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heat recovery processes
Renewable Energy 64 (2014) 26e33
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A novel integrated solar desalination system with multi-stage evaporation/heat recovery processes Zhen-hua Liu*, Ren-Lin Hu, Xiu-juan Chen School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2013 Accepted 25 October 2013 Available online
A novel small-sized integrated solar desalination system with multi-stage evaporation/heat recovery processes is designed and tested in this study. The system consists of four linked collecting units and operates under barotropic and atmospheric pressure. Each of the four units contains a seawater tank and at least one solar collecting/desalination panel mainly comprising a simplified CPC (Compound Parabolic Concentrator) and an all-glass evacuated tube collector. In the last three units, heat exchangers made of copper tubes are inserted concentrically into the all-glass evacuated tubes to recover heat. In each unit, an independent desalination process including solar collecting, heat recovery (no heat recovered in the first unit) and seawater evaporation can be carried out completely. The experimental results show that the freshwater field of the designed system can reach as high as 1.25 kg/(h m2) in the autumn and the system total efficiency is close to 0.9. Both experimental results provide a striking demonstration that the designed solar desalination system has outstanding performance in solar collecting, heat recovery and seawater evaporation. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Solar Desalination Heat recovery Collector
1. Introduction Water has always been mankind’s most precious resource. There are no substitutes and the struggle to control water resources has shaped human political and economic history. The amount of water deemed necessary to satisfy basic human needs is 1000 cubic meters per capita annually. By 2050, it is projected that the availability of potable water will fall below this level for 1.7 billion people in 39 countries [1]. On the other hand, about 97% of the earth’s water is salt water in the oceans, and a tiny 3% is freshwater. It would be feasible to address the water-shortage problem through seawater desalination [2]. Seawater desalination techniques are mainly divided into two categories, namely, the thermal processes, for which either multistage flash (MSF), multiple-effect boiling (MEB), vapor compression (VC), freezing or solar distillation are used [3e14] and the membrane processes, for which reverse osmosis (RO) or electrodialysis (ED) is applied [15,16]. In the thermal processes, the distillation of seawater is achieved by utilizing a thermal energy source. The thermal energy may be obtained from a conventional fossil-fuel source, nuclear energy or from a non-conventional solar thermal energy, geothermal energy, etc. [2,17,18].
* Corresponding author. Tel.: þ86 21 34206568. E-mail addresses: [email protected], [email protected] (Z.-h. Liu). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.10.040
In terms of energy consumption, desalination of seawater is an energy intensive process. The installed capacity of desalinated water systems in the year 2000 is about 22 million m3/day and the production of this water requires about 203 million tons of oil per year [2,19,20]. With the total installed capacity expected to increase drastically in the coming decades, the energy consumption for desalination will continue to rise and hence the required amount of conventional hydrocarbon fuels required will go up substantially [21]. Meanwhile, the world’s production of conventional hydrocarbons will soon decline. Hydrocarbon shortages are inevitable unless radical changes occur in demand, or in the supply of nonconventional hydrocarbons [22]. Renewable energy is the alternative solution to the decreasing reserves of fossil fuels [23]. At present, there are already several kinds of renewable energy applied to water desalination. Garcıa-Rodriguez presented an economic analysis of wind-powered RO desalination technology [15]. Marcos S. Miranda and David Infield used a 2.2 kW wind turbine generator powering a variable-flow RO desalination unit which was considered suitable for the application in remote areas [16]. Karim Bourouni et al. experimentally investigated a desalination plant functioning by aero-evapo-condensation. The work was carried out on a geothermal desalination plant installed in the south of Tunisia [24]. The use of solar energy in thermal desalination processes is one of the most promising applications of the renewable energies. In terms of solar thermal energy usage, solar desalination can either be direct; using solar energy to distillate directly in the solar
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Nomenclature ACPC h Ir Gm n PR P Q
F htol ht
lighting area of a CPC plate (m2) enthalpy (J/g) solar radiation intensity (W/mP2) freshwater yield per second (kg/s), number of the CPC plates in the system performance ratio net energy absorbed by the collector (W) power used to evaporate seawater in one unit (W) heat collecting power of one panel (W) system total efficiency system collecting efficiency
Subscripts I UnitI II UnitII III UnitIII IV UnitIV collector, or indirect; combining conventional thermal processes desalination techniques with solar collectors for heat generation [25]. Many different systems of direct and indirect solar desalination have been proposed and implemented. ArefY. Maalej studied the performance ratio and efficiency of a solar still operating under different conditions [3]. Fath reviewed the various designs of solar stills and studied the suitability of solar stills for providing potable water [4]. Xiao et al. categorized the solar stills into six sorts based on the design guidelines used in each device. The properties of these design guidelines were detailed and evaluated in terms of enhancing the productivity of solar stills [5]. Arunkumar et al. designed a tubular solar still with a rectangular basin for water desalination and studied the effect of cooling air and water flowing over the cover [6]. Nematollahi et al. conducted an experimental and theoretical energy and exergy analysis for a solar desalination system consisting of a solar collector and a humidification tower, the results showed that the overall exergy efficiency increased by a decrease in humidification tower length, a decrease in inlet air temperature, and an increase in tower diameter [7]. Reddy et al. developed a novel multi-stage evacuated solar desalination system utilizing latent heat recovery. The effect of various design and operating parameters on the system performance were studied to optimize the configuration and suggested that the designed multi-stage evacuated solar desalination system was a viable option to meet the needs of rural and urban communities [8]. Mohamed and El-Minshawy theoretically investigate the principal operating parameters of a proposed desalination system based on air humidificationedehumidification principles [9]. Rajvanshi proposed a scheme to desalinate seawater using solar energy. The seawater heated by solar energy was flash evaporated in a multistage flash evaporator (MSF) unit to yield freshwater. Economic analysis of the scheme showed that it compared favorably with the existing fossil fuel fired desalination plants of the equivalent capacity [10]. Zejli et al. designed a multi-effect desalination system operating with an adsorption heat pump with an open cycle and using zeolite as the solid vapor adsorbent. A theoretical model was used to study the water production and energy consumption of the system [11]. Hawlader et al. designed a single effect desalination unit connected to an existing solar assisted heat pump and conducted a series of experiments on the system under different operating and meteorological conditions of Singapore [12]. Abdalla Hanafi analyzed the design and performance of the solar MSF desalination systems and carried out a stage-to-stage design of the
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MSF chamber and the system transient performance [13]. Joseph et al. carried out an experimental study of a single stage solar desalination system. The input parameters such as solar irradiance and vacuum pressure in the flash evaporator were varied to find its influence on the system efficiency and yield of potable water per day [14]. Mahmoud Shatat et al. presented an economic and comparative evaluation study for a small scale solar powered water desalination system [26]. A. Eslamimanesh and M.S. Hatamipour made an economical study of humidificationedehumidification desalination (HDD) pilot plant in order to estimate the economic benefits of the process in comparison with a small-scale reverse osmosis (RO) system [27]. A.E. Kabeel and Emad M.S. El-Said presented a hybrid solar desalination system consisting of a humidificationedehumidification unit and single stage flashing evaporation unit and numerically studied the system [28]. Kyaw Thu et al. have made a detailed study on an adsorption desalination cycle with evaporatorecondenser heat recovery circuit [29e32]. In this paper, we put forward a novel small-sized solar desalination system which incorporates the solar collecting, seawater evaporation, heat recovery and freshwater condensation processes within the common evacuated tubular solar collector to produce freshwater directly. The system operates under barotropic and atmospheric pressure and adopts a stepwise heat recovery method to recycle the latent heat of the steam generated. It consists of four linked collecting units. Each of the four units contains a seawater tank and at least one solar collecting/desalination panel mainly comprising a simplified CPC (Compound Parabolic Concentrator) and an all-glass evacuated tube collector. In the last three units, heat exchangers made of copper tubes are inserted concentrically into the all-glass evacuated tubes for heat recovery. In each collecting unit, an independent desalination process including solar heat collecting, heat recovery (no heat recovered in the first unit) and seawater evaporation can be carried out completely. This novel solar desalination system is easy to assemble just like building blocks and can work with the absorption refrigerator to carry out the combined cycle of desalination and air-conditioning. The cost of manufacturing and running the designed system is low and its footprint is also small. Unlike the common multiple-effect boiling (MEB) process where the steam from one effect acts the role of heat source in the following effect, the majority of the heat used in each unit (effect) of the present system to evaporate the seawater comes directly from the solar energy collected by the all-glass evacuated tube, while the rest from the latent heat of the steam passing through the unit. Instead of using a vacuum pump to provide pressure gradient between each unit, the present system simply adopts several pressure regulating valves to control the working pressure which ranges from the atmospheric pressure to barotropic and neither vacuum pump nor delivery pump is needed. In the Multiple Effect Stack (MES) evaporator which is considered to be the most appropriate type for solar energy application, the thin seawater film evaporates or boils simultaneously on the outside of the tube bundles, the steam generated is likely to carry the seawater, while in the present system, the heat transfer mode is mainly the natural convection of the seawater immerged in the glass tube and the evaporation only occurs on the surface of the vaporeliquid, therefore the possibility of seawater carried by steam is smaller than that in MES system. Due to the temperature differences between the four units, the method of multi-stage heat recovery is applied to the system to fully utilize the sensible and latent heat of the steam generated. All these designs aim at simplifying the structure of solar desalination system, improving the thermal performance and increasing the freshwater yield. Though some manual operations such as brine draining are needed to run the system, this new type of solar desalination
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system with multi-stage evaporation/heat recovery process has fulfilled our expected goals and provides useful guidelines for future applications.
2. Experimental apparatus and procedure 2.1. Experimental apparatus Fig. 1 gives out a schematic view of the experimental system and Fig. 2 shows a photograph of the actual experimental apparatus before it is fully packaged with the insulation material. As is shown in Fig. 1, the experimental system mainly consists of four units linked together for solar collecting/desalination, a cooling pool (condenser) and a data acquisition system. The solar collecting/desalination units are the core component of the experimental system. The first unit (UnitI) contains a seawater tank, a pressure regulating valve and one solar collecting/desalination panel which consists of a simplified CPC (Compound Parabolic Concentrator), an all-glass evacuated tube and a steam outlet pipe. Since there is no heat recovery process happens in this unit, heat used to evaporate the seawater only comes from the solar energy. The second and third units (UnitII and UnitIII) have the same structure containing a seawater tank, a pressure regulating valve and one solar collecting/desalination panel which comprises a simplified CPC, an all-glass evacuated tube and a heat exchanger made of copper tubes mounted concentrically into the all-glass evacuated tube for heat recovery. The last unit (UnitIV) consists of a seawater tank, a pressure regulating valve and two parallel solar collecting/desalination panels mounted with heat exchangers which is the same as the one mentioned above. Since the mass of steam gradually accumulates when passing through these units, two panels are mounted in the last unit to provide more heat transfer area to recover heat. By such arrangement, the latent heat of the steam generated can
Fig. 2. A photograph of actual experimental apparatus.
be recovered almost completely in the last three units in order to evaporate the seawater outside the copper tubes. In each of the evacuated tubes, there is a steam outlet pipe to conduct the steam generated into the following unit. The bending head of the steam outlet pipe is inserted with a 30 mm long stainless steel mesh used as a vaporeliquid separator. A detailed view of a typical unit is shown in Fig. 3. In the present study, the cooling pool is used to fully condense the steam generated in the last unit. All the freshwater produced is collected in a graduated cylinder and the cumulative freshwater influx is measured every 15 min to get the mean freshwater yield. As this study mainly focuses on the solar collecting and heat recovery performances of the solar collecting/desalination units, the steam generated in the cooling pool has not been taken into consideration. The data acquisition system collects and processes data such as the solar radiation intensity, steam pressures, temperatures and
Fig. 1. A schematic view of the small-sized integrated solar desalination system.
Z.-h. Liu et al. / Renewable Energy 64 (2014) 26e33
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stainless steel mesh(200 mesh) steam
sea water locations of thermalcouples
steam outlet pipe Common concentric tube
simplified CPC
all-glass evacuated tube rubber seal
steam inlet locations of thermalcouples pressure/temperature sensor sea water inlet
pressure balance pipe steam outlet pressure insulation material regulating valve
Fig. 3. A detailed view of a typical collecting unit.
wall temperatures of the tubes, etc. Data are logged using Agilent 34970A data-logger connected to the computer. Fig. 4. Profiles of standard CPC and simplified CPC; (a) standard CPC (b) simplified CPC.
2.2. The simplified CPC structure CPC is capable of collecting the solar radiation for a long period during the day without tracking the sun. As a kind of highly efficient high temperature solar collector, CPC solar collector has been widely studied in the design and analysis [33e36]. In the present study, a CPC with a tubular absorber is designed by the method proposed by Khonkar and Sayigh [37]. The tubular absorber is an all-glass evacuated solar tube, which consists of two concentric glass tubes sealed at one end with an annular vacuum space and a selective absorbing coating layer on the outer surface (vacuum side) of the inner tube. Fig. 4(a) shows the standard CPC profile for an all-glass evacuated tube. For the purpose of easy manufacturing and cost reduction, the standard CPC is simplified with a flat curve bottom instead of the involute shape bottom. Meanwhile, the upper parts of concentrator are cut out to form a truncated CPC. Fig. 4 (b) shows a simplified CPC after truncation and cutting involute out. According to the calculated and test results [38], the concentrating efficiency of the simplified CPC with a flat curve bottom decreases by about 15% compared to that of the truncated CPC with an involute bottom. The simplified CPC is made of stainless steel mirror (2 mm thick, 330 mm wide and 146 mm high) welded directly on the steel shelf. For a CPC collector with a flat-plate absorber, the concentration ratio is defined as ratio of lighting area to flat-plate area. For the CPC collector with a tubular absorber, however, the concentration ratio is defined as ratio of lighting area to peripheral area of tubular absorber. According to this definition, the concentration ratio of the present CPC is 2.16. The characteristics of the present collector system in the present study are shown in Table 1.
2.3. Concentric tube heat exchanger In the present study, the system uses the concentric tube to recover heat from the steam generated. As being shown in Fig. 3, the concentric tube is made of two copper pipes welded at one end, the sizes of which are also shown in Table 1. The concentric tube is inserted concentrically into the inner glass tube and the annular space thus formed is filled with the seawater supplied by the seawater tank. The steam coming from the former unit enters the inlet of the concentric tube, goes into the annular section, then arrives at the inlet of the inner tube located at the top, and finally exits from the outlet of the inner copper tube. 2.4. Experimental procedure and experimental error In the present experimental system, the temperature or pressure of the saturated steam in each unit is controlled by the pressure regulating valve. All these valves are regulated only once at the time when the intensity of solar radiation is about 1 kW/m2 and then the highest temperature of the steam in each unit varies on their own without any further adjustment of the valves. Since this solar desalination system is expected to run all year round, the highest temperature being set should not be too high and be achieved even in the winter. Therefore, the highest steam temperature being set in UnitI is 130 C and decreased by 10 C at each following unit, so the steam temperature in UnitIV is 100 C corresponding to the atmospheric pressure. By preliminary adjustments, the maximum temperature difference between two neighboring units
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Table 1 Characteristics of the simplified CPC collector system. Parameter Simplified CPC Width of CPC Effective length of CPC Height of CPC r: radius of absorber g: distance of CPC cusp and absorber qA: acceptance half angle Reflectivity of CPC Concentrating ratio All-glass evacuated tube Diameter of inner glass tube Diameter of cover glass tube Concentric tube Diameter of outer copper tube Diameter of inner copper tube Effective length of concentric tube
Value 330 mm 1600 mm 146 mm 24 mm 6 mm 65 0.85 2.16 58 mm 47 mm 36 mm 24 mm 1500 mm
1. concentric tube, 2. all-glass evacuated tube, 3. simplified CPC, 4. seawater supply inlet, 5. valve, 6. seawater tank, 7. vaporeliquid separator, 8. pressure balance pipe, 9. steam outlet pipe, 10. pressure regulating valve, 11. seawater inlet pipe, 12. cooling pool, 13. cylinder, 14. freshwater tank, 15. solar radiation meter, 16. Agilent data-logger, 17. computer.
is set at 10 K. According to our former calculations, the concentric tubes are capable of recovering all latent heat of evaporation from the steam even at the temperature difference of 3 K. In the present study, the steam temperature/pressure, wall temperature at the center of inner glass tube, wall temperature at the outlet of concentric tube in each unit are measured to investigate the solar collecting and heat recovery performance of the unit. In each one of the all-glass tubes, four thermocouples located at the center (axial direction) of the inner glass tube are equally distributed along the circumference. The arithmetic mean value of the four detected temperatures is taken as the mean wall temperature of the glass tube. The same arrangement of another four thermocouples is applied to the concentric tube at the outlet of the inner copper tube to measure the wall temperature, which can be regarded as the fluid outlet temperature inside the concentric tube. A pressure/temperature sensor is inserted into the all-glass evacuated tube to measure the steam pressure and temperature in each unit. The locations of the thermocouples and the pressure/temperature sensors are shown in Fig. 3. All the thermocouples adopted in our experimental apparatus are calibrated by an accurate thermometer and the maximum measurement error is 0.2 K. Pressure sensors with accuracy class 0.5 are used. Radiation meter is used to measure the intensity of solar radiation with the measurement error less than 4%. Since this experiment is an initial study on the proposed original solar desalination system, deionized water is used instead of actual seawater and corrosion problem is not taken into consideration. In order to find out whether the water and the steam are well separated, the deionized water is dyed red before the preliminary test begins. According to our observations, the water collected in the graduated cylinder is clean demonstrating that no liquid is carried out by the steam and thus the vaporeliquid separators work well.
Fig. 5 shows the steam temperatures/pressures, wall temperatures of inner glass tubes, wall temperatures of concentric tubes of the four units on a sunny day. These data were collected on Nov. 13, 2012. Each of the Roman numerals labeled on the curves represents one of four units. As can be seen in Fig. 5, in the morning, deionized water stored in these units is being heated from a relatively low temperature to the saturated temperature, so there is no steam generation during this period and the pressure in each unit stays at the atmospheric pressure. Soon the pressures in the former three units rise sharply due to the generation of steam. At noon, steam temperatures and pressures tested in each unit are close to the values preset. Then, in the afternoon, the steam temperatures and pressures in the former three units begin to decrease gradually. Meanwhile, both the steam temperature y, and the steam temperature differences between two neighboring units are about 8 Ce9 C. These steam temperatures and their differences are quite close to the values preset. In the meantime, the steam temperature is 4 Ce5 C lower than the wall temperature of the inner glass tube and 2 Ce3 C lower than the wall temperature of the concentric tube in each unit. As mentioned above, the wall temperature of the concentric tube in the each unit can be considered as the fluid outlet temperature inside the concentric tube. As being shown in Fig. 5, this fluid outlet temperature is much lower than the steam temperature in the former unit even though the solar radiation changes in a wide range during the whole test period. This implies that the steam flowing pass each concentric tube has been completely condensed to the subcooled water when arriving at the outlet of the concentric tube. Since the pressure inside the concentric tube is very close to that in the former evacuated glass tube, the steam temperature in the concentric tube should also be close to that in the former evacuated glass tube. The fact shows that the present structure design of the solar collecting unit can provide enough heat recovery capacity to recover the latent heat of the steam generated. Fig. 6 shows the steam temperature/pressure, wall temperature of the inner glass tube, the wall temperature at the outlet of the concentric tube of each unit on a cloudy day. These data were collected on Nov. 6, 2012. The trends of the temperatures and pressures in the four units are quite similar to those on Nov. 13, 2012 as being shown in Fig. 5. The main difference is that all the steam temperatures and pressures in the cloudy day are somewhat lower than those in the sunny day. At noon, the highest steam temperature in each of the four units reaches 122 C, 114 C, 107 C and 101 C, respectively, and the temperature differences between two neighboring units are about 6 Ce7 C. These steam temperatures and the temperature differences are lower than the values preset. In the latter three units, however, the wall temperatures of
3. Experimental results and discussion The outdoor experiments were carried out in the autumn and winter in 2012 and a few representative experimental results are shown in this paper. Considering that the deionized water accumulated in the system may exert bad influence on the experimental accuracy, each test was carried out continuously for two days and the data collected on the second day are used for analysis.
Fig. 5. Temperature and pressure values of each unit in a sunny day.
Z.-h. Liu et al. / Renewable Energy 64 (2014) 26e33
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the heat recovery effect of the system, since the total energy also includes the energy recovered from the steam generated and it equals 1 if there is no heat recovery effect.
PR ¼
Fig. 6. Temperature and pressure values of each unit in a cloudy day.
the concentric tube are still much lower than the steam temperatures in the former units throughout the whole test period. It again confirms that steam flowing pass the concentric tube has been completely condensed to subcooled water when arriving at the outlet of the concentric tube. Fig. 7 shows the freshwater yield per hour per square meter of the lighting area in 7 test days. Here, the lighting area of each panel is equal to the product of effective length and width of the CPC. The total lighting area of the present system is 2.64 m2, the projection of which on the horizontal plane represents the floor area of the collector. As can be seen in Fig. 7, the highest value of freshwater yield exceeds 1.26 kg/(h m2) on a sunny day in the autumn and reaches 0.7 kg/(h m2) even though it was cloudy. Considering that these experiments were conducted in October and November, it is reasonable to predict that the freshwater yield will be doubled in the summer. These results demonstrate that on the basis of well solar collecting and heat recovery, the present solar desalination system shows an outstanding performance of producing freshwater and provides a solid foundation for future applications. It should be emphasized that the freshwater used to calculate the yield is only generated in the four units, while the freshwater generated in the cooling pool has not been added. If the cooling pool were also designed as a still device, the total freshwater yield would increase further. As another important performance factor to evaluate the desalination system, the performance ratio (PR) is defined as the ratio between the total energy used to evaporate saline water into freshwater and the net energy absorbed by the collector. PR reflects
(1)
where,Gm is the freshwater yield per second, Dh is the enthalpy difference between subcooled water and the saturated steam, and P is the net energy absorbed by the collector. The PR can be bigger than 1 due to the heat recovery effect and the bigger the value is, the more the heat is recovered. The theoretical value of PR is determined by the system design. In the present system, the theoretical value of PR is 2.2 and is calculated as follows: Assuming that the collecting power of each panel is the same and the latent heat of vaporization of water is fully recovered, then the power used to evaporate the seawater in each unit is as follows:
QI ¼ F
(2a)
QII ¼ F þ QI ¼ 2F
(2b)
QIII ¼ F þ QII ¼ 3F
(2c)
QIV ¼ 2F þ QIII ¼ 5F
(2d)
where, QI , QII , QIII andQIV are the power used to evaporate seawater in unitI, unitII, unitIII and unitIV, respectively. F is the heat collecting power of one panel. Therefore, the theoretical value of the PR for the present system is calculated as follows:
PR ¼
QI þ QII þ QIII þ QIV 11F ¼ ¼ 2:2 5F 5F
(3)
As is mentioned above, the present experimental system exhibits an outstanding capability in heat recovery. Therefore, it is reasonable to regard the actual PR close to the theoretical value of 2.2. Besides the PR, the system total efficiency,htol , is also an important parameter and is defined as the ratio between the total energy used to evaporate saline water into freshwater and the total energy incident on the solar collector. This parameter can be seen as an integrated indicator to judge the solar collecting and heat recovery performance of the collector.
htol ¼
Fig. 7. Freshwater yields in 5 test days.
Total energy used for evaporation Gm Dh ¼ Net energy absorbed by collector P
Total energy used for evaporation Gm Dh ¼ Total energy incident on collector ACPC nIr
(4)
where, ACPC is the lighting area of a CPC plate, n is the number of the CPC plates in the system and Ir is the intensity of the solar radiation that is perpendicular to the CPC plate and measured by a radiometer. Fig. 8 shows the system total efficiency curves in the 5 test days. As can been seen in Fig. 8, the maximum experimental value of system total efficiency reaches as high as 0.9 in sunny days and exceeds 0.8 in the cloudy. This value would rise if more heat recovery processes happened, i.e., more collecting/heat recovery units were adopted. As the maximum working temperature of the solar collector should not be too high, if more heat recovery processes happen, the temperature differences between two neighboring units will decrease correspondingly, meanwhile, the mass of the steam will increase correspondingly when passing through these units. So how to recover more heat from gradually increasing
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being set at 130 C, the system collecting efficiency can reach about 0.4, which is an advanced value among related researches. 4. Conclusions
Fig. 8. System total efficiency curves in 5 test days.
steam mass at a relatively low temperature difference, that is, how to improve the heat transfer performance of the concentric tube is our priority in the following researches. For common solar collectors used for heating air and water, the collecting efficiency of the collector is an important indicator to judge the effectiveness in using the solar energy. The collecting efficiency of the collector is defined as the ratio of the net energy absorbed by the selective absorbing coating on the inner glass tubes to the total energy incident on collector. In order to get the collecting efficiency of the present system, the heat recovered from the steam should not be added to net energy absorbed and it is calculated as follows:
h Net energy absorbed by collector P ht ¼ ¼ ¼ tot Total nenrgy incident on collector ACPC nIr PR (5) Fig. 9 shows the system collecting efficiency in 5 test days. The value of PR used to calculate the collecting efficiency is the theoretical value of 2.2. The system collecting efficiency is mainly affected by the temperature of the working fluid, the intensity of the solar radiation and the structure design of the system. In the autumn when solar radiation usually ranges from 700 W/m2 to 800 W/m2 and with the maximum temperature of working fluid
Fig. 9. System collecting efficiency curves in 5 test days.
A novel small-sized solar desalination system with integrated multi-stage evaporation/heat recovery process is designed and tested in this study. The system consists of four collecting units linked together. Each of the four units contains a seawater tank and at least one solar collecting/desalination panel. The solar collecting/ desalination panel mainly comprises a simplified CPC (Compound Parabolic Concentrator) and an all-glass evacuated tube collector. In the last three units, heat exchangers made of copper tubes are mounted concentrically in the all-glass evacuated tubes for heat recovery. The heat collecting and freshwater yielding performances of the whole system and each unit are investigated under different operating conditions. According to the experimental results, freshwater yield in the solar collector can reach as high as 1.25 kg/h/ m2 even though the experiments are carried out in the autumn or winter and the system total efficiency can be close to 0.9. All these performance indexes match our expected goals and the experimental study provides useful guidelines for future application. The present solar desalination system can also be developed easily into a large scale. Acknowledgments This work was supported by the national natural science foundation of China under grant No. 51276113. References [1] Hoffman AR. Water security: a growing crisis and the link to energy. In: AIP Conf Proc 2008. p. 55. [2] Kalogirou SA. Seawater desalination using renewable energy sources. Prog Energy Combust Sci 2005;31:242e81. [3] Maalej A. Solar still performance. Desalination 1991;82:197e205. [4] Fath HES. Solar distillation: a promising alternative for water provision with free energy, simple technology and a clean environment. Desalination 1998;116:45e56. [5] Xiao G, Wang X, Ni M, Wang F, Zhu W, Luo Z, et al. A review on solar stills for brine desalination. Appl Energy 2013;103:642e52. [6] Arunkumar T, Jayaprakash R, Ahsan A, Denkenberger D, Okundamiya MS. Effect of water and air flow on concentric tubular solar water desalting system. Appl Energy 2013;103:109e15. [7] Nematollahi F, Rahimi A, Gheinani TT. Experimental and theoretical energy and exergy analysis for a solar desalination system. Desalination 2013;317:23e31. [8] Reddy KS, Kumar KR, O’Donovan TS, Mallick TK. Performance analysis of an evacuated multi-stage solar water desalination system. Desalination 2012;288:80e92. [9] Mohamed AMI, El-Minshawy NA. Theoretical investigation of solar humidification Cdehumidification desalination system using parabolic trough concentrators. Energy Convers Manag 2011;52:3112e9. [10] Rajvanshi AK. A scheme for large scale desalination of sea water by solar energy. Solar Energy 1980;24:551e60. [11] Zejli D, Benchrifa R, Bennouna A, Bouhelal OK. A solar adsorption desalination device: first simulation results. Desalination 2004;168:127e35. [12] Hawlader MNA, Dey PK, Diab S, Chung CY. Solar assisted heat pump desalination system. Desalination 2004;168:49e54. [13] Hanafi A. Design and performance of solar MSF desalination systems. Desalination 1991;82:165e74. [14] Joseph J, Saravanan R, Renganarayanan S. Studies on a single-stage solar desalination system for domestic applications. Desalination 2005;173:77e82. [15] Garcia-Rodriguez L, Romero-Ternero V, Gomez-Camacho C. Economic analysis of wind-powered desalination. Desalination 2001;137:259e65. [16] Miranda MS, Infield D. A wind-powered seawater reverse-osmosis system without batteries. Desalination 2003;153:9e16. [17] El-Ghonemy AMK. RETRACTED: water desalination systems powered by renewable energy sources, review. Renew Sustain Energy Rev 2012;16:1537e56. [18] Li C, Goswami Y, Stefanakos E. Solar assisted sea water desalination: a review. Renew Sustain Energy Rev 2013;19:136e63. [19] Sagie D, Feinerman E, Aharoni E. Potential of solar desalination in Israel and in its close vicinity. Desalination 2001;139:21e33. [20] Mesa AA, Gómez CM, Azpitarte RU. Energy saving and desalination of water. Desalination 1997;108:43e50.
Z.-h. Liu et al. / Renewable Energy 64 (2014) 26e33 [21] Maroo SC, Goswami DY. Theoretical analysis of a single-stage and two-stage solar driven flash desalination system based on passive vacuum generation. Desalination 2009;249:635e46. [22] Bentley RW. Global oil & gas depletion: an overview. Energy Policy 2002;30: 189e205. [23] Delyannis E. Historic background of desalination and renewable energies. Solar Energy 2003;75:357e66. [24] Bourouni K, Deronzier JC, Tadrist L. Experimentation and modelling of an innovative geothermal desalination unit. Desalination 1999;125:147e53. [25] Qiblawey HM, Banat F. Solar thermal desalination technologies. Desalination 2008;220:633e44. [26] Shatat M, Worall M, Riffat S. Economic study for an affordable small scale solar water desalination system in remote and semi-arid region. Renew Sustain Energy Rev 2013;25:543e51. [27] Eslamimanesh A, Hatamipour MS. Economical study of a small-scale direct contact humidificationedehumidification desalination plant. Desalination 2010;250:203e7. [28] Kabeel AE, El-Said EMS. A hybrid solar desalination system of air humidification Cdehumidification and water flashing evaporation: part I. A numerical investigation. Desalination 2013;320:56e72. [29] Ng KC, Thu K, Saha BB, Chakraborty A. Study on a waste heat-driven adsorption cooling cum desalination cycle. Int J Refrig 2012;35:685e93. [30] Thu K, Chakraborty A, Kim Y-D, Myat A, Saha BB, Ng KC. Numerical simulation and performance investigation of an advanced adsorption desalination cycle. Desalination 2013;308:209e18.
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[31] Thu K, Saha BB, Chakraborty A, Chun WG, Ng KC. Study on an advanced adsorption desalination cycle with evaporatorecondenser heat recovery circuit. Int J Heat Mass Transf 2011;54:43e51. [32] Thu K, Yanagi H, Saha BB, Ng KC. Performance analysis of a low-temperature waste heat-driven adsorption desalination prototype. Int J Heat Mass Transf 2013;65:662e9. [33] Oommen R, Jayaraman S. Development and performance analysis of compound parabolic solar concentrators with reduced gap losses C oversized reflector. Energy Convers Manag 2001;42:1379e99. [34] Tripanagnostopoulos Y, Yianoulis P, Papaefthimiou S, Souliotis M, Nousia T. Cost effective asymmetric CPC solar collectors. Renew Energy 1999;16: 628e31. [35] Eames PC, Norton B. Thermal and optical consequences of the introduction of baffles into compound parabolic concentrating solar energy collector cavities. Solar Energy 1995;55:139e50. [36] Rabl A. Solar concentrators with maximal concentration for cylindrical absorbers. Appl Optics 1976;15:1871e3. [37] Khonkar H, Sayigh A. Optimization of the tubular absorber using a compound parabolic concentrator. Renew Energy 1995;6:17e21. [38] Liu Z-H, Hu R-L, Lu L, Zhao F, Xiao H-s. Thermal performance of an open thermosyphon using nanofluid for evacuated tubular high temperature air solar collector. Energy Convers Manag 2013;73:135e43.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
A novel integrated solar desalination system with multi-stage evaporation/heat recovery processes Zhen-hua Liu*, Ren-Lin Hu, Xiu-juan Chen School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2013 Accepted 25 October 2013 Available online
A novel small-sized integrated solar desalination system with multi-stage evaporation/heat recovery processes is designed and tested in this study. The system consists of four linked collecting units and operates under barotropic and atmospheric pressure. Each of the four units contains a seawater tank and at least one solar collecting/desalination panel mainly comprising a simplified CPC (Compound Parabolic Concentrator) and an all-glass evacuated tube collector. In the last three units, heat exchangers made of copper tubes are inserted concentrically into the all-glass evacuated tubes to recover heat. In each unit, an independent desalination process including solar collecting, heat recovery (no heat recovered in the first unit) and seawater evaporation can be carried out completely. The experimental results show that the freshwater field of the designed system can reach as high as 1.25 kg/(h m2) in the autumn and the system total efficiency is close to 0.9. Both experimental results provide a striking demonstration that the designed solar desalination system has outstanding performance in solar collecting, heat recovery and seawater evaporation. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Solar Desalination Heat recovery Collector
1. Introduction Water has always been mankind’s most precious resource. There are no substitutes and the struggle to control water resources has shaped human political and economic history. The amount of water deemed necessary to satisfy basic human needs is 1000 cubic meters per capita annually. By 2050, it is projected that the availability of potable water will fall below this level for 1.7 billion people in 39 countries [1]. On the other hand, about 97% of the earth’s water is salt water in the oceans, and a tiny 3% is freshwater. It would be feasible to address the water-shortage problem through seawater desalination [2]. Seawater desalination techniques are mainly divided into two categories, namely, the thermal processes, for which either multistage flash (MSF), multiple-effect boiling (MEB), vapor compression (VC), freezing or solar distillation are used [3e14] and the membrane processes, for which reverse osmosis (RO) or electrodialysis (ED) is applied [15,16]. In the thermal processes, the distillation of seawater is achieved by utilizing a thermal energy source. The thermal energy may be obtained from a conventional fossil-fuel source, nuclear energy or from a non-conventional solar thermal energy, geothermal energy, etc. [2,17,18].
* Corresponding author. Tel.: þ86 21 34206568. E-mail addresses: [email protected], [email protected] (Z.-h. Liu). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.10.040
In terms of energy consumption, desalination of seawater is an energy intensive process. The installed capacity of desalinated water systems in the year 2000 is about 22 million m3/day and the production of this water requires about 203 million tons of oil per year [2,19,20]. With the total installed capacity expected to increase drastically in the coming decades, the energy consumption for desalination will continue to rise and hence the required amount of conventional hydrocarbon fuels required will go up substantially [21]. Meanwhile, the world’s production of conventional hydrocarbons will soon decline. Hydrocarbon shortages are inevitable unless radical changes occur in demand, or in the supply of nonconventional hydrocarbons [22]. Renewable energy is the alternative solution to the decreasing reserves of fossil fuels [23]. At present, there are already several kinds of renewable energy applied to water desalination. Garcıa-Rodriguez presented an economic analysis of wind-powered RO desalination technology [15]. Marcos S. Miranda and David Infield used a 2.2 kW wind turbine generator powering a variable-flow RO desalination unit which was considered suitable for the application in remote areas [16]. Karim Bourouni et al. experimentally investigated a desalination plant functioning by aero-evapo-condensation. The work was carried out on a geothermal desalination plant installed in the south of Tunisia [24]. The use of solar energy in thermal desalination processes is one of the most promising applications of the renewable energies. In terms of solar thermal energy usage, solar desalination can either be direct; using solar energy to distillate directly in the solar
Z.-h. Liu et al. / Renewable Energy 64 (2014) 26e33
Nomenclature ACPC h Ir Gm n PR P Q
F htol ht
lighting area of a CPC plate (m2) enthalpy (J/g) solar radiation intensity (W/mP2) freshwater yield per second (kg/s), number of the CPC plates in the system performance ratio net energy absorbed by the collector (W) power used to evaporate seawater in one unit (W) heat collecting power of one panel (W) system total efficiency system collecting efficiency
Subscripts I UnitI II UnitII III UnitIII IV UnitIV collector, or indirect; combining conventional thermal processes desalination techniques with solar collectors for heat generation [25]. Many different systems of direct and indirect solar desalination have been proposed and implemented. ArefY. Maalej studied the performance ratio and efficiency of a solar still operating under different conditions [3]. Fath reviewed the various designs of solar stills and studied the suitability of solar stills for providing potable water [4]. Xiao et al. categorized the solar stills into six sorts based on the design guidelines used in each device. The properties of these design guidelines were detailed and evaluated in terms of enhancing the productivity of solar stills [5]. Arunkumar et al. designed a tubular solar still with a rectangular basin for water desalination and studied the effect of cooling air and water flowing over the cover [6]. Nematollahi et al. conducted an experimental and theoretical energy and exergy analysis for a solar desalination system consisting of a solar collector and a humidification tower, the results showed that the overall exergy efficiency increased by a decrease in humidification tower length, a decrease in inlet air temperature, and an increase in tower diameter [7]. Reddy et al. developed a novel multi-stage evacuated solar desalination system utilizing latent heat recovery. The effect of various design and operating parameters on the system performance were studied to optimize the configuration and suggested that the designed multi-stage evacuated solar desalination system was a viable option to meet the needs of rural and urban communities [8]. Mohamed and El-Minshawy theoretically investigate the principal operating parameters of a proposed desalination system based on air humidificationedehumidification principles [9]. Rajvanshi proposed a scheme to desalinate seawater using solar energy. The seawater heated by solar energy was flash evaporated in a multistage flash evaporator (MSF) unit to yield freshwater. Economic analysis of the scheme showed that it compared favorably with the existing fossil fuel fired desalination plants of the equivalent capacity [10]. Zejli et al. designed a multi-effect desalination system operating with an adsorption heat pump with an open cycle and using zeolite as the solid vapor adsorbent. A theoretical model was used to study the water production and energy consumption of the system [11]. Hawlader et al. designed a single effect desalination unit connected to an existing solar assisted heat pump and conducted a series of experiments on the system under different operating and meteorological conditions of Singapore [12]. Abdalla Hanafi analyzed the design and performance of the solar MSF desalination systems and carried out a stage-to-stage design of the
27
MSF chamber and the system transient performance [13]. Joseph et al. carried out an experimental study of a single stage solar desalination system. The input parameters such as solar irradiance and vacuum pressure in the flash evaporator were varied to find its influence on the system efficiency and yield of potable water per day [14]. Mahmoud Shatat et al. presented an economic and comparative evaluation study for a small scale solar powered water desalination system [26]. A. Eslamimanesh and M.S. Hatamipour made an economical study of humidificationedehumidification desalination (HDD) pilot plant in order to estimate the economic benefits of the process in comparison with a small-scale reverse osmosis (RO) system [27]. A.E. Kabeel and Emad M.S. El-Said presented a hybrid solar desalination system consisting of a humidificationedehumidification unit and single stage flashing evaporation unit and numerically studied the system [28]. Kyaw Thu et al. have made a detailed study on an adsorption desalination cycle with evaporatorecondenser heat recovery circuit [29e32]. In this paper, we put forward a novel small-sized solar desalination system which incorporates the solar collecting, seawater evaporation, heat recovery and freshwater condensation processes within the common evacuated tubular solar collector to produce freshwater directly. The system operates under barotropic and atmospheric pressure and adopts a stepwise heat recovery method to recycle the latent heat of the steam generated. It consists of four linked collecting units. Each of the four units contains a seawater tank and at least one solar collecting/desalination panel mainly comprising a simplified CPC (Compound Parabolic Concentrator) and an all-glass evacuated tube collector. In the last three units, heat exchangers made of copper tubes are inserted concentrically into the all-glass evacuated tubes for heat recovery. In each collecting unit, an independent desalination process including solar heat collecting, heat recovery (no heat recovered in the first unit) and seawater evaporation can be carried out completely. This novel solar desalination system is easy to assemble just like building blocks and can work with the absorption refrigerator to carry out the combined cycle of desalination and air-conditioning. The cost of manufacturing and running the designed system is low and its footprint is also small. Unlike the common multiple-effect boiling (MEB) process where the steam from one effect acts the role of heat source in the following effect, the majority of the heat used in each unit (effect) of the present system to evaporate the seawater comes directly from the solar energy collected by the all-glass evacuated tube, while the rest from the latent heat of the steam passing through the unit. Instead of using a vacuum pump to provide pressure gradient between each unit, the present system simply adopts several pressure regulating valves to control the working pressure which ranges from the atmospheric pressure to barotropic and neither vacuum pump nor delivery pump is needed. In the Multiple Effect Stack (MES) evaporator which is considered to be the most appropriate type for solar energy application, the thin seawater film evaporates or boils simultaneously on the outside of the tube bundles, the steam generated is likely to carry the seawater, while in the present system, the heat transfer mode is mainly the natural convection of the seawater immerged in the glass tube and the evaporation only occurs on the surface of the vaporeliquid, therefore the possibility of seawater carried by steam is smaller than that in MES system. Due to the temperature differences between the four units, the method of multi-stage heat recovery is applied to the system to fully utilize the sensible and latent heat of the steam generated. All these designs aim at simplifying the structure of solar desalination system, improving the thermal performance and increasing the freshwater yield. Though some manual operations such as brine draining are needed to run the system, this new type of solar desalination
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system with multi-stage evaporation/heat recovery process has fulfilled our expected goals and provides useful guidelines for future applications.
2. Experimental apparatus and procedure 2.1. Experimental apparatus Fig. 1 gives out a schematic view of the experimental system and Fig. 2 shows a photograph of the actual experimental apparatus before it is fully packaged with the insulation material. As is shown in Fig. 1, the experimental system mainly consists of four units linked together for solar collecting/desalination, a cooling pool (condenser) and a data acquisition system. The solar collecting/desalination units are the core component of the experimental system. The first unit (UnitI) contains a seawater tank, a pressure regulating valve and one solar collecting/desalination panel which consists of a simplified CPC (Compound Parabolic Concentrator), an all-glass evacuated tube and a steam outlet pipe. Since there is no heat recovery process happens in this unit, heat used to evaporate the seawater only comes from the solar energy. The second and third units (UnitII and UnitIII) have the same structure containing a seawater tank, a pressure regulating valve and one solar collecting/desalination panel which comprises a simplified CPC, an all-glass evacuated tube and a heat exchanger made of copper tubes mounted concentrically into the all-glass evacuated tube for heat recovery. The last unit (UnitIV) consists of a seawater tank, a pressure regulating valve and two parallel solar collecting/desalination panels mounted with heat exchangers which is the same as the one mentioned above. Since the mass of steam gradually accumulates when passing through these units, two panels are mounted in the last unit to provide more heat transfer area to recover heat. By such arrangement, the latent heat of the steam generated can
Fig. 2. A photograph of actual experimental apparatus.
be recovered almost completely in the last three units in order to evaporate the seawater outside the copper tubes. In each of the evacuated tubes, there is a steam outlet pipe to conduct the steam generated into the following unit. The bending head of the steam outlet pipe is inserted with a 30 mm long stainless steel mesh used as a vaporeliquid separator. A detailed view of a typical unit is shown in Fig. 3. In the present study, the cooling pool is used to fully condense the steam generated in the last unit. All the freshwater produced is collected in a graduated cylinder and the cumulative freshwater influx is measured every 15 min to get the mean freshwater yield. As this study mainly focuses on the solar collecting and heat recovery performances of the solar collecting/desalination units, the steam generated in the cooling pool has not been taken into consideration. The data acquisition system collects and processes data such as the solar radiation intensity, steam pressures, temperatures and
Fig. 1. A schematic view of the small-sized integrated solar desalination system.
Z.-h. Liu et al. / Renewable Energy 64 (2014) 26e33
29
stainless steel mesh(200 mesh) steam
sea water locations of thermalcouples
steam outlet pipe Common concentric tube
simplified CPC
all-glass evacuated tube rubber seal
steam inlet locations of thermalcouples pressure/temperature sensor sea water inlet
pressure balance pipe steam outlet pressure insulation material regulating valve
Fig. 3. A detailed view of a typical collecting unit.
wall temperatures of the tubes, etc. Data are logged using Agilent 34970A data-logger connected to the computer. Fig. 4. Profiles of standard CPC and simplified CPC; (a) standard CPC (b) simplified CPC.
2.2. The simplified CPC structure CPC is capable of collecting the solar radiation for a long period during the day without tracking the sun. As a kind of highly efficient high temperature solar collector, CPC solar collector has been widely studied in the design and analysis [33e36]. In the present study, a CPC with a tubular absorber is designed by the method proposed by Khonkar and Sayigh [37]. The tubular absorber is an all-glass evacuated solar tube, which consists of two concentric glass tubes sealed at one end with an annular vacuum space and a selective absorbing coating layer on the outer surface (vacuum side) of the inner tube. Fig. 4(a) shows the standard CPC profile for an all-glass evacuated tube. For the purpose of easy manufacturing and cost reduction, the standard CPC is simplified with a flat curve bottom instead of the involute shape bottom. Meanwhile, the upper parts of concentrator are cut out to form a truncated CPC. Fig. 4 (b) shows a simplified CPC after truncation and cutting involute out. According to the calculated and test results [38], the concentrating efficiency of the simplified CPC with a flat curve bottom decreases by about 15% compared to that of the truncated CPC with an involute bottom. The simplified CPC is made of stainless steel mirror (2 mm thick, 330 mm wide and 146 mm high) welded directly on the steel shelf. For a CPC collector with a flat-plate absorber, the concentration ratio is defined as ratio of lighting area to flat-plate area. For the CPC collector with a tubular absorber, however, the concentration ratio is defined as ratio of lighting area to peripheral area of tubular absorber. According to this definition, the concentration ratio of the present CPC is 2.16. The characteristics of the present collector system in the present study are shown in Table 1.
2.3. Concentric tube heat exchanger In the present study, the system uses the concentric tube to recover heat from the steam generated. As being shown in Fig. 3, the concentric tube is made of two copper pipes welded at one end, the sizes of which are also shown in Table 1. The concentric tube is inserted concentrically into the inner glass tube and the annular space thus formed is filled with the seawater supplied by the seawater tank. The steam coming from the former unit enters the inlet of the concentric tube, goes into the annular section, then arrives at the inlet of the inner tube located at the top, and finally exits from the outlet of the inner copper tube. 2.4. Experimental procedure and experimental error In the present experimental system, the temperature or pressure of the saturated steam in each unit is controlled by the pressure regulating valve. All these valves are regulated only once at the time when the intensity of solar radiation is about 1 kW/m2 and then the highest temperature of the steam in each unit varies on their own without any further adjustment of the valves. Since this solar desalination system is expected to run all year round, the highest temperature being set should not be too high and be achieved even in the winter. Therefore, the highest steam temperature being set in UnitI is 130 C and decreased by 10 C at each following unit, so the steam temperature in UnitIV is 100 C corresponding to the atmospheric pressure. By preliminary adjustments, the maximum temperature difference between two neighboring units
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Table 1 Characteristics of the simplified CPC collector system. Parameter Simplified CPC Width of CPC Effective length of CPC Height of CPC r: radius of absorber g: distance of CPC cusp and absorber qA: acceptance half angle Reflectivity of CPC Concentrating ratio All-glass evacuated tube Diameter of inner glass tube Diameter of cover glass tube Concentric tube Diameter of outer copper tube Diameter of inner copper tube Effective length of concentric tube
Value 330 mm 1600 mm 146 mm 24 mm 6 mm 65 0.85 2.16 58 mm 47 mm 36 mm 24 mm 1500 mm
1. concentric tube, 2. all-glass evacuated tube, 3. simplified CPC, 4. seawater supply inlet, 5. valve, 6. seawater tank, 7. vaporeliquid separator, 8. pressure balance pipe, 9. steam outlet pipe, 10. pressure regulating valve, 11. seawater inlet pipe, 12. cooling pool, 13. cylinder, 14. freshwater tank, 15. solar radiation meter, 16. Agilent data-logger, 17. computer.
is set at 10 K. According to our former calculations, the concentric tubes are capable of recovering all latent heat of evaporation from the steam even at the temperature difference of 3 K. In the present study, the steam temperature/pressure, wall temperature at the center of inner glass tube, wall temperature at the outlet of concentric tube in each unit are measured to investigate the solar collecting and heat recovery performance of the unit. In each one of the all-glass tubes, four thermocouples located at the center (axial direction) of the inner glass tube are equally distributed along the circumference. The arithmetic mean value of the four detected temperatures is taken as the mean wall temperature of the glass tube. The same arrangement of another four thermocouples is applied to the concentric tube at the outlet of the inner copper tube to measure the wall temperature, which can be regarded as the fluid outlet temperature inside the concentric tube. A pressure/temperature sensor is inserted into the all-glass evacuated tube to measure the steam pressure and temperature in each unit. The locations of the thermocouples and the pressure/temperature sensors are shown in Fig. 3. All the thermocouples adopted in our experimental apparatus are calibrated by an accurate thermometer and the maximum measurement error is 0.2 K. Pressure sensors with accuracy class 0.5 are used. Radiation meter is used to measure the intensity of solar radiation with the measurement error less than 4%. Since this experiment is an initial study on the proposed original solar desalination system, deionized water is used instead of actual seawater and corrosion problem is not taken into consideration. In order to find out whether the water and the steam are well separated, the deionized water is dyed red before the preliminary test begins. According to our observations, the water collected in the graduated cylinder is clean demonstrating that no liquid is carried out by the steam and thus the vaporeliquid separators work well.
Fig. 5 shows the steam temperatures/pressures, wall temperatures of inner glass tubes, wall temperatures of concentric tubes of the four units on a sunny day. These data were collected on Nov. 13, 2012. Each of the Roman numerals labeled on the curves represents one of four units. As can be seen in Fig. 5, in the morning, deionized water stored in these units is being heated from a relatively low temperature to the saturated temperature, so there is no steam generation during this period and the pressure in each unit stays at the atmospheric pressure. Soon the pressures in the former three units rise sharply due to the generation of steam. At noon, steam temperatures and pressures tested in each unit are close to the values preset. Then, in the afternoon, the steam temperatures and pressures in the former three units begin to decrease gradually. Meanwhile, both the steam temperature y, and the steam temperature differences between two neighboring units are about 8 Ce9 C. These steam temperatures and their differences are quite close to the values preset. In the meantime, the steam temperature is 4 Ce5 C lower than the wall temperature of the inner glass tube and 2 Ce3 C lower than the wall temperature of the concentric tube in each unit. As mentioned above, the wall temperature of the concentric tube in the each unit can be considered as the fluid outlet temperature inside the concentric tube. As being shown in Fig. 5, this fluid outlet temperature is much lower than the steam temperature in the former unit even though the solar radiation changes in a wide range during the whole test period. This implies that the steam flowing pass each concentric tube has been completely condensed to the subcooled water when arriving at the outlet of the concentric tube. Since the pressure inside the concentric tube is very close to that in the former evacuated glass tube, the steam temperature in the concentric tube should also be close to that in the former evacuated glass tube. The fact shows that the present structure design of the solar collecting unit can provide enough heat recovery capacity to recover the latent heat of the steam generated. Fig. 6 shows the steam temperature/pressure, wall temperature of the inner glass tube, the wall temperature at the outlet of the concentric tube of each unit on a cloudy day. These data were collected on Nov. 6, 2012. The trends of the temperatures and pressures in the four units are quite similar to those on Nov. 13, 2012 as being shown in Fig. 5. The main difference is that all the steam temperatures and pressures in the cloudy day are somewhat lower than those in the sunny day. At noon, the highest steam temperature in each of the four units reaches 122 C, 114 C, 107 C and 101 C, respectively, and the temperature differences between two neighboring units are about 6 Ce7 C. These steam temperatures and the temperature differences are lower than the values preset. In the latter three units, however, the wall temperatures of
3. Experimental results and discussion The outdoor experiments were carried out in the autumn and winter in 2012 and a few representative experimental results are shown in this paper. Considering that the deionized water accumulated in the system may exert bad influence on the experimental accuracy, each test was carried out continuously for two days and the data collected on the second day are used for analysis.
Fig. 5. Temperature and pressure values of each unit in a sunny day.
Z.-h. Liu et al. / Renewable Energy 64 (2014) 26e33
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the heat recovery effect of the system, since the total energy also includes the energy recovered from the steam generated and it equals 1 if there is no heat recovery effect.
PR ¼
Fig. 6. Temperature and pressure values of each unit in a cloudy day.
the concentric tube are still much lower than the steam temperatures in the former units throughout the whole test period. It again confirms that steam flowing pass the concentric tube has been completely condensed to subcooled water when arriving at the outlet of the concentric tube. Fig. 7 shows the freshwater yield per hour per square meter of the lighting area in 7 test days. Here, the lighting area of each panel is equal to the product of effective length and width of the CPC. The total lighting area of the present system is 2.64 m2, the projection of which on the horizontal plane represents the floor area of the collector. As can be seen in Fig. 7, the highest value of freshwater yield exceeds 1.26 kg/(h m2) on a sunny day in the autumn and reaches 0.7 kg/(h m2) even though it was cloudy. Considering that these experiments were conducted in October and November, it is reasonable to predict that the freshwater yield will be doubled in the summer. These results demonstrate that on the basis of well solar collecting and heat recovery, the present solar desalination system shows an outstanding performance of producing freshwater and provides a solid foundation for future applications. It should be emphasized that the freshwater used to calculate the yield is only generated in the four units, while the freshwater generated in the cooling pool has not been added. If the cooling pool were also designed as a still device, the total freshwater yield would increase further. As another important performance factor to evaluate the desalination system, the performance ratio (PR) is defined as the ratio between the total energy used to evaporate saline water into freshwater and the net energy absorbed by the collector. PR reflects
(1)
where,Gm is the freshwater yield per second, Dh is the enthalpy difference between subcooled water and the saturated steam, and P is the net energy absorbed by the collector. The PR can be bigger than 1 due to the heat recovery effect and the bigger the value is, the more the heat is recovered. The theoretical value of PR is determined by the system design. In the present system, the theoretical value of PR is 2.2 and is calculated as follows: Assuming that the collecting power of each panel is the same and the latent heat of vaporization of water is fully recovered, then the power used to evaporate the seawater in each unit is as follows:
QI ¼ F
(2a)
QII ¼ F þ QI ¼ 2F
(2b)
QIII ¼ F þ QII ¼ 3F
(2c)
QIV ¼ 2F þ QIII ¼ 5F
(2d)
where, QI , QII , QIII andQIV are the power used to evaporate seawater in unitI, unitII, unitIII and unitIV, respectively. F is the heat collecting power of one panel. Therefore, the theoretical value of the PR for the present system is calculated as follows:
PR ¼
QI þ QII þ QIII þ QIV 11F ¼ ¼ 2:2 5F 5F
(3)
As is mentioned above, the present experimental system exhibits an outstanding capability in heat recovery. Therefore, it is reasonable to regard the actual PR close to the theoretical value of 2.2. Besides the PR, the system total efficiency,htol , is also an important parameter and is defined as the ratio between the total energy used to evaporate saline water into freshwater and the total energy incident on the solar collector. This parameter can be seen as an integrated indicator to judge the solar collecting and heat recovery performance of the collector.
htol ¼
Fig. 7. Freshwater yields in 5 test days.
Total energy used for evaporation Gm Dh ¼ Net energy absorbed by collector P
Total energy used for evaporation Gm Dh ¼ Total energy incident on collector ACPC nIr
(4)
where, ACPC is the lighting area of a CPC plate, n is the number of the CPC plates in the system and Ir is the intensity of the solar radiation that is perpendicular to the CPC plate and measured by a radiometer. Fig. 8 shows the system total efficiency curves in the 5 test days. As can been seen in Fig. 8, the maximum experimental value of system total efficiency reaches as high as 0.9 in sunny days and exceeds 0.8 in the cloudy. This value would rise if more heat recovery processes happened, i.e., more collecting/heat recovery units were adopted. As the maximum working temperature of the solar collector should not be too high, if more heat recovery processes happen, the temperature differences between two neighboring units will decrease correspondingly, meanwhile, the mass of the steam will increase correspondingly when passing through these units. So how to recover more heat from gradually increasing
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Z.-h. Liu et al. / Renewable Energy 64 (2014) 26e33
being set at 130 C, the system collecting efficiency can reach about 0.4, which is an advanced value among related researches. 4. Conclusions
Fig. 8. System total efficiency curves in 5 test days.
steam mass at a relatively low temperature difference, that is, how to improve the heat transfer performance of the concentric tube is our priority in the following researches. For common solar collectors used for heating air and water, the collecting efficiency of the collector is an important indicator to judge the effectiveness in using the solar energy. The collecting efficiency of the collector is defined as the ratio of the net energy absorbed by the selective absorbing coating on the inner glass tubes to the total energy incident on collector. In order to get the collecting efficiency of the present system, the heat recovered from the steam should not be added to net energy absorbed and it is calculated as follows:
h Net energy absorbed by collector P ht ¼ ¼ ¼ tot Total nenrgy incident on collector ACPC nIr PR (5) Fig. 9 shows the system collecting efficiency in 5 test days. The value of PR used to calculate the collecting efficiency is the theoretical value of 2.2. The system collecting efficiency is mainly affected by the temperature of the working fluid, the intensity of the solar radiation and the structure design of the system. In the autumn when solar radiation usually ranges from 700 W/m2 to 800 W/m2 and with the maximum temperature of working fluid
Fig. 9. System collecting efficiency curves in 5 test days.
A novel small-sized solar desalination system with integrated multi-stage evaporation/heat recovery process is designed and tested in this study. The system consists of four collecting units linked together. Each of the four units contains a seawater tank and at least one solar collecting/desalination panel. The solar collecting/ desalination panel mainly comprises a simplified CPC (Compound Parabolic Concentrator) and an all-glass evacuated tube collector. In the last three units, heat exchangers made of copper tubes are mounted concentrically in the all-glass evacuated tubes for heat recovery. The heat collecting and freshwater yielding performances of the whole system and each unit are investigated under different operating conditions. According to the experimental results, freshwater yield in the solar collector can reach as high as 1.25 kg/h/ m2 even though the experiments are carried out in the autumn or winter and the system total efficiency can be close to 0.9. All these performance indexes match our expected goals and the experimental study provides useful guidelines for future application. The present solar desalination system can also be developed easily into a large scale. Acknowledgments This work was supported by the national natural science foundation of China under grant No. 51276113. References [1] Hoffman AR. Water security: a growing crisis and the link to energy. In: AIP Conf Proc 2008. p. 55. [2] Kalogirou SA. Seawater desalination using renewable energy sources. Prog Energy Combust Sci 2005;31:242e81. [3] Maalej A. Solar still performance. Desalination 1991;82:197e205. [4] Fath HES. Solar distillation: a promising alternative for water provision with free energy, simple technology and a clean environment. Desalination 1998;116:45e56. [5] Xiao G, Wang X, Ni M, Wang F, Zhu W, Luo Z, et al. A review on solar stills for brine desalination. Appl Energy 2013;103:642e52. [6] Arunkumar T, Jayaprakash R, Ahsan A, Denkenberger D, Okundamiya MS. Effect of water and air flow on concentric tubular solar water desalting system. Appl Energy 2013;103:109e15. [7] Nematollahi F, Rahimi A, Gheinani TT. Experimental and theoretical energy and exergy analysis for a solar desalination system. Desalination 2013;317:23e31. [8] Reddy KS, Kumar KR, O’Donovan TS, Mallick TK. Performance analysis of an evacuated multi-stage solar water desalination system. Desalination 2012;288:80e92. [9] Mohamed AMI, El-Minshawy NA. Theoretical investigation of solar humidification Cdehumidification desalination system using parabolic trough concentrators. Energy Convers Manag 2011;52:3112e9. [10] Rajvanshi AK. A scheme for large scale desalination of sea water by solar energy. Solar Energy 1980;24:551e60. [11] Zejli D, Benchrifa R, Bennouna A, Bouhelal OK. A solar adsorption desalination device: first simulation results. Desalination 2004;168:127e35. [12] Hawlader MNA, Dey PK, Diab S, Chung CY. Solar assisted heat pump desalination system. Desalination 2004;168:49e54. [13] Hanafi A. Design and performance of solar MSF desalination systems. Desalination 1991;82:165e74. [14] Joseph J, Saravanan R, Renganarayanan S. Studies on a single-stage solar desalination system for domestic applications. Desalination 2005;173:77e82. [15] Garcia-Rodriguez L, Romero-Ternero V, Gomez-Camacho C. Economic analysis of wind-powered desalination. Desalination 2001;137:259e65. [16] Miranda MS, Infield D. A wind-powered seawater reverse-osmosis system without batteries. Desalination 2003;153:9e16. [17] El-Ghonemy AMK. RETRACTED: water desalination systems powered by renewable energy sources, review. Renew Sustain Energy Rev 2012;16:1537e56. [18] Li C, Goswami Y, Stefanakos E. Solar assisted sea water desalination: a review. Renew Sustain Energy Rev 2013;19:136e63. [19] Sagie D, Feinerman E, Aharoni E. Potential of solar desalination in Israel and in its close vicinity. Desalination 2001;139:21e33. [20] Mesa AA, Gómez CM, Azpitarte RU. Energy saving and desalination of water. Desalination 1997;108:43e50.
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