Parameswar Hari

Selective spectral filtration with nanoparticles for concentrating solar collectors

Abstract. A spectral fluid filter for potential use in hybrid photovoltaic/thermal concentrating solar collectors has been developed, targeting maximum absorption above and transmission below a desired wavelength. In this application, the temperature-dependent bandgap of the potential solar cell is used in the optimization of the filter. Dispersing a mix of colloidal nanoparticles in a heat transfer fluid is shown to absorb 86% of sub-bandgap insolation while absorbing only 18% above bandgap insolation. Transmission above bandgap light would be directly absorbed into the photovoltaic (PV) cell while absorbed photons transfer energy directly into the heat transfer fluid ultimately reducing the number of heat transfer steps. Placement of a filter in front of the PV cell is shown to decrease losses by converting an additional 2% of the total solar energy into thermal energy since it allows recollection of light reflected off the receiver.

Plasmonic nanoparticle based spectral fluid filters for concentrating PV/T collectors

We propose a design for a concentrating PV/T collector utilizing plasmonic nanoparticles directly suspended in the working fluid to spectrally filter the incoming solar flux. This liquid filter serves two purposes: the direct capture of thermal energy as well as filtering off of key portions of the spectrum before transmission to the PV cell. Our device builds upon the current Cogenra T14 system with a two-pass architecture: the first pass on the back side of the PV cell pre-heating the fluid from any thermalization losses, and the second pass in front of the PV cell to achieve the spectral filtering. Here we present details on the selection of plasmonic nanoparticles for a given cell bandgap as well as the impact to the overall system pumping power and cost.

Performance Evaluation of a High-Speed Inertial Exercise Trainer

Caruso, JF, Hari, P, Coday, MA, Leeper, A, Ramey, E, Monda, JK, Hastings, LP, and Davison, S. Performance evaluation of a high-speed inertial exercise trainer. J Strength Cond Res 22(6): 1760-1768, 2008-A high-speed, low-resistance inertial exercise trainer (IET, Impulse Training Systems, Newnan, Ga) is increasingly employed in rehabilitative and athletic performance settings. Repetitions on an IET are done through a large range of motion because multijoint movements occur over more than one plane of motion, with no limitation on velocities or accelerations attained. The current study purpose is to assess data reproducibility from an instrumented IET through multiple test-retest measures. Data collection methods required the IET left and right halves to be fitted with a TLL-2K force transducer (Transducer Techniques, Temecula, Calif) on one of its pulleys, and an infrared position sensor (Model CX3-AP-1A, automationdirect.com) located midway on the underside of each track. Signals passed through DI-158U signal conditioners (DATAQ Instruments, Akron, Ohio) and were measured with a four-channel analog data acquisition card at 4000 Hz. To assess data reproducibility, college-age subjects (n = 45) performed four IET workouts that were spaced 1 week apart. Workouts entailed two 60-second sets of repetitive knee- and hip-extensor muscle actions as subjects were instructed to exert maximal voluntary effort. Results from multiple test-retest measures show that the IET elicited reproducible intra- and interworkout data despite the unique challenge of multiplanar and multijoint exercise done over a large range of motion. We conclude that future studies in which IET performance measurement is required may choose to instrument the device with current methodology. Current practical applications include making IET data easier to comprehend for the coaches, athletes, and health care providers who use the device.

A Parametric Investigation of a Concentrating Photovoltaic/Thermal System With Spectral Filtering Utilizing a Two-Dimensional Heat Transfer Model

A 2D heat transfer model of a hybrid photovoltaic/thermal (PV/T) system has been created. This paper investigates the impact of ideal filters to best accommodate for a nonuniform PV temperature along the length of the receiver. The proposed configuration consists of a GaAs cell laminated to an aluminum extrusion. The working fluid, a transparent high-temperature heat transfer fluid with suspended nanoparticles, flows through the hollow extrusion where it cools the PV cell before it is redirected in front of the cell acting as an optical filter. The model accounts for PV cell efficiency, temperature, and bandgap dependence, the details often neglected in prior works.

Design and feasibility of high temperature nanoparticle fluid filter in hybrid thermal/photovoltaic concentrating solar power

A nanoparticle fluid filter used with concentrating hybrid solar/thermal collector design is presented. Nanoparticle fluid filters could be situated on any given concentrating system with appropriate customized engineering. This work shows the design in the context of a trough concentration system. Geometric design and physical placement in the optical path was modeled using SolTrace. It was found that a design can be made that blocks 0% of the traced rays. The nanoparticle fluid filter is tunable for different concentrating systems using various PV cells or operating at varying temperatures.

Cobalt Doped ZnO Nanorods Fabricated by Chemical Bath Deposition Technique

ZnO nanorods are currently studied for variety optoelectronic applications. Typically, thin film and bulk ZnO show a strong light absorption in the ultra violet (UV) range. For devices that operate in the visible and infrared range such as optoelectronic sensors and photovoltaic cells, it is necessary to modify the absorption profile from UV to higher wavelengths in the visible region. In this study we investigate optical absorption of ZnO nanorods doped with cobalt using a modified chemical bath deposition technique. The light absorption properties of Cobalt doped ZnO nanorods were studied using Photoluminescence (PL) and Raman Spectroscopy. For doping of Cobalt ranging from 3 to 10 percentage of the total weight, the PL intensity shows a suppression of the prominent UV peak at 383 nm with increase in doping concentration. This reduction in PL intensity at 383 nm is accompanied by an increase in the PL intensity at 429 nm and 469 nm. We will discuss details of ZnO-Cobalt structures using Raman spectroscopy on cobalt doped ZnO nanorod samples of 1 -20% doping concentration.

Indirect (J) coupling of inequivalent 75As nuclei in crystalline and glassy As2Se3 and As2S3

We present the results of nuclear quadrupole resonance (NQR) measurements in crystalline and glassy As2Se3 and As2S3. As a function of pulse separation, the decays of the NQR Hahn echoes following a 90°–180° pulse sequence exhibit damped oscillations superimposed on an exponential decay. These damped oscillations can be explained by an indirect coupling (J coupling) between two As nuclei through the polarized electrons on the chalcogen atom. Experimental values of the J couplings were obtained from the periods of the oscillations and calculations of the most probable transitions using second-order perturbation theory. The value estimated by this method for the 2J(75As–S–75As) in crystalline As2S3 compares well with empirical estimates, which are obtained using an existing value of 2J(31P–S–31P) and known scalings with atomic number from the literature. Unfortunately, there is insufficient experimental data on 2J(X–Se–X) couplings to make any empirical estimate of 2J(75As–Se–75As).

Variation of index of refraction in cobalt doped ZnO nanostructures

One dimensional zinc oxide (ZnO) nanostructures were fabricated using a low temperature chemical bath deposition technique. The ZnO nanorods were doped with cobalt using cobalt nitrate with cobalt concentration varying from 0% to 9%. The scanning electron microscope images of the nanostructures indicate that the diameter of ZnO nanorods increased with the increase in cobalt doping concentration. The optical characterizations of the doped and undoped samples were performed by investigating the variation in the band gap, the Urbach energy, the index of refraction, and the extinction coefficient with cobalt concentration. The dispersion of index of refraction in cobalt doped ZnO nanostructures was modeled based on the Wemple DiDomenico single oscillator model. The interband oscillator energy and the dispersion energy were estimated for different cobalt doped ZnO nanorod samples based on this model.One dimensional zinc oxide (ZnO) nanostructures were fabricated using a low temperature chemical bath deposition technique. The ZnO nanorods were doped with cobalt using cobalt nitrate with cobalt concentration varying from 0% to 9%. The scanning electron microscope images of the nanostructures indicate that the diameter of ZnO nanorods increased with the increase in cobalt doping concentration. The optical characterizations of the doped and undoped samples were performed by investigating the variation in the band gap, the Urbach energy, the index of refraction, and the extinction coefficient with cobalt concentration. The dispersion of index of refraction in cobalt doped ZnO nanostructures was modeled based on the Wemple DiDomenico single oscillator model. The interband oscillator energy and the dispersion energy were estimated for different cobalt doped ZnO nanorod samples based on this model.

INVESTIGATIONS ON THE ELECTRICAL PROPERTIES OF ZnO NANORODS AND COMPOSITES FOR PHOTOVOLTAIC AND ELECTROCHEMICAL APPLICATIONS

ZnO nanorods grown by hydrothermal technique on glass, Zinc, and Indium tin oxide (ITO) substrates exhibit both open and closed hexagonal structures. On the nanoscale, closed ZnO nanostructures exhibit two types of ion conduction regions as revealed by AC-impedance spectra collected through the tip of an atomic force microscope (AFM). One region has higher impedance values (apparent values of approximately 107 ohms) with two semicircles. Two semicircles are indicative of a ZnO structure composed of bulk and grain boundary conduction. Other regions were found to have impedance values that were two orders of magnitude lower (apparent values of 105 ohms). This indicates that these ZnO films have two conduction pathways. The polyethylene oxide (PEO)–ZnO nanorod composite was made by spin-coating the ZnO rods growing from the ITO substrate with PEO. In the PEO–ZnO composite film, only the AC impedance values of 105 ohms were observed. This is higher than PEO electrolyte without ZnO nanorods. Since regions of higher impedance were not seen in the PEO–ZnO nanorod composites, the polymer electrolyte either dominated the conduction of the system or suppressed the first pathway of higher impedance in the ZnO rods.

Low Temperature Growth of ZnO Nanorods by Chemical Bath Method

ZnO nanorods grown by chemical bath methods are of great interest in photovoltaic and electronic device applications because they offer low cost, low temperature deposition techniques compared to conventional molecular beam vapor deposition and sputtering methods. Our previous studies of ZnO nanorods grown by chemical bath technique on indium tin oxide (ITO) coated glass substrates at 90 C for 8-10 hours resulted in uniform growth of hexagonally shaped closed nanorod structures. We used scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques to map changes in surface morphology of nanorods grown on various substrates. Morphology of ZnO nanorods at temperatures 80C, 90C, 95C and 100C for 9 hours of hydrothermal growth also resulted in hexagonal shaped nanorods of various sizes and surface roughness. In addition, we studied the changes in surface morphology of ZnO nanorods on indium tin oxide coated glass, aluminum coated glass, and conducting tin oxide glass substrate. In this paper, we present quantitative data on changes in cluster size and shape of nanorods as the growth substrate and deposition temperature are varied. We will also discuss conductivity changes of ZnO nanorods deposited on various substrates.