AL-HINAI, Hilal; AL-NASSRI, M S ; JUBRAN, B A Effect of climatic, design and operational parameters on the yield of a simple solar still Energy Conversion and

Prévia do material em texto

Effect of climatic, design and operational parameters on
the yield of a simple solar still
H. Al-Hinai, M.S. Al-Nassri, B.A. Jubran *
Department of Mechanical and Industrial Engineering, Sultan Qaboos University, P.O. Box 33, Al-Khoud 123, Muscat,
Oman
Received 24 February 2001; accepted 15 June 2001
Abstract
This paper reports the use of a mathematical model to predict the productivity of a simple solar still
under different climatic, design and operational parameters in Oman. The shallow water basin, 23� cover
tilt angle, 0.1 m insulation thickness and asphalt coating of the solar still were found to be the optimum
design parameters that produced an average annual solar still yield of 4.15 kg/m2 day.
A cost analysis is performed to shed some light on the potential of utilizing an array of simple solar stills for
the production of drinking water in remote areas in Oman. It was found that the unit cost for distilled water
obtained from such an array of solar stills is $74/1000 gal. � 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Solar still; Design and operational parameters; Numerical modeling
1. Introduction
The presence of water is an important element in the development of the economy and the
welfare of any nation. One of the major concerns in the third world at present is to find new
resources and new processes of providing cheap fresh water, especially for people in remote areas.
Desalination systems use traditional fuels in many countries of the world and in particular in the
Middle East and the Gulf states where water resources are very scarce. The use of solar energy in
desalination systems is gaining more momentum especially in the Gulf region where the solar
radiation intensity is very high.
Energy Conversion and Management 43 (2002) 1639–1650
www.elsevier.com/locate/enconman
*Corresponding author. Tel.: +968-515-315; fax: +968-513-416.
E-mail address: bassamj@squ.edu.om (B.A. Jubran).
0196-8904/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.
PII: S0196-8904(01)00120-0
Numerous experimental and numerical investigations have been done on classical types of solar
stills, such as those of Mowla and Karimi [1], Dunkle [2], Abu-Hijleh [3], Al-Abbasi et al. [4],
Hamdan et al. [5], Badran and Hamdan [6] and Singh and Tiwari [7]. An extensive review paper
on solar desalination systems has been published by Kalogirou [8].
The aim of the present paper is to report a parametric study on the performance of a simple
solar still under the Sultanate of Oman climate. Moreover, a cost analysis is performed to shed
some light on the feasibility of using an array of such type of solar stills for production of drinking
water in remote areas.
Nomenclature
C specific heat (J/kgK)
h convection heat transfer coefficient (W/m2 K)
I hourly incident solar radiation (W/m2 h)
L length (m)
k thermal conductivity (W/mK)
Nu Nusselt number
P pressure (Pa)
Pr Prandtl number
q heat flux (W/m2)
Q heat (W)
Ra Rayleigh number
Re Reynold number
T temperature (�C)
U overall heat transfer coefficient (W/m2 K)
a absorptivity
d declination angle
h angle between direction of air flow and normal to surface
U relative humidity
e emissivity
m kinematic viscosity (m2/s)
s transmissivity
Subscripts
a air
b basin, beam component of solar radiation
c convection
d diffuse component of solar radiation
ev evaporation
g glass
r radiation
w water
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2. Description of the solar still
A simple basin type solar still has been numerically investigated. A computer program has been
developed to conduct a parametric study of the solar still under the climatic conditions in the
Sultanate of Oman.
The basin type solar still consists of a basin that accommodates the brackish water and is
covered by two sloping covers, symmetrical at the center of the basin. The operation of the still is
very simple. The incident solar radiation is transmitted through the transparent glass cover to the
water and heats it, so that it will evaporate and condense on the inside of the glass cover and run
down the cover, where it will be collected at the cover end (Fig. 1).
3. Mathematical modeling
Modeling of a simple basin type solar still is well established, and the authors claim no con-
tribution to the development of the mathematical model. The mathematical model used here is
similar to that developed by Sartori [9].
The transient energy balance equation for the glass cover per unit area of cover, using Fig. 1,
can be written as
mgCg
dTg
dt
¼ ðagIb þ agdIdÞ þ ðqev þ qr;w–g þ qc;w–gÞ � qr;g–a � qc;g–a ð1Þ
The heat transfer between the water and the cover by evaporation, qev is found from [2]:
qev ¼ 16:28hcðpw � pgÞ ð2Þ
where pw and pg are the vapor pressures at the water and glass temperatures, respectively, and are
given by [10]:
Fig. 1. Energy balance for the solar still.
H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650 1641
pw ¼ eð25:317�5144=TwÞ ð3Þ
pg ¼ eð25:327�5144=TgÞ ð4Þ
The heat transfer from the water to the cover by radiation is given by Dunkle [2]
qr;w–g ¼ 0:9rðT 4w � T 4g Þ ð5Þ
The heat transfer between the basin and the cover and the radiative heat lost from the glass cover
to the atmosphere are given, respectively, by
qc;w–g ¼ hcðTw � TgÞ ð6Þ
qr;g–a ¼ 0:9rðT 2g � T 4skyÞ ð7Þ
The temperature of the sky is very much dependent on the amount of water vapor and dust in the
atmosphere, as this will result in an increase in the reflected heat from the ground. Yellot [11] and
Clark and Berhal [12] proposed a relation for the temperature of the sky as
Tsky ¼ Tair½0:74þ ð0:006tdpÞ�0:25 ð8Þ
where tdp is the dew point temperature of the ambient air and can be computed from [13]
tdp ¼
237:3 ln/ þ 17:27TairTairþ237:3
� �h i
½17:27� ln/ þ 17:27TairTairþ237:3�
ð9Þ
The convective heat transfer between the cover and the air is given by
qc;g–a ¼ hgðTg � TairÞ ð10Þ
where the heat transfer convection coefficient between the cover and the air is computed from the
following relations [11] by
hg ¼ ðNuL þ NufÞ1=3k=Lg ð11Þ
NuL ¼ 0:288Re0:731Pr1=3 ð12Þ
Nuth ¼ 0:037Re4=5Pr1=3 ð13Þ
Nuf ¼ NuL cos2 h þ ð1� cos2 hÞNuth ð14Þ
The energy balance equation for the basin liner per unit area is computed from
mbCb
dTb
dt
¼ qabs þ qc;w–b � ðqk þ qx0 Þ ð15Þ
where qabs, qc;w–b and ðqk þ qx0 Þ are the absorbed solar heat radiation by the basin liner, the
convective heat transfer between the water and the basin liner and the heat loss from the basin
liner to the insulation by conduction, respectively, and are given by
qabs ¼ abðIbswb þ IdswdÞ ð16Þ
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qc;w–b ¼ hbðTw � TbÞ ð17Þ
ðqk þ qx0 Þ ¼ kb=LbðTb � TinsulationÞ ð18Þ
The heat transfer coefficient is calculated similarly to that recommended by Ref. [14]. The energy
balance equation for the insulation, per unit area of basin, is
minCin
dTin
dt
¼ ðqk þ qx0 Þ � qloss ð19Þ
where ðqk þ qx0 Þ is the conduction heat gained by the insulation from the basin liner. The heat loss
from the insulation may be written as
qloss ¼ UinðTin � TaÞ ð20Þ
The overall heat transfer coefficient is calculated from Fourier’s conduction equation
Uin ¼ ðLin=kin þ 1=hinÞ�1 ð21Þ
4. Results and discussion
A computer program has been developed based on the equations outlined above. The equations
have been solved using a one-dimensional implicit finite difference method that is unconditionally
stable. The computer program was used to conduct numerical experiments to investigate the
various design and operating parameters that affect the performance of the simple solar still under
the climatic conditions of the Sultanate of Oman.
The effect of the predicted solar radiation on the yield of the solar still on an average day of
February in the Seeb area with a daily global insolation of 16.4 MJ/m2 day is shown in Fig. 2. The
maximum yield of 0.5 kg/m2 h occurred at 13:50 pm, while the maximum insolationoccurred at
12:00 noon. The difference is due to the time lag of the system. The variations of the daily solar
Fig. 2. The variation of solar intensity and still output on February 14.
H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650 1643
still yield and the average solar radiation for a typical year is shown in Fig. 3 for a still having a
5 cm water depth and 5 cm thickness of insulation. The maximum still yield occurred in June with
a daily yield of 6.78 kg/m2 day, while the minimum still yield of 3.17 kg/m2 day occurred in
December.
The effects of the ambient temperature and the wind speed are shown in Fig. 4. It can be seen
from Fig. 4 that increasing the ambient temperature and the wind speed tend to increase the yield
of the solar still. Fig. 4(a) shows that increasing the ambient air temperature from 23�C to 33�C
increases the still yield by 8.2%. Fig. 4(b) indicates that the effect of increasing the wind speed on
the solar still is more significant than the effect of the ambient temperature. For instance, in-
creasing the wind speed from 1 to 3 m/s results in an increase of 8%. This may be explained by the
fact that increasing the wind speed results in a higher heat transfer coefficient, which results in a
lower cover temperature and higher condensation rate inside the still and, hence, a higher yield of
the still.
The effects of the design parameters, such as the tilt angle of the cover, the thickness of the
insulation and the basin liner material, on the yield of the solar still are shown in Figs. 5–8. Fig. 5
indicates that during the winter months, increasing the cover tilt angles tends to increase the yield
of the still with an opposite effect during the summer where increasing the tilt angles resulted in a
decrease in the yield of the still. This might be explained by the fact that the declination angle d
has negative values in the winter and positive values in the summer. Positive declination angles
result in an increase in the reflected radiation of the cover as the cover angle decreased. The
optimum tilt angle of the cover is obtained by taking the average still yield when various cover
tilt angles are used over the whole year. The optimum tilt angle in this study is found to be 23�
(Fig. 6).
Fig. 7 shows the effect of the thickness of insulation on the still yield. The figure indicates that
increasing the insulation thickness of the still increases the still output, especially for the thickness
up to 0.13 m, where the effect is more significant on the still than that at higher thickness values. It
was found in the present work that the optimum thickness of insulation for a simple solar still
under the Omani climatic conditions is in the range of 0.09–0.13 m with an optimum value of
0.1 m, after which the increase in the still yield does not justify the additional insulation cost. The
Fig. 3. The variation of an average solar intensity and single still output in 12 months.
1644 H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650
Fig. 4. Effect of ambient conditions on single still output (a) temperature and (b) wind velocity.
Fig. 5. The effect of the cover slope angle on the single still output for winter and summer months.
H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650 1645
Fig. 6. The average single still output for different slopes of the cover.
Fig. 7. The insulation thickness effect on the still output for simple solar still.
Fig. 8. Effect of basin liner material on the simple solar still.
1646 H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650
effect of the basin liner material on the yield of the solar still is shown in Fig. 8. The figure in-
dicates that the best basin liner material to be used in the still is that of asphalt, which gives a still
yield of 4.26 kg/m2 day in an average day in April.
The effect of feed water preheating at different temperatures is shown in Fig. 9. It is clear from
the figure that the effect is not very significant. As the temperature of the initial brine of water is
increased from 20�C to 50�C, the increase in daily output of the still is 9%. It is interesting to note
that as the temperature of the brine water increases beyond 40�C, the effect starts to diminish. This
increase in the still output is due to the difference between the glass cover and brine water tem-
peratures. The effect of water depth increase in the basin on the daily yield of the still is shown in
Fig. 10. It is evident from the figure that as the depth of the water increases, the daily still output is
decreased. This decrease in the yield of the still is due to the fact that as the depth decreases, the
Fig. 9. Effect of feed water on the solar still output.
Fig. 10. Water effect depth on the solar still output.
H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650 1647
brine will have a lower heat capacity, which results in a higher temperature in the basin and, thus,
higher evaporation rate. The decrease of the water depth from 0.1 to 0.005 m resulted in an in-
crease of 19.6% in the still output, while a decrease in the depth from 0.29 to 0.1 m resulted only in
a 6.3% increase in the still yield. The recommended brine water depth is in the range of 0.02 to
0.06 m.
The average daily output of the solar still when the cover tilt is at the optimum angle of 23�, for
one typical year is shown in Fig. 11. The maximum solar still yield occurs in June where the
Fig. 11. The average daily output of still for different months of the year.
Table 1
Solar still cost analysis
Installed cost summary Item cost ($) Contingency (%) Total ($)
Land 1000 10 1100
Civil work 500 10 550
Solar still 12 500 10 13 750
Auxiliary system 1250 10 1375
Pretreatment system 1000 10 1100
Postreatment system 1000 10 1100
Total capital cost (TCC) 18 975
Construction management (5% of TCC) 949
Total facility cost (TFC) 19924
Interest during construction (8% of TFC) 1594
Preproduction (10% of TFC) 1992
Spare parts (10% of TFC) 1992
Total installed cost (TIC) 25 502
Operating and maintenance cost (10% of TIC) 2550
Inflation of (5% of TIC) 1275
Total cost 29 328
Treated water cost ($/m3) 16.3
Treated water cost ($/1000 gal) 74.1
1648 H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650
insolation is also maximum. The average annual solar still yield per day is 4.15 kg/m2 day. This
average yield will be used next to conduct a simple cost analysis of the solar still under the Omani
climate.
A simple cost analysis, based on a procedure developed by Kelly and De Laqui [15] is per-
formed to estimate the distiller unit cost in USA $/1000 gal (4546 l). The analysis assumes that
distilled water will be supplied to a remote community of 100 people using a cluster of 250 simple
solar stills. The water consumption per person is assumed to be 10 l/day. A summary of the cost
analysis is shown in Table 1. The cost of land for construction of the solar water distillation
system is estimated at USA $2000 per 1000 m2 and the solar still cost at $50/m2, based on the
present yield of 4.15 l/m2 and production of 1000 l/day. The cost of the fittings, tubes and pumps is
taken to be 10% of the total cost of the solar still. The operating and maintenance cost is taken to
be 10% of the total installed cost. The plant is expected to operate a full week and 52 weeks per
year with a life expectancy of five years. Major maintenance work is performed once every year. It
can be seen from Table 1 that the gallon of distilled water will cost 0.074 $/gal (16.3 $/m3). This
cost is competitive with conventional desalination systems for remote areas, with the advantage
that solar stills could be locally produced.
5. Conclusions
A mathematical model has been used to predict the productivity of a simple solar still under
various climatic, operational and design parameters in Oman. The shallow water basin, 23� cover
tilt angle, 0.1 m insulation thicknessand asphalt coating of the solar still were found to be the
optimum design parameters for a simple solar still operation in Oman. The optimum design
conditions tend to give an average annual solar still yield of 4.15 kg/m2 day.
The simple cost analysis indicated that the unit cost for distilled water using an array of simple
solar stills is $74/1000 gal (16.3 $/m3). The cost analysis shows clearly that such a system of simple
solar stills may be used to provide fresh water to people in remote areas at reasonable cost.
References
[1] Mowla D, Karimi G. Mathematical modeling of solar still in Iran. Solar Energy 1995;55:389–93.
[2] Dunkle RV. Solar water distillation: the roof type still and multiple effect diffusion still. Fifth International
Conference of Development in Heat Transfer, U. Colorado, 1961.
[3] Abu-Hijleh BAK. Enhanced solar still performance using water film cooling of the glass cover. Desalination
1996;107:235–44.
[4] Al-Abbasi MA, Al-Karaghouli AA, Minasian AN. Photochemically assisted solar desalination of saline water.
Desalination 1992;86:317–24.
[5] Hamdan MA, Musa AM, Jubran BA. Performance of solar still under Jordanian climate. Energy Convers Mgmt
1999;40:495–503.
[6] Badran AA, Hamdan MA. Inverted trickle solar still. Int J Solar Energy 1995;17:51–60.
[7] Singh AK, Tiwari GN. Thermal evaluation of regenerative active solar distillation under thermosyphon mode.
Energy Convers Mgmt 1993;34(8):697–706.
[8] Kalogirou S. Survey of solar desalination systems and system selection. Energy 1977;22:69–81.
H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650 1649
[9] Sartori E. Solar still versus evaporator: a comparative study between their thermal behaviors. Solar Energy
1996;56(2):199–206.
[10] Fernandez JL, Chargoy N. Multi-stage indirect heated solar still. Solar Energy 1990;44:215–23.
[11] Yellott JI. Passive and hybrid cooling research. Advances in solar energy, vol. 1. NewYork: Plenum Press; 1982.
p. 241–63.
[12] Clark E, Berhal P. Radiative cooling: resources and applications. Proceedings of the Passive-Cooling Workshop,
Amherst, MA; 1980. p. 177–212.
[13] Murray F. On computation of saturation vapour pressure. J Appl Meteorol 1967;16:203–4.
[14] Incropera FP, Dewitt DP. Introduction to heat transfer, second edition. New York: Wiley; 1990.
[15] Kelly BD, De Laqui P. Conceptual design of a photocatalytic wastewater treatment plant. Solar Engineering 1992,
The 1992 ASME, JSES, KSES International Solar Energy Conference, Maui, Hawaii, vol. 1; 1992.
1650 H. Al-Hinai et al. / Energy Conversion and Management 43 (2002) 1639–1650