Effect of Lowering Condensing Surface Temperature on the Performance of Solar Still

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Effect of Lowering Condensing Surface Temperature on the Performance of Solar Still
  International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME   41   EFFECT OF LOWERING CONDENSING SURFACE TEMPERATURE ON THE PERFORMANCE OF SOLAR STILL Ajeet Kumar Rai*, Vivek Sachan*, Vinay Tripathi # , Pramod Kumar*, Abhishek Tripathi* * Mechanical Engineering Department, SSET, SHIATS –DU Allahabad # MED, United College of Engineering and Research, Allahabad ABSTRACT In the present work an attempt has been made to study the effect of increasing temperature difference between evaporating surface and condensing surface on the performance of solar distillation system. An indoor simulation study has been performed on a constant temperature bath. In order to increase the temperature difference between evaporating water surface and the condensing surface, the condensing surface temperature has been reduced by putting ice on the glass cover. It is observed that a maximum of 205 % rise in distillate is obtained by 54 % reduction in the condensing surface temperature for constant temperature of the evaporating water at 50 0 C. INTRODUCTION Single basin solar still is a very simple solar device used for converting available brackish water into potable water. This device can be fabricated easily with locally available materials. This device can be a suitable solution to solve drinking water problems. Because of its low productivity it is not popularly used. Many theoretical and experimental works were performed by many researchers to improve the productivity of the still. The performance prediction of a solar distillation unit mainly depends on accurate estimation of the basic internal heat and mass transfer relations. The oldest, semi- empirical internal heat and mass transfer relation is given by Dunkle [1]. Then to predict the hourly and daily distillate output from the different designs of solar distillation units, numerous empirical relations were developed. Most of these are based on the simulation studies. Malik et al. [2] has considered the values of C = 0.075 and n = 0.33 for Gr > 3.2 x 10 5  as proposed by Dunkle.   INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME: Journal Impact Factor (2014): 7.5377 (Calculated by GISI)   IJMET   © I A E M E    International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME   42   However, the relation developed by Dunkle has the following limitations: (1)   It is valid only for a low operating temperature range (45-50 0 C). (2)   It is independent of the cavity volume, i.e. the average spacing between the condensing and evaporative surfaces. (3)   It is valid for cavities that have parallel condensing and evaporative surfaces. Clark [3] developed a model for higher operating temperature range ( ≥  55 0 C) in a simulated condition for small inclinations of the condensing surface ( β   ≤  15 0 C). Clark [3] has observed that the coefficient of convective mass transfer becomes half that given by Dunkle [1]. Tiwari et. al. [4] developed a modified Nusselt number, precisely for a trapezoidal cavity, for evaluation of convective mass transfer in a solar distillation. A theoretical expression developed was validated by experiments but only for temperatures greater than 60 0 C. Later on Kumar and Tiwari [5] developed a thermal model to determine convective mass transfer for different Grashof numbers for solar distillation on a passive and active solar distillation system for only summer climatic conditions. Then Tiwari and Tripathi [6] developed a model for a high temperature range of the order of 80 0 C but for an opaque, metallic, semi-cylindrical condensing cover made of Aluminium, which is not suitable practically for passive solar distillation in the field. The condensing cover developed may be suitable for either active solar distillation or for multi-source distillation units. Instead of indoor simulation, many researchers have performed experimental studies to find real in-situ performance of the passive and active solar stills. As Heat transfer rate increases with increase in temperature difference between the evaporating and condensing surfaces, many researchers have attempted to reduce the condensing surface temperature by spraying water over the glass cover in order to increase the temperature difference between the evaporating and condensing surfaces, and hence the distillate output. In the present work an attempt has been made to find the effect of reducing glass cover temperature on the distillate output of the solar still. EXPERIMENTAL SET-UP A constant temperature bath filled with water (having capacity of 40 litres) was used as a basin for the distillation unit. The temperature range of the bath was from 30 0 C to 110 0 C (lest count temperature was 5 0 C). The steady state temperature and heating of water was controlled by an electronic controlled panel. The condensing chambers were made of GRP sheet with glass as the condensing surface. The chamber was double walled on four sides with air inside the 2.5 cm thick cavity between the two GRP surfaces, which acts as an air insulation. The hollow base of the condensing chamber is of the same size as the constant temperature bath opening, and it can be placed over it to form an air tight enclosure. Channels have been made along the length and the breadth of condensing chamber in order to collect the condensate. The inclination of the glass cover with the horizontal β  =30 0 . Copper – constant an thermocouples are used, along with a digital temperature indicator, to recorded the glass temperature, water temperature and water vapor temperature in the experimental stop. These thermocouples, over a prolonged usage period, tend to deviate from the actual temperature. Therefore, they were calibrated with respect to a standard thermometer. The glass cover (condensing surface for water vapor) is 3mm thick. In order to conduct the heat released by the condensing water (latent heat) from the inner surface of the glass to the outer surface, a temperature gradient exists across the thickness of the glass. Therefore, the inner and outer temperature of the glass cover is not equal, the inner temperature being higher. The average inner (T G )and outer (T g ) glass cover temperature have been obtained experimentally as well as theoretically, match with each other within an accuracy of 5%. In our experiment, it was possible to  International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME   43   record the outer glass temperature only. This was changed to the average inner glass temperature by using the previously obtained steady state values for the inner outer glass temperature. A view of the condensing chamber and photograph of the experimental set up are shown in figure1. Fig.1: Photograph showing experimental work Table 1: Observation Table  Serial No. T w  T v  T g  T w -T g  m ew  (p) Operating condition (Glass Cover) 1 40 36 30 10 0.036 open to atmosphere 2 45 41 33 12 0.046 open to atmosphere 3 50 45 37 13 0.053 open to atmosphere 4 55 49 41 14 0.070 open to atmosphere 5 60 52 44 16 0.090 open to atmosphere 6 40 30 15 25 0.085 Covered with ice 7 45 35 16 29 0.095 Covered with ice 8 50 40 17 33 0.162 Covered with ice 9 55 42 18 37 0.212 Covered with ice 10 60 50 22 38 0.240 Covered with ice  International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 8, August (2014), pp. 41-48 © IAEME   44   ANALYSIS OF CONVECTIVE MASS TRANSFER The moist air above the water surface is freely converted to the condensing cover by the action of a buoyancy force caused by density variation due to the difference between the water surface and condensing cover. This process within the unit always happens in natural mode. However the external heat transfer from condensing cover to the atmosphere takes place outside the still and can either be under the natural or forced mode depending on ambient conditions   The rate of heat transfer from the water surface to glass cover ( cw Q ) by convection in the upward direction through humid fluid can be given by )( gwcwcw  T T hq  −=  (1) The coefficient h cw can be determined form the relation ncw  pr Gr C k d h Nu ).( ==  (2) The expression for Gr and Pr are given as 223 '.... 3  f  f r   T g xG  µ  β  ρ   ∆=  (3)  f  f  pr  k C P  µ  . =  (4) It is clear from the above equation that the value of cw h   depends upon the values of two coefficients namely, C and n. It had been observed from the different values of C and n for given models, for a particular range of Grashof number, that experimental and theoretical values closely agree with a reasonable accuracy only for indoor simulation. However, for outdoor experiments the deviation was more prominent between theoretical and experimental values. Dunkle (1961), gave following expression for cw h  for normal operating temperature range, 3 / 13 )109.268( )273)(( )(884.0  −×+−+−= wwgwgW cw  pT  p pT T h  (5) The expression for h cw  cannot be use for the situations not fulfilling conditions i.e. for operating temperature of 75 ° C, spherical, conical and higher inclined solar stills etc. Hence new values of C and n need to be developed. In the present work, a thermal model will be developed and methodology is proposed to evaluate values of C and n. These are found by using experimental data of distillate output (m w ), water temperature (T w ) and glass temperature (T g ). Malik et. al. (1982) have assumed that water vapour obeys the perfect gas equation and have given the expression for evaporative heat transfer rate (q ew ) as, = ew q )(0163.0 gW cw  PPh  −  (6)


Jul 23, 2017
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