Experimental and Theoretical Investigation on a Solar Chimney System for Ventilation of a Living Room

In the architecture of classical buildings in the Mediterranean or tropical regions, the use of passive ventilation by large openings to preserve appropriate thermal comfort is a well-known and old concept. In temperate climates where the architecture is generally not designed for hot climatic conditions, the use of natural ventilation is seasonal and it’s done at the initiative of the occupants by their action on the ventilation openings. Mechanical ventilation systems have undesirable energy involvement as they further require operating electricity. For some cities, air conditioning requirements use almost the full capacity of their electricity grids [1]. Also, mechanical ventilation systems are related to high carbon emission, which has a negative impact on the environment. On the other hand, natural ventilation is expected to provide cooling energy savings of approximately 10% and provide annual energy consumption of 15% when climate operating conditions are appropriate [2].

Therefore, natural ventilation is being offered as a means of saving energy and providing adequate thermal air quality for inhabitants inside the buildings as well as for offices and industrial spaces. Natural ventilation is generated by thermal buoyancy forces which creates pressure differences for the airflow (or wind) [3].

The solar chimney system is an excellent example, as it was designed to increase the effect of ventilation by increasing the solar gain [4], thus creating an adequate temperature difference between the inside and outside of the building to drive adequate airflow. The solar chimney is a thermo-syphonic air channel in which the main driving mechanism of air circulation is natural convection [5]. There are different configurations of solar chimney design, which are affected by multiple criteria such as location, climate, size of the space to be ventilated, and indoor heat gain [6]. However, basic elements such as solar collectors, clear cover, and openings are part of each design.

The first fundamental study on the solar chimney was performed by Bansal et al. [3]. The study is analysis of the improvement of ventilation in living spaces by using a solar chimney. The authors have developed steady-state equations to model chimney heat transfer processes. They found that the ventilation rates obtained vary between 140 and 330 m³/h, for solar radiation varying from 220 to 1000 W/m². Several theoretical and experimental research projects followed those of Bansal et al. [3]. These studies are mainly aimed to improve the energy performance of the solar chimney systems [4-32]. For illustration, Afonso and Oliveira [7] have showed the importance of thermal storage in chimneys. They reported that solar-assisted efficiency will be reduced by more than 60% if insulation is not provided on the outside of the brick wall of a solar chimney. It was mentioned that the optimum storage thickness for solar chimney depends on building use patterns. A small thickness was recommended for daytime ventilation, whereas, the greater thickness was suggested for night-time ventilation. An insulation thickness of 5 cm was considered optimum. Nouanégué et al. [8] numerically examined the idea of solar wind tower ventilation. They considered the mixed ventilation case: forced convection in a tower system resulting from negative pressure created at the tower outlet by the venture effect and natural convection due to the buoyancy effect. The Nusselt number and dimensionless volume flow rate were calculated as a function of the dimensionless conductivity of the solid medium, the Rayleigh number, the Reynolds number (or Richardson number) and geometric parameters such as aspect ratio, exit port size, and wall thickness. The result revealed that the wall thickness has a minor degree of influence on ventilation performance than the other parameters. Bassiouny and Koura [9] developed a correlation for the absorber average temperature and the average air exit velocity as a function of solar intensity. They suggested that the maximum absorber temperature could increase by a factor of 2.25 when the solar intensity increased by a factor of 5. Haghighi and Maerefat [10] examined the capability of a solar chimney to reach the required thermal and ventilation needs of individuals on winter days. In their analysis, the heat transfer by natural convection and surface radiation in a 2D vented room in contact with a cold external ambient is studied numerically. The dependence of the system performance on air gap depth of the solar chimney, size of openings, outdoor air temperature, and solar radiation have been investigated to determine the appropriate operating conditions and thermal comfort criteria. The findings show that the system is able to provide good indoor air conditions during the daytime; even with a poor solar intensity of 215 W/m² and a low ambient temperature of 5℃. Imran et al. [11] experimentally and numerically studied the performance of a solar chimney. The steady two-dimensional turbulent flow model is proposed. The results showed that for solar intensity between 150 and 750 W/m², 4-35 air changes per hour were provided, and the airflow went rises by increasing the air gap. The purpose of Hosien et al. [12] is to investigate the performance of solar chimney used for natural ventilation of closed enclosure in order to save energy. To achieve this purpose, a mathematical model simulating the performance of solar chimney was developed. It’s developed a computer program to solve the governing conservation equations. The results showed that the air change rate per hour (ACH) was significantly dependent on the wind speed as well as the dimensions (height, gap, and width). The effect of chimney height was not significant enough to be cost-effective. It’s showed that the air change rate per hour (ACH) throughout the year was more than the desired ventilation standards in the enclosure space. Khanal and Lei [13] performed a numerical simulation of flow behavior due to natural convection of air inside a solar chimney with an imposed heat flux on a vertical absorber wall. Three distinct flow regimes are identified.

It appears from the literature review of the previous works that the performance of the solar chimney used for natural ventilation depends mainly on two groups of parameters. One is the geometrical parameters such as chimney height, width, and gap, while the other is the operational parameters such as solar radiation, wind speed, and ambient temperature, etc. However, there is a lack of information about using other materials as an external wall (glazing cover) for the chimney, this is because the previous researches have been chiefly concerned with using glass as an external cover. Moreover, little attention has been given to study the influence of wind speed on the performance of the solar chimney.

The present work relates to an experimental and numerical study of a solar chimney under Algerian climatic conditions. The energy performance of the chimney according to the geometrical and environmental parameters will be determined experimentally and numerically. An in-house FORTRAN code is developed for the numerical solution.  Effects of incidental solar radiation on the instant temperature distributions in different components, the mass flow of air circulation inside the chimney, the Air Changes Per Hour (ACH), and the efficiency are examined.

The schematic representation of the solar chimney is given in Figure 1.

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Figure 1. Schematic representation of the solar chimney passive ventilation system

Figure 4 shows the physical model used in this study, which consists of a solar chimney with an air inlet by its opening lower than an intake temperature, Tf_,i, assumed equal to the temperature of the room, Tr. The hot air flows through the chimney top at an outlet temperature Tf_,0. The walls of the chimney consist of the glazing on the left side and the absorber on the right side.

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Figure 4. Physical model used

The mathematical model is based on equations of thermal balances at the glazing, the absorbent wall and along the chimney channel. The basic assumptions used are: the inlet air temperature in the chimney is assumed to be uniform and equal to that of the room; the surface temperatures of the absorber and the glazing are supposed to be uniform; the air frictions forces at the walls of the chimney are supposed to be negligible compared to the air pressure forces generated by the thermal gradient; the storage capacities of the glazing and the absorber are supposed to be negligible.

The corresponding equations are:

$\rho_{g} V_{g} C p_{g} \frac{\partial T_{g}}{\partial t}=S_{1} A_{g}+U_{t} A_{g}\left(T_{a}-T_{g}\right)$$+h_{g} A_{g}\left(T_{f}-T_{g}\right)+A_{w}\left(T_{w}-T_{g}\right)$   (1)

$\rho_{f} V_{f} C p_{f} \frac{\partial T_{f}}{\partial t}+q^{\prime \prime}$$=h_{g} A_{g}\left(T_{g}-T_{f}\right)+h_{w} A_{w}\left(T_{w}-T_{f}\right)$  (2)

$\rho_{w} V_{w} C p_{w} \frac{\partial T_{w}}{\partial t}=S_{2} A_{w}+U_{b} A_{w} T_{r}+h_{r w g} A_{w} T_{g}$$+h_{w} A_{w} T_{f}-\left(h_{w}+h_{r w g}+U_{b}\right) A_{w} T_{w}$   (3)

where: U_t is the overall exchange coefficient between the glazing and the atmosphere, given by:

U_t=hw_ind+‏h_rs

The convective heat transfer coefficient, h_Wind, is relative to the environment. It’s given by the expression [14, 15]:

h_Wind =2.8+3V_w         (4)

where: V_w, is the air ambient wind speed.

The radiative heat transfer coefficient, h_rsbetween the surface of the glass and the sky is expressed by:

$h_{r s}=\frac{\sigma \varepsilon_{g}\left(T_{g}+T_{S}\right)\left(T_{g}^{2}+T_{S}^{2}\right)\left(T_{g}-T_{S}\right)}{\left(T_{g}-T_{a}\right)}$  (5)

The temperature of the sky, T_s, is expressed by:

T_s=0.0552T_a^1.5  (6)

The solar radiation absorbed by the glazing is given by:

S₁=a₁H     (7)

The radiative heat transfer coefficient, h_rwg, between the absorber and the glazing is expressed by:

$h_{r w g}=\frac{\sigma\left(T_{g}^{2}+T_{w}^{2}\right)\left(T_{g}-T_{w}\right)}{\left(\frac{1}{\varepsilon_{g}}+\frac{1}{\varepsilon_{w}}-1\right)}$  (8)

The convective heat transfer coefficient, h_g, between the glazing and the air of the channel is given by [14]:

$h_{g}=\frac{N u K_{f}}{L_{g}}$  (9)

with: Nu = 0.60 (G_r×P_r×cos $\theta$)^0.2 and $G_{r}=g \beta_{f} \Delta T L_{g}^{3} / v_{f}^{2}$.

The thermal conductivity of the air is given by:

 K_f=0.00263+0.000074(T_f–300)   (10)

The useful gain of heat can be calculated by:

$q^{\prime \prime}=\dot{m} C_{p, a}\left(T_{f, 0}-T_{f, i}\right)$   (11)

where: $\dot{m}$ and C_p,a, respectively denote the mass flow rate and the specific heat of the air.

The average temperature of the air flowing in the chimney is:

$T_{f}=\gamma T_{f, 0}+(1-\gamma) T_{f, i}$  (12)

The coefficient g is determined experimentally [14, 26]; it’s equal to 0.74. In addition, the temperature T_f,i was taken equal to the ambient temperature, T_r. From where:

$q^{\prime \prime}=\frac{\dot{m} C_{p, a}\left(T_{f}-T_{r}\right)}{\gamma}$  (13)

The mass flow of air in the chimney [3] is given by the expression:

$\dot{m}=C_{d} \frac{\rho_{f, 0} A_{0}}{\sqrt{\left(1+A_{r}\right)}} \sqrt{\frac{2 g L\left(T_{f}-T_{r}\right)}{T_{r}}}$  (14)

The value of the coefficient C_d is taken equal to 0.57 [3, 14, 15].

The convective heat transfer coefficient between the absorber wall and the air channel is:

$h_{w}=\frac{N u \cdot K_{f}}{L_{W}}$  (15)

The dynamic viscosity of the fluid is expressed by:

$\mu_{f}$=[1.846+0.00472(T_f–300)].10^-5

The mass density of the fluid is:

$\rho_{f}$=1.1614–0.00353(T_f –300)

The specific heat of the fluid is given by:

C_p,a=[1.007+0.00004(T_f –300)]×10³

The coefficient of volume expansion in the air channel is expressed by:

$\beta$=1/T_f

The overall exchange coefficient of the insulating board, located on the back part of the absorbent wall is expressed by:

$U_{b}=\frac{K_{\text {ins }}}{\Delta w_{\text {ins }}}$  (16)

The radiative flux absorbed by the absorbent wall is:

S₂=a₂H     (17)

The airflow velocity in the chimney is expressed by:

$v_{0}=\frac{\dot{m}}{\rho_{f} A_{0}}$  (18)

The instant efficiency, η_i, relative to the energy gain through the chimney, will be deduced:

$\eta_{i}=\frac{\dot{m} C_{p, a}\left(T_{f, 0}-T_{f, i}\right)}{W M \times H} \times 100 \%$  (19)

The hourly rate of renewal of air in the room, ACH, is defined by:

$A C H=\frac{Q_{V} \times 3600}{\text { Total volume of the room }}$  (20)

Figures 9 and 10 show the evolutions of the average theoretical temperatures of the different elements of the chimney obtained by our code and those obtained by Ong and Chow [14] as a function of chimney width for H=200 W/m² and H=650 W/m². It is noted at first sight that the temperature profiles are little influenced by the width of the chimney. A good agreement is observed between our obtained results and those of Ong and Chow [14]. As an indication, for a solar intensity of 200W/m², the average absorber wall temperature is about 45.5℃ for our model and it is 45℃ for Ong and Ong and Chow [14]. And we have registered 75℃ at H = 650W/m², while it is 76℃ for Ong and Chow et al. [14]. It should also be emphasized that the temperature of the absorber and higher than that of air flowing in the chimney and glazing, which is predictable. In fact, the absorption of thermal radiation raises the temperature of the absorber wall and contributes to the elevation of the temperature of the air in contact with these walls.

In addition, Table 1 shows that the temperatures of the different elements of the chimney and the air speed at the outlet are proportional to the solar intensity. A good agreement between the results obtained and these obtained by Ong and Chow [14].

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Figure 9. Comparison of the average theoretical temperatures of the different elements of the chimney with those obtained by Ong and Chow [14] as a function of chimney width at H=200w/m²

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Figure 10. Comparison of the average theoretical temperatures of the different elements of the chimney with those obtained by Ong and Chow [14] as a function of chimney width at H=650w/m²

Table 1. Comparison of our numerical and experimental results with these of Ong and Chow [14]