Effect of air flow on tubular solar still efficiency
- Arunkumar Thirugnanasambantham^1Email author,
- Jayaprakash Rajan^2,
- Amimul Ahsan2 and
- Vinothkumar Kandasamy^3
© Thirugnanasambantham et al.; licensee BioMed Central Ltd. 2013
Received: 9 January 2013
Accepted: 13 January 2013
Published: 15 April 2013
An experimental work was reported to estimate the increase in distillate yield for a compound parabolic concentrator-concentric tubular solar still (CPC-CTSS). The CPC dramatically increases the heating of the saline water. A novel idea was proposed to study the characteristic features of CPC for desalination to produce a large quantity of distillate yield. A rectangular basin of dimension 2 m × 0.025 m × 0.02 m was fabricated of copper and was placed at the focus of the CPC. This basin is covered by two cylindrical glass tubes of length 2 m with two different diameters of 0.02 m and 0.03 m. The experimental study was operated with two modes: without and with air flow between inner and outer tubes. The rate of air flow was fixed throughout the experiment at 4.5 m/s. On the basis of performance results, the water collection rate was 1445 ml/day without air flow and 2020 ml/day with air flow and the efficiencies were 16.2% and 18.9%, respectively.
The experimental study was operated with two modes: without and with air flow between inner and outer tubes. The rate of air flow was fixed throughout the experiment at 4.5 m/s.
On the basis of performance results, the water collection rate was 1445 ml/day without air flow and 2020 ml/day with air flow and the efficiencies were 16.2% and 18.9%, respectively.
Ahsan and Fukuhara  studied on a new heat and mass transfer tubular solar still and found that the heat and mass transfer coefficients can be expressed as functions of the temperature difference between the saline water and the cover. Ahsan et al. , experimentally studied on the evaporation, condensation and production of a tubular solar still and found that the relative humidity of the humid air was definitely not saturated and the hourly evaporation, condensation and production fluxes were proportional to the humid air temperature and relative humidity. Ahsan and Fukuhara [3–5] analyzed on a tubular solar still and on a quasi steady heat and mass transfer due to evaporation and condensation taking an account of humid air properties inside the still and found that the analytical solutions derived from the present model could reproduce the experimental results perfectly. Arunkumar et al. , studied on a hemispherical solar still.
Numerous research activities have been done with CPCs such as cost-effective asymmetric CPCs , non-modified absorbers , solar powered adsorption refrigerator with CPC collection system , non-imaging solar collector , non-tracking solar concentrators , and non-evacuated solar collectors .
Singh et al. , studied on the design parameters for concentrator assisted solar distillation system. Analytical expressions for the water and glass cover temperatures, the rate of heat flux due to evaporation, the rate of distillate output and the instantaneous thermal efficiency have been derived in terms of the system design and climatic parameters. It is analytically shown that the efficiency of the system with a concentrator was higher than that with a collector.
Chaochi et al. , designed and built a small solar desalination unit equipped with a parabolic concentrator and observed that the maximum efficiency corresponds to the maximum solar lighting obtained towards 14:00. At that hour, the boiler was nearly in a horizontal position, which maximizes the offered heat transfer surface. Mohamad and El-Minshawy  dealed with the status of solar energy as clean and renewable energy applications in desalination.
A novel idea proposed in this work is the CPC’s absorber which is acting as a saline water basin in a tubular arrangement. This specially designed CPC-CTSS required only 3 minutes warming up, as opposed to a typical warm-up period of an hour or more for basin type stills with no reflectors. The inner tube was cooled by flowing air at the rate of 4.5 m/s.
Design of the CPC
The bottom region of the profile can be modified by incorporating a V-shaped reflector portion just below the absorber.
Materials and methods
Still technical and operation details
Solar radiation (W/m2)
652 - 1159
Ambient temperature (°C)
26.2 - 34
Relative humidity (%)
55 - 21
Average wind velocity (m/s)
Basin absorption (αb)
Absorbtivity of cover (αg)
Reflectance of cover
Transmittance of cover (τg)
Specific heat of water (Cw)
Radius of the receiver (m)
Radius of the envelope (m)
2.2 x 10-2 m
Water quality analysis
Tested water quality results
The inlet and outlet temperatures  of the cooling air are shown in Figure 8. The extracted heat from the CPC-CTSS would allow further production of distilled water in a single slope solar still. With a 25°C rise and 4.5 m/s flow for 8 hours, the amount of heat was 140 Wh. With a 50% efficient single slope still, this could produce an additional 110 ml of distilled water, roughly 5% increase in output and efficiency. Air flow meter was kept near by the system to supply the cooling air. Its temperature was influenced by the ambient temperature and the solar radiation falling on it, which explains the variation in input temperature.
This research article summarized a new compound parabolic concentric-tubular solar still (CPC-CTSS), which has been designed for and tested under the climatic conditions of Coimbatore, India. The effect of cooling air flowing over the condensation surface was studied. The daily yield of CPC-CTSS was found 1445 mL/day and 16.2% efficiency without air flow and 2020 mL/day and 18.9% efficiency with air flow at a constant flow rate of 4.5 m/s. To bound in a nutshell, this innovative approach of concentrator assisted tubular solar still with air flow augments the performance with enhanced rate of evaporation and condensation with safer operation procedures.
This study has received funding from Indian Ministry of New and Renewable Energy (MNRE). NREF/TU/2010/001. The Authors are grateful to Professor Dr. S. Jayaraman, for providing facilities to carry out experimental work at the institute.
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