# Design Limitations of Laboratory Solar Chimney for Engineering Education
## 2 A Brief Overview of the Existing Solutions
Since this study aims to investigate a solar chimney power plant mock-up for experimental study in the laboratory class under the requirements of the proStudent project, its geometry must be appropriate for a fit on a classroom table. Therefore, it is worth exploring previous research on small-scale solar chimney power plants. [PERSON] and [PERSON] constructed the smallest solar chimney power plant recorded in the literature, which is 150 cm tall with a chimney radius of 8 cm [19]. The primary objective of this experiment was to achieve the maximum outlet velocity, which was measured at 5.12 m/s. This value is considered very high relative to the small scale of the chimney. High speed was achieved using mirrors as intensifiers, which were designed to follow the position of the Sun. In addition, a novel feature of the chimney was the container that was placed underneath the collector. The addition of a container feature was more cost-effective than expanding the collector area. The collector radius, that is, the canopy of the container, was only 46 cm, but its height was 110 cm, which allowed a considerable air volume relative to the geometry of the chimney. [PERSON] et al. constructed a solar chimney power plant with 200 cm height and 150 cm collector radius that achieved a maximum air velocity of 1.3 m/s [20]. The materials used to construct a collector were found to greatly improve the efficiency of the solar chimney. The collector roof was carried by a steel structure, the roof itself was made of soda-lime glass sheets, whereas the chimney was made of polycarbonate. [PERSON] et al. constructed a 12 m high solar chimney power plant with a collector radius of 5 m. Their study reported two important features, low cost and simple solution to increase solar absorption [21]. The chimney was made out of inexpensive polyethylene sheets. To improve the absorption of solar radiation, the roof was sloped as suggested in [22]. Experimental analysis showed that the air input velocity at the collector inlet was approximately zero, while the maximum air velocity was reached at 2.9 m/s. Reducing the collector's inlet size improves the power efficiency of the solar chimney power plant. The literature suggests that the sloped collector increases the efficiency of the solar chimney power plant by minimizing dust accumulation and maximizing the effective surface. For example, the 0.5\({}^{\circ}\) incline increased the efficiency by 10%, while the 5\({}^{\circ}\) incline quadrupled its power [23]. That said, slope also creates turbulence in the air which can have a negative effect. [PERSON] et al. showed that dividing the collector roof into partitions increased the efficiency of the solar chimney power plant, with the optimal number of partitions being 12 [24]. Next, introducing layers on the collector roof separates two functions of the collector, cooler air input and warmed air rising, positively affecting power production as in [25]. [PERSON] et al. investigated four tower shapes, two cylindrical and two divergent, both hyperboloid and conical, via the CFD method. It was concluded that efficiency is directly affected by divergent shapes, while hyperboloid is optimal when the chimney produces significant output power [26]. As the product of this work is intended to educate college-level engineers, we based our investigation on a simplest model. Even recent ANSYS analyses of solar chimney power plants, for example, [27; 28], apply what is fundamentally, the model developed for the Manzanares prototype, which explains the spontaneous flow of the warm air in terms of buoyancy due to the Greenhouse effect. There are two assumptions in these analyses: viscosity is negligible and heat conduction is natural. Therefore, the model can be derived from [PERSON]'s principle, and even the complex Navier-Stokes system of equations is somewhat simplified, as in [27], due to the mentioned approximations. However, if buoyancy exists, then the conduction cannot be natural, rather it is mixed, which does not allow us to neglect viscosity.