In moderate climatic regions, such as Germany, the buildings are exposed to the relatively cold winters but very warm summers. An optimal glazing system should have relatively high total solar transmittance in winter to enhance passive solar energy utilisation, but in summer a significant decrease in transmission in the solar infrared region is desirable to avoid overheating or high cooling loads. Currently no such superior glazing systems are commonly available on the market. However new technologies of electrochromic or thermochromic glazing are emerging. These glazing types are capable of changing their optical properties in order to control solar radiant heat flux into the building.
This study was commissioned by the Frauenhofer Institute for Solar Energy Systems, Freiburg, Germany who designed alternations to the central atria in the Rhoen Klinik, a neurological clinic in Bad Neustadt. Currently air temperatures in the central atria are too high during summer. Also the system of natural ventilation through stack effect from doors held open continuously, causes very uncomfortable draughts in occupied areas. The proposed alternation concentrates mainly on changing the atrium's glazing systems. It was decided to use the computer simulation to help select the optimal glazing system and predict effect of the new natural ventilation strategy including openings at ground level.
The computer model of the central atria and four adjoin shading wings was created from geometrical and material data provided by the client (see Figure 1). Of particular interest in the Rhoen Klinik are the glazing systems. The main optical characteristics of the current glazing as well as the alternative glazing are indicated in Table 1. In respect to the novel thermochromic glazing system, the parameter called critical temperature is very important. We have modelled a set of optimistic and pessimistic optical properties for glazing with a critical temperature of 20 ° C and 30 ° C. This glazing temperature determines when the optical properties are being changed - switched.
Figure 1. Model of the central atria in Rhoen-Klinik viewed from Northeast.
| Glazing | Direct transmission | U-value |
|---|---|---|
| (-) | (W.m-2.K-1) | |
| Current glazing | 0.29 | 1.76 |
| Sunprotective, wall | 0.30 | 1.20 |
| Sunprotective, roof | 0.29 | 1.20 |
| Thermochromic, wall | 0.41 | 1.25 |
| Thermochromic, roof | 0.37 | 1.24 |
| Thermochromic, wall | 0.10* | 1.25* |
| Thermochromic, roof | 0.03* | 1.24* |
Table 1. Main optical and thermal characteristics of the glazing systems.
The natural ventilation created by the stack effect plays an important role during summer conditions in the atrium. A multi-zone mass flow network was used to model this feature. In the atrium, natural ventilation takes place via entrance and balcony doors as well as through openings located in the atrium roof.
For this simulated case, the client kindly provided hourly climate data in the form of A Test Reference Year for Wuerzburg (latitude 49.80° N; , longitude 9.90° E; and altitude 256 m). It was decided to carry out simulations over 7 warm days in July. Figure 2. illustrates air temperature frequency distribution for the case with sunprotective glazing. For the comparison Figure 3. illustrates the same temperature statistic for the thermochromic glazing with a critical temperature of 20 ° C.
Figure 2. Atria air temperature statistic for sunprotective glazing.
Figure 3. Atria air temperature statistic for thermochromic glazing (critical temperature of 20°C).
It was found that in the terms of reducing top (4th) floor and overall atrium air temperatures the thermochromic glazing (20 ° C, optimistic) has the best performance followed by other thermochromic glazing. The client has been provided with results (see Figure 2 and 3) for all considered alternative glazing. We believe that this information is adequate to select the optimal glazing system for the atria retrofit.
During the simulation study of glazing systems, the client initiated another study to investigate the influence of a new natural ventilation system comprising additional airflaps in the top and the bottom of the atrium to avoid usage of entrance and balcony doors, which create severe draught. The new four ducts of 1 m² cross sectional area are introduced at the basement level to supply outdoor air (see Figure 4).
Figure 4. Sketch of the new natural ventilation system at the basement level.
The client requested that the following four alternatives be simulated and the atrium air temperature frequency distribution predicted:
Simulation results indicate that the current glazing will outperform sunprotective glazing in terms of the reduction in summer overheating . This could be mainly attributed to the fact that both glazings have a similar total solar heat coefficient (0.33 vs. 0.39) but quite different U-values (1.76 vs. 1.2). Therfore, sunprotective glazing has a higher insulation effect. It was also found that opening 4th floor windows will have a significant effect on summer conditions in the central atria (see Figure 5).
Figure 5. Atrium air temperature statistic for current glazing with opened and closed windows (4th floor only).
