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