Experimental studies of the performance of adsorbed natural gas storage system during discharge

https://doi.org/10.1016/j.applthermaleng.2004.07.002Get rights and content

Abstract

In this work, comparative experiments were carried out to study the thermal effect of the adsorption heat on the discharge performance of an adsorbed natural gas (ANG) storage system. A storage vessel with u-shaped heat exchanging pipe round the central region was designed according to the temperature field determined by the finite element analysis of the adsorbent bed within 1 L cylinder. Discharge performances of the ANG on the activated carbon within the storage vessel were tested on a volumetrically built experimental unit. Results show that the central region of the adsorbent bed suffers from the severest temperature fluctuation in a short period of the initial discharge state; introduction of the hot water whose temperature is similar to that of the cooling water of a vehicular engine can significantly moderate the temperature fluctuation of the adsorbent bed, shorten the discharge process for about 60 % in comparing with that without the application of the supplemental heat. Conclusions are drawn that the heat from the cooling water of a vehicular engine should be a consideration to improve the discharge performance of the ANG storage system.

Introduction

Mobile applications of natural gas (NG) at present are limited mainly due to its low volumetric energy density [1]. NG stored as an adsorbed phase in porous materials is referred to as ANG, which has the potential for applying on the natural gas vehicles (NGVs) because of its lower storage pressure (about 3.5 MPa) [2]. However, how to effectively manage the thermal effects resulted from the adsorption heat on both charge and discharge processes of the ANG is still a bottleneck for developing the vehicular ANG storage technology [3], [4], [5], [6], [7], [8], [9], [10].

From the relevant literatures to the ANG storage, we can find that most studies whether by experiments or from theoretical approaches focused on the adsorption equilibrium or the performance in the charge process [11], [12], [13], [14], [15], [16]. Generally speaking, while ANG is released from a storage system, the adsorbent bed will be cooled down since desorption is an endothermic process and a considerable amount of the gas will accordingly be retained in the system under the depletion pressure in comparing with that of the isothermal operation. For vehicular applications, the discharge performance of the ANG should be paid more concerns since the discharged amount is directly demanded by the output power in need, therefore the discharge time cannot be widely varied to moderate the impact of cooling down during discharging.

Some measures have been suggested to increase the conductivity and the thermal capacity of the ANG storage system for moderating the temperature fluctuation of the adsorbent bed [9], [10]. In general, the re-supplied heat can be from the heat of adsorption transferred by the phase change material or the heat from surroundings. However, putting the phase change material into the storage system will consume some of the capacity and reduce the volume available for storage; mechanical way to enhancing the heat transfer from the radial instead of the axial will make the storage system much complicated, and this sometimes cannot supply sufficient heat due to the low heat capacity of the methane molecules surrounding the storage vessel.

In this work, performances of discharging ANG are studied by comparative experiments. Firstly, a storage vessel with u-shaped pipe round its central region is designed by the temperature field of an adsorbent bed determined by the results from the finite element analysis. Secondly, discharge performances of ANG in the designed vessel were tested on a volumetrically built experimental unit, the effect of the hot water whose temperature is similar to that of the cooling water of a vehicular engine has also been evaluated. Finally, comments are made on the improvement of the discharge performance of the adsorbent bed by the introduction of the cooling water of the engine.

Section snippets

General scheme

As a general rule, under normal working conditions, the temperature of the cooling water outlet the cooling space of an automobile is about 343–353 K, which is maintained by the heat exchange between the cooling water and the surrounding air in a radiator tank [17]. For utilizing the heat of the cooling water, here we lead some part of the cooling water into the central region of the ANG storage vessel by the water pump through the heat exchange pipe, this on one hand lowers down the

Thermal effect under different charge and discharge pressures

During the experiment, we firstly charged the storage vessel to a certain equilibrium pressure, then discharged under different back pressures. For comparing, here we select one of the whole processes under charge and discharge pressures respectively 4.6 MPa and 0.1 MPa. Variation of the temperature in the very central region of the storage vessel is shown in Fig. 5, from which we can find that the temperature quickly rise to the maximum in a short period after the quick charge of the methane

Conclusions

To limit the impact of the adsorption heat on the discharge performance of the ANG storage system, the temperature field of the adsorbent bed, the flow rate and the accumulated discharge amount of the ANG are studied by comparative experiments. The following conclusions can be drawn:

  • 1.

    The central region of the adsorbent bed suffers from the severest temperature fluctuation during discharging the ANG; application of the supplemental heat should be emphasized in this region for limiting the

Acknowledgments

The author would like to thank Prof. A.Z. Gu, Prof. X.S. Lu and Dr. Q.R. Zheng for their constructive comments and suggestions on this manuscript.

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