performance of a solar still with clothes moving wick - International ...

Fifteenth International Water Technology Conference, IWTC ٢٠١١, Alexandria, Egypt

PERFORMANCE OF A SOLAR STILL WITH CLOTHES MOVING WICK Helmy E. Gad 1, Safya M. El-Gayar 2 and Hisham E. Gad3 1

Mechanical Power Engineering Department, Faculty of Engineering, Mansoura University, El-Mansoura 35516, Egypt, E-mail: [email protected] 2

Senior researcher, Agric. Eng. Res. Inst., A. R. C, Giza, P.O. 256, Egypt E-mail: [email protected] 3

Computers and Systems Department, Faculty of Engineering, Mansoura University, El-Mansoura 35516, Egypt, E-mail: [email protected]

ABSTRACT Solar distillation presents a promising alternative for saline water resources that can partially support arid remote areas needs form fresh water with free energy, simple technology and a clean environment. The problem of low daily productivity of the solar stills triggered scientists to investigate various means of improving its productivity and thermal efficiency in order to reduce the distilled water production cost. This paper investigates experimentally the performance of a simple solar still with clothes moving wick driven with a DC motor via a control circuit. The clothes wick is immersed in water when the motor is ON, and the wet clothes is subjected to solar radiation when the motor is in OFF period. The setup is suitably instrumented to measure solar radiation data on tilted surface, amount of distilled water, ambient air temperature, the inner glass and bottom box inside temperatures. Results show that an ON period 30 seconds is suitable and an OFF period of 25 minutes yields maximum thermal efficiency. This research work proves that it is possible to operate such solar still with the computer to reduce the cost of production of distilled water. Results are given in graphical form. KEYWORDS: Performance - Solar still - moving - clothes wick

INTRODUCTION Clean water is a basic human necessity, and without water the life will be impossible. Supplying fresh and healthy water is still one of the major problems in different parts of the world especially in arid remote areas [1]. Sources of fresh water are the great oceans and seas that can be desalinated by various methods including solar energy. Most existing desalination plants use fossil fuel as a source of energy. Although a few techniques such as multi-effect evaporation, multi-stage flash distillation, thin film distillation, vapor compression, reverse osmosis and electro dialysis are energy intensive and operating cost is high. These technologies are expensive, however, for the production of small amount of fresh water. The direct use of solar energy represents a promising option for eliminating the major operating cost. Solar distillation can provide the most attractive solution for those areas where plenty of solar energy is available and water demand is not too much. Solar still can be used


for producing drinking water with a low cost and maintenance, but the problem of this device is the low productivity [2]. The development of solar distillation has demonstrated its suitability for saline water desalination when the weather conditions are favorable and the demand is not too large, i.e., less than 200 m3/d [3]. The basin-type solar stills are the most popular solar distillation systems from a technical standpoint. However, it is known that, for the traditional solar stills, there are three serious shortcomings; the latent heat of condensation is not reused in the evaporation–condensation processes; small evaporation surface and limited condensation process. The basin-type solar still is tested by many researchers [4-8] concluding that the maximum productivity is about 2-3 L/m2 day. Ohshiro et al. [9] indicated that after theoretically and experimentally analyzing a wick/poly (tetrafluoroethylene) net/wick layered unit, they constructed a ten unit theoretical model of a multi-wick solar still. Compared to a conventional 10 mm gap unit, their internal temperature drops are found to be drastically lower, at the cost of only small decreases in productivity. The net-unit stills allow a much more compact design and have greater productivity than a still consisting of ten conventional 10 mm gap units. While, Fath [10] reported that the following design and operational parameters should be considered in future developments in solar stills: higher basin temperature (lower water level, use of wick, adding black dyes, additional external heating-collector, concentrator, waste heat recovery), lower cover temperature (cover cooling, multi-effect, overnight with basin energy storage, additional condenser), large evaporation and condensation surface areas re-utilization of the latent heat of condensation (multi effect) minimize heat losses (good side and bottom insulation), utilization of the shaded area (additional condenser, combined stills, etc.). Continuous research will ultimately lead to a water production cost that can compete with other technologies, in addition to the basic advantages of solar distillation. Moreover, Badran [11] used that the inverted trickle solar still, is improved mainly by adding a heat exchanger inside the condenser. The exchanger recovers part of the heat released in the condensation process and utilizes it in heating the saline water feed. This improvement, in addition to insulating the sides of the still and increasing the flow rate of the saline water feed, resulted in increasing the productivity from about 2.5 l/d to 2.8 l/d during the month of May. This improvement, which amounts to about 12%, is related mainly to the condensate production. Theoretical analysis and experimental investigation of heat and mass transfer mechanisms inside the capillary film solar distiller have been developed by Bouchekima et al [12]. The experimental results obtained show the significant superiority of this type of distiller over the conventional basin type solar still. In addition Naim and Abd El Kwui [13] showed that charcoal granules can be used as absorber medium successfully instead of wick-type, black butyl rubber, or asphaltic absorbers. A 15% improvement in productivity over wicktype stills has been attained. The still is non-conventional, cheap, simple, easy to construct and operate, and is of low thermal capacity. They also showed that coarse charcoal granules give acceptable results at high flow rates followed by intermediate then fine granules. Additionally, Nafey et al. [14] concluded that solar still with floating aluminum perforated black plate. They mentioned that, using the floating aluminum perforated black plate in the productivity by 15% for a water depth of 3 cm and 40% for a water depth of 6 cm. While, Abu-Hijleh and Rababah [15] reported


that, the use of sponge cubes in the basin water resulted in a significant improvement in still production, up to 273%.Where, the sponge cubes increase the surface area over which water evaporation occurs. The optimal combination was: sponge cubes with 6 cm sides, 20% sponge to basin water volume ratio and 7 cm basin water depth. They found that, black colored material tends to be better than materials with other colors in absorbing the incident radiation. An experimental study for an inverted trickle solar still was performed by Badran et al. [16]. The still was basically composed of an inclined absorber plate painted black on the top. Saline water flowed at the backside of the plate and was kept attached to the plate. The water flow rate was kept low so that its temperature was raised to produce vapor. Condensation took place in another compartment where a heat exchanger was placed to provide heat recovery. They concluded that the productivity of the inverted trickle solar still is moderately improved by using brackish water. The productivity increased from 2.5 to 2.8 L/d when the salinity of the water was reduced from (35000 ppm) to brackish water (6000 ppm). Badran and Al-Tahaineh [17] integrated a conventional flat plate collector with a solar still to augment the production rate. They found that, the mass of distilled water production was increased by 52%, when the still was coupled with flat plate collector. Moreover, Tanaka and Nakatake [18] investigated the effect of vertical flat plate external reflector on the productivity of the tilted wick solar still. They proposed a geometrical method to calculate the solar radiation reflected by the external reflector and absorbed on the evaporating wick, and also performed numerical analysis of heat and mass transfer in the still to predict the distillate productivity on four days (spring and autumn equinox and summer and winter solstice days) at 30 oN latitude. They found that the external reflector can increase the distillate productivity in all but the summer seasons, and the increase in the daily amount of distillate averaged over the four days is predicted to be about 9%.While, Sadineni et al. [19] indicated that there is a significant reduction in the performance with a double-pane glass compared with a single-pane glass. Due to the reduced temperature difference between the evaporating water and condensing glass in a still with double-pane glass used both as transparent cover and condensing surface, the productivity reduced significantly. They also observed that the proposed design is superior in productivity (20% improvement) compared with a conventional basin-type solar still. In addition, Abdallah et al. [20] investigated the performance of a basin type solar still system with three different design modifications. Installing internal reflecting mirrors gave an average of 30% increase in the amount of distilled water produced when compared with a classical fixed solar still system. Modifying the still design from a flat basin into step-wise basin gave a higher production rate with an average increase of 180%. Also, coupling the modified still design with a sun tracking system gave further improvement, reaching up to a 380% increase in the production rate of distilled water. Whereas, Abdallah et al. [21] reported that the addition of the absorbing materials improves the thermal performance of the solar still. Both coated and uncoated metallic wiry sponges have increased the water collection gain by 28% and 43% respectively. One of main drawbacks noticed on the metallic wiry sponges that corrosion started to appear in certain parts of the sponge. Whereas, black rocks gave around 60% gain with no corrosion problems. So, they concluded that the black rocks


absorb, store and release the incident solar energy better than the coated and uncoated metallic wiry sponges and can enhance the productivity by nearly 20%. Kaushal and Varun [22] indicated that, solar radiation is used for the energy available by the sun, which means this system works on solar energy. There are many methods for desalination of brackish water in to potable water. Therefore, different types of solar stills are discussed for the production of pure water. A proper combination of cooling film parameters enhanced the still efficiency by 20%. In multi-effect diffusion model the productivity decreases about 15% with an increase in diffusion gaps between partitions from 5 mm to 10 mm. In the present work, a new technique is used and experimentally tested. A black clothes moving wick which is intermittently moves on two rollers with a DC motor via a control circuit. The control circuit works by a computer program by fixing the ON period at a suitable value, and changing the OFF periods to different values to produce the maximum thermal efficiency of the system.

EXPERIMENTAL SETUP AND PROCEDURE Figure 1 shows a schematic diagram of the setup which consists of the solar still with the control circuit. The stills basin is a wooden box measured 80 cm long, 60 cm width with a depth of 15 cm from inside. The box is painted with oil white several times from inside to prevent leakage. The still is 3 mm thick glass covered and fixed on an iron frame with an inclination angle of 30o facing south. The distillate is collected at the lower edge of the glass cover via a thin iron trough to a calibrated Jar. The compensate water is added from the still top as shown in Fig. 1. The black clothe is fixed on a two copper rollers as a belt. The lower roller is free, while the upper roller is fixed with a high torque DC motor. The lower roller and the cloth are immersed in the water. The setup is instrumented to measure the inner glass surface and inside bottom temperatures. The ambient air temperature and solar radiation on tilted surface are also measured. The solar radiation data and distilled water were collected nearly each 30 minutes. The experimental work is carried out on the roof of the Thermal engineering laboratory, Mechanical power department, Faculty of Engineering, Mansoura University, latitude 31.04083o N and longitude 31.4861o E. The experiments were performed during the different days from months of October and November 2010 on sunny days. Figure 2 shows a block diagram of the control circuit. This circuit is used also for monitoring and sending the measured temperatures from the system to the computer. The main component of the circuit is the PIC18F2455, which it is a USB (universal serial bus) microcontroller form Microchip Company. This microcontroller has been chosen, since it provides the capability of connecting it to the computer via a USB (version 2.0) interface. The USB 2.0 is a high speed serial interface, which it provides a communication speed up to 12 Mb/s, making it the most suitable interface for transferring high speed data between the computer and the microcontroller [23].


Fig. 1 The experimental setup. Also, this microcontroller has a large numbers of A/D converter channels (up to 10 channels) to convert the analog signals from the temperature sensors into a digital form. However, only three channels have been used for the temperature sensors. Additionally, the internal memory size of this microcontroller (24 KB program memory and 2 KB data memory) and its processing power are high making it the most suitable device for controlling and monitoring the system in real time [24]. Moreover, it is readily available in the market with a low cost compared to the digital signal processors and the PLC controller. The temperature sensors are LM35DZ from National Semiconductor, which it is a high precision integrated circuit temperature sensor. The output voltage of this sensor is linearly proportional to the Celsius temperature. The LM35DZ does not require any external calibration. Also, it provide a typical accuracies of ± 0.25 °C at room temperature and ± 0.75 °C over a full −55 to +150°C temperature range. Moreover, it has a very low self-heating, which it is less than 0.1°C in stagnant air [25]. However, a noise suppression circuit must be added to the output of each sensor to reduce the effect of the electromagnetic noise from the DC motor as shown in Fig. 2. The DC motor interface circuit, which consists of a high power transistor, is used as an electronic switch to provide the necessary current and voltage to drive the DC


motor. The transistor is controlled by a small control signal coming from the microcontroller as shown in the same figure. The computer program is designed and written by using Csharp.net software version 9.0. This program is responsible for storing the measured temperatures in an Excel file (with a specific sampling rate) and controlling the DC motor ON-OFF timing rates.

Fig. 2 A block diagram of The control circuit.

The three temperature sensors are fixed in the experimental setup, one is fixed to measure the inner glass cover temperature, the second is fixed to measure the inner still bottom temperature and the third is fixed under the setup to measure the ambient air temperature. The sampling rate of recorded data is adjusted via the computer program by 5 minutes. The DC motor is also adjusted by the same program to be ON for 30 seconds, and OFF for a specific period of time. Results are recorded in an Excel file. The OFF time is taken as 5, 10, 15, 20, 30, 40, 50 and 60 minutes. The ON time (30 seconds) is sufficient to keep the cloth wet. The OFF times are tested experimentally. Figure 3 shows a photograph of the setup where the computer, the control circuit and the solar meter are fixed in the shadow under the solar still. The motor is fixed to the upper roll via a flexible coupling. The system is totally driven via the computer program which decides the ON and OFF periods of operation. The sampling rate of recording readings is also decided by 5 minutes. The computer program is subjected to some problems in the beginning of experiment because of the higher ambient temperature. For a stable operation, the microcontroller must work in a moderate ambient temperature.


Fig. 3 A photograph of the experimental setup.

RESULTS AND DISCUSSION Figures from 4 to 11 show some collected data of the experiments which are carried out in different days from October and November, 2010. The global solar radiation intensity on the tilted surface (w/m2) of the still and the amount of distilled water (ml) are shown in the left part of the figures. The inner surface glass temperature (°C), box bottom inside temperature (°C) and ambient air temperature (°C) are shown in right part of the figures. The experiments are performed at different motor OFF periods; 5, 10, 15, 20, 30, 40, 50 and 60 minutes. In general, the solar radiation intensity on tilted surface increases to noon hours and then decreases again. The maximum solar radiation changes from day to another from 710 w/m2 to 835w/m2. The distilled water has the same trend with a little time delay. The cloud conditions and ambient temperature change also from day to another as usual during autumn season.

Fig. 4 Solar radiation, amount of distilled water and temperatures vs. time (OFF period 5 min).










Therefore, to compare between results, it is better to use the thermal efficiency since the solar radiation differ from day to another. The thermal efficiency of the still during a certain period,  is defined by,  

mw L t

A  G dt 0

Where, mw is the total mass of distilled water collected during this period, Kg L Latent heat of evaporation at the glass temperature, KJ/kg, G Solar radiation on tilted surface, w/m2 A Area of the clothes wick, m2


and

 Operation period, seconds t is the time, seconds.

A MATrix LABoratory (MATLAB) computer program is developed to calculate the above equation with a time step 0.01 seconds. The best fit equation is obtained for the solar radiation intensity, G and is used to integrate it. The amount of distilled water (ml) is simply the total amount of water collected during the experiment (Kg). The subjected area of the clothes, A is taken as an average 0.25 m2 since it contracts during the experimental run. The operation period changes from 7 to 8 hours as shown in the above figures. Results show the solar still thermal efficiency as a function of OFF period (min) as given on Fig. 12. The continuous line represents the best fit for the experimental results (stars). It is clear that the thermal efficiency increases to a maximum of 0.43 at 25 OFF period according to the best fit curve. This research work proves that it is possible to operate such solar still with the computer to reduce the cost of production of distilled water.

Fig. 12 The solar still thermal efficiency vs. the OFF period, min

CONCLUSION The problem of low daily productivity of the solar stills has to be investigated by various means of improving its productivity and thermal efficiency in order to reduce the distilled water production cost. This paper investigates experimentally the performance of a simple solar still with clothes moving wick driven with a small DC motor via a control circuit. The clothes wick is immersed in water when the motor is ON, and the wet clothes is subjected to solar radiation when the motor is in OFF


period. The setup is suitably instrumented to measure solar radiation data on tilted surface, amount of distilled water, ambient air temperature, the inner glass and bottom box inside temperatures. The solar radiation data and distilled water were collected nearly each 30 minutes. Results show that an ON period 30 seconds is suitable and an OFF period of 25 minutes yields maximum thermal efficiency. Results show also that, the solar radiation intensity on tilted surface increases to noon hours and then decrease again. The maximum solar radiation changes from day to another from 700 w/m2 to 800 w/m2. The distilled water has the same trend with a little time delay. This research work proves that it is possible to operate such solar still with the computer to reduce the cost of production of distilled water. Results are given in graphical form.

REFRENCES [1] Tiwari G.N, "Solar energy: fundamentals, design, modeling and application", New York/New Delhi: CRC Press/Narosa Publishing House; 2003. [2] Duffie J.A and BeckmanW.A, "Solar engineering of thermal process", New York, USA: Wiley; 1991. [3] Sukhamte S.P, "Solar energy: principle of thermal collection and storage", New Delhi: Tata-McGrawth-Hill; 1987. [4] Hilal H and Nassri, B.A, “Parametric investigation of a double-effect solar still in comparison with a single-effect solar still”, Desalination 150 (2002) 75-83. [5] Murugavel Chockalingam K.K.S.K and K. Srithar, “Progresses in improving the effectiveness of the single basin passive solar still”, Desalination 220 (2008) 677–686. [6] Spiegler K.S and El_Sayed Y.M, A, "desalination primer Italy" : Balaban Desalination Publications; 1994. [7] Malik M, Tiwari G.N, Kumar A, Sodha M, "Solar distillation. Oxford, UK: Pergamon Press; 1982. [8] Rahul D, G.N. Tiwari, “Characteristic equation of a passive solar still”, Desalination 245 (2009) 246–265. [9] Ohshiro K, Nosoko T and Nagata T, " A compact solar still utilizing hydrophobic poly (tetrafluoroethylene) nets for separating neighboring wicks", Desalination 105 (1996)207-217. [10] Fath H. E.S," Solar distillation: a promising alternative for water provision with free energy, simple technology and a clean environment", Desalination 116 (1998) 45-56. [11] Badran A.A, "Inverted trickle solar still: effect of heat recovery", Desalination 133 (2001) 167-I 73.


[12] Bouchekimaa B.B, Grosb, R. Ouahesc, and M. Dibouna, "The performance of the capillary film solar still installed in South Algeria", Desalination 137 (2001) 31-38. [13] Naim, M.M and M.A Abd El Kwui" Non-conventional solar stills Part 1. Nonconventional solar stills with charcoal particles as absorber medium", Desalination 153 (2002) 55-64. [14] Nafey, A. S., M. Abdelkader, A. Abdelmotalip and A.A. Mabrouk" Enhancement of solar still productivity using floating perforated black plate" Energy Conversion and Management 43 (2002) 937–946. [15] Abu-Hijleh, B.A/K., and H.M. Rababah "Experimental study of a solar still with sponge cubes in basin "Energy Conversion and Management 44 (2003) 1411– 1418. [16] Badran A.A, Assaf K.S Kayed, Ghaith F.A and M. I. Hammash, "Simulation and experimental study for an inverted trickle solar still", Desalination 164 (2004) 77-85. [17] Badran A.A, and H.A Al-Tahaineh,"The effect of coupling a flat-plate collector on the solar still productivity", Desalination 183 (2005) 137–142. [18] Tanaka H. and Y. Nakatake, " Improvement of the tilted wick solar still by using a flat plate reflector", Desalination 216 (2007) 139-146. [19] Sadineni S.B, Hurt R, Halford C.K, and R.F. Boehm, "Theory and experimental investigation of a weir-type inclined solar still", Energy 33 (2008) 71–80. [20] Abdallah S, Badran O, and M. M. Abu-Khader, "Performance evaluation of a modified design of a single slope solar still" Desalination 219 (2008) 222–230. [21] Abdallah S, Abu-Khader M.M and O. Badran, "Effect of various absorbing materials on the thermal performance of solar stills, "Desalination 242 (2009) 128–137. [22] Kaushal, A., and Varun, "Renewable and Sustainable Energy Reviews", Renewable and Sustainable Energy Reviews 14 (2010) 446–453 447 [23] Jan Axelson, “USB Complete”, Lakeview Research, Forth edition, 2009. [24] PIC18F2455 datasheet, Microchip Technology Inc., 2007. [25] LM35DZ datasheet, National Semiconductor, Nov. 2000.