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Reuse of the retired/aged supercapacitors plays an important role in production sustainability and environmental concerns. Therefore, appropriate health characterization methods along with proper guidelines to identify the remaining useful capacitance of the retired supercapacitor cells are required. This paper focuses on conventional approach based capacitance diagnosis of retired supercapacitors, since there is a lack of technological development for the same. In particular, we evaluate the effectiveness of Maxwell six-step process-based time domain approach and electrochemical impedance spectroscopy based frequency domain approach in identifying the remaining capacitance of the retired supercapacitors. This work considers a retired cell of the commercially available Maxwell supercapacitors to demonstrate the approach. The results obtained from the conventional approaches via cyclic voltammetry test is validated. Subsequently, the effectiveness of the test methods are experimentally verified by considering a new specimen of the Maxwell 100 F supercapacitor. The obtained results confirm that both the approaches are showing promising results, however, for different operating voltage range. This clearly indicates the need for future research towards second-life application of retired supercapacitors. Finally, the importance of choosing proper capacitance characterization scheme for reliable use of retired supercapacitors considering a wireless sensor network based realistic second-life application is discussed.

To expand the adoption of battery-operated portable consumer products and reduce the fear of running out of charge, the manufacturers and researchers have developed various health-aware fast-charging protocols. However, the different health-conscious fast charging algorithms only generates a pre-defined charging profile based on initial operating boundary conditions without having the option of real-time controls. In this work, a real-time optimal fast charging protocol is implemented by using Pontryagin’s Minimum Principle (PMP) to solve the optimal control framework having a trade-off between charging time and ohmic heat generation. Modifying the control concepts of costate jump conditions and from extensive offline optimization results, the real-time optimal fast charging protocol is examined for varying operating constraints. The effect of different boundary conditions, mainly due to charging behaviours, on the charging profile and other sensitive parameters are also investigated and compared with a standard CCCV charging algorithm. Finally, the comparison between a typical optimal fast charging profile and a standard 2C CCCV protocol is experimentally examined.

The popularity of supercapacitors (SC) as an energy storage device is growing along with the development of the electrical and electronics applications demanding high operating power. Consequently, the number of retired SC cells is increasing leading to the recycling problem. Supercapacitor reaches its end-of-life (EOL) when its nominal capacitance is decreased by 20% and the initial equivalent series resistance (ESR) is increased by 100%. Under this scenario, the SC is no-longer suitable for its original purpose as a source of high power. However, the aged supercapacitors often retain enough capacity to serve the applications demanding low operating power. This paper investigates the possibility to re-purpose the aged supercapacitors for low power second-life applications. Specifically, we propose a capacitance prognosis model as a function of the operating power of second-life applications. We consider three aged Maxwell supercapacitors to illustrate the proposed scheme. Subsequently, we identify their effective capacitance considering MICAz wireless sensor module as the low power second-life application. Next, we evaluate the sensitivity and robustness of the proposed model. Finally, we study the second-life aging of the aged supercapacitors under low-power charge–discharge cycling.

The present study discusses the results pertaining to eduction of coherent structures and assesses their contribution in promoting mixing between low and high momentum fluid streams to enhance thermal convection. The analysis has been carried out numerically in a periodic parallel-plate-channel containing built-in longitudinal vortex generators on the bottom wall. The Reynolds number of the investigation is 6000 based on the channel height and bulk mean velocity. The computational geometry resembles heat exchanger surfaces on which the vortex generators create strong longitudinal vortices that enhance heat transfer on the plane channel wall. At high Reynolds numbers, the enhancement of convective heat transfer near the wall largely depends on the large scale turbulent structures (coherent structures) that disappear and reappear frequently and exhibit typical correlated scales in time or space. Proper orthogonal decomposition (POD) is an efficient tool to identify these coherent turbulent structures from instantaneous flow field data. Despite geometrical simplicity, the complex swirling motion behind the vortex generators demands highly accurate computational method, like the large eddy simulation (LES) of turbulence for capturing the nuances of flow. The instantaneous velocity data at a cross-flow plane have been assembled over a long time-window and analyzed deploying POD technique to extract the large- and small-scale structures based on the turbulent kinetic energy of the structures. Reconstructions of the velocity field using different combinations of eigenmodes have yielded different patterns of the flow. Large-scale structures have been visualized using first few energetic modes, while remaining less energetic modes have provided small-scale structures. The technique enables us to identify coherent structures hidden in the random flow field by filtering the small scale effects. The analysis yields as large as 15 dominant POD modes to cumulatively capture as low as 25% of the total turbulent kinetic energy of the flow, indicating broad range of energy spectrum that shares a large number of POD modes and thereby concludes highly complex turbulent fields with numerous spatial and temporal turbulent scales. Reconstructed velocity fields considering the leading dominant modes reveal a cut-off of 43% turbulent kinetic energy to clearly expose the coherent structures, which are consistent with published literature on similar turbulent channel flows. Additionally, quadrant analysis on the reconstructed velocity fields has clearly revealed that sweep and ejection modes are the only dominating Reynolds-stress-producing events for the current flow of interest, where the vortices lift the low-momentum fluid away from wall through the ejection mechanism. The high-momentum fluid impinges on the wall in sweep mode.

The mixed convective flow and heat transfer of nanofluids past a rotating cylinder placed in a uniform cross stream is investigated via SUPG (streamline upwind Petrov–Galerkin) based finite element method. Nano sized copper (Cu) particles suspended in water are used with Prandtl number ( Pr ) = 6.9. The computations are carried out at a representative Reynolds number ( Re ) of 100. The dimensionless cylinder rotation rate ( α ) is varied between 0 and 2. The range of nanoparticle volume fractions ( ϕ ) considered is 0 ≤ ϕ ≤ 5%. Effect of aiding buoyancy is brought about by considering two fixed values of the Richardson number ( Ri ) as 0.5 and 1.0. The thermophysical properties of nanofluid is predicted using a model which considers the fundamentals of interparticle interaction kinetics according to the details of agglomeration–deagglomeration of nanoparticles, variations in the effective particulate dimensions and agglomerate size ( n ), as well as the volume fractions of the particulate system. Increase in cylinder rotation and nanoparticle agglomeration shows suppression of vortex shedding. Completely steady solution is obtained at α = 2 . At Ri = 0.5, ϕ = 5 % , and n = 5, vortex shedding disappears and the flow achieves a steady state. A von Karman street is seen at α = 1.5 , for n > 1 and ϕ ≤ 3%. For ϕ = 4 % and 5%, vortex shedding ceases at n = 5. Characterisation of vortex shedding by POD (proper orthogonal decomposition) shows that increase in the values of n institutes coherence in the wake vortical structures. The average Nusselt number decreases with increasing α and increases with increasing n . At n = 5, a significant enhancement in heat transfer is seen.

The objective of the present investigation is to assess the performance of a plate-fin heat exchanger with an emphasis on acquiring fundamental understanding of the relation between local flow behavior and heat transfer augmentation mechanism. Numerical simulations are performed in a rectangular channel containing built-in longitudinal vortex generators on the bottom wall arranged periodically both in the streamwise and spanwise directions. Two types of vortex generators, namely, rectangular winglet pair (RWP) and delta-winglet pair (DWP) with two different flow arrangements, common-flow-up (CFU) and common-flow-down (CFD) have been explored to assess the influence of shape and flow arrangements on heat transfer enhancement. The basic mechanisms of flow structure and heat transfer characteristics have been examined with the help of secondary velocity vectors, streamlines, and temperature contours. Additionally, the mechanism of the local heat transfer augmentation has been explained using a novel concept called the field synergy principle. The performance of the vortex generators has been compared based on integral quantities such as Nusselt number, pressure loss, performance evaluation factor and domain averaged synergy angle. The computations reveal enhanced mixing of fluid between the wall layer and the core due to strong secondary flows produced by vortex generators. The performance analysis indicates that the RWP is more effective in terms of heat transfer enhancement as compared to DWP. The field synergy analysis has shown that the sites with higher Nusselt number are associated with smaller synergy angle or better coordination between the velocity vector and the temperature gradient.

Accurate information of supercapacitor (SC), also called electric double layer capacitor, leakage current is vital for effective State-of-Charge (SOC) estimation in Wireless Sensor Network (WSN) applications having long rest phase. In addition to improving accuracy of SOC estimation, real-time information on leakage current is highly beneficial for SC health monitoring. On the other hand, accurate mapping of SC open circuit voltage (OCV) vs. SOC significantly contributes towards accurate SOC estimation. Inaccuracies in either of these two information, i.e. leakage and OCV-SOC map, lead to inaccuracies in estimated SOC. In this paper, we propose a real-time estimation framework for accurate estimation of SOC under uncertain leakage and OCV-SOC map. Specifically, the proposed approach co-estimates leakage and part of OCV-SOC map in real-time along with SOC. The estimation framework utilizes Unscented Kalman Filter (UKF) along with an Equivalent Circuit Model (ECM) which captures SC leakage phenomenon. We identify the ECM parameters based on a Maxwell 25 F commercial SC. The experimentally identified ECM is subsequently used to perform simulation and experimental studies to validate the proposed framework. Finally, the robustness of the proposed framework with respect to parametric and measurement uncertainties is verified.

The mixed convective flow stability of nanofluids past a square cylinder is investigated by Dynamic Mode Decomposition (DMD). The energy content in the individual modes for Cu–water nanofluids is found to be higher than that of Al2O3–water nanofluids. DMD results showed the fact that Cu–water nanofluids have more small-scale structures of higher frequency modes compared to that of Al2O3–water nanofluids. The most dominant temporal dynamic mode corresponds to the lower-frequency eigenvalue λ = ( 0.99374 , ± 0.1117 ) for Al2O3–water nanofluids and λ = ( 0.99451 , ± 0.10464 ) for Cu–water nanofluids. Energy content in the mean flow of the base fluid at Richardson number of −0.5 is found to be maximum compared to that of nanofluids.

A subgrid-scale model pertaining to Large Eddy Simulation (LES) was developed by modifying the standard Smagorinsky model in order to take into account the inhomogeneities of the mean flow. According to this model, the magnitude of mean strain-rate is subtracted from the magnitude of the resolved strain-rate tensor for the calculation of eddy-viscosity. In this work, we perform large eddy simulation of turbulent channel flows at low and moderate Reynolds numbers. The predicted results compare well with the DNS data and results due to dynamic LES closure. The focus of this study has been on the assessment of capabilities of the model pertaining to the description of flow physics for the Reynolds numbers of interest. Also, the results are intended to establish the dominant effects of shear-length-scale near the wall. The simulations highlight other statistical features and turbulence characteristics too in order to broaden the applicability of the model.