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...]

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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....