The concept:
As per the literature the two different strategies mentioned above are planned to employ at the same time. Additionally, the prototype is also to be designed with intention for evaluation of more conventional hydrogen storage materials like LaNi5 for its performance. Enhancing the thermal transfer of metal hydride beds during hydrogen absorption/desorption cycles are expected to achieve by incorporating additives to Mg based systems (improve thermal conductivity) and including internal/external heating/cooling (heat exchangers) systems. Further, stacking the powder bed for larger thickness is not recommended as it would induce pressure difference and add one more barrier of hydrogen diffusion through the stack to reach the bottom layer of the stack. Thus, to avoid this, different powder-bed units with limited thickness is proposed. As the storage tanks are intended to be use in portable applications, non-displacement of powder bed is highly desirable and therefore the covered powder-bed units with opening for hydrogen. This opening is secured with Ni-gasket to avoid escape of powder through the opening. As heat exchanger, the main gas and powder storing part is connected with axial and outer heat exchange units.
Schematic
of prototype tank.
The prototype tank design
1. Powder-bed unit module:
Initially
the powder bed unit was designed with constraints of the possible minimum
thickness of aluminium (>1.5mm) and the desired powder stack thickness (<5mm span="" style="mso-spacerun: yes;"> The parts were also modified to incorporate
the axial heat-exchanger unit.
Additionally,
to constraint any displacement of powder bed out of unit part during its
application in portable devices the unit-cover part is also designed. This
unit-cover part will keep the powder in bottom unit and will allow the hydrogen
to go in through the drilled holes. The holes are protected with Ni-gasket
(~0.5µm) to avoid the powder particles to escape through the hydrogen
inlet/outlet holes.
The
dimensions of cover part and powder bed bottom part are so selected that they
get firmly fixed. The support for gasket part is supposed to fix under the
cover part and must be supported from inside.
For higher temperature operated tanks (Mg based composites as active
material) the rigid support while for lower temperature operated tanks (LaNi5
as active material) spring for support can be used. Both kind of support and
the resulting fixture.
Moreover,
for improved thermal exchange between the hydride bed and heat exchanger can be
achieved by employing the metal foams. In that case the support for gasket is
replaced with metal foam part with depressed spots, where the gasket parts can
be fixed (Figure G.5).
2. Axial heat exchanging part:
The axial
heat exchanging part has to serve dual purpose. Firstly it would allow the
extract heat (as name suggest) and secondly it should also act shield for entire
gas/ powder bed unit module storing part. The easiest way to achieve this is to
design an axial tube passing through all of the assembly. However, it will
remove make the whole design as hanging device, as the top and bottom part of
design will have opening for heat-exchanging fluid to pass. To avoid this
situation, the co-axial tubular design for axial heat exchange is designed. This part also will have opening for the main inlet/outlet of
hydrogen. The co-axial tube will allow the heat-exchanging fluid to flow in and
out of system using same space.
Two parts were fitted co-axially to obtain co-axial cover, which
serves the desired dual purpose. The side tube (off-centre) presents
inlet/outlet for hydrogen. Reddish central tube will allow the heat-exchanging
fluid to enter the system. Upon collision at the bottom of yellowish part the
heat-exchanging fluid is diverted up-side to allow the heat-exchanging fluid to
come out of the outer-central tube (horizontal yellow tube). At the top, where
the yellow and red tubes are shown connected is sealed permanently by welding
to avoid any leakage of heat-exchanging fluids.
3. Gas/powder bed unit modules storing part and outer heat exchange part:
Basically, Gas/powder bed unit modules
storing part acts as container of hydrogen and powder bed unit modules and
therefore its gas tightness and rigidness (thickness) is of the very important.
In addition to that this part must be of least tolerance to dis-allow any
lateral movement of unit modules. However, the tight tolerance of the
dimensions of unit modules and this container might impose lengthwise pressure
difference of hydrogen while absorption/desorption. Therefore, to avoid this
situation the inner surface of container is modified for easy passage of
hydrogen gas. To ensure the gas
tightness of the upper surface, i.e. cover part (axial heat exchange part)
receiving surface, was channeled to press fit the graphite/asbestos based
gasket. The graphite/asbestos based gasket allows the gas
tightness even at higher temperature without any deformations.
The face of
this container has twelve drilled hole to receive the six screws each from
upper cover part and from outer heat exchange part. The outer heat exchange
part (jacket) would serve the purpose by allowing heat exchange fluid from
bottom inlet to top outlet upon fixing with container. When jacket part is
fixed with the container, the heat exchanging fluid is then constrained to go
out form the upper opening in the jacket.
To ensure the
fluid-proof sealing of jacket and the container, the upper receiving surface
was channeled to press fit the polymer gasket. The jacket part is meant to be
used with low temperature operated prototype tanks, where the heat is released
during the hydrogen cycling and thereby needs to be extracted. However, for
high temperature operated tanks, the jacket part will be replaced by insulating
and heating element part.
The total
assembly with detailed dimensions is shown in following figure.
The prototype tank fabrication
The powder-bed unit module was designed with constraints of the least possible thickness of Al alloy. Purpose of each part designed in Powder-bed unit module is described below:
- The bottom part – the active material is to be stored inside this part
- Cover part – To avoid spilling off of the powder (active material) from bottom part
- With drilled holes to press fit the gaskets – provide inlet/outlet for hydrogen
- Bottom + cover – Provide discrete stacking of powder-beds in limited thickness
- Stacked units – stacking of unit modules i.e. “Bottom + cover” part
- Unit modules can be stacked about the axial heat exchanging unit
Powder-bed unit module: parts and their stacking.
Axial heat
exchanging part, gas/powder bed unit modules storing part and outer heat
exchange part are fabricated according dimensions in figure G9-G11. ‘cover+
cover lead’ part were welded together to have to secure any possible leakages
of heat exchange fluids. Figure G.14
shows photos of these fabricated parts.
Photograph of fabricated axial heat exchanging part, gas/powder bed unit modules storing part and outer heat exchange part, and purchased asbestos/graphite and polymer based ‘O’ rings.
Photograph of tank parts ready for assembly.
Photograph of assembled tank with the directions of heat exchanging
fluids and hydrogen inlet/outlet port.
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