In 2003 ENEA realized the PCS experimental Facility at Casaccia Research Centre (Rome, Italy), in order to test in real operating conditions the components of a parabolic trough solar plant, and to evaluate the technical feasibility of using the solar molten salts mixture (60% NaNO₃, 40% KNO₃, melting point 220÷240°C) in such a type of plant. ENEA also had the need to assess the behaviour of the solar receiver tubes during abnormal operating situations (wrong operation, pump block, power failure, etc.), when a block of the circulation of the molten salts may occur and cause the cooling or, worse, the freezing of the salts mixture inside the pipes. Some experimental tests have been performed, aimed to examine what happens in such a cases. In fact, without quick maneuvers to restart the circulation of the molten salts or to readily empty the receiver tubes, the molten salts contained in them may cool down to temperatures near/below the solidification. In this report are shown the results of the experimental tests carried out on the receiver tubes of the PCS Facility by cooling the process fluid down to temperatures near or below its freezing point. The tests show that the solidification of the salts does not damage the components of the plant but it is manageable and reversible, provided you apply the correct procedures.

Among the concentrating solar power (CSP) technologies, the parabolic trough (PT) solar collector is a proven technology mainly used to produce electricity and heat for industrial processes. Since 2003, a stand-alone Molten Salt Parabolic Trough (MSPT) experimental plant, located in the ENEA research centre of Casaccia (PCS plant), has been in operation. In this paper a brief description of the plant, the main plant operation figures, and a report of the main results obtained during the experimental test campaigns are presented. The aim of the tests was the evaluation of the thermal power collected as a function of DNI, mass flow rate, and inlet temperature of molten salt; experimental data were compared with simulation results obtained using a heat transfer software model of the solar receiver.

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.