Scholarly article on topic 'Influence of Water Depth on Internal Heat and Mass Transfer in a Double Slope Solar Still'

Influence of Water Depth on Internal Heat and Mass Transfer in a Double Slope Solar Still Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — M.R. Rajamanickam, A. Ragupathy

Abstract In this work, an attempt has been made to study the effect of water depth on the internal heat and mass transfer in the single basin double slope (DS) solar still. The experimental setup was fabricated from Galvanized iron sheet. The bottom and all sides of the still are made from same material. The cover is made from a transparent glass of 3 mm thickness. The solar still was sealed to reduce the leakage of vapor to the surroundings. The study covers the influence of different environmental and operational parameters on the still productivity. Environmental parameters include solar intensity, ambient temperature and wind speed. Operational parameters are feed water quantity, water depth and orientation. The influence of operational parameters on the internal heat and mass transfer, and hence on the productivity of the still of a DS solar still was studied and compared with that of a single slope (SS) solar still. Both SS and DS solar stills of the same area are fabricated and tests conducted by maintaining the depth of water in the basin at 0.01m, 0.025m, 0.05m and 0.075m after fixing the orientation. The maximum distillate output of 3.07 L/m2/day was obtained with water depth in still basin 0.01m in the DS solar still with north-south orientation. The results indicated that decrease in depth of basin water, resulted with increase in productivity of the still.

Academic research paper on topic "Influence of Water Depth on Internal Heat and Mass Transfer in a Double Slope Solar Still"

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Energy Procedía 14 (2012) 1701 - 1708

Energy

Procedía

2011 2nd International Conference on Advances in Energy Engineering (ICAEE2011)

Influence of Water Depth on Internal Heat and Mass Transfer

in a Double Slope Solar Still

M.R. Rajamanickam,a* A. Ragupathyb

"Assistant Professor, Mechanical Engineering, Annamalai university, Annamalai nagar, Tamil nadu, India.

mrrmanickam@yahoo.co.in bAssociate Professor, Mechanical Engineering, Annamalai university, Annamalai nagar, Tamil nadu, India.

aragupathy_au@hotmail.com

Abstract

In this work, an attempt has been made to study the effect of water depth on the internal heat and mass transfer in the single basin double slope (DS) solar still. The experimental setup was fabricated from Galvanized iron sheet. The bottom and all sides of the still are made from same material. The cover is made from a transparent glass of 3 mm thickness. The solar still was sealed to reduce the leakage of vapor to the surroundings. The study covers the influence of different environmental and operational parameters on the still productivity. Environmental parameters include solar intensity, ambient temperature and wind speed. Operational parameters are feed water quantity, water depth and orientation. The influence of operational parameters on the internal heat and mass transfer, and hence on the productivity of the still of a DS solar still was studied and compared with that of a single slope (SS) solar still. Both SS and DS solar stills of the same area are fabricated and tests conducted by maintaining the depth of water in the basin at 0.01m, 0.025m, 0.05m and 0.075m after fixing the orientation. The maximum distillate output of 3.07 L/m2/day was obtained with water depth in still basin 0.01m in the DS solar still with north-south orientation. The results indicated that decrease in depth of basin water, resulted with increase in productivity of the still.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of 2nd International Conference on Advances inEnergy Engineering (ICAEE).

Keywords: Desalination; solar still; orientation; basin water depth;

* Corresponding author. Tel.: +91 94425238447; fax: +91 4144-239733. E-mail address: mrrmanickam@yahoo.co.in.

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of 2nd International

Conference on Advances in Energy Engineering (ICAEE).

doi:10.1016/j.egypro.2011.12.887

1. Introduction

Need of pure water is important in day-to-day life. The chronic shortage of potable water is the most important issues in the developing countries and using water for survival from contaminated sources cause serious damage of health. The availability of drinking water per capita is shrinking because of population growth. The possible water sources are the bore wells, rainwater, and river or lake water. This surface water, however, must be purified for human consumption. This may involve removal of undissolved substances, dissolved substances and harmful microbes. Popular methods are filtering with sand which only removes undissolved material while chlorination and boiling kill harmful microbes only.

Solar distillation does all these three functions._

Nomenclature

Ac Area of cover, m0

As Area of basin liner, m0

how Convective heat transfer coefficient from water to cover, W/ m0 °C

hrw Radiative heat transfer coefficient from water to cover, W/ m0 °C

hew Evaporative heat transfer coefficient from water to cover, W/ m0 °C

I(t) Total solar radiation, W/ m0

Pg Partial pressure at cover temperature, N/m0

Pw w Partial pressure at basin water temperature, N/m0

qcw Convective heat transfer from water to cover, W/ m0

qrw Radiative heat transfer from water to cover, W/ m0

qew Evaporative heat transfer from water to cover, W/ m0

Ta 1 a Ambient temperature, °C

Tg Glass cover temperature, °C

Tw w Basin water temperature, °C

V Wind speed, m/sec

Seff Effective emissivity, dimensionless

£g Emissivity of cover, dimensionless

£w Emissivity of water, dimensionless

G Stefan-Boltzmann constant, W/m0 K4

2. Solar Distillation

A water distiller captures the process of evaporation and condensation in a chamber, leaving behind all impurities, such as inorganic materials and chemicals. It can even purify seawater. Distillation is one of the mankind's earliest forms of water treatment, and it is still a popular treatment solution throughout the world today. In ancient times, the Greeks used this process in their ships to convert sea water to drinking water and also to treat water in other area that are fouled by natural and unnatural contaminants. Solar still having the advantage of low capacity and self reliance is best suited as, they can produce pure water by using solar energy only, and do not need other expensive energy sources such as fuel or electricity. Among the various types of solar stills, the single passive solar distillation system is the most simple and least expensive. This system is complete and requires no additional infrastructure except the necessary raw water and sufficient sunlight.

3. Identification of Problem

H.P.Garg et al [1] studied the effect of climatic conditions, thermo physical properties of the materials used in the construction of the still, its orientation, tilt angle of cover glasses, water depth in still, vapor

tightness and operating parameters on the distillate output. The ultimate aim of their study was to increase the productivity of the stills by varying the parameters. Tanaka et al. [2] found that a tilted wick type solar still can increase the productivity by 20-50% more than basin-type solar stills. The distillate system can be classified into two categories: passive and active. Malik et al. [3] reviewed the work on passive solar distillation. Later on Tiwari et al. [4] carried out a study on the present status of research work on both passive and active solar distillation systems. They have recommended that only passive solar stills can be economical to provide potable water and active solar distillation system can be economical from a commercial point of view. H.N.Singh et al [5] elevated the monthly performance of passive and active solar stills for different Indian climatic conditions and they found that the yield significantly depends on water depth.

Many investigations have been conducted over the years towards improving the daily distillate output per unit basin area of solar stills. This include studies on the effects of various parameters like orientation/glass cover inclination by A.K.Singh et al [6], addition of dyes to the water body to increase absorptivity [7], evaporation area [8] and high-rate condensation using condensers [9] to enhance the yield. Zaki et al. [9] was studied an active system of conventional single-slope solar still integrated with a flat plate collector under thermosyphon mode of operation and found that the maximum increase in the yield was up to 33% when the water in the still was preheated in the collector. Delyannis[10] presented the status of the solar assisted desalination that were installed in commercial and semi-commercial plants during that time. M.K.Phadatare et al. [11] studied the influence of water depth on internal heat and mass transfer in a plastic solar still and found that maximum distillate output of 2.1 L/m2/day, at water depth of 0.02m in still basin, could be achieved.

The objective of this work is to study the effects of orientation and depth of water in the basin of the still on the productivity of a DS solar still and compare the same with that of a SS solar still.

4. System description

Fig.1. Schematic diagram of double slope solar still

Fig. 1 shows the schematic representation of a solar still with a simple basin double slope cover. The overall dimensions of the still are: length = 1m, breadth = 1m. Height = 0.15m, basin liner area = 1.0m2 and the cover area = 1.2m2. The setup comprised of a storage tank, basin, steel frame structure, glass cover, insulation, measuring jar, temperature measuring devices and piping. The plastic storage tank of capacity 50 litres was used to avoid corrosion. Water flows through the flexible hoses and a valve (V)

controls the mass flow rate. Poly Vinyl Chloride (PVC) hoses was used for greater flexibility. The black painted still basin was made up of 18 gauge galvanized iron sheet, this was placed inside the steel frame structure at a pre determined height. The area below the basin was filled with saw dust for insulation purpose. A small plastic piece obstruction was fixed in the inside surface of the glass cover, to facilitate the deflection of the condensate return into the collection channel, which was affixed with the steel frame structure. The gliding water from the channel was transferred into the measuring jar through the flexible piping. For comparison, a SS solar still of the same dimensions was fabricated and is shown in Fig.2.

This experimental setup was designed, installed and tested in Energy Laboratory, Department of Mechanical Engineering, Annamalai University, Annamalai Nagar, Tamil Nadu, India. The experimental setup was kept in the north-south and then in east-west orientation, with the inclination of 11°, which is the latitude of Annamalai Nagar.

Fig.2. Schematic diagram of a single slope solar still

Copper Constantan thermocouple was used for temperature measurement. These thermocouples were fixed at still basin plate, water, inside and outside surface of the glass cover. Temperature were measured at more than one location and averaged was considered for the case of base plate temperature and basin water temperature. Thermocouples were integrated with a temperature indicator and selector switch. To measure the solar radiation a calibrated Eppley Pyranometer was used. The accuracies of various measuring instruments used in these experiments are given in table 1. The experiments were conducted from 11.02.2011 to 19.02.2011, hourly measurements were made for ambient temperature, and basin water temperature and glass cover temperature from 10.00 am to 5.00 pm during the trial period. These temperatures were measured with the aid of copper constantan thermocouples and noted from digital temperature indicator. The hourly weather data, i.e., solar radiation and wind speed were measured with the aid of Eppley Pyranometer and anemometer. The heat transfers co-efficient were estimated for the two orientations (east-west, north-south) of the DS solar still. Table 1. Accuracies and ranges of measuring instruments

Sl No. Instrument Accuracy Range % Error

1. Thermocouple ± 1°C 0-100°C 0.25%

2. Eppley Pyranometer ± 1W/m2 0-2800 W/m2 0.25%

3. Measuring beaker ± 10 ml 0-1000 ml 10%

0. Anemometer ± 0.1 m/s 0-15 m/s 10%

5. Results and Discussion

Fig.3 represents the variation of east-west, north-south orientation and solar intensity for DS solar still at a depth of 0.02 m of water maintained in the basin. For both orientations solar intensity was nearly constant and the distillate output was higher in north-south as compared with east west orientation. Every morning at 10.00 am the water is fed in the still basin up to a desired level and the level is maintained throughout the trial period.

Fig.4 shows the variation of evaporative heat transfer co-efficient for DS and SS solar still in north-south orientation at a depth of 0.01m maintained in the basin. The values are higher for the north-south orientation as compared with east-west orientation. The evaporative heat transfer is calculated for measured solar radiation on same day for SS and DS solar still. The peak values were recorded at 1.00 pm., every day and it decreases in the afternoon in both DS and SS solar stills.

From fig.5 it was inferred that the same pattern is obtained for convective heat transfer co-efficient. It was clear from fig.6 that the radiation heat transfer coefficient is almost same for both DS and SS solar stills.

Fig.7 shows the variation of evaporative, convective and radiation heat transfer rate for both stills. The evaporative heat transfer rate, at all the hourly data is high in north-south orientation in DS solar still compared with SS solar still.

Fig.8 shows the hourly variation of condensate collection for DS slope and SS still in north-south orientation at a depth of 0.01m. It was higher in DS solar still.

Fig. 9 shows the variation of condensate collection for DS solar still in north-south orientation at different depths viz., 0.01m, 0.025m, 0.05m and 0.075m. As the water depth increases the condensate collection decreases. At a depth of 0.01m the yield was 3.07 Lit/m2/day in north-south orientation of DS slope solar still. This was the highest yield of all the experimented depths.

Fig.10, Fig.11 and Fig.12 shows the variation of maximum evaporative, convective and radiation heat transfer co-efficient for DS solar still in north-south orientation at different depth. All the heat transfer coefficient values are higher in the minimum depth of 0.01m.

sinrn 1 a

L«ij wail i: bf rtn

MO i»

S S S H S 5 î £ ï S

! S n M u s SSflBflSflS Tna Fhi.ii

si8R3R8iiaKs

Fig. 3. Variation of distillate output and solar intensity for DS solar still in east-west, north-south orientation (water depth = 0.02m)

Fig. 4. Variation of evaporative heat transfer coefficient for DS and SS solar still in north-south orientation at a depth of 0.01m

1.1 1«

-*- — — K™UB) Ink MHttil

/7 - * j

r vy \

e » M s s M s i t ti s » i

Tfî- Itouil

Fig. 5. Variation convective heat transfer coefficient for DS and SS Fig. 6. Variation of radiative heat transfer coefficient for solar still in north-south orientation at a depth of 0.01 m DS and SS still in north-south orientation at a depth of 0.01

JO) as

HQ 1S0 1«

fji* IdWÎÛTI

T — CorKSc-aaJo |ÛÊj -i I \

/V / A p.

/ / \ V /T / A t K N

SBESSB8SSBSBBBS

PPrrÎfsSSSiSSÏÎS

fn», noun:

Fig. 7. Variation of evaporative, convective and radiative heat transfer rate for DS and SS solar still in north-south orientation at a depth of 0.01 m

Fig. 8. Variation of condensate collection for DS and SS solar still in north-south orientation at a depth of 0.01 m

Cat HU2BI1

■ . • h-rA ' HI I

- —• - hTWII^JTl

--«--l«r{ni|

m. _ _ -m

Dwptl "H

Fig. 9. Variation of condensate collection for DS solar still in Fig. 10. Variation maximum evaporative, maximum convective

and maximum radiative heat transfer coefficient for DS solar

north-south orientation at different depth

still in north-south orientation at different depths

--» — T. i-mj \ - - Tj ¡rtn

r X1*-*. * \ \ Jv

11K 1WI

lug 6» no № HC

Dtpn I»

Fig. 11. Variation of maximum evaporative, maximum convective and maximum radiative heat transfer rate for DS solar still in north-south orientation at different depths.

Fig. 12. Variation of maximum water temperature, glass cover temperature and ambient temperature for different depth at north-south orientation

5. Economic analysis

The payback period of the experimental setup depends on overall cost of fabrication, maintenance cost, operating cost and cost of feed water. The cost of feed water is negligible. Overall fabrication cost to be considered = Rs.10000 ($200) Cost per litre of distilled water = Rs.10 ($0.2) Productivity of the solar still = 3.070 l/m2/day Cost of water produced per day = cost of water / litre x productivity

= 10 x 3.070 = Rs.30.7 ($0.614)

Maintenance cost = Rs.5/day ($0.1)

Net earnings = Cost of water produced - maintenance cost

= 30.7 - 5 = Rs.25.7 ($0.514) Payback period = Investment/Net earning =10000/25.7 = 389 days

6. Conclusion

1. The maximum distillate output of 2.760 L/m2/day was obtained with a water depth of 0.02m in still basin of DS solar still at north-south orientation.

2. The maximum distillate output of 3.07 L/m2/day was obtained with a water depth of 0.01m in still basin on DS solar still at north-south orientation in comparison with SS solar still which resulted with an output of 2.34 L/m2/day under the same conditions.

3. Increase in depth of water in the basin resulted with a decrease in productivity of the still. While the output was 3.07 L/m2/day at 0.01m depth, it was only 0.69 L/m2/day at a depth of 0.075m.

4. The radiation heat transfer co-efficient is higher than the convective heat transfer co-efficient. The evaporative heat transfer co-efficient is higher at 0.01m depth and gradually decreases with increasing depth of water in the basin. The values of radiative heat transfer co-efficient and convective heat transfer co-efficient falls in the same range for all depths.

5. DS solar still with north-south orientation has an edge over the SS solar still for the same depth

and orientation at lower water level maintained in the still.

Acknowledgements

The authors acknowledge with thanks the support extended by Energy laboratory of Mechanical Engineering, Annamalai University, India in conducting the experiments.

References

[1] H.P.Garg and H.S.Hann, Effect of climatic, operational and design parameters on the year round performance of single sloped and double sloped solar still under Indian arid zones conditions. Solar Energy 1976; 1:159.

[2] K.Tanaka, A.Yamashita and K.Watanabe, Experimental and analytical study of the tilted wick type solar still, in. Solar world Forum, Pergamon Press, Oxford, 1982.

[3] M.A.S.Malik, G.N.Tiwari, A.Kumar and M.S.Sodha, Solar Distillation, Pergamon Press, UK, 1982.

[4] G.N.Tiwari, H.N.Singh and R.Tripathi, present status of solar distillation. Solar Energy 2003; 75 (5): 367-373.

[5] H.N.Singh and G.N.Tiwari, Monthly performance of passive and active solar stills for different Indian climatic conditions, Desalination 2004; 168: 145-150.

[6] A.K.Singh, G.N.Tiwari, P.B.Sharma and E.Khan, Optimization of orientation for higher yield of solar still for a given location. Energy Conservation Management 1995; 36(3): 175-187.

[7] A.K.Rajvanshi, Effect of various dyes on solar distillation, Solar Energy 1981; 27: 51-65.

[8] H.S.Kwatra, Performance of a solar still: predicted effect of enhanced evaporation area on yield and evaporation temperature. Solar Energy 1996; 56(3): 261-266.

[9] Zaki.G.M., Dali, T.El., Shafie, H.EI., 1983.Improved performance of solar still. In: Proceedings of 1st. Arab International Solar Energy Conference, Kuwait, p.331.

[10] Delyannis, E., Status of solar assisted desalination. Desalination 1987; 67: 3.

[11] M.K. Phadatare and S.K. Verma, Influence of water depth on internal heat and mass transfer in a plastic solar still, Desalination 2007; 217: 267-275.

Appendix

Various relations for heat transfer co-efficient, heat transfer rate and for partial vapor pressure are [11] Pw = exp [25.317--——1

W 1 L (Tw+273.15)1

Pg = exp Î25.317--——1

g r L (Tg+273.15)J

hcw = 0.884 [(Tw - Tg) + (Pw-pg>(Tw +273)f CW w g (268.9x10 —Pw)J

qcw _ hcw (Tw - Tg)

hrw = Eeff. o [(Tw + 273)2 + (Tg + 273)2] • [Tw + Tg + 546]

o = 5.669 X 10"s W/m2K4

Seff = i4- + -W - ll_1

L-- -w J

sg = sw = 0.9

qrw hrw (Tw - Tg)

hew = 16.273 x 10-3 x hcw(Pw"Pg)

ew (Tw-Tg)

qew hew (Tw Tg)