The paper deals with climate change and global warning and concentrates on the various forms of renewable energy, mainly Photovoltaics, Solar Thermal, Biomass, Geothermal, Wave, Hydro and Wind Energy. Specific examples will be placed on Photovoltaic applications and the future prospects of this discipline to meet 20% of the electricity generated on a global scale by the year 2020. The paper discusses the various fundings and programs available around the world in renewable energy and concludes that renewable energy is the best energy strategy for any country.

A number of devices have been constructed which directly convert light into electrical work or into a flow of chemical products with a free energy higher than the starting materials. Solar Concentrators have been developed which increase the irradiance on the converter material to levels above that used without the device even if the incident light is diffuse. In contrast to man made converter systems, natural photosynthesis seems, at first, to rely on entirely different conversion mechanisms and physics. In this paper is presented a unification of the physics and the thermodynamics of all quantum solar converters. It will be shown that the Planck equation may describe the upper limit to the performance of systems as diverse as the Fluorescent Solar Concentrator, the novel Ruthenium dye sensitized TiO₂ photoelectrochemical cells of Dr. Graetzel, silicon solar cells, Chlorophyll, and, perhaps, the unique `protonic' solar conversion found in the purple membrane of Halobacterium halobium.

The sol-gel method which features a low-temperature wet-chemical process opens vast possibilities to incorporating organic dyes into solid matrices for various optical applications. In this paper we present our experimental results on the sol-gel derived photochromic organic- inorganic composite (Ormocer) materials following an introductory description of the sol-gel process and a brief review on the state of the art of the photochromic solids prepared using this method. Our photochromic spirooxazine-Ormocer gels and coatings possess better photochromic response and color-change speed than the corresponding photochromic polymer coatings and similar photochemical stability to the latter. Further developments are proposed as to tackle the temperature dependence problem and further tap the potentialities of the photochromic dye-Ormocer material for practical applications.

Nb₂O₅ prepared by a sol-gel process in form of coatings and aerogels are new materials which present interesting properties: (a) the coatings present electrochromic properties and exhibit a blue coloration under Li⁺ insertion with 100% reversible variation of the optical transmission in the visible and near infrared range between 80% and 20% and have a high chemical stability (tested up to 2000 cycles), (b) they are semiconductor and present a photoelectric effect when illuminating in the UV region ((lambda) < 360 nm). These films are therefore very promising to be used in electrochromic devices, as electrodes for photoelectrochemical purpose and the development of nanocrystalline solar cell, and (c) when prepared in aerogel form, the high BET surface area of the powders is a promising asset to use these new materials for catalytic purposes.

A novel synthesis of a sol-gel precursor for WO₃ thin films is presented. Through addition of oxalic acid dihydrate to the precursor solution, films with different microstructures may be obtained; and these microstructures are characterized using TEM, FESEM, and SAXS. Without the additive, the films are relatively homogeneous and amorphous; with the additive, the films are multiphase, containing amorphous and crystalline regions. In addition, the latter films contain numerous small regions of apparently much higher electron density, which likely correspond to a dispersed W-rich phase. Optoelectrochemical analysis of the films under galvanostatic conditions demonstrates that the homogeneous, amorphous films display a pronounced cycling effect in both the response of the electrode potential and the optical efficiency. In contrast, the multiphase films show no significant cycling effect.

The characterization of primary physical and chemical properties of various thin films prepared by plasma assisted deposition techniques for selective absorber layers is discussed with special emphasis on photoelectron spectroscopy. The materials of interest here consist of an amorphous hydrogenated carbon matrix in which metallic or carbidic clusters are embedded.

Results from a comprehensive case study on accelerated life testing of some selective solar collector absorber coatings for DHW systems are reviewed. The study was conducted within Task X `Solar Materials Research and Development' of the IEA Solar Heating and Cooling Program from 1987 to 1992 and is unique due to its quantitative and systematic approach for durability assessment. The work of case study involved the development of both experimental and theoretical tools to aid the assessment of service life or absorber coatings. This entailed performance analysis, failure analysis, microclimate characterization, environmental resistance testing and life date analysis. Predicted in-service degradation of coatings from accelerated life testing was found to be in fairly good agreement both qualitatively and quantitatively with what was actually observed on coatings installed and tested for three years in solar collectors working under typical DHW conditions.

Tungsten and chromium containing hydrogenated amorphous carbon films were deposited in a process which combines plasma activated chemical vapor deposition of methane and r.f.- sputtering of a metallic target. The metal content of the deposits can be adjusted by the ratio of gas flow between argon and methane and was determined by X-ray Photoelectron Spectroscopy (XPS). For the a-C:H/W composites the XPS data are presented in detail and supply information about the chemical state of carbon and tungsten. The presence of W in carbidic state could be proven. Furthermore the optical constants n and k of a-C:H/W were obtained in the wavelength range between 0.4 and 2.6micrometers and for W concentrations of up to 33 at.%. The morphology of the deposits, determined by atomic force microscopy, ranges from very porous to compact and can be controlled by the substrate bias voltage. Accelerated ageing investigations were performed in air in order to characterize the deterioration mechanisms of a-C:H/W and a-C:H/Cr films deposited on different substrate materials. The significance of the morphology of the coating and the roughness of the substrate for the ageing mechanisms could be shown. Very promising results concerning the film stability were obtained for a-C:H/Cr.

In general vacuum tube collectors are used in solar process heat systems. Another possibility is to use transparent insulated flat plate collectors. A critical point however, is that most of the common transparent insulating materials can not withstand high temperatures because they consist of plastics. Thus, temperature resistive collector covers combining a high tranmisivity with a low U-value are required. One possibility is to use capillaries made of glass instead of plastics. Measurement results of collector efficiency and incident angle modifier will be presented as well as calculated energy gains for three different collectors: a vacuum tube collector (Giordano Ind., France), a CPC vacuum tube collector (microtherm Energietechnik Germany) and a new flat plate collector using glass capillary as transparent insulation (SET, Germany).

W-WO_x Cermets have been prepared by chemical vapor deposition of W(CO)₆ in the presence of oxygen and by the subsequent reduction of the WO_x with H₂. The vapor was produced by sublimation; the vapor pressure of W(CO)₆ is a function of the temperature of the source and the flow rate of the carrier gas. During film formation the substrate temperatures were between 300 and 400 degree(s)C, the pressure range was 1 to 10⁵ Pa, the gas flow rates ranged from 0,2 to 4500 sccm. The deposition rate as a function of pressure and flow rate is best described according to a theory by Holman and Huegel. The surface geometry of the films depends on pressure, temperature and gas velocities. The reduction treatment was done with a mixture of 10% H₂ and 90% N₂ for 30 minutes at 490 degree(s)C. The properties of the cermet films were analyzed by reflection spectroscopy in the range 0,3 to 25 micrometers , x-ray and SEM investigations. A model for the change of the structure by ageing in the temperature range up to 800 degree(s)C is suggested.

Thin films of CR-SiO-cermets produced by evaporation on metal substrates show spectral selectivity. This property depends not only on the substrate material,m the average composition and the thickness of the film but also on the conditions of the preparation. In particular the temperature of the substrate during preparation has a strong influence on the change of selectivity during ageing. Ageing processes have been investigated in the temperature range 400 to 800 degree(s)C in Ar-atmosphere for time periods up to 42 days by means of optical measurements, X-ray analysis and other properties. The main effect is the decrease of the thickness of the film and a change of the structure. By dissociation of the SiO into Si and SiO₂ all chromium-silicides have been observed which are known from the binary Cr-Si-constitution diagram with the exception of CrSi. Some formation of Cr₂O₃ was observed not because of the development of oxygen by the reduction of SiO₂ but by residual oxygen in the Ar-atmosphere.

In solar collectors used for low temperature applications such as water or air heating the primary heat losses are reradiation in the infrared, convection and conduction losses to the air. To improve the performance of these collectors considerable amount of work has been undertaken over the past few decades in the development of selective coatings which could be applied to the absorber plate surface to reduce the effective emittance of the absorber plate. Many candidate materials exist which offer selective optical properties. These include electroplated, chemically deposited, vacuum coated and paint coatings. Electroplating technique has been industrially used for the production of selective black coatings as it offers a number of advantages over its rivals. This paper reports on the development of nickel-copper selective coating from an electrolyte containing 40 g/l nickel sulphate, 20 g/l copper sulphate, 15 g/l ammomium acetate operated at a pH of 5.5 at 35 degree(s)C at a current density of 4 A/dm² for a deposition time of 30 seconds. Optimized coating is characterized by solar absorptance ((alpha) ) of 0.98 and thermal emittance ((epsilon) ) of 0.10.

Silicon-monoxide (SiO) film on aluminum (Al) substrate is known as one of the spectral selective radiating materials that utilizes selective infrared emission through the atmospheric window (8-13 micrometers ). Nevertheless, the radiative cooling power of them is rather low comparing to that of black body which is ranging from 60 to 100 W/m², depending on the model of the atmosphere and the humidity. In other words, the sky radiator must have a considerably large are film with homogeneous thickness. This is the reason why it is difficult to put the radiative cooling idea to practical use. To attempt a solution of this problem, we have examined some silicon composite films and/or their multilayers on aluminum substrate by numerical calculation. The main results are following: (1) (SiO + Si)/SiO film with adequate thickness shows the best result to obtain the high power radiative cooling. The radiative cooling power at room temperature (300 K) is expected to be about 20% larger, and the obtainable temperature drop to be about 10% larger than that of SiO single film on Al. (2) The difficulty of deposition of the coating with large area and homogeneous thickness can be overcome by use of porous SiO stacked film whose optical properties are not so influenced by the film thickness. If we couple the above mentioned materials with cover films of high reflectance for solar radiation and high transmittance in atmospheric window region, the radiative cooling may be more realistic even under direct daylighting.

The Drude-like behavior of the group IVB metal nitrides: TiN, ZrN and HfN furnishes the physical basis for the use of these hard, inert materials as replacement for noble metals in optically selective multilayers. A low value of the refractive index, n, in the visible region and rapidly increasing extinction coefficient, k, when the wavelength increases into the infrared, is characteristic for these nitrides, although to a lesser extent than for the noble metals. A screened Drude model can be fitted to the experimental dielectric function over the near infrared and at least part of the visible spectrum to determine the parameters: plasma resonance energy hv_p and relaxation time (tau) . Systematic studies of TiN and ZrN films show that n increases with decreasing film thickness below 60 nm when the film transmits. This increase can be modelled with a increasing Drude parameter (tau) and has previously been explained as an extrinsic effect from defects etc. It is argued that most of this change can be understood as an effect of diffuse scattering against the back surface of the film and is therefore not cured by improvements in deposition technology.

TiN_xO_y-Al and SiO₂/TiN_xO_y-Al selective absorbers, prepared in an ARE (Activated Reactive Evaporation) deposition process, have been investigated in respect of effects induced by heat treatment. Samples were exposed to residual gas atmospheres of 10^-4hPa and 10hPa respectively, in a temperature range of 225 degree(s)C - 470 degree(s)C. Three different processes were observed. They were analyzed investigating optical properties, element concentration and structure of the compounds. Reflectance measurements are in agreement with calculations of the spectral reflection from known optical constants.

Thin films were deposited at an oblique angle of incidence by electron-beam evaporation as well as reactive and non-reactive sputtering. We have produced evaporated Cr films and sputtered metallic and partially oxidized films of Al, Cr and Ti. The optical transmittance of the films was measured as a function of the angle of incidence, and pronounced angular selectivity was found in many cases. The evaporated films show the highest angular selectivity in the luminous transmittance, but also some of the sputtered ones display promising optical properties. The optical properties of the sputtered films could be modeled with effective medium theories.

This paper reports on the development of pigmented polyethylene foils for radiative cooling. The optical properties of the foils were optimized for applications in day-time radiative cooling and water condensation. We first study highly scattering foils used as convection shields. These cover foils combine a high solar reflectance and a high transmittance in the atmospheric window region in the infrared. Different pigment materials were studied and ZnS was the only one that could prevent heating of an underlying blackbody at noon, with the sun in its zenith. A 400 micrometers thick ZnS pigmented polyethylene foil with a pigment volume fraction of 0.15 was tested in Tanzania. At noon the observed temperature of the covered blackbody was only 1.5 K above the ambient. Secondly, we study the potential for condensation of water in an arid region. Pigmented foils for this purpose should combine a high solar reflectance and a high infrared emittance, in order to promote condensation by the radiative cooling effect. Titanium dioxide is a fairly good infrared emitter, but the emittance can be improved by using a mixture of TiO₂ and BaSO₄ pigments or only employing a composite SiO₂/TiO₂. Field tests with a 390 micrometers thick polyethylene foil with TiO₂ and BaSO₄ pigments gave encouraging results.

The optical constants of thin sputtered molybdenum layers, embedded in a ceramic-metal composite, which was produced by a batch sputtering deposition system, were analyzed. This was done by assuming a multilayer system for a tin oxide-molybdenum cermet and calculating the optical constants from angular and polarization dependent reflection and transmission spectra. These optical constants were found to differ strongly from those for sputtered bulk material obtained ellipsometrically. Good agreement between measured effective refractive indices for cermets and effective medium calculation was found, if these optical constants for molybdenum were used in the effective medium calculations. Differences to the optical constants of the cermet determined ellipsometrically were explained by the birefringence of the cermet. The size and the shape of the embedded particles were investigated with an atomic force microscope.

This paper aims to present a representative rather than exhaustive review of recent research on thermotropic materials, which have the potential for being produced on the large scale required for solar energy system applications. Thermochromic and variably light scattering materials are treated, with brief discussions of the physical switching mechanisms, the spectra of the materials in the low-temperature, high-transmittance and high-temperature, low- transmittance states, long-term stability and system simulation. The integrated values of visible (luminous) and solar transmittance above and below the switching temperature are given to allow a qualitative comparison of the different materials and identification of appropriate applications.

The performance factors which will influence the market acceptance of electrochromic windows are reviewed. A set of data representing the optical properties of existing and foreseeable electrochromic window devices was generated. The issue of reflective versus absorbing electrochromics was explored. This data was used in the DOE 2.1 building energy model to calculate the expected energy savings compared to conventional glazings. The effects of several different control strategies were tested. Significant energy and peak electric demand benefits were obtained for some electrochromic types. Use of predictive control algorithms to optimize cooling control may result in greater energy savings. Initial economic results considering annual savings, cooling equipment cost savings, and electrochromic window costs are presented. Calculations of thermal and visual comfort shows additional benefits from electrochromics but more work is needed to quantify their importance. The design freedom and aesthetic possibilities of these dynamic glazings should provide additional market benefits, but their impact is difficult to assess at this time. Ultimately, a full assessment of the market viability of electrochromics must consider the impacts of all of these issues.

Electrochromism is sol-gel deposited TiO₂ films and films containing TiO₂ and WO₃ has been observed. The films are deposited by dip-coating from a precursor containing titanium isopropoxide in ethanol or titanium propoxide in ethanol, and after deposition the films are heat treated to between 250 degree(s)C and 300 degree(s)C. The films do not show any signs of crystallinity. However substantial coloration is observed using Li⁺ ions in a non-aqueous electrolyte, both in pure TiO₂ films and in mixed metal oxide films (WO₃:TiO₂), although the voltage required to produce coloration is different in the two cases. Results will be presented detailing the optical switching and charge transport properties of the films during cyclic voltammetry. These results will be used to compare the performance of the TiO₂ films with other electrochromics. The TiO₂ and mixed metal films all color cathodically, and the colored state is a neutral greyish color for TiO₂, while the bleached state is transparent and colorless, Results on coloration efficiency and the stability under repeated electrochemical cycling will also be presented. The neutral color of the TiO₂ films and mixed-metal films means that electrochromic windows based on TiO₂ may have significant advantages over WO₃-based windows. A detailed analysis of the optical properties of the colored state of the films will be presented. The dynamics of coloration for these films is also under investigation, and preliminary results will be presented.

This paper deals with preparation of WO₃ thin films and study of their optical and structural properties. Films were obtained using the following technological steps: first, the basic process is chemical vapor deposition (CVD) of W(CO)₆. When pyrolitical decomposition at 400 degree(s)C of this material takes place in Ar atmosphere W thin films on quartz substrate were obtained. This step was followed by hydrogen reduction at 740 degree(s)C and by further oxidizing process. Some of the as-deposited films were directly submitted to oxidizing process without heating in H₂ atmosphere. They are also WO₃ structures and exhibit good transmittance. Using integrated sphere Perkin-Elmer spectrophotometer we measured the spectral transmittance and reflectance in the range of the solar spectrum. Films have high transmittance reaching in some samples 80% and reflectance in the range from 20 to 40%. We studied the structure of W oxide films using Reflection high energy electron diffraction (RHEED) method. The d-spacing values show that as-prepared films are WO₃ structures.

We investigated the light switching properties of non-mechanical shading devices, namely liquid crystal films and thermochromic layers. These systems can be switched between a transparent and a scattering state either by application of an electric voltage or surpassing a characteristic switching temperature. This paper presents data on the light scattering properties of such materials, i.e. the change in diffuse transmittance due to switching, the light scattering efficiency and the size of the scattering entities. Radiative transport calculations were performed in order to describe the light scattering properties of the liquid crystal films and to determine the optimization potential of these layers. For comparison data of an electrochromic layer, which has a transparent and an absorbing state, are shown.

An electrochromic device using tungsten oxide as an electrochromic electrode, cerium- titanium oxide as an optically passive counter electrode, laminated by a polymer based ion conductor has been fabricated and characterized. The fabrication technology as well as the optical switching behavior and cyclability show that this device is a promising system for smart windows in automotive and building application.

Thin films of CeO₂-TiO₂ have been prepared by a sol-gel process involving 2 alcoxides (Ce(OBu²)₄ and Ti(OBuⁿ)₄ in BuOH). The films are formed in an amorphous matrix of titanium oxide including nanocrystallites of cerium oxide whose size increases considerably (10 to 50 angstroms) for compositions with more than 50% CeO₂ in the film. The voltammetric data are in perfect concordance with the structural model previously described and deduced from X-ray Scattering: (i) the TiO₂ amorphous matrix accepts only a small amount of lithium up to the ITO reduction limit. The lithium ion mores in this matrix with a mobility similar to that of lithium in TiO₂ on its own. (ii) in the presence of very small separate grains of CeO₂ equivalent to pinpoint sites, the lithium passes through the TiO₂ matrix and occupies these pinpoint sites very quickly according to a homogeneous diffusion process in very small particles. This behavior appears for low values of CeO₂ contents. The global mobility should be close to that of Li⁺ in TiO₂. (iii) when the grain size increases, CeO₂ occupation of sites in the grain bulk is not instantaneous: the behavior of the intercalation in CeO₂ tends to the semi-infinite diffusion mode. On the other hand, TiO₂ content decreases, reducing the possible paths necessary to reach the deepest CeO₂ grains. The global mobility of Li⁺ will decrease for these 2 reasons and reaches that of Li⁺ in pure CeO₂.

The technique of Fourier Transform Infrared (FTIR) Spectroscopy has been used to study tungsten oxide (WO₃) thin films. The effect of coloration with lithium has been investigated and leads to the conclusion that the coloration mechanism for lithium in WO₃ requires a structural reorganization induced by the co-insertion of the lithium ion and an electron. This can produce a valence change in one of the tungsten atoms, necessary for absorption of radiation by the inter-valence charge transfer mechanism. There is a clear redistribution of the infra-red active modes when the material is colored with lithium or sodium, and this can be used to characterize the film to some extent. Furthermore, the FTIR spectral changes in anomalously coloring WO₃ as reported previously, have been investigated. No change is seen in the FTIR spectra until the onset of the electrochromic coloration, indicating the lithium ion initially occupies a different site or performs a different role to that seen in normal coloration. This evidence, when combined with other information, leads to the conclusion that the lithium may react with excess oxygen in the WO₃ forming a second chemical phase, perhaps of lithium oxide.

This paper describes the deposition of nickel-cobalt-phosphorous coatings by the electroless deposition technique for use as solar control coatings in architectural glazing of buildings. Electroless deposition is characterized by the autocatalytic deposition of a metal/alloy from an aqueous solution of its ions by interaction with a chemical reducing agent. The reducing agent provides electrons for the metal ions to be neutralized. The reduction is initiated by the catalyzed surface of the substrate and continued by the self catalytic activity of the deposited metal/alloy as long as the substrate is immersed in the electroless bath and operating conditions are maintained. Electroless nickel-cobalt-phosphorous thin films were deposited from a solution containing 15 g/l nickel sulphate, 5 g/l cobalt sulphate, 60 g.l ammonium citrate and 25 g/l sodium hypophosphite operating at 30 degree(s)C, at a pH of 9.5 for two minutes. Electroless nickel-cobalt-phosphorous coatings are found to satisfy the basic requirements of solar control coatings. Autocatalytic deposition technique offers the possibilities of producing large area coatings with low capital investment, stability and good adhesion to glass substrates.

The development of full solid-state electrochromic (EC) devices on polymeric substrate is under way within a CEC BRITE-EURAM project (Project `FREDOPS', BE-4137) carried out by four industries, two universities and two research centers from Belgium, Denmark, France and Italy. Specific goal of the project is to develop a Fast Response Electrochromic Device on Polymeric Substrate (FREDOPS); in order to satisfy the required range of specifications in terms of fast response, long term performance and high contrast ratio, several systems based on different materials have been tested. The full cells consist of an electrochromic material layer and a counter-electrode, inserted between two PET/ITO layers and separated by a polymeric electrolyte. Different types of polymeric electrolytes, counter-electrodes and electrochromic layers have been developed, studied and checked. Full devices have been assembled using different combinations. Voltammetric and spectrophotometric measurements have been executed to check the electrochromic behavior of the developed layers in half and full cells. Comparison of the electrochromic performances of different materials based cells has led to the rejection of several solutions due to poor performance and incompatibilities between layers. The present paper discusses these results in order to indicate the best foreseen solutions.

Here are presented the results of electrochemical and optical measurements on tungsten trioxide, molybdenum doped (WO₃:Mo) thin films, electrochemically deposited, assembled with Niafion-H^R in order to realize an electrochromic (EC) device. The polymer that permits the ionic exchange has a residual viscosity, so that our device cannot be defined as a really solid state device. A good reason for the use of this polymer is its very high proton storage capability, so that, in our devices, it is possible to avoid the presence of a specific ion storage counterelectrode. In this work we compare also the electrochromic behavior of devices in dependence on the thermal treatment operated on the electrochemical films after deposition. Analysis has been carried on to reveal morphological characteristics of surface, thickness and stoichiometry of EC compound. Particular attention is paid to the potential waveform used to bias the devices, in order to optimize their performance and to avoid problems of stability and degradation.

Thermochromic VO₂ films were prepared by reactive magnetron sputtering under various conditions of substrate temperature, total sputter pressure and oxygen flow ratio and characterized by XRD, RBS, AFM and spectrophotometry. Films with VO₂ single phase were formed from a fairly low substrate temperature of 300 degree(s)C by precisely controlling the oxygen flow ratio. The use of vanadium-nucleated substrates significantly improved the crystallinity of VO₂. Tungsten doped V_1-xW_xO₂ films with x equals 0 approximately 0.26 were formed by dual-target sputtering and the thermochromism of films was evaluated. The tungsten doping linearly hysteresis loop width.

Tungsten-doped vanadium oxide (V_1-xW_xO₂) films were prepared by concurrent reactive dc magnetron sputtering of vanadium and tungsten in an Ar + O₂ plasma with a controlled oxygen partial pressure. Films were deposited onto glass substrates at 400 degree(s)C. The films had a metal-semiconductor transition at a temperature (tau) _t that was depressed when x was increased. Rutherford Back Scattering was used to determine x. X- ray diffraction was employed to confirm the monoclinic low-temperature VO₂ phase. The relation between x and (tau) _t was studied and compared with results from the literature. It was shown that (tau) _t could be set to a value between 17 and 65 degree(s)C by proper choice of x. The optical and electrical properties of the films were investigated around the metal-semiconductor phase transition. The luminous transmittance was rather unaffected by the temperature, whereas the near infrared transmittance showed lower values above (tau) _t. The degree of thermochromic modulation decreased for increased x. Electrical measurements showed that the ratio of the resistance above and below (tau) _t decreased with increasing x.

Oxyfluoride films based on W and Ti were prepared by reactive sputtering in plasmas containing O₂ + CF₄. The deposition rate was large, particularly when chemical sputtering was promoted by heating the target. The films could show large charge insertion/extraction, high coloration efficiency, and good cycling durability.

The paper presents the results of a study investigating the energy performance of electrochromic windows under a variety of state-switching control strategies. We used the DOE-2.1E energy simulation program to analyze the annual cooling, lighting, and total electricity use and peak demand as a function of glazing type, size, and electrochromic control strategy. We simulated a prototypical commercial office building module located in the cooling-dominated location of Blythe, California. Control strategies analyzed were based on daylight illuminance, incident total solar radiation, and space cooling load. Our results show that when a daylighting strategy is used to reduce electric lighting requirements, control algorithms based on daylight illuminance results in the best overall annual energy performance. If daylighting is not a design option, controls based on space cooling load yield the best performance through solar heat gain reduction. The performance of incident total solar radiation control strategies varies as a function of the switching setpoints; for small to moderate window sizes which result in small to moderate solar gains, a large setpoint-range was best since it provides increased illuminance for daylighting without much cooling penalty; for larger window sizes, which provide adequate daylight, a smaller setpoint-range was best to reduce unwanted solar heat gains and the consequential increased cooling requirement. Of particular importance is the fact that reduction in peak electric demand was found to be independent of the type of control strategy used for electrochromic switching. This is because the electrochromics are generally in their most colored state under peak conditions, and the mechanism used for achieving such a state is not important.

Tantalum oxide films were prepared by sol-gel process using tantalum ethoxide Ta(OC₂H₅)₅. The dependence of deposition conditions (i.e. composition of polymeric solutions and spinning rate) on ionic conductivities for tantalum oxide films were studied. The best results achieved for films fabricated by the spin coating technique from clear polymeric solutions. These films had low packing density (rho) equals 3.2 g/cm³ and good proton conductivity (about 10^-6 (Omega) ^-1/cm^-1). X-ray photoelectron spectroscopy (XPS) was used for studying the compositions of the tantalum oxide films. We report on the use of tantalum oxide films as ion conductors in devices consisting of WO₃/Ta₂O₅/H⁺ ion storage polymer structure. We found tantalum oxide to have very good properties for proton device applications.

We have studied the photochromic effect of tungsten oxide thin films, i.e. their coloration by exposure to ultraviolet light, in order to investigate the basic mechanisms of the chromogenic behavior of this material. We have measured the absorptivity of our samples by means of optically excited surface plasmons which enabled us for the first time to resolve the photochromism of layers 10 nm thick on a scale as short as seconds. In contrast to the widely accepted double-charge-injection model, we found that in the case of photochromism, oxygen is exchanged between the sample and the surrounding atmosphere. On the basis of this model, a simple nonlinear rate equation is presented which accounts both for the temporal behavior and for the atmosphere dependence of the absorbance of the films during and after ultraviolet light exposure.

A number of different thermotropic materials are now known, which operate on the principle of a reversible change between a homogeneous, transparent state at lower temperatures and a heterogeneous, scattering state at higher temperatures. Depending on the switching temperature, which is between 20 and 100 degree(s)C for the samples investigated, these materials can be used to prevent overheating in different solar energy applications, e.g. overhead glazing, transparently insulated walls and flat-plate collectors. In this paper, such materials are characterized experimentally by the temperature dependence of the direct-direct and direct-hemispherical transmittance and reflectance spectra over the solar spectral range. The results from multiple scattering theory for Mie scatterers and models for scattering in random media are compared to determine their correctness in describing the material structure in its scattering state. On the basis of the results obtained (diameter of scattering center or typical correlation length for a refractive index domain and difference between the component dielectric constants), suggestions are made on the changes required for a further optimization of the solar radiation switching performance.

Holographic optical elements are utilized in daylighting systems as light directing elements. The holograms can be fabricated on thin foils which are laminated between glass panes. The function of the holograms is limited by dispersion. Especially for large angles of incidence only a small portion of the solar spectrum is diffracted by a single hologram. Thus the redirected sunlight changes color. In this paper we show how the color changes can be minimized by using a stack of volume holograms. Each hologram diffracts a different portion of the solar spectrum into the same direction. The diffracted waves are superimposed in order to generate white light according to the additive color theory. The case of two holograms operating in the blue and red portion of the visible spectrum is analyzed theoretically and realized experimentally. Measurements of the diffraction efficiency as a function of wavelength are presented for different angles of incidence. From these measurements the color performance and the angular sensitivity of the stack is inferred.

The redirection of light by holographic glazing is based on diffraction. The intensity and the color of the redirected light depend strongly on the direction of the diffracted waves. The paper presents a model for the design of volume holograms as light directing elements which takes into account the 3D character of the diffraction process. Formulas for the Bragg- wavelength, the bandwidth, and the diffraction efficiency are given. It is shown how to use conventional photometric formulas in order to calculate the illuminance and the color distribution on surfaces illuminated by the diffracted light. Finally, the design method is demonstrated for a single volume grating incorporated into a window as a light directing element.

Multilayer systems of thin silver films and dielectric layers are used for example in low-e windows. The optical properties of the silver films differ markedly from the optical properties of bulk silver and are influenced by the deposition of the top dielectric layer. Recently, we presented a dual-ion-beam sputtering apparatus with in-situ monitoring of reflectance and dc- resistance. The dielectic, Si₃N₄, was produced by sputtering silicon and assisting with an energetic nitrogen beam (200 eV). A sharp reduction in reflectance and a corresponding increase in dc-resistance of the silver film was observed during the first stage of the deposition. It was speculated that a thickness reduction of the silver film due to sputtering by the energetic nitrogen ions might cause some of the changes. In the present paper, we report measurements of film thicknesses by grazing incidence x-ray reflectometry (GIXR), which allow to exclude thickness reduction. We show that the optical absorptance of the sandwich system increases during deposition of the top dielectric layer. Different elements (Al, Si, Ta, and Ti) were sputtered onto the silver film without assisting, causing similar changes as dual- ion-beam sputtered Si₃N₄. Furthermore, Si3N4 was sputtered directly from a dielectric target. In contrast to the other materials, we observed only small changes in reflectance and dc-resistance.

Thermal conversion of non-concentrated solar radiation at high temperatures can be achieved by a selective absorber with high absorptance in the energy regions where solar irradiance exceeds thermal absorber radiance and with low absorptance (emittance) in the other regions. In the simplest case optimum absorptance (emittance) is one above a certain threshold energy and zero below. While this optimum design requires infinitely steep transitions, it is shown that the loss due to a broadened transition is of second order with regard to the transition width, at least for sufficiently smooth solar spectra and for a given type of transition being a function of the ration of the energy deviation from the threshold to the width. Hence comparatively large widths can be used without significantly deteriorating performance. It is shown that the slope at the threshold is not a sufficient measure to quantify losses by a non- ideal transition, since radiative losses increase with the distance form the threshold. For converting 800 Wm^-2 solar irradiance at 500 degree(s)C, a width in the order of the thermal energy can be accepted. This is confirmed for different types of transitions. For thermal conversion at high temperatures it is more important to choose the right threshold and to achieve low emittance away from the threshold than to pursue a steep transition.

Thin silver layers are frequently used as the infrared reflecting component of low-e coatings. The optimum thickness of the layer must be found by maximizing the IR-reflectance and the visible transmittance. Since the optimum demands a very thin silver layer, the condensation and nucleation of the silver on the respective substrate or interface is very important. The subject of this study was the sputter deposition of silver on different substrate materials: float glass, tin oxide and bismuth oxide. The optical properties of the layers were measured spectroscopically from UV to IR. The topography of the layers was investigated by scanning force microscopy. The measurements exhibit significant differences depending on the substrate material.

Fluorescent Planar Concentrators have attracted the interest of a large number of scientists and engineers all over the world from the late seventies up to now. Although the main goal, to develop a concentrator for photovoltaic energy conversion on a large scale, has not been achieved so far, research on these interesting devices is continuing. In this paper, an overview of the history, the operation principle, the energy loss mechanism of the systems and the properties of the materials involved is given. The thermodynamic limits on the maximum concentration ratio are discussed. Some applications will be presented. Finally, information on some new lines of development in this field is given.

Solar collectors using non-imaging mirror troughs (e.g. CPC-troughs) significantly alter the angular and spatial distribution of incident radiation from the aperture to the absorber surface. For flat plate collectors the transmission of transparent covers (including honeycomb transparent insulation materials) can be modelled with one incidence angle modifier symmetrical to the collector's normal. After multiple reflections in a mirror trough however, this symmetry is not maintained and a more complex treatment is required. For three examples of mirror troughs, the angular distribution of gained solar radiation was calculated using a ray- tracing program, taking into account the angular dependence of the transmission of a cover and of the absorptance of a selective coating. The solar radiation at the aperture was modelled using the continuous diffuse radiation model of A. Brunger with radiation data from Freiburg.

Economic operation of high-efficiency concentrator solar cells requires solar concentration ratios which up to now can only be achieved with two-axis tracking. In this paper we present a two-stage concentrator approaching concentration ratios up to 300X while being tracked around only one polar axis. Its principle is as follows: A parabolic trough focusses the direct solar radiation onto a line parallel to the polar tracking axis. The half rim angle of this first concentrating stage is chosen to be equal to the sun's maximum declination of 23.5 degree(s). The second stage consists of a row of dielectric, nonimaging 3D-concentrators, which couple the concentrated light directly into square solar cells. The proposed design makes use of the limited divergence of +/- 23.5 degree(s)) in the NS-direction which still can be concentrated by a factor of n/sin(23.5 degree(s)). The performance of the system depends sensitively on how well the angular acceptance characteristic of the second stage matches with the square-shaped angular irradiance distribution in the focal line of the parabolic trough. A new concentrator profile has been found that exhibits an almost ideal square acceptance characteristic with a very sharp cut-off. It is longer than the standard CPC and its slope is reduced towards the exit of the concentrator. A prototype two-stage concentrator has been constructed with a total geometrical concentration of 214X. In outdoor measurements a total optical efficiency of 77.5% was obtained.

Decreasing the aperture of a solar central receiver operating at high temperatures contributes significantly to the increase of the efficiency of energy absorption. However, decreasing the aperture also decreases the collection efficiency. A simple solution is using a 3D- CPC as secondary element for augmentating the energy collection, while the aperture can remain relative small. Nevertheless, the receiver aperture as well as the secondary concentrator are usually rather large. For practical considerations an approximate solution may be chosen at times, designing the concentrator by a series of truncated cones[1}. However, in particular cases, the solution of truncated cones remains expensive and unpractical and therefore we designed a CPC approximated by a series of trapezoidal planar facets. Under technical restrictions, there is an optimum for choosing the partition of the purecPc in truncated pyramids and the paper presents the method for a good solution. The design of the concentrator is such that it can accomodate two different receiver apertures (60 cm and 47 cm respectively). The final design of the concentrator provides an entrance diameter of 121 cm and consists of four truncated pyramids, each composed of 12 trapezoidal facets, when connected to the 60 cm diameter aperture of the receiver. A fifth ring of facets is added when the concentrator is used in conjunction with the second receiver. . Some considerations about the materials used in construction are presented in the final section of the paper.

The efficiency of low concentration CPC type collectors depends greatly on the quality of the reflecting films used in the case of evacuated glass tub, specially shaped to become a low concentration CPC type collector, it is of great importance to have a mirror of good quality, namely with good reflectivity and adhesion as well as low degasing of the deposited films. In this paper a technique to obtain such reflecting film is described. Magnetron sputtering of an aluminum post cathode was used. Several configurations were tested and the design was optimized taking into account both discharge stability and industrial constraints, like equipment volume and energy consumption. Discharge characteristic, deposition rate and general features are shown. Film reflectivity dependence on discharge pressure is discussed. The results obtained are encouraging and foreseen to be easily industrialized.

A new non-evacuated solar collector of the CPC type, developed and manufactured in Portugal, is now commercially available. Its design features are unique and deserve a careful study, both of its optical and of its thermal characteristics. The optics is interesting given the unusual shape and the opportunity to test different convection suppression schemes and determine their impacts on the collector's optical performance. As for the collector's thermal behavior it is very interesting to test how simple (and potential marketable) different convection suppression ideas can improve an already very good collector from the heat loss point of view (FU_L equals 4.0 W/( degree(s)C.m²)). In the course of the paper a brief description of the collector is given and testing results are presented for the testing carried out in the following situations: (1) (i) measurement of its optical and thermal performance (instantaneous efficiency curve) measured both in E.W. and N.S. collector orientation (the collector has a very wide acceptance angle allowing it to work in N.S. orientation and, thus, function in a thermosyphon mode like any regular flat plate collector, (ii) measurement of its angular acceptance function; (2) measurement of the instantaneous efficiency curve after the introduction of (i) a thin Teflon high transmissivity film below the glass cover, (ii) transparent insulation of the capillary type, inserted also under the glass cover, (iii) measurement of the acceptance angle function in this last situation. In this paper it is shown that the addition of the film reduces the heat loss coefficient by a factor of 1.3 W/( degree(s)C.m²) and the transparent insulation leads only to an improvement of 1.0 W/( degree(s)C.m²) in that same coefficient.

A ray tracing study has been performed on the optical properties of cylindrical nonimaging concentrators with linear corrugated reflectors. The corrugations are assumed to be V-formed and to have an extension parallel to the meridian plane of the concentrators. It is shown that the acceptance angle for radiation incident in the meridian plane can be increased for moderate corrugations. This increased acceptance is balanced by a decreased acceptance of radiation from other directions. Calculations of angular acceptance for a 2X compound parabolic concentrator is presented. It is shown that the annual irradiation on a solar collector with booster reflector can be increased if corrugated reflectors are used instead of smooth reflectors.

The optical and thermal properties of aerogels play an important role for their application in transparent insulation (TI) or daylighting systems. In this paper the directional-directional transmission t_dd as well as the directional-hemispherical transmission t_dh are presented for 20 mm thick aerogel specimens. For optimized monolithic aerogels we obtained solar averaged values of ^-t_dh,sol equals (91 +/- 1/4)% and ^-t_dh,vis equals (84 +/- 2)% in the solar and visible spectral region, respectively. The investigated granular aerogels show a visual transmission of ^-t_dh,vis equals (58 +/- 4)% for waterglass as a precursor and of ^-t_dh,vis equals (78 +/- 4)% for tetramethoxysilane. In order to correlate the structural build-up of the SiO₂-network of the aerogels with their light scattering properties angular-dependent light and X-ray scattering measurements are performed. To characterize the visual image transmission the modulation transfer function (MTF) was measured for 20 mm thick monolithic layers. In order to explain the measured data a Monte Carlo based simulation model was developed. The analysis yields that the MTF is mainly effected by surface scattering.

After more than ten years of research and development on a new generation of transparent insulation materials in the more restricted sense, namely honeycomb type and other geometric structures, granular and monolithic aerogel, a critical overview of existing materials is presented. During the last years considerable progress has been made, and still more is to be expected as well on the theoretical understanding of the physical properties as on the actual material development. On the other hand there still are problems when one looks at the application aspect, e.g. temperature stability, inflammability, optical appearance, irregularities of the structures, and production costs. The development objectives in the past have been either to physically optimize a material for an application, or to produce a lower-quality cheap material. An example of the first strategy is the recent development of glass capillary structures. The question here, of course, is, whether production, handling and transport can be made easy enough to meet the target of reasonable costs. It should be emphasized, that material costs are not the key issue, but system costs. A continuous and intensive collaboration between production engineer, system or design engineer, scientist and salesman is probably the only chance to meet the requirements of high quality products, adapted to the application, with reasonable costs. An engineer designing an application with transparent insulation materials without understanding the physics of the material type used may run the risk of deteriorating the performance severely.

This paper discusses the performance limitations of evacuated glazing. Evacuated glazing up to at least 1 m² in area can be rapidly evacuated. The mechanical tensile stresses in evacuated glazing are strongly dependent on the level of insulation achieved, particularly with respect to the pillar array during manufacture. Highly insulating evacuated glazing contains stresses, in regions which can lead to fracture, that are comparable with conventional double glazing. It appears likely that such glazing can withstand temperature differentials of at least 40 degree(s)C. Evacuated glazing consisting of two 1 m² sheets of 4 mm thick glass, with internal pyrolytically deposited low emittance coatings, has been manufactured with an air-to- air mid-device thermal conductance of approximately 0.85 Wm^-2K^-1, of which approximately 2/3 is due to radiation and 1/3 to pillar conductance. These samples have negligible gas conductance and an extremely stable internal vacuum at room temperature. Substantially lower values of radiative heat flow would require sputtered transparent low emittance coatings that can withstand the high temperature currently used in the solder glass edge sealing process. Lower pillar conductance can only be achieved using thicker glass sheets, or tempered glass.

Evacuated glazing is a form of double glazing, utilizing the internal vacuum between the two glass sheets to eliminate heat transport by gas conduction. To achieve the target thermal conductance, the internal gas pressure must be <EQ 0.1 Pa. To estimate the useful service life of the glazing in terms of vacuum stability, it is necessary to establish the rate of degradation of the vacuum as a result of outgassing processes. We have measured the gas pressure of several samples of evacuated glazing which have been deliberately degraded by baking at high temperature. These measurements were made using two different methods of measuring the thermal conductance of the samples, on of which can be used while the glazing is at high temperature. The insulating properties of the glazing can be considerably degraded (gas pressure > 10 Pa) by baking at temperatures close to those used in the evacuation procedure during the construction of the glazing. Such samples show an improvement in the vacuum with time if stored at temperatures significantly below the bakeout temperature. The experimental data are consistent with a qualitative model of the outgassing of evacuated glazing which involves diffusion of molecules within the volume of the glazing, and adsorption/desorption of gas molecules on the surfaces.

The precise optical characterization of transparent insulation materials used in windows, flat- plate collectors or for transparent insulation of buildings, is an important step to design solar collector and daylighting systems with these materials and to estimate energy benefits, peak loads, efficiencies, and different potential risks such as overheating, thermal damage or glare. Physically the aim is clear: Angle-dependent transmittance and reflectance properties for the solar and visible wavelength ranges yield the necessary information for the engineer to enable him to design a good system. However, it is far from trivial to obtain these data with sufficient precision for the rather different materials. The class of TIMs poses mainly the following problems, originating in their special character. TIMs often: (a) have a rather coarse structure, (b) show considerable scattering, (c) are relatively thick, (d) are spectrally selective, (e) and are not always rotationally symmetric. Therefore the optical measurement process has to: (a) integrate over a relatively large sample area (b) be able to detect intensity scattered in the sample (c) take into account the complex structure of the sample (d) and weight the different spectral bands correctly. We have set-up a set of radiation sources and integrating detector spheres which are able to measure directional-hemispherical and hemispherical-hemispherical reflectance and transmittance (hence also absorptance) for the visible and the solar wavelength range. This was possible by applying a PTFE-based coating to the spheres, having a unique spectrally flat response over the whole range, and using non-selective broadband detectors. Careful design tried to optimize integrating sphere geometry. Moreover, spectral measurements between 285-1100 nm are possible with an optical multichannel analyzer utilizing glass fiber optics. The whole experimental set-up will be presented and discussed together with representative results.

Integral as well as spectral methods to characterize the thermal properties of Silica Aerogel and Xerogel samples are presented. A new method to determine the effective thermal conductivity with the help of an infrared camera was developed, which allows for measuring small samples. Keeping the lower surface of the samples at a constant temperature, the IR camera monitors the upper surface temperature. With the help of a reference sample of known thermal conductivity, the total heat flux through the samples can be calculated. This is done under different pressure conditions in order to observe the Knudsen effect. As there is a complex coupling between the different modes of heat transport, the spectrum of the emitted radiation is of interest. Emission spectroscopy on Silica Aerogel has not been previously investigated in detail. An emission chamber was constructed, which permits measuring of the emitted spectra due to an applied temperature gradient using a Perkin Elmer Infrared Grating spectrometer. Different boundary emissivities and varying pressure conditions can be applied. For a correct interpretation of the measured spectra, a numerical treatment of the underlying heat transport mechanism is necessary. Measurements as well as computer simulations are presented. Computer simulations show that a variation of the total thermal conductivity in the range between 1 mW/mK and 30 mW/mK leads to a characteristic damping of the emitted radiation between 500 cm^-1 and 2000 cm^-1. This characteristic damping of the emitted radiation can be observed by measuring the emitted spectra of Silica Aerogel/Xerogel at varying pressure conditions. Therefore, in principle, it should be possible to extract the thermal conductivity through the gas pores and through the silica skeleton. Boundary effects, however, prevent this data from being extracted. Hopefully these problems will be solved in the near future.

An apparatus for measuring angle-dependent transmission and reflection of large (40 cm X 40 cm) samples is presented and details of the optical, mechanical and computer aspects given. The apparatus consists of two fixed light sources, an adjustable sample holder and a movable solar cell as the detector. All angle positions are computer-controlled, using a workstation to achieve automatic measurements. Data-processing steps for large and small samples are presented.

Errors in performing optical measurements of scattering samples are analyzed. Measurements are described from a Perkin-Elmer Lambda-9 spectrophotometer with an integrating sphere of 15 cm diameter for spectral data between 300 and 2500 nm and from a 65 cm integrating sphere for broadband data on larger samples. The influence of the size of the illuminated sample area, the size of the sphere ports and sample thickness are investigated. Results are compared with Monte Carlo simulations.

Double glazing units filled with granular aerogel open up new applications in the fields of daylighting systems and passive solar architecture. Silica aerogel has ideal characteristics for solar thermal applications. High transparency for solar radiation is combined with extremely low thermal conductivity. The chemical company BASF (D) is developing a granular form of aerogel, which will be introduced to the market in the near future. The application potential of this material in window systems was tested as part of a CEC-JOULE project in co-operation with four other industrial companies and two research institutes under the leadership of the Fraunhofer-Institut fur Solare Energiesysteme (FhG-ISE) in Freiburg. A typical u-value for a double glazed window with a 16 mm thick layer of granular aerogel is 1.0 W m^-2 K^-1, the solar transmission for diffuse light is about 45%. Both parameters are variable over a wide range depending on the particle size distribution, thickness of the layer and choice of the filling gas. First window elements were tested in different applications. Realized installations are presented.

A new type of transparent insulation with glass capillaries allows the construction of high temperature flat-plate solar collectors reaching stagnation temperatures of 260 degree(s)C and more. In the temperature range above 100 degree(s)C, the radiant heat is the dominant heat loss mechanism. For the plastic honeycomb materials used up to now, the heat losses were described adequately with a non-spectral `grey' model. To describe the radiant heat losses for the new glass capillary collector covers, a spectral analysis is necessary, due to the strong spectral dependence of the absorptance in glass. A new experimental set-up allows the measurements of the spectral radiant heat emitted by large samples, not only integrated over a hemisphere but also with angular resolution. A theoretical model for the calculation of the spectral, angle-dependent total emission of a Tl/absorber construction has been developed. The presented results of the calculations for glass capillaries show good agreement with the measurements.

Solar control coatings used in architectural glazings of buildings in warm climates should exhibit controlled optical transmittance (10 to 50%) in visible region and must reflect efficiently (>= 90%) in IR region to create a cool interior buildings. CuInS₂ coatings deposited by the electron beam evaporation technique are found to satisfy basic requirements of SCC and appear superior to the metallic solar control coatings and PbS coatings.

In this paper, experimental results have been reported on proposed window systems comprising of cavities filled with air and water. Experiments have been performed on smaller laboratory samples with the help of Perkin-Elmer Lambda-9 double beam spectrophotometer. The shading co-efficient of water filled system alone has been found to be appreciably lower than that of an air-filled system; the luminous efficacy has also found to be higher. However, because of higher thermal conductivity, water filled glazing systems have been found to have higher thermal transmittance values. Therefore, an optimal system combining air and water columns has been fabricated and studied. In this paper, an attempt has also been made to analyze the energy and environmental benefits for possible applications of solar efficient glazing systems.

Mathematical modelling can be a powerful tool in the design and optimalization of glazing. By calculation, the specifications of a glazing design and the optimal design parameters can be predicted without building costly prototypes first. Furthermore, properties which are difficult to measure, like for instance solar and visible light properties for oblique or diffuse irradiation, can be determined accurately by calculation. At the TNO Institute of Applied Physics, a glazing model for simulation and optimalization is being developed, which allows to determine according to international standards, glazing specifications, ranging from optical properties (solar energy, visible light, color appearance) to thermal properties (U-value, thermal shock resistance, fire control properties). The model is connected to a data base containing optical constants and other properties of glasses, coatings and other materials, used in glazing. The paper discusses the Glazing Model which consists of an Optical Transfer Model simulating the optical behavior, and a Thermal Transfer Model which simulates the thermal behavior. The paper gives an overall description of the Glazing Model. A detailed description is given of the matrix formulation which forms the basis for the optical transfer model. An example of a spectrally selective double glazing illustrates how the model works. Present and future developments are also discussed.

The light diffraction properties of holographic diffractive structures present an opportunity to improve the daylight performance in side-lit office spaces by redirecting and reflecting sunlight off the ceiling, providing adequate daylight illumination up to 30 ft (9.14 m) from the window wall. Prior studies of prototypical holographic glazings, installed above conventional `view' windows, have shown increased daylight levels over a deeper perimeter area than clear glass, for selected sun positions. In this study, we report on the simulation of the energy performance of prototypical holographic glazings assuming a commercial office building in the inland Los Angeles climate. The simulation of the energy performance involved determination of both luminous and thermal performance. Since the optical complexity of holographic glazings prevented the use of conventional algorithms for the simulation of their luminous performance, we used a newly developed method that combines experimentally determined directional workplane illuminance coefficients with computer-based analytical routines to determine a comprehensive set of daylight factors for many sun positions. These daylight factors were then used within the DOE-2.1D energy simulation program to determine hourly daylight and energy performance over the course of an entire year for four window orientations. Since the prototypical holographic diffractive structures considered in this study were applied on single pane clear glass, we also simulated the performance of hypothetical glazings, assuming the daylight performance of the prototype holographic glazings and the thermal performance of double-pane and low-e glazings. Finally, we addressed various design and implementation issues towards potential performance improvement.

The dispersion of the solar radiation into different spectral bands which are focused onto spectrally matched solar cells improves the electric efficiency of photovoltaic collectors in comparison with conventional systems. Holographic lenses are able to disperse and to focus solar radiation at the same time. They may be reproduced from a master and are suitable for cheap mass production. The paper presents the fabrication and test of a 50 X 50 cm² PV-concentrator composed of a stack of two holographic lens arrays. Each lens array consists of 49 single lenses of dimension 7 X 7 cm². One array operates in the long wave spectral band, the other in the short wave range. The solar radiation is focused on spectrally matched solar cells of size 1 X 1 cm².

Within a comparatively short time the research on thin film solar cells has led to photovoltaic conversion efficiencies exceeding 16% which makes this technology a viable candidate for widespread applications. The contribution focusses on major issues of the design and implementation of cells based on Cu-III-Vi₂ chalcopyrite absorber thin films. The flexibility of this system leads to a large degree of freedom for the preparation as well as the electronic structure of the cell. Characterization of films and cells is not straightforward and there are still several important aspects that are not yet fully understood. Nevertheless, useful models have been derived for growth mechanisms, surface properties, interface formation, recombination paths and photo current collection using a combination of several independent characterization methods and numerical simulations. The substrate and back contact as well as secondary phases, mainly binary copper chalcogenides and copper poor ternaries, are having a significant influence. Different compounds have been investigated for the buffer layer between absorber and TCO (transparent conductive oxide) front contact. The highest efficiencies have been obtained with absorber band gaps less than 1.4 eV using a (very thin) CdS buffer and a ZnO TCO. The performance of various thin film solar cells will be briefly summarized.

Thin Si films used for solar energy purposes are commonly treated by relatively slow thermal annealing on a time scale of seconds to obtain the proper electrical behavior. We investigate a different approach, in which the films are annealed and/or molten by a frequency doubled Q- switched Nd:YAG laser pulse on a nanosecond time scale. We studied thin polycrystalline Si films of thickness between 43 nm and 259 nm on fused silica and on sapphire substrates. The different thermal conductivities of these substrates lead to different quench rates for the molten Si films. The optical and electrical properties of the Si films were systematically characterized during, respectively after the various annealing conditions. In addition we monitored the solidification process in situ by time-resolved optical measurements. At low energy densities the film is not completely molten by the laser pulse and resolidification takes place at the moving liquid-solid interface. Above a thickness-dependent threshold energy density complete melting is observed and nucleation in the supercooled melt prevails. In the latter case Sameshima and Usui showed that amorphization can be observed for Si films on fused silica up to thicknesses of 36 nm. We found that Si films on sapphire even with a thickness of 80 nm can be amophized. The reproducible threshold values suggest the possibility of lateral structuring.

p-CuInSe₂ single crystals and films are produced. Charge carriers mobility and carriers density reaches as high as 6 - 10 cm²/V sec and 4 - 5 X 10¹⁷ cm^-3, respectively, in the films. A longwave exponential edge of quantum efficiency is 1.0 - 1.15 eV, photo-e.m.f. being invariable up to 2.5 eV. Thin film solar cells based on p-CuInSe/n-CdS heterojunctions are produced on the glass and mica substrates. The best HJ gave V_oc equals 0.85 V; J_sc equals 9.7 mA/cm at 300 K under illumination by 50 mW/cm, with 6 - 7% light efficiency.

Optical properties of some aerogel samples were investigated. The samples were provided by Japan, Norway and Sweden as part of a collaborative program of research on behalf of the International Energy Agency. For each sample the total near-normal hemispherical spectral transmittance and reflectance as well as the diffuse near-normal hemispherical spectral transmittance and reflectance were measured for the solar spectral range using a Beckman integrating sphere. The ratio of diffuse to total transmittance was calculated at each wavelength. Solar optical parameters as well as chromaticity coordinates were determined for both the total and the diffuse components. The influence of bulk and surface scattering is discussed and the effect of sample thickness and incident beam size in relation to sample area in respect of measured optical properties was also investigated and significant differences were observed which are relevant for good practice aspects of the measurement technique. No significant specular reflectance component was detected for any sample. The investigated samples vary considerably in density, but it was found that the visible scattering was nearly the same for all samples. Much stronger absorption is evident in the near infrared spectral range for the Norwegian sample and this is attributed to enhanced water content and higher density.

Sb:SnO₂ films (body resistance 1.2 X 10^-2 (Omega) cm) which were deposited by dip-coating technique via the sol-gel route using SnCl₄(DOT)5H₂O and SbCl₃ precursors have been found to exhibit mixed electronic conductivity and ion-storage characteristics. The Li⁺ and H⁺ ion (de)insertion properties of the films were determined from the cyclic voltammetric (CV) measurements from solvents containing LiClO₄ (0.1 M) in acetonitrile or water, LiOH (0.1 M) and HClO₄ (0.001 - 0.01 M). The particle grain size of Sb:SnO₂ and SnO₂ films has been determined from the XRD measurements and was found to be larger for undoped (55 - 71 angstroms) than for the Sb doped films (46 - 58 angstroms). The Sb:SnO₂ films exhibit effective charges per film thickness (Q_(deint)/d) in the range 0.01 - 0.02 mC/cm²nm and outrange by about 3 times the Q/d values obtained for undoped films. Combining the CV results with the in-situ spectroelectrochemical measurements it was concluded that the intercalation process of Li⁺ ions in Sb:SnO₂ films is governed by the layer charging of the inner grain surfaces which overwhelms the underlaying redox changes involving the Sn⁴⁺ ions. The performance of the transmissive electrochromic device made of WO₃/H₃PO₄-polyvinyl alcohol (PVA)/Sb:SnO₂ was tested and the results presented and discussed.

Electrochromic-ion conductive gels ((sigma) equals 1.7 X 10^-4 S/cm) and thin solid films composed of phosphotungstic acid (PWA) incorporated in titanium oxide xerogel (PWA/Ti equals 0.07) were made via the sol-gel route by the dip-coating technique. The electrochromism of the as - deposited film was studied with the help of cyclic voltammetric (CV) measurements in HClO₄ electrolyte, in-situ UV-VIS spectroelectrochemical measurements and ex-situ Near-Grazing Incidence Angle (NGIA) reflection spectroscopy. It was demonstrated that the color change from transparent to blue in the electrolytic cell is about 40% and is accompanied with effective inserted charges up to 35 mC/cm², giving rise to H_xPW₁₂^y+O₄₀^3- species with x equals 4.7 and y equals 5.6. Ex-situ NGIA FT-IR reflection measurements revealed that proton injection decreases the intensity of the longitudinal optical (LO) modes corresponding to the intra vW-O_c-W and inter vW-O_b-W vibrations of the Keggin's ions while the vW-O_d modes remain unaffected. An intensity decrease of the LO modes of Ti-O vibrations was also detected in the ex-situ NGIA reflection spectra of cathodically colored pure Ti-xerogel films. It was found that the proton insertion process is accompanied by the hydroxylation of titanate ions, while no such an effect was noted for PWA/Ti and Ti xerogel films aged in water and the corresponding electrolytes without the applied potential. Electrochromic-ionic conductive properties of mixed PSA/Ti gels were demonstrated in a semi-liquid electrochromic cell (EC) with reflectance modulation and electrochromism of PWA/Ti solid films was shown by the in- situ UV-VIS transmittance measurements of an all-solid state device with PWA/Ti / H₃PO₄ doped polyvinyl alcohol (PVA) / Sb:SnO₂ configuration.

Iron oxide films which were synthesized via the sol-gel route from the iron (III) chloride precursor exhibit electrochromism (T_b - T_c equals 55% at 400 nm) in LiOH (0.01 M) electrolyte. The structure of the XRD amorphous films was identified with the help of near- normal reflection absorption (6 degree(s)) (IRRA) and near-grazing incidence angle (NGIA) FT-IR spectroscopy to correspond to the nano-crystalline (gamma) -Fe₂O₃. Ex-situ NGIA FT-IR spectra of bleached and re-colored films were measured and it was found that the Fe-oxide films irreversibly change to the new phase in which `amorphous iron oxide' is admixed with the characteristic LO mode at 545 cm^-1. Electrochemical stability of the Fe-oxide film was modified by admixing other non-absorbing Ti and Si-oxides. The structure of the mixed oxide films was identified from the corresponding IR spectra and was described as translatory disordered solid solution in which the phonon modes exhibit one mode (Fe/Ti-oxide) and two-mode (Fe/Si-oxide) behavior. Electrochemical investigations revealed that the films are able to uptake reversibly Li⁺, Na⁺, K⁺ ions with Q/d values in the range 0.1 - 0.31 mQ/cm²nm. The electrochromic properties of the films investigated were established from the measured in-situ UV-VIS spectroelectrochemical measurements which revealed that the electrochromic efficiencies ((Delta) OD/Q_(deint)) are in the range of 6 - 14 cm²/C. The nature of the electrochromic process is discussed and correlated to the absorption edges of the various iron oxygenated compounds.

Ion storage CeO₂ and CeO₂/SnO₂ coatings were prepared by sol-gel dip-coating method using aqueous-based process. The influence of added SnO₂ in the CeO₂ oxide coatings on the inserted/extracted charge was determined by chronocoulometric measurements. It was found that for 60 nm thick film the inserted/extracted charge was twice larger (Q equals 10 mC/cm²) for films containing 17 mol % SnO₂ if compared to pure CeO₂. The addition of SnO₂ to the mixed oxides coatings on their optical properties and structural characteristics were studied.

Because of their high theoretical conversion efficiencies, narrow band gap semiconductors (e.g. Si, GaAs, InP) are most suitable for photoelectrochemical solar energy conversion. unfortunately, they are destroyed by anodic dissolution (photocorrosion) in aqueous electrolytes parallel to photooxidation of electrolyte components. The coating of the semiconductor by a transparent, conductive film is one possibility for corrosion protection. We investigated the suitability of indium tin oxide (ITO) films deposited by different sputter techniques onto n-silicon and n-gallium arsenide substrates. First, the influence of the preparation conditions and of the post deposition annealing treatment on the photovoltaic properties is discussed. Second, the photoelectrochemical properties of the n- semiconductor/ITO electrodes in different aqueous electrolytes containing different redox systems are investigated. n-Si/ITO samples produced by the reactive magetron sputtering technique show the expected corrosion protection behavior, especially in electrolytes with reversible redox systems (e.g. Fe(CN)₆^3-/4-). ITO films deposited by different sputter techniques (reactive magnetron sputtering, d.c. sputtering in different atmospheres) onto n-GaAs are not suitable for corrosion protection purposes. The low Schottky barrier at the n-GaAs/ITO interface and pinholes in the ITO layer are responsible for this behavior. The differences in photoelectrical and photoelectrochemical behavior and in corrosion stability are discussed.

Highly qualitative indium tin oxide (ITO) films were deposited by reactive dc-sputter technique on glass and quartz substrates. Both pure H₂O vapor and pure O₂ have been used as reactive sputtering atmosphere. The optical constants refractive index and absorption coefficient were calculated from reflection and transmission spectra in the infrared (IR) region. Using the Drude theory the electrical parameters free carrier concentration and carrier mobility of the ITO-films were contactless determined from IR reflection spectra. The dc-H₂O-sputtered ITO-films show improved optical and electrical properties. For application as transparent window material they distinguish by high visual (VIS) transmittance and high IR- reflectance. Moreover these ITO-films show a high free carrier concentration of N equals 6 X 10²⁰ cm^-3 directly after deposition compared with N equals 4 X 10²⁰ cm^-3 for dc-O₂-sputtered ITO-films after thermal annealing. The differences between optical and electrical properties of de-O₂- and dc-H₂O-sputtered ITO-films are discussed in regard to their crystal structure and surface morphology.

Forward biasing of transparent nanocrystalline TiO₂ (anatase) films in lithium ion containing organic electrolytes leads to rapid and reversible coloration due to electron accumulation and Li⁺ intercalation in the anantase lattice. Absorption of > 90 percent light throughout the visible and near IR can be switched on and off within a few seconds. The nanocrystalline morphology of the film plays a primordial role in enhancing the electrochromic process. Preliminary results from the reduction of a surface attached viologen molecule leading to a blue coloration of the film are reported.

The unique property of silica aerogel, to be transparent with a low thermal conductivity and the safety of the CO₂ producing mode, provides the motivation for European Laboratories to work together in the field of transparent insulation materials under the framework of European projects (Joule II, Human Capital and Mobility). A couple of problems have to be solved to achieve the goal of manufacturing a double glazing filled with monolithic carbogel. To improve the mechanical and optical properties of carbogel for its application in double glazing, one has to know the relationship between the structure and the physical properties of the carbogel. Therefore, one has to analyze the structure of carbogel in a detailed manner. This paper discusses the change in particle size as a function of the polyethoxydisoloxane precursor to water ration. We present Tapping Mode and Contact Mode atomic Force Microscope Images of carbogel which supply 3D information about the carbogel surface. The measured images provide information about the particle's size in three dimensions in nanometer scale range.

This paper describes some essential properties of all-solid-state transmissive electrochromic devices based on a combination of polyaniline (PANI) and tungsten trioxide (WO₃) layers on ITO sandwiching a proton conducting polymeric layer. The single electrochromic layers were prepared by electrochemical deposition onto ITO/glass electrodes. Proton conducting polymeric electrolytes were prepared by mixing protonic acids with (poly)vinyl alcohol. The fabricated all solid-state electrochromic devices exhibit electrochromic response times with color contrasts of about 50% in the range between 0.1 and 1 s, depending on the thickness of the single electrochromic layers, on the cell voltage, on the ion conductivity of the polymeric layer and on the electronic conductivity of the ITO layers. The observed color can be changed from transparent clear yellow to deep blue by applied voltages in the range between 0.5 - 2 V. The response time of the single investigated electrochromic layers is governed by the rate of proton transport within the layers. The response times of single PANI/ITO/glass half cells in acid aqueous electrolytes show assymetric characteristics and can be less than 0.2 s.

A redox molecule (acceptor) is attached, using a surface chelate (spacer), to a semiconductor electrode (donor). Such donor-spacer-acceptor complexes, referred to as heterodyads, offer the prospect of testing important aspects of the theory of heterogeneous electron transfer (ET) at the semiconductor electrode-liquid electrolyte interface (SLI). Specifically, potentiostatically controlled ET from the conduction band of the semiconductor electrode to a redox species held at a fixed distance and orientation with respect to the SLI is possible. Extending the above approach, a modified SLI has been prepared at which potentiostatically controlled vectorial electron flow leading to long-lived charge trapping is possible. Specifically, a spacer-acceptor I-acceptor II complex is adsorbed at a semiconductor electrode to form a heterotriad. Application of a potential more negative than the potential of the conduction band at the SLI results in acceptor I mediated reduction of acceptor II. The reduced form of acceptor II is stabilized and long-lived charge trapping results. Efficient light induced charge separation by vectorial electron flow at the above modified SLI is also possible.

Dye-sensitized micro -or, to be more precise, nanoporous - Ti02 film electrodes for solar cells1' 2 are currently under investigation in our laboratory35. We have earlier presented methods to gain information about fundamental properties of semiconducting thin film electrodes from front-side (EE) and backside (SE) illumination of Ti02 electrodes by analysis of the action spectra3' 6 (For the experimental set up and the definition of SE and EE, see Fig. 1.) We are here extending our experimental work into the visible region and give preliminary result from an investigation of the action spectra of dye-sensitized Ti02-film electrodes.

Thin substoichiometric WO_x films were produced by reactive sputtering with variation of the oxygen partial pressure and the temperature of the substrate to get samples with different degrees of crystallinity and colorations. Measurements were done on the conductivity, the transmittance and reflectance from 100 K up to 300 K. The dielectric constant was evaluated by ellipsometry. As observed in earlier works, with increasing coloration a sudden increase in conductivity and a decrease of the temperature dependence of the conductivity takes place, which is attributed to an Anderson transition. But based on Hall measurements and the ellipsometrically evaluated dielectric constants the formation of Drude electrons with a plasma edge in the visible or NIR range is unlikely. Rather the measured dielectric constant suggests a strong Lorentz oscillator with a weak bond of the coloring electrons. Reflectance and transmittance did not change when cooling the samples from 300 K to 100 K. At very high colorations a second qualitative change of the conductivity and the optical properties was observed. Here perhaps the first metallic clusters are formed.