Double-stage Systems Research Articles

CO2 and H2S binary sorption on polyamine modified fumed silica

Low cost fumed silica modified by amine addition with the ability to remove both carbon dioxide (CO2) and hydrogen sulfide (H2S) was developed. The effect of amine type (polyethyleneimine with a molecular weight of 800 (PEI800), aminoethylethanolamine, N-(3-trimethoxysilypropyl) diethylenetriamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine), loading level and sorption temperature were investigated. The selected sorbent was then evaluated in a single- and double-stage systems. The polymeric amine PEI800 gave the best compromise between the highest CO2 and H2S sorption capacities with minimal amine leaching, and was optimal at a 40% loading level onto fumed silica, in terms of giving the highest breakthrough and saturation capacity. The amount of available amine groups in the sorbent and the interaction between amine groups and CO2 or H2S were the two major factors that affected the sorption capacity and amine efficiency. The sorption of H2S was predominately determined by thermodynamics, while a high temperature helped the diffusion of CO2 molecules from the surface into the bulk of PEI800. The sorbent favored CO2 sorption and the sorbed CO2 hindered the sorption of H2S. However, the sequential sorption of CO2 followed by H2S in a double-stage system was found to solve this problem.

Performance comparison between a conventional vapor compression and compression-absorption single-stage and double-stage systems used for refrigeration

This study reports a comparison from the first and second law of thermodynamics of a conventional vapor compression cooling system, a compression-absorption single-stage (CASS) system, and a compression-absorption double-stage (CADS) system operating with CO2 and R134a in the compression cycle and H2O/LiBr in the absorption cycles. The CADS system is being by the first time proposed in the literature. The performance of the systems were analyzed as function of diverse operating parameters. It was found that the electrical energy consumption in the refrigeration cycles was about 45% lower than in the classical compression refrigeration cycles using CO2 and R134a as refrigerants under the same operating conditions. The results showed that the COP for the CADS could be 50% higher than those obtained with the CASS system. The systems operating with R134a always achieved higher COP than those obtained using CO2. From the exergy analysis it was clear that the highest irreversibilities occurs in the absorber and the evaporator for both mixtures. It was also found that the irreversibilities of the proposed system using R134a in the compression cycle were 17% lower than those obtained with the system using CO2

Single-stage and double-stage photovoltaic driven regeneration for liquid desiccant cooling system

Liquid desiccant cooling system (LDCS) is a novel air-conditioning system with good energy saving potential. However, the present LDCS has a poor performance, mainly because the conventional thermal regeneration method wastes too much energy during the regeneration process. To improve that, photovoltaic-electrodialysis (PV-ED) regeneration method is introduced: it has a higher performance by using solar photovoltaic panels to drive an electrodialysis regeneration process. To further explore the PV-ED method, both single-stage and double-stage photovoltaic-electrodialysis regeneration systems are presented in this paper. Analysis is made on these two systems and some influential factors are investigated. It reveals that the concentration difference between the desiccant solution before and after regeneration has a strong impact on system performance. Moreover, comparison is conducted between the single-stage and the double-stage systems, the results show that the double-stage system is more energy-efficient and it can save more than 50% energy under optimized working conditions.

Performance of three-heat-reservoir absorption cycles with external and internal irreversibilities

The ideal three-heat-reservoir (THR) model for absorption refrigeration cycles is extended to include external and internal ireversibilities. The operating point of the cycle is determined by equating the internal entropy generation to the net entropy transfer from the working fluid. The main features of the entropy transfer function are presented. Three empirical functions are used to model the internal entropy generation of the cycle. The parameters of the these functions are estimated by fitting data obtained by simulation to the predictions of the THR model. The THR model using a linear function or a logarithmic function for the internal entropy generation is able to reproduce performance data for single-stage and double-stage absorption systems with good accuracy. The THR model provides a convenient means for representing performance characteristics of absorption cooling systems over a wide range operating temperatures. It can be used in preliminary design studies and in larger simulation programmes in which the absorption machine is a sub-component.