Thermal Effects During Discharge of the Reservoir

During discharge of the reservoir the desorption of the gas consumes energy. This consumption gives rise to a decrease in the temperature of the reservoir. If the gas discharge rate is not sufficiently slow, or if the heat of adsorption is not resupplied to the reservoir, the temperature drops (Figure 6), thereby increasing the residual amount of gas that remains in storage a depletion pressure [7]. The net deliverable capacity will decrease. It should be noted that this problem cannot be solved by manipulating the discharge flow rate, because this variable is controlled by the power requirements of the engine.

Probably the best way to solve this problem is to employ the energy content of the exhaust gas, which leaves the engine at high temperature, in order to compensate for the heat of adsorption consumed during discharge [11]. The usual course of the exhaust gas is altered, before it is expelled to the environment, by forcing it to flow along a jacket that raps the reservoir, along which the hot gas transfers heat to the carbon bed in contact with the internal wall of the reservoir (Figure 7). From the environmental point of view, this process has the added advantage of reducing the temperature of the expelled exhaust gas.

In traditional storage cylinders the gas flows axially. This means that radial heat transfer across the carbon bed takes place solely by conduction. In this case, the low thermal conductivity of the carbon bed acts as a heat

Dimensionless radial position, r / R

Figure 6. History of radial temperature profile during discharge of a methane adsorptive storage cylinder. Symbols represent experimental temperature measurements at radial locations in the reservoir, taken every 20 min; the curves are the prediction of a simulation model of the reservoir. Cylinder dimensions: L = 74 cm, D = 20 cm; amount of carbon = 15.78 kg; discharge flow rate = 6.7 L/min.

Dimensionless radial position, r / R

Figure 6. History of radial temperature profile during discharge of a methane adsorptive storage cylinder. Symbols represent experimental temperature measurements at radial locations in the reservoir, taken every 20 min; the curves are the prediction of a simulation model of the reservoir. Cylinder dimensions: L = 74 cm, D = 20 cm; amount of carbon = 15.78 kg; discharge flow rate = 6.7 L/min.

Figure 7. Virtual image of a natural gas storage reservoir with temperature control system. The heat of adsorption consumed during discharge is compensated by the energy of the exhaust gas. Heat transfer to the carbon be takes place whilst the gas is flowing along the jacket wrapping the cylinder. (•: natural gas; •: exhaust gas.)

transfer resistance, thereby limiting the usefulness of the thermal capacity of the cylinder wall and the energy of the exhaust gas. This problem can be somewhat reduced by introducing internal fins that increase the conductive heat transfer and change the flow direction from axial to radial (Figure 8). Convective heat transfer to the center of the reservoir is, thereby, enhanced.

Figure 8. Temperature profiles inside a natural gas adsorptive storage cylinder with external recirculation of the exhaust gas and internal finning; the profiles refer to the end of the discharge. If the gas flows into the jacket at a temperature of 80°C, the net storage capacity of the system is identical to that of an isothermal system. L = 74 cm, D = 20 cm, thickness of annular space of the jacket = 5 mm, fin thickness = 2.5 mm.

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