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

0

5

10

15

20

25

30

35

40

1995 1997 1999 2001 2003 2005

Year

% o

f N

ew

Off

ice

Bu

ildin

gs

RFUFAD

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

]