Strazzullo, Paolo (2025) HYBRID PHOTOVOLTAIC-THERMAL FLAT PLATE COLLECTORS IN HIGH VACUUM Optimization of thermo-optical properties of High-Vacuum PV-T collectors and implementation of a novel measurement system for their thermal characterisation. [Tesi di dottorato]

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Abstract

This thesis investigates the potential of High-Vacuum Photovoltaic-Thermal (HV PVT) collectors as a novel technology to exploit the solar spectrum to its fullest potential. In the best case, the photoelectric conversion occurring in photovoltaic (PV) modules has an intrinsic limit of 33% efficiency, with the remaining portion of solar energy being dissipated as heat. This thermal energy from PV modules has historically been harvested as a minor, derived output in hybrid photovoltaic-thermal (PVT) collectors to match low-temperature applications like HVAC or water heating systems. In this thesis, however, the harvestable thermal energy becomes the primary output in the exploitation of the solar spectrum. The objective is to pave the way for HV PVT technology to enter the industrial sector, satisfying medium-temperature processes previously precluded due to excessive convective and radiative losses. To overcome this limitation, this thesis presents a novel approach based on the synergistic integration of high-vacuum insulation to eliminate convective losses and low-emissivity transparent conductive oxide (TCO) coatings to reduce radiative heat transfer. The methodology followed to advance this new technology in its early stages can be summarised in three steps: (1) modelling of the device in the MATLAB environment; (2) a study of the materials to be adopted; and (3) the development of an innovative measurement chain to characterise the thermal properties at the single-cell level (2x2 cm²). The modelling phase provided crucial guidance on the emissivity required for the HV PVT to offer a concrete advantage over stand-alone solar thermal and photovoltaic solutions. This analysis revealed that HV PVT technology is particularly competitive for industries with processes around 100 °C (e.g., dairies and breweries) with limited area for renewable technology installation. The key advantage of this technology is its potential to save up to 20% of space compared to stand-alone solutions, but to achieve this benefit, TCOs with an emittance below 0.20 must be fabricated. Consequently, the materials investigation focused primarily on TCOs, which were deposited in the laboratory via Radio-Frequency Magnetron Sputtering. Materials such as ITO (Tin-doped indium oxide), ITMO (Molybdenum-Tin doped indium oxide) and IZO (Tin-doped indium oxide) were deposited onto glass and silicon substrates to enable their characterisation in the infrared region of the electromagnetic spectrum, where the thermal emission of a body at 100 °C is most significant. IZO was also deposited on Perovskite PV cells, preserving a normalised efficiency of around 95%. Although Perovskite devices are not the primary focus of this work, their integration is relevant in the broader context of low-e HV PVT development. Perovskite absorbers, with their higher bandgap and partial NIR transparency, naturally lend themselves to hybrid configurations in which sub-bandgap photons can be redirected toward the underlying thermal absorber. In parallel, the metrological challenge of quantifying the thermal performance of the 2x2 cm² cells was addressed. In the literature, the state-of-the-art for measuring the thermal performance of hybrid devices in high vacuum involves the construction of entire modules, whose dimensions are closer to those of a collector than a laboratory-scale cell. This metrological gap is filled by proposing a measurement chain capable of quantifying the heat collected by a single photovoltaic cell under vacuum and illumination. The core of this chain is a heat flux meter (HFM) that converts a thermal flux into a voltage. The primary objective was to verify their functionality in a vacuum and evaluate their calibration constant. The characterisation of these heat flux meters in high vacuum demonstrated that they maintain a calibration constant similar to the factory-provided value in air, paving the way for further developments of the measurement chain, including heating to operating conditions (100 °C).

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