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Solar still with weirs and baffles(new design)

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  ScienceDirect   Available online at www.sciencedirect.com ww.elsevier.com/locate/procedia Energy Procedia 157 (2019) 1071–1082 1876-6102 © 2019 The Authors. Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18.10.1016/j.egypro.2018.11.274   Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18, 19–21 September 2018, Athens, Greece  Numerical Analysis of Thermal Performances for a Novel Cascade Solar Desalination Still Design M. Bouzaid a,* , O. Ansari  b , M. Taha-Janan a , N. Mouhsin a , M. Oubrek  a   a  PCMT, Laboratoire de Mécanique Appliquée et Technologies, Centre de Recherche en Sciences et Technologies Industrielles et de la Santé,  ENSET, Mohammed V University in RABAT, Avenue de l’Armée Royale, BP 6207 Rabat-Instituts, MOROCCO b  Equipe de Recherche en Thermique et Energie, Centre de Recherche Energie, ENSET, Mohammed V University in RABAT, Avenue de l’Armée  Royale, BP 6207 Rabat-Instituts, MOROCCO *mariambouzaid.91@gmail.com Abstract As it is the case of many countries in the world, Morocco suffers from a shortage of drinking water; water resources are also limited and unequally distributed. Solar desalination is thought to be the least expensive method to produce drinking water from  brackish water. Different ways, technologies and techniques exist. Solar stills, in various forms present the basic technology that could be used. The single basin type is widely used as it is the simplest and the least expensive process. However the production rate of this technology is very low in comparison with other desalination techniques. For this reason, and in the framework of the development of a novel design in which absorbing new plate was proposed and tested. This work presents a theoretical and numerical study of an inclined cascade solar still with baffles. A mathematical model and thermal analysis were developed to evaluate the temperatures of different levels. The saline water temperature, basin plate temperature and glass cover temperature are achieved by solving the heat exchange equations of the solar still using an Euler explicit method. The energy balance equations have been solved numerically by iteration for a duration of 13 hours, using a C ++ program. The performance of the new still was investigated and compared with the ordinary model. The results indicate that the new design allows increasing the  production. © 2019 The Authors. Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientic committee of Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18.    Keywords: Solar desalination, brackish water, stepped solar still, cascade solar still, heat transfer;    1072  M. Bouzaid et al. / Energy Procedia 157 (2019) 1071–1082   1.   Introduction Water covers approximately 70% of the earth’s surface. However only 3% of Earth’s water is fresh water, where 2.15% of this water is trapped as glaciers and ice caps and 97% is saline water. Since Saline water covers the greater  part of the planet. Purifying brackish water is one of the best solutions to water shortage issue. Morocco has an important solar energy potential, which is clean and has no adverse effects on the environment, whence our interest for the question.  Nowadays, using single basin solar stills is a suited process to product drinkable water for different reasons. First, there construction is very simple and easy to fabricate. Second, they are cheaper than other processes. However the single basin solar still produces a lower yield than other desalination methods. For this reason our objective, as well as lot of researchers is to increase the efficiency and the performance of this type of devices. There is lot of factors affecting the productivity of solar stills. The metrological parameters cannot be controlled. In contrast the water-glass temperature difference, free surface area of water, absorber plate area, temperature of inlet water, glass angle and depth of water can be varied to enhance the production. According to SafwatNafeya et al. [1] the depth of saline water on the solar still is an important factor affecting the productivity. For this reason a still with minimum depth of water will start producing earlier than others of greater thickness. For this reason to increase the production rate of the single solar still several designs have been carried out. The stepped solar stills have higher productivity compared with basin-type stills, because the absorber  plate is made of number of steps, offering minimum depth of brine water. According to Tiwari and Madhuri [2], the still productivity is inversely proportional to the water deepness. Maintain the least deepness in a solar still is very difficult. Stepped solar still is a well fitted technology to purifying saline water and used by lot of researchers to achieve a minimum depth of water in the basin. Radhwan [3] presents a transient analysis of a stepped solar still with five basins. The efficiency of this still was about 63% and can  produce about 4.92 2 ml   per day. Velmurugan et al. [4] fabricated and tested a stepped solar still with 50 trays. Using fins and pebbles allowed enhancing the production rate by nearly 98%. Kabeel et al. [5] used a modified stepped solar still that consists of a number of trays with different widths and depths to improve the production. In comparison with the single basin type, stepped still reach almost of 57.3% of productivity. In this paper, a novel construction of cascade solar still absorber plate was proposed, developed and tested. The new conception of the still consisted of a stepped absorber plate with sloped surfaces and baffles. The performance of the new stepped solar still was investigated throw a mathematical and a thermal analysis. The proposed design of absorber plate was evaluated and compared with the simple type presented by El-Sebaii et al. [6]. This comparative study was done under the same climate conditions and physical properties. The design, physical and operating  parameters used in theoretical calculation are shown in table 1. Table 1.Design, physical and operating parameters used in theoretical calculation. Specific Heat Cp(J/ kg K) Thermal Conductivity λ (W/m K) Density ρ (kg/m3) Thickness e (m) Glass Cover 800 1.02 2530 0.003 Brackish Water 4190 0.67 1022 0.04 Absorber Plate 896 73 2700 0.002 Insulation Materiel 670 0.059 200 0.05   M. Bouzaid et al. / Energy Procedia 157 (2019) 1071–1082 1073 Nomenclature A Area, m   Cp Specific heat, J Kg  K   h Heat transfer coefficient, W m  K    I   Incident solar power, W m   Q Heat flux density, W m   m Mass, kg t Temps, hour dt   Calculation step, hour T Temperature, °C dT  Incremental rise, °C ∆T  Temperature difference, °C Greek letters ε  Emissivity α  Absorptivity τ  Transmissivity λ   Thermal conductivity, W m  K    ρ  Density, Kg.m   Subscipts a Ambient  b Absorber c Convection cd Conduction e Evaporation e,is External face of insulation g Glass is Insulation i,is Internal face of insulation r Radiation sky Sky w Brackish water 2.   System description The present work is an extension of first attempt of modeling the new device by Bouzaid et al. [7]. As shown in Fig.1, we used in our pattern inclined glass cover with angle of 30° to facilitate the runoff of condensed water to the collector. The absorber plate is made of number of steps, with horizontal and inclined surfaces with the angle of 35° according to latitude of Rabat-Morocco (Latitude: 34° 01'N, Longitude: 6° 49'W). Therefore baffles were added to the surface of absorption in the new sloped solar still in order to minimize the flow velocity of the saline water.  1074  M. Bouzaid et al. / Energy Procedia 157 (2019) 1071–1082  Fig. 1.Novel cascade absorber plate design (with slope surface and baffles). (a) 2-dimensional prototype drawing and (3) 3-dimensional prototype drawing 2.1.   Solar still process As indicated in [7]. During the solar still process the solar radiation received by a sloping glass cover is absorbed  by a black surface and heat is transferred to the water in the basin. The water temperature increases and raises the rate of evaporation. At the end of the process the water vapor raised upward and condenses on the inner surface of the glass cover. The new design increases the absorber plate and the basin water temperatures for two reasons. The absorber plate is split into several small basins and has inclined surfaces which offering minimum depth of water, heating water quickly and a better orientation relative to the sun. See Fig.2. Fig.2. Schematic sketch of the stepped absorber plate (with slope surface and weirs) [7].   M. Bouzaid et al. / Energy Procedia 157 (2019) 1071–1082 1075 3.   Mathematical and Thermal Analysis The valuation of the condensing glass cover, brackish water and absorber plate temperature is based on the following hypotheses:    Heat losses from the sides of the solar still are negligible.    There is no air escape from the still.    The cover is assumed that it is clean.    The temperature of every component is uniform.    At the beginning of the process, the temperatures of all surfaces are equal to the ambient temperature.    The condensation takes place only on the cover.    The glass has good moisture.    The concentration of the brine is not involved in the heat and mass transfer.    The basin is leak-free. The numerical results are obtained by solving the energy balance equations for glass cover, saline water, absorber  plate and insulation of the solar still. In the subsequent equations, Tg, Tw, Tb and Tis are average glass cover temperature, saline water temperature, absorber plate temperature and insulation temperature, respectively, all in terms of °C. 3.1.   Thermal energy balance equation for different components of solar still 3.1.1.   Thermal energy balance equation for the condensing glass cover The thermal energy balance for the glass cover is calculated according to the following expression: ag cgargskyewg cwg rwg   g  g  pg   P QQQQQ dt dT mC     (1) The radiative heat flux density exchanged between the glass cover and the sky is expressed by: )(  sky g  g rgsk rgsky T T  AhQ    (2) Where rgsky h  the radiative heat exchange coefficient existed between the glass cover and the sky is given by Stefan Boltzmann law: ))(( 22  sky g  sky g wrgsky T T T T h      (3) The sky temperature is expressed by:   5.1 0552.0 a sky T T     The convective heat flux density between the glass cover and the ambient air is expressed as follows: )( a g  g cgacga T T  AhQ    (4) Where cga h  is the convective heat exchange coefficient exchanged between the glass cover and the ambient air is given by Kreith [8]: V h cga 8.38.5    (5) V used to express the wind velocity average. The radiative heat flux density between the brackish water and the glass cover is expressed by: )(  g wwrwg rwg  T T  AhQ    (6) Where rwg  h , the radiative heat exchange coefficient between the brackish water and the glass cover is :
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