Multilayer passive radiative selective cooling coating based on Al/SiO2/SiNx/SiO2/TiO2/SiO2 prepared by dc magnetron sputtering

A multilayer passive radiative selective cooling coating based on Al/SiO2/SiNx/SiO2/TiO2/SiO2 prepared by dc magnetron sputtering is presented. The design was first theoretically optimized using the optical constants, refractive index and extinction coefficient, of thin single layers. The spectral optical constants in the wavelength range from 0.3 to 27 µm were calculated from the transmittance and reflectance data of thin single layers deposited on silicon and glass substrates. The samples were characterized by Scanning Electron Microscopy, X-ray diffraction, Fourier-transform Infrared Spectroscopy and UV–VIS–NIR spectroscopy. It is shown that the TiO2 layer presents a partially rutile phase polycrystalline structure and a higher refractive index than amorphous SiO2 and SiNx layers in the spectral range from 0.3 to 2.5 μm. The cooling device was deposited on copper substrates and a thin low-density polyethylene foil with high transmittance in the 8 to 13 µm spectral range was used as convection cover material. The device is characterized by both low reflectance (high emittance) in the sky atmospheric window (wavelength range from 8 to 13 µm) and high hemispherical reflectance elsewhere, allowing for temperature drops of average 7.4 °C at night-time in winter, which corresponds to a net cooling power of ~43 W m−2. Further, a temperature drop of 2.5 °C was obtained during winter daytime.FCT in the framework of the Strategic Funding UID/FIS/04650/2013 and the financial support of FCT, POCI and PORL operational programs through the project POCI-01-0145-FEDER-016907 (PTDC/CTM-ENE/2892/2014), co-financed by European community fund FEDE

Numeryczne rozwiązania równań niestacjonarnej warstwy przyściennej uogólnionego płynu drugiego rzędu

Unsteady, incompressible boundary layer equations for a modified power- law fluid of the second grade are considered. The model is a combination of the power-law and second grade fluid in which the fluid may exhibit normal stresses, shear thinning or shear thickening behaviour. The equations of motion are formulated for two-dimensional flows, and from which the boundary layer equations are derived. By using the similarity transformation, we reduce the boundary layer equations to system of non-linear ordinary differential equation. The ordinary differential equations are numerically integrated for classical boundary layer conditions. Effects of the power-law index and second grade coefficient on the boundary layers are shown.W pracy omówiono równania niestacjonarnej i nieściśliwej warstwy przyściennej zmodyfikowanego modelu płynu drugiego rzędu typu potęgowego. Rozważany model stanowi kombinację koncepcji płynu drugiego rzędu i opisu potęgowego, która pozwala na odzwierciedlenie zjawiska występowania naprężeń normalnych w płynie oraz efektu zmiany grubości warstwy pod wpływem naprężeń stycznych. Sformułowano równania ruchu dla przepływu dwuwymiarowego i na ich podstawie wyprowadzono równania warstwy przyściennej. Używając przekształcenia przez podobieństwo, uproszczono równania warstwy do układu nieliniowych równań różniczkowych zwyczajnych. Następnie równania te scałkowano numerycznie, stosując klasyczne warunki brzegowe. W dalszej części przeanalizowano wpływ wykładnika potęgowego modelu oraz współczynnika drugiego rzędu na zachowanie się płynu w warstwie przyściennej

Comparison of PV Power Production Estimation Methods Under Non-homogeneous Temperature Distribution for CPVT Systems

The way to increase energy generation in a standard photovoltaic (PV) or photovoltaic/thermal (PV/T) system is the tracking of the sun and/or concentrating to increase the solar energy coming into the field. As the radiation is increased in both concentrated PV and PV/T systems, both PV power output and PV module temperature increase. The fact that the PV module temperature increases and exceeds the reasonable level reduces the life of solar cells and permanently damages the cells. The way to prevent this is to cool the PV modules. In other words, thermal energy is absorbed by integrating the thermal system. Thus, both electrical and thermal energy needs will be met easily, and a concentrating photovoltaic thermal (CPVT) system produces both electricity and thermal energy from the sun. Electrical and thermal behavior analyzes of CPVT systems are important issues in order to robust and accurate deciding for electrical and thermal power production. In a previous study, finite volume methods were applied for thermal analysis of the CPVT system. Temperature distribution of the PV modules and CPVT surfaces was done. In the numerical analysis; power/temperature coefficient-based method was used for electrical power estimation. In this chapter, power/temperature coefficient-based and five parameter models of PV modules were presented and discussed for forecasting of electrical power production. Decided to PV module temperature in power/temperature coefficient model and temperature distribution applications on diode model were discussed. Power/temperature-based power estimation methods are depending on first, medium, and end PV module temperature. However, different case studies for CPVT electrical power production forecasting methods were investigated. © 2023, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd

The effect of temperature distribution on parabolic triangular-based CPVT system performances: Electrical and thermal perspectives

The performance of photovoltaic (PV) and photovoltaic-thermal (PVT) systems is affected by environmental parameters and working conditions such as, partial shading, refrigerant and operating temperature. This study focuses on the investigation of the operating conditions and performance of a low concentrated parabolic-trough PVT (CPVT) system. Electro-thermal (electrical and thermal) analysis of the CPVT system was investigated. In the electrical analysis, the effect of series, serial-parallel (SP), total cross-tied (TCT) and grouped connection forms were investigated using a single diode model. In thermal analyses, temperature distribution of the CPVT system was investigated using finite volume methods considering different fluid inlet temperatures. Firstly, thermal analysis was carried out under certain conditions of the CPVT system. Then the obtained temperature distribution was applied to the electrical model. Thus, the electrical and thermal performance of the CPVT system under certain conditions was estimated. Obtained results show that series connection is negligibly better than TCT connections. According to identical mass and pressure and under operating conditions, fluid inlet and outlet temperature differences is 8.77 °C for water. PV module temperature increases up to 110 °C, when fluid inlet temperature is 50 °C for R134a. So, the PV module output power significantly decreases. When the electrical and thermal efficiencies of the CPVT system were evaluated, water obtains better results than the other fluids due to its high specific heat. Therefore, fluid, fluid mass and pressure have to be carefully selected and designed before system's experimental design. Finally, fluid mass should be selected at more and more rates in high temperature applications