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CNG TECHNOLOGY

Gas Containment System

EnerSea achieves its cost-effective design by optimizing the pressure and temperature conditions of gas stored in its VOTRANS™ system, which offers the following key advantages:

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  • 50%+ lower operating pressure than other CNG concepts

    • Lower compression horsepower and fuel requirements

  • 60%+ greater gas storage efficiency than other CNG concepts

    • Less pressure and lower temperature means more gas stored per ton of steel

  • Use of carbon steel pressure vessels allows conventional pressure vessel fabrication

    • No coiling or fiberglass winding

    • No single-sourced fabrication

  • Less containment steel also reduces total ship weight and related fuel use​

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The practical application of Boyle’s Law for real (non-ideal) gases provides the basis for EnerSea’s optimization process. The law states that PV = NrTZ; where P is the gas pressure, V is the containment volume, N is the mass (moles) of gas, T is the temperature of the gas and R is the gas constant. The empirical “compressibility factor” (or “Z factor”) of non-ideal gases is sensitive to both temperature and pressure.

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As indicated in the above diagram, gases stored at lower temperatures also have lower Z factors. For a given pressure, lower temperatures (with associated lower Z factors) allow more gas to be packed into the storage container. Also, for a given gas temperature there is a distinct Z factor minima and associated pressure on the curve that represents the maximum compressibility effect for the gas.

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Designing the VOTRANS system to operate at or near the minimum compressibility factor for a specific gas means that a denser “volume-optimized” gaseous state can be achieved, with a much more commercially attractive “specific capacity” (ratio of the volume of gas at ambient conditions to the volume of gas at storage conditions) at a relatively low pressure. The gas will be compressed and cooled prior to entering the gas containment system. As an example, EnerSea can store a typical natural gas onboard at approximately 125 bar (1815 psi) and -30oC (-22oF).

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Bottom-line, the overall cost to load and store CNG in EnerSea’s VOTRANS system is significantly lower than alternative CNG designs.


More details regarding comparisons with other technologies are available.

Regional Opportunities

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