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.     

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.     

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.     

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.     

Effect of the condensing cover''s slope on internal heat and mass transfer in distillation: an indoor simulation

The objective of the study was to determine a relation for predicting convective and evaporative heat transfer coefficients for all three condensing surfaces inclined at 15°, 30° and 45° under indoor simulation. The condensing covers were made of the same flat transparent glass as found in any solar distillation unit. The operating temperature range for the experiment was maintained at steady state from 40°C to 80°C by using a constant temperature bath. The temperatures and yields obtained for a 10-min interval were used to determine the values of constants C and n and consequently convective and evaporative heat transfer coefficients. It was found that a higher yield was obtained with an increase in temperature for a 30° slope compared to 15° and 45° slopes of the condensing cover.     

Progresses in improving the effectiveness of the single basin passive solar still

Single basin solar still is a very simple solar device used for converting available brackish or waste water into potable water. This device can be fabricated easily with locally available materials. The maintenance is also cheap and no skilled labor is required. This device can be a suitable solution to solve drinking water problem. Because of its low productivity it is not popularly used. Numbers of works are undertaken to improve the productivity of the still. In this work progresses in the works done on single basin passive types still to improve its productivity are reviewed. The orientation and inclination are optimized to receive maximum radiation and lower the condensation loss. Different materials are used in the basin along with water to improve the heat capacity, radiation absorption capacity and enhance the evaporation rate. Rubber is the best basin material to improve absorption, storage and evaporation effects. The effect of varying the depth of the basin water is also studied.     

Study of heat and mass transfer in indoor conditions for distillation

An indoor simulation study was carried out to evaluate heat and mass transfer relation for a semi-cylindrical metallic (opaque) condensing cover for higher yields at different operating temperatures under free and forced modes of operation. The objective is to design a distillation unit for higher yield for commercialization, particularly in India. Experiments have been conducted for the operating temperature range of 40°C to 80°C in a steady-state condition by using constant temperature bath. Data (temperature and yield) obtained from experimentation have been used to determine the values of coefficient C and n and, consequently, convective as well as evaporative heat transfer coefficients. It is inferred that a higher yield is obtained with an increase of temperature in the forced mode of operation as compared to that in the natural mode of operation due to a fast release of heat from the condensing cover.     

Effect of water depth on daily yield of the still

The effect of water depth on daily yield of a still has been studied using a transient analysis of a solar still. It is found that the dependence of yield on water depth is a strong function of initial temperature of the brine in the basin of the still. It is concluded that the daily yield increases with depth for an initial temperature of the brine ≥45°C and decreases with depth for an initial temperature of the brine ≤40°C. This has also been verified experimentally at IIT Delhi.     

黏性耗散效应对微石英管内部对流换热的影响

以去离子水为工质,对流过内径分别为45 μm、92 μm及141 μm的微石英管内的黏性耗散对换热特性的影响进行了实验研究。通过对紧密缠绕在管外的细铜丝通电以加热管外壁,得到了Reynolds数在100～2000之间变化时的Nusselt数,同时在考虑到电双层效应的基础上计算出黏性耗散所产生的热量。实验结果表明,Re较低时,黏性耗散效应对微管内部对流换热的影响较小;随着Re的增加,黏性耗散对对流换热的影响增大,并随着微管直径的减小而明显增强。对于内径为45 μm的管,当Re达到2000左右时,黏性耗散效应对对流换热的影响超过14.1%;对于内径为141 μm的管,层流黏性耗散效应对微管内部对流换热的影响较小,基本可以忽略。     

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.     

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%.     

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%.     

Performance of solar still with intermittent flow of waste hot water in the basin

The effect of intermittent flow of waste hot water in the basin of solar stills on its performance has been discussed as well as the effect of various parameters — duration of flow of hot water, flow rate and water depth. It is concluded that for higher yield, the waste hot water should be fed into the basin during off-sunshine hours. The results have also been compared with those of Tiwari and Malik [1] and Sodha et al. [3].     

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%。     

Experimental study of the evaporation of a falling film in a closed cavity

This article concerns the experimental study of heat and mass transfer in a distillation cell. This latter is a parallelepiped, of large form factor, whose active walls are vertical. The cell is fed with salt water, and pure water is evaporated from a thin film that falls along a heated wall while the opposite wall is maintained at a lower temperature and is used as a condensation surface. The experimental results show that the heat transfer in the distillation cell is dominated by the latent heat transfer associated with evaporation. A parametric study of the behavior of the distillation cell has been performed. A convenient choice of the operating parameters is suggested to optimize the distillation yield.     

降膜蒸发内回热型太阳能海水淡化装置的实验研究

对一台利用太阳能驱动的横管降膜蒸发内回热型海水淡化装置进行了实际天气条件下的动态测试。系统采用了多效内回热措施,在蒸发及冷凝过程中,大部分水蒸汽的凝结潜热被重复利用于海水的预热及蒸发过程,在相同的天气条件下,该装置的产水率比单级盘式太阳能蒸馏器提高了2～3倍。太阳辐射越强,产水率越大;启动温度越高,产水量越大。对影响淡水产量的其他因素也进行了分析讨论。     

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.     

Integrated performance of stepped and single basin solar stills with mini solar pond

An attempt is made to enhance the productivity of the solar stills by connecting a mini solar pond, stepped solar still and a single basin solar still in series. Experiment is also carried out by replacing the single basin solar still into a wick type solar still. For further augmentation of the yield, baffle plate, pebble, fins, wicks and sponges are added. Day and night productivity of the solar stills for these modifications is studied. Daily efficiency and percentage increase in productivity for these modifications are also studied. Industrial effluent water is used as feed. Theoretical analysis gives very good agreement with experimental results.     

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.     

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.