In the context of Concentrated Solar Power (CSP) technology, this paper presents a comparison between the Parabolic Trough Collector (PTC) and the Linear Fresnel Collector (LFC), considering both evacuated and non-evacuated receiver tubes. The comparison was carried out in terms of the Levelized Cost of Electricity (LCOE) considering a reference year and four locations in the world, characterized by different levels of direct normal irradiation (DNI) from 2183 kWh/m²/year to 3409 kWh/m²/year. The LCOE depends on economic parameters and on the net energy generated by a plant on an annual basis. The latter was determined by a steady-state 1D model that solved the energy balance along the receiver axis. This model required computing the incident solar power and heat losses. While the solar power was calculated by an optical ray-tracing model, heat losses were computed by a lumped-parameter model developed along the radial direction of the tube. Since the LFC adopted a secondary concentrator, no conventional correlation was applicable for the convective heat transfer from the glass cover to the environment. Therefore, a 2D steady-state CFD model was also developed to investigate this phenomenon. The results showed that the PTC could generate a higher net annual energy compared to the LFC due to a better optical performance ensured by the parabolic solar collector. Nevertheless, the difference between the PTC and the LFC was lower in the non-evacuated tubes because of lower heat losses from the LFC receiver tube. The economic analysis revealed that the PTC with the evacuated tube also achieved the lowest LCOE, since the higher cost with respect to both the LFC system and the non-evacuated PTC was compensated by the higher net energy yield. However, the non-evacuated LFC demonstrated a slightly lower LCOE compared to the non-evacuated PTC since the lower capital cost of the non-evacuated LFC outweighed its lower net annual energy yield. Finally, a sensitivity analysis was conducted to assess the impact on the LCOE of the annual optical efficiency and of the economic parameters. This study introduces key technical parameters in LFC technology requiring improvement to achieve the level of productivity of the PTC from a techno-economic viewpoint, and consequently, to fill the gap between the two technologies.

The expected increase in energy production from VRE (Variable Renewable Energy) requires a significant increase in energy storage capacity, with thermal storage potentially offering a key contribution. However, heat transfer mechanisms can limit the maximum power instantaneously transferable to the storage medium, posing a significant operational challenge. An analysis is presented here of the power limitations that arise when molten salt thermal storage adopting Solar Salt (NaNO₃/KNO₃, 60/40%wt) is heated by electrical resistances (Joule heating), and a possible alternative—the volumetric heating of the salt mass by microwaves—is discussed. Results show that microwave heating is an interesting path to overcome the power limitations of Joule heating. A first, theoretical analysis indicates a potential increase of more than 10 times in the maximum power transferable per unit area. Thermal-fluid-dynamic and electromagnetic models have been developed to numerically test the performance of a one-tank thermocline system endowed with a microwave heater. The proposed heating system showed limitations in terms of the maximum power that can be transferred to the salt because of the high temperatures established in the boundary layer. Finally, it performs in a comparable way with respect to an (ideal) heating system based on the Joule effect. However, many design improvements can still be adopted to enhance the performance of the proposed technology, likely overcoming the performance reachable using Joule heating systems.

This work aims at enhancing the photo-thermal performance of a parabolic trough collector (PTC) system by implementing multiple coatings arrayed along the receiver tube. For this purpose, a lumped-parameter model was developed in the radial direction of the receiver tube to compute absorber tube wall temperature and heat losses at various heat transfer fluid (HTF) temperatures. The HTF is a mixture of molten salt (60%wt. NaNO₃ + 40%wt. KNO₃). The lamped-parameter model was exploited by a 1D model developed in the axial direction to determine the HTF temperature profile along the tube. The 1D model was employed to calculate photo-thermal efficiency at different HTF temperatures considering six selective coating formulations. Consequently, the most photo-thermally efficient configuration of the PTC system was determined, encompassing three HTF temperature ranges characterized by three different selective coating formulations. These temperature ranges were 290–436 °C (low temperature), 436–517 °C (medium temperature) and 517–550 °C (high temperature). The respective tube lengths were computed to be 792 m, 566 m and 293 m, considering the reference operational conditions. The optimal configuration enhanced the overall photo-thermal efficiency by 0.5–1.9% compared to the single-coated configurations. Furthermore, receiver cost could be reduced because of the employment of the more expensive coating only at the final segment.