Sunlight passing through facades (facilities)
In ESP-r direct solar radiation passing through a facade is treated as a vector while diffuse radiation is assumed to arrive at an angle of 57 degrees.
For transparent surfaces solar transmission and absorption characteristics at five angles are taken into account. These characteristics are typically imported into an ESP-r Optics database from reports generated via third party software such as WIS [rev] and Window 6 [rev]. Instances of these optical properties are then held in model zone files for access at simulation time. Solar absorbed within the glass is accounted for in the surface energy balance and if PV cells are associated with a layer conversion to electrical power is also handled.
Where there are multiple optical states these are held separately and can be pointed to as part of a control regime. Some data within WIS or Window 5/6 reports do not match the ESP-r data structure but are retained as documentation.
Bookkeepping for reflections within constructions is also handled as is the impact of solar radiation exiting rooms. These are included in the surface energy balance. There are also reports of solar entering zones, solar absorbed in zones.
For projects where spectro-gonometer measurements are available (5 degree increments across horizontal and vertical axis) these bi-directional scattering distribution function (BSDF) data can be associated with surfaces in ESP-r.
An alternative scheme is call CFC (complex fenistration components) where the solar, optical and long-wave properties of each layer are supplied along with bind attributes and the solution to the optics and transmission is resolved as the simulation progresses. A separate CFC database holds collections of CFC entities.
ESP-r has no concept of facade devices or glazing which alters the direction of the direct solar vector arriving at the model site with normal optical property sets. In this case BSDF or CFC are needed.
ESP-r does not include the concept that glazing might diffuse arriving direct solar radiation.
In ESP-r solar radiation standard transmission values at each layer of the construction at five angles [normal, 40 55 70 80 degrees] are used. Each layer of the construction also has absorption at the five angles and visual transmission at five angles. Reflection values at the inside and outside faces are derived. BSDF data or a CFC definition over-rides this. All materials are assumed to be opaque for long-wave radiation.
Usage cases
In the case of an exterior roller blind (shutter) a simple representation might make use of an alternative optical transmission and absorption properties. But this assumes a non-ventilated air space. For higher resolution the air space could be represented as a thin thermal zone with optional inclusion within a mass flow network to track leakage within the space. Having pass though the thin zone all radiation arriving in the occupied space would be considered diffuse.
For blinds which are embedded between glass the blinds are a material layer of the construction and an alternative optical transmission set would be used to represent the blind in another state. Alternatively blind definitions are included as attributes of CFD.
Blinds or curtains inside rooms could be represented as back-to-back surface pairs which take part in the energy balance. It has proven difficult to find information about the thermophysical and optical properties of curtain materials. In order to approximate local build-up of heat one could define a separate zone for the space between the blind and facade and use an air flow network to manage air exchanges. This level of resolution is rarely employed.
If blinds are implied within an optical transmission set imported from WIS or Window 6 the blind slats have no specific shape. CFC do include a number of geometrical attributes of slats.It is, however also possible to represent blinds as many pairs of back-to-back surfaces floating within a zone. This approach does not scale well but allows the temperature and energy balance at the blind to be reported.
The following Control actions for blinds or curtains are supported:
The zone solver uses the surface temperatures and convective exchanges from all surfaces in the zone. Each surface energy balance includes solar absorbed at the zone and other face. At this point the direct and diffuse portions will have been combined.
The solution of transmission, reflection and absorption characteristics at each layer is based on the following assumptions:
Solar radiation bookkeepping is carried out at the following locations in the model: Total of solar arriving from ambient into each zone, arriving from adjacent zones and the total absorbed within the zone. Each face of each surface includes an absorbed solar term.
Geometric dependencies
In order to carry out shading and insolation calculations all surfaces associated with the zone must be flat and they must conform to the edge-ordering rules. Distances between parallel edges in a polygon of less than 10mm are discouraged. Parent/child surface relationships are not taken into account. Zones can take any arbitrary form as long as they are fully bounded. Openings between zones must be represented by a surface (typically attributed with a fictitious construction with high conductivity and optical transmission). Surfaces floating within the zone bounds (e.g. furniture, fittings, explicit blinds) must be defined as back-to-back surface pairs in order to take part in shortwave and longwave radiant exchanges.
A surface must be marked as transparent and must face the outside to be considered an insolation source.
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