Mass and heat transfer model of Tubular Solar Still (2023)

Multi-stage solar desalination running under vacuum is a low-cost, effective and sustainable way to mitigate fresh water shortage. However, questions remain on the operation pressure ($P_{op}$) and stage number of the multi-stage solar still. To address these questions, a predictive model was developed for the yield of multi-stage tubular solar still under vacuum (MSSV) and validated against field experiments. A 3-stage tubular still under typical weather (i.e. 19.75 MJ/m${}^2$ cumulative solar radiation) produced maximum water yield (7.15 kg/m${}^2$) with optimal $P_{op}$ (48kPa), much higher than under normal (5.81 kg/m${}^2$) and lower pressure (1.58 kg/m${}^2$ with $P_{op}=20\mathrm{kPa}$). At optimal $P_{op}$, a delicate balance exists between the energy adsorbed and dissipated. Stronger solar intensity leads to more water yield and higher optimal $P_{op}$, while increasing the stage number improved the fresh water yield with diminishing marginal effect and slightly shifted the optimal $P_{op}$ to a lower value. Using water immersion cooling other than air cooling significantly increased the water yield from 7.15 kg/m${}^2$ to 10.51 kg/m${}^2$. The MSSV model enriched the fundamental understanding of energy utilization for solar desalination and could serve as a design tool to maximize the performance of multi-stage solar still by optimizing still parameters and geometry.

Modern designs of solar still desalination technology are reported to tackle the cost and energy issues. However, the low productivity of this technology is the main challenge that inhibits its commercialization uses. In this study, a triangular solar still (TrSS) integrated with an external PVC pipe solar heater and an internal separated condenser is designed for the first time to enhance water desalination under Malaysian climate conditions. The daily water production of active still was 24% higher than that of passive one. The temperatures of water and ambient air, relative humidity inside the still and the intensity of solar radiation affect the daily distillate output of both stills. Besides, a few strong relationships were developed between the intensity of solar radiation and water productivity, between the daily average ambient temperature and water productivity, and between the daily average water temperature and water productivity. The desalinated water cost per liter is estimated and is reasonable. Finally, the water quality tests of raw seawater and product water were performed and analyzed. A significant improvement in the desalinated water quality is realized when compared with raw seawater quality. The product water was considered as safe drinkable water while comparing its quality with the World Health Organization (WHO) standards.

Tubular solar still (TSS) has emerged significantly since the last decades for freshwater production especially in small communities and remote areas away from the main infrastructures. In this review, the recent developments, and the future of the various TSS designs and techniques have been discussed in detail. The study covers the parameters affecting the TSS performance concentrating on important achievements and developments in the last ten years with future perspective on the TSS potential in the future desalination market. Moreover, the study analyzed the pre-existing knowledge and highlights the knowledge gaps of the TSS technologies. Results and analysis of this review showed a great potential of the TSS design which still has many opportunities for further development. Some recent TSS applications have been analyzed such as: atmospheric water harvesting (AWH), thermal photovoltage (PVT), recent promising geometries, heat storage materials, and nanomaterials and nanofluids. Moreover, an economic evaluation of the recent TSS technologies has been conducted based on the cost per liter (CPL) of the produced freshwater. At the end, the authors introduced their perspective and outlook on the future of the TSS technologies.

Solar still is widely used for supplying fresh water to small communities in remote areas. One drawback of this technique lies in the low freshwater yield. Recent studies on stills of multi-effect and vacuum design proved their potential for high yield. However, such systems suffer from high electricity consumption and insufficient cooling. In this study, a novel system with a periodic pressure control scheme and water immersion cooling has been proposed to mitigate these defects. A prototype was constructed and associated with a 0.19-m² solar panel. A 5-day outdoor experiment was conducted to evaluate the overall performance. Results indicated that the highest yield during the test was 9.8kg/m² at operating pressure of 40kPa. A significant performance ratio of 1.87 was achieved with immersion cooling, i.e., 0.42 higher than that with air cooling. Thermal analysis showed that the heat transfer coefficient of water immersion cooling was 15–50 times higher than that of air cooling. Compared with previous vacuum-operated systems, the specific electricity consumption of maintaining vacuum was greatly reduced, i.e., from 21.6kJ/kg to 1.7kJ/kg for the case at 60kPa. The forecast cost of the distilled water is $0.012/kg, representing an affordable desalination technique for off-grid communities.

Both basin and tubular solar stills have been used for solar desalination for long time. Using solar energy for the freshwater production is a big challenge due to their low productivity with relatively high cost. However, in some specific situations like rural and isolated areas, it is considered suitable. The present study aims to analyze the latest situation of the solar water desalination based on the two typical solar stills (basin and tubular). The potential of both types for leading the future development of the solar desalination has been evaluated. Various techniques and developments of solar stills such as multi-effect, wick, heat storage materials, condenser surface and photovoltaic thermal (PVT) systems and atmospheric water harvesting (AWH) have been reviewed. The most promising solar still techniques and the future expectations have been discussed at the end of the study.

Tubular solar still is characterized by having a large condensing and receiving surface area compared to traditional single slope solar still. The present study aims to enhance the performance of tubular solar still through the mathematical and experimental investigation of proposed modifications applied to the conventional tubular solar still. The modification was done by using the v-corrugated absorbers and inclusion of wick materials to maximize the rate of evaporation, in order to achieve the maximum possible utilization of the large condensing surface area that characterize the tubular solar stills to increase the distillate water productivity. This goal was achieved by forming the absorbent surface in a v-corrugated form and using black jute clothes that are installed on the v-corrugated surface so that they take the same corrugated form and the lower half of the corrugated black jute clothes was immersed in water and the rest of the clothes are saturated with water by the capillary property. The performance of modified tubular still with v-corrugated wick materials and conventional tubular still with flat absorber surface was compared under the weather Egyptian conditions to characterize the performance enhancement of the modified design. The results show that the conventional tubular still gives the accumulated distillate production of 4150ml/m² while the v-corrugated wick materials utilization improves the distillate production to 6010ml/m², with 44.82% improvement in the productivity. Also, the average efficiency along the test days for the conventional tubular still reached 35%, while the v-corrugated wick materials utilization improves the average efficiency to51.4%, with 46.86% improvement in average efficiency.

Desalination, Volume 375, 2015, pp. 10-20

This paper presents an experimental investigation and performance analysis of a novel triple-effect vertical concentric tubular solar desalination unit. The productivity and thermal efficiency of the device for three operational modes (e.g., single-, double- and triple-effect) were tested and compared under the fixed heating power and controlled heating temperature conditions, respectively. The temperature in different measured points in the device was tested. Also, Gain Output Ratio (GOR) of the system was calculated for three operational modes. The experimental results indicate that the fresh water production for the triple-effect mode can reach 1.162kg/h at the heat input of 400W. The GOR of the device for the double-effect and triple-effect modes can reach about 1.21 and 1.89 under the condition of fixed heat input. The experimental results are then compared with the numerical results, and it is found that the trend of the experimental results is consistent with the theoretical water production rate. The economic analysis results show that the production cost of water is about 6.6$/ton, which implies an excellent application prospects of the designed desalination device.

Renewable and Sustainable Energy Reviews, Volume 81, Part 1, 2018, pp. 136-148

Adequate quality and reliability of drinking water is vital for all inhabitants and for agriculture and industrial applications. Solar desalination is impactful method for getting potable water from brackish/wastewater in hot climatic condition and/or remote area where the scarcity of water as well as for electricity. In recent years, attention has been focused on development of various designs of solar still in order to overcome limitations possesses by conventional single basin single slope solar still. Pyramid solar still is one of the outcomes of such a development. The present paper reviews the development in the field of pyramid solar still as well as the various techniques to improve the performance of still. From the review on research carried out by the various researchers, it has been found that pyramid solar still is more efficient and economical in compare to conventional single slope single basin still. Thus, the review paper will assist the researchers to understand the fundamentals of pyramid solar still with the need, developments and challenges in pyramid solar still to improve its thermal performance and to make it more and more economic.

Desalination, Volume 314, 2013, pp. 67-72

This paper presents a modification of stepped solar still through internal reflectors. A comparison study between modified stepped solar still with trays (5mm depth×120mm width) and conventional solar still was carried out to evaluate the developed desalination system performance under the same climate conditions. The effect of installing a reflecting mirror on the vertical sides of the steps of stepped still on the distillate productivity was investigated. An experimental as well as theoretical investigation is carried out. The results indicate that, during experimentation the productivity of the modified stepped solar still with and without internal reflectors is higher than that for conventional still approximately by 75% and 57%, respectively. Also the daily efficiency for modified stepped still with and without internal reflectors and conventional solar still is approximately 56%, 53% and 34%, respectively.

Desalination, Volume 500, 2021, Article 114880

The present study developed a novel PCM-tubes geometry to be used inside a tubular solar still with parabolic solar concentrator. Paraffin wax is filled inside 12-aluminum tubes working as a PCM material. Copper rods are installed inside the paraffin wax to enhance thermal properties. Parabolic concentrator solar tracking system is used to intensify solar radiations on the tubular solar still. Experimental work was executed under the climatic conditions of Ha'il city (27.64 °N, 41.75°E) 965m above sea level in Saudi Arabia. The 12‑aluminum tubes were black painted and placed inside the trough. Results show a great enhancement of the freshwater productivity and device performance with relatively low production cost. Using PCM-tubes enhances the productivity and efficiency of the developed device by 40.51% and 38.25% respectively. The yield, efficiency and cost per liter are 5.55L/m²day, 44.1% and 0.00782 USD for device with PCM-tubes compared to 3.95L/m²day, 31.9% and 0.0163 USD for device without PCM-tubes. The developed device was able to reduce the freshwater production cost by 21.8% achieving a very competitive cost compared to the available solar desalination techniques. Further development of the tubular solar still is required in the future research effort.

Desalination, Volume 329, 2013, pp. 68-77

A computer program has been developed to simulate the effect of the orientations (east–west and south–north), on the performance of a symmetric double slope solar still compared to those of an asymmetric solar still with a double effect, in order to obtain the optimum design parameters for both types, under climatic conditions of Constantine (Eastern area of Algeria), using meteorological data of 1st August of 2005 as the hottest day selected from the last decade (2002–2012). The obtained results show that10° is the optimum inclination angle that allows receiving a maximum solar radiation for both stills by increasing the evaporation condensation processes. A water depth of 0.02m gives higher daily production. The stability of solar radiation seen in a south–north orientation leads to high reception of solar radiation by both stills. As a consequence, the daily productivity of an asymmetric still, with a double effect process is found higher than that corresponding to the symmetric one with a double slope system, and where at the south–north orientation we notice an increase of 22.57% and at the east–west orientation it is of 16.23%. It is shown also that a south–north orientation enhances the performance of the asymmetric still with a double effect by 16.76%.

Desalination, Volume 443, 2018, pp. 44-55

In this study, two types of solar stills, triangular and tubular one, have been experimentally tested under a real weather condition. Following the same procedure, the experiments were carried out over seven typical winter days and the effects of solar radiation and ambient temperature on water productivity and total efficiency of the stills has been experimentally investigated. Furthuremore, to understand the detail structures of the air flow inside the enclosures, the fluid flow has been numerically simulated using computational fluid dynamics. Having the details of the fluid flow, the values of local entropy generation in the chamber have been obtained. The results indicated that the tubular still showed a better performance by 20% compared to the triangular one. The results of numerical simulation showed that the greater strength of the recirculating zones and the lower entropy generation are the main reasons to have a better water production in the tubular still. Furthermore, the cost of water production by the triangular solar still was found to be lower due to its lower manufacturing cost compare to that of tubular one. Based on the experimental results, two new correlations have been proposed to predict the water productivity in these systems.