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ABSTRACT
The low freshwater output is always a matter of concern regarding a solar still. It is observed from the current literature that evacuated tubes are directly connected to the still basin for productivity enhancement. The main limitation of integrating evacuated tubes directly to solar still is rapid scaling on the inner surfaces of tubes due to the presence of dissolved impurities in water. This increases thermal resistance and decreases the overall performance of the still. In the present experimental study, a copper tube heat exchanger is installed in still basin and waste engine oil is utilised as a working fluid in the evacuated tube collector (ETC) and heat exchanger circuit. This modification increases the evaporation rate of basin water and boosts freshwater output. The modified still is experimentally investigated at distinct water depths of 4 cm, 5 cm and 6 cm, and the performance is simultaneously compared with a conventional still. Experimental results showed maximum daily productivity of 7.38 L/m2 day and daily efficiency of 30.5% achieved for modified still at 4 cm water depth. These values are 138.9% and 2.1% higher than that of conventional still respectively at same water depth.
Disclosure statement
No potential conflict of interest was reported by the authors.
Nomenclature
| = | Coefficient of evacuated tube collector efficiency. | |
| = | Still basin area (m2). | |
| A | = | Heat exchanger area (m2). |
| = | Area of evacuated tube collector (m2). | |
| = | Inner area of ETC (m2). | |
| = | Outer area of ETC (m2). | |
| = | Water specific heat (J/kg K). | |
| = | Oil specific heat (J/kg K). | |
| dt | = | Time interval, s. |
| = | Heat exchanger outer dia (m). | |
| = | Heat exchanger inner dia (m). | |
| = | Overall heat loss coefficient of still basin (W/m2K). | |
| = | Total heat loss coefficient of water surface (W/m2 K). | |
| = | Total heat loss coefficient of glass surface (W/m2 K). | |
| = | Latent heat of water vapour (J/kg). | |
| = | Solar radiation intensity falling on evacuated tube collector (W/m2). | |
| = | Solar radiation intensity at a given time (W/m2). | |
| K | = | Thermal conductivity of heat exchanger (W/mK). |
| L | = | Length of heat exchanger (m). |
| = | Water mass in still basin (kg). | |
| = | Rate of water circulation by free convection trough one evacuated tube (kg/s). | |
| = | Oil flow rate in heat exchanger pipe (kg/s). | |
| = | Water mass present in evacuated tubes (kg). | |
| = | Hourly freshwater productivity (kg/h). | |
| = | Total number of evacuated tubes used. | |
| = | Partial pressure of saturated water (N/m2). | |
| = | Partial pressure of saturated glass cover (N/m2). | |
| = | Conduction heat loss (W/m2). | |
| = | Convection heat loss from basin water to glass cover (W/m2). | |
| = | Evaporation heat loss from basin water to glass cover (W/m2). | |
| = | Radiation heat loss from basin water to glass cover (W/m2) | |
| = | Evaporation loss rate from water surface (W/m2). | |
| = | Ambient temperature (K). | |
| = | Time (s). | |
| = | Temperature of glass cover (K). | |
| = | Heat loss coefficient inside heat exchanger (W/m2 K). | |
| = | Heat loss coefficient for water surface (W/m2 K). | |
| = | Heat loss coefficient outside heat exchanger (W/m2 K). | |
| = | Evaporative heat loss coefficient for water surface (W/m2 K) | |
| = | Radiative heat loss coefficient for glass surface (W/m2 K). | |
| = | Convective heat loss coefficient for glass surface (W/m2 K). | |
| = | Convective heat loss coefficient of still basin to water (W/m2 K). | |
| = | Radiative heat loss coefficient of basin water (W/m2 K). | |
| = | Temperature of sky (K). | |
| = | Overall energy coefficient (W/m2 K). | |
| U | = | Overall heat loss coefficient of heat exchanger (W/m2 K). |
| = | Overall heat loss coefficient of top glass cover (W/m2 K). | |
| = | Standard uncertainty. | |
| = | Overall heat loss coefficient (W/m2 K). | |
| = | Standard uncertainty. |
Greek symbols
| = | Standard deviation. | |
| = | Stefan Boltzmann constant, W/m2 K4. | |
| = | Standard deviation. | |
| = | Overall daily efficiency. | |
| = | Still basin absorptivity. | |
| = | Basin water absorptivity. | |
| = | Glass cover absorptivity. | |
| = | Glass cover transmissivity. | |
| = | Basin water emissivity. | |
| = | Instantaneous efficiency of solar still. | |
| = | Daily efficiency of passive still. | |
| = | Daily efficiency of active still. | |
| = | Emissivity of glass cover. | |
| = | Total effective emissivity. |
Additional information
Notes on contributors
Mohit Bhargva
Mohit Bhargva, Corresponding Author, is a Ph.D. research scholar in Mechanical Engineering Department, National Institute of Technology Kurukshetra, India. His current interests are in solar water desalination using solar still. His research work mainly concentrates on the performance enhancement of a solar still using evacuated tube collector and different heat exchangers. His previous publications have appeared in reputed International Journals.
Avadhesh Yadav
Dr. Avadhesh Yadav, Co-Author, is an Assistant Professor in Mechanical Engineering Department, National Institute of Technology Kurukshetra, India. His primary research interests include solar cooking (both indoors and outdoors), solar distillation, solar drying and water extraction from atmospheric air using solar energy. He has published more than 50 research papers in reputed International Journals in the field of solar energy and others.