Research on preparation and thermodynamic properties of chloride molten salt materials

doi:10.11949/0438-1157.20191541

Abstract

Abstract:

Six chloride molten salt materials, NaCl-CaCl₂, NaCl-KCl-CaCl₂, NaCl-CaCl₂-MgCl₂, KCl-CaCl₂-MgCl₂, NaCl-KCl-MgCl₂ and NaCl-KCl-CaCl₂-MgCl₂, were prepared. Then the DSC method was used to determine the eutectic temperature and composition of those molten salts, and the other thermophysical properties were also measured, which were heat capacity, density and viscosity. Also, the mass loss of molten salt materials was investigated to determine the upper limit of working temperature. Correspondingly, energy storage density was calculated for each salt. The results shows that NaCl-KCl-CaCl₂-MgCl₂ has an eutectic temperature of 380.3℃ with good fluidity. The operating temperature range is 430—700℃, and the energy storage density is 625.1 J?cm^-3. By comparison, NaCl-KCl-CaCl₂-MgCl₂ has the lowest eutectic temperature and the highest energy storage density, which is suitable to be used as heat transfer and heat storage material. The eutectic temperature of NaCl-KCl-CaCl₂ is 503.8℃, and the working temperature range is 550—850℃. The energy storage density is 559.9 J?cm^-3, that means NaCl-KCl-CaCl₂ is suitable for high-temperature heat storage material.

Key words: solar energy, chloride molten salts, preparation, thermo-physical properties, thermal stability, energy storage density, heat transfer, heat storage

摘要：

制备了NaCl-CaCl₂、NaCl-KCl-CaCl₂、NaCl-CaCl₂-MgCl₂、KCl-CaCl₂-MgCl₂、NaCl-KCl-MgCl₂、NaCl-KCl-CaCl₂-MgCl₂六种氯化物熔盐材料。采用差示扫描量热法确定它们的低共熔点和组成，测量其比热容、密度、黏度等热物性。测试熔盐材料的质量损失曲线确定工作温度上限，根据测试的结果，对其储能密度进行计算。研究结果表明：NaCl-KCl-CaCl₂-MgCl₂熔盐材料熔点为380.3℃，流动性较好，工作温度范围为430~700℃，储能密度为625.1 J·cm^-3，是六种熔盐中熔点最低、储能密度最大的熔盐，适合作为传热储热材料。NaCl-KCl-CaCl₂熔盐熔点为503.8℃，工作温度范围为550~850℃，储能密度为559.9 J·cm^-3，储能密度仅次于NaCl-KCl-CaCl₂-MgCl₂熔盐，适合作为高温储热熔盐材料。

关键词: 太阳能, 氯化物熔盐, 制备, 热物性, 热稳定性, 储能密度, 传热, 储热

CLC Number:

TQ 013

Xiaolan WEI, Pei XIE, Xuechuan ZHANG, Weilong WANG, Jianfeng LU, Jing DING. Research on preparation and thermodynamic properties of chloride molten salt materials[J]. CIESC Journal, 2020, 71(5): 2423-2431.

魏小兰, 谢佩, 张雪钏, 王维龙, 陆建峰, 丁静. 氯化物熔盐材料的制备及其热物理性质研究[J]. 化工学报, 2020, 71(5): 2423-2431.

Figures/Tables 11

Fig.1 DSC curves of eutectic composition of six kinds of molten salts

Table 1 Eutectic temperature and composition of six kinds of molten salt

样品	NaCl/%(mol)	CaCl₂/%(mol)	KCl/%(mol)	MgCl₂/%(mol)	低共熔点温度/℃	相变潜热/(J·g^-1)
NaCl-CaCl₂	49.03	50.97			499.2	152.6
NaCl-CaCl₂-KCl	41.72	52.16	6.12		503.8	178.2
NaCl-CaCl₂-MgCl₂	45.10	26.30		28.60	431	241.9
KCl-CaCl₂-MgCl₂		11.63	59.79	28.58	427.3	166.9
KCl-NaCl-MgCl₂	33.70		17.80	48.50	383.5	199.3
KCl-NaCl-CaCl₂-MgCl₂	31.80	6.00	16.80	45.70	380.3	228

Fig.2 Heat capacity of six kinds of molten salts as function of temperature

Table 2 Experimental values of heat capacity of six kinds of molten salts in different temperatures/(J?g-1?K-1)

温度T/℃	Na/Ca-Cl	Na/Ca/K-Cl	Na/Ca/Mg-Cl	K/Ca/Mg-Cl	Na/K/Mg-Cl	Na/K/Ca/Mg-Cl
420					1.044	1.061
440					1.034	1.056
460			1.028	1.032	1.046	1.059
480			1.040	1.049	1.059	1.062
500			1.045	1.068	1.071	1.066
550	0.758	0.865	1.045	1.108	1.097	1.076
600	0.870	0.887	1.053	1.121	1.137	1.090
650	0.923	0.971	1.059	1.116	1.176	1.105
平均比热容 $c ˉ p$ /(J?g^-1?K^-1)	0.850	0.908	1.045	1.082	1.083	1.072

Table 2 Experimental values of heat capacity of six kinds of molten salts in different temperatures/(J?g-1?K-1)

温度T/℃	Na/Ca-Cl	Na/Ca/K-Cl	Na/Ca/Mg-Cl	K/Ca/Mg-Cl	Na/K/Mg-Cl	Na/K/Ca/Mg-Cl
420					1.044	1.061
440					1.034	1.056
460			1.028	1.032	1.046	1.059
480			1.040	1.049	1.059	1.062
500			1.045	1.068	1.071	1.066
550	0.758	0.865	1.045	1.108	1.097	1.076
600	0.870	0.887	1.053	1.121	1.137	1.090
650	0.923	0.971	1.059	1.116	1.176	1.105
平均比热容 $c ˉ p$ /(J?g^-1?K^-1)	0.850	0.908	1.045	1.082	1.083	1.072

Fig.3 Densities of six kinds of molten salts as function of temperature

Table 3 Fitting formulas of densities of six kinds of molten salts as function of temperature

样品	ρ/(g·cm^-3)	R²
NaCl-CaCl₂	ρ=2.17-4.02×10^-4T	0.99351
NaCl-CaCl₂-KCl	ρ=2.20-4.41×10^-4T	0.98246
NaCl-CaCl₂-MgCl₂	ρ=2.22-5.03×10^-4T	0.99794
NaCl-KCl-MgCl₂	ρ=2.21-7.12×10^-4T	0.99408
KCl-CaCl₂-MgCl₂	ρ=1.89-3.83×10^-4T	0.99466
KCl-NaCl-CaCl₂-MgCl₂	ρ=2.22-4.87×10^-4T	0.98266

Fig.4 Viscosities of six kinds of molten salts as function of temperature

Table 4 Fitting formulas of viscosities of six kinds of molten salts as function of temperature

样品	μ/(mPa·s)	R²
NaCl-CaCl₂	$μ = 60.45986 - 0.1902 T + 0.000158 T 2$	0.99153
NaCl-CaCl₂-KCl	$μ = 67.97221 - 0.20979 T + 0.000176 T 2$	0.99032
NaCl-CaCl₂-MgCl₂	$μ = 47.39481 - 0.15309 T + 0.000133 T 2$	0.99347
NaCl-KCl-MgCl₂	$μ = 24.65093 - 0.07961 T + 0.000069 T 2$	0.97933
KCl-CaCl₂-MgCl₂	$μ = 33.71382 - 0.10975 T + 0.000093 T 2$	0.99877
KCl-NaCl-CaCl₂-MgCl₂	$μ = 31.86677 - 0.10261 T + 0.000090 T 2$	0.99146

Table 4 Fitting formulas of viscosities of six kinds of molten salts as function of temperature

样品	μ/(mPa·s)	R²
NaCl-CaCl₂	$μ = 60.45986 - 0.1902 T + 0.000158 T 2$	0.99153
NaCl-CaCl₂-KCl	$μ = 67.97221 - 0.20979 T + 0.000176 T 2$	0.99032
NaCl-CaCl₂-MgCl₂	$μ = 47.39481 - 0.15309 T + 0.000133 T 2$	0.99347
NaCl-KCl-MgCl₂	$μ = 24.65093 - 0.07961 T + 0.000069 T 2$	0.97933
KCl-CaCl₂-MgCl₂	$μ = 33.71382 - 0.10975 T + 0.000093 T 2$	0.99877
KCl-NaCl-CaCl₂-MgCl₂	$μ = 31.86677 - 0.10261 T + 0.000090 T 2$	0.99146

Fig.5 Mass loss curves of molten salts under isothermal conditions

Table 5 Properties and thermal energy storage density

样品	c_p/(J?g^-1?K^-1)	ρ/(g·cm^-3)	T_m/K	T_lim/K	E/(J?cm^-3)
NaCl-CaCl₂	0.850	$ρ = 2.17 - 4.02 × 10 - 4 T$	772.3	1073	460.0
NaCl-CaCl₂-KCl	0.908	$ρ = 2.20 - 4.41 × 10 - 4 T$	776.8	1123	559.9
NaCl-CaCl₂-MgCl₂	1.045	$ρ = 2.22 - 5.03 × 10 - 4 T$	704	973	505.5
KCl-NaCl-MgCl₂	1.083	$ρ = 2.21 - 7.12 × 10 - 4 T$	656.5	923	475.6
KCl-CaCl₂-MgCl₂	1.082	$ρ = 1.89 - 3.83 × 10 - 4 T$	700.3	973	463.1
KCl-NaCl-CaCl₂-MgCl₂	1.072	$ρ = 2.22 - 4.87 × 10 - 4 T$	653.3	973	625.1

Table 5 Properties and thermal energy storage density

样品	c_p/(J?g^-1?K^-1)	ρ/(g·cm^-3)	T_m/K	T_lim/K	E/(J?cm^-3)
NaCl-CaCl₂	0.850	$ρ = 2.17 - 4.02 × 10 - 4 T$	772.3	1073	460.0
NaCl-CaCl₂-KCl	0.908	$ρ = 2.20 - 4.41 × 10 - 4 T$	776.8	1123	559.9
NaCl-CaCl₂-MgCl₂	1.045	$ρ = 2.22 - 5.03 × 10 - 4 T$	704	973	505.5
KCl-NaCl-MgCl₂	1.083	$ρ = 2.21 - 7.12 × 10 - 4 T$	656.5	923	475.6
KCl-CaCl₂-MgCl₂	1.082	$ρ = 1.89 - 3.83 × 10 - 4 T$	700.3	973	463.1
KCl-NaCl-CaCl₂-MgCl₂	1.072	$ρ = 2.22 - 4.87 × 10 - 4 T$	653.3	973	625.1

Fig.6 Mass loss curves of NaCl-KCl-CaCl2和NaCl-KCl-CaCl2-MgCl2 molten salts

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