Window Model

Overview

Windows and glass doors are simulated as embedded objects in their host surface. The window model computes two parallel effects: the heat transmission (transmission heat loss) and the solar transmission through the glazing system. The data basis is the assigned window component and its glazing system.

Heat transmission

The heat flow between the two adjacent sides (zone/outdoor climate) is computed as a parallel connection of glazing, frame and muntins:

• Glazing — in the “Simple” model type via the U-value of the glazing system (ThermalTransmittance [W/m²K]) applied to the glass area; the surface heat transfer resistances of both sides come from the boundary conditions of the host surface.
• Frame and muntins — if defined in the window component, each via material thermal conductivity and thickness: $\alpha = \lambda/d$, in series with the surface heat transfer resistances on both sides, multiplied by the frame or muntin area.

The glass area results from the window area minus the frame and muntin area. If one side is not connected (no outdoor climate/no zone), the window behaves adiabatically. Result quantities: FluxHeatConductionA/B [W] and the surface temperatures SurfaceTemperatureA/B [C] (glass surface).

Solar gains with angle-dependent SHGC

For the incident radiation, the climate load model provides the direct, isotropic diffuse and horizon-brightening components together with the angle of incidence — already reduced by the pre-computed shading factors of the surroundings (result quantities DirectShadingFactor, DiffuseShadingFactor, HorizonShadingFactor, combined as ExternalShadingFactor).

The transmitted solar flux through the glass area is computed in the “Simple” model type as

$$q_{\mathrm{solar}} = \mathrm{SHGC}(\theta) \cdot q_{\mathrm{direct}} \;+\; \mathrm{SHGC}_{\mathrm{hemis}} \cdot q_{\mathrm{diffuse}}$$

where $\mathrm{SHGC}(\theta)$ is the angle-dependent solar heat gain coefficient from the SHGC curve of the glazing system (evaluated at the current angle of incidence $\theta$) and $\mathrm{SHGC}_{\mathrm{hemis}}$ is the hemispherical value for diffuse radiation. The currently effective value is available as the output quantity SHGCValue, the angle of incidence as IncidenceAngle.

The transmitted gains are then distributed to the room air node and interior surfaces via the solar distribution model.

Solar shading (window shading model)

In addition to the surroundings shading, the window can have solar shading (assign solar shading). Three model types:

• Constant — fixed reduction factor $z$ (remaining share of the solar gains when the shading is closed).
• Pre-computed — time-dependent reduction factor from a schedule.
• Controlled — the shading control provides a control signal $F_z \in [0,1]$ (radiation-threshold-based with hysteresis between minimum and maximum radiation intensity). The effective factor results in

$$z_{\mathrm{eff}} = 1 - (1 - z)\cdot F_z$$

Direct and diffuse radiation are reduced equally by the factor (assumption: the shade acts on both components equally). The combined effective shading factor is available as the output quantity SolarShadingFactor.