Determination of optimal compositions and properties for phase change materials in a solar electric generating station
Abstracts:Thermal Energy Storage (TES) systems coupled to thermodynamic power cycles capable of generating electrical work are becoming strategic technologies of the future. The efficiency of TES fluids is a function of several thermodynamic and physical properties such as their heat capacity, latent enthalpy of melting and thermal conductivity. The stored thermal energy in these materials can be delivered to heat transfer fluids either by sensible heat or latent heat interactions. Challenges in the design of TES-based technologies are linked to the thermal stability and corrosivity of the selected heat storage fluid that needs to be encapsulated in metallic tubes/containers as well as the heat transfer efficiency. Latent heat storage materials, also known as Phase Change Materials (PCMs) offer high specific heat storage capacity and can operate at a constant temperature if their chemistry is adjusted so that they represent minima on liquidus surfaces. Working at a constant and minimal temperature is highly desirable from an engineering perspective as it limits corrosion degradation and temperature cycling stresses experienced by the container materials. Up to now, the identification of optimal PCMs has been mostly done via experimental trial-and-error based on limited amounts of thermodynamic data. Being able to theoretically identify PCM candidates and fine-tune their thermo-physical behavior would drastically improve their design. We present here an efficient tool specifically developed for the design of PCMs. This tool was used to identify 30 PCM candidates found in high-order anhydrous chloride-based salt systems that can operate at a temperature of 390±10∘ C.
Development of heat transfer enhancement of a novel composite phase change material with adjustable phase change temperature
Keywords:Composite phase change material;Thermal conductivity;Melting point modifier;Heat transfer mechanism;Adjustable melting point
Abstracts:Latent thermal energy storage plays an indispensable role in exhaust heat recycles in various industries which is useful for reducing energy consumption. Different phase change temperatures and high thermal conductivities of composite phase change materials were required for various domains. In the present work, Alum as phase change material and Ala as melting point modifier, a series of phase change materials with adjustable melting temperatures were prepared by physical mixing method. Moreover, EG as porous skeleton and thermal conductive enhancer, composite with high thermal conductivity was obtained with simple physical adsorption method. The XRD and SEM were applied to characterize the phase crystal and microstructure of the composite phase change material. DSC results revealed that phase change temperatures of Alum-Ala mixtures could be adjustable from 91.11 °C to 74.61 °C and thermal conductivity of the composite was improved to 10.93 W·m−1·K−1, which is 21.9 times of pristine Alum-Ala mixture. The infrared thermography and temperature-time curves were obtained to represent the heat transfer performance and the subcooling degree intuitively. The subcooling degree was decreased from 2.238 K to 1.866 K. The result of TGA indicated that the composite possessed favorable thermal stability during the operating temperature. From the result, the composite obtained in the present work is a prospective candidate for exhaust heat recycles.
Structural, optical and excitonic properties of MAxCs1-xPb(IxBr1-x)3 alloy thin films and their application in solar cells
Keywords:MAxCs1-xPb(IxBr1-x)3;Stable perovskite solar cells;Time-dependent photoluminescence
Abstracts:The structural, optical and excitonic characteristics of MAxCs1-xPb(IxBr1-x)3 crystal thin films were investigated by using the X-ray diffraction patterns, scanning electron microscopic images, absorbance spectra and photoluminescence (PL) spectra. The time-dependent PL spectra shows that the MA0.8Cs0.2Pb(I0.8Br0.2)3 crystal thin film is a photo-stable light absorbing material, which results in an efficient and stable perovskite solar cell without the additional encapsulation. The highest power conversion efficiency (PCE) of MA0.8Cs0.2Pb(I0.8Br0.2)3 solar cells is 10.02%. In addition, the PCE decreases slowly toward a stable value (5.71%) for more than 120 days under moderate environment conditions (55–60 RH%).
Sputtered indium tin oxide as a recombination layer formed on the tunnel oxide/poly-Si passivating contact enabling the potential of efficient monolithic perovskite/Si tandem solar cells
Keywords:Passivating contacts;Polycrystalline silicon;Transparent conductive oxide;TOPCon;Perovskite;Tandem solar cell
Abstracts:We focus on utilizing sputtered indium tin oxide (ITO) as a recombination layer, having low junction damage to an n-type silicon solar cell with a front-side tunnel oxide passivating electron contact, thereby enabling the development of a high efficiency monolithic perovskite/Si tandem device. High transparency and low resistivity ITO films are deposited via low power DC magnetron sputtering at room temperature onto a front-side thin SiOx/n+ poly-Si contact in a complete Cz n-Si cell with a back-side Al2O3/SiNx passivating boron-diffused p+-emitter on a random pyramid textured surface. We report the cell characteristics before and after ITO sputtering, and we find a cure at 250 °C in air is highly effective at mitigating any sputtering induced damage. Our ITO coated sample resulted in an implied open-circuit voltage (iVoc) of 684.7 ± 11.3 mV with the total saturation current density of 49.2 ± 14.8 fA/cm2, an implied fill factor (iFF) of 81.9 ± 0.8%, and a contact resistivity in the range of 60 mΩ-cm2 to 90 mΩ-cm2. After formation of a local Ag contact to the rear emitter and sputtered ITO film as the front-side contact without grid fingers, the pseudo-efficiency of 20.2 ± 0.5% was obtained with the Voc of 670.4 ± 7 mV and pseudo FF of 77.3 ± 1.3% under simulated one sun with the calculated short-circuit current density of 30.9 mA/cm2 from the measured external quantum efficiency. Our modelling result shows that efficiency exceeding 25% under one sun is practically achievable in perovskite/Si tandem configuration using the ITO recombination layer connecting a perovskite top cell and a poly-Si bottom cell.
Improved emitter performance of RIE black silicon through the application of in-situ oxidation during POCl3 diffusion
Keywords:Phosphorus diffusion;In-situ oxidation;Field effect passivation;Emitter recombination;Black silicon
Abstracts:Nano-texture has the potential to reduce the optical losses of crystalline silicon solar cells. RIE fabricated black silicon enables near zero reflectance across a broad range of wavelengths and the angular dependence has been shown to be superior to existing technologies. However, in front-contact cells which are the current industrial mainstream architecture, the emitter is located on the front textured side and is typically realized by POCl3 diffusion. The interaction of this process with the nano-texture is complex, which makes it challenging to optimise the electrical performance of the phosphorus emitter. This paper studies the impact of in-situ oxidation during emitter formation to the electrical performance of a POCl3 diffused RIE nanotextured emitter surface. Additional corona charge was applied on the ALD SiO2/Al2O3 stack to avoid the limitation on the emitter performance due to non-ideal surface passivation conditions. After saturation with surface charge, the results demonstrate in-situ oxidation to be an effective technique to improve the electrical performance. An emitter recombination factor of 147 fA/cm2 was achieved for a 127 Ω/□ emitter formed on reactive-ion etched sample with surface area enhancement factor and effective slope index of 4.19 and 1.63, respectively. Further paths for improvement are identified, particularly relating to the collection of carriers generated by short wavelength light and how that relates to the shape of the texture used.
Effect of SiO2 nanoparticle addition on the wetting and rheological properties of solar salt
Keywords:Molten salt;Nanofluid;Contact angle;Viscosity;Phase transition;Thermal energy storage
Abstracts:Molten salt based nanofluids are involved in Thermal Energy Storage (TES) Systems, both as heat transfer fluids and energy storage materials. Rheological and wetting properties play an acute role in the handling and storage of materials and are closely related with pumping and corrosion issues. In this study the effect of nanoparticle addition on the aforementioned properties of molten salt nanofluids is investigated. The solar salt (60% NaNO3 – 40% KNO3), as well as its individual components NaNO3 and KNO3, are mixed with various concentrations of SiO2. The contact angle and viscosity are measured throughout the liquid phase. Addition of a small percentage of nanoparticles, significantly alters the contact angle and viscosity of the nanofluid. In the absence of silica all the molten salts display a linear behavior with respect to temperature. However, in the presence of nanoparticles the solar salt retains an elevated value until 300 °C in the case of the contact angle and 260 °C in the case of the viscosity, after which a steep reduction occurs. With larger concentrations of nanoparticles, this effect is shifted to higher temperatures. Similar behavior, however, is not present in the case of the NaNO3 and KNO3 individually, both of which, with the addition of nanoparticles, retain curve trends similar to their baseline cases. Further investigation, involving differential scanning calorimetry, suggests that nanoparticles delay the liquid/solid phase transition process of the molten salt mixture, which in turn affects the rheological and wetting behavior of the molten mixture.
Light-induced-degradation defect independent of the boron concentration: Towards unifying admittance spectroscopy, photoluminescence and photoconductance lifetime spectroscopy results
Keywords:Boron-oxygen defect;Lifetime spectroscopy;Light-induced-degradation;Recombination;Defect;Silicon
Abstracts:Recent photoluminescence and admittance spectroscopy measurements point towards the defect responsible for Light-Induced Degradation (LID) being a shallow level. Dopant dependent lifetime spectroscopy and temperature dependent lifetime spectroscopy on the other hand reveal the existence of a deep level driving the recombination activity of the LID defects. There is an apparent disagreement between admittance spectroscopy results and lifetime spectroscopy results. Here we overcome this apparent disagreement and we show that a negative-U defect with one shallow level can explain both the doping dependent lifetime data and injection dependent data. The critical consequence of this model is that it readily explains the dependence of the LID defect on the hole concentration rather than the Boron concentration.
Ammoniated aqueous precursor ink processed copper iodide as hole transport layer for inverted planar perovskite solar cells
Keywords:Halide perovskite;Copper iodide;Interface recombination;Capacitance spectra
Abstracts:The inorganic carrier transport layers are robust and stable to the environment compared to the organic hole transport layer (HTL). Here, we report on the fabrication of the halide perovskite solar cells (HaPSCs) employing CuI deposited with ammoniated aqueous solution ink as HTL and explored the material properties and device characteristics. The film morphology of CuI is found to have an influence on the HaP film growth and hence effects on the device parameters. The HaPSC with CuI has demonstrated the power conversion efficiency of ~14.21% with high reproducibility. The capacitance spectra analysis shows that the deep trap states are induced in the perovskite absorber layer. The results obtained from the temperature-dependent open circuit voltage implicates that the recombination activities in the perovskite bulk are dominant. Furthermore, the HaPSC with CuI has revealed better air stability compared to the device with PEDOT:PSS. This work suggests that the CuI processed with aqueous precursor is a promising alternative HTL to PEDOT:PSS for efficient and stable perovskite solar cells as well as in a tandem device.
Synthesis of hollow copper sulfide nanocubes with low emissivity for highly efficient solar steam generation
Keywords:Photothermal conversion;CuS;Solar steam generation;Emissivity
Abstracts:Interfacial solar steam generation is an eco-friend and energy-efficient way of harvesting solar energy for desalination and wastewater treatment. For the solar steam generation, the primary focus is to design new materials that are fully sunlight absorption, fast photothermal conversion and low heat emissivity. Here, a novel hollow copper sulfide (CuS) nanocubes with concave surface is used for rapid and efficient solar steam generation through the micro thermal management. Interestingly, the emissivity of CuS foam is as low as 0.64, whose absorption is more than 97% and strongly dependents on the structure and morphology. The unique concave structure of hollow CuS can trap the sunlight by reabsorption as well as recover radiative and convective heat loss. As a result, the CuS based generator exhibits a high evaporation efficiency of 91.5% under one sun illumination. Furthermore, the device can maintain a stable performance of solar steam generation during the cyclic seawater desalination. The novel structure design based on the micro thermal management offers a new insight into the future development of highly efficient solar steam generation.
Effects of post bonding annealing on GaAs//Si bonding interfaces and its application for sacrificial-layer-etching based multijunction solar cells
Keywords:Epitaxial lift-off;Surface activated bonding;Sacrificial layer etching;Low temperature annealing;GaAs//Si double junction cells
Abstracts:By using the sacrificial layer (SL) etching, GaAs substrates are separated from III–V epi substrate//Si substrate junctions that are made by surface activated bonding (SAB) technologies. The post-bonding low-temperature (300- ∘C) annealing plays an essential role in achieving a promising ( ∼90%) bonding yield. The effects of the post-bonding annealing are investigated by hard X-ray photoemission spectroscopy and current–voltage measurements of GaAs//Si bonding interfaces. It is found that the concentration of oxygen atoms at interfaces is reduced and the resistance decreases to 1.6–2.1 m Ωcm2 by the low-temperature annealing. Aluminum fluoride complexes are not observed by X-ray photoelectron spectroscopy on the exposed surfaces of separated GaAs substrates. The roughness average of the surfaces is ≈0.25–0.30 nm. The characteristics of double junction cells fabricated on the GaAs//Si junctions prepared by the SL etching are almost the same as those of cells fabricated by dissolving GaAs substrates after bonding. These results indicate that multijunction cells could be fabricated in a process sequence compatible with reuse of GaAs substrates by combining the SL etching and SAB.