Accepted Manuscript Experimental study on the air velocity effect on the efficiency and fresh water production in a forced convective double slope solar still Margarita Castillo-Téllez, Isaac Pilatowsky-Figueroa, Áaron Sánchez-Juárez, José Luis Fernández-Zayas PII:
S1359-4311(14)00903-X
DOI:
10.1016/j.applthermaleng.2014.10.032
Reference:
ATE 6046
To appear in:
Applied Thermal Engineering
Received Date: 21 December 2013 Revised Date:
8 October 2014
Accepted Date: 10 October 2014
Please cite this article as: M. Castillo-Téllez, I. Pilatowsky-Figueroa, Á. Sánchez-Juárez, J.L. FernándezZayas, Experimental study on the air velocity effect on the efficiency and fresh water production in a forced convective double slope solar still, Applied Thermal Engineering (2014), doi: 10.1016/ j.applthermaleng.2014.10.032. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Experimental study on the air velocity effect on the efficiency and fresh water production in a forced convective double slope solar still Margarita Castillo-Télleza*, Isaac Pilatowsky-Figueroab, Áaron Sánchez-Juárezb, José Luis Fernández-Zayasc a
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Centro del Cambio Global y Sustentabilidad del Sureste, S.A., Calle Centenario del Instituto Juárez, S/N, Col. Reforma, Villahermosa, 86080, Tabasco. b Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Cerrada Xochicalco s/n. Colonia Centro,Temixco, Morelos, 62580, México. c Instituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Av. Universidad 3000, Distrito Federal, 04510, México *
[email protected],
[email protected],
[email protected],
[email protected]
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Abstract
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The fresh water production in single and double slope solar stills (DSSS) depends on the rates of simultaneous processes of evaporation and condensation, where optical material properties, solar irradiance, temperature, velocity and air direction and the operating mode, natural or forced convection, are involved. Scarce experimental studies of the behavior of air velocity on water production on DSSS have been published, but mostly, the applied methodology is not clear. We analyzed the effect of the air velocity on the water production, temperature distributions and efficiency in a modified double slope solar still and compared the results with another with the same characteristics operating at natural convection. The influence of the other climatic parameters was minimized, placing a transparent wind tunnel on the solar still external cover. We tested the solar still at different average air velocities; 2.5, 3.5, 5.5 and 6.9 m/s. During the test period, the climatic parameters such as daily average solar irradiance (750 to 850Wm-2), and the maximal ambient temperature (33 to 37 ºC), had small variations. We experimentally demonstrated that the thermal efficiency and production increment when the air velocity increases up to the value limit around 5.5 m/s and it then decreases at higher velocities and the velocity of 3.5 m/s is considered to be the optimum. The experimental uncertainty and error analysis was also presented. These results can be applied to any type of solar still; single or double slope, operating at natural or forced convection and improve both efficiency and production within the domain of the tested air velocities.
Key words: solar energy, passive solar still, solar desalination, drinking water production. _____________________ *Corresponding author
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1. Introduction
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The greatest challenge mankind faces is the supply of fresh water. The problems associated with the growing demand of the big cities, the irrational usage and pollution problems limit its availability. In the rural areas, the contamination is partly due to the salt infiltrations in aquifers, the high content of heavy metals, and use of chemicals (pesticides, fertilizers, etc.) in the agricultural activities. There is a large availability of water in the oceans (97.5% of total water on the planet); however, the content of dissolved salts (3.5%) does not make it suitable for human consumption
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There are many technologies for seawater desalination, such as distillation, vapor compression, reverse osmosis and freezing among others: Most of them are highly intensive in energy consumption: thermal energy (evaporation) and electricity (compression and reverse osmosis).
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The current situation requires saving and efficient use of energy within a sustainability framework, where the application of renewable energy such as solar, could have an important role for application in the desalination of seawater.
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Solar distillation for seawater purification can be an useful application for farming and fishing rural communities. The single slope solar still (SSSS) and double-slope solar still (DSSS) are simple and economical technologies; however, the studies of the evaporation and condensation processes are very complex due to the variation of environmental parameters and the material properties. For instance, the vapor produced is condensed by direct contact with the cold internal surface of the transparent cover (below dew point), and this one with the ambient temperature. The transparent cover should be inclined to facilitate drainage of attached condensed water on its inner surface. Its accumulation in the inner surface can decrease the solar irradiance transmission and the fresh water could return to basin for the small slope covers
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The capacity of water purification production in a DSSS can improved if its operational parameters are optimized and, mainly its ability to absorb the maximum amount of solar energy radiation, and to improve the heat transfer rate of the simultaneous evaporation and condensation processes. 1.1 Effect of climatic and geometrical parameters The efficiency analysis of a solar still involves material properties and geometric parameters. Concerning the operational orientation an east-west orientation was proposed in order to get the best production [1]; however, these authors do not justify that this orientation is better than a north-south. In the DSSS, the production of fresh water increases when the solar irradiance increases [2, 3, 4]. The influence of the cover slope angle has been extensively studied experimentally. Different works have proposed several cover inclination angles; 4° [5], 10° [6,7], which include the use of reflective surfaces 15°[8], 16° [9], 40° [3], and 23° [10,11]. Nevertheless an optimal angle of 23° was
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proposed[10], allowing more uniform production during the year, because in winter, (when, the angle increases), the efficiency increases, a whereas for the summer (when the angle increases), the efficiency decreases.
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Concerning the basin, it has been built with different materials, such as asphalt [10], mild steel sheet [12], or galvanized iron [13, 14], whereas the transparent using cover, glass or plastic [1, 9, 13]. Regarding the cover thickness, mathematical and experimental models have shown that it does not have effect on the water production [9]. However, in several transparent materials, the solar radiation transmission decreases when thickness increases. In fact, some authors suggest a thickness of 3 mm [3]. In relation to thermal insulating materials, polyurethane, polystyrene [1], wood [9] and polyurethane foam [13] have been utilized.
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About the depth of water to distillate, most authors conclude that at shallower depths, the production increases and decreases up to 14%, and when the depth increases from 2 to 7 cm [2]. An optimal depth range between 2 and 6 cm has been proposed [7]. Another study concluded that, if the depth decreases from 6 cm to 0.5 cm, the production improves up 19% [9]. Further experimental studies carried out with depths of 0.5, 2, 3 and 4 cm also concluded that at shallower depths the production improves substantially; however, no specific value was reported [3]. In an experimental study on an active solar still with preheated water, operating at different depths (0.5, 10 and 15 cm), the authors found a significant variation in the convective heat transfer coefficient [15].
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1.2 Effect of air velocity
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To calculate the thermal efficiency of a solar still ( ), equation 1 is proposed [16,18], where (V) is the distillate volume, (hfg) is the latent heat of vaporization, (Iav)is the average irradiance in W/m2day and (t) is the solar exposition time in hours.
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The amount of condensed water in DSSS depends on the temperature and air velocity that flows on the external cover surface. Such parameters change during daytime. Air velocity influences on the rate of condensation; however, the analysis of this effect on production has technical difficulties in order to establish an uniform and constant air velocity over the transparent cover. Some studies have analyzed the effect of air velocity and most of them are theoretical and only an of these were experimental. A theoretical model proposed [10], predicts that the DSSS production depends on climatic and operational parameters. The effect of air velocity up to a maximum of 6 m/s has been found more significant than a change in temperature. An increase of 1 to 3 m/s resulted in an 8% increment of production, meanwhile an increase of 23 to 33oC results in an 8.2% increment.
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Other studies show a small or even an unfavorable effect of the air velocity on the fresh water production. Regarding these a mathematical model was proposed, which analyzes the effect of different parameters on the efficiency of a single slope solar still [2, 18]. It was found that an increase in the air velocity, gradually decreases the water production, and at 9 m/s, a reduction of 13% was observed.
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A theoretical and experimental study on a DSSS was also developed by Abu-Qudais et al.,[19]. In this work, an initial increase in water production was observed when the air velocity was greater than 1.5 m/s, and decreases for an air velocity of 3 m/s. This means that any additional cooling reduced the inner water temperature in the basin, i.e. a slight breeze was sufficient to improve the efficiency.
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A mathematical model shows that an increase in air velocity at 8 m/s, have a decrease effect of 10% on the water production [20]. Other authors have proposed mathematical models for DSSS, which have been also validated by using experimental studies [9].
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Different types of solar stills have been theoretically analyzed and it has been found that air velocity has an important effect on water production, for velocities below 8 m/s, but it decreases and is almost negligible at 10 m/s. Additionally, the effects of water temperature, air velocity and angle of inclination on the still performance have been also studied[10]. In these studies the inclination angle was varied at 10, 20 and 40º and the air velocity ranged at 2, 4 and 6 m/s. The results showed that increasing the water temperature and the air velocity, increases the water production. In this work, the experimental methodology was not described in detail [21].
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Due to the limited number of experimental works concerning the influence of air velocity on the performance of DSSS, in this work an experimental study was carried out under controlled conditions for analyzing the effect of air velocity on the production of fresh water. The main results are outlined in detail.
2. Experimental study
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Because of the lack of technical information about the optical properties of the transparent covers, in particular their transmittances, in this work they were determined experimentally.
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2.1 Experimental determination of the optical properties for the transparent covers. To assess the amount of solar energy incident on the basin surface of solar still it is very important to determine the optical properties of transparent covers and their variations with respect to the thickness and angle of incidence of the radiation. Because of the lack of technical information about the optical properties of the transparent covers, in the present work the transmittances of these materials and reflectance of these materials were determined experimentally. 2.1.1 Materials The optical properties of the transparent materials used in solar energy: glass (3 and 6 mm) acrylic (1.5, 2 and 3, and 6 mm) and polycarbonate, (2 mm) were determined. 2.1.1
Methods
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To determine the optical properties a double beam spectrophotometer, Shimadzu 3101 PC with SR-260. Integrating sphere attachment was used for transmission and reflection measurement using a wavelength (240 to 800 nm), and 190 to 3200 nm, with an accuracy of 0.3%T. Measurements in the UV-VIS range (from 0.1 a 7.5 nm) and near infrared region (from 0.4 to 30 nm, with resolution length range of 0.1 nm), were also carried out. The instrument was always calibrated using suitable standards before the sample measurements were logged.
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The spectrum for transmission and reflection as a function of wavelength, were obtained in the interval from 290 to 2500 nm. The specular and diffuse reflection were measured. We can be observed that the diffuse reflection is independent of the thickness, and only depend of the properties of the material surface properties. The spectrum of diffuse reflection is a function of the wavelength and its maximum value is founded for all transparent covers tested, between 1% for the ultraviolet region and 2 % for the visible region. The absorptance property was calculated by difference between the optical properties. 2.1.3 Results
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The experimental results of the total transmittance, total reflectance (spectral and diffuse) and absorptance (calculated) are shown in Table1. It can be observed that, the transmittance values are greater than 80% ± 0.3%. Acrylic with a thickness of 1.5 mm exhibits the maximum value. Glass with 3mm has a transmittance at normal incidence of 81.4% ± 0.3%. In this experiment, the solar irradiance received by the seawater basin is attenuated by the acrylic and glass covers, with a total transmittance at normal incidence of 65%.
2.2 Salinity evolution
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Table 1. Experimental results of optical properties of transparent covers
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In general, the experimental study was carried out with distilled water, in order to limit the number of the operating parameters; consequently, the effect of the salinity was not analyzed. However, during two selected days of tests, the evolution of salinity versus solar irradiance and water production was observed. A volume of five liters of sea water, with an initial sodium chloride concentration of 24 g/l ± 0.1, was placed in one of the solar stills. During the solar distillation, the concentration of the sodium chloride was measured by an electric conductimetry device . To determine the salinity of sea water a Thermo Scientific Orion Star A212 conductivity Benchtop meter, with a wide range 0.001 µS to 3000 mS, was utilized. with a resolution of the 0.001 µS minimum and relative accuracy 0,5% reading ± 1 digit. Figure 1; shows the evolution of the production of water distillated as function of solar radiation for two selected days of tests. It can observer that the distilled water production from sea water was almost the same and slight independent of solar iradiance.
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Figure 1. Evolution of the production of distilled water as function of solar irradiance for the two days of tests
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Figure 2. Concentration of NaCl as function of the solar irradiance
As can be seen in Figure 2, saline solution concentration increases as solar irradiance increases. The evaporation process continues despite the unavailability of solar irradiance. Initially the evaporation rate was slow (8:00 to 11:00 h), then it increases until reaching a value of 26.8 g/l ± 0.1, at 18:00 h, then increases
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more slowly reaching a maximum value of 27.4 g/l ± 0.1 at 20:00 h
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2.3 Experimental study of the effect of the air velocity on the production de distilled water. The aim of the experimental study is to analyze the effect of the air velocity on the DSSS operating at natural convection and to compare it with other DSSS at forced convection, also. Based on the above, two DSSS with the same features were designed, constructed and evaluated. 2.3.1 Description of the experimental device.
The experimental device consists of two DSSS, each one with a copper tray with 0.25 m2 of solar collection
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area and 12.5 L ± 0.2 L of capacity. The trays were coated with black paint (high temperature resistant), for better absorption of solar radiation, which functions as an absorber of solar radiation. The transparent covers are made of common glass (3 mm thickness) with an inclination of 23°, which results in an optimal transmittance, according to the local latitude. The walls and the bottom were thermally isolated with polyurethane foam (10 mm of thickness), which was covered with a metallic protective shield. The covers and the enclosure were sealed in order to prevent any losses of vapor and condensate, which are collected by a container placed at the bottom. All the experiments were carried out keeping the water depth at 2 cm (5.0 l). Figure 3 shows a general view of the experimental equipment.
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A central distributor allows a constant water level to be treated for each one of the solar stills, which have a separate collector for condensate, where the weight of fresh water was continuously recorded. In order to analyze the influence of the air velocity on the fresh water production, a transparent plastic wind tunnel, was placed above the transparent glass cover of one of the stills, in order to control and stabilize the flow and air direction at different velocities. The double cover (wind tunnel) also allowed isolating the influence of external parameters. Is important to note, that, during the test period, small changes in the climatic parameters were observed; such as daily global solar irradiance range from750 to 850Wm-2 , and the maximum ambient temperature ranged from 33 to 37 ºC. The air velocity specific range is produced inside the tunnel by three fans, which were operated by a variable voltage source, thus allowing the control and stability of the air velocity. During the tests, an on/off control timer was integrated. The experimental study was carried with distilled water, in order to limit the number of the operating parameters; consequently, the effect of the salinity was not analyzed.
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Figure 3. General view of the experimental device: (1) passive DSSS, (2) active DSSS, (3) fans, (4) saline solution distributor , (5) passive DSSS fresh water collectors , (6) variable voltage source, (7) container of 4 automatic weight recording of water produced by data acquisition equipment, condensate collection and active DSSS. 2.3.3 Operating mode
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In the nighttime, the central distributor supplies the water to the solar stills for their daily initial operation and then the constant liquid level is established.
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The experimental studies were carried out in the Solar Platform of the Instituto de Energías Renovables (IER) of the Universidad Nacional Autónoma de México (UNAM) which is located in Temixco, Morelos State in Mexico. The location coordinates of this place are 18 º 54` Latitude North and 99 ° 13' Longitude West, at 1230 meters above sea level. The period of test was carried out from January 2012 to February 2013.
2.4 Instrumentation
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The daily test period was established, between 8:00 to 20:00 h, for each air velocity selected; the internal and external temperatures of the still, ambient air temperature, solar irradiance, distillate water collected were recorded with an automatic acquisition system. Daily, before the start of the test, the transparent covers were cleaned.
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2.4.1 Temperature. Thermocouples type K were used for temperature measurements, which were previously calibrated using a AMETEK Jofra Instruments Temperature Calibrator, D55SE, with a range from -12 to 110 °C and accuracy ± 0.05 °C. Thermocouple reference polynomial linearization error was typically < ± 0.5 °C, from – 35 to 50 °C. Figure 4 shows the location of the temperature sensors.
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Figure 4. Location of temperature sensors
2.4.2 Distilled water. The daily fresh water produced was continuously measured (every 2 min.) by a TorRey electronic balance, model L-EQ 5/10 with accuracy of 1mg, coupled to a computer by a specific software. Figure 5 shows a view of the distillated collection and the weight recording system. The measurements of volume were performed using a glass test tube with an accuracy of ∓5%.
Figure 5. View of the condensate collection and weight recording systems
2.4.3 Air velocity. In order to have the best control and an air velocity distribution, a wind tunnel of transparent plastic was designed, constructed and placed over the glass cover of one of the stills. To achieve
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the required velocity range, three fans were used. Each fan was operated with a maximum direct current of 0.94 A, and a nominal voltage of 48V. The required voltage was supplied by a regulated source of DC (2 x 30 V), Matrix, MPS-3005l-3 model. The voltage accuracy was ± 0.25 V. The voltage and amperage consumed by fans were recorded and during the test the output was 5V and 3A, respectively.
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Due to the instability of the source at high voltages and the air interactions with the walls of the wind tunnel, it was necessary to establish several measuring points of the air velocity throughout the entire length and breadth of the tunnel, in order to obtain average values.
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To measure and to log the air velocity, flow and temperature measurements were carried out using two hot wire thermo-anemometers with data loggers, (Extech SDL350). The established measure range was from 0.2 at 25 m/s, resolution 0.01 m/s and basic accuracy of ± 5% rdg. The temperature measure range was from 0 to 50 °C with accuracy of ± 0.8 °C. The first thermo-anemometer was used to measure the velocities near to the cover surface (glass) and the second one to measure the velocity distribution in the wind tunnel. Within the established experimental domain, four average air velocities were measured: 2.5 ± 5%, 3.5 ± 5%, 5.5 ± 5%, and 6.9 m/s. ± 5%. Figure 6 show the transparent wind tunnel.
Figure 6.View of the transparent wind tunnel
2.4.4 Solar radiation.
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The measurement of global solar radiation was performed by a Eppley precision spectral pyrometer (PSP). With accuracy of ± 0.5% from 0 to 2800 W/m2 .Meteorological data; wind velocity and direction, ambient temperature, and relative humidity were provided by the local meteorological station of the IER. Figure 7, shows the experimental equipment utilized and the meteorological station. Table 2, presents the description; calibration and accuracy of the main measuring instruments.
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The data acquisition system consisted in a Data Logger, Campbell Scientific, model CR10X. Program execution was set-up with a real time of 64 Hz, with 6 differential or 12 single ended analog inputs, individually configured at full scale ± 2.5 at ± 2500 mV. Including measurements of time and the possibility to convert the engineering units. Thermocouple reference polynomial linearization error: was < ± 0.5 °C, from – 35 to 50 °C. Table 2. Description and characteristics of measuring instruments
Figure 7. Graphic representation of the experimental equipment and meteorological station
3. Results
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In order to determine experimentally the influence of air velocity on the water production, the following mode of operation of the solar stills were established: a) natural convection (passive), b) forced convection with wind tunnel c) forced convection (active) without wind tunnel and variable velocity.
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Figure 8, shows, for a representative day of test, the evolution of the temperatures in a DSSS, operating at natural convection.
Figure 8: Inside and outside temperature evolutions in the solar still, operating at natural convection
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It can be observed that the inside and outside temperatures of glass surfaces was practically the same, with a maximum at 58°C ± 0.5%. The basin temperature reaches a maximum of 77°C ± 0,5%, while the temperature of the surface of the water was 69°C. In this kind of solar still, in a typical winter day with a solar irradiance of 4.2 kWh/m2 , an average value of production of 0.58 L was obtained. Figures 9,10, 11 and 12 show the variations of temperature for the different zones of the DSSS as function of the solar irradiance at different average air velocities (2.5 , 3.5, 5.5 and 6.9 m/s respectively). Figure 9 shows maximum values: basin temperature of 70°C; water surface and indoor air temperatures 65º and 63°C, respectively; outer transparent cover at 50°C, whereas the ambient temperature reaches a maximum value of 35°C and air velocity of 2.5 m/s.
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In Figures 10 and 11, it can be observed, that for air velocities of 3.5 and 5.5 m/s, the outer glass temperature is maintained close to the ambient. High temperature values, in the basin and water were founded, between 55º and 52°C respectively. As shown in Figure 11, despite this temperature decrease in most areas of the still, the daily production of water continues to increase as long as the air velocity value of 5.5 m/s remains.
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However, in Figure 12, for an air velocity of 6.9 m/s, there is a marked difference between the ambient temperature and the one corresponding to the outer glass cover, yielding a decrement in the water production. In general, it can be observed that as the air velocity increases, the temperatures inside the still, surface water and basin decrease, but the overall effect was a decrement in the water production.
Figure 9.Temperature evolutions in the DSSS at air velocity of 2.5 m/s
Figure 10.Temperature evolutions in the DSSS at air velocity of 3.5 m/s
Figure 11.Temperature evolutions in the DSSS at air velocity of 5.5 m/s
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Figure 12.Temperature evolutions in the DSSS at air velocity of 6.9 m/s
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. Table 3. Summary of the experimental results of different test days for each selected average velocity of air.
Table 3.Main experimental results in the solar still operating at forced convection
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In order to know the effect of air velocity on normal operating conditions in a natural convection solar still, tests were carried out without the wind tunnel. In this case, the velocity of 5.5 m/s was selected, which obtained the higher water production. Figure 13, presents the thermal behavior of the solar still operating without the wind tunnel and forced convection (February 15, 2013).
Figure 13. Solar still operating without the wind tunnel and forced convection
Under these conditions the temperature of the basin, the water and the indoor air decreased, from 65 to 49ºC and from 63 to 49ºC, respectively.
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In general, if the air velocity over the outer cover is increased, the temperature of the glass cover decrease from 51ºC (2.5 m/s) to a value equal to ambient temperature (5.5 m/s). In the range of the considered velocities, the amount of distilled water obtained was increased due to an increment in the rate of condensation, reaching a maximum value, when the outer cover temperature is equal to the ambient and at the same time, the production decreases because the glass cover is colder than the ambient air. Table 4, shows the DSSS efficiency, operating at natural and forced convection, for the established conditions, [22].
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Table 4. Summary of the main experimental results
As it can be seen in Table 4, when comparing data from 2 August and 24 July, the forced convection at 5.5m/s maximizes the water production, despite having less solar energy available. Figure 8 shows that for energy received on the basin of 0.8 kWh in the collector, the production was 0.52 L, thus it is improved due to forced convection. In the case of the forced convection without wind tunnel, the distilled water obtained was 0.66 L at 4.01 kWh/m2 of solar irradiance, while solar energy (Es) received at the basin was 0.816 kWh. At an average air velocity of 5.5 m/s, and with similar solar irradiance values, the water production was 13% lower than that obtained with a double cover. Without forced convection the water production was 21% higher, obtaining 0.52 L at 0.82 kWh of energy received.
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When the tunnel was removed and only one of the slopes of the still is cooled at a rate of 5.5m/s, the distillate production was reduced compared with the cooling of the two slopes, but the water production was higher than that obtained without forced convection. Consequently, the forced cooling on both slopes improves the yield of distilled water.
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Average efficiency values are practically independent of the season of the year, taking an average value between summer and winter (44.8 at 44.3% respectively) of 44.5%, within the domain of efficiency of the solar still with a single slope. The maximum theoretical value reached is 60% and the experimental does not exceed 50%.
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Table 4, shows that the wind tunnel integration causes an average decrease of 36% in its efficiency compared with 40% without it. Due to the decrease of the transmittance of incident solar radiation (34%) of the water temperature, of the processes of evaporation and condensation and, therefore, production of water, an overall efficiency decrement of 10% was observed.
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In the case of natural and forced convection operations, when the solar irradiance values are very similar, the production of water is increased in the case of forced convection (1 and 6, Table 4). Concerning the forced convection it increases the volume by 33.9%, having as a consequence an increment in the efficiency of 58% for an air velocity of 3.5 m/s. This substantiates the hypothesis presented in the experimental study.
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Increasing the air velocity up to 5.5 m/s, the production of water was increased to a maximum value, where thermal equilibrium between the glass cover and the environment was achieved. For higher velocities, the temperature of the glass was lower than ambient, decreasing the water production and at the same time the maximum efficiency was obtained. In the test, a volume of 0.76 L of water with a solar irradiance of 4.9 kWh/m2 and an efficiency of 62.3% higher than the estimated for one-slope solar stills was obtained [4].
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Increasing the air velocity up to 5.5 m/s, the production of water was increased to a maximum value, where thermal equilibrium between the glass cover and the environment was achieved. For higher velocities, the temperature of the glass was lower than ambient, decreasing the water production and at the same time the maximum efficiency was obtained.
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When the double cover is removed (9 in Table 4), the volume of water increases up to 15.8% compared to operation at natural convection (1 in Table 4), showing that the cooling of the glass cover increases both water production and the efficiency.
4. Conclusions
The experimental study shows that for similar values of solar irradiance within the range of velocities from 2.5 to 5.5 m/s, the amount of distilled water and the efficiency increase up to a maximum value of 62.3%
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A double slope solar still with an integrated wind tunnel, operating at 5.5 m/s with a basin area of 0.25 m2 and an average daily irradiance of 5.1 Kwh/m2, produces 0.76 L of fresh water and 0.66 L operating at natural convection.
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The main experimental results have shown that for velocities above a certain limit value (5.5 m/s) the water production and efficiency decrease, which agrees with some previous works. This conclusion is explained by the cooling effect produced within the still when the air velocity increases. The thermal convective effects between hot updrafts and cold downdrafts produce the cooling of the water contained in the basin, reducing the water production and also increasing the rate of condensation.
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Experimental analysis of the effect of the air velocity at forced convection on the efficiency and production of fresh water in a double slope solar still was presented. The range of air velocities was set from 2.5 to 6.9 m/s. The increments of efficiency and production were obtained by increasing the air velocity up to the value limit of 5.5 m/s. The velocity of 3.5 m/s was considered to be the optimum, if the velocity distribution is homogeneous and parallel to the surface of the transparent cover. According to the results, the air velocity has a reduced effect on the still thermal performance and water production, and the efficiency increases up to 37.9% when the air velocity increases up to a maximum value of 5.5 m/s and, and then it begins to decrease affecting the fresh water production.
Acknowledgment
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The results of this experimental study can be applied either to single or double slope solar stills, and improve both efficiency and production within the domain of the analyzed air velocities.
References
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The authors gratefully acknowledge to academic technical; Oscar Gómez Daza for technical support during the experimental determination of the optical properties of transparent covers and to Eng. José de Jesus Quiñones Aguilar for providing in the climatic parameters and solar irradiance database.
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[4] Cooper P. 1. Some factors affecting the absorption of solar radiation in solar stills, Solar Energy, 13, (1972) 373-381. [5] Porta Gándara, Fernández Zayas, Chargoy del Valle, Influencia de la distancia vidrio-agua en
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destiladores solares de caseta, Memorias de la XVIII Reunión Nacional de Energía Solar, (1994) 105. [6] El-Sebaii A.A., Effect of wind speed on active and passive solar stills, Energy Conversion and Management, 45, (2004), 1187 1204.
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[8] Sanjay Kumar, G. N. Tiwari, Estimation of convective mass transfer in solar distillation systems,
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[10] Hinai-Al, MS. Nassri-Al, Jubran B.A.,Parametric investigation of a double-effect solar still in comparison with a single-effect solar still, Desalination,150, (2002) 75-83. [11] Hinai-Al H., Nassri-Al MS, Jubran B. A. Effect of climatic, design and operational parameters on the yield of a simple solar still. Energy Convers Mgmt. 1639, (2002) 43 50.
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[12] K. Vinoth Kumar, R. KasturiBai, Performance study on solar still with enhanced condensation. Desalination, 230, (2008)51-61.
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[13]Kamal W. A. A., theoretical and experimental study of the basin-type solar still under the Arabian Gulf climatic conditions. Solar & Wind Technology, 5, (2) (1998)147-157. [14] A. E. Kabeel, Performance of a solar still with a concave wick evaporation surface, Energy, 34, (2009)1504-1509. [15] Rajesh Tripathi, G.N. Tiwari, Effect of water depth on internal heat and mass transfer for active solar distillation. Desalination, 173, (2005)187 200. [16] Dunkle RV,Solar water distillation, the roof type still and multiple effect diffusion still, Fifth International, Conference of Development in Heat Transfer, U. Colorado, (1961) 206. [17] Mahdi J.T., B.E. Smith, and A.O. Sharif, An experimental wick-type solar still system: design and construction, Desalination, 267, (2011) 233 238.
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[18] Malik M. A. S. and Tran Van Vi A simplified mathematical model for predicting the nocturnal output of a solar still. Solar Energy, 14, (1973) 371-385.
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[19] Mohamad Abu-Qudais, Bassam A/K Abu-Hijleh and Othman N., Experimental study and numerical simulation of a solar still using an external condenser, Energy, 21 (10), (1996) 851-855. [20] Samy M. El-Sherbiny and Hassan E. S. Fath, Solar distillation under climatic condition in Egypt. Renewable Energy, 3, (1) (1993), 63-65. [21] S. H. Soliman, Effect of wind on solar distillation, Solar Energy, 13, (4), (1972)403-415.
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[22] Castillo Téllez Margarita, Estudio teórico-experimental de un destilador solar de doble caseta
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con convección forzada, Tesis Doctoral en Ingeniería, Universidad Nacional Autónoma de México, (2013).
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Tables captations
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Table 1. Experimental results of optical properties of transparent covers.
Table 2. Description and characteristics of measuring instruments.
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Table 3. Main experimental results in the solar still operating at forced convection
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Table 4. Summary of the main experimental results
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Tables
Table 1
81.4 81.7
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Absorptance (%) (Calculated)
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Total Reflectance (%) ± 0.3% 10.3 9.6 9.9 11.1 11.1 10.0 10.3
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Acrylic (1.5 mm) Acrylic (3 mm) Acrylic (6 mm) Acrylic (2 mm) Polycarbonate celular (2 mm) Glass (3 mm) Glass (6 mm)
Total Transmittance (%) ± 0.3% 86.5 80.7 80.6 86.0 85.5
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Material
3.2 9.7 9.5 2.9 3.4
8.6 7.8
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Table 2
Model
Constant of calibration
Global solar irradiance
Precision Spectral Pyranometer (Eppley)
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Annual (K=7.80x10 V/Wm-2 sensor Campbell
Temperature in the DSSS
Thermocoupl es type K
Distilled water produced Volume
Electronic balance Test tube
AMETEK Jofra Model D55SE Tor-Rey series L-EQ 5/10
Data Logger Campbell Scientific
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Acquisition data
Spectrophoto meter Shimadzu
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Optical properties: trasmission and reflection (especular and diffuse)
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Required voltage to vary the velocity
EXTECH Instruments Mod.SDL35O
MATRIX MPS-3005L-3
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Measure and register of the velocity
Hot wire thermoanemometer with datalogger Regulated source of DC
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Accuracy
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Description
solar irradiance: ±0.5 W/m2
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Parameter measured
± 0.05 °C
0.001 kg
± 5% ± 5%rdg (velocity) ±0.1 m/s (velocity) ± 0.8 % (temperature)
± 0.25 V Tracking error
