Stepped Solar Still

Stepped coaxial pipe solar still with parabolic trough

Hamza Javed, Abdullah Sarfaraz, Nouman Iqbal

Department of Mechanical & Manufacturing Engineering, National University of Sciences & Technology (NUST), Islamabad 44000, Pakistan

Keywords:                                                 A B S T R A C T 

                                                                                    ________________________________________      

Coaxial pipes                                                A new configuration of solar still using

Stepped solar still                                         coaxial pipes as heat absorbers arranged in Theoretical analysis                                          step is theoretically analyzed. A parabolic Cylindrical parabolic trough                         trough is also in co-operated to enhance heat

transfer rate to water through cylindrical coaxial pipes. Geometrical and operational parameters of the solar still for heat transfer phenomena including evaporation and condensation, are taken into account in this overview, giving a comprehensive structure and classification to the study of solar stills from the thermodynamic point of view. Energy balance is included to account for thermodynamic imperfections in the several processes inside the solar still. Detailed thermodynamic analysis is performed on each component of the solar still. Simulations are carries out on a software to verify the results. The present paper describes a comprehensive review on modifications done on solar stills to improve the productivity. Also, the percentage improvement in the productivity of the suggested model is shown in this article.

1. Introduction

The worldwide demand for potable water is continuously growing because of industrial, population and agricultural growth; the result is insufficient supply in many places of the world.

To meet water demands various advanced technology desalination techniques using fossil fuels or electrical energy derived from fossil fuels, are used worldwide, such as reserve osmosis, electro dialysis, ozone, UV, vapor compression and active carbon filtration to obtain potable water. Although these techniques are capable of desalinating water, they directly contribute to global warming and incur high costs. Additionally, these technologies are commercially suited for large cities and have high efficiencies and productivity rates when non-natural energy sources are used. Therefore, to provide fresh water to arid and semi-arid areas where sunshine is readily available, relative to non-natural energy, these methods are not suitable.

A solar still is a valuable device that can be used for purifying brackish water and salt water for drinking water purpose. But the main drawback of solar still is a low productivity device. Heat exchange mechanisms play an important role to enhance the productivity of the solar still systems. The productivity of any solar distillation system depends on the basin water temperature. The productivity increases with increasing water temperature in the basin. Many authors have researched solar stills to improve the performance by various methods.

In this context, the endeavors are likewise made to improve the performance of still. The present study aims to enhance the productivity of the solar still, by using a new design of the modified solar still with multi-groups of two coaxial pipes in basin. The productivity of the modified still are compared to conventional still to calculate the improvement in the productivity under the same ambient conditions.

In the previous studies, radiation shape factor between saline water and glass cover for a stepped solar still was not considered. Therefore, the present study will focus on a new accurate simulation for stepped solar stills taking into account the radiation shape factor between saline water and glass. Actually, it is well established that the optimum glass cover inclination should be equal to the place latitude angle to receive high radiation.

Nomenclature[pic 1]

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Fig. 1. Complete analytical model

      [pic 4]                              [pic 5] 

       Fig. 2. Coaxial pipe with rotating PCM at the center                                     Fig. 3. Energy balance of coaxial pipe

[pic 6]    [pic 7]

 Fig. 4. Energy balance of the saline water                                                                       Fig. 6. Energy balance of glass cover                                                                                              

2. Mathematical model

The energy balance for the stepped coaxial solar still will be applied on three regions: cylindrical absorbing body, saline water, and glass cover. The absorbing body temperature, saline water temperature, and glass cover temperature can be evaluated at every instant using temperature sensors. In the present study, the following assumptions are considered for the solar still energy equations:

• Steady state conditions exists.

• The glass cover is assumed to be thin ie    no heat is absorbed by the glass.

• The solar still is vapor leakage proof.

• Negligible change in temperature when heat flow through thickness of pipe (proof in Appendix B)

•Temperature changes axially in pipes only not radially

• Flow remains fully developed laminar flow (proof in Appendix A)

• Constant cp = 4200 Jkg-1K-1 (taking average of cp at 100 and 25 ⁰C)

• Temperature of outer coaxial pipe remains constant at 400 ⁰C (proof in Appendix C)

Calculations for 1st pipe are as follwows

Net heat which salty water get

                                      [pic 8]