Design of new molten salt thermal energy storage material for solar thermal power plant

Applied Energy 112 (2013) 682–689

Contents lists available at SciVerse ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Design of new molten salt thermal energy storage material for solar thermal power plant Qiang Peng a,⇑, Xiaoxi Yang a,c, Jing Ding b, Xiaolan Wei a, Jianping Yang a a Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China b School of Engineering, Sun Yat-sen University, 132 Waihuan Dong Road, Guangzhou High Education Mega Center, Panyu, Guangzhou 510006, PR China c Guangdong Provincial Key Laboratory of Distributed Energy Systems, Dongguan University of Technology, Dongguan 523808, PR China

h i g h l i g h t s " New quaternary reciprocal system (K, Na/NO2, Cl, NO3) is prepared. " This molten salt has a lower melting point. " This new salt has excellent thermal stability. " This salt mixture has a reduced cost.

a r t i c l e

i n f o

Article history: Received 28 August 2012 Received in revised form 21 October 2012 Accepted 22 October 2012 Available online 17 November 2012 Keywords: Molten nitrate salt Heat transfer fluid (HTF) Eutectic mixture Phase diagram calculation Conformal ionic solution (CIS) theory Solar thermal power

a b s t r a c t In order to obtain molten salt with lower melting point, higher thermal stability and reduced cost relative to previously available materials, a variety of molten salt mixtures of alkali nitrates are investigated by experimental methods. However, since measurements are generally expensive and time-consuming, it is of interest to be able to predict melting point and the component of multi-component systems by using the numerical methods. In this paper, eutectic point and component of a new kind of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) are determined firstly by conformal ionic solution theory. Then thermal stability of the mixtures that show a lower melting point is measured by thermogravimetric analysis device. Experimental results show the agreement between measurements and calculations is found to be very good. This kind of molten salt has a lower melting point, 140 °C. It is thermally stable at temperatures up to 500 °C, and may be used up to 550 °C for short periods. Besides, this molten salt has a reduced cost relative to previous low-melting nitrate mixtures due to the elimination of cesium nitrate and lithium nitrate. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Thermal energy storage (TES) technologies is a key factor in solar thermal power plants. Concentrating solar power (CSP) plants with TES can generate electricity when sunlight is not available, for example, during momentary cloud transients, which otherwise disrupt electricity generation and cause widely varying power output, and during evening hours when electricity is highly valued [1]. There are several types of HTF in TES system, among which molten salt has been widely studied due to its higher working temperatures (more than 500 °C) and heat capacity, lower vapor pressure and corrosivity, good thermal and physical properties even at elevated temperatures. For instance, increasing the maximum ⇑ Corresponding author. Tel.: +86 20 39332320; fax: +86 20 39332319. E-mail address: [email protected] (Q. Peng).

fluid output temperature of current CSP plants from 390 °C to 450–500 °C would increase the Rankine cycle efficiency of the power block steam turbine to the 40% range, compared to 393 °C with the current high-temperature oil and a cycle efficiency of 37.6%, thereby reducing the levelized energy cost by 2 cents/ kW h [2,3]. At present, molten salts for heat transfer and energy storage mainly include nitrates, chlorides, fluorides and carbonates. The eutectics of fluoride salts may be utilized in space solar power and molten salt nuclear reactor because of their high heat storage capacity, but with the disadvantage of cost, material compatibility and toxicity [4–6]. Chlorides are attractive due to their high heat fusion and low cost although they are less attractive in terms of high corrosiveness [7]. Carbonates can be used for high temperature latent heat storage applications such as central receiver system, but with high viscidity and easy to decomposition [8,9]. The eutectics of nitrate or nitrite salts have the advantages

0306-2619/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.10.048

Q. Peng et al. / Applied Energy 112 (2013) 682–689

683

Nomenclature CIS CSP DSC

conformal ionic solution concentrating solar power differential scanning calorimeter

of low chemical reactivity, low corrosiveness and low cost [10]. Therefore, nitrates or nitrites are suitable for heat transfer and thermal storage material in solar thermal power plants. There are several commercially available molten salt formulations, mixtures of nitrates or nitrites, and they also have been used for solar thermal systems. The binary solar salt mixture (60 wt.% NaNO3–40 wt.% KNO3) is used at the 10 MWe solar two central receiver project in California [11] and the indirect TES system for the Andasol plant in Spain [12]. It has the higher thermal stability (600 °C) and the lower cost, but also the higher melting point (220 °C). Hitec (53 wt.% KNO3–7 wt.% NaNO3–40 wt.% NaNO2) has been used for decades in the heat treating industry. This salt has the lower melting point (142 °C). It is thermally stable at temperatures up to 454 °C, and may be used up to 538 °C for short periods [13]. However, a drawback of these molten salts as HTF is their relatively high melting point and limits the practical applications in CSP applications. A straightforward approach to identifying an improved HTF would be to add constituents to solar nitrate salt that depress the melting point significantly without compromising its properties. The eutectic temperature of LiNO3, NaNO3 and KNO3 is 120 °C and a Hitec XL mixture of Ca(NO3)2, NaNO3 and KNO3 melts at about 133 °C [14]. Recently, some work have been done on more complex salt mixtures. Melting temperatures of the quaternary nitrate salts with Li, Na, K, and Ca cations are below 100 °C [15,16]. Eutectic points of salt mixtures with Li, Na, and K cations and nitrate/nitrite anions are below 80 °C [17–19]. The advanced mixtures consisting of a mixture of nitrate salts of lithium, sodium, potassium, cesium, and calcium have a low melting point of 65 °C [20]. A novel mixtures of inorganic salts with multi-component system (Li, Na, K, Ca/NO3, NO2, Cl) show a lower melting point of 53 °C [21]. However, the problems of these salts are the high proportion of nitrate salts of lithium, cesium improves the cost considerably and the addition of calcium nitrate generally increased density and viscosity. As we know, very large quantities (millions of kilograms) of HTF are required for energy storage in 100– 200-MW power plants and entail high capital investment costs, so minimizing that cost while maximizing the HTF performance is paramount. Generally speaking, phase behavior of higher order salt mixtures has been focused largely on experimental methods and directly measuring the phase transitions of a system of salts. However, since measurements are generally expensive, timeconsuming and the difficulty of study increases very rapidly with the number of components, it is of interest to be able to predict thermodynamic properties of higher order systems from known theories by using available data for lower order systems. Thus that also is hope that the development of analytical equations which express the thermodynamic properties of multi-component solutions as simple polynomial functions in terms of mole fractions has permitted sophisticated thermodynamic calculations to be performed with the aid of digital computers [22]. Besides, the lack of information for the molten salt at high temperatures, as well as existing databases of thermodynamic salt properties are incomplete, some detailed material properties of the component may be estimated roughly here. In this paper, in order to maximizing HTF performance while minimizing material cost, the quaternary reciprocal system (K,

HTF TES TGA

heat transfer fluid thermal energy storage thermal gravimetric analysis

Na/NO2, Cl, NO3) is investigated to identify low melting (low liquidus temperature) mixtures. Firstly, the liquidus temperatures of the quaternary reciprocal system can be calculated a priori using the equations derived on the basis of the CIS theory [23,24]. Then, the mixtures that exhibit a lower melting point are determined to further testing for thermal stability using a thermogravimetric analysis (DSC–TGA) device, the Q600 SDT from TA Instruments (New Castle, Delaware). Approximately 15 mg of each mixture is loaded onto a graphite pan for TGA testing. A TGA heats a sample in a nitrogen environment and continuously measures the sample weight, which typically decreases at higher temperatures as the sample decomposes into gaseous products. 2. Theory 2.1. Calculation of phase diagram A reciprocal quaternary system [25,26] is defined as one containing two different cations and three different anions (A, B/X, Y, Z). Even though such a system contains five different ions (A+, B+, X, Y, Z) and has six constituent salts (AX, AY, AZ, BX, BY, BZ). It is one restriction on the system, namely, the electroneutrality condition. Thus, one has six constituents, but only four independent components. The choice of the independent components can be arbitrarily made and should not affect the final expressions for the thermodynamic properties of mixing as long as the components contain the five different ions. The composition of a reciprocal quaternary system A, B/X, Y, Z is conveniently plotted on a composition prism as shown in Fig. 1. The two triangular bases of the prism represent compositions in

Fig. 1. Composition prism of a reciprocal quaternary system illustrating the geometrical basis of the CIS equation.

684

Q. Peng et al. / Applied Energy 112 (2013) 682–689

the additive ternary system A/X, Y, Z and B/X, Y, Z while the three square faces represent compositions in the reciprocal ternary systems A, B/X, Y; A, B/X, Z and A, B/Y, Z. The axes of the prism are the cationic or anionic mole fractions XA, XB, XX, XY, XZ defined as Xi (i = X, Y, Z) = ni/(nx + ny + nz), Xi (i = A, B) = ni/(nA + nB), where ni is the number of moles of ion i. The dotted triangular section in Fig. 1 represents a plane of constant cationic fraction XA (or XB). The Gibbs energy of a reciprocal quaternary system A, B/X, Y, Z at composition P in Fig. 1 can be estimated from its values at points 1, 2, 3, 4 and 5 via the Quaternary CIS equation. Thus, the expression derived for the total excess free energy of mixing (DGE) of the three salts AX, AY, AZ and BX is [27].

DGE ¼ X B X Y DG0I þ X B X Z DG0II þ X X DG014 þ X Y DG025 þ X Z DG036 þ X A DG0123 þ X B DG0456 þ X A X B X X X Y KI þ X A X B X X X Z KII þ X A X B X Y X Z KIII

ð1Þ

where the Xi (i = A, B, X, Y, Z) are ion fractions of ion i. For example, the cation fraction of i (i = A, B) is Xi = ni/(nA + nB) and the anion fraction of i (i = X, Y, Z) is Xi = ni/(nX + nY + nZ), where the n’s are the number of moles of the ions indicated (n = nA + nB = nX + nY + nZ). DGoi ði ¼ I; II; IIIÞ is the standard molar Gibbs free energy change for the metathetical reaction:

AYðlÞ þ BXðlÞ $ BYðlÞ þ AXðlÞ BXðlÞ þ AZðlÞ $ BZðlÞ þ AXðlÞ

ð2Þ

AYðlÞ þ BZðlÞ $ BYðlÞ þ AZðlÞ DGEij is the excess free energy of mixing of the binary mixture of the salts i and j where AX is salt 1, AY is 2, AZ is 3, BX is 4, BY is 5 and BZ is 6. (For the quaternary reciprocal system K, Na/NO2, Cl, NO3, NaCl is salt 1, NaNO2 is 2, NaNO3 is 3, KCl is 4, KNO2 is 5 and KNO3 is 6.) DGEijk is the excess free energy of mixing of the ternary mixture of the salts i, j and k. Because it has been shown that the terms in DGEij are general, in this paper we will include only second-order terms so that

DGE14 ¼ X A X B k14 ; DGE12 DGE46

¼

DGE25 ¼ X A X B k25 ;

X X X Y k12 DGE13

¼ X X X Z k13 ;

DGE36 ¼ X A X B k36 ;

DGE45 ¼ X X X Y k45 ;

ð3Þ

¼ X X X Z k46

where kij is an energy parameter depending solely on the properties of the binary systems. However, experimental results for KNO3– KNO2 and NaNO3–NaNO2 indicate their thermodynamic properties are consistent with sub-regular solutions theory and kij is dependent of composition [14,28]. Therefore, the excess free energy of mixing for KNO3–KNO2 is

DGE56 ¼ X Y X Z ½I0 þ ðX Y  X Z ÞI1 þ ðX 2Y  4X Y X Z þ X 2Z ÞI2

ð4Þ

K¼

ð5Þ

ð8Þ

where Z is a ‘‘coordination number’’ which should be between 4 and 6. In the calculations presented here a value of 6 has been chosen for Z. The calculation of excess chemical potentials of a salt species of the quaternary reciprocal system is now straightforward from the thermodynamic view. Thus, the activity coefficients of any component can be calculated from Eq. (1) with the aid of the relation:

RT ln ci ¼

@nDGEm @nDGEm @nDGEm ¼ þ @ni @nþ @n

ð9Þ

where n+ and n denote the number of moles of cations and anions of the component i. Besides, for an ideal mixture of non-electrolytes, the liquidus temperature Tij of the phase field of a salt species ij may be calculated from expressions such as:

R ln aij ¼ R ln X i X j cij ¼ DHf ðijÞ ðT f ðijÞ Þð1=T ij  1=T f ðijÞ Þ þ DC pðijÞ ðT f ðijÞ =T ij  1  lnðT f ðijÞ =T ij ÞÞ Tij, T oij

ð10Þ

DHofðijÞ

where and are liquidus temperature, the melting point and the enthalpy of fusion of salt ij, respectively; DC pðijÞ ¼ C lpðijÞ  C spðijÞ is the difference between the heat capacities of the pure liquid and solid. This difference is rather small for most salts and we shall assume that it is practically independent of temperature and obtain it from the published values of heat capacities at the melting point. In order to calculate liquidus temperature Tij, a value of DG° and four values of T oij ; DHofðijÞ ; DC pðijÞ and cij are needed for each system. Table 1 lists the values of T oij ; DHofðijÞ ; DC pðijÞ used [14,29–32]. Values of DG° are calculated from values of the standard molar Gibbs free energies of formation, DfG°. Values of DG° and DfG° are also showed in Tables 2 and 3 [32]. Because potassium nitrite is very sensitive to temperature, humidity and atmosphere, the value of DCp for KNO2 is estimated referring to the nitrate salts and nitrite salts. Values of the cij coefficients are calculated from known binary phase diagrams in a manner described previously. To reproduce the binary liquidus temperature, the kij parameters are usually chosen so as to give a best fit of the eutectic temperature and composition under the simplified assumptions that for a given binary system ij.

RT ln ci ¼ kij X 2j

ð11Þ 2

RT ln cj ¼ kij ð1  X j Þ

ð12Þ

From Eqs. (10)–(12), we obtain the equations:

kij ¼

For NaNO3–NaNO2

DGE23 ¼ X Y X Z ½I00 þ ðX Y  X Z ÞI01 

ðDG0 Þ2 2ZRT

¼

ðDHf ;j =T f ;j ÞT  RT ln X j  DHf ;j þ DC p;j ðT f ;j  T  T lnðT f ;j =TÞÞ ð1  X j Þ2 ðDHf ;i =T f ;i ÞT  RT lnð1  X j Þ  DHf ;i þ DC p;i ðT f ;i  T  T lnðT f ;i =TÞÞ X 2i ð13Þ

At the same time, according to the CIS theory, the expression for the total excess free energy of mixing, DGE, of the ternary mixture for AX–AY–AZ is deduced:

DGE123 ¼ DGE12 þ DGE13 þ DGE23 ¼ X X X Y k12 þ X X X Z k13 þ X Y X Z ½I00 þ ðX Y  X Z ÞI01 

ð6Þ

For BX–BY–BZ

DGE456 ¼ DGE45 þ DGE46 þ DGE56 ¼ X X X Y k45 þ X X X Z k46 þ X Y X Z ½I0 þ ðX Y  X Z ÞI1 þ ðX 2Y  4X Y X Z þ X 2Z ÞI2 

ð7Þ

The term K cannot be calculated from theory but is proportional to (DG°)2.

Table 1 Physical parameters for the single salts. Component

T oij (K)

DHofðijÞ (J mol1)

DC pðijÞ (J mol1 K1)

NaNO3 KNO3 NaCl KCl NaNO2 KNO2

582 610 1073 1043 557 711

14588.2 11704 27964.2 26501.2 14937 16720

0.879 2.887 2.158 6.658 1.505 4.5

685

Q. Peng et al. / Applied Energy 112 (2013) 682–689 Table 2 The standard molar Gibbs free energies of formation for pure salt. Component

NaNO3 NaNO2 NaCl

DfG° (T/K) = a + bT a

b

451.0527 347.855 411.565

0.299 0.205 0.093

Component

KNO3 KNO2 KCl

Table 6 Calculated eutectic point of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) referring to NaCl, NaNO2, NaNO3, KCl.

DfG° (T/K) = a + bT

Xþ Na

a

b

489.368 371.492 437.223

0.321 0.211 0.0963

0.1 0.3 0.5 0.7 0.9

Eutectic point Cl (mol.%)

NO 2 (mol.%)

NO 3 (mol.%)

Tf (K)

– – 0.01895 0.02265 0.02576

– – 0.8449 0.5589 0.4397

– – 0.1362 0.4184 0.5346

– – 447 446 468

Table 3 The standard molar Gibbs free energy change for the metathetical reaction. Reaction

DG° (T/K ) = a + bT

KCl + NaNO3 ? KNO3 + NaCl KCl + NaNO2 ? KNO2 + NaCl NaNO3 + KNO2 ? KNO3 + NaNO2 KNO3 + NaCl ? KCl + NaNO3 KNO2 + NaCl ? KCl + NaNO2 KNO3 + NaNO2 ? NaNO3 + KNO2

2.2. Experiment section

a

b

12.657 2.0208 114.678 12.657 2.0208 114.678

0.0181 0.003 0.0151 0.0181 0.003 0.0151

Table 4 Interaction coefficient (k) and eutectic composition (X) for binary system. Component

KNO3–NaNO3 KCl–NaCl KCl–KNO3 NaCl–NaNO3 KCl–KNO2 NaCl–NaNO2 KNO2–NaNO2

kij (J mol1)

2825.06 8604.036 1275.244 443.1042 1843.878 1959.365 651.2703

Eutectic point Calculated

Experimental

Tf (K)

X2 (mol.%)

Tf (K)

X2 (mol.%)

496 931 587 570 657 544 498

0.5 0.437 0.912 0.938 0.794 0.9306 0.693

496 931 581 571 – 543.48 481.13

0.49 0.5 0.905 0.934 – 0.9306 0.657

Potassium nitrate, sodium nitrite, sodium nitrate, potassium chloride and sodium chloride are bought from chemical company with A.R. grade and they are dried in an oven at 120 °C for 48 h. Then, molten salts are prepared by mechanical rolling. statically heating, natural cooling and mechanical grinding. The details of the preparation are given in an earlier paper [34]. The high temperature simultaneous thermal analyser (Q600 SDT) supplied by America TA companies is used in this study. In this apparatus, the temperatures are measured with a Pt/(Pt + 10%Rh) thermocouple. The thermocouple is calibrated using the following reference samples of Zn and Ag. The Al2O3pan is used to make calibration in order to avoid occurring chemical reaction. Table 5 shows the deviation analysis of the measurements for two samples. The relative errors of melting point for the samples Zn and Ag are 0.01% and 0.07% respectively, these meet the test requirement. The DSC is calibrated with thermal analysis standards samples and K2C2O4 for temperature and energy, and the sapphire for heat capacity. Small samples (<15 mg) are encapsulated in the graphite pans. Different DSC profiles of the salt mixtures are obtained using heating rates from 1.25 to 20 K min1, sample sizes of 1–20 mg, DSC energy scales from 0.5 to 5 mcal s1 and nitrogen flow of 100 mL s1 as the shielding gas.

Table 5 Melting point of Zn and Ag. Component

Measured value (K)

Literature value (K)

Relative error (%)

Zn Ag

692.73 1233.09

692.65 1233.95

0.01 0.07

3.1. Liquidus temperature of the quaternary reciprocal system (K, Na/NO2, Cl, NO3)

From which kij can be calculated from the measured value of the eutectic temperature. The calculations from Eq. (13) for each of the four binary systems are done on a computer using the measured eutectic temperature as a fixed point and the eutectic composition and kij as variables. This led to calculated values of Xi (given in Table 4) for the composition of each of the binary eutectics. These differ from the measured values [14,28] of Xi by less than 2 mol.%, which is within the experimental uncertainties of the measurements. For kij, the calculated values obtained are unique and in reasonable agreement with other available thermodynamic data. These values of kij are also listed in Table 4. Here, eutectic point of KCl–KNO2 is determined by using a simple ‘hard-sphere’ ionic interaction model [33], and that for NaCl–NaNO2 is measured by thermogravimetric analysis device. Meanwhile, referring to phase diagrams [14,28] and thermodynamic properties for KNO3–KNO2 and NaNO3–NaNO2, the interaction energy parameter kij for KNO3–KNO2 and NaNO3–NaNO2 is respectively given:

k56 ¼ 9520:2246  1078:2072  ðX A  X B Þ þ 1964:3884  ðX 2A  4X A X B þ X 2B Þ k23 ¼ 2712:955  735:2891  ðX A  X B Þ

3. Results and discussion

ð14Þ ð15Þ

An interactive computer program has been written to perform the calculation of the phase diagram. The program calculates liquidus temperatures over a grid of liquid compositions. It does this by calculating phase equilibrium of solid–liquid and partial molar excess free energies referring to the CIS theory with sets of input parameters Df Go ; DGo ; DC pðijÞ ; T oij and DHofðijÞ relevant to the salt species. Additional input parameters are the two kij values deduced from experimental data. The input variables are Xi and Xj. The iterative procedure gives Tij values consistent within 0.01 K. Taking NaCl, NaNO2, NaNO3 and KCl as the research object, the calculated phase diagrams of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) are presented graphically in Fig. 2, and the composition of eutectic point are shown in Table 6. These indicate the difference of liquidus temperatures is very small with increasing ratio of X Naþ from 0.5 to 0.9, but eutectic point is not obvious when the ratio of X Naþ is 0.1 and 0.3. At the same time, referring to KCl, KNO2, KNO3 and NaCl, phase diagrams and corresponding eutectic point of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) are also indicated in Fig. 3 and Table 7. These express a decreasing trend in the liquidus temperatures from 445 K to 543 K with increasing ratio of X Kþ from 0.3 to 0.9. Eutectic point is not obvious when the ratio of X Kþ is 0.1, so the lowest melting point mixture will exist when X Kþ is around 0.5. Experimental results also show the

686

Q. Peng et al. / Applied Energy 112 (2013) 682–689

XNa+ = 0.3

XNa+ = 0.1

XNa+ = 0.7

XNa+ = 0.5

XNa+ = 0.9 Fig. 2. Phase diagram of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) referring to NaCl, NaNO2, NaNO3 and KCl.

eutectic mixture will appear when X Kþ is 0.48, but eutectic point cannot be obtained when X Kþ is 0.54 and 0.51. Thermal stability of the mixtures with potassium ratio (X Kþ ) of 0.48 will be researched at next section.

3.2. Thermal stability Thermal stability of the mixture with potassium ratio (X Kþ ) of 0.48 is shown in Fig. 4. That indicates the weight percent of the

687

Q. Peng et al. / Applied Energy 112 (2013) 682–689

XK+ = 0.3

XK+ = 0.1

XK+ = 0.5

XK+ = 0.7

XK+ = 0.9 Fig. 3. Phase diagram of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) referring to KCl, KNO2, KNO3 and NaCl.

eutectic mixture is 98.8% at 500 °C, and that is 93% at 550 °C. This eutectic mixture is thermally stable at temperatures up to 500 °C, and may be used up to 550 °C for short periods. DSC curve shows melting point of this eutectic mixture is about 140 °C. Besides, thermal cycling of 1000 cycles at 50–550 °C for this mixture is pre-

sented in Fig. 5. That shows the fluctuate of melting point is between 134.08 °C and 136.07 °C, and the reduction rate is 25.36% from 70.83 J g1 to 52.87 J g1 for latent heat. Thermal cycling of eutectic mixture is better and the fluctuate of melting point is also small, so that is suitable for heat transfer and thermal storage

688

Q. Peng et al. / Applied Energy 112 (2013) 682–689

Table 7 Calculated eutectic point of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) referring to KCl, KNO2, KNO3, NaCl. Xþ K

0.1 0.3 0.5 0.7 0.9

Eutectic point Cl (mol.%)

NO 2 (mol.%)

NO 3 (mol.%)

Tf (K)

– 0.1760 0.2674 0.3708 0.5299

– 0.0440 0.0598 0.0772 0.1088

– 0.7797 0.6728 0.5520 0.3613

– 445 491 521 543

Table 8 Price of storage media. Component

Cost ($ kg1)

Component

Cost ($ kg1)

Therminol VP-1 Hitec Solar salt LiNO3 CsNO3

7.6 1.7 1.3 6.4 48.0

NaNO3 KNO3 NaNO2 NaCl KCl

0.5 0.8 0.45 0.1 0.4

nitrate and cesium nitrate can reduce the liquidus temperature and have a minor effect on viscosity and maximum operating temperature of molten salt mixtures, they have a disadvantage of the cost, especially for cesium nitrate. Our prepared eutectic salt shows a significant cost advantage as heat transfer fluids and storage media and the price of this mixture will be reduced by about 70 $ ton1 compared to Hitec and Solar Salt. Therefore, the chlorides can be considered as a energy storage material since they have the lowest price in this table. 4. Conclusion

Fig. 4. TG and DSC curves for decomposition of eutectic mixture.

Mixed molten salt is considered a promising medium for both heat transfer and energy storage in solar thermal power plants. Liquidus temperature of a new molten salt consisting of the quaternary reciprocal system (K, Na/NO2, Cl, NO3) is determined by the CIS theory and that shows this molten salt has a lower melting point relative to previously available materials. At the same time, experimental results also indicate this kind of molten salt has a lower melting point, 140 °C. It is thermally stable at temperatures up to 500 °C, and may be used up to 550 °C for short periods. These properties produce a broad operating range molten salt and enable effective thermal storage for parabolic trough concentrating solar power plants. Besides, this new salt has a reduced cost relative to previous low-melting nitrate mixtures due to the elimination of cesium nitrate and lithium nitrate. Acknowledgments This work is supported by National Basic Research Program of China (No. 2010CB227103), National Nature Science Foundation of China (No. 50930007) and National High technology R&D Program (2012AA050604). References

Fig. 5. Thermal cycling of eutectic mixture.

material. This data represent a laboratory screening measurement and may not provide a definitive prediction of the HTF long-term thermal stability in a commercial application. 3.3. Economic impact Generally speaking, there are a large number of molten salt for energy storage in solar thermal power plants, so the cost of constituent molten salt is specially important because it will directly affect the overall capital investment of thermal energy storage systems. Table 8 shows the price of the commonly used commercial storage media. That indicates Therminol VP-1 is relatively expensive comparing to Hitec and Solar Salt, and the price of sodium nitrate, potassium nitrate and sodium nitrite is also higher than sodium chloride and potassium chloride. Although lithium

[1] Glatzmaier GC, Gomez J, Ortega J, Starace A, Turchi C. High temperature phase change materials for thermal energy storage application. SolarPACES 2011, Granada, Spain; September 2011. [2] Kearney D, Herrmann U, Nava P, et al. Assessment of a molten salt heat transfer fluid in a parabolic trough solar field. J Sol Energy – Trans ASME 2003;125:170–6. [3] Kolb GJ. Conceptual design of an advanced trough utilizing a molten salt working fluid. SolarPACES 2008, Las Vegas, NeVada; March 2008. [4] Phillips WM, Stearns JW. Advanced latent heat of fusion thermal energy storage for solar power systems. In: Proceedings of the 20th intersociety energy conversion engineering conference, vol. 2; 1985. p. 384–91. [5] Fujiwara M, Sano T, Suzuki K, Watanabe S. Thermal analysis and fundamental tests on heat pipe receiver for solar dynamic space power system. In: Proceedings of the 23rd intersociety energy conversion engineering conference, vol. 2; 1988. p. 195–200. [6] Christian LB. Molten salts and nuclear energy production. J Nucl Mater 2007;360(1):1–5. [7] Kenisarin MM. High temperature phase change materials for thermal energy storage. Renew Sust Energy Rev 2010;14(3):955–70. [8] Shin BC, Kim SD, Park WH. Ternary carbonate eutectic (lithium, sodium and potassium carbonates) for latent heat storage medium. Sol Energy Mater 1990;21(1):81–90. [9] Mamantov G, Braunstein J, Mamantov CB. Advances in molten salt chemistry. New York: Plenum Press; 1981. [10] Kamimoto M, Tanaka T, Tani T, Horigome T. Investigation of nitrate salts for solar latent heat storage. Sol Energy 1980;24(6):581–7.

Q. Peng et al. / Applied Energy 112 (2013) 682–689 [11] Pacheco JE. Final test and evaluation results from the solar two project. Sandia National Laboratories. SAND2002-0120; January 2002. [12] Geyer M, Herrmann U, Sevilla A, Nebrera JA, Zamory AG. Dispatchable solar electricity for summerly peak loads from the solar thermal projects andasol-1 and andasol-2. Solar millennium, SolarPACES, Seville, Spain; 2006. [13] Badger Energy Corp. Design, handling, operation and maintenance procedures for Hitec molten salt. Sandia National Laboratories, SAND81-8179; January 1981. [14] Janz GJ et al. Physical properties data compilations relevant to energy storage. NSRDS-NBS 1981;61. Parts I, II, and IV. [15] Bradshaw RW, Cordaro JG, Siegel NP. Molten nitrate salt development for thermal energy storage in parabolic trough solar power systems. ASME Proc Energy Sustain 2009. [16] Bradshaw RW, Brosseau DA. Improved molten salt formulations for heat transfer fluids in parabolic trough solar power systems. SolarPACES 2008, Las Vegas, NeVada; March 2008. [17] Gladen H, Wasserscheid P, Medved M. Multinary salt system for storing and transferring thermal energy. WO 2008/071205 A1; 2008. [18] Cordaro JG, Bradshaw RW. Low-melting point heat transfer fluid. US patent no. 7828990; 2010. [19] Cordaro JG, Rubin NC, Bradshaw RW. Multicomponent molten salt mixtures based on nitrate/nitrite anions. J Sol Energy – Trans ASME 2011;133(1):011014/1–4/4. [20] Raade JW, Padowitz D. Development of molten salt heat transfer fluid with low melting point and high thermal stability. J Sol Energy – Trans ASME 2011;133(3):031013/1–3/6. [21] Raade JW, Padowitz D, Vaughn J. Low melting point molten salt heat transfer fluid with reduced cost. SolarPACES 2011, Granada, Spain; September 2011.

689

[22] Lin PL, Pelton AD, Bale CW. An interactive computer program for calculating ternary phase diagrams. Calphad 1980;4(1):47–60. [23] Blander M, Yosim SJ. Conformal ionic mixtures. J Chem Phys 1963;39(10):2610–6. [24] Reiss H, Katz JL, Kleppa OJ. Theory of the heats of mixing of certain fused salts. J Chem Phys 1962;36(1):144–8. [25] Gaune-Escard M. Calculation of phase diagrams of quaternary reciprocal systems. Calphad 1979;3(2):119–27. [26] Pelton AD, Lin PL. Calculation of phase diagrams of the reciprocal quaternary systems lithium, sodium, potassium/carbonate, sulfate; lithium sodium/ carbonate, sulfate, hydroxide; lithium potassium/carbonate, sulfate, hydroxide and sodium, potassium/carbonate, sulfate, hydroxide. Calphad 1983;7(4):295–303. [27] Saboungi ML. Calculation of thermodynamic properties of multicomponent ionic reciprocal systems. J Chem Phys 1980;73(11):5800–6. [28] Levin EM, Robbins CR, Mcmurdie HF. Phase diagrams for ceramists, vols. I– VIII. Columbus, Ohio: American Ceramic Society; 1964. [29] Janz GJ. Molten salts handbook. New York: Academic Press; 1967. [30] David RL. Handbook of chemistry and physics. 84th ed. Florida: CRC Press; 2004. p. 1038–65. [31] Chase Jr MW. NIST-JANAF thermochemical tables. 4th ed. Washington DC: American Chemical Society; 1998. [32] Barin I. Thermochemical data of pure substances. 3rd ed. New York: VCH Publishers; 1995. p. 1380–91. [33] Forland T. Properties of some mixtures of fused salts. Norg Tek Vitenskapsakad 1957;4. [34] Peng Q, Ding J, Wei XL, et al. The preparation and properties of multicomponent molten salts. Appl Energy 2011;87(9):2812–7.