Enhancement of distillate output of double basin solar still with vacuum tubes

Abstract

The latent heat of condensation is lost to the atmosphere; hence it is not utilized to increase distillate output of single basin solar stills. This difficulty was overcome by attaching an additional basin to the main basin. The performance of the double basin solar still was also increased by attaching vacuum tubes to the lower basin; hence the lower basin possessed a higher temperature throughout the day. The latent heat of condensation of the bottom basin was also utilized to increase distillate. But the distillate output of the top basin was even lower compared with that of the bottom basin. This paper proposed a novel approach to increase the distillate output of the double basin solar still attached with vacuum tubes by introducing different sensible energy storage materials like pebbles, black granite gravel and calcium stones to increase the basin area. Experiments were conducted in climate conditions of Mehsana (23.6000° N, 72.4000° E) Gujarat from April to September 2013 with a constant water depth of 2 cm in the top basin with and without the use of basin materials. The results showed that the distillate output of basin material with calcium stones is greater (74%) compared with that of black granite gravel and pebbles. The integration of vacuum tubes with solar still greatly increases the distillate output of the solar still by providing hot water at the lower basin.

Keywords

double basin solar still / calcium stones / pebbles / granite gravel / distillate output

Introduction

Water is essential for all life forms on the earth, plants, animals and humans. However, the lack of fresh water is always a trouble for humanity. The rapid industrial growth and worldwide population explosion have resulted in a huge rising demand for freshwater, both for household needs and for crops to produce adequate quantities of food. Added to this is the problem of the pollution of rivers and lakes by industrial waste and the large amounts of sewage discharge. On a global scale, man-made pollution of natural sources of water is becoming one of the greatest causes for freshwater shortage. In most places, ground water is brackish and not suitable for drinking and cooking. In arid areas, clean and potable water is extremely limited and the establishment of a human habitat in these areas strongly depends on the amount of such water which can be made accessible [1].

The distillate output of a single basin single slope solar still was very low and efforts were made by many researchers to increase the rate of production. Inferences were made by researchers that the decrease in water depth increased the distillate output of the still [1-4]. The addition of dye in the basin water raised the absorption of solar radiation and distillate output [5-7]. Various energy absorbing materials were used to increase the distillate output of the solar still [8]. Madani and Zaki [9] put rubber mate for the enhancement of the efficiency, and Naim and Abd El Kawi [10] used charcoal as the absorbing material in the basin to raise the absorption of solar radiation. Furthermore, Nafey et al. [11] and Abdel-Rehima and Lasheen [12] increased the distillate output by using black materials, glass, rubber, and black gravels. Kalidasa Murugavel et al. [13] put the spreading materials with a thin layer of water to broaden the water in the entire basin area of the basin. Arjunan et al. [14] made requirements to store excess heat energy in solar stills at daytime and to store energy at night for increasing nocturnal production using a blue metal stone as the energy storage medium. Kalidasa Murugavel et al. [15] made an attempt to raise distillate output by using a layer of water and different sensible heat storage materials like quartzite rock, red brick pieces, cement concrete pieces, washed stones, and irons scraps. Kalidasa Murugavel and Sridhar [16] put various energy storage materials like light cotton cloth, sponge sheet, coir mate and waste cotton pieces, and aluminum rectangular fin arranged in different configurations in the basin. Abdullah et al. [17] used various energy storage materials like black coated and uncoated wiry sponges and quartzite rocks to increase distillate output of solar still. Recently Rajaseenivasan et al. [18] used various different wick and porous materials like black cotton cloth, jute cloth, waste cotton pieces, clay pots facing up and down and mild steel pieces in double basin and single basin solar still. They found that the double basin solar still with mild steel plates was more productive compared with other materials.

Various types of evacuated tubes and their thermal analysis were explained by Tiwari [19]. Morrison et al. [20] found from the studies on water in glass evacuated tube water heater that that the glass evacuated tube water heater is a novel concept due to its simplicity and low manufacturing cost. Dev and Tiwari [21] conducted an experiment with an evacuated tubular collector integrated with solar still for increasing distillate output of solar still. The one-year experiment proved that there was a considerable increase in distillate output. Sampathkumar et al. [22] conducted experiments with the evacuated tube integrated with single basin solar still with gravel as an energy storage medium and found that there was an increase in distillate output with or without the use of energy storage medium with the integrated glass tube solar still. Xiong et al. [23] made a novel multi effect solar still with evacuated tube solar still and tested it in climate conditions in China. They found good agreement between the theoretical model and experimental results.

In this paper, a double basin solar still with vacuum tubes made in locally available materials for increasing distillate output of top basin was tested experimentally with various sensible energy storage materials like pebbles, black granite gravel and calcium stones for six months from April to September 2013 in climate conditions of Mehsana, Gujarat.

Experimental setup

The experimental setup is installed at the Terrace of Gujarat Power Engineering & Research Institute, Mehsana Gujarat, India. Mehsana is a city with more than 250 days’ of sunshine in a year and an average solar insolation of more than 800 W/m². Hence, it is the best location for solar energy experiments. The experimental setup mainly consists of a double basin solar still with vacuum tubes, as shown in Fig. 1. The data logger attached to the solar still is demonstrated in Fig. 2. The hourly temperatures recorded in the computer with the help of the data logger are illustrated in Fig. 3. The overall size of the inner basin used is 1006 mm × 325 mm × 380 mm, and the outer basin is 1006 mm × 536 mm × 100 mm. The absorber plates used in the inner and outer basins were made of aluminum sheet with black chrome paint for increasing absorptivity of solar radiation. An insulation of 4 cm in thickness was provided at the bottom and the sides of the outer basin to prevent heat losses. Here polyurethane foam (PUF) with a thermal conductivity of 0.025 W/(m²·K) was used for the present experiment. The evaporated water inside the inner and the outer basins was condensed by toughened glasses of 3 cm in thickness. The condensed water of the inner and outer basins was collected by hanging jars. A silicone rubber sealant was provided to hold the toughened glass in contact with solar still surfaces. A total of 4 holes was made on the inner and outer basins for the location of thermocouples. Here, 14 vacuum tubes were coupled with a hole of 6 cm in diameter in the lower side of the inner basin. The vacuum tubes were connected to the still stand at an angle of 35° with respect to the horizontal axis. Rubber gaskets were provided to fix the vacuum tubes attached to the inner basin of the solar still. The bottom portion of the vacuum tubes was connected to a sponge material to prevent breakage of vacuum tubes. The instruments used in the present experiment with their accuracy, range and percentage of error are listed in Table 1.

Procedure of experiment

Solar radiation was transmitted through the toughened glass cover to the saline or brackish water in the basin. Thus, the water in the basin got heated and evaporated. The evaporated water particles condensed on the inner side of the glass cover. The condensed water flew down the cover due to slope and reaches the distillate channel, where it was collected by the flask. At the beginning of the experiment, the water level inside both basins was maintained at 2 cm in depth. The experiment was commenced after 24 h, from 7 am to 7 am the next day with assuming steady-state conditions built at every hour. For each experiment, the glass cover was cleaned to avoid dust concentration on the top of the glass cover of the outer basin solar still. The experiments were conducted on sunny days from April to September 2013. The variables measured in the present experiments were the water temperature of the top basin (T_w1), the inner glass cover temperature of the top basin (T_gi1), ambient temperature (T_a), vacuum tube temperature (T_b), solar radiation on evacuating tubes (I_e(t)), solar radiation with glass cover (I_g(t)) wind speed (v) and distillate output (m_w). Here, all the experiments were conducted during sunshine hours only with the help of the data logger. Hence, cloudy days were not preferred. The average velocity of wind is 2-3 m/s

Result and discussion

Figure 4 presents the comparison of distillate water output from the double basin solar still with a constant water depth and different basin materials inside the top basin of the solar still. It shows that distillate output is higher with calcium stones compared with black granite gravel and pebbles as basin materials during the heating period. For all basin materials, the maximum distillate output is obtained during 1:00 pm. Figure 4 clearly shows that, during the cooling period, distillate output of the top basin is also higher; hence the nocturnal distillate output of calcium stones as basin material is higher compared with that of black granite gravel and pebbles as basin materials.

Figure 5 exhibits the comparison of cumulative distillate output of the top basin with various materials. Here, all observations are made by constant water depth and moderate sizes of basin materials; hence the readings gained in the research are clear and well understood. Figure 6 clearly shows that distillate output is higher with calcium stones as basin materials during the heating as well as the cooling period. Black granite gravel is also a good basin material to increase distillate output after calcium stones and at least the cumulative distillate output is achieved by pebbles as basin material.

For deep basin solar still with different wick type materials, porous materials and energy storage materials are used by many researchers around the world to enhance distillate output of solar still [9-11]. When the water layer inside the solar still is low and different basin materials are used to spread the water throughout the basin for increasing surface area, good result is obtained. There are many other basin materials available, which lead to further study of such materials to increase distillate output of solar still.

Figure 6 shows the comparison of top basin water temperature with different basin materials. The temperature rise is higher for the still with calcium stones as basin material during the heating and cooling period. During the heating period, black granite gravel as basin material reaches its highest temperature at around 1:00 pm. For other basin materials, the water temperature rise in the top basin is uniform but it reaches the water temperature at around 2:00 pm and 3:00 pm respectively. During the cooling period, the top basin with calcium stones and black granite gravel maintains higher water temperatures. Similar effects are obtained for inner glass cover temperature during the heating and cooling periods for the top solar still with the same basin materials, as shown in Fig. 7.

From the above analysis, it is evident that the distillate output from the solar still increases with the water temperature rise during the heating period for all basin materials. During the heating period, as the water temperature increases, the glass cover temperature also increases. This is a very normal behavior for each solar still but if the maximum water temperature condition is taken into consideration, at that time, a lower water temperature could also gain a remarkable distillate output when black granite gravel is used as basin material. During the cooling period, the solar still with a higher water and glass cover temperature increases the distillate output while a lower water and glass cover temperature decreases the distillate output. Hence, calcium stones gain remarkable distillate output during the cooling period due to the higher water and glass cover temperature and the pebbles gain a lower distillate output due to the low water and glass cover temperature.

The distillate output of the top basin solar still increases with the increase in temperature difference between water and glass cover temperature. Figures 8, 9 and 10 show the comparison of water temperature (T_w1), glass cover temperature (T_g1), difference of water and glass cover (T_w1-T_g1) temperature and distillate output of the top basin with different basin materials. It is clearly demonstrated that for each top basin material, it reaches its highest distillate output just before its water temperature reaches its highest valueFor all basin materials, the distillate output increases with the increase in the temperature difference between water and glass cover temperature (T_w1-T_g1). This value arrives at its maximum when the water temperature reaches 50 °C to 60 °C during the heating period only, and decreases gradually after the completion of the heating period. This is the versatile behavior of the solar still noticed in every experiment. One more good behavior of the solar still is that during the above period for certain duration, distillate output decreases with the increase in basin water temperature, but when solar still reaches its maximum water temperature at that time, the solar still distillate output becomes lower than the maximum.

It is observed that for calcium stones in the basin, there exists a remarkable temperature difference between water and glass cover temperature. Due to that difference, the distillate output increases linearly with the increase in temperature difference during bright sunshine hours, and the minimum water depth in the basin is maintained by the constant water depth in the top basin by suitable arrangement, which enhances the remarkable distillate output due to the low thermal capacity of the water in the basin. Sampathkumar Karupassamy [22] used gravel as storage material in vacuum tube integrated solar still. He found a great increase in distillate output.

Table 2 tabulates the density, thermal conductivity and specific heat capacity of basin materials used in the present experiment. Of all basin materials, calcium stones have a higher specific heat capacity than any other basin materials. Hence a large amount of solar energy utilized and stored during sunshine hours is relieved during the absence of the sun (night time). Calcium stones have pore holes. Hence they allow the saline water to store inside and energy stored during sunshine hours is gained by the saline water inside pore holes. As a result, during the absence of the sun, distillate output is higher.

Calcium stones having a remarkable specific heat capacity of 910 J/(kg·K) are the best basin material compared with 3/4 inches quartzite rock with a specific heat capacity of 775 J/(kg·K) [15] and blue metal stones [14] in the basin for efficient distillation.

Table 3 presents the average distillate output gained by the top basin solar still from April to September 2013 with a constant water depth of 2 cm in the top basin. It is clearly indicated that the highest average distillate output is gained by calcium stones as a basin material in the top basin solar still (4.1 kg/h) and the lowest is gained by pebbles as storage material (2.9 kg/h). Black granite gravel is also a good basin material, with an output of 3.4 kg/h. Table 4 is the comparison of researchers on active solar still with vacuum tubes.

Conclusions

In this paper, a novel approach was proposed to increase the distillate output of the double basin solar still attached with vacuum tubes by introducing different sensible energy storage materials like pebbles, black granite gravel and calcium stones to increase the basin area. The inner basin is 1006 mm × 325 mm × 380 mm, and the outer basin is 1006 mm × 536 mm × 100 mm which is fabricated by locally available materials. And the test was conducted in climate conditions of Mehsana with a water depth of 2 cm in the top basin. For maintaining a constant layer of water in the top basin, the water should be spread in the entire basin so that various basin materials like pebbles, black granite gravel and calcium stones used. The variation of distillate output, cumulative distillate output, water and glass cover temperatures, difference of water and glass cover temperatures are analyzed for various basin materials. The following results were obtained.

1) Calcium stones are one of the best basin materials to increase distillate output during the daytime as well as the nighttime.

2) Calcium stones have a higher specific heat capacity compared with other basin materials. Hence it is responsible for getting a higher distillate output for the top basin water.

3) The constant water depth of 2 cm inside the top basin has significant influence on the thermal performance of the top basin solar still.

4) There is a greater difference of basin water temperature and inner glass cover temperature between calcium stones and pebbles and black granite gravel when used as basin materials.

5) Calcium stones possess good pore holes, which help to store the solar energy as well as saline water at night. Therefore, the distillate output of calcium stones as basin material is remarkable.

6) Average six months distillate output of Calcium stones as basin material is higher of 4.1 kg/h compared with black granite gravel and pebbles like 3.4 kg/h and 2.9 kg/h.

7) Distillate output of solar still greatly depends on attachment of vacuum tubes, because it produces hot water and hence, distillate output of solar still.

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