• Energy Research

Shape-stabilized phase change materials (ss-PCMs) based on silica and butyl stearate were thermally enhanced via the addition of different hexagonal boron nitride particles (BN) to the in situ sol-gel synthesis. The dataset is used in conjunction with the experimental data of the influence of the particle size and surface area of BN on the thermal and mechanical properties of ss-PCMs discussed in Marske et al. (2021). To study the effect of the different BN particles on the hydrolysis degree of the silica network and on the chemical nature of the porogens sodium dodecyl sulfate and poly(vinyl alcohol) used for the ss-PCM synthesis, the ss-PCM samples are measured via High Power Decoupling (HPDEC) Magic Angle Spinning (MAS) 29Si NMR and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, respectively. Additionally, data of the influence of BN on the thermal properties is presented as thermogravimetric analysis (TG). The 29Si MAS NMR spectra are referenced to tetramethylsilane and show the different silica species in ppm. The different value of wavenumber and intensity of each reference and ss-PCM sample is listed in the IR spectra. The decomposition points of the ss-PCMs are calculated from the TG data via OriginLab. The spectra and data can be used as a reference for other researchers and engineers to use in synthesizing ss-PCMs based on silica and other polymeric materials or as reference for pure BN, SDS, stabilized silica sol and PVA.

Self-Organized Macrostructures in Anodically Formed Mesoporous Silica S. Frey, B. Gresillon, F. Ozanam, J.-N. Chazalviel,* J. Carstensen, H. Foll,* and R. B. Wehrspohn* Laboratoire de Physique de la Matiere Condensee, Ecole Polytechnique, 91128 Palaiseau, France Faculty of Engineering, Christian-Albrechts-University of Kiel, 24143 Kiel, Germany Physik Department, University of Paderborn, 33098 Paderborn, Germany

AbstractPhoton management is a key component in the development of efficient solar cells. Especially light-trapping concepts have a high potential to realize enhanced efficiencies. Here, we give an overview over several light trapping concepts for photon management in solar cells. These include basic as well as advanced light-trapping concepts. The theoretical limits of light path enhancement of the different concepts are given and experimental work on these topics is presented. The potential of 3D photonic crystals is discussed in the context of the corresponding approaches as well.

Abstract Monolithic shape-stabilized phase change materials (ss-PCMs) are among the most promising materials to store large amounts of solar energy, waste heat and off-peak electricity as heat in energy-saving buildings, battery- and water-heating systems. Generally, graphite-like additives, such as hexagonal boron nitride platelets (BN), are used to increase the thermal conductivity of ss-PCMs. However, it is still unknown which particle size and surface type of graphite-like additives increase the thermal conductivity of ss-PCMs to the highest extent. Here, we present the first experimental study about the influence of BN with different particle sizes and specific surface areas on the physicochemical properties of ss-PCMs synthesized via the sol–gel route. With a BN particle size of 25 µm, the average thermal conductivity of ss-PCMs containing 5–20 wt% BN increased by 57% when compared to the same ss-PCMs prepared with smaller BN particles (3 µm). This effect possibly results from the stronger exfoliation of the larger BN particles during synthesis and a larger amount of BN particles ordered in the same alignment in the silica structure. BN particles larger than 25 µm hinder the formation of mechanically stable silica structures. Smaller BN particles lead to higher viscous reaction mixtures with shorter gelation times and, thus, result in smaller silica macropores with diameters below 1 µm and higher compressive strengths of ss-PCMs up to 2.4 MPa at 10 °C and 1.7 MPa at 30 °C. An increasing specific surface area of BN up to 20 m2/g decreases the thermal conductivity by 0.11 W/mK (30 °C) and the compressive strength by 0.28 MPa (30 °C) of ss-PCMs with 20 wt% BN because of more BN agglomeration. Therefore, ss-PCMs containing 20 wt% BN with a particle size of 25 µm and a specific surface area of 3 m2/g have the highest thermal conductivity of 1.14 W/mK at 10 °C and 0.62 W/mK at 30 °C, but a 0.7–0.3 MPa lower compressive strength (10 °C) than the ss-PCM synthesized using 3 µm sized BN particles and specific surface areas of 12–20 m2/g.

AbstractOptically black silicon nanostructures show excellent anti-reflection and light trapping properties minimizing reflection losses to less than 1.6% between 300 – 1100nm. Our light-trapping scheme enables an absorption enhancement factor of ∼10 at the band edge of silicon (1150nm) as compared to a simulated perfect ARC, where the Yablonovitch limit corresponds to a factor of 15. Just recently it was shown that similar wet-chemically black etched silicon surfaces can be exploited to fabricate high efficiency solar cells [1]. Towards the integration of our structures into a solar cell device, the passivation performance of atomic layer deposited thin Al2O3 films is investigated on a variety of black etched structures. The coatings lead to measured surface recombination velocities of less than 13cm/s on bifacially black structured as well as 12cm/s on polished 1 – 5Ωcm p-type Si CZ wafers. Thinner layers promise to be even more effective. This technology will enable high efficiencies on various solar cell concepts.

The recycling of lithium‐ion batteries (LIBs) through extractive pyrometallurgy is widely used, but a significant drawback is the loss of lithium to the slag. To address this, lithium‐bearing slag from an industrial LIB recycling plant is analyzed using wavelength dispersive X‐ray fluorescence, inductively coupled plasma optical emission spectroscopy, X‐ray diffraction (XRD), and thermogravimetry coupled infrared. The slag's chemical composition is complex, best described by the ternary system CaO–SiO2–Al2O3, with additional major components being Na2O, Fe2O3, MgO, V2O5, Mn2O3, and Cr2O3. The slag cone shows little chemical zonation and a relatively constant lithium content of Ø 0.82 mass%. The recycling slag shows a mineralogical composition typical of nonferrous slags (e.g., melilite, clinopyroxene, nepheline). Lithium is either bound in β‐eucryptite or, to a lesser extent, in lithium metasilicate. β‐eucryptite contains up to 5.51 mass% lithium stoichiometrically, which is more than typical lithium ores contain. Moreover, β‐eucryptite has potential for the engineering of artificial minerals strategy as an easily implementable lithium phase. β‐eucryptite forms in slags with lower overall lithium content, allowing for the use of slag modifiers that reduce the process temperature. Hence, β‐eucryptite could prove as efficient and feasible option for improving lithium recovery from smelting processes.

AbstractUpon inductive coupled plasma reactive ion etching (ICP-RIE) of Si surfaces needle-like nanostructures with aspect ratios up to 10 emerge showing excellent anti-reflection and light-trapping properties with absorption over 97% throughout the UV and VIS spectral range. In addition, the absorption at the band edge of silicon is enormously enhanced due to scattering. In this work we report on the feasibility to deposit conformal Al2O3 dielectric layers on these very rough silicon surfaces which enable adequate surface passivation. Lifetimes over 170μs have been measured on deep structured b-Si substrates. The optical properties of black silicon (b-Si) and the possible influence of applied alumina passivation layers as well as their passivation performance are discussed.

In thin-film silicon tandem solar cells consisting of amorphous and microcrystalline silicon, efficiency enhancements can be achieved by increasing the current density in the a-Si:H top cell. Therefore, it is necessary to provide an optimized current matching at high current densities for both junctions. We present a numerical method to obtain suitable intermediate reflections achieving optimal current-matched tandem cells. We used idealized and measured reflection spectra of spectrally-selective intermediate filters for the current matching. For the optical design of such intermediate reflective layers a prediction of the spectral properties towards optimal current matching is presented.