• Energy Research

The thermal emissivity of selective solar absorbers is a critical determinant of their operational efficiency. This paper details the computation of hemispherical emissivity for multi-layered coatings, specifically designed and optimized for solar thermal applications, particularly within high vacuum flat collectors at temperatures up to 200°C. We employed a MATLAB script, underpinned by the transfer matrix method, to calculate the hemispherical emissivity. The calculated emissivity values aligned well with experimental measurements derived from sputter-deposited samples. Our findings underscore the efficacy of the designed multi-layered coatings in relation to their thermal emissivity characteristics, thus highlighting their potential for solar thermal applications.

The orientation of solar collector arrays significantly influences the efficiency and performance of solar thermal systems. However, in some instances, practical limitations may prevent the optimal orientation of these collector arrays. This research investigates the performance of a field of High Vacuum Flat Collectors (HVFPCs) under suboptimal conditions.Our test facility features 25 arrays located on both sides of a building's roof. Each array forms a 6° angle with the horizontal plane. Of these, fourpanels are on the east-facing surface,and threeare on the west-facing surface. All collectors within the arrays have a tilt angle of 15° and an azimuth angle of 0°.We began performance assessments using a single pyranometer placed between the two roof sides to gauge solar irradiation. Later, we used two pyranometersone on each roof side. Both had identical tilt and azimuth as the collectors, ensuring independent irradiation measurements.When comparing these measurements to numerical results from a dynamic simulation, we discovered that a comprehensive plant performance evaluation must account for the varied orientations of collectors in the arrays. These results underscore the need to consider collector orientation's impact on performance, even if ideal orientations are unattainable due to installation challenges. Understandinghow non ideal collector array orientation affects system efficiency leadsto improved flow rate regulation techniques, optimizing performance and energy use

In the quest for more efficient solar thermal systems, accurately determining the thermal emittance of low-emissive materials is crucial in determining the power losses. This paper describes the calorimetric method designed to precisely measure the thermal emittance of Selective Solar Absorbers (SSAs) to be used in High Vacuum Flat Plate Collectors (HVFPCs). The method’s capability is demonstrated through the successful correction of thermal emittance values for copper samples of varying sizes, including dimensions down to 49 cm2. Results highlight the method’s potential to significantly reduce measurement errors associated with small-size and/or low-emittance samples, providing a path forward to improve the design and efficiency of SSAs. This research marks a significant step in advancing solar thermal technology by enabling emittance measurements with a precision better than 0.003, which is essential for the development of high-performance solar thermal absorbers. The method has also been applied to correct the thermal emittance value of SSA measured in previous measurement campaigns, and it allows a better estimation of the SSA efficiency conversion curve.

We investigate the performance of a novel flat photovoltaic-thermal (PV-T) module under high-vacuum through a 1D numerical model based on steady-state energy balance, with the aims of optimizing the simultaneous production of thermal and electrical energy. In the proposed design, the photovoltaic (PV) cell is positioned directly above the selective solar absorber (SSA), in a multilayer or fully integrated PV-SSA structure, which allows full exploitation of spectral solar radiation. In fact, in this configuration the losses related to non-absorption of low-energy photons and thermalization, typical of a classical single-junction PV cell, are reduced. The present study is conducted as the emittance and energy bandgap of the PV layer varied, thus admitting a wide variety of materials into the analysis. The dependence of the temperature coefficient, β(%/K), on the energy bandgap of the PV cell is also included. In the last part of the work, we discuss the performance of the proposed evacuated PV-T equipped with a SSA layer and thin film solar cells, namely those made of CdTe, CdS and GaAs. Overall, the paper highlights the great advantage of using high vacuum insulation, which suppresses conductive losses, and the versatility of the proposed system, which could be adapted to the user's needs simply by choosing the appropriate material for the photovoltaic layer.

Abstract Thermal piping insulation of implants is crucial for heat delivery, production, collection, or storage at high temperature values. It is currently obtained by enveloping low thermal conductivity materials such as rockwool, fiberglass, polyurethane, polystyrene, and aerogel. However, better performances can be reached by adopting vacuum technology. In this case, conductive losses are annihilated, and the radiative heat transfer mechanism represents the only loss mechanism. Here, we compare a high vacuum-based novel solution and the traditional insulation for heat delivery applications. We propose a high vacuum- based solution consisting of an evacuated gap that surrounds the hot pipe coated by a thin aluminium foil. Experimental results using this novel solution show a fivefold reduction of the thermal radiation losses compared to the traditional solutions when in the temperature range between 100 °C and 250 °C.