Radiative diffusion in a time-dependent outflow: a model for fast blue optical transients

Fast Blue Optical Transients (FBOTs) are luminous transients with fast evolving (typically trise < 12 days) light curve and blue color (usually−0.2 > g−r > −0.3)that cannot be explained by a supernova-like explosion. We propose a radiative diffusion in a time-dependent outflow model to interpret such special transients. In this model, we assume a stellar-mass black hole is formed from stellar core-collapse. As a central engine, the black hole accretes the infalling stellar envelope material via an accretion disk. Due to the extremely super- Eddington accretion rate, the disk ejects continuous outflow during a few days. We consider the ejection of the outflow to be time-dependent. The outflow is optically thick initially and photons are frozen in it. As the outflow expands over time, photons gradually escape, and our work is to model such an evolution. Numerical and analytical calculations are considered separately, and the results are consistent. We apply the model to three typical FBOTs: PS1-10bjp, ZTF18abukavn, and ATLAS19dqr. The modeling finds the total mass of the outflow (∼ 1M⊙), and the total time of the ejection (∼ a few days) for them, leading us to speculate that they may be the result of the collapse of massive stars.

Feasibility Study of Building Integrated Photovoltaic (BIPV) as a Building Envelope Material in Europe

Buildings play a vital role as regards the energy efficiency of urban areas since they are responsible for a significant portion of the energy demand of urban areas. In Europe, building energy use accounts for 41% of the total energy consumption of the cities [1]. Urban energy transition has recently come about by intensifying the endeavour towards promoting distributed or decentralised energy generation (DG) and realign the energy production and consumption of buildings. One of the leading solutions which can be of great assistance to contribute towards such an approach is building integrated photovoltaic (BIPV) systems. BIPV is a PV system on the building skin serving as both a building envelope material and a power generator. An alternative that is not covered here is PV systems nearly – in the landscape or garden. There is a tendency currently in the market to use BIPV systems in the part of the building skins with the highest incident solar radiation and, therefore, higher electricity production as an output. These areas in the northern hemisphere are roof and south façade. However, employing other facades and areas of building skins also results in many advantages. The possibility to achieve zero energy buildings (ZEB) or even plus energy building goals, using different facades and orientations of buildings to have a distributed electricity generation during the day, and the system's contribution in reinforcing the energy performance of the building skin are some advantages. To place PV modules so that they deliver energy when the energy need in the building is highest is also of importance as it reduces the need for storage. Therefore, this thesis focuses on building integrated photovoltaic systems (BIPV) and their feasibility as a building envelope material in Europe. The main research question is defined as follows: Is the BIPV system as an alternative for the more usual building envelope materials feasible for the entire skin of buildings in Europe? The goal is to investigate the technical and economic aspects of such a solution in two steps. Finally, the project seeks to briefly discover the potential and challenges of such a solution in the energy transition of cities. Both qualitative and quantitative methodologies are employed in this project, and most of the analyses are based on the data obtained from the Photovoltaic Geographical Information System (PVGIS) and the Surface Solar Radiation Data Set - Heliosat (SARAH) dataset. The results are expected to help the end-users, architects and urban planners to acknowledge the BIPV system as a suitable option for the building skins in Europe and steer governments or decision-makers to promote the technology by rational subsidies and incentives (where it is needed). This can contribute towards making cities as well as more rural areas into “power stations”.

Levelised Cost of Electricity (LCOE) of Building Integrated Photovoltaics (BIPV) in Europe, Rational Feed-In Tariffs and Subsidies

Building integrated photovoltaics is one of the key technologies when it comes to electricity generation in buildings, districts or urban areas. However, the potential of building façades for the BIPV system, especially in urban areas, is often neglected. Façade-mounted building integrated photovoltaics could contribute to supply the energy demand of buildings in dense urban areas with economic feasibility where the availability of suitable rooftop areas is low. This paper deals with the levelised cost of electricity (LCOE) of building integrated photovoltaic systems (BIPV) in the capitals of all the European member state countries plus Norway and Switzerland and presents a metric to investigate a proper subsidy or incentive for BIPV systems. The results showed that the average LCOE of the BIPV system as a building envelope material for the entire outer skin of buildings in Europe is equal to 0.09 Euro per kWh if its role as the power generator is considered in the economic calculations. This value will be 0.15 Euro per kWh if the cost corresponding to its double function in the building is taken into the economic analysis (while the average electricity price is 0.18 Euro per kWh). The results indicate that the BIPV generation cost in most case studies has already reached grid parity. Furthermore, the analysis reveals that on average in Europe, the BIPV system does not need a feed-in tariff if the selling price to the grid is equal to the purchasing price from the grid. Various incentive plans based on the buying/selling price of electricity from/to the main grid together with LCOE of the BIPV systems is also investigated.

Mechanical characterization of a woven multi-layered hyperelastic composite laminate under uniaxial loading

We characterize the material properties of a woven, multi-layered, hyperelastic composite that is useful as an envelope material for high-altitude stratospheric airships and in the design of other large structures. The composite was fabricated by sandwiching a polyaramid Nomex® core, with good tensile strength, between polyimide Kapton® films with high dielectric constant, and cured with epoxy using a vacuum bagging technique. Uniaxial mechanical tests were used to stretch the individual materials and the composite to failure in the longitudinal and transverse directions respectively. The experimental data for Kapton® were fit to a five-parameter Yeoh form of nonlinear, hyperelastic and isotropic constitutive model. Image analysis of the Nomex® sheets, obtained using scanning electron microscopy, demonstrate two families of symmetrically oriented fibers at 69.3°± 7.4° and 129°± 5.3°. Stress-strain results for Nomex® were fit to a nonlinear and orthotropic Holzapfel-Gasser-Ogden (HGO) hyperelastic model with two fiber families. We used a linear decomposition of the strain energy function for the composite, based on the individual strain energy functions for Kapton® and Nomex®, obtained using experimental results. A rule of mixtures approach, using volume fractions of individual constituents present in the composite during specimen fabrication, was used to formulate the strain energy function for the composite. Model results for the composite were in good agreement with experimental stress-strain data. Constitutive properties for woven composite materials, combining nonlinear elastic properties within a composite materials framework, are required in the design of laminated pretensioned structures for civil engineering and in aerospace applications.

The Effect of Climate on the Solar Radiation Components on Building Skins and Building Integrated Photovoltaics (BIPV) Materials

The business model of building-integrated photovoltaics (BIPV) is developing expeditiously and BIPV will soon be recognised as a building envelope material for the entire building skins, among other alternatives such as brick, wood, stone, metals, etc. This paper investigates the effect of climate on the solar radiation components on building skins and BIPV materials in the northern hemisphere. The selected cities are Stavanger in Norway, Bern in Switzerland, Rome in Italy, and Dubai in the UAE. The study showed that for all the studied climates, the average incident radiation on the entire building skins is slightly more than the average incident radiation on the east or west facades, regardless of the orientations of the building facades. Furthermore, the correlation between solar radiation components and different BIPV technologies is discussed in this paper. It is also found that when it comes to the efficiency of different BIPV cells, the impact of the climate on some of the BIPV technologies (such as DSC and OSC) is much more significant than others (such as c-Si, mc-Si and CIGS). The evidence from this study suggests that in climates with higher diffuse radiation-or with more overcast days per year-the contribution of IR radiation decreases. Therefore, the efficiency of BIPV materials that their spectral responses are dependent on the IR radiation (like Si and CIGS) in such a climate would drop down meaningfully. On the other hand, the DSC and OSC solar cells could be a good option for cloudy climates since they have more stable performance, even in such a climate. Although, their efficiency compared to other BIPV materials such as Si-based BIPV solar cells is still significantly less thus far.

Performance and Manufacturing of Silicon-Based Vapor Chambers

This paper reviews recent progress in the development of silicon-based vapor chambers for heat spreading in electronic packages. Effective hotspot mitigation is an increasingly challenging issue in electronics thermal management, and the use of silicon vapor chambers creates opportunities for thermal-expansion matched, high performance heat spreaders that can be directly integrated with the semiconductor die. While silicon microheat pipes have been extensively studied as one-dimensional heat transport mechanisms for heat routing in semiconductor substrates, silicon vapor chambers require special consideration and different manufacturing approaches due to the different heat transport configurations involved. The following review therefore provides an overview on the evolution of silicon vapor chambers in terms of fabrication strategies and performance characterization. Particular focus is given to opportunities and challenges associated with using silicon as the vapor chamber envelope material rather than more traditional metal-based vapor chambers, such as the ability to optimize the wick geometry with greater fidelity and issues with manufacturing scalability.

Mechanical properties of UN-5100 envelope material for stratospheric airship

ABSTRACT Stratospheric airships are promising aircraft, usually designed as a non-rigid airship. As an essential part of the non-rigid airship, the envelope plays a significant role in maintaining its shape and bearing the external force load. Generally, the envelope material of a flexible airship consists of plain-weave fabric, composed of warp and weft fibre yarn. At present, biaxial tensile experiments are the primary method used to study the stress–strain characteristics of such flexible airship materials. In this work, biaxial tensile testing of UN-5100 material was carried out. The strain on the material under unusual stress and the stress ratio were obtained using Digital Image Correlation (DIC) technology. Also, the stress–strain curve was corrected by polynomial fitting. The slope of the stress–strain curve at different points, the Membrane Structures Association of Japan (MSAJ) standard and the Radial Basis Function (RBF) model were compared to identify the stress–strain characteristics of the materials. Some conclusions on the mechanical properties of the flexible airship material can be drawn and will play a significant role in the design of such envelopes.