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.     

Horizons and future of water desalination in Libya

A premature study has been conducted to overlook the viability and feasibility of the future of desalination in Libya, by estimating the water situation in several locations in the country. A simplified review of the spectrum of well-known desalination techniques and a presentation of these analysis of the water situation would be highlighted to guideline the planners on how, why and when to embark on desalination to resolve the water shortage problem that would be faced at any moment in the future. Finally the methods to resolve shortage problem alongside their economical evaluation would be considered briefly.     

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.     

Pilot plant study of desalination by direct contact heat transfer

A pilot plant was constructed to study desalination by direct contact liquid-liquid heat transfer. A refined mineral oil was used as the heat carrier.The system could be used successfully to produce fresh water while eliminating corrosion and wear to a large extent. Trace quantities of oil dissolved in the product water could be removed by adsorption over activated charcoal, which could subsequently be activated by live steam.The by-product brine has a high concentration offering greater value and less problems for disposal.     

Water desalination: An imperative measure for water security in Egypt

Water Desalination is an indispensable industry for the most of the Arab countries. In the last four decades, the number and capacities of desalination units have increased dramatically (45% Multi-Stage Flash (MSF) and 42% Reverse Osmosis (RO) of world capacity); especially in the Gulf States. Almost all available conventional water resources in Egypt - represented by the Nile water, renewable groundwater, and some scant annual precipitation- have been exhausted. Further development measures require review of current water allocations in order to raise efficiencies and protect against pollution, in addition to exploring new options of non-conventional water resources to narrow the gap between water supply and demand. These measures are the pillars of Egypt's integrated water policy and have been clearly postulated in its National Water Resources Plan 2017. The objective of this paper is to study and investigate water desalination as a solution for water scarcity in Egypt. Moreover, the present work demonstrates the significance of seawater desalination for national development in Egypt. At present, Egypt is encouraging, not only the public sector but also the private sector, to apply modern technologies for desalination, which historically started with Distillation then Electrodyalisis and followed by RO. The great achievements in desalination technology have now moved the costs for desalting in many applications from the realm of "expensive" to "competitive". Current technology is feasible for tourist villages in the north coast and the Red Sea, due to its far distances from conventional sources that makes the cost of water conveyance very high and subject to pollution problems. The results indicated that, in spite of research and developments, still the energy requirement and membrane know-how are limiting factors. Thus, Egypt's future vision is non-traditional in the field of desalination. It is based on a real breakthrough towards the use of renewable energy, namely, solar energy to be harnessed for operating high compression pumps needed for reverse osmosis modular systems. The reasons are obvious, since Egypt has great potential of brackish water wells, immense amounts of solar radiation in remote areas and future integrated development projects are located at a distance from the Nile water. This trend is what Egypt is focusing on as a prospective future for wide applications of desalination. Finally, this research concluded that, the water desalination as a conventional water resource should be considered as an imperative measure for water security in Egypt. The future use of such resource for different purposes will largely depend on the rate of improvement in the technologies used for desalination and the cost of needed power.     

Operational water cost and productivity improvements for small-size RO desalination plants

This detailed study was carried out for the determination of the water cost in small size RO desalination plants in remote areas. Data from desalination plants in three Greek islands were used for a period of three years. The actual expenses for these plants were examined thoroughly and as a result the real cost per cubic meter was estimated. An evaluation of the efficiency of two different energy recovery systems was conducted by using factual operating data. The introduction of new technologies, automation, data acquisition and remote operation can be used to reduce the labor and maintenance cost for small size RO desalination plants. The increase in labor productivity has proved to be the direct result of the introduction of a low cost SCADA system to the RO plant.     

Fabrication and characterization of hydrophobic PVDF hollow fiber membranes for desalination through direct contact membrane distillation

The mixture of inorganic salt LiCl and soluble polymer polyethylene glycol (PEG) 1500 as non-solvent additive was introduced to fabricate hydrophobic hollow fiber membrane of polyvinylidene fluoride (PVDF) by phase inversion process, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. Compared with other three membranes from PVDF/DMAc, PVDF/DMAc/LiCl and PVDF/DMAc/PEG 1500 dope solution, it can be observed obviously by scanning electron microscope (SEM) that the membrane spun from PVDF/DMAc/LiCl/PEG 1500 dope had longer finger-like cavities, ultra-thin skins, narrow pore size distribution and porous network sponge-like structure owing to the synergistic effect of LiCl and PEG 1500. Besides, the membrane also exhibited high porosity and good hydrophobicity. During the desalination process of 3.5 wt% sodium chloride solution through direct contact membrane distillation (DCMD), the permeate flux achieved 40.5 kg/m² h and the rejection of NaCl maintained 99.99% with the feed solution at 81.8 °C and the cold distillate water at 20.0 °C, this performance is comparable or even higher than most of the previous reports. Furthermore, a 200 h continuously desalination experiment showed that the membrane had stable permeate flux and solute rejection, indicating that the as-spun PVDF hollow fiber membrane may be of great potential to be utilized in the DCMD process.     

An exergetic analysis of a membrane desalination system

In this paper, the exergetic analysis of a seawater membrane-based desalination plant has been carried out. The desalination plant has been described in detail, then the exergy of the various saline water streams has been determined and a comprehensive analysis towards the exergy distribution of the major process components has been conducted. The examination of the exergy losses throughout the plant revealed that exergy destruction was mainly due to pressure drops in the membrane modules, valves and brine lines. Moreover, 12.9% of the exergy input to the system was supplied by the heater. Therefore, the most reasonable way to reduce power input to the plant, thus improving its performance and cost, has been shown (i) to be replacing the valves on the reverse osmosis brine stream by an energy recovery system, and (ii) to have thermal energy available in the plant. With the identified technical changes, energy consumption decreased from 18.3 to 2.05 kWh/m³, resulting in an annual saving of 0.17$/m³.     

Transient analysis of a new humidification-dehumidification solar still

A numerical study was carried out to investigate the transient thermal performance of a new humidification-dehumidification solar still. The still body is a relatively thin rectangular box with a top glass cover and bottom condensing sheet. The still body is divided (by a central insulated stepped sheet carrying a group of basins) into two chambers: the upper evaporation chamber and lower condensation chamber. Air is circulated between the upper evaporation chamber (where it is heated and humidified) and the lower condensation chamber (where it is cooled and dehumidified for water production). The influence of different environmental, design, and operational parameters on the still productivity was investigated. The results indicated that increasing the solar intensity, ambient temperature, basin absorbitivity, and initial saline water temperature increases the system productivity. On the other hand, increasing wind velocity, basin insulation thickness, evaporation and condensation surface areas, condenser emissivity, and saline water mass has little effect on productivity. The interesting result is that the decreasing airflow rate has insignificant influence on system productivity. Decreasing airflow rate from 0.1 to 0.001 kg/s partially increases the productivity from 5.2 to 5.3 L/m². The physical explanation and implications of these findings are explained.     

Exergy analysis of a reverse osmosis desalination plant in California

The exergy analysis of a 7250 m³/d reverse osmosis (RO) desalination plant in California was conducted by using actual plant operation data, and an alternative design was investigated to improve its performance. The RO plant is described in detail, and the exergies across the major components of the plant are calculated and illustrated using exergy flow diagrams in an attempt to assess the exergy destruction distribution. The primary locations of exergy destruction were the membrane modules in which the saline water is separated into the brine and the permeate, and the throttling valves where the pressure of liquid is reduced, pressure drops through various process components, and the mixing chamber where the permeate and blend are mixed. The largest exergy destruction occurred in the membrane modules, and this amounted to 74.07% of the total exergy input. The smallest exergy destruction occurred in the mixing chamber. The mixing accounted for 0.67% of the total exergy input and presents a relatively small fraction. The second law of efficiency of the plant was calculated to be 4.3%, which seems to be low. The analysis of the alternative design was based on the exergy analysis. It is shown that the second law of efficiency can be increased to 4.9% by introducing a pressure exchanger with two throttling valves on the brine stream, and this saved 19.8 kW electricity by reducing the pumping power of the incoming saline water.     

Optimal design of an electrodialysis brackish water desalination plant

This paper considers the optimal design and operation of electrodialysis (ED) desalination plants. In general an ED plant aims to produce potable water from a high salinity source, like brackish water or high salinity water. The system is modelled mathematically as mixed-integer non-linear programming (MINLP) optimization problem, determining the number of desalination stages, the membrane area, the total required energy so as to minimise the total annualised cost of the investment accounting for both infrastructure and operating costs. Two examples from the literature illustrate the applicability of the proposed approach and evaluate the quality of the results obtained.     

Simulation of fresh water production using a humidification-dehumidification seawater greenhouse

A thermodynamic simulation study was performed on the influence of greenhouse-related parameters on a desalination process that combines fresh water production using humidification-dehumidification with the growth of crops in a greenhouse. With the system under study, surface seawater trickles down a porous front wall evaporator through which air is drawn into the greenhouse. The saturated air passes through a condenser, which is cooled using cold deep seawater or cool seawater coming out of the evaporators. Thermodynamic modeling of the seawater greenhouse system in our laboratory has shown that the dimension of the greenhouse had the greatest overall effect on water production and energy consumption. A wide shallow greenhouse, 200 m wide by 50 m deep gave 125 m³.d⁻¹ of fresh water. This was greater than a factor oftwo compared to the worst-case scenario with the same area (50 m wide by 200 m deep), which gave 58 m³.d⁻¹. Low power consumption went hand-in-hand with high efficiency. The wide shallow greenhouse consumed 1.16 kWh.m⁻³, while the narrow deep structure consumed 5.02 kWh.m⁻³ . The benefits of the development of the seawater greenhouse for arid regions are discussed.     

Exergy analysis of a combined RO, NF, andEDR desalination plant

A brackish water desalination plant in California that incorporates RO, NF, and EDR units was analyzedthermodynamically using actual plant operation data. Exergy flow rates were evaluated throughout the plant, and the exergy flow diagrams were prepared. The rates of exergy destruction and their percentage are indicated on the diagram so that the locations of highest exergy destruction can easily be identified. The analysis shows that most exergy destruction occurs in the pump/motor and the separation units. The fraction of exergy destruction in the pump/motor units is 39.7% for the RO unit, 23.6% for the NF unit, and 54.1 % for the EDR unit. Therefore, using high-efficiency pumps and motors equipped with VFD drives can reduce the cost of desalination significantly. The plant was determined to have a Second Law efficiency of 8.0% for the RO unit, 9.7% for the NF unit, and 6.3% for the EDR unit, which are very low. This indicates that there are major opportunities in the plant to improve thermodynamic: performance by reducing exergy destruction and thus the amount of electrical energy supplied, making the operation of the plant more cost effective.     

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.     

Renewable energy desalination plants for the Greek islands—technical and economic considerations

For the majority of the Greek islands, water resources are quite restricted, limiting the economic development of the local societies. To face increased potable water requirements, more than 2,500,000 m of clean water is transferred annually to these islands at a cost approaching the value of 7 /m³ On the other hand, the final cost of the locally produced water from renewable energy sources (RES) based desalination plants is expected to be quite lower than this value. The main purpose of the present study is to examine the economic viability of several representative desalination plant configurations based on the available renewable energy sources using an integrated cost-benefit analysis. In the proposed analysis all the cost parameters of the problem are taken into consideration, including the capital cost of the desalination plant, the annual maintenance and operation cost, the energy consumption cost, the local economy annual capital cost index and the corresponding inflation rate. The calculation results obtained definitely support the utilization of RES-based desalination plants as the most promising and sustainable method to satisfy the fresh, potable water demands of the small- to medium-sized Greek islands at a minimal cost, without disregarding the considerable environmental and macro-economic benefits.     

Analysis of water production costs of a nuclear desalination plant with a nuclear heating reactor coupled with MED processes

A nuclear heating reactor (NHR) was designed with the required inherent safety and simplified design features. Power capacity of the NHR-200 (200 MW(th), with steam production of 380 t/h) is compatible with reasonably sized desalination plants. Thermal-hydraulic parameters of the produced steam (2.4 bar and 124°C) are suitable for coupling with distillation processes. Economic competitiveness of the NHR desalination plant is the key point to which the public and decision-makers are paying good deal of attention. Coupling of the NHR with selected MED processes and design parameters of an integrated desalination plant are described. Results of analyses of water production costs are presented as well. Based on the economic evaluation, the average energy cost of the nuclear plant may reach 5.44 $/t of steam, and the provided water production cost may reach 0.72 $/m³ and 0.76 $/m³ for coupling with HT–VTE–MED and LT–HTE–MED processes, respectively.     

Feasibility study and performance assessment for the integration of a steam-injected gas turbine and thermal desalination system

This study proposes a systematic approach for retrofitting a steam-injection gas turbine (SIGT) with a multi-effect thermal vapor compression (METVC) desalination system. The retrofitted unit's product cost of the fresh water (RUPC) was used as a performance criterion, which comprises the thermodynamic, economic, and environmental attributes when calculating the total annual cost of the SIGT–METVC system. For the feasibility study of retrofitting the SIGT plant with the METVC desalination system, the effects of two key parameters were analyzed using response surface methodology (RSM) based on a central composite design (CCD): the steam air ratio (SR) and the temperature difference between the effects of the METVC system (?T_METVC) on the fresh water production (Q_freshwater) and the net power generation (W_net) of the SIGT–METVC system. Multi-objective optimization (MOO) which minimizes the modified total annual cost (MTAC) and maximizes the fresh water flow rate was performed to optimize the RUPC of the SIGT–METVC system. The best Pareto optimal solution showed that the SIGT–METVC system with five effects is the best one among the systems with 4–6 effects. This system under optimal operating conditions can save 21.07% and 9.54% of the RUPC, compared to the systems with four and six effects, respectively.     

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.