New experimental aspects of the carrier gas process (CGP)

The carrier gas process (CGP) based on humidification and dehumidification is a new interesting process (with respect to previous conventional processes such as multistage flash and reverse osmosis) for water desalination. The CGP contains several advantages such as flexibility in capacity, moderate installation and operating cost, possibility of using a wide range of thermal energies (geothermal, solar, recovered, direct fossil fuel, etc.) and simplicity (atmospheric pressure). The aim of this paper is to present the principal and characteristics of this technique based on experimental investigation. The present pilot plant unit consists of two packed columns, humidification and dehumidification, one heat exchanger and one air pre-heater. Most investigators have used solar thermal energy as the source for heating the saline water, but in this work electrical energy was used for heating the air stream. Besides this point most investigators have used a coil heat exchanger for condensation of fresh water but in the present work a packed column was used instead to do the same job. The experimental results of the work that was carried out at Bushehr Port, southern Iran, were: the effect of air pre-heater and coolant water temperatures, air, saline water, recirculating fresh water and coolant water flow rates on the amount of produced fresh water per unit of heat duty and fresh water production flow rate. It was found that the performance of the system strongly depended on the temperature and flow rate of the air pre-heater and the temperature and flow rate of the coolant water. However, it depended weakly on the flow rate of the saline water and fresh water re-circulation. It is expected that the unit would be of great potential for saline water desalination in arid areas and isolated islands.     

Parametric analysis to improve the performance of a solar desalination unit with humidification and dehumidification

A solar desalination unit with humidification and dehumidification, characterized by reusing some of somewhat concentrated saline water after evaporation, recovering condensation heat, and forced air flow, was expected to produce more fresh water. A mathematical model of the unit is presented. The model was experimentally validated for numerical simulation. Parametric analysis was conducted in order to optimize the unit performance. The effect of some of the operating conditions such as flow rates, temperatures of feed water, air and cooling water, etc., was studied in detail. The daily solar productivity corresponding to unit square meter of collector area is about 6 kg/m²/d with 20 MJ solar energy input a day under given conditions. The unit has proven to be an efficient device to utilize solar energy for obtaining fresh water from saline water.     

Theoretical study on multi-effect solar distillation system driven by tidal energy

Based on an analysis of the characteristics of solar and tidal energy, an innovative, multi-effect solar distillation unit for seawater desalination utilizing solar and tidal energy has been developed. The uniqueness of the system is that without being transferred to electricity, tidal energy is utilized to supply power for water supply and drainage, and vacuum extraction instead of pumps powered by electricity. So the cost can be greatly reduced. The system is based on multi-effect evaporation-condensation processes and operates under vacuum condition, so low grade solar heat can be used with a high thermal efficiency. Hydrodynamic and thermal analysis is carried out for the water supply and drainage system driven by tidal energy. The operating mechanism of the vacuum extraction system driven by tidal energy is presented. A parametric study of the behavior of the solar desalination unit has been performed.     

Experimental investigation of a baffled shell and tube desalination column using the humidification-dehumidification process

The humidification-dehumidification process is an interesting technique that has been adapted for water desalination. Most previous work performed the humidification-dehumidification desalination process in two separate columns, which increases the complexity of the system and limits the humidification effect of the carrier gas as well as the thermal efficiency of the process. In this work, a baffled shell and tube desalination unit was built to perform humidification and dehumidification simultaneously at the tube and shell side of the column, respectively. The effects of several operating conditions on the production and thermal efficiency of the process were investigated, including water flow rate, inlet water temperature, carrier gas flow rate and external steam flow rate. The results show that the productivity and thermal efficiency of the process both increase with increasing inlet water temperature. Suitable flow rates exist for both the water and carrier gas, which are 6–30 kg h⁻¹ and 4–20 kg h⁻¹ for the present column, respectively. The increase of external steam flow rate increased the productivity but decreased thermal efficiency of the process. A further performance comparison with a previous unbaffled desalination unit indicated that the baffle plates significantly enhance the productivity of the column, which is usually 3–6 times that of the unbaffled one. Meanwhile, the salinity of the produced water was determined to be in the range of 20–30 mg/L with the feed water containing 10,000 mg/L NaCl.     

First experimental results of a new hybrid solar/gas multi-effect distillation system: the AQUASOL project

From 2002 to 2006, a combined R&D project named AQUASOL has been carried out at the facilities of the Plataforma Solar de Almería (Spain). Main objective of this project has been the development of a hybrid solargas desalination system based on multi-effect distillation process that meets at the same time the requirements of low-cost, high efficiency and zero discharge.

The final AQUASOL plant, implemented at the Plataforma Solar de Almería for its evaluation under real meteorological conditions, is composed of: (i) a 14-cell forward-feed vertically-stacked MED unit, (ii) a 500m2 stationary CPC (compound parabolic concentrator) solar collector field, (iii) a 24 m3 thermal storage system based on water, (iv) a new advanced prototype of double-effect absorption (LiBr-H2O) heat pump, (v) a smoketube gas boiler to guarantee 24-h operation

This paper shows the first experimental results obtained during the test campaign of the project. The performance ratio reached by the distillation plant in different operational modes is evaluated, as well as the issues related with the operation of the subsystems that compose the AQUASOL desalination system.     

Solar desalination with a humidification-dehumidification technique — a comprehensive technical review

Major desalination processes consume a large amount of energy derived from oil and natural gas as heat and electricity. Solar desalination, although researched for over two decades, has only recently emerged as a promising renewable energy-powered technology for producing fresh water. Solar desalination based on the humidification-dehumidification cycle presents the best method of solar desalination due to overall high-energy efficiency. This paper provides a comprehensive technical review of solar desalination with a multi-effect cycle providing a better understanding of the process. Discussion on methods to improve system performance and efficiency paves the way towards possible commercialisation of such units in the future.     

Experimental study of a multiple-effect humidification solar desalination technique

The object of this research is to experimentally investigate the principal operating parameters of a new desalination process working with an air multiple-effect humidification-dehumidification method. A test set-up was designed and constructed to carry out and optimize this technique. The main parts of the present set-up consist of a heat equipment device (heat exchanger), a spray humidifier and a dehumidifier system. This equipment was used to simulate the seawater desalination process experimentally with an eight-stage air solar collector heating-humidifying system. The outlet temperature of the air solar collector was correlated for use in the desalination process as a solar heating device. The operating conditions studied were: ratio of water to dry air mass flow rate through the system, humidifier inlet absolute humidity, dry air mass flow rate through the system and solar irradiation or humidifier inlet air temperature. The experimental results obtained were used to put stress and correlate the influence of the different operating conditions on the behavior of the eight-stage air heating-humidifying desalination process. The ratio of water to dry air mass flow rates was optimized, precisely 45%. The value of dry air mass flow rate through the system can be also varied with solar radiation in order to have a maximum of humidity content at the end of the system and though working in an adiabatic humidification process.     

Experimental investigation of a two-stage solar humidification–dehumidification desalination process

This paper experimentally evaluates a two-stage technique to improve the humidification–dehumidification process in fresh water production from brackish water. According to modeling results of multi-stage process and on the basis of construction cost estimation, using a two-stage process is the most suitable choice that can improve important parameters such as specific energy consumption, productivity and daily production per solar collector area and thus, investment cost. A pilot plant was designed and constructed in an arid area with 80 m² solar collector area to evaluate the two-stage process. This unit was tested on cold and hot days. The effect of main parameters on fresh water production of the unit is studied. Experimental results show that two-stage HD desalination unit can increase heat recovery in condensers and hence, reduce thermal energy consumption and investment cost of the unit. Moreover, productivity can be increased by 20% compared with the single-stage unit.     

Performance analysis of a new type desalination unit of heat pump with humidification and dehumidification

Desalination with humidification and dehumidification process is deemed as an efficient and promising means of utilizing the condenser and evaporator of heat pump to produce freshwater from seawater. This paper presents a new type desalination unit driven by mechanical vapor compression pump which was designed and fabricated by the Institute of Air-Conditioning & Solar Energy of the Northwestern Polytechnical University. The unit utilized the heat from condenser and the cold from evaporator of heat pump adequately, and reclaimed most latent heat. The air, firstly, was humidified in the humidifier with the alveolate structure, and then was cooled in the precondenser and the evaporative condenser to produce freshwater. A mathematical model of the unit is presented, in which the hydrokinetics method was used to study the flow and the heat and mass transfer inside the alveolate humidifier. The effects of some of the operation such as flow rates, temperatures of cooling water and air, and etc., were studied in detail. The comparison between the numerical and experimental results was accepted. The desalination unit that is considered in the study produces freshwater 60 kg/day with the less electric power that is 500W and is proven to be an efficient desalination device to obtain freshwater.     

Economical study of a small-scale direct contact humidification-dehumidification desalination plant

Economical study of humidification-dehumidification desalination (HDD) pilot plant was made in order to estimate the economic benefits of the process in comparison with a small-scale reverse osmosis (RO) system. The energy recovery of the unit was investigated to be 75%. Some theoretical modifications were made to the HDD system in order to reduce the energy costs of the unit. Exact and clear economic analysis results were obtained using COMFAR ΙΙΙ software including fixed investment costs, production costs, internal rate of return on investment, operating costs, energy costs and some other economical parameters. Based on the energy prices in Iran, the total fresh water production cost was calculated to be $ 6.4/m³ that was nearly the same as produced by an imported RO plant. Finally some points were recommended whether to choose HDD or RO plant in different cases and capacities.     

Efficiency of a Liquid Desiccant Dehumidification System Regenerated by Using Solar Collectors/ Regenerators with Photovoltaic Fans

Abstract

A hybrid solar dehumidification air-conditioning system was used to study the absorption of water vapor from moist air by contacting the air with aqueous solutions that contained from 90 to 94% triethylene glycol (TEG). For the packings of 2-inch polypropylene Jaeger Tri-Packs, which have a surface-to-volume ratio of 157 m²/m³ (48 ft²/ft³), the efficiency of dehumidification can reach 93.3%. The environmental air was introduced into the dehumidifier cocurrently flowing with the liquid desiccant, and the liquid desiccant was sprayed on the top of the packing material. The air-to-liquid mass flow ratio was controlled in a range of 0.46 to 1.36. As the moisture was absorbed from air by the TEG solution, the solution was diluted. The regeneration of the solution was carried out in 20-piece (38.8 m²) basin-type solar collectors/regenerators whose regeneration coefficients of performance are above 0.2. Air generated by photovoltaic fans was blown into the solar collectors/regenerators and carried away the water vapor from the evaporation of the aqueous desiccant solution. On the basis of the experimental results, the system performance is acceptable for most applications.     

低温多效海水淡化装置调试问题的分析及处理

介绍了目前世界上工艺中级数最多、单套出力最大的某电厂低温多效技术(MED)海水淡化装置的工艺流程及特点,叙述了系统调试的主要步骤,即蒸汽管道吹扫、控制系统和辅助系统调试、初次注水查漏、真空气密性试验、海水整体循环、启动制水等;试验运行结果表明,4套海水淡化装置的运行指标满足设计要求,性能试验合格,具备商业运行条件。介绍了调试过程中遇到问题进行分析及处理方法,提出了完善系统的建议,认为在我国MED海水淡化工艺运行经验有待不断摸索和积累。     

Seawater integrated desalination plant without brine discharge and powered by renewable energy systems

Environmental effects are one of the main concerns of massive desalination facilities. To reach the objective of no brine discharge the salt from seawater must be completely separated and obtained as a secondary and valuable product. If no CO₂ emission increase is desired, the power source must be a combination of renewable energy systems (RES). This paper presents an analysis of an integrated desalination scheme consisting of two sequential systems: a multi-effect distillation (MED) plant and a mechanical vapour compression (MVC) system based on evaporator equipment. The energy is obtained by several wind turbines (WT) and a thermal solar collector (TSC) field. Separation of salt and water is achieved in a coupled multi-effect distillation-mechanical vapour compression (MED+MVC) two step process. The MED stage is driven by thermal solar collectors, whereas the energy consuming mechanical compression of the vapour (MVC) is fuelled by wind-powered turbines. Interestingly, the final products of this process are dry salt and fresh water. Such a system has been designed and dimensioned for a throughput of 100 m³/h of desalted water A preliminary study of the investment, amortization and exploitation costs of a combined MED+MVC+WT+TSC installation with these dimensions has been done. The price of desalted water, after considering the profits due to the sale of salt and electricity has been estimated at 0.59 ?/m³. If the initial investment has a 35% subsidy, a final price of 0.41 ?/m³ could be ensured, which is near the price associated to conventional energy sources. An outline of the solar collector system and the technical requirements of the wind turbines in needed to meet the energy demand of the MED+MVC system are also included.     

海水淡化系统的性能优化与评价准则

可持续发展要求海水淡化系统不仅具有优良的经济性,还要具有优良的环境性能,目前综合考虑这二方面因素的研究还很少。文中从海水淡化系统的综合性能优化和评价出发,建立了统一性能指标(亦称淡水综合成本),该指标由淡水生产的经济成本和环境成本构成,以其为目标函数优化得到的系统具有最佳能源、经济、环境综合性能。淡水的综合成本、经济成本、环境成本还构成了海水淡化系统的多准则性能评价体系,据此可实现对不同海水淡化方法和装置的更全面、合理的评价。基于前面建立的性能准则,以近年来受到较多关注的低温多效蒸发海水淡化系统为例进行了计算、分析和讨论,研究结果验证了准则的合理性和有效性。     

采用增湿-去湿工艺的竖直列管式脱盐装置的实验研究

A vertical tubular desalination unit with shell and tube structure was built to perform humidification and dehumidification simultaneously on the tube and shell side of the column, respectively. The effects of several operating conditions on the productivity and thermal efficiency of the column were investigated. The results show that both the productivity and thermal efficiency of the column enhance with the elevation of the inlet water temperature. The flow rates of water and carrier gas both have optimal operating ranges, which are 10-30kg·h-1 and 4-7kg·h-1 for the present column, respectively. Meanwhile, the increase of external steam flow rate will promote the productivity of the column but reduce its thermal efficiency.     

Solar desalination using humidification—dehumidification technology

A numerical study has been carried out to investigate the performance of a simple solar desalination system using humidification—dehumidification processes. The desalination system consists of a solar air heater, humidifier, dehumidifier and a circulating air-driving component. The study covers the influence of different environmental, design, and operational parameters on the desalination system productivity. Environmental parameters include solar intensity, ambient temperature and wind speed. Design parameters include the solar heater base insulation, the humidifier and the dehumidifier effectiveness. Operational parameters include air circulation flow rate, feed water rate and temperature. The results indicated that the solar air heater (energy source) efficiency significantly influences system productivity. Increasing the solar intensity and ambient temperature and decreased wind velocity increases system productivity. Increasing the air flow rate up to 0.6 kg/s increases the productivity, after which it has no significant effect. The feed water flow rate has an insignificant influence on system productivity. The surprising result is that the dehumidifier effectiveness has an insignificant influence on system productivity, which has a very important implication for the system's economy. The physical explanation of this finding is given.     

Empirical model for the condenser of the seawater greenhouse

Seawater greenhouses (SWGH) utilize seawater or saline/brackish groundwater for cooling the microclimate and providing freshwater for irrigation through a humidification–dehumidification desalination process. The overall effectiveness of the SWGH greatly depends on the effectiveness of its condenser. The present study provides a good review on the available simulation models of the SWGH condenser and proposes a multiple linear regression model to predict the dehumidification rate of the condenser in the Oman SWGH. Four climatic and operational input variables were considered, including solar irradiance, inlet moist air temperature, inlet humidity ratio, and inlet air mass flow rate. The results showed that the model accurately predicts the dehumidification rate when compared against experimental values [a mean predictive error (PE)?=??0.127?kg/h and root mean square error (RMSE)?=?4.691?kg/h]. The model also outperformed some other model in several accuracy indicators such as PE, mean absolute predictive error, RMSE, R², index of agreement and fractional variance.     

Studies of scale formation and optimization of antiscalant dosing in multi-effect thermal desalination units

Scaling of heat transfer surfaces is of great concern for the performance of the thermal desalination process working on seawater, whether it is for multistage flash or multi-effect evaporation. Lot of work has already been carried out for the development of antiscalants for inhibiting/minimizing the scaling. The normal concept of scale formation is due to inverse solubility of hardness salts in the seawater with temperature. Apart from temperature, reasons for scales differ by some factors in case of multi-effect desalination units than that of multi-flash units as experienced in operation of six multi-effect desalination units. As such for taking care of such additional scaling factors, dosing rates of the same antiscalant product is almost double or sometimes even more, in multi-effect evaporators than that in multistage flash units.

The purpose of presentation of this paper is to identify these additional factors responsible for scale formation in multi-effect desalination units, other than temperature, with illustrations wherever possible and case study of optimization of dosing rate of antiscalant in 5 million gallon per day capacity multi-effect thermal desalination unit at one of the thermal desalination plants of Federal Electricity and Water, U.A.E. The paper is concluded with the general suggestions for minimizing scales in multi-effect thermal desalination units.     

太阳能海水淡化与空调装置控制系统研究

阐述了太阳能海水淡化与组合式空调系统的原理,该系统具有高效的海水淡化功能和空调除湿能力.比较了多个现有的控制器,对该装置的控制系统做出了初步设计并进行了实验验证.最终的实验结果证实该控制器完全可行.     

新型闭式太阳能海水淡化装置及其性能模拟

本文提出了一种闭式海水淡化装置,阐述了系统的工作原理,建立了相应的数学模型。就冷却水流量、海水喷淋量、集热器面积、蒸发器尺寸等因素对系统淡水产量的影响情况进行了计算机模拟,根据实验测得的气象数据对一天内系统淡水产量随时间变化情况进行了模拟,并根据典型的月平均日气象数据对系统一年内每个月的系统性能进行了分析。分析表明,不计系统夜间运行时的淡水产量,在西安地区系统淡水产量可以达到4.6kg/d.m2。