Energy-efficient buildings
description
Transcript of Energy-efficient buildings
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 Modern buildings
• Well shaded• Tall interior spaces• Heavyweight• Loose construction
• 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 consideredat 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 single-sided ventilation
Positive pressures on windward side
Negative pressures on leeward side and roof
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 requirescooling
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 sidenormal full
open
Leeward sidenormal 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
gdz
dp
In a fluid at rest the weight of the fluid produces an increase in pressure with depth
Air is well represented as a perfectgas
RTp
Pressure in air at rest is hydrostatic, so pressure gradient is
The neutral level
RT
gp
dz
dp
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
warm
height
neutral level
pressureNeutral level is the height where internal and external pressures are same
The neutral level
warm
height
neutral level
pressurep4
p3
p2
p1 p1 p2
p3 p4
p4 > p3 - pressure difference drives inflow
p2 > p1 - pressure difference drives outflow
To stratify or not to stratify …
Minimum flow rate
Maximum outlet temperature
Maximum flow rate
Minimum outlet temperature
Displacement ventilation
Mixing ventilation
QT
QT
Q
T+T
T
QT
T+T
Displacement Mixing
Filling box – Baines & Turner (1969)Caulfield & Woods (2001)
Mixing flow – draining a hot space
1 window and 1 skylight
2 skylights
Mixing flow – draining a hot space
Displacement flow – draining a hot space
inflow
Single plume with displacement ventilation
inflow
outflowLinden, Lane-Serff & Smeed (1990)
Single source of buoyancy with displacement ventilation
•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
QT
QT
Q
T+T
T
TT
T
ub
ut
h
H
T
Tgg
'
Flow rate Q u A u At t b b * *
)(222 hHguu bt
AA A
A A
t b
t b
** *
* *
22 2
2
1* )]([ hHgAQ
**bt AA ** 2 tAA →
local control
Turbulent plume
wue eu
B
b
z
Plume width grows by entrainment
w
Morton, Taylor & Turner (1956)
Entrainment constant α ≈ 0.1
buoyancy flux
volume flux
reduced gravity
B G Q
Q cB z1
3
5
3
G c B z1
2
3
5
3
3
23
1
10
9
5
6
c
Steady state
Match draining flow with MTT plume
buoyancy flux
volume flux
reduced gravity
At z = h equate
B G Q
Q cB z1
3
5
3
G c B z1
2
3
5
3
- volume fluxes
- densities
g G c B hz h
12
3
5
3
3
23
1
10
9
5
6
c
3
5
3
1
2
1* )]([ hcBhHgA
2
1
2
5
22
3
*
1
Hh
Hh
Hc
A
Children’s Museum, San Diego
Underfloor air distribution (UFAD)
• Cooling part of the space• Effect on IAQ• Plenum flow
Technology Overview - UFAD ConceptUFAD – the conceptual design
heat transfer from room into plenum causes supply air to warm up
Market Trends- USA
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stratificationlayer
Under Floor Air DistributionUFAD
Heat sourceCooling vent
Initial case1 heat source and 1 cooling vent
outQ
Q MB
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
Tem
perature [F]
Temperatures in plenum T
emperature [F
]