Heat Transfer Through Fenestrations

HEAT TRANSFER THROUGH FENESTRATIONS

The heat energy transfer through fenestration takes place by the following ways (Fig. 1.12).

• The short-wave (<25 μm) solar radiation incident on the fenestration system.

• The part of the incident solar energy is transmitted through opening and eventually absorbed by the room surfaces.

• The part of the incident solar energy is absorbed by the fenestration and reradiated as thermal energy.

• The net long-wave radiation (>2.5 μm wavelength) radiative exchange between fenestration surroundings.

• The conductive and convective heat transfer caused by inside/outside temperature difference.

Total heat transfer

The total heat transfer through fenestration consists of two main components:

Q_solar = heat transfer from solar radiation.

This is always a positive number.

Q_thermal = heat transfer between indoor and outdoor air.

This is positive or negative depending on temperature.

The total heat transfer through fenestration

Q_total = Q_solar +  Q_thermal

The first component viz solar heat gain, is present only during the day when the fenestration (or) building openings are exposed to the solar radiation and it is therefore related to the intensity of that radiation.

The second component, the transmission gain, occurs whenever the temperature difference exits, whether the sun is shining or not, i.e, even during cloudy (or) rainy days.