| There are serious mismatches in space,time and intensity between supply and demand of renewable energy,such as wind power,solar photovoltaic,solar thermal power.Heat storage materials and large-scale energy storage technologies with high performance are urgently needed to solve the mismatches and improve energy utilization efficiency.Molten salts phase change materials are potential heat storage mediums for large-scale thermal energy storage,especially the heat storage materials whose phase change temperature are in the range of 550～720°C,they are expected to improve the operation stability,power generation efficiency and reduce the levelized cost of electricity(LCOE).However,there are few researches on the design and selection of molten salt with low-cost and high latent heat,and also their heat storage characteristics.Based on the development and demand of renewable energy,the contents studied are summarized and listed as follows:1)The candidate single-component salts were screened based on the cost of unit energy(￥/kJ),energy storage density(kJ/m~3),thermal stability and safety.The fluoride and chloride salts of Na-K-Mg-Ca were the potential single-component salts that could form molten salts with low-cost and high-latent heat.An algorithm was developed for the design of binary fluoride and chloride mixture molten salts based on artificial neural network(ANN).The algorithm has a good accuracy,and the prediction error of it was less than 1.5%.The fluoride and chloride mixture molten salt systems were designed by ANN algorithm and CALPHAD thermodynamic calculation.Combined with the composition,latent heat and cost obtained from literature retrieval and empirical formula estimation,a series low-cost and high latent heat molten salts which could meet the requirement of the existing and next generation solar concentrated power(CSP)were recommended,such as NaF-NaCl-KCl molten salt systems.2)In order to evaluate performance of the molten salt phase change materials,heat storage characteristics for latent heat storage of NaF-NaCl-KCl molten salt systems were characterized and predicted.The relationship of molten salt composition and thermophysical properties were explored,and the economy of NaF-NaCl-KCl molten salt systems for latent heat storage was analyzed based on experimental results.Melting points of NaCl-KCl,Na F-KCl,NaF-NaCl and NaF-NaCl-KCl were 660°C,649°C,679°C and 604°C,respectively.It basically covers the working temperature range of the next generation CSPs,and has a reasonable temperature gradient.The latent heat of these molten salts are 308.7,454.8,555.0 and 398.4 kJ/kg,respectively.The high latent heat indicate they are very suitable as phase change heat storage materials.The specific heat of these molten salts increases with the increase of temperature,while the density decreases with the increase of temperature.The specific heat and density can be linearly regressed to the relationship of composition and temperature.The four kinds of molten salts in NaF-NaCl-KCl system almost have no weight loss at 720°C,and the temperature at which weight loss more than 3wt%is above 770°C.Based on the above experimental data,the economic analysis and comparison were conducted,which showed that molten salts in NaF-NaCl-KCl system have obvious advantages over the molten salts adopted latent heat reported in literature and the existing typical molten salts with sensible heat.The suitable temperature,low cost,high latent heat and thermal stability for the molten salts in NaF-NaCl-KCl system make them can meet the development needs of renewable energy such as solar thermal power generation stations.3)The corrosion resistance of structural materials in molten salt determines the safety and cost of CSPs.In order to evaluate the corrosion resistance of structural materials,the corrosion behaviors of 316 stainless steel in NaF-NaCl-KCl molten salt was studied by immersion corrosion test,and the corrosion mechanism was also discussed.After immersion corrosion,an oxide layer was found on the sample,the weight gain increased with the increase of temperature and time.From the profile morphology analysis,the corrosion rate(including Cr poor layer)is about 60μm/a.The NaF-NaCl-KCl molten salt has a very good compatibility with 316 stainless steel.The corrosion of 316stainless steel in NaF-NaCl-KCl is mainly caused by Cr diffusion loss and selectively corrosion.A corrosion zone with three-layer structure is formed after corrosion,it contains a surface layer which rich of Fe and Ni,a middle layer which rich of Cr and O,and a bottom layer which Cr is poor.The reason for this structure may be that the corrosion layer formation and its thickness increase inhibit the outward diffusion rate of Cr in the alloy,while the diffusion of corrosive substances into the alloy is not affected.The molten salts in NaF-NaCl-KCl system has the advantages of suitable phase change temperature,high latent heat,good thermal stability,quite economical and good compatibility with structural materials.It make them can meet the requirements of renewable energy such as solar thermal power generation stations and deserves further study. |
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