# Design Limitations of Laboratory Solar Chimney for Engineering Education
## 4 Mock-Up Design and Experimental Measurements  
The numerical results revealed that spontaneous airflow would not be established in the analyzed small-scale geometry. Therefore, the next step in the study was to investigate whether airflow can be established if the structure is modified to improve efficiency and enhancers are used. To opt for the initial parameters, analyses were carried out in CAD and CFD software. Based on these and the literature overview presented in the previous sections, the following was determined:  
* The tower is as high as possible with a small relative radius and divergent shape;
* The collector has partitions;
* The roof of the collector is as large as possible, preferably inclined and double-layered;  
Figure 3: Numerical results presented with bold line and analytical results present with dashed line: (**a**) Analysis of mass flow rate, (**b**) Analysis of pressure difference. (**c**) Analysis of speed of air in the chimney. * The inlet of the collector is as low as possible, in order to avoid heat losses and reduce the influence of wind. The slope of the collector roof was chosen according to [23], and is 5\({}^{\circ}\). In AutoCAD software, a realistic layout of the elements was finely tuned based on the previously listed features. The 2D drawings were then transferred to SOLIDWORKS, where the assembly of the parts into a single unit and the general appearance of the would-be mock-up were tested. The created 3D models were used for 3D printing of the parts and forwarded to the Cinema 4D software, where colors and effects were added for a realistic display of the model. Figure 4a,b show the 3D model of the solar chimney power plant in Cinema 4D software. As seen in Figure 4a,b, the solar chimney power plant has the following characteristics: divergent shape of the tower, collector roof divided into eight partitions and slightly inclined, with the collector also layered. As the blade profiles of classic wind turbines, as well as those obtained by the inverse method, are made to serve larger systems [29], they are not suitable for a mock-up of these dimensions. Therefore, the final profile was based on fans for personal computers, specifically the NACA 7407 airfoil. NACA airfoils are marked with several numbers that determine profile features. For four-digit profiles, the first digit indicates the maximum slope as a percentage of the chord length, the second digit indicates the distance of the maximum pitch from the leading edge of the airfoil in tenths of the chord, and the last two digits indicate the maximum airfoil thickness as a percentage of the chord. A specialized online tool [30] was used to generate and modify the NACA 7407 airfoil that was used in the experiment (Figure 4c). Two modifications were made: The cross-sectional surfaces of the blades were increased by flattening the lower curve of the blade by 7% as seen in Figure 5). The number of blades was increased from 7 to 9 to compensate for the loss of the effective surface so that the modified turbine abides the Betz's coefficient as closely as possible. Arbitrary physical measurements were performed to determine an optimal number of blades in accordance with [PERSON]'s law. As the turbine was made on a 3D printer, tolerance fields make it impossible to accurately determine deviations from the curve shown in Figure 5). Due to low wind speeds, nine blades have been shown to best utilize wind energy. After 3D printing the plastic structure and the turbine, the collector was made. The collector base was constructed out of 2.5 cm thick extruded polystyrene (XPS). The plastic structure is supported by this base. The collector roof was constructed out of glass instead of plastic because of the smaller thermal expansion coefficient, but the tower was made of PVC pipe because the temperatures there were expected to be lower. Figure 4: Visualization of the final 3D model. (**a**) Solar chimney power plant model in full.