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


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Solar Energy

Volume 84, Issue 7,

July 2010

, Pages 1147-1156

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(Video) CFD Solar Still -Simultaneous Heat and Mass Transfer Model- rights and content


In this paper, a new mass and heat transfer model of a Tubular Solar Still (TSS) was proposed incorporating various mass and heat transfer coefficients taking account of the humid air properties inside the still. The heat balance of the humid air and the mass balance of the water vapor in the humid air were formulized for the first time. As a result, the proposed model enabled to calculate the diurnal variations of the temperature, water vapor density and relative humidity of the humid air, and to predict the hourly condensation flux besides the temperatures of the water, cover and trough, and the hourly evaporation flux. The validity of the proposed model was verified using the field experimental results carried out in Fukui, Japan and Muscat, Oman in 2008. The diurnal variations of the calculated temperatures and water vapor densities had a good agreement with the observed ones. Furthermore, the proposed model can predict the daily and hourly production flux precisely.


In remote and coastal areas of Bangladesh, India, Kenya, Ethiopia, Peru, Nigeria and United States, many people have been suffering from the shortage of drinking or cooking water. Many remote and coastal areas in underdeveloped and developing countries do not have an enough resource of electric power for producing distilled water using any conventional water desalination techniques; namely multi-effect distillation, multi-stage flash, reverse osmosis and vapor compression. A high initial installment cost of these conventional techniques is far beyond the fiscal resources solvency in most local regions. In addition, waterworks are not installed in these regions, and the road network and transportation system are insufficient to carry/transport a large amount of water regularly.

In view of such social backgrounds, we designed a new Tubular Solar Still (TSS) so as to produce distilled water near a house for the purpose of reducing time and labor involved in carrying of drinking and cooking water for families or small society. Moreover, the new TSS should be made of cheap, durable and locally acquisitioned materials to reduce the initial cost and to keep using it for many years. The change of the materials facilitated the assembly, set-up and maintenance of TSS.

Numerical modeling of mass and heat transfer in a solar still is very significant to design a economically optimal still and to enhance the production performance for a given cost. Many researchers (e.g. Dunkle, 1961, Chaibi, 2000, Nafey et al., 2000, Hongfei et al., 2002, Paul, 2002, Al-Karaghouli and Alnaser, 2004, Kalogirou, 2005, Tiwari and Tiwari, 2008) have focused their research on conventional basin-type stills rather than tubular stills. Dunkle (1961) first proposed theoretical relations of the convective heat transfer coefficient and evaporative heat flux for basin-type still. Tiwari and Kumar, 1988, Kumar and Anand, 1992 used these correlations for analyzing the performance of a tubular still and a multiwick tubular still, respectively.

In order to predict the distilled water output, i.e. production, most of the production models of basin-type still have the evaporative mass and heat transfer correlations using the temperature and vapor pressure on the water surface and still cover without noting the presence of intermediate medium, i.e. humid air. In fact, the humid air was assumed to be saturated, because of the easiness of the calculation of evaporation from the water surface, we suppose. Recently from our field experiments (Fukuhara et al., 2002, Islam et al., 2005, Fukuhara and Islam, 2006, Ahsan, 2009) we found that the relative humidity of the humid air is definitely not saturated in the daytime. Therefore, the incorporation of the humid air properties in numerical modeling should be necessary to predict not only the evaporation but also the condensation.

In a solar still, distilled water is produced through evaporation from the water surface and the subsequent condensation on the inner surface of the cover (Fig. 1). Therefore, there exists a time lag between the occurrence of the evaporation or condensation and the beginning of the production. Since the time lag of the latter is smaller than that of the former, a condensation theory would give a better result to predict the hourly production than an evaporation theory.

This paper proposes a new mass and heat transfer model of the TSS incorporating various mass and heat transfer coefficients taking account of the humid air properties to calculate the temperature, water vapor density and relative humidity of the humid air, and to estimate the condensation flux additionally. The validity of the proposed model is verified using the field experimental results carried out in Fukui, Japan and in Muscat, Oman.

Section snippets

Mechanism of pure water production

The mechanism of pure water production in a TSS is illustrated in Fig. 2. The TSS consists of a transparent tubular cover and a blackened rectangular trough inside the cover. The solar radiant heat after transmitting the cover is mostly absorbed by the saline water in the trough. The rest is absorbed by the cover and the trough. Thus, the saline water is heated up and evaporates. The water vapor density of the humid air increases associated with the evaporation from the water surface and then

Mass and heat transfer in TSS

Fig. 3 shows the mass and heat transfer inside and outside a TSS. Inside the TSS, two types of mass transfer including phase changes occur, i.e. evaporation from the water surface to the humid air and condensation from the humid air to the cover.

Many sorts of heat transfer occur in TSS, e.g. evaporative heat transfer from the water surface to the humid air, Qew, condensation heat transfer from the humid air to the cover, Qcdha, convective heat transfer between the water surface and the humid

Novelty of proposed model

Fig. 4 explains the comparison between the proposed mass and heat transfer model and past models. The past models (Dunkle, 1961, Chaibi, 2000, Nafey et al., 2000, Hongfei et al., 2002, Paul, 2002, Al-Karaghouli and Alnaser, 2004, Tiwari and Tiwari, 2008) contain the heat balance of the water, trough and cover, and the mass balance of the water. In this paper, the heat balance of the humid air and the mass balance of the water vapor in humid air were formulized newly. Consequently, the proposed

Evaporative MTC

The evaporation flux, me (kg/m2/s), is expressed asme=hew(ρvw-ρvha)where hew is the evaporative MTC (m/s) and is theoretically derived by a dimensional analysis (Ahsan and Fukuhara, 2008) as follows:hew=γeMvRv(Tw+Tha)2RgThagβDv2(Tw-Tc)ν1/3

Condensation MTC

The condensation flux, mc (kg/m2/s), is defined asmc=hcdha(ρvha-ρvc)where hcdha is the condensation MTC (m/s) and is theoretically formulated by a film-wise condensation theory (Ahsan and Fukuhara, 2009) ashcdha=0.996γc3/4gρl(ρl-ρvha)λl3a3(Tha-Ta)3μlDhfg3(ρvha-

Evaporative HTC

The heat flux associated with the evaporation is given by the product of latent heat of vaporization, hfg, and me, i.e.qew=hfgme=hfghew(ρvw-ρvha)

Condensation HTC

The heat flux due to the condensation can be expressed as the product of hfg and mc, i.e.qcdha=hfgmc=hfghcdha(ρvha-ρvc)

Convective HTC between water surface and humid air

Using Newton’s law of cooling, the convective heat flux between the water surface and the humid air, qcw, is denoted asqcw=hcw(Tw-Tha)

qcw is also expressed by the form of sensible heat flux (or form of enthalpy), i.e.qcw=veρaCa(Tw-Tha)

Discretization and numerical solutions

An explicit finite difference scheme was used to discretize the basic Eqs. (1), (3), (5), (6) and (24), (25), (26), (27).

Experimental work

To support the validity of the proposed mass and heat transfer model, field experiments were carried out in Fukui, Japan and Muscat, Oman in 2008.

Results and discussions

Two typical one-day data, of July 5, 2008 (in Fukui, Japan) and of July 13, 2008 (in Muscat, Oman) are presented in this paper.


In this study, a new mass and heat transfer model of a Tubular Solar Still (TSS) was presented by taking account of the humid air properties inside the still. The proposed model contains the equations of heat balance of the humid air and of mass balance of the water vapor in the humid air, which were formulized newly besides the conventional equations of heat balance of the water, trough and cover, and of mass balance of the water. Consequently, the proposed model was made possible to provide


Authors gratefully acknowledge Dr. Rashid Al Maamari, Dr. Kazuo Okamura and Mr. Mark Sueyoshi for their kind cooperation during staying in Muscat, Oman. The financial support provided by Shimizu Corporation, Japan and Japan Cooperation Center, Petroleum (JCCP) for this study is also acknowledged.

References (22)

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    Progress in Energy and Combustion Science


  • Z. Hongfei et al.A group of improved heat and mass transfer correlations in solar stills

    Energy Conversion and Management


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    Applied Energy


  • Ahsan, A., 2009. Production Model of New Tubular Solar Still and Its Productivity Characteristics. PhD Thesis, Dept. of...
  • A. Ahsan et al.

    Evaporative mass transfer in tubular solar still

    Journal of Hydroscience and Hydraulic Engineering, JSCE


  • A. Ahsan et al.

    Condensation mass transfer in unsaturated humid air inside tubular solar still

    Annual Journal of Hydraulic Engineering, JSCE


  • (Video) CFD simulations of a single slope solar still (Part-1)

    Cited by (107)

    • Performance of multi-stage tubular solar still operating under vacuum

      2022, Renewable Energy

      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 (Pop) 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/m2 cumulative solar radiation) produced maximum water yield (7.15 kg/m2) with optimal Pop (48kPa), much higher than under normal (5.81 kg/m2) and lower pressure (1.58 kg/m2 with Pop=20kPa). At optimal Pop, a delicate balance exists between the energy adsorbed and dissipated. Stronger solar intensity leads to more water yield and higher optimal Pop, while increasing the stage number improved the fresh water yield with diminishing marginal effect and slightly shifted the optimal Pop to a lower value. Using water immersion cooling other than air cooling significantly increased the water yield from 7.15 kg/m2 to 10.51 kg/m2. 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.

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      2022, Sustainable Energy Technologies and Assessments

      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 stills: Recent developments and future

      2022, Solar Energy Materials and Solar Cells

      (Video) Awesome Solar Air Heater for Off Grid Living, Tiny Houses, RVs and more..

      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.

    • Experimental study on three-effect tubular solar still under vacuum and immersion cooling

      2021, Desalination

      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.

    • Basin and tubular solar distillation systems: A review

      2021, Process Safety and Environmental Protection

      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.

    • Performance improvement of a tubular solar still using V-corrugated absorber with wick materials: Numerical and experimental investigations

      2021, Solar Energy

      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.

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