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Mini solar pond
RenewableEnergyVoh I No. 3/4. pp. 463--467, 1991 Printed in Great Britain.
0960-1481/91 $3.00+.00 Pergamon Press plc
MINI SOLAR POND
C. O. AKOSHILE Physics Department
, University of llorin, Nigeria.
llorin
(Received 7 August 1990 ; accepted 7 March 1991)
ABSTRACT An attempt has been made to construct a Mini_Solar Pond in ordinary laboratory environment. Most of literature'~eports are based on natural lakes turned into solar pond to harness solar energy. This experiment was done with the aim of producing mobile, experimental and domestic solar pond. NaCI solution was used and temperature difference as high as 18°C within shallow depth of 28cm was attainable in ordinary darkened clay pot. Temperature gradient of 0.46 *C/cm was obtained. INTRODUCTION This study is based on the importance attached to solar energy which can be harnessed from radiation from the sun freely bathing the earth. Experiment Three nearly hyperbolic identical pots were filled to same depth with water, sodium chloride solution and magnesium chloride solution respectively. The salt solution concentration varies with depth. The inner surface of each port had been coated with black paints. Since each solution is transluscent, solar radiation can get to the underneath black absorption layer of the pot. While the NaCI solution was saturated at the base, the MgCI 2 solution was not so saturated being more costly to obtain. A cascade of themometers 7cm apart located from botom to top was used to measure the temperature as a function of depth. (See Fig. i). The t e m p e r a t u r ~ w e r e sampled hourly from 800 hours to 1700 hours local time for a number of days.
463
464
C.O. AKOSHILE Thermometer Blackened
~ E D
Salt solution
/
B
clay pot
Thermometer
/ Fig. i.
inner
surface
/
C
mount
Mini Solar Pond and Temperature
Sensors
Results The result of the data collected for a day is plotted in Figure 2. Curves i, 3 and 4 were obtained from hourly temperature sampling of NaCI at depths 0 cm, 21 cm and 28 cm respectively. The curves I and 2 were obtained for water at depth 0 and 28 cm respectively. To avoid clumsiness, the vertical curves are provided which show the variation of NaCI temperature with depth for other intervening depths of 7 cm and 14 cm every hour from 800 to 1700 hours local time. Change in temperature d anain of water is shaded in comparison to the overall of brine. The ambient temperature profile isthe dotted underneath curves. Temperature difference of up to 20°C was attained in shallow NaCI Temp(°C)
20
i'0
12
Fig. 2 Temperature
14
16
t (OOhrs)
versus time (hours of day)
Mini solar pond pond depth of 20cm (0.28 m) as compared to less than 3°C in water of equal depth. A Geographer showed that variation of temperature in sea water of up to i000 m is between 0 and 5°C. In this case salt content could be considered as essentially uniform. This temperature difference is convenient for the survival of sea animals as large temperature difference would affect them when moving from the deep to the surface. This shows that temperature variations in uniformly mixed salt solution is much smaller than in concentration gradient solution. The behaviour of unadulterated water is therefore similar to that of salt solution of uniform concentration. It is evident that the much higher temperature difference between top and bottom of salt solution pond in comparison to water is due to controlled convectional current. Otherwise an overall temperature difference would have resulted with accompany higher average temperature than obtained in water due to temperature mixing. Higher surface temperature would imply higher evaporation and greater loss of the harnessed solar energy. A plot of the average temperature on the hour for five data days is shown in Figure 3. It peaks at about 1430 hours. This point does not correspond to the time when the sun is directly overhead. An exponential fit to the temperature rise portion of the curve (A to B) gives bt e T = a .......... (I) where
a=
26.5°C, b =0.46/hr for temperature T in °C and time t in hours.
Similar plots are obtainable for other depths (not shown here). The result shows that a lot of radiation passes through the liquid and gets to the black absorber coating below. Extra insulation was not provided for the outside of the pot. The peak temperature occurs about an hour after the sun has passed overhead in the pond. The ambient temperature dropped to 0.895 peak temperature in 1 hour while it t o o k 1 ~ hours to drop to same fraction of peak temperature at h = 21cm. This implies that if extra insulation is provided, higher peak temprature will be obtained in the pond and it will be retained for longer time, especially by insulating the high temperature domain. Variation of Salt Concentration with depth The variation of salt concentration with depth is shown in Figure 4. To obtain this plot, the salt is sampled at a particular depth h by withdrawing some solution with a pipette at this depth and titrating against silver nitrate solution using appropriate indicator and employing standard chemistry technique. The curve shows a gentle rise in concentration to a depth of h = 21cm followed by a sharp rise between h = 21cm and h = 28cm.
465
466
C.O. AKOSFIILE B
Temp(°C)
h = 21cm
36
32
30
A 26
J I
i
i
~
,~
1 00
I
i
|
1400
I
'
800
i000
1600
t (hrs)
Fig. 3.
Plot of Temp. vs time (hours) of the day
5 Conc(x I0 kg/m 3) Temp(°C) 15.00 15
50
/
//I 12
40
9
30
6 3 0
J
14.oo
///
13.oo 1,.OOa.m
~ _ / ~ - . ~ ' ~
,9.00a.m
~ / " - - / ~ " -.- ' ~
' -8.00a.m
20 I0
|
|
!
7
14
21
I
i
h(em)
Fig. 4. concentration of NaCI vs depth.
16.00
7
I
14
I
21
2~
i h(cm)
Fig. 5. Variation of temperature with depth for hours of the day.
Mini solar pond Temperature
467
variation with depth at Chan$in$ hours
The plot of temperature against depth as the hours of the day changes is shown in Fig. 5. This is the same as the near vertical curves crossing the temperature-time profile of Figure 2. This plot enables one to observe the effect of concentration on temperature as a function of depth. Initially, the curves slant slightly and curves downwards between the hours of 0800 and If00. It becomes inclined and linear at 1300 hours and does not show any curving up or down. Beyond this hour however, the curves turn upward. What is happening is that thermal loss rate compares favourably with absorption rate at the early hours of the day until equilibrium is attained about 1300 hours. Beyond this absorption exceeds loss rate until the rate is reversed later in the day into the night. Maximum absorption is expected when the sun is overhead since one can deduce that the straight line occurs about when the sun is overhead. The other deduction is that solar absorption is highest where the salt concentration is highest. Hence there is a direct dependence between the solar radiation absorped per unit time and the salt concentration. Since the temperature - depth curve changes with available incident radiation, at this stage it is difficult to obtain a mathematical relationship between concentration and temperature and more work needs to be done in this area. In addition, this technique significantly controls the temperature mixing observed in pure water where the concentration is uniform. Conclusion The results obtained from this laboratory - mini solar pond has shown that the technique is feasible for harnessing solar radiation and storing it as thermal energy for immediate or delayed application. An average temperature gradient of higher than 0.411°C/cm (41.1°C/m) and a temperature difference of 20°C in 28cm solution depth was achieved in comparison to 0.089°C/m and 2.5°C in water under the same conditions. The variation in the salt concentration in the pond has evidently controlled convectional current temperature and particle mixing in the solution. There is a direct dependence between the temperature and salt concentration at any depth. Maximum radiation is absorbed when the sun is directly overhead.lf it is possible to make the pond face-track the sun, more radiation will be aborbed and higher temperature attained. The plots of figure 5 enables one to locate the position of the sun being overhead when the curve is linear showing a phase change. It is hoped that improved material, system more efficient.
design and insulation will make
REFERENCES: S. M. Patel and C.L. Gupta (1981), 'Experimental himid climate~ Sun World, 5(4), 115-118. H. Tabour 181-194.
(1981),
'Review Article,
Solar Pond in a hot
Solar Ponds,'Solar
Energy,
27(3),
0960-1481/91 $3.00+.00 Pergamon Press plc
MINI SOLAR POND
C. O. AKOSHILE Physics Department
, University of llorin, Nigeria.
llorin
(Received 7 August 1990 ; accepted 7 March 1991)
ABSTRACT An attempt has been made to construct a Mini_Solar Pond in ordinary laboratory environment. Most of literature'~eports are based on natural lakes turned into solar pond to harness solar energy. This experiment was done with the aim of producing mobile, experimental and domestic solar pond. NaCI solution was used and temperature difference as high as 18°C within shallow depth of 28cm was attainable in ordinary darkened clay pot. Temperature gradient of 0.46 *C/cm was obtained. INTRODUCTION This study is based on the importance attached to solar energy which can be harnessed from radiation from the sun freely bathing the earth. Experiment Three nearly hyperbolic identical pots were filled to same depth with water, sodium chloride solution and magnesium chloride solution respectively. The salt solution concentration varies with depth. The inner surface of each port had been coated with black paints. Since each solution is transluscent, solar radiation can get to the underneath black absorption layer of the pot. While the NaCI solution was saturated at the base, the MgCI 2 solution was not so saturated being more costly to obtain. A cascade of themometers 7cm apart located from botom to top was used to measure the temperature as a function of depth. (See Fig. i). The t e m p e r a t u r ~ w e r e sampled hourly from 800 hours to 1700 hours local time for a number of days.
463
464
C.O. AKOSHILE Thermometer Blackened
~ E D
Salt solution
/
B
clay pot
Thermometer
/ Fig. i.
inner
surface
/
C
mount
Mini Solar Pond and Temperature
Sensors
Results The result of the data collected for a day is plotted in Figure 2. Curves i, 3 and 4 were obtained from hourly temperature sampling of NaCI at depths 0 cm, 21 cm and 28 cm respectively. The curves I and 2 were obtained for water at depth 0 and 28 cm respectively. To avoid clumsiness, the vertical curves are provided which show the variation of NaCI temperature with depth for other intervening depths of 7 cm and 14 cm every hour from 800 to 1700 hours local time. Change in temperature d anain of water is shaded in comparison to the overall of brine. The ambient temperature profile isthe dotted underneath curves. Temperature difference of up to 20°C was attained in shallow NaCI Temp(°C)
20
i'0
12
Fig. 2 Temperature
14
16
t (OOhrs)
versus time (hours of day)
Mini solar pond pond depth of 20cm (0.28 m) as compared to less than 3°C in water of equal depth. A Geographer showed that variation of temperature in sea water of up to i000 m is between 0 and 5°C. In this case salt content could be considered as essentially uniform. This temperature difference is convenient for the survival of sea animals as large temperature difference would affect them when moving from the deep to the surface. This shows that temperature variations in uniformly mixed salt solution is much smaller than in concentration gradient solution. The behaviour of unadulterated water is therefore similar to that of salt solution of uniform concentration. It is evident that the much higher temperature difference between top and bottom of salt solution pond in comparison to water is due to controlled convectional current. Otherwise an overall temperature difference would have resulted with accompany higher average temperature than obtained in water due to temperature mixing. Higher surface temperature would imply higher evaporation and greater loss of the harnessed solar energy. A plot of the average temperature on the hour for five data days is shown in Figure 3. It peaks at about 1430 hours. This point does not correspond to the time when the sun is directly overhead. An exponential fit to the temperature rise portion of the curve (A to B) gives bt e T = a .......... (I) where
a=
26.5°C, b =0.46/hr for temperature T in °C and time t in hours.
Similar plots are obtainable for other depths (not shown here). The result shows that a lot of radiation passes through the liquid and gets to the black absorber coating below. Extra insulation was not provided for the outside of the pot. The peak temperature occurs about an hour after the sun has passed overhead in the pond. The ambient temperature dropped to 0.895 peak temperature in 1 hour while it t o o k 1 ~ hours to drop to same fraction of peak temperature at h = 21cm. This implies that if extra insulation is provided, higher peak temprature will be obtained in the pond and it will be retained for longer time, especially by insulating the high temperature domain. Variation of Salt Concentration with depth The variation of salt concentration with depth is shown in Figure 4. To obtain this plot, the salt is sampled at a particular depth h by withdrawing some solution with a pipette at this depth and titrating against silver nitrate solution using appropriate indicator and employing standard chemistry technique. The curve shows a gentle rise in concentration to a depth of h = 21cm followed by a sharp rise between h = 21cm and h = 28cm.
465
466
C.O. AKOSFIILE B
Temp(°C)
h = 21cm
36
32
30
A 26
J I
i
i
~
,~
1 00
I
i
|
1400
I
'
800
i000
1600
t (hrs)
Fig. 3.
Plot of Temp. vs time (hours) of the day
5 Conc(x I0 kg/m 3) Temp(°C) 15.00 15
50
/
//I 12
40
9
30
6 3 0
J
14.oo
///
13.oo 1,.OOa.m
~ _ / ~ - . ~ ' ~
,9.00a.m
~ / " - - / ~ " -.- ' ~
' -8.00a.m
20 I0
|
|
!
7
14
21
I
i
h(em)
Fig. 4. concentration of NaCI vs depth.
16.00
7
I
14
I
21
2~
i h(cm)
Fig. 5. Variation of temperature with depth for hours of the day.
Mini solar pond Temperature
467
variation with depth at Chan$in$ hours
The plot of temperature against depth as the hours of the day changes is shown in Fig. 5. This is the same as the near vertical curves crossing the temperature-time profile of Figure 2. This plot enables one to observe the effect of concentration on temperature as a function of depth. Initially, the curves slant slightly and curves downwards between the hours of 0800 and If00. It becomes inclined and linear at 1300 hours and does not show any curving up or down. Beyond this hour however, the curves turn upward. What is happening is that thermal loss rate compares favourably with absorption rate at the early hours of the day until equilibrium is attained about 1300 hours. Beyond this absorption exceeds loss rate until the rate is reversed later in the day into the night. Maximum absorption is expected when the sun is overhead since one can deduce that the straight line occurs about when the sun is overhead. The other deduction is that solar absorption is highest where the salt concentration is highest. Hence there is a direct dependence between the solar radiation absorped per unit time and the salt concentration. Since the temperature - depth curve changes with available incident radiation, at this stage it is difficult to obtain a mathematical relationship between concentration and temperature and more work needs to be done in this area. In addition, this technique significantly controls the temperature mixing observed in pure water where the concentration is uniform. Conclusion The results obtained from this laboratory - mini solar pond has shown that the technique is feasible for harnessing solar radiation and storing it as thermal energy for immediate or delayed application. An average temperature gradient of higher than 0.411°C/cm (41.1°C/m) and a temperature difference of 20°C in 28cm solution depth was achieved in comparison to 0.089°C/m and 2.5°C in water under the same conditions. The variation in the salt concentration in the pond has evidently controlled convectional current temperature and particle mixing in the solution. There is a direct dependence between the temperature and salt concentration at any depth. Maximum radiation is absorbed when the sun is directly overhead.lf it is possible to make the pond face-track the sun, more radiation will be aborbed and higher temperature attained. The plots of figure 5 enables one to locate the position of the sun being overhead when the curve is linear showing a phase change. It is hoped that improved material, system more efficient.
design and insulation will make
REFERENCES: S. M. Patel and C.L. Gupta (1981), 'Experimental himid climate~ Sun World, 5(4), 115-118. H. Tabour 181-194.
(1981),
'Review Article,
Solar Pond in a hot
Solar Ponds,'Solar
Energy,
27(3),