1. Introduction

People in the world are increasingly facing the lack of clean water for living nowadays. Every year, millions of people die from lack of clean water and from the diseases relating to drinking and living water. There is a lot of technology in the world to produce fresh water from sea water or brackish water. However, these technologies are mostly expensive, not suitable for the poor and developing countries and communities where most of the water shortages are occurring. In addition, most of industrial-scale distillation technologies and equipment consume a lot of energy, contributing to the depletion of fossil fuel energy sources and increasing environmental pollution. It is therefore necessary to promote cheap, less energyefficient, and environmentally friendly distillation methods. The methodology and technology of solar distillation almost meet all three criteria: simple equipment with low-cost, no use of fossil fuels, and no contribution to the environmental pollution.

Solar water distillation is a method of using solar energy, a source of clean and endless energy, to produce clean water from impure water. In solar water distillation equipment, evaporation and purification of pure water occur, thereby removing salts and impurities that are harmful to human health from marine and brackish water resources to give out drinking water. In many solar water distillation

technologies and devices, the solar stills are widely used because they are designed and operated in a manner that is consistent with the technological level and economic conditions of the poor and developing countries and communities.

measured data or data generated from a sub-computer program developed by the

The processes of heat and mass transfer in a passive solar still are indicated in Figure 1. In order to develop the formulae for the energy and mass balances in the

• The lost amount of water through evaporation is small compared to the amount

• The energy required to heat up the water from outside temperature before adding to the still to the basin temperature is negligible as compared to the latent energy required to evaporate the same amount of water. In other words,

• The areas of the cover, the water surface, and the still basin are equal.

remaining will increase the temperature of the basin water Mw dTw

• The temperature gradients along the cover thickness and the water depth are

As can be seen in Figure 1, the heat and mass transfer inside the solar still occurs as follows: the solar incidence QT from the sun reaches the glass, part of it will be reflected Qr, part will be absorbed by the glass Qα, and the remaining Q' will transfer through the glass and reach the basin water. Then, Q' absorbed into the basin water will be partially reflected back to the glass under convection qcw, evaporation qew, and radiation qrw, partially transfer to the basin qw-b, and the

dt . The basin, in its

<sup>T</sup>), partially from the water (qw-b).

author and linked to the main program [9].

DOI: http://dx.doi.org/10.5772/intechopen.83228

still, the following assumptions are made:

Cpwð Þ Tw � Ta ≪ hw.

ignored.

Figure 1.

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of water in the basin and can be ignored.

The Mathematical Model of Basin-Type Solar Distillation Systems

• There is no leakage in a well-designed still.

turn, gains the energy partially from the sun (αQ″

The heat and mass transfer processes in a conventional solar still.

The most popular solar stills are passive type, in which distillation process occurs within the still through evaporation and condensation [1]. They are simple in design and manufacture, easy to operate, usually small, and reasonably cheap. Passive solar stills only use solar energy to remove the salts or impurities in saline or brackish water; thus, it is environmentally friendly and saving energy. Therefore, it is still of value to study in this type of stills to continue improving its efficiencies and designs. This is the main aim of this chapter.

The main drawback of this type of solar distillation system is low energy efficiency and distillate productivities. Hence, many active distillation systems such as solar still coupled with flat plate or evacuated tube collectors, solar still coupled with parabolic concentrator, solar still coupled with heat pipe, solar still coupled with hybrid PV/T system, multistage active solar distillation system, multi-effect active solar distillation system, etc. have been developed theoretically and experimentally [2]. However, a forced circulation solar still with enhanced water recovery has not been researched and presented. Therefore, this type of solar still has been developed and modeled, both theoretically and experimentally, and will be presented in this chapter as well.

In terms of numerical analyses of passive and active solar distillation systems, there are several models presented in literatures [2–8]. Sampathkumar et al. [2] comprehensively reviewed mathematical models applied to predict the performances of active solar distillation systems and concluded that Kumar and Tiwari's model [3] was most suitable for evaluating the internal heat transfer coefficients and hourly yield accurately except in extreme cases. However, Dwivedi and Tiwari [4] observed from their studies in passive solar still that Dunkle's model [5] gave better agreement between theoretical and experimental results. Madhlopa and Johnstone [6] numerically modeled a passive solar still with separate condenser and claimed that the distillation productivity of their still was 62% higher than that of the conventional passive solar still. Ahsan et al. [7] reviewed a few numerical models of a tubular solar still and compared them with Dunkle and Ueda models. Recently, Edalatpour et al. [8] reviewed the latest developments in numerical simulations for solar stills including the use of computational fluid dynamics (CFD) simulations, MATLAB.

Based on the above literature review, it is obvious that although Dunkle's model is one of the oldest thermal model for predicting the internal heat transfers of solar stills, it still can be used to accurately present the performance of heat transfers inside the solar stills. However, there is no research found in the literature review that consistently uses Dunkle's equations to develop the numerical models for both passive and active solar stills. Therefore, this chapter will use this approach to develop the mathematical models for a conventional solar still and a forced circulation solar still with enhanced water recovery.
