MAINTENANCE STRATEGY FOR A GRADIENT SOLAR COUPLED WITH AN

Khairy R. Agha, Saleh M. Abughres, and Abd-ghani M. Ramadan Center for Solar Energy Studies, (CSES). P.O.Box 12932, Tripoli - Libya, Phone (+218-21) 360-1107, Fax (+218-21) 360-1108 E-mail: [email protected]

ABSTRACT - In this paper, all the results presented were predicted for the first three years of operation. The daily variations of brine concentration in the Evaporation Pond (EP) of Tajoura’s Experimental Solar Pond (TESP) and those based on different designs were predicted and discussed under different scenarios. The quantities of brine provided by the evaporation pond and that required by SGSP were predicted for both cases of surface water flushing (fresh water and seawater) under the different design conditions. The quantities of salt that can be contributed by (EP) were predicted to be in the range of 20% to 40% during the first year and 45% to 95% during the third year depending on the design selected. Comparing the percentage of salt provided for different designs, it can be clearly seen that the Autumn design presents a favorable condition. It provides a reasonable percentage reaching 79% in the case of fresh water surface flushing and 93% in the case of seawater surface flushing. Under the prevailing weather conditions of Tripoli, the results have shown that in addition to the higher flushing quantity required during the Summer, flushing is needed more frequent. It was predicted that the number of flushing varies between five times per month during the summer to two or three times per month during winter. Also, the study predicted that the quantity of seawater surface flushing is bigger than that of fresh water.

1. INTRODUCTION LCZ to UCZ, wind mixing, evaporation, dust and dirt falling on pond surface. Some of these problems were first Salt gradient solar (SGSP) present an attractive investigated by (Tabor, 1963). Tabor, 1981 and Weinberger, method of collecting and storing solar energy on a large 1965 addressed the physics of pond’s stability. More scale. A SGSP consists of three distinct zones, the Upper recently, Hassab et al, 1989 presented a field report on a Convective Zone (UCZ) of thickness varying between 0.15 solar pond constructed in the State of Qatar. They reported and 0.30 meter, which has a low and nearly uniform salt the problems encountered in operating SGSPs in the Arabian concentration. Beneath the (UCZ) is the Non-Convective Gulf region, characterized as a windy and dusty Zone (NCZ) of thickness, which varies between 1.0 and 1.5 environment. These problems are excessive erosion of the meter and has a salt concentration increasing with depth, and gradient zone, the formation of sizable localized convective it is therefore a zone of variable properties. The bottom layer zones, the deterioration of pond water clarity and high rates is the Lower Convective Zone (LCZ), also called the storage of surface evaporation. These weather related problems zone, which has a thickness varying between 1.0 and 2.0 severely impair the pond operation and performance. meter and has a nearly uniform high salt concentration. The The in the UCZ increases due to convective mixing salt gradient zone (NCZ) is the key to the working of a (wind, evaporation. .) with NCZ and salt diffusion from the SGSP. It allows solar radiation to penetrate into the storage bottom. In a typical case this diffusion amounts to about 60 zone while prohibiting the propagation of long wave radiation (tons/km2 Day). Weinberger, 1965, estimated the annual rate because water is opaque to infrared radiation. The zone of this natural diffusion of , to be in the range of 20 - 30 suppresses global convection due to the imposed density Kg/m2, depending on the thickness of NCZ, the temperature stratification. It offers an effective conduction barrier because profile and the concentration difference between the UCZ of the low thermal conductivity and the zone thickness, and the LCZ. Newell et al, 1994, estimated the salt which averages over 1.0 meter. transported per year from the LCZ to the UCZ for 2000 m2 A (SGSP) located in Tajoura to the east of Tripoli has been solar pond at the University of Illinois, in the range of 25 to designed and constructed by (CSES) in joint cooperation with 50 tons. a Swiss company as an experimental facility. Tajoura’s In addition to wind mixing, evaporation can cause a local

Experimental Solar Pond (TESP) consists of a main SGSP with concentration of salt and possibly local reversal of the density a surface area of 830 m2 and a total depth of 2.5 m and an gradient. Surface washing can eliminate the effect of evaporation pond (EP) with a surface area of 105 m2 and 1.5 m evaporation. Schladow, 1984, showed that evaporation can be depth. The salt concentration profile is constructed with three the dominating mechanism in surface layer mixing under light zones, the (UCZ) of 0.30 m thickness and a salt concentration winds. Whereas in the case of strong wind, the evaporation is of about 41 Kg/m3, the (LCZ) of 1 m thickness and salt of secondary importance compared to direct wind stirring. concentration of 256.94 Kg/m3. Separating these two zones is The common method of maintaining the salt gradient is to the (NCZ) of 1.2 m thickness and variable salt concen-tration. flush fresh water to the pond surface and inject saturated

The pond, fully equipped with systems to monitor all relevant brine, or salt, at the bottom to offset the salt that diffused to parameters, is designed as an experimental facility enabling the the surface. In locations like North Africa, where fresh water investigation of various aspects of pond performance. is very short, the surface can be washed by seawater. Also, There are a number of difficulties and limitations that affect North Africa has very high rates of evaporation and very low the performance of solar ponds, and in some locations limit rates of rain, and therefore recycling of salt by evaporation is their use, were recognized and several schemes for solution practical and economical. have been proposed to eliminate or minimize their effect. To over come the salt supplies problem and to ensure the These problems include, among others, salt diffusion from continuous high performance operation of solar ponds with a

1 minimum environmental damage caused by the salt disposal, ± The effects of the local weather conditions on the a salt recycling system is a necessity. Based on the a above thermal stability of the SGSP, and those considerations, it is felt essential to demonstrate the recommendations on how to minimize their effects on capability of a long term, closed cycle salt management future designs. facility by evaporative brine concentration, which is the topic addressed in this paper. ± The best method for salt gradient maintenance. Several schemes have been proposed for preserving a ± A day to day operation for solar ponds. salinity gradient against diffusion. Tabor, 1963, suggested the falling pond i.e. a three- terminal flow system in which ± A guide for a systematic operation and maintenance density profile is nearly exponential. More recently Rabl and of solar ponds. Nielsen, 1975, suggested the use of a linear gradient which results from a four terminal flow system. The common 2. MATHEMATICAL MODELING: method of maintaining the salt gradient suggested by Tabor, A major concern related to the use of solar ponds is salt 1963, is to flush fresh water to the pond surface and inject gradient maintenance. Over time, salt diffuses from the LCZ saturated brine, or salt, at the bottom to offset the salt which to the UCZ. To maintain the salinity gradient surface brine diffused to the surface. has to be removed and replaced with fresh water ( or sea A mathematical model has been developed by Batty, Riley water) consequently, more salt has to be added to the LCZ . and Panahi, 1987, to estimate the area of salt gradient solar One way of recycling the salt is to re-concentrate the ponds, associated evaporation ponds, and storage reservoirs removed surface brine in an evaporation pond. that could be maintained with a given water source. They Operating SGSP starts with building the required salt concluded that salt and water requirements might be the gradient profile and filling the EP with seawater. During the physical factor that limits solar pond applications in many heating up period, the storage zone temperature increases locations. Alagao et al, 1994, presented the theoretical basis gradually which increases the salt diffusion rate and thus and its validation of a Closed Cycle Salt Gradient Solar Pond leading to the problem of thermal stability and keeping the (CCSGSP). Results from the initial operation of the solar salt gradient profile within a certain margin. pond show that wind action and convective mixing caused For the purpose of analysis, an energy balance model was some erosion of the gradient layer thereby increasing the used based on the one-dimensional transient model surface layer thickness. formulated by Pancharatnam (1972) and mentioned by In locations like North Africa, where fresh water is very Newell, T.A. et al, (1994). In this study, the model was short, the surface can be washed by sea-water. Also, North modified to accommodate the measured evaporation rates. Africa has very high rates of evaporation and very low rates The model used in this paper can be written as; of rain, and therefore recycling of salt by evaporation is dT practical and economical. In order to generate the initial salt mc = a * Aep *Qs - hr * Aep (T - Tsky ) - h* Aep (T - Ta ) - Efw *aep * L*rw concentration gradient required to prevent convection, a dt usual practice is to fill the pond with several layers of salt the coefficient of convective heat transfer (h ) in ( w / m2. ok) solution. This is normally start with near saturation layer at is based on the following correlation; the bottom followed by layers lower in concentration up to h = 5.6779 (1+ 0.671*V ) the top layer, which nearly reaches fresh water conditions. The initial quantity of salt required is estimated to be about the radiation heat transfer coefficient (hr) is taken to be equal 2 o 500 kgs of salt per square meter in a pond 3 m deep, and 800 to 5.7 ( w/m . k ) for shallow solar ponds . Kgs of salt per square meter in a pond 4m deep Newell et al, The model is applied to the evaporation pond of TESP, with 2 1994. This huge amount of salt can be a problem for a large surface area of 105 (m ), the block diagram below scale solar ponds in areas short of natural resources of salt. (Figure(1)) illustrates the method of programming and Solar ponds also require large quantities of water to replace solution procedure. evaporation loss and flush away salt diffusing to the surface from the concentrated brines below. 3. RESULTS AND DISCUSSION: In a previous study, the authors have shown that the (EP) of (TESP) is under sized and can provide only a bout 30% of 3.1. The Growth of Upper Convective Zone and its Control: the salt required by the SGSP. The anticipated size of EP The UCZ thickness and its concentration are known to were estimated and presented in that study under different affect the overall performance of SGSP, and it must be design conditions. The design conditions considered in that closely monitored and controlled to minimize its effect. As study include Summer, Autumn and designs, while mentioned above, the growth of the UCZ depends on many the winter design was excluded due to the low rates of net parameters including, among others, the net evaporation rate, evaporation during the winter season. The objective of this wind speed, and salt diffusion rate. paper is to establish a physical understanding of the This section presents the results regarding the quantities of problems associated with the operation and maintenance of surface water flushing, overflow water, for both cases (fresh solar ponds. And then develop a guide for a systematic water or seawater) and their timing with the assumption that operation and maintenance of such systems. 4% and 5% constitute the lower and upper limits of salt The results of this paper will help the designers of solar concentration in the UCZ respectively. It should be noted pond systems and the operation engineers of such systems by that these limits might not be representing the actual giving the following parameters: happenings in practical applications. Figure (2) shows the

2 Input initial conditions and meteorological data increase exponentially with increasing C1. The figure also of Tripoli - Libya shows that the quantity of seawater (C1=3.5%) required for surface flushing is about 8 times that of fresh water, while, Calculate ambient air temperatures (Tair), and the over flow increases rapidly to about 55 times. The large Insolation (Qs) over flow quantity for the case of seawater has resulted due to the fact that flushing is expected to replace the evaporation Calculate the amount of evaporation from fresh and wash the top surface of the pond. The quantity of water (Efw), reduction of evaporation coeff. (aep) evaporation to be replaced is the same regardless of flushing , and then amount of evaporation rate from brine water concentration. water (E ) ep æ Q1, C1ö Figure (4b) shows that the ratio ç ÷ is constant for all è Q1, F ø Solve the ordinary differential equation governing the brine temperature (T) and brine year round for the same concentration and it increases as the concentration (S %) with time flushing water concentration increases. Therefore, this ratio depends only on the value of surface flushing water Compute new brine temperature (T), new brine concentration concentration C1, and the overflow water concentration C4. (S %), new volume of fresh water (VOLW) new volume of It is independent of prevailing conditions. For example, for salt (VOLS) and new height of brine water of evaporation æ Q1, C1ö seawater surface flushing (C1=3.5%), the ratio ç ÷ as pond (H) è Q1, F ø stated above is about 8 and remains constant throughout the Calculate the above parameters for the next time year. The quantity of water used for flushing away the salt changes according to the variations occurring in the Check stopping criteria, End the calculations if evaporation rate of the solar pond. These changes do not it reaches the desired conditions affect the overall value of the above ratio as clarified by the following example: Figure (1) Flow chart of the mass and energy balance model For the month of March Net Evaporation from Solar Pond (Esp-R) = 130.8 m3 estimated daily variations in the UCZ concentration during Quantity of Fresh water used for flushing away the salt the first year of operation under the meteorological 3 2 (Q4,F) = 33.4 m conditions of Tripoli- Libya for a SGSP of 830 m surface Quantity of sea water used for flushing away the salt area, 0.30 m UCZ initial thickness, and 3.5% initial (Q4,S) = 1154 m3 concentration (concurrent with TESP). The figure also shows For the month of June the quantities of surface water flushing and overflow water Net Evaporation from Solar Pond (Esp-R) = 260.6 m3 and their timing for both cases (fresh water and seawater). Quantity of Fresh water used for flushing away the salt Figures (3a, b & c) show the same results for the months of (Q4,F) = 27.65 m3 January, May and September respectively. These results Quantity of sea water used for flushing away the salt clearly show that comparing Winter and Summer seasons, (Q4,S) = 1995 m3 flushing is required more frequent during the Summer season And Therefore; resulting in higher flushing quantities, which is attributed to the high rates of net evaporation in the Summer season. It was æ Q1,C1ö 130.8 +1154 ç ÷ = = 7.82 predicted that the number of flushing varies between five è Q1, F ø 130.8 + 33.4 times per month during the Summer to two or three times per March month during the Winter. It also shows the huge differences between the quantities required for fresh water flushing and æ Q1,C1ö 260.65 +1995 ç ÷ = = 7.82 those of seawater flushing. This is due, as mentioned earlier, è Q1,F ø 260.65 + 27.65 to fact that in addition to replacing the evaporation losses, June washing the surface is expected to flush away salt diffusing to the surface from the concentrated brines below. And remains constant throughout the year as depicted in As explained above, the higher the flushing water Figure (4b). concentration (C1), the higher quantities of flushing water æ Q4, C1ö Figure (4c) shows that the ratio ç ÷ increases and overflow. The effect of C1 on the quantities of flushing è Q4, F ø water and overflow can be further investigated by estimating during Summer months for the same flushing water the ratio of the quantity at the desired concentration to that of concentration and increases with the flushing water æ ö concentration. This is due to the fact that it depends on the Q1, C1 Q4, C1 fresh water ç and ÷ . The results of such an prevailing conditions and on other parameters known to ç Q1, F Q4, F ÷ è ø affect the growth of the UCZ (C1, C4, C7, …etc). estimation is presented in Figures (4a, b, c) for the TESP (Asp=830 m2). æ Q1, C1ö Figure (4a) shows the effect of increasing C1 on ç ÷ è Q1, F ø æ Q4, C1ö and ç ÷ . It can be clearly seen that both ratios è Q4, F ø

3 7 900 6 Q1,F Q4,F Q1,S Q4,S Conc. 800 700 5 )

600 3 4 500 ( m 4 3 400 ,Q

300 1 2 Q Co ncen tr a t i on,% 200 1 100 0 0 1 31 61 91 121 151 181 211 241 271 301 331 361 Days Fig. 2: Daily variations of UCZ concentration, quantities of flushing water, Q1 for SP and overflow water, Q4 to EP for first year of operation. 6 600 6 Q1,F Q4,F Q1,S Q4,S Conc. 5 500 5

4 400 ) 4 3 (m

3 300 4 3 ,Q 1 2 2 200 Q Concentration, % Concentration, % 1 1 100 0 0 0 1 3 5 7 9 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 11 13 15 17 19 21 23 25 27 29 31 Days Days Fig.3b: Daily variations of UCZ concentration, quantities of Fig.3a: Daily variations of UCZ concentration, quantities of flushing water, Q1 for SP and overflow water, Q4 to EP in flushing water, Q1 for SP and overflow water, Q4 to EP in MAY for first year of operation. JAN (under the assumption of first of January as the starting day). 60 6 600 ,F

4 Q4,C1/Q4,F Overflow water Q1,F Q4,F Q1,S Q4,S Conc. 50 5 500 /Q

1 Q1,C1/Q1,F ,C 40 ) 4 4 400 3

30 (m 3 300 4 ,F or Q ,Q 1 20 1 Q

/Q 2 200 1

Makup water Concentration, % ,C 10 1 1 100 Q 0 0 0

0 0.5 1 1.5 2 2.5 3 3.5 1 4 7 10 13 16 19 22 25 28 Flushing Water Concentration, C1 (% age) Days Fig. 4a: The effect of Flushing Water Concentration on the Fig. 3c: Daily variations of UCZ concentration, quantities of make up water and over-flow quantities. flushing water, Q1 for SP and overflow water, Q4 to EP in SEP for first year of operation.

100 10 ,F ,F 4 1 /Q /Q

1 10 1 ,C ,C 4 1 Q Q

1 1 Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec C1=0.5% C1=1% C1=1.5% C1=2% C1=0.5% C1=1% C1=1.5% C1=2% C1=2.5% C1=3% C1=3.5% C1=2.5% C1=3% C1=3.5% Fig. 4c: The effect of Flushing Water Concentration on the Fig. 4b: The effect of Flushing Water Concentration on the overflow water quantities for different months under the make up water quantities for different months under the prevailing conditions of Tripoli – Libya. prevailing conditions of Tripoli – Libya.

4 3.2. The Evaporation Pond Potential: 55 In this section, the daily variations of brine concentration in 50 Spring Design 45 Autumn Design the EP of TESP and those based on the design conditions 40 Summer Design defined in reference (Agha et al, 2000) are presented and 35 TESP Design 30 discussed for the first three years of operation under different 25 scenarios. 20 Figures (5a, b, c and d) show the variation of salt 15 st CONCENTRATION,% 10 concentration in the EP, with the 1 of January as the starting 5 day, for both types of surface water flushing, fresh water and 0 1 61 121 181 241 301 361 421 481 541 601 661 721 781 841 901 961 sea water, starting from primary salt concentration of 1021 1081 First year Second year Third year seawater (C1=3.5%) and RO-Plant brine reject concentration DAYS (C1=7%), under different design conditions. As stated above, it should be noted that these results are based on the Figure (5a): Variation of salt concentrations in EP started at assumption of Closed Cycle Salt Gradient Solar Pond JAN with primary salt concentration (3.5%), seawater under (CCSGSP), i.e. the evaporation pond is coupled with the three years of operation for Fresh water surface flushing. solar pond. This case was carried out to simulate the real 55 case scenario. 50 45 Spring Design These figures clearly show that the maximum concentration 40 Autumn Design levels occur at the months of August and September, while, 35 Summer Design 30 TESP Design the minimum concentration levels occur at the months of 25 December and January. 20 It seems that the difference in the initial concentration 15 CONCENTRATION,% 10 (3.5% to 7%) vanishes by the end of the first year, and no 5 difference exists during the second and third years of 0 1 61 121 181 241 301 361 421 481 541 601 661 721 781 841 901 961 operation. All the figures show maximum concentration First year Second year Third year 1021 1081 levels for the Spring design and minimum concentration DAYS levels for TESP design. This is, of course, due to the high Figure (5b): Variation of salt concentrations in EP started at area ratio for Spring design and low area ratio for TESP as JAN with primary salt concentration (3.5%), seawater under given in Table (1). three years of operation for Seawater surface flushing. A Comparison between the results for the uncoupled 55 (separate) evaporation pond (Figure (6)) with those for the 50 Spring Design coupled evaporation pond (Figures (5a, b, c and d) clearly 45 Autumn Design shows the effect of coupling on the concentration level of the 40 Summer Design 35 TESP Design evaporation pond. For the case of uncoupled evaporation 30 pond, a concentration level of about 35% were achieved by 25 20 the end of June, while the end of June concentration level in 15 the coupled evaporation pond was only 6%. CONCENTRATION,% 10 5 For the Spring design, the maximum concentration levels 0 1 for fresh water surface flushing were 27%, 43% and 43% for 61 121 181 241 301 361 421 481 541 601 661 721 781 841 901 961

st nd rd 1021 1081 the 1 , 2 and 3 years of operation, respectively. For First year Second year Third year flushing by seawater, these concentration levels were 22%, DAYS 40% and 40% during the same years of operation. For the Figure (5c): Variation of salt concentrations in EP started at particular case of Summer design, the big difference in area JAN with primary salt concentration (7%), brine reject from ratio shown in Table (1) seems to affect the rate of change of RO-Plant under three years of operation for Fresh water concentration level in the EP. This effect is not so clear in surface flushing. the other designs despite the difference in area ratio. 55 50 As mentioned in the previous sections, the EP of TESP is Spring Design 45 under-sized, this is very clear from the results given in Autumn Design 40 Figures (5a, b, c and d) regardless of the type of surface 35 Summer Design water flushing. 30 TESP Design 25 3.3. The Salt Concentration Profile Maintenance: 20 15

The quantities of brine required by the SGSP of TESP (830 CONCENTRATION,% 10 m2) and that provided by the EP based on different design 5 0 conditions were predicted for both cases of surface water 1 61 121 181 241 301 361 421 481 541 601 661 721 781 841 901 961 flushing (fresh water and seawater) for three years of First year Second year Third year 1021 1081 operation. It should be noted that these predictions simulate DAYS the real case scenario by assuming the following conditions; ¨ The EP and the SGSP are completely coupled together Figure (5d): Variation of salt concentrations in EP started at through Q4 (over-flow from SGSP) and Q7 (high JAN with primary salt concentration (7%), brine reject from concentrated brine injection). RO-Plant under three years of operation for Sea-water st surface flushing. ¨ The 1 of January was assumed as the starting operating day.

5 ¨ The natural rate of salt diffusion was assumed as 16.6 the other two design conditions (Summer and TESP) can kg/m2-year (as assumed by TESP team). provide a maximum of about 60% at the end of summer rd ¨ The LCZ concentration is allowed to decrease from season of the 3 year. This re-emphasizes the results 26% to 22%. These were taken as the upper and lower presented by the authors in a previous study concerning the limits of marginal stability. size of EP of TESP [ ]. Figure (7) also show the quantity of brine to be injected into Figures (7a, b, c and d) show the results of the fresh water 3 surface flushing for the Spring, Autumn, Summer and TESP the SGSP (Q7). An amount of about 3.4 m with a brine design conditions respectively. These figures clearly concentration of 26% is being drawn from the EP and demonstrate the ability of salt recycling scheme by re- injected into the LGZ of SGSP every 49 days (corresponding evaporation in the EP. It can be seen that during the first year to the marginal stability conditions of the pond, stated of operation non-of the design conditions considered is able above). In cases of high concentration in the EP (higher than to provide a complete quantity of the salt required by the 26%, refer to Figure (5), the required quantity of salt can be SGSP. obtained by withdrawing lower amount of brine from the EP. For the first five months of operation, all the design On the other hand, in times of low concentration in the EP, a conditions seem to provide the same quantity of salt ranging fresh salt must be added to the brine drawn from the EP to from 10% to 17% of that required. At the start of the summer make-up the required quantity of salt. season of the 1st year, a difference in the amount of salt Figures (8a, b, c d) shows the predicted salt quantities for the provided by the design conditions start to appear. The case of seawater surface flushing for Spring, Autumn, highest quantities of salt are for the Spring design and the Summer and TESP design conditions, respectively. As in the lowest quantities are for TESP. This is of-course due to the case of fresh water surface flushing, the quantity of salt high area ratio (Ar) for the Spring design (refer to Table (1). provided by the EP during the first five months of operation By the end of the 1st year the quantity of salt contributed by appear to be the same for all the design conditions and the Spring and Autumn design conditions are 75% and 50 ranging from 10% to 17% of that required. Although, the %respectively, while, for summer and TESP design difference in the amount of salt provided by the different conditions, it is only about 30%. design conditions appears at the start of the summer season The Spring design can provide a complete quantity of salt for the case of fresh water surface flushing, this difference is required by the beginning of the summer season of 2nd year, delayed to the end of the summer season if seawater is used whereas, the Autumn design can provide the complete for surface flushing. This is attributed to the high overflow quantity by the end of this summer season. At the same time, quantities for the case of seawater surface flushing.

2 40 JAN MAY SEP 35

1.5 30

25

1 20

15 Bri ne Leve l, (m) 0.5 10 Bri ne Concent rat i on , % 5

0 0

1

21 41 61 81

101 121 141 161 181 201 221 241 261 DAYS Figure (6): Daily variations in brine concentration and depth for actual EP (Aep=105 m2, Depth=1.5 m) for concentration to reach (35%) starting from seawater concentration (3.5%) for different starting months. Table (4.2): Comparison between the quantities of salt that can be provided by the Evaporation Pond for the first three years of operation under different design conditions. Area Ratio %age of salt provided by EP Design Overall (%age) (Ar=Aep/Asp) fresh water flushing sea water flushing Fresh water Sea water 1st yr 2nd yr 3rd yr 1st yr 2nd yr 3rd yr Fresh water Sea water Spring 0.65 28.40 44 89 87 39 92 95 74 76 Autumn 0.32 20.20 30 73 79 32 85 93 61 71 Summer 0.17 14.40 21 36 48 26 69 88 36 62 TESP 0.13 (constructed) 20 33 44 19 33 44 33 32

6 5 5 1400 ) 1400 3 IN MSSP MSEP MSAD Q7 MSSP MSEP MSAD Q7 7

(m 1200 4 1200 7 4 1000 1000 3

3 ) 800 800 3

(m 2 600 2 600 400 400 1 MASS OF SALT (kg)

1 MASS OF SALT (kg) 200 200 0 0 QUANTIT Y OF BRINE , Q QUANTIT Y OF BRINE, Q 0 0 4/8 9/2 5/5 1/5 6/1 9/7 2/18 5/27 7/15 12/9 1/27 3/17 6/23 8/11 9/29 2/23 4/13 7/20 10/21 11/17 10/26 12/14 4/8 9/2 5/5 1/5 6/1 9/7 2/18 5/27 7/15 12/9 1/27 3/17 6/23 8/11 9/29 2/23 4/13 7/20

10/21 11/17 10/26 12/14 First year Second year Third year First year Second year Third year

DATE OF BRINE INJECTION DATE OF INJECTION Figure (7b): Quantity of salt and brine required to SP and Figure (7a): Quantity of salt and brine required to SP and that provided by EP for Autumn Design (Fresh water surface that provided by EP for Spring Design (Fresh water surface flushing). flushing). 5 1400 ) 3 MSSP MSEP MSAD Q7 (m

7 4 1200

1000 3 800

2 600

400 1 MASS OF SALT(kg) 200 QUANTIT Y OF BRINE, Q 0 0 4/8 9/2 5/5 1/5 6/1 9/7 2/18 5/27 7/15 12/9 1/27 3/17 6/23 8/11 9/29 2/23 4/13 7/20 First year 10/21 Second year 11/17 Third year 10/26 12/14

DATE OF INJECTION Figure (7d): Quantity of salt and brine required to SP and Figure (7c): Quantity of salt and brine required to SP and that provided by EP for TESP Design (Fresh water surface that provided by EP for Summer Design (Fresh water surface flushing). flushing). Figure (10) presents an overall comparison between all the cases It seems that the type of water used for surface flushing does considered. The least contribution comes from the TESP, which not have a noticeable effect on the quantity of salt provided is, as stated above, due to the low EP area. by the EP for all the design conditions (except Summer Table (1) summarizes the predicted quantities of salt that can design condition). Comparing Figure (7c) (Summer design be contributed by (EP) (as, a percentage). It can be seen that condition and fresh water surface flushing) with Figure (8c) (EP) provide 20% to 40% during the first year and 45% to 95% (Summer design condition and seawater surface flushing) during the third year depending on the design selected. show that the quantity of salt provided in the case of Comparing the percentage of salt provided for different designs, seawater surface flushing is almost double that provided by it can be clearly seen that the Autumn design presents a the case of fresh water surface flushing. This is believed to favorable condition. It provides a reasonable percentage be due to the high area ratio for the case of seawater surface reaching 79% in the case of fresh water surface flushing and flushing under the condition of Summer design as compared 93% in the case of seawater surface flushing with a relatively with that of fresh water surface flushing. Changing the type low area ratio. of flushing water from fresh water to seawater is expected to increase the overflow quantity by about 55 times (as shown 3.4. Comparison between Evaporation Ponds and Evaporation in Figure (4a)). While, the area ratios are expected to Surfaces: increase by about 44, 63 and 85 times for the design This section investigates the effect of EP depth on the area conditions of Spring, Autumn and Summer respectively. ratio and the related performance of EP. In order for the EP to The overall year contribution of EP under different design provide the required amount of salt, decreasing the EP depth conditions in providing the required salt quantities for both leads to increasing the EP area. The idea of using large surface types of surface water flushing is shown in figures (9a, b, c areas is very attractive, especially in areas of low to moderate and d). These figures clearly show the differences existing net evaporation rates. between the design conditions considered for the first three Figures (11a and b) show the effect of decreasing the EP depth years of operation. The highest contribution comes from the on the area ratio for both cases of surface water flushing and Spring design for both types of surface water flushing. Also, under different design conditions. Noting that a depth of 0.2 m for each design condition, the contribution increases by the was treated in this study as an evaporation surface (ES) year. condition. None of the design conditions can provide a complete Figures (12a and b) show the daily variations in salt quantity of the salt required for the three years. This is, of- concentration for various EP depths under the Autumn design course, due to the reduction in the concentration levels condition. It can be seen that the improvement in salt resulting from the precipitation rate during the winter season, concentration increases for the ES, especially during the first and can be overcame by storing salt from the Summer season year where the concentration has reached 35% at the start of the for use during the winter season. summer months.

7 5 5 ) )

1400 3 1400 3 MSSP MSEP MSAD Q7

(m MSSP MSEP MSAD Q7 (m 7 7 4 1200 4 1200

1000 1000 3 3 800 800 2 600 2 600 400

1 MASS OF SALT(kg) 400 MASS OF SALT(kg) 200 1 QUANTIT Y OF BRINE, Q QUANTIT Y OF BRINE, Q 200 0 0

4/8 9/2 5/5 1/5 6/1 9/7 0 0 2/18 5/27 7/15 12/9 1/27 3/17 6/23 8/11 9/29 2/23 4/13 7/20 10/21 11/17 10/26 12/14 4/8 9/2 5/5 1/5 6/1 9/7 2/18 5/27 7/15 12/9 1/27 3/17 6/23 8/11 9/29 2/23 4/13 7/20

First year Second year Third year 10/21 11/17 10/26 12/14 First year Second year Third year DATE OF INJECTION DATE OF INJECTION Figure (8b): Quantity of salt and brine required to SP and Figure (8a): Quantity of salt and brine required to SP and that provided by EP for Autumn Design (Seawater surface that provided by EP for Spring Design (Seawater surface flushing). flushing). 5 5 1400 1400 ) MSSP MSEP MSAD Q7 )

3 MSSP MSEP MSAD Q7 3 (m (m

7 4 1200 4 1200 7

1000 1000 3 3 800 800

2 600 2 600

400 400 MASS OF SALT(kg) MASS OF SALT(kg) 1 1 200 200 QUANTIT Y OF BRINE, Q QUANTIT Y OF BRINE, Q

0 0 0 0 4/8 9/2 5/5 1/5 6/1 9/7 4/8 9/2 5/5 1/5 6/1 9/7 2/18 5/27 7/15 12/9 1/27 3/17 6/23 8/11 9/29 2/23 4/13 7/20 2/18 5/27 7/15 12/9 1/27 3/17 6/23 8/11 9/29 2/23 4/13 7/20 10/21 11/17 10/26 12/14 10/21 11/17 10/26 12/14 First year Second year Third year First year Second year Third year

DATE OF INJECTION DATE OF INJECTION Figure (8d): Quantity of salt and brine required to SP and Figure (8c): Quantity of salt and brine required to SP and that provided by EP for TESP Design (Seawater surface that provided by EP for Summer Design (Seawater surface flushing). flushing). 10000 10000 9000 MSEP MSAD 9000 MSEP MSAD 8000 8000 95% 93% 7000 7000 6000 79% 6000 85% 5000 89% 92% 73% 5000 4000 87% 4000 3000 3000 MASS OF SALT (kg)

2000 30% 32% MASS OF SALT (kg) 2000 44% 39% 1000 1000 0 0 Year YE FirstAR(1), YEsecoAR(2),nd YE ThirdAR(3), YEFirstAR(1), Second YEAR(2), ThirdYEAR(3), fresh water fresh water fresh water seawater seawater seawater Year YE FirstAR(1), YE secondAR(2), YE ThirdAR(3), FirstYEAR(1), Second YEAR(2), Third YEAR(3), fresh water fresh water fresh water seawater seawater seawater

Fresh Water Sea Water Fresh Water Sea Water Figure (9b): Quantity of salt provided by EP and that added Figure (9a): Quantity of salt provided by EP and that added to SP for both types of surface water flushing under three to SP for both types of surface water flushing under three years of operation for Autumn Design. years of operation for Spring Design.

10000 10000 9000 MSEP MSAD 9000 MSEP MSAD 8000 8000 7000 7000 88% 6000 6000 5000 5000 69% 4000 4000 48% 3000 44% 3000

44% MASS OF SALT (kg) MASS OF SALT (kg) 2000 33% 2000 36% 33% 26% 19% 1000 21% 1000 20% 0 0 YEAR(1), YEAR(2), YEAR(3), YEAR(1), YEAR(2), YEAR(3), Year YE A FirstR(1), YE secondAR(2), YE ThirdAR(3), YE FirstAR(1), SecondYEAR(2), ThirdYEAR(3) Year First second Third First Second Third fresh water fresh water fresh water seawater seawater seawater fresh water fresh water fresh water seawater seawater sea water

Fresh Water Sea Water Fresh Water Sea Water Figure (9d): Quantity of salt provided by EP and that Figure (9c): Quantity of salt provided by EP and that added to SP for both types of surface water flushing under added to SP for both types of surface water flushing three years of operation for TESP Design. under three years of operation for Summer Design.

8 30000 MSEP MSAD 25000

20000

15000 % % % 74 76 71 % 10000 % 61 MA SS OF S AL T (k g) 62 % % 5000 % 36 33 32

0 Spring Summer Autumn TESP Spring Summer Autumn TESP Design Design Design Design Design Design Design Design

Fresh Water Sea Water Figure (10): Quantity of salt provided by EP and that added to SP for both types of surface water flushing under different design conditions. 45 450 40 400 Ar (Spring Design) 35 350 Ar (Summer Design) 30 300 Ar (Autumn Design) 25 250 20 200 15 Depth =1.5 m 150

10 Depth =1.0 m AREA RATIO , Ar CONCENTRATION , % Depth =0.5 m 100 5 Depth =0.2 m 50 0 1

61 0 121 181 241 301 361 421 481 541 601 661 721 781 841 901 961 First year Second year Third year 1021 1081 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

DAYS DEPTH (m) Figure (11b): Variation of Area Ratio with Depth for Figure (11a): Variation of Area Ratio with Depth for Different Design conditions for Seawater surface Flushing. Different Design conditions for Fresh water surface Flushing. 45 12 40 Ar (Spring Design) 10 35 Ar (Summer Design) 30 8 Ar (Autumn Design) 25 20 6 Depth =1.5 m 15 Depth =1.0 m 4

10 AREA RAT I O , Ar

CONCENTRAT ION, % Depth =0.5 m 5 Depth =0.2 m 2 0 0 1 61 121 181 241 301 361 421 481 541 601 661 721 781 841 901 961

1021 1081 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 First year Second year Third year DAYS DEPTH (m)

Figure (12b): Daily variation of salt concentrations with Figure (12a): Daily variation of salt concentrations with depth in EP started at JAN with primary salt concentration depth in EP started at JAN with primary salt concentration (7%), brine reject from RO-Plant for seawater surface (7%), brine reject from RO-Plant for fresh water surface flushing under Autumn design for three years of operation. flushing under Autumn design for three years of operation.

9 4. CONCLUSIONS: Tabor H., (1981), “ Solar Ponds”, Solar Energy, Vol. 27, No. Considering the results presented for each set of 3, pp. 181-194. circumstances, the following conclusions were drawn: Tabor, H., (1963), “ Large Area Solar Collectors for Power 1. The net evaporation rate is the dominant factor Production”, Solar Energy, Vol. 7, No.4, pp. 189-194. during summer months while salt diffusion rate is the dominant factor during winter season. Weinberger H., (1965), “ The physics of solar ponds”, Solar 2. In all cases, the required salt can not be met during Energy 8, PP. 45-56. winter months. 3. In all cases, the EP can not be used for the first six months of operation due to very low concentration levels. 4. Although the model used was developed to be used under the closed loop condition, but due to the high quantities of flushing by sea-water part of the overflow was rejected. 5. Very large EP areas are required for seawater surface flushing. 6. Due to high net evaporation rates in summer months, the required number of surface flushing the SP is five times in summer, while it is estimated to be from two to three times in winter months. 7. EP can provide 20% to 40% of the salt required by SGSP during the first year of operation and 45% to 95% during the third year of operation depending on the design selected. 8. Under the assumption of 16.6 (kg/m2-year) for the natural rate of up ward salt diffusion (assumed by TESP team), the brine must be injected from EP to the LCZ every 49 days or earlier to keep the pond stable. 9. For EP saturated brine of (26%) salt concentration, the quantity of the brine to be injected to LCZ is about 3.4 (m3). 10. Reducing the depth of EP improves the capability of EP for brine re-concentration.

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, vol 53, PP. 343 –351. Claire Batty, J., Paul Riley, J., and Panahi, Z., (1987), “A Water Requirement Model for salt gradient solar ponds”, Solar Energy 39, PP. 483-489. Hassab et al, (1989), “Problems encountered in operating Salt Gradient Solar Ponds in The Arabian Gulf Region”, Solar Energy, Vol. 43, No. 3, pp. 169-181. Newell, T.A. et al, (1994), “ Characteristics of a Solar Pond Brine Re-concentration System”, ASME, J. of Solar Energy, Vol. 116, pp. 69-73. Rable A. and Neilsen C., (1975), “Solar Ponds for Space Heating”, Solar Energy 17, PP. 1-12. Schladow S. G., (1984), “The Upper Mixed Zone of a Salt Gradient Solar Pond: its Dynamics, Prediction and Control”, Solar Energy, Vol.33, No.5, pp.417-426.

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