Daylighting Calculations
Daysim 2.1
ABS 731 Spring 2006
Daysim 2.1
• Developed by National Research
Council of Canada, Institute for
Research Construction
• Available for free download from:
http://irc.nrccnrc.gc.ca/ie/lighting/daylight/daysi
m_e.html
Key Points
• DAYSIM is an expert daylighting
analysis software that offers a
comparative analysis of the annual
amount of daylight available in
arbitrary buildings as well as the
lighting energy performance
of automated lighting controls
(occupancy sensors, photocells)
compared to standard on/off
switches.
C:\DAYSIM\html\overview.html (html manual)
Key Points
• Combines the backward ray tracing
software Radiance, developed by
Greg Ward at the Lawrence Berkeley
National Laboratory, with a daylight
coefficients approach.
• The underlying sky model to calculate
annual illuminance profiles is the Perez
all weather sky model.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html
Key Points
• Indoor illuminances based on hourly
mean direct and diffuse irradiance
values.
• Annual Illuminance Profiles are
coupled with user occupancy data to
predict the annual use of electric
lighting in a building zone depending
on the lighting and blind control
strategy.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html
Key Points
• Underlying manual lighting control
model LIGHTSWITCH is based on
monitored occupancy behavior in
several field studies.
• The method is especially suitable for
people who already have experience
in simulating with RADIANCE as
DAYSIM requires the same input files
and simulation parameters as
RADIANCE.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html
Key Points
• Download the tutorial and case study files
from the download site.
• This overview is from those materials!
• The manual and help files can be accessed
from the Daysim menus.
• DAYSIM includes Radiance software
(http://radsite.lbl.gov/radiance) developed
by the Lawrence Berkeley National
Laboratory (http://www.lbl.gov).
• The JAVA 2 Runtime Environment has been
developed by Sun Microsystems, Inc.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html;
C:\DAYSIM\html\acknowledgment.html (html manual)
Daylight Automony
• The daylight autonomy at a point in a
building is defined as the percentage
of occupied hours per year, when the
minimum illuminance level can be
maintained by daylight alone. In
contrast to the more commonly used
daylight factor, the daylight
autonomy considers all sky conditions
throughout the year.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Daylight Automony Example
• A daylight autonomy of 70% for a work
place with working hours on weekdays
between 8 a.m. and 6 p.m. and a
minimum illuminance levels of 500 lux
implies that the occupant can–in
principle – work 70% of the year by
daylight alone.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Daylight Automony Limitations
• The daylight autonomy is a relatively new
daylight performance indicator and no
recommended performance ranges have
been established as of yet.
• The main advantage of the daylight
autonomy over the daylight factor is that it
takes facade orientation and user
occupancy profiles into account and
considers all possible sky conditions
throughout the year.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Daylight Automony Limitations
• The daylight autonomy characterizes
the daylighting potential of a space.
• As it is independent of the installed
electric lighting power density and
lighting control, a high daylight
autonomy is a necessary requirement
but not a guarantee for lighting
energy savings due to daylight.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Using Daylight Simulation Tools
• Generally advisable if a design:
– makes extensive use of daylight,
– involves non-ordinary material surfaces such as
glazed blinds, translucent materials, etc.
– features a complicated shading situation due to
surrounding buildings or landscape,
– includes automated lighting/shading controls
– includes innovative daylight elements and/or
– requires a careful management of solar gains
due to a reduced HVAC system.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Using Daylight Simulation Tools
• The description of a building for a
daylight simulation requires a three
dimensional model of the building
which contains information on the
geometry of the building and its
surroundings as well as optical
properties of the all material surfaces.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Using Daylight Simulation Tools
• Generally, you should only provide the
geometric detail that your analysis
requires.
• 3-dimensional model preparation
tends to be the most time-consuming
part of a daylight simulation.
• Only simulate what you have to and
recycle whatever you can from
existing models (if they exist).
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Climate Data
• To describe the annual amount of daylight
available inside a building, you first need to
know the amount of solar radiation at the
building site over the course of the year.
• This kind of information is usually provided in
the form of test reference years (TRY).
• TRYs provide typical annual profiles of
exterior climate data such as ambient
temperature, wind direction and velocity,
precipitation and direct and diffuse
irradiances.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Climate Data
• A free source of TRYs is the US
Department of Energy’s site at
http://www.eere.energy.gov/buildings
/energyplus/weatherdata.html.
• The site provides hourly climate data
for over 660 locations worldwide in the
so-called EPW format.
• Daysim directly imports EPW files and
extracts the information required for
an annual daylight simulation.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Simulation Model
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Daysim Subprograms
C:\DAYSIM\html\subprogram_index.html (HTML manual)
Radiosity
• Radiosity was originally developed to solve
problems involving radiative heat transfer in various
forms between surfaces based on form factors.
• Since the 80’s it is also applied to computer
graphics to calculate illuminance levels due to
artificial lighting or daylight.
• A form-factor defines the fraction of energy leaving
a given surface to that which arrives at a second
surface directly.
• In radiosity each surface is treated like a perfectly
diffuse reflector with a constant luminance so that
the radiation exchange between two surfaces can
be described by a single number which depends
on the reflective properties of the surfaces and the
scene geometry.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Raytracing
• The idea behind (backward) raytracing is to
simulate individual light rays in space to calculate
the luminous distribution in a room from a given
viewpoint.
• Therefore, rays are emitted from the point of
interest and traced backwardly until they either hit
a light source or another object.
• In the former case the luminance distribution
function of the light source determines the
luminance contribution at the view point.
• If a ray hits an object other than a light source, the
luminance of the object needs to be calculated by
secondary rays which are emitting from the object.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Raytracing vs Radiosity
• An advantage of radiosity compared to
raytracing is that it requires less calculation
times for straightforward geometries which
do not contain too many surface elements.
This advantage of radiosity diminishes with
rising model complexity.
• A radiosity calculation yields the total
luminance distribution in a room
independent of the point of view of the
spectator.
• A walk-through a scene can be faster
realized with radiosity than with raytracing
as each new viewpoint requires a new
raytracing run.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Raytracing vs Radiosity
• A decisive advantage of raytracing over
radiosity is that only the former approach is
able to simulate specular and partly
specular materials. This aspect is less crucial
if only visual impressions of a given scene
are desired, but if physically correct results
are needed only raytracing based methods
can succeed as most real surfaces exhibit
specular components.
• Some daylighting elements including blinds,
light-shelves or prisms exhibit extremely nondiffuse surface properties and their correct
modeling is crucial as all incoming daylight
passes through them.
http://irc.nrc-cnrc.gc.ca/ie/lighting/daylight/daysim_e.html (tutorial)
Radiance
• RADIANCE is a physically based, backward
raytracing rendering tool that has been
developed by Greg Ward at Lawrence
Berkeley National Laboratory.
• RADIANCE is able to predict internal
illuminance and luminance distributions in
complex buildings under arbitrary sky
conditions.
• RADIANCE uses raytracing in a recursive
evaluation of the luminance integral in a
room.
Radiance Resources
• The Rendering with Radiance book written
by Ward and Shakespeare is an excellent
source of further information which may
serve as both, an introduction into daylight
simulations with RADIANCE as well as a
reference guide for detailed descriptions of
the underlying simulation algorithms.
• Another valuable source of advice is the
Radiance online mailing list at
http://www.radiance-online.org/.
What is a simulation parameter?
• RADIANCE is a backward raytracer, i.e. light
paths are traced backward from the
spectator’s eye to the light sources. In
principle, forward raytracing could be
employed just the same, but for a great
number of scenes the former approach is
more economical considering the required
calculation times.
• The Radiance simulation parameters are a
set of parameters that can be individually
set for each simulation.
What is a simulation parameter?
• The parameters guide Radiance how to carry out a
simulation. The most intuitive parameter is the
number of “ambient bounce” (ab).
• The parameter instructs Radiance how many
surfaces a ray can bounce of or transmit through
before it is discarded by Radiance.
• A detailed description of all simulation parameters
for the Radiance program “rtrace” can be found
under
http://radsite.lbl.gov/radiance/man_html/rtrace.1.
html.
Scene Complexity
Annual Illumination Profiles
• RADIANCE has been primarily
developed to simulate luminances
and illuminances under selected sky
conditions.
• DAYSIM uses the RADIANCE simulation
algorithms to efficiently calculate
illuminance distributions under all
appearing sky conditions in a year.
Daysim & Radiance
• In order to calculate annual illuminance
profiles, one could in principle also use the
standard Radiance programs and start
thousands of individual raytracing runs for
all sky conditions of the year.
• This approach is not practical as a
Radiance simulation for a single sky
condition can take hours so that an hourly
annual simulation would literally require
years of calculation time. To keep
simulation times short,
• Daysim uses the Radiance algorithm
coupled with a daylight coefficient
approach.
Daylight Coefficients
• A daylight coefficient is not to be confused
with a daylight factor. The concept of
daylight coefficients was originally
proposed by Tregenza as a method to
calculate indoor illuminance levels due to
daylight under arbitrary sky conditions.
• The underlying idea is to theoretically divide
the celestial hemisphere into disjoint sky
patches.
• Afterwards the contribution to the total
illuminance at a point in a building is
calculated for each sky patch individually.
Daylight Coefficients
• The key advantage of using a daylight
coefficient approach is that once the
daylight coefficients for all segments
of the sky have been calculated for a
reference point, the illuminance or
luminance at the reference point can
be calculated within seconds for any
possible sky condition by combing the
daylight coefficients with the luminous
distribution of the sky.
Tregenza P R, Waters I M, Daylight Coefficients, Lighting Research & Technology
15(2), 65-71, 1983.
Creating & Converting 3D
Models
• Depending on what CAD modeler
you are using, you will either have to
export your model into a 3rd party file
format – such as 3ds and then convert
the 3ds file into Radiance/Daysim or
you might be able to export the
model directly into Radiance format.
Creating & Converting 3D
Models
Example Run
• Office building from Sketch-up
example overview of program.
• Student teams do the run!
List of References (from manual)
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Reinhart C F, Voss K, “Monitoring Manual Control of Electric Lighting and
Blinds.” Lighting Research & Technology 35:3, pp. 243-260, 2003
Abromeit A, Entwicklung eines architekturbezogenen Planungswerkzeuges
zur Bestimung der Tageslichtautonomie in Gebäuden, Master Thesis, Faculty
of Architecture, Technical University of Karlsruhe, Germany (2002), For a copy
of the thesis, please contact the author at arne@fbta.uni-karlsruhe.de.
Walkenhorst O, Luther J, Reinhart C F, Timmer J, “Dynamic Annual Daylight
Simulations based on One-hour and One-minute Means of Irradiance Data.”
Solar Energy 72:5 pp. 385-395, 2002.
Reinhart C F, Walkenhorst O, ”Dynamic RADIANCE-based Daylight Simulations
for a full-scale Test Office with outer Venetian Blinds”, Energy & Buildings, Vol.
33 pp. 683-697, 2001
Reinhart C F , Daylight Availability and Manual Lighting Control in Office
Buildings – Simulation Studies and Analysis of Measurements, Ph.D. thesis at
the Faculty of Architecture of the Technical University of Karlsruhe, Germany,
2001
Reinhart C F, Herkel H, “The Simulation of Annual Daylight Illuminance
Distributions- A state of the art comparison of six RADIANCE based methods”,
Energy & Buildings, Vol. 32 pp. 167-187, 2000
Reinhart C F , “LIGHTSWITCH 2002: A Model for Manual Control of Electric
Lighting and Blinds”, submitted to Solar Energy, December 2003