Desalination 332 (2014) 1–6
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Experimental investigation of a two-stage solar humidification– dehumidification desalination process M. Zamen a,b, S.M. Soufari a, S. Abbasian Vahdat a, M. Amidpour b,⁎, M.A. Zeinali a, H. Izanloo c, H. Aghababaie d a
Iranian Institute of Research and Development in Chemical Industries (ACECR), Karaj, Iran Department of Mechanical Eng., K.N. Toosi University of Technology, Tehran, Iran Research Center for Environmental Pollutants and Department of Environmental Health Engineering, Health Faculty, Qom University of Medical Sciences, Qom, Iran d Qom Payame Noor University, Qom, Iran b c
H I G H L I G H T S • • • • •
A multistage HD process was investigated to improve efficiency of HD desalinator. 2-Stage process is the best choice from cost and performance point of view. A 2-stage solar HD desalination unit constructed and tested in an arid area Production is presented and compared during hot and cold days. Improvement in productivity and reduction of solar collector area were obtained.
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Article history: Received 3 May 2013 Received in revised form 7 October 2013 Accepted 18 October 2013 Available online xxxx Keywords: Solar desalination Humidification–dehumidification Two-stage process
a b s t r a c t This paper experimentally evaluates a two-stage technique to improve the humidification–dehumidification process in fresh water production from brackish water. According to modeling results of multi-stage process and on the basis of construction cost estimation, using a two-stage process is the most suitable choice that can improve important parameters such as specific energy consumption, productivity and daily production per solar collector area and thus, investment cost. A pilot plant was designed and constructed in an arid area with 80 m2 solar collector area to evaluate the two-stage process. This unit was tested on cold and hot days. The effect of main parameters on fresh water production of the unit is studied. Experimental results show that two-stage HD desalination unit can increase heat recovery in condensers and hence, reduce thermal energy consumption and investment cost of the unit. Moreover, productivity can be increased by 20% compared with the singlestage unit. © 2013 Elsevier B.V. All rights reserved.
1. Introduction With diminishing water resources, the problem of drinking water seems to be the most important issue in the near future. It is predicted that more than half of the world's population will suffer from water shortage by 2025 [1]. One of the best solutions to this problem is brackish/seawater desalination. Air humidification–dehumidification (HD) desalination is a suitable choice for producing fresh water when demand is decentralized. Conventional desalination methods such as MSF (Multi-Stage Flash), ME (Multi-Effect), VC (Vapor Compression) and RO (Reverse Osmoses) are suitable for large and medium capacities of fresh water production. But most remote arid areas need low capacity desalination systems. ⁎ Corresponding author at: Energy system Engineering Department, Mechanical Engineering Faculty, K.N. Toosi University of Technology, No. 15 Pardis Street, Mollahsadra Ave., Tehran, Iran. Tel.: +98 21 88677272; fax: +98 21 88677273. E-mail address:
[email protected] (M. Amidpour). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.10.018
Simplicity of the process and availability of high levels of solar energy in most of the areas that need fresh water make this system suitable for these areas. Most of investigations on HD desalination process are concerned about productivity and efficiency improvement and have been done after 1990 and several units have been built which are similar in base but different in type of equipment. Farid and Hamad [2] constructed an HD desalination unit in Basrah, south of Iraq. The unit produced 12 l/day·m2 of solar collector surface which was about three times the production of a single-basin solar still under similar solar conditions. However, the pressure drop in the condenser and the humidifier was too high, increasing the electrical power consumption by the blower to a level that makes such a process uneconomic. Then, two units of different sizes were constructed and operated in Jordan described by Farid et al. [4] and Al-hallaj et al. [3]. They found that the effect of water flow rate on heat and mass transfer coefficient is more significant than air flow rate.
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Ben Bacha et al. [5] presented a perfect study including modeling, simulation, and experimental validation of a solar HD called SMCEC (Solar Multiple Condensation Evaporation Cycle). They concluded that perfect insulation of the unit, high water temperature and flow rate at the entrance of the evaporation tower, low temperature of water at the entrance of the condenser and hot water recycling by injection at the top of the evaporation tower can improve operation and production of the system. To consider the simultaneous effect of parameters, Hou et al. [6] used pinch technology to optimize the performance of the HD process. They maximized condenser heat recovery through composite curves. They found that there is an optimum value for water to air flow rate ratio (L/G). But the effects of humidifier inlet temperature and solar collector efficiency were not studied. In another study, Hou [7] evaluates a twostage solar HD desalination process using pinch technology. A global optimization of the HD process was developed by Soufari et al. [8]. Effects of different parameters were analyzed and a mathematical programming model was presented to optimize the process with different objective functions. Then, the model was developed by adding the solar part and finally a low-cost design for solar HD desalination was obtained [9]. In the next step, a unit with the capacity of 10 l/hr, which has been located at the Iranian Research and Development Center for Chemical Industries (IRDCI), Karaj, Iran, was constructed according to optimization results [10]. In continuation of the previous work, Zamen et al. [11] developed a mathematical model to study the effect of multi-stage technique on process parameters. Their results show that multi-stage technique has good improvements in comparison with the single-stage technique. They concluded that 2stage unit will be the best choice for a multi-stage HD desalination unit. McGovern et al. [12] investigated the effect of air extraction– injection on the performance and energy recovery of an HD system through developing the saturation curve and pinch methodology. They found that a two-stage system would be improved by decreasing the temperature range of the cycle and pinch point temperature. In this paper, experimental results of a two-stage desalination unit with solar collectors designed and constructed on the basis of mathematical modeling and optimization will be represented.
2. Process description The single-stage HD unit is distillation under atmospheric condition by an air loop saturated with water vapor, and has three main sections: Humidifier, dehumidifier and heat source as shown in Fig. 1. First, salty water heated by an external heat source such as solar collectors (points: 2–3 on Fig. 1) enters to the humidifier section. In the humidifier (points: 3–4 on Fig. 1) hot water is in contact with air in a packed bed and a certain quantity of vapor is extracted by air (points: 5–6 on Fig. 1). Then, hot humid air leaves the humidifier and enters to the dehumidifier. In this section, water vapor is distilled by bringing the humid air in contact with a cooled surface which causes condensation of vapor in the air and production of fresh water (points: 6–5 on Fig. 1). Generally, the latent heat of condensation is used for preheating the salty feed water (points: 1–2 on Fig. 1). As mentioned in the previous study [11], the process can be improved by increasing energy recovery between air and hot water and consequently reducing external heat requirement. This can be achieved by reducing minimum temperature approach through dividing air loop to some smaller loops where each loop may be considered as a single stage. Based on this fact, Fig. 2 shows the new process with a number of closed air loops named multi-stage HD desalination. Feed salt water enters to the first stage (right hand side in Fig. 2) and after passing through the first condenser, enters to the second stage and so on. Salt water temperature increases after each stage. The outlet of the last stage enters to the solar water heater system and then, hot water enters to the last stage and cascades in lower stages
Solar Collector
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Fig. 1. A schematic diagram of a single HD process.
to exit from the first stage. Temperature of the hot water outlet could be controlled via the design of packed bed and condenser. The main reason for low thermal energy recovery rate of a singlestage HD unit is the shape of the saturated air curve. If the air enthalpy curve could be flattened, hot and cold water lines could be closer, and the thermal energy recovery rate could be higher. This issue is extensively discussed in the previous study [11]. The shape of air curve could be improved if the mass flow rate of air would vary. The mass flow rate of air could be changed for each stage and set to an optimum value. Hence, the heat recovery section is extended and energy consumption will be reduced. More details of this issue are presented in the previous study [11]. In this paper, at first, brief results from modeling the multi-stage process described in the previous study [11] are presented. Related equations used in previous studies for single-stage HD [8–10] are used for multi-stage process modeling. After that, experimental results of a 2-stage unit constructed in Qom, Iran will be presented. 3. Modeling results A multi-stage process with separated closed air loops and solar collector as heat source was selected as the base case. Main equations of each stage are based on heat and mass balance in humidifier and dehumidifier towers presented in [8]. Also, additional equations for solar collector and packing are used according to relations presented by Zamen et al. [9]. As shown in Fig. 2, the hot water outlet of each humidifier is the humidifier inlet of the next (lower) stage and the condenser outlet of each stage is the inlet of the upper stage. These conditions are inserted to the model as new constraints. By adding related constraints to the closed air cycle (such as top and bottom temperature limit and flow rate of air in each loop) and inlet and outlet conditions of hot and cold water between stages, the mathematical model of the system would be completed. The mathematical programming model is solved for one- to fourstage processes with the mentioned constraints to achieve the minimum energy consumption per kg of fresh water production. The temperature of the humidifier inlet water and cooling water entering the dehumidifier is considered to be 70 (°C) and 20 (°C), respectively.
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Solar Collectors
Third Stage
Second Stage
First Stage
Salt Water
Distillate Water
Drain Fig. 2. A simple multi-stage HD desalination unit (with 3stages).
Fig. 3a shows the effect of increasing the number of stages on minimum specific thermal energy consumption of the process and daily production per solar collector area. It is observed that increasing the number of stages from one to two reduces energy consumption by more than 35%. But, this reduction will be small for systems with more than two stages. The same effect will be seen in productivity (ratio of fresh water production to salty feed water) and total distillate water. As shown in Fig. 3b, daily production per unit solar collector area increases by more than 40% in the two-stage process, but this increment will be 4% and 1% for 3- and 4-stage processes, respectively. The reason why three-stage and higher processes contribute less to productivity is that the temperature of inlet water of each stage, which is the outlet of the previous stage, gradually decreases. The less the temperature of inlet water, the less the productivity of each stage. Generally when the number of stages increases, energy consumption per kg of fresh water, productivity and daily production per solar collector area are enhanced, but the main enhancement occurs from one- to two-stage process. On the basis of the mentioned modeling, more results are obtained that can be summarized as follows:
4. Pilot plant description A two-stage HD desalination unit utilizing solar collectors was designed to produce 500 l/day distillate water required for 100 people residing in an arid village near Qom. A simple process flow diagram of the two-stage HD is shown in Fig. 4. Salty water from domestic resources is pumped from feed water tank to the first and second condensers successively and heated to the required temperature of process in heat exchange with the hot water of solar collectors. Then, hot salty water is sprayed on the second humidifier packed bed. The outlet of this humidifier could be pumped either to the humidifier of the first stage or to the hot water storage tank (for daily demand of hot water). If the temperature of the outlet stream of the first humidifier is low enough, a section of that can be recycled to Tank-1 to reduce the amount of rejected water and also feed the water required. Two electrical fans with low and medium speeds facilitate air circulation in the second and first stages, respectively. The power consumption of these fans is 100 and 150 W, respectively that can be supplied by solar PV panels. Fig. 5 presents some pictures from the pilot plant constructed in an arid village in Qom, Iran.
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Daily production per collector area (L/d/m2)
Energy Consumption (kJ/kg)
- Stages with higher temperature have higher productivity. - In the multi-stage process, higher temperature stages need lower air mass flow rates. In other words, liquid to gas flow rate ratio (L/G) reduces with reduction of the humidifier inlet water temperature. In low temperature stages, therefore, a forced draft made by a fan may be required.
It should be noted that when the number of stages increases, the cost of desalination unit increases. According to the above explanations, the two-stage process seems to be the most suitable choice. And finally on the basis of these results, a two-stage HD desalination unit was designed and constructed in an arid village near Qom at the center of Iran.
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Fig. 3. Specific thermal energy consumption: (a) and daily production per collector area, (b) as a function of the number of stages.
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70°
Tank3 Solar Heater Tank 3
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PU-101 Fig. 4. Process flow diagram of the two-stage HD unit.
According to modeling results for the required amount of fresh water production and solar radiation of the selected site, dimension and capacity of equipments were obtained. The desalinator was constructed as two adjacent towers, each of which count as one stage that comprises of humidifier and dehumidifier sections. The humidifier consists of a polypropylene packed bed with a specific surface area of 240 m2/m3. Cross section area of the humidifier in the first and second stages is 0.3 and 0.5 m2, respectively. The height of both stages is equal to 2.4 m. The dehumidifier is a finned tube condenser with copper tube and aluminum fins with a heat exchange surface area of 30 m2 for both of the condensers. A small
low speed fan is placed at the top of the condenser of each stage (as showed in Fig. 4). The salty feed water was heated in a tank by solar heated water circuits. Solar collectors were divided into two sections, some directed to south and others to south–east to gain solar heat almost uniformly during all day and benefit afternoon sun heat. An automatic control system was used to control the operation of the unit according to changes in the temperature of hot water. When temperature reaches 70 °C in the morning, the system starts and when temperature drops under 50 °C, the system stops because of low production. The system is designed to work with various flow rates and a maximum of 1400 l/h. Digital
Fig. 5. Pilot plant of the two-stage HD unit constructed in Qom.
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Fig. 6. Storage tank temperature variation during a hot and a cold day.
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Inlet Hot Water Temperature (ºC) Fig. 8. Effect of inlet hot water temperature on production rate (water flow rate: 1350 lit/h).
thermocouples are used in various points (as depicted in Fig. 4) for measuring temperature and two rotameters are used for feed line of each stage. Flow rate is regulated manually by means of a regulating valve in the entering line of the second stage. 5. Experimental results The pilot plant was tested in a certain period of time including summer (hot) and winter (cold) days. More than 200 tests were performed starting in the morning and continuing to evening. Examining the performance of setup for each test takes one day in real conditions of various days and in each test, about 15 parameters of water and air streams were recorded. Therefore, more than 3000 data was gathered during the test period on hot and cold days. In early hours of the day, solar collectors heated the water of storage tank. Desalination unit starts when water in the storage tank reaches an appropriate temperature and fresh water production begins. In summer, production can start early in the morning before 11 o'clock when the hot water temperature of the tank reaches 70°C. But in winter, it takes longer till about 12 o'clock or even later. The temperature variation of the storage tank for a hot and a cold day are shown in Fig. 6. The hourly production rate of the unit on winter and summer days is shown in Fig. 7. On both of these days, the unit starts at 11 o'clock. Because of higher solar intensity and length of hot days, the production rate on these days is higher than that on cold days. Fresh water production depends on hot water temperature of the storage tank and reduces when it drops. On hot days, the temperature of the storage tank does not change considerably during daylight hours and therefore,
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production is almost uniform during the operation of the unit, but on cold days, because of lower solar intensity and thus, reduction of the temperature of the storage tank, production sharply reduces after about 2 h from the start time. The total production of the unit during these two days was measured. On summer days, the total production of the unit is more than twice that of winter production and reached about 580 l/day. In this pilot plant, 80 m2 solar collectors were used and therefore, production reached 7.25 l/day·m2 solar collector that is about 40% higher than the previous pilot plant (Soufari et al., 2009 b). Hence, this result confirms the theoretical calculation and benefits of the twostage process in reduction of specific energy consumption and thus, in reduction of the initial investment cost. Fig. 8 shows the effect of the inlet hot water temperature on production rate. It can be seen that this temperature has an important effect on production rate. On summer days, the inlet hot water temperature may exceed 80 °C. This increases fresh water production. Also on winter days, this temperature has the most effect in decreasing fresh water production. The effect of inlet water flow rate is presented in Figs. 9 and 10 at the inlet water temperature of 70 °C. As shown in Fig. 9, increase in water flow rate increases the production rate. But, when water flow rate is more than 1200 l/h, its increasing rate will decrease. Also, change of productivity (ratio of the fresh water production rate to the inlet salt water flow rate) has been shown in Fig. 10 as a function of inlet water flow rate. It can be seen that an increase in
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Time Fig. 7. Hourly production rate on hot and cold days.
Fig. 9. Effect of inlet water flow rate on fresh water production (inlet water temperature: 70 °C).
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Production Rate (%)
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in using even two-stage process. Designing a suitable controller and using sufficient sensors such as level switches for storage tanks or thermocouples in various points of the system can help to decrease operational problems and increase the reliability of the unit. Cost of these instruments can be easily compensated with reduction of investment cost of solar collector area.
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Inlet Water Flow Rate (Lit/hr) Fig. 10. Effect of inlet water flow rate on productivity (inlet water temperature: 70 °C).
water flow rate decreases productivity. Productivity changes from 3.6 to 4.8 depending on water flow rate. On the basis of the last experimental results (Soufari et al. [10]), productivity of the single stage unit can reach 3.5–4% for inlet hot water temperature of 70 °C. Therefore, using two-stage process can enhance productivity by up to 20% depending on water flow rate. Fig. 11 presents the contributions of the first and second stages to total production of the unit in various salty feed water flow rates. As depicted in this diagram, more than 75% of the total production occurs in the stage with higher temperature (the second stage). Mass flow rate of air circulated in this stage is about half of that in the first stage which has lower temperature. Results show that 50% of the required thermal energy can be supplied by heat recovery in condensers to preheat the inlet feed water: 35% in the second stage condenser and 15% in the first stage condenser. Because of this, fewer heat sources are needed for the same production and that is why multi-stage system has higher productivity in comparison to single-stage unit. Since production is more than the desired value and the temperature of the outlet water of the second humidifier is more than 50 °C in summer, it may be possible to use this stream as hot water required in the village. Hence, this method can be used as a combined system for simultaneous production of fresh water and hot water. Total dissolve solids (TDS) of salty feed water in village was more than 4500 ppm and TDS of fresh water during the test period was less than 100 ppm. This shows the ability of this process in desalination of brackish water to produce potable water in arid area. Although using multi-stage process can decrease the investment cost of the system, more complexity of the system may be a problem
Salt water flowrate(lit/hr)
First Stage production
Second stage production
1350 1200 1080 780 720 0%
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Fig. 11. Contribution of each stage to the production of fresh water.
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The multi-stage HD process for desalination was introduced. Theoretical results show that important parameters of the process such as specific energy consumption, productivity and daily production per solar collector area improved when multi-stage process was used instead of single-stage process. But, this improvement is negligible when the number of stages is more than two. According to construction cost, a two-stage unit seems to be the most suitable choice. Hence, a two-stage pilot plant was designed and constructed in an arid area with 80 m2 solar collector. This unit was tested on cold and hot days. In summer, the total production of the unit is more than twice that of winter production and reached about 580 l/day. According to these results, fresh water production can reach 7.25 (l/day·m2) that was about 40% higher than single-stage unit tested in the previous study [10]. This led to reduction of required solar collectors and thus, reduction in investment cost. Also, productivity can reach 4.8% that is at least 20% higher than the rate obtained in the single-stage unit [10]. These results confirm the theoretical calculation and the benefits of the two-stage process compared with the single-stage process. Moreover, quality control of inlet and outlet water shows the ability of this process in desalination of brackish water in arid area to produce potable water. Acknowledgment The authors acknowledge the financial support provided for this work by the Rural Water & Wastewater Company of Qom province. References [1] C. Mertes, Seawater desalination as a chance for water supply, Water Desalination Technologies Seminar, Tehran, Iran, 2006. [2] M.M. Farid, F. Hamad, Performance of a single-basin solar still, Renew. Energy 3 (1993) 75–83. [3] S. Al-Hallaj, M.M. Farid, A.R. Tamimi, Solar desalination with humidification– dehumidification cycle: performance of the unit, Desalination 120 (1998) 273–280. [4] M.M. Farid, N.K. Nawayseh, S. Al-Hallaj, A.R. Tamimi, Solar desalination with humidification dehumidification process: studies of heat and mass transfer, Proceeding of the Conference: SOLAR 95, Hobart, Tasmania, 1995, pp. 293–306. [5] H. BenBacha, M. Bouzguenda, M.S. Abid, A.Y. Mallej, Modeling and simulation of a water desalination station with solar multiple condensation evaporation cycle technique, Renew. Energy 18 (1999) 349–365. [6] S. Hou, S. Ye, H. Zhang, Performance optimization of solar humidification– dehumidification desalination process using pinch technology, Desalination 183 (2005) 143–149. [7] S. Hou, Two-stage solar multi-effect humidification dehumidification desalination process plotted from pinch analysis, Desalination 222 (2008) 572–578. [8] S.M. Soufari, M. Zamen, M. Amidpour, Performance optimization of humidification– dehumidification desalination process using mathematical programming, Desalination 237 (2009) 305–317. [9] M. Zamen, M. Amidpour, S.M. Soufari, Cost optimization of a solar humidification– dehumidification desalination unit using mathematical programming, Desalination 239 (2009) 92–99. [10] S.M. Soufari, M. Zamen, M. Amidpour, Experimental validation of an optimized solar humidification–dehumidification desalination unit, Desalin. Water Treat. 6 (2009) 244–251. [11] M. Zamen, S.M. Soufari, M. Amidpour, Improvement of solar humidification– dehumidification desalination using multi-stage process, Chem. Eng. Trans. 25 (2011) 1091–1096. [12] R.K. McGovern, G.P. Thiel, G. Prakash Narayan, S.M. Zubair, J.H. Lienhard, Evaluation of the performance limits of humidification–dehumidification desalination systems via a saturation curve analysis, Appl. Energy 102 (2013) 1081–1090.
