Solar Optical Materials Applications & Performance of Coatings & Materials in Buildings & Solar Energy Systems Proceedings of the Conference, Oxford, UK, 12-13 April 1988 Edited by M. G. Hutchins Solar Energy Materials Research Laboratory Oxford Polytechnic, Oxford, UK Published for INTERNATIONAL SOLAR ENERGY SOCIETY, UK SECTION by PERGAMON PRESS OXFORD · NEW YORK · BEIJING · FRANKFURT SAO PAULO · SYDNEY · TOKYO · TORONTO Other Pergamon Titles of Related Interest ALAWI et al Solar Energy & the Arab World ALAWI & AYYASH Solar Energy Prospect in the Arab World BOWEN & YANN AS Passive & Low Energy Ecotechniques CARTER & DE VILLIERS Principles of Passive Solar Building Design ISES Intersol 85 ISES Advances in Solar Energy Technology MCVEIGH Sun Power, 2nd Edition SODHA et al Solar Passive Building Pergamon Journals (free specimen copy gladly sent on request) Energy Energy Conversion & Management Heat Recovery Systems & CHP International Journal of Hydrogen Energy Materials Research Bulletin Solar & Wind Technology Solar Energy UK Pergamon Press pic, Headington Hill Hall, Oxford OX3 OBW, England USA Pergamon Press Inc, Maxwell House, Fairview Park, Elmsford, New York 10523, USA PEOPLE'S REPUBLIC Pergamon Press, Room 4037, Qianmen Hotel, Beijing, OF CHINA People's Republic of China FEDERAL REPUBLIC Pergamon Press, Hammerweg 6, OF GERMANY D-6242 Kronberg, Federal Republic of Germany BRAZIL Pergamon Editora, Rua Eca de Queiros, 346, CEP 04011, Sao Paulo, Brazil AUSTRALIA Pergamon Press Australia, PO Box 544, Potts Point, NSW 2011, Australia JAPAN Pergamon Press, 5th Floor, Maisu:- ι Central Building, 1-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160, Japan CANADA Pergamon Press Canada, Suite 271, 253 College Street, Toronto, Ontario, Canada M5T1R5 Copyright © 1988 International Solar Energy Society, UK Section (UK-ISES) All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the copyright holders. First edition 1988 ISBN 0-08-036613-9 UK-ISES is not responsible for the accuracy of any statements or claims made by the authors in this publication. FOREWORD Spectrally selective solar absorber surfaces are the best known example of optical materials use for enhancing the thermal per­ formance and efficiency of solar energy systems. The use of heat mirrors to reduce thermal losses in glazings is also well known. In more recent years considerable research interest has developed in areas such as electrochromic cells for the dynamic control of window transmittance, transparent insulation, and the service lifetime of solar optical materials. New application areas such as automotive glazing and daylighting control have been found. Measurement techniques for the determination of optical proper­ ties have been refined and improved but problem areas still exist, e.g., in the spectral directional measurement of infrared properties. The Solar Optical Materials conference provided an opportunity to assemble a number of leading European experts to discuss progress and areas of concern relevant to materials use in solar energy and buildings applications. These Proceedings contain the texts of the papers presented at the conference. The papers are ordered within the structure of the four ses­ sions used for the conference. The sessions are entitled: I. Transparent media for advanced window applications. II. Optical switching films and novel materials for the controlled conversion of solar radiation. HI. Selective absorber surfaces, durability and service lifetime prediction. IV. Measurement techniques for surface charaterisation. It has been a pleasure to be involved in the organisation of this conference and on behalf of UK-ISES I should like to express my thanks to Oxford Polytechnic's Conference Office and to Pergamon Press for their invaluable assistance throughout. Dr M.G. Hutchins Ox fο rd Po1ytechn i c March 1988 vi i . PRINCIPLES AND PROPERTIES OF HEAT MIRROR COATINGS FOR DOMESTIC WINDOWS R.P. HOWSON PHYSICS DEPARTMENT, LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY LOUGHBOROUGH LEICS. LE11 3TU REQUIREMENTS Transfer of energy by radiation is characterised by the temper­ ature of the source and the receiver, and their emission and absorption characteristics. The amount of radiative energy emitted is proportional to the absolute temperature,T, to the fourth part, through Stefans constant,^ and the emissivity, ε. Mg = ε3σΤ4. S is the area. This energy is distributed in wavelengths with a maximum given by T A = 2. 9xlO~*^m°K. The m ax amount of energy absorbed by the receiver is dependent 0.1 the incident radiant flux and the integrated energy abs rptan.3e,A. Δ receiver then intercepts energy over one wavelength range, characteristic of the source temperature, and radiates it over a range characteristic of its own temperature. Because the radiation from the sun is characteristic of a source at about 5,800°K, while energy is radiated from the receiver at around room Temperature, say 300°K, it is possible to adjust the optical properties of a window to act as an energy trap. The receiver window passes energy at short wavelengths through to the energy absorber, i.e., the room, with little attenuation. The visible window, which is in general opaque to heat/infra-red radiation, will radiate this heat, but characteristic of a much longer wavelength. This radiation can be limited by adjusting the surface properties to be highly reflective in that spectral region. From heat balance considerations the emissivity, ε, = 1-R and it will be low and the ridiation inhibited. The heat mirror is created. It requires to have high transmittance for visible wavelengths, those of the sun's energy, and high reflectance, and hence low emissivity, for heat-infra-red wave­ lengths. This distribution is shown in fig. 1. A window is also to see through. The response of the human eye is also shown in fig. 1. Generally, inhibition of radiant energy loss is best done within a double or more glazed enclosure. A way of passing solar energy into an enclosure to raise its temperature and inhibiting loss of energy is thus created. The gaining of energy for a window system is not always desirable, In some cases visual contact is required but minimum energy transfer is desired to heat an already hot room. In this case windows with good visual, but poor solar, transparency are required, and are obviously narrow band filters around the peak of the eye1s sensitivity in the green region of the spectrum, at 550nm, with high reflectance elsewhere, though absorption could be tolerated with good window cooling. Solar radiation 1 . Wavelength (microns) Fig. 1: The emission spectra of the sun (5,800°K,A) and for surfaces at room temperature (300°K,B) normalised to the same peak value. Reflectances (R) and transmittances (T) for an ideal filter are shown together with the response of the eye (E). is received over large areas and domestic windows are designed to be as large as possible to utilise visual and heat energy. Any coated window must be available in large areas and at a cost which will justify their use. Materials and techniques have recently emerged to meet these requirements, and are commonly used in domestic windows. The background to this development can be described to indicate where future progress is likely to take place. MATERIALS Established optical filter manufacturers use absorbing dyes or multiple layers of alternating dielectrics in an interference array. The former does not give the required properties, the latter can give the performance using a large number of layers, which makes it very expensive. A filter requires to be of a small number of layers, of little thickness, made with a tech­ nique that gives high uniformity and high rates. They need to be surface layers which cannot be protected so that high durability is required. The only way this performance can be achieved is to use the intrinsic properties of a material. It turns out that some metals have the selected properties required and they can be enhanced in a simple way. The theory of metals explains that their optical properties are related to the density of free carriers and their relax­ ation time to give a reflection which is a function of wavelength. It is dielectric in nature at low wavelengths, falls to a minimum and rises to close to 1 for longer wavelengths (fig. 2). This reflectance is associated with the highly absorbing nature of the interaction of electrons with electro­ magnetic radiation. The transparent region at low wavelengths is not easily seen with many materials because of the simul- 2. β.3 0.5 0.7 0.9 2 4 6 8 10 NRVELENGTH/MICRONS Fig. 2: The reflectance and transmittance curve for a thin film (60nm) of an "ideal" metal. taneous molecular type absorption of the system. Elemental metals, which have a small molecular absorption with lon^ relaxation times, to have the potential for selective reflec­ tance include silver, gold and copper. Unfortunately the wavelength at which the transition to higher reflecting surfaces occurs is too low, generally in the visible or UV. However, if a high index optical matching layer is used either side of the thin metal layer, then the transition waveband can be moved to the infra-red and the visible transparency can be increased to a higher value. This is done with very thin layers of each, about 30nm for all layers. (Fig. 3). Compound metals such as the nitrides of Ti, Zr or Hf also show selective properties and may be used to give more durable but less effective filters. (Fig. 4). The metallic oxides of indium, tin, cadmium and their alloys show the optical properties associated with less-well-conducting metals, with a transition from transparent to reflecting occur- ing around 2μπι, and remain free of molecular absorption in the visible region. They show a strong dielectric reflection associated with their high refractive index of around 2. This can be eliminated using anti-reflection layers of intermediate index. Of more concern is the fact that these materials have to be of consdierably greater thickness, than the conventional metals, to achieve similar IR reflectances and hence low emissivity. This thickness leads to interference colours being seen in reflection in the visible which changes with viewing angle. This is regarded as unacceptable for domestic windows so that material properties have either to be the best possible, to allow films of thickness below that which reflected inter­ ference colours are seen, or thick films used, hwich give multiple wavelength reflection peaks which are seen as "white". 3. 100 Ι » » 1—ι—ι—π 1 r Wavelength (/cm ) 4. Unfortunately, very thick films often have rough surfaces leading to a "milky" appearance. (Fig. 5). Fig. 5: The optical performance of elect­ rically conducting oxide films. C.3 1.0 I G 32 WRv'L'LCNGTH (microns) 10 ( >, ITO ( ), CTO C ) The opposite problerr occurs with metal films which are required in thicknesses which are so small as to be close to the level at which they become discontinuous and lose their selective properties. The technique of providing them can be an important influence on their properties. The trade in properties that is done is shown in fig. 6, calculated for "bulk" properties and also those actually realised. .80- Fig. 6: The theoretical performance of simple silver • 70- films ( ) and 3-layer sandwiches ( ) compared with 60- actual performances (Δ Ο Σ: Δ ζ respectively) Δ § 50- i ί.01— JL 100 *0 60 80 REFLECTANCE in % α τ 12 microns TECHNIQUES Oxide coatings are traditionally made by the pyrolytic decomp­ osition of a vapour, solution in a spray, of a metal compound in the presence of oxygen. Doping to produce the required properties can be done with the addition of the dopant in a similar way. Typical films made in this way are Sn (:F1)02 and In (:Sn)203. These processes require high temperatures of 5 . around 400°C and problems are encountered with the diffusion of sodium ions from soda-lime glass into the films, which destroy their electrical conducting characteristics and hence their IR reflectivity. Barrier layers have to be used. Within the last few years, the technique of planar magnetron sputtering has emerged to create a method of producing both thin metal and metal oxide films with the desired properties onto large area substrates at a higher rate. This technique is used at the moment to produce oxide-Ag-oxide filters which form the basis of the current commercial market and is being used for indium- tin oxides as well. Planar magnetron sputtering is a vacuum technique which is very inefficient in its use of energy. It requires expensive capital equipment and some skill to control. It can, however, provide a large area film at sufficient rate to make the financial and energy costs small, making the coating an essential feature of any window being replaced for some other reason. The add-on cost is justified. Sputtering is a process which provides good adhesion of coatings to unheated substrates and can be used to coat both rigid and flexible polymers with coatings to give desirable electrical and optical properties, but also to provide abrasion resistance which may give greater impetus to their wider use. EXTENSIONS Double glazing incorporating heat mirrors is now commonplace, but this is only the beginning. Many climates and applications require solar energy blocking whilst maintaining visual trans­ parency; after all, the highest energy consumption in many parts of the USA occurs in summer due to the use of air conditioning. Such a filter uses a combination of Fabry-Perot techniques with the intrinsic properties of a thin metal layer which can be realised in a 5-layer coating of oxide-metal-oxide-metal-oxide (fig. 7). The centre layer provides the Fabry-Perot transmission selection of whichever order and wavelength is chosen; the metal provides the IR reflection. More and more features are required of windows of glass, E-M radiation rejection, ability to be tempered, heating of the surfaces to prevent condensation etc. These properties need to be extended to polymers. CONCLUSIONS Current window coatings for heat mirrors generally consist of Sn02-Ag-Al-Sn02. The tin oxide coatings are about 400Â thick and are produced by reactive planar magnetron sputtering; silver and aluminium are directly sputtered, the aluminium being made very thin, and it is there to protect the silver during the final deposition process for the oxide. Such coatings show very little colour in reflection, good transmittance and an IR reflectance of up to 90%, i.e. an emissivity of 0.1. These properties are all that are needed essentially to eliminate the radiation component part of the heat conduction of a double glazed window when it is placed on one internal surface. The U value is dominated by convection and edge loss through the sealant. These coatings are, however, soft and sensitive 6 .