Advantages of Low Pressure Storage

For the NG vehicle to stir up a strong and sustainable interest of the transportation market, it must compete with conventional vehicles in terms of economy, performance and ease of use, because the price of fuel is not the only determining market factor. For this aim to be achievable, the system for storage, distribution, and utilization of NG must be economic, light, compact, and efficient.

Natural gas is a gaseous fuel because its main component - methane -cannot be liquified at ambient temperature. As a consequence, the energy content of natural gas on a volume basis is low. Currently, NG must be stored under compressed form at pressures in the range 180-240 atm, to be stored compactly and dispensed quickly.

The equipment employed in high-pressure compression storage systems is costly, bulky, heavy, and complex, compared to that of storage and distribution of liquid fuels from oil. The cost of the compression equipment, storage reservoirs and high-pressure metering equipment, corresponds to about 50% of the total investment required to convert a fleet of vehicles to NG operation.

The high cost of private refueling stations has discouraged fleet operators to opt for NG. On the other hand, the individual consumer is not interested in converting his vehicle to dual-fuel operation, and even less to NG only, unless there is a substantial network of NG refueling stations. Lastly, the private sector is apprehensive to invest on the construction of public NG refueling stations as long as the NGV market stays small.

The need to substantially reduce the maximum storage pressure, for safety reasons and the high-pressure compression cost, has led to the search for competitive storage alternatives for NG at pressures well below that employed in traditional compression. The maximum operating pressure value that has reached a large consensus is of the order 35-40 atm, although a few researchers suggest even lower values (21 atm). The maximum operating pressure in this range of values reduces considerably the cost of building and operating a NG refueling station. In places close to the NG residential network, the gas can be compressed to 35 atm by a single-stage compressor, which allows for the construction of small refueling stations at a reduced cost. In the outskirts of metropolitan areas, the servicing stations can refuel the vehicles directly from the high-pressure gas distribution network without the need for compression equipment.

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