Thermal Effects During Refueling of the Reservoir

The adsorption of a gas is highly temperature dependent: the amount of adsorbed gas decreases with increasing temperature. Therefore, the storage capacity of an adsorptive reservoir decreases with increasing temperature. On the other hand, physical adsorption is an exothermic process: the adsorption of a gas releases heat. In the specific case of NG adsorption on activated carbon, there is a significant amount of heat (17 kJ/mol) that is released.

If the reservoir is quickly charged, the bed temperature will increase considerably. Less gas will be stored, which has a negative impact on capacity. Computer simulations have shown that in a quick charge the temperature inside the carbon bed can increase by 55°C, reducing the capacity by about 25% with respect to an isothermal charge [9]. This prediction has been confirmed experimentally [10].

The ideal charge should be carried out with a sufficiently small feed flow rate in order to allow the heat of adsorption to dissipate to the environment. This way, one would prevent the undesirable heating of the reservoir and its consequent capacity loss. Unfortunately, this process cannot be widely applicable in practice because the duration of a refueling operation cannot be excessively long (below 10 min), otherwise NG would loose competitiveness against liquid fuels which require very short refueling times.

There are, however, a few transportation sectors where the above-mentioned procedure can be applied. These are fleets whose normal driving range is predefined, as is the case of public transportation. Usually, the dimension and consumption level of such type of fleets justifies the investment on the construction of small private refueling stations. In these cases, the reservoirs can be charged overnight which would provide enough time to dissipate the heat of adsorption.

A solution that can be applicable to a quick refueling consists of an external recirculation of the charged gas to remove the heat of adsorption. During the charge of the reservoir, the gas at 35 atm is continuously recirculated through the reservoir in a closed loop. The heat released by the adsorption process is removed from the reservoir by the circulating gas and released to the environment through a heat exchanger and a cooler (Figure 5).

Figure 5. Schematic drawing of a natural gas refueling station. The gas supplied by the residential network flows across a filter, to retain the heavier components and prevent the poisoning of the carbon bed, and is charged into the reservoir. The heat released by the adsorption process is removed by the recirculating gas and dissipated to the environment.

Figure 5. Schematic drawing of a natural gas refueling station. The gas supplied by the residential network flows across a filter, to retain the heavier components and prevent the poisoning of the carbon bed, and is charged into the reservoir. The heat released by the adsorption process is removed by the recirculating gas and dissipated to the environment.

An alternative method to control temperature is to introduce into the reservoir an encapsulated material (usually an inorganic salt) that changes phase at ambient temperature and that has a high heat of fusion. With the phase change form solid state to liquid, the material absorbs the heat of adsorption, acting like a surge for temperature changes [7].

The use of a phase-change material has, however, the disadvantage of reducing the available volume for the carbon bed. Consequently, there is an optimum amount of phase-change material that maximizes the storage capacity. This method has been studied both experimentally and computationally. Although it is appealing, it has some disadvantages such as the high cost of the phase-change material and its possible degradation when subject to a large number of cyclic phase-changes.

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