Matteo Chiesa

High-performance flat-panel solar thermoelectric generators with high thermal concentration

The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. Photovoltaic cells are deployed widely, mostly as flat panels, whereas solar thermal electricity generation relying on optical concentrators and mechanical heat engines is only seen in large-scale power plants. Here we demonstrate a promising flat-panel solar thermal to electric power conversion technology based on the Seebeck effect and high thermal concentration, thus enabling wider applications. The developed solar thermoelectric generators (STEGs) achieved a peak efficiency of 4.6% under AM1.5G (1 kW m(-2)) conditions. The efficiency is 7-8 times higher than the previously reported best value for a flat-panel STEG, and is enabled by the use of high-performance nanostructured thermoelectric materials and spectrally-selective solar absorbers in an innovative design that exploits high thermal concentration in an evacuated environment. Our work opens up a promising new approach which has the potential to achieve cost-effective conversion of solar energy into electricity.

Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid

This study investigates the thermal conductivity and viscosity of copper nanoparticles in ethylene glycol. The nanofluid was prepared by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. Volume loadings of up to 2% were prepared. The measured increase in thermal conductivity was twice the value predicted by the Maxwell effective medium theory. The increase in viscosity was about four times of that predicted by the Einstein law of viscosity. Analytical calculations suggest that this nanofluid would not be beneficial as a coolant in heat exchangers without changing the tube diameter. However, increasing the tube diameter to exploit the increased thermal conductivity of the nanofluid can lead to better thermal performance.

A frequency-domain thermoreflectance method for the characterization of thermal properties

A frequency-domain thermoreflectance method for measuring the thermal properties of homogenous materials and submicron thin films is described. The method can simultaneously determine the thermal conductivity and heat capacity of a sample, provided the thermal diffusivity is greater, similar3x10(-6) m(2)/s, and can also simultaneously measure in-plane and cross-plane thermal conductivities, as well the thermal boundary conductance between material layers. Two implementations are discussed, one based on an ultrafast pulsed laser system and one based on continuous-wave lasers. The theory of the method and an analysis of its sensitivity to various thermal properties are given, along with results from measurements of several standard materials over a wide range of thermal diffusivities. We obtain specific heats and thermal conductivities in good agreement with literature values, and also obtain the in-plane and cross-plane thermal conductivities for crystalline quartz.

An optical pump-probe technique for measuring the thermal conductivity of liquids.

We present a pump-probe optical technique for measuring the thermal conductivity of liquids. The technique uses a reflective geometry which does not depend on the optical properties of the liquid and requires as little as a single droplet to produce a result. An analytical solution is given for bidirectional heat flow in layered media, including the effects of radial heat flow from coaxial Gaussian laser spots, thermal interface resistances, and the accumulation of multiple laser pulses. In addition, several experimental improvements over previous pump-probe configurations are described, resulting in an improved signal to noise ratio and smaller errors at long stage delay times. The technique is applied to a range of liquids and solids. Results are in good agreement with literature values.

Photovoltaic-thermoelectric hybrid systems : A general optimization methodology

The present work outlines a general optimization methodology for hybrid systems consisting of photovoltaic (PV) and thermoelectric (TE) modules. Exemplarily, hybrid systems with hydrogenated microcrystalline silicon, hydrogenated amorphous silicon, and bulk heterojunction polymer thin-film solar cell for different solar TE generator efficiencies are evaluated. The proposed methodology optimizes the partitioning of the solar spectrum in order to yield the maximum conversion efficiency of a PV-TE hybrid system with a solar cell operating at ambient temperature.

A method to provide rapid in situ determination of tip radius in dynamic atomic force microscopy

We provide a method to characterize the tip radius of an atomic force microscopy in situ by monitoring the dynamics of the cantilever in ambient conditions. The key concept is that the value of free amplitude for which transitions from the attractive to repulsive force regimes are observed, strongly depends on the curvature of the tip. In practice, the smaller the value of free amplitude required to observe a transition, the sharper the tip. This general behavior is remarkably independent of the properties of the sample and cantilever characteristics and shows the strong dependence of the transitions on the tip radius. The main advantage of this method is rapid in situ characterization. Rapid in situ characterization enables one to continuously monitor the tip size during experiments. Further, we show how to reproducibly shape the tip from a given initial size to any chosen larger size. This approach combined with the in situ tip size monitoring enables quantitative comparison of materials measurements between samples. These methods are set to allow quantitative data acquisition and make direct data comparison readily available in the community.

Single element spectral splitting solar concentrator for multiple cells CPV system

Shockley Read Hall equation poses a limit to the maximum conversion efficiency of broadband solar radiation attainable by means of a single bandgap converter. A possible approach to overcome such a limit is to convert different parts of the broadband spectrum using different single junction converters. We consider here a different modus operandi where a single low-cost optimized plastic prismatic structure performs simultaneously the tasks of concentrating the solar light and, based on the dispersive behavior of the employed material, spatially splitting it into its spectral component. We discuss its approach, optical simulations, fabrication issues and preliminary experimental results demonstrating its feasibility for cost effective high efficiency Concentrated Photovoltaic Systems (CPV) systems.

Characterization of thin metal films via frequency-domain thermoreflectance

Frequency-domain thermoreflectance is extended to the characterization of thin metals films on low thermal diffusivity substrates. We show how a single noncontact measurement can yield both the thickness and thermal conductivity of a thin metal film with high accuracy. Results are presented from measurements of gold and aluminum films 20–100 nm thick on fused silica substrate. The thickness measurements are verified independently with atomic force microscope cross sections, and the thermal conductivity measurements are verified through electrical conductivity measurements via the Wiedemann–Franz law. The thermoreflectance thermal conductivity values are in good agreement with the Wiedemann–Franz results for all the films at least 30 nm thick, indicating that our method can be used to estimate electrical conductivity along with thermal conductivity for sufficiently thick films.

Quantifying charge carrier concentration in ZnO thin films by Scanning Kelvin Probe Microscopy.

In the last years there has been a renewed interest for zinc oxide semiconductor, mainly triggered by its prospects in optoelectronic applications. In particular, zinc oxide thin films are being widely used for photovoltaic applications, in which the determination of the electrical conductivity is of great importance. Being an intrinsically doped material, the quantification of its doping concentration has always been challenging. Here we show how to probe the charge carrier density of zinc oxide thin films by Scanning Kelvin Probe Microscopy, a technique that allows measuring the contact potential difference between the tip and the sample surface with high spatial resolution. A simple electronic energy model is used for correlating the contact potential difference with the doping concentration in the material. Limitations of this technique are discussed in details and some experimental solutions are proposed. Two-dimensional doping concentration images acquired on radio frequency-sputtered intrinsic zinc oxide thin films with different thickness and deposited under different conditions are reported. We show that results inferred with this technique are in accordance with carrier concentration expected for zinc oxide thin films deposited under different conditions and obtained from resistivity and mobility measurements.

Experimental investigation of nanofluid shear and longitudinal viscosities

Dilute nanoparticle suspensions of alumina in decane and isoparaffinic polyalphaolefin (PAO) exhibit thermal conductivity and shear viscosity that are enhanced compared to continuum models that assume well-dispersed particles. An optical technique has been used to measure the longitudinal viscosity of these suspensions at frequencies from 200to600MHz and evaluate an effective hydrodynamic particle size. The measurements indicate that for the decane-based nanofluids the nanoparticles do not form clusters. In the case of PAO nanofluids, the measurements of longitudinal viscosity and the corresponding values of the particle size are consistent with a picture of nonclustered particles in a weakly shear-thinning viscous oligomeric oil.Dilute nanoparticle suspensions of alumina in decane and isoparaffinic polyalphaolefin (PAO) exhibit thermal conductivity and shear viscosity that are enhanced compared to continuum models that assume well-dispersed particles. An optical technique has been used to measure the longitudinal viscosity of these suspensions at frequencies from 200to600MHz and evaluate an effective hydrodynamic particle size. The measurements indicate that for the decane-based nanofluids the nanoparticles do not form clusters. In the case of PAO nanofluids, the measurements of longitudinal viscosity and the corresponding values of the particle size are consistent with a picture of nonclustered particles in a weakly shear-thinning viscous oligomeric oil.