Computer modeling of passive/active solar stills by using inner glass temperature

Expressions for water and glass temperatures, hourly yield and instantaneous efficiency for both passive and active solar distillation systems have been derived. The analysis is based on the basic energy balance for both the systems. A computer model has been developed to predict the performance of the stills based on both the inner and the outer glass temperatures of the solar stills. In the present work two sets of values of C and n(C_inner, n_inner and C_outer, n_outer), obtained from the experimental data of January 19, 2001 and June 16, 2001 under Delhi climatic conditions, have been used. It is concluded that (i) there is a significant effect of operating temperature range on the internal heat transfer coefficients and (ii) by considering the inner glass cover temperature there is reasonable agreement between the experimental and predicted theoretical results.     

An experiment with a plastic solar still

A solar still is a device which allows obtaining fresh water from seawater or brackish water. It utilizes the greenhouse effect by using solar energy. In a conventional solar still the production of fresh water in bright sunny weather and with warm air temperature is about 5-5.5 L m⁻² d⁻¹, according to the depth of the water in the solar still. In some devices it is possible to obtain efficiencies of up to 0.50 and 0.60. The aim of this research is to increase distillation productivity by utilizing the latent heat released by the condensing water steam. For this purpose the author built a solar still characterized by two basins (B₁ and B₂) superimposed upon each other. The building materials were a sheet of black Plexiglas for the bottom of the solar still, a sheet of transparent Plexiglas for all boxes, and a sheet of expanded polystyrene, used as insulating material. The solar still was hermetically sealed to reduce the leakage of vapor to the surroundings. The greatest quantity of fresh water obtained by the tested solar still was 1.7-1.8 L m⁻² d⁻¹. This result was achieved in the third week of July when solar radiation was 27-28 MJ m⁻² d⁻¹. The efficiency of the tested solar still was about 0.16. This low efficiency is probably due to the low temperature of the water contained in the still (about 50°C). The solar still has only been used in experiments for some months, during which it has not been possible to study the deterioration of the material (Plexiglas). These results show that an elaborate design and the increased costs for such design and construction do not always improve the water yield.     

Parametric investigation of a basin-type-multiple-effect coupled solar still

Parametric analysis was performed for the basin-type multiple-effect coupled solar still with a triangle cross-section consisting of a horizontal basin liner and a number of vertical parallel partitions in contact with saline-soaked wicks with narrow gaps between the partitions, under dry weather conditions at 26.1 °N latitude. The productivity of the still increased in winter with an increase in the angle between the glass cover and the basin and decreased in summer. In the spring and autumn seasons productivity had a gentle peak between a 40° and 45° angle. Increasing supply rates of saline water to the wicks and increasing initial mass of the basin water decreased productivity. With a decrease in diffusion gaps between partitions, productivity exponentially increased, and it also showed exponential increases with an increase in the number of distillation cells between the partitions. The productivity of the still of 13 partitions with 5-mm gaps and a 40° angle of the glass cover was four times more than the basin-type stills, and the still was more productive than the conventional multiple-effect stills by about 40% or more.     

A vertical multiple-effect diffusion-type solar still coupled with a heat-pipe solar collector

We proposed a newly designed, 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. The solar collector and the vertical multiple-effect diffusion-type solar still can be folded or separated when it is carried, so that the still would be easy to carry and shipping cost would be very cheap. The solar energy absorbed on the solar collector is transported as latent heat of working fluid to the vertical multiple-effect diffusion-type still where the energy is recycled to increase the productivity of distillate. The performance of the proposed still is analyzed theoretically, and the still is predicted to produce 21.8 kg/m²d distilled water on a sunny autumn equinox day of 22.4 MJ/m²d solar radiation, and the productivity is greater than that of a vertical multiple-effect diffusion-type still coupled with bulky basin type still.     

A highly productive basin-type-multiple-effect coupled solar still

A newly designed multiple-effect solar still with a triangle cross-section consisting of a horizontal basin liner, a tilted double glass cover and the vertical parallel partitions in contact with saline-soaked wicks is presented and theoretically analyzed. Solar radiation is absorbed in the basin and in the first partition, and the partition section recycles the energy from the basin as well as solar energy, which is directly absorbed by the first partition. A single distillation cell with a 5-mm diffusion gap between vertical partitions has been experimentally explored. No contamination of distillate with saline water was experimentally detected in the single distillation cell, and the experimental observation of the wick and the measurements of the temperature drop through the cell with the 5-mm gap showed that dry patches hardly appear on the wick. The proposed still with 5-mm diffusion gaps is theoretically predicted to produce distillate of 15.4kg⁻² m⁻²d⁻¹ on a sunny day of G_ti=22.4 MJ/m⁻²d⁻¹ solar radiation, and its efficiency is about 3.5 times larger than the average experimental value for the conventional basin type stills by Cooper [20] and 1.2-1.6 times larger than the experimental maximum values of the conventional multiple-effect stills [3,9].     

Parametric study on a vertical multiple-effect diffusion-typesolar still coupled with a heat-pipe solar collector

A parametric investigation was theoretically performed for the vertical multiple-effect diffusion-type solar still, which consists of a number of vertical partitions in contact with saline-soaked wicks with narrow gaps between the partitions, coupled with a heat-pipe solar collector. The proposed still has some advantages: the still's size is compact, the still can produce distilled water without electricity, and the productivity is greater than that of conventional multiple-effect diffusion-type solar stills. We theoretically predicted the optimum angle of the solar collector on the spring and autumn equinox and the summer and winter solstice days, and also performed parametric investigations of the design and operation conditions; it was found that productivity increased with an increase in the number of partitions and the temperature of the saline water fed to the wicks, and with a decrease in the ratio of the solar collector area to each partition area, the thickness of the diffusion gaps between partitions, and the feeding rate of saline water to the wicks.     

The effect of using different designs of solar stills on water distillation

Solar distillation is one of the important methods of utilizing the solar energy for the supply of potable water to small communities where the natural supply of fresh water is inadequate or of poor quality, and where sunshine is abundant. Solar energy utilization in two different types of solar stills is considered, and factors that influence the productivity of solar stills are discussed. The present investigation showed that the productivity of asymmetric greenhouse type still (ASGHT) having mirrors on its inside walls was higher than that of the symmetric greenhouse type still (SGHT) and more efficient. It was found that the distilled water output of the asymmetrical greenhouse type was 20% higher than that of symmetric greenhouse type. Performance characteristics of the two stills showed that the temperature at the water surface is closely related to the incident solar radiation, and the productivity of the stills can be increased with decreasing water depth, and by the addition of dye.     

Experimental study of basin-type, multiple-effect, diffusion-coupled solar still

Basin-type, multiple-effect, diffusion-coupled solar stills consisting of a tilted double glass cover, a horizontal basin liner and a number of closely spaced vertical partitions in contact with saline-soaked wicks were constructed and then examined in an outdoor experiment in fall and winter at Okinawa, Japan. This new type of still is greatly improved by narrowing diffusion gaps between the partitions, increasing the number of partitions, sandwiching small spacers between partitions and insulating the frame of the glass cover. The daily performances of the improved still are in good agreement with predictions calculated from the theoretical model developed by Tanaka et al. [1]. The still with 5-mm diffusion gaps between 11 partitions produces 14.8-18.7 kg/d distillate per unit effective area of the glass cover at 20.9-22.4 MJ m⁻² d⁻¹ solar radiations incident on the glass cover and 19-30°C ambient air temperatures and is highly productive in comparison with the 7-effect diffusion-type still with 8-mm gaps developed by Okamura et al. [2] and the 3-effect diffusion-type still with 19-mm gaps of Cooper and Appleyard [3].     

A solar still augmented with a flat-plate collector

A single-stage, basin-type solar still and a conventional flat-plate collector were connected together in order to studythe effect of augmentation on the still under local conditions. 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. Measurements of various temperatures, solar intensities and distilled water productions were taken for several days at various operating conditions. Several modes of operation were studied: still connected to collector for a 24-h period; still connected only during sunlight hours from 8 am to 5 pm, and still operating alone for a 24-h period. These tests were performed using tap water and saline water as a feed. It was found that the mass of distilled water production using augmentation was increased by 231% in the case of tap water as a feed and by 52% in the case of salt water as a feed.     

A simple and highly productive solar still: a vertical multiple-effect diffusion-type solar still coupled with a flat-plate mirror

This paper presents a newly designed simple solar still: a vertical multiple-effect diffusion-type solar still consisting of a flat-plate mirror, a number of vertical parallel partitions in contact with saline-soaked wicks with narrow air gaps between partitions, and casters for azimuth tracking by human power. The proposed still is designed so that non-skilled persons can construct it with common materials in developing countries. We theoretically analyzed the dependence of the solar absorption on the first (or heated) partition on the angle of the flat-plate mirror and the azimuth angle of the still, and found that the solar absorption on the first partition can be significantly increased by rotating the still at southing of the sun just once a day, and the daily solar absorption would be about 85% or 99% of the daily solar radiation on a horizontal surface on the spring equinox or winter solstice, respectively. We also theoretically analyzed the heat and mass transfer in the proposed still, and found that a proposed still of 10 partitions with 10 mm diffusion gaps between partitions was predicted to produce 29.2 34.5 kg/m²d on sunny spring equinox and winter solstice days respectively at the equator.     

Experimental and theoretical method for the determination of the daily output of a solar still: input-output method

In the present work the thermal behavior of a typical solar still is examined experimentally and theoretically and its basic characteristics are analyzed. It is concluded from a larger number of experimental measurement that the operation of such stills during daytime is characterized by three phases: (1) starting, (2) pseudo steady-state, and (3) saturation. A mathematical equation is formed for the instantaneous simulation of the solar still during the pseudo steady-state phase. From this the equation in its integrated form determines satisfactorily the daily output by the daily solar radiation (solar energy input), the average ambient air temperature during the day, T_a(av), and the temperature of the water in the basin at the beginning of the day, T_win, as parameters. This method is called the “input-output” method.     

Thermal performance of a triple-basin solar still

A transient mathematical model is presented for a triple-basin solar still. It is based on an analytical solution of the energy-balance equations for the various elements of the still. The energy-balance equations are solved analytically using the elimination technique. System performance was investigated by computer simulation. Numerical calculations were performed on typical summer and winter days in Tanta (latitude 30°47′ N) for different water masses in each effect of the still and also for various wind speeds (V) to study the effect of these parameters on the daily productivity of the system. It was observed that the daily total productivity of the still decreases with the increase of water mass in each basin. The total productivity was a maximum for the least water mass in both the lower and middle basins without dry spots over the base of each effect. Moreover, it was found that the daily total productivity of the still increases with the increase of V up to a typical velocity (V_t), beyond which the increase in productivity becomes insignificant. The value of V_t is independent of the water mass in each effect, but it showed some seasonal dependence. On a typical summer day, the daily total productivity of the still was found to be 12.635 kg/m²/d, which agrees well with the results reported in the literature for triple-effect solar stills.     

Tilted wick solar still with external flat plate reflector: Optimum inclination of still and reflector

We present a theoretical analysis of a tilted wick solar still with an external flat plate reflector. The reflector is assumed to be able to be inclined forwards or backwards according to the seasons. We theoretically predicted the daily amount of distillate of the still throughout the year, which varies with the inclination angle of both the still and reflector. We found the optimum inclination angle of the still as well as the optimum inclination angle of the reflector for each month at 30°N latitude. The daily amount of distillate of the still can be increased by adjusting the inclination of both the still and reflector for any season, thus producing an average of about 21% more than a conventional tilted wick still throughout the year.     

Experimental study of a basin type solar still with internal and external reflectors in winter

A basin type solar still with internal and external reflectors was constructed and then examined in outdoor experiments in winter in Kurume, Japan. The external reflector was inclined slightly forward to make the reflected sunrays hit the basin liner of the still effectively. The daily productivity of a basin type still can be increased about 70% to 100% with a very simple modification using internal and external reflectors. The experimental results and the theoretical predictions are in fairly good agreement, especially on clear days.     

Mathematical modeling of an inclined solar water distillation system

An inclined solar water distillation (ISWD) system, which generates distilled water (i.e., condensate) and hot water at the same time, was modeled and simulated. In the parametric studies, the effects of feed water mass flow rate and solar intensity on the system parameters were investigated. Finally, the system was simulated using actual deviations of solar intensity and environment temperature during a typical summer day in North Cyprus. The system can generate 3.5–5.4 kg (per m² absorber plate area) distilled water during a day (i.e., 7 am till 7 pm). The temperature of the produced hot water reached as high as 60°C, and the average water temperature was about 40°C, which is good enough for domestic use, depending on the type of feed water. The simulation results are in good agreement with the experimental results.     

Sun tracking system for productivity enhancement of solar still

A sun tracking system was deployed for enhancing the solar still productivity. A computerized sun tracking device was used for rotating the solar still with the movement of the sun. A comparison between fixed and sun tracked solar stills showed that the use of sun tracking increased the productivity for around 22%, due to the increase of overall efficiency by 2%. It can be concluded that the sun tracking is more effective than fixed system and it is capable of enhancing the productivity.     

Evaporative heat loss and heat transfer for open- and closed-cycle systems of a floating tilted wick solar still

An analytical expression for the thermal efficiency of evaporative heat loss and heat transfer for a open- and closed-cycle systems of floating tilted wick solar stills in terms of system design and climatic parameters has been derived. The expression for open- and closed-cycle systems was validated by performing experiments for both systems. Optimization of the design of the still for evaporative cooling (open cycle) and the distillation unit (closed cycle) was obtained using the derived analytical expression for large-scale installation. Numerical calculations were also carried out for a typical summer day at the Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore, India, to predict the performance of the still.     

Experimental validation of thermal model of a double slope active solar still under natural circulation mode

In this paper, thermal modeling of a double slope active solar still has been carried out on the basis of energy balance of east and west glass covers, water mass and basin liner under natural circulation mode. The thermal model of distillation system has been validated for hourly data for inner and outer glass cover temperatures, water temperature and the yield. The hourly thermal and exergy efficiency of active solar still have also been evaluated for 0.03 m water depth. It has been observed that the thermal efficiency of double slope active solar still is lower than the thermal efficiency of double slope passive solar still. However, the exergy efficiency of double slope active solar still is higher than the exergy efficiency of double slope passive solar still. All numerical calculations have been performed for a typical day in the month of March 2008 for the composite climate of Ghaziabad (28°40′N, 77°25′E), U.P, India.     

An experimental study on an inclined solar water distillation system

An inclined solar water distillation system was designed and tested under actual environmental conditions of northern Cyprus. Unlike solar still systems, the feed water falls down on the solar absorber plate, and the system produces fresh water and hot water simultaneously. It was suspected that the longer the flowing water is held on the absorber plate, the greater the rate of evaporation, leading to an increase in the amount of distilled water. Therefore, the system was tested with three variants: bare plate, black-cloth wick, and black-fleece wick. As was expected, the wicks increased the fresh water generation by two or three times of a bare plate. The hot water temperature was good enough for domestic usage.     

Experimental and numerical analysis of a tube-type networked solar still for desert technology

A tube-type solar still is proposed to integrate a conventional still and a water distribution network suitable to our concept of desert plantation. This still is directly set up on ground-like pipelines connecting brackish water or seawater ponds. The distilled water is immediately supplied to the plants under the ground by penetration without any extra water pipelines. The still is made of metal-free materials, e.g., plastic as vinyl chloride or PET resin which are inhibited from corrosion by seawater. Experimental data measured in our laboratory using infrared lamps showed the effectiveness of the method for productivity, the design of the basin tray and thermal efficiency up to 12.5%. Numerical simulations with experimental data as the temperature profiles at the tube walls estimated heat convection occurring within the still for modification of the Dunkle's model. Active convection over the basin was predicted in the half-cut tube type of the basin as compared with a conventional flat basin. The secondary convective circulations under the basin in case of the flat basin promote bulk flow from the upper part of the basin to the lower part, which suggests the necessity of insulating the lower part of the still. On the other hand, the tube basin enlarges the evaporative area over the basin and restricts the vapor circulation under the basin. Simulated results show that the performance of this type is not necessary for the insulation of a still and possible for setting up the lower part of the still under the ground. The tube-type solar still is very simple and has been researched, but further research is needed to make it a suitable technology for desert plantations.