- Email: [email protected]
Design methodology for a salt gradient solar pond coupled with an evaporation pond
Pergamon
PII:
Solar Energy Vol. 72, No. 5, pp. 447–454, 2002 2002 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 2 ) 0 0 0 2 1 – X All rights reserved. Printed in Great Britain 0038-092X / 02 / $ - see front matter
www.elsevier.com / locate / solener
DESIGN METHODOLOGY FOR A SALT GRADIENT SOLAR POND COUPLED WITH AN EVAPORATION POND , , K. R. AGHA† *, S. M. ABUGHRES* ** and A. M. RAMADAN* *Center for Solar Energy Studies, P.O. Box 12932, Tripoli, Libya **Department of Mechanical & Industrial Engineering, Faculty of Engineering, P.O. Box 13589, Tripoli, Libya Received 15 May 2000; revised version accepted 21 November 2001 Communicated by TY NEWELL
Abstract—This paper presents the results of a simple mathematical model for predicting the ratio of the evaporation pond (EP) area to that of a Salt Gradient Solar Pond (SGSP) area. The EP idea provides a very attractive method of salt recycling by evaporation, especially in areas of high rates of evaporation and low rates of rain as it is the case for North Africa. The model is applied for two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli-Libya (Lat. 5 32.688N) and for measured evaporation rates. Under the summer conditions and for the case of surface flushing by fresh water, the area ratio was estimated at about 0.17. While for the case of using seawater this ratio increases enormously to about 14.4. The time required for the salt concentration to increase from seawater concentration to a high concentrated brine, which can be injected at the bottom of the solar pond, is also presented. It was estimated that the time required to increase the salt concentration from 3.5 to 35% is about 120 to 250 days during the summer months and about 200 to 220 days during the winter months. 2002 Elsevier Science Ltd. All rights reserved.
centration. The salt gradient zone (NCZ) is the key to the working of a SGSP. It allows solar radiation to penetrate into the storage zone while prohibiting the propagation of long wave radiation because water is opaque to infrared radiation. The zone suppresses global convection due to the imposed density stratification. It offers an effective conduction barrier because of the low thermal conductivity of water and the large thickness of the NCZ, which averages over 1.0 m. The salinity in the UCZ increases due to convective mixing (wind, evaporation...) with NCZ and salt diffusion from the bottom. The annual rate of this natural diffusion of salts from high to low concentration was estimated by Tabor (1975) and Weinberger (1964) to be in the range of 20–30 kg / m 2 . This rate depends on the thickness of NCZ, the temperature profile and the concentration difference between the UCZ and the LCZ. A common method of maintaining the salt gradient is to flush fresh water to the pond surface and inject saturated brine, or salt, at the bottom to substitute for the salt, that is diffused to the surface. In locations like North Africa, where fresh water is very limited, the surface can be washed by seawater. Also, North Africa has very high rates of evaporation and very low rates of
1. INTRODUCTION
An extensive amount of work on SGSP as a cost-effective method of collecting and storing solar energy on large scale is available in the literature. SGSP use has demonstrated success in many applications such as in space heating (Nielsen, 1982), desalination (Tabor, 1975), low temperature industrial heating process (Swift and Golding, 1992) and power production (Denius and Batton, 1984). A SGSP consists of three distinct zones as shown in Fig. 1. The Upper Convective Zone (UCZ) of thickness varying between 0.15 and 0.30 m which has a low and nearly uniform salt content. Beneath the UCZ is the Non-Convective Zone (NCZ) of thickness that varies between 1.0 and 1.5 m and has a salt content increasing with depth, and it is therefore a zone of variable properties. The bottom layer is the Lower Convective Zone (LCZ), also called the storage zone, which has a thickness varies between 1.0 and 2.0 m and has a nearly uniform high salt con-
†
Author to whom correspondence should be addressed. Current address: Civil Engineering Department, Dalhousie University, Halifax, NS B3J 2X4, Canada. Fax: 11-902494-3108; e-mail: [email protected] 447
448
K. R. Agha et al.
precipitation, and therefore recycling of salt by evaporation is practical and economical. This, in addition, overcomes the salt shortage problem and ensures the continuous high performance operation of solar ponds with a minimum environmental damage caused by salt disposal. Based on the above considerations, it is felt essential to demonstrate the capability of a long term, closed cycle salt management facility by evaporative brine concentration, which is the topic dealt within this paper. The current study has been performed to determine the relationship between the area ratio (ratio between the EP area and SGSP area) and some parameters known to affect the thermal stability of the SGSP. The relative monthly quantities of flushing water at the surface and those of high salt concentration to be injected at the bottom were also presented. For the effective operation of the system coupling the SGSP and EP, the transient brine re-concentration phenomena were discussed. The paper also presents the design, construction and the method used in the establishment of salt concentration profile of Tajoura’s Experimental Solar Pond (TESP). 2. TAJOURA’S EXPERIMENTAL SOLAR POND (TESP)
TESP is an artificial solar pond located to the east of Tripoli and is designed and constructed, in joint cooperation with a Swiss company, with SGSP surface area of about 830 m 2 and EP area of about 105 m 2 . The salt concentration profile is constructed with three zones, the Upper Convective Zone (UCZ) of 0.30 m thickness and a salt concentration of about 41 kg / m 3 , the Lower Convective Zone (LCZ) of 1 m thickness and salt concentration of 260 kg / m 3 . Separating these two zones is the Non-Convective Zone (NCZ) of 1.2 m thickness and variable salt concentration. The
pond, fully equipped with systems to monitor all relevant parameters, is designed as an experimental facility enabling the investigation of various aspects of pond performance. 3. COUPLING THE SGSP AND EP
Fig. 1 shows a schematic diagram of a SGSP coupled to an EP. To estimate the required evaporation pond area that will provide the quantity of salt required for the maintenance of salt concentration profile in the SGSP, the mass balance model presented by Alagao et al. (1994) and Batty et al. (1987) was used in this study. In order to operate the pond effectively, it is important to know the time required to substitute for the removed surface brine (from SGSP) by the brine that has to be injected at the bottom of the SGSP. The hourly variation of temperature and concentration of the brine in the EP is of paramount importance in accounting for the loss of energy from the LCZ. An energy balance model (Newell et al., 1994) was used to predict this time with a little modification to the terms representing the sky radiation and evaporation energy. The model used in this study becomes as follows: dT mCP ] 5 tA ep Is h r A ep [T 2 T sky ] dt 2 h c A ep [T 2 T a ] 2 aep Efw Lrw The term on the left-hand side represents the energy stored in the body of the evaporation pond. The first term on the right-hand side represents the solar energy penetrating the surface of the evaporation pond. The last three terms represent the energy losses by radiation, convection and evaporation, respectively. 4. RESULTS AND DISCUSSION
The mathematical model described above was applied by using the prevailing weather conditions
Fig. 1. Schematic diagram of SGSP/ EP system showing the various mass and energy balance components.
Design methodology for a salt gradient solar pond coupled with an evaporation pond
449
Fig. 2. Monthly average measured weather data for Tripoli-Libya Lat. (32.688N) over 30-year period (1961–1990).
in Tripoli-Libya (Lat. 32.688N) shown in Fig. 2, averaged over a 30-year period (1961–1990). Fig. 2 clearly shows the high rates of net evaporation and therefore the practical viability of evaporation ponds, also compares the measured evaporation rates with those calculated. Fig. 3a and b show the effect of ambient air temperature on the evaporation rates from both fresh water and evaporation pond water (concentration 26%), respectively; for various values of surface water temperatures. It is shown that as the ambient air temperature increases, the evaporation rates increase. This is quite obvious since the air temperature at constant relative humidity works as a driving force for evaporation. The effect could be further manifested by the plots of the resulting evaporation rates for different surface water temperatures, as shown on the same diagram. Again, the higher the water temperature, the higher the evaporation rate. Changes in ambient air temperature does not seem to affect the value of aep which represents the ratio of evaporation from the evaporation pond to that from fresh water. This is attributed to the fact that the evaporation pond has a very high salt content (26%). Owing to the intermittent nature of solar energy, solar energy systems will have to be sized according to a pre-specified time. For this purpose, Table 1 shows the required area ratios under the prevailing conditions in Tripoli for different design conditions and both types of surface water
flushing. The winter design was excluded because of its low rate of net evaporation † from the evaporation pond. These results show that large area evaporation ponds are needed if seawater is used for surface flushing, as higher quantities of seawater are required to wash the surface of the SGSP compared with those quantities of fresh water. It can also be seen that the summer design conditions resulted in the smallest area ratio, due to the high rates of net evaporation during the season as depicted in Fig. 2. A further investigation of the effect of the prevailing conditions at the design stage for various cases is shown in Fig. 4. These figures show a graphical presentation of the effect of various parameters on the area ratio for Summer, Autumn, and Spring, respectively. The parameters analyzed in the figure are the net evaporation from the evaporation pond, the type of water used for surface flushing and the rate of salt diffusion. In the case of fresh water surface flushing, the resulting evaporation does not contribute to the increase of concentration in the evaporation. Similar results are obtained for Summer, Autumn and Spring designs as shown in Fig. 4a–c, respectively, when fresh water is used for surface flushing. In other words, in the case of fresh water surface flushing, the quantity of water evaporated is equal to the decrease in the quantity of brine †
Net evaporation rate is defined as the evaporation rate minus the precipitation rate.
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K. R. Agha et al.
Fig. 3. (a) The effect of ambient air temperature of Efw for different surface temperatures: (b) The effect of ambient air temperature on aep for different surface temperatures.
flushed with a corresponding change in salt concentration from C2 to C3.
4.1. Applying the model to TESP As mentioned earlier, for the purpose of high performance operation of solar ponds, it is important to keep the pond thermally stable. This requires the injection of high salt concentration brine at the bottom to compensate for the natural diffusion of salts upwards, it also requires washing the surface with low concentration water to Table 1. Estimated area ratios under different design parameters Area ratio, A r Fresh water Spring 0.65 Summer 0.17 Autumn 0.32 TESP Project 0.13 ‘Constructed’
Seawater 28.40 14.40 20.20
replace evaporation losses and flush away the salt diffusing to the surface from the concentrated brines below. Fig. 5a shows the predicted quantity of salt provided by the EP each month as compared to that required by the SGSP (A SP 5830 m 2 ) under the prevailing conditions, for different design scenarios with measured evaporation rates for the case of fresh water surface flushing. Fig. 5b compares the same quantities when seawater is used for surface flushing. Fig. 6 shows monthly quantities of salt that can be provided by the constructed evaporation pond (105 m 2 ) and (1.5 m) depth at the TESP project compared with that required by the SGSP (830 m 2 ). It is clearly shown that the EP area was rather underestimated even if fresh water is used for surface flushing. The quantity of salt required by SGSP to maintain its salt concentration profile was estimated at about 1150 kg / month
Design methodology for a salt gradient solar pond coupled with an evaporation pond
451
Fig. 4. The variation of Area ratio (A r ) with net evaporation rate from EP for both types of surface water flushing and different salt transport rates.
(16.6 kg / m 2 –year), which could be under-estimated compared to previous studies (Alagao et al., 1994; Swift and Golding, 1992). Fig. 7 shows the quantity of water in (m 3 / month) required for surface flushing using fresh water and seawater flushing with measured evaporation rates. This graph clearly shows the large quantities of flushing water required during the summer, which is attributed to the high rates of evaporation in the summer season. It also shows the big differences between the quantities
required for fresh water flushing and that for seawater flushing. This comes as a result of the fact that in addition to replacing the evaporation losses, washing the surface is expected to flush away salt diffusing to the surface from the concentrated brines below. Fig. 8 shows the required time for the concentration in an EP to reach (35%) starting from sea water (3.5%) under different prevailing conditions. It is clearly shown that the shortest period for the increase of concentration occurs during the
452
K. R. Agha et al.
Fig. 5. Mass of salt provided by the EP as compared to that required by the SGSP of TESP (A SP 5830 m 2 ) under different scenarios.
summer months. This is attributed to the high net evaporation rates in the summer season. Presenting the daily amounts of concentration
as shown in Fig. 9 could further manifest the brine re-concentration phenomena. The figure also shows the daily decrease in brine depth that
Fig. 6. Mass of salt provided and required by the TESP (Constructed pond), It should be noted that for the case of seawater surface flushing the real quantity of salt is equal to the shown values divided by ten. (A ep 5105 m 2 , Depth51.5 m) (A sp 5830 m 2 ).
Design methodology for a salt gradient solar pond coupled with an evaporation pond
453
Fig. 7. The quantity of water required for surface flushing in (m 3 / month) for TESP.
Fig. 8. The time required (in days) to increase the salt concentration from seawater concentration (3.5%) to (35%) for different starting months for the TESP, and the temperature of brine and ambient at the last day.
corresponds to the daily increase in concentration. It should be noted that the results given in Figs. 8 and 9 are based on the assumption that no withdrawal or feeding to EP has occurred once it’s started. In order to operate the SGSP effectively and minimize the energy losses during the high concentration injection, it is important to use the highest possible temperature for the brine to be injected. Fig. 10 is constructed for this purpose, it shows the hourly temperature variation of the
high concentration brine (at the last day of different starting months). It can be seen from this figure that the maximum temperature of the concentrated brine occurs at about (2:30 p.m.) of the last day, which is the preferred time to inject the concentrated brine from EP to SGSP. 5. CONCLUSIONS
A mathematical model describing the relationships between the evaporation pond and the salt
Fig. 9. The daily variation of brine concentration and depth in the evaporation pond of TESP.
454
K. R. Agha et al.
Fig. 10. The temperature variation of the available high concentration brine (35%) at the last day of different starting months for TESP.
gradient solar pond with the associated stability parameters has been presented. Considering the results of this model for each set of circumstances, the following conclusions were drawn: 1. Salt re-concentration by evaporation can be an effective method of providing salt to the main solar ponds. 2. The time required to increase the salt concentration from seawater (3.5–35%) was estimated to be about 120–250 days during the summer months and about 200–220 days during the winter months. 3. The salt concentration of water used for surface flushing significantly affects the area ratios and the quantities of water used for surface flushing. 4. The best time for high concentration brine injection to the SGSP was estimated at about (2:30 p.m.) to insure minimum thermal energy losses from the storage zone. 5. Because of the low rates of net evaporation during the winter season, a scheme must be devised to collect high concentrated brines during the summer season and store it for later use in winter. 6. The model has demonstrated that the constructed evaporation pond of TESP is undersized.
NOMENCLATURE A ep , A sp Ar Cp C 1 ,C 2 & C 3 EP Efw , Esp , Eep
EP area and SGSP area (m 2 ) Area ratio (A r 5 A ep /A sp ) Specific heat of brine (kJ / kg 8C) Brine concentrations of Q 1 , Q 2 & Q 3 (kg salt / m 3 ) Evaporation pond Evap. rates, from fresh water, from SP,
hr hc Isolar,sp Isolar,ep L M Q1, Q2 & Q 3
R sp , R ep SGSP ST TESP T T T a ,T sky
aep rw t
and from EP (m / period) Radiative heat transfer coefficient (W/ m 2 8C) Convective heat transfer coefficient (W/ m 2 8C) Incident solar radiation on the surface of the solar pond and that of EP Latent heat of vaporization Mass of brine to be evaporated (kg) Quantities of water used for surface flushing, over flow to the EP, and saturated brine injection to the SGSP (m 3 / period) Precipitation rates, into solar pond, into evaporation pond (m / period) Salt gradient solar pond Salt transport (kg / m 2 —period) Tajoura’s experimental solar pond Temperature (8C) Time (period) Ambient air temperature and sky temperature (8C) Ratio of evaporation from EP to that from fresh water (aep 5 Eep /Efw ) Density of fresh water Transmissivity of water surface
REFERENCES Alagao F. B., Akbarzadeh A. and Johnston P. W. (1994) The design, construction, and initial operation of a closed cycle, Salt Gradient Solar Pond. Solar Energy 53(4), 343–351. Batty J. C., Paul Riley J. and Panahi Z. (1987) A water requirement model for salt gradient solar pond. Solar Energy 39(6), 483–489. Denius, M. and Batton, W. (1984) A solar pond organic rankine cycle power generating system. ASME Paper 84WA / SOL-15. Newell T. A. et al. (1994) Characteristics of a solar pond brine re-concentration system. ASME J Solar Energy 116, 69–73. Nielsen (1982) Experience with a prototype solar pond for space heating. Sharing the Sun 5, 169–182. Swift A. and Golding P. (1992) Topics in gradient maintenance and salt recycling in an operational solar pond. ASME J. Solar Energy 114, 62–69. Tabor H. (1975) Solar ponds as heat source for low temperature multi effect distillation plants. Desalination 17, 289– 302.
PII:
Solar Energy Vol. 72, No. 5, pp. 447–454, 2002 2002 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 2 ) 0 0 0 2 1 – X All rights reserved. Printed in Great Britain 0038-092X / 02 / $ - see front matter
www.elsevier.com / locate / solener
DESIGN METHODOLOGY FOR A SALT GRADIENT SOLAR POND COUPLED WITH AN EVAPORATION POND , , K. R. AGHA† *, S. M. ABUGHRES* ** and A. M. RAMADAN* *Center for Solar Energy Studies, P.O. Box 12932, Tripoli, Libya **Department of Mechanical & Industrial Engineering, Faculty of Engineering, P.O. Box 13589, Tripoli, Libya Received 15 May 2000; revised version accepted 21 November 2001 Communicated by TY NEWELL
Abstract—This paper presents the results of a simple mathematical model for predicting the ratio of the evaporation pond (EP) area to that of a Salt Gradient Solar Pond (SGSP) area. The EP idea provides a very attractive method of salt recycling by evaporation, especially in areas of high rates of evaporation and low rates of rain as it is the case for North Africa. The model is applied for two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli-Libya (Lat. 5 32.688N) and for measured evaporation rates. Under the summer conditions and for the case of surface flushing by fresh water, the area ratio was estimated at about 0.17. While for the case of using seawater this ratio increases enormously to about 14.4. The time required for the salt concentration to increase from seawater concentration to a high concentrated brine, which can be injected at the bottom of the solar pond, is also presented. It was estimated that the time required to increase the salt concentration from 3.5 to 35% is about 120 to 250 days during the summer months and about 200 to 220 days during the winter months. 2002 Elsevier Science Ltd. All rights reserved.
centration. The salt gradient zone (NCZ) is the key to the working of a SGSP. It allows solar radiation to penetrate into the storage zone while prohibiting the propagation of long wave radiation because water is opaque to infrared radiation. The zone suppresses global convection due to the imposed density stratification. It offers an effective conduction barrier because of the low thermal conductivity of water and the large thickness of the NCZ, which averages over 1.0 m. The salinity in the UCZ increases due to convective mixing (wind, evaporation...) with NCZ and salt diffusion from the bottom. The annual rate of this natural diffusion of salts from high to low concentration was estimated by Tabor (1975) and Weinberger (1964) to be in the range of 20–30 kg / m 2 . This rate depends on the thickness of NCZ, the temperature profile and the concentration difference between the UCZ and the LCZ. A common method of maintaining the salt gradient is to flush fresh water to the pond surface and inject saturated brine, or salt, at the bottom to substitute for the salt, that is diffused to the surface. In locations like North Africa, where fresh water is very limited, the surface can be washed by seawater. Also, North Africa has very high rates of evaporation and very low rates of
1. INTRODUCTION
An extensive amount of work on SGSP as a cost-effective method of collecting and storing solar energy on large scale is available in the literature. SGSP use has demonstrated success in many applications such as in space heating (Nielsen, 1982), desalination (Tabor, 1975), low temperature industrial heating process (Swift and Golding, 1992) and power production (Denius and Batton, 1984). A SGSP consists of three distinct zones as shown in Fig. 1. The Upper Convective Zone (UCZ) of thickness varying between 0.15 and 0.30 m which has a low and nearly uniform salt content. Beneath the UCZ is the Non-Convective Zone (NCZ) of thickness that varies between 1.0 and 1.5 m and has a salt content increasing with depth, and it is therefore a zone of variable properties. The bottom layer is the Lower Convective Zone (LCZ), also called the storage zone, which has a thickness varies between 1.0 and 2.0 m and has a nearly uniform high salt con-
†
Author to whom correspondence should be addressed. Current address: Civil Engineering Department, Dalhousie University, Halifax, NS B3J 2X4, Canada. Fax: 11-902494-3108; e-mail: [email protected] 447
448
K. R. Agha et al.
precipitation, and therefore recycling of salt by evaporation is practical and economical. This, in addition, overcomes the salt shortage problem and ensures the continuous high performance operation of solar ponds with a minimum environmental damage caused by salt disposal. Based on the above considerations, it is felt essential to demonstrate the capability of a long term, closed cycle salt management facility by evaporative brine concentration, which is the topic dealt within this paper. The current study has been performed to determine the relationship between the area ratio (ratio between the EP area and SGSP area) and some parameters known to affect the thermal stability of the SGSP. The relative monthly quantities of flushing water at the surface and those of high salt concentration to be injected at the bottom were also presented. For the effective operation of the system coupling the SGSP and EP, the transient brine re-concentration phenomena were discussed. The paper also presents the design, construction and the method used in the establishment of salt concentration profile of Tajoura’s Experimental Solar Pond (TESP). 2. TAJOURA’S EXPERIMENTAL SOLAR POND (TESP)
TESP is an artificial solar pond located to the east of Tripoli and is designed and constructed, in joint cooperation with a Swiss company, with SGSP surface area of about 830 m 2 and EP area of about 105 m 2 . The salt concentration profile is constructed with three zones, the Upper Convective Zone (UCZ) of 0.30 m thickness and a salt concentration of about 41 kg / m 3 , the Lower Convective Zone (LCZ) of 1 m thickness and salt concentration of 260 kg / m 3 . Separating these two zones is the Non-Convective Zone (NCZ) of 1.2 m thickness and variable salt concentration. The
pond, fully equipped with systems to monitor all relevant parameters, is designed as an experimental facility enabling the investigation of various aspects of pond performance. 3. COUPLING THE SGSP AND EP
Fig. 1 shows a schematic diagram of a SGSP coupled to an EP. To estimate the required evaporation pond area that will provide the quantity of salt required for the maintenance of salt concentration profile in the SGSP, the mass balance model presented by Alagao et al. (1994) and Batty et al. (1987) was used in this study. In order to operate the pond effectively, it is important to know the time required to substitute for the removed surface brine (from SGSP) by the brine that has to be injected at the bottom of the SGSP. The hourly variation of temperature and concentration of the brine in the EP is of paramount importance in accounting for the loss of energy from the LCZ. An energy balance model (Newell et al., 1994) was used to predict this time with a little modification to the terms representing the sky radiation and evaporation energy. The model used in this study becomes as follows: dT mCP ] 5 tA ep Is h r A ep [T 2 T sky ] dt 2 h c A ep [T 2 T a ] 2 aep Efw Lrw The term on the left-hand side represents the energy stored in the body of the evaporation pond. The first term on the right-hand side represents the solar energy penetrating the surface of the evaporation pond. The last three terms represent the energy losses by radiation, convection and evaporation, respectively. 4. RESULTS AND DISCUSSION
The mathematical model described above was applied by using the prevailing weather conditions
Fig. 1. Schematic diagram of SGSP/ EP system showing the various mass and energy balance components.
Design methodology for a salt gradient solar pond coupled with an evaporation pond
449
Fig. 2. Monthly average measured weather data for Tripoli-Libya Lat. (32.688N) over 30-year period (1961–1990).
in Tripoli-Libya (Lat. 32.688N) shown in Fig. 2, averaged over a 30-year period (1961–1990). Fig. 2 clearly shows the high rates of net evaporation and therefore the practical viability of evaporation ponds, also compares the measured evaporation rates with those calculated. Fig. 3a and b show the effect of ambient air temperature on the evaporation rates from both fresh water and evaporation pond water (concentration 26%), respectively; for various values of surface water temperatures. It is shown that as the ambient air temperature increases, the evaporation rates increase. This is quite obvious since the air temperature at constant relative humidity works as a driving force for evaporation. The effect could be further manifested by the plots of the resulting evaporation rates for different surface water temperatures, as shown on the same diagram. Again, the higher the water temperature, the higher the evaporation rate. Changes in ambient air temperature does not seem to affect the value of aep which represents the ratio of evaporation from the evaporation pond to that from fresh water. This is attributed to the fact that the evaporation pond has a very high salt content (26%). Owing to the intermittent nature of solar energy, solar energy systems will have to be sized according to a pre-specified time. For this purpose, Table 1 shows the required area ratios under the prevailing conditions in Tripoli for different design conditions and both types of surface water
flushing. The winter design was excluded because of its low rate of net evaporation † from the evaporation pond. These results show that large area evaporation ponds are needed if seawater is used for surface flushing, as higher quantities of seawater are required to wash the surface of the SGSP compared with those quantities of fresh water. It can also be seen that the summer design conditions resulted in the smallest area ratio, due to the high rates of net evaporation during the season as depicted in Fig. 2. A further investigation of the effect of the prevailing conditions at the design stage for various cases is shown in Fig. 4. These figures show a graphical presentation of the effect of various parameters on the area ratio for Summer, Autumn, and Spring, respectively. The parameters analyzed in the figure are the net evaporation from the evaporation pond, the type of water used for surface flushing and the rate of salt diffusion. In the case of fresh water surface flushing, the resulting evaporation does not contribute to the increase of concentration in the evaporation. Similar results are obtained for Summer, Autumn and Spring designs as shown in Fig. 4a–c, respectively, when fresh water is used for surface flushing. In other words, in the case of fresh water surface flushing, the quantity of water evaporated is equal to the decrease in the quantity of brine †
Net evaporation rate is defined as the evaporation rate minus the precipitation rate.
450
K. R. Agha et al.
Fig. 3. (a) The effect of ambient air temperature of Efw for different surface temperatures: (b) The effect of ambient air temperature on aep for different surface temperatures.
flushed with a corresponding change in salt concentration from C2 to C3.
4.1. Applying the model to TESP As mentioned earlier, for the purpose of high performance operation of solar ponds, it is important to keep the pond thermally stable. This requires the injection of high salt concentration brine at the bottom to compensate for the natural diffusion of salts upwards, it also requires washing the surface with low concentration water to Table 1. Estimated area ratios under different design parameters Area ratio, A r Fresh water Spring 0.65 Summer 0.17 Autumn 0.32 TESP Project 0.13 ‘Constructed’
Seawater 28.40 14.40 20.20
replace evaporation losses and flush away the salt diffusing to the surface from the concentrated brines below. Fig. 5a shows the predicted quantity of salt provided by the EP each month as compared to that required by the SGSP (A SP 5830 m 2 ) under the prevailing conditions, for different design scenarios with measured evaporation rates for the case of fresh water surface flushing. Fig. 5b compares the same quantities when seawater is used for surface flushing. Fig. 6 shows monthly quantities of salt that can be provided by the constructed evaporation pond (105 m 2 ) and (1.5 m) depth at the TESP project compared with that required by the SGSP (830 m 2 ). It is clearly shown that the EP area was rather underestimated even if fresh water is used for surface flushing. The quantity of salt required by SGSP to maintain its salt concentration profile was estimated at about 1150 kg / month
Design methodology for a salt gradient solar pond coupled with an evaporation pond
451
Fig. 4. The variation of Area ratio (A r ) with net evaporation rate from EP for both types of surface water flushing and different salt transport rates.
(16.6 kg / m 2 –year), which could be under-estimated compared to previous studies (Alagao et al., 1994; Swift and Golding, 1992). Fig. 7 shows the quantity of water in (m 3 / month) required for surface flushing using fresh water and seawater flushing with measured evaporation rates. This graph clearly shows the large quantities of flushing water required during the summer, which is attributed to the high rates of evaporation in the summer season. It also shows the big differences between the quantities
required for fresh water flushing and that for seawater flushing. This comes as a result of the fact that in addition to replacing the evaporation losses, washing the surface is expected to flush away salt diffusing to the surface from the concentrated brines below. Fig. 8 shows the required time for the concentration in an EP to reach (35%) starting from sea water (3.5%) under different prevailing conditions. It is clearly shown that the shortest period for the increase of concentration occurs during the
452
K. R. Agha et al.
Fig. 5. Mass of salt provided by the EP as compared to that required by the SGSP of TESP (A SP 5830 m 2 ) under different scenarios.
summer months. This is attributed to the high net evaporation rates in the summer season. Presenting the daily amounts of concentration
as shown in Fig. 9 could further manifest the brine re-concentration phenomena. The figure also shows the daily decrease in brine depth that
Fig. 6. Mass of salt provided and required by the TESP (Constructed pond), It should be noted that for the case of seawater surface flushing the real quantity of salt is equal to the shown values divided by ten. (A ep 5105 m 2 , Depth51.5 m) (A sp 5830 m 2 ).
Design methodology for a salt gradient solar pond coupled with an evaporation pond
453
Fig. 7. The quantity of water required for surface flushing in (m 3 / month) for TESP.
Fig. 8. The time required (in days) to increase the salt concentration from seawater concentration (3.5%) to (35%) for different starting months for the TESP, and the temperature of brine and ambient at the last day.
corresponds to the daily increase in concentration. It should be noted that the results given in Figs. 8 and 9 are based on the assumption that no withdrawal or feeding to EP has occurred once it’s started. In order to operate the SGSP effectively and minimize the energy losses during the high concentration injection, it is important to use the highest possible temperature for the brine to be injected. Fig. 10 is constructed for this purpose, it shows the hourly temperature variation of the
high concentration brine (at the last day of different starting months). It can be seen from this figure that the maximum temperature of the concentrated brine occurs at about (2:30 p.m.) of the last day, which is the preferred time to inject the concentrated brine from EP to SGSP. 5. CONCLUSIONS
A mathematical model describing the relationships between the evaporation pond and the salt
Fig. 9. The daily variation of brine concentration and depth in the evaporation pond of TESP.
454
K. R. Agha et al.
Fig. 10. The temperature variation of the available high concentration brine (35%) at the last day of different starting months for TESP.
gradient solar pond with the associated stability parameters has been presented. Considering the results of this model for each set of circumstances, the following conclusions were drawn: 1. Salt re-concentration by evaporation can be an effective method of providing salt to the main solar ponds. 2. The time required to increase the salt concentration from seawater (3.5–35%) was estimated to be about 120–250 days during the summer months and about 200–220 days during the winter months. 3. The salt concentration of water used for surface flushing significantly affects the area ratios and the quantities of water used for surface flushing. 4. The best time for high concentration brine injection to the SGSP was estimated at about (2:30 p.m.) to insure minimum thermal energy losses from the storage zone. 5. Because of the low rates of net evaporation during the winter season, a scheme must be devised to collect high concentrated brines during the summer season and store it for later use in winter. 6. The model has demonstrated that the constructed evaporation pond of TESP is undersized.
NOMENCLATURE A ep , A sp Ar Cp C 1 ,C 2 & C 3 EP Efw , Esp , Eep
EP area and SGSP area (m 2 ) Area ratio (A r 5 A ep /A sp ) Specific heat of brine (kJ / kg 8C) Brine concentrations of Q 1 , Q 2 & Q 3 (kg salt / m 3 ) Evaporation pond Evap. rates, from fresh water, from SP,
hr hc Isolar,sp Isolar,ep L M Q1, Q2 & Q 3
R sp , R ep SGSP ST TESP T T T a ,T sky
aep rw t
and from EP (m / period) Radiative heat transfer coefficient (W/ m 2 8C) Convective heat transfer coefficient (W/ m 2 8C) Incident solar radiation on the surface of the solar pond and that of EP Latent heat of vaporization Mass of brine to be evaporated (kg) Quantities of water used for surface flushing, over flow to the EP, and saturated brine injection to the SGSP (m 3 / period) Precipitation rates, into solar pond, into evaporation pond (m / period) Salt gradient solar pond Salt transport (kg / m 2 —period) Tajoura’s experimental solar pond Temperature (8C) Time (period) Ambient air temperature and sky temperature (8C) Ratio of evaporation from EP to that from fresh water (aep 5 Eep /Efw ) Density of fresh water Transmissivity of water surface
REFERENCES Alagao F. B., Akbarzadeh A. and Johnston P. W. (1994) The design, construction, and initial operation of a closed cycle, Salt Gradient Solar Pond. Solar Energy 53(4), 343–351. Batty J. C., Paul Riley J. and Panahi Z. (1987) A water requirement model for salt gradient solar pond. Solar Energy 39(6), 483–489. Denius, M. and Batton, W. (1984) A solar pond organic rankine cycle power generating system. ASME Paper 84WA / SOL-15. Newell T. A. et al. (1994) Characteristics of a solar pond brine re-concentration system. ASME J Solar Energy 116, 69–73. Nielsen (1982) Experience with a prototype solar pond for space heating. Sharing the Sun 5, 169–182. Swift A. and Golding P. (1992) Topics in gradient maintenance and salt recycling in an operational solar pond. ASME J. Solar Energy 114, 62–69. Tabor H. (1975) Solar ponds as heat source for low temperature multi effect distillation plants. Desalination 17, 289– 302.