# Design Limitations of Laboratory Solar Chimney for Engineering Education
## 4 Mock-Up Design and Experimental Measurements
(**b**) Cross-section at the roof-tower junction with emphasis on the location of the wind turbine and generator. (**c**) Isolated and magnified view of the turbine, a modified NACA 7407 airfoil, used in the physical model. As the mock-up would be used primarily for education purposes, the first option for the enhancer was to introduce artificial sunlight that would cause the Greenhouse effect, as illustrated in Figure 6a. Based on the models (Figure 4), the surface area of the mock-up is approximately 0.45 m\({}^{2}\). The value of solar radiation in the Republic of Croatia ranges from 250 to 900 W/m\({}^{2}\), which yields that the equivalent value of source dissipation should be approximately 300 W. This corresponds to a simple halogen reflector. The expected useful power on the surface is approximately 16 W, as the balance in Figure 6b suggests. However, a halogen reflector develops temperatures above 250 \({}^{\circ}\)C, as the thermogram in Figure 7 shows, and most of the radiation is in the infrared part of the spectrum. Therefore, the dissipation value from such reflectors would create lighter air above the chimney, which would prevent buoyancy. Artificial sunlight as a source is not a realistic option due to the limitations of physical dimensions. Additionally, dissipation using a heater was briefly considered, but that would lead to degradation of the base material, which has to be made of lighter materials for transport and, therefore, has a smaller heat capacity. It was decided to utilize two PC fans with speed regulation at the top of the tower, which would regulate the negative pressure and ensure a variable airflow to rotate the solar chimney turbine. PC fans were used because they are cost effective and widely available, which makes the mock-up easier to reproduce. Figure 5: Cross-section (red line) of the modified NACA 7407 profile made in the AirFoil Tools software to fit the mock-up geometry. Figure 6: Visualization of the energy balance of a solar chimney power plant. **(a)** Basic overview of the Greenhouse effect occurring in the solar chimney power plant in the case of a halogen light reflector or infrared heater applied as an energy source. **(b)** Detailed energy balance of transformation of energy carried by light waves into heat, causing air flow. Figure 7: Temperature distribution of a halogen reflector detected by a thermographic camera in the long-wave infrared region. Taking into account the spatial limitations set by the proStudent project and the geometry limitations of the physical model, air flow was induced with two PC fans, shown in Figure 8, in accordance with the Nyquist criterion for realization. Despite the air flow speed of 3.39 m/s per fan, the turbine did not rotate. Therefore, the LAB setup was utilized to turn the drive unit into a fan and a generator. Up to an airflow speed of 2.58 m/s, the generator remained blocked. For higher speeds, it overcame the resistance of the magnet and started, as seen in Table 3. Then, a vacuum cleaner was used to determine the minimum flow for which the turbine began to rotate. The results are presented in Table 4. However, after reaching maximum speed, the turbine separated from the shaft and was sucked out by