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

New and renewable energy and environmental engineering

There is an acute scarcity of potable water in many parts of the world, and especially in most of the Middle East region. Important advances have been made in desalination technology but its wide application is restricted by relatively high capital and input energy costs, even when solar energy is used. Until recently, flat-plate solar collectors have usually been employed to distill water in compact desalination systems. Currently, it is possible to replace these collectors by the more advanced evacuated tube collectors, which are now available on the market at a similar price. The research which is concerned with the development of a novel small scale solar water desalination technology, consists of experimental and theoretical investigations of the operation of a multi stage solar still desalination system coupled with a heat pipe evacuated tube solar collector with an aperture area of about 1.7 m(^2). The multi stage still was tested to recover latent heat from the evaporation and condensation processes in each of its four stages. A number of experimental tests were carried out using a laboratory rig to investigate its water production capacity. Solar radiation (insolation) during a mid-summer day in the Middle East region was simulated by an array of 110 halogen flood lights. Computational Fluid Dynamics (CFD) modeling of the evaporation and condensation processes in one of the still's stages was conducted using FLUENT 6.2 software. The simulation results demonstrate the importance of the various parameters affecting the total production rate of the solar still and provide detailed information on the temperature distribution and condensate formation inside the solar still. However, it was found that the CFD technique at this stage does not provide accurate quantitative predictions and results obtained can be used only for qualitative analysis. Hence, the use of a lumped parameter mathematical model was preferred for analysis and design purpose. A lumped parameter model has been developed to describe the system's operation. It consists of a system of ordinary differential equations of energy and mass conservation written for each stage of the still. A MATLAB computer program was written to solve the system of governing equations to simulate the evaporation and condensation processes and the experimental results were used to validate numerical predictions. The experimental and theoretical values for the total daily distillate output were found to be closely correlated. The test results demonstrate that the system produces about 9 kg of clean water per day and has a distillation efficiency of 90%. The overall efficiency is 33% due to the presence of heat losses in the system. However, this level of efficiency is greater of that for conventional solar stills. Following the experimental calibration of the lumped parameter model, this was used for determination of rational design parameters of the still and it was demonstrated that the performance of the system could be considerably improved to produce 11 kg/m(^2) of water per day if the number of stages and evaporation area were 4 and 1 m(^2), respectively. A water quality analysis was performed for the distilled water and the levels of total dissolved solids, electrical conductivity and pH were well within the range defined by the World Health Organization guidelines for drinking water. An economic study was also conducted for the system and it was shown that the distilled water costs of 0.016 US$/litre with a payback period of 6 months in the Middle East region conditions. This research demonstrates, empirically and theoretically, the potential role in the field of solar desalination of the multistage solar still coupled to the evacuated tube solar collector. Not only is this system a promising new technology but it could prove to be particularly appropriate in remote and rural areas. Simultaneously this system also uses a completely clean energy source and contributes to tackling environmental pollution, global carbon emissions and climate change problems

Feasibility analysis and simulation of reverse electrodialysis for the regeneration of power in desalination plants

This document presents a multi-faceted parametric review on the reverse electrodialysis process regarding ideal and real feed solutions. A hybrid forward osmosis - reverse electrodialysis model is presented for the optimal utilisation of desalination brine effluent for electrical power generation. The power generated is recommended to offset the high energy requirements of seawater reverse osmosis desalination due to relative proximity of brine. The proposed large scale reverse electrodialysis plant has the potential to minimise both capital and operating expenditure of an up-scaled system while maximising net power output. A multi-variable optimisation of the process is achieved using the simulation model derived within the study, concluding with a 10.3% reduction in the specific energy consumption of Perth’s Seawater Desalination Plant located in Kwinana when utilising a series-parallel arrangement of reverse electrodialysis units. However, the model is not specific to a Western Australian context and can be used wherever an opportunity for salinity gradient power generation exists

Effect of membrane performance including fouling on cost optimization in brackish water desalination process

Membrane selection is a crucial step that will affect the economic feasibility of the membrane water treatment process. A comprehensive evaluation consisting of Verberne Cost Model, assessment of membrane performance and fouling propensity, osmotic pressure differential (OPD) and specific energy consumption (SEC) was employed to determine the potential of nanofiltration (NF 270, NF 90 and TS 80) and low pressure reverse osmosis (XLE) membranes to be used in brackish water desalination process. The aim was to save costs by replacing the typical brackish water reverse osmosis (BW 30) membrane. Verberne Cost Model showed that higher flux NF membranes resulted in lower overall costs. However, after assessing the membrane performance, NF 270 and TS 80 were excluded due to their high fouling propensity and their failure to reduce total dissolved solids (TDS) in the solution. Instead, NF 90 membrane which produced water with acceptable TDS and has moderate permeability ended up to be more cost competitive compared to BW 30 membrane, with 17%-21% lower total costs and 13%-17% lower water costs. Apart from this, OPD and SEC were applied to justify the selection of optimal membrane recovery rate based on the water costs calculated. It was determined that the optimal recovery rate was 80% where the SEC and water costs were close to available water treatment plants. Overall, this study showed that the selection of membrane can be carried out by using Verberne Cost Model assisted by assessment of membrane performance and fouling propensity, OPD and SEC

Thermodynamic analysis of humidification dehumidification desalination cycles

Humidification–dehumidification desalination (HDH) is a promising technology for small-scale water production applications. There are several embodiments of this technology which have been investigated by researchers around the world. However, from a previous literature [1], we have found that no study carried out a detailed thermodynamic analysis in order to improve and/ or optimize the system performance. In this paper, we analyze the thermodynamic performance of various HDH cycles by way of a theoretical cycle analysis. In addition, we propose novel high performance variations on those cycles. These high-performance cycles include multi-extraction, multi-pressure and thermal vapor compression cycles. It is predicted that the systems based on these novel cycles will have gained output ratio in excess of 5 and will outperform existing HDH systems.King Fahd University of Petroleum and MineralsCenter for Clean Water and Clean Energy at MIT and KFUP

Modeling and Optimization of Renewable Energy Systems

This book includes solar energy, wind energy, hybrid systems, biofuels, energy management and efficiency, optimization of renewable energy systems and much more. Subsequently, the book presents the physical and technical principles of promising ways of utilizing renewable energies. The authors provide the important data and parameter sets for the major possibilities of renewable energies utilization which allow an economic and environmental assessment. Such an assessment enables us to judge the chances and limits of the multiple options utilizing renewable energy sources. It will provide useful insights in the modeling and optimization of different renewable systems. The primary target audience for the book includes students, researchers, and people working on renewable energy systems

Simulation of reverse osmosis process: Novel approaches and development trends

Reverse osmosis is an essential technological separation process that has a large number of practical applications. The mathematical simulation is significant for designing and determining the most effective modes of membrane equipment operation and for a deep understanding of the processes in membrane units. This paper is an attempt at systematization and generalizing the results of the investigations dedicated to reverse osmosis simulation, which was published from 2011 to 2020. The main approaches to simulation were analyzed, and the scope of use of each of them was delineated. It was defined that computational fluid dynamics was the most used technique for reverse osmosis simulation; the intensive increase in using of molecular dynamics methods was pointed out. Since these two approaches provide the deepest insight into processes, it is likely that they will further be widely used for reverse osmosis simulations. At the same time, for the simulation of the membrane plant, it is reasonable to use the models that required the simplest solutions methods. The solution-diffusion model appears to be the most effective and flexible for these purposes. Therefore, this model was widely used in considering the period. The practical problems solved using each of the considered approaches were reviewed. Moreover, the software used for the solution of the mathematical models was regarded