Storage Requirements

· To determine the H₂O tank's material and dimensions that stores water before the electrolysis takes place
· To devise the H₂ tank's capacity required for storing the 400kg/day quantity of Hydrogen
· To determine the H₂ tank's material and dimensions.
· To examine the potential of having more than one tanks for Hydrogen storing
· To determine the O₂ tank's material and dimensions
· The storing infrastructure is taking place underground

Water tank material selection and dimensions:

Water is stored in a tank before the electrolysis process is taking place to extract the hydrogen as a direct product. It is considered appropriate to select steel sheets in manufacturing the water tank. These sheets will be welded together to form a box-shape vessel and additional steel bars will be welded in consequent distances in order to enhance stability and withstanding to the water pressure exerted on the tank walls. An additional water-filtering system will be employed in order to eliminate any impurities.

Water Tank

· Water volume storage demand: 22 m³
· Water tank dimensions: 3*3*2.5 (m*m*m)
· Water tank's actual storage capacity: 22.5 m³ water
· Steel sheets thickness: 3 mm
· Steel bars dimensions: 50*100 (mm*mm) rectangular shape
· Steel bars thickness: 4mm
· Number of steel bars welded on each side of the tank: 3
· Distance between welded steel bars (3 m facet): 733 m
· Distance between welded steel bars (2.5 m facet): 566 mm

The electrolysis is performed at 80 °C and pressure of 30 bar. The produced volume of hydrogen is 191.883 m³/day. The storage of hydrogen is achieved under a pressure application of 200 bar. In order to find the appropriate volume the hydrogen consumes in this pressurised environment it is essential to identify the compression process thermodynamically and to perform additional calculations. The process taking place during hydrogen's compression from the 30 bar lower limit to the 200 bar upper limit is a polytropic process. The following equations apply:

P₁*V₁^γ= P₂*V₂^γ ,

where P₁= 30 bar
P₂= 200 bar
V₁= 191.883 m³/day
γ= 1.4

The resulting value V₂= 50 m³/day and denotes the hydrogen's volume at the 200 bar pressure application.

Two hydrogen storage tanks will be used to store this volume of hydrogen. The application of two tanks instead of one for hydrogen storing satisfies performance and safety incentives: One tank can be used for back-up in the case of the other's under performance because of a leak or any sudden malfunction.

Hydrogen tanks' materials and configurations available:

· Fiberglass-wrapped aluminium cylinder
Pressure limit: 248 bar
Volumetric storage density: 12 kg/m³ of hydrogen
Gravimetric density: 2 wt % (grams of H₂/grams of system weight)

· Carbon fibre-wrapped polymer cylinder
Pressure limit: 290 bar
Volumetric storage density: 15 kg/m³ of hydrogen
Gravimetric density: 5 wt % (grams of H₂/grams of system weight)

· Advanced light weight pressure vessels (using lightweight bladder liners that act as inflatable mandrels for composite overwrap)
Pressure limit: 338bar
Volumetric storage density: 15 kg/m³ of hydrogen
Gravimetric density: 12 wt % (grams of H₂/grams of system weight)

· Stainless Steel pressure vessels
Pressure limit: 390 bar
Volumetric storage density: 17 kg/m³ of hydrogen
Gravimetric density: 12 wt % (grams of H₂/grams of system weight)

Hydrogen tanks' materials and configurations selection:

The material selection for the two hydrogen tanks is stainless steel. The main reason for choosing this configuration lies on the fact that it provides all the necessary warranties for withstanding the 200 bar pressure effectively. It is expected, for safety reasons, a tank that is indicated to withstand such a high pressure limit for an explosive gas like hydrogen to provide a limit of withstanding close to the double of the indicated one in order to avoid any unexpected accidents. Under this segment, both the advanced light-weight pressure vessels and the stainless steel pressure vessels configurations are capable for application. A secondary assessment of the manufacturing cost of both though, indicates that the stainless steel pressure vessels are more affordable.

Hydrogen tanks' dimensions:

The two hydrogen tanks are expected to have a cylindrical shape and to possess a volumetric capacity of 25 m³ of hydrogen each. A sensitivity analysis performed taking consideration of optional dimensions and volumetric capacity revealed that the most appropriate dimensions for the cylindrical hydrogen tanks should be:

Hydrogen Tank

· Inner Radius: 1000 mm
· Length: 8500 mm
· Thickness: 154 mm

At this dimensions the volumetric capacity for each hydrogen tank is 26.70 m³.

Oxygen tank' s materials selection:

Oxygen is a direct product of the electrolysis performed at the hydrogen production stage. In this particular filling station study though, the volume of oxygen occurring is collected and stored under pressure in a vessel for later utilization. The oxygen storage pressure is 30 bar and the oxygen volume occupied at this condition is 96 m³/day. The options for tank material selection are the ones already mentioned for the hydrogen tanks. Again the best option lies in the selection of the stainless steel pressure vessels because of their high pressure withstanding limits and their most affordable manufacturing costs.

Oxygen tank' s dimensions:

The oxygen tank is expected to have a cylindrical shape and to possess a volumetric capacity of 96 m³. A sensitivity analysis performed taking consideration of optional dimensions and volumetric capacity revealed that the most appropriate dimensions for the cylindrical oxygen tank should be:

Oxygen Tank

· Inner Radius: 2000 mm
· Length: 8000 mm
· Thickness: 40.5 mm

At this dimensions the volumetric capacity of the tank is 100.48 m³.