Natural Ventilation Enhancement of a Roof Solar Chimney with Wind-Induced Channel

Abstract

A novel roof solar chimney with wind-induced channel was designed herein to augment indoor natural ventilation under combined action of wind and solar energy. Compared with the traditional solar chimney, the new rooftop solar chimney improves the air flow due to the wind-induced channel. The effects of channel width ratio, chimney inclination at different outdoor wind speeds on the natural ventilation performance of the roof solar chimney were studied by numerical simulation. Finite-volume method was used for the numerical calculation. It was found that the trends of ventilation rate are different when the channel widths increase. The mass flow rate of the new structure increases with the increase in the inclination angle. When the inclination angles increase from 30° to 90°, the ventilation rate increases by 212% for U = 1.0 m·s⁻¹, 166% for U = 2.0 m·s⁻¹, and 127% for U = 3.0 m·s⁻¹ under the condition of and the solar radiation I = 600 W·m⁻².

1. Introduction

Deducing building energy consumption is required to solve some energy problems, so the use of renewable energy has attracted more and more attention. Solar energy has been widely used in buildings to reduce building energy consumption. It is worth mentioning that solar chimney can promote indoor ventilation and save energy for building ventilation by using the solar radiation to heat the air in the channel.

Many studies have been conducted on the solar chimney. Mathur et al. [1] showed that optimal inclination and dimensions of inclined solar chimney varied with the latitude, and the optimum inclination of the absorber was discovered for the maximum ventilation rate during summers in India. Sakonidou et al. [2] developed a mathematical model, which can predict the temperature and flow rate of air in the chimney, to determine the inclination of a solar chimney that obtained the maximum natural air flow. Susanti et al. [3] investigated experimentally the performance of air flow in a roof cavity, which reduced the roof solar heat gain. The results indicated that the opening size had a big effect on air flow and heat transfer in the cavity. Du et al. [4] studied the influence of chimney physical parameters on air flow rate. The results showed the optimal length to width ratio is 12:1 for maximum air mass flow rate, and the optimal inclined angle of the second-floor chimney was found to be 4° from the horizontal. Khanal et al. [5] conducted an experiment to study the performance of an inclined passive wall solar chimney. It was found that the inclination angle of the passive wall had significant influence on the air flow velocity across the air gap width. Imran et al. [6] investigated experimentally and numerically a roof solar chimney in Iraq; the ventilation performance under varying inclination of solar chimney, air gap thickness, and solar heat flux were studied. The results showed that the rate of maximum ventilation appeared at the chimney inclination angle of 60°, which is 20% higher than 45°. Li et al. [7] analyzed the effect of the solar radiation intensity on indoor air temperature and velocity in a classroom with a solar chimney. The results show that the ventilation rate increases with the increase in solar radiation intensity. Lei et al. [8] proposed a novel roof solar chimney, and the effects of parameters were investigated by three-dimensional numerical simulations. The results showed that solar chimney with a perforated absorber plate augments the natural ventilation compared with the traditional roof solar chimney. Xuan et al. [9] proposed a calculation method that can predict the ventilation rate in the inclined solar chimney. The effects of solar chimney inclination on ventilation rate were studied. The results showed that the ventilation rate of inclined solar chimney with an inclined angle of 50° is 11.52% higher than vertical solar chimney. Al-kayiem et al. [10] conducted an experiment and a numerical simulation to study the performance of a roof solar chimney with different inlet configurations. The results show that the optimum performance is the inlet configuration with vertical cross section. Abdeen et al. [11] conducted the investigation to increase the indoor air velocity and improve thermal comfort by optimizing the solar chimney. They concluded that the chimney width had great influence, followed by inclination angle and then air gap. Khosravi et al. [12] introduced a funnel form for improvement of ventilation performance and investigated numerically the performance optimized solar chimney. Compared to conventional designs, the new design augments the natural ventilation rate. Zhao et al. [13] carried out a study of the temperature distribution of inclined solar chimney at different ambient temperatures, tilt angles, and heat fluxes. The results showed that the temperature distribution along the width direction of the chimney is extremely uneven. The temperature of the hot wall is the highest, the temperature near the hot wall drops sharply, the temperature away from the hot wall drops gently, and the temperature at the glass side rises slightly. Kong et al. [14] developed a method to determine the optimal inclination of roof-inclined solar chimneys. It was found that inclination angle influences the ventilation performance significantly under a given heat flux. Wang et al. [15] investigated the impacts of external wind on the optimum designing parameters of a wall solar chimney by numerical simulations. The result showed that the optimum cavity depth tends to increase from 0.2–0.3 m to 0.4–0.5 m with the consideration of external wind. Villar-Ramos et al. [16] studied the energy behavior of a single-channel solar chimney system through a parametric study. The results showed that more significant temperature differences can be obtained by variation of the inclination. Dhahri et al. [17] simulated four different configurations to determine the optimal configuration of solar chimney-shaped channels. The result indicated the benefits of triangle corners were preferable to other forms. Vazquez-Ruiz et al. [18] proposed the double-channel vertical roof solar chimney and analyzed the flow patterns and temperature field of the air under six kinds of configurations with or without a heated wall. It was found that the chimney position importantly influences the thermal efficiency. Padhi et al. [19] simulated and analyzed the influence of solar chimney geometry dimensions on thermodynamic characteristics of air. The results showed that it is beneficial to increase the height and diameter of the chimney. Additionally, the suitable location of the turbine was given.

As can be seen from the above documents, researchers have mainly carried out the research on the ventilation performance of the roof solar chimney with different thermal pressure effects, structural parameters, and meteorological parameters, but there is no research on the optimization and improvement of the wind pressure effect. A novel roof solar chimney with wind-induced channel is proposed to enhance the indoor natural ventilation. Solar chimney is an effective facility by renewable energy to achieve indoor natural ventilation. Optimizing the structure of solar chimney to enhance natural ventilation is a hot research topic. This paper focuses on the wind-induced effect through structural optimization to enhance the performance of the roof solar chimney.

In the following, the physical model of the roof solar chimney was showed first. Next, the simulation method was introduced in the paper. Finally, the influences of channel width ratio, chimney inclination at different outdoor wind speed were discussed.

2. Calculation Basis

2.1. Physical Model

Figure 1 shows the structure of a solar chimney with wind-induced channel. It is composed of glass cover plate, absorber plate, air channel I, and air channel II (wind-induced channel), which is installed on the south outer wall of the building. The roof solar chimney height h₁ is 1500 mm, the height between the top of the external wall opening and the roof h₂ is 300 mm, the external wall opening height h₃ is 150 mm, the width of channel I is w₁, the width of channel II is w₂, and the inclination angle is θ. Its working principle is as follows: the indoor air enters the solar chimney through channel I, the sunlight shines on the heat absorbing plate through the glass cover plate, and the absorber plate absorbs the solar radiation, so that the air density in the channel of the solar chimney increases and flows upward; the outdoor wind enters channel II through the opening of the building’s outer wall, then enters the solar chimney at a certain speed, and induces the air in it to flow upward and pass through the roof. The air flow is enhanced by the opening of the part. Under the joint action of outdoor wind speed and solar radiation energy, the structure induces the air in the room into the solar chimney, and the natural ventilation of the building is strengthened.

2.2. Calculation Domain

Figure 2 shows the calculation domain of the roof solar chimney with wind-induced channel. The room length is CK = 3000 mm and height is KJ = 3000 mm. In order to ensure that the reasonable calculation sizes of AB, BC, and KL, those can accurately obtain the calculation results and save the calculation resources; the wind field calculation domain with different sizes are simulated, and a reasonable size is obtained, which satisfies the calculation results error of less than 5%. It can not only satisfy the reliability of the calculation results, but also the calculation area is not large. Finally, the reasonable calculation domain is determined as AB = 8000 mm, BC = 2800 mm, and KL = 1100 mm.

2.3. Boundary Conditions

Figure 2 displays the boundary conditions for numerical simulation. AB is the velocity inlet boundary condition. AM and ML are pressure outlet boundary conditions. CD, EF, DG, GH, IJ, JK, and KL are wall boundary conditions. In the calculation region, the glass cover plate and heat absorber plate are constant heat fluxes, which are determined by solar radiation, material absorptivity, and transmissivity. All solid surfaces are no-slip condition boundary. The absorptivity of glass cover plate is 0.06, the transmissivity is 0.84, and the absorptivity of heat absorber plate is 0.95.

3. Simulation

3.1. Governing Equations and Calculation Methods

The air flow in the channel was turbulent in this research. The governing equations of three-dimensional steady-state turbulence are as follows:

Equation of continuity:

$$\frac{\partial u_{\mathrm{i}}}{\partial x_{\mathrm{i}}}=0$$

(1)

Momentum equation:

$$\frac{\partial (u_{\mathrm{i}}{u}_{\mathrm{j}})}{\partial x_{\mathrm{j}}}=-\frac{1}{\rho }\frac{\partial P}{\partial x_{\mathrm{i}}}+\frac{\partial }{\partial x_{\mathrm{j}}}\left[(\nu +{\nu }_{\mathrm{t}})\frac{\partial u_{\mathrm{i}}}{\partial x_{\mathrm{j}}}\right]-g_{\mathrm{i}}\beta \left(T-T_{\infty }\right)$$

(2)

Energy equation:

$$\frac{\partial \left(u_{\mathrm{j}}T\right)}{\partial x_{\mathrm{j}}}=\frac{\partial }{\partial x_{\mathrm{j}}}\left[\Gamma \frac{\partial T}{\partial x_{\mathrm{j}}}\right]$$

(3)

$$\Gamma =\frac{\nu }{\mathit{Pr}}+\frac{{\nu }_{\mathrm{t}}}{{\sigma }_{\mathrm{t}}}$$

(4)

K equation of turbulent kinetic energy of turbulence model:

$$\frac{\partial \left(\kappa u_{\mathrm{i}}\right)}{\partial x_{\mathrm{i}}}=\frac{1}{\rho }\frac{\partial }{\partial x_{\mathrm{j}}}\left[\left(\nu +\frac{{\nu }_{\mathrm{t}}}{{\sigma }_{\mathsf{\kappa }}}\right)\frac{\partial \kappa }{\partial x_{\mathrm{j}}}\right]-\epsilon +G_{\mathrm{k}}$$

(5)

ε equation of dissipation rate of turbulent model:

$$\frac{\partial \left(\epsilon u_{\mathrm{i}}\right)}{\partial x_{\mathrm{i}}}=\frac{1}{\rho }\frac{\partial }{\partial x_{\mathrm{j}}}\left[\left(\nu +\frac{{\nu }_{\mathrm{t}}}{{\sigma }_{\mathsf{\epsilon }}}\right)\frac{\partial \epsilon }{\partial x_{\mathrm{j}}}\right]+\frac{\epsilon }{\kappa }\left(c_1{G}_{\mathrm{k}}-c_2\epsilon \right)$$

(6)

$$G_{\mathrm{K}}=\frac{{\nu }_{\mathrm{t}}}{\rho }\frac{\partial u_{\mathrm{i}}}{\partial x_{\mathrm{j}}}\left(\frac{\partial u_{\mathrm{i}}}{\partial x_{\mathrm{j}}}+\frac{\partial u_{\mathrm{j}}}{\partial x_{\mathrm{i}}}\right)$$

(7)

where u_i is average velocity component in x_i direction, m·s⁻¹; x_i represents the coordinate axis X, Y, Z (i = 1, 2, 3); P is average pressure, pa; ρ is air density, kg m⁻³; ν_t, ν are turbulent and laminar viscosity coefficients; g_i is gravitational acceleration in x_i direction, m·s⁻²; β is air expansion coefficient, 1 K⁻¹; T, T_∞ are average temperature and reference point temperature, respectively, K; Γ is generalized diffusion coefficient; Pr is Prandtl number; c₁, c₂ are empirical coefficients; G_k is a production term of turbulent kinetic energy; and σ_t, σ_k, σ_ε, are empirical coefficients, respectively.

Air generated the buoyant force owing to the absorber plate and heated the air in the channel. Boussinesq hypothesis was used to deal with the buoyancy term in the momentum equation. This was based on the following assumptions:

(1)

The energy loss caused by the viscous force will be neglected;

(2)

The density is not constant, while the values of other physical property parameters will be constant;

(3)

The density involved in the volume force in the momentum equation is not treated as a constant, but the density in the remaining expressions related to density is taken as a constant.

The density involved in the gravity term is expressed by the following expression:

$$\rho ={\rho }_0\left[1-\beta \left({\mathrm{T}-\mathrm{T}}_0\right)\right]$$

(8)

where T₀ is working environment temperature, and ρ₀ is density corresponding to T₀.

The air flow problem in the channel was solved with CFD directly. The above governing equations were discretized by Finite-volume method. Pressure-velocity coupling was realized by SIMPLE, which is a widely used numerical method for solving incompressible flow. The algorithm obtained the velocity field by solving the momentum equation in discrete form at a given pressure field. Second-order upwind discretization scheme was used for the discretization scheme of the convection term. Based on Boussinesq hypothesis, the calculation was simplified, and the convergence speed was accelerated.

3.2. Grid Generation and Independence

Due to the complexity of the physical model and the existence of the wind field area, the calculation area adopted the combination of unstructured grids and structural grids. Figure 3 shows the local calculation grids. The grid independence test was carried out for the accuracy of simulation and saving computing resources. Five sets of grids were established so that the number of the grids was 9721, 10,587, 11,938, 13,707, and 16,582. The calculated ventilation rate under different grid numbers is shown in Figure 4. It was found that the ventilation rate deviation between the fourth set of grids and the fifth set of grids was less than 1%.

3.3. Validation

The ventilation rate for the same structure and boundary condition of the experimental model in reference [20] was calculated numerically. Under the experimental conditions, the height of solar chimney was 1025 mm, the length was 925 mm, the heat flux was 600 W·m⁻², and the width ranged from 20 mm to 110 mm. Figure 5 shows the comparison between calculation and experiment results. It was found that the calculation results were consistent with the experiment results, and the deviation was less than 14%. It was proved by the correctness of the calculation method adopted.

4. Result and Discussion

4.1. Influence of Channel width Ratio w₂/w₁

Figure 6 shows the variation of ventilation rate Q of the roof-inclined solar chimney with channel width ratio w₂/w₁ under different outdoor wind speed U. The channel width ratio w₂/w₁ of the roof-inclined solar chimney ranges from 0.05 to 0.3, and the solar chimney inclination angle θ is 60°.

It is observed from Figure 6 that with the increasing of channel width ratio, the varying trend of ventilation rate of the roof solar chimney is different at different wind speeds.

For U = 1.0 m·s⁻¹ and U = 1.5 m·s⁻¹, the ventilation rate decreases first, then increases with the increasing of w₂/w₁ and reaches the maximum 0.378 kg·s⁻¹ and 0.411 kg·s⁻¹ appearing at w₂/w₁ = 0.2. For U = 2.0 m·s⁻¹, the ventilation rate increases at first and then decreases with the increase in w₂/w₁ and reaches the maximum around w₂/w₁ = 0.2, which is 0.471 kg·s⁻¹. For U = 2.5 m·s⁻¹ and U = 3.0 m·s⁻¹, the ventilation rate increases with the increase in w₂/w₁ and reaches the maximum around w₂/w₁ = 0.267, which is 0.535 kg·s⁻¹ and 0.591 kg·s⁻¹, respectively. It is not difficult to see from the figure that the outdoor wind speed is beneficial to indoor ventilation under channel width ratio w₂/w₁; the ventilation rate of the roof solar chimney increases with the increase in the wind speed. For w₂/w₁ = 0.2, the ventilation rate increases by 8.7% for U = 1.5 m·s⁻¹, 24.6% for U = 2 m·s⁻¹, 38.6% for U = 2.5 m·s⁻¹, and 50.5% for U = 3 m·s⁻¹ compared with U = 1 m·s⁻¹.

4.2. Influence of Chimney Inclination θ

Figure 7 shows the variation of ventilation rate Q of the roof-inclined solar chimney with the chimney tilt angle θ for different outdoor wind speed U. The inclination angle of the roof solar chimney ranges from 30° to 90°, and the channel width ratio is w₂/w₁ = 0.2.

As seen in Figure 7, the ventilation rate tends to increase as the inclination angle increases at the same outdoor wind speed, and it reaches the maximum when the inclination angle θ is 90°. When the inclination angle increases from 30° to 90°, the ventilation rate of the roof solar chimney increases by 212% for U = 1.0 m·s⁻¹, 166% for U = 2.0 m·s⁻¹, and 127% for U = 3.0 m·s⁻¹, as solar radiation intensity I is 600 W·m⁻². When the outdoor wind speed increases from 1 m·s⁻¹ to 3 m·s⁻¹, the ventilation rate increases by 116% for θ = 30°, 53.6% for θ = 45°, 50.5% for θ = 60°, 51.8% for θ = 75°, and 57.5% for θ = 90°. It can be attributed to the increase in the inclination angle of the chimney and the decrease in the resistance of the chimney passage, thus increasing the ventilation volume.

5. Conclusions

A roof solar chimney with wind-induced channel was proposed. A mathematical model of a roof solar chimney with wind-induced channels was developed. Compared with the traditional solar chimney, the new rooftop solar chimney enhances the air flow due to the wind-induced channels. The effects of channel width ratio and inclination angle on the ventilation performance were investigated by numerical simulation. The main results are as follows:

(1)

With the increase in channel width ratio w₂/w₁, the trend of the mass rate changes differently as wind speed varies. When the ventilation rate reaches the maximum, the channel width ratio w₂/w₁ is also different. When the outdoor wind speed is U = 1.0 m·s⁻¹ and the channel width ratio is w₂/w₁ = 0.2, the mass flow rate reaches its maximum, which is 0.378 kg·s⁻¹. When the outdoor wind speed is U = 3.0 m·s⁻¹ and the channel width ratio is w₂/w₁ = 0.267, the mass flow rate reaches its maximum, which is 0.591 kg·s⁻¹.

(2)

The ventilation rate increases as the inclination angle θ increases under different outdoor wind speeds. For solar chimneys with different inclination angles, the ventilation rate is greatly influenced by the external wind speed. When the solar radiation is I = 600 W·m⁻² and the inclination angles increase from 30° to 90°, the ventilation rate increases by 212% for U = 1.0 m·s⁻¹, 166% for U = 2.0 m·s⁻¹, and 127% for U = 3.0 m·s⁻¹. For the given structural parameters of the solar chimney, the outdoor wind speed has a positive effect on indoor air quality.