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Experimental and numerical study of stepped solar still integrated with a passive external condenser and its application

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Solar still is used widely to produce clean water from any brackish water. The efficiency of the still depends on the design of the still with respect to the prevailing local ambient conditions. A number of literature discussed about different designs of evaporator on improving the overall efficiency of the still but not on the condenser design. Hence, the present work focuses on the improvement in efficiency by providing a passive external condenser with a stepped design of evaporator. The passive external condenser has shown an increase in the overall efficiency of still by 10.6% in summer and 12.2% in winter. Higher performance was observed in winter than summer when the passive external condenser was added with stepped evaporator (integrated system) and found inverse for the stepped solar still without condenser. Heat transfer analysis was also made to determine the effectiveness of the external condenser in summer and winter. The correlations developed by Dunkle (in: Proceedings of the ASME International Heat Transfer Conference. Part V, International Developmental in Heat Transfer. University of Colorado, Boulder, Colorado, 1961), Hongfei et al. (Energy Convers Manag 43:2469–2478, 2002) and Tsilingiris (Sol Energy 83:420–431, 2009. were used to fit the experimental data obtained from the coupled system, and temperature correction factors were introduced along with the correlations to improve the predictions. The application of the external condenser as a heat source was also summarized.

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Conventional solar


External condenser


Number of transfer units


Stepped solar


Stepped solar still with external condenser

\(A_{\rm{still}}\) :

Area of the still, m2

\(A_{\rm{ec}}\) :

Cross-sectional area of external condenser, m2

\(C_{\rm{hot}}\) :

Heat capacity of hot fluid, kW K−1

\(C_{\rm{cold}}\) :

Heat capacity of hot fluid, kW K−1

\(C_{\rm{p,v}}\) :

Specific heat capacity of vapour, J kg−1 K−1

\(C_{\rm{p,a}}\) :

Specific heat capacity of air, J kg−1 K−1

\(C_{\rm{p,m}}\) :

Specific heat capacity of air, J kg−1 K−1

\(D_{\rm{wa}}\) :

Diffusivity coefficient of water in air, m2 s−1

\(g\) :

Acceleration due to gravity, m s−2

\(h_{\rm{fg}}\) :

Latent heat, J kg−1 K−1

\(h_{{{\rm{conv,w}} - {\rm{gi}}}}\) :

Convection heat transfer coefficient of air between evaporator and condenser

\(h_{{{\rm{evap,w}} - {\rm{gi}}}}\) :

Evaporation heat transfer coefficient of vapour between evaporator and condenser, W m−2 K−1

\(I_{\rm{solar}}\) :

Solar radiation, W m−2

\(k_{\rm{m}}\) :

Thermal conductivity of moisture, W m−1 K−1

\(l_{\rm{c}}\) :

Characteristic length, m

\(L\) :

Length of the external condenser, m

\(M_{\rm{a}}\) :

Molecular mass of dry air, kg mol−1

\(M_{\rm{w}}\) :

Molecular mass of water vapour, kg mol−1

\(\dot{m}_{\rm{ew}}\) :

Mass of water evaporated, kg s−1

\(\dot{m}_{\rm{ew,ec}}\) :

Mass of water evaporated in the external condenser, kg s−1





\(P_{\rm{w}}\) :

Saturation vapour pressure of water vapour at evaporator surface, N m−2

\(P_{\rm{gi}}\) :

Saturation vapour pressure of water vapour at condenser surface, N m−2

\(P_{\rm{o}}\) :

Total pressure, N m−2

\(R_{\rm{a}}\) :

Gas constant for dry air, J kg−1 K−1

\(R_{\rm{w}}\) :

Gas constant for water vapour, J kg−1 K−1

\(R\) :

Universal gas constant, J mol−1 K−1

\({\rm{Ra}}^{{\prime }}\) :

Modified Rayleigh number

\(T_{\rm{w}}\) :

Temperature of water, K

\(T_{\rm{gi}}\) :

Temperature of glass inner surface, K

\(T_{{{\rm{a}} - {\rm{v}}}}\) :

Temperature of air–vapour, K

t :


Y F :

Distillate yield at the front, mL

Y R :

Distillate yield at the rear, mL

\(\alpha_{\rm{m}}\) :

Thermal diffusivity of humid air, m2 s−1

\(\beta\) :

Thermal expansion coefficient, K−1

\(\rho_{\rm{m}}\) :

Density of humid air, kg m−3

\(\mu_{\rm{m}}\) :

Dynamic viscosity of humid air, N s m−2

\(\varepsilon\) :


\(\eta\) :

Efficiency, %


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The author acknowledges the financial support provided by the Carbon Zero Challenge—Indian Institute of Technology Madras, India, towards the design and fabrication of the stepped solar still with an external condenser (W2-Suryaneer). The authors acknowledge the instrumentation facilities provided by National Institute of Technology Tiruchirappalli, India. Authors, P M Sivaram and S Dinesh Kumar acknowledge the institute fellowship provided by Ministry of Human Resources and Development (MHRD), Govt. of India.

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Correspondence to A. Arunagiri.

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Sivaram, P.M., Dinesh Kumar, S., Premalatha, M. et al. Experimental and numerical study of stepped solar still integrated with a passive external condenser and its application. Environ Dev Sustain 23, 2143–2171 (2021).

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