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Performance assessment of a magnesium chloride saturated solar pond
Renewable Energy 78 (2015) 35e41
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance assessment of a magnesium chloride saturated solar pond Ismail Bozkurt a, Sibel Deniz b, Mehmet Karakilcik b, *, Ibrahim Dincer c a
Department of Mechanical Engineering, Faculty of Engineering, University of Adiyaman, Adiyaman 02040, Turkey Department of Physics, Faculty of Sciences and Letters, University of Cukurova, Adana 01330, Turkey c Faculty of Engineering and Applied Science, University of Ontario Institute of Technology (UOIT), 2000 Simcoe Street, North Oshawa, Ontario L1H 7K4, Canada b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2014 Accepted 27 December 2014 Available online
This paper deals with the experimental investigation of a magnesium chloride saturated solar pond and its performance evaluation through energy and exergy efficiencies. The solar pond system is filled with magnesium chloride containing water to form layers with varying densities. A solar pond generally consists of three zones, and the densities of these zones increase from the top convective zone to the bottom storage zone. The incoming solar radiation is absorbed by salty water (with magnesium chloride) which eventually increases the temperature of the storage zone. The high-temperature salty water at the bottom of the solar pond remains much denser than the salty water in the upper layers. Thus, the convective heat losses are prevented by gradient layers. The experimental temperature changes of the solar pond are measured by using thermocouples from August to November. The densities of the layers are also measured and analysed by taking samples from at the same point of the temperature sensors. The energy and exergy content distributions are determined for the heat storage zone and the nonconvective zone. The maximum exergy destructions and losses appear to be 79.05 MJ for the heat storage zone and 175.01 MJ for the non-convective zone in August. The energy and exergy efficiencies of the solar pond are defined as a function of solar radiation and temperatures. As a result, the maximum energy and exergy efficiencies are found to be 27.41% and 26.04% for the heat storage zone, 19.71% and 17.45% for the non-convective zone in August, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Magnesium chloride Solar pond Heat storage Energy Exergy Efficiency
1. Introduction Renewable energy resources and technologies have a key role to play in meeting current and future energy needs. They cause less environmental impact than the conventional energy sources [1]. Turkey, situated on the sunny belt between 36 N and 42 N latitude, is located in a relatively more advantageous geographical location for harvesting solar energy. Especially, Mediterranean and Aegean Sea coastal zones have very high potential for utilization of solar energy [2]. During the past decades, solar energy sources have gained greater importance to meet the energy demands for various sectors. Solar energy systems have used for several applications like heating water, warming greenhouses, drying, water desalination,
* Corresponding author. Tel.: þ90 322 338 60 84; fax: þ90 322 338 60 70. E-mail addresses: [email protected] (I. Bozkurt), fzksibel.deniz@ hotmail.com (S. Deniz), [email protected] (M. Karakilcik), [email protected] (I. Dincer). http://dx.doi.org/10.1016/j.renene.2014.12.060 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
power generation and so on. One of these systems is solar pond where the main goal is to heat a large mass of water [3]. In this regard, various studies on solar ponds have been undertaken and their heat transfer and thermodynamic aspects have been investigated by various researchers [4e14]. Singh et al. [15] designed and tested a combined system of thermosyphon and thermoelectric modules for the generation of electricity from low-grade thermal sources like solar pond. Bozkurt and Karakilcik [16] investigated an integrated solar pond where heat collected from the flat-plate solar collector is transferred to the storage zone through a heat exchanger system. The system performance was determined both experimentally and theoretically according to the number of flatplate collectors. Karakilcik et al. [17] studied a solar pond performance with and without shading effect and comparison of the energy efficiencies of the experimental solar pond. Thus, the shading effect and its ratios were determined to study the efficiency of the solar pond. Most of these studies aimed to investigate the performance of the solar ponds and study the effects of varying
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operating conditions and thermophysical properties on the pond performance. Sodium chloride has been the most commonly used salt in most of these studies. In a few studies, however, magnesium chloride salt has been used. Subhakar and Murthy [18e20] studied saturated solar pond by using magnesium chloride and potassium nitrate salts for the experimental testing and performance assessment. A comparison was also made with an unsaturated solar pond. The temperature and concentration gradients were developed by heating the pond from the bottom and adding finely powdered salt from the top. Most of the experimental studies presented in the literature focuses on energetic performance evaluation of the ponds. In this study, an experimental magnesium chloride saturated solar pond is constructed, built and employed for experimental investigations, and its performance is investigated and evaluated through energy and exergy efficiencies. Although the energetic performance of the magnesium chloride solar pond was studied previously, the exergetic based analysis and assessment of a magnesium chloride saturated solar pond is the original contribution of this study. Since exergy analysis and assessment provide a true and meaningful picture of the system, we aim to compare the results with the energetic results where the first-law of thermodynamics (based on conservation law) is used. This is in fact the key motivation behind the present work. 2. Experimental system and procedure A solar pond is of various regions as types and sizes. It generally consists of two main regions as outer and inner regions. First, the outer region is called insulation region to prevent the heat losses by conduction from inner region to surrounding of the pond. Second, the inner region is a large body of salty water with a salinity gradient to prevent heat loss by convection. The body of the region generally consists of three zones (e.g., surface zone, middle zone and bottom zone). The surface zone is called as upper convective zone (UCZ). UCZ is the fresh water layer at the top of the pond. The middle zone is called as non-convective zone (NCZ). NCZ is composed of different salty water layers whose density gradually increases toward bottom of the pond. This zone plays a key role in the solar pond because this zone constitutes a transparent insulating layer to prevent convective heat losses from bottom zone to UCZ. In this regard, the size of NCZ is very important to increase the performance of a solar pond so that Husain et al. [21] developed a rational analytical insight for judicious selection of NCZ size considering optimum thermal performance as well as stability aspects. Finally, bottom zone of the pond is composed of salty water with highest density. Thanks to this feature, it is absorbed the solar radiation that reaches the bottom of the pond and converted as heat, and stored as heat storage zone (HSZ). In this study, an experimental solar pond with the area of 0.72 m2 and a depth of 1.10 m was built in Cukurova University in Adana, Turkey (i.e., 35180 E longitude, 37 050 N latitude). The pond's bottom and side-wall was insulated by using 0.10 m thickness glass wool. The pond temperature was measured at 7 points, starting from the bottom, at 0.25, 0.40, 0.55, 0.65, 0.75, 0.85 and 1.05 m heights by using thermocouples with an accuracy of about ±1 C. The pond was filled with the different densities magnesium chloride water to set up the density gradient of the pond in August 2012 and worked. Fig. 1 shows a schematic representation of the experimental solar pond system that built in salt production system. In the inner region of the pond, the ranges of magnesium chloride water density in UCZ, NCZ and HSZ are 1000e1020 kg/m3, 1030e1150 kg/m3 and 1170e1200 kg/m3, and the thicknesses of the zones are 0.10, 0.50, 0.50 m, respectively. The density distributions are also measured
Fig. 1. A schematic representation of the magnesium chloride saturated solar pond.
and analysed by taking samples from at the same point of the temperature sensors. As seen in Fig. 1 the salt gradient protection system is used to protect the density gradient against erosion of the magnesium chloride water in the inner zones of the pond. The protection system was a system based on the natural circulation of water caused by density difference, as it was first proposed by Akbarzadeh and Mac Donalds [22]. 3. Energy and exergy analyses The energy and exergy values of the solar pond are calculated using conservation of mass and energy principles as well as second law analysis. Therefore, in this section, energy and exergy analyses of the solar pond, through the balance equations, are presented. The energy balance equations are useful to know the thermal behaviour of solar ponds. In this study, we focus on both energy and exergy fluxes in HSZ and NCZ, because the useful thermal energy is not stored in UCZ. 3.1. Energy analysis of heat storage zone The energy balance equation of HSZ is written as follows:
Q stored ¼ Q solar;HSZ Q loss;HSZ ¼ bEAHSZ ½ð1 FÞhðx dÞ ks A ksw 2prLHSZ ðT TNCZ Þ þ ðTHSZ Ta Þ DxHSZNCZ HSZ Dxside ksw A ðT Ta Þ þ Dxdown down (1) where E is the solar energy reaching the surface, AHSZ is the area of the HSZ, F is the fraction of energy absorbed at a region of dthickness, h is the solar radiation ratio, A is the surface area, Ta is the air temperature, ksw is the thermal conductivity of the walls, ks is the thermal conductivity of the salty water, LHSZ is the thickness of the HSZ, r is inner radius of the cylindrical solar pond, Dxdown is the thickness of the down wall, Dxside is the thickness of the side wall, DxHSZ-NCZ is the thickness of the HSZ's middle point and the NCZ's
I. Bozkurt et al. / Renewable Energy 78 (2015) 35e41
37
middle point, and b is the fraction of the incident solar radiation and is given by Hawlader [23] as follows:
3.3. Exergy analysis of heat storage zone
sinqi sinqr 2 tanqi tanqr 2 b ¼ 1 0:6 0:4 sinqi þ sinqr tanqi þ tanqr
The exergy content distributions are shown in Fig. 2. The exergy balance equation of HSZ is written as:
(2)
DExstored ¼ Exr;NCZ Exd;HSZ þ Exl;HSZ þ Exside;HSZ þ Exdown;HSZ ( THSZ ¼ bExsolar AHSZ ½ð1 FÞhðx dÞ T0 DSnet;HSZ þ mHSZ Cp;HSZ THSZ Tm;NCZ T0 ln Tm;NCZ " !# " !#) THSZ THSZ þ mHSZ Cp;HSZ THSZ Tside;HSZ T0 ln þ mHSZ Cp;HSZ THSZ Tdown;HSZ T0 ln Tside;HSZ Tdown;HSZ
Here, qi and qr are the incidence and refraction angles. h represents the ratio of the solar energy reaching the depth in the layer I and is given by Bryant and Colbeck [24] as
h ¼ 0:727 0:056ln
ðxI dÞ cosqr
where Exr,NCZ is the recovered exergy from NCZ to HSZ, Exd,HSZ is the exergy destruction in HSZ, Exl,HSZ is the exergy loss from HSZ to NCZ, Exside,HSZ is the exergy loss through side walls. Exdown,HSZ is the exergy loss through bottom wall, Exstored is the exergy stored in HSZ. The exergy of solar radiation can be expressed as [25]:
(3)
( Exsolar ¼ Enet
where xI is the thickness of the layer, d is the thickness of the layer in the UCZ where long-wave solar energy is absorbed. The energy efficiency of the magnesium chloride saturated solar pond for HSZ is defined as
(8)
destructions, within HSZ can be written as follows:
ks A
DxHSZNCZ ðTHSZ TNCZ Þ þ
ksw 2prLHSZ Dxside
ksw A ðTHSZ Ta Þ þ Dx ðTdown Ta Þ down
bEAHSZ ½ð1 FÞhðx dÞ
3.2. Energy analysis of non-convective zone
(4)
Q g;HSZ Q side;HSZ T Exd;HSZ ¼ T0 mHSZ Cp;HSZ ln HSZ þ T0 THSZ T0 Q down þ T0
The energy balance equation of NCZ is written as follows:
Q stored ¼ Q solar;NCZ þ Q down;NCZ Q loss;NCZ ks A ¼ bEANCZ ½ð1 FÞhðx dÞ þ ðT TNCZ Þ DxHSZNCZ HSZ ks A ksw 2prLNCZ ðTNCZ TUCZ Þ þ ðTNCZ Ta Þ DxNCZUCZ Dxside (5) where Qstored is the stored energy in NCZ, Qsolar,NCZ is the amount of net solar energy which absorbs in NCZ, Qdown,NCZ is the heat from HSZ, Qloss,NCZ is the heat loss from NCZ to UCZ and the side wall. The energy efficiency of the magnesium chloride solar pond for NCZ can be defined as
hNCZ
) 4T0 1 T0 4 þ 1 3 T 3T
DSnet,HSZ is the net entropy change of HSZ which is defined DSnet,HSZ ¼ DSsysþDSsurr. Then, the exergy losses, including exergy
hHSZ ¼ 1
(7)
ks A ksw 2prLNCZ ðT T Þ þ ðT T Þ a NCZ UCZ NCZ Dxside DxNCZUCZ ¼1 ks A bEANCZ ½ð1 FÞhðx dÞ þ DxHSZNCZ ðTHSZ TNCZ Þ (6)
(9) where mHSZ ¼ rHSZVHSZ is the mass of salty water in HSZ; rHSZ is the averaged density of HSZ, and VHSZ is the volume of the salty water in HSZ. Cp,HSZ is the specific heat of HSZ; T0 is the reference air temperature; THSZ is temperature of HSZ and Tm,NCZ is the average temperature of NCZ. The specific heat capacity was determined by using an empirical equation [26]:
Cp ¼ ð0:0044s þ 4:1569Þ1000
(10)
where C is heat capacity, s is salinity. The density difference at low temperature takes place approximately in linear relationship between density and salinity. We used an empirical correlation as given below [26] to determine the salinity of the zones:
s¼
ðr 998:24Þ 0:756
(11)
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I. Bozkurt et al. / Renewable Energy 78 (2015) 35e41
Fig. 3. Density distributions of the solar pond.
jNCZ ¼ 1
Exd;NCZ þ Exside;NCZ þ Exl;NCZ Exr;UCZ þ Exg;NCZ
(14)
4. Results and discussion
Fig. 2. The exergy flux of HSZ of the solar pond.
The exergy efficiency is written for HSZ as follows:
jHSZ ¼
Exd;HSZ þ Exl;HSZ þ Exside;HSZ þ Exdown;HSZ Exstored ¼ 1 Exr;NCZ Exr;NCZ (12)
3.4. Exergy analysis of non-convective zone The exergy balance equation of NCZ is written as
DExstored ¼ Exr;UCZ þ Exg;NCZ Exd;NCZ þ Exside;NCZ þ Exl;NCZ ¼ bExsolar AUCZ ½ð1 FÞhðx dÞ THSZ þ mHSZ Cp;HSZ THSZ Tm;NCZ T0 ln Tm;NCZ ( Q side;NCZ Q T g;NCZ þ T0 mNCZ Cp;NCZ ln NCZ T0 TNCZ T0 " !# TNCZ þ mNCZ Cp;NCZ TNCZ Tside;NCZ T0 ln Tside;NCZ ) TNCZ þ mNCZ Cp;NCZ TNCZ Tm;UCZ T0 ln Tm;UCZ (13) where Exr,UCZ is the recovered exergy from UCZ to NCZ, Exg,UCZ is the exergy gain from HSZ. Exd,NCZ is the exergy destruction in NCZ, Exl,NCZ is the exergy loss from NCZ to UCZ, Exside,NCZ is the exergy loss through side walls. The exergy efficiency of NCZ is defined as follows:
The density variation is considered an important part of the maintenance of the solar pond, as it affects the heat storage performance. Fig. 3 shows the variation of the experimental magnesium chloride water densities with height from bottom to surface of the pond, throughout the four months. In Fig. 3, the density differences are observed between the densities variations measured for different months, essentially due to increase in the temperature of inner zones of the solar pond. The density gradient in the solar pond is kept approximately stable. Some erosion is observed at the top of the layer of HSZ. The density erosion is prevented by using salt gradient protection system. The salinity of the zones was calculated by using Eq. (11) and list in Table 1. Table 2 lists the mass, volume and specific heat capacity of the zones. The mass was calculated by using volume and average density. The specific heat capacity is calculated by Eq. (10). Fig. 4 shows the variations of solar energy and exergy contents and air temperature in Adana, Turkey. The maximum solar energy and exergy contents are 713.91 MJ/m2 and 666.32 MJ/m2 in August, respectively. The minimum solar energy and exergy contents are 218.48 MJ/m2 and 204.77 MJ/m2 in January, respectively. Furthermore, the maximum and minimum average air temperatures are 30.80 C in August and 11.30 C in January, respectively. This
Table 1 The salinity of the zones (g/kg).
August September October November
HSZ
NCZ
UCZ
227.64 221.99 225.87 227.64
119.74 114.81 113.76 104.18
19.96 47.96 36.72 16.44
Table 2 The mass, volume and specific heat capacity of the zones.
Mass (kg) Volume (m3) Specific heat capacity (J/kg C)
HSZ
NCZ
UCZ
426.60 0.36 3163.45
392.40 0.36 3659.16
73.08 0.072 4023.71
I. Bozkurt et al. / Renewable Energy 78 (2015) 35e41
39
Table 3 Proximate composition of the experimental temperature data with ± standard error for the magnesium chloride saturated solar pond.
Fig. 4. Variations of monthly solar energy and exergy contents and air temperature in Adana, Turkey.
location has high solar radiation intensity. Moreover, the temperature does not fall below zero even in winter months. The average experimental temperature distributions are shown in Fig. 5. As seen here, the average temperature of HSZ is observed to be a maximum of 52.42 C in August, a minimum of 24.46 C in November. Similarly, the average temperature of NCZ is observed to be a maximum of 44.29 C in August, a minimum of 23.17 C in November. The average temperature of UCZ is observed to be a maximum of 30.31 C in August, a minimum of 15.16 C in November. The temperature of the solar pond increases toward to the bottom like the density distribution. The average temperature of UCZ closes to the ambient temperature because this zone is surface zone. The experimental temperature data of the magnesium chloride saturated solar pond was analysed by using SPSS 15.0 [27]. These tests were performed to determine the significant differences between the means. Table 3 shows the experimental temperature with ± standard errors. Fig. 6 shows both averaged energy and exergy distributions from August to November. As shown here, the energy and exergy contents for HSZ are determined to be a maximum of 114.77 MJ and 106.88 MJ in August, a minimum of 54.90 MJ and 51.33 MJ in November, respectively. The differences of the energy and exergy are due to the fact that exergy is not conserved. The losses and irreversibility cause exergy losses and destructions. Also, the energy and exergy contents for NCZ are determined to be a maximum
Month
HSZ X±SE
August September October November
52.42 48.85 38.62 24.46
± ± ± ±
0.277a 0.588b 0.641c 0.237d
NCZ X±SE 37.38 40.15 29.95 23.42
± ± ± ±
0.096b 0.557a 0.359c 0.294d
UCZ X±SE 30.31 27.50 21.92 15.16
± ± ± ±
0.467a 0.466b 0.374c 0.793d
of 265.57 MJ and 211.99 MJ in August, a minimum of 127.04 MJ and 101.80 MJ in November, respectively. Fig. 7 shows the alteration of the exergy input, stored and destruction and losses from August to November for HSZ. The exergy content distributions were calculated by using the temperature distribution of the solar pond and the reference air temperature. As seen in Fig. 7, the exergy inputs are equivalent to the summation of exergy stored, destruction and losses. The exergy stored appear to be maximum 27.84 MJ in August and minimum 6.48 MJ in November, respectively. The exergy destruction and losses appear to be maximum 79.05 MJ in August and minimum 44.84 MJ in November, respectively. Fig. 8 shows the alteration of the exergy input, recovered, and destruction and losses for NCZ. The exergy stored appear to be maximum 36.98 MJ in August and minimum 5.17 MJ in November, respectively. The exergy destruction and losses appear to be maximum 175.01 MJ in August and minimum 96.63 MJ in November, respectively. The energy and exergy efficiency variations are given in Fig. 9 for comparison purposes. As seen in the figure, the maximum and the minimum energy efficiencies of HSZ are observed in August as
Fig. 6. Averaged input energy and exergy contents for HSZ and NCZ.
Fig. 5. Average temperatures of the solar pond's layers.
Fig. 7. Changes in averaged exergy contents for HSZ.
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I. Bozkurt et al. / Renewable Energy 78 (2015) 35e41
found to be 27.41% and 26.04% for the heat storage zone, 19.71% and 17.45% for the non-convective zone in August, respectively. The total exergy destructions of the storage zones are determined and compared. The maximum exergy destructions and losses appear to be 79.05 MJ for the heat storage zone and 175.01 MJ for the non-convective zone in August. Acknowledgement The authors are thankful to University of Cukurova for financial support of the present work (Grant No. FEF2009D2, FEF2010BAP5, FEF2012YL13).
Fig. 8. Changes in averaged exergy contents for NCZ.
Fig. 9. Variations of energy and exergy efficiencies of the solar pond.
27.41% and 12.64% in November, respectively. Also, the maximum and the minimum exergy efficiencies of HSZ are observed in August as 26.04% and 12.62% in November, respectively. Furthermore, as shown in Fig. 9, the maximum and the minimum energy efficiencies of NCZ are observed in August as 19.71% and 5.91% in November, respectively. Also, the maximum and the minimum exergy efficiencies of NCZ are observed in August as 17.45% and 5.07% in November, respectively. Especially in October and November, the energy and exergy efficiencies become closer to each other, due to the decrease of temperature difference between pond and surrounding area. The energy and exergy efficiencies increased with the temperature of the solar pond. The results show that the difference between energy and exergy analyzes is small because the solar pond system is a low temperature system. The exergy analysis takes into account the true magnitudes of the destructions and losses and these should be minimized for performance improvement of the solar pond.
5. Conclusions In this study, we have built and tested an experimental magnesium chloride saturated solar pond and analysed and assessed to improve heat storage performance of the solar pond. Some concluding remarks can be extracted as follows: Energy and exergy efficiencies of the thermal zones are determined and compared. The energy and exergy efficiencies of the solar pond are defined as a function of solar radiation and temperatures. The maximum energy and exergy efficiencies are
Nomenclature A C E Ex F h HSZ k L m NCZ Q r S T UCZ V
surface area, m2 specific heat, J/kg C total solar energy reaching to the pond, MJ/m2 exergy absorbed energy fraction at a region of d-thickness solar radiation ratio heat storage zone thermal conductivity, J/m C thickness of the inner zones, m mass, kg non-convective zone heat, J inner radius, m entropy, J/K temperature, C upper convective zone volume, m3
Greek letters h energy efficiency d thickness where long wave solar energy is absorbed, m b incident beam entering rate into water q angle, rad r density, kg/m3 j exergy efficiency Subscripts a ambient b bottom d destruction dw down wall g gained i incident in energy input m mean net net irradiation out energy output r refraction rec recovered surr surrounding sw side-wall sys system References [1] Dincer I, Rosen M. Exergy energy, environment and sustainable development. London: Elsevier; 2007. [2] Stritih U, Ostermana E, Evliya H, Butala V, Paksoy H. Exploiting solar energy potential through thermal energy storage in Slovenia and Turkey. Renew Sustain Energy Rev 2013;25:442e61.
I. Bozkurt et al. / Renewable Energy 78 (2015) 35e41 [3] Bozkurt I, Mantar S, Karakilcik M. A new performance model to determine energy storage efficiencies of a solar pond. Heat Mass Transf 2015;51:39e48. [4] Li XY, Kanayama K, Baba H, Maeda Y. Experimental study about erosion in salt gradient solar pond. Renew Energy 2001;23:207e17. [5] Ramakrishna Murthy GR, Pandey KP. Comparative performance evaluation of fertiliser solar pond under simulated conditions. Renew Energy 2003;28: 455e66. [6] Husain M, Patil PS, Patil SR, Samdarshi SK. Computer simulation of salt gradient solarpond's thermal behaviour. Renew Energy 2003;28:769e802. [7] Sozen A. Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond's temperature. Renew Energy 2003;29:501e15. [8] Karakilcik M, Dincer I, Rosen MA. Performance investigation of a solar pond. Appl Therm Eng 2006;26:727e35. [9] Karakilcik M, Dincer I. Exergetic performance analysis of a solar pond. Int J Therm Sci 2008;47:93e102. [10] Bernad F, Casas S, Gibert O, Akbarzadeh A, Cortina JL, Valderrama C. Salinity gradient solar pond: validation and simulation model. Sol Energy 2013;98: 366e74. [11] Karakilcik M, Bozkurt I, Dincer I. Dynamic exergetic performance assessment of an integrated solar pond. Int J Exergy 2013;12:70e86. [12] Dehghan AA, Movahedi A, Mazidi M. Experimental investigation of energy and exergy performance of square and circular solar ponds. Sol Energy 2013;97:273e84. [13] Bozkurt I, Karakilcik M, Dincer I, Atiz A. Transparent covers effect on the performance of a cylindrical solar pond. Int J Green Energy 2014;11:404e16. [14] Ranjan KR, Kaushik SC. Thermodynamic and economic feasibility of solar ponds for various thermal applications: a comprehensive review. Renew Sustain Energy Rev 2014;32:123e39.
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[15] Singh R, Tundee S, Akbarzadeh A. Electric power generation from solar pond using combined thermosyphon and thermoelectric modules. Sol Energy 2011;85:371e8. [16] Bozkurt I, Karakilcik M. The daily performance of a solar pond integrated with solar collectors. Sol Energy 2012;86:1611e20. [17] Karakilcik M, Dincer I, Bozkurt I, Atiz A. Performance assessment of a solar pond with and without shading effect. Energy Convers Manag 2013;65: 98e107. [18] Subhakar D, Murthy SS. Experiments on a magnesium chloride saturated solar pond. Renew Energy 1991;(5e6):655e60. [19] Subhakar D, Murthy SS. Saturated solar ponds: 2. Parametric studies. Sol Energy 1993;50:307e19. [20] Subhakar D, Murthy SS. Saturated solar ponds: 3. Experimental verification. Sol Energy 1994;53:469e72. [21] Husain M, Sharma G, Samdarshi SK. Innovative design of non-convective zone of salt gradient solar pond for optimum thermal performance and stability. Appl Energy 2012;93:357e63. [22] Akbarzadeh A, MacDonald RWG. Introduction of a passive method for salt replenishment in the operation of solar ponds. Sol Energy 1982;29:71e6. [23] Hawlader MNA. The influence of the extinction coefficient on the effectiveness of solar ponds. Sol Energy 1980;25:461e4. [24] Bryant HC, Colbeck I. A solar pond for London. Sol Energy 1977;19:321e2. [25] Petala R. Exergy of undiluted thermal radiations. Sol Energy 2003;74:469e88. [26] Sun H, Feistel R, Koch M, Marko A. New equations for density, entropy, heat capacity, and potential temperature of a saline thermal fluid. Deep Sea Res Part I: Oceanogr Res Pap 2008;55:1304e10. [27] SPSS. Computer program, MS for Windows. version 15.0. USA: SPSS Inc.; 2006.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance assessment of a magnesium chloride saturated solar pond Ismail Bozkurt a, Sibel Deniz b, Mehmet Karakilcik b, *, Ibrahim Dincer c a
Department of Mechanical Engineering, Faculty of Engineering, University of Adiyaman, Adiyaman 02040, Turkey Department of Physics, Faculty of Sciences and Letters, University of Cukurova, Adana 01330, Turkey c Faculty of Engineering and Applied Science, University of Ontario Institute of Technology (UOIT), 2000 Simcoe Street, North Oshawa, Ontario L1H 7K4, Canada b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2014 Accepted 27 December 2014 Available online
This paper deals with the experimental investigation of a magnesium chloride saturated solar pond and its performance evaluation through energy and exergy efficiencies. The solar pond system is filled with magnesium chloride containing water to form layers with varying densities. A solar pond generally consists of three zones, and the densities of these zones increase from the top convective zone to the bottom storage zone. The incoming solar radiation is absorbed by salty water (with magnesium chloride) which eventually increases the temperature of the storage zone. The high-temperature salty water at the bottom of the solar pond remains much denser than the salty water in the upper layers. Thus, the convective heat losses are prevented by gradient layers. The experimental temperature changes of the solar pond are measured by using thermocouples from August to November. The densities of the layers are also measured and analysed by taking samples from at the same point of the temperature sensors. The energy and exergy content distributions are determined for the heat storage zone and the nonconvective zone. The maximum exergy destructions and losses appear to be 79.05 MJ for the heat storage zone and 175.01 MJ for the non-convective zone in August. The energy and exergy efficiencies of the solar pond are defined as a function of solar radiation and temperatures. As a result, the maximum energy and exergy efficiencies are found to be 27.41% and 26.04% for the heat storage zone, 19.71% and 17.45% for the non-convective zone in August, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Magnesium chloride Solar pond Heat storage Energy Exergy Efficiency
1. Introduction Renewable energy resources and technologies have a key role to play in meeting current and future energy needs. They cause less environmental impact than the conventional energy sources [1]. Turkey, situated on the sunny belt between 36 N and 42 N latitude, is located in a relatively more advantageous geographical location for harvesting solar energy. Especially, Mediterranean and Aegean Sea coastal zones have very high potential for utilization of solar energy [2]. During the past decades, solar energy sources have gained greater importance to meet the energy demands for various sectors. Solar energy systems have used for several applications like heating water, warming greenhouses, drying, water desalination,
* Corresponding author. Tel.: þ90 322 338 60 84; fax: þ90 322 338 60 70. E-mail addresses: [email protected] (I. Bozkurt), fzksibel.deniz@ hotmail.com (S. Deniz), [email protected] (M. Karakilcik), [email protected] (I. Dincer). http://dx.doi.org/10.1016/j.renene.2014.12.060 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
power generation and so on. One of these systems is solar pond where the main goal is to heat a large mass of water [3]. In this regard, various studies on solar ponds have been undertaken and their heat transfer and thermodynamic aspects have been investigated by various researchers [4e14]. Singh et al. [15] designed and tested a combined system of thermosyphon and thermoelectric modules for the generation of electricity from low-grade thermal sources like solar pond. Bozkurt and Karakilcik [16] investigated an integrated solar pond where heat collected from the flat-plate solar collector is transferred to the storage zone through a heat exchanger system. The system performance was determined both experimentally and theoretically according to the number of flatplate collectors. Karakilcik et al. [17] studied a solar pond performance with and without shading effect and comparison of the energy efficiencies of the experimental solar pond. Thus, the shading effect and its ratios were determined to study the efficiency of the solar pond. Most of these studies aimed to investigate the performance of the solar ponds and study the effects of varying
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operating conditions and thermophysical properties on the pond performance. Sodium chloride has been the most commonly used salt in most of these studies. In a few studies, however, magnesium chloride salt has been used. Subhakar and Murthy [18e20] studied saturated solar pond by using magnesium chloride and potassium nitrate salts for the experimental testing and performance assessment. A comparison was also made with an unsaturated solar pond. The temperature and concentration gradients were developed by heating the pond from the bottom and adding finely powdered salt from the top. Most of the experimental studies presented in the literature focuses on energetic performance evaluation of the ponds. In this study, an experimental magnesium chloride saturated solar pond is constructed, built and employed for experimental investigations, and its performance is investigated and evaluated through energy and exergy efficiencies. Although the energetic performance of the magnesium chloride solar pond was studied previously, the exergetic based analysis and assessment of a magnesium chloride saturated solar pond is the original contribution of this study. Since exergy analysis and assessment provide a true and meaningful picture of the system, we aim to compare the results with the energetic results where the first-law of thermodynamics (based on conservation law) is used. This is in fact the key motivation behind the present work. 2. Experimental system and procedure A solar pond is of various regions as types and sizes. It generally consists of two main regions as outer and inner regions. First, the outer region is called insulation region to prevent the heat losses by conduction from inner region to surrounding of the pond. Second, the inner region is a large body of salty water with a salinity gradient to prevent heat loss by convection. The body of the region generally consists of three zones (e.g., surface zone, middle zone and bottom zone). The surface zone is called as upper convective zone (UCZ). UCZ is the fresh water layer at the top of the pond. The middle zone is called as non-convective zone (NCZ). NCZ is composed of different salty water layers whose density gradually increases toward bottom of the pond. This zone plays a key role in the solar pond because this zone constitutes a transparent insulating layer to prevent convective heat losses from bottom zone to UCZ. In this regard, the size of NCZ is very important to increase the performance of a solar pond so that Husain et al. [21] developed a rational analytical insight for judicious selection of NCZ size considering optimum thermal performance as well as stability aspects. Finally, bottom zone of the pond is composed of salty water with highest density. Thanks to this feature, it is absorbed the solar radiation that reaches the bottom of the pond and converted as heat, and stored as heat storage zone (HSZ). In this study, an experimental solar pond with the area of 0.72 m2 and a depth of 1.10 m was built in Cukurova University in Adana, Turkey (i.e., 35180 E longitude, 37 050 N latitude). The pond's bottom and side-wall was insulated by using 0.10 m thickness glass wool. The pond temperature was measured at 7 points, starting from the bottom, at 0.25, 0.40, 0.55, 0.65, 0.75, 0.85 and 1.05 m heights by using thermocouples with an accuracy of about ±1 C. The pond was filled with the different densities magnesium chloride water to set up the density gradient of the pond in August 2012 and worked. Fig. 1 shows a schematic representation of the experimental solar pond system that built in salt production system. In the inner region of the pond, the ranges of magnesium chloride water density in UCZ, NCZ and HSZ are 1000e1020 kg/m3, 1030e1150 kg/m3 and 1170e1200 kg/m3, and the thicknesses of the zones are 0.10, 0.50, 0.50 m, respectively. The density distributions are also measured
Fig. 1. A schematic representation of the magnesium chloride saturated solar pond.
and analysed by taking samples from at the same point of the temperature sensors. As seen in Fig. 1 the salt gradient protection system is used to protect the density gradient against erosion of the magnesium chloride water in the inner zones of the pond. The protection system was a system based on the natural circulation of water caused by density difference, as it was first proposed by Akbarzadeh and Mac Donalds [22]. 3. Energy and exergy analyses The energy and exergy values of the solar pond are calculated using conservation of mass and energy principles as well as second law analysis. Therefore, in this section, energy and exergy analyses of the solar pond, through the balance equations, are presented. The energy balance equations are useful to know the thermal behaviour of solar ponds. In this study, we focus on both energy and exergy fluxes in HSZ and NCZ, because the useful thermal energy is not stored in UCZ. 3.1. Energy analysis of heat storage zone The energy balance equation of HSZ is written as follows:
Q stored ¼ Q solar;HSZ Q loss;HSZ ¼ bEAHSZ ½ð1 FÞhðx dÞ ks A ksw 2prLHSZ ðT TNCZ Þ þ ðTHSZ Ta Þ DxHSZNCZ HSZ Dxside ksw A ðT Ta Þ þ Dxdown down (1) where E is the solar energy reaching the surface, AHSZ is the area of the HSZ, F is the fraction of energy absorbed at a region of dthickness, h is the solar radiation ratio, A is the surface area, Ta is the air temperature, ksw is the thermal conductivity of the walls, ks is the thermal conductivity of the salty water, LHSZ is the thickness of the HSZ, r is inner radius of the cylindrical solar pond, Dxdown is the thickness of the down wall, Dxside is the thickness of the side wall, DxHSZ-NCZ is the thickness of the HSZ's middle point and the NCZ's
I. Bozkurt et al. / Renewable Energy 78 (2015) 35e41
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middle point, and b is the fraction of the incident solar radiation and is given by Hawlader [23] as follows:
3.3. Exergy analysis of heat storage zone
sinqi sinqr 2 tanqi tanqr 2 b ¼ 1 0:6 0:4 sinqi þ sinqr tanqi þ tanqr
The exergy content distributions are shown in Fig. 2. The exergy balance equation of HSZ is written as:
(2)
DExstored ¼ Exr;NCZ Exd;HSZ þ Exl;HSZ þ Exside;HSZ þ Exdown;HSZ ( THSZ ¼ bExsolar AHSZ ½ð1 FÞhðx dÞ T0 DSnet;HSZ þ mHSZ Cp;HSZ THSZ Tm;NCZ T0 ln Tm;NCZ " !# " !#) THSZ THSZ þ mHSZ Cp;HSZ THSZ Tside;HSZ T0 ln þ mHSZ Cp;HSZ THSZ Tdown;HSZ T0 ln Tside;HSZ Tdown;HSZ
Here, qi and qr are the incidence and refraction angles. h represents the ratio of the solar energy reaching the depth in the layer I and is given by Bryant and Colbeck [24] as
h ¼ 0:727 0:056ln
ðxI dÞ cosqr
where Exr,NCZ is the recovered exergy from NCZ to HSZ, Exd,HSZ is the exergy destruction in HSZ, Exl,HSZ is the exergy loss from HSZ to NCZ, Exside,HSZ is the exergy loss through side walls. Exdown,HSZ is the exergy loss through bottom wall, Exstored is the exergy stored in HSZ. The exergy of solar radiation can be expressed as [25]:
(3)
( Exsolar ¼ Enet
where xI is the thickness of the layer, d is the thickness of the layer in the UCZ where long-wave solar energy is absorbed. The energy efficiency of the magnesium chloride saturated solar pond for HSZ is defined as
(8)
destructions, within HSZ can be written as follows:
ks A
DxHSZNCZ ðTHSZ TNCZ Þ þ
ksw 2prLHSZ Dxside
ksw A ðTHSZ Ta Þ þ Dx ðTdown Ta Þ down
bEAHSZ ½ð1 FÞhðx dÞ
3.2. Energy analysis of non-convective zone
(4)
Q g;HSZ Q side;HSZ T Exd;HSZ ¼ T0 mHSZ Cp;HSZ ln HSZ þ T0 THSZ T0 Q down þ T0
The energy balance equation of NCZ is written as follows:
Q stored ¼ Q solar;NCZ þ Q down;NCZ Q loss;NCZ ks A ¼ bEANCZ ½ð1 FÞhðx dÞ þ ðT TNCZ Þ DxHSZNCZ HSZ ks A ksw 2prLNCZ ðTNCZ TUCZ Þ þ ðTNCZ Ta Þ DxNCZUCZ Dxside (5) where Qstored is the stored energy in NCZ, Qsolar,NCZ is the amount of net solar energy which absorbs in NCZ, Qdown,NCZ is the heat from HSZ, Qloss,NCZ is the heat loss from NCZ to UCZ and the side wall. The energy efficiency of the magnesium chloride solar pond for NCZ can be defined as
hNCZ
) 4T0 1 T0 4 þ 1 3 T 3T
DSnet,HSZ is the net entropy change of HSZ which is defined DSnet,HSZ ¼ DSsysþDSsurr. Then, the exergy losses, including exergy
hHSZ ¼ 1
(7)
ks A ksw 2prLNCZ ðT T Þ þ ðT T Þ a NCZ UCZ NCZ Dxside DxNCZUCZ ¼1 ks A bEANCZ ½ð1 FÞhðx dÞ þ DxHSZNCZ ðTHSZ TNCZ Þ (6)
(9) where mHSZ ¼ rHSZVHSZ is the mass of salty water in HSZ; rHSZ is the averaged density of HSZ, and VHSZ is the volume of the salty water in HSZ. Cp,HSZ is the specific heat of HSZ; T0 is the reference air temperature; THSZ is temperature of HSZ and Tm,NCZ is the average temperature of NCZ. The specific heat capacity was determined by using an empirical equation [26]:
Cp ¼ ð0:0044s þ 4:1569Þ1000
(10)
where C is heat capacity, s is salinity. The density difference at low temperature takes place approximately in linear relationship between density and salinity. We used an empirical correlation as given below [26] to determine the salinity of the zones:
s¼
ðr 998:24Þ 0:756
(11)
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Fig. 3. Density distributions of the solar pond.
jNCZ ¼ 1
Exd;NCZ þ Exside;NCZ þ Exl;NCZ Exr;UCZ þ Exg;NCZ
(14)
4. Results and discussion
Fig. 2. The exergy flux of HSZ of the solar pond.
The exergy efficiency is written for HSZ as follows:
jHSZ ¼
Exd;HSZ þ Exl;HSZ þ Exside;HSZ þ Exdown;HSZ Exstored ¼ 1 Exr;NCZ Exr;NCZ (12)
3.4. Exergy analysis of non-convective zone The exergy balance equation of NCZ is written as
DExstored ¼ Exr;UCZ þ Exg;NCZ Exd;NCZ þ Exside;NCZ þ Exl;NCZ ¼ bExsolar AUCZ ½ð1 FÞhðx dÞ THSZ þ mHSZ Cp;HSZ THSZ Tm;NCZ T0 ln Tm;NCZ ( Q side;NCZ Q T g;NCZ þ T0 mNCZ Cp;NCZ ln NCZ T0 TNCZ T0 " !# TNCZ þ mNCZ Cp;NCZ TNCZ Tside;NCZ T0 ln Tside;NCZ ) TNCZ þ mNCZ Cp;NCZ TNCZ Tm;UCZ T0 ln Tm;UCZ (13) where Exr,UCZ is the recovered exergy from UCZ to NCZ, Exg,UCZ is the exergy gain from HSZ. Exd,NCZ is the exergy destruction in NCZ, Exl,NCZ is the exergy loss from NCZ to UCZ, Exside,NCZ is the exergy loss through side walls. The exergy efficiency of NCZ is defined as follows:
The density variation is considered an important part of the maintenance of the solar pond, as it affects the heat storage performance. Fig. 3 shows the variation of the experimental magnesium chloride water densities with height from bottom to surface of the pond, throughout the four months. In Fig. 3, the density differences are observed between the densities variations measured for different months, essentially due to increase in the temperature of inner zones of the solar pond. The density gradient in the solar pond is kept approximately stable. Some erosion is observed at the top of the layer of HSZ. The density erosion is prevented by using salt gradient protection system. The salinity of the zones was calculated by using Eq. (11) and list in Table 1. Table 2 lists the mass, volume and specific heat capacity of the zones. The mass was calculated by using volume and average density. The specific heat capacity is calculated by Eq. (10). Fig. 4 shows the variations of solar energy and exergy contents and air temperature in Adana, Turkey. The maximum solar energy and exergy contents are 713.91 MJ/m2 and 666.32 MJ/m2 in August, respectively. The minimum solar energy and exergy contents are 218.48 MJ/m2 and 204.77 MJ/m2 in January, respectively. Furthermore, the maximum and minimum average air temperatures are 30.80 C in August and 11.30 C in January, respectively. This
Table 1 The salinity of the zones (g/kg).
August September October November
HSZ
NCZ
UCZ
227.64 221.99 225.87 227.64
119.74 114.81 113.76 104.18
19.96 47.96 36.72 16.44
Table 2 The mass, volume and specific heat capacity of the zones.
Mass (kg) Volume (m3) Specific heat capacity (J/kg C)
HSZ
NCZ
UCZ
426.60 0.36 3163.45
392.40 0.36 3659.16
73.08 0.072 4023.71
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39
Table 3 Proximate composition of the experimental temperature data with ± standard error for the magnesium chloride saturated solar pond.
Fig. 4. Variations of monthly solar energy and exergy contents and air temperature in Adana, Turkey.
location has high solar radiation intensity. Moreover, the temperature does not fall below zero even in winter months. The average experimental temperature distributions are shown in Fig. 5. As seen here, the average temperature of HSZ is observed to be a maximum of 52.42 C in August, a minimum of 24.46 C in November. Similarly, the average temperature of NCZ is observed to be a maximum of 44.29 C in August, a minimum of 23.17 C in November. The average temperature of UCZ is observed to be a maximum of 30.31 C in August, a minimum of 15.16 C in November. The temperature of the solar pond increases toward to the bottom like the density distribution. The average temperature of UCZ closes to the ambient temperature because this zone is surface zone. The experimental temperature data of the magnesium chloride saturated solar pond was analysed by using SPSS 15.0 [27]. These tests were performed to determine the significant differences between the means. Table 3 shows the experimental temperature with ± standard errors. Fig. 6 shows both averaged energy and exergy distributions from August to November. As shown here, the energy and exergy contents for HSZ are determined to be a maximum of 114.77 MJ and 106.88 MJ in August, a minimum of 54.90 MJ and 51.33 MJ in November, respectively. The differences of the energy and exergy are due to the fact that exergy is not conserved. The losses and irreversibility cause exergy losses and destructions. Also, the energy and exergy contents for NCZ are determined to be a maximum
Month
HSZ X±SE
August September October November
52.42 48.85 38.62 24.46
± ± ± ±
0.277a 0.588b 0.641c 0.237d
NCZ X±SE 37.38 40.15 29.95 23.42
± ± ± ±
0.096b 0.557a 0.359c 0.294d
UCZ X±SE 30.31 27.50 21.92 15.16
± ± ± ±
0.467a 0.466b 0.374c 0.793d
of 265.57 MJ and 211.99 MJ in August, a minimum of 127.04 MJ and 101.80 MJ in November, respectively. Fig. 7 shows the alteration of the exergy input, stored and destruction and losses from August to November for HSZ. The exergy content distributions were calculated by using the temperature distribution of the solar pond and the reference air temperature. As seen in Fig. 7, the exergy inputs are equivalent to the summation of exergy stored, destruction and losses. The exergy stored appear to be maximum 27.84 MJ in August and minimum 6.48 MJ in November, respectively. The exergy destruction and losses appear to be maximum 79.05 MJ in August and minimum 44.84 MJ in November, respectively. Fig. 8 shows the alteration of the exergy input, recovered, and destruction and losses for NCZ. The exergy stored appear to be maximum 36.98 MJ in August and minimum 5.17 MJ in November, respectively. The exergy destruction and losses appear to be maximum 175.01 MJ in August and minimum 96.63 MJ in November, respectively. The energy and exergy efficiency variations are given in Fig. 9 for comparison purposes. As seen in the figure, the maximum and the minimum energy efficiencies of HSZ are observed in August as
Fig. 6. Averaged input energy and exergy contents for HSZ and NCZ.
Fig. 5. Average temperatures of the solar pond's layers.
Fig. 7. Changes in averaged exergy contents for HSZ.
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I. Bozkurt et al. / Renewable Energy 78 (2015) 35e41
found to be 27.41% and 26.04% for the heat storage zone, 19.71% and 17.45% for the non-convective zone in August, respectively. The total exergy destructions of the storage zones are determined and compared. The maximum exergy destructions and losses appear to be 79.05 MJ for the heat storage zone and 175.01 MJ for the non-convective zone in August. Acknowledgement The authors are thankful to University of Cukurova for financial support of the present work (Grant No. FEF2009D2, FEF2010BAP5, FEF2012YL13).
Fig. 8. Changes in averaged exergy contents for NCZ.
Fig. 9. Variations of energy and exergy efficiencies of the solar pond.
27.41% and 12.64% in November, respectively. Also, the maximum and the minimum exergy efficiencies of HSZ are observed in August as 26.04% and 12.62% in November, respectively. Furthermore, as shown in Fig. 9, the maximum and the minimum energy efficiencies of NCZ are observed in August as 19.71% and 5.91% in November, respectively. Also, the maximum and the minimum exergy efficiencies of NCZ are observed in August as 17.45% and 5.07% in November, respectively. Especially in October and November, the energy and exergy efficiencies become closer to each other, due to the decrease of temperature difference between pond and surrounding area. The energy and exergy efficiencies increased with the temperature of the solar pond. The results show that the difference between energy and exergy analyzes is small because the solar pond system is a low temperature system. The exergy analysis takes into account the true magnitudes of the destructions and losses and these should be minimized for performance improvement of the solar pond.
5. Conclusions In this study, we have built and tested an experimental magnesium chloride saturated solar pond and analysed and assessed to improve heat storage performance of the solar pond. Some concluding remarks can be extracted as follows: Energy and exergy efficiencies of the thermal zones are determined and compared. The energy and exergy efficiencies of the solar pond are defined as a function of solar radiation and temperatures. The maximum energy and exergy efficiencies are
Nomenclature A C E Ex F h HSZ k L m NCZ Q r S T UCZ V
surface area, m2 specific heat, J/kg C total solar energy reaching to the pond, MJ/m2 exergy absorbed energy fraction at a region of d-thickness solar radiation ratio heat storage zone thermal conductivity, J/m C thickness of the inner zones, m mass, kg non-convective zone heat, J inner radius, m entropy, J/K temperature, C upper convective zone volume, m3
Greek letters h energy efficiency d thickness where long wave solar energy is absorbed, m b incident beam entering rate into water q angle, rad r density, kg/m3 j exergy efficiency Subscripts a ambient b bottom d destruction dw down wall g gained i incident in energy input m mean net net irradiation out energy output r refraction rec recovered surr surrounding sw side-wall sys system References [1] Dincer I, Rosen M. Exergy energy, environment and sustainable development. London: Elsevier; 2007. [2] Stritih U, Ostermana E, Evliya H, Butala V, Paksoy H. Exploiting solar energy potential through thermal energy storage in Slovenia and Turkey. Renew Sustain Energy Rev 2013;25:442e61.
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