Radiation which strikes a surface has just three options:
- radiation is reflected
- radiation is absorbed
- radiation is transmitted
The sum of these three is always 100%. Absorption and transmittance help to increase the g-value as both of these mean that sun or heat radiation enter the room. If a sunblind has low reflectance, the solar radiation will either get caught (be absorbed) in the material and converted into heat which is emitted to the room, or it will be transmitted and then heat up the room once it has been absorbed by the floor, for example. Generally, energy can never disappear. It is merely converted into heat sooner or later.
Once the sun is in the room, the heat is in there as well – or is it?
Internal sun blinds are effective as long as they are highly reflective for both short-wave and long-wave radiation. Short-wave radiation passes through the window without changing wavelength. When radiation strikes the highly reflective surface, it is not absorbed (as with an ordinary textile curtain) but is reflected; that is to say, the radiation wavelength is not changed here either. The reflecting radiation can then pass back out through the window – and hence it does not help to increase the temperature in the room.
If, on the other hand, the sunblind is a standard textile curtain, most of the short-wave radiation will be absorbed by the curtain (the part that is not transmitted), resulting in an increase of the temperature of the curtain. The curtain then starts to give off heat in the form of long-wave radiation.
The sun heats objects – the objects heat the air
Only when the rays of the sun are absorbed by the floor, walls or other objects in the room is heat formed. The short-wave radiation passes through the air and is absorbed by the floor, for example. The short-wave radiation contains a lot of energy and so the floor is heated up; and as the temperature of the floor rises, the excess heat is emitted from the floor as long-wave heat radiation. This heat radiation is what makes the temperature rise in the room. Window glass allows only short-wave radiation to pass through. Heat radiation is not transmitted: it is reflected back into the room or absorbed by the glass and emitted as heat (both indoors and out, ratio depending on the temperature difference indoors and out). The temperature in the room increases as the heat is not released; this is what is known as the greenhouse effect.
The g-value indicates the total energy transmission of sun rays through a window or a combination of a window and sunblind. Energy can enter the room either by direct transmission (designated Tsol or primary transmission) or by first being absorbed by the window (or a sunblind) and is then given off as heat radiation. Thus the g-value is the sum of direct transmission and the heat radiation emitted to the room after first having been absorbed.
It is not possible to specify a constant g-value for a particular sunblind. According to the standard, the sunblind is always measured together with a specific window, and so the value changes depending on which window is used. The season – i.e. the height of the sun – also has an influence. The rays strike the window at different angles depending on the height of the sun. The higher the angle of the sun, the greater the reflection in the window, which means that less radiation passes through the window, giving a lower g-value. In winter, the opposite is true; lower reflection in the window due to a lower sun angle means that a higher proportion of the rays enter through the window, thereby giving a higher g-value. The g and T-values specified in the product data sheets are for the winter criteria; that is to say, a worst-case scenario.
Energy is saved for both cooling and heating
Just as our Solar control collection reflects solar radiation back through the window in summer, it can also be used in winter to keep the heat in the room.
The unique surface of the aluminum means that our solar control can also reflect long-wave heat radiation and so prevent heat losses through the window. In winter it is therefore a good idea during evenings and at weekends to pull down the solar control; this keeps the heat in the room and helps keep heating costs down.