Wind turbine-inclined still collector integration with solar still for brackish water desalination

Wind turbine-inclined still collector integration with solar still for brackish water desalination

Desalination 249 (2009) 490–497 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m ...

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Desalination 249 (2009) 490–497

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Wind turbine-inclined still collector integration with solar still for brackish water desalination Mohamed A. Eltawil a,b,⁎, Zhao Zhengming a a b

The State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084, China Agricultural Engineering Department, Kafrelsheikh University, Box 33516, Egypt

a r t i c l e

i n f o

Article history: Received 11 December 2007 Accepted 19 June 2008 Available online 4 October 2009 Keywords: Wind turbine Solar stills Water desalination Hot water Water quality

a b s t r a c t This paper presents a new hybrid desalination system that constitutes of wind turbine (WT) and inclined solar water distillation (ISWD) integrated with main solar still (MSS). The new developed system is designed, fabricated and evaluated under actual environmental conditions. A small wind turbine is used to operate a rotating shaft fitted in the MSS to break boundary layer of the basin water surface. Also, an ISWD system which consists of an inclined flat solar absorber plate covered with black-wick medium is attached to the exit of MSS. The system can produce distilled and hot water. The heating and evaporating processes take place in MSS as well as ISWD, and then the water are condensing on the glass covers. The system was tested at different water depths (0.01, 0.02 and 0.03 m), different water flow rates (25.0, 41.7 and 58.3 ml/min) and two modes of operation as due south and tracking the sun. Variation of ambient conditions, and water temperatures and outputs were used to evaluate each parameter. It was found that, increasing water depths at the same flow rate caused a decrease in the distilled water productivity. The amount of fresh water per square meter from the ISWD could be higher than the MSS with a range of 26.55 to 29.17% when the system is due south, while it ranged from 27.1 to 32.93% when the system is tracking the sun. The average daily efficiency of MSS and ISWD ranged from 67.21 to 69.59 and 57.77 to 62.01% when the system was due south, while it ranged from 66.81 to 69.01 and 57.08 to 62.38% when the system was tracking the sun, respectively. The water product cost is found to be 0.662 and 0.552 RMB/l (1 US $ = 7.43 RMB) when the system was due south and tracking the sun, respectively. The electricity annual savings is found to be 195.22 RMB/kWh/m2. The distilled water quality as well as hot remaining water is good enough for domestic usage. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Large quantities of fresh water are required in many parts of the world for agricultural, industrial and domestic uses. A very small fraction, about 0.3%, of the available water resources is available as fresh water [1]. In the arid areas of the world, small and remote communities have critical problems associated with providing safe water supplies. Specific water quality problems include salinity, iron, manganese, fluoride, heavy metals, bacterial contamination, and pesticide/herbicide residues [2]. Lack of fresh water is a prime factor in inhibiting regional economic development. A chronic drinking water shortage is one of the most important issues in the developing countries, and drinking water from dirty water sources causes serious damage to health. Desalination processes consume significant amounts of energy, and many countries in the world, particularly those suffering from severe ⁎ Corresponding author. The State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084, China. Tel.: +86 10 62773237; fax: +86 10 6279415. E-mail address: [email protected] (M.A. Eltawil). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.06.029

water shortages, cannot afford the energy required for desalination. Fortunately, many of those countries lie in areas with high insolation rates. Therefore, solar distillation of water can be a practical alternative which offers life to those regions where the lack of fresh water hinders development. Solar energy can be used to produce fresh water directly in a solar still or indirectly where the thermal energy from a solar energy system is supplied to a desalination unit. It has been shown that solar distillation remains the most favourable process for the supplying of water to small communities. Renewable energies are expected to have a flourishing future and an important role in the domain of brackish and seawater desalination in remote areas. Conventional technologies to treat water supply systems consist of simple settling, disinfection with chlorine or iodine, reverse osmosis water systems or ion exchange water softeners. Simple disinfection reduces harmful bacteria, but does not reduce salinity or remove heavy metals. Several types of solar stills exist, the simplest of which is the singlebasin type. Solar stills could, however, be considered attractive for domestic purposes, especially in areas having no access to the electric grid and low labor cost. An improved version with enhanced efficiency needs more complex constructional, operational and maintenance

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standards and, as such, is not appropriate for installation in rural areas of developing countries with limited economic and technical resources [3]. Solar stills are characterized by the ease of construction performed by local people from locally available materials, simplicity in operation by unskilled personnel, direct free solar energy, no hard maintenance requirements and almost no operation cost. However, the low yield and large areas of land required for installation offset these advantages. A number of efforts have been made to develop and improve the performance of solar desalination systems, particularly solar stills. To increase the temperature of the water inside the still, some researchers [4–9] suggested coupling the still to solar collectors. The results showed an improvement in the still's performance. One of the main reasons behind the low efficiency of solar stills, which is about 30–40% [1], is the loss of the latent heat of condensation to the environment and the sensible heat carried away by the condensate. The use of latent heat of condensation to preheat the feed water has shown good improvement in the still's performance [10,11]. The use of latent heat of condensation of one stage to evaporate water in another stage, as in multieffect stills, has been studied by many researchers showing very good improvement in the still's performance [12,13]. Al-Kharabsheh and Goswami [14] presented a theoretical analysis and preliminary experimental results for an innovative water desalination system using low-grade solar heat. The system utilizes natural means (gravity and atmospheric pressure) to create a vacuum under which water can be rapidly evaporated at much lower temperatures and with less energy than conventional techniques. Aybar et al. [15] designed and tested an inclined solar water distillation system under actual environmental conditions of northern Cyprus. Unlike solar still systems, the feed water falls down on the solar absorber plate, and the system produces fresh water and hot water simultaneously. In a basin-type solar wick still, the feed water is absorbed slowly by a porous radiation-absorbing pad (the wick). Two advantages are claimed over basin stills. First, the wick can be tilted, so that the feed water presents a more suitable angle to the sun and enlarge the effective area. Second, less feed water is in the still at any time, and thus the water is heated more quickly and to a higher temperature. Simple basin-type solar wick stills are more efficient than common basin-type solar stills, and some designs are claimed to be less costly than a basin-type solar still of the same output [16]. Benjemaa et al. [17] evaluated the potential of renewable energy development for water desalination in Tunisia. Different technologies of water desalination systems have been carried out by Safi and Korchani [18]. They presented a thermo-economic model to measure and compare the efficiency of solar desalination system and two desalination low-temperature technologies: multi-stage flash and multieffect desalination in dual purposes power/desalination plant based on gas turbines. In this experimental study, a small wind turbine is designed and attached with the main solar still (MSS) to operate rotating shaft provided with impellers. The impeller is used to break boundary layer of the basin water surface, thus increasing the water vaporization and condensation. An ISWD system that consists of an inclined flat solar absorber plate with black-wick medium and covered with glass is attached to the exit of MSS. Water dripping onto the absorber plate creates a continuous film of water. The heating and evaporating processes take place on MSS as well as ISWD, and then the water is condensing on the glass covers. There are two outputs of the distilled water and one outlet for hot water. Also, the system is installed on the movable trolley to track the sun during the day-time, by adjusting its positions twice daily. It was due east from morning until 11:30 am, due south until 1:30 pm and then due west until late evening. The most important feature of the system is the fact that the system produces hot water while it produces fresh drinking water. The

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heated water can be used as domestic hot water if it is not too briny. 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 wind and solar energies. 2. System description The outdoor phase of the study is conducted at the campus of Tsinghua University (Beijing, China) with the test facility located on the roof of Electrical Engineering Department. Experiments were carried out on clear days during Sept. and Oct. 2007. Fig. 1 shows different components of the hybrid desalination system. 2.1. Main solar still (MSS) This design is a single-basin still of 94.0 cm × 59.0 cm overall, with a basin area of 3600 cm2 (80.0 cm × 45.0 cm). High-side wall depth is 40.0 cm and the low-side wall depth is 17.0 cm (outside dimensions). The bottom and side frame was made and constructed from iron sheets. The inside basin (pan) of the still was made of galvanized steel sheets. The whole basin is coated with black paint from inside and outside for greater absorptivity and insulated from the bottom and side walls with fiber glass of 7.0 cm thick (Fig. 1). The main basin is covered with a glass sheet of 3 mm thick, which is inclined by 36° horizontally. A constant head tank of 20 × 20 × 15 cm is used to control the brine level of water inside the MSS by a float type regulating valve installed at the tank's inlet. A feeding cylindrical tank of 48,825 cm3 was used to compensate the still water on a daily basis. The still is operated on a batch basis with fresh feed water introduced to the main basin on a daily basis. 2.2. Wind turbine (WT) Wind turbine is a device for converting wind energy into mechanical (windmill) or electrical energy. The main purpose of the WT is to utilize wind induction to operate a horizontal rotating hollow shaft of 1.8 cm diameter located at the middle of the MSS. The shaft is made from aluminum, fixed close to the water surface and provided with four impellers. The turbine is consisted of 3 cups (semi-stainless steel hollow balls of 15.5 cm diameter each, light in weight which fixed on the top of vertical steel bar of 1.0 cm diameter and 43 cm height). The semi-balls rotate horizontally on a vertical pivot bar making a rotor diameter of 44 cm. Two bevel gears of 25 teeth each are used to transmit the horizontal rotation of the vertical bar to vertical rotation on the horizontal impeller shaft as shown in Figs. 1 and 2(a). Bevel gears have conical faces that operate on intersecting right axes with a 1:1 ratio. The vertical and horizontal bars are supported by ball bearings, and care is taken to ensure that the turbine can work with low wind velocity. 2.3. Inclined solar water distillation (ISWD) The ISWD consists mainly from an absorber plate, water distribution pipe and a glass cover that creates a cavity. The cavity length, width, and height are 40 cm, 40 cm, and 10 cm, respectively. The absorber plate is made of galvanized sheet, and painted to form a matt black surface (absorptivity of about 0.96 and, emissivity of about 0.08). A black-wick approximately 2-cm-long is laid on the absorber plate in order to distribute water evenly on the plate and to increase the thickness of the water film as shown in Fig. 2(b). Also, care is taken to make the water distribution pipe horizontal to assure even water distribution. The absorber plate is insulated from the bottom by a fiber glass of 7 cm thick to prevent heat losses. The cavity is covered with 3mm glass (transmissivity of about 0.88). The system is inclined to an angle of 38° in order to make the dripping water run down on the

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Fig. 1. Photograph of different components of the hybrid desalination system.

absorber plate and to increase the amount of solar radiation reaching the surface aperture more frequently during the day. The feed water comes from the main solar still (MSS) that heated by direct solar radiation and goes into a distribution pipe of 2.54 cm diameter, which has longitudinal holes of 1 mm each. The rate of water flow is adjusted by a control valve. The water then falls through these holes onto the black absorber plate or onto the black-wick, creating a layer of water all over the absorber plate. Solar energy warms the absorber plate. Some of the water evaporates and condenses as it touches the cool glass cover. The condensate flows into a condensate channel and is taken out from the side of the cavity. The rest of the feed water, which is hot water, flows into another collection channel downward the ISWD called the remaining water channel. The hot water is taken out from the bottom center of the remaining water channel and goes to an intermediate tank for possibility of recycling by the dc water pump or for domestic use. The fresh water and hot water are collected in separate tanks. 2.4. Factors and variables of the experiment The storage effect was studied by filling the basin of the solar still with water at different depths: 0.01, 0.02 and 0.03 m. Water flow rates

from MSS to ISWD are 25.0, 41.7 and 58.3 ml/min. It is decided to run the experiments with low water flow rates because the initial temperature of the water is low, in addition to smaller area of the ISWD, meanwhile this flow rates can be increased as the area increased. The hybrid system was tested in two positions as due south and tracking the sun. The hourly variation of solar intensity, wind velocity, water, glass and ambient temperatures, and hourly output for different depths and flow rates of water in solar stills were used to evaluate average values of each parameter.

2.4.1. Daily still performance The following parameters were measured at hourly interval for different depths and different modes of experimentations: ambient air, brackish water (input), hot water, glass and vapor temperatures of MSS as well as ISWD were recorded with the help of calibrated K type thermocouples and a digital multimeter. Total insolation is measured on horizontal, and on the same level of MSS and ISWD glass covers with the help of a Datalogging solar power meter. Wind speed is measured with the help of a van type anemometer. The data are then handled on a PC. The distilled water is measured by a 250 ml graded cylinder with an accuracy of 5 ml.

Fig. 2. Photographs of (a) wind turbine and (b) inclined solar water distillation.

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The performance of a collector is determined by its efficiency (η) defined as the ratio of the amount of heat collected by the collector fluid (water) to the amount of solar radiation intercepted by the absorber plates of the collector. This efficiency can be expressed as [19]: η = 0:76−4:36

  0:5ðTiw + Tow Þ−T1 Ins

ð1Þ

where Tiw and Tow are the collector inlet and outlet water temperatures (°C), T1 is the ambient temperature (°C) and Ins is the insolation (W/m2) on the absorber area. 2.5. Water quality A number of tests, e.g., physical–chemical tests, bacteriological tests are carried out on a number of samples obtained from the hybrid desalination plant. In these experiments, the city main water (about 16–23 °C) served as the feedstock. 3. Results and discussion 3.1. Effect of water depth Figs. 3–5 show the comparison between the hourly variation in water productivity from MSS and ISWD, ambient and water tempera-

Fig. 4. Comparison between variations of water productivity as affected by ambient and water temperatures, for 0.02 m water depth and 25 ml/min flow rate with tracking the sun.

Fig. 3. Comparison between variations of water productivity as affected by ambient and water temperatures, for 0.01 m water depth and 25 ml/min flow rate with tracking the sun.

tures for 25 ml/min water flow rate at water depths of 1, 2 and 3 cm, respectively, with respect to time of the day when the system is tracking the sun. The corresponding results when the system was due south are summarized and presented as average daily in Table 1. It was found that the distilled water changed significantly with different measured parameters. The water productions were increased from zero value in the morning and reached the maximum values at noon. This increase in water productivity would arise due to high insolation heating with the consequent low heat transfer from the solar stills to the ambient, and a high ambient temperature. The low heat transfer from the still refers to the well wall insulations. For this reason, minimum distilled water was recorded in the mornings when the water has not yet been heated up, while maximum productivity was recorded at noon. Also, the higher water production was observed after noon in comparison with that before noon. The reason for that may be due to the low temperature of the water that feed to the system at early morning which needs time to warm up. From these figures it is clear that the hourly water temperature and yield are also a strong function of water depth. It is clear that increasing water depths at the same flow rate caused a decrease in the distilled water productivity due to a decrease in water temperature. It was observed that, the designed glass angles (inclination) of MSS and ISWD systems have improved the desalination process, since the condensate water run (downward) fast towards the separator. Hence it does not affect much the fallen incident solar radiation on the still surfaces. In MSS provided with wind turbine, the fresh water productivity is increased due to an increase of water evaporation and

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3.2. Effect of water flow rate Experiments were carried out for several days to study the effect of water flow rate on the fresh water productivity for both modes of operations. For example, Figs. 3, 6 and 7 show the comparison between different water flow rates at the same water depth (0.01 m) when the system is tracking the sun. It was found that, increasing the water flow rates at the same water depth caused a decrease in the distilled water. The best performance of the system was achieved at 0.01 m water depth and 25 ml/min flow rate. Also, with tracking the sun the distilled water is increased in comparison with due south. 3.3. Effect of operating mode

Fig. 5. Comparison between variations of water productivity as affected by ambient and water temperatures, for 0.03 m water depth and 25 ml/min flow rate with tracking the sun.

condensation, since the rotating shaft (impellers) breaks the boundary layer of saline water. Also, the wind turbine during operation caused a little vibration of the system, hence the condensate water run fast downward towards the outlets. The system is performed well when the wind velocity remains in the range up to 2.0 m/s, where it does not affect much the insolation and temperature.

Table 1 shows the average daily values of distilled water output from MSS and ISWD as affected by insolation (Ins), temperatures and wind velocity (WV) with respect to time of the day during the experimentations for both modes of operation. The analysis of recorded data indicated that, the water output of desalination system changed significantly with insolation and temperature. The average daily insolation (Ins) measured on horizontal, MSS and ISWD ranged from 2.91 to 4.06, 4.012 to 5.82 and 4.47 to 6.01 kWh/m2/d, respectively when the system was due south. While for tracking the sun, the insolation ranged from 3.01 to 5.12, 4.16 to 5.75 and 4.61 to 6.26 kWh/m2/d, respectively. The average daily ambient air temperature and wind velocity ranged from 18.1 to 24.1 °C and 0.5 to 4 m/s, respectively, during the experimentation days. The average daily hot water temperature ranged from 44.4 to 55.4 °C for both modes of operations. The temperature of hot water is good enough for domestic usage. The comparisons between the accumulative variation of fresh water productivity for the two modes of operations (MSS and ISWD) as affected by water depths, flow rates are depicted in Figs. 8 and 9. It was found that the amount of fresh water from ISWD is lower than that of MSS this is due to the smaller area of ISWD in comparison with MSS. With respect to the square meter the amount of fresh water from the ISWD could be higher than the MSS with about 26.55 to 29.17% when the system is due south, while it could be as high as 27.1 to 32.93% when the system is tracking the sun. The reason for that is that the water temperature fed to the ISWD is higher than the water fed to the MSS. The developed system could produce fresh water of about

Table 1 Average daily distilled and hot water produced by the hybrid system at different conditions and parameters. Water depth, m

South facing

Tracking the sun

0.01 0.01 0.01 0.02 0.02 0.02 0.03 0.03 0.03 0.01 0.01 0.01 0.02 0.02 0.02 0.03 0.03 0.03

Water flow rate, ml/min

25 41.7 58.3 25 41.7 58.3 25 41.7 58.3 25 41.7 58.3 25 41.7 58.3 25 41.7 58.3

Insolation, W/m2

Temperature, °C Ambient

Main solar still (MSS)

Inclined solar water distillation (ISWD)

T1

T2

T3

T4

T5

T6

T7

InsH

InsMSS

InsISWD

22.4 23.3 22.3 20.4 24.1 20.9 23.0 19.9 22.3 24.0 23.0 22.4 24.0 22.3 22.3 20.9 18.1 19.8

51.1 54.3 51.8 51.3 47.0 48.3 53.9 47.5 50.5 56.8 51.6 54.9 57.3 55.0 49.8 55.3 49.1 49.4

48.9 51.1 51.3 47.5 46.0 46.4 53.4 45.8 49.0 55.5 51.9 51.9 56.3 50.6 47.1 50.1 47.1 47.3

39.1 40.1 38.3 38.3 37.1 34.3 40.5 33.5 37.0 44.6 40.1 41.1 44.4 38.8 36.1 40.8 32.5 34.1

46.5 51.6 49.4 50.6 43.1 43.3 53.1 48.6 49.3 55.8 50.9 51.6 55.8 53.8 46.3 56.8 42.0 47.8

46.1 52.1 48.4 47.4 44.4 44.6 53.3 45.5 47.4 55.0 51.0 51.8 55.4 54.8 48.3 50.1 44.9 45.9

37.0 40.8 36.5 38.0 34.5 31.8 40.8 33.4 36.0 44.0 37.9 38.0 43.8 37.8 34.0 39.9 28.5 32.1

477.5 522.4 579.3 516.5 493.1 415.4 534.6 506.4 525.3 543.0 491.6 527.5 564.5 535.4 429.9 732.1 641.0 621.5

617.1 659.3 831.1 701.6 588.4 573.1 679.0 766.9 657.5 666.9 602.8 716.0 699.3 784.6 594.6 813.4 821.0 801.6

658.5 710.1 858.4 791.8 638.3 639.4 725.9 770.1 715.0 720.1 660.6 763.4 749.9 836.9 658.3 894.3 893.4 843.9

Wind velocity, m/s

Water productivity (MSS), ml

Water productivity (ISWD), ml

Remaining hot water, ml

ηMSS, %

ηISWD, %

1.7 1.1 2.2 1.3 1.5 2.9 0.7 2.7 1.2 0.8 2.5 2.9 0.5 1.1 2.9 1.1 4.0 2.2

1235 1157 1104 1173 1125 1064 1090 1034 905 1410 1261 1219 1346 1283 1185 1304 1174 1158

709 661 623 670 638 599 617 583 509 826 745 692 785 736 670 758 673 654

9795 16,856 23,865 9834 16,878 23,889 9885 16,935 23,979 9675 16,772 23,798 9718 16,781 23,818 9746 16,844 23,836

67.7 68.1 69.1 68.2 69.6 67.2 67.4 69.2 68.1 67.2 66.8 67.9 67.3 68.9 67.9 68.8 68.6 69.0

59.4 58.6 62.0 61.1 61.6 59.2 57.8 61.4 60.2 57.1 57.2 59.2 57.5 60.2 59.2 61.8 62.4 62.0

T1 = ambient air temp. T2 = MSS air temp. T3 = MSS water temp. T4 = MSS glass temp. T5 = ISWD air temp. T6 = ISWD water temp. (Hot water temp.) T7 = ISWD glass temp.

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Fig. 6. Comparison between variations of water productivity as affected by ambient and water temperatures, for 0.01 m water depth and 41.7 ml/min flow rate with tracking the sun.

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Fig. 7. Comparison between variations of water productivity as affected by ambient and water temperatures, for 0.01 m water depth and 58.3 ml/min flow rate with tracking the sun.

3.5. Water quality test 2.51 to 3.43 and 3.18 to 4.43 l/m2/d from MSS and ISWD, respectively, when the system is due south. While it could produce fresh water of about 3.22 to 3.92 and 4.09 to 5.16 l/m2/d from MSS and ISWD, respectively, when the system is tracking the sun. In this study, the tests were performed for 7 h (from 9.00 am to 4.00 pm). If we consider the day-time from sunrise to sunset, daytime will increase to 10, even 12 h for a day in the summer conditions. It should be noted that the desalination process continued till late evening due to thermal storage, but this amount of water is not considered for the present calculations. Further improvement to the standard solar still configuration can be added, as suggested by some works found in the open literature. The most feasible ones could be the use of nocturnal production in the solar still [20]. It is expected that as the hot remaining water recycled the output distilled water will increase. Also the developed system could produce more water during the hot summer season compared with winter season. 3.4. Solar still efficiency The average daily efficiency of MSS ranged from 67.21 to 69.59 and 66.81 to 69.01% when the system was due south and tracking the sun, respectively. While it ranged from 57.77 to 62.01 and 57.08 to 62.38% for ISWD when the system was due south and tracking the sun, respectively (Table 1). The ISWD efficiency is lower than that of MSS due to the higher temperature of water input to the ISWD. Also, lower efficiencies of the stills were recorded in case of tracking mode compared with due south mode and this refers to the higher temperature recorded with tracking.

Water analyses are done at the Department of Environmental Science and Engineering, Tsinghua University and the results are given in Table 2. From the presented results it was found that the water quality lies in the acceptable range according to [21,22]. The hot remaining water is in good conditions that can be either useful for domestic usage or for recycling through the desalination system. 3.6. Cost analysis of the solar still plant In order to make an assessment of the cost effectiveness of a proposed plant of hybrid solar stills, a simple cost analysis was conducted. If C is the capital cost of the system and CRF is the capital recovery factor, then the first annual cost of the system, can be determined by the following formula: Ac = C × CRF

ð2Þ

where CRF =

rð1 + rÞn ð1 + rÞn −1

ð3Þ

where r is the interest rate (assumed to be 8%) of landing banks and n is the life of the system (assumed to be 10 years). The salvage value of the system is considered as 50% cost of usable material (with the exception of glass sheets where 25% of the cost is considered) saved even after the system life is over. If S is the salvage value of the system, the first annual salvage value (As) can be determined as: As = SðSFF Þ

ð4Þ

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Fig. 8. Comparison between water accumulative variation as affected by water depths, flow rates for MSS when due south (S) and tracking the sun (T). f1, 2, 3 = flow rates of 25, 41.7 and 58.3 ml/h, respectively. Fig. 9. Comparison between water accumulative variation as affected by water depths, flow rate for ISWD when due south (S) and tracking the sun (T). f1, 2, 3 = flow rates of 25, 41.7 and 58.3 ml/h, respectively.

where SFF =

r ð1 + rÞn −1

ð5Þ

To make the hybrid desalination system perform well it requires some maintenance, and the annual maintenance cost should considered. For this system, maintenance is required frequently due to the following reasons: (a) continuous water supply into the stills, (b) replacement of broken or damaged parts, (c) cleaning of solar stills, and (d) checking the wind turbine bearings and lubricating gears. Keeping this in view, the annual maintenance cost has been taken as 12% of the first annual cost (Ac). Therefore, actual annual cost of the system is given by:

The total estimated cost of the hybrid solar still pilot plant is 3000 Chinese RMB (1 US $ = 7.43 RMB) including 12% labor charges, without land values, as land cost is insignificant for remote areas or the system can be set on the roof of the residence building. Based on the above mentioned assumption, the calculated costs are summarized in Table 3. Since the current developed system can provide the consumer with hot water as well as distillate water, therefore the total electricity annual savings should be considered and this could minimize the annual cost of the system. The actual load is calculated as the power required for heating up main water to the specified hot water temperature. If m is the

Annual cost = First annual cost + Annual maintenance cost −Annual salvage value

ð6Þ

If the annual yield of the system is Y, then Product cost per liter = Annual cost = Y 2

ð7Þ

Annual cost per m = Annual cost = Area of system

ð8Þ

Yield per RMB = Annual yield = Annual cost

ð9Þ

Assuming that the average sunny days in Beijing is about 240 d (the sunny days could be 300 d or even more in some tropical countries); average daily distilled water is 2.5 and 3 l/day when the system is due south and tracking the sun, respectively.

Table 2 Water quality characteristics in the pilot waterworks during the study. Items

Input water

Distilled water

Remaining water

pH Turbidity, NTU Dissolved organic carbon, mg/l Conductivity, µS/cm Total dissolved solid, mg/l Salinity, % Total bacteria, cfu/ml Ca, mg/l Cu, mg/l Fe, mg/l K, mg/l Mg, mg/l Mn, mg/l

8.09 0.975 3.37 586 317 0.3 27 19.29 0.003 0.006 8.277 90.41 0.001

6.78 0.204 2.18 13.29 6.5 0 10 1.396 0.003 0.005 0.4871 0.3751 0.011

7.75 0.618 0.14 563 304 0.4 49 50.27 0.010 0.022 2.118 29.2 0.006

M.A. Eltawil, Z. Zhengming / Desalination 249 (2009) 490–497 Table 3 Average estimated costs of the hybrid developed desalination system. Item

Cost, RMB (1 US $ = 7.43 RMB)

Capital cost Annual cost Product cost per liter

3000 (403.77 US $) 397.2 0.662 for due south and 0.552 for tracking 763.85 1.51 for due south and 1.813 for tracking 101.52 195.22

Annual cost per m2 Yield per RMB Electricity annual savings, RMB/kWh Electricity annual savings, RMB/kWh/m2

mass flow rate of refill water (0.695 × 10− 3 kg/s as average) and Th is the required hot water temperature (45 °C), both specified by the user, then the power required (Qload) to heat the fresh water (kW) to the intended temperature is expressed as: Q load = Cp mðTh −Tc Þ

ð10Þ

where: Cp is the specific heat capacity of water (4.18 kJ/kg °C), Th is the required hot water temperature and Tc is the cold or main water temperature (19 °C). The heating process takes place at the daily peak period (9:00 am to 4:00 pm). The average energy required to heat the water to the required temperature is found to be 0.529 kWh. The cost of electricity is about 0.8 RMB/kWh (according to Beijing rate), therefore the average electricity annual savings by the developed system could be 101.52 RMB/kWh, while it could be 195.22 RMB/kWh/m2. 4. Conclusions – Solar distillation presents a promising alternative for saline water desalination that can partially support humanity's needs for fresh water with free energy, simple technology and a clean environment. Technologically, desalination is a proven, effective mechanism for providing a new source of water, particularly in rural areas. – It was found that increasing water depths at the same flow rate caused a decrease in the distilled water productivity due to a decrease in water temperature. – The well designed glass angles (inclination) of MSS and ISWD systems improved the desalination process, since the condensate water run (downward) fast towards the separator; hence it does not affect much the fallen incident solar radiation on the still surfaces. – The amount of fresh water per square meter from the ISWD could be higher than the MSS with about 26.55 to 29.17% when the system is due south, while it could be as high as 27.1 to 32.93% when the system is tracking the sun. – Using wind turbine with the system has two advantages, and they are: i) Operating the rotating shaft (impellers) breaks the boundary layer of saline water in MSS, and the fresh water productivity is increased due to an increase in water evaporation and condensation. ii) The wind turbine during operation caused a little vibration of the system, hence the condensate water run fast downward towards the outlets. – The system is performed well when the wind velocity remains in the range up to 2.0 m/s, since it does not affect much the insolation and temperature. – The average daily efficiency of MSS and ISWD ranged from 67.21 to 69.59 and 57.77 to 62.01% when the system was due south, while it ranged from 66.81 to 69.01 and 57.08 to 62.38% when the system was tracking the sun, respectively.

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– The annual cost of the developed system is about 397.2 RMB while, the water product cost is found to be 0.662 and 0.552 RMB/l when the system was due south and tracking the sun, respectively. The electricity annual savings of the hybrid system is found to be 101.52 RMB/kWh. – The distilled water quality as well as hot remaining water is good enough for domestic usage. – Also, it is expected that as the hot remaining water recycled the output distilled water will increase. – The developed solar distillation system has demonstrated its suitability for the desalination process when weather conditions are suitable and demand is not too great.

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