PERFORMANCE ANALYSIS OF A SINGLE BASIN DOUBLE SLOPE PASSIVE SOLAR STILL

Transcription

1 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: PERFORMANCE ANALYSIS OF A SINGLE BASIN DOUBLE SLOPE PASSIVE SOLAR STILL Vidya Sagar Gupta ABSTRACT An experimental study has been carried out to evaluate the internal heat transfer coefficient, namely convective and evaporative in a double slope solar still under the free modes of operation. For the present study, experiments have been conducted for 24 hours during September- February months for different water depths in the basin (.1,.2 and.3 m) of double slope as well as single slope solar still. The objective of the present paper is to study the effect of different water depths in a solar still of double slope on the internal heat and mass transfer coefficients. It is inferred that the convective heat transfer coefficient between water and inner condensing cover depends significantly on the water depth in the basin. It is also observed that more yields are obtained during the off shine hours as compared to daytime for higher water depths in solar still due to storage effect. The temperature dependent physical properties of enclosed vapour were considered. Observations (temperature and yield) obtained from experimentation have been used to determining the values of coefficient C and n by using linear regression analysis and, consequently, the convective as well as evaporative heat transfer coefficient. Keywords: Distillation; Heat and mass transfer coefficients, Solar still INTRODUCTION The process of solar distillation is used to distill brackish/saline water by using solar energy. The systems involved in solar distillation operate under two modes: passive and active. Tiwari et al. [1] reviewed the present status of research work on both passive and active solar distillation systems. Most of the work done on solar stills has used the expressions for internal heat transfer coefficients as developed by Dunkle [4] under simulated conditions. However, these expressions having some limitations and are independent of the average distance between the condensing and evaporating surfaces. Kumar and Tiwari [7] have recently developed a model, based on regression analysis, to determine the values of C and n using the experimental data obtained from the stills. This method does not impose any limitations on the determination of expressions for internal heat transfer coefficients. In this paper, an attempt has been made to study the effect of different water depths in a solar still on the heat and mass transfer coefficients for the passive mode. The heat transfer is within the distillation unit, which is regarded as the internal heat transfer. It is due to three modes of heat transfer, radiation, convection and evaporation. Convective heat transfer occurs among the different layers of water inside the distillation unit. From the water surface to the condensing cover, heat transfer is accompanied by transport of water vapor formed above the water surface through an air-vapor mixture. Both convective and evaporative heat transfers occur simultaneously and are independent of radiative heat transfer. The air-vapor mixture is convected from

2 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: the water surface to the condensing cover due to the difference in temperature between the water surface and the glass cover. IDENTIFICATION OF THE PROBLEM The double slope solar distillation unit were designed and manufactured to use in study. Experiments were carried out to study the effect of different water depths in the solar still on the internal heat and mass transfer coefficients. Solar still is placed in east-west direction and the inclination of the condensing cover is 15 o. To study the heat transfer coefficients, experiments was conducted for 24 hours during the September-February months for different water depths in the basin (.1,.2 and.3 m). THERMAL MODEL Heat transfer occurs across humid air in the distillation unit by free convection, which is caused by the effect of buoyancy, due to the density variation in the humid fluid. This occurs due to the temperature gradient in the fluid. The rate of heat transfer is from the water surface to the glass cover is in the upward direction. The general equation of convective heat transfer is, cw s cw Q h A T T h A T (1) Where, h cw is the convective transfer coefficient and is a function of the following parameters: Operating temperature range Geometry of the condensing cover Physical properties of the fluid at the operating temperature Flow characteristics of the liquid. The following relation gives the nondimensional Nusselt number related by the convective heat transfer coefficient: hcw Lv Nu C Gr Pr n K or, h cw v Kv C Gr Pr n (2) L v Where, Gr and Pr are the Grashof and Prandtl numbers, respectively. The unknown C and n are constants given in Eq. (2) to be determined by regression analysis using experimental data. The Gr and Pr are given by the following expressions: 3 2 glv T Gr 2 C Pr K v p

3 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: here, T T T w g Pw Pg Tw P w Table 1 Temperature dependent physical properties of vapor: Quantity Symbol Expression Specific heat C p *T i +1.11*1-4 *T i *1-8 *T i 3 Density ρ /(T i ) Thermal Conductivity K v *1-4 *T i Viscosity μ 1.718* *1-8 *T i Latent heat of vaporization of L *1 6 *[1-(7.616*1-4 *T i )] ; for T i >7 C water *1 6 *[1-(9.4779*1-4 T i *1-7 *T 2 i *1-9 *T 3 i ] ; for T i <7 C Partial saturated vapor pressure P ci Exp [ /(T ci +273)] at condensing cover temp. Partial saturated vapor pressure P w Exp [ /(T w +273)] at water temp. Expansion factor β 1/(T i ) and, evaporative heat transfer coefficient is given as: h ew.16 hcw Pw Pg T T w g The temperature-dependent physical properties of vapor are given in Table 1.The linear regression analysis is used to evaluate C and n, which is explained in the section on numerical computation. By evaluating C and n, the convective heat transfer coefficient (h cw ) is evaluated, which is free from various limitations as contained in the relation obtained by Dunkle [4], which is given by, 1 3 P P T 273 w ci w hcw.884 Tw Tci 3 (3) Pw However, this expression has the following limitations: 1. This equation is valid only for normal operating temperature 5 C in a solar still and equivalent temperature difference of T = 17 C. 2. This is independent of cavity volume, i.e., the average spacing between the condensing and evaporating surfaces, due to n = 1/3.

4 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: This is valid only for upward heat flow in horizontal enclosed air space, i.e. for parallel evaporative and condensing surface. EXPERIMENTAL SET-UP, PROCEDURE AND OBSERVATIONS Solar still is an air tight basin, constructed of fibre reinforced plastic (FRP) with a top cover of transparent material like glass, glass is considered to be best for most long-term applications. The inner surface of the base known as basin liner is blackened to efficiently absorb the solar radiation incident on it. There is a provision to collect distillate output at lower ends of top cover. The brackish or saline water is fed inside the basin for purification using solar energy. Fig.1. Cross sectional view of a symmetrical double slope solar still Fig.2. Photograph of a Double Slope Solar Still

5 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: Experiments are conducted for different water depths (.1,.2, and.3) on single slope and double sloped solar distillation unit during the September-February months, Table 2 Dimensions of solar distillation unit. Sr. No. Parameters Dimensions 1. Area of Basin 2m 2 2. Height of Basin.22m, at sides.48m, at center 3. Area of Glass cover 1.2x1.2m 2 4. Thickness of Glass Cover.4m 5. Angle of Glass cover 15 o 6. Thickness of Insulation.15m 7. Height of Still From Ground 1.38m and the following parameters were measured every hour for a period of 24 hour for different depths: Water temperature Inner glass temperature Outer glass temperature Vapor temperature Total radiation on the glass cover Diffuse radiation on the glass cover Total and diffuse radiation on horizontal Ambient temperature Distillate output Copper-constantan thermocouples are used with a digital temperature indicator, having a least count of.1 C, to record the water, vapor and condensing cover temperatures. Initially the thermocouples are prepared by soldering the junctions of two dissimilar metals and Zeal thermometer is used for the calibration of thermocouples, which gives accurate temperature readings. The ambient temperature and the distillate output were recorded with the help of a calibrated mercury thermometer having a least count of 1 C and with a measuring cylinder of a least count 1 ml. The solar intensity was measured on both side of the glass cover (east and west side) and also on the horizontal surface with the help of a calibrated solarimeter, having, least count of 2mW/cm 2. The hourly variation of solar intensity, water, glass and ambient temperatures and hourly output for different depths of water in solar still were used to evaluate average values of each parameter for numerical computation. The values of each measured parameter are given in Tables 3 a-c.

6 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: Time hrs Table 3a various measured temperatures and yield in Double Slope for.1m water depth in the basin for each hour interval, in the month of September. T amb. T og T ig T v T w I t E I d E I t W I d W I t h I d h m w (kg)

7 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: Time hrs Table 3b Various measured temperatures and yield in Double Slope for.2m water depth in the basin for each hour interval, in the month of September. T amb. T og T ig T v T w I t E I d E I t W I d W I t h I d h m w (kg)

8 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: Time hrs Table 3c various measured temperatures and yield in Double Slope for.3m water depth in the basin for each hour interval, in the month of September. T amb T og T ig T v T w I t E I d E I t W I d W I t h I d h m w (kg) NUMERICAL COMPUTATION The distillate output (in kg) from the distiller unit can be obtained by the relation: qew Aw t mew (4) L where, ew ew w ci q h T T (5) and, h ew Pw Pci.1623 hcw (6a) Tw Tci

9 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: By substituting the expression for h cw from Eq. (2) into Eq. (6a), we get, Kv.1623 Pr n Pw Pci hew C Gr (6b) Lv Tw Tci Further, substituting the value of h ew into Eq. (5) and hence q ew into Eq. (4), we get,.1623 Kv m Pr n ew Aw t Pw Pci C Gr L L (7) v The non-dimensional numbers, Gr and Pr, have been evaluated using standard expressions in the temperature ranges. Eq. (7) can be written as, mew or, m ew R C Gr Pr n C Gr Pr n R (8) where,.1623 Kv R Aw t Pw Pci L L v Taking the logarithm to both sides of Eq.(8) and comparing it with the straight line equation, y mx C (9) we get, y ln, R C ln C, x ln Gr Pr m ew And m n By using linear regression analysis, the coefficients in Eq. (9), m and Co, can be obtained by the following expressions: m and C 2 N x x 2 N xy x y 2 y x x xy 2 N x x 2 Where N is the number of experimental observations. The constants m and Co can be calculated from Eqs. (1) and (11) by using the data of Tables 3 a-c. Further, the value of m and Co is used to evaluate constants C and n by using Eqs. (12a,b). C expc (12a,b) n m (1) (11)

10 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: RESULTS AND DISCUSSION The values of x and y in Eq. (9) have been computed at various temperatures by using the experimental data from Tables 3 a-c, and expressions for physical properties of vapor, Table 1. These x and y values were used in Eqs. (1) and (11) to evaluate m and C and finally C and n from Eq. (12a,b). Further, the values of convective and evaporative heat transfer coefficients were calculated for different operating temperature and different water depth. And based on readings the graphs are plotted which is shown in Fig The results were also compared with the Dunkle model [4], which is shown in the same graph. The variation of the convective and evaporative heat transfer coefficients for present model and Dunkle model are indicated as present model (PM) and Dunkle model (DM), respectively. hcw(pm) hcw(dm) 3 hcw (W/m 2 o C) Time (hours) Fig. 3a. Hourly variation of convective heat transfer coefficient for.1m water depth in the basin, in the month of September.

11 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: hew(pm) hew(dm) 3 hew (W/m 2 o C) Time (hours) Fig. 3b. Hourly variation of evaporative heat transfer coefficient for.1m water depth in the basin, in the month of September. hcw(pm) hcw(dm) hcw (W/m 2 o C) Time (hours) Fig. 4a. Hourly variation of convective heat transfer coefficient for.2m water depth in the basin, in the month of September.

12 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: hew(pm) hew(dm) hew (W/m 2 o C) Time (hours) Fig. 4b. Hourly variation of evaporative heat transfer coefficient for.2m water depth in the basin, in the month of September. hcw(pm) hcw(dm) hcw (W/m 2 o C) Time (hours) Fig. 5a. Hourly variation of convective heat transfer coefficient for.3m water depth in the basin, in the month of September.

13 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: hew(pm) hew(dm) 6 hew (W/m 2 o C) Time (hours) Fig. 5b. Hourly variation of evaporative heat transfer coefficient for.3m water depth in the basin, in the month of September. CONCLUSIONS On the basis of the present studies, the following conclusions were made: The present study indicates the importance of a variation of the convective heat transfer coefficient with water depth in the basin. This will be useful to design efficient solar distillation systems. The values obtained for the convective and evaporative heat transfer coefficients by Dunkle s [4] relation do not agree with the values obtained by the present experiment. This is definitely because of the violation of Dunkle's [4] assumptions encountered by the real experiments. The present model gives more accurate and realistic values. On the basis of results we can say that the convective heat transfer coefficient between water and inner condensing cover depends significantly on the water depth in the basin. It is also observed that more yields are obtained during the off shine hours as compared to daytime for higher water depths in solar still due to storage effect. SYMBOLS A Surface area, m 2 A w Area of water surface, m 2 C Unknown constant in Nusselt number expression. C p Specific heat, J/Kg C g Acceleration due to gravity, m/s 2 Gr Grashof number h cw Convective heat transfer coefficient from water to condensing cover, W/m 2 C

14 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: h ew Evaporative heat transfer coefficient, W/m 2 C I t Total solar radiation, W/m 2 I d Diffuse solar radiation, W/m 2 L Latent heat of vaporization of water, J/kg L v Characteristic dimension of condensing cover, m K v Thermal conductivity of humid air, W/m 2 C m ew Distillate output, kg n Unknown constant in Nusselt number expression P ci Partial saturated vapor pressure at condensing cover temperature, N/m 2 Pr Prandtl number P w Partial saturated vapor pressure at water temperature, N/m 2 q ew Rate of evaporative heat transfer, W/m 2 t Time, s T ig Inner temperature of condensing cover, C T og Outer temperature of condensing cover, C T s Evaporative surface temperature, C T v Vapor temperature, C T w Water temperature, C Greek β Coefficient of volumetric thermal expansion, K -1 ρ Density of humid air, kg/m 3 μ Dynamic viscosity of humid air, N.S/m 2 ΔT Temperature difference, C REFERENCES [1] G.N. Tiwari, H.N. Singh, Rajesh Tripathi, Present status of solar distillation. Solar Energy 75 (23); [2] G.N. Tiwari, Rajesh Tripathi, Study of heat and mass transfer in indoor conditions for Distillation. Desalination 154 (23); [3] Rajesh Tripathi and G.N. Tiwari, Effect of water depth on internal heat and mass Transfer for active solar distillation. Desalination 174 (25); [4] R.V. Dunkle, Solar water distillation; the roof type still and multiple effect diffusion still, International Developments in Heat Transfer, ASME, in Proc. International Heat Transfer, Part V, University of Colorado, [5] G. N. Tiwari, Solar Energy: Fundamentals, Design, Modeling and Applications, New Delhi, Narosa Publishing House; 22.

15 (IJAER) 212, Vol. No. 3, Issue No. II, February ISSN: [6] Sanjeev Kumar, G.N. Tiwari, H.N. Singh, Annual performance of an active solar Distillation system. Desalination 127 (2); [7] S. Kumar and G.N. Tiwari, Estimation of convective mass transfer in solar distillation Systems. Solar Energy 57 (1996); [8] G. N. Tiwari, Md. Emran Khan, R.K. Goyal, Experimental study of evaporation in distillation. Desalination 115 (1998); [9] Anil Kr. Tiwari, G. N. Tiwari, Effect of the condensing cover slope on internal heat and mass transfer in distillation: an indoor simulation. Desalination 18 (25);