Enhancement of single solar still integrated with solar dishes: An experimental approach
Introduction
Energy and water problems are the two most basically things for the sustaining of life. There is an acute shortage of both energy and water, especially in the third World countries [1]. The earth is known as the water planet, but there is a shortage of fresh water due to the increasing the world population [2]. Many desalination systems have been developed to overcome the water shortage problem, but most of these techniques require intensive energy [3], [4], [5]. Most of the energy needs in the World are met by fossil and nuclear power plants. A small part is met by renewable energy technologies, such as the solar, wind, geothermal, biomass and the ocean. The consumption of fossil fuel and nuclear power sources increases with a high rate, so using the renewable energy sources is the best solution for energy problem. Today’s due to population growth and unsustainable consumption rates; the shortage of potable water is expected to be one of the biggest problems of the World. Also water resources (rivers, lakes and underground water) are polluted by industrial wastes, organisms, wastes etc. Most desalination techniques require intensive energy, so using renewable energy for desalination techniques is considered the best economical method for producing potable water with low cost. Solar desalination systems are classified into direct and indirect collection systems [6]. Indirect collection systems, solar energy is used to produce distillate directly with solar collectors, but in indirect type, two sub-systems are employed, one for solar energy collection and the other one for desalination [7]. The conventional solar still is a representative example of the direct collection system. It consists of a water basin and a glass cover. The sun rays pass through the glass cover and are absorbed by the absorber of the basin. The water is heated and its vapor pressure increases and hence the vapor rises. The water vapor is condensed on the cold glass cover and runs down into the troughs to the distilled water reservoir. The glass cover prevents losses and the wind from cooling the water in the basin [8]. The productivity and the system efficiency of the conventional solar still is very low compared with other types. So different enhancement methods and modifications were performed to increase the productivity and the system efficiency [9], [10].
The solar still output depends on many parameters such as the climatic parameters, the design parameters and the operational parameters [11], [12]. The climatic parameters include solar radiation, ambient temperature, and wind speed, outside humidity and sky conditions. The design parameters include glazing material, water depth, bottom insulation, still orientation, inclination of glazing and the spacing between water and glazing. The operational parameters include water preheating, coloring and salinity.
Omara et al. [13] investigated the traditional solar still integrated with a mechanical system; this system consists of wind turbine geared with still under water fan, to increase the water evaporation. The results showed that the productivity increased by 17% at a water depth of 3 cm, rotation speed of 30 RPM and the efficiency of 39.8%. Pakdel et al. [14] modified the shape of the conventional solar still by side toughs to the side glass walls; thermosiphon collector was used. The results showed that the new design improved the still productivity by 31.59% and the system thermal efficiency increased to 81.72%. Sharshir et al. [15] studied the effect of using nanoparticles and micro particles; copper oxide or graphite on the still performance. They concluded that using nanoparticles and micro particles increase the still efficiency to 38.61% and 41.18% respectively.
Kumar et al. [16] compared between the active and passive solar stills at different water depths of 0.05, 0.1 and 0.15 m. Nickel-chromium heater powered by solar photovoltaic was used to increase the productivity in the still. They concluded that the daily productivity from the solar still is 6 times more than the conventional passive still and the overall thermal efficiency increased by 25% higher than the conventional one. Al-Harahsheh et al. [17] studied the effect of using phase change material (PCM) on the solar still performance. The results showed that the solar still produced 4300 ml/day.m2 and 40% was produced after sunset. Salem et al. [18] studied experimentally the performance of the double acting solar still incorporated with parabolic trough collector at different medium of saline water. Pure saline water, steel wire mesh and sand are three different mediums had different impact on yielding distilled water with and without trough. The results obtained in the winter and the summer showed that wire mesh and sand with saline water improved the system efficiency by 3.9% and 13.8% respectively in winter and 3.3% and 15.3% respectively in summer. Fathy et al. [19] studied experimentally the effect of coupling parabolic trough collector with a double slope solar still on its performance. The results indicated that the daily efficiency of the conventional solar still, solar still with tracking PTC and a solar still with fixed PTC are 36.87, 29.81 and 23.26% respectively.
In general solar still productivity was improved by different methods and many researches such as, changing the shape of the glass and basin [20], [21], using sponge liner [22], using double basin [23], [24], [25], [26], [27], [28], and the utilization of solar water heater [29], using compound parabolic concentrator [30], [31], [32], [33] and using an air blower [34]. These modifications include also the solar radiation intensity with ambient temperature [35], [36], wind speed [37], glass cover angle and thickness [38], still insulation [39], [40], [41], water depth and free surface area [42], [43], [23], [44] and the temperature difference between water and glass cover [45]. On the other hand many scientists used different techniques to increase the water temperature such as using nanofluids [46], [47], [48], solar air-heater [49], different new absorber configurations [50] photovoltaic modules with heater [51], phase change energy storage in different designs [52], [53] and phase change material (PCM) [54], [17], [55]. Also different techniques were developed to increase and improve the still productivity like sponge cubes in the basin [56], using sensible storage medium [57], using floating absorber [58] and using black granite gravel as an energy storage medium [59]. The enhancement methods also contain the various absorbing materials with different geometrical dimensions [60], [61], [62], [63].
Sabiha et al. [64] reviewed the evacuated solar collector used as a heat source. Arunkumar et al. [65] reviewed how to increase the efficiency and the productivity of the solar stills.
Mokhiamar et al. [66] designed a small and moderate parabolic solar dish concentrator; 5, 10 and 20 m diameter; focal point/dish diameter is 0.3. The concentrator aimed to generate the electricity. The available materials in local market and design optimization reduced the cost. The frame stress analysis showed that the same technique can be applied to the larger dish diameters.
Although much research has been done on ways to increase efficiency and the productivity of solar stills, especially on the external solar collectors integrated with a solar still, the present work present a way to increase the daily productivity with no input power. To the author’s best information, the majority of the present work is the integrated of one or two conical tanks on the solar parabolic dishes with the use of the control unit for the first time to achieve the greatest benefit of the power output from the PV panels. The study is conducted at two different water basin depths in the solar still.
Section snippets
Experimental setup description
Fig. 1, Fig. 2 illustrate the main components of the proposed system integrated with a single solar still. The main components of the integrated desalination system are a single solar still, four modules of PV, two identical parabolic solar dishes with two conical tanks, ½ hp centrifugal pump for each solar dish and a control unit. Three sets of experimental are conducted from the proposed system. First, the experiments are conducted with the conventional solar still. The second, one parabolic
Control unit description
Three codes were designed and programmed using the Arduino program for different purposes:
- 1-
Maximum power tracking for the two solar parabolic dishes to obtain the maximum power gain during the solar radiation period.
- 2-
The water level inside the solar still is maintained constant at the desired level.
- 3-
Operating the solenoid valves and pumps.
All the codes are joined through the control unit. Fig. 8 illustrates the inputs and the outputs to and from the control unit. The inputs to the control units
Experimental procedure
All experiments were carried out at the Suez Canal University in Ismailia city, Egypt (30°35′N 32°16′E) during the three summer months from June to August 2018.
Three groups of experimental tests were carried during the three summer months. The first group was conducted with the traditional solar still. The second group was carried with one solar dish. The third was conducted with two solar parabolic dishes. The level of the saline water was examined at two levels 100 and 200 mm.
The reading of
Error analysis
The following variables were measured for the period of the experiments, total solar radiation, wind velocity, ambient temperature, glass temperature, water basin temperature, water inside cone tank, hot water flow rate for each water tank, water level inside the water basin and the amount of fresh water. The errors in the measuring devices were calculated and their effect on results accuracy were considered. The accuracy for different measuring instruments is summarized in Table 2.
Based on the
Results and discussion
The experimental work was performed and the solar stills were evaluated during three months (From June to August 2018). The measured weather characteristics data started in the month of June and finished at August, but a cloud sky condition was occurring during some experimental days in these months. So the experimental working days were selected based on a clear sky conditions to study the performance of these solar stills. The selected number of experiments were 18 for the three cases were
Cost analysis
The main objective for solar stills design for a primary application is to produce potable water in remote isolated locations with minimum cost. In this study for conventional and proposed solar stills and agreeing to the accessibility in the local Egypt market, the constant price was shown in details in Table 4.
Omara et al. [13] estimated the variable price per year, which is the operational and maintenance price to be
Conclusion
A new modified technique of water desalination is applied in this work to the conventional solar still to increase the distilled water productivity. The integration of one or two conical tanks on the focal points of solar parabolic dishes using the control unit and the power output from the PV panels to operate a heater in sunset time results in high productivity. The study was conducted at the two different water basin depths in the solar still. The experimental results are recorded at the
Declaration of Competing Interest
None.
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