Experimental and numerical analysis of sodium-carbonate salt gradient solar-pond performance under simulated solar-radiation

APPLIED ENERGY

Applied Energy 83 (2006) 324–342

www.elsevier.com/locate/apenergy

Experimental and numerical analysis of sodium-carbonate salt gradient solar-pond performance under simulated solar-radiation Hu¨seyin Kurt a

a,*

, Mehmet Ozkaymak a, A. Korhan Binark

b,1

Zonguldak Karaelmas University, Technical Education Faculty, 78200 Karabuk, Turkey Marmara University, Technical Education Faculty, 34722 Kuyubasi-Istanbul, Turkey

b

Available online 13 June 2005

Abstract The objective of this study is to investigate experimentally and theoretically whether sodium carbonate (Na2CO3) salt is suitable for establishing a salinity gradient in a salt-gradient solar-pond (SGSP). For this purpose, a small-scale prismatic solar-pond was constructed. Experiments were conducted in the laboratory under the incident radiation from two halogenlamps acting as a solar simulator. Furthermore, a one-dimensional transient mathematical model that describes the heat and mass transfer behaviour of the SGSP was developed. The differential equations obtained were solved numerically using a finite-difference method. It was found from the experiments that the density gradient, achieved using sodium carbonate salt, can suppress convection from the bottom to the surface of the pond.
2005 Elsevier Ltd. All rights reserved. Keywords: Salt-gradient solar-pond; Sodium-carbonate; Indoor experiment; Solar simulator; Transient heat-and-mass transfer model

*

1

Corresponding author. Tel.: +90 370 4338200; fax:+90 370 4338204. E-mail address: [email protected] (H. Kurt). Tel.: +90 216 3365770/666; fax: +90 216 3378987.

0306-2619/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2005.03.001

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Nomenclature a Ca Cp D hc he I I(x) I0 Ir Is J k L N P1 Patm Ps q Qc Qe Qr Qt QU Rh t T T1 TLCZ Ts Tsky TUCZ Vr x XLCZ XNCZ XUCZ q_ b Dt

reflection factor of pond
s surface (Albedo) (%) specific heat of air (J/kg C) specific heat of the solution (J/kg C) coefficient of salt diffusion (m2/s) convective heat-transfer coefficient (W/m2 C) latent heat of evaporation of water (J/kg) solar-radiation intensity (W/m2) solar-radiation intensity at depth x (W/m2) available solar-energy below surface after reflection from pond
s surface (W/m2) reflected solar-radiation intensity from the pond
s surface (W/m2) incident solar-radiation intensity at the pond
s surface (W/m2) diffusion flux (kg/m2s) thermal conductivity of the solution (W/m C) depth of the pond (m) number of cells in the NCZ partial pressure of water vapor in ambient air (Pa) atmospheric pressure (Pa) vapor pressure of water at the surface
s temperature Ts (Pa) heat flux (W/m2) heat loss by convection (W/m2) heat loss by evaporation (W/m2) heat loss by radiation (W/m2) total heat-losses from the pond
s surface (W/m2) heat extracted from the storage zone relative humidity (%) time (s) temperature (C) ambient temperature (C) LCZ temperature (C) pond
s surface temperature (C) sky temperature (C) UCZ temperature (C) wind velocity (m/s) depth (m) thickness of the LCZ (m) thickness of the NCZ (m) thickness of the UCZ (m) internal heat-generation rate (W/m3) fraction of energy absorbed at the pond
s surface (%) time step (s)

326

Dx ew l hg hk q r

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thickness of layer (m) emissivity of water absorption coefficient (m
1) radiation incident-angle over pond
s surface () angle of refraction at the pond
s surface () density of solution (kg/m3) Stefan–Boltzman constant (W/m2 K4)

1. Introduction A salt-gradient solar-pond (SGSP) is an inexpensive solar-energy collection and storage system for low-temperature heat-sources. It has a shallow, large body of water in which a stable salinity-gradient is artificially established in order to prevent thermal convection induced by the absorption of solar radiation. Thus, the pond acts as a trap for solar radiation. Thermal energy is collected and stored in the lower layers of the pond, and the capacity for long-term energy storage is a major attractive feature of a SGSP. This long-term store provides an alternative for conventional energy-sources [1–3]. The SGSP generally consists of three distinct zones: the upper convective zone (UCZ), non-convective zone (NCZ), and the lower convective zone (LCZ), as shown in Fig. 1. The UCZ is the topmost layer and usually a thin layer of fresh water. The NCZ is just below the UCZ and has linearly increasing salinity gradient downwards.

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It acts as transparent insulation to prevent heat loss due to convection from the LCZ. The LCZ is the bottom layer, with a nearly constant and uniform high density. Because of serving as the solar-energy collection and heat storage medium, it is also called the storage zone. Solar radiation is transmitted through the UCZ and NCZ and then trapped in the LCZ. As a result of solar-radiation absorption, a gradient of temperature is established. In the NCZ, the density decreases due to the temperature gradient producing an upward buoyancy force. This force in counterbalanced by the increase in the density due to the salinity gradient increasing in the downward direction. Thus convection currents are suppressed and prevent convection heat-loss from the LCZ by the artificially established salinity gradient. Heat stored in the LCZ only escapes by conduction. Since water has a low thermal-conductivity, the NCZ acts as a transparent insulator, so allowing a considerable amount of incident solar radiation being trapped and stored in the form of heat in the LCZ [1–3]. SGSPs have been extensively studied because of their excellent heat collection and storage performances. There have been considerable theoretical and experimental studies [3–12] on SGSPs, which include analytical and numerical model treatments, laboratory testing and construction and economic analyses, to gain a better understanding of the mechanism of their operation and applications. Many experimental solar ponds [13–19] have been constructed, instrumented and operated, and various numerical models [20–30] have been developed for analysing SGSP performance in the literature. The numerical models initially were generally onedimensional (1D) and treated the problem of transient heat conduction and mass diffusion. The 1D transient heat-and-mass transfer equations were solved using a finite-difference method to predict the time-dependent temperature and density. Tasdemiroglu [1] reported salt availability and solar pond utilization in Turkey. Kurt et al. [2,3], Bozdemir and Kayali [4] investigated the performance of the sodium-chloride SGSP experimentally and theoretically. Kanayama et al. [5] have analyzed practical-scale sodium chloride SGSP performance. Leshuk et al. [6] investigated experimentally the stability of the salinity gradient, with established potassium nitrates salt, under a solar simulator. Keren et al. [7] carried out an indoor experiment and a numerical analysis on a small-scale model of a magnesium-chloride SGSP under a solar simulator. Xiang et al. [8,9] conducted an indoor experiment and a numerical analysis of a small-scale SGSP using NaCl salt, and examined the erosion phenomenon on the NCZ under incident radiation from a solar simulator. Kho et al. [10] studied the design and performance evaluation of a solar pond, containing sodium chloride salt, for industrial process-heating experimentally and numerically. Tahat et al. [11] investigated experimentally and theoretically the performance of a portable mini solar-pond. Kumar and Kishore [12] constructed a 6000 m2 solar pond for a milk-processing dairy plant to supply process heat, and demonstrated the technical and economic viability of solar-pond technology in India. Subhakar and Murthy [13,14] investigated a saturated solar-pond, with magnesium chloride (MgCl2) and potassium nitrate (KNO3) salts, theoretically and experimentally. Banat et al. [15] studied experimentally the temperature and salinity

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profiles in the pond in which a salinity gradient is established using carnalite salt. Pawar and Chapgaon [16], as well as Murthy and Pandey [17] have evaluated experimentally the performances of solar ponds using fertilizer salt under simulated conditions. Hassairi et al. [18] experimentally investigated the performance of small-scale solar pond of natural brine. Lund et al. [19] measured the spectral transmittances of magnesium chloride (MgCl2), sodium sulphate (Na2SO4), sodium nitrate (NaNO3), potassium nitrate (KNO3) and sodium carbonate (Na2CO3) saltsolutions under a solar simulator. Mansour et al. [20] investigated numerically the transient behaviours of the thermal and salinity fields, and the stability of the SGSP. Jubran et al. [21] developed a three-dimensional finite-volume method for modelling the convective layers in the solar pond. Hongfei et al. [22] studied the performance of heat collection and storage of a SGSP based on similar methods to analyse and calculate the flat-plate solar-collector performance. Angeli and Leonardi [23] developed a 1D transient mathematical model for investigating the salt diffusion and stability of the density gradient in a solar pond. Husain et al. [24] studied the estimation of radiation flux in solar ponds and proposed a simple empirical formulation. E1-Refaee et al. [25] developed a 1D transient mathematical-model for predicting the thermal performance of the SGSP and the obtained results from the model are compared with those from an experimental study. Alkhalaileh et al. [26] developed a computer simulation model, and analysis of a solar-pond floor heating system. Hawlader et al. [27] solved the basic energy-equation numerically and studied the pond
s behaviour. Antonopoulos and Rogdakis [28] developed simple correlations that express the maximum useful-heat received from a SGSP throughout the year. Subhakar and Murthy [29] described a 1D simulation procedure for a saturated solar-pond. Alagao [30] developed a 1D simulation model, which simulates the transient behaviour of the pond using a finite-difference method, for a closed-cycle SGSP. Solar ponds normally employ sodium-chloride salt (NaCl). Various salts, like magnesium chloride (MgCl2), potassium nitrate (KNO3), ammonium nitrate (NH4NO3), sodium nitrate (NaNO3), fertilizer salts as urea (NH2CO Æ NH2), satisfy the stability criterion and hence are considered suitable for a solar pond. After reviewing the literature, it is seen that establishing the SGSP
s densitygradient with sodium carbonate salt has not been tested. Hence, for this purpose, a small-scale pond in laboratory conditions was constructed for the experimental work and also a 1D mathematical model for the SGSPs heat and mass transfer was developed.

2. A mathematical model for the SGSP A model of transient behaviours of the heat and mass transfers in a SGSP was developed. Fig. 2 shows the configuration of the pond under consideration. The mathematical model is based upon energy and mass balances over a horizontal fluid layer in the vertical direction. Because of various processes occurring in and out of

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Fig. 2. Salt-gradient solar-pond model configuration for heat flux.

the pond, the operation of the pond is usually complicated. Therefore, some assumptions were made for simplifying the analysis as follows:  The temperature variation along the y-direction is considered small enough so that it is negligible. Therefore, the temperature and salinity distributions within the pond are 1D.  The pond has three distinct zones, which are the LCZ, NCZ, and UCZ, and the coordinates of the zone boundaries are fixed.  Heat losses through the pond
s sidewalls are considered small enough, due to all the sides of the pond being well insulated, to be considered negligible. Heat loss only occurs from the pond
s surface due to convection, evaporation and radiation.  The bottom surface is blackened in order to maximize the radiation absorption. Therefore, the radiation energy reaching the LCZ is completely absorbed by the solution and the bottom of the pond.  The pond is artificially stabilized by a density gradient, so that the convection currents can be considered negligible and remain as such during the period of operation.  The physical properties of the salt solution like density, specific heat, thermal conductivity and salt diffusivity, do not vary with temperature and salinity.  Due to the presence of convection, the temperatures of the UCZ and LCZ are likely to be uniform. Therefore, the UCZ and LCZ are considered as a single cell, and which have thicknesses of XUCZ and XLCZ. The NCZ is divided into five equal finite cells, each of size Dx. Conservation of energy and mass apply for each cell. The total depth of the pond is L.

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3. Heat-transfer model Under the prescribed assumptions, application of the energy balance for a small layer in the NCZ, then oT ðx; tÞ Dx; ot
 
  dqx dIðxÞ oT ðx; tÞ Dx ¼ qC p Dx; qx
q x þ Dx þ IðxÞ
IðxÞ þ dx ot dx ½ðqx Þi
ðqxþDx Þi  þ ½ðI x Þi
ðI xþDx Þi  ¼ qC p

and


 o oT ðx; tÞ oT ðx; tÞ k ; þ q_ ¼ qC p ox ox ot

ð1Þ ð2Þ

ð3Þ

_ internal heat-generation term, which represents the absorption rate of the where q, solar radiation per unit volume at a depth x from the pond
s surface. It is given by oIðxÞ q_ ¼
; ð4Þ ox where I(x) is the solar energy radiation intensity at depth x, in the solution, which is a time and depth dependent function. Radiation attenuation in the pond is calculated as an exponential decay, following a simplified equation of Beer
s Law IðxÞ ¼ ð1
aÞð1
bÞI s e
lx

for x > 0;

ð5Þ

where I(x) is the solar-radiation intensity at depth x, Is is the incident solar-radiation intensity at the pond
s surface, a is the reflectance of solar radiation at the surface, b represents the long-wave fraction of Is which is absorbed very close to the surface, and where l is the attenuation or extinction coefficient. The extinction coefficient normally depends on the wavelength of the radiation. A single extinction coefficient is used to describe the absorption of radiation. It is assumed that a fraction, b, of the radiation, is absorbed within a depth d (=5 mm) from the surface of the pond and, the remaining radiation is absorbed within a depth d from the surface of the pond and the remaining radiation follows an exponential decay. For the present tests, a = 3–10%, b = 0.5 and l = 0.7 were used. The thermal process in the SGSP can be treated as a 1D unsteady-conduction problem with heat generation in to the proposed mathematical model. The solution of this equation requires an initial and two boundary conditions. The initial condition is the initial pond-temperature, which equals the ambient temperature at the time of initiating the pond
s operation. The first boundary-condition is specified at x = L1 (UCZ–NCZ interface) and the second boundary-condition is specified at x = L2 (NCZ–LCZ interface). For both boundary-conditions, an energy balance was applied at each pond depth. The first boundary-condition is obtained from energy balance over the UCZ as    oT ðx; tÞ oT ðx; tÞ xUCZ ; k þ Ijx¼0
Ijx¼L1
Qt ¼ qC p ð6Þ  ox x¼L1 ot

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where Qt is the heat loss from the pond
s surface by convection, evaporation and radiation. The second boundary condition is obtained from an energy balance over the LCZ as given by    oT ðx; tÞ oT ðx; tÞ xLCZ .
k þ Ijx¼L2
Ijx¼L3 ¼ qC p ð7Þ  ox x¼L2 ot

4. Heat-loss calculation The model solar-pond is assumed to be well insulated: heat loss from the pond
s surface occurs due to convection, evaporation and radiation. Thus, the total heatloss can be calculated as follows [2,3,20]: Qt ¼ Qc þ Qe þ Qr .

ð8Þ

The convective heat-loss is given by Qc ¼ hc ðT y
T 1 Þ;

ð9Þ

where hc is the wind convection heat-transfer coefficient, which depends on the velocity of wind, is given by hc ¼ 5.7 þ 3.8V r .

ð10Þ

The heat loss due to evaporation is proportional to the wind-induced convective heat-transfer coefficient hc and the difference between the vapour pressure of the free surface and the partial pressure of the water vapour in the atmosphere. The evaporative heat loss can be expressed as follows: Qe ¼

he hc ðP s
P 1 Þ ; 1.6C a P atm

where Ps is the vapour pressure evaluated at the surface temperature
 3885 P s ¼ exp 18.403
T s þ 230

ð11Þ

ð12Þ

and P1 is the partial pressure of water vapour in the ambient air obtained at the ambient temperature
 3885 P 1 ¼ Rh exp 18.403
. ð13Þ T 1 þ 230 Heat loss due to radiation from the pond
s surface to the sky can be calculated from the following expression:   4 4 Qr ¼ ew r ðT s þ 273.15Þ
ðT sky þ 273.15Þ . ð14Þ The sky temperature is estimated as follows: pffiffiffiffiffiffiffi 0.25 T sky ¼ T 1 þ ð0.55 þ 0.704ð P 1 ÞÞ .

ð15Þ

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5. Mass-transfer model There are several physical processes occurring in the operation of a solar pond. Convective mass-transfer occurs in the LCZ and UCZ and diffusive mass-transfer in the NCZ. The density gradient could develop by molecular diffusion. In this model, the total mass of the system in the control volume is constant, and the mass transfer takes place as a result of molecular diffusion. The mass-transfer processes are independent of the thermal processes. Based upon these assumptions, 1D mass diffusion in the x-direction for a differential volume-element of thickness, Dx, as illustrated in Fig. 6, is given as follows:
 oqðx; tÞ ðJ x
J xþDx Þ ¼ Dx; ð16Þ ot



 ðJ jxþDx
J jx Þ oqðx; tÞ ; ¼ ot Dx

ð17Þ




oJ oqðx; tÞ ¼ . ox ot

ð18Þ

By Fick
s law of diffusion, the diffusion flux J is related to the density gradient by J ¼
D

oqðx; tÞ . ox

ð19Þ

Substituting from Eq. (19) into Eq. (18) and assuming a constant D, the following equation. can be obtained,
 o oqðx; tÞ oqðx; tÞ D . ð20Þ ¼ ox ox ot The solution of the mass diffusion equation needs an initial condition and two boundary conditions. The initial condition is specified by a linear density-gradient of the form, as follows: q ¼ q1 ;

0 6 x < L1 ;

ð21Þ

q ¼ q2 ;

L2 < x 6 L3 .

ð22Þ

The first boundary-condition is specified at x = L1 (UCZ–NCZ interface) and the second at x = L2 (NCZ–LCZ interface). For both boundary-conditions, a mass balance was applied at each point. A mass balance at the UCZ gives the first boundary condition, as follows: D

oqðx; tÞ oqðx; tÞ ¼ X UCZ ; ox ot

x ¼ L1 ; t > 0.

ð23Þ

A mass balance at the LCZ gives the second boundary-condition, as follows:
D

oqðx; tÞ oqðx; tÞ ¼ X LCZ ; ox ot

x ¼ L2 ; t > 0.

ð24Þ

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333

Fig. 3. Salt-gradient solar-pond model configuration for mass transfer.

Equation (24) is of exactly the same form as the 1D unsteady heat-conduction equation: the same mathematical techniques are applicable for its solution. The equations obtained from the heat and mass transfers are solved numerically to determine the temperature and density profiles within the pond. The method used in generating solutions to the 1D temperature and density finite-difference equations is explicit because unknown nodal variables for a new time are calculated using the known values of the parameters at a previous time. A finite-difference form of the differential equation is derived by integration over the control volume surrounding the typical node i, as shown in the grid of Figs. 2 and 3. A Fortran computerprogram has been developed for the aforementioned reasons. A layer increment of 5 cm and a time step of 1 h were used in the model. The stability criterion of explicit formulation is Dt < qCp(Dx)2/2k.

6. Experimental study Experiments were carried out under the laboratory conditions using a scale solarpond of dimensions 60 · 50 cm2 and 60 cm deep as shown in Fig. 4. This pond was constructed from 1.5 mm galvanized metal sheet. Inside of the pond was painted black to ensure absorption of the radiation, while the outside was insulated with 20 mm thick glass–wool and 30 mm thick styrofoam to reduce the rate of heat loss. The pond was subjected to a simulator solar-radiation spectrum close to that of solar radiation. A low-cost solar simulator, which has 2 · 1000 W, 220–230 V, 6.5 A and 25,000 ml Philips halogen lamps, adjustable on the vertical axis above the pond
s surface, was designed. The simulator was installed 35 cm above the pond
s surface. The incident radiation intensity was measured with a Solar-130 type pyranometer of accuracy ±1.5 W/m2. The corresponding radiant-flux intensity is 750 W/m2.

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Fig. 4. Cross-section of experimental solar pond.

The temperatures were measured by chromel–alumel (K-type) thermojunctions, fixed at 8 points on a vertical side wall of the pond, with an accuracy of ±0.3 C. The thermojunctions are spaced 10 cm apart from top to bottom of the pond: they provide a clear temperature profile by connecting the thermocouples to a digital multimeter (Mastech MY-64 type). Density profiles were determined by analyzing the densities of small samples extracted from the solar pond at the same level as the thermojunctions. Flexible plastic tubes, attached 10 cm apart, acted as sampling vents at the other sidewall of the pond. The density of the withdrawn solution was determined by measuring the mass of a given volume to an accuracy of ±10
4 g. The volume was measured with a 10 ml pycnometer to an accuracy of ±0.2 ml. Density profiles were taken twice a day while the pond was subjected to radiation, and at regular intervals while cooling. The pond was filled layer by layer, starting with the layer of highest concentrated solution to fill the LCZ. Next, the NCZ was established by painstakingly pouring slowly a decreasingly less-concentrated solution from a floating plastic can. The NCZ is formed of five layers. Lastly, the UCZ is filled with fresh water on top of the NCZ in the same way as the NCZ. The thicknesses the UCZ, NCZ and LCZ are 10, 25 and 25 cm, respectively. Then the pond was covered by a non-transparent plastic sheet to prevent radiation from heating up the solution. The pond remained covered for three days to allow molecular diffusion of the salt to take place and to achieve a linear salt-gradient. Subsequently, the pond was subjected to solar simulator radiation.

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7. Results and discussion After reviewing the literature, it is seen that achieving the SGSP density-gradient with sodium-carbonate salt has not previously been tested. So, four experiments with the solar pond having different ranges of density gradient have been conducted. In Tables 1–4, the distributions of salinity and density of sodium carbonate solutions

Table 1 The distributions of salinity and density in the pond zones for the first experiment Density (kg/m3)

Pond zone

Pond zone thickness (cm)

Salinity (%)

UCZ

10

0

998

NCZ

25

2 4 4 6 6

1036.8 1052.6 1052.6 1068.3 1068.3

LCZ

25

8

1081

5 5 5 5 5

Table 2 The distributions of salinity and density in the pond zones for the second experiment Pond zone

Pond zone thickness (cm)

UCZ

10

NCZ

25

LCZ

25

Salinity (%)

5 5 5 5 5

Density (kg/m3)

0

998

2 4 4 6 8

1036.8 1052.6 1052.6 1068.3 1081

10

1108

Table 3 The distributions of salinity and density in the pond zones for the third experiment Pond zone

Pond zone thickness (cm)

UCZ

10

NCZ

25

LCZ

25

Salinity (%)

5 5 5 5 5

Density (kg/m3)

0

998

3 5 5 7 7

1042 1063 1063 1079.5 1079.5

12

1120

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Table 4 The distributions of salinity and density in the pond zones for the fourth experiment Pond zone

Pond zone thickness (cm)

UCZ

10

NCZ

25

LCZ

25

Salinity (%)

5 5 5 5 5

Density (kg/m3)

0

998

3 5 8 10 12

1042 1063 1081 1108 1120

16

1157

that filled the pond are seen. As seen from the tables, the salinity difference between the surface and the bottom of the pond amount to 8% for first experiment, 10% for second experiment, 12% for third experiment and 16% for last experiment. A higher salinity range cannot be achieved because thin crystal solid layers, then form at the base of the pond. The density and temperature profiles as functions of the pond
s depth are shown in Figs. 5 and 6, respectively. In Fig. 5, the density profile initially looks like stair steps, then it starts to turn to a shape like the SGSP density profile after the first day. After the fourth day, a stable density-gradient was formed as a result of salt diffusion from the bottom to the surface. The temperature profile after the pond is subjected to the solar simulator radiation for a day is illustrated in Fig. 6. Figs. 7 and 8 show the density and temperature profiles for the second experiment. In the first experiment, the density gradient with a 10% of salinity range cannot be enough to suppress convection currents that occur as a result of the radiation absorption in the LCZ. However, when a comparison has made between the first and fourth day
s experimental results, the temperature profile has tended to the SGSP temperature profile.

Fig. 5. Density profile in experiment 1.

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Fig. 6. Temperature profile in experiment 1.

Fig. 7. Density profile in experiment 2.

Fig. 8. Temperature profile in experiment 2.

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H. Kurt et al. / Applied Energy 83 (2006) 324–342

The density and temperature profiles provided from the third experiment are given in Figs. 9 and 10. It is more stable than the first and second density profiles and more suitable for a SGSP. A similar situation was observed in the fourth experiment. A salinity range with a 12% density gradient is enough to store the radiation absorbed in the form of heat at the LCZ. After seven days
observations, the temperature difference was measured between the pond
s bottom and its surface as 10 C. The experiment ceased at the seventh day due to the storage temperature remaining unchanged. The density gradient with 12% salinity range prevented convection from the bottom to the surface. The pond
s temperature-profile during the cooling period, which shows a similar characteristic as with the SGSP was kept for a long time. From the results, at least a 12% salinity range to establish a worthwhile density gradient between the pond
s bottom and surface is needed to store heat in the SGSP containing a sodium-carbonate solution.

Fig. 9. Density profile in experiment 3.

Fig. 10. Temperature profile in experiment 3.

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339

Fig. 11 shows a stable density-gradient. The temperature profile remained unchanged after the sixth day and a 12 C temperature difference ensued on the seventh day (see Fig. 12). The comparisons of density and temperature profiles for the fourth experiment with the results of the model are shown in Figs. 13 and 14. The results are qualitatively in good agreement, but a small difference was seen quantitatively due to the physics of the solar pond. The model storage temperature was higher than the measured storage temperature due to the temperature values measured in the NCZ and UCZ being larger than the corresponding temperatures provided from the model. This is a result of solar simulator radiation being used as the energy source in the experiments. The simulator spectrum has a bias towards the long-wavelength (infrared) radiation compared with that of the Sun. The long wavelength range of

Fig. 11. Density profile in experiment 4.

Fig. 12. Temperature profile in experiment 4.

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Fig. 13. The comparison of model-experiment temperature profiles.

Fig. 14. The comparison of model and experiment density profiles.

simulator radiation is mostly absorbed by the UCZ, and the temperature rise of the UCZ occurs quickly. On the contrary, only small part of the simulator radiation is transmitted to the LCZ, so that the temperature rise of the LCZ occurs slowly.

8. Conclusions Sodium-carbonate salt has been used in the establishment of salinity gradients in the SGSP. Four different density-gradients with sodium-carbonate solution were initially established in the pond with salinity ranges of 8%, 10%, 12% and 16%. In the first experiment, a temperature-gradient similar to that of SGSP in the pond was not obtained. However, a slow conversion to a temperature gradient similar to the SGSP

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temperature-gradient was observed in the second experiment. In the third experiment, temperature differences were observed between the bottom and the surface of the pond of around 10 C. In fourth experiment, this difference became 12 C. In order to be able to store heat in the SGSP with sodium carbonate solution, a density gradient with a salinity range of 12% between the bottom and surface of the pond is necessary. In order to verify the validity of the experimental results, a 1D time-dependent mathematical model for heat-and-mass transfers, based on energy and mass balances, was developed. Differential equations constituting the model were solved using a finite-difference method. The numerical results were compared with the experimental results. It is seen that experimental and numerical results were well correlated. The results are in harmony, but small differences between the surface and bottom of the pond in the model became higher than the experimental temperature differences. The solar simulator radiation that is used as the energy source in the experiments causes this dissimilarity the between model and experiment profiles. If a suitable density-gradient with sodium-carbonate salt is established, a considerable amount of the incident solar radiation can be trapped and stored as heat energy in the LCZ over a long time-period.

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