Computation of solar radiation distribution in a solar still with internal and external reflectors

A model that calculates the distribution of solar radiation inside a basin-type solar still with plane reflectors has been proposed. In this model, both surface finish and optical view factors are taken into consideration in the computation of solar radiation that effectively reaches the surface of saline water in the distillation system. The model was applied to a conventional solar still and an advanced solar still with separate condenser. The conventional solar still was tested at New Delhi (28°35′N, 77°12′E) in previous work while the advanced distillation system was developed in this investigation and it comprised three basins with saline water. Basin 1 was fitted horizontally in the evaporator chamber, with the condenser chamber housing basins 2 and 3. The top part of the evaporator unit was covered with glass to allow solar radiation to reach water in basin 1. In addition, the internal surfaces of the walls of the evaporator and external surface of the front wall of the condenser unit reflected part of the incoming solar radiation onto the surface of saline water in basin 1. The distillation system was tested outdoors at the Malawi Polytechnic (15°48′S, 35°02′E). Under the prevailing meteorological conditions, it is found that the computed solar load on the surface of saline water in the evaporator basin is lower than that observed on a horizontal plane outside each energy system. The new model exhibited a higher degree of accuracy than the previous one. Other results are presented and discussed in this paper.     

Analysis of a parallel double glass solar still with separate condenser

A solar still designed and built at Physics department Hacettepe University, Ankara, Turkey (39° 57′N), utilizes direct and reflected (from a reflector) solar radiation incident on a parallel double glass cover, to evaporate sea or brackish water. Water vapor purges from the evaporator and diffuses to an integrated condenser due to pressure difference exists as a result of the volume ratio and temperature difference between the evaporator and condenser. A theoretical analysis based on the energy balance for different components of the unit was performed. Graphical analysis were presented to show the variations of solar radiation, ambient temperature, basin water temperature, vapor temperature and other important temperatures at different locations in the solar still. The productivity was in good agreement with what obtained by other authors and the efficiency was increased from 48% to more than 70% when the condenser cover was cooled down.     

Cascade solar still for distilled water production

This paper describes a new design for a tilted double-sided solar still with cascade water trays made of corrugated aluminium which are positioned 1·25 in. (3·2 cm) below the glass cover on insulated supports. The mean effective water depth is 0·7 in. (1·8 cm) which leads to high water temperatures (60° – 78°C) and a low thermal inertia (15 min) but is sufficient to ensure two days of maximum production before dry spots develop. Temperatures increase from the outer edges to the apex, hence the apex glass support is hollow and preheats the feed water when the still is fed continuously. At vapour temperatures of over 55°C the glass cover no longer condenses all the vapour so a secondary surface is positioned beneath the cascade, increasing the condensing ratio (ratio of condensing area to horizontal sunlight collector area) from about 1:1 to about 2:1 and allowing distillate collection from both surfaces. The cover produces 70–100 per cent of the distillate depending upon the vapour temperature. With batch feeding, thermal efficiencies of 60–75 per cent are usually obtained. These high efficiencies are due to the short distillation gap, the high condensing ratio, the low thermal inertia, and the fact that most heat transfer from the cascade to the condensing surfaces is accompanied by vapour transfer since radiative and conductive heat losses from the cascade are small.     

Thermal modelling of high temperature distillation

This communication presents a comparative study of a single basin solar still under various modes of operation. A simple transient analysis of all the modes under the same meteorological conditions has been presented. The water depth in the solar still and the absorptivity of the basin liner along with the water flow over the glass cover of the still has been found to affect the daily distillate production of the system considerably. The thermosyphon mode enhances the daily distillate production. It has also been observed that the evaporative heat transfer coefficient is a very strong function of temperature.     

Effect of inclination of external flat plate reflector of basin type still in winter

This paper presents a theoretical analysis of a basin type solar still with an internal reflector (two sides and back walls) and an inclined flat plate external reflector on a winter solstice day at 30° N latitude. We are proposing a new geometrical method for calculating the solar radiation reflected by the inclined external reflector and then absorbed on the basin liner. Using this method, we performed a numerical analysis of heat and mass transfer in the still in order to determine the effectiveness of the inclination of the external reflector. We found that the benefit of a vertical external reflector would be smaller or even negligible for a still with a larger value for the glass cover angle, while an inclined external reflector can increase the distillate productivity of the still at any glass cover angle, and the external reflector angle should be set at about 15° from vertical on a winter solstice day. The daily amount of distillate of the still with the inclined external reflector would be about 16% greater than that with the vertical external reflector, and about 2.3 times as large as that of the still with neither the internal nor the external reflector on a winter solstice day.     

Theory and experimental investigation of a weir-type inclined solar still

A weir-type solar still is proposed to recover rejected water from the water purifying systems for solar hydrogen production. This consists of an inclined absorber plate formed to make weirs, as well as a top basin and a bottom basin. Water is flowed from the top basin over the weirs to the bottom collection basin. A small pump is used to return the unevaporated water to the top tank. Hourly distillate productivity of the still with double- and single-pane glass covers was measured and the latter showed higher production rates. The average distillate productivities for double- and single-pane glass covers are approximately 2.2 and 5.5 l/m²/day in the months of August and September in Las Vegas, respectively. Mathematical models that can predict the hourly distillate productivity are developed. These compared well with the experimental results. Productivity of the weir-type still with a single-pane glass was also compared with conventional basin types tested at the same location. The productivity of the weir-type still is approximately 20% higher. The quality of distillate from the still is analyzed to verify the ability of the still to meet the standards required by the electrolyzers.     

一种改进的太阳能吸附式空气取水器

介绍了一种透光良好、冷凝迅速、吸附彻底、集热高效的太阳能吸附式空气取水器。冷凝罩在上布置, 便于和空气分层的水蒸气及时冷凝;冷凝罩夹层内所填相变材料的蓄冷,加速冷凝过程。透光罩在下布置,避 免了水蒸气在其内表面的凝结,以维持原有透光率。金属网球既可对吸附床内的吸附剂导热、均温,从而提高 集热效率;又可在吸附床内均布气道,加速吸,脱附过程。解决了沙漠地区的淡水补给问题,避免了淡水运输的 困难。     

Transient analysis of double basin solar still

The effect of water flowing over the upper glass cover of a double basin solar still on its transient performance has been presented. A comparative study of the daily distillate production of a double basin solar still with and without water flowing over the upper glass cover has been made, and some interesting conclusions have been drawn. Numerical calculations have been made for a typical hot day (viz 2 May 1980) in Delhi.     

Modeling thermal asymmetries in double slope solar stills

Solar distillation literature reports a generalized use of stills with single and double slope condensing covers, where modeling equations describe a solution approach that considers the condenser as a single element. The solution for a double slope still analysis is generally forced to the case of one with a single flat condensing cover. This paper proposes a new lumped parameters mathematical model to study the asymmetries that arise in the temperature and distillate yield in double slope solar stills. The condenser is studied as a two-element system and non-simplified equations for heat transfer and optical transmission characteristics are used. The model is tested for the case with the strongest thermal differences and validated with experimental data. Overall results show a good correlation between predictions and experimentation.     

Thermal performance of the condensing covers in a triangular solar still

An experimental investigation has been carried out into an attic-shaped solar still subject to natural conditions of Grashof numbers up to 1.7×10⁸. The performance of the condensing covers was studied under two still orientations, east–west and north–south. A continuous recording of the main parameters affecting productivity was made, and significant thermal and distillate differences between the two condensing plates were found. A procedure to estimate each cover production in triangular solar stills as a function of still temperature and area fraction is proposed as an extension to the model proposed by Dunkle. Predictions made with this procedure show closeness to experimental data for the cases studied.     

Recent advancements in condensers to enhance the performance of solar still: A review

Pure drinking water sources are continuously decreasing day by day in the world and in contrast requirement is continuously increasing. Solar still is a device, which converts the saline water into pure form at a lower cost and is useful in the remote areas. But its less productivity and low efficiency is a significant issue to use it as potable water provider. Solar still productivity can be increased by using flat plate collectors, fins, evacuated tubes, different absorbing, and nanomaterials, but the major problem on its productivity is heat loss from glass cover to the ambient. Condenser reduces the temperature of the water vapor and maintain the condensation to enhance the productivity. This review shows different modification in the design of condenser attached with solar still to enhance the yield. At last the future work on the condenser is also discussed here.     

ETUDE EXPERIMENTALE ET COMPARATIVE DE L'INFLUENCE DE L'HUILE DE PARAFFINE ET DE LA COUVERTURE AUX LACS SOLAIRES SANS EAU SALÈE

A simple solar still coupled to an external condenser, to enhance the yield, is modeled and analytically investigated. The analysis is general in nature and not limited to a particular condenser features. The present analysis, based on actual meteorological data, shows that the yield improvement depends upon both brine level and the surface area provided for heat removal. For an ideal still with negligible thermal inertia (water depth approaches zero) an auxiliary condensing surface at a temperature below the glass temperature decreases the daily yield. The still yield is improved for brine levels up to 10 cm above this level additional heat removal from the still does not significantly improve the daily productivity. The productivity for the limiting condition when the auxiliary condensing surface is maintained at the ambient temperature is also presented and discussed.     

Performance of a double condensing multiple wick solar still

Because of high insolation on hot days, the condensing glass cover of a simple multiple wick solar still becomes reasonably hot, and hence, the yield is not very high. To increase the yield under this situation, a double condensing, multiple wick solar still has been proposed. In this case, the area of the condensing surface can be increased by introducing an additional G.I. sheet just below the blackened wet jute cloth with a slight spacing around the absorbing surface, such that the excess of vapor can also be condensed below the G.I. tray G₁ and above the G.I. tray G₂.     

Analytical expression for thermal efficiency of a passive solar still

Using energy balance equations for the different components of a passive solar still (namely, glass cover, water mass and basin liner), the effect on its thermal efficiency of water flowing over the glass cover with uniform velocity has been studied analytically. The efficiency is found to increase due to the water flow at higher depths of water in the basin. The result is validated by the experimental results of other authors.     

Monthly optimum inclination of glass cover and external reflector of a basin type solar still with internal and external reflector

In this report, we present a theoretical analysis of a basin type solar still with internal and external reflectors. The external reflector is a flat plate that extends from the back wall of the still, and can presumably be inclined forwards or backwards according to the month. We have theoretically predicted the daily amount of distillate produced by the still throughout the year, which varies according to the inclination angle of both the glass cover and the external reflector, at 30°N latitude. We found the optimum external reflector inclination for each month for a still with a glass cover inclination of 10-50°. The increase in the average daily amount of distillate throughout the year of a still with inclined external reflector with optimum inclination in addition to an internal reflector, compared to a conventional basin type still was predicted to be 29%, 43% or 67% when the glass cover inclination is 10°, 30° or 50° and the length of external reflector is half the still’s length.     

Performance analysis of double basin solar still with evacuated tubes

Solar still is a very simple device, which is used for solar distillation process. In this research work, double basin solar still is made from locally available materials. Double basin solar still is made in such a way that, outer basin is exposed to sun and lower side of inner basin is directly connected with evacuated tubes to increase distillate output and reducing heat losses of a solar still. The overall size of the lower basin is about 1006 × 325 × 380 mm, the outer basin is about 1006 × 536 × 100 mm Black granite gravel is used to increase distillate output by reducing quantity of brackish or saline water in the both basins. Several experiments have conducted to determine the performance of a solar still in climate conditions of Mehsana (latitude of 23°59′ and longitude of 72°38′), Gujarat, like a double basin solar still alone, double basin solar still with different size black granite gravel, double basin solar still with evacuated tubes and double basin solar still with evacuated tubes and different size black granite gravel. Experimental results show that, connecting evacuated tubes with the lower side of the inner basin increases daily distillate output of 56% and is increased by 60, 63 and 67% with average 10, 20 and 30 mm size black granite gravel. Economic analysis of present double basin solar still is 195 days.     

Performance of a multiple-wick solar still with condenser

In a conventional still, because the water condenses underneath a glass cover, its temperature becomes quite high. During the period of maximum sunshine, the glass temperature is higher than the dew point of the air-vapour mixture inside the still. Thus the yields of these kinds of stills are low. In this communication we have investigated experimentally the performance of a still with a condenser. Most of the condensation now takes place in the condenser, consequently the glass temperature remains low causing less heat loss to the ambient environment. The yield of the condenser-type still is higher than that of the non-condenser-type still.     

Theoretical and experimental studies of an ethanol basin solar still

A transient-state mathematical model for an ethanol basin solar still based on Spalding's work was developed. Driving force B was defined based on the mass balance between the evaporating (S) and condensing (G) surfaces. Mass transfer conductance (g) was obtained from an indoor experiment. Then productivity could be calculated. In order to validate the model an ethanol basin solar still was tested under outdoor conditions. The model had RMSEs of 4% and 23% of the measured mean temperature and productivity. The mean productivity was 0.33 kg/h when the mean solar radiation input was 1.95 MJ/m²/h. The simulated distillate concentrations were 74, 59 and 24%v/v for ethanol solution concentrations of 50, 30 and 10%v/v. The monthly means of the simulated daily productivity and total daily solar radiation were linearly correlated.

An indoor experimental equipment of the same type as that used for the outdoor experiments was constructed. Ethanol solutions with concentrations of 10–100%v/v were distilled. The ethanol solution temperature varied between 40 and 70 °C. The experimental data from the still was then used to find the g used for the above mathematical model. The still height had a slight effect on the productivity. Increasing the ethanol solution concentration by not more than around 80% v/v could improve the still productivity.     

Stand alone triple basin solar desalination system with cover cooling and parabolic dish concentrator

A stand-alone triple basin solar desalination system is experimentally tested and the results are discussed in this paper. This system mainly consists of a triple basin glass solar still (TBSS), cover cooling (CC) arrangement, parabolic dish concentrator (PDC) and photovoltaic (PV) panel. Four triangular hollow fins are attached at the bottom of the upper and middle basin in order to increase the heat transfer rate and place the energy storing materials. The performance of the system is studied by, conventional TBSS system, integrating the TBSS with CC, TBSS with PDC, and TBSS with CC and PDC. Also, each configuration is tested further by using fins without energy storing material, fins filled with river sand, and fins filled with charcoal. The results of the test reveal that, TBSS with charcoal and TBSS with river sand enhance the distillate by 34.2 and 25.6% higher than conventional TBSS distillates. TBSS with cover cooling reduces the glass temperature to about 8 °C compared to the conventional TBSS. The presence of concentrator increases the lower basin water temperature upto 85 °C. The maximum distillate yield of 16.94 kg/m².day is obtained for TBSS with concentrator, cover cooling and charcoal in fins.     

A numerical model and experimental investigation for a solar still in climatic conditions in Abidjan (Côte d''Ivoire)

A solar distillation in a single basin is studied theoretically and experimentally. We present a mathematical model to describe the energy balances for the glass cover, water in the basin and base plate. The model neglects the thermal capacity of the base plate and the temperature gradient through the width of the glass. The energy equations for the glass, water and absorber are simultaneously solved. The effects of water depth, wind speed and glass cover thickness on still productivity are evaluated. The daily total production increases with decreasing water depth. A small increasing of productivity occurs with the increase of wind speed. The thickness of the glass cover has no effect on the production. An experiment has been conducted to validate the mathematical model. The relative difference between experiment and theory is 5% for temperatures, and 15% for productivity.