استیل خورشیدی با حوضه جذب بخار: تجزیه و تحلیل عملکرد

In this work, a vapor adsorption type solar still was designed, fabricated and tested at Thiagarajar College of Engineering, Madurai, India. A vapor adsorbent pipe network comprising activated carbon–methanol pair was integrated with the basin. Losses from the bottom of the still are considerably reduced due to sensible heat absorption by the activated carbon and latent heat of vaporization by methanol. Also water circulated through the inner tube of the adsorbent bed is used as a feed to basin, thus enhancing the evaporation rate during day time. The increase in temperature of the basin due to adsorbent bed and condensation of methanol vapor, augments the evaporation rate during the night time also. Sponges, gravels, sand and black rubbers were used in the vapor adsorption type solar still for improving the yield. Experimental results were compared with ordinary conventional basin type still. The governing energy balance equations for both conventional and vapor adsorption type solar still were solved analytically and compared with experimental results. Theoretical analysis gave very good agreement with experimental results

As population of this planet has rapidly grown, we have increasingly tapped deeper into our planets fresh water resources. The trend of population growth is quite obvious. The availability of water in sufficient quantities and quality is a challenge of significant importance in many regions as it is scarce and unevenly distributed resource. Water scarcity is a function of supply and demand and an indicator of the gap between them. The increased demand associated with uncontrolled development, inadequate management practices, overpopulation, mass tourism, intensive agriculture, and over consumption results in a complexity of interrelated problems affecting social, economic and natural aspects of everyday life. As a result many regions are now in a perpetual state of demand consistently exceeding supply. Solar distillation extracts potable water from a salty or polluted source using a simple process which is particularly well-suited for use on small scale in remote or developing regions. Solar stills of diverse designs, many equipped with numerous improvements, have been widely studied and put into use. Tiwari et al. [1] reviewed the present status of solar distillation. The basic heat and mass transfer relation responsible for developing, testing procedure for various designs of solar stills have also been discussed. Tanaka and Nakatake [2] have proposed a novel compact multiple-effect diffusion-type solar still consisting of a heat-pipe solar collector and a number of vertical parallel partitions in contact with saline-soaked wicks to increase the evaporation rate. Abu-Hijleh and Rababa'h [3] have done the experimental study of a solar still with sponge cubes in basin. The effects of sponge cube size, volume ratio of sponge, water depth, water salinity and the use of black coal and black steel cubes were also investigated. The study showed that the daily output of such a still could be greatly enhanced using sponge cubes. Badran et al. [4] designed a solar still augmented with flat plate collector. The still inlet was connected to a locally made fin-tube collector such that its outlet was fed to the still basin instead of the common storage tank. Velmurugan and Srithar [5] integrated a mini solar pond for enhancing the productivity of solar still. Also they reviewed [6] various applications of solar pond. Fins [7] and [8], stepped solar still [9] were also used to enhance the productivity of the solar still. Also an extensive review on solar desalination system has been carried out by Velmurugan and Srithar [10]. Leite and Daguenet [11] have proposed a predictive model for an adsorption solar cooling system using the activated carbon and methanol pair and found its numerical simulation. Wang et al. [12] have done a research of a combined adsorption heating and cooling system. The experiments showed the potentials of the application of the solar powered hybrid water heater and refrigerator. Dai and Sumathy [13] investigated the heat and mass transfer in the adsorbent of a solar adsorption cooling system. A mathematical model, which accounts for the heat and mass transfer of sorption (adsorption and desorption) processes as well as the effects of non-equilibrium and non-uniform temperature and pressure distribution, was developed and experimentally validated. Liu et al. [14] proposed a new adsorption refrigeration system, without refrigerant valves, and solved the problem of mass transfer resistance resulting in pressure drop along refrigerant passage in conventional systems when methanol or water is used as a refrigerant. Anyanwu and Ogueke [15] proposed the thermodynamic design procedure for solid adsorption solar refrigeration using activated carbon/methanol, activated carbon/ammonia and zeolite/water adsorbent/adsorbate pairs. Sumathy and Zhongfu [16] described the operation of a solar-powered ice-maker with the solid adsorption pair comprising activated carbon and methanol. The literature survey showed that higher water temperatures resulting in the increase in evaporation rate are achieved by decreasing the saline water depth in the basin, by increasing the exposure area or by preheating the saline water by some other sources before it enters the still or by combining all. The vapor adsorption solar refrigeration techniques using activated carbon–methanol or activated carbon–ammonia pairs gave good result in solar cooling process because of the higher sensible heat of activated carbon and the latent heat of methanol/ammonia. The objective of this work is to increase the temperature of saline water by utilizing an adsorbent bed pipe network with activated carbon–methanol pair embedded in the basin of the solar still. In an inner tube of the adsorbent pipe network, saline water is circulated from the storage tank. This preheated water is used as a feed to the still. An outer pipe of the adsorbent pipe network is enclosed with activate carbon–methanol pair and sealed at both ends. The temperature of the saline water is further augmented by means of adding the sensible heat storage materials like sponge, sand and rubber on the basin. Also adding sponges in the basin will increase the exposure area to solar radiation and thus evaporation rate is increased. Results showed that there is good improvement in distillate production in both day and night times. Theoretical modeling is also carried out to validate the experimental results. 1.1. Experimentation The vapor adsorption type solar still system comprises vapor adsorbent bed is shown in Fig. 1 and Fig. 2. The solar still consists of a 25 mm thick plywood box with dimensions depicted in Fig. 1 and Fig. 2. Holes are provided for the distilled water output and for the brackish water input. Outer sides of the wooden box are covered by the metal sheet in order to protect the box from solar radiation and rain. The basin of the still is made up of 2 mm galvanized iron (GI) sheet selected due to its good conductivity and low cost. The dimensions of the basin are 1 × 1 m2. A 0.2 m gap between the side of the basin tray and the wooden box is filled with saw dust. This acts as insulation and prevents the side loss of heat through conduction. The basin is coated with black paint to increase the radiation absorption. Full-size image (39 K) Fig. 1. Schematic diagram of the vapor adsorption type solar still. Figure options Full-size image (29 K) Fig. 2. Top view of the adsorbent bed. Figure options The condensing surface in the still is simply a 1.1 × 1.1 m2 sloping glass cover. Distilled water condensed on the glass surface is collected by a piece of gutter attached at the bottom of the sloping cover and directed to a measuring jar. The slope was selected as 10° angle, which is the latitude of Madurai, Tamil Nadu, India. The still is positioned that the glass cover faced south direction during all experiments. The only modification made in the basin plate of the vapor adsorption type solar still is the integration of a concentric pipe line network in the basin as shown in Fig. 1 and Fig. 2. An inner pipe of this network is 0.025 m in diameter and is enclosed with an outer pipe (diameter of 0.05 m) sealed at both ends as shown in Fig. 2. The space between the inner and the outer pipes contains an activated carbon–methanol pair acting as an adsorbent bed. During the day time, brackish or saline water is circulated from a storage tank through the inner tube where water is preheated and collected in collection tank. It is pumped back to the storage tank for continuous circulation. The storage tank and the collection tank each is made of PVC (poly vinyl chloride plastic) and having the capacity of 30 L. PVC is selected due to its ability to handle high salinity water. Therefore preheated water is supplied as a feed to the basin during the day time. During the night time heat will be released to saline water in the basin through the liberation of latent heat from the methanol vapor and the sensible heat from the activated carbon. Hence both day and night time the evaporation will be enhanced. The working principle is as follows: The saline water from the storage tank is allowed to flow through the inner tube of the adsorbent bed through the valve V1. The water will be preheated and returned to the storage tank through a collection tank by the pump. Feed to the still is supplied from the storage tank through the valve V3. When solar radiation falls on the basin and adsorbent water evaporates and condenses on the bottom side of the glass. The evaporation rate in the vapor adsorption still is increased because water is preheated before entering the basin and also because of the increase in effective exposure area of the adsorbent bed, which act as a fin. The condensed water is collected in the measuring jar through a pipe provided at the bottom of the still. As the evaporation takes place, the saline water level in the solar still decreased. Make-up water is added hourly to the still from the storage tank through valve V3. For comparison, a conventional still without any modification was also made run in parallel. This vapor adsorption solar still system along with a conventional solar still had been fabricated and tested at Thiagarajar College of Engineering, Madurai, India during January 2010–June 2010 where year round maximum solar radiation is obtained during this period. Copper-constantan thermocouples integrated with a temperature indicator and selector switch were used for temperature measurements. The thermocouples were fixed at the following locations: still basin plate, adsorbent bed, water at basin plate, inlet and outlet of saline water in the adsorbent bed and inside of the glass cover. To measure solar radiation a calibrated Kipp–Zonen pyranometer was used. Wind velocity was measured by vane type digital anemometer and distilled water output was measured by the measuring jar. The following modifications were made in the vapor adsorption still while conventional still was running throughout the periods without any change for comparison purpose. 1.1.1. Still with sponges Sponges were made to flow over the saline water surface in the basin in order to increase the free water surface area. In this experimental setup, 150 sponges were used. All the sponges are 35 mm by 35 mm by 20 mm in size with the volumetric ratio between sponge and water of 20% [3]. Due to capillary action, sponges sucked saline water thereby increasing the evaporation rate. 1.1.2. Still with sponge and gravels The experiments were repeated with sponges and small quantity of sensible heat storage materials like gravels added in the still basin. Gravels with a total mass of 0.5 kg were used in the vapor adsorption type solar still. 1.1.3. Still with sponge and sand Sand as a heat storage material was filled on the basin surface and sponges were made to flow over the saline water surface. Two kg of fine river sand was used. 1.1.4. Still with sponge, sand and black rubber Black rubbers were added in addition to the sand and sponge in the vapor adsorption type solar still, as a heat storage medium. A larger fraction of solar radiation was absorbed resulting in enhanced productivity of the solar still. 1.2. Theoretical simulation The basin plate temperature, water temperature and glass temperature can be evaluated at every instant by solving the energy balance equations for the absorber plate, saline water and glass of the solar still respectively. In the case of conventional solar still, the solar energy absorbed by the basin plate is equal to the summation of heat stored in the basin plate, energy lost by convective heat transfer between basin and water and side losses. This can be written as [5], [7], [8] and [9], equation(1) View the MathML sourceI(t)Abαb=mbCp,b[ⅆTbⅆt]+Qc,b-w+Qloss Turn MathJax on For vapor adsorption solar still the energy balance equation can be rewritten by including the sensible heat gain by the activated carbon and latent heat gain by the methanol. So Eq. (1) is modified as, equation(2) View the MathML sourceI(t)Abαb=(mbCp,b+mactCp,act)(ⅆTbⅆt)+mmethfg,met+Qc,b-w+Qloss Turn MathJax on Energy received by the saline water in the still (from sun and base) is equal to the summation of energy lost by convective heat transfer between water and glass, radiative heat transfer between water and glass, evaporative heat transfer between water and glass and sensible energy gained by the saline water. This can be written as [5], [7], [8] and [9], equation(3) View the MathML sourceI(t)αwAw+Qc,b-w=Qc,w-g+Qr,w-g+Qe,w-g+mwCp,w(ⅆTwⅆt) Turn MathJax on Energy gained by the glass cover (from sun and convective, radiative and evaporative heat transfer from water to glass) is equal to the summation of energy lost by radiative and convective heat transfer between glass and sky, and energy gained by glass [5], [7], [8] and [9]. equation(4) View the MathML sourceI(t)αgAg+Qc,w-g+Qr,w-g+Qe,w-g=Qr,g-sky+Qc,g-sky+mgCp,g(ⅆTgⅆt) Turn MathJax on Initially water temperature, glass temperature and plate temperature were taken as ambient temperature and the change in basin temperature (dTb), increase in saline water temperature (dTw) and glass temperature (dTg) were computed for every time interval (dt) of 5 s by solving Eqs. (3) and (4) respectively in the case of conventional still and the temperatures obtained by solving Eqs. (3) and (4) respectively for vapor adsorption type solar still. For evaluation of above said temperatures in the simulation, the experimentally measured values of solar radiation and the ambient temperature of the corresponding day and hour were used. This iteration was performed for total duration from 9am to 5pm of a day. The mass of water in the still was taken as 3.75 kg. The mass of water equivalent to the condensate (m con), was added every half an hour and the constant level of water was maintained in the stepped solar still. The area of saline water (A w) was equal to area of the basin. Mass of the glass (m g) was taken as 12.5 kg. The absorptivity of the still, αbαb was taken [7] and [8] as 0.95. The absorptivity of the water, αwαw and absorptivity of the glass, αgαg were taken as [7] and [8] 0.05. For the next time step, the parameter was redefined as, equation(5) Tw=Tw+ⅆTwTw=Tw+ⅆTw Turn MathJax on equation(6) Tg=Tg+ⅆTgTg=Tg+ⅆTg Turn MathJax on equation(7) Tb=Tb+ⅆTbTb=Tb+ⅆTb Turn MathJax on The total condensation rate was given in Refs. [5], [7], [8] and [9], equation(8) View the MathML source(ⅆmconⅆt)=he,w-g(Tw−Tghfg) Turn MathJax on The total solar flux on an inclined surface was obtained from Refs. [5], [7], [8] and [9], equation(9) View the MathML sourceI(t)=(Ig−Id)(cosθicosθh)+Id(1+cosβ2) Turn MathJax on where θiθi and θhθh were the incidence angle on an inclined surface and horizontal surface respectively and were obtained from Refs. [8] and [9]. The convective heat transfer between basin and water was taken from Refs. [5], [7], [8] and [9] as, equation(10) View the MathML sourceQc,b-w=hc,b-wAb(Tb−Tw) Turn MathJax on The convective heat transfer coefficient between basin and water, hc, b-w was taken from Refs. [5], [7], [8] and [9] as 135 W/m2 K. The heat loss from basin to ambient was calculated from Refs. [5], [7], [8] and [9], equation(11) Qloss=UbAb(Tb−Ta)Qloss=UbAb(Tb−Ta) Turn MathJax on where Ub was taken from Refs. [5], [7], [8] and [9] as, 14 W/m2 K. The convective heat transfer between water and glass was given in Refs. [5], [7], [8] and [9], equation(12) View the MathML sourceQc,w-g=hc,w-gAw(Tw−Tg) Turn MathJax on where the convective heat transfer coefficient between water and glass was given in Refs. [5], [7], [8] and [9], equation(13) View the MathML sourcehc,w-g=0.884{(Tw−Tg)+[Pw−Pg][Tw+273.15][268.9×103−Pw]}1/3 Turn MathJax on The radiative heat transfer between water and glass was determined by Refs. [5], [7], [8] and [9], equation(14) View the MathML sourceQr,w-g=hr,w-gAw(Tw−Tg) Turn MathJax on The radiative heat transfer coefficient between water and glass was given in Refs. [5], [7], [8] and [9], equation(15) View the MathML sourcehr,w-g=εeffσ[(Tw+273)2+(Tg+273)2](Tw+Tg+546) Turn MathJax on equation(16) View the MathML sourcewhereεeff=(1εw+1εg−1)−1 Turn MathJax on The evaporative heat transfer between water and glass was given in Refs. [5], [7], [8] and [9], equation(17) View the MathML sourceQe,w-g=he,w-gAw(Tw−Tg) Turn MathJax on The evaporative heat transfer coefficient between water and glass was given in Refs. [5], [7], [8] and [9], equation(18) View the MathML sourcehe,w-g=(16.273×10−3)hc,w-g(pw−pg)(Tw−Tg) Turn MathJax on The radiative heat transfer between glass and sky was given in Refs. [5], [7], [8] and [9], equation(19) View the MathML sourceQr,g-sky=hr,g-skyAg(Tg–Tsky) Turn MathJax on The radiative heat transfer coefficient between glass and sky was given in Refs. [5], [7], [8] and [9], equation(20) hr,g-sky=εσ[4(Tg+273)−4(Tsky+273)]/(Tg−Tsky)hr,g-sky=εσ[(Tg+273)4−(Tsky+273)4]/(Tg−Tsky) Turn MathJax on The effective sky temperature was taken from Refs. [5], [7], [8] and [9], equation(21) Tsky=Ta−6;Tsky=Ta−6; Turn MathJax on The convective heat transfer between glass and sky, Qc,g-sky was given in Refs. [5], [7], [8] and [9], equation(22) View the MathML sourceQc,g-sky=hc,g-skyAg(Tg–Tsky) Turn MathJax on where hc,g-sky was taken from Refs. [5], [7], [8] and [9], equation(23) View the MathML sourcehc,g−sky=2.8+3.0V Turn MathJax on The partial pressure of water vapor in the air in N/m2, is calculated for given temperature (°C) using the following correlation [5], [7], [8] and [9]. equation(24) p=7235–431.43T+10.76T2p=7235–431.43T+10.76T2 Turn MathJax on The latent heat of evaporation of water in J/kg, at given basin water temperature (°C) is given by the following correlation [5], [7], [8] and [9]. equation(25) hfg=(2503.3–2.398×T)×1000hfg=(2503.3–2.398×T)×1000 Turn MathJax on The specific heat capacity, in J/kg K, of the water vapor inside the still is calculated using the following correlation [5], [7], [8] and [9]. equation(26) View the MathML sourceCp,w=999.2+(0.14339×Tw)+(0.0001101×Tw2)–(0.000000067581×Tw3) Turn MathJax on The mathematical simulation was performed for different type of operations as discussed below: 1.2.1. Still with sponges To augment the evaporation rate, sponges of uniform sizes, were added. In simulation, the area of the free surface water Aw was taken as 1.35 m2 including sponge exposure area. The equations and other parameter remain the same. 1.2.2. Still with sponge and gravels Gravel is one of the high solar thermal energy storage materials. Addition of gravels in the basin surface increases the water temperature and thereby increasing the evaporation rate. Approximately 0.5 kg of gravels were placed along with the sponges of the same quantity as in Section 1.1.1. The mathematical model of the simple basin still discussed in Section 1.2, was used for this modification also, except the area of the free surface water Aw was taken as 1.35 m2 including sponge exposure area. Also Eq. (2) was modified as follows by considering mass and specific heat of the gravels. equation(27) View the MathML sourceI(t)Abαb=(mbCpb+mactCp,act+mgrCp,gr)(ⅆTbⅆt)+mmethfg,met+Qc,b-w+Qloss Turn MathJax on 1.2.3. Still with sponge and sand The same area Aw as in Section 1.2.1 was used and Eq. (2) was modified by considering sensible energy of the sand, giving: equation(28) View the MathML sourceI(t)Abαb=(mbCpb+mactCp,act+msCp,s)(ⅆTbⅆt)+mmethfg,met+Qc,b-w+Qloss Turn MathJax on 1.2.4. Still with sponge, sand and black rubber In addition to sensible energy of the sand, here sensible energy of the black rubber is added in Eq. (2). equation(29) View the MathML sourceI(t)Abαb=(mbCpb+mactCp,act+msCp,s+mbrCp,br)(ⅆTbⅆt)+mmethfg,met+Qc,b-w+Qloss Turn MathJax on The other equations and parameter were similar to the previous modifications. Theoretical simulation was numerically solved by Runge–Kutte method of fourth order using MATLAB software at Australian National University, Canberra, Australia during April–June, 2011. 1.3. Error analysis Accuracies of various measuring instruments used in the experiments are shown in Table 1. Solar radiation, temperatures at various places, distilled water collection and wind velocity were measured in hourly basis. Table 1. Accuracies and ranges of measuring instruments. Sl. No. Instrument Accuracy Range % Error 1. Thermocouple ±1 °C 0 °C–100 °C 3% 2. Solarimeter ±0.025 W/m2 0–2000 W/m2 3.5% 3 Anemometer ±0.1 m/s 0–15 m/s 10% 4 Measuring beaker ±10 ml 0–1000 ml 10% Table options 1.4. Results and discussion Experiments were carried out in both vapor adsorption solar still and conventional still during January 2010 and June 2010. The basin of the vapor adsorption type still was modified by adding sponges, sand, gravel and black rubber and the performance of the system was compared with that of the conventional solar still. Comparison of theoretical and experimental results is also conducted in this section. Efficiencies of the various modifications are tabulated. 1.4.1. Comparison of theoretical and experimental performance Fig. 3 shows the production rate variations for the vapor adsorption type still and the conventional solar still. The cumulative evaporation rate in the vapor adsorption still with sponge and gravels is 3.96 kg/m2 for 8 h duration while the yield in conventional solar still is only 2.1 kg/m2 for the same duration. Increase in exposure area by the addition of sponge and increase in sensible heat due to gravels, accelerated the evaporation rate of the vapor adsorption solar still by 47% than conventional solar still. Full-size image (35 K) Fig. 3. Theoretical and experimental comparisons. Figure options The comparison between theoretical and experimental performance was also plotted in Fig. 3. For simulation, actual experimentally determined operational and meteorological parameters were used. Theoretical analysis agrees well with experimental analysis. The maximum deviation between experimental and theoretical was 2.3%. 1.4.2. Effect of various modifications in solar still on distillate production Integration of vapor adsorption bed at the basin plate causes increase in solar energy exposure, decrease in side loss and thereby increase in saline water temperature. Addition of sponges enriches the free water surface area and thus saline water temperature increases. Due to high volumetric capacity (ρCp) of gravels, sand and black rubber, more thermal energy was stored and so saline water temperature increased. Results showed that the evaporation rate was increased, when vapor adsorption solar still was modified with sponge, gravel, sand and black rubber. Fig. 4 shows the variation of evaporative heat transfer for all modifications. Result shows that evaporation rate increased at an average value of 95%, when sponge, sand and rubber were added in fin type solar still. Also during night time, the evaporation rate of conventional still is 0.4 kg/m2 while vapor adsorption solar still gives 0.7 kg/m2 which is about 57% rise in distillate production. Full-size image (52 K) Fig. 4. Variation in distillate production rate with various modifications. Figure options 1.4.3. Daily efficiency and increase in productivity The daily efficiency, ηd, is defined as energy out-put and energy input. The energy output is obtained by summing up the hourly distillate production mcon, multiplied by the latent heat of vaporization hfg. Energy input is the product of daily solar radiation I(t) over the whole area A of the device. Mathematically it can be written as: equation(30) ηd=∑mconhfg/∑AI(t)ηd=∑mconhfg/∑AI(t) Turn MathJax on The daily efficiency and percentage increase in productivity for the various modifications are given in Table 2. It was found that maximum efficiency was obtained when sponges, sand and black rubber were added with the vapor adsorption still basin. Table 2. Daily efficiency. Sl. No. Modifications Conventional solar still Vapor adsorption solar still Daily efficiency Daily efficiency 1 Without any modifications 61 75 2 With sponge – 78 3 With sponge and gravel – 79 4 With sponge and sand – 86 5 With sponge sand and black rubber – 93 Table options 1.5. Economic analysis The payback period of the experimental setup depends on overall cost of fabrication, maintenance cost, operating cost and cost of feed water. The overall fabrication cost was Rs. 10,000 ($ 223). The maintenance cost and the cost of feed water were negligible. The cost per kg of distilled water was Rs. 10 ($0.22) and the average productivity of the vapor adsorption type solar still is 3.7 kg/m2. Thus the cost of water produced per day was Rs. 37.0 ($0.83) and payback period was calculated as 270 days.

A novel solar still integrated with vapor adsorption bed at the basin was designed, fabricated and characterized. To enhance the productivity of the still, sponge, gravels, sand, black rubbers and some of their combinations were used. The performance of the novel still was compared with that of a conventional solar still. It was found that distillate production rate in the vapor adsorption was ranged between 3.1 and 4.3 kg/m2 while the conventional still distillate production rate was between 1.9 and 2.3 kg/m2. The night time distillate production rate was also augmented in the vapor adsorption type solar still by two times than the conventional solar still. The maximum distillate production rate and daily efficiency was obtained in the vapor adsorption solar still with sand, sponge and black rubber combinations. The maximum deviation between theoretical experimental analyses was less than 6%.