Effect of Temperature Dependent Internal Heat Transfers on the Performance of a Solar Still

In this paper, an effect of the temperature dependence of internal heat transfer coefficients on the performance of a single basin solar still has been studied by incorporating the change of water mass in the basin with time, initial basin water temperature etc. On the basis of numerical calculations based on simple transient analysis, the following conclusions have been drawn:

i) the evaporative heat transfer coefficient is a strong function of brine temperature, and

ii) the change of brine mass with time has insignificant effect on the performance of the still in nocturnal production which is in accordance with the assumption made by Malik and Tran (1973). This is true for other cases also.

These have also been validated by experimental results.     

Experimental study on conventional solar still integrated with inclined solar still under different water depth

This study primarily focuses on comparative experimental analysis on standalone conventional solar still (CSS), inclined solar still (ISS), and integrated conventional and inclined solar still (CSS‐ISS) for different parameters that affect the freshwater yield. For enhancing the freshwater yield only a few studies are available on still‐still integration. The present novel study provides a greater improvement in improving the freshwater yield by integrating ISS with CSS. This experimental work mainly concentrates on the importance of water depth (d _w) and mass flow rate of water ( m _w) in the solar still. Water depth inside the conventional still varied from 0.02 to 0.06 m whereas, water is constantly flown with a mass flow rate of 8.33 kg/hour in an ISS with baffles. The experimental result shows that the accumulated freshwater yield from CSS‐ISS, ISS, and CSS were 6.2, 5.04, and 4.24 kg, respectively. CSS‐ISS and ISS produced 46.23% and 18.87% higher productivity than the CSS. From the experimental investigation, it is also identified that the water temperature is significantly improved by 20% using integration as compared with CSS without integration under the same water depth of d _w = 0.02 m. The overall improvement in yield was higher in the case of CSS‐ISS. The deviations between experimental and theoretical values of yield from the conventional and modified solar still were in the range of ±7%.     

Thermal modeling of passive and active solar stills for different depths of water by using the concept of solar fraction

This communication presents the thermal analysis of passive and active solar distillation system by using the concept of solar fraction inside the solar still with the help of AUTOCAD 2000 for given solar azimuth and altitude angle and latitude, longitude of the place. Experiments have been conducted for 24 h (9 am to 8 am) for New Delhi climatic conditions (latitude 28°35′N, longitude 77°12′E) during the months of November and December for different water depths in the basin (0.05, 0.1 and 0.15 m) for passive as well as active solar distillation system. Analytical expressions for water and glass cover temperatures and yield have been derived in terms of design and climatic parameters. It is observed that

(i) the solar fraction plays a very important role at lower values of solar altitude angle;

(ii) the internal convective heat transfer coefficient decreases with the increase of water depth in the basin due to decrease in water temperature;

(iii) there is a fair agreement between the experimental observation and theoretical prediction during daytime as compared to that during the night.

Experimental and theoretical study of a single-basin solar sill in Jordan

This paper presents experimental results of a single-basin solar still using various absorbing materials. The still has equal angle double-sloped covers with an effective basin area of 3 m². The experiments were conducted in Amman, Jordan during the months of April and May. The materials used to enhance the absorptivity of water for solar radiation include dissolved salts, violet dye, and charcoal. The salts were potassium permanganate (KMnO₄) and potassium dichromate (K₂Cr₂O₇). Significant increase in still efficiency and productivity was obtained. For example, the addition of potassium permanganate resulted in 26% improvement in efficiency. Also, a comparison between theoretical and measured water productivity is presented. It was found that there is a good agreement between theory and experiment.     

EFFECT OF ADDING A PASSIVE CONDENSER ON SOLAR STILL PERFORMANCE

A theoretical and experimental study was carried out to investigate the effect of adding a passive condenser on the performance of the single slope, basin type solar still. A theoretical model based on Dunkle [1] mass transfer (evaporation) rate was developed. The model assumes that the transfer of water vapour from the still to the condenser is due to one or more of the following mass transfer modes; (i) diffusion, (ii) purging and (iii) natural circulation. The theoretical results indicate that diffusion contribution is relatively small. The contribution through purging represents the fraction [V(cond)/(V(cond) + V(s))] of the still yield, while it represents 75% of the still yield through natural circulation. An experimental study that simulates the purging mass transfer mode was investigated. The experimental results show good agreement with the theoretical predictions and an increase of 45% in solar still efficiency was obtained.     

Performance of a greenhouse cum solar still for the climatic condition of Port Moresby

Thermal modelling, based on heat and mass transfer relations, of a greenhouse integrated with a solar still has been discussed in detail. The effect of the system (viz. heat capacity of plants/pot mixture, water mass, and orientation, etc.) as well as climatic parameters (solar insolation, ambient air temperature and ventilation due to wind, etc.) have been incorporated in the energy balance for various components of the system in order to validate the theoretical results. An experiment was carried out for a typical greenhouse in Port Moresby. The following observations were made: (i) there is a reasonable agreement between theoretical and experimental results, and (ii) the amount of distilled water obtained is sufficient to grow the plants inside the greenhouse.     

Effect of energy storage medium (black granite gravel) on the performance of a solar still

In this study, a detailed experiment has been conducted on a single‐basin solar still which is modified with energy storage medium of black granite gravel. An attempt has been made to utilize the maximum amount of solar energy and to reduce the heat loss from the sides and bottom of the still. The conventional still is modified with an energy storage medium of black granite gravel of 6 mm size which is provided in the basin for different (quantity) depths. The black granite gravel functions as energy storage medium and also as an insulation layer to reduce the bottom and side loss coefficients. The black gravel is used for absorbing the excess heat energy from solar radiation during the noon hours. Due to this, the heat accumulated in the space between the water and glass surface is reduced and hence the temperature difference between the water and glass surfaces increases. The depth (quantity) of the gravel layer in the basin will influence the performance of the still and some of the parameters like basin temperature, water temperature, glass temperature and still productivity. This study deals with the effect of aforesaid parameters on the performance of the still. An attempt has been made to optimize the still performance for the above‐mentioned parameters. A mathematical model is developed to estimate the water, gravel, and inside glass temperatures theoretically and to estimate the hourly and daily yield. To show the effectiveness of the modification, its performance is compared with the conventional still under the same climatic condition. It is found that the still yield is increased by 17–20% with almost no cost for this modification as black granite gravel is very cheap. Error analysis was done by comparing the theoretical and experimental results to show the validity of the mathematical model. It is found that the maximum percentage of discrepancy for all the parameters is about ±18%. Theoretical value of yield per day has 8% discrepancy over experimental value. Copyright © 2007 John Wiley & Sons, Ltd.     

Solar distillation under climatic conditions of Egypt

Solar distillation in a single basin was studied theoretically under the climatic conditions of Alexandria, Egypt. The unsteady energy equations for the glass cover, water and basin were simultaneously solved. The effects of saline water depth, insulation thickness and wind speed on the still productivity were evaluated. A year-round study showed that the productivity of the still strongly depends on the solar radiation and ambient temperature. The daily still productivity varies from 1.1 to 5.2 kg/m² of basin area with an annual average of 3.16 kg/m². The solar still efficiency variesfrom 0.34 to 0.49 with an annual average of 0.42. The daily total still production increased with decreasing the water depth and increasing the insulation thickness. Increasing the wind speed resulted in a relatively small reduction in still productivity. The maximum production rate occurred after the peak in solar radiation and the time lag increases with the increase in water depth.     

Experimental study on performance of passive and active solar stills in Indian coastal climatic condition

This present work is aimed to examine the effect of mass flow rate on distillate output and performance of a solar still in active mode. Outdoor experiments were conducted at the coastal town, Kakinada (16°93′N/83°33′E), Andhra Pradesh, India. A solar still with a 30° of fixed cover inclination, 1m² of effective basin area, and a flat-plate collector (FPC) with an effective area of 2 m² were used. An attempt was also made earlier in passive mode to optimize the water depth for the same solar still for maximum yield and distillation efficiency. For the passive still, it is observed that the capacity of heat storage and heat drop are significant parameters that affect the still performance. For the selected still design, the study reveals that 0.04 m water depth is the optimum value for specific climatic conditions. In the active solar still, with the optimum water depth, different flow rates of 0.5, 1 and 1.5 L/min are considered through FPC. It is observed that both the mass flow rate and the variation of internal heat transfer coefficients with the mass flow rate have a significant effect on the yield and performance of the still. The experimental results show that the combination of 1.5 L/min mass flow rate and an optimum water depth of 0.04 m leads to a maximum yield for the active solar still. The enhanced yield of the active solar still is 57.55%, compared with that of the passive solar still, due to increase in area of radiation collection and more heat absorption rate.     

Experimental investigation on the effect of photovoltaic panel partially and fully submerged in water

This study presents an experimental analysis of improving the thermal, electrical efficiency, and yield of a conventional solar still (CSS). The photovoltaic (PV) efficiency decreases with increase in water depth inside the basin while the still efficiency is higher in the case of fully submerged condition. The maximum water production was about 8 kg/m ²/day with PV under fully submerged condition; and during off‐shine hours the still efficiency was higher when compared with the partially submerged condition. Similarly, with a decrease in water temperature the panel efficiency is increases. The maximum hourly water production with and without the PV was found to be 1.3 and 0.45 kg/m ², respectively. The main outcome of this study is that this mechanism can be used in isolated locations where there is a scarcity of current and distilled water.     

Effect of dye on the performance of a double basin solar still

The use of single basin solar stills for water distillation has been well demonstrated. However, the amount of distilled water produced per unit area is fairly low which makes the single basin solar still unacceptable in situations where space is a limitation. Malik has suggested a new conceptual design of a double basin solar still to overcome the above problem partially. The relative results obtained on a double basin as opposed to a single basin solar still are reported here with reference to (a) its performance, (b) the effect of dye in the lower basin, and (c) the effect of maintaining the raw water level in the basins.     

Optimization of number of collectors for integrated PV/T hybrid active solar still

The aim of this paper is to optimize the number of collectors for PV/T hybrid active solar still. The number of PV/T collectors connected in series has been integrated with the basin of solar still. The optimization of number of collectors for different heat capacity of water has been carried out on the basis of energy and exergy. Expressions of inner glass, outer glass and water temperature have been derived for the hybrid active solar system. For the numerical computations data of a summer day (May 22, 2008) for Delhi climatic condition have been used. It has been observed that with increase of the mass of water in the basin increases the optimum number of collector. However the daily and exergy efficiency decreases linearly and nonlinearly with increase of water mass. It has been observed that the maximum yield occurs at N = 4 for 50 kg of water mass on the basis of exergy efficiency. The thermal model has also been experimentally validated.     

Cavity geometry influence on mass flow rate for single and double slope solar stills

An experimental setup was built in order to evaluate the distillate yield for a double slope laboratory still under controlled conditions for basin water and collector temperatures within typical operating ranges. Additionally, experimental data previously gathered are correlated to propose a new empirical model to estimate mass flow rates in single slope solar stills. This model is compared with those from other published relations and with the double slope still experimental data. It was found that the proposed model reproduces in good agreement shallow stills experimental data and that there is not significant difference in productions between single and double slope cases subject to the same water and cover temperature. Dunkle’s model [R.V. Dunkle, Solar water distillation: the roof type still and a multiple effect diffusion still, International Development in Heat Transfer (Part 5) (1961) 895–902] is found to slightly underestimate production in shallow solar stills.     

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

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

Estimation of internal heat transfer coefficients of a hybrid (PV/T) active solar still

In this paper, an attempt is made to estimate the internal heat transfer coefficients of a deep basin hybrid (PV/T) active solar still. The estimation is based on outdoor experimental observation of hybrid (PV/T) solar still for composite climate of New Delhi (latitude 28°35′N and longitude 77°12′E). The internal heat transfer coefficients are evaluated by using thermal models proposed by various researchers. The comparison of hourly yield predicted using various thermal models to the experimental has also been carried out by evaluating the correlation coefficient and percentage deviation. It is observed that, Kumar and Tiwari model (KTM) better validate the results than the others model. The average annual values of convective heat transfer coefficient for the passive and hybrid (PV/T) active solar still are observed as 0.78 and 2.41 W m⁻² K⁻¹, respectively at 0.05 m water depth.     

Transient analysis of double basin solar still with intermittent flow of waste hot water in night

A transient analysis of a double basin solar still has been presented in this paper, incorporating the effect of intermittent flow of waste hot water into the lower basin at a constant rate during off sunshine hours. The waste hot water can be obtained either from a thermal power plant or any other industry. The effect of various parameters, e.g. inlet temperature, flow rate, water mass, etc., on the distillate output of the still has been investigated in detail. It has been found that the yield increases with flow rate if the inlet waste hot water temperature is above its optimum value. If the inlet temperature is below its optimum value, the yield decreases as the flow rate increases. The yield also decreases with an increase of water mass in the lower basin.     

Evaluation of thermal efficiency of double-pass solar collector with porous–nonporous media

The double-pass solar collector with porous media in the lower channel provides a higher outlet temperature compared to the conventional single-pass collector. Therefore, the thermal efficiency of the solar collector is higher. A theoretical model has been developed for the double-pass solar collector. An experimental setup has been designed and constructed. The porous media has been arranged in different porosities to increase heat transfer, area density and the total heat transfer rate. Comparisons of the theoretical and the experimental results have been conducted. Such comparisons include the outlet temperatures and thermal efficiencies of the solar collector for various design and operating conditions. The relationships include the effect of changes in upper and lower channel depth on the thermal efficiency with and without porous media. Moreover, the effects of mass flow rate, solar radiation, and temperature rises on the thermal efficiency of the double-pass solar collector have been studied. In addition, heat transfer and pressure drop relationships have been developed for airflow through the porous media. Close agreement has been obtained between the theoretical and experimental results. The study concluded that the presence of porous media in the second channel increases the outlet temperature, therefore increases the thermal efficiency of the systems.     

Exergy Analysis of Solar Still with Sand Heat Energy Storage

This paper studies the experimental and exergy analysis of solar still with the sand heat energy storage system. The cumulative yield from solar still with and without energy storage material is found to be 3.3 and 1.89 kg/m², respectively for 8-h operation. Results show that the exergy efficiency of the system is higher with the least water depth of 0.02 m (m_w = 20 kg). Competitive analysis of second law efficiency shows that the exergy efficiency improves the system by 30% than conventional single slope solar still without any heat storage. The maximum exergy efficiency with energy storage material is found as 13.2% and it is less than the conventional solar still without any material inside the basin.     

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

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

Thermal modelling of high temperature distillation

This communication presents a comparative study of a single basin solar still under various modes of operation. A simple transient analysis of all the modes under the same meteorological conditions has been presented. The water depth in the solar still and the absorptivity of the basin liner along with the water flow over the glass cover of the still has been found to affect the daily distillate production of the system considerably. The thermosyphon mode enhances the daily distillate production. It has also been observed that the evaporative heat transfer coefficient is a very strong function of temperature.