Maddali Ramgopal

Theoretical performance of an adsorbed natural gas storage system subjected to variable charge–discharge conditions

A transient heat and mass transfer model is developed to simulate the performance of an adsorbed natural gas (ANG) storage system subjected to variable charge and discharge conditions. The obtained results are applicable for internal combustion engines with natural gas as the fuel. The ANG system performance is presented in terms of delivery capacity. Results are obtained for the cases of (i) constant pressure charging with a constant flow discharging and (ii) constant flow charging and discharging. Results show the importance of improving the heat transfer performance of the reactor, particularly at high specific gas flow rates. From the second law analysis of the system, it is seen that significant internal entropy generation occurs at high specific flow rate and low heating fluid temperatures. Based on the minimum specific entropy generation criterion, the optimum heating fluid temperatures for different ranges of specific flow rate of the adsorbate are found.

Development of thermodynamic and reaction kinetic data on BaCl2-NH3 system with and without expanded natural graphite

BaCl2 as sorbent in ammonia-based solid sorption refrigeration systems offers advantage due to its lower heat of reaction. This article presents the experimentally obtained material, thermodynamic and kinetic properties of BaCl2 and BaCl2/expanded natural graphite composites suitable for refrigeration. Values of enthalpy and entropy of reaction of BaCl2 obtained from the present study compare favorably with the literature reported values. Results showed that the reaction enthalpy and entropy of BaCl2/expanded natural graphite are lower than that of pure BaCl2. Using the measured reaction rate data and regression analysis, values of relevant kinetic coefficients for absorption and desorption are obtained for BaCl2-NH3 and BaCl2-expanded natural graphite-NH3. BaCl2-expanded natural graphite is seen to offer favorable properties in terms of lower enthalpy of reaction and better kinetics. It is expected that this study which demonstrates the influence of additives on thermodynamic and reaction kinetics is useful in the synthesis and evaluation of new sorbents and additives suitable for refrigeration and air conditioning.

A new tank configuration for large scale storage of natural gas in adsorbed form

Natural Gas (NG) is an inexpensive and clean burning fuel. Hence it has attracted the attention of the policy makers in several countries including China and India for its commercial and industrial use.1 However the challenge of safe and compact storage of NG, due to its low energy density need to be addressed before it can be considered for its large scale use in various portable as well as stationary applications. Among the various storage methods available at present such as compressed natural gas (CNG), liquefied natural gas (LNG) and adsorbed natural gas (ANG), the last option (i.e., ANG) offers many benefits interalia safety, design flexibility of the storage tank, low cost etc..2–7 On the other hand the exothermic and endothermic nature of the adsorption and desorption processes with poor transport properties of the adsorbent particles strongly affects the storage and delivery capacity, thereby reducing the driving range of the vehicles substantially, when ANG is used for transport applications. Chang & Talu7 observed a reduction of 35% storage capacity under adiabatic conditions, as compared to an isothermal charge case. In view of the importance of effective heat and mass transfer, many researchers have carried out studies on heat and mass transfer management of the adsorbent bed undergoing charge and discharge cycles. Different heat transfer enhancement techniques such as provision of internal and external fins, insertion of tubes carrying hot water during desorption, multi cylinder ANG tanks with centrally located heat pipes etc. have been proposed.8–17 Studies are also carried out on prediction of the structural, thermo-physical as well as adsorption characteristics of different activated carbons.18,19 Several cities in India and elsewhere have already introduced CNG based buses and other vehicles. There is also a growing interest in running locomotives using NG. Attempts were also made to use natural gas for fishing boat engines.20 An important requirement of large scale storage of NG is seen in the filling stations. In most of these applications, design of ANG reactors with fast charge/discharge characteristics is essential. Since charging is exothermic and discharge is endothermic, use of suitable thermal energy storage system can minimize the costs associated with cooling and heating during charge-discharge processes. However, no studies on such ANG systems are available in open literature. In this paper a shell and tube type reactor suitable for large scale storage of NG is considered. Due to high storage pressure (35 bar) of NG, the adsorbent (Activated carbon) and adsorbate (NG) are confined to the tube side, while the external fluid (water) flows on the shell side in a closed loop. The water on the shell side acts as an energy carrier absorbing heat of adsorption from the bed during charging and supplying the same to the bed during discharging. A mathematical model based on reactor heat transfer and kinetics is developed to simulate the performance of this ANG system under variable charge-discharge conditions. Effects of important design and operating parameters on the performance of the reactor are studied in detail so as to suggest means for controlling the system performance. It is expected that this study will be useful in the practical design and evaluation of ANG systems for large scale storage of NG.

Thermodynamic Analysis and Optimization of a CO2 Based Transcritical Refrigeration System with an Ejector

In the present paper some results obtained from the thermodynamic analysis of a CO2 based transcritical refrigeration system are presented. The system studied uses an ejector as an expansion device in place of the conventional throttle valve. Results are obtained considering constant pressure mixing process in the mixing chamber of the ejector. The compressor discharge pressure and ejector entrainment ratio are optimized for maximum COP. The required compressor displacement rate per ton of refrigeration (specific piston displacement rate, PD) is calculated at the optimum conditions. A correlation is obtained for PD in terms of the evaporator, gas cooler exit and ejector efficiencies. From the sensitivity analysis it is seen that of all the parameters, the PD is most sensitive to evaporator temperature.