable thermoplastic hydrogen
storage tanks explored options
for the design and production of
a new hydrogen storage tank. The
programme's charter was to investigate alternative technologies
that could be used as a drop-in
replacement for current hydrogen
tank technology. Initial team

members and their responsibilities included:
- EPL Composite Solutions:
project leadership and vessel
consolidation;
- Celanese Corp.: development
and supply of vessel materials;
- Crompton Mouldings Ltd.:
design and production of rotomoulding tool and rotomoulding
cylinder liners;
- CTG Ltd.: filament winding
vessel reinforcements;
- Delta Motorsport Ltd.: market
research and case study design;
- Element Materials Technology
Hitchin Ltd.: testing;
- Oxford Brookes University: lifecycle analysis and recycling study.
Unusual for a research project,
the team members were selected
not only for their technical expertise, but also their willingness to
form a viable supply chain should
the project output lead to commercialization.
Work began with a study to
understand issues and opportunities in this market and to develop
preliminary specifications for a
cylinder design. A patent survey
showed that most intellectual
property in high-pressure storage

More information

Fig. 1: Final design of rotomoulded acetal pressure vessel liners with
connection hardware

vessels involved thermoset composites. This news, coupled with
experience by one team member
who had previously investigated
the permeation resistance of lowcost thermoplastic tank liners,
convinced the team to explore
thermoplastic options. Such
matrices permit reuse of scrap
and facilitate end-of-life recycling
vs. hybrid systems, plus process
faster than thermoset composites
(providing cost reduction opportunities), and offer higher impact
strength and durability at lower
weight than metals. Thermoplastics also provide the opportunity
to produce liner and overwrap
separately, then join them for a
monolithic tank that avoids the
fatigue issues of hybrid systems.

By eliminating all metals (except
for coupling hardware, which can
be isolated and is necessary to
get gas into and out of the tanks),
issues with hydrogen embrittlement and galvanic corrosion
(from contact between metal
and the electrically-conductive
carbon fibre overwrap) can be
reduced or eliminated.
Three semi-crystalline thermoplastics (polybutylene
terephthalate (PBT), polyphenylene sulphide (PPS), and
polyoxymethylene (POM, or
acetal)) known for their good
mechanicals and high chemical
and permeation resistance were
evaluated. Acetal was selected
based on the experience of several

Hydrogen is a preferred fuel source for many fuel-cell vehicles. It is non-toxic, non-corrosive, and non-poisonous. Industry has safely handled it for
decades, but it is challenging to store and this is especially true in the confined spaces available in most passenger cars and especially when programme goals are to reduce vehicle mass in order to increase the driving range. Here are some of the challenges:
1. Low energy density by volume - While hydrogen has 3 times greater energy density by weight than conventional automotive fuels, it also
has far less energy density by volume. Therefore, to store sufficient fuel to ensure an equivalent driving range vs. conventional cars, hydrogen
must be compressed at high pressures (the current preferred form for the fuel) or liquefied and stored at cryogenic temperatures (not currently
deemed practical for passenger cars) to achieve sufficient energy density/unit volume. Each operation is energy-intensive and requires big,
heavy, bulky tanks for safe storage - tanks that do not package well in most passenger cars and can significantly increase the vehicle mass,
thereby reducing fuel efficiency.
2. Extremely light gas - H2 is an extremely light gas that is 14 times lighter than air. To achieve sufficient energy density by volume to reach an
acceptable driving range, it must be either compressed or stored at cryogenic temperatures to pack more molecules into a given space
(increasing its energy density by unit volume - see above).
3. Extremely small molecule - H2 is the smallest molecule known. Therefore, the permeation resistance of storage materials has to be
rigorously tested to ensure the fuel will not leak out.
4. Embrittles some metals and metal alloys - Certain metals and metal alloys are prone to embrittlement when exposed long-term to high
levels of hydrogen. This shortens the life/durability of tanks using these materials.
5. Hybrid storage vessels are hard to recycle and prone to fatigue issues - Tanks using combinations of different materials in order to reduce
weight (e.g. aluminium and steel, or steel or aluminium and composite, or unreinforced plastic or rubber and composite) are difficult to
separate and recycle at the end of tank life. Worse, these hybrid tanks tend to be prone to fatigue issues at the interface of the different
materials, which also shortens tank life/durability.

20 jec composites magazine / No95 March 2015

JEC COMPOSITES MAGAZINE - Issue #95 - Special JEC Europe - March 2015

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