Multi-effect Desalination System Research Articles

A low-temperature multi-effect desalination system powered by the cooling water of a diesel engine

This paper introduces a medium-size low-temperature multi-effect desalination (LT-MED) system. Powered by the cooling water of a 1000kW diesel power generator set, the system can produce 60tons of fresh water per day. First, the thermodynamic and heat transfer model of the LT-MED system is derived. Second, an integrated four-effect tower distiller is designed and built. The system is installed in the Guishan Island, Zhuhai, Guangdong, China, and tested comprehensively. In the operation, the evaporation temperature of each effect is linearly proportional to the heat load of the power generator. Additionally, the evaporation temperature in each effect distiller rose linearly when the heat load was gradually increased. The variation of evaporation pressure was consistent with the evaporation temperature. When the heat load of the power generator increases from 300kW to 530kW, the fresh water production rate increases from 1.26m3/h to 2.30m3/h. The conductivity of the fresh water is usually <100μS/cm. Because of electricity and freshwater co-production, the system is environmental friendly and is particularly useful for islands and offshore platforms.

Thermo-economic comparison of three configurations of combined supercritical CO2 refrigeration and multi-effect desalination systems

This paper proposed a novel combination of the supercritical carbon dioxide refrigeration and Boosted-multi-effect-desalination system. Comparative study of the combined system and two different modifications of it is performed from thermo-economic viewpoint. Thermodynamic and economical optimization is also carried out for the two modified forms; equipped with the expander and two-stage compression. It is found that an acceptable recovery ratio is gained from the introduced system compared to that of the conventional steam-driven MED. For both modifications, thermodynamic optimization results showed that COP and exergy efficiency improve about 12% and 8% respectively, at the expense of 18% increment in the TAC rate, compared to the optimal economical design case. On the other hand, the most enhancement resulted from the thermodynamic optimization was associated with the case in which the recovery ratio was increased up to 200% compared to that of the economical one. However, the TAC rate for the two compressors modification is 2.2% lower than that of the expander one. While, the first modification would significantly decreases the recovery ratio (about 41%). Finally, by the exergo-economic factor definition, a reasonable decision about the stability between the exergy destruction of the components and their capital investment would be achieved.

Multi-objective optimization of MED-TVC-RO hybrid desalination system based on the irreversibility concept

The mathematical model for the performance prediction of a multi-effect desalination system with thermal vapor compression (MED-TVC) and a reverse osmosis (RO), was presented. To develop the mathematical model, the energy and concentration conservation laws were applied by considering the Boiling Point Elevation and various thermodynamic losses. This analysis led to the determination of the thermodynamic properties at different points and the Gain Output Ratio value. The exergy analysis of a sea water RO desalination plant was performed and its performance was investigated. A computational model based on the diffusion and convection transport mechanisms as well as the concentration polarization concept, was developed to predict the performance of the RO membrane. Then, the hybrid MED-RO desalination system was studied, the irreversibility analysis was performed and the exergy destruction (considering chemical and physical exergy), exergetic efficiency and system performance were assessed. To obtain the optimum point of the system, the multi-objective optimization with the genetic algorithm tool was used. The determination of the best tradeoff between the exergetic efficiencies of MED and RO, was the final goal of this optimization. The optimum design led to the selection of a MED-RO hybrid system with the highest exergetic efficiency.

Conceptual design and experimental investigation involving a modular desalination system composed of arrayed tubular solar stills

A novel conceptual design of a low temperature multi-effect desalination (LT-MED) system is presented in this paper. The LT-MED system operates at vacuum pressure and is composed of several modules of tubular solar still (TSS) cells, each of which can independently produce freshwater. To verify the performance of the modules in different stages of the system, an experimental TSS was fabricated and tested under different vacuum pressures and heating conditions from 20 to 80kPa and 100 to 300W, respectively. The results showed that the energy utilization efficiency exceeded 0.7 in most cases, with a maximum value of approximately 0.81, which was nearly double that of a basin still operated at normal pressure. A 5-effect modular MED system for practical application with a capacity of 86kg/d was proposed according to the presented theoretical and experimental work. A structural design and control scheme were also presented. Performance and economic analyses indicated a wide applicability of the system, especially for remote and desert regions.

Performance assessment and multi-objective optimization of an integrated organic Rankine cycle and multi-effect desalination system

Integrated energy systems have attracted ample attention due to the fact that they increase the efficiency of the system and consequently reduce the environmental impacts. Among integrated energy systems, integration of a gas turbine (GT) with a multi-effect desalination (MED) and an organic Rankine cycle (ORC) has introduced itself as a potential option. In this paper, a comprehensive thermodynamic modeling of an integrated system consisting of a micro gas turbine, a single pressure heat recovery steam generator (HRSG), an ORC and an MED is conducted. Exergy and economic analyses are performed. In addition, a multi-objective optimization is performed to determine optimized design parameters. Moreover, an optimization is performed for different working fluids including R123, R134a and R245fa. The distilled water production and the total cost rate (power generation price) are considered as two objective functions. The fast and elitist Non-dominated Sorting Genetic Algorithm II (NSGA-II) is applied to optimize the objective functions simultaneously. The results show that MED evaporator approach temperature has a significant effect on the optimum selected power and distilled water capacity. Furthermore, by increasing compressor pressure ratio, the exergy efficiency increases and the cost rate decreases. Moreover, R134a shows the best exergy efficiency compared with other fluids.

DESIGNING PREDICTIVE CONTROLLER BASED ON NONLINEAR MODEL FOR MULTI-STAGE MED DESALINATION SYSTEMS

http://dx.doi.org/10.5902/2179460X19422Due to the use of fossil energy and environmentally pollutant fuels, groundwater and surface water pollution has led to a more progressive movement towards the technology of desalination systems in recent decades so that the issue of the desalination of seawaters and ocean waters has received great attention from most countries. In the meantime, the use of distillation desalination systems has been welcomed more than other systems because they take advantage of the sun's energy and do not negatively influence the environment. The PDE and nonlinear model of multi-effect desalination systems constitutes the structure under study in this paper. In order to control the variables of this model, a nonlinear predictive model controller (NPMC) has been proposed. The results obtained from the conduct of this controller on multi-effect desalination system in MATLAB Software is indicative of the acceptable performance of the controller.

Heat transfer characteristics in horizontal tube bundles for falling film evaporation in multi-effect desalination system

Horizontal tube falling film evaporation finds wider applications in multi-effect distillation system in recent years. The latent heat released inside the tube due to condensation of vapor is transferred to the falling film on the outer surface of the tube resulting in convective evaporation of water film. The evaporator consists of multiple rows and columns of horizontal tubes. Heat transfer from condensing film inside the tube bundle is more or less uniform while there is a large variation in heat transfer outside the tube bundle. This paper focuses on Computational Fluid Dynamics (CFD) analysis of falling film evaporation of seawater on a single tube and bundle of tubes using ANSYS Fluent 13.0®. CFD results are validated with published data available in the literature and also with experimental studies carried out. The effect of feed rate, tube diameter, wall temperature, etc. on the heat transfer is studied. Local film coefficient around the tubes of 19.05, 25.4 and 50.8mm ∅ for different film Reynolds numbers is discussed. It is observed that convective evaporation heat transfer performance increases with feed rate, but decreases with tube diameter. In-line tube configuration is found to be better compared to staggered tube configuration.

Economic optimization of the number of effects for the multieffect desalination plant

The multieffect desalination (MED) process is one of the most efficient thermal desalination processes due to its low energy consumption. Due to the recent developments in the desalination industry, latest desalination plants are more cost effective than the older technology-based plants. The primary goal of this paper is to investigate a mathematical model of an MED system in such a way that, it is able to predict the optimum number of effects. The number of effects for a specific plant is a major factor in determining thermal economy and capital amortization. Also the gain output ratio of an MED unit is totally dependent on steam economy and is directly proportional to the number of effects. The total cost of water is dependent on two chief costs: steam cost and capital cost. In this paper, a compromise has been made between these two costs to find optimum number of effects that would produce the minimum sum of the two cost components.

Experimental studies on thermal spray-coated horizontal tubes for falling film evaporation in multi-effect desalination system

Multi-effect distillation is an important principle in desalination technology, in which the thin film evaporation of sea water takes place at the outer surface of bundle of tubes, normally arranged in horizontal. Heat of condensation inside the tube is transferred to the falling water film by conduction, thereafter convective evaporation of film takes place. Heat transfer on the outside is unpredictable due to the uncertainty in nature of film, film thickness, film dryout, etc. and film coefficient is considerably less compared to inside. Thermal spray coating on metallic surface with molten metal particles create porous surface which enhances heat transfer due to higher turbulence and nucleation sites, if sufficient temperature difference available. This paper discusses the experimental studies carried out on a 3 × 5 bundle of 25.4 mm ∅ Al horizontal tubes with thermal spray coating of aluminum. Influences on temperature profile, heat transfer coefficient, mass evaporation, etc. are studied. A comparison of sea water and fresh water is carried out for the performance of the system. The experimental setup is established at IIT Madras, Chennai, India.

Experimental Studies on a Solar Low Temperature Multi-Effect Distillation Desalination System

Solar energy has been widely used in desalination systems. A low-temperature multi-effect desalination system driven by solar is constructed for a series of experimental studies. The results show that water production rate grows with solar radiation, and maintains at a high level between 12am to 4pm. The optimized heat water flow is 1400 kg/h and appropriate cooling water temperature is 24 °C, respectively.

Performance of a two-stage multi-effect desalination system based on humidification–dehumidification process

This paper presents a two-stage multi-effect desalination system based on humidification–dehumidification process. The sketch of two-stage multi-effect humidification–dehumidification desalination process is given. The good quality of distilled water obtained by this novel concept favors its use for producing water for drinking and irrigation. A mathematical model based on mass and energy balances in each component of the system is developed. The numerical model is used to investigate the performance of this kind of installation exposed to a variation of the control parameters. The fresh water production can reach 72.6kg/h and the gained output ratio (GOR) can reach 2.44, because it recycles and reutilizes latent heat of condensation and the residual heat in the brine. A series of experiments was conducted and compared with the simulation results to validate the developed models. As a result, the proposed models can be used for describing the behavior of such a type of desalination system.

Current status and future directions of MED-TVC desalination technology

Desalination has now become one of the major water treatment processes in several countries around the world where shortage of water is a serious problem. Energy consumption is a vital economic factor in selecting the type of desalination processes because current desalination processes require large amount of energy which is costly. Multi-effect desalination system with thermal vapor compression (MED-TVC) is particularly more attractive than other thermal desalination systems due to its low energy consumption. MED-TVC is characterized by high performance ratio, easier operation, low maintenance requirements and simple geometry. These attractive features make MED-TVC highly competitive to other well-established desalination techniques that include the reverse osmosis and multi-stage flash desalination. The primary goal of this paper is to present a preview of some aspects related with the theory of the technology, parametric study of the MED-TVC systems and its development. It will analyze the current and future aspects of the MED-TVC technology in view of latest installed plants.

Experimental Assessment of a Multi-Effect Solar Desalination System in Southern China

This paper presents the thermal performance of a multi-effect distillation (MED) solar desalination unit located in southern China. The system was designed and installed in April 2011, and has been in operation since that time till now. The seawater desalination system is driven by 60 m2 all-glass vacuum tubes solar collector with an additional rainwater utilization system to produce fresh water on both sunny and rainy days. The preliminary experimental results show that the water production of the system for per unit of solar collector area could reach up to more than 12.5kg/m2·day under the fine weather conditions. Water production of the system was stable in long period and the annual production could reach to 387.5 m3 in southern China. The economic performance of the system is also discussed. The cost of water production is estimated approximately 4.15 $/m3 for the 15-year service life.

Thermodynamicmodellingof an integrated solid oxide fuel cell and micro gas turbine system for desalination purposes

SUMMARY In this article, an integrated solid oxide fuel cell (SOFC) and micro gas turbine (MGT) with a multi-effect desalination (MED) system is proposed, and its comprehensive thermodynamic modelling, through energy and exergy analyses, is conducted. In addition, the effects of some design parameters on the hybrid system are investigated. The results show that fuel cell stack pressure has a significant effect on the combined system power and distilled water capacity. It also increases the SOFC-MGT energy efficiency. Moreover, the pressure of the inlet heating steam to the multieffect desalination system affects the pure water production in a positive way. An increase in the steam pressure results in a lower steam mass flow rate generated in the heat recovery steam generator. However, it increases desalination product capacity. Copyright © 2011 John Wiley & Sons, Ltd.

Numerical investigation of the effect of inlet skew angle on the performance of mechanical vapor compressor

Mechanical vapor compressor is an essential component that governs the system performance and stability in the multi-effect desalination (MED) system. The importance of improving mechanical vapor compressor's compressing ability in enhancing the efficiency of multi-effect desalination (MED) system has been stressed for a long time. This study was aimed to investigate the influences of inlet skew angle of the impeller on performance improvement. Four designs of inlet skew angles were attempted at the first stage impeller with 8°, 10°, 15°, and 20° respectively. Three dimensional fluid flows were simplified using periodic model to reduce the computational cost and time required. A good performance was expected as which effectively reduce the losses caused by secondary flow in the flow passage of the impeller. The results show detail flow pattern on decreasing the secondary flow due to incidence angle at the impeller eye and impeller passage. The numerical solutions were performed and allowed the better design.

Simulation of Thermo Vapor Compressor Performance

Thermo vapor compressor (TVC) is simply a steam ejector employed in multi-effect desalination (MED) system. A well understanding of flow phenomena inside an ejector is important for improving the ejector performance. To optimize the performance of the ejector used in thermo vapor compressor, a three dimensional model for the computational fluid dynamics (CFD) was conducted. The results due to the CFD were verified with the results data obtained by a new analytical model developed using the equations of continuity, momentum and energy. The present results investigated recent patents of the effect of the condenser, evaporator and boiler pressures and also the effect of ejector geometry on the thermo vapor compressor performance. A comparison between the present CFD and the analytical results at the same operating conditions, gave good agreement between them with an acceptable deviation ranges from -5 to 16 %. Keywords: analytical model, CFD, ejector geometry, multi-effect desalination (MED), steam ejector, thermo vapor compressor, geometrics and grids, condenser, nozzle, desalination system

Analysis and design of a multi-effect desalination system with thermal vapor compression and harvested heat addition

The design and optimization of a concentrated solar power–desalination of seawater (CSP–DSW) plant is based on the accurate characterization and better integration of sub-components, such as the multiple effect distillation (MED). We set out to design and fabricate two pilot MED systems that consist of high performing components and involve a high degree of thermal integration with the rest of the system. An MED system with a series of thermal vapor compressors (TVCs) driven by heat harvested from the thermal storage subsystem of the CSP–DSW is proposed. An algorithm is presented which optimizes the gain output ratio (GOR) by varying of the number and entrainment ratio of TVCs. The use of modular parallel plate falling film heat exchangers and overall process thermal management increase the flexibility and overall efficiency of MED. Experience with the design and fabrication of a transparent single-effect experimental MED aimed to quantify the performance of the parallel plate falling film heat exchanger and to allow visualization of permeate vapor is discussed. A 10 kWt four-effect MED design is also described for use in a proof-of-principle CSP–DSW study to be performed at the Cyprus Institute.

Exergetic performance of eco friendly absorption heat transformer for seawater desalination

Vapour Absorption Heat Transformers (VAHT) have shown good potential for upgrading waste heat to useful temperature levels. VAHT can be effectively coupled with Multi Effect Desalination (MED) systems for water purification. In this study an exergetic analysis is carried out for VAHT integrated with MED. A simulation model has been developed to evaluate the exergetic efficiency, exergy losses in the components and exergy balance of the system. The effects of operating parameters such as heat source temperature, sink temperature, effectiveness of solution heat exchanger are evaluated on the performance of the system. Exergy efficiency varies from 11.14% to 35.35% depending upon the temperatures of heat source and sink. It is found that the highest exergy loss is in the condenser of VAHT, followed by second effect of MED.

Optimal model for multi effect desalination system integrated with gas turbine

Integrated thermal desalination unit and gas turbine is a new achievement in energy engineering considerations. In this research, two study cases have been computed for mentioned system. For the first case [Y. Wang, N. Lior, Desalination 214 (2007) 306–326], grass root design of desalting plant has been applied by code developing in EES (Engineering Equation Solver) software. Then, changes in quantitative and qualitative parameters have been investigated for different types of systems. In order to find an economical amount, an optimization code has been developed in GAMS (Generalized Algebraic Modeling System) software. According to results, a desalting system with three effects is the best choice in this case at all. {0.52 (kg/s) distillate production, 113 (m 2) total heat transfer area, 76.69 (kW) total exergy destruction and 1.98 ($/m 3) unit product cost}. In the second case, feasibility of integration has been investigated by retrofit viewpoint for Lavan Island. As a result of simulation, thermal energy recovered by HRSG (Heat Recovery Steam Generator) has been equal to 4728 (kW). It shows a great potential for motive steam generation plus to 2.77 (kg/s) distillate productions. The amount of 105,494 ($/yr) cost reductions is concluded optimization procedure for substitution of boiler with HRSG.

Numerical investigation of entraining performance and operational robustness of thermal vapor compressor having swirled motive steam inflow

The importance of improving TVC's entraining ability in enhancing the efficiency of a multi-effect desalination (MED) system has been stressed for a long time. Moreover, the side effect detrimental to the system's operational stability when TVC is adopted for the MED plant has been frequently reported. In this study, an iterative CFD analysis was conducted to discover the TVC's shape that maximize its entraining performance, but we included a new design parameter of the motive flow's swirl intensity, which has never been previously considered as a TVC design parameter. Under the given reference operating pressures, the performance map was created in wide ranges of design parameters including the motive flow's swirl intensity. We tested whether the TVC will work in a stable double-choking range through an analysis of performance changing patterns for various operating conditions. Adopting the motive flow's swirl effect, a high entraining performance at reference condition as well as a good operational stability in wide operating ranges was achieved. This was impossible in the past when only the exteriorly geometrical parameters (like the mixing tube diameter and the primary nozzle exit diameter) were simply adjusted.