Valeria Russo

The objective of this study is to present updated estimates of the cost of concentrated solar power (CSP) projects that are relevant for the current Italian market. To this end, a comparative techno-economic analysis of different CSP plant types based on parabolic trough (PT) collectors was conducted, considering various scenarios with respect to heat transfer fluids, location and storage size. The scenarios were defined in accordance with a number of requirements, including the utilization of technologies produced in Europe (in alignment with the EU's Net-Zero Industry Act), the availability of performance and cost data from existing literature, and/or the authors' own experience, as well as the eligibility for incentives according to the initial drafts of the new support measure. The analysis employed two distinct modelling tools, developed by ENEA and DLR, respectively. These offered varying levels of detail in the description of the plant behavior. Thus, cross-checking of the results was enabled. Additionally, insights were gained concerning the impact of the chosen modelling approach on performance assessment.

Parabolic trough collector (PTC) plants that use solar salt as a heat transfer fluid face operational challenges due to the salt’s relatively high solidification temperature of around 240 °C, which can compromise reliability if solidification occurs within receiver tubes or piping. While electric tracing cables are typically used to heat piping, they cannot be installed on PTC receivers due to the presence of external glass covers. As an alternative, impedance heating can be employed, applying voltage directly to the steel receivers, which act as resistive heaters. This study presents experimental results on the phase-change behavior of solar salt within receivers, focusing on melting and solidification times. Tests were conducted using two dedicated receivers under vacuum and non-vacuum conditions. Under vacuum, complete melting was achieved at 4.5 V and 1.43 kW in 5.5 h, while solidification from 270 °C took about 4 h, progressing inward from the tube connections. For non-evacuated receivers, 7 V and 3.2 kW were needed for melting in 5.6 h, and solidification at 270 °C was completed in 1.45 h. These outcomes illustrate that non-evacuated tubes require nearly twice the power and have a 2.8-fold increase in heat loss rate, offering quantitative guidance for vacuum loss detection in PTC systems.

Concentrated Solar Power (CSP) plants integrated with Thermal Energy Storage (TES) represent a promising renewable energy source for generating heat and power. Binary molten salt mixtures, commonly referred to as Solar Salts, are utilized as effective heat transfer fluids and storage media due to their thermal stability and favorable thermophysical properties. However, these mixtures pose significant challenges due to their high solidification temperatures, around 240 °C, which can compromise the longevity and reliability of critical system components such as pressure sensors and bellows seal globe valves. Thus, it is essential to characterize their performance, assess their reliability under various conditions, and understand their failure mechanisms, particularly in relation to temperature fluctuations affecting the fluid’s viscosity. This article discusses experimental tests conducted on a pressure sensor and a bellows seal globe valve, both designed for direct contact with molten salts in CSP environments, at the ENEA Casaccia Research Center laboratory in Rome. The methodology for conducting these experimental tests is detailed, and guidelines are outlined to optimize plant operation. The findings provide essential insights for improving component design and maintenance to minimize unplanned plant downtime. They also offer methodologies for installing measurement instruments and electrical heating systems on the components.

Concentrated Solar Power (CSP) plants integrated with Thermal Energy Storage (TES) represent a promising renewable energy source for generating heat and power. Binary molten salt mixtures, commonly referred to as Solar Salts, are utilized as effective heat transfer fluids and storage media due to their thermal stability and favorable thermophysical properties. However, these mixtures pose significant challenges due to their high solidification temperatures, around 240 °C, which can compromise the longevity and reliability of critical system components such as pressure sensors and bellows seal globe valves. Thus, it is essential to characterize their performance, assess their reliability under various conditions, and understand their failure mechanisms, particularly in relation to temperature fluctuations affecting the fluid’s viscosity. This article discusses experimental tests conducted on a pressure sensor and a bellows seal globe valve, both designed for direct contact with molten salts in CSP environments, at the ENEA Casaccia Research Center laboratory in Rome. The methodology for conducting these experimental tests is detailed, and guidelines are outlined to optimize plant operation. The findings provide essential insights for improving component design and maintenance to minimize unplanned plant downtime. They also offer methodologies for installing measurement instruments and electrical heating systems on the components.

In the framework of the European Optimised Microturbine Solar Power system (OMSoP) project, a novel energy system for solar electricity production was developed, based on the integration of the solar dish technology with Micro Gas Turbines (MGT). A pilot plant with a capacity of 5–7 kW_e was realized and installed at the ENEA Casaccia site (Rome) and went under testing to validate the feasibility of the technology and improve the current design. The present work deals with the development of a quasi-state system model, built in the Engineering Equation Solver environment, composed of different modules that correspond to the main system components. The system model was used to define the optimal system parameters, to help the elaboration on an operational strategy to maximize the overall plant efficiency, and to guide the improvement of the single components in view of their optimised design. From the analysis it emerged that the system in design conditions is able to generate, in nominal conditions, 4.5 kW_e instead of the expected 5 kW_e due to the limitation of the stator current to 13 A, while maximum levels of 5.6 kW could be achieved by “overcharging” the high-speed generator up to 15 A and operating the MGT at the very high speed of 150 krpm. From the transient simulation of the demo system on an annual basis, the maximum average output power is 3.58 kW_e. Regarding the cycle efficiency, the annual averaged value is about 17%, whereas the target value is 21%. The improvement of the generator only does not seem to significantly increase the power output on the annual basis (3.75 kW_e vs. 3.58 kW_e). Differently, the improvement of the solar dish, with the upgrade of the other system components, would significantly increase the system power output to around ~10 kW_e.

Thermal energy storage (TES) optimization for Concentrated Solar Power (CSP) plants is a key component to improve dispatchability and power production, regardless of sunlight availability. ORC-PLUS, in the frame of Horizon2020, aims to deepen this aspect for an existing CSP plant coupled with a 1 MWe Organic Rankine Cycle (ORC) system, located in a desert area in Ben Guerir in Morocco and using linear Fresnel collectors as well as thermal oil as heat transfer fluid (HTF). In this context, it has been planned the implementation of a TES based on only one tank with molten salt and two integrated heat exchangers. This storage will be powered by an additional dedicated solar field. Aim of this paper is the analysis of the behaviour of the TES under different operating conditions using a proper computational model.