Three different types of glazings with various U-values and transmittances are studied in this paper: one "standard” clear double-glazed window (clear DG), one low-e coated double-glazed window (Low-e DG) and one low-e plus AR-coated double-glazed window (AR Low-e DG), Table 1.

The energy simulation tool ParaSol v2 was used to simulate the monthly average direct and total solar energy transmittance (Ts0| and g-value) as well as the annual energy demand. The solar transmittance, Tsol is the transmittance of the glazing for the entire solar spectrum. ParaSol simulates the monthly average value of Tsol, taking into consideration the actual climate and solar angles. The g-value (total solar energy transmittance) is the solar transmittance plus the absorbed heat in the window panes emitted to the inside. Consideration of potential overheating problems was done by looking at the number of overheating hours. ParaSol simulates a room module with only one wall and one window that abuts to the outside climate. The other three walls, floor and ceiling abuts to other rooms with the same indoor temperature i. e. adiabatic walls. ParaSol is a freeware developed by The Division of Energy and Building Design, Lund University (Bulow-Hube H. and Wall M). It is available at (http://www. parasol. se).

Each glazing type was studied in three different Nordic climates: Copenhagen (DK), Stockholm (SWE) and Helsinki (FIN). The Parasol simulations were also done for all 4 major directions (N, E, S, W) and the average was then taken from these four simulations. The ParaSol simulations were used to evaluate the potential of energy savings by using AR — coating on a low-e DG.

The room size investigated was 20 m2 (L x W x H = 5.0 m x 4.0 m x 2.7 m) and can be regarded as a typical living room. The temperature set point was 20°C for heating. No consideration of the cooling demand was done in this paper, since air-conditioning is not common in residential buildings in Scandinavia. The ventilation rate was set to a constant value of 0.6 ach. The internal heat load was 5 W/m2 both day and night. The glass area was assumed to be 70% of the window area, or approximately 2.1 m2. See Table 2.

Table 2 Input data used in the simulations of the room.

Exterior wall U-value

Room size L x W x H

Window measurement incl. frame

Clear DG U-value excl. frame / incl. frame

Low-e DG U-value excl. frame / incl. frame

AR Low-e DG U-value excl. frame / incl. frame

Ventilation (00-24)

Internal heating load (00-24)

To simulate the daylight availability we used Rayfront v1.04. Rayfront is a user interface to the lighting simulation software Radiance which is the industry standard raytracing engine for physically correct lighting simulations.

The Rayfront simulations were performed to obtain the daylight factor, which is a measure of the ratio between the interior illuminance and the exterior illuminance from an unobstructed overcast sky, see equation 1. The daylight factor was calculated 0.8 m above the floor level. Since the daylight factor is independent of orientation and latitude, it was only studied for one location (Stockholm) and one orientation. The simulations were made for the standard CIE overcast sky with the reference illuminance value of 13826.73 lux (default value in Rayfront) for the 21st of June.

The daylight factor is defined as:

Ei

DF = *100% Eq.1

Eo

Ei= daylight illuminance on indoor working plane

Eo= simultaneous outdoor daylight illuminance on a horizontal plane from an obstructed hemisphere of overcast sky

In Rayfront the transmittance of the window is given by the transmissivity parameter. The transmissivity is calculated from the light transmittance of the window according to the following formulas:

The total light transmittance of the window with two panes (index 1 and 2) was obtained from:

1 — Rsi* Ris2

Where index 1 and 2 refer to the transmittance T and reflectance R of the two panes. The transmissivity was then calculated from the light transmittance in Eq.2:

vis

Equation 3 was found in the Radiance manual. The equation recalculates the light transmittance to transmissivity because Radiance uses the transmissivity parameter instead of the transmittance.

The properties of the different panes with and without coatings are described in Table 3.