• Energy Research

The development of heat transfer fluids (HTF) and heat storage materials (HSM) is crucial to design concentrating solar plant (CSP). Binary alkaline nitrate mixtures are currently used as sensible thermal energy storage materials. However, multi-component nitrate/nitrite systems were proposed as possible better candidates. In particular, ternaries mixtures containing sodium, potassium, and calcium are extremely promising as thermal fluids, given their reduced toxicity and greater cost-effectiveness. Nevertheless, very few data are present in the scientific literature regarding the correspondent phase diagram, and only the properties of specific compositions are reported. For this reason, a regular solution model was developed and employed in this work, and validated by comparing the simulation results with experimentally obtained phase change values. In particular, given that the common calorimetric techniques are impracticable for detecting the transition temperatures of calcium containing nitrate mixtur...

Molten salts eutectics are promising candidates as phase change materials (PCMs) for thermal storage applications, especially considering the possibility to store and release heat at high temperatures. Although many compounds have been proposed for this purpose in the scientific literature, very few data are available regarding actual applications. In particular, there is a lack of information concerning thermal storage at temperatures around 600 °C, necessary for the coupling with a highly efficient Rankine cycle powered by concentrated solar power (CSP) plants. In this contest, the present work deals with a thermophysical behavior investigation of a storage heat exchanger containing a cost-effective and safe ternary eutectic, consisting of sodium chloride, potassium chloride, and sodium carbonate. This material was preliminarily and properly selected and characterized to comply with the necessary melting temperature and latent enthalpy. Then, an indirect heat exchanger was considered for the simulation, assuming aluminum capsules to confine the PCM, thus obtaining the maximum possible heat exchange surface and air at 5 bar as heat transfer fluid (HTF). The modelling was carried out setting the inlet and outlet air temperatures at, respectively, 290 °C and 550 °C, obtaining a realistic storage efficiency of around 0.6. Finally, a conservative investment cost was estimated for the storage system, demonstrating a real possible economic benefit in using these types of materials and heat exchange geometries, with the results varying, according to possible manufacturing prices, in a range from 25 to 40 EUR/kWh.

When designing a concentrating solar power (CSP) system, selection of a proper heat transfer fluid (HTF) material is a key, especially when employed in parabolic trough CSP plants. In particular, the use of low melting mixtures as an alternative to the widely commonly used “solar salt” can increase the CSP manageably and, as a result, several innovative nitrite/nitrate mixtures have been proposed. However, very few thermodynamics data are available for these compounds, especially regarding ternary compositions. One of the most interesting low freezing mixture is prepared with sodium and potassium nitrate together with sodium nitrite. The aim of this work is to investigate the thermodynamics properties of this ternary system, starting from its binary subunits, studying the phase diagram of this compound both experimentally and by a regular solution model. At this purpose, the literature phase diagrams of the binary subsystem were simulated in order to obtain the fitting parameters necessary for the employed semi-predictive tool. Then, the ternary system was modeled and the results showed very good agreement with the experimental points. It is quite interesting to note that both the theoretical and experimental results showed a low melting zone presenting greater sodium nitrate molar fractions with respect to sodium nitrite than previously reported in literature. This would lead to a decrease in the HTF price and an improvement regarding the fluid toxicity.

Abstract The production of electric energy from solar radiation is nowadays one of the most investigated and developed “carbon free” technology. A throughout investigation of the ceramics most commonly used as heat transfer fluids and/or heat storage materials for concentrating solar power systems (i.e. alkaline and heart alkaline nitrate/nitrite mixtures) is here reported. The study stems by the need to base materials selection on an accurate and critical knowledge of all their characteristics, including their thermophysical, environmental compatibility, and economic features. At this purpose, a rating criterion have been established, to readily show the advantages and disadvantages of each material, and to highlight which characteristics of the examined materials need to be further investigated and improved. Nitrate/nitrite mixtures have also been compared with other ceramics used for thermal storage, such as solid fillers, liquid metals, other salt mixtures, or phase change materials.

Different fluid compositions have been considered as heat transfer fluids (HTF) for concentrating solar power (CSP) applications. In linear focusing CSP systems synthetic oils are prevalently employed; more recently, the use of molten salt mixtures in linear focusing CSP systems has been proposed too. This paper presents a comparative assessment of thermal oils and five four nitrate/nitrite mixtures, among the ones mostly employed or proposed so far for CSP applications. The typical medium-size CSP plant (50 MWe) operating with synthetic oil as HTF and the “solar salt” as TES was considered as a benchmark. In the first part of the paper, physical properties and operation ranges of different HTFs are reviewed; corrosion and environmental issues are highlighted too. Besides an extensive review of HTFs based on data available from the open literature, the authors report their own obtained experimental data needed to thoroughly compare different solutions. In the second part of the paper, the impact of the different HTF options on the design and operation of CSP plants are analyzed from techno-economic perspectives.

Thermal energy storage technology with Phase Change Materials (PCM) is an attractive option to optimise energy resources and to recover and promote excess heat. The phase change behaviour of PCM requires advanced research to understand and better control the thermal energy storage using PCM, which is a crucial step to develop a powerful latent storage system. This paper aims to analyse the multiphysics phenomena of three regenerator configurations, horizontal case and two injection direction of Heat Transfer Fluid (HTF): top and bottom in vertical case. The study is done for the charge and discharge cycles of the solid-solid and solid-liquid phase transitions of PCM. First, the temperature dependence of the thermal and physical properties of paraffin as PCM is characterised. Second, an experimental study of an annular latent storage system was carried out. Also, an experimental mesh method was introduced to compare the energy behaviour of the three cases. Third, a numerical analysis of the experimental storage unit with low thermal diffusion is performed. The experimental results are confronted with the numerical results obtained with ANSYS Fluent and COMSOL Multiphysics commercial software. Last, the three configurations are compared to a reference case without gravitational field. The results show that specific mechanisms control the thermal and energetic behaviour of the regenerator. Furthermore, several parameters, such as storage density, distribution of energy storage rate in the different regenerator components (PCM, HTF, and heat exchanger), were analysed. Altogether, the results supply important information to understand the dynamics of passive storage systems.

Abstract The selection of a proper heat transfer (HTF) and storage (HSM) material is a key parameter in the design of a CSP system, especially in the employment of parabolic trough concentrating solar power plants (CSP). The use of nitrates as HTF, alternatively to thermal oils, can increase the CSP efficiency and several mixtures have been proposed at this aim. The most employed molten nitrate salt is the so called “solar salt” that presents several advantages but also a relative high liquidus temperature point of 238 °C. The use, either as HTF or HSM, of a low melting mixtures is currently considered a promising alternative to decrease the investment cost and improve the manageability of CSP plants. In this work, a low melting fluid was selected, composed of calcium, sodium and potassium nitrate, and proposed to be used both as HTF and HSM in a direct and active storage system. The electric LCOEs was calculated for a medium size (50 MWe) plant located at the Priolo Gargallo site (Sicily-Italy). In order to evaluate the obtained outcome, the identical procedure was carried out considering the same size and location, and the “solar salt” and a thermal oil as heat transfer fluids, with the former as storage medium. The comparison between the results show that there is a clear, although limited, economic benefit in using the ternary mixture.