iThemba LABS
Theoretical modeling and
experimental verification of a passive
downdraught evaporative
cooling tower
Stellenbosch University
SAHPA® Energy Postgraduate Conference 2013
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
Introduction
• HVAC accounts for substantial
amount of energy use in
buildings (4000 gWh annually
in S.A.)
• Greenhouse gases (GHG)
emissions from HVAC account
for up to 22% of total GHG
emissions.
• Reducing the total HVAC load
would result in a reduction in
operating costs, GHG emissions
and related environmental
impacts
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
2
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
Motivation
• Adequate ventilation for buildings is needed
• Air-conditioning and mechanical ventilation expensive
running cost
•
Natural ventilation cheap or with no running cost
•
PDEC (Passive Downdraught Evaporative Cooling) efficient
method of passive cooling in arid regions
• Design Strategies include:
• CFD
• Empirical Methods
• 2D Axisymmetric Theoretical Model
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
3
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
PDEC Principle
Psychrometric Chart
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
4
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
PDEC Principle
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
5
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
PDEC in practice
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
6
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
2D Model of PDEC
Control Volume Grid
Inlet Domain
r
PDEC Inlet
Water saturated Solid insulated wall
medium
Outside air
(1,1)
Axis of
symmetry
(2,2)
(m,n)
z
PDEC Outlet
rpor rwall renv
Outlet Domain
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
7
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
2D Model of PDEC
• Conservation of Mass
• Mass diffusion
• Conservation of energy
• Conservation of momentum
  mC pT 
(mvz )
(
i , jmom
)
(mSvm
)

(
m
v
)

mom


 zEmh
 Nmh
  mh  S zW(i , j ),inmh
zS ( i , j ),out
zNW( i , j ),in
( i , j ),
outN
 E  t
v t
 
 
 
mdiffvrS),(Wi , jA) Wm
(  vr ) E 
AE m
 (diff
 vNz,()i ,Sj )A
( diff
vzW) N,( i ,Aj )N 
 mdiffVE ,(i , 0j )
S  (
Acond
tE ,Em(i , j ) g
PN A
 ,SrzW( i, j ) Q
AWcond,W rzE( i, j )Q
t PS AS   zzN( i, j ) AQNcond , NzzS( i , j ) AQScond
N 

i, j

i, j
i , j 
i , j 
i, j
i, j
i , j 

i , j 

SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
8
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
CFD Model of PDEC
Transport Equations
In the air
 (  )

   v        S
t
Change = Convection + Diffusion + Source
where  may be
• 1 (continuity)
• velocity components u, v
• turbulent kinetic energy k
• turbulent dissipation rate 
• temperature
• water vapour concentration
The standard solver can
be customized to meet
the requirements unique
to a particular
application by using the
user-defined functions
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
9
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
CFD Model of PDEC
PDEC laminar flow in room
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
10
Introduction
iThemba LABS
Motivation
PDEC
2D Model
CFD
Conclusion
Conclusions
•
Important to understand the fundamentals of the downdraft evaporative cooling
•
PDEC tower can’t maintain a constant performance due to
a strong climatic dependency
•
PDEC towers may not be suitable to humid climates
without significant advancement
•
CFD tools have been extensively used to simulate flow
and thermal fields for a variety of applications.
• Climatic data with Theoretical models could benefit the
design and implementation of PDEC towers
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
11
iThemba LABS
Thank you
SAHPA®
South African Heat Pipe Association
Energy Postgraduate Conference EPC2013, 11-14 Aug 2013
12