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Experimental study of a basin type solar still with internal and external reflectors in winter
Desalination 249 (2009) 130–134
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Experimental study of a basin type solar still with internal and external reflectors in winter Hiroshi Tanaka ⁎ Mechanical Engineering Department, Kurume National College of Technology, Komorino, Kurume, Fukuoka 830-8555, Japan
a r t i c l e
i n f o
Article history: Accepted 5 February 2009 Available online 4 October 2009 Keywords: Solar desalination Solar still Solar distillation Basin Reflector Outdoor experiment
a b s t r a c t A basin type solar still with internal and external reflectors was constructed and then examined in outdoor experiments in winter in Kurume, Japan. The external reflector was inclined slightly forward to make the reflected sunrays hit the basin liner of the still effectively. The daily productivity of a basin type still can be increased about 70% to 100% with a very simple modification using internal and external reflectors. The experimental results and the theoretical predictions are in fairly good agreement, especially on clear days. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Internal and external reflectors can increase the distillate productivity of a basin type still, and many reports about the effect of internal reflectors [1–4] and external reflector [5,6] on the distillate productivity of basin type stills have been presented. However, a detailed and quantitative analysis of the effect of an external reflector on a basin type still had not been presented. Therefore, we have added to this research by presenting a geometrical model to evaluate the effect of internal reflectors as well as a vertical external flat plate reflector on the distillate productivity of the basin type still [7]. We found that an internal reflector and a vertical external flat plate reflector can remarkably increase the distillate productivity of the still during the spring and autumn seasons. But during the summer and winter seasons, the effect of the vertical external reflector would be less than during spring and autumn, or would be even negligible. In summer, the vertical reflector would not effectively reflect the sunrays onto the basin liner around noon since the sunrays would be nearly vertical. In winter, the solar altitude angle would be so small that a considerable amount of the sunrays from the vertical reflector would escape to the ground without hitting the basin liner. Therefore, the arrangement of the external reflector has to be changed from vertical for these seasons. During the winter season, inclining the external reflector slightly forward would enable the reflected sunrays to hit the basin liner even when the solar altitude angle is small. Therefore, we have presented
⁎ Tel.: +81 942 35 9359; fax: +81 942 35 9321. E-mail address: [email protected]. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.02.057
an additional analysis to calculate the solar radiation reflected from an external reflector inclined forward and then absorbed onto the basin liner [8], and found that distillate productivity was predicted to be increased by inclining the external reflector. Besides the use of reflector, many modifications have been done to increase the productivity of distillate of a basin type still (reviewed by Tiwari et al. [9]), such as reducing water depth, use of dye, reducing bottom loss coefficient and so on. However, in this study, we used a typical basin type still, which does not take into consideration of these modifications, since we focused the effect of the reflector. Therefore, the structure of a basin type still we constructed is basically the same with other basin type stills tested by many researchers such as Cooper [10], and so on [1,3,4]. In this paper, we present the results of outdoor experiments of a basin type still with internal reflectors and an external reflector inclined forward during the winter season in Kurume, Japan (33.2°N latitude and 130.2°E longitude). The purpose of the study is to investigate whether the basin liner of basin type still with internal and external reflectors can absorb solar radiation adequately as predicted by the geometrical models described in our previous papers [7,8], since such outdoor experiments on basin type still had not been reported.
2. Experimental apparatus and procedure A schematic diagram and a snap shot of an outdoor experimental apparatus are shown in Figs. 1 and 2. The still consists of a basin liner, a 5-mm-thick glass cover, and an external reflector inclined forward. The angle of the glass cover was set at 20° from horizontal. The angle of the external reflector was set at 10°, which was predicted as an optimum reflector angle when glass cover angle is 20° [8]. The width
H. Tanaka / Desalination 249 (2009) 130–134
131
Nomenclature Symbols Gdf, Gdr diffuse and direct solar radiations on a horizontal surface, (W m− 2) global solar radiation on a horizontal surface, (W m− 2) Gh daily global solar radiation on a horizontal surface ΣGh during experimental period, (MJ m− 2 day− 1) extra-terrestrial solar radiation, (W m− 2) I0 length of still, (m) ls heat capacity, (J/K) mcp calculated daily productivity during experimental md,cal period, (kg m− 2 day− 1) experimental result of daily productivity, (kg m− 2 day− 1) md,exp convective heat transfer rate, (W) Qc conductive heat transfer rate, (W) Qd heat transfer rate by mass transfer, (W) Qe radiative heat transfer rate, (W) Qr solar radiation absorbed on basin liner, (W) Qsun,b solar radiation absorbed on glass cover, (W) Qsun,g Qsun,df, Qsun,dr diffuse and direct solar radiations absorbed on basin liner, (W) Qsun,ext, Qsun,int solar radiation reflected by external and internal reflectors and absorbed on basin liner, (W) T temperature, (°C) t time, (s) w width of still, (m) α absorptance of basin liner β incident angle of sunrays to glass cover, (°) ϕ solar altitude angle, (°) θ angle of glass cover, (°) transmittance of atmosphere τatm transmittance of glass cover τg transmittance of glass cover for diffuse radiation (τg)df
Subscripts a surroundings b basin liner g glass cover
and length of the basin liner and external reflector are 355 mm × 343 mm and 355 mm × 348 mm, respectively, so both of the areas are almost the same. The basin liner and the walls of the still were made of plywood. The basin liner was coated with black silicone sealant, and the side and back walls were covered with the reflector. A 1.8-mmthick mirror-finished stainless steel plate (reflectance is 0.7 [11]) was used for the internal and external reflectors. The bottom and all walls of the still were insulated using a 50-mm-thick urethane foam board. A partition was placed in front of the basin liner to make a channel for collecting the distilled water. The direct and diffuse solar radiations as well as the reflected radiation are transmitted through the glass cover and absorbed on the basin liner. The absorbed solar energy heats up and evaporates the basin water, and the water vapor condenses on the glass cover. The distilled water flows down to the channel and is gathered through the collecting pipe placed at the bottom of the channel. The experimental procedure is as follows: the still was oriented toward due south. Tap water was poured into the basin liner to form a water pool approximately 10-mm deep. Condensate was collected almost every hour and measured with a flask and a digital balance. Global solar radiation on a horizontal surface was measured on the roof of the neighboring building with an actinometer at a recording interval of 10 s. Since the place at which the experiments were
Fig. 1. Schematic diagram of experimental apparatus.
performed was shaded by the neighboring buildings during early morning and late evening, the experiments were performed when solar radiation would not be obstructed and could be absorbed onto the basin liner each day. 3. Theoretical analysis Theoretical analysis of the proposed still was described in detail in our previous papers about the geometrical model to calculate the solar radiation absorbed on the basin liner [8] as well as heat and mass transfer in the still [7].
Fig. 2. Snap shot of experimental apparatus.
132
H. Tanaka / Desalination 249 (2009) 130–134
The major differences in assumptions and conditions between the theoretical analysis described in our previous papers and that employed in the calculation in this paper are as follows: (1) Direct and diffuse radiations on a horizontal surface, Gdr and Gdf, were calculated with Bouguer's and Berlage's equations [12] with transmittance of atmosphere, τatm, of 0.7 and a solar constant of 1370 W m− 2 in the previous paper as Gdr =
1 = sinϕ I0 sin ϕ⋅τatm
ð1Þ 1 = sinϕ
Gdf = 0:5I0 sin ϕ⋅ð1−τatm
Þ = ð1−1:4 ln τatm Þ
ð2Þ
where I0 is extra-terrestrial solar radiation and ϕ is solar altitude angle. In the experiments, global solar radiation on a horizontal surface, Gh, was measured with an actinometer, and the global solar radiation should be divided into direct and diffuse radiations for the calculations. Therefore, transmittance τatm which satisfies the equation ð3Þ
Gh = Gdr + Gdf
was calculated with the measured value of the global solar radiation on a horizontal surface Gh and a solar constant of 1370 W m− 2, and the calculated τatm was substituted in Eqs. (1) and (2) to determine the direct and diffuse solar radiations on a horizontal surface at a given time each day. (2) The thickness of the glass cover was assumed to be 3 mm in the previous paper, while a 5-mm-thick glass cover was used in the outdoor experiments, and the transmittance of the glass cover, τg, was determined as a function of the incident angle of sunrays to the glass cover, β, thus [13]: 4
3
2
τg ðβÞ = 0:674 cos β−0:300 cos β−2:029 cos β + 2:478 cos β:
ð4Þ (3) Tap water was used in the outdoor experiments instead of saline water to determine the productivity of distillate and avoid possible troubles such as corrosion of experimental apparatus and so on. So the salinity of the saline water supplied to the basin liner is determined as 0 wt.% in these calculations while that was assumed to be 3.44 wt.% in the previous paper. The saturated water vapor pressure of saline water is lower than that of pure water because of the boiling point elevation which is a function of salt concentrations and temperature. The decrease in distillate productivity caused by the lower saturated water vapor pressure was estimated to be 3% or less [14]. (4) Transmittance of the glass cover for the diffuse radiation, (τg)df, can be calculated by integrating transmittance of the glass cover for diffuse radiation from all directions in the sky dome. In previous papers [7,8], the effect of obstruction by the external reflector for diffuse radiation was neglected when transmittance (τg)df was calculated since all the calculations were performed for clear days and the ratio of the diffuse radiation to the global solar radiation was enough small. In this paper, some outdoor experiments were performed on cloudy days and the ratio of the diffuse radiation to the global radiation would be greater than on a clear day. Therefore, the transmittance of a 5-mm-thick glass cover for the diffuse radiation (τg)df was calculated as following, taking into account the effect of obstruction by the external reflector in this paper. −5
ðτg Þdf = −3:29 × 10
2
× θ + 3:01 × 10
−3
× θ + 0:313:
ð5Þ
Theoretical analysis of the proposed still was described in detail in our previous papers [7,8] as mentioned above. Therefore, the only
major governing equations are described here. Heat transfer from the basin liner to glass cover was caused by radiation (Q r,b-g), convection (Q c,b-g) and mass transfer (Q e,b-g). Thermal energy escapes to the ground from basin liner through bottom insulation (Q d,b-a). Heat transfer from glass cover to the surroundings was caused by radiation (Q r,g-a) and convection (Q c,g-a). Therefore, the energy balance for the basin liner and the glass cover can be expressed as Q sun;b = Q r;bg + Q c;bg + Q e;bg + Qd;ba + ðmcp Þb
dTb
.
ð6Þ
dt
Q sun;g + Q r;bg + Q c;bg + Q e;bg = Q r;ga + Q c;ga + ðmcp Þg
dTg
. dt
ð7Þ where Q sun,b and Q sun,g are absorbed solar radiation by the basin liner and the glass cover, mcp is heat capacity, T is temperature and t is time. In the calculation, we theoretically analyzed three types of stills: IES (a still with both internal and external reflectors), IS (one with an internal reflector only) and NRS (one with no reflector). Q sun,b in Eq. (6) was determined for IES, IS and NRS as Q sun;b = Q sun;dr + Q sun;df + Q sun;int + Q sun;ext
ð8Þ
Q sun;b = Q sun;dr + Q sun;df + Q sun;int
ð9Þ
Q sun;b = Q sun;dr + Q sun;df
ð10Þ
where, Q sun,dr and Q sun,df are direct and diffuse solar radiations absorbed on the basin liner, and Q sun,int and Q sun,ext are solar radiation reflected by internal and external and then absorbed on the basin liner. Q sun,dr and Q sun,df can be expressed as Q sun;dr = Gdr τg ðβÞαb × wls
ð11Þ
Q sun;df = Gdf ðτg Þdf αb × wls
ð12Þ
where αb is absorptance of basin liner (=0.9), and w and ls are width (=355 mm) and length (=343 mm) of basin liner. 4. Results Hourly variations of the global solar radiation on a horizontal surface (Global), the experimental results and the theoretical predictions of the distillate production rates of the stills on 15 Feb. 2007 are shown in Fig. 3a. The experimental period on this day is 8:45 a.m. to 5:00 p.m. Here, IES represents a still with both internal and external reflectors, IS represents one with an internal reflector only and NRS represents one with no reflector. The outdoor experiments were performed with an IES still (a still with both internal and external reflectors) only. The distillate production rate for each still peaks about 1 h later than that of the global solar radiation because of the heat capacity of the still. The distillate production rate can be increased by using internal and/or external reflector(s). The experimental results (IES (exp.)) was in good agreement with the theoretical predictions (IES (cal.)), and this indicates that the solar radiation reflected from the reflectors and then absorbed onto the basin liner can be adequately calculated using the geometrical model described in our previous paper [8]. Hourly variations of direct and diffuse solar radiations on a horizontal surface calculated from the measured global solar radiation on a horizontal surface on the same day are shown in Fig. 3b, and the solar radiation absorbed on the basin liner caused by direct and diffuse radiations as well as the radiation reflected from internal and external reflectors and then absorbed on the basin liner on the same day is shown in Fig. 3c. Here, the values for direct and diffuse radiations in Fig. 3c are smaller than those in Fig. 3b, since the
H. Tanaka / Desalination 249 (2009) 130–134
Fig. 3. a. Hourly variations of global solar radiation on a horizontal surface (Global), theoretical predictions (cal.) of distillate production rates of NRS, IS, IES and experimental results (exp.) of IES on 15 Feb. 2007. b. Hourly variations of global, direct and diffuse solar radiations on a horizontal surface on 15 Feb. 2007. c. Hourly variations of absorption of direct and diffuse solar radiations and reflected radiation from internal and external reflectors on a basin liner on 15 Feb. 2007.
133
Fig. 4. a. Hourly variations of global solar radiation on a horizontal surface (Global), theoretical predictions (cal.) of distillate production rates of NRS, IS, IES and experimental results (exp.) of IES on 22 Jan. 2007. b. Hourly variations of global, direct and diffuse solar radiations on a horizontal surface on 22 Jan. 2007. c. Hourly variations of absorption of direct and diffuse solar radiations and reflected radiation from internal and external reflectors on a basin liner on 22 Jan. 2007.
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H. Tanaka / Desalination 249 (2009) 130–134
Table 1 Overall result of outdoor experiments. Date
Temperature
Experimental period
ΣGh
md,exp (IES)
md,cal (IES)
md,cal (IS)
md,cal (NRS)
md,exp / md,cal (IES)
19 19 22 25 13 15 19
6 °C–19.5 °C 10 °C–18 °C 12 °C–17 °C 12 °C–21 °C 11 °C–22 °C 9 °C–22 °C 11 °C–23 °C
8:20–17:00 10:15–16:23 10:26–16:00 10:18–17:00 8:50–17:00 8:45–17:00 8:40–17:00
9.9 8.3 7.1 8.8 13.4 14.6 12.9
1.77 1.61 1.19 2.05 3.31 4.01 2.99
2.55 2.14 1.58 2.25 3.75 4.10 3.15
1.90 1.53 1.17 1.63 2.82 3.09 2.50
1.31 1.08 0.80 1.17 2.14 2.31 1.91
0.69 0.75 0.75 0.91 0.88 0.98 0.95
Dec. 2006 Jan. 2007 Jan. 2007 Jan. 2007 Feb. 2007 Feb. 2007 Feb. 2007
absorption of solar radiation on the basin liner would be less than the solar radiation on a horizontal surface because of the reflection and absorption of the glass cover and the reflection of the basin water. The amount of daily cumulative absorption of solar energy caused by the internal or external reflector is greater than that provided by diffuse radiation, and about 30% or 40% of that provided by direct radiation. Fig. 4a to c shows the same results as Fig. 3a to c on 22 Jan. 2007 which was a cloudy day. The experimental period for this day was 10:26 a.m. to 4:00 p.m. The global solar radiation on a horizontal surface is unstable during the day, and the theoretical predictions for distillate production rate as well as the solar radiation absorbed on the basin liner fluctuate in accordance with the global solar radiation. The experimental results of distillate production rate are lower than the predictions, and the daily productivity in the experimental result (1.19 kgm− 2 day− 1) is about 75% of the predictions (1.58 kgm− 2 day− 1). This is mainly because the thermal losses related to frequent heating up and cooling down of the still during fast transition of solar radiation would cause reductions of distillate efficiency. Other reasons may be that: (1) The diffuse solar radiation would normally make up a large amount of the global solar radiation on a cloudy day, while the ratio of diffuse radiation was estimated at comparatively small values throughout the day in these calculations. (2) The heat transfer through insulation of side and back walls would be neglected in the calculation, while small amount of thermal energy would escape to the surroundings through these walls. The overall results of the outdoor experiments are listed in Table 1, and the variations in experimental results of the daily productivity of IES and the theoretical predictions of the daily productivity of IES, IS and NRS in relation to the daily global solar radiation on a horizontal surface are shown in Fig. 5. Here, the daily global solar radiation as
well as the daily productivity is the cumulative results for the experimental period of each day. Cooper [10] has experimentally tested a conventional basin type still in detail. The daily productivity of a conventional basin type still (NRS) predicted in the present model is in good agreement with the experimental values presented by Cooper. The experimental results of the daily productivity of IES are more than about 70% of the predictions. Especially on sunny days (on 13, 15 and 19 Feb. 2007), the discrepancy between the experimental results and the theoretical predictions would be averaged as about 6%. This indicates that the radiation reflected from an inclined external reflector and then absorbed on the basin liner can be adequately calculated with the geometrical model described in our previous paper [8]. The daily productivity of IES is predicted to be about 1.7 to 2.0 times as great as that of NRS, and this indicates that the distillate productivity of a conventional basin type still can be dramatically increased by a very simple modification using internal and inclined external flat plate reflectors in winter. 5. Conclusions We performed outdoor experiments of a basin type solar still with internal and external reflectors in winter season in Kurume, Japan. The results of this work are summarized as follows: 1. The experimental results and the theoretical predictions of the daily productivity of the still are in fairly good agreement, and especially on clear days, the discrepancy between the experimental results and theoretical predictions would be averaged as about 6%. This indicates that the radiation reflected from the reflectors and then absorbed onto the basin liner can be adequately calculated with the geometrical model described in our previous paper [8]. 2. The daily productivity of a conventional basin type still predicted in the present model is in good agreement with the experimental results of Cooper [13]. 3. A very simple modification using internal and external reflectors can increase the daily productivity of a basin type still by about 70% to 100%. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Fig. 5. Daily global solar radiation on a horizontal surface and daily productivity of IES, IS and NRS.
[14]
A. Tamimi, Solar & Wind Technology 4 (1987) 443. M.E. El-Swify, M.Z. Metias, Renewable Energy 26 (2002) 531. R. Tripathi, G.N. Tiwari, Desalination 169 (2004) 69. I. Al-Hayek, O.O. Badran, Desalination 169 (2004) 121. M.A.S. Malik, G.N. Tiwari, A. Kumar, M.S. Sodha, Solar Distillation, Pergamon Press Ltd, UK, 1982, p. 98. A. El-Bahi, D. Inan, Renewable Energy 17 (1999) 509. H. Tanaka, Y. Nakatake, Desalination 197 (2006) 205. H. Tanaka, Y. Nakatake, Solar Energy 81 (2007) 1035. G.N. Tiwari, H.N. Singh, R. Tripathi, Solar Energy 75 (2003) 367. P.I. Cooper, Solar Energy 15-3 (1973) 205. Japan Society of Mech. Eng., JSME Data Book, Heat Transfer, 4th ed, Japan Society of Mech. Eng, Tokyo, 1986, p. 184. Japan Solar Energy Society, Solar Energy Utilization Handbook, Japan Solar Energy Society, Tokyo, 1985, p. 20. Central Glass Corp, Architectural Glass Products, Central Glass Corp, Tokyo, 2002, p. 177. H. Tanaka, T. Nosoko, T. Nagata, Desalination 130 (2000) 279.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Experimental study of a basin type solar still with internal and external reflectors in winter Hiroshi Tanaka ⁎ Mechanical Engineering Department, Kurume National College of Technology, Komorino, Kurume, Fukuoka 830-8555, Japan
a r t i c l e
i n f o
Article history: Accepted 5 February 2009 Available online 4 October 2009 Keywords: Solar desalination Solar still Solar distillation Basin Reflector Outdoor experiment
a b s t r a c t A basin type solar still with internal and external reflectors was constructed and then examined in outdoor experiments in winter in Kurume, Japan. The external reflector was inclined slightly forward to make the reflected sunrays hit the basin liner of the still effectively. The daily productivity of a basin type still can be increased about 70% to 100% with a very simple modification using internal and external reflectors. The experimental results and the theoretical predictions are in fairly good agreement, especially on clear days. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Internal and external reflectors can increase the distillate productivity of a basin type still, and many reports about the effect of internal reflectors [1–4] and external reflector [5,6] on the distillate productivity of basin type stills have been presented. However, a detailed and quantitative analysis of the effect of an external reflector on a basin type still had not been presented. Therefore, we have added to this research by presenting a geometrical model to evaluate the effect of internal reflectors as well as a vertical external flat plate reflector on the distillate productivity of the basin type still [7]. We found that an internal reflector and a vertical external flat plate reflector can remarkably increase the distillate productivity of the still during the spring and autumn seasons. But during the summer and winter seasons, the effect of the vertical external reflector would be less than during spring and autumn, or would be even negligible. In summer, the vertical reflector would not effectively reflect the sunrays onto the basin liner around noon since the sunrays would be nearly vertical. In winter, the solar altitude angle would be so small that a considerable amount of the sunrays from the vertical reflector would escape to the ground without hitting the basin liner. Therefore, the arrangement of the external reflector has to be changed from vertical for these seasons. During the winter season, inclining the external reflector slightly forward would enable the reflected sunrays to hit the basin liner even when the solar altitude angle is small. Therefore, we have presented
⁎ Tel.: +81 942 35 9359; fax: +81 942 35 9321. E-mail address: [email protected]. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.02.057
an additional analysis to calculate the solar radiation reflected from an external reflector inclined forward and then absorbed onto the basin liner [8], and found that distillate productivity was predicted to be increased by inclining the external reflector. Besides the use of reflector, many modifications have been done to increase the productivity of distillate of a basin type still (reviewed by Tiwari et al. [9]), such as reducing water depth, use of dye, reducing bottom loss coefficient and so on. However, in this study, we used a typical basin type still, which does not take into consideration of these modifications, since we focused the effect of the reflector. Therefore, the structure of a basin type still we constructed is basically the same with other basin type stills tested by many researchers such as Cooper [10], and so on [1,3,4]. In this paper, we present the results of outdoor experiments of a basin type still with internal reflectors and an external reflector inclined forward during the winter season in Kurume, Japan (33.2°N latitude and 130.2°E longitude). The purpose of the study is to investigate whether the basin liner of basin type still with internal and external reflectors can absorb solar radiation adequately as predicted by the geometrical models described in our previous papers [7,8], since such outdoor experiments on basin type still had not been reported.
2. Experimental apparatus and procedure A schematic diagram and a snap shot of an outdoor experimental apparatus are shown in Figs. 1 and 2. The still consists of a basin liner, a 5-mm-thick glass cover, and an external reflector inclined forward. The angle of the glass cover was set at 20° from horizontal. The angle of the external reflector was set at 10°, which was predicted as an optimum reflector angle when glass cover angle is 20° [8]. The width
H. Tanaka / Desalination 249 (2009) 130–134
131
Nomenclature Symbols Gdf, Gdr diffuse and direct solar radiations on a horizontal surface, (W m− 2) global solar radiation on a horizontal surface, (W m− 2) Gh daily global solar radiation on a horizontal surface ΣGh during experimental period, (MJ m− 2 day− 1) extra-terrestrial solar radiation, (W m− 2) I0 length of still, (m) ls heat capacity, (J/K) mcp calculated daily productivity during experimental md,cal period, (kg m− 2 day− 1) experimental result of daily productivity, (kg m− 2 day− 1) md,exp convective heat transfer rate, (W) Qc conductive heat transfer rate, (W) Qd heat transfer rate by mass transfer, (W) Qe radiative heat transfer rate, (W) Qr solar radiation absorbed on basin liner, (W) Qsun,b solar radiation absorbed on glass cover, (W) Qsun,g Qsun,df, Qsun,dr diffuse and direct solar radiations absorbed on basin liner, (W) Qsun,ext, Qsun,int solar radiation reflected by external and internal reflectors and absorbed on basin liner, (W) T temperature, (°C) t time, (s) w width of still, (m) α absorptance of basin liner β incident angle of sunrays to glass cover, (°) ϕ solar altitude angle, (°) θ angle of glass cover, (°) transmittance of atmosphere τatm transmittance of glass cover τg transmittance of glass cover for diffuse radiation (τg)df
Subscripts a surroundings b basin liner g glass cover
and length of the basin liner and external reflector are 355 mm × 343 mm and 355 mm × 348 mm, respectively, so both of the areas are almost the same. The basin liner and the walls of the still were made of plywood. The basin liner was coated with black silicone sealant, and the side and back walls were covered with the reflector. A 1.8-mmthick mirror-finished stainless steel plate (reflectance is 0.7 [11]) was used for the internal and external reflectors. The bottom and all walls of the still were insulated using a 50-mm-thick urethane foam board. A partition was placed in front of the basin liner to make a channel for collecting the distilled water. The direct and diffuse solar radiations as well as the reflected radiation are transmitted through the glass cover and absorbed on the basin liner. The absorbed solar energy heats up and evaporates the basin water, and the water vapor condenses on the glass cover. The distilled water flows down to the channel and is gathered through the collecting pipe placed at the bottom of the channel. The experimental procedure is as follows: the still was oriented toward due south. Tap water was poured into the basin liner to form a water pool approximately 10-mm deep. Condensate was collected almost every hour and measured with a flask and a digital balance. Global solar radiation on a horizontal surface was measured on the roof of the neighboring building with an actinometer at a recording interval of 10 s. Since the place at which the experiments were
Fig. 1. Schematic diagram of experimental apparatus.
performed was shaded by the neighboring buildings during early morning and late evening, the experiments were performed when solar radiation would not be obstructed and could be absorbed onto the basin liner each day. 3. Theoretical analysis Theoretical analysis of the proposed still was described in detail in our previous papers about the geometrical model to calculate the solar radiation absorbed on the basin liner [8] as well as heat and mass transfer in the still [7].
Fig. 2. Snap shot of experimental apparatus.
132
H. Tanaka / Desalination 249 (2009) 130–134
The major differences in assumptions and conditions between the theoretical analysis described in our previous papers and that employed in the calculation in this paper are as follows: (1) Direct and diffuse radiations on a horizontal surface, Gdr and Gdf, were calculated with Bouguer's and Berlage's equations [12] with transmittance of atmosphere, τatm, of 0.7 and a solar constant of 1370 W m− 2 in the previous paper as Gdr =
1 = sinϕ I0 sin ϕ⋅τatm
ð1Þ 1 = sinϕ
Gdf = 0:5I0 sin ϕ⋅ð1−τatm
Þ = ð1−1:4 ln τatm Þ
ð2Þ
where I0 is extra-terrestrial solar radiation and ϕ is solar altitude angle. In the experiments, global solar radiation on a horizontal surface, Gh, was measured with an actinometer, and the global solar radiation should be divided into direct and diffuse radiations for the calculations. Therefore, transmittance τatm which satisfies the equation ð3Þ
Gh = Gdr + Gdf
was calculated with the measured value of the global solar radiation on a horizontal surface Gh and a solar constant of 1370 W m− 2, and the calculated τatm was substituted in Eqs. (1) and (2) to determine the direct and diffuse solar radiations on a horizontal surface at a given time each day. (2) The thickness of the glass cover was assumed to be 3 mm in the previous paper, while a 5-mm-thick glass cover was used in the outdoor experiments, and the transmittance of the glass cover, τg, was determined as a function of the incident angle of sunrays to the glass cover, β, thus [13]: 4
3
2
τg ðβÞ = 0:674 cos β−0:300 cos β−2:029 cos β + 2:478 cos β:
ð4Þ (3) Tap water was used in the outdoor experiments instead of saline water to determine the productivity of distillate and avoid possible troubles such as corrosion of experimental apparatus and so on. So the salinity of the saline water supplied to the basin liner is determined as 0 wt.% in these calculations while that was assumed to be 3.44 wt.% in the previous paper. The saturated water vapor pressure of saline water is lower than that of pure water because of the boiling point elevation which is a function of salt concentrations and temperature. The decrease in distillate productivity caused by the lower saturated water vapor pressure was estimated to be 3% or less [14]. (4) Transmittance of the glass cover for the diffuse radiation, (τg)df, can be calculated by integrating transmittance of the glass cover for diffuse radiation from all directions in the sky dome. In previous papers [7,8], the effect of obstruction by the external reflector for diffuse radiation was neglected when transmittance (τg)df was calculated since all the calculations were performed for clear days and the ratio of the diffuse radiation to the global solar radiation was enough small. In this paper, some outdoor experiments were performed on cloudy days and the ratio of the diffuse radiation to the global radiation would be greater than on a clear day. Therefore, the transmittance of a 5-mm-thick glass cover for the diffuse radiation (τg)df was calculated as following, taking into account the effect of obstruction by the external reflector in this paper. −5
ðτg Þdf = −3:29 × 10
2
× θ + 3:01 × 10
−3
× θ + 0:313:
ð5Þ
Theoretical analysis of the proposed still was described in detail in our previous papers [7,8] as mentioned above. Therefore, the only
major governing equations are described here. Heat transfer from the basin liner to glass cover was caused by radiation (Q r,b-g), convection (Q c,b-g) and mass transfer (Q e,b-g). Thermal energy escapes to the ground from basin liner through bottom insulation (Q d,b-a). Heat transfer from glass cover to the surroundings was caused by radiation (Q r,g-a) and convection (Q c,g-a). Therefore, the energy balance for the basin liner and the glass cover can be expressed as Q sun;b = Q r;bg + Q c;bg + Q e;bg + Qd;ba + ðmcp Þb
dTb
.
ð6Þ
dt
Q sun;g + Q r;bg + Q c;bg + Q e;bg = Q r;ga + Q c;ga + ðmcp Þg
dTg
. dt
ð7Þ where Q sun,b and Q sun,g are absorbed solar radiation by the basin liner and the glass cover, mcp is heat capacity, T is temperature and t is time. In the calculation, we theoretically analyzed three types of stills: IES (a still with both internal and external reflectors), IS (one with an internal reflector only) and NRS (one with no reflector). Q sun,b in Eq. (6) was determined for IES, IS and NRS as Q sun;b = Q sun;dr + Q sun;df + Q sun;int + Q sun;ext
ð8Þ
Q sun;b = Q sun;dr + Q sun;df + Q sun;int
ð9Þ
Q sun;b = Q sun;dr + Q sun;df
ð10Þ
where, Q sun,dr and Q sun,df are direct and diffuse solar radiations absorbed on the basin liner, and Q sun,int and Q sun,ext are solar radiation reflected by internal and external and then absorbed on the basin liner. Q sun,dr and Q sun,df can be expressed as Q sun;dr = Gdr τg ðβÞαb × wls
ð11Þ
Q sun;df = Gdf ðτg Þdf αb × wls
ð12Þ
where αb is absorptance of basin liner (=0.9), and w and ls are width (=355 mm) and length (=343 mm) of basin liner. 4. Results Hourly variations of the global solar radiation on a horizontal surface (Global), the experimental results and the theoretical predictions of the distillate production rates of the stills on 15 Feb. 2007 are shown in Fig. 3a. The experimental period on this day is 8:45 a.m. to 5:00 p.m. Here, IES represents a still with both internal and external reflectors, IS represents one with an internal reflector only and NRS represents one with no reflector. The outdoor experiments were performed with an IES still (a still with both internal and external reflectors) only. The distillate production rate for each still peaks about 1 h later than that of the global solar radiation because of the heat capacity of the still. The distillate production rate can be increased by using internal and/or external reflector(s). The experimental results (IES (exp.)) was in good agreement with the theoretical predictions (IES (cal.)), and this indicates that the solar radiation reflected from the reflectors and then absorbed onto the basin liner can be adequately calculated using the geometrical model described in our previous paper [8]. Hourly variations of direct and diffuse solar radiations on a horizontal surface calculated from the measured global solar radiation on a horizontal surface on the same day are shown in Fig. 3b, and the solar radiation absorbed on the basin liner caused by direct and diffuse radiations as well as the radiation reflected from internal and external reflectors and then absorbed on the basin liner on the same day is shown in Fig. 3c. Here, the values for direct and diffuse radiations in Fig. 3c are smaller than those in Fig. 3b, since the
H. Tanaka / Desalination 249 (2009) 130–134
Fig. 3. a. Hourly variations of global solar radiation on a horizontal surface (Global), theoretical predictions (cal.) of distillate production rates of NRS, IS, IES and experimental results (exp.) of IES on 15 Feb. 2007. b. Hourly variations of global, direct and diffuse solar radiations on a horizontal surface on 15 Feb. 2007. c. Hourly variations of absorption of direct and diffuse solar radiations and reflected radiation from internal and external reflectors on a basin liner on 15 Feb. 2007.
133
Fig. 4. a. Hourly variations of global solar radiation on a horizontal surface (Global), theoretical predictions (cal.) of distillate production rates of NRS, IS, IES and experimental results (exp.) of IES on 22 Jan. 2007. b. Hourly variations of global, direct and diffuse solar radiations on a horizontal surface on 22 Jan. 2007. c. Hourly variations of absorption of direct and diffuse solar radiations and reflected radiation from internal and external reflectors on a basin liner on 22 Jan. 2007.
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H. Tanaka / Desalination 249 (2009) 130–134
Table 1 Overall result of outdoor experiments. Date
Temperature
Experimental period
ΣGh
md,exp (IES)
md,cal (IES)
md,cal (IS)
md,cal (NRS)
md,exp / md,cal (IES)
19 19 22 25 13 15 19
6 °C–19.5 °C 10 °C–18 °C 12 °C–17 °C 12 °C–21 °C 11 °C–22 °C 9 °C–22 °C 11 °C–23 °C
8:20–17:00 10:15–16:23 10:26–16:00 10:18–17:00 8:50–17:00 8:45–17:00 8:40–17:00
9.9 8.3 7.1 8.8 13.4 14.6 12.9
1.77 1.61 1.19 2.05 3.31 4.01 2.99
2.55 2.14 1.58 2.25 3.75 4.10 3.15
1.90 1.53 1.17 1.63 2.82 3.09 2.50
1.31 1.08 0.80 1.17 2.14 2.31 1.91
0.69 0.75 0.75 0.91 0.88 0.98 0.95
Dec. 2006 Jan. 2007 Jan. 2007 Jan. 2007 Feb. 2007 Feb. 2007 Feb. 2007
absorption of solar radiation on the basin liner would be less than the solar radiation on a horizontal surface because of the reflection and absorption of the glass cover and the reflection of the basin water. The amount of daily cumulative absorption of solar energy caused by the internal or external reflector is greater than that provided by diffuse radiation, and about 30% or 40% of that provided by direct radiation. Fig. 4a to c shows the same results as Fig. 3a to c on 22 Jan. 2007 which was a cloudy day. The experimental period for this day was 10:26 a.m. to 4:00 p.m. The global solar radiation on a horizontal surface is unstable during the day, and the theoretical predictions for distillate production rate as well as the solar radiation absorbed on the basin liner fluctuate in accordance with the global solar radiation. The experimental results of distillate production rate are lower than the predictions, and the daily productivity in the experimental result (1.19 kgm− 2 day− 1) is about 75% of the predictions (1.58 kgm− 2 day− 1). This is mainly because the thermal losses related to frequent heating up and cooling down of the still during fast transition of solar radiation would cause reductions of distillate efficiency. Other reasons may be that: (1) The diffuse solar radiation would normally make up a large amount of the global solar radiation on a cloudy day, while the ratio of diffuse radiation was estimated at comparatively small values throughout the day in these calculations. (2) The heat transfer through insulation of side and back walls would be neglected in the calculation, while small amount of thermal energy would escape to the surroundings through these walls. The overall results of the outdoor experiments are listed in Table 1, and the variations in experimental results of the daily productivity of IES and the theoretical predictions of the daily productivity of IES, IS and NRS in relation to the daily global solar radiation on a horizontal surface are shown in Fig. 5. Here, the daily global solar radiation as
well as the daily productivity is the cumulative results for the experimental period of each day. Cooper [10] has experimentally tested a conventional basin type still in detail. The daily productivity of a conventional basin type still (NRS) predicted in the present model is in good agreement with the experimental values presented by Cooper. The experimental results of the daily productivity of IES are more than about 70% of the predictions. Especially on sunny days (on 13, 15 and 19 Feb. 2007), the discrepancy between the experimental results and the theoretical predictions would be averaged as about 6%. This indicates that the radiation reflected from an inclined external reflector and then absorbed on the basin liner can be adequately calculated with the geometrical model described in our previous paper [8]. The daily productivity of IES is predicted to be about 1.7 to 2.0 times as great as that of NRS, and this indicates that the distillate productivity of a conventional basin type still can be dramatically increased by a very simple modification using internal and inclined external flat plate reflectors in winter. 5. Conclusions We performed outdoor experiments of a basin type solar still with internal and external reflectors in winter season in Kurume, Japan. The results of this work are summarized as follows: 1. The experimental results and the theoretical predictions of the daily productivity of the still are in fairly good agreement, and especially on clear days, the discrepancy between the experimental results and theoretical predictions would be averaged as about 6%. This indicates that the radiation reflected from the reflectors and then absorbed onto the basin liner can be adequately calculated with the geometrical model described in our previous paper [8]. 2. The daily productivity of a conventional basin type still predicted in the present model is in good agreement with the experimental results of Cooper [13]. 3. A very simple modification using internal and external reflectors can increase the daily productivity of a basin type still by about 70% to 100%. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Fig. 5. Daily global solar radiation on a horizontal surface and daily productivity of IES, IS and NRS.
[14]
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