A solar desalination plant for domestic water needs in arid areas of South
A solar desalination plant for domestic water needs in arid areas of South Algeria
A solar desalination plant for domestic water needs in arid areas of South Algeria
Laboratory of Renewable Energy Development University of Ouargla, 30000 Ouargla, Algeria
Tel. +213 (29) 712468; Fax +213 (29) 715161; email: email@example.com
Received 16 April 2002; accepted 30 April 2002
The shortage of drinking water in many areas of the world is now a serious problem. At least 80% of arid and semiarid countries, like Algeria, where 40% of the world's population live, have serious periodic droughts. To resolve this crisis, different methods of solar desalination have been used in several countries. Among these is solar distillation — a process where solar energy is used to distil fresh water from saline or brackish water for drinking purposes and other applications. It is generally classified into passive and active distillation systems. The work on passive solar distillation has been reviewed by Malik et al., the work on active solar distillation system was carried out by other researchers. It is well known that in solar distillation, the radiation from the sun evaporates water inside a chamber at a temperature higher than the ambient. The principle of operation is the greenhouse effect provided by the glass cover. Energy balances are made for each element of still; solar time, direction of beam radiation, clear sky radiation, optical properties of the cover, convection outside the still, convection and evaporation inside are accounted. The most studied model is the solar distiller, which has the following advantages: low installation cost, independent water production at the point of use and no need for trained maintenance staff. But it has important disadvantages, such as low efficiency and problems associated with the deposit of salt, deposit of scale and corrosion. The aim of our research work is to develop a highly efficient solar distillation system of simple construction, applicable in small units of production. It concerns a device of a solar distiller with heat recovery; it is a solar distiller for arid zones that should be very simple, handy, and easy to maintain and repair by every village artisan with limited technical means. The study on the performance of this solar still has been conducted under the actual insulation at the South of Algeria, where the feed water is brackish underground geothermal water.
Theoretical analyses of the heat and mass transfer mechanisms inside this solar distiller have been developed. The measured performance was then compared with the results obtained by theoretical analysis of the heat and mass transfer processes. The results clearly show that the efficiency is increases with the increase of solar radiation flux and with the increase of brine temperature. There is an optimum value of feed water (brine) when the efficiency is maximum. The efficiency increases when there is a recovery of latent heat of condensation.
Keywords: Solar desalination; Heat and mass transfer; Evaporation-condensation; Direct solar distillation; Multipleeffect distiller; Modeling-simulation; Water resources; Remote arid lands
Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries:
Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean.
Sponsored by the European Desalination Society and Alexandria University Desalination Studies and Technology
Center, Sharm El Sheikh, Egypt, May 4–6, 2002.
Large areas of our planet suffer from lack or pollution of fresh water. These include traditionally dry regions, such as deserts and modern industrial areas. The same problem also exists in remote areas and islands where in many cases fresh water supply by means of transport is expensive.
On the other hand, seawater exists in large amounts, together with the free and in large amounts of solar energy. It is really very fortunate that in times of high water demand solar radiation is also intense. It is therefore beneficial to exploit
solar energy directly by installing solar stills. Two major advantages favor the use of solar stills: clean and free energy, and friendly to the environment.
Solar distillation seems to be a promising method and alternative way for supplying small communities in remote areas and islands with water. In fact, it has been reported that for such places solar distillation could be the most favorable way for water supply. Indeed, several solar still designs have been proposed, and many of them have found significant applications worldwide.
Nevertheless, solar desalination systems have low operation and maintenance costs and require large installation areas and high initial investments.
However, this is the best solution for remote areas and small communities in arid regions with lack of drinking water. These regions generally have a great solar energy potential. This potential can probably be best developed by solar desalination concepts and methods specifically suited to supply dry regions with fresh water.
Solar distillation for production of potable water from saline water has been practiced for many years, but little attention has been paid to the basic mechanisms involved and to the possibility of significant improvements in water output per unit area of solar absorber. It has been demonstrated that it is possible to produce fresh water, but the problem is to produce large amounts of water at a reasonable capital cost and utilizing a minimum amount of land surface.
This paper concerns a study of a solar distillation system. It has been designed and installed in an experimental station in South Algeria. It consists of many devices of single-effect and multi-effect solar stills and a flat-plate collector field. The thermal behavior of the system was investigated in the solar still unit, heated by a solar collector field.
Moreover, it can use available latent heat of condensation by using a feed of geothermal water, optimizing exploitability of any available heat sources.
2. Principle of the solar still
Solar stills are used to produce fresh water from sea and brackish water by directly utilizing sunshine. Construction and operation principles of solar stills are simple. A black-painted basin contains brackish or seawater. This is enclosed in a completely airtight area formed by a transparent cover. Incident solar irradiance passes through the transparent cover and is absorbed by the water and mainly by the black basin surface. Consequently, water contained in the basin is heated up and evaporates in the saturated conditions inside the still. Water vapors rise until it comes in contact with the cooler inner surface of the cover.
There it condenses in pure water, runs down along the cover surface and water gutters due to gravity and is collected in vessels nearby.
Solar stills are characterized by ease of construction performed by local people from locally available materials, simplicity in operation by unskilled personnel, no hard maintenance requirements and almost no operation cost. However, they require large areas of land for installation and have low output. This is related, amongst others, to the fact that their productivity rate depends on the available solar radiation. If there is no sunshine, the productivity is almost zero.
Moreover, they have a high initial cost.
Research activities nowadays aim to strengthen the position of solar stills as a means for supplying water in the above-mentioned cases.
This can be only done by taking actions that lead to increased water productivity from the still, increased reliability and reduced initial cost. Such actions involve new designs of solar distillation systems that increase output through the increase of water temperature in the still. This can be achieved by heat recovery in multi-effect solar still or by a coupling solar still with a heat storage tank, heated by any source nearby.
In this paper, evaluation of a solar distillation system is presented, which involves a multi-effect solar still, the capillary film distiller, with heat recovery thermal storage, heated by a flat plate solar collector. The operation of the system has been investigated through a series of tests, showing the benefits of the system concerning water productivity and exploitation of solar energy.
3. The capillary film solar distiller: principle and construction (Fig. 1)
As a multiple effects distiller, the capillary film solar distiller, presents the advantage of heat recovery: it is the reusing the vapor condensation heat to evaporate another quantity of water. This type of distiller was designed and patented by Ouahes et a. . Instead of a thick spongy fabric, the authors propose a very thin fabric comprising
Fig. 1. Principle of capillary film distiller (DIFICAP).
a single, finely woven layer. This fabric is held in contact with the overhanging plate through the interfacial tension which is much greater than the force due to gravity. A capillary film is formed at the plate fabric interface. This device is a robust, rustic apparatus, which can be installed in villages in developing countries having only minimal technical capabilities. The still should only require maintenance and repairs within the technical capabilities of local craftsmen and it should even be possible for men to make copies and improvements to the apparatus.
The system developed to provide different constant low flow rates of saline solution is based on the use of hypodermic syringe needles which are readily available in pharmacies at low cost and in a range of diameters. Concerning the stabilization of the flowing films, it is well known that, due to the Marangoni effect, a thin flowing film undergoing evaporation tends to tear and be reduced to a set of independent rivulets separated by dry zones. However, on all plates except the first there is a fortunate auto-stabilization effect on the liquid flow. This is because the wetted zones under the flowing streams undergoing evaporation are in fact colder and therefore condense vapor better on the other side of that same plate.
4. Experimental investigation
The experiments were carried out under solar irradiation at an experimental station in near
Touggourt (South Algeria: in summer, with an ambient mean temperature of 40°C). In these experiments, the feed is the groundwater (its temperature is between 65 and 75°C at the source). In order to assess the influence of solar radiation and temperature of feed water in the productivity of the solar still, a series of tests were carried out in the system. This system operated as a usual solar distiller for several consecutive days with different radiation levels. Measurements of temperatures by thermocouples were taken during this series of tests in which the water flows in capillary film.
5. Experimental results
Experimental results are presented in Figs. 2 to 5.
A solar distillation system was developed and its performance was studied in South Algeria. It is a capillary film distiller. Several prototypes of this device operate now with the aim of optimization of this type of distiller. It is an economic means to provide drinking water for population
of remote arid lands. Theoretical analysis of the heat and mass transfer mechanisms inside this solar distiller has been developed. Experimental investigations on the distillation performance of this solar still have been carried out. The experimental results obtained show the significant superiority of this type of distiller over the conventional basin type solar still. The productivity of this system increases with the intensity of solar flux and the temperature of feed water. Significant raises in distilled water productivity have been obtained not only during the day but mainly during night operation of the system because the feed water is geothermal.
The total efficiency of a solar still is determined by the product of the efficiency rate of absorption of solar irradiation and that of utilization of the absorbed energy. Different researchers have proved from knowledge of the equations governing internal and external heat transfer that a total efficiency of about 60% is the upper limit of an ideal solar still. The efficiency of solar still devices rarely exceeded 50% at application and thus outlined the features required to attain high efficiencies.
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