Exergy analysis of a high-temperature-steam-driven, varied-pressure, humidification–dehumidification system coupled with reverse osmosis

In this study, exergy analysis of a novel desalination system is presented and discussed. The water desalination is carried out using combined humidification–dehumidification and reverse osmosis technologies. Six system performance parameters are examined: overall exergetic efficiency, equivalent electricity consumption, specific exergy destruction, specific exergy lost, and total true specific exergy lost, as well as the exergy destruction ratios of the main components. The total true specific exergy lost is a new parameter presented in this study. It is a function of summation of total the exergy destruction rate and loss per total mass flow rate of the total pure water produced. This parameter is found to be a useful parameter to assess the exergetic performance of the system considered. By contrast, use of overall exergetic efficiency as an assessment tool can result in misleading conclusions for such a desalination system and, hence, is not recommended. Furthermore, this study reveals that the highest exergy destruction occurs in the thermal vapor compressor, which accounts for 50% of the total exergy destruction of the system considered. This study, in addition, demonstrates that the specific exergy destruction of the dehumidifier and TVC are the parameters that most strongly affect the performance of the system.Center for Clean Water and Clean Energy at MIT and KFUP

Variable Pressure Humidification Dehumidification Desalination System

Nature uses air as a carrier gas to desalinate seawater through evaporation and rain. Several investigators have previously studied desalination cycles based on carrier gas processes. However, single pressure carrier gas cycles suffer from low energy recovery and hence low performance. Here we discuss a novel carrier gas cycle which operates under varied pressure. This cycle operates the evaporation process under a reduced pressure and the condensation process at an elevated pressure to enhance energy recovery. The pressure is varied by using a mechanical compressor. This cycle has been found to be several times as efficient as the existing carrier gas cycles. In this paper, the salient features of this cycle are analyzed in an on-design sense by defining a component effectiveness for the simultaneous heat and mass exchange components and an isentropic efficiency for the compressor and the expander. Based on this study, ways to improve the cycle are proposed. The possibility of using a throttle valve instead of an expander and the effect this would have on the overall performance is reported. Comparison of the new desalination cycle with existing ones is also performed in terms of specific work consumption.Center for Clean Water and Clean Energy at MIT and KFUP

Use of multiple extractions and injections to thermodynamically balance the humidification dehumidification desalination system

Humidification dehumidification (HDH) desalination systems are well suited for small scale, off-grid desalination. These systems are very robust and can tolerate a wide range of feed salinities, making them a good candidate for treating produced water from hydraulically fractured natural gas wells. A primary engineering challenge for these systems is their high thermal energy consumption. In this study, we examine the use of multiple air extractions and injections to thermodynamically balance the HDH system, so as to make it more energy efficient. The effect of the number of extractions on several performance parameters is studied. In addition, we study the effect of the enthalpy pinch, which is a measure of performance for a heat and mass exchanger, on these performance parameters. Finally, we present results that can be used as guidelines in designing HDH systems. These results include the identification of appropriate temperatures for the extracted/injected air streams, the division of the heat duty between stages, and the value of the mass flow rate ratio in each stage at various values of enthalpy pinch.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Thermodynamic balancing of a fixed-size two-stage humidification dehumidification desalination system

Humidification dehumidification (HDH) is a desalination technology that has shown promise in small scale, decentralized applications. Previous studies on the multi-staging of HDH have used fixed-effectiveness models which do not explicitly account for transport processes in the components. However, to fully understand the effect of the variation of the mass flow rate ratio, it is necessary to implement heat and mass transfer models of the HDH system. In this paper, we model an HDH system consisting of a packed-bed humidifier and a multi-tray bubble column dehumidifier. We study the effect of the mass flow rate ratio on the performance of a fixed-size system, and we consider its effect on the entropy generation and the driving forces for heat and mass transfer. In addition, we define a generalized energy effectiveness for heat and mass exchangers. We also implement an air extraction/injection and simulate a wide range of operating conditions. We define criteria for the best system performance, and we study the effect of the distribution of available area between separate stages. We also present a thorough explanation of why the direction of extraction should always be from the humidifier to the dehumidifier.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Thermodynamic Balancing of the Humidification Dehumidification Desalination System by Mass Extraction and Injection

Humidification dehumidification (HDH) is a promising technology for small scale seawater desalination and has received widespread attention in recent years. The biggest roadblock to commercialization of this technology is its relatively high energy consumption. In this paper, we propose thermodynamic balancing of the humidifier or the dehumidifier through mass extraction and injection as a potential means of reducing the energy consumption of these systems. Balancing minimizes the entropy generation caused by imbalance in driving temperature and concentration differences. We outline a procedure to model the system, using on-design component variables, such that continuous or discrete extraction and/or injection of air from the humidifier to the dehumidifier or vice versa can be analyzed. We present an extraction profile (mass flow rate ratio versus non-dimensional position) in the dehumidifier and the humidifier for attaining close to complete thermodynamic reversibility in an HDH system with a 100% effective humidifier and dehumidifier. Further, we have examined in detail the effect of having finite-sized systems, of balancing the humidifier versus the dehumidifier, and that of the number of extractions.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08)United States. Dept. of State (International Fulbright Science & Technology Award

Estimation Of Demand For Fish In Delhi And NCR, India

Fast population growth, increase in per capita income and increase in level of awareness among the people regarding health are the main causes of increase in demand for nutritional and protein rich food. Fish is very good source of protein as well as vitamins. Fish may play a vital role to ensure the nutritional security in rural areas. Fish production and consumption has however undergone major uneven changes in the past four decades. It is found that at higher ends of the income distribution, the consumption of milk, eggs, meat, fish and processed foods have risen