Evaluation of 15-m-Height Solar Chimney Model Integrated with TES under Tropical Climate

The integration of renewable energy resources into the global power generation portfolio is imperative for mitigating carbon emissions and fostering environmental sustainability. Solar chimney power plants (SCPPs) are CO₂-neutral systems that leverage solar radiation to generate electricity without chemical reactions, thus contributing to the stabilization of supply-demand dynamics and the safeguarding of the global environment.

The pioneering SCPP prototype was constructed in Manzanares, Spain, in 1982, consisting of a 244-meter-diameter collector, a 196-meter-tall chimney, and a turbine, achieving a 50-kW output with an air velocity of 15 m/s at the chimney’s base. Subsequent studies sought to address the inherent low efficiency and the challenges associated with the scale of the collector and chimney through a plethora of experimental, computational, and mathematical approaches. Comprehensive reviews have synthesized advancements in SCPP technology, as in Chikere et al. [1], Al-Kayiem and Chikere [2], Kasaeian et al. [3], and more recent analyses of hybrid systems by Ahmed et al. [4], and evaluations of SCPP concepts and performance by Cuce et al. [5].

At Universiti Teknologi PETRONAS (UTP), a scaled-down SCPP model with a 6.0-meter-diameter collector and a 6.3-meter chimney has been the subject of extensive research. Investigations into collector geometry’s impact on performance, as detailed by Al-Azawiey et al. [6], have demonstrated that larger collector areas significantly enhance air velocity within the chimney. Modifications to the SC model, including adjustable canopy height, were reported by Al-Azawiey et al. [7] to optimize efficiency across various collector inlet heights, with a 0.05-meter inlet height showing superior performance.

Further experimental studies include the 8-meter chimney and 10-meter diameter collector model by Zhou et al. [8], which registered a maximum updraft velocity of 2.13 m/s in Wuhan City, China. In Iran, an SCPP model with a 12-meter chimney and a 10-meter collector diameter was evaluated by Kasaeian et al. [9], focusing on air inversion phenomena and temperature variations within the system. A notable outlier in reported velocities is the claim of a 12.2 m/s air velocity at the chimney base by Lal et al. [10] from a model in Rajasthan Technical University Kota, India, a finding that stands in contrast to established literature. Rao [11] analyzed a stack shaft in a building, indicating that increased chimney heights correlate with more reliable hydrothermal process measurements.

Considering these findings, the scale of the SC model at UTP was increased to a 15-meter chimney to enhance the reliability of velocity and temperature measurements. This paper aims to present a comprehensive evaluation of the scaled-up system through experimental and numerical simulations under tropical conditions. The assessment spans various solar irradiances and three distinct configurations of the collector and sensible thermal energy storage (TES), denoted as Case-1 (4.9 m collector diameter), Case-2 (6.6 m collector diameter), and a novel Case-3, incorporating an extended diameter of 6.6 + 2 m for the sensible TES.

A 6.3-m-height SC experimental model was modified to an updated version of a 15-m-height SC model to acquire experimental measurement data. The collector was also modified in terms of size and absorbing ground materials. The experimental setup is in the solar research site of UTP at 4° 23' 6.59" N 100° 58' 28.19" E.

The SC model, shown in Figure 1, has a chimney with a 15-m height above the ground and a collector with a 6.6-m diameter. The chimney is made of a PVC pipe with a total of 14.35 mm long and 0.152 mm diameter installed above the central metal fixture of the collector. The chimney’s friction losses are relatively low due to its short length. The system was investigated at two collectors’ diameters, 4.9 and 6.6 m. The collector was made of metal frames supported by legs anchored to the ground and covered by a canopy made of Perspex sheets.

The collector has double slopes. The inner part has a 20° slope, and the outer part has a 5° slope. Black materials are more efficient for solar photothermic conversion. A 100-mm thick layer of black-painted pebbles covered the absorbing ground of the collector. The black-painted pebbles increase solar absorption and create higher heat flux inside the collector. Also, it operates as sensible thermal energy storage to store the energy during solar time and discharge it after sunset.

The measurements were performed at three different collector sizes, including Case-1 with a 4.9-m-diameter, Case-2 with a 6.6-m-diameter, and Case-3 with a 6.6-m-diameter + 1 m extra sensible TES outside the canopy.

The measured variables in this experiment are temperatures, air velocity, solar irradiation, and humidity. The measuring instrument used is the Extech Model 45160 3-in-1 Humidity, Temperature, and Airflow Meter. It could measure air velocity in a range of 0.4 to 30 m/s with 0.1 resolution and ±0.9% m/s accuracy. It has an accuracy of ±1.2℃ for temperature measurement in the range of 0℃ to 50℃. The data obtained could be displayed directly on the device’s LCD screen. For the solar irradiance, a solarimeter KIMO-SL 200 model is used. Solar irradiation ranges from 1.0 to 1300 W/m², with a 5% measurement accuracy and an operating temperature range from -10 to 50℃. One of the essential parameters to be measured is the outlet air temperature at the top of the chimney. A K-type thermocouple probe is installed at the top of the chimney around 200 mm below the exit to avoid the inflow effect and mixing with the ambient air. The probe is connected to a digital Data logger brand GRAPHIC GL820.

The experimental work was started in early Oct. 2021 with preparation, instrument calibration, and preliminary runs for checking. The data acquisition commenced on 12 Nov. 2021. For each case, the experimental measurements were performed three days from 8:00 AM to 6:00 PM. Repeatability is important to reduce the uncertainty of measurements and possible system and human errors. Then, the average was considered for the presentation and discussion of results. The details of the experimented cases are shown in Table 1.

For each case, temperatures, velocities, and solar irradiance were recorded at each hour, starting from 8.00 AM till 6:00 PM. The temperature probe was placed at different points on the collector. The inlet temperature is measured at four locations of the canopy's circumferential and repeated many times at North, South, West, and East. The average of the four readings is considered for calculations and analysis. The measuring device typically used for incident solar radiation is a solarimeter that measures direct and diffuse combined solar radiation. The recorded data was clustered in an Excel file for further analysis.

The selected data was able to characterize the SCPP performance and permit comparison between the investigated cases. The Performance Indicator (P.I.) is a parameter that represents the amount of increased air temperature of the generated air mass flow rate in the system. The P.I. is a highly recommended parameter to compare various design and operational cases of solar updraft power.

The term ( $T_{\text {chinney, out}}-T_{\text {amb}}$ ) represents the air temperature rise across the system, from the inlet to the ambient to the outlet at the top of the chimney. The mass flowrate is predicted using the measured velocity, $V_{\text {chimsey}}$ at a plane 1.0 m above the chimney inlet to ensure fully developed flow, and the chimney cross-sectional area, as:

The simulation was carried out assuming steady-state, turbulent, 2-D flow. The incompressible flow assumption is justified since the maximum Mach number is very small, < 0.1. The buoyancy effects were considered to occur due to temperature gradient, according to the Boussinesq criteria presented in Eq. (3). Air density change in the momentum equation is a function of temperature difference. This model treats density as a constant value in all solved equations except for the buoyancy term in the momentum equation. This model accounts for the full bouncy effect, where the density varies with temperature, and the flow is motivated by the force of gravity, which influences the change in density.

Eq. (3) is valid for $\frac{\Delta \rho}{\rho_0} \leq 0.1$.

The model’s fluid dynamics prediction was achieved through computational modeling, simulation, and numerical solution of the mass, momentum, and energy conservation equation. The numerical solution was performed utilizing ANSYS Fluent software.

The experimental setup has been modeled in the computational fluid dynamic (CFD) simulation. The computational model with details is shown in Figure 2.