Showing papers by "Shankar Narayanan published in 2020"

In situ healing of dendrites in a potassium metal battery

Abstract: The use of potassium (K) metal anodes could result in high-performance K-ion batteries that offer a sustainable and low-cost alternative to lithium (Li)-ion technology. However, formation of dendrites on such K-metal surfaces is inevitable, which prevents their utilization. Here, we report that K dendrites can be healed in situ in a K-metal battery. The healing is triggered by current-controlled, self-heating at the electrolyte/dendrite interface, which causes migration of surface atoms away from the dendrite tips, thereby smoothening the dendritic surface. We discover that this process is strikingly more efficient for K as compared to Li metal. We show that the reason for this is the far greater mobility of surface atoms in K relative to Li metal, which enables dendrite healing to take place at an order-of-magnitude lower current density. We demonstrate that the K-metal anode can be coupled with a potassium cobalt oxide cathode to achieve dendrite healing in a practical full-cell device.

A concise review on performance improvement of solar stills

Abstract: Conversion of saline water into freshwater by the use of solar thermal energy is known as solar desalination and the devices used for carrying out solar desalination are known as solar stills. The major problem faced by the solar stills is that they have low productivity and therefore unable to meet the high demand for freshwater. Over the years, several researchers have worked on improving the productivity of solar stills. Preheating of feed water, forced convection and energy storage are some of the methods used to increase their efficiencies. Some of these methods are discussed in the present review. The main aim of the current review is to provide the researchers with an idea to select the appropriate method for the improvement in the performance of currently available solar stills.

Moving boundary model for dynamic control of multi-evaporator cooling systems facing variable heat loads

Abstract: Two-phase systems comprising of a single pump or compressor, a condenser, and multiple microchannel evaporators can provide the unique advantage of handling multiple heat sources while being compact and lightweight. Such systems also present challenges with regards to handling asynchronous transient heat loads, maintaining fixed evaporator temperatures, enabling high system efficiencies, and avoiding the critical heat flux. The overall objective of this study is to dynamically control the performance of a two-phase system comprising of two evaporators experiencing transient heat loads. This study simulates a pumped liquid cycle comprising of two microchannel evaporators using the moving boundary model, which considers phase-change heat transfer and pressure drop occurring in the evaporators. The moving boundary model predicts the overall performance corresponding to different system settings and evaporator heat loads to determine the optimum operating conditions. By knowing these conditions in advance, the study presents a strategy combining feedforward and feedback control to ensure system operation close to the optimum operating condition. The approach discussed in this study is generally applicable and allows maintaining the desired evaporator temperatures and high system efficiency in the presence of transient heat loads.

Nickel-Infused Nanoporous Alumina as Tunable Solar Absorber

Abstract: Solar energy can alleviate our dependence on traditional energy sources like coal and petroleum. In this regard, the design and performance of solar absorbers are crucial for capturing energy from sunlight. Specifically, for applications relying on solar-thermal energy conversion, it is desirable to construct solar absorbers using scalable techniques that also allow a variation in optical properties. In this study, we demonstrate the ability to tune the spectral absorptance of nickel-infused nanoporous alumina using a scalable and inexpensive fabrication procedure. With simple variations in the geometry of the nanostructures, we enable broadband absorption with a net solar absorptance of 0.96 and thermal emittance of 0.98 and spectrally-selective absorption with a net solar absorptance of 0.83 and thermal emittance of 0.22. The simple manufacturing techniques presented in this study to generate nanoengineered surfaces can lead to further advancements in solar absorbers with well-controlled and application-specific optical properties.

Quantifying the evaporation rate of sessile droplets using a quartz crystal microbalance

Abstract: This study quantifies the evaporation rate of sessile droplets using a quartz crystal microbalance (QCM). Specifically, we analyze the evaporation of water droplets on a gold-coated flat surface exposed to dry nitrogen at different temperatures. In this approach, we use the QCM as a radius sensor and determine the contact angle by droplet imaging, which allows calculating the instantaneous volume and the evaporation rate. For comparison, we quantify evaporation using computational modeling and an experimental technique based on droplet imaging alone. In general, the QCM-based approach was found to provide higher accuracy and a better agreement with the model predictions compared to the approach using imaging only. With modeling and experiments, we also elucidate the role of droplet self-cooling, vapor advection, and diffusion on the net rate of evaporation of sessile droplets. For all the conditions analyzed in this study, the evaporation rate was found to decrease monotonically. We found this reduction to take place even in the presence of a steadily increasing droplet temperature due to a shrinking evaporation area. Considering the vapor transport mechanisms occurring in the ambient, we find diffusion to be the rate-limiting process controlling the net evaporation rate of the droplet.

An In-situ and Direct Confirmation of Super-Planckian Thermal Radiation Emitted From a Metallic Photonic-Crystal at Optical Wavelengths.

Abstract: Planck’s law predicts the distribution of radiation energy, color and intensity, emitted from a hot object at thermal equilibrium. The Law also sets the upper limit of radiation intensity, the blackbody limit. Recent experiments reveal that micro-structured tungsten can exhibit significant deviation from the blackbody spectrum. However, whether thermal radiation with weak non-equilibrium pumping can exceed the blackbody limit in the far field remains un-answered experimentally. Here, we compare thermal radiation from a micro-cavity/tungsten photonic crystal (W-PC) and a blackbody, which are both measured from the same sample and also in-situ. We show that thermal radiation can exceed the blackbody limit by >8 times at λ = 1.7 μm resonant wavelength in the far-field. Our observation is consistent with a recent calculation by Wang and John performed for a 2D W-PC filament. This finding is attributed to non-equilibrium excitation of localized surface plasmon resonances coupled to nonlinear oscillators and the propagation of the electromagnetic waves through non-linear Bloch waves of the W-PC structure. This discovery could help create super-intense narrow band thermal light sources and even an infrared emitter with a laser-like input-output characteristic.