Electromagnetic and thermal modeling of passive radiative cooling for photovoltaic systems

In recent years, the sharp rise in energy consumption and the growing concern about climate change have increased the demand for innovative technologies that can accelerate the path towards a sustainable future. The goal of reaching a greener economy can only be achieved by re-designing most of the traditional technologies and industrial processes, with the purpose of mitigating their environmental impact. Cooling systems are one of these technologies. They are the only relief against unstoppable global warming, yet they are energy-intensive and contribute to air pollution generating a climate feedback loop. In this context, the thesis’ work aims to investigate an alternative and greener technique, i.e., daytime passive radiative cooling. Every body on Earth emits heat by radiation, whose spectral density depends on Planck’s law and the emissivity of the object. For the typical temperatures found on Earth, the spectral density is concentrated in the atmospheric transparency window, i.e., λ=8÷13μm, where the atmosphere is almost transparent. Kirchhoff’s law of thermal radiation ensures that absorptivity and emissivity spectra of a body coincide, hence all thermal radiation goes to outer space without any absorption from the atmosphere, leading to a cool-down of the material. In particular, the work examines the potential of such technology applied to photovoltaic systems. It is well known that part of the Sun radiation absorbed by solar cells is converted into heat instead of electricity, yielding typical operating temperatures of about 50°C or higher in terrestrial applications, under 1-sun illumination, with local variations. However, both efficiency and reliability of the photovoltaic system deteriorate at high temperatures, limiting in practice the annual energy yield and lifespan. Therefore, the goal is to study the application of the radiative cooler as heat sinks for the solar cell to maximize efficiency. To this aim, the development of an electromagnetic-thermal self-consistent model for simulating the performance of new materials based on radiative cooling technology and their impact on solar cell efficiency is proposed. The first part of the thesis focuses on the analysis of the theory underlying the radiative cooling mechanism and on the elaboration of a thermal model based on a steady-state heat flux balance equation. It evaluates the performance of the radiative cooler, i.e., its temperature at equilibrium. Then, the development of a model that estimates the enhancement of the solar cell efficiency due to the radiative cooler is presented. More precisely, the device under test is composed of a radiative cooler below the solar cell based on crystalline semiconductors. The model is composed of two parts: a script based on the detailed-balance method presented by Shockley-Queisser for the computation of the power density produced by a photovoltaic cell at a certain temperature, and the above mentioned thermal model for the evaluation of the temperature of the entire structure. The second part of the work is dedicated to the development of an electromagnetic model for multi-layer analysis based on the transmission line technique. It evaluates the dielectric properties of stratified materials, such as the reflection coefficient. This model provides the possibility of testing different nanostructures to find the right emissivity for radiative cooling capability. Finally, the complete simulation tool is obtained from the combination of the electromagnetic and thermal models.