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.     

Design of a commercial solar-powered greenhouse

A completely solar-powered greenhouse complex has been designed for climate controlled food production in hot, humid environments. The concept uses passive design features to provide irrigation water from saline water sources. This paper discusses the overall system concept and the solar still subsystem which has been designed to provide a continuous fresh water supply. While incorporating many innovative features, the use of proven technologies and passive design results in a system that should compete economically with current, more traditional designs.     

Transient performance of a single basin solar still with water flowing over the glass cover

A simple transient theory of a single basin solar still which includes the effect of water flowing over the glass cover is presented. The daily distillate production of the system is almost doubled by lowering the temperature of the glass cover by water flowing over it at a uniform velocity. Dependence of various operating parameters on the performance of the system has been studied.     

Performance of floating cum tilted-wick type solar still with the effect of water flowing over the glass cover

A simple transient performance of floating cum tilted-wick type solar still has been presented by incorporating the effects of water flowing over a glass cover, heat capacity of tilted-wick water surface and floating-wick water surface. Explicit expressions for flowing water, glass, tilted-wick water surface and floating-wick water surface temperature and efficiency of the system have been derived. Numerical calculations have been carried out for a typical day in the month of March 2004. The relative standard deviations between the numerical and experimental results of different temperature components of the proposed still have been found. The results indicate that the relative standard deviations between theoretical and experimental results are less than 8% (glass cover), 2% (tiltedwick water surface), 1% (floating-wick water surface) and 2% (flowing water at the lower end of the glass cover) an average for the working hours of the day. Moreover, based on the numerical results, the following conclusions have been drawn: (i) glass cover temperature decreases significantly; (ii) the effect of water flowing over the glass cover has a fascinating effect on the production of distillate output during peak sunny hours; (iii) water flow rate of 1.5 m/s is optimum, and beyond it the efficiency decreases. Experimental investigations have been performed at Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore, India.     

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.     

Effect of water depth on internal heat and mass transfer for active solar distillation

In this communication, an attempt has been made to find out the convective heat transfer coefficient for active solar distillation system. It is a well-known fact that the distillate output (the yield) decreases significantly with the increase of water depth in the basin of the solar still. It is also known that more yield is obtained in case of active solar distillation system as compared to passive solar still due to higher temperature difference between the water and inner glass cover temperatures in the active mode. For the present study, experiments have been conducted for 24 hours during winter months for different water depths in the basin (0.05, 0.1 and 0.15 m) for passive as well as active solar distillation system. The objective of the present paper is to study the effect of different water depths in the basin on the heat and mass transfer coefficients. It is inferred that the convective heat transfer coefficient between water and inner condensing cover depends significantly on the water depth in the basin. It is also observed that more yield is obtained during the off shine hours as compared to daytime for higher water depths in solar still (0.10 m and 0.15 m) due to storage effect.     

Modeling and performance analysis of a solar desalination unit with double-glass cover cooling

In this study modeling and performance analysis of a single-basin solar still with the entering brine flowing between a double-glass glazing were investigated. The base area of the solar still is 1 m². The function of this arrangement is to lower the glass temperature and thus increase the water-to-glass temperature difference. This results in improved performance represented by a faster rate of evaporation from the basin. The performance of the still is compared with that of a conventional single-glass cover solar still under identical weather conditions. The results show that the relative performance of the stills depends on the level of insulation used. For perfectly insulated stills the conventional solar still is superior while the double glass is superior when heat loss exceeds a certain value. The hourly and daily productivities of the stills and the temperatures of the water and the glass covers were also predicted under the meteorological conditions of Muscat, Oman.     

Thermal performance of a single-sloped basin still with an inherent built-in additional condenser

This paper presents a theoretical study of the thermal performance of a single-sloped basin still with enhanced evaporation and a built-in additional condenser. The still was built so that one of its cover sides (the condenser) is tilted to be parallel to the sun rays and, therefore, be in the shaded area. The condenser heat capacity and surface area (finned) could be increased so that it will always be cooler than the other glass cover sides and act as a more effective heat and mass sink. Enhancing evaporation is carried out by adding black dyes in the still basin to improve basin absorbitivity and increase evaporation surface area. The proposed still design is simple and is not more complex than the conventional design. The effect of different design, operational, and environmental parameters on still performance are studied. The most influencing parameters are found to be solar intensity, base insulation effectiveness, basin mass, evaporation surface area and condenser inner reflectivity. Other parameters such as condenser material, mass, outer emissivity and surface area, cover angle, and wind speed were found to be of less importance. The effect of glass cover frequent instantaneous cooling (up to once every hour) was also investigated as one of the operational parameters and was found to have insignificant effect. Factors having the most influence were combined together and found to produce a yield of about 55% over the base case.     

Effect of condensing cover material on yield of an active solar still: an experimental validation

An attempt has been made to evaluate inner and outer glass temperature and its effects on yield. Numerical computations have been performed for a typical day in the month of December, 2005, for the climatic condition of New Delhi (latitude: 28°35´ N; longitude: 77°12´ E and an altitude of 216 m above mean sea level). Higher yield was observed for an active solar distillation system as compared to the passive mode due to higher operating temperature differences between water and inner glass cover. The parametric study has also been performed to find out the effects of various parameters, namely thickness of condensing cover, collector absorbing surface, wind velocity and water depth of the still. It is observed that there is significant effect on daily yield due to change in the values of collector absorbing surface, wind velocity and water depth. For all the cases, the correlation of coefficients (r) between predicted and experimental values have been verified and they showed fair agreement with 0.90 < r < 0.99 and root mean square percent deviation 3.22% < e < 22.64%. Effect of condensing cover materials, namely copper and polyvinyl chloride (PVC), on daily yield have also been investigated and compared.     

Transient performance of a stepped solar still withbuilt-in latent heat thermal energy storage

The transient performance of a stepped solar still with built-in latent heat thermal energy storage was studied. Thestill was designed for heating and humidification of agriculture greenhouses (GH) in remote areas. The solar still consists of five stepped basins with an inclined glass cover and is insulated on the bottom. The basin was placed on a slab filled with a layer of paraffin wax (phase change material, PCM) that acts as a latent heat thermal energy storage system (LHTESS). Air from GH enters the still from the bottom, flows between the basins and glass cover where it is heated and humidified, and then flows back into the GH. The still performance parameters investigated were analyzed, and the results compared with the case of a still without the LHTESS. The results showed that the still with LHTESS has an efficiency of 57%, and the total daily yield is about 4.6 L/m². The still temperature as well as outlet air temperature and GH heat load are more uniform compared to the sinusoidal trends for the still without LHTESS. It was found that the relative humidity of circulating air increased along the still and always leaves at saturation conditions. The results indicated that decreasing the air flow rate has an insignificant influence on the still yield, while the GH heat load experiences a decrease. For a selected design and operational parameters, the still was able to provide heat for the GH for 24 h/d. This finding is important since heat could be provided to the GH at night and when it is most needed.     

Development of empirical relation to evaluate the heat transfer coefficients and fractional energy in basin type hybrid (PV/T) active solar still

The simple empirical relation is developed to estimate the glass cover temperature for known values of water and ambient temperatures in basin type hybrid (PV/T) active solar still. The empirical relation developed is based on outdoor experimental results of water and ambient temperature in the range of 14 °C to 92 °C, and 14 °C to 36 °C respectively. The results obtained for glass cover temperature using proposed relation are validated with the experimental as well as using a numerical results (obtained by numerical solution of heat balance equation) of solar still. The proposed glass cover temperature is obtained with a maximum relative error of 1.12% compared to the value obtained through a numerical solution. The maximum relative error in evaporative mode of energy transfers from water surface is obtained as 1.2%.     

Transient analysis of a new humidification-dehumidification solar still

A numerical study was carried out to investigate the transient thermal performance of a new humidification-dehumidification solar still. The still body is a relatively thin rectangular box with a top glass cover and bottom condensing sheet. The still body is divided (by a central insulated stepped sheet carrying a group of basins) into two chambers: the upper evaporation chamber and lower condensation chamber. Air is circulated between the upper evaporation chamber (where it is heated and humidified) and the lower condensation chamber (where it is cooled and dehumidified for water production). The influence of different environmental, design, and operational parameters on the still productivity was investigated. The results indicated that increasing the solar intensity, ambient temperature, basin absorbitivity, and initial saline water temperature increases the system productivity. On the other hand, increasing wind velocity, basin insulation thickness, evaporation and condensation surface areas, condenser emissivity, and saline water mass has little effect on productivity. The interesting result is that the decreasing airflow rate has insignificant influence on system productivity. Decreasing airflow rate from 0.1 to 0.001 kg/s partially increases the productivity from 5.2 to 5.3 L/m². The physical explanation and implications of these findings are explained.     

An experimental study on single basin double slope simulation solar still with thin layer of water in the basin

Single basin solar still is a popular solar device used for converting available brackish or waste water into potable water. Because of its lower productivity, it is not popularly used. Numbers of works are under taken to improve the productivity of the still. The still productivity depends on parameters like solar radiation intensity, atmospheric temperature, basin water depth, glass cover material, thickness and its inclination, wind velocity and the heat capacity of the still. When compared with other parameters, the basin water depth is the main parameter that affects the performance of the still. For a particular still, the basin water temperature is the function of depth with day variation of solar radiation intensity. In this work a double slope single basin passive type still with basin area of 1.75 m² is fabricated and tested under laboratory conditions for a thin layer of water in the basin. For maintaining thin layer of water basin, it is necessary to spread the water through out the basin by some kinds of wick material or porous materials. In this work, performance of the still is compared by using wick materials like light cotton cloth, light jute cloth and sponge sheet of 2 mm thickness and porous materials like washed natural rock of average size 3/8" × 1/4" and quartzite rock of average size 3/8" as spread materials. The actual solar radiation condition is simulated by 2 kW electrical resistance heater placed below the inner basin. The results show that the still with black light cotton cloth as spread material is found to be more productive. At higher water and glass temperature region, for all basin materials, the production rate increases with the decrease of the difference between the water and glass temperatures for certain period and also the productivity decreases with the increase of water temperature for certain time during this period. Hence the production rate is a complex function of water, glass and the difference between the water and glass temperatures, basin volumetric heat capacity of the material and its porosity.     

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.     

Parametric investigation of a double-effect solar still in comparison with a single-effect solar still

This paper reports the use of two mathematical models to compare the productivity of single-effect and double-effect solar stills under different climatic, design and operational parameters in Oman. The shallow water basin, 23° cover tilt angle, 0.1 m insulation thickness, and asphalt coating of the solar still were found to be the optimum design parameters that produced an average annual solar still yield of 4.15 kg/m²/d and 6 kg/m²/d for single and double effect solar stills, respectively. A cost analysis is carried out to shed some light on the potential of utilizing an array of single-effect solar stills or double-effect solar stills for the production of drinking water in remote areas in Oman. It has been found that the unit cost for distilled water using an array of single-effect solar stills is $74/1000 gal (16.3 $/m³) or $62.4/1000 gal (13.7 $/m³) when using a double-effect solar still. The cost saving is 15.7%.     

Micro-climatic environmental conditions inside a greenhousewith a built-in solar distillation system

A numerical study was carried out to investigate the micro-climatic environmental conditions inside a greenhouse-distillation system, self-sufficient for irrigating water. The greenhouse consists of the planting cavity, circulating air channels and roof solar distiller for the production of a rather modest rate of irrigating water. A turbulent, steady-state flow, energy and humidity concentration equations were solved using the numerical code FLUENT 6.1. Velocity vectors, steam function, isotherms and temperature and humidity distribution inside the greenhouse present the resultant: micro-climatic environmental conditions. The results are presented for hot days where cold and humid air (from the evaporative cooler) enters the greenhouse from one side and is circulated through the partially porous cavity (representing the plants) and flows through air flow channels and leaves from a vertical thermal chimney. The results show that, with the selected inlet flow conditions, the flow velocity, temperature, and relative humidity can be within the comfort values for plant growth. The effect of some important environmental, design, and operational parameters on greenhouse micro-climatic conditions has also been highlighted.     

Performance study on an acrylic mirror boosted solar distillation unit utilizing seawater

A solar still is a simple desalination unit, but typical configuration is a sealed box with an angled glass top. Sunlight shining into the box heats water or liquid, causing it to evaporate. The moisture condenses on the relatively cool glass cover and runs down the sloped surface for collection. In this paper, the performance of solar still in terms of collection of distilled water have been analyzed and a booster mirror (acrylic) is attached with just above the glass cover of solar still, which will reflect solar radiation in excess to water and it is possible to adjust the booster mirror for perfect reflection depending upon the sun moving angle. Low rates of distillation have been observed with the existing unit. A notable result has been observed with a boosted distillation unit (4.2 L/m²/d at 890 W/m² max.). The arrangements have been made by commercial Al sheet material and insulated with a thermocol sheet.     

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.     

负压条件管式太阳能蒸馏空腔的传热传质特性

针对目前管式太阳能蒸馏装置在常压运行时产水率较低的问题，提出了一种可在真空负压条件下连续补水运行的管式蒸馏方法，并在定温及定功率加热条件下，测试了实验样机负压运行时的温度及产水率变化，证明了所提出的负压条件下运行的管式蒸馏方法优于常压运行的一般蒸馏方法。在P_T=40 kPa、Q_h=200 W定功率加热实验中，空腔传热温差比常压运行时降低约40%；装置全天累计产水量为1.9 kg，比常压运行时产水量增加22.5%。基于稳态实验数据得到了负压修正的空腔传质计算关系式，在此基础上构建了管式蒸馏装置在负压条件运行时的产水率预测模型，其对实验样机全天累计产水预测误差和最大产水率误差分别为2.1%和4%。     

Transient analysis of a stepped solar still for heating and humidifying greenhouses

This paper presents the transient analysis of a stepped solar still for heating and humidifying agriculture greenhouses (GH). The still body consists of five stepped basins insulated from the bottom with an inclined glass cover. Air from the GH enters the still from the bottom and flows between the basins and glass cover where it is heated and humidified, and then re-enters the GH from the still top. The still performance parameters, i.e., still components heat rates, air temperatures and humidity, still yield and efficiency are analyzed and presented. The results show that (1) the still daily average efficiency (η_av) is about 63%; (2) total daily yield is about 4.92 L/m² out of which 0.81 L/m² is condensed on the glass cover and the rest (4.11 L/m²) enters the GH as humidity carried by the circulating air; (3) temperature distribution for still components increases along the still and follows the sinusoidal trends as the solar intensity; (4) circulating air relative humidity increases along the still and always leaves the still at saturation conditions; and (5) decreasing air flow rate has an insignificant influence on the system productivity; however, it does influence the GH heat load.