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.     

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.     

Low temperature flash vaporization for desalination

The feasibility of desalination by low pressure vaporization of seawater at temperatures between 26°C and 32°C was demonstrated in a pilot plant. The plant operated at vacuum pressures between 1.3 KPa and 2.3 KPa. The saline water was sprayed into the vaporizer as fine droplets using a swirl nozzle and evaporated at the low pressure. The maximum flow rate was 1,000 1/h. Condensation of the vapor so formed, was carried out in a shell and tube heat exchanger. The yield of fresh water was up to 4% as predicted by theory. The quality of the condensed water was excellent. The investigation demonstrated a novel concept of devising a desalination system by using warm water from the upper strata of the ocean for flash vaporization at low pressures and condensing the vapors using cold water from the lower strata of the ocean. The performance of the pilot plant was determined for different feed water temperatures, vacuum and water injection pressures.     

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.     

Theoretical and experimental investigation of humidification-dehumidification desalination unit

A theoretical and experimental investigation of humidification-dehumidification desalination system is presented. The system is based on an open cycle for water and a closed cycle for the air stream. The air is circulated either by natural or forced circulation. The system modeling is based on various heat and mass balance equations and their numerical solution. The effect of operating parameters on the system characteristics has been investigated. An experimental test set-up has been fabricated and assembled. The set-up has been equipped with appropriate measuring and controlling devices. Detailed experiments have been carried out at various operating conditions and using several packing materials. The heat and mass transfer coefficients have been obtained experimentally and fitted in forms of empirical correlations.The results of the investigation have shown that the system productivity increases with the increase in the mass flow rate of water through the unit. Water temperature at condenser exit increases linearly with water temperature at humidifier inlet and it decreases as water flow rate increases. The higher water temperature at humidifier inlet or water flow rate, the higher is the air temperature and humidity ratio at condenser inlet and exit. A maximum productivity of 5.8 liter/h has been obtained using wooden slates packing and with forced air circulation. No significant improvement in the performance of the desalination unit has been achieved by forced circulation of air at high water temperatures. The average relative deviation of theoretical predictions from measurements is (− 0.9%) in the air temperature at condenser inlet, (3.8%) in the humidity ratio at condenser exit and (− 1%) in the water temperature at condenser exit.     

A small solar desalination plant for the production of drinking water in remote arid areas of southern Algeria

The common methods of desalination salt water for production of fresh water by distillation, reverse osmosis and electrodialysis are intensive energy techniques. However, in remote arid areas, the desalination needs not exceed a few cubic meters per day. This decentralised demand favours local water production by developing other desalination processes, especially those using renewable or recovered energy (solar, geothermal, etc.). Solar desalination process is one of these methods used to resolve the scarcity of fresh water. Several reviews have been published by different authors. Small production systems as solar stills can be used if fresh water demand is low and the land is available at low cost. To supply the population of remote arid lands of South Algeria with drinkable water, solar distillation of brackish waters is recommended. It satisfies some of theses demands. Solar stills are used to produce fresh water from brackish water by directly utilising sunshine. These stills represent the best technical solution to supply remote villages or settlements in South Algeria with fresh water without depending on high-tech and skills. The production capacity indicates a possible daily production of far more than 15 l/m²d. Therefore, the still has a place in the upper range of known comparable products with regards to production output. This depends on the material used and the price of the solar stills and their accessories. The best working temperature solves most problems. Small, modular high-performance stills with features like the possibility of decentralised use, less maintenance and robust construction can help to reduce fresh water scarcity. The recent development of stills based on new concepts and heat recovery has been successful. The technical optimization is still in process today, it aims to improvement of the efficiency of these distillers. In our research work, a plant for brackish water distillation by directly sunshine and heat recovery was constructed and investigated experimentally and theoretically in South Algeria. This study aims the improvement of the performance of this solar distillation plant, conducted under the actual insulation, for brackish underground geothermal water desalination.     

Experimental study of the performance of a cooling tower used in a solar distiller

In this work, we investigated experimentally the thermal performance of a forced cooling tower used in a solar desalination system based on humidification-dehumidification of air. The cooling tower is a counter flow wet one filled with film packing materials.The measured variables were obtained for wide ranges of mass flow rates of air and water as well as for several inlet water temperatures; the tower characteristic and efficiency were then evaluated and expressed in terms of water to air mass flow rate ratio.     

Comparison of classical solar chimney power system and combined solar chimney system for power generation and seawater desalination

An alternative method of heat and moisture extraction from seawater under the collector of a solar chimney system for power generation and seawater desalination is investigated with the aim of estimating the output of power and fresh water when used in seawater desalination using one-dimensional compressible flow model. It is found that the temperature and velocity of the airflow inside the chimney in the combined plant is less than that inside the chimney in the classic plant due to the release of vapor latent heat as the air rises up the chimney. Additionally, the power output from air turbine generators and water generators in the combined plant is less than that of the classic plant. Furthermore, a revenue analysis based on the price of fresh water and electric power in Dalian, China shows that the chimney less than 445 m high for the proposed combined solar chimney power plant having a collector 3000 m in radius is more economical than for the classic plant. The critical chimney height is found to depend on the local price of fresh water and electricity.     

新型太阳能海水淡化技术研究进展

总结了微纳尺度下毛细驱动的局域热法太阳能海水淡化的相关机制,包括毛细驱动水分传输、太阳能热量传递机理。叙述了界面太阳能局域热法光蒸汽系统最近发展,并归纳了在1个太阳辐照下突破单级产水量理论极限的新型研究思路,指出了太阳能局域热法海水淡化的当前挑战,包含盐分结晶和蒸汽冷凝效率低等问题。认为界面太阳能局域热法海水淡化技术在效率和成本上都具有极大的优势,系统材料制备简单、结构紧凑、系统创新明显,特别适用于偏远山区和海岛地区电力缺乏的家庭生活淡水制备。随着研究者更多地转向保证蒸汽高效凝结和收集、水蒸汽潜热回收利用,预计这种新型海水淡化技术的未来应用范围将不断扩大。     

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.     

Solar desalination with a humidification-dehumidification cycle: mathematical modeling of the unit

Solar desalination is gradually emerging as a successful renewable energy source of producing fresh water. Solar Multi-Effect Humidification (MEH) units based on the humidification-dehumidification principle are considered as the most viable among solar desalination units. A simulation study of these units leads to a better understanding of the performance of such type of desalination units. This study therefore focuses on studying and analysing the effects and performance of various components involved in the process along with the study of the effect of water feed flow rate on the desalination production. To our knowledge, there is no such comprehensive model available in the literature. This study could lead a step further in the commercialisation of solar desalination units based on the humidification-dehumidification principle.     

Experimental study of forced circulation evaporator in zero discharge desalination process

Zero discharge desalination process is the most promising technology to prevent salinity and thermal shocks to ecosystem by effluent streams of desalination unit drained into the sea. Pretreatment, solar pond and forced circulation crystallizer are the major steps of one option to provide the purposes of zero discharge desalination process. Reduction of total hardness of wastewater occurs in pretreatment unit and the solar pond is proposed for effluent concentrated brine wastewater of pretreatment unit which seeks production of potable water and concentrated brine.The effluent stream from solar pond is conveyed to one forced circulation evaporator in order to produce salt and distilled water. This work presents the effective optimum parameters of one forced circulation evaporator such as occupied capacity of crystallizer main body, residence time, optimum temperature of heat exchanger, flow rate of cooling water, color and size distribution of salt crystals, amount of distilled water and energy consumption. Results show that the suitable range of crystal growth (700–830 μm) occurs when the occupied capacity of crystallizer is 70%. Also, the flow rate of cooling water in condenser is 10 kg/min and the efficient residence time is 4 h.     

Energy and exergy analyses of seawater desalination system integrated in a solar heat transformer

Among the numerous options to improve the energy efficiency of desalination plants stands out the absorption heat transformer. A heat transformer is a device, which can deliver heat at a higher temperature than the temperature of the fluid by which it is fed. Solar thermal energy can be used as heat input for single effect heat transformer while the high grade thermal energy delivered by the heat transformer can be used as heat source for water desalination.In this paper, an attempt has been made to study the combination: flat plate solar collectors, a single effect heat transformer and desalination system (distillation process) used to provide a beach house located in Skikda (East of Algeria; Latitude 36.52°N, Longitude 6.57°E) with drinking water. This system produces about 500 l of drinking water per day in July.Mathematical models of the solar flat plate collectors (FPC), absorption heat transformer (AHT) operating with the water/lithium bromide solution and the overall desalination system (WP) were developed to simulate the performance of this combination system. The energy and exergy analyses are carried out for each component of the system. All exergy losses that exist in this solar desalination system are calculated. Energy and exergy efficiencies are estimated.     

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.     

Aquifer composition and the tendency toward scale-deposit formation during reverse osmosis desalination — examples from saline ground water in New Mexico, USA

Desalination is expected to make a substantial contribution to water supply in the United States by 2020. Currently, reverse osmosis is one of the most cost effective and widely used desalination technologies. The tendency to form scale deposits during reverse osmosis is an important factor in determining the suitability of input waters for use in desalination. The tendency toward scale formation of samples of saline ground water from selected geologic units in New Mexico was assessed using simulated evaporation. All saline water samples showed a strong tendency to form CaCO₃ scale deposits. Saline ground water samples from the Yeso Formation and the San Andres Limestone showed relatively stronger tendencies to form CaSO₄·2H₂O scale deposits and relatively weaker tendencies to form SiO_2(a) scale deposits than saline ground water samples from the Rio Grande alluvium. Tendencies toward scale formation in saline ground water samples from the Dockum Group were highly variable. The tendencies toward scale formation of saline waters from the Yeso Formation, San Andres Limestone, and Rio Grande alluvium appear to correlate with the mineralogical composition of the geologic units, suggesting that scale-forming tendencies are governed by aquifer composition and water-rock interaction.     

膜蒸馏海水淡化研究进展及发展趋势

海水淡化是解决我国水资源短缺的重要措施之一。膜蒸馏海水淡化技术可以充分利用太阳能等低品位热源，具有成本低、设备简单、操作容易、能耗低等优点，在海水及苦咸水淡化方面应用前景广阔。     

The effect of air on condensation on a roped tube

With an increasing interest in the use of enhanced heat transfer surfaces being shown in connection with desalination, experiments have been carried out to compare the effect on different tubes of non-condensable gases. The tests were made on two roped and one plain tube each tube having a nominal length and inside diameter of 690 mm and 11 mm respectively. Cooling water velocities ranged from 0.8 m/s to 4.2 m/s and bulk air concentrations at inlet of up to 4.25% by weight were investigated.It was found that although the effect of air on the condensation process was greater on the roped tubes at low cooling water velocities, at the velocities commonly associated with desalination plants there was little difference between the effect of non-condensables on roped and plain tubes. It was concluded, therefore, that this type of fouling was no worse on roped tubes compared to plain tubes.     

Operation experience with solar powered 10 M3 per day MSF-plant in Kuwait and results of upscaling experiments

This paper reports on the progress which was made in the field of solar desalination in order to obtain reliable equipment capable to both operate under the severe requirements associated with the use of solar energy as a variable source of heat and to yield favourable economic conditions in view of larger scale utilisation of the undepletable source of energy for the generation of fresh water.The results of a 2 year experience with a selfregulating solar MSF desalination plant under the extreme environmental conditions in the middle east will be discussed. A new type of a solar selfregulating fluidized bed MSF desalination unit will be presented for the first time. The new development which has been carried to a mass-production-level is the result of the experience gained in this field over the last years.The operation of the Atlantis desalination unit with different types of solar collection systems, in particular the salt gradient solar pond energy collection and storage system, will be discussed from the technical and economical point of view to demonstrate the feasibility of future larger scale solar desalination.