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.     

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.     

A solar desalination system using humidification–dehumidification process: experimental study and comparison with the theoretical results

In this study, influence of the different system operating conditions on the performance of a solar desalination system using humidification-dehumidification process have been investigated experimentally under the climatological conditions of Ankara (40°N, 33°E), Turkey. An experimental set-up that consists of a double-pass flat plate solar air heater with two glass covers, pad humidifier, dehumidifying exchanger and water storage tank was designed and manufactured. Working principle of the set-up is based on the idea of closed water and open-air cycles. A series of tests were performed on it in outdoor environment, in order to assess the effect of mass flow rate of the feed water, process air and cooling water, double-pass flat plate solar air heater, initial water temperature and amount of the water inside the storage tank on the productivity of the system. Additionally, an evacuated tubular solar water heater unit was integrated to the existing system and the effect of this integration on the performance of the system was examined. Solar radiation, wind speed, relative humidity, mass flow rate of the feed water, process air and cooling water, mass of condensate water and temperatures at various locations were obtained during the experiments.

The results of the experimental study showed that under certain operating conditions, the system productivity decreases about 15% if double-pass solar air heater is not used and significant improvement on the productivity of the system is achieved by increasing the initial water temperature inside the storage tank. In addition, productivity of the system increases with increasing the feed water mass flow rate and quantity of water inside the storage tank. However, productivity of the system remains approximately the same when the air mass flow rate is increased. Moreover, increasing the cooling water mass flow rate results in the improvement on the productivity of the system investigated. Finally, results obtained from the present investigation were compared with the theoretical study and a good agreement between them is observed.     

Modeling and performance analysis of a solar desalination unit with double-glass cover cooling

In this study modeling and performance analysis of a single-basin solar still with the entering brine flowing between a double-glass glazing were investigated. The base area of the solar still is 1 m². The function of this arrangement is to lower the glass temperature and thus increase the water-to-glass temperature difference. This results in improved performance represented by a faster rate of evaporation from the basin. The performance of the still is compared with that of a conventional single-glass cover solar still under identical weather conditions. The results show that the relative performance of the stills depends on the level of insulation used. For perfectly insulated stills the conventional solar still is superior while the double glass is superior when heat loss exceeds a certain value. The hourly and daily productivities of the stills and the temperatures of the water and the glass covers were also predicted under the meteorological conditions of Muscat, Oman.     

Myth and reality of the hybrid desalination process

The paper discusses some misconceptions that have contributed to the continued use of thermal desalination processes and promotion of the hybrid desalination process for new plants being built or considered at Middle East locations. The misconceptions are examined both on the basis of fundamental thermodynamic principles and in terms of practical engineering parameters. The analysis shows that there is no economic or performance advantage in the installation of greenfield hybrid power/thermal desalination/ seawater reverse osmosis (SWRO) plants in preference to power/SWRO plants, because the latter would produce water more cheaply under all conditions and at all fuel costs, and would provide more operational flexibility than the former. The paper identifies situations where the hybrid desalination process can be fully justified: in existing power/desalination plants, where aging boilers and multistage flash (MSF) units need to be repaired or replaced, through retrofitting and repowering. In such situations, abandonment of the MSF process would result in a reduction in the power output of the plant. The paper refers to previous work which showed that the repowering of a typical existing power/desalination station with refurbishment/replacement of the MSF units, together with the addition of SWRO units, would result in a several-fold increase in the water and power output and a dramatic improvement in the fuel efficiency, without any additions to the existing seawater intake system. The paper emphasizes the importance of test stations/demonstration plants at existing power/desalination stations in the Middle East in order to obtain data and make improvements in the technology of higher temperature SWRO, with the feed obtained from the cooling water returning from the power plant condenser and the thermal desalination plant. The paper shows that the potential benefits would easily justify the investment in research and development required to validate this concept.     

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.     

Performance optimization of solar multi-stage flash desalination process using Pinch technology

This paper presents a performance optimization of solar multi-stage flash (MSF) desalination process using Pinch technology. Three different situations were studied by using pinch analysis in this paper, respectively. Firstly, the distilled water is not discharged in each middle stage. Secondly, the distilled water is discharged in each stage. Thirdly, the distilled water is discharged every 5 stages. Pinch charts at different situations are given. At the same stage temperature difference (2 k) and pinch point temperature difference (2 k), the first situation has a higher gain output rate (GOR), about 17.5, and the second and third have lower GOR, around 9. However, GOR is easily influenced by abnormal stage temperature difference at the first situation, and but not in the second and third. The GOR rests on the working temperature range of MSF and the sum of both the maximum stage temperature difference and pinch point temperature difference. To enhance the GOR, it is suggested that we should use the wide working temperature range of MSF, not pumped out the distilled water at middle stages, and keep the same stage temperature difference without fluctuate.     

Comparison of solar thermal technologies for applications in seawater desalination

This paper deals with a global analysis of the use of solar energy in seawater distillation under Spanish climatic conditions. Static solar technologies as well as one-axis sun tracking were compared. Different temperature ranges of the thermal energy supply required for a desalination process were considered. At each temperature range, suitable solar collectors were compared in some aspects as: (1) fresh water production from a given desalination plant; (2) attainable fresh water production if a heat pump is coupled to the solar desalination system; (3) area of solar collector required for equivalent energy production. Results showed that direct steam generation (DSG) parabolic troughs are a promising technology for solar-assisted seawater desalination.     

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

The closed air cycle humidification-dehumidification process was used for water desalination using solar energy. The circulated air by natural or forced convection was heated and humidified by the hot water obtained either from a flat plate solar collector or from an electrical heater. The latent heat of condensation was recovered in the condenser to preheat the saline feed water. Two units of different sizes were constructed from different materials. The productivity of these units was found to be much higher than those of the single-basin stills. Moreover, these units were capable of producing a large quantity of saline warm water for domestic uses other than drinking. No significant improvement in the performance of the desalination units was achieved using forced air circulation at high temperatures. While at lower temperatures, a larger effect was noticed. This can be related to the low heat and mass transfer coefficients at low temperatures and to the non-linear increase in the water vapor pressure with temperature.     

Experimental study of basin-type, multiple-effect, diffusion-coupled solar still

Basin-type, multiple-effect, diffusion-coupled solar stills consisting of a tilted double glass cover, a horizontal basin liner and a number of closely spaced vertical partitions in contact with saline-soaked wicks were constructed and then examined in an outdoor experiment in fall and winter at Okinawa, Japan. This new type of still is greatly improved by narrowing diffusion gaps between the partitions, increasing the number of partitions, sandwiching small spacers between partitions and insulating the frame of the glass cover. The daily performances of the improved still are in good agreement with predictions calculated from the theoretical model developed by Tanaka et al. [1]. The still with 5-mm diffusion gaps between 11 partitions produces 14.8-18.7 kg/d distillate per unit effective area of the glass cover at 20.9-22.4 MJ m⁻² d⁻¹ solar radiations incident on the glass cover and 19-30°C ambient air temperatures and is highly productive in comparison with the 7-effect diffusion-type still with 8-mm gaps developed by Okamura et al. [2] and the 3-effect diffusion-type still with 19-mm gaps of Cooper and Appleyard [3].     

Experimental assessment of connection of an absorption heat pump to a multi-effect distillation unit

Theoretical analysis of integrating an absorption heat pump cycle in a multi-effect distillation (MED) process has shown better performance than with other types of heat pumps conventionally used as thermocompressors. However, to date, only two pilot facilities have been implemented worldwide. Both of them have been developed and tested in the framework of two different research and demonstration projects carried out at the Plataforma Solar de Almería (Spain). Two different double-effect absorption (LiBr-H₂O) heat pump (DEAHP) prototypes were coupled to an existing 14-effect MED unit. This paper reports the results of the experimental assessment of integrating the second prototype in the process. Although the initial design of the DEAHP prototype was based on fitting it to the MED unit power demand and their direct connection, the prototype was unable to achieve steady operation in this configuration. However, the indirect connection of both units by means of two auxiliary tanks was successful. An overall performance ratio of 20 was measured; therefore, integration of the DEAHP doubles the performance ratio of the MED unit alone, although the temperature of the external heat input required is increased from 70 °C to 180 °C.     

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.     

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.     

Validating the performance simulation program “SOLDES” using data from an operating solar desalination plant

A computer program (named SOLDES) was developed to simulate the operation of solar desalination plants which utilize evacuated tube collectors, heat accumulators and multiple-effect distillation (MED) systems. The heat accumulator used is of the thermally stratified type using pure water as the storage fluid. The procedure was written in Fortran language and consists of a main program, 22 sub-programs, two system data files and four meteorological data files. The absorber area of the solar collector field can be varied between 500 m² and 20,000 m²; the storage capacity per unit collector area of the heat accumulator can vary between 0.05 and 1.00m³/m²; the capacity of the evaporator can be varied between 100 m³/d to 2000 m³/d. The heat collecting system uses a bypass circuit to allow the heat collecting fluid (pure water) to recirculate back to the solar collector field when the outlet temperature from the collector field is below a set-point. When the collector outlet temperature rises above the set-point, operation is switched over to the accumulator side. A solar-cell-type controller is used to start and stop the water circulating pump of the collector field. The operation of the MED evaporator is controlled by the state of charge of the heat accumulator by the use of set-point switches which allow the evaporator to start up when the accumulator water temperature is above a set-point and to shut down if the water temperature drops below the set point. In order to validate the SOLDES program, a comparison was made between the predicted results of the program and the actual measured data from a solar plant of similar design features to the simulation program. The selected plant was the one in actual operation in Abu Dhabi, UAE, which has almost identical design features as the simulation program and has been in operation since 1984. The data from the plant collected during 1985 were used to compare the simulation results for the months of January and June. These two months were found to be typical of a winter month (January) and of summer months (June). Except for days when a plant interruption took place, such as a power failure, the agreement between the measured and simulation data appears to be quite good.     

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.     

Applicability of flashing desalination technique for small scale needs using a novel integrated system coupled with nanofluid-based solar collector

The present study introduces an attempt for the application of flash desalination technique for small scale needs. An integrated system uses a flashing desalination technique coupled with nano-fluid-based solar collector as a heat source has been made to investigate both the effect of different operating modes and that of the variation of functioning parameters and weather conditions on the fresh water production. The flashing unit is performed by similar construction design technique of commercial multi-stage flashing (MSF) plant. The thermal properties of working fluid in the solar collector have been improved by using different concentrated nano-particles. Cu nano-particle is used in the modeling to determine the proper nano-fluid volume fraction that gives higher fresh water productivity. An economic analysis was conducted, since it affects the final cost of produced water, to determine the cost of fresh water production. Although a system may be technically very efficient, it may not be economical. The effect of different feed water and inlet cooling water temperatures on the system performance was studied. The mathematical model is developed to calculate the productivity of the system under different operating conditions. The proposed system gives a reasonable production of fresh water up to 7.7 l/m²/day under the operation conditions. Based on the cost of energy in Egypt, the estimated cost of the generated potable water was 11.68 US$/m³. The efficiency of the system is measured by the gained output ratio (GOR) with day time. The gained output ratio (GOR) of the system reaches 1.058. The current study showed that the solar water heater collecting area is considered a significant factor for reducing the water production cost. Also, the produced water costs decrease with increasing the collecting area of the solar water heater. The volume fractions of nano-particle in solar collector working fluid have a significant impact on increasing the fresh water production and decreasing cost.     

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.     

A vertical multiple-effect diffusion-type solar still coupled with a heat-pipe solar collector

We proposed a newly designed, compact multiple-effect diffusion-type solar still consisting of a heat-pipe solar collector and a number of vertical parallel partitions in contact with saline-soaked wicks. The solar collector and the vertical multiple-effect diffusion-type solar still can be folded or separated when it is carried, so that the still would be easy to carry and shipping cost would be very cheap. The solar energy absorbed on the solar collector is transported as latent heat of working fluid to the vertical multiple-effect diffusion-type still where the energy is recycled to increase the productivity of distillate. The performance of the proposed still is analyzed theoretically, and the still is predicted to produce 21.8 kg/m²d distilled water on a sunny autumn equinox day of 22.4 MJ/m²d solar radiation, and the productivity is greater than that of a vertical multiple-effect diffusion-type still coupled with bulky basin type still.     

An approach to improve the economy of desalination plants with a nuclear heating reactor by coupling with hybrid technologies

In order to investigate the possible approach to increase thermal efficiency of desalination plants, decrease water production costs and further optimize the coupling design of a nuclear heating reactor (NHR) with the desalination process, the coupling schemes of NHR reactors with hybrid desalination technologies were investigated. The cogeneration operation mode was adopted in this investigation. Two coupling schemes were selected for the cogeneration mode: NHR + low-temperature MED+RO and NHR + low-temperature MED+MED/VC. Technical specifications and economic aspects of the investigation are briefly presented in this paper.