Overview of solar radiation and daylighting in ESP-r
The underlying mathmatics for
deriving the solar radiation arriving at a surface are
straightforward and validation studies have shown the
values are as expected. Normal hourly weather presents the
same constraints for rapidly changing weather patterns.
As an alternative, short term weather data can be imported
and held in so-called temporal files for use at simulation
time.
Sun position
[Describe how the sun position is calculated]
Sky radiation model
[Describe the sky radiation model]
Solar from weather data
[Describe relationship between weather data and solar processing]
Solar radiation at facades
The ESP-r data model and numerical facilities approximate aspects of the
patterns of shade and shadow and insulation distribution we can observe in the
built environment. Some things we observe are less well approximated
and some assessment issues cannot be accommodated. more...
ESP-r models include site location attributes (latitude
and longitude) and all zone surfaces and site details and facade
details are defined within a single 3D coordinate system.
In ESP-r facades are composed of zone surfaces and represent
the glazing, frames, reveals, as well as walls and roofs. Such surfaces can be
of arbitrary shape. more...
Shading calculations are based on the number,
form and composition of solar obstructions associated with
each zone. Obstruction blocks or bodies are included in models
exported to Radiance. more...
Model resolution can be increased by
requesting shading
and/or insolation
calculations to be carried out.
Sunlight falling on facade elements is absorbed based on
the current position of the sun, the surface orientation and the
surface properties.
Sunlight passing through facades
Direct solar radiation passing through facades
is treated as time-dependent vectors while diffuse radiation is assumed
to be at a nominal fixed angle. Solar radiation absorption within
layers of a construction (e.g. glass or blind materials)
is tracked..
As ESP-r does not usually account for specular reflections in rooms
some aspects of light-shelves will not be captured more....
ESP-r can import optical property sets from WIS [ as well
as from Window 5/6. Imported optical properties become part of the
model's optical database as named entities. more...]
Sky and ground interactions
In ESP-r a user defined portion of the sunlight arriving at the
facade is reflected from the ground near the building. This can be
augmented by schedules of snow cover.
Solar distribution within rooms
If no directives have been issued the default assumption is for diffuse
distribution of solar radiation entering the zone from the outside
or adjacent zones. Users can increase the model resolution by
pre-calculating insolation patterns via
the ish module. The patterns
are recorded to a binary access file for each zone which are then
accessed by the simulation engine.
Direct solar radiation become diffuse at the first intersection
and diffuse solar is then iteratively distributed. All reflections
are considered diffuse.
All zone surfaces (including those representing furniture and
fittings) take part in insolation calculations. more...
Where solar radiation falls on a transparent partition
the equivalent diffuse radiation is added to the adjacent
zone at the next timestep (adjusted for the glass transmission).
Daylighting
Natural light offsets artificial lighting [general discussion...]
[discuss options for representing daylighting...]
[discuss method used to determine Lux or glare etc. ...]
Blind or translucent façade elements are treated as [discuss...]
...
Controls
Occupants often need to control light levels. ESP-r offers
so called casual-gain controls which define one or more sensors
and one or more lighting zones and uses this to control a casual
gain in the room. It is also possible to commission Radiance
assessments at each timestep to access light levels.
Use of thermochromic or electrochromic glazing is also a
minor variant of optical controls. The time-delays observed
in reality would not be captured.
Validation
Validation studies indicate good agreement with theory on how
solar positions are calculated as well as the solar radiation absorbed
on surfaces.
Modelling approaches
In ESP-r we might approach architectural situations as follows:
A building in an urban setting with a park on one side: solar obstruction
blocks and rectilinear bodies would be used to represent the form of
adjacent buildings, trees, sidewalks and major land forms. If they were
appropriately attributed they would also provide a useful context within
Radiance views of the model. As an alternative, adjacent buildings might
be represented as additional thermal zones for purposes of visual assessments.
A building on the north slope of a hill: in addition to the above use
of obstructions the topography of the hill would need to be approximated
via collections of solar obstructions in order to constrain the solar
radiation arriving at the facade.
A facade with shading provided by sets of small perforated fins:
although each fin could be defined via a copy transform operation it
would also be necessary to set the opacity of each fin to reflect
the perforations. Large numbers of fins will increase computational
time.
A stone facade with deep set windows: The user needs to decide if the
surfaces of the zone are following the inside or outside face of the
facade. Window reveal obstructions would need to be defined in order
to reduce the solar aperture. Thermal bridge calculations and attributes
would be recommended. It is possible to represent
really thick facades as thermal zones so that the full inside and outside
area are explicitly represented. High heat transfer coefficients would
need to be set to transfer heat between the faces. Such augmented 1D
modelling of 3D forms is an experts task.
Office space facing an atria two levels below a glass roof: the
atria would likely be subdivided and include in a mass flow network.
The solar radiation arriving at the office would be diffuse. Alternatively
the atria could be represented as a CFD domain in which case the atria
is a single zone and an insolation analysis in it will setup the
radiation to pass into the office.
A commercial facade with a deep (150mm) extrusion separating vision panels.
Here the actual exposed area is greater than the projected area. The extrusion
would be observed to intercept some of the solar radiation. This is a
classic 3D issue which is only partially resolved via increased
geometric resolution. There is no specific constraint to the
representation of facade frames explicitly as zones (other than
the difficulty of scaling such an approach). The more usual approach
is to associate the frame with a construction which approximates the
heat flow potential of the actual frame.
A facade with electrochromic glazing for vision panels: this
requires an optical control to be applied.
Text looking for a place...
For visual assessments ground topography can be defined but is not yet recognised by shading utility.
Optical controls for blinds within constructions....
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©Copyright 2017 Energy Systems Research Unit, Glasgow, Scotland. License: GPL V2. Last edited by JWH, 30 Oct 2016