• Energy Research

AbstractThis paper presents a project named CoMETHy (Compact Multifuel-Energy to Hydrogen converter) co-funded by the European Commission under the Fuel Cells and Hydrogen Joint Undertaking (FCH JU). The project is developing an innovative steam reformer for pure hydrogen production to be powered by Concentrating Solar Power (CSP) plants using molten salts as heat transfer fluid. Due to the limitations in the molten salts maximum operating temperature of 550°C, the reformer should operate at lower temperatures than conventional steam reforming processes. This implies the development of an innovative system, involving different R&D topics presented in this paper.

One of the most interesting perspectives for the development of concentrated solar power (CSP) is the storage of solar energy on a seasonal basis, intending to exploit the summer solar radiation in excess and use it in the winter months, thus stabilizing the yearly production and increasing the capacity factor of the plant. By using materials subject to reversible chemical reactions, and thus storing the thermal energy in the form of chemical energy, thermochemical storage systems can potentially serve to this purpose. The present work focuses on the identification of possible integration solutions between CSP plants and thermochemical systems for long-term energy storage, particularly for high-temperature systems such as central receiver plants. The analysis is restricted to storage systems potentially compatible with temperatures ranging from 700 to 1000 °C and using gases as heat transfer fluids. On the basis of the solar plant specifications, suitable reactive systems are identified and the process interfaces for the integration of solar plant/storage system/power block are discussed. The main operating conditions of the storage unit are defined for each considered case through process simulation.

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...

The blast furnace slags are a by-product of the production process of cast iron, during which large amounts of liquid slags are formed. The composition of the blast furnace slags depends on the actual quality and proportion of the minerals and fluxes present in the blast furnace charge. Every year the steel industry in Europe produces 2900 tons of slags that, if left untreated, represent an industrial waste to be sent to landfills with serious environmental impact. In the RESLAG (Turning waste from steel industry into valuable low cost feedstock for energy intensive industry) project, the use of waste products deriving from iron and steel plants as new feedstock in different fields is considered: recovery of precious metals, thermal energy storage systems for steel-making and CSP industry, production of innovative refractory ceramic compounds. Within this framework, ENEA is investigating the possibility of using pebbles made with slags produced by the steel industry as a filler in high-temperature packed-bed thermocline TES systems, using a binary mixture of molten salts (60% NaNO3 40% KNO3) as HTF. More specifically, the pebbles are obtained by processing and sintering Electric Arc Furnace (EAF) slags produced during the manufacture of crude steel. Here, the lessons learned and the first experimental results collected during the commissioning phases of the PBTTES pilot plant developed within the RESLAG project are reported. In particular, the paper firstly reports about a preliminary set of tests carried out to check the chemical compatibility of the slags with the molten salts. Subsequently, the pilot plant is described and the results of the first commissioning tests, which were aimed at flooding the packed-bed with molten salts and checking the installed instrumentation, are reported.

The NEXTOWER H2020 EU project investigates the possibility of using liquid lead as heat storage medium for high-temperature Thermal Energy Storage (TES) in concentrated solar power plants. To that end, within such project, a demonstration TES unit named SOLEAD (SOlar LEAd Demonstrator) is being developed to be coupled with an open volumetric air receiver in a solar tower CSP system. The SOLEAD demonstrator will use pure lead as working fluid and will be tested stand-alone, to address structural material behaviour at very high temperature, without coupling with air receiver. The tests are planned by the ENEA Brasimone R.C. in Italy. The introduction of the paper provides a general summary of the TES and CSP technology. Then a section is devoted to the conceptual design of SOLEAD. In the lead stand-alone experiment, the focus is on the materials corrosion in lead environment. Although the thermal stratification in the main vessel cannot be reproduced in a stand-alone experiment, the thermal cycle of the facility in the range 600-750°C will be properly reproduced in the experimental test. To this aim, external heating cables will heat up the system from 600°C to 750°C, while a proper Air Cooling System (ACS) will cool down the lead pool. A section of the paper contains the design criteria and calculation of the ACS with the air flowing on an annular gap between the vessel and the insulation. The power provided and extracted by the two systems (heating cables and ACS) is around 30 kW, so that the temperature range 600-750°C can be covered in about 8 hours and a complete cycle can be carried out in one day. Finally, a brief summary of the operational procedures needed from lead melting to the materials inspection is provided. The plan is to test the vessel for 4 months with daily thermal cycles to assess the resistance of FeCrAl materials exposed to very high temperature lead.

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 A linear receiver able to achieve temperatures up to 800 °C is presented. The high-temperature resistance is achieved by avoiding critical aspects (vacuum, glass-metal joints, surface films) that limit the temperature in usual receivers; the thermal insulation is obtained by enclosing the receiver tube in an elliptic reflecting cavity. The tube is placed near a focus of the cavity, and the primary collector concentrates the radiation on the other focus, where the cavity has a small opening: the ellipse reflects the radiation toward the tube and largely contains the reflected radiation and thermal emission, thus acting both as a secondary reflector and as a cavity receiver. Optical and thermal simulations show that temperatures up to 800 °C can be achieved, with optical efficiency above 70% and thermal efficiency in the range 45–85% for temperatures in the range 500–800 °C; the local overall efficiency ranges from about 40% to 66%, depending on the receiver tube emissivity and fluid temperature. In this way, the field of applicability of the linear collector technology can be significantly extended to include a vast amount of processes such as thermochemical cycles for hydrogen production, and solar fuel production processes, which require temperatures above 700 °C.