Energy performance and performance calculation

The energy performance of buildings with transparent insulation systems can be calculated in a similar manner to other buildings with monthly methods like the one proposed in the European standard EN832 (1998). First, the system parameters have to be determined from product characteristics; then the solar gains must be calculated for each month depending upon orientation; and, third, a utilization factor for these gains must be determined. For the last task, according to the standard, all gains (internal and through windows and solar walls) are summed up. However, due to the different dynamic storage effects, in reality, there should be a difference in calculating gains. The night-time discharging of solar wall heating, in particular, should have much higher utilization than instantaneous gains through windows. However, this can only be proven through dynamic simulation.

System characterization

For solar wall heating and solar insulation, the same methodology can be used for characterizing the wall construction. The U-value can be calculated according to ISO 6946 (EN ISO 6946, 1996). In general, the wall construction is an inhomogeneous building element consisting of wall material, transparent insulation product and frame part. Similar to a window, the solar gains are proportional to a total solar energy transmittance (g-value) of the complete system gSWH - or, in collector terminology, a solar efficiency nSWH. The basis for the determination of this parameter is the thermal resistance network of Figure 9.4.6: the solar energy transmitted is proportional to the g-value of the transparent insulation gTI and the relation of the resistance from the wall surface to the exterior Rtii + R to the total resistance R :

e sys

It is sometimes confusing that there are three different g-values: the one of the building product gB as tested, the one of the TI construction, including absorber colour and air gaps gTI, and the one of the total system, including the wall gSWH. The calculation of the second quantity from the tested product quantities is different for the two types T and O, described in detail in (Platzer, 2000).

Moreover, the g-value for transparent insulation is very much dependent on incidence angle. In order to calculate the monthly effective values for different vertical orientations, only the values gn,B and gh,B for normal and diffuse hemispherical irradiation are needed (Platzer, 2000).

Source: Fachverband TWD e.V. Gundelfingen, Germany

Figure 9.4.6 Thermal resistance network for system consisting of transparent insulation cover and wall (Ri, Re: interior and exterior surface coefficients)

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