A study on luminous efficacy of global radiation under clear sky conditions in Athens, Greece

The current interest in energy efficient design has led to a demand for methods of predicting daylight availability in buildings. However, basic daylight data, which are the basis of these prediction methods, are not available for most regions of Greece, as well as for many areas worldwide. For that purpose luminous efficacy can be used in order to generate daylight illuminance from solar radiation data, which are much more widely recorded.This paper concentrates on the analysis of global luminous efficacy in Greece under clear sky conditions. The study involves the evaluation of existing models with respect to daylight data recorded in Athens, Greece, as well as the development of new luminous efficacy models for global radiation.The study was based on climatic data provided by the National Observatory of Athens during the years 1996–2000 and tested according to CIE. recommendations. The proposed luminous efficacy models were derived by the analysis, correlation and linear regression of the accepted values of measured data. Statistical analysis techniques were used for the assessment of their performance, as well as for the evaluation of the accuracy in predicting global illumination provided by the proposed and the existing models.The new proposed models can offer engineers and researchers a more reliable and accurate approach for estimating the global illumination levels under clear skies. Nowadays, the knowledge of daylight levels is considered highly important in local architecture, the energy conservation and environmental policies in general, particularly in the case of Mediterranean regions, where clear sky conditions dominate.     

Optical characterisation of materials and systems for daylighting

The measurement of BRTF (Bi-directional reflectance and transmittance function) is described using a new instrument which is capable of supplying BRTF data and algorithms for use in computer simulations directly on diffuse materials and indirectly on large samples and sub-systems. A high sensitivity and dynamic range is needed to achieve low minimum observable BRTF and the role of angular resolution are discussed with examples. Forward scattering with extended tails is found to dominate pigmented polycarbonate. Slatted blinds are discussed as examples of systems where azimuth is important.     

Maximising the light output of a Luminescent Solar Concentrator

Luminescent Solar Concentrators (LSC's) are currently being developed so that sunlight may be more fully utilised in place of conventional lighting sources. In order to optimise the light output of an LSC, various collector properties including fluorescent dye concentration, collector dimensions and matrix quality have been investigated. This study, involving measurement and computer modelling of the light transport in a single colour LSC, shows that luminous output is highly sensitive to weak extinction processes in the emission spectral band. These fine details in the loss spectrum play a key role in determining the output and efficiency of an LSC system. The separation of dye and matrix losses enables a quantitative study of the effect of matrix optical quality on performance.     

Tubular guidance systems for daylight: Achieved and predicted installation performances

Tubular daylight-guidance systems are linear devices that channel daylight into the core of a building. The development, over the last decade, of materials with high specular reflectance has led to a large number of passive zenithal systems; the most commercially successful type of daylight guidance being installed in many parts of the world. The rapid change in technology has not been matched by the development of either reliable and standardised design methods or design criteria against which the systems may be evaluated. This paper presents the results of several surveys of daylight guidance systems in 13 working buildings. These give an indication of the conditions created, which are used as the bases of suggested design criteria. A critical review of existing performance prediction methods notes that these lag far behind comparable methods for electric lighting and conventional glazing. A number of improved methods of prediction, currently under consideration by the CIE Technical Committee TC3-38, are presented and each is tested against measured data from the installation surveys.     

Lightswitch-2002: a model for manual and automated control of electric lighting and blinds

A simulation algorithm is proposed that predicts the lighting energy performance of manually and automatically controlled electric lighting and blind systems in private and two-person offices. Algorithm inputs are annual profiles of user occupancy and work plane illuminances. These two inputs are combined with probabilistic switching patterns, which have been derived from field data, in order to predict the status of the electric lighting and blinds throughout the year. The model features four different user types to mimic variation in control behavior between different occupants.An example application in a private office with a southern facade yields that––depending on the user type––the electric lighting energy demand for a manually controlled electric lighting and blind system ranges from 10 to 39 kW h/m² yr. The predicted mean energy savings of a switch-off occupancy sensor in the example office are 20%. Depending on how reliably occupants switch off a dimmed lighting system, mean electric lighting energy savings due to a daylight-linked photocell control range from 60% to zero.     

Analysis of daylight factors and energy saving allowed by windows under overcast sky conditions

The aim of this research is to quantify the daylight factors produced inside a room for different models of windows, and to conduct an analysis of the results obtained. All trials were performed under overcast sky conditions, as these represent the worst case scenario for calculation. The shape, size and position of the window are variable, as is the reflectance of the inner surfaces of the room. A total of 28 simulations are provided by the lighting simulation program Daylight Visualizer 2.6, validated by the CIE test cases. After trials it was concluded that square windows produce daylight factors slightly higher than those obtained with horizontal windows and noticeably higher than those measured with vertical windows, considering the same surface of openings. It is confirmed that the daylight factors are directly proportional to the glass surface, except in the area near the window. It is also concluded that the windows in the upper position allow higher luminance at the back of the room than those in centered locations. Finally, the energy savings produced by the different models of windows is calculated.     

An analysis of energy-efficient light fittings and lighting controls

Electric lighting is one of the major energy consuming items in many non-domestic buildings. Using appropriate energy-efficient light fittings with dimming controls and proper daylighting schemes can help reduce the electrical demand and contribute to visual comfort and green building development. This paper presents a study on the energy and lighting performances for energy-efficient fluorescent lamps associated with electronic ballasts and high frequency photoelectric dimming controls installed in a school building. Electricity expenditures and indoor illuminance levels for a workshop and a classroom employing high frequency dimming controls were analyzed. Simple prediction methods were used to illustrate the lighting savings. The findings provide the operational and performance information, which would be applicable to other spaces with similar building layouts and lighting schemes.     

Study of a novel sunlight concentrating and optical fibre guiding system

This paper presents the experimental investigation of a novel sunlight concentrating and optical fibre guiding system for daylighting applications. The key part of the system is a sunlight concentrator unit which includes a novel mirror image co-focus compound parabolic concentrator. The structure and working principle of an experimental sunlight concentrating and optical fibre guiding system are introduced. Under a real sky condition, the experimental system was investigated to determine its performance and characteristics. The relationship curves between the internal illuminance of the integrating box and external illuminance are given for different tracking accuracy values and optical fibre geometries. The transmittance of the experimental system is calculated from the measured illuminance values. The results demonstrate the feasibility of the novel sunlight concentrating and optical fibre guiding system for daylighting applications.     

Optimization of antireflection coating for VO2-based energy efficient window

Vanadium dioxide (VO₂) films have been proposed as energy efficient window coatings for their thermochromism, with which the solar energy transmission in the IR region may be controlled passively. These coatings suffer from low visible (or luminous) transmission (380–760 nm in wavelength), which hinders their practical uses. We here consider an antireflection (AR) coating for the VO₂-based window. Optical calculation was first performed upon a basic structure for thermochromic window composed of a VO₂ layer on glass with an AR layer of refractive index n and thickness d. Optimization was carried out on n and d for a maximum integrated luminous transmittance (T_lum). The calculation demonstrates that the optimal n value changes with thickness of VO₂, and at n≈2.2 it gives the highest T_lum enhancement from 32% (without AR coating) to 55% for 50-nm VO₂. Experiment was done on a structure of 50-nm VO₂ on quartz glass using ZrO₂, of which n≈2.2 matching the best n value, as AR coating. Formation of an optimized structure, ZrO₂ (56 nm)/VO₂ (50 nm)/quartz, was done by sputtering, and its optical properties were characterized with spectrophotometry. An improvement of T_lum from 32.3% to 50.5% was confirmed for the semiconductor phase with similarity also for the metallic one. The IR switching properties were not much deteriorated.     

Silica aerogel granulate material for thermal insulation and daylighting

Silica aerogel granulate is a nanostructured material with high solar transmittance and low thermal conductivity. These properties offer exciting applications in building envelopes. One objective of the joint R&D project ISOTEG at ZAE Bayern was to develop and characterize a new glazing element based on granular silica aerogel. Heat transfer coefficients of less than 0.4 W/(m² K) and a total solar energy transmittance of 35% for the whole glazing unit were achieved. The glazing has a thickness of less than 50 mm. Another application for granular silica aerogel is, for example, in solar collectors.The thermal properties of the glazing as well as the optical and thermal properties of the granular aerogels are presented here. The solar transmittance of a 10 mm packed bed of silica aerogel was 53% for semi-translucent spheres and 88% for highly translucent granulate. In our heat transfer experiments the gas pressure, external pressure load, temperature and gas filling were varied. The various thermal conductivity values measured for the glazing and collector applications were compared to the values calculated using two different packed bed models. For the gas-dependent measurements the intergranular voids in the granulate were 1.0 ± 0.1 mm before loading the packed bed, 0.3 ± 0.1 mm at an external load of 3.2 bar (3.2 × 10⁵ Pa) and 0.6 ± 0.1 mm after release.A direct radiative conduction of λ_direct = 4.5 ± 0.5 × 10⁻³ W m⁻¹ K⁻¹ was obtained.     

Recommendations for energy efficient and visually acceptable street lighting

While designing new street lighting installations or dealing with reconstructions of the existing ones, lighting designers usually do not take into consideration all of the available means for energy savings and optimal performance. This paper offers a set of the most important recommendations regarding the relevant influencing factors for energy savings in street lighting, the majority of which represent the results and conclusions of original research. Recommendations which result from user needs and regard visual quality are also briefly presented. Taking all of these recommendations into account provides improvement of appearance and sense of security, as well as energy and cost savings.     

Light transmission efficiency of daylight guidance systems: An assessment approach based on simulations and measurements in a sun/sky simulator

New and more advanced guidance systems are nowadays available, allowing bringing natural light into buildings and offering potentials for energy savings associated to well-being for occupants. From a design point of view, the key factor is the knowledge of their photometric performances in terms of global light transmission efficiency, so as to predict the daylight availability in an interior space due to an array of guidance systems (or, the other way around, to predict the number of pipes needed to produce a minimum natural light illuminance according to standard requirement) through known analytical methods such as the lumen method. In spite of this, determining the global light transmission efficiency of advanced guidance systems is a quite complicate matter because of the redirecting optical properties these elements rely on even in the case of simple typologies (for instance, passive domes with micro-prismatic profiles and micro-prismatic or holographic films used as pipe’s coating).This paper presents an approach to characterize photometric performances of tubular daylight guidance systems in terms of light transmission efficiency: the global system efficiency is the result of the product of the efficiencies of the three individual components (collector, pipe and diffuser) and each efficiency is determined as the ratio of the flux emitted through the output window to the flux hitting the input window, accounting for both the beam and the diffuse efficiency. The approach, based on both measurements on physical models and simulations, was applied to different typologies of pipes and passive collectors and the obtained data were used to eventually calculate the global efficiency for the analyzed system.     

汽车轻量化材料的应用

本文主要介绍了目前已经应用的汽车轻量化材料的现状,包括高/超高强度钢板、变截面薄板、轻金属材料以及其它轻量化材料的应用状况,表明我国汽车轻量化材料应用与国外汽车强国还存在差距。     

A simple clear skies model for the luminous efficacy of diffuse solar radiation on inclined surfaces

In the present work we study the luminous efficacy of diffuse solar radiation incident on vertical surfaces for a clear sky and mean hourly values of diffuse irradiance and diffuse illuminance. We develop a model easy to use, similar to a model previously obtained for horizontal surfaces. To develop the present model for vertical surfaces we assume that the slope of the surface influences diffuse illuminance and diffuse irradiance in the same way. As a consequence of this hypothesis, the luminous efficacy of diffuse solar radiation is assumed to be the same for both horizontal and inclined surfaces.     

Reference luminous solar constant and solar luminance for illuminance calculations

A recently proposed extraterrestrial solar spectrum is used to derive reference values of the luminous solar constant. Two slightly different photopic curves, CIE’s V(λ) and V_M(λ), are used for the needed convolution with the spectrum. A historical perspective about these two curves is presented, with the conclusion that both curves can be useful, depending on application. Two reference values (one for each photopic curve) for the luminous solar constant, the Sun’s luminous efficacy and the average Sun’s luminance are proposed, consistent with the most recent determination of the solar constant. Comparisons are made with earlier determinations of these quantities, as well as of the average Sun’s radiance and the Sun’s effective temperature.     

Impact of window blinds on daylight-linked dimming and automatic on/off lighting controls

This paper presents the field-measured performance of two commercial photocontrolled lighting systems, continuous dimming and automatic on/off, as a function of various configurations of manual and photocontrolled automatic venetian blinds. The performance indicators considered were the space illumination and the electric lighting consumption as a function of blind slat angle and retraction area. The results showed that under clear sky and without blinds both lighting control systems reduced the lighting energy consumption on average by 50–60% when compared to lights fully on from 6 AM to 6 PM. These savings, however, dropped by 5–45% for the dimming system, and by 5–80% for the automatic on/off system with the introduction of various static window blind configurations. The savings in lighting energy were more significant when the lighting control systems were used with photocontrolled blinds. This was due to the capability of the blinds to adjust their position automatically in direct response to the variable daylight levels.     

Passive solar systems for heating, daylighting and ventilation for rooms without an equator-facing facade

Spaces without northerly orientations have an impact on the ‘energy behaviour’ of a building. This paper outlines possible energy savings and better performance achieved by different zenithal solar passive strategies (skylights, roof monitors and clerestory roof windows) and element arrangements across the roof in zones of cold to temperate climates typical of the central and central-southern Argentina. Analyses were undertaken considering daylighting, thermal and ventilation performances of the different strategies. The results indicate that heating, ventilation and lighting loads in spaces without an equator-facing facade can be significantly reduced by implementing solar passive strategies. In the thermal aspect, the solar saving fraction reached for the different strategies were averaged 43.16% for clerestories, 41.4% for roof monitors and 38.86% for skylights for a glass area of 9% to the floor area. The results also indicate average illuminance levels above 500 lux for the different clerestory and monitor arrangements, uniformity ratios of 0.66–0.82 for the most distributed arrangements and daylighting factors between 11.78 and 20.30% for clear sky conditions, depending on the strategy. In addition, minimum air changes rates of 4 were reached for the most extreme conditions.     

Photovoltaic product form and importance of colour

Abstract

Uptake of photovoltaic (PV) devices is increasing rapidly despite difficult economic conditions. The majority of this uptake consist of rigid, rectangular, glass encapsulated cSi modules, which have been retro fitted to roofs. As the PV market grows, more varied products will be developed for a wider range of end uses (both indoor and outdoor). There is therefore a need to consider how product form (e.g. colour, transparency, shape, integration) may be influenced by PV end use. This paper considers these issues for dye sensitised solar cells (DSC). Thus, while DSC devices do not exhibit the power conversion efficiencies of cSi PV, this paper considers how using different dyes either alone or in combination by co-sensitisation can produce a huge range of colours, which is a consistently important customer issue depending on the final end use.     

An analysis of geometrical shapes for PV module glass encapsulation

In this work we simulate structured-surface covers of photovoltaic modules with different patterns in order to compare the optical performance (light trapping) of these patterns at different conditions. To achieve this, we use a new software application with an optical calculus engine. Although the primarily reflected light represents a small percentage of the total incoming energy at normal incidence and some part of that fraction will be lost due to Fresnel and absorption losses, there is a substantial increase of the final energy reaching the photocell, specially for large angles of light incidence. In particular, we analyze two different covers: an abrupt-shaped pattern of triangular form and a smoothed-shaped pattern of sinusoidal form in order to evaluate and compare their light transmission properties. We study both structures by varying their geometrical parameters, and also by considering different angles of the incoming light, and different absorption coefficients of the materials. We conclude that the use of these types of covers can improve the performance of regular photocells molding these structures in their encapsulation.     

Low cost data acquisition module for evaluating the quantitative performance of daylight systems

The search for efficient, auto-sustainable constructions that allows the user a contact with the outer environment has stimulated the development of advanced strategies, in various devices, for the exploitation of the daylight. A low cost data acquisition system was developed in this study, to observe the distribution of the natural light inside a prototype and to evaluate the quantitative performance for redirecting systems. The luminous sensor is a light dependent resistor, that responds to the illuminance with a reduction in the resistance when illuminated, through a log–log dependence. Calibration curves are set up to relate the change of resistance to absolute illuminance. It therefore provides a continuous investigation of the illuminance for various sampled points in the interior test space with a 0.03% digital error due to the 12-bit resolution. The final measured error of 5% is mainly due to the system calibration and resistance memory history. The circuit connects to a standard parallel port of any Personal Computer and supplies 64 analog inputs, one for each luminous sensor. The circuit can be easily modified to attend different quantities of analog inputs or communication ports.