Optimized spectral transmittance of sun protection glasses

A spectral optical transmission function τ(λ) for sun protection insulating glasses is proposed in order to reduce the solar energy load of a building’s interior and thus reducing overheating. τ(λ) is based on standard functions such as spectral distribution of solar radiation S_λ(λ), spectral photopic luminous efficiency V(λ), standard illuminant D₆₅(λ), and the CIE color-matching functions . In the framework of the present approach an optimized spectral transmittance τ_min(λ) with a minimal normalized solar energy load (i.e., solar direct transmittance normalized to light transmittance, τ_e/τ_v) has been obtained on the condition of color neutrality of the transmitted light. A comparison with the performance of actual commercial sun protection glasses on the market shows that the present model for an optimized spectral transmittance could reduce the solar energy load by roughly one third for equal light transmittance τ_v.     

Optical characterization of tandem absorber/reflector systems based on titanium oxide–carbon coatings

The use of chemical methods to produce coatings with good spectrally selective properties is described. Sol–gel and dipping techniques were combined to fabricate thickness-sensitive solar-selective absorbing coatings based on titanium oxide combined with carbon black (TiO₂/CB) and carbon nanotubes (TiO₂/CNT) on top of stainless-steel substrates. The low to high reflectance transition (λ_T) observed in the tandem system based on TiO₂/CB films was smooth and red shifted when compared to the sharp transition observed in systems based on mesoporous TiO₂/CNT films. Mesoporosity of the oxide was controlled by the use of polymeric surfactants with different molecular weights, causing a significant blue shift in λ_T as the molecular weight increases. These films were characterized by X-ray diffraction and UV–vis light absorption spectroscopy.     

Optical properties of higher and lower refractive index composites in solar selective coatings

The focus of attention in this study was the choice of material for optically solar selective coatings on the basis of their optical constants. A computer programme which calculates the optical constants, solar absorptance at air mass (AM)-2, α, and thermal emittance at 300 K, , of the 200-nm-thick selective coating on the assumption of both the Maxwell Garnett and Bruggeman theories for the metallic volume fraction below and above 0.3 respectively, was used to design the structure of the composite films. Two systems of composite thin films of metal and dielectric were investigated experimentally, fabricated by RF and DC sputter coater and were verified with computer simulations. One system consist of lower refractive index composites such as Ni : SiO₂ and the other of higher refractive index composites such as V : Al₂O₃ in the spectral range of 0.3–20 μm. These films were fabricated on infrared reflective substrates such as nickel plated copper or aluminium. Results of the copper substrates are being presented here. For comparison and verification, tungsten, cobalt and chromium based composites, having different refractive indices, were also investigated which validated the concept of the choice of material in selective coatings. It was observed that high refractive index composites have lower reflective properties by choosing suitable metallic volume fraction in dielectric and antireflection coating. The higher value of the imaginary part of refractive index, k, is responsible for higher absorption by a factor α_λ=4πk/λ. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.05 thermal emittance for 200 nm thick composites of V : Al₂O₃. It results that higher values of both n and k of the material are more suitable in solar selective coatings.     

Sensitization of an iron-thiazine photogalvanic cell to the blue: An improved match to the insolation spectrum

Sensitization of a totally illuminated thin layer (TI-TL) photogalvanic cell based on the iron-thiazine photoredox reaction has been demonstrated by both current action spectra and enhanced white light output. Singlet sensitization of thionine (λ_max = 601 nm) and of methylene blue (λ_max = 660 nm) to wavelengths below 400 nm has been achieved. Photogalvanic output has been obtained upon illumination of a single solution containing two photoredex dyes and three sensitizers with monochromatic light throughout the wavelength range 375–700 nm. Sensitization with rhodamine 6 G considerably enhances cell output under illumination with white light. With some sensitizers, however, white light output is inhibited.     

The use of the self-calibration method for accurate determination of silicon solar cell internal quantum efficiency

The application of the self-calibration method as a means of increasing the accuracy of spectral response, SR, and internal quantum efficiency, Q(λ), measurements is discussed. The principle of the method is the precise calculation of Q(λ_m) of a test cell for light at λ_m≈0.8 μm, where the response is weakly dependent on the emitter and base parameters. The ratio of the calculated and measured Q(λ_m) values gives the corresponding factor for shifting the experimental spectral response curve. The sequence of calculations is described, and an algorithm of the necessary operations for a computer is developed. Several examples of the use of the self-calibration method for correction of SR measurements of solar cells with low shunt resistance demonstrate its very high effectiveness. The corrected Q(λ) values follow the respective actual data with very high accuracy even when the measured SR is decreased by factor 2–3 due to low shunt resistance of the solar cell.     

Infrared radiative heat transfer in highly transparent silica aerogel

The radiative heat transfer in silica aerogel is determined by measuring the spectral infrared extinction (absorption) E(Λ) between 2.5 and 14 μm. From E(Λ) we calculate a temperature-dependent Rosseland mean for extinction. Using a suitable diffusion model for the infrared radiation the temperature-dependent radiative conductivity λ_r(T) is determined. For T = 280 K, a mean temperature within the insulation, a value as low as λ_r 0.002 W/(m·K) results. Between T = 250 K and 450 Kλ_r increases roughly ∝ T^α, with α = 5.6.     

Solar radiation data for Beer Sheva, Israel

A number of years of data on the global irradiation incident on a horizontal surface in Beer Sheva, Israel (lat. = 31°15′N, long. = 34°48′E, elevation 240 m) have been correlated. It is apparent from these data that this locale possesses a relatively high abundance of sunshine. The average cumulative annual irradiation is 6722MJ/m² and the average daily irradiation is 18.43 MJ/m². The percentage frequency of days possessing irradiation rates greater than 20 MJ/m² is 46 per cent, whereas that possessing less than 10 MJ/m²d is 11.9 per cent. The percentage frequency of cloudy days (K_T < 0.34) is low, 7.5 per cent, whereas that for clear days (K_T > 0.65) is 29.2 per cent.     

Photochemical storage potential of azobenzenes

The potential for storage of solar energy using photochemical trans-cis isomerizations of substituted azobenzenes has been studied. Polar substituents are found to res-shift the π-π^* absorption spectrum of these compounds without destroying the endothermicity of the photoisomerization. Highly red-shifted compounds such as azonaphthalenes revert thermally to the trans isomer within seconds. Methyl orange has the most favorable energy storage potential of the compounds studied and has a storage efficiency of 16 per cent for 436 nm irradiation and 2 per cent for AM-1 isolation. Because of limited solubilities and rapid thermal reversion rates in polar solvents, it is concluded that azobenzenes are not favorable compounds for photochemical solar energy storage.     

Single-layer heat mirror films and an improved method for evaluation of its optical and radiative properties in infrared

A technique for preparing single heat mirror films with high quality is described. We also present an improved method for evaluating its optical and thermal radiative properties in the infrared region (2.5<λ<25 μm). The calculated results agree well with that obtained by experimental measurements. The results calculated by using the simple Hagen–Rubens relation are also given. It is shown that the Hagen–Rubens relation is not accurate to predict the thermal radiative properties for the transparent semiconducting oxide films, such as indium tin oxides, prepared in our studies.     

Photoelectrical properties of doped cadmium sulphide powders

Photosensitive powders of CdS were prepared with different concentrations of dopants. Doses of donors (Cl⁻) and acceptors (Cu²⁺, Ag⁺) varied from 0 to 41 and from 0 to 9 mg/g CdS, respectively. Reflectance, absorption coefficient and resistance dependence on illumination intensity and voltage at the wavelength of about λ=630 nm and photovoltage spectra in the range 450–900 nm were measured on layers prepared from the powders. The value of absorption coefficient grew with the increasing dopant concentrations; acceptors appeared more efficient than donors. Reflectance decreased with growing acceptor dose. Using the values of reflectance, absorption coefficient and resistance the corrected photovoltage, as the measure of the concentration photogenerated charge carriers, was calculated. The ratio (σ_IGB) of the corrected photovoltage and photocurrent was used as the criterion of intergrain barrier conductivity. All doped samples exhibited similar value of σ_IGB which was about three orders of magnitude lower than that of undoped sample.     

Calculation of the photocurrent-potential characteristic for regenerative, sensitized semiconductor electrodes

The steady state anodic photocurrent for sensitized semiconductor electrodes, where the sensitizer is regenerated by a redox electrolyte, has been modeled taking into account the rate of light absorption by the sensitizer S, the rate of electron injection from the excited state S^* of the sensitizer to the conduction band of the semiconductor, the rate of decay of S^* (radiative and non-radiative) and the rate of reductive regeneration of the sensitizer by the redox electrolyte. In this model the rate of recombination between the conduction band electron and the oxidized sensitizer, S⁺, and the reactions between S^* and the redox couple have been assumed to be negligible. The rate constant for injection, k_inj, the injection efficiency, φ_inj, the photocurrent density, J_P, and the steady state concentrations of S^* and S⁺, have been calculated as a function of light intensity, Helmholtz potential, λ_max and halfwidth, ΔE_0.5, of the sensitizer absorption spectrum, and the semiconductor band gap and electron affinity both for monochromatic light and for AM1.5 sunlight simulated by radiation from a 5600K black body. For the calculation of k_inj as a function of Helmholtz potential, the Gurney -Gerischer-Marcus (GGM) model has been used. Allowance for the distribution of electrode potential between the semiconductor and the electrolyte has been introduced in principle. The steady state concentrations of S^* and S⁺ were used in the Nernst equation to calculate the S^*/S⁺ quasi-reversible potential. It is shown that the short-circuit current density of the cell is a maximum for a sensitizer with a λ_max of about 1550 nm. Inter-relationships between variables involving the sensitizer have been used to show that only four sensitizer parameters are needed when considering the effects of changing the sensitizer. These parameters are the reorganization energies and the standard reduction potentials for the couples S⁺/S^* and S⁺/S. For a related series of sensitizers, such as obtained by changing a substituent group, only the two standard reduction potentials are required to predict the effects of changing the sensitizer.     

A comparative study of SPCTRAL2 and SMARTS2 parameterised models based on spectral irradiance measurements at Valencia, Spain

Results obtained using the parametric models SPCTRAL2 and SMARTS2 for the urban area of Valencia, Spain, have been analysed and compared with experimental measurements at ground level obtained with two Li-cor 1800 spectroradiometers with a 6 nm resolution. The study used two different input parameters in both models for the aerosol characterisation: the aerosol optical thickness at 0.5 μm, τ_aλ(0.5), and the Angstrom turbidity coefficient β. The results obtained show that both algorithms reproduce quite correctly the spectral irradiance experimental values when an urban aerosol model parameterised by the τ_aλ(0.5) value is considered. In all the cases the deviations are lower when SMARTS2 code is used.     

Shallow borehole heat exchanger: Response test and charging–discharging test with solar collectors

Possible heat sources for seasonal Underground Thermal Energy Storage (UTES) can be divided into two main groups—renewable energy and waste heat. The design of a ground heat exchanger for UTES applications requires knowledge of the thermal properties of the soil (thermal conductivity λ and borehole thermal resistance R_b), which can be done performing the so-called Thermal Response Test (TRT). The present article describes the results of a cooperative work between research groups of Chile and Argentina, which led to the realization of a TRT and of a charge/discharge experiment with solar collectors on a shallow single borehole. The tests are the first of its kind performed in South America. An installation for realizing the Borehole Thermal Energy Storage (BTES) was prepared at the ‘Solar Energy Laboratory’ of the Technical University Federico Santa Maria, Valparaiso, Chile. A comparison by means of different methods in order to evaluate λ and R_b was made. Time evolution of measured and predicted temperatures during charging and discharging was performed. The experiments done are a good precondition prior to using UTES for heating and cooling in different regions of Chile and Latin America (Argentina, Brazil) and to apply the BTES technology in the same region.     

Design and test of non-evacuated solar collectors with compound parabolic concentrators

The intermediate range of concentration ratios (1.5X–10X) which can be achieved with CPCs without diurnal tracking provides both economic and thermal advantages for solar collector design even when used with non-evacuated absorbers. The present paper summarizes more than 3 yr of research on non-evacuated CPCs and reviews measured performance data and critical design considerations. Concentrations in the upper portions of the practical range (e.g. 6X) can provide good efficiency (40–50 per cent) in the 100–160°C temperature range with relatively frequent tilt adjustments (12–20 times per year). At lower concentrations (e.g. 3X) performance will still be substantially better than that for a double glazed flat plate collector above about 70°C and competitive below, while requiring only semi-annual adjustments for year round operation. In both cases the cost savings associated with inexpensive reflectors, and the optimal coupling to smaller, simple inexpensive absorbers (e.g. tubes, fins, etc.) can be as important an advantage as the improved thermal performance.The design problems for non-evacuated CPC collectors are entirely different from those for CPC collectors with evacuated receivers. For example, heat loss through the reflector can become critical, since ideal CPC optics demands that the reflector extend all the way to the absorber. Recent improvements in reflector surfaces and low cost antireflection coatings have made practical a double-glazed non-evacuated CPC design. It is calculated that a 1.5X version of such a collector would have an optical efficiency η_o = 0.71, a heat loss coefficient U = 2.2 W/m²°C and a heat extraction effciency factor F′ ≥ 0.98, while requiring no tilt adjustments.     

Contact resistivities of sputtered TiN and Ti---TiN metallizations on solar-cell-type-silicon

Contact resistivities of TiN and Ti---TiN contacts on a shallow junction solar-cell-type silicon substrate have been investigated. The contact materials were sputter-deposited. The method of the transmission line model was applied for contact resistivity measurements. The contact resistivity of the n⁺Si---TiN contact system was 2 × 10⁻³ Ωcm² ± 50 per cent and remained constant after annealing up to 700°C for 30 min. For the n⁺Si---Ti---TiN system, the contact resistivity of 9 × 10⁻⁴ Ωcm² ± 50 per cent was measured. A heat treatment of 700°C. 30 min decreases this value by one order of magnitude and the interposed Ti fully reacts with Si and forms a TiSi₂ layer. The voltage drop caused by the n⁺Si---TiN contact system in a standard non-concentrator solar cell is negligible. The n⁺Si---TiSi₂---TiN contact system should be acceptable for Si solar cells used at up to 100 times solar concentration.     

Self-calibration as a tool for high-accuracy determination of silicon solar cell internal quantum efficiency

Light nonuniformity, uncertainty in the illuminated photoactive area, and relative, but not absolute radiometric data for the reference detector, can be the reasons for the inaccuracy or impossibility of solar cell spectral response and quantum efficiency determination. The use of a self-calibration principle permits minimization of the errors caused by the above factors. This principle consists of quite precise calculation of the internal quantum efficiency Q(λ_m) of the test cell at λ_m≈0.8 μm, where the cell response is weakly dependent on emitter and base parameters. Experimentally determined short- and long-wavelength internal quantum efficiencies, Q(0.4) and Q(0.95), respectively, based on relative radiometric data for a reference detector, are used as starting data for the Q(λ_m) calculation. The ratio of the calculated to measured Q(λ_m) values gives the correction factor for shifting the experimental quantum efficiency curve. Computer modeling supports the assumption that uniform deviation of measured Q(λ) can be precisely corrected by calculation. Analysis of the accuracy of the self-calibration method demonstrates very small uncertainties in the corrections of quantum efficiency measurements, attainable for many practical situations. Confirmation of correctness of the proposed method is shown by analysis of the results of spectral response measurements of several solar cells.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

Effect of annealing on the electrical, optical and structural properties of hydrogenated amorphous silicon films deposited in an asymmetric R.F. plasma CVD system at room temperature

This paper reports the effect of annealing on hydrogenated amorphous silicon films (a-Si : H) deposited by r.f. self-bias technique on cathode in an asymmetric r.f. plasma CVD system at room temperature. Detailed study of the variation of the dark and photoconductivity (σ_D and σ_ph) as a function of temperature and light intensity, surface morphology, hydrogen evolution, optical absorption, subgap absorption and related parameters, thermal and structural disorder on the optical-absorption edge, IR vibrational modes and bonded hydrogen content have been carried out on unannealed and annealed samples at different temperatures (T_a) from 100°C to 550°C. It is found that the values of σ_ph increase and that of Urbach energy (E_o), subgap defect density (N_d) and the polyhydride to monohydride ratio decrease upto T_a=250°C and beyond 250°C the values of σ_ph decrease and that of E_o, N_d and the polyhydride to monohydride ratio increase. The best opto-electronic properties with much improved σ_ph and σ_ph/σ_D and dominant monohydride bonding are obtained after annealing the room temperature deposited film at 250°C for 1 h. The σ_D data obeys a Meyer Neldel rule in annealed a-Si : H films. The value of optical band gap is found to be related to the E_o and the hydrogen content. The Urbach energy (E_o) which is a measure of the disorder is the sum of structural and thermal disorder. The structural disorder part decreases with the annealing temperature upto 300°C and thereafter it increases. The curves of optical absorption coefficient versus photon energy at different T_a converge to a common point.     

Characterization of a-Si:H films via analysis of multiple photocurrent spectra

The spectral response of photoconductive materials applicable to solar cells and photosensors, is commonly characterized using different experimental approaches, such as photocurrent spectra normalized to incident light intensity, photoconductivity spectra measured at constant photon flux, the constant photocurrent method (CPM) [3–5], and the dependence of the photocurrent on the photon flux.In this work, a procedure is proposed, which allows the complementary results of the above mentioned approaches to be gained by analysing several photocurrent spectra, obtained by routine measurements at different illumination levels (Multiple Spectra). This procedure is used for investigation of two a-Si:H samples. The spectra, corresponding to a constant photon flux and a constant photocurrent, as well as the dependencies of the photocurrent on the photon flux at various wavelengths I−Φ spectral map) are derived from the Multiple Spectra and are subsequently used to obtain the following characteristics of the investigated samples: the subbandgap optical absorption spectra; the bulk density of “defect” states; the Urbach tail width; and the power index of the I−Φ)_λ dependencies for the bulk and for the interface regions.     

The parameterisation of the atmospheric aerosol optical depth using the Ångström power law

We have analysed the ability of the Ångström power law to model the spectral aerosol optical depth, τ_aλ, for the 400–670 nm band, obtained from spectral direct irradiance measurements at normal incidence. The spectra were registered at ground level in Valencia, Spain, using a Li-cor 1800 spectroradiometer. The results obtained showed that the fitting method that introduces lower errors in the determination of the Ångström power law coefficients is to adjust directly the spectral experimental data. In this way the errors obtained for the turbidity coefficient, β, were about 0.004 and for the wavelength exponent, α, 0.07. The correlation coefficient was always greater than 0.95. These values of the correlation coefficient could be improved by parameterisation of τ_aλ using an alternative function of the wavelength to the Ångström power law. But this may not be justified for the turbidity values attained in Valencia.