High performance domestic solar desalination system

This paper presents a newly designed, high performance solar desalination system suitable for domestic use. In this system the evaporating and condensing zones, unlike in conventional stills, are separated in two different units. The advantages of the force condensing technique and shallow basin still are integrated together to improve the system performance. A new approach of storing excess solar heat energy during the day for the continuation of the desalination process at night is introduced. Also heat energy absorbed by the evaporator-zone back-plate wall is utilized to improve the system efficiency. Exhaustive data collected during 11 months are analyzed and presented to show that the performance of the system is higher.     

Cost analysis of different solar still configurations

The enhancement of the productivity of the solar desalination system, in a certain location, could be attained by a proper modification in the system design. Therefore, different design configurations could be found in literatures. However, the increase in the system productivity with high system cost may increase also the average annual cost of the distillate. Cost analysis of different design configurations of solar desalination units is essential to evaluate the benefit of modification from the economical point of view. The main objective of this work is to estimate the water production cost for different types of solar stills. In this paper 17 design configurations are considered. Systems with higher and lower values of productivity are considered in this investigation. A simplified model for cost analysis is applied in this study. The results show that, the best average and maximum daily productivity are obtained from solar stills of single-slope and pyramid-shaped. The higher average annual productivity for a solar still is about 1533 l/m² using pyramid-shaped while the lower average annual productivity is about of 250 l/m² using modified solar stills with sun tracking. The lowest cost of distilled water obtained from the pyramid-shaped solar still is estimated as 0.0135 $/l while highest cost from the modified solar stills with sun tracking is estimated as 0.23 $/l.     

Solar stills for community use—digest of technology

The many factors that influence the productivity of solar stills are discussed in three categories: atmospheric variables, design features and operational techniques. Data on the large solar stills which have been operated are tabulated, and productivity curves are given for several basin-type stills. The economics of solar distillation is also considered, and an equation is presented to calculate the cost of producing fresh water. A primary area for further work is identified, that of proving the durability of improved materials by the successful long-term operation of large solar stills. Solar distillation appears well suited for the supply of potable water to small communities where the natural supply of fresh water is inadequate or of poor quality, and where sunshine is abundant. The capital cost of large permanent-type solar stills can be as low as $1 per ft² of basin area, which is equivalent to $10 to $15 per daily gallon output, depending on the yearly amount of solar radiation and rainfall collection. The corresponding distilled water cost is between $3 and $4 per 1000 gal. These water costs are generally lower than those associated with other types of desalination equipment in plant sizes of up to, perhaps, 50,000 gpd.     

New method to store heat energy in horizontal solar desalination still

A new approach is proposed to store excess heat energy in horizontal solar desalination stills during daytime for the continuation of the process at night. This technique divides the horizontal still into evaporating and heat storing zones and combines the advantages of shallow and deep stills. The performance of heat storing zone was studied over one year and exhaustive data were collected, analyzed and presented. To show the effectiveness of the system, its performance was compared with that of the shallow still. The heat storing capacity of the system during the daytime was found to be an average of 35.7% of the total amount of solar energy entering the system. The efficiency of recovering process, in the form of portable water produced at night, was found to be an average of 47.2% of the total amount of energy stored during the day. Furthermore, this technique does not require any kind of external power for storing and recovering processes.     

A multistage solar still with photovoltaic panels and DC water heater using a pyramid glass cover enhanced by external cooling shower and PCM

A novel multistage solar desalination system with a photovoltaic heater was manufactured. The base of the down basin of the solar still had a layer of paraffin wax with a mass of 13 kg as a phase change material. The system has been studied to evaluate the enhancement of freshwater. Saltwater was heated by solar radiation and by a direct current water heater. The surfaces of condensation vapor, such as the pyramid glass cover and lower surface of two stacked trays, were designed. This is to improve the productivity of freshwater by decreasing the resistance of condensation. The high temperature of the glass cover is modified by using a cooling water shower, especially at the highest intensity. The study includes parameters, such as cooling water shower flow rate, down basin water level, and the effect of the heater. It is observed that the novel solar desalination is proportional to solar radiation, paraffin wax, the heat input from a heater, cooling water shower flow rate, and down basin water level. The Multiple Stage Effect Photovoltaic Heater (MSEPVH) can produce 15 L/day of distilled water. The excellent flow rate of cooling water, the total freshwater, and the efficiency of MSEPVH for the optimal day were mathematically and experimentally determined.     

Numerical and experimental investigation of parabolic trough collector with focal evacuated tube and different working fluids integrated with single slope solar still

This study deals with the design and fabrication of parabolic trough solar collectors (PTCs) used to increase the yield of a single slope solar still. The designed parabolic trough solar collector is investigated numerically using Ansys Fluent 18.2. The proposed solar still is coupled with a parabolic trough solar collector with an evacuated tube receiver in its focal axis using different working fluids. The working fluids are water (case 1), oil (case 2), and nano-oil (CuO/mineral oil 3% vol; case 3). In the case when the working fluid is not water, then a heat exchanger serpentine should be used in the solar still basin. The PTC has a rim angle of 82° and an aperture width of 0.9 m and length of 2.8 m. An assessment of the performance for the studied systems was accomplished under the weather conditions of Ismailia, Egypt, during summer months, June, July, and August 2019. The outcomes of closed-loop working fluids different flow rates are investigated. The experimental results of the accumulated freshwater productivities record 2.955, 3.475, 4.29, and 5.04 L m⁻² d⁻¹ for the traditional solar still and the modified cases 1 to 3 solar stills, respectively. The modified solar still in case 3 has the highest daily accumulated freshwater productivity with a percentage increase of 71.2% than the traditional solar still. The maximum daily efficiency is 46% and 26.9% for the traditional and modified (case 3) solar stills, respectively. The cost of 1 L of fresh water is 0.057 and 0.062 $/L for the traditional and the modified (case 3) solar stills, respectively.     

DESIGN STUDY OF A MULTIWICK-CUM-CONVENTIONAL SOLAR STILL-CUM-SOLAR HOT WATER SYSTEM

There has been considerable effort as to the manner in which the productivity of solar stills is affected by many of the designs and operating variables. To assist in designing the stills of improved performance, design parameters involved in the operation of the system have been considered in this paper. Curves showing the magnitude of the effects of design changes on the productivity are presented. Numerical computations, which are based on energy balances of different components of this system, confirm the productivity improvement in between 23.6% to 51.2% depending on the water-flow-rate from the vertical water column on the multiwicks. This design incorporates a multiwick solar still and a conventional basin type solar still. Hot water at considerably higher than the ambient temperature may be obtained at different flow rate from this design as well.     

Plastic solar stills: Past, present, and future

Over 400,000 plastic solar stills have been sold commercially. According to the literature, highest yields for large-scale solar stills were obtained from those with plastic covers. The technology of plastic stills is changing; that of glass covers has remained basically the same for the past hundred years.

A review of technical and patent literature on plastic solar-still elements is presented. Recent experiences with plastic covers are summarized with emphasis on causes for reported failures and explanations of acknowledged successes. The advantages of plastic solar still covers are listed with details on the properties of various plastics. Trends and influences which assure better future performance are also mentioned.

Shortcomings to the present approach to desalination of community water supplies by solar energy are compared with the advantages for individual family water supplies. The integration of desalination with other tasks, such as air conditioning, is recommended. By having the solar still replace the ceiling and roof of a room, it may be possible to eliminate all capital costs for the still.     

Utilization of solar earth-water stills for desalination of groundwater

Three solar earth-water stills of single-sloped type, designed for producing drinking water in remote areas, have been investigated. These stills were installed over an insulated hole, a hole with insulated base, and a hole with insulated walls. Multiple linear regression equations, relating to ambient air temperature, wind speed, and solar radiation, were developed to estimate the productivity of these stills. The study showed that condensation process inside these stills is achieved during the period between sunset and sunrise. Results showed also that the average wall's contribution in supplying fresh water is about 56%, whereas base contribution is about 31%. It is concluded, therefore, that setting many stills on a number of separated holes will give higher output rather than setting a single still on one large hole of the same volume.     

Performance of a self-regulating solar multistage flash desalination system

A 10,000-1 solar multistage flash desalination system was designed and tested at Kuwait Institute for Scientific Research. The system consisted of a 220 m² solar line-concentrating collector field, 7000-1 thermal storage tank and a self-regulating 12-stage multistage flash desalination subsystem. The collector field, equipped with closed-loop tracking system, was installed with individual troughs oriented in the north-south direction. The thermal storage subsystem was useful in leveling off the thermal energy supply and allowing the production of desalinated water to continue during periods of low radiation and nighttime. The self-regulating capability of the desalination subsystem allowed for the adjustment of the flow rate and pressure, in the various stages of the desalination subsystem, according to the relative difference between the hot brine and input sea water temperatures. This allowed for maintaining a relatively high overall efficiency.     

Improvement of basin type solar still performance : Use of various absorber materials and solar collector integration

Experimental measurements to determine conditions necessary for efficient solar desalination are given. The effects on performance of using various different absorber materials together with the integration of flat-plate collectors with storage systems in basin type solar stills are investigated. Correlations between daily yield (Y, in litres) and solar insolation (I, in MJ/m²day) are found to be, Y = 0.152I − 0.706 (for black-paint absorbers), Y = 0.180I − 0.987 (for charcoal absorbers), Y = 0.225I − 0.467 (for integrated solar collector and storage system, with black-paint absorbers). The calculated daily-average monthly yields for each case are also given.     

Comparison of free and forced condensing systems in solar desalination units

Solar desalination methods to produce potable water are gaining attention in these days of energy crisis. This paper deals with the utilization of solar energy for evaporation on a stationary brackish water surface. Exhaustive data collection and graphical analysis are presented to compare the performance of free and forced condenser systems in solar desalination units. The efficiency of the existing system is improved by introducing a thermally insulated flat plate to separate the evaporation zone from the condensation zone. The efficiencies of the above systems are calculated and compared with those of a shallow basin solar system for which data are available (A. I. Kudish, J. Gale and Y. Zarm; 1982, Energy Conversion and Management22, 269–274).     

Experimental analysis and exergy efficiency of a conventional solar still with Fresnel lens and energy storage material

This study focuses on the experimental investigation and exergy analysis of a modified solar still (MSS) with convex lenses on glass cover to collect the solar radiation at the focus on surface water. A comparative analysis of the performance and yield of the MSS with convex lenses and the conventional single slope SS were carried out for the same climatic condition of Tanta (Egypt). Similarly, the effect of modification in the SS using convex lenses, with or without black stones, on the freshwater yield is experimentally investigated. The results indicated that the lenses focus the solar radiation to the water placed in the basin and increase the water‐glass temperature difference (T _w – T _g). The yield of freshwater from the MSS with the convex lenses is comparatively higher than that of the conventional SS (26.64%). In addition to convex lenses in the inner cover surface, freshwater yield improved by 35.55% by adding blue stones as energy material inside the basin under constant water mass of 30 kg. The maximum exergy efficiency of the SS with lenses and blue stones was 11.7%, while the SS with lenses alone was 4%. The quality of freshwater produced after desalination was well within the World Health Organization standards. The total dissolved solids and pH after desalination were 22 mg/L and 8.08, respectively.     

Optimum exergy efficiency of single-effect ideal passive solar stills

There is a complex heat and mass transfer phenomenon in the solar stills. It is desired to examine the ways of maximizing the efficiency with the help of an effective thermodynamic tool, i.e., energy and exergy analysis. In this paper, a thermodynamic model has been developed to estimate the overall instantaneous exergy efficiency of the single-effect horizontal basin-type ideal passive solar stills. Theoretical overall instantaneous exergy efficiency of a passive solar still having 30° tilt angle of glass cover and water depth of 0.04 m on a typical day in June is evaluated and found in the range 0.06 to 5.9 % for the variation of experimental results of energy efficiency from 8 to 87.2 %. The daily energy and exergy efficiency of the solar still is 20.7 and 1.31 %, respectively. An optimum exergy efficiency of the ideal solar still is found to be 21.11 % corresponding to 80 % ultimate energy efficiency and at a typical operating condition. A feasible target of optimum exergy efficiency has been set under assumed ideal conditions to achieve in the future for the real working passive solar stills. It is also confirmed that the overall exergy efficiency increases with the increase of water temperature and decreases with the increase of ambient temperature.     

Analysis of assisted coupled solar stills

The potential of coupled solar stills as a mean for improving solar distillation yield is investigated. A model for a simple solar still assisted by an external solar collector is presented. The governing coupled heat and momentum balance equations are solved for a quasi-steady-state condition with temperature dependent physical properties. The results show that for coupled stills the fresh water productivity increases as the solar collector area of the assisting device increases. For an ideal system, neglecting the thermal inertia, the yield is linearly dependent upon the solar insolation. The thermal inertia causes a significant drop in the system yield and deviation from linearity. The net efficiency of the coupled system is higher than that of a similar simple still by a value that depends mainly upon the system configuration and independent of the meteorological conditions. Comparison of the analysis with experimental data is satisfactory.     

Experimental evaluation of a single-basin solar still using different absorbing materials

Single-basin solar stills can be used for water desalination. Probably, they are considered the best solution for water production in remote, arid to semi-arid, small communities, where fresh water is unavailable. However, the amount of distilled water produced per unit area is somewhat low which makes the single-basin solar still unacceptable in some instances. The purpose of this paper is to study the effect of using different absorbing materials in a solar still, and thus enhance the productivity of water. Experimental results show that the productivity of distilled water was enhanced for some materials. For example, using an absorbing black rubber mat increased the daily water productivity by 38%. Using black ink increased it by 45%. Black dye was the best absorbing material used in terms of water productivity. It resulted in an enhancement of about 60%. The still used in the study was a single-basin solar still with double slopes and an effective insolation area of 3 m².     

Performance evaluation of v-trough solar concentrator for water desalination applications

The working principle and thermal performance of a new v-trough solar concentrator are presented in this paper. Compared with the common parabolic trough solar concentrators, the new concentrator has two parabolic troughs which form a V-shape with the focal line at the bottom of the troughs. This is beneficial for the installation and insulation of the receiver, and the shadow on the reflective surface is avoided. The new v-trough collector does not require high precision tracking devices and reflective material. And therefore the cost of the system could be significantly reduced. Various experimental tests were carried out both outdoor and indoor using different types of receiver tubes. The results show that the collector system can have thermal efficiency up to 38% at 100 °C operating temperature. System modelling was used to predict the rate of fresh water produced by four different solar collector systems which include both static and one-axis solar tracking technologies. Comparison of the solar collectors at different temperature ranges for humidification/dehumidification desalination process using specific air flow rate were considered. At each temperature range, suitable solar collectors were compared in the aspect of fresh water production and area of solar collector required. Results showed that the new v-trough solar collector is the most promising technology for small to medium scale solar powered water desalination.     

Solar still versus solar evaporator: A comparative study between their thermal behaviors

This paper presents a theoretical comparison between the thermal behavior of a basin type solar still and that from a solar evaporator. The analyses are twofold: (a) comparing the system temperatures and the heat and mass transfer rates in the transient mode; (b) obtaining parametric representations from both systems using their heat and mass transfer equations. Such comparisons had never been done before. It is shown, among other things, that the evaporation in solar stills is much less than that in open evaporation despite the higher water temperatures in the former system. This is also true even when the water temperature of both systems is the same. It is also observed that the distillation and evaporation rates increase with the increasing water temperature and temperature difference. For relatively high water temperatures of each system the evaporative fraction is equivalent to more than 50% of the corresponding total heat transfer rate.     

Performance evaluation of solar distillation using vacuum tube coupled with photovoltaic system

The distillation is one of the important methods of getting distilled water using the free energy supply from the sun. This study presented a short review on solar still distillation. Also, the study is to enhance the thermal performance of stills through coupling the still with the Super Heat Conduction Metal Vacuum Tube (SHCMV). A SHCMV process achieved a continuous operating mode fully operated using flat plat photovoltaic system. The SHCMV has produced 12 l/m²/day, whereas the traditional solar still produced around 1 l/m²/day of distilled water. The SHCMV increased the yield to around 90%. The SHCMV gave more than 60% efficiency for more than 5 h during the daylight.     

Determination of rational design parameters of a multi-stage solar water desalination still using transient mathematical modelling

The paper describes the experimental investigations of the performance of a multi-stage water desalination still connected to a heat pipe evacuated tube solar collector with aperture area of 1.7 m². The multi-stage solar still water desalination system was designed to recover latent heat from evaporation and condensation processes in four stages. The variation in the solar radiation during a typical mid-summer day in the Middle East region was simulated on the test rig using an array of 110 halogen floodlights covering the area of the collector. The results of tests demonstrate that the system produces about 9 kg of fresh water per day and has a solar collector efficiency of about 68%. However, the overall efficiency of the laboratory test rig at this stage of the investigations was found to be at the level of 33% due to excessive heat losses in the system. The analysis of the distilled water showed that its quality was within the World Health Organization guidelines. The still's operation was numerically simulated by employing a mathematical model based on a system of ordinary energy and mass conservation differential equations written for each stage of the still. A computer program was developed for transient simulations of the evaporation and condensation processes inside the multi-stage still. Experimental results obtained and theoretical predictions were found to be in good agreement. The results on the determination of rational design dimensions and number of stages of the still for a given aperture of the solar collector are also presented in this work.