Daylight simulation: validation, sky models and daylight coefficients

The application of lighting simulation techniques for daylight illuminance modelling in architectural spaces is described in this thesis. The prediction tool used for all the work described here is the Radiance lighting simulation system. An overview of the features and capabilities of the Radiance system is presented. Daylight simulation using the Radiance system is described in some detail. The relation between physical quantities and the lighting simulation parameters is made clear in a series of progressively more complex examples. Effective use of the interreflection calculation is described. The illuminance calculation is validated under real sky conditions for a full-size office space. The simulation model used sky luminance patterns that were based directly on measurements. Internal illuminance predictions are compared with measurements for 754 skies that cover a wide range of naturally occurring conditions. The processing of the sky luminance measurements for the lighting simulation is described. The accuracy of the illuminance predictions is shown to be, in the main, comparable with the accuracy of the model input data. There were a number of predictions with low accuracy. Evidence is presented to show that these result from imprecision in the model specification - such as, uncertainty of the circumsolar luminance - rather than the prediction algorithms themselves. Procedures to visualise and reduce illuminance and lighting-related data are presented. The ability of sky models to reproduce measured sky luminance patterns for the purpose of predicting internal illuminance is investigated. Four sky models and two sky models blends are assessed. Predictions of internal illuminance using sky models/blends are compared against those using measured sky luminance patterns. The sky model blends and the Perez All-weather model are shown to perform comparably well. Illuminance predictions using measured skies however were invariably better than those using sky models/blends. Several formulations of the daylight coefficient approach for predicting time varying illuminances are presented. Radiance is used to predict the daylight coefficients from which internal illuminances are derived. The form and magnitude of the daylight coefficients are related to the scene geometry and the discretisation scheme. Internal illuminances are derived for four daylight coefficient formulations based on the measured luminance patterns for the 754 skies. For the best of the formulations, the accuracy of the daylight coefficient derived illuminances is shown to be comparable to that using the standard Radiance calculation method. The use of the daylight coefficient approach to both accurately and efficiently predict hourly internal daylight illuminance levels for an entire year is described. Daylight coefficients are invariant to building orientation for a fixed building configuration. This property of daylight coefficients is exploited to yield hourly internal illuminances for a full year as a function of building orientation. Visual data analysis techniques are used to display and process the massive number of derived illuminances

Editorial: Evaluating research: Do we know what ‘works’?

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Aperture-based daylight modelling: Introducing the ‘View Lumen’

This paper presents a new way to determine measures of view at the building aperture. It introduces the concept of the view lumen – this is the illumination effect received at the building aperture from a visible external entity (e.g. ground, sky, obstruction, etc.) which is made self-luminous for this purpose. The paper describes the fundamental principle behind the view lumen, and gives some preliminary illustrations of the technique to show its potential. The examples employ the British Standard framework to define categories of view. That is, three layers named upper, middle and lower comprising, respectively, sky, natural or man-made objects (e.g. buildings) and ground. The proposal is an extension of the recently introduced sunlight beam inde

Rethinking daylighting and compliance

Daylight in buildings is the natural illumination experienced by the occupants of any man-made construction with openings to the outside. Our attempts to formulate some measure of daylight provision in buildings can be traced back over a century, and the daylight factor as we know it today is over 50 years old. Still the most common measure found in guidelines and recommendations worldwide, the daylight factor is used routinely and, it is fair to say, often rather uncritically. The consideration of daylight in buildings has received a new impetus from the accumulation of evidence on the wider benefits of daylight exposure. But it is continuing to prove difficult to advance beyond daylight factors towards a more realistic quantification of daylighting performance that would allow us to accommodate these new considerations in an evaluative schema. This paper examines the basis of current practice with respect to daylight evaluation, and suggests a few ways in which it can be improved with relatively modest additional effort. The paper also critiques some of the recent attempts to advance daylight evaluation by incremental means using so-called “clear sky options”

Climate-based daylight modelling and its discontents

In 2013 the UK Education Funding Agency (EFA) made climate-based daylight modelling (CBDM) a mandatory requirement for the evaluation of designs submitted for the Priority Schools Building Programme (PSBP). School designs submitted to the PSBP must achieve certain ‘target’ criteria for the useful daylight illuminance metric. This is believed to be the first major upgrade to mandatory daylight requirements since the introduction of the daylight factor more than half a century ago. In the US, a climate-based daylight metric approved by the IESNA has appeared in the latest version of LEED. Perceived as long overdue in some quarters, in others the EFA decision was seen as controversial and is not without its critics. Whilst it may appear that the case for CBDM has effectively been made, and that wider adoption in standards and guide- lines is likely, it is important not to ignore or dismiss out-of-hand the critics of CBDM. Nor should it be overlooked that CBDM and the metrics derived using it are both still evolving. This paper: reviews the recent developments; the reactions to them; and, forecasts what might be expected in the near future. Attention is given to the formulation of the PSBP requirements for daylight and how the various stakeholders have responded to this major new development in building codes

An image-based analysis of solar radiation for urban settings

This paper describes a novel approach for evaluating the total annual/monthly irradiance incident on building facades in urban settings. The analysis is founded on a physically-based rendering approach and uses datavisualisation techniques to generate ‘maps’ (i.e. false-colour images) of annual/monthly irradiance. The irradiance ‘maps’ are derived from hourly time-series data for one year and take accurate account of shading by, and inter-reflection from, other buildings and surfaces. The sun and sky irradiance are evaluated separately. The sky contribution is calculated using realistic, non-isotropic models for the sky radiance distribution. The ‘maps’ can be used to confidently identify facade-locations where there is high irradiance, for example to aid the siting of photo-voltaic panels. The technique can be applied to scenes of arbitrary complexity from a single building to fully ‘worked-up’ city models. The results of the analysis have been linked to a GIS-based solar energy planning system. The system is targeted at city planners and one of its aims is to encourage the consideration of solar energy in the urban planning process

Envelope first / Inside later: Aperture sunlight and skylight indices

This paper describes a back-to-basics rethinking for quantifying the sunlight and skylight potential of building apertures. The recently formulated sunlight beam index (SBI) approach has been conflated with a complementary metric called the aperture skylight index (ASI). The sunlight beam index is a measure of an aperture’s ‘connectedness’ to all of the annually occurring possible sun positions where sunlight can be incident on the aperture. In a complementary fashion, the aperture skylight index is a measure of an aperture’s ‘connectedness’ to the hemispherical sky vault. In the absence of any localised shading/obstructions, the sunlight beam index depends on the location, orientation (i.e. azimuth angle), aspect (i.e. zenith angle) and size of the aperture. The SBI is the cumulative measure of the cross-sectional area of sunbeam that can pass through a window aperture. In contrast, the ASI depends only on the size and aspect of the aperture in addition to shading/obstructions – it has no dimensions of time and it is not dependent on the azimuth orientation of the aperture relative to the hemisphere of sky. Obstruction of the aperture’s ‘connectedness’ to either the sky or the entirety of possible sun positions can be due to: features integral to the facade (e.g. external window reveal); shading devices; or, any external structures. The effect of obstructions, whatever their origin or complexity, is automatically accounted for in the computation of the indices – the only requirement is that they are included in the 3D model of the scene. The approach provides designers and product specifiers with an intuitively simple “aperture rating system” to evaluate and compare the in-situ performance of building apertures in combination with any shading system. Application of the rating system is demonstrated for a residential dwelling

Daylighting buildings: Standards and the needs of the designer

Despite widespread research on daylighting, there are insufficient data to justify a definitive statement on daylighting design criteria. This paper reviews the requirements for daylighting codes and guidelines, doing so from two different viewpoints. The first considers standards and regulations, the second is focused on development and the the scope of climate-based daylight modelling

The sunlight beam index: a new metric to quantify the sunlight potential of arbitrarily complex building apertures

This paper describes a fundamental rethinking of the basis for the evaluation of the sunlight potential of spaces. It aims to provide a methodology to answer the question: how much sunlight can enter a room? The measure proposed is the cross-sectional area of sunlight beam that: (a) passes through a window; and, (b) enters the main volume of the internal space. The new measure – called the sunlight beam index – is described, and examples are given for a realistic residential house. The sunlight beam index (SBI) is determined for all full year on a timestep basis (e.g. 15 minutes) but can be aggregated into monthly or yearly totals. The annual total provides a single measure for: one window; a group of windows; or, all the windows for an entire dwelling. Thus the measure is ideally suited for rating, planning and/or guideline purpose

The sunlight beam index

This paper describes a fundamental rethinking of the basis for the evaluation of the sun- light potential of spaces. It provides a robust methodology to answer the question: how much sunlight can enter a room? The measure proposed is the cross-sectional area of beam sunlight that passes through a window. The new measure { called the sunlight beam index { is described, and examples are given for a realistic residential dwelling. The sunlight beam index is determined for a full year on a time-step basis (e.g. every 15 minutes), but it can be aggregated into monthly or yearly totals. The annual total provides a single measure for: one window; a group of windows; or, all the windows for an entire dwelling