Effect of Operational Parameters on the Production of a Solar Distiller Coupled to a Hybrid Photovoltaic Thermal Collector

It is universally recognized that the lack of water on the one hand, and the depletion of fossil fuels on the other hand are one of the major challenges of our century, the face these critical issues, desalination of salt water and/or brackish water appears as one of the possible solutions to the survival of humanity. Among the techniques used in this field, and needs relatively low drinking water, solar distillation can be a very good solution especially for arid and desert zones. In order to improve the production of solar stills, our work focuses on the coupling of a flat plate solar distiller with a flat collector to ensure preheating distilled water. We prepared heat balances at the distiller and describe their sensor transient thermal behavior. Next, we used the RK4 method to solve systems of equations obtained. The numerical results clearly show the influence of various parameters on the daily production of this system.


Introduction
Problems of environmental pollution and energy lack in the world have become much serious recently. Solar energy as a renewable and environmentally friendly energy has the potential to meet global energy demand in the future. Methods for the conversion of solar energy can be classified into two types: a thermal option, that converts solar energy into heat, subsequently transformed into electricity, and the photovoltaic methods that converts solar energy directly into electrical energy. Therefore, there is a need to develop an ingenious method of solar energy conversion systems and then to substitute it where applications of fossil fuels are most vulnerable. One of the ingenious methods of solar energy conversion systems is the photovoltaic thermal solar collector or hybrid solar collector, which converts solar radiation directly to both thermal and electrical energies. It is very attractive for solar applications in which limited space and area related installation cost are of primary concern. The hybrid collector is also attractive when the space needed to install side-by-side solar thermal and photovoltaic collectors is not readily available.
Need of drinking water in the world is increasing continuously, while underground reserves are contently reduced. More than a third of humanity, mostly belonging to third world countries, do not have access to drinking water. The drinking water supply is one of the most serious problems of our time, as water demand reached believes that natural resources tend to diminish or disappear. Desalination of seawater or darkish can contribute to increased water potential; however, this method requires a lot of energy which is involved in the cost of water. In areas with strong sunlight solar distillation then appears as a very attractive solution. However, the problem is the low efficiency of this type of process. And also to overcome this drawback is we are planning in the context of this work to couple the solar still with a flat plate solar thermal collector. We still consider the numerical simulation of an active solar. Particular attention is paid to the effect of some parameters (ambient temperature, the thickness of the layers of water ….) on the cumulated production of distilled water.

The Mathematical Model
The studied system is a solar still coupled with a solar collector. The model is established by taking the following assumptions [1][2][3].
The sun is assimilated to a black body. The temperature of each component is uniform because the temperature gradient is negligible with respect to the space. The side walls are assumed adiabatic.
The condensation takes place only on the cover (there is no condensation on the sides)

The energy Balance
Equations that characterize the running of flat plate solar distiller in transient regimes are given respectively by:

Calculation of the Coefficient of Heat Transfer by Convection
Between the ambient environment and the outer surface of the glass [4][5][6]: Between the absorber and the water mass [10]: Gr: Grashof number [11]: Pr:Brandt number:

Calculation of the Coefficient of Heat Transfer by Radiation
The surface solar power absorber by the water mass [12]: Solar power absorber by the surface of the water mass: Solar power absorber by the absorber: Between the glass and the water mass:

Calculation of the Coefficient of Heat Transfer by Evaporation
Between the glass and water mass [10,13,14]:

Calculation of the Coefficient of Heat Transfer by Conduction
Between the insulator and the absorber (inside): Between the interior insulator and the exterior insulator:

Results and Discussion
After establishing the balances reflecting thermal behavior of the hybrid sensor photovoltaic thermal and the distiller in transient state, non-linear systems and coupled differential equations obtained are solved by the Runge-Kutta 4TH order method. For the evaluation of operating parameters, a computer program is developed in FORTRAN language for solving heat balance equations for different elements. The input parameters of the computer program include weather for the city of Constantine, Algeria (latitude 36°,17 north, longitude 6°,36 east, and a time difference of one hour), 18 august was chosen as the calculation date (date of the month representative) under normal conditions. The thermo physical parameters and settings are displayed in Table 1. It is evident that the production of distiller depends closely on the quantity of heat received thus an improving of evaporation process and therefore a higher production Fig. 3.   4 Shows the variation in the cumulative production of distilled water with room temperature, allowing to note the increase in production until the value of 43°C, too high beyond the ambient temperature leads to an increase in glass temperature which will have an adverse effect on the condensation and therefore on production. The cumulative production of distilled water is zero during the first hours of the day (before 8 am) for both systems (with and without preheating), we note that the running temperature of the flat plate distiller with preheating starts before temperature of distiller without preheating. Fig. 5 from this moment it appears a difference in production between the distiller and the coupled system, which increases with the growth of the solar radiation reaching a maximum at 13h for the coupled system and 14h for the flat plate distiller. Where the hourly production of plate distiller is 1.6 Kgm -2 h -1, while that of the coupled system reaches 2.3 Kgm -2 h -1 . It is easy to see in Fig. 6 when the temperature between the water and glass (T w -T v ) increases, there is an increase in production resulting from increased evaporation rate. In the Fig. 7 the effect of inlet temperature on the hot feed distillate stream is four feed rates, to the configuration of the air gap. Thank you to the logarithmic scale of the y-axis, a clear exponential trend is observed for all flow rates. This was expected due to the dependence of the flow of distillate to the difference in vapour pressure, which in turn depends exponentially on the temperature on the temperature according to a modified Antoine equation. The effect of the thickness of the brine on production, a flat plan distiller plane is highlighted in Fig. 8 and shows clearly that in all cases the reduction of the thickness of the layer water leads to the increase of the characteristics of distillers. The experimental results obtained by Badran. O. O [6], shows that the evolution of the thickness of the sheet of water, cause the decrease of the production of distilled water.

Conclusion
Solving the system of equations governing the running of the solar stiller coupled with a flat plate collector was discussed by the order 4 Runge-Kutta method. The results obtained allow us to advance the following conclusions: Solar radiation is the parameter most affecting the running of a solar stiller. The increase of the ambient temperature leads to an increase in production. And increasing the temperature difference between the glass and the brine leads better production of distilled water. Increasing the water layer decreases the production of distilled water. The preheating improves the production of flat plan distiller 37%.