Daily exergy analysis of passive solar still for maximum yield

1. Background The need for potable water for the survival of human beings is well known. There is an acute shortage of fresh drinking water in the remote as well as rural areas of many countries. At many places enough saline water is present but it is not suitable for the drinking and domestic as we as agricultural uses. Advanced desalination systems namely vacuum distillation [1], multi-stage flash distillation [2,3], reverse osmosis [4,5], electrodialysis membrane [6], etc are based on fossil fuel-based grid electricity which is not friendly with environment and climate. In this scenario, the solar distillation method is one of the economical and best ways to have potable water from brackish/hard/saline water [7]. Broadly solar distillation system is classified as the passive and active system and lots of research work have been carried out in the area of passive and active solar distillation [8]. Basically, most of the research work on passive and active solar still has been carried out based on energy conservation to evaluate hourly yield. It is also well known that passive solar distillation operates at a low operating temperature range between 15°C–80°C depending upon climatic conditions [9]. Hence the performance of solar distillation can be carried out based on either first law of thermodynamics (energy conservation) [10,11] or the second law of thermodynamics (exergy analysis). Exergy [12–14] is elucidated as the maximum available work that can be taken out from a system during a process that brings the system into equilibrium with its corresponding environment (i.e., ambient temperature) [14–16]. The specific property of exergy that is in contrast to the energy is that exergy can be destroyed during a process due to irreversibility within the given system during the process [17]. Such an analysis helps to identify which components of the thermal energy system are responsible for irreversibility. So the exergy analysis for the passive solar still with all its components/parts is an effective way to design economically viable solar still [18]. Exergy analysis can be further applied as a powerful thermodynamic technique for estimating and optimizing the performance of energy systems [19]. Gude [14] has used the exergy tool to calculate the thermodynamic efficiency of various desalination processes which are supported by different renewable energy sources. R.G. Singh, G.N. Tiwari / Desalination and Water Treatment 204 (2020) 1–9 2 He identified different processes that contributed to the exergy destruction and also suggested suitable working conditions to mitigate the exergy losses. Vaithilingam and Esakkimuthu [20] have done an energy and exergy analysis of single slope solar still experimentally and found that irrespective of the water depth and in comparison to other components, the highest exergy destruction is observed in the basin liner of the solar still. Mohamed et al. [21] have experimentally observed the role of stone particles placed in solar still over the exergy efficiency. It has been observed that the exergy efficiency of the solar till increases from 65% to 123% when the stone size is doubled from 1 to 2 cm. Recently, Hedayati-Mehdiabadi et al. [22] have studied the exergy performance of basin type double solar still equipped with the phase changed materials and photovoltaic thermal collector on a sample winter and summer days. It has been observed that the exergy efficiency was 48% lower in winter condition than the summer condition. The exergy analysis takes into account the role of ambient temperature, which has a significant effect on the performance of solar stills. It has been seen that many authors have carried out exergy analysis of the solar distillation system by using Carnot efficiency [23–29], which is valid for higher operating temperatures under the second law of thermodynamics as mentioned earlier. However, the performance of the solar distillation system should be analyzed from daily performance in terms of daily exergy to minimize the daily destruction exergy for maximum yield. There can be two types of destruction exergy, namely known exergy and unknown exergy. Unknown exergy may be due to the presence of heat capacity of condensing cover, absorber plate, internal heat energy generated and quasi-steady-state conditions. It is very difficult to control this unknown exergy, but known exergy can be minimized to have maximum daily exergy output. Since solar radiation is generated from the sun which is a very high temperature, say about 6,000 K and hence there is not much change in the exergy of solar radiation, therefore, one may consider solar radiation as exergy. However, exergy factor (ψ) for solar radiation coming from the sun has also been derived by Neri et al. [30] which is given as: ψ = − −       1 4 3 3 4