Conformable Compressed Natural Gas Tank | REL Inc.

Conformable Compressed Natural Gas Tank

Creating great opportunities for American motorists

Natural gas presents a great opportunity for American motorists. It’s clean, it’s low cost, and it’s plentiful. Thanks to fracking and horizontal drilling technologies, America is now a global leader in natural gas production. Plus, it’s distributed to 60 million homes in America.

So, why isn’t it used as a vehicle fuel today?Natural gas isn’t used for two reasons: refueling and storage. There aren’t fueling stations serving natural gas, often delivered under high pressure as in compressed natural gas (CNG). While the infrastructure in CNG fuel stations grows, the fuel container still consumes too much space. Natural gas vehicles (NGV’s), are fairly common across the world with as many as 20 million in use today. The packaging of the cylindrical storage vessel isn’t pretty, nor is it practical, as shown in the photograph to the right.

Convenience is king in America, and many other developed countries. Americans won’t buy a vehicle with so many compromises, on trunk space, range, or places to fill up. Until these issues are solved, CNG will not be a mainstream fuel for the USA.

REL's CNG Tank Under Truck
REL's Conformable Tank Infographic

REL’s Efforts to Solve the Problem

REL is tackling the issue of CNG tank packaging. Nature’s shape for high pressure storage is a sphere, or more practically, a cylinder. However, the enclosed volume around a cylinder is wasted seeing as it is unable to be utilized for gas storage. The non-cylindrical tank is called a “conformable” tank. The conformable tank stores high pressure compressed natural gas in the same place on a vehicle where gasoline is stored today, yet still has enough gas to meet the desired driving range of the vehicle.

Studies have shown an increase from 20-35% more CNG in the same package envelope. Our compressed natural gas tank will meet the needs for storage and driving distance!

The Technology

REL is under a cost-share contract from the Department of Energy’s Advanced Research Project Agency (ARPA-e) initiative to solve the challenge of CNG storage in a non-cylindrical tank. To overcome this challenge, REL is borrowing the shape of a sea urchin from nature. This shape is extremely efficient, one that is structurally and volumetrically efficient for high pressure gas storage. The shape is defined as a Schwarz P-surface. REL designed the tank using the P-surface structure; then manufactured the inner core structure of this shape, and is using its proprietary casting technologies to develop a high integrity casting with a very complex inner geometry.

A Foseco MDU-300 portable degassing unit has been added to the equipment used in the manufacturing of the tanks. This unit allows for the efficient removal of hydrogen gas in the melt. Along with the vacuum test REL now has complete confidence in the quality of the metal prior to the casting process.

REL's Schwarz P surface

Tank Testing Capabilities

In Hydrostatic Burst Testing, the chamber allows for the containment of the tank and control dispersion of any and all energy associated with the hydrostatic burst, as well as a burst with gas pressure should the tank fail while filled with compressed gas.

Cycle testing is a key factor in determining the service life of a tank. REL designed and built a servo controlled cycle tester capable of cycling pressures between ambient and 8000 psi. NGV-2 specifications require the tank to be pressurized from less than 360 psi to 4500 psi 33,750 times for a 15-year life tank and 45,000 times for a 20-year life tank. The rate should be 10 cycles per minute.

burst pressure chamber
REL's penetration test of CNG Tank
During Gas Permeable Testing, NGV-2 only requires gas permeation testing on Type IV containers, with that being said it was determined that it would be in the best interest of REL to ensure no permeability in the tanks prior to sending to third party testing. REL’s test cell is equipped with a Phill home refueling appliance, this gives REL the capability to conduct other tests along with the gas permeability tests using natural gas. With the use of an accumulator tank fast fill processes can be evaluated, the temperatures during the filling and discard of the tank will be recorded to validate previous mathematical models.
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The Applications

REL's CNG applications
Generally speaking, the worse the mileage, the better the payback for low cost natural gas. Today, the Class 8 trucks including refuse trucks, are great candidates for natural gas because diesel fuel is a leading cost, and they can have structured routes and/or come home each night. Fleets will develop their private fueling station; will equip the vehicles with natural gas; and will see a return on investment in as little as two years.

The next application is for smaller vehicles, the airport shuttle buses, snow plows and other delivery vehicles. Next are the contractor pickups, a heavy duty pickup that gets 10-13 mpg and is driven regularly. To these contractors, the cargo bed space is premium space, and to use as much as 40% of it for fuel is not acceptable, and is a significant hurdle to mass market acceptance of CNG.

REL’s initial target application is the pick-up truck, to replace the cylinder in the bed, to allow contractors to use the cargo space for cargo.


REL has accepted the ARPA Challenge, and is working to make it real. In this process, REL has developed unique structures and processes, and has filed four patents. The goal of the ARPA program is a 9 gasoline gallon equivalent (GGE) tank that is tested for vehicle applications. Through various process improvements the target burst pressure of 8100 psi was achieved. Additional work is ongoing to further optimize the weight, energy density, and specific energy of the tank. The test standards are designed for cylindrical tanks, so REL is working with the tank test community to determine a fair test parameter that ensures the safety and performance of the conformable tank design.

REL is seeking financial and commercial partners to bring resources and investment for future product development, manufacturing scale-up, sales, marketing and distribution. If you are interested, please contact REL today.

REL Inc.