International Conference of WREC-Asia & SuDBE2011, Chongqing, China 28-31 October 2011 Simulation of the performance of a solar still in Tehran climate E. Taghdiri1* (1. Khaneye Kaargar University of Applied Science and Technology, Ghazvin Unit, Iran) Abstract This paper presents a transient analysis and simulation of a solar still. The modeling is carried out based on equations of energy conservation which are written from different components of the system. Since these equations are coupled, they need to be solved simultaneously. Weather data for Tehran is taken from Ashraee 2007 source. Based on these data, first simulation is carried out for January 1st and variations of various parameters during the hours of the day are analyzed and reviewed. Then, simulation is carried out for the whole year and performance criteria are evaluated for different seasons and months, leading to recommendations on best times of the year to employ the solar still system. Keywords: Solar still, efficiency, energy, desalination, heat transfer 1 Introduction Countries of the Persian Gulf region including Iran have more or less the problem of potable water. Low annual rainfall and contamination of rivers and lakes and other resources of fresh water has made it a worthy undertaking to desalinate the brackish seawater. There are a number of different methods of desalination but solar desalination systems as a result of being environmentally friendly and requiring no fossil fuel, are particularly attractive. Solar stills are the simplest and most reliable among different methods of desalination of the seawater and because of their simplicity, they have a great potential to become a prominent solution in developing countries, such as Iran. The first noteworthy findings related specifically to the solar still and its heat transfer mechanisms belong to Dunkle 1 whose equations are still valid tools for analysis of solar stills. Farid and Shawaqfeh 2 further presented a new theory for solar desalination systems. Malik et al 3 presented an equation for evaporative heat transfer coefficient in low operational temperatures. Several other researchers including Chen et al. 4, Adhikari et al. 5 and Clark 6 established equations for computing internal heat transfer coefficients, which unfortunately suffer from the flaw of depending on constant values of C and n, resulting in lack of generality. Tiwary and Kumar 9 tried to overcome the limitations of Dunkle model using a model based on linear regression analysis to evaluate internal heat transfer coefficients for different values of Grashof number. Zheng et al. 10 proposed some heat transfer equations for the solar still system which they claimed to have been improved compared to older equations. Tiwari and Dwivedi 11 compared the results obtained using different analytical models and the experimental results for experimental solar stills. In this paper, the simple solar still system is simulated using equations of energy conservation and its performance is evaluated in Tehran climate throughout the year. Energy conservation equations are written and solved in transient state. Results can be used as a tool for making decisions related to best times of the year to operate solar still systems. Also, insight is provided into variations of their performance in different times of the day, with or without solar radiation. 2 The Model A simple solar still can be divided into four main components. There are the insulated walls, the insulated bed the floor of which is died black to absorb as much solar energy as possible, the glass covering which traps the energy from solar radiation and is tilted in order to help the water droplets which form on its inner surface easily flow downward and finally be gathered in the fourth component, a channel which is properly dimensioned and placed to collect this distillate and guide it to the fresh water output. The best angle for this glass covering is known from experience to be between and . A schematic of a typical simple solar still system is shown in figure 1. International Conference of WREC-Asia & SuDBE2011, Chongqing, China 28-31 October 2011 Fig.1 Schematic of a typical solar still The transparent cover on the top is supposed to trap the thermal energy of the solar radiation. That is why glass is probably the most suitable material for this part of the solar still. Besides being cheap and easy to install, it transmits the thermal wavelengths of the sun to the inside area of the solar sump. The thermal radiation produced by the water inside the solar sump is however of a larger wavelength which does not pass through the glass. Therefore, glass is capable of producing what is commonly known as the green house effect to heat up inside of the solar sump. The solar radiation after passing through the glass roof, is primarily absorbed in two stages: first, b