Design and Fabrication of a Multistage Solar Still With ...

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Mar 13, 2018 - Multistage Solar Still With Three. Focal Concentric Collectors. This research is intended to design and manufacture a multilayer solar distiller at ...
Design and Fabrication of a Multistage Solar Still With Three Focal Concentric Collectors Fayadh M. Abed1 College of Engineering, Tikrit University, SalahAdeen-Tikrit, Al-Qadysia 34001, Iraq e-mail: [email protected]

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This research is intended to design and manufacture a multilayer solar distiller at a promising cost. The solar distiller manufactured has the same design as simple water distillers, which are based on the principle of evaporation and condensation with a different energy cycle, where the processes of evaporation and condensation are completely isolated. The obtained results showed that the amount of produced water has increased by 60% compared to the traditional solar distillers, where the system is not isolated. No catalysts were used, and the areas of the evaporation and condensation have also been increased leading to the production of distilled water under natural conditions and low cost. A comparison between the theoretical and experimental results is performed. The productivity was as follows: 8.45, 11.04, 12.20, 21.44, 18.69, 16.15, and 14.49 L/day in January, February, March, September, October, November, and December, respectively. [DOI: 10.1115/1.4039351]

Introduction

Sustainability of life for all creatures on earth is associated with the existence of water. All human activities (such as domestic, agricultural, or industrial uses) depend on water. Seas and oceans are the major sources of water, where 97.5% of global water is provided. The remaining part (2.5%) is provided by surface and underground waters. Rivers, lakes, and aquifers provide only 0.5% of the total fresh waters [1]. The scarcity of potable waters in our region represents an eradicative imminent danger that must be encountered and curbed by states represented by their research and scientific centers in order to find appropriate solutions [2]. Such issue has been considered as the biggest problem of the current century. Facts indicate that 85% of the world populations inhabit the drier part of the earth’s hemisphere, 6  106 to 8  106 people die annually due to causes related to water shortage, 80% of diseases in third-world countries are related to the poor quality of potable water, and 6.3% of the Middle East population and North Africa of the world and their quota are only 1.4% of the freshwater available across the world. Also, other two reasons in decreasing freshwater available are the global warming and the growing need for water. The expected percentage of the increased demand on freshwater will be 19% by 2050 compared to the current need. Furthermore, around 75% of freshwater sources in the Middle East and North Africa are contaminated. Solar distillers are considered a significant unit for providing a potable water meeting human consumption demands and basically utilize a renewable energy, thus utilizing the sun as the energy source. The difference in the design technology in terms of the distiller type, size, and materials incorporated in the manufacturing process and the degree of simplicity and complexity are priorities, where the researcher resorts to achieve the targets and produce potable water as much as possible by adopting the simplest design that can be implemented by the available raw materials at the lowest cost. The latest studies indicated that oceans’ saline seawater accounts for 97% of waters on earth, and due to the need for 1 Corresponding author. Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received November 3, 2017; final manuscript received February 4, 2018; published online March 13, 2018. Assoc. Editor: Gerardo Diaz.

Journal of Solar Energy Engineering

freshwater, people looked forward throughout history to make use of this permanent resource [3]. Owing to the problems encountered related to high water salinity, it becomes a community critical demand to find a solar technology in desalinating water through designing and manufacturing multilayer solar distiller in order to provide potable waters. Renewable energy sources have become increasingly important since the energy crisis in 1970. Since the sun is the main energy source, serious efforts were exerted to transform solar energy into other forms of energy. With the growing need for fresh water, it becomes crucial to desalinate saline water and use it for human needs and irrigation or any other application where saline water is adequate. There are an infinite number of uses of waters since the early ages of human being. Most of populations were looking for water and specified where water was found. As the world currently faces an increasing shortage for waters needed to fulfill essential needs in all aspects of modern life, the trend today is to desalinate seawaters and treat sewage water, in addition to reuse of water for those living in areas that suffer water scarcity. There are various methods toward producing potable waters, thermal and nonthermal, and most of them depend principally on the use of nonclean energy sources. The use of these sources creates environmental problems; therefore, modern science is exploring the possibility of exploiting renewable energy in all industrial fields including water desalination [4,5]. Solar distiller was made up of a thermally insulated basin with all sides locked and has a glass cover. The glass cover is normally slanted in order to allow vapor condensation over it to drip down along a side conduit and then to a trough for collecting pure water. The basin can be manufactured from inexpensive materials that are not corroded by water. It is also necessary to cover the bottom and sides of the basin with thermal insulators to reduce the loss of energy from the basin water to the surroundings and, in turn, raise the efficiency of the distiller. It is normal to coat the bottom of the basin with black paint or any other appropriate coating materials in order to increase the absorption rate of sun radiation. In some designs, the internal vertical walls of the basin are coated with reflecting paints in order to reflect incident rays in water, and it is essential to tighten seals along the basin sides to reduce leakage of air saturated with vapor from the inside to the outside and to reduce heat transfer through air leakage openings. As water vapor pressure increases with increasing temperature, the pressure of water vapor on the water surface directly is at a higher

C 2018 by ASME Copyright V

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temperature than the surface in contact with the air inside the basin. As a result of this pressure difference between the vapor layer touching the surface of the basin water and the vapor existing in the air, the basin water starts to evaporate in order to balance the vapor pressure inside the basin. And as a result of thermal convection factors, saturated water moves upward and is replaced by air that is less saturated with vapor [6,7]. Solar distillation is a natural phenomenon that has the same principle as a real phenomenon when sun rays fall of rivers, lakes, oceans, and seawaters; it will heat up this water, which evaporates and rise upward. Afterward, the vapor is transferred through the wind until it reaches a cooler location where it condenses and forms clouds, then it turns into either rain or ice. The solar distiller works on the same principle where solar radiation heats up saline water in the distiller basin, and then the vapor is formed and transferred to the glass ceiling of the distiller through convection, on which it condenses and forms water droplets that angle down toward especially prepared troughs to collect potable water that is adequate for human consumption [8]. These distillers are relatively simple compared to other types, and they are made up of a basin filled with saline water and coated from the inside with black paint in order to attract and absorb as much solar radiation as possible during daylight. The basin is covered by a transparent glass panel, which is inclined to a particular direction to allow vapor to condense on the internal surface of the cover and water drops to slide along the inclined surface to be collected in a special container designed for this purpose through pipes and paths. The productivity of distiller depends on a number of factors: the inclination of the transparent glass cover, the distance between the glass cover and the saline water surface, the depth of the saline water in the basin, the temperature difference between the transparent cover and saline water surface, and the salinity ratio of water. Such simple distillers are the most common in the world, and they are characterized by their low manufacturing and maintenance cost compared to other types of distillers [9]. Below are some common types of distillers that shall be mentioned without further elaboration and one may go to references for further details: single-slope (single collector) solar distiller [9], symmetrical doubleslope (double collector) solar distiller [10], unsymmetrical doubleslope solar distiller [11], and spherical distiller with a scanner [12]. In addition, there are several models of this type of solar distillers containing several stages of evaporators. A brief survey addressing different types of distillers follows in this study: multitier solar distiller [11,13], a distiller operated by capillary criteria [14], conical solar distiller [15], vertical solar distiller [4], V-type solar distiller [16], and inclined basin solar distiller [17]. Satcunanathan and Hansen [18] studied the effect of the following three variables on the performance of solar distiller: the distance from the saline water basin to the transparent cover, the transparent cover inclination, and cooling of the transparent cover. Practical experiments conducted by the two researchers showed that reducing the distance and placing the transparent cover in a vertical position over sun rays’ path and cooling cover improve the still performance [18]. In 2007, Badran and Abu-Khader [19] studied the effect of using two types of internal factors on solar distiller productivity: an insulator with different thicknesses (1 cm, 2 cm, and 5 cm) and different depths of water (2 cm, 3.5 cm). Experiments showed that productivity increases with increasing insulator thickness and decreases with increasing water depth. Abdallah and Badran [20] have conducted a practical study in which they used a device to track sun movement and rotate the solar distiller in accordance with sun movement to increase the efficiency and productivity of the distiller. The results of using a tracking system showed that the productivity and efficiency are higher compared to using a fixed type by 22% and 2%, respectively. Abdallah and Badran [20] and Hashim [4] presented a study that aimed at improving the productivity of solar distiller. A new type of double-basin solar distiller was designed and manufactured. The results of tests showed an identical productivity of water in both single and double basins. The area of the single horizontal basin was 0.25 m2, 041003-2 / Vol. 140, AUGUST 2018

with the double-basin system having the same area divided by 0.1 m2 for the horizontal basin and 0.15 m2 for the vertical part. Improvement in daily productivity of the new distiller was noticed. The advantage of the vertical part of the basin is that the potential energy for evaporation inside the distiller was used for heating water inside the vertical part, where energy is stored, so that the evaporation process continues after sunset. In addition, the vertical basin facilitates the process of sunray’s concentration by using any reflector to boost productivity. Hashim [4] and Gawande et al. [21] have conducted tests on a new type of multilayer solar distillers to study the impact of water depth on its productivity. Three different depths of water in the distiller basin (10 mm, 7.5 mm, and 5 mm) were considered and tested under different operating and weather conditions. The results showed that the productivity of distiller decreases with the increase in water depth. It has been observed that the productivity at 5 mm depth is higher than that obtained at 7.5 mm depth by 14.15%. In addition, the productivity at 7.5 mm depth is higher than that at 10 mm depth by 22.26% under the same operational conditions. Abed et al. [22] have carried out an experimental work including a comparison between two distillers by changing water depth. The walls of one distiller were insulated, while the walls of the other were not insulated. The study has also added some chemical materials like thymol solution and orange methyl solution to the well-insulated solar distiller. The performance of distiller in the case of the presence of chemical materials was compared. The practical tests showed more efficiency and productivity when the walls were insulated and in the presence of chemical materials [22]. Based on the points presented above, it has been noticed that most of the studies focused on the effect of the depth of water in the basin, the influence of inclination of transparent glass cover facing sun rays and its height on the productivity of the distiller, and the use of improving materials to increase the quantity of water evaporated through the use of dyes and chemical solutions. Other studies concentrated on studying the effect of using thermal insulators with different thicknesses for distiller walls, while others showed an interest in cooling the glass cover in order to condense as much water vapor as possible. Some researchers have concentrated their studies on the atmospheric conditions and wind speed factor of distiller productivity. All the previous cases have been taken into account in the present research along with a number of additional improvements that would lead toward boosting the efficiency and productivity of the solar distiller. In the present study, a multilayer solar distiller system was designed and manufactured.

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Mathematical Model

The main variables influencing the design of the multilayer solar distillate and the distillation of distilled water were determined by solving mathematical equations (energy conservation, thermal balance of evaporation, and condensation of water for each distilled layer) to determine the water productivity of each stage and then to determine all required dimensions of surface area of evaporation and condensation areas. 2.1 Model Assumption. The system of energy equations was solved theoretically to determine the performance of the solar distill. The system of energy equations was modeled and solved by MATLAB language. Solving such system of equations, the following assumptions were made in this study. The system is air and vapor. Water is at a uniform temperature at each stage, and the heat lost from the sides and heat lost to ambient through evaporation have been neglected. In addition, the system is well insulated so that the losses to the environment are negligible. 2.2 Model Formulation. The multistage water desalination system is shown in Fig. 1. The area of the solar collector, the number of stages, the flow rate of the oil, and the type of oil used Transactions of the ASME

dMsi ¼ m_ e dt

(4)

Latent heat coefficient and latent heat (corrected) are used to evaporate the saline in a distilled device. Equations (5) and (6) below were used to calculate the latent heat coefficient [23] and the corrosive latent heat [24] hfgi ðTi Þ ¼ 1000  ½3161:5  2:40741  ðTi þ 273Þ

(5)

hfg ¼ hfgi þ 0:68cpi ðTsi Tci Þ

(6)

The surface area of the evaporation water in each stage is calculated by the following equation: Fig. 1

Multistage water desalination system processing

Asi ¼ 0:4  L

have been determined by maximizing the productivity depending on the operational conditions and weather variables. Energy conservation equation per unit area and thermal balance for all stages are written as follows [22]: Energy conservation equation and thermal balance for the first stage are Q_H  m_ e1 hfg1 þ Cp Tc1



dTs1 ¼ Ms1 Cp þ 䉭Q_ losses1 dt

(1)

Energy conservation equation and thermal balance for the second stage are  dTs2 þ 䉭Q_ losses2 m_ e1 hfg1  m_ e2 hfg2 þ Cp Tc2 ¼ Ms2 Cp dt

(2)

Energy conservation equation and thermal balance for the third stage are  dTs3 þ 䉭Q_ losses3 m_ e2 hfg2  m_ e3 hfg3 þ Cp Tc3 ¼ Ms3 Cp dt

(3)

The symbols for the above Eqs. (1)–(3) are shown in the nomenclature list; all the terms are explained in Fig. 2, and the symbols of TC1, TC2, and TC3 are in fact represent temperature difference as stated in Eqs. (18)–(21). Equations (1)–(3) are presented in Fig. 2. The mass conservation equation for each stage is Latent heat coefficient and latent heat (corrected) are used to evaporate the saline in a distilled device

(7)

The average heat input to the solar distillatory by hot fluid is represented by the following equation: Q_H ¼ m_ e Cp ðTSCinlet Tscoutlet Þ

(8)

2.2.1 The Average Heat Losses in Each Stage are as Follows  dTs1 DQ•losses1 ¼ Q•H  m•e1 hfg1 þ cpTc1  Ms1 cp dt

(9)

 dTs2 DQ•losses2 ¼ m•e1 hfg1  m•e2 hfg2 þ cpTc2  Ms2 cp dt

(10)

 dTs3 DQ•losses3 ¼ m•e2 hfg2  m•e3 hfg3 þ cpTc3  Ms3 cp dt

(11)

The vapor pressure, fresh water, and heat capacity are calculated, respectively, as Pi ¼ eð25:3175144Þ=ðTsþ273Þ

(12)

mfresh ¼ m_ e  time

(13)

Cpi ¼ 1000  ½4:2101  0:0022  Ti þ 5  105  Ti2  3  107  Ti3 

(14)

Heat transfer coefficient by convection for each stage is estimated empirically. This coefficient is calculated in the evaporation region between the surface of the saline water and the lower surface of the condenser at each stage [25–27] hsci ¼ 0:884  ððTsi  Tci Þ þ ðTsi þ 273Þ  ðPsi  Pci Þ=ð268:9  1000  psi Þ^ð1=3Þ

(15)

2.2.2 Evaporation Mass Transfer Coefficient. This coefficient is a function of the convective heat transfer coefficient and can be calculated using the following equation: hewi ¼ 16:273  0:001  hsci  ðPsi  Pci Þ=ðTsi  Tci Þ

(16)

Equation (17) calculates average evaporation mass for all stages m_ ei ¼ ðTsi  Tci Þ  hewi  Asi =hfg

(17)

The temperature of condenser surface for each stage can be empirically estimated as shown in the following equations: Fig. 2 Multistage water still thermal losses and gain process

Journal of Solar Energy Engineering

TC1 ¼ TS2  2K

(18)

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Table 1 Laboratory results for eight types of light industrial oils Oil type

TOL

TO

Code no. ISO grade Viscosity at 40  C Viscosity at 100  C Viscosity index Flash point C

4501 70 11–15 2.9–3 85 140

4001 16 12–16 3–3.2 95 170

4002 20 19–23 3.5–4 95 190

CO Oil type Code no. ISO grade Viscosity at 40  C Viscosity at 100  C Viscosity index

Fig. 3 Mean monthly variation of ambient temperature and relative humidity in Kirkuk

TC2 ¼ TS3  2:7K

(19)

TC3 ¼ TS4  1:11K

(20)

3 2 TC4 ¼ TS4  ð0:00007Ts4  0:015Ts4 þ 0:9763Ts4  10:324Þ (21)

The diagram of thermal losses and gain process is presented in Fig. 3.

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Experimental Work

3.1 Research Methodology and Limitations. The experimental method employed in this research considered the operational variables and variables related to the weather conditions. The research plan was based on these variables taking into account the design variables aiming at meeting the actual demands. Hereby, the procedure was based on the following criteria:  Controlling the area of the solar collectors and the type of reflectors, which are used to obtain the greatest value of thermal energy to ensure that the evaporation of salt water is optimal.  Controlling the depth of salt water inside, taking into consideration that the salt water rises above the heat exchanger by 2 cm.  Studying the effect of weather conditions, ambient temperature, wind speed, and solar radiation.  Testing different lengths for saltwater storage boxes, while the distillate height is set according to the proposed shape.  Surveying the available energy parameters such as solar radiation, sunshine duration, air temperature, and humidity. Based on these parameters, theoretical calculations are made to determine the optimal design of the solar distiller and the number of layers according to the equilibrium equations between the distiller layers and then estimate the area and the size of each layer.  Tests are conducted according to the standards of Ref. [28]. Then, the operational variables are determined to calculate the optimal productivity through determining the required flow rate of heat transfer fluid (HTF) in the exchanger, then estimating the optimal fluid type, and finally determining the distillate efficiency. The proposed system consisted of two parts: a stationary, which was made up of three iron boxes, the saline water box, and the 041003-4 / Vol. 140, AUGUST 2018

5001 15 13–17 3.2–3.5 95 150

4003 33 28–33 4.5–5 95 200 ATF

5002 22 20–24 4–4.5 95 180

5003 32 29–35 5–5.5 95 195

402 50 35 7 170 160

Note: TOL—transformer oil light; TO—turbine oil; CO—circulating oil; ATF—automatic transmission fluid.

thermal exchanger (evaporator), which was completely insulated from the surroundings. The second part is a mobile one consisting of three solar reflectors covered with cellophane sheets, which have been tested for heat and light reflection. These reflectors are fixed on a remotely controlled movable metal frame to follow sun movement and to ensure that sunrays fall continuously vertically on solar collectors, and a cylindrical-shaped artificial oil reservoir in addition to the plastic pipes employed to transfer hot oils to the evaporator for heating and evaporating the saline water of the first box. Afterward, the oil returned to the reservoir to be heated again. The process continues until the test is over; consequently, it would be possible to obtain the highest value of solar energy during test periods, which will lead to an increase in the quantity of saline water, evaporated inside a box, and will considerably increase the efficiency and productivity of the solar distiller. 3.2 Materials and Equipment 3.2.1 Oil. The following oils are available in the local markets:  Transformer oil (light) combined with antioxidants, rust, and very high and very low electrical insulation.  Turbine oil: this type of oils is of high purity added to some of the enhancers such as antioxidants, foam, and rust.  Circular oil: this type of oils is of a high purity without additives.  Automatic transmission oil: automatic transmission oils combined with antioxidants, rust, and foam optimizers, used in the transmission system. In order to select the best fluid for the developed system that would give a high conductivity and low viscosity, these oils were examined and the results obtained are shown in Table 1. Based on these results, the transformer oil was selected because it is a very light, conductive, high heat capacity, and low viscosity oil. 3.2.2 Solar Reflector. In this research, a moving metal structure is designed in the form of three triangles with equal sides as shown in Fig. 4. In each triangle, a solar reflector of 60 cm diameter was used. Each of the three reflectors can be moved independently of each other to track the movement of the sun during the day, from sunrise to sunset. The cellophane paper available in local markets adhered to the reflector surface to obtain the best solar reflection. 3.2.3 Oil Tank. Two reservoirs of equal size were used in this study with different geometries: one is cubic and the other is circular; both were set at the center of the focal length of the three Transactions of the ASME

Fig. 4

Tracking and collector system

concentrators. Both tanks have vertical mobility to ensure calibration and reception of the reflected radiation from all reflectors. The tests were conducted for each tank, and it was found that the circular reservoir is the most suitable by increasing the oil speed temperature in order to obtain the largest quantity of water productivity. 3.2.4 Thermal Insulation of the Distill. Screws tightly bound the three boxes. Plastic and silicone isolation was used between every two stages of boxes. The distiller was isolated with two layers of 3 cm thickness compressed foam and 5 cm glass wool. The pipes and other joints convey the heated fluid between the box and the heat exchanger, which are isolated.

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temperatures were 39  C and 15  C in September and January, respectively. 4.2 Effect of Distiller Length on Productivity. Different lengths of the solar distiller from 40 cm to 200 cm were selected to determine the length that gives the highest productivity, while the other dimensions and the flow rate of the heat transfer fluid are specified. Results of calculations showed that the highest productivity occurs at 120 cm, and a higher productivity can be achieved in the first stage compared to other stages. Table 2 and Fig. 6 show the effect of changing the distillate length on the productivity.

Results and Discussion

Practical tests were conducted under weather conditions in Kirkuk city, which is located at 35.4656  N, 44.3804  E. Tests were conducted during January, February, March, September, October, November, and December. The temperatures recorded during the rest of the year were above 40  C, which is overheated solar radiation, and it was expected that the tests will not give a precise evaluation of the proposed design performance. The impacts of the operational weather conditions were discussed in Secs. 4.1, 4.7, and 4.8. 4.1 The Effect of Global Solar Radiation and Ambient Temperature. The year-round global solar radiation and the ambient temperature data for the site of the test are taken from Ref. [22]. Data distribution of humidity, global solar radiation, and air temperature is shown in Figs. 3 and 5, respectively. The maximum radiation occurs in the month of September with 25 MJ/m2 day and the minimum radiation occurs with 10 MJ/m2 day in January, whereas the maximum and minimum ambient Journal of Solar Energy Engineering

Fig. 5 Mean monthly variation of solar radiation and ambient temperature in Kirkuk

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Table 2 Length of distill (cm) 40 50 60 70 80 90 100 110 120 130 140

Effect of multistage length on production per day using transformer oil (HTF) and circular storage tank Production (L/day)

Stage no. 1 (L/day)

Stage no. 2 (L/day)

Stage no. 3 (L/day)

8.40 10.25 13.35 15.40 17.65 19.20 20.10 21.15 22.25 21.08 19.56

5.80 6.55 7.65 8.55 9.30 9.90 10.50 10.80 11.20 10.75 9.92

1.75 2.50 3.45 4.50 5.25 5.70 5.90 6.50 6.65 6.20 5.88

0.85 1.20 2.25 2.35 3.10 3.60 3.70 3.85 4.40 4.13 3.76

of stages, and there is no improvement in the production of fresh water beyond three stages as shown in Fig. 6. 4.4 Effect of Surface Area of Solar Concentrates on Productivity. Figure 7 shows the relation between the time and the productivity. The tests were performed on a device using three solar concentrators with a total area of 1.5 m2. Two types of fluid were used as a heat transfer medium (transformer oil and water). The output of the solar distiller in September 2016 was 21.44 L/ day when using oil and 12.1 L/day when using water, while, in the case of using one-third of the area and using water as an intermediary, productivity decreased to 5 L/day. Such behavior can be attributed to the amount of thermal energy gained by the effect of both surface area of the concentrates and the type of heat transfer fluid.

4.3 Effect of the Number of Stages on the Productivity. Many parameters such as dimensions of the desalination units, the gap between the stages, flow rate, and the volume of the evaporation and condensation were taken into account to determine the number of stages. The results showed that the temperature difference between the stages decreases with the increase in the number

4.5 Cumulative Distillate Yield and Stage Temperature. The variation of cumulative distillate yield is calculated every hour as shown in Fig. 8. The production rate after 2 h is the sum of the hourly product at the first and second hours. In this way, the total is found out throughout the day for all the stages. Further, the cumulative yield from all the stages was collected to get the total cumulative distillate yield from the multistage desalination system. Even after 13:00, beyond which the solar radiation drops, the cumulative yield is slightly increased because of the existence of stage temperature difference, which is due to hot collector outlet temperature. The cumulative distillate yield at the end of the day for the individual stages is 11 L/m2 day for the first stage, 6.0 L/m2 day

Fig. 7 Effect of collector’s area and HTF on productivity

Fig. 8 Variation of distillate productivity with time

Fig. 6 Effect of distill length on productivity

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Transactions of the ASME

4.6 Evaporation–Condensation Temperatures Difference Affecting the Distillate Productivity. The temperature difference and hourly distillate productivity for all the stages were computed. Figure 10 shows the effect of the temperature difference between the evaporating and condensing surfaces on the hourly distillate production of the first stage. When the temperature difference between the evaporating and condensing surfaces increases, the distillate production increases. It was found that the maximum hourly distillate production for the first stage is 1.4 L/h for a temperature difference of 2.01  C at 13:30. The high production is obtained owing to the temperature difference between two phases in addition to the high rate of heat.

Fig. 9 Hourly variation of temperature in the stages

4.7 Effect of Weather Conditions on Productivity. Tests were conducted to determine the effect of weather conditions on distillate production for different seasons in the study area. The results showed that the highest productivity has been found in September and is lowest in January. The production per day is as follows: 8.45, 11.04, 12.20, 21.44, 18.69, 16.15, and 14.49 liters in January, February, March, September, October, November, and December, respectively. The results are shown in Table 3, where the productivity and its relationship to the intensity of solar radiation and air temperature gradually begin to increase until it reaches the maximum rate in the middle of the day and then declines gradually, which means that more intensity of solar radiation at the midday and the temperature of air increases the productivity of solar distillers. In addition, the highest productivity was obtained in September (21.44 liter per day) and the lowest rate of production is in January (8.45 liter per day). 4.8 Effect of Solar Radiation on Productivity. The solar energy was utilized for evaporation and condensation processes in this research. The oil heated and evaporated the saline water through a heat exchanger. The solar radiation intensity starts low in the morning, then increases gradually until it reaches the highest value at the midday, and then gradually decreases as shown in Fig. 5. The change in the intensity of solar radiation with time is a major factor in the process of energy acquisition as well as the evaporation and condensation process. The productivity is highest in the first stage as the source of evaporation and then gradually decreases in the other stages. The results of this effect are presented in Table 4. It was found that the higher the intensity of solar radiation received, the higher the productivity obtained.

Fig. 10 Effect of temperature difference between the evaporator and condenser surfaces on the hourly distillate for the first stage

for the second stage, and 4.1 L/m2 day for the third stage, respectively. The overall cumulative distillate yield at the day for the multistage solar desalination system is found to be 21.1 L/m2 day. The variation of temperature in the stages with time is computed and shown in Fig. 9.

4.9 Effect of Water and Evaporation Temperatures on Productivity. One of the interesting findings in the present study is the observation that when the temperatures of the water, steam, and the surface of the condenser were close to each other, the productivity decreased. In the case of a thermal balance between the two processes, evaporation and condensation, the productivity stopped. The second observation was when the water temperature raised and the evaporation occurred; at the same time, the temperature of the condenser surface was equal to the temperature of the steam. The steam leaked without water productivity. This process continued until the temperature of the second and third stages was reduced upon adding water to the device.

Table 3 Effect of weather parameters on distill water production Daily production (L) 21.44 18.69 16.15 13.49 8.45 11.04 12.20

Water condensation temp. rate ( C)

Water evaporation temp. rate ( C)

Air temp. rate ( C)

Daily solar radiation (w/m2)

Months

70 68 66 58 53 62 66

76 73 70 63 58 67 70

37 32 19 15 14 15 24

662 579 489 395 364 429 564

September October November December January February March

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Table 4 HTF flow rate (L/h)

Multistage production per day with transformer oil (HTF) and circular storage tank Production (L/day)

Stage no. 1 (L/day)

Stage no. 2 (L/day)

Stage no. 3 (L/day)

16.33 19.42 21.28 22.25 20.56 18.29 15.16

9.10 10.64 11.04 11.20 10.68 9.85 8.73

4.65 5.43 6.28 6.65 6.12 5.74 4.61

2.58 3.35 3.96 4.40 3.76 2.69 1.82

90 100 110 120 130 140 150

Table 5 Effect of salinity on distill water production Salinity (ppm)

1156

1040

1008

913

Production (L/day), stage no. 1

9.20

9.55

9.75

10.2

Note: ppm—part per million, conditions: transformer oil (HTF) and circular storage tank, HTF flow rate 90 L/h.

Table 6 Effect of tank shape (cubic and circular shape) on productivity Time (h) 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 Total production stage no. 1

Production (L/day) (cubic)

Production (L/day) (circular)

Hourly solar radiation (w/m2)

0 0 0.85 1.54 1.78 1.85 1.80 1.18 1.10 10.10

0 0.18 1.05 1.46 1.64 2.00 2.15 1.56 1.16 11.20

187 330 530 564 593 597 527 452 342

Fig. 11 Effect of mass flow rate on the distillate production

4.10 Effect of Salinity on Productivity. Table 5 illustrates that the increasing of water salinity leads to increasing boiling point of water; therefore, the productivity of fresh water decreases with increase in water salinity. 4.11 Effect of Tank Shape on Productivity. Two types of HTF equal size tanks were used in this study. The first type is a cubic tank and the second is cylindrical. The tests were conducted in very close atmospheric conditions, using the oil as a fluid. The test results showed that the cylinder-shaped oil tank was more productive than the cubic tank as shown in Table 6. This is because the solar energy received by the cylindrical tank is higher than that of the cubic tank, and the sharp edges in the cubic tank are dispersing part of the reflected energy, thus reducing productivity. 4.12 Effect of Mass Flow Rate on the Distillate Production. In order to study the effect of mass flow rate on the distillate yield, the flow rate is regulated to control the quantity with different mass flow rates. The effect of increasing the mass flow rate from 90 L/m2 day to 120 L/m2 day leads to an increase in the temperature of water at the first stage, and this will increase the production in all stages, but the increase further leads to a decrease in the production due to a decrease in the heat gain from the heat exchanger. The effect of mass flow rate on the distillate production is shown in Fig. 11 and Table 4. 4.13 Comparative Productivity and Temperature. The hourly profile for both experimental and predicted results during 041003-8 / Vol. 140, AUGUST 2018

Fig. 12 Comparison of hourly productivity of the system during Sept. 14, 2016

Sept. 13, 2016 is shown in Fig. 12. A reasonable difference of 7% is observed between the predicted and experimental results. Figure 13 shows the temperature distribution of evaporation phase during the day and a good agreement between the predicted and experimental results was obtained. Transactions of the ASME

Tsc ¼ collector temperature,  C Tsi ¼ water bed temperature at the ith stage,  C

References

Fig. 13 Comparison of hourly space temperature of the system at the first during Sept. 14, 2016

5

Conclusions

The design of the solar distiller employed in this investigation has two advantages: the mobile distiller is developed and placed outdoor to receive solar energy, and the reduction of the environmental pollution was achieved. The experimental tests showed that the use of three-concentric-concentrator collector covered with the golden sheets is more productive than using a single solar collector, and using oil as a heat transfer medium is more productive than using water as an intermediary. The experiments indicated that the cylindrical tank is more efficient as a heat transfer medium than the cubic-shape tank. The light transformer oil showed good efficiency in transferring the thermal energy. The present design contributed to the reduction of the size of the distiller and increased performance to 22 L/m2 of drinking water in the atmospheric conditions of the testing area. The optimum design conditions give a maximum yield of 22 L/m2 day and a minimum one of 5 L/m2 day. The multistage solar desalination system can meet the needs for drinking water in rural and urban areas and offer a productivity of 10–30 liters per day.

Nomenclature CP ¼ hew ¼ hfg ¼ hsc ¼ hfg ¼ i¼ Ms ¼ mfresh ¼ m_ e ¼ N¼ Pc ¼ Ps ¼ Q_H ¼ Te ¼ Tamb ¼ Tci ¼

water heat capacity, J/kg K evaporation heat transfer coefficient, W/m2 K latent heat, J kg free heat transfer coefficient, W/m2 K corrected latent heat of vaporization of water, J/kg stage indicator mass of saline water, kg fresh water mass, kg evaporation mass rate, kg/s number of days partial pressure between two stages, Pa vapor pressure at the surface of water, Pa heat energy input, W water vapor temperature,  C air temperature,  C condenser surface temperature at the ith stage,  C

Journal of Solar Energy Engineering

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