Energy-efficient buildings
Paul Linden
Department of Mechanical and Aerospace Engineering
University of California, San Diego
Outline
• Wind-driven flow
–
–
–
–
Historical perspective
Environmental perspective
Flow through an orifice
Wind-driven flow through a building
• Stack-driven flow
–
–
–
–
The neutral level
Thermal plumes
Displacement ventilation produced by a single heat source
Mixing ventilation
• Underfloor air distribution
– Non-uniform cooling
– Flow in the plenum
Wind-driven flow
– Historical perspective
– Environmental perspective
– Wind-driven flow through a building
Yazd, Iran
Traditional wind tower, Iran
Al Arish, UAE
Jame Mosque Isfahan, Iran
Sheik Lotfollaf Mosque, Isfahan, Iran
Mai Hong Song, Thailand
Namwam banquet hall, Korea
Energy usage
Over 10% of total annual energy consumption in
the US is used in heating and cooling of buildings
– at a cost > $100B per annum
In LA, more energy is used in buildings than in
transport
Built environment is responsible for > 30% of GHG
emissions in US
Traditional buildings
•
•
•
•
Well shaded
Tall interior spaces
Heavyweight
Loose construction
Modern buildings
•
•
•
•
Highly glazed
Low interior spaces
Lightweight
Tight construction
Ventilation requirements
• For breathing and general fresh air require about
10 ls-1 per person
For a typical one-person office (5 m X 3 m X 2.5
m) ⇒ 1/6 ACH
This is a very low ventilation rate – to remove the
heat (100 W) generated by 1 person this flow
rate would require an interior temperature about
10 K above the ambient.
Ventilation strategies
• Natural ventilation
– flow driven by wind and temperature
• Forced air – mechanical ventilation
– fan-driven through ducts
• Traditional HVAC
– mechanical cooling, overhead distribution
• Unconventional HVAC
– mechanical cooling, unconventional distribution
• Hybrid ventilation
– combinations of the above systems
Low-energy strategies
• Low-energy ventilation
• Night cooling
• Thermal storage
These have implications for the building
forms and structure – need to be considered
at an early stage in the design
Natural Ventilation
Ventilation driven by natural pressure
forces
• wind
• buoyancy - due to temperature differences;
the ‘stack effect’
A temperature difference of 50C across a
doorway 2m high will give a flow of 0.1ms -1
Wind-driven ventilation
cross ventilation
Positive pressures on windward side
Negative pressures on leeward side and roof
single-sided ventilation
Cross ventilation rules of thumb
• Codes allow a zone to be considered “naturally ventilated” if
within 6m of an operable window
Thermal zoning rules of thumb
6m glazed perimeter
zone is affected by
external environment
Stable interior zone
always requires
cooling
ASHRAE field research: Brager & deDear
• Occupants in controllable naturally ventilated offices accept a wider
range of comfort as acceptable
San Francisco Federal Building
Building geometry in the
naturally ventilated floors
•
•
The building will be naturally cross-ventilated (C-V)
in most of the floor plan in floors: 6-18.
The building volume with C-V
measures:
107x19x52 m and starts at an elevation of 20 m.
Windward side
normal full open
Leeward side
normal full open:
2- BMS + Informed Users
3- BMS + No Night Cooling
4- BMS + Uninformed Users
5- No BMS + Uninformed users
Stack-driven ventilation
– The neutral level
– Thermal plumes
– Displacement ventilation produced by a single heat
source
– Mixing ventilation
Ionica, Cambridge
Portland Building, UK
BRE low energy office building
Inland Revenue Building, UK
Architect: Michael Hopkins &
Partners
Naturally ventilated office block –
control at towers and fans at each
vent opening allow outdoor air to
cool the indoor space. Exposed
concrete ceiling, daylighting
Hydrostatic pressure gradient
In a fluid at rest the weight of the
fluid produces an increase in
pressure with depth
dp
  g
dz
Air is well represented as a perfect
gas
p  RT
The neutral level
Pressure in air at rest is
hydrostatic, so pressure
gradient is
dp
gp

dz
RT
Thus pressure increases downwards and the
gradient is larger when the air is cooler
For a warm building the pressure gradient inside is
larger than outside
The neutral level
height
warm
neutral level
Neutral level is the height
where internal and external
pressures are same
pressure
The neutral level
p1
height
p1
p2
p2
warm
neutral level
p3
p4
p4 > p3 - pressure difference drives inflow
p2 > p1 - pressure difference drives outflow
p3
p4
pressure
To stratify or not to stratify …
Displacement ventilation
Mixing ventilation
Minimum flow rate
Maximum flow rate
Maximum outlet
temperature
Minimum outlet
temperature
Displacement
Mixing
QDT
QDT
T+DT
T+DT
QDT
T
Q
Filling box – Baines & Turner (1969)
Caulfield & Woods (2001)
Mixing flow – draining a hot space
1 window and 1 skylight
Mixing flow – draining a hot space
2 skylights
Displacement flow – draining a hot space
inflow
Single plume with displacement ventilation
Linden, Lane-Serff & Smeed (1990)
outflow
inflow
Single source of buoyancy with displacement ventilation
QDT
T+DT
QDT
T
Q
•Upper layer has a uniform temperature
•Temperature of upper layer is temperature of plume at
level of interface
•Flow through space is volume flux in plume at level of the
interface
ut
H
T  DT
ut2  ub2  2 g ( H  h)
h
g' g
T
Q  ut At*  ub Ab*
Flow rate
ub
Q  A*[ g ( H  h)]
*
A 
*
2 At Ab
*2
1
2
*
*
At  Ab → A*  2 At*
*
At  Ab
DT
T
*2
local control
Turbulent plume
Morton, Taylor & Turner (1956)
z
b
Plume width grows by entrainment
u e  w
w
ue
B
buoyancy flux
volume flux
reduced gravity
Entrainment constant α ≈ 0.1
B  G Q
1
3
Q  cB z
1
3
6  9  3
c

 
5  10 
5
3
2
3
G  c 1B z

5
3
2
Steady state
Match draining flow with MTT plume
buoyancy flux
B  G Q
6  9  3
c
  
5  10 
1 5
3 3
volume flux
Q  cB z
reduced gravity
2
2
5

3
3
G  c1B z
1
3
1
2
- volume fluxes
At z = h equate
1
3
A*[ g ( H  h)]  cB h
2
3
- densities
g   G  z  h  c 1 B h
5
2
h
 
A*
H

3
1
c 2 H 2 1  h  2


 H
5
3
5
3
Children’s Museum, San Diego
Underfloor air distribution (UFAD)
• Cooling part of the space
• Effect on IAQ
• Plenum flow
Technology Overview UFADUFAD
– the conceptual
design
Concept
heat transfer from room into
plenum causes supply air to
warm up
Market Trends- USA
% of New Office Buildings
40
35
30
25
RF
UFAD
20
15
10
5
0
1995
1997
1999
2001
Year
2003
2005
Under Floor Air Distribution
UFAD
stratification
layer
Initial case
1 heat source and 1 cooling vent
Qout
Cooling vent
Q
M
Heat source
B
Flow in the plume
Heat source
The diffuser flow
diffuser
UFAD
To be used in the new HQ building for the New York Times in
Manhattan
Measurements in plenum
• 75 temperature loggers installed in
underfloor plenum
• Produced color contour plots of hourly
plenum temperature distributions
– September 2 – hot day, night flushing
– September 25 – cooler day, no night flushing
Temperatures in plenum
Movie
Temperature [F]
Temperatures in plenum
Temperature [F]