# Experimental Investigation and Optimization of a Glazed Transpired Solar Collector
## 2 Materials and Methods
### Optimization of the GTC Design  
#### 2.1.1 Experimental Set-Up and Protocol  
The GTC configuration considered is presented in Figure 1. The working principle of this GTC is almost identical to that of TSCs. The air is supplied to the bottom part (see Figure 1), passing both through the cavity between the glass layer and the absorber plate and through the orifices of the absorber plate. Finally, the heated air is collected at the top of the GTC, in a plenum, and then extracted through a circular duct (160 mm) by a variable-speed fan (type EBM-Papst). The frame of the GTC was built of the following materials: a wooden structure and OSB (oriented strand board) plates. The space between the OSB plates was filled with thermal insulation--rock wool (thickness of 50 mm). The GTC absorber (2000 mm \(\times\) 1050 mm) was made of a steel plate (thickness of 2 mm), painted black. On the other hand, the main change in the GTC design, compared to previous solar air collectors analyzed at the CAMBI Research Center, was related to the perforations of the absorber. The size, shape, and pitch of the absorber perforations could be important influence factors on the efficiency of transpired solar collectors [52]. [PERSON] et al. [53] reported that about 28% of the air's heat gain is due to the plate perforations. On the other hand, [PERSON] et al. [33] determined, based on experimental data, that the solar collector efficiency is not significantly influenced by reducing the dimensions of the absorber perforations (only a 0.56% increase for decreasing the diameter of the perforations from 20 mm to 10 mm). Consequently, the new proposed  
Figure 1: GTC design details: 1—transpired steel plate; 2—plenum; 3—fan; 4—inner cavity; 5—glazing; 6—air inlet; 7.9—thermal insulation; 8—air duct; 10—air layer between glass and absorber; 11—air outlet. design of the GTC was based on larger and simpler perforations (50 \(\times\) 50 mm) uniformly distributed on the absorber plate (Figure 2). This allows considerably simplifying the manufacture of the GTC and obtaining a low-cost solar air collector model. It is worthwhile to mention that this simplified absorber configuration of the GTC prototype represents one of the major novelties of this study because more complicated absorber solutions (both regarding the perforations and their arrangement) are usually investigated in the literature, as shown above. Consequently, the study of this basic configuration is even more interesting to be able to determine if it allows obtaining satisfactory results. Normal window glass (thickness of 4 mm) was used as the glazing for the GTC (Figure 2). This GTC glazing cover can be mounted, due to the experimental set-up configuration, at variable distances from the absorber plate (30, 50, 70, or 90 mm). Finally, the entire GTC structure was painted in black, using ordinary paint (Figure 2). The different measurements of temperatures within the experimental set-up are carried out using 19 K-type thermocouples (Figure 1):  
* 4 thermocouples on the air GTC inlet (T\({}_{\text{1-4 in}}\));
* 5 thermocouples on the air GTC outlet (T\({}_{\text{1-5 out}}\));
* 9 thermocouples on the GTC absorber plate (T\({}_{\text{1-9p}}\));
* 1 thermocouple on the inner surface of the GTC rear wall (T\({}_{\text{w}}\)). It is worth mentioning that before performing the experimental study and starting the measurements, all temperature sensors mentioned above were calibrated using a thermostatic bath (Lauda RE 1225 S). A flow meter (FlowFinder mk2) was used to determine the airflow through the GTC. The installation of the flow meter is shown in Figure 3. This set-up allows precise and reliable airflow measurements. Figure 2: GTC picture with square perforations on the absorber plate. In addition, meteorological data were logged during the GTC experimental study. Ambient temperature, solar radiation intensity, and wind speed and direction were recorded using the Almemo weather station with different sensors, positioned at the top of the GTC (Figure 2).