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1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of RAAR 2016.doi: 10.1016/j.egypro.2017.03.064

Energy Procedia 109 ( 2017 ) 447 – 455

ScienceDirect

International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India

Modelling and Performance Enhancement of Single Slope Solar Still using CFD

Vaibhav Rai Kharea, Abhay Pratap Singhb*, Hemant Kumarc, Rahul Khatrib

a Centre for Energy and Environment, Malaviya National Institute of Technology, Jaipur 302017, India b Department of Mechanical Engineering, Manipal University, Jaipur 303007, India

c Department of Mechatronics, Manipal University, Jaipur 303007, India

Abstract

With rising population, development, and environmental pollution, availability of potable water is shrinking fast. Thus, it is required to focus on the distillation of available water. Solar Still is one of the promising technologies available to purify water because of its low cost, energy, and skill requirement. However, the efficiency of present technology is low, so it is required to advance the designs of existing solar stills. In this study, a multi-phase three-dimensional CFD model of a simple solar still developed for simulation with using ANSYS FLUENT. The simulation has been done for transient state to validate the results with experimental data for climate conditions of Jaipur (26º13’N, 75º49’E). Within the scope of this study, simulation results were found to be in good agreement with the experimental data. It is also examined that thermal efficiency of the Solar Still is higher from 16:00 to 17:00 hrs. Parametric analyses has been done to enhance the productivity of Solar Still. Different materials were used in the basin to increase the heat capacity, absorption capacity and the evaporation rate. The impact of varying the depth of the basin water was also studied. It has been found that the Solar Still have more productivity for low water depth.

© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of RAAR 2016.

Keywords: Solar Still; Solar Desalination; CFD; Multiphase Model; Productivity

* Corresponding author. Tel.: +91-8829004587

E-mail address: [email protected]

Available online at www.sciencedirect.com

© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of RAAR 2016.

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448 Vaibhav Rai Khare et al. / Energy Procedia 109 ( 2017 ) 447 – 455

1. Introduction

The continued deficiency of consumable water is an significant issue in developing countries, and contaminated water can result in various diseases, which are often lethal. According to World Health Organization (WHO) report, about 30,000 people die every day, due to water-borne diseases [1]. As per UNICEF, globally, 1 billion people are currently without access to potable water supply and 2.6 billion have no form of sanitation services (figures for 2004) [2]. Therefore, purification of available water is essential for the general well-being of the masses. Solar energy can be used as an important source for purifying water for its low manufacturing expenses; and its usage has no adverse impact on the environment. Hence, application of solar stills for distillation of salty water to produce fresh water is economical in terms of energy, but the distillate rate is little low. [3] The solar distillation involves all the three modes: conduction, convection, and radiation of heat transfer. Heat flows from inside the solar still to the environment through the transparent glass cover and the walls by conduction. Heat from the basin to the water, from vapors to the glass cover and from glass cover to the environment is transferred by convection. While heat flows from the sun to the solar still through radiations [4]. F.F. Tabrizi, A.Z. Sharak [5], used inbuilt sandy heat reservoir under climate conditions of Iran. He showed that integrated sandy heat reservoir increases the productivity of solar still during cloudy day and night, and it also does not require any pumping element for night mode usage. K. Kalidasa Murugavel et.al [6] worked on double slope basin solar still with mild steel plates with a lower mass of water and different wick materials like light cotton cloth, sponge sheet, waste cotton pieces, coir mate pieces in basin with aluminum fins. He found from an experiment that, the light black cotton cloth is effective wick material compared with other wick materials as well as aluminum fin covered with cotton cloth and arranged in lengthwise was more efficient.

The objective of this study is to develop a 3D CFD model of Simple Solar Still to understand the evaporation and condensation phenomena in solar still. The model has been developed with the help of ANSYS Workbench and then simulated with Fluent. Water temperature and production rate of fresh water from the simulation results compared with the actual results. Further comparison has been made between simulation result and experimental results of water temperature, glass cover temperature. Parametric analyses have been done to enhance the productivity of Solar Still. Different materials were used in basin to improve the heat and absorption capacity to increase the evaporation rate. The impact of varying the depth of the basin water was also studied.

2. Mathematical Modelling

The performance of solar still based on productivity as well as internal heat and mass transfer coefficient was studied. Internal heat and mass transfer coefficient in the solar still based on convection, radiation, and evaporation. Its effectiveness is directly proportional to internal heat transfer coefficient and distillate output. Heat transfer coefficients of different types are convective heat transfer coefficient, radiative heat transfer coefficient and evaporative heat transfer coefficient. Single slope solar still is preferable for higher than 20° altitude. For north latitude places the single slope still with south facing cover and for south latitude places with north facing cover are used [7]. A 3-dimensional three-phase model was developed in the mixture model for air, liquid water and water vapor system at transient state condition which means only surface evaporation of liquid occurs. Energy and mass transfer have been considered in this work. For each phase, the time and volume-average continuity, energy and mass equations were numerically solved.

2.1. Governing Equations

Equations follow steady state condition that is modeled based on the continuity, momentum, energy and mass transfer conservation principles. When solar energy is incident inside the basin water, heat transfer mechanism starts. Energy balance equation can be written with taking following assumption [9]: 1. There is no vapor leakage in solar still. 2. The heat capacity of absorbing material, insulation, and cover is negligible.

Vaibhav Rai Khare et al. / Energy Procedia 109 ( 2017 ) 447 – 455 449

3. There is no temperature gradient across the basin water and glass cover of solar still. 4. The water level is maintained at constant level inside the basin. Figure 1 shows the energy flow in a solar still.

Fig 1: Energy flow in a solar still

2.1.1 Energy equation

The energy equation for the mixture is given below:

(1)

Where is the effective conductivity , where is the turbulent thermal conductivity according to turbulence model.

2.1.2 Continuity equation

The continuity equation for the mixture is:

(2) Where, is the mass-averaged velocity: (3)

2.1.3 Momentum equation

The momentum equation for the mixture can be attained by adding the each momentum equations for all the phases. It can be expressed as:

(4)

2.1.4 Volume conservation equation

This is simply the constraint that the volume fractions add up to unity.

rL + rG = 1 (5)

2.1.5 Mass transfer equation

The energy source in a cell for phase p and phase q are:

(6) (7)


Where are the formation enthalpies of species i of phase and species j of phase q respectively and is the enthalpy of species i of phase p. [8]

3. Experimental Setup

A sectional view of a simple solar still is shown in Figure 2 made with an insulated metallic GI sheet basin with a base area of 0.8 m x 1.0 m. A double walled 1” thick thermocol sheet sand-witched between the walls of basin to reduce heat losses from walls and painted with black epoxy paint from inside to increase its absorptivity and with white enamel paint from outside to reduce thermal losses. The sidewalls covered with a mirror to increase the reflectivity of solar radiation to the bottom surface. A transparent glass of 5 mm thickness concealed the top of the still with an inclination angle of 26° towards the south, which is the same as the latitude of Jaipur to ensure that solar still captures maximum average radiations during the year. The basin and the glass cover sealed with 3 mm foam so that vapors generated in the basin did not escape from the still without condensing. [10]

Fig. 2: Experimental Setup of Single Slope Solar Still

4. The CFD Model

4.1 Geometry Creation and Meshing Detail

The 3D geometry of solar still was modeled using ANSYS Design Modeler with all the geometrical constraints same as in the experimental setup. The three-dimensional model geometry of Solar Still along with its unstructured meshes is shown in figure 3. The physical model of the building was then meshed using 3D hexahedral meshing consist of total 1.5 million cells (elements) at a growth rate of 1.2, in ANSYS’s workbench MESHING.

Fig. 3: Mesh Geometry of Model


A Grid Independence Test has been done to check the mesh size for the model. The convergence criteria were achieved for continuity, velocity, k- (1e-3) and the energy equation (1e-6) at each time step. The grid size was determined by increasing the number of meshes until the criterion [p-(p+1)]/p < 10-3 was satisfied. Here ‘p’ represents the calculated temperature using current mesh size and ‘p+1’ resemble temperature using the next mesh size. Air temperature inside the still was evaluated for the grid independence study.

4.2 Boundary Conditions and Initial Conditions

Solar insolation incident on solar still is the most critical factor inside a still. First, it is incident upon glass cover, due to absorptivity and transmissivity of the glass; it is then absorbed by absorber plate, which increases the temperature of water. The initial water level inside the solar still was assumed to be 30 cms for simulation purpose. Water and air volume fraction were considered as 0.1 and 0.9, respectively. The initial temperature of water and amount of radiation received to the system used according to experimental the data in every hour.

Appropriate boundary conditions were specified at all boundaries to solve the continuity and momentum equations. CFD simulation had run time of 10 hours because of a high number of time steps and computer time restrictions. It was assumed that for 1 hour, the received solar radiation by the basin as well as water and glass temperatures were based according to the solar calculator in fair weather condition. Constant temperature boundary conditions were enforced on glass, bottom, and collecting surface. The experiment carried out from 0730 hours until 1830 hours. During each 1-hour time interval, an average temperature was set as the boundary condition. Solar intensity was based upon absorption factor and emissivity of glass, water, and bottom. Heat transfer coefficient of sidewalls was calculated and maintained constant for the overall process. A no-slip wall boundary condition specified for the liquid phase and free-slip boundary condition was used for the gas phase.

4.3 Solution Initialization

ANSYS FLUENT v14.0 was used in the study that uses the finite volume method to convert the governing equations into numerically solvable algebraic equations. The numerical studies were based on the following assumptions:

Thermo-physical properties of aluminium, glass, and air remain constant during process. Thermal contact between glass, solar still basin walls and its surrounding is perfect. Solar still wall temperature is considered equal and undisturbed.

5. Simulation Results

Solver ANSYS FLUENT v14.0 was used to carry out CFD analysis for solving the equations of using two 3.00 GHz CPU processors as parallel run. The solution gets converged according to the criteria mentioned above and then an unsteady simulation has been carried out for the specific time. Figure 4 shows the volume weighted average volume fraction of all three phases. It has been shown that initially, the volume fraction of air was high because of the absence of vapor. With the increase in temperature, the volume fraction of water vapors increases significantly, and that of air decreases significantly.


Fig. 4: Overall Volume Fraction of Different Phases

In the following Figure 5, the relative volume fraction of water at any time is shown. The red color is showing more volume fraction of water and the blue color is showing less volume of a fraction of water.

Fig. 5: Overall Volume Fraction of Water at a particular Time

Figure 6 (a), (b) respectively shows the temperature of water vapor mixture and temperature of the water inside the Solar Still respectively. It follows the trend of the intensity of solar radiations, as expected.

Fig. 6 (a) Temperature of Water Vapor of Simulated and Experimental Results; (b) Temperature of Water Liquid of Simulated and Experimental Results

The simulation outcomes have been compared with the experimental data. Although the readings do not exactly match, they follow similar patterns. The most likely reason for the variation is that the intensity of solar radiations taken in the simulation doesn’t account for natural attenuation. Figure 7 shows the solar intensity of simulated and experimental data.

T = 0800 Hrs T = 1200 Hrs T = 1600 Hrs


Fig. 7: Solar Intensity Variation of Simulated and Experimental Data with respect to Time

The maximum efficiency of solar still was 65% at 14:00 hrs. As the intensity of solar radiation increases, more distilled water is produced due to a higher temperature. After reaching its maximum, efficiency falls as the intensity of solar radiations drops. The efficiency of solar still is shown in Figure 8.

Fig. 8: Graph between Efficiency of Solar Still and Time

From the simulation of solar still, it is found that results obtained are as expected. The simulation results were compared with the experimental data, and it is observed that experimental and simulated data follow the same pattern.

6. Parametric Analysis of Solar Still

The productivity of solar still depend on several parameters such as solar radiation intensity deviation and ambient temperature variation from morning to evening for the day, latitude and longitude of the place and solar still parameters such as cover plate thickness, orientation and inclination, depth of water and properties of basin materials used for improving the energy absorption, energy storage and evaporation heat transfer. The solar still parametric optimization is done for the given atmospheric parameters variations for a particular place.

6.1 Varying the depth of water

The functions of the basin are to collect the radiation passing into the still through the cover with minimum reflectance loss and conduction loss to atmosphere [11]. The evaporation of water depends on the natural convection circulation of air mass inside the still, which is the water and glass temperature difference. This difference is the main force for the flow of air and water [12]. Also, the evaporation rate of water depends on the area of contact of basin water inside the still [13].


The water depth is having a significant effect on the productivity of the basin. Investigations show that the water depth is inversely proportional to the productivity of still [14]. In this study different water depth in the basin has been simulated. Figure 9 shows that the efficiency of solar still decreases with increasing water level depth in the day time so as productivity.

Fig. 9: Graph between Efficiency of solar still at different water depth

The heat stored in the water is released during the absence of daylight and production continuous even during the night. The volumetric heat capacity of water is less for shallow basin; thus, the temperature of the water is high which increases the evaporation rate and productivity of the solar still. 6.2 Material of basin

There are different black materials which store greater amount of heat energy and increase the heat capacity of the basin in adding to increasing the absorption. Glass, rubber, and gravel are some material having these properties. Simulation results show that black rubber with small size increases the productivity of the still by 20% while black gravel increases the productivity by 19%. Figure 10 shows the graph of productivity for the different material used in the basin.

Fig. 10: Graph between productivity for various material of basin

6.3 Maximize the effect of solar radiation

However, the glass cover inclination is optimized to receive the sunrays, a part of sun rays impinge on the back wall and side walls of the basin which reduces the amount of radiation available to the basin water for heating. To reflect the sun rays fall on the side plates onto the basin reflecting mirror is added in the model. Simulation results show


that the production rate is increased by 22% for stills with reflecting mirrors on the vertical walls compared with conventional still.

7. Conclusion

The main aim of the study was to develop a CFD model of a Simple Solar Still and use it for performance enhancement by some parametric analysis. A multi-phase model has been developed in ANSYS which accounts for all the three phases present in the solar still, i.e. air, liquid water and water vapors. The CFD simulation carried out for the unsteady system using this model. The simulation results have been linked with the experimental data. The water output rate is also verified by varying few parameters for the basin to enhance the efficiency of solar still. The main conclusions of the study are:

The temperature of air & water vapor mixture is uniform until 14:00 hrs and then it becomes non-uniform. Higher solar intensity results in quick evaporation-condensation rate, hence the distillate from the solar still.

It was found that the productivity of the still decreases with an increase in depth of water during daytime and the reverse after dusk. The overall efficiency of solar still was calculated 32%, 29% and 27% for 5 L, 10 L, and 15 L water quantity respectively.

Other basin materials like rubber, gravel are having the properties of absorbing solar radiation in different amounts with increasing the exposed area for evaporation of water. Rubber is found to be the best basin material to improve absorption, storage, and evaporation effects.

The still efficiency is further increased when the basin is modified with a blackened baseliner and when increased the incident solar radiation by use of reflective glasses.

Simulated results show that computational fluid dynamics is a powerful tool for design, parametric analysis and difficulties removal for a simple solar still. Future work can be done with changing the orientation of Solar Still and considering other different design parameters.

References

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Desalination 253 (2010) 195-199. [6] K. Kalidasa Murugavel, K Shrithar, “ Performance study on basin type double slope solar still with different wick materials and minimum

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and Environment, 2(6), 985-998. [9] O.O. Badran (2007). Experimental study of the enhancement parameters on a single slope solar still productivity. The 9th Arab

International Conference on Solar Energy (AICSE-9), Kingdom of Bahrain. Desalination 209, 136–143. [10] Khalid, M. S. (2011). Design & Fabrication of Double Condensing Chamber Solar Still and Comparison with Single Slope Solar Still. [11] M.A.S. Malik, G.N. Tiwari, A. Kumar and M.S. Sodha, Solar Distillation, Pergamon Press, UK, 1982. [12] P.I. Cooper, Maximum efficiency of single effect solar stills, Solar Energy, 15 (1979) 205. [13] Harpreet S. Kwatra, Performance of a solar still: predicted effect of enhanced evaporation area on yield and evaporation temperature, Solar

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