A Subambient Open Roof Surface under the Mid-Summer Sun

A novel material open to warm air stays below ambient temperature under maximum solar intensities of mid-summer. It is found to be 11 °C cooler than a commercial white cool roof nearby. A combination of specially chosen polymers and a silver thin film yields values near 100% for both solar reflectance, and thermal emittance at infrared wavelengths from 7.9 to 13 μm

Five layer narrow band position variable filters for sharp colours and ultra low emittance

A simplified approach to creating narrow visible and near IR transmission bands with thin films is outlined utilising just five layers on glass, three of which are thin silver. These films have very high reflection at most wavelengths except for a narrow anti-reflection band where reflectance can be very close to zero and transmittance is close to 70%. In addition these properties are combined with IR reflectance approaching 99% for a very small thermal signature. Spatial variation of narrow band colour is easily achieved and is demonstrated with production of a simple wedge filter covering the full visible spectrum. Measured CIE colour contours in transmittance and reflectance are presented and spectral data on experimental films is compared with ideal models. Potential sources of small departures from ideal models are examined to assess the scope for future improvements. © 2008 Springer-Verlag

Is enhanced radiative cooling of solar cell modules worth pursuing?

© 2016 Elsevier B.V. All rights reserved. Recent suggestions that worthwhile additional cooling of 1.0-1.5 °C below what glass covers in solar cell modules already achieve, hence raised power output, will occur via enhanced thermal radiation to the sky with special nanostructures, is examined. Rigorous thermal models indicate these observations require a much lower hemispherical emittance (EH) for the benchmarks of silica and glass covers near 0.75. If the currently accepted value for EH of glass of 0.84 applied even EH=1.0 would provide inadequate extra cooling. An accurate angular emittance profile for glass does predict this lower EH. Complete models include solar heating, heating by atmospheric radiation, cooling by convection and side/base losses. Unfortunately any large lift in radiative output from raised EH at normal cell temperatures is mostly annulled by the accompanying fall in convective cooling. The link of EH to angular IR response points the way to novel coating approaches which may achieve the desired cooling gains. This has wider implications for buildings and other solar technologies. Direct power gains from accompanying anti-reflectance add value

Angular selectivity: Impact on optimised coatings for night sky radiative cooling

To achieve strong net thermal radiation emission from surfaces whose temperature is at or below ambient it is important to have high absorptance between 7.9 μm to 13 μm where the atmosphere is most transparent. Outside of this band the atmosphere behaves like a black body emitter and hence at these wavelengths net radiant heat loss is normally not possible at sub-ambient temperatures. It becomes possible using two types of angular electivity, which also improve emission between 7.9 μm to 13 μm. One is coating based, and one uses external heat mirrors. In the latter low emittance mirrors replace the higher emitting segments of the atmosphere. The coating's net gain is a result of its reflectance rise countering the atmosphere's drop in transparency as ray angles to the zenith approach the horizontal. These ideas are examined in the context of experimental data on coatings which rely on nanostructure to largely limit their spectral absorption to the atmosphere's transparent band. The angular selective coating becomes possible in two multilayer types (a) one nano-layer is strongly reflective (b) one layer has much higher index than the other. Type (a) materials as nanoparticles provide surface phonon resonance in the desired absorption band. © 2009 SPIE

Optimized cool roofs: Integrating albedo and thermal emittance with R-value

For cool roofs the combined effect of the three parameters that define heat gain and loss from a roof, namely solar albedo α, thermal emittance E, and sub-roof R-value, must be considered. An accurate contribution of night sky cooling, and hence humidity and total down-welling atmospheric radiation is needed. A systematic analysis of the contribution of a roof to average cooling load per day and to peak load reductions is presented for a temperate climate zone over 6 cooling months using an hour-by-hour analysis. Eighteen 3-parameter sets (α,E,R) demonstrate the over-riding importance of a high α, while sensitivity to R-value and E drops away as albedo rises. Up-front cost per unit reductions in peak demand or average energy use per day always rises strongly as R rises unless albedo is low. A moderate R∼1.63 is superior to high R unless a roof is dark, or winter heating demand is high. We indicate briefly why the roof typically does not present a dominant influence on average winter heating needs in most temperate zones, enhancing the benefits of cool roofs. © 2011 Elsevier B.V. All rights reserved

Discharge amplified photo-emission from ultra-thin films applied to tuning work function of transparent electrodes in organic opto-electronic devices

A novel photoemission technique utilising localised discharge amplification of photo-yield is reported. It enables fast, accurate measurement of work function and ionisation potential for ultra-thin buffer layers vacuum deposited onto single and multilayer transparent conducting electrodes for organic solar cells and OLED's. Work function in most traditional transparent electrodes has to be raised to maximise charge transfer while high transmittance and high conductance must be retained. Results are presented for a range of metal oxide buffers, which achieve this goal. This compact photo-yield spectroscopy tool with its fast turn-around has been a valuable development aid since ionisation potential can vary significantly as deposition conditions change slightly, and as ultra-thin films grow. It has also been useful in tracking the impact of different post deposition cleaning treatments along with some storage and transport protocols, which can adversely reduce ionisation potential and hence subsequent device performance. © 2013 Elsevier B.V. All rights reserved

Managing and monitoring radiant and convective heat flows in temperate zone urban environments

Modeling and experimental techniques for studying the impact of facades and roofing materials plus building layouts, on the UHI and interior energy use have been carried out for Sydney, which has a temperate zone climate. Cool roofing is treated as a multi-parameter sensitivity problem where changes in one of: solar reflectance, sub-roof R-value, and roof thermal mass (via material type or with phase change layers), plus change from cooling to heating season, alter the sensitivity of energy use and external surface temperatures to all of the other design settings. For high-rise layouts the issue of aspect ratio (height/spacing) plus spacing cover were considered. Special attention was paid to the night-time as an indicator of UHI problems and hence to flows of thermal radiation from the sky and from surfaces. Models needed to be accurate on this influence while a low cost, compact and portable technique for monitoring thermal radiation flows has been developed and calibrated using simultaneous full sky data from a pyrgeometer. An additional aspect of cool roofing has recently emerged involving modification to convective flow and cool air flow towards ground from varieties of multilayer roofing, including proliferation of solar modules on urban roofs, and additional layers involving polymer mesh or another solid layer

Modes of interaction between nanostructured metal and a conducting mirror as a function of separation and incident polarization

The optical resonances that occur in nanostructured metal layers are modulated in thin film stacks if the nanostructured layer is separated from a reflecting conducting layer by various thicknesses of thin dielectric. We have measured and modeled the optical response of interacting silver layers, with alumina spacer thickness ranging from a few nm to 50 nm, for s- and p-polarized incident light, and a range of incident angles. Standard thin film models, including standard effective medium models for the nanostructured layer, will break down for spacer thickness below a critical threshold. For example, with polarisation in the film plane and some nano-islands, it may occur at around 10 nm depending on spacer refractive index. Of particular interest here are novel effects observed with the onset of percolation in the nanolayer. Hot spot effects can be modified by nearby mirrors. Other modes to consider include (a) a two-particle mode involving a particle and its mirror image (b) A Fano resonance from hybridisation of localized and de-localised plasmon modes (c) a Babinet's core-(partial) shell particle with metal core-dielectric shell in metal (d) spacing dependent phase modulation (e) the impact of field gradients induced by the mirror at the nano-layer. © 2013 Copyright SPIE

Anomalously strong plasmon resonances in aluminium bronze by modification of the electronic density-of-states

© 2016 IOP Publishing Ltd. We use a combination of experimental measurements and density functional theory calculations to show that modification of the band structure of Cu by additions of Al causes an unexpected enhancement of the dielectric properties. The effect is optimized in alloys with Al contents between 10 and 15 at.% and would result in strong localized surface plasmon resonances at suitable wavelengths of light. This result is surprising as, in general, alloying of Cu increases its DC resistivity and would be expected to increase optical loss. The wavelengths for the plasmon resonances in the optimized alloy are significantly blue-shifted relative to those of pure Cu and provide a new material selection option for the range 2.2-2.8 eV