entropy generation analysis of the solar chimney … the objective of this paper is to determine the...

Tewfik CHERGUI1 , Amor BOUHDJAR1, Hocine BOUALIT1 & Salah LARBI2

1Centre de Développement des Energies Renouvelables -Alger – Algérie

2L G M D, Ecole Nationale Polytechnique Alger

[email protected]

World Renewable Energy Forum Denver May 13-17, 2012

mailto:[email protected]; [email protected]

Objective: The objective of this paper is to determine the optimal

geometrical configuration of Solar Chimney used for

a Power Plant (SCPP) through the analysis of the

effect of some geometrical forms and physical

parameters

To this purpose, the concept of entropy generation

minimization was investigated for a finding of optimal

geometrical configuration .


Solar chimney power plant Description

Fig.1 Schéma description of solar power plant

Tower

turbine

Collector

Solar chimney Prototype

1. Solar collector

2. Tower

3. Turbine

2

A typical solar chimney power plant is made of : • air solar collector in which by greenhouse effect, the air gets high energy level

• of a chimney tower permitting air circulation because of density gradient

• and an aero generator in order to produce electrical energy


Operational principle

Solar radiation

Chimney

Turbine

Fresh air inlet

Collector

Fresh air inlet

Operational principles

1. Solar collector

2. Tower

3. Turbine

Solar chimney is a natural power generator by using solar radiation in order to increase the enthalpy of the air circulating in the system and this produces an ascendant air movement. The kinetic energy is then transformed in electrical energy.

3


A global analysis developed for the system generates,

among other results, for steady-state laminar natural convection, the velocity of the fluid:

And this is dependent on the system geometry, the

environmental data and the physical properties of

the air.


Several methods of optimization have been considered

by authors:

As it was done by some authors for different systems,

the evaluation of the entropy generation might be

considered as an optimal design criteria.

In these criteria, the total entropy generation in the

designed systems can be minimized under some

physical and geometric arrangements, and an optimal

configuration with minimum loss of available energy

may be obtained.


The development of improved thermal designs is

enhanced by the ability to identify clearly the source

and the location of entropy generation.

In order to study the flow field, equations governing a

2-D laminar natural convection flow in cylindrical

coordinates with Boussinesq approximations,

negligible compressibility effects and viscous

dissipation, are considered through this generic

conservation equation.

S

yyrr

rrv

yru

rrt

∂

∂

∂

∂

∂

∂

∂

∂1

∂

∂

∂

∂1

∂

∂


The existence of a thermal gradient between the ground of

the collector and the moving air sets the fluid in a non-

equilibrium state which causes entropy generation in the

system which can be expressed, in a 2-D cylindrical

coordinate system, by:

222222

22

y

v

r

u

Ty

v

r

u

r

u

Ty

T

r

T

T

kSgen


In our study and since the flow is regarded as

incompressible, the interest is the determination of the

maximum velocity, any configuration can generate.

Considering the collector height and the driving force

of the flow i.e. the temperature difference through the

Rayleigh number, the velocity field through the

system is evaluated.

So the investigation considers mostly the geometrical

configuration of the system.


A. Straight Junction B. Curved junction

D. Slanted canopy with curved junction

C. Slanted canopy, curved junction with diffuser

Fig. 2 Basic configurations


A. Straight Junction B. Curved junction

Fig.3 Dimensionless iso velocity lines for

configurations A and B, at Ra=105


C. Slanted canopy with curved junction D. Slanted canopy, curved

junction with diffuser

Fig.3 Dimensionless iso velocity lines for

configurations C and D, at Ra=105


Curved junctions generate well-distributed

temperature fields, recirculation-free flow (Fig. 3b),

as well a higher mass flows (table 2).

Straight junction configurations show corner point

which cut a stream line at the base of the chimney

(Fig. 3a).

Curved junctions with a diffuser show no

recirculation (Fig. 3d).

Inclined covers may facilitate the appearance of

recirculation patterns similar to Bernard cells.


Case Hc1 Rin Rex Vmax

Straight junction 0.01 - - 0.24 10-3

Curved junction 0.01 0.025 - 1.6 10-3

Slanted /curved 0.07 0.025 - 1.5 10-3

Slanted / diffuser 0.07 0.025 0.05 0.6 10-3

Table 2 configurations and maximum velocity for simulated

cases (Ra=105)


Slanted configuration gives a large local/global entropy generation

Straight junction presents the lowest value of both local and global generation.

Entropy generation rate follows the evolution of the velocity.

High velocity goes along with high entropy generation rate

Global entropy generation increases with the Rayleigh number increase.

Global entropy generation changes significantly with respect to the Rayleigh number at some values.


Table 3 Local and global entropy generation rate (Ra=108)

Case Ycent Xcent Smax,Local SGlobal

Straight junction 0.019 0.73594 1.71 10-5 0.01269

Curved junction 0.019 0.74483 1.64 10-5 0.01347

Slanted /curved 0.0197 0.73225 2.34 10-5 0.02234

Curved/diffuser 0.020 0.753 1.65 10-5 0.01291


Concluding remarks:

A straight form for the cover chimney junction gives smaller flow rate,

due to the occurrence of junction flow recirculation.

The use of a curved junction allowed higher flow rates.

The introduction of a deflector did not bring major thermal or

hydrodynamic improvements.

As this study deals with a laminar flow, more investigation is needed to

tackle the turbulent flow.

Other configurations are to be considered in order to improve the

performance of the system.

Question : how to minimize the entropy generation and to increase the

flow rate?


entropy generation analysis of the solar chimney … the objective of this paper is to determine the...

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