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Energy Procedia 36 (2013) 628 - 637

Terragreen13 International Conference

Thermo-fluid aspect analysis of passive cooling system case using solar chimney in the south regions of Algeria

Salah Larbia, Adel El Hellaa, a*

aLGMD- Department of Mechanical Engineering, Ecole Nationale Polytechnique 10, Avenue Hassen Badi, BP 182, El-Harrach, 16200, Algiers, Algeria

Abstract

The purpose of this present work is related to thermo-fluid flow and energy performance aspect analysis of solar chimneys versus geometrical parameters and environmental sites. Governing equations are solved numerically using finite volume method. Adrar site located at the south western region of Algeria is chosen according to its important energy potential compared to other regions of Algeria. Obtained results are related to fluid flow temperature and velocity distributions along the chimney, the mass flow rate and air change per hour (ACH). A good agreement is obtained between our results and those of the literature.

© 2013 The Authors. Published by Elsevier Ltd.

Selection and/or peer-review under responsibility of the TerraGreen Academy

Keywords: Solar Chimney; Thermo-fluid; Energy performances; Passive ventilation and cooling; Numerical simulation, Adrar site.

1. Introduction

Passive ventilation systems are regarded as an alternative solution to mechanical ventilation systems according to their operational cost, energy requirement and carbon dioxide emission which has a negative impact on the environment.

In hot countries such as the Middle East regions and the south of Algeria, large parts of energy are used for air conditioning, despite the significant solar energy potential of these countries. Some passive cooling systems [1] can be used to reduce mechanical air conditioning requirements in areas where cooling is a dominant problem. Solar chimney technology is used to circulate air flow and heat throughout by changes in density. This air may be used for drying, power generation and other uses such as air ventilation for dwelling.

* Corresponding author. Tel.: + 213 21 52 27 35; fax: +213 21 52 29 73. E-mail address: larbisalah@yahoo.fr

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.

Selection and/or peer-review under responsibility of the TerraGreen Academy

doi: 10.1016/j .egypro .2013.07.072

Most of studies dealing with solar chimneys are based essentially on drying purposes, ventilation and power production. In recent years scores are mainly devoted to optimize the design of these systems through the maximization of the output power and minimization of the installation's cost. Considerable efforts have been made these recent decades on sizing and estimating the energy performance of solar chimneys to demonstrate their feasibility as well as their profitability.

Bansal et al. [2] can be considered as the pioneers to investigate solar chimney configurations and performances for ventilation in buildings. The authors presented a steady state mathematical model for a solar chimney which is used to enhance the effect of thermally induced ventilation in buildings. The model takes into consideration different sizes of the openings of a solar chimney with varying values of the discharge coefficients.

Significant research works were achieved on solar chimneys both theoretically and experimentally [310]. These researches were conducted using natural ventilation for different applications including passive solar heating and cooling in buildings, ventilation, and power generation.

Many studies based on flowing air modelling in solar chimneys using computational fluid dynamics (CFD) techniques were developed these last decades. The computational methods were used to predict flow patterns, pressure variation and temperature distributions along the chimney channel. Bassiouny and Koura [4] presented a numerical study on the chimney width influence on space ventilation. The flow patterns inside the room and in the chimney were presented in their study. The assumptions used in their model are similar to those presented by Bansal et al. [2], Ong and Chow [3]. Zamora and Kaiser [5] analysed fluid flows in channels with solar chimney configuration using Phoenics code. The effect of air gap width over the thermal and dynamic behaviour of the thermally induced flow was investigated. Nouanegue and Bilgem [6] studied numerically the mixed ventilation problem in a tower system. The authors showed that the wall thickness has lesser degree of influence on ventilation performance of the device.

The purpose of this present work is related to thermo-fluid flow and energy performance aspect analysis of a passive cooling system based on solar chimney according to geometrical parameters and environmental sites. Adrar site located at the south western region of Algeria is chosen according to its important energy potential compared to other regions of Algeria.

Nomenclature

A Area, m2

V Velocity, m.s-1

Cp Specific heat, J.kg-1.K-1

Cd Discharge coefficient = 0.57

K Thermal conductivity, W.m-1.K-1

ACH Air change per hour, h-1

Ra Rayleigh number

Pr Prandtl number

Nu Nusselt number

P Density, kg.m-3

^ Dynamic viscosity, kg.m-1.s-1

£ Emissivity

Subscripts

g Glass

f Fluid

w Wall

wind Wind

ins Insulation

c Convection

rs Radiative between glass and sky

rwg Radiative between wall and glass

2. Physical and mathematical models

Figs. 1a and 1b show respectively the schematic representation of the passive system using solar chimney device and the physical model used.

Air enters the chimney at the inlet temperature, Tfi , which is assumed equal to the room air, Tr. Warm air exits at the outlet temperature, Tfo , from the top of the chimney. Temperatures at the surfaces of the glass, Tg, and wall, Tw, and mean air temperature in the flow channel, Tf, are all assumed to be uniform.

Fig. 1. (a). Schematic representation of the passive ventilation system; (b). Physical model used for the solar chimney.

2.1. Solar chimney performance analysis model

The mathematical model used for the performance analysis of the chimney is based on heat balance equations at the glazing, the absorber and along the flow channel [1, 3].

Heat balance at the glazing, on the air flow channel and on the wall absorber are given respectively by the following equations [1, 3]:

SA + UtAg (Ta - Tg ) = hcgAg (Tg - Tf ) + hrWgAw (Tg - Tw )

Í • ^ mC

hcgAgTg -

f hcgAg + hwAw +m Cp^

Tf + hwAwTw =-

hrwgAwTg -hwAJf +(hcA + hwgAw + UbAw)Tw = SWAW + UbAwTr

Where: Ut is the overall heat transfer coefficient between air and glazing given by

Ut = hcwind + K

Convective heat transfer coefficient, hcwind, is defined by ^[1] hcmnd - 2.8 + 3.Va

Radiative heat transfer coefficient, hrs, between glazing and sky is given by

a* (Ts + Ts)(Tg + T?)(T -Ts)

(Tg - Ta )

With: Ts = 0.0552 Tla5

Ub is the overall heat transfer coefficient between insulation panel and room given by

Solar radiation heat flux absorbed by glazing is Sg - agH

Solar radiation heat flux absorbed by the absorber wall is

Sw = ™wH

Where: H is the Incident solar radiation, a, the absorptivity and x the transmitivity.

(1) (2)

Radiative heat transfer coefficient, hrws, between glazing and wall is given by

Salah Larbi and Adel El Hella /Energy Procedia 36 (2013) 628 - 637

fc+TW fc - Tw)

f \ -1 +-1 -1

Convective heat transfer coefficient, hcg, is defined by ^[1] Nu.Kt

hcg =■

With: Nu = 0.68 + (0.67.RaV4 )/[l + (0.492/Pr )9/16 J^

Nu = "0.825 + (0.387 RaU6 )/[l + (0.492/Pr )9/16 f27 j2 for turbulent flow case (Ra>109). Kf = 0.00263 + 0.000074.(7} -300), fif = [1.846 + 0.00472.(7} - 300)]10 pf = 1.1614- 0.00353.(Tf - 300), pf = 1/ Tf, Cp,a = ¡1.007+ 0.00004(rf -300J103.

The mean air temperature in the flow channel is given by

Tf = rTffi +(1 -r)Tf,0 (12)

^The coefficient, y, is determined experimentally [5] and the temperature Tfi is assumed equal to the room air, Tr , as it is specified previously.

Convective heat transfer coefficient, hcw, is defined by ^[1] Nu.K,

hcw =■

The mass flow rate is given by

f ,oP f ,0 A0

An = C,

Pf ,0 A0

2gLw (Tf - Tr )

The air change per hour (ACH) is defined by

Qv *3600

ACH = -

Room Total volume Room Total volume The instantaneous efficiency of the chimney, can then be deduced as

m Cp,a (Tf,0 " Tf i ) .

7i =■

-x100%

2.2. Thermo-fluid Governing Equations

For fluid dynamics aspect of flows in the chimney, the mathematical model developed is based on steady state laminar flow and prescribed boundary conditions for temperature and velocity. The assumptions used are related to an incompressible and Newtonian fluid, with constant fluid properties, and without heat source and non viscous dissipation. The fluid flow model used is based on balance equations, mass, energy and momentum equations. These equations are:

d (m ) + ô(v) = 0

Ou du d T

uT"+ vT"= gß(T ~ ) + ox dy dy

PCv\u — + v —

d2 T ~dyT

(18) (19)

The buoyancy term, gfi(T - Tx), which appears in the momentum conservation equation results from the density variation, and is obtained by using the Boussinesq approximation.

2.2.1. Boundary conditions

Fig. 2 shows the domain of study and boundary conditions used. Tg is the cover temperature. The chimney wall is considered as adiabatic (5T/5x=0 at wall). The non-slip condition is imposed on the wall. At the upper part of the chimney, the fully developed flow condition is considered on the velocity and temperature. At the inflow boundary, the entrance temperature Tr is assumed constant. The mass flow at the entrance caused by buoyancy forces is unknown in the beginning. The u-component of the velocity at the entrance is updated at each iteration by the values of the neighbouring velocities.

du=^v JT=0 n-4-T

dy dy dy

u = v - 0 T = T

— = 0

u=ue,v=0^

L-î-JT=t

Fig. 2. Domain of study and boundary conditions.

3. Results and discussion

Heat balance equations at the glazing, the absorber and along the flow channel, given by equations (1), (2) and (3) were solved numerically using Gauss- Seidel iterative method with a relaxation factor in Fortran [11]. Commercial computational fluid dynamics software FLUENT version 6.2 was used in the numerical simulation of air flow and heat transfer through the chimney. Technical data related to the geometrical prototype of the chimney used in the numerical simulation are those of Ong and Chow [3].

Fig. 3 a gives the monthly evolution of average solar radiation and instantaneous solar chimney efficiency of Adrar site. We can note that the minimum of radiation is in December month with a value of approximately 219 W/m2 and the best solar radiation is in June with a value of approximately 588 W/m2. The corresponding instantaneous solar chimney efficiency varies between 15 and approximately 40%.

Fig. 3b shows the monthly mean temperature evolution of the glazing, the fluid flow and the absorber wall. We must underline that the mean absorber wall temperature is higher than that of the fluid flow and glazing, which is predictable, because thermal radiation absorption induces an increase in absorber wall temperature which affect the air flow in contact with these walls.

Months of the year Months of the year

Fig. 3. (a). Monthly evolution of average solar radiation and instantaneous solar chimney efficiency; (b). Monthly mean temperature evolution of the glazing, the fluid flow and the absorber wall.

Months of the year Months of the year

Fig. 4.(a). Monthly evolution of air change per hour for different values of z/Lg and for: d/W= 0.1; (b). Monthly evolution of mass flow rate for different values of z/Lg and for: d/W= 0.1.

Figs. 4a et 4b illustrate the monthly evolution of the air change per hour (ACH) and mass flow rate parameters for different values of z/Lg and for a fixed value of d/W. As it is shown in these figures, their maximum corresponds to the maximum value of incident solar radiation. These parameters increase with the flow rate increasing following the absorber wall aperture size and no difference between curves can be observed when z/Lg trends to be equal to 0.3. This result is also confirmed by [4].

Fig. 5 shows the monthly evolution of the outlet air flow velocity for different values of d/W for Adrar region. We can note that the maximum magnitude is obtained between May and July months owing to incident solar radiation intensity during this period. The maximum of airflow velocity is obtained for small values of the width, d, of the channel because for one fixed flow rate, the velocity increases when the section decreases.

Fig.5. Monthly evolution of air flow outlet velocity for different values of d/W and for: z/Lg= 0.1.

Fig. 6. (a). Iso-velocity lines for: d=0.6m and H= 500W/m ; (b). Flow velocity vectors distributions for: d=0.6m and H= 500W/m.

Fig. 6a shows the iso-velocity lines for the chimney width of 0.6 m and the incident solar radiation of 500W/m2. We must underline that the maximum of velocity is located near the main inlet area. The sudden contraction increases the air velocity in this zone due to the vena-contracta effect. Fig. 6b shows the flow velocity vectors for the chimney width equals to 0.6 m and for the incident solar radiation equals to 500W/m2. The air gap between the absorber and the glass cover, d, plays an important role in the ventilation rate.

Figs. 7 and 8 show the flow patterns comparison between the results obtained in this study and those of [4] for different values of width and for fixed incident solar radiation value. As it is shown, a good agreement is obtained between the results.

Fig.7. Stream functions for d=0.3m and H= 300W/m2. Results comparison. (a)- Results obtained in this study; (b)- Results of [4].

Fig. 8. Stream functions for d=0.2m and H= 300W/m2. Results comparison. (a). Results obtained in this study; (b). Results of [4].

4. Conclusions

The work presented in this study is related to an energy system analysis based on passive cooling system for dwellings. ^ZezIt consists to solar chimney energy performances determination versus geometrical and environmental considerations. Adrar site located in the southern region of Algeria is chosen for this study according to ambient temperature and solar irradiance technical data availability. The glazing temperature distributions, the chimney mass flow rate, the internal wall temperatures and the air room change per hour (ACH) parameter are presented and discussed. Obtained results show that:

- The maximum of airflow velocity is obtained for small values of the width of the channel because for one fixed flow rate, the velocity increases when the section decreases.

- The influence of the incident solar radiation is the important parameter on energy performances analysis of the chimney and an optimum design between the width of the chimney and the aperture of the absorber wall may exist for increasing ACH parameter.

- The air gap between the absorber and the glass cover plays an important role in the ventilation rate.

- The maximum of velocity is located near the main inlet air flow area. The sudden contraction increases the air velocity in this zone due to the vena-contracta effect.

- A good agreement is observed between the results obtained in this study and those of [3] and [4].

References

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