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In our earlier papers on thermocontrolled tank storage system, we presented the basic concept of storing natural gas and the results from optimization of the vessel design. In this paper we are presenting optimization results of the entire storage system, assuming that the vehicle is running in a certain pre-scheduled pattern. Two kinds of vehicles have been considered for investigation, a city bus with a continuous running schedule and a car with a short run-park schedule. The initial waiting time, parking time, safe pressure ratio and range of the two vehicles have been investigated and then optimized to define the performance requirements.

To increase the operational range of the vehicle equipped with direct injection gas supply and to eliminate the problems associated with the cryogenic pumps, a composite high pressure vessel is proposed for use as a thermocontrolled tank in vehicles. The test results showed that the liquefied natural gas can be brought from atmospheric pressure to the high pressure required for direct injection in a reasonable time period. Based on these results a mathematical model for the heat transfer to the natural gas was developed and simulated. Three different aspects of gas storage, namely, the initial operational temperature, the gas to vessel mass ratio and the gas weight to vessel volume ratio have been investigated and formulated as objectives for optimization. Due to the presence of different conflicting objectives, the problem was formulated as a nonlinear multi objective optimization problem and then solved.