Experimental study on modified single slope single basin active solar still
Introduction
Water is very essential for sustainability of mankind. We do need this precious water in applications such as drinking, cooking, washing, irrigation, etc. Potable water scarcity has been a common situation around the globe especially in developing countries. Unfortunately we are continuously losing fresh water reserves mainly due to industrialization. So, it is not only necessary to preserve fresh water but also to develop an eco-friendly technology to get the distilled water from saline/contaminated water. Among various water treatment technologies, ‘solar distillation’ is one which is based on renewable energy, easy to operate and low cost technology to produce distilled water by using solar energy and it can be a solution to problems of shortage of drinking water especially in rural areas.
Solar distillation technique has the advantage of being eco-friendly, zero fuel cost and low maintenance cost. But on the other hand, it has the disadvantage of occupying a large space and being a slow process leading to less distillate output per unit of time. For the last many decades, efforts are continuously being made to make this technology more efficient, economical and faster. Delyannis [1] extensively reviewed different desalination processes using renewable energy resources. According to him, the first reported solar distillation plant was fabricated by a Swedish engineer, Carlos Wilson in Las Salinas, Chile in 1872 for supplying fresh water to a nitrate mining community. That plant produced potable water for about 40 years until the mines were exhausted. Velmurugana & Srithar [2] also reviewed various researches done on improvement of productivity of solar stills.
Distillation through a solar still depends upon various design parameters (e.g. shape of still, size of still, its orientation, tilt angle of condensing cover, etc.), climatic parameters (e.g., ambient temperature, amount of solar radiation, wind velocity, etc.) and operational parameters (e.g. water depth, heat absorbing material, cooling of condensing cover, etc.). Many researchers worked on these parameters and proposed number of designs and theories. Zurigat & Abu-Arabi [3] modeled a regenerative desalination unit consisting of two basins with provision for cooling water to flow in and out. This arrangement had the advantage of increasing the temperature difference between water and glass cover in the first effect and utilized the latent heat of water vapor condensing on the glass of the first effect to produce more fresh water in the second effect. Dwivedi & Tiwari [4] analyzed the performance of a still by evaluating internal heat transfer coefficients of single and double slope passive solar stills in summer as well as winter climatic conditions for different water depths (0.01, 0.02 and 0.03 m) by various thermal models. Dwivedi & Tiwari [5] also compared two stills on the basis of life cycle cost.
Salah et al. [6] used different types of absorbing materials to examine their effect on the yields of solar stills. Also Akash et al. [7] studied the effect of using different absorbing materials in a single basin double slope solar still to enhance the productivity. A black rubber mat and black ink were reported to increase the yield by 38% and 45% respectively. Boubekri & Chaker [8] proposed internal and external reflectors on a single slope solar still to increase the rate of received solar radiation and found increase in overall productivity by 72.8% in the winter. Tiris et al. [9] integrated a flat plate collector with the solar still to increase the temperature of the basin water. Also Badran & Al-Tahaineh [10] made some experimental investigations to study the effect of coupling a flat plate collector on the productivity. It was found that with the collector the productivity was increased by 36%. Boukar & Harmim [11] compared passive and active solar stills.
Kumar & Bai [12] applied water cooling system on the side walls to achieve more condensation and found efficiency to be 30%. Khalifa [13] investigated the cover tilt angle as the most crucial parameter which affects the performance of a still. He found a relationship between latitude and cover tilt angle for various seasons. Badran [14] experimentally studied different operational parameters of a single slope solar still. The still productivity increased up to 51% when combined enhancers such as asphalt basin liner and sprinkler were applied to the still.
Tiwari & Tiwari [15] made an attempt to analyze the effect of water depth on evaporative mass transfer coefficient on a passive single slope distillation system in summer climatic conditions. El-Nashar [16] analyzed that performance of a solar desalination plant is influenced by the ability of the glazing system to transmit solar radiation to the collector absorption surface. Cleaning the surface with jets of compressed air improves its productivity. Al-Hussaini & Smith [17] studied the effect of applying vacuum inside the solar still. A solar still's productivity was found to increase by 100% with the vacuum. Khalifa et al. [18] made an arrangement of condensers to enhance the productivity of the solar still.
In the present work, experiments have been conducted to analyze the performance of a modified basin type solar still, consisting of a secondary condensing cover. The performance of the modified still has been compared, by perfectly synchronized experiments, with that of the conventional basin type single slope solar still of identical dimensions and material. Effects of shading the secondary cover of a modified still with wooden frame, covering it with wet cotton cloth and replacing it with aluminium have also been analyzed.
Section snippets
Principle and system description
The principle behind the working of a basin type solar sill is water evaporation and condensation. Impure water fed into the still and solar radiation entering the still through a transparent top cover are entrapped within the still and raises the temperature of the water. At its corresponding vapor pressure, water from the surface evaporates and leaves all contaminants and microbes behind in the basin and sticks to the inner side of the top cover. This is called internal heat transfer. Due to
Comparison of conventional still and Model-1
Comparison between the conventional still and Model-1 of proposed modified still has been made on the basis of different observations which were recorded for the two stills running parallel.
Parameter of comparison of two stills was water depth inside the basin. Results are shown for the particular days of April '12 (on 1st April '12 with 0.01 m depth, on 3rd April '12 with 0.02 m depth and on 5th April '12 with 0.03 m depth). Fig. 3 indicates solar radiation falling on south & north facing covers
Conclusions
In this study a modified solar still is proposed which significantly enhances the fresh water output. Based upon the present experimental study, it is also evident that the behavior of the modified still is a complex function of various operational and design parameters. The following conclusions are deducted:
- 1.
Proposed modified solar still (Model-1) produces 25.4% higher yield than the conventional still under the same climatic conditions. The secondary cover enhances external heat transfer rate
Nomenclature
- m
mass of distillate (kg)
- I
solar radiation intensity (W/m2)
- UI
internal uncertainty
- σ
standard deviation
- S
south direction
- N
north direction
- w
water
Subscripts
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2021, Solar EnergyCitation Excerpt :A variety of geometries have been used to make the top cover of PSS such as double slope, hemispherical, and pyramid covers which increases the available area for the condensation of water vapour and improves the condensation rate (Ahsan et al, 2014; Andrew et al, 2014; Arunkumar et al, 2012; Bait and Si-Ameur, 2017; Bhardwaj et al, 2015; Ismail, 2009; Pinheiro et al, 2018; Sathyamurthy et al, 2014; Tiwari et al, 1986; Watercone Inc., 2020). Directing some proportion of water vapour to an external or built-in condenser characterising a larger area and lower temperature also improves the condensation rate (Fath and Elsherbiny, 1993; Kabeel et al., 2017a; Kumar and Dwivedi, 2015; Moh’d A and Al-Ammari, 2016; Ahmed et al., 2018). In addition, studies explored different approaches to reduce the temperature of the condensation surface and prepare greater temperature differences with bulk water (Andrew Jones et al., 2014; Kabeel et al., 2019a; Pinheiro et al., 2018; Rahbar et al., 2015; Sathyamurthy et al., 2014; Sharshir et al., 2017).