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Solar still distillation enhancement through water surface perturbation
Solar Energy 196 (2020) 312–318
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Solar still distillation enhancement through water surface perturbation a,⁎
b
M.A. Porta-Gándara , J.L. Fernández-Zayas , N. Chargoy-del-Valle a b
T
b
Engineering Group, Centro de Investigaciones Biológicas del Noroeste, Instituto Politécnico Nacional s/n, Playa Palo de Santa Rita Sur, 23096 La Paz, B.C.S., Mexico Instituto de Ingeniería, Universidad Nacional Autónoma de México, Torre de Ingeniería, Ciudad Universitaria, Coyoacán 04510, CDMX, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Experimental solar stills Basin single-slope solar still Evaporative surface improvement Improve distillation efficiency
Solar still water production enhancement was measured in experimental single-slope basin-type solar stills (BTSS) by perturbing the water surface of the still. The perturbation is achieved by the injection of air bubbles into the water basin, which produces surface ripples, thus increasing the overall evaporative surface area and stimulating the mass transfer coefficient. Overall, distilled water production is therefore enhanced as the evaporation improves. The position and flowrate of the air bubble injectors do not seem to affect the amount of evaporation increase. Two very low-tilt covered, shallow BTSS were operated simultaneously side by side. Only one of them was subjected to mass transfer enhancement, in order to produce conclusive mass transfer enhancement results. These results can be accommodated in a computer simulation program with the introduction of a simple intensification factor. Experimental work was carried out in the semi-desert, water-starved, highly insolated region of La Paz, BCS, Mexico.
1. Introduction The pressing need for high-quality natural water, suitable for human consumption and for preserving the ecosystems, with a view on stopping noxious gas emissions, has increased water prices. Consequently, there is a current quest for improving solar distillation systems (Dunkle, 1961; Liao et al., 2018). Efficient combinations of photovoltaic (PV) systems with advanced reverse-osmosis water purifiers can produce important amounts of high-quality water from sea water; although mounting worries about sustainability issues have complicated the basic technology (Muslih et al., 2010). The pursuit of an even more efficient and economic sea water distillation system is analyzed in various reviews of solar still enhancement techniques (Kabeel et al., 2017). Two basic groups of distillation enhancement techniques in BTSS can be distinguished: on one side, the passive improvement of distillate yield by means of improving the use of still geometry, materials and practice. On the other side, the employment of heat recovery and other industrial active techniques, such as the supply of additional heat to the still. However, the authors of the present work believe that simple, unsophisticated solar stills will be best suited to supply drinkable water in isolated regions of the planet, due mostly to their straightforwardness and simplicity (Marimuthu et al., 2017). In developing societies passive techniques are mandatory due to local scarcity of skilled labor. These challenge is faced preferably by revamping the traditional BTSS,
⁎
although important contributions are made in both groups of techniques (Gordes and McCracken, 1985; Eltawil and Omara, 2014). A promising pathway can be found in studies related to humidification-dehumidification (HDH) technologies. These combine solar energy and salty or brackish water with enhanced methods for air HDH, and result in improved rates of desalination (Elminshawy et al., 2016). The present work reports on still enhancement resulting from an increase in evaporation in a real, well-instrumented BTSS. Published research in HDH is abundant and concludes that the technique can improve BTTS markedly (Eltawil and Omara, 2014; Sharon and Reddy, 2015; El-Agouz, 2010). Some work related to water bubbling in a solar still reveals that the HDH process can enhance distilled water production (Halima et al., 2019). This evidence has been gathered under laboratory controlled conditions. The present work is performed in openair prototypes, where variable control is less precise, but results are more conclusive. Comparisons and evaluations reported in the work herein are made with respect to the overall energy efficiency, which is calculated as the ratio of energy effectively employed to produce distilled water to the solar energy received over the external surface of the still during the day. This method includes the account of the small amount of water distillated during the night in efficient shallow BTSS (Marimuthu et al., 2017; Duffie and Beckman, 1991).
Corresponding author.
https://doi.org/10.1016/j.solener.2019.12.028 Received 3 June 2019; Received in revised form 28 November 2019; Accepted 8 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 196 (2020) 312–318
M.A. Porta-Gándara, et al.
Nomenclature
qp qi Tamb Te Tw Tg Ts Tsup t Ud Uext
List of symbols
Ap , Ac BTSS d wood d wool F HDH H hr , hc ,he hc2 hc3
hr2 hr3 IF k wood k wool L Pw PV qsol qu
heat loss, heat input transfer area, m2 basin type solar still, or shallow solar still plywood thickness, m insulation thickness, m (and inches) dimensional complement of ec (3) humudification-dehumidification process larger height of sloped solar still, m unit heat transfer coefficients, kW m−2 K−1 convective heat transfer coefficient from outer glazing to ambient, kW m−2 K−1 convective heat transfer coefficient from outer basin to ambient, kW m−2 K−1 radiative heat transfer coefficient from outer glazing to ambient, kW m−2 K−1 radiative heat transfer coefficient from outer basin to ambient, kW m−2 K−1 distillation mass intensification transfer factor (1, 2… 16) plywood thermal conductance, kW m−1 K−1 insulation thermal conductance, kW m−1 K−1 sloping length of solar still, m Pg , water vapor partial pressure, Nm−2 photovoltaic solar global irradiance, kW m−2 useful heat flow, W
Uge
Uk Uwg Uwe Vwind α η ε w , εg εout σ (τα )
2. Distillation enhancement strategy
and quickly splashing onto the water surface (Kumar-Rao et al., 2017). This process is repeated constantly, disrupting the water-air boundary layer and improving the evaporative heat and mass transfer.
For more than half a century, Dunkle’s approach to heat and mass transfer in solar stills (Dunkle, 1961) has been unchallenged. Water distillation in passive solar stills is represented by a single equation of distillate production as a function of water-cover temperature difference, assuming that whatever takes place in the water-cover realm is properly accounted for by the Dunkle formula. However, the physical processes that take place between the basin water surface and the condensing glazing can be analyzed separately. The water-to-cover evaporation-condensation (distillation) heat transfer coefficient can be written as:
Ud = (he−1 + hc−1)−1
heat lost flow, W internal heat flow (=qsol), W ambient temperature, °C external ambient temperature, °C water surface temperature, °C glass cover internal temperature. °C equivalent sky temperature. °C external basin temperature, °C time, s overall distillation heat transfer coefficient, Wm−2 K−1 overall heat transfer coefficient from basin external surface to ambient, Wm−2 K−1 glass cover to external ambient heat transfer coefficient, Wm−2 K−1 combined conduction heat transfer coefficient, Wm−2 K−1 water surface to glass cover heat transfer coefficient, Wm−2 K−1 water surface to external ambient heat transfer coefficient, Wm−2 K−1 wind average speed, ms−1 thermal radiation absorbance overall heat transfer efficiency heat radiation emittance for water, glass surface BTSS outer surface radiation emittance Stefan Boltzmann radiation constant, 5.6697 × 10-8 Wm2 −4 K transmittance absorbance factor for limiting the effective heat input, non-dimensional
3. Materials and methods The present work was done in two stages, with stills of the basic design shown in Fig. 2. First, a BTSS is instrumented to record temperatures in the water basin, condensing glass cover and surrounding environment, and it is provided with a pyranometer to measure solar energy continuously, as well as a wind speed and direction measurement device. This arrangement allowed us to become familiar with the unenhanced solar still for a year, and to identify a representative set of
(1)
The suffix e denotes evaporation and c stands for condensation. The radiation component is usually assumed to be small (Tsilingiris, 2015) and is generally taken as independent of the distillation process. If the evaporation component is smaller than the condensation part, as it is generally calculated (Agrawal et al., 2017; Badran, 2007), then the overall mass transfer coefficient Ud will be controlled by the evaporation part of the process. Since evaporation resistance is generated mainly by the stable water and air laminar boundary layers at the interphase, the enhancement strategy is aimed at disrupting them. In this work, air bubbles are injected in the bulk of the water basin to induce mixing and disruption at the air-water interphase, as shown in Fig. 1. Preliminary tests were conducted to gain some understanding as to how bubble size and frequency would induce better evaporation. However, no relationship could be established between injected air flow and evaporation increase, although the available knowledge on bubble size and interactions is wide (Castell et al., 2008). This feature is further discussed with measured results. As Fig. 1 illustrates, raising air bubbles approach the water surface from below and produce a small bump before bursting, hence enlarging the heat transfer surface. After they burst, a thin micro layer of water is severely disturbed by the succession of water droplets thrown upward
Fig 1. Schematic diagram of air bubbles reaching the water surface. 313
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water surface. This device was only used in the BTSS on the right of Fig. 2. The other still was employed as a witness. The injected air was taken from the BTSS air space, as illustrated in Fig. 4. No thermal imbalance was introduced by the bubbling. Several extraction and injection sites as well as various air flow rates were experimented with. No discernible effects among them were found. In a preliminary fashion, we propose a fairly even application of the bubble injections across the whole evaporation surface, in order to increment the distilled water production notably. This phenomenon occurs due to the nature of small waves or ripples, which travel freely in all directions and collide with one another often, thus increasing the surface area. The research procedure consisted in various experimental tests, where both stills were operated simultaneously and their typical temperatures and yield were recorded. The tests were repeated for various positions and air flowrates. Results were then compared to those calculated with a simple lumped-parameter mathematical model, which allowed for the exploration of the sensitivity of the still production to design variables, and thus, the interpretation of the process as offered in the Conclusions section was made possible.
Fig. 2. Schematic diagram of a typical shallow basin type solar still (BTSS).
data to report, as Table 1 shows. Effective insolation area of the first still was 1.22 (L) × 1.45 = 1.77 m2, and cover tilt was around 4°, the taller side H = 0.28 m, and the opposite side, 0.20 m. Typical water volume to be distilled was 15 kg at the onset of the experiments, with a typical distillate production of 4–5 kg daily. Environmental data was recorded by means of a data logger meteorological station equipped with a solar radiation pyranometer S-LIB-M003, with an accuracy of ± 10 W/m2, and a spectral range of 300–1100 nm. Electronic temperature sensors model TMCX-HD were used, with accuracy of ± 0.25 °C and an anemometer S-WS/DA-M003 with ± 1.1 m/s for wind speed and ± 5° for wind direction accuracy, as per accepted standards of accuracy (Tiwari et al., 1994). The logger was programed to acquire data every 900 s. During that period, distillate yield was captured in calibrated test tubes and recorded manually. The strategy was to gather enough experimental data to calibrate the accepted mathematical model (Tamik and Hasson, 1971). In the second stage, two small solar stills were employed, which were built with identical specifications and materials. One of them was kept unenhanced, and measurements were employed as reference values. The other one was equipped with a bubble injection system, adapted from an aquarium air supply, and moveable air injectors. For both stages, the basic geometry is shown in Fig. 2 and a picture of stills for stage two are shown in Fig. 3. It must be stressed that such a small tilt angle at the cover could result in undesirable dripping of condensate, unless very thoroughly cleaned at the onset. The appropriate degreasing technique is described in (Gordes and McCracken, 1985). A small 2.5 W PV powered air pump was employed to inject air bubbles in the water basin to perturb the stillness of the evaporative
4. Experimental results Typical experimental results for unenhanced still are shown in Table 1 and Fig. 5. Measurements chosen are for June 29th, on a particularly sunny day, which yielded a daily insolation of 5.7 kW h/m2, or 20,520 kJ/m2. Since total distillate yield was 6.137 kg/m2, energy conversion efficiency was 0.676 at hfg = 2, 260 kJ/kg. This experimental run was repeated several times over several days with similar results. Solar irradiance and distillate output, shown in Fig. 5, where time lag in basin temperature is the cause for the yield curve to lean to the right of the graph. Since distillation mass flow rate is fairly constant, at about 0.22 kg/15′, between 11:00 and 14:00, Table 1 is now reduced, with that information outlined in the shaded area. 5. Basis for a numerical model The mathematical model that best describes the time evolution of a BTSS is detailed in (Agrawal et al., 2017). The basis of the model is hc , the heat transfer coefficient for natural convection between the hot brine water surface at temperature Tw , and the inside of the transparent cover, at Tg , as proposed initially by Dunkle (1961):
Table 1 Selected values for 11:00–14:00 h. Measured values of qsol and distillate (shaded). Calculated values for Tw , Tg , Uwe and efficiency η .
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Fig. 3. Outdoor setting for monitoring shallow solar still operation. Undisturbed still on left. Bubble sources are moved around for different tests.
Fig 4. Schematic diagram of electric-powered air bubble pump operation.
Fig. 6. Flowchart diagram for mathematical model solving.
Fig 5. Measurements of distillate production in 900 s intervals and global solar irradiance.
hc = 0.884[Tw − Tg + Tw (Pw − Pg )/(268.9 ∗ 103 − Pw )]1/3
(2)
The distillation heat transfer coefficient is usually calculated by
Fig 7. Reflectance-absorbance coefficient (τα ) calculated for June 29. Time in hours from noon, positive in the afternoon. A slight adjustment to effective transmittance is made to account for condensate inside the transparent cover (Duffie and Beckman, 1991).
(3)
he = Fh wc
where F = 16.273 ∗ 10−3 (Pw − Pg )/(Tw − Tg ) . The heat transferred between the water surface and the cover is increased by radiation, depicted by hr as
hr =
[s (Tw2
+
Tg2 )(Tw
+
Tg )]/(∈−w1
+
∈−g 1
− 1)
water surface is not perturbed, and in the perturbed occurrence. In the second case, Eq. (5) is written as
(4)
Uwg = IF (hc + he + hr )
The overall heat transfer between water surface and cover, Uwg , can thus be written as
Uwg = hc + he + hr
(5’)
where IF is a non-dimensional intensification factor, the result of the combination of evaporation surface increment and the ripple-effect enhancement arising from bubble injection in the basin. This factor is taken as unity when no bubbling is done.
(5)
Eqs. (2) and (3) are deemed to be valid in the standard case, when 315
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Ts = 0.0552Te1.5
hc3 = 2.8 + 3.0Vwind Additionally, the conductive thermal conductance Uk is calculated by
Uk = 1/(d wood / k wood )
(8)
Finally, thermal flow from the still cover to the surroundings is calculated as the addition of radiated heat to the sky and the convective heat to the surroundings:
Uge = hr 2 + hc 2
(9)
where
Fig 8. Distillate production (kg/m2 in 900 s intervals) for a typical day in March.
hr 2 = εg σ (Tg2 + Ts2)(Tg + Ts ) hc 2 = 2.8 + 3.0Vwind The thermal balance of the BTSS can now be solved with these expressions. Since absolute pressure P and other fluid and material properties will vary with temperature, the equations system is nonlinear, and the solution of the heat balance must be made by numerical approximations. A computer program was written to produce the simulation results with the help of the flowchart in Fig. 6. 6. Calculation of the principal variables Calculated values for Tw and Tg are now included in Table 1. From the measured distillate yield in periods of 900 s each, and with the measured insolation, the overall thermal conversion efficiency η can be directly calculated (last column, Table 1). The calculated mean efficiency for the three-hour period is hence about 0.65. Calculated glass cover temperature Tg and water temperature Tw are also included, and since ambient air temperature Te was stable at 31 °C during the period, the overall thermal loss coefficient Uwe can be calculated in each interval, as shown. The small differences in this coefficient, around the mean value of 0.00655 kW/m2 K, can be explained in terms of wind velocity variations. The measurements made around solar noon, when insolation and ambient temperature vary insignificantly, can be interpreted with the aid of a basic thermal balance. Since in that three-hour period, variables such as heat transfer coefficients Uwg , Uge and Uwe stay basically constant, and many days as the one reported are usually found in the southernmost region of BCS, Mexico, many registries were made with similar results. These are correlated by the basic heat balance equations in such a fashion that they yield special understanding of the expected values of local heat transfer coefficients. Notice that values for solar absorbance and transmittance are basically constant in that time interval as well, as presented in Fig. 7. A procedure to evaluate how reliable the model is, as compared to the measured data, is exemplified with the calculation of the distilled water produced. With calculated variables, the expected distillate for each 900 s period is computed. When all the 3 h interval calculated products are added together, the resulting yield is 2.914 kg/m2, which compares well with the total measured of 2.883 kg/m2, a difference of about 1%. The mathematical model is taken to be reliable.
Fig 9. Comparison between measured and calculated un-enhanced still production in kg/m2 in 900 s intervals for June 29.
Fig 10. Calculated daily production for still enhancement of 1 ≤ IF ≤ 16. Weather data for June 29th. Thermal insulation thickness increases from 0 to 2″ rock wool.
The overall setup of the mathematical simulation model employs the preceding expressions to solve the heat balance. During the almost constant temperature period around noon in Table 1, steady-state conditions can be assumed, and dynamic effects neglected. The solving of the heat balance requires the calculation of thermal losses from the water basin to the surroundings, characterized by the overall heat transfer coefficient Uwe , which can be estimated as −1 Uwe = 1/(Uext + Uk−1)
(6) 7. Air bubbling in distilled water tray
The heat transfer coefficients Uext and Uk are employed to describe thermal flow from the outer surface of the still basin to the environment, and the thermal conduction from the basin water to the outer basin surface, hence
Uext = hr 3 + hc3
Evaporation processes can be expedited if the evaporation surface is extended, as (Tamik and Hasson, 1971) proposed. One way of extending the evaporative surface without altering the condensation capacity of a BTSS is by injecting a small air jet into the distilland. As stated above, pumping power is produced by a small 2.5 W electrical pump powered by a photovoltaic cell. This apparatus only works when a threshold of irradiance is reached, which fits well with the intended application. Air is aspired from the air cavity within the BTSS, hence no
(7)
where 2 hr 3 = εσ (Text + Ts2)(Text + Ts )
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the evaporation surface enhancement. However, the results gathered could be very much improved with proper testing in more sophisticated still arrangements. As a complement of this research, it was verified that proper still box thermal insulation is essential to achieve high production rates. Results in very clear summer days in Baja California Sur, Mexico, support the possibility of attaining high production rates announced several years ago by Horace MacCracken (Gordes and McCracken, 1985). Now it is clear that the substantial insulation effort he insisted in employing, coupled with a minimal inside volume of water in the solar still, are necessary to achieve such impressive results (η ̴ 0.65). However, the authors are persuaded that further work is needed to better understand the nature of the water surface perturbation.
mass nor temperature changes are introduced in the brine deposit. Each air bubble will increase substantially the contact water-to-air surface area (Yoshikawa et al., 2010; Dietzel et al., 2019). Distillation area is also extended due to the wavy surface motion that the bubbles produce (Amirnia et al., 2013). While switching air extraction/injection positions in preliminary tests, no variation due to bubbling positioning was perceived. A general appraisal of a sample of basic experimental results using this enhancement technique is illustrated in Fig. 8. These results are confirmed in several other experiments. The measurement of state and operational variables was performed during March in the reported fashion, and it was consistently found that the enhanced device produced roughly 12% more distillate than the standard BTSS (IF = 1.12) . Nevertheless, no optimal enhancement arrangement has been found yet.
Declaration of Competing Interest 8. Results and discussion There is no conflict of interest. This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors.
8.1. Unenhanced still results Distillate prediction accuracy can be tested under extreme insolation conditions, as recorded on June 29th. Results of measured vs calculated distillate yields are plotted in Fig. 9. The total distillate collected over the day was measured as 6.137 kg/m2, whereas the similar calculated value was 6.187 kg/m2, which is 1.0081 times higher. This kind of experiments confirm the mathematical model credibility, which allows the inference of further results of the bubble injection enhancement.
References Dunkle, R.V., 1961. Solar water distillation: the roof type still and a multiple effect diffusion still, Commonwealth scientific and industrial research organization, C.S.I.R.O., 108, 895-902., Victoria, Australia. Liao, H., Sarver, E., Krometis, Y.L., 2018. Interactive effects of water quality, physical habitat, and watershed anthropogenic activities on stream ecosystem health. Water Res. 130, 69–78. https://doi.org/10.1016/j.watres.2017.11.065. Muslih, I., Abdallah, S., Husain, Y.W., 2010. Cost comparative study for new water distillation techniques by solar energy using, solar power plants and their application. Appl. Sol. Energy 46 (1). https://doi.org/10.3103/S0003701X10. pp. ISSN 0003–701X, 8–12. Kabeel, A., Arunkumar, T., Denkenberger, D., Sathyamurthy, y R., 2017. Performance enhancement of solar still through efficient heat exchange mechanism – A review. Appl. Therm. Eng. 114, 815–836. https://doi.org/10.1016/j.applthermaleng.2016. 12.044. Marimuthu, T., Atnaw, S.M., Mardarveran, P., Yi, S.S., Usop, M.A.B., Md Gapar, M.K.B., Rusdan, S.A.B., Ramli y, R.B.M., Sulaiman, S.A., 2017. Design and development of solar desalination plant, de MATEC Web of Conferences 131, 02004. Gordes, J., McCracken, H., 1985. Understanding solar stills. Volunteers in Technical Assistance, in Technologies for Development, p46. Eltawil, M., Omara, Z., 2014. Enhancing the solar still performance using solar photovoltaic, flat plate collector and hot air. Desalination 349, 1–9. https://doi.org/10. 1016/j.desal.2014.06.021. Elminshawy, Nabil A.S., Siddiqui, Farooq R., Addas, Mohammad F., 2016. Development of an active solar humidification-dehumidification (HDH) desalination system integrated with geothermal energy. Energy Convers. Manage. 126, 608–621. https:// doi.org/10.1016/j.enconman.2016.08.044. Sharon, H., Reddy, K.S., 2015. A review of solar energy driven desalination technologies. Renew. Sustain. Energy Rev. 41, 1080–1118. https://doi.org/10.1016/j.rser.2014. 09.002. El-Agouz, S.A., 2010. A new process of desalination by air passing through seawater based on humidification dehumidification process. Energy 35, 5108–5114. https://doi.org/ 10.1016/j.energy.2010.08.005. Halima, Hanen Ben, Frikha, Nader, Gabsi, Slimane, 2019. Numerical and experimental investigation of a modified solar basin using as a bubbler humidifier in humidification–dehumidification desalination system. Environ. Progr. Sustain. Energy 38 (2), 518–526. https://doi.org/10.1002/ep.12980. Duffie, J.A., Beckman, Y.W.A., 1991. Solar Engineering of Thermal Processes, 2nd ed. John Wiley and Sons, New York. Tsilingiris, P., 2015. Parameters affecting the accuracy of Dunkle's model of mass transfer phenomenon at elevated temperatures. Appl. Therm. Eng. 75, 203–212. https://doi. org/10.1016/j.applthermaleng.2014.09.010. Agrawal, Abhay, Rana, R.S., Srivastava, Pankaj K., 2017. Heat transfer coefficients and productivity of a single slope single basin solar still in Indian climatic condition: Experimental and theoretical comparison, Resour.-Efficient Technol. 3, 466–482. Badran, O., 2007. Experimental study of the enhancement parameters on a single slope solar still productivity. Desalination 209, 136–143. https://doi.org/10.1016/j.desal. 2007.04.022. Castell, Albert, Solé, Cristian, Medrano, Marc, Roca, Joan, Cabeza, Luisa F., García, Daniel, 2008. Natural convection heat transfer coefficients in phase change material (PCM) modules with external vertical fins. Appl. Therm. Eng. 28 (13), 1676–1686. https://doi.org/10.1016/j.applthermaleng.2007.11.004. Kumar-Rao, D.C., Syam, S., Karmakar, S., Joarder, R., 2017. Experimental investigations on nucleation, bubble growth, and microexplosion characteristics during the combustion of ethanol/Jet A-1 fuel droplets. Exp. Therm. Fluid Sci. 89, 284–294. Tiwari, G.N., Thomas, J.M., Khan, E., 1994. Optimization of glass cover inclination for maximum yield in a solar still. Heat Recovery Syst. CHP 14, 4. Tamik, A., Hasson, D., 1971. Evaporation and condensation coefficient of water, Chem.
8.2. Distillation enhancement extrapolation The mathematical model is now solved with standard heat transfer equations to calculate heat balances on both water and glass cover, and the relative importance of distillation enhancement can be explored by using an enhancement factor IF on the Eq. (5) to calculate Uwg . All other external factors being constant, the expected distillation rate can be plotted for IF values 1, 2… 16 as shown in Fig. 10. It can be noticed that lower values of IF result in higher value change of net enhancement, and distillation enhancement tends to an invariant for higher values. Notice that same-insulation curves are almost parallel to one another. It seems to be confirmed that, in properly insulated BTSS, top thermal losses (radiation and convection) are small compared with evaporative heat transfer. 8.3. Cost consideration The cost of the air injection system is calculated at about $ 25 US. This includes a suitable PV solar panel, an air pump, a battery, control electronics, and assorted connecting parts. Those components can be assembled from readily available consumer electronics offered on-line. It can be then calculated that if the added cost contributes to improve production up to 15% or more, the increased water production would cost about one half of the direct cost of solar distilled water. 9. Conclusions The possibility of enhancing the distillate product of a shallow single-slope solar still by injecting air bubbles into the water basin was investigated. The expected controlling factor of evaporation in the distilling process was confirmed, since evaporation can be very much incremented without having overall distillation attaining a proportional response; on the opposite extreme, evaporation enhancement can be induced at over one order of magnitude, while the overall distillation improvement stays practically invariant. These results are similar to those found in HDH by other authors Elminshawy et al., 2016; Sharon and Reddy, 2015). Simple experimental tests allow us to verify the important effect of 317
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growth and interaction of vapour bubbles in superheated liquid jets. Int. J. Multiphase Flow 121, 103112. Amirnia, S., deBruyn, J., Bergougnou, M.A., Margaritis, A., 2013. Continuous rise velocity of air bubbles in non-Newtonian biopolymer solutions. Chem. Eng. Sci. 94, 60–68.
Eng. J. 2. Yoshikawa, H.N., Mathis, C., Maïssa, P., Rousseaux, G., Douady, S., 2010. Pattern formation in bubbles emerging periodically from a liquid free surface. Eur. Phys. J. E 33, 11–18. Dietzel, D., Hitz, T., Munz, C.D., Kronenburg, A., 2019. Numerical simulation of the
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Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Solar still distillation enhancement through water surface perturbation a,⁎
b
M.A. Porta-Gándara , J.L. Fernández-Zayas , N. Chargoy-del-Valle a b
T
b
Engineering Group, Centro de Investigaciones Biológicas del Noroeste, Instituto Politécnico Nacional s/n, Playa Palo de Santa Rita Sur, 23096 La Paz, B.C.S., Mexico Instituto de Ingeniería, Universidad Nacional Autónoma de México, Torre de Ingeniería, Ciudad Universitaria, Coyoacán 04510, CDMX, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Experimental solar stills Basin single-slope solar still Evaporative surface improvement Improve distillation efficiency
Solar still water production enhancement was measured in experimental single-slope basin-type solar stills (BTSS) by perturbing the water surface of the still. The perturbation is achieved by the injection of air bubbles into the water basin, which produces surface ripples, thus increasing the overall evaporative surface area and stimulating the mass transfer coefficient. Overall, distilled water production is therefore enhanced as the evaporation improves. The position and flowrate of the air bubble injectors do not seem to affect the amount of evaporation increase. Two very low-tilt covered, shallow BTSS were operated simultaneously side by side. Only one of them was subjected to mass transfer enhancement, in order to produce conclusive mass transfer enhancement results. These results can be accommodated in a computer simulation program with the introduction of a simple intensification factor. Experimental work was carried out in the semi-desert, water-starved, highly insolated region of La Paz, BCS, Mexico.
1. Introduction The pressing need for high-quality natural water, suitable for human consumption and for preserving the ecosystems, with a view on stopping noxious gas emissions, has increased water prices. Consequently, there is a current quest for improving solar distillation systems (Dunkle, 1961; Liao et al., 2018). Efficient combinations of photovoltaic (PV) systems with advanced reverse-osmosis water purifiers can produce important amounts of high-quality water from sea water; although mounting worries about sustainability issues have complicated the basic technology (Muslih et al., 2010). The pursuit of an even more efficient and economic sea water distillation system is analyzed in various reviews of solar still enhancement techniques (Kabeel et al., 2017). Two basic groups of distillation enhancement techniques in BTSS can be distinguished: on one side, the passive improvement of distillate yield by means of improving the use of still geometry, materials and practice. On the other side, the employment of heat recovery and other industrial active techniques, such as the supply of additional heat to the still. However, the authors of the present work believe that simple, unsophisticated solar stills will be best suited to supply drinkable water in isolated regions of the planet, due mostly to their straightforwardness and simplicity (Marimuthu et al., 2017). In developing societies passive techniques are mandatory due to local scarcity of skilled labor. These challenge is faced preferably by revamping the traditional BTSS,
⁎
although important contributions are made in both groups of techniques (Gordes and McCracken, 1985; Eltawil and Omara, 2014). A promising pathway can be found in studies related to humidification-dehumidification (HDH) technologies. These combine solar energy and salty or brackish water with enhanced methods for air HDH, and result in improved rates of desalination (Elminshawy et al., 2016). The present work reports on still enhancement resulting from an increase in evaporation in a real, well-instrumented BTSS. Published research in HDH is abundant and concludes that the technique can improve BTTS markedly (Eltawil and Omara, 2014; Sharon and Reddy, 2015; El-Agouz, 2010). Some work related to water bubbling in a solar still reveals that the HDH process can enhance distilled water production (Halima et al., 2019). This evidence has been gathered under laboratory controlled conditions. The present work is performed in openair prototypes, where variable control is less precise, but results are more conclusive. Comparisons and evaluations reported in the work herein are made with respect to the overall energy efficiency, which is calculated as the ratio of energy effectively employed to produce distilled water to the solar energy received over the external surface of the still during the day. This method includes the account of the small amount of water distillated during the night in efficient shallow BTSS (Marimuthu et al., 2017; Duffie and Beckman, 1991).
Corresponding author.
https://doi.org/10.1016/j.solener.2019.12.028 Received 3 June 2019; Received in revised form 28 November 2019; Accepted 8 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Nomenclature
qp qi Tamb Te Tw Tg Ts Tsup t Ud Uext
List of symbols
Ap , Ac BTSS d wood d wool F HDH H hr , hc ,he hc2 hc3
hr2 hr3 IF k wood k wool L Pw PV qsol qu
heat loss, heat input transfer area, m2 basin type solar still, or shallow solar still plywood thickness, m insulation thickness, m (and inches) dimensional complement of ec (3) humudification-dehumidification process larger height of sloped solar still, m unit heat transfer coefficients, kW m−2 K−1 convective heat transfer coefficient from outer glazing to ambient, kW m−2 K−1 convective heat transfer coefficient from outer basin to ambient, kW m−2 K−1 radiative heat transfer coefficient from outer glazing to ambient, kW m−2 K−1 radiative heat transfer coefficient from outer basin to ambient, kW m−2 K−1 distillation mass intensification transfer factor (1, 2… 16) plywood thermal conductance, kW m−1 K−1 insulation thermal conductance, kW m−1 K−1 sloping length of solar still, m Pg , water vapor partial pressure, Nm−2 photovoltaic solar global irradiance, kW m−2 useful heat flow, W
Uge
Uk Uwg Uwe Vwind α η ε w , εg εout σ (τα )
2. Distillation enhancement strategy
and quickly splashing onto the water surface (Kumar-Rao et al., 2017). This process is repeated constantly, disrupting the water-air boundary layer and improving the evaporative heat and mass transfer.
For more than half a century, Dunkle’s approach to heat and mass transfer in solar stills (Dunkle, 1961) has been unchallenged. Water distillation in passive solar stills is represented by a single equation of distillate production as a function of water-cover temperature difference, assuming that whatever takes place in the water-cover realm is properly accounted for by the Dunkle formula. However, the physical processes that take place between the basin water surface and the condensing glazing can be analyzed separately. The water-to-cover evaporation-condensation (distillation) heat transfer coefficient can be written as:
Ud = (he−1 + hc−1)−1
heat lost flow, W internal heat flow (=qsol), W ambient temperature, °C external ambient temperature, °C water surface temperature, °C glass cover internal temperature. °C equivalent sky temperature. °C external basin temperature, °C time, s overall distillation heat transfer coefficient, Wm−2 K−1 overall heat transfer coefficient from basin external surface to ambient, Wm−2 K−1 glass cover to external ambient heat transfer coefficient, Wm−2 K−1 combined conduction heat transfer coefficient, Wm−2 K−1 water surface to glass cover heat transfer coefficient, Wm−2 K−1 water surface to external ambient heat transfer coefficient, Wm−2 K−1 wind average speed, ms−1 thermal radiation absorbance overall heat transfer efficiency heat radiation emittance for water, glass surface BTSS outer surface radiation emittance Stefan Boltzmann radiation constant, 5.6697 × 10-8 Wm2 −4 K transmittance absorbance factor for limiting the effective heat input, non-dimensional
3. Materials and methods The present work was done in two stages, with stills of the basic design shown in Fig. 2. First, a BTSS is instrumented to record temperatures in the water basin, condensing glass cover and surrounding environment, and it is provided with a pyranometer to measure solar energy continuously, as well as a wind speed and direction measurement device. This arrangement allowed us to become familiar with the unenhanced solar still for a year, and to identify a representative set of
(1)
The suffix e denotes evaporation and c stands for condensation. The radiation component is usually assumed to be small (Tsilingiris, 2015) and is generally taken as independent of the distillation process. If the evaporation component is smaller than the condensation part, as it is generally calculated (Agrawal et al., 2017; Badran, 2007), then the overall mass transfer coefficient Ud will be controlled by the evaporation part of the process. Since evaporation resistance is generated mainly by the stable water and air laminar boundary layers at the interphase, the enhancement strategy is aimed at disrupting them. In this work, air bubbles are injected in the bulk of the water basin to induce mixing and disruption at the air-water interphase, as shown in Fig. 1. Preliminary tests were conducted to gain some understanding as to how bubble size and frequency would induce better evaporation. However, no relationship could be established between injected air flow and evaporation increase, although the available knowledge on bubble size and interactions is wide (Castell et al., 2008). This feature is further discussed with measured results. As Fig. 1 illustrates, raising air bubbles approach the water surface from below and produce a small bump before bursting, hence enlarging the heat transfer surface. After they burst, a thin micro layer of water is severely disturbed by the succession of water droplets thrown upward
Fig 1. Schematic diagram of air bubbles reaching the water surface. 313
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water surface. This device was only used in the BTSS on the right of Fig. 2. The other still was employed as a witness. The injected air was taken from the BTSS air space, as illustrated in Fig. 4. No thermal imbalance was introduced by the bubbling. Several extraction and injection sites as well as various air flow rates were experimented with. No discernible effects among them were found. In a preliminary fashion, we propose a fairly even application of the bubble injections across the whole evaporation surface, in order to increment the distilled water production notably. This phenomenon occurs due to the nature of small waves or ripples, which travel freely in all directions and collide with one another often, thus increasing the surface area. The research procedure consisted in various experimental tests, where both stills were operated simultaneously and their typical temperatures and yield were recorded. The tests were repeated for various positions and air flowrates. Results were then compared to those calculated with a simple lumped-parameter mathematical model, which allowed for the exploration of the sensitivity of the still production to design variables, and thus, the interpretation of the process as offered in the Conclusions section was made possible.
Fig. 2. Schematic diagram of a typical shallow basin type solar still (BTSS).
data to report, as Table 1 shows. Effective insolation area of the first still was 1.22 (L) × 1.45 = 1.77 m2, and cover tilt was around 4°, the taller side H = 0.28 m, and the opposite side, 0.20 m. Typical water volume to be distilled was 15 kg at the onset of the experiments, with a typical distillate production of 4–5 kg daily. Environmental data was recorded by means of a data logger meteorological station equipped with a solar radiation pyranometer S-LIB-M003, with an accuracy of ± 10 W/m2, and a spectral range of 300–1100 nm. Electronic temperature sensors model TMCX-HD were used, with accuracy of ± 0.25 °C and an anemometer S-WS/DA-M003 with ± 1.1 m/s for wind speed and ± 5° for wind direction accuracy, as per accepted standards of accuracy (Tiwari et al., 1994). The logger was programed to acquire data every 900 s. During that period, distillate yield was captured in calibrated test tubes and recorded manually. The strategy was to gather enough experimental data to calibrate the accepted mathematical model (Tamik and Hasson, 1971). In the second stage, two small solar stills were employed, which were built with identical specifications and materials. One of them was kept unenhanced, and measurements were employed as reference values. The other one was equipped with a bubble injection system, adapted from an aquarium air supply, and moveable air injectors. For both stages, the basic geometry is shown in Fig. 2 and a picture of stills for stage two are shown in Fig. 3. It must be stressed that such a small tilt angle at the cover could result in undesirable dripping of condensate, unless very thoroughly cleaned at the onset. The appropriate degreasing technique is described in (Gordes and McCracken, 1985). A small 2.5 W PV powered air pump was employed to inject air bubbles in the water basin to perturb the stillness of the evaporative
4. Experimental results Typical experimental results for unenhanced still are shown in Table 1 and Fig. 5. Measurements chosen are for June 29th, on a particularly sunny day, which yielded a daily insolation of 5.7 kW h/m2, or 20,520 kJ/m2. Since total distillate yield was 6.137 kg/m2, energy conversion efficiency was 0.676 at hfg = 2, 260 kJ/kg. This experimental run was repeated several times over several days with similar results. Solar irradiance and distillate output, shown in Fig. 5, where time lag in basin temperature is the cause for the yield curve to lean to the right of the graph. Since distillation mass flow rate is fairly constant, at about 0.22 kg/15′, between 11:00 and 14:00, Table 1 is now reduced, with that information outlined in the shaded area. 5. Basis for a numerical model The mathematical model that best describes the time evolution of a BTSS is detailed in (Agrawal et al., 2017). The basis of the model is hc , the heat transfer coefficient for natural convection between the hot brine water surface at temperature Tw , and the inside of the transparent cover, at Tg , as proposed initially by Dunkle (1961):
Table 1 Selected values for 11:00–14:00 h. Measured values of qsol and distillate (shaded). Calculated values for Tw , Tg , Uwe and efficiency η .
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Fig. 3. Outdoor setting for monitoring shallow solar still operation. Undisturbed still on left. Bubble sources are moved around for different tests.
Fig 4. Schematic diagram of electric-powered air bubble pump operation.
Fig. 6. Flowchart diagram for mathematical model solving.
Fig 5. Measurements of distillate production in 900 s intervals and global solar irradiance.
hc = 0.884[Tw − Tg + Tw (Pw − Pg )/(268.9 ∗ 103 − Pw )]1/3
(2)
The distillation heat transfer coefficient is usually calculated by
Fig 7. Reflectance-absorbance coefficient (τα ) calculated for June 29. Time in hours from noon, positive in the afternoon. A slight adjustment to effective transmittance is made to account for condensate inside the transparent cover (Duffie and Beckman, 1991).
(3)
he = Fh wc
where F = 16.273 ∗ 10−3 (Pw − Pg )/(Tw − Tg ) . The heat transferred between the water surface and the cover is increased by radiation, depicted by hr as
hr =
[s (Tw2
+
Tg2 )(Tw
+
Tg )]/(∈−w1
+
∈−g 1
− 1)
water surface is not perturbed, and in the perturbed occurrence. In the second case, Eq. (5) is written as
(4)
Uwg = IF (hc + he + hr )
The overall heat transfer between water surface and cover, Uwg , can thus be written as
Uwg = hc + he + hr
(5’)
where IF is a non-dimensional intensification factor, the result of the combination of evaporation surface increment and the ripple-effect enhancement arising from bubble injection in the basin. This factor is taken as unity when no bubbling is done.
(5)
Eqs. (2) and (3) are deemed to be valid in the standard case, when 315
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Ts = 0.0552Te1.5
hc3 = 2.8 + 3.0Vwind Additionally, the conductive thermal conductance Uk is calculated by
Uk = 1/(d wood / k wood )
(8)
Finally, thermal flow from the still cover to the surroundings is calculated as the addition of radiated heat to the sky and the convective heat to the surroundings:
Uge = hr 2 + hc 2
(9)
where
Fig 8. Distillate production (kg/m2 in 900 s intervals) for a typical day in March.
hr 2 = εg σ (Tg2 + Ts2)(Tg + Ts ) hc 2 = 2.8 + 3.0Vwind The thermal balance of the BTSS can now be solved with these expressions. Since absolute pressure P and other fluid and material properties will vary with temperature, the equations system is nonlinear, and the solution of the heat balance must be made by numerical approximations. A computer program was written to produce the simulation results with the help of the flowchart in Fig. 6. 6. Calculation of the principal variables Calculated values for Tw and Tg are now included in Table 1. From the measured distillate yield in periods of 900 s each, and with the measured insolation, the overall thermal conversion efficiency η can be directly calculated (last column, Table 1). The calculated mean efficiency for the three-hour period is hence about 0.65. Calculated glass cover temperature Tg and water temperature Tw are also included, and since ambient air temperature Te was stable at 31 °C during the period, the overall thermal loss coefficient Uwe can be calculated in each interval, as shown. The small differences in this coefficient, around the mean value of 0.00655 kW/m2 K, can be explained in terms of wind velocity variations. The measurements made around solar noon, when insolation and ambient temperature vary insignificantly, can be interpreted with the aid of a basic thermal balance. Since in that three-hour period, variables such as heat transfer coefficients Uwg , Uge and Uwe stay basically constant, and many days as the one reported are usually found in the southernmost region of BCS, Mexico, many registries were made with similar results. These are correlated by the basic heat balance equations in such a fashion that they yield special understanding of the expected values of local heat transfer coefficients. Notice that values for solar absorbance and transmittance are basically constant in that time interval as well, as presented in Fig. 7. A procedure to evaluate how reliable the model is, as compared to the measured data, is exemplified with the calculation of the distilled water produced. With calculated variables, the expected distillate for each 900 s period is computed. When all the 3 h interval calculated products are added together, the resulting yield is 2.914 kg/m2, which compares well with the total measured of 2.883 kg/m2, a difference of about 1%. The mathematical model is taken to be reliable.
Fig 9. Comparison between measured and calculated un-enhanced still production in kg/m2 in 900 s intervals for June 29.
Fig 10. Calculated daily production for still enhancement of 1 ≤ IF ≤ 16. Weather data for June 29th. Thermal insulation thickness increases from 0 to 2″ rock wool.
The overall setup of the mathematical simulation model employs the preceding expressions to solve the heat balance. During the almost constant temperature period around noon in Table 1, steady-state conditions can be assumed, and dynamic effects neglected. The solving of the heat balance requires the calculation of thermal losses from the water basin to the surroundings, characterized by the overall heat transfer coefficient Uwe , which can be estimated as −1 Uwe = 1/(Uext + Uk−1)
(6) 7. Air bubbling in distilled water tray
The heat transfer coefficients Uext and Uk are employed to describe thermal flow from the outer surface of the still basin to the environment, and the thermal conduction from the basin water to the outer basin surface, hence
Uext = hr 3 + hc3
Evaporation processes can be expedited if the evaporation surface is extended, as (Tamik and Hasson, 1971) proposed. One way of extending the evaporative surface without altering the condensation capacity of a BTSS is by injecting a small air jet into the distilland. As stated above, pumping power is produced by a small 2.5 W electrical pump powered by a photovoltaic cell. This apparatus only works when a threshold of irradiance is reached, which fits well with the intended application. Air is aspired from the air cavity within the BTSS, hence no
(7)
where 2 hr 3 = εσ (Text + Ts2)(Text + Ts )
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the evaporation surface enhancement. However, the results gathered could be very much improved with proper testing in more sophisticated still arrangements. As a complement of this research, it was verified that proper still box thermal insulation is essential to achieve high production rates. Results in very clear summer days in Baja California Sur, Mexico, support the possibility of attaining high production rates announced several years ago by Horace MacCracken (Gordes and McCracken, 1985). Now it is clear that the substantial insulation effort he insisted in employing, coupled with a minimal inside volume of water in the solar still, are necessary to achieve such impressive results (η ̴ 0.65). However, the authors are persuaded that further work is needed to better understand the nature of the water surface perturbation.
mass nor temperature changes are introduced in the brine deposit. Each air bubble will increase substantially the contact water-to-air surface area (Yoshikawa et al., 2010; Dietzel et al., 2019). Distillation area is also extended due to the wavy surface motion that the bubbles produce (Amirnia et al., 2013). While switching air extraction/injection positions in preliminary tests, no variation due to bubbling positioning was perceived. A general appraisal of a sample of basic experimental results using this enhancement technique is illustrated in Fig. 8. These results are confirmed in several other experiments. The measurement of state and operational variables was performed during March in the reported fashion, and it was consistently found that the enhanced device produced roughly 12% more distillate than the standard BTSS (IF = 1.12) . Nevertheless, no optimal enhancement arrangement has been found yet.
Declaration of Competing Interest 8. Results and discussion There is no conflict of interest. This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors.
8.1. Unenhanced still results Distillate prediction accuracy can be tested under extreme insolation conditions, as recorded on June 29th. Results of measured vs calculated distillate yields are plotted in Fig. 9. The total distillate collected over the day was measured as 6.137 kg/m2, whereas the similar calculated value was 6.187 kg/m2, which is 1.0081 times higher. This kind of experiments confirm the mathematical model credibility, which allows the inference of further results of the bubble injection enhancement.
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8.2. Distillation enhancement extrapolation The mathematical model is now solved with standard heat transfer equations to calculate heat balances on both water and glass cover, and the relative importance of distillation enhancement can be explored by using an enhancement factor IF on the Eq. (5) to calculate Uwg . All other external factors being constant, the expected distillation rate can be plotted for IF values 1, 2… 16 as shown in Fig. 10. It can be noticed that lower values of IF result in higher value change of net enhancement, and distillation enhancement tends to an invariant for higher values. Notice that same-insulation curves are almost parallel to one another. It seems to be confirmed that, in properly insulated BTSS, top thermal losses (radiation and convection) are small compared with evaporative heat transfer. 8.3. Cost consideration The cost of the air injection system is calculated at about $ 25 US. This includes a suitable PV solar panel, an air pump, a battery, control electronics, and assorted connecting parts. Those components can be assembled from readily available consumer electronics offered on-line. It can be then calculated that if the added cost contributes to improve production up to 15% or more, the increased water production would cost about one half of the direct cost of solar distilled water. 9. Conclusions The possibility of enhancing the distillate product of a shallow single-slope solar still by injecting air bubbles into the water basin was investigated. The expected controlling factor of evaporation in the distilling process was confirmed, since evaporation can be very much incremented without having overall distillation attaining a proportional response; on the opposite extreme, evaporation enhancement can be induced at over one order of magnitude, while the overall distillation improvement stays practically invariant. These results are similar to those found in HDH by other authors Elminshawy et al., 2016; Sharon and Reddy, 2015). Simple experimental tests allow us to verify the important effect of 317
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