Performance of the TI wall heating system

During the heating periods 2002/03 and 2003/04 excellent results were obtained for the realized TI facade. The maximum temperatures of the absorber and of the indoor wall surface were about 75 and 30°C, respectively. During a cold and sunny week in January 2002 heat gain fluxes of up to 50 W/m2 were measured. The evaluation of the optimized TI wall according to van Dijk et al. [6] is illustrated in Fig. 5, showing performance charts, on a weekly and 4-weekly (monthly) basis, obtained from August 2002 until April 2003 (2002/03) and from August 2003 until March 2004 (2003/04). The equivalent U-value (Ueq), which is the ratio of the net heat flux through the wall divided by the temperature difference between indoor and outdoor is plotted versus the degree-day related irradiation, which is the daily sum of irradiation on the wall divided by the degree-day. Only in December 2002 a small monthly heat loss was obtained. Due to relatively high solar irradiation in December 2003, for all months of the heating period 2003/04 monthly heat gains were obtained. The linear regression of the data points yielded a solar energy efficiency of about 43% and a U-value ranging from 0.74 to 0.79 W/(m2K), which are outstanding values compared to data of various TI walls given in [7]. Comparing the TI wall performance of the two successive heating periods it can be concluded, that the air-openings of the TI elements work properly and that no adverse pollution of the TI facade or material

2003/04 (right)

Special attention was given to the evaluation of the hygro-thermal performance of the TI wall system. Although CTA absorbs a high amount of water no adverse condensation phenomena within the TI facade was observed visually during the investigated heating periods. These findings may be attributed to the diffusion-open construction of the TI facade based on timber profiles with minimized material fraction, to the large airgap between glass pane and TI structure and to the fact, that the pre-painted wall was in use two years without transparent insulation, which enabled the wall to get dry.

Fig. 6 shows the weekly averaged values of the ambient and room temperature (top layer), the calculated weekly maximum values of the relative humidity at the glass pane (middle

layer) and the weekly averaged values of the ambient and air gap water vapour pressure (bottom

layer). The maximum weekly averaged values of the room temperature (28°C) were observed in August 2003. These findings are related to the fact, that efficient shading of the TI facade is provided from May to July. In August the shading effect is relatively poor. Furthermore, August 2003 was an unusual warm and sunny month. The calculated weekly maximum values of the relative humidity at the glass pane clearly indicate, that during the whole measuring period never 92% were exceeded. Thus, no adverse condensation was observeable. A detailled analysis showed, that the maximum values of the relative humidity at the glass pane (above 90%) mainly occur during clear and cold nights at the end of cloudy periods with very poor irradiation in winter time (e. g., December 2002). The plot of the weekly averaged ambient water vapour pressure and the water vapour pressure in the air gap of the diffusion-open TI facade indicates, that the moisture exchange with the environment is quite good. Furthermore, it can be seen, that the maximum values of ambient water vapour pressure exceed the vapour pressure in the air gap. This phenomenon can be related to the fact, that the testing field is positioned in the middle of the TI facade element. Humidity entering the TI element at the bottom is absorbed by the cellulose acetate TI structure at the bottom. Whereas in sunny periods the desorption of water vapour from the TI structure is rather quick, in periods without irradiation the water vapour adsorption takes a long time.

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