4 Thermal Performance of Roofs - J. Adams

Thermal Performance of Roofs –

REFLECT OR ABSORB.Jamie Adams

BlueScope Steel Research

July 2007

2BlueScope Steel Research July, 2007

Thermal Performance of Roofs – REFLECT OR ABSORB.

• Influence of Roof.

• Saving Energy in Australian Buildings.– Influence of Climate.

– Potential savings.

• COOL Roofs

• Tailoring Solar Reflectance

• Smart Roofs– Air heaters / coolers.

– Water heaters.

– PV.


Influence of the Roof – Energy Efficient Design.

Important Design Strategies:

• Orientation - Northerly Aspect - catch winter sun- exclude summer sun (eaves)

• Shading - eaves + western side shading

• Ventilation - catch prevailing cooling summer breezes- exclude prevailing cold winter winds

• Insulation - minimise heat loss & gain through roof, walls, & floor.

• Roofing - most exposed element- appropriate selection compliments other design strategies.


Influence of the Roof – Ratio of External Fabric.

Influence < 10 %> 60 %


Influence of the Roof – Heat Flow.

Solar Radiation

RadiationConvection

Keep Heat OutHigh Solar ReflectanceHigh Thermal Emittance

Keep Heat In – vice versa

CLIMATE


Saving Energy – Influence of Climate.

hot humid desert

alpinewarm temperate



hot humid

alpine

Most people live in either

warm humid,

warm temperate, or

mild temperate.

(All have average maximum summer temperatures of between about 24°C to 30 °C)



Residential Buildings

Heating and Cooling Saving From Higher Solar Reflectance(Assumptions:- Base SR=0.05, E = 0.9, R3.35 Roof Insulation, Electrictity @ 15 c/kWh, Conditioner Efficiency 2.0)

$0.0

$0.2

$0.4

$0.6

$0.8

$1.0

$1.2

$1.4

$1.6

$1.8

$2.0

0 1 2 3 4 5 6 7 8 9 10

Heating Degree Days / Cooling Degree Days

Re

du

ce

d H

ea

tin

g a

nd

Co

olin

g C

os

t .

($ p

er

square

mete

r per

year)

.

SR=0.25

SR=0.35

SR=0.45

SR=0.55

SR=0.65

SR=0.70

SR=0.75

SR=0.80

SR=0.85

Brisbane

Pert

h

Sydney

Melb

ourn

e

Black to Pure White.

warm humid > $ 1/ m2.annum (€ 0.63)

warm temperate $0.15 to $0.50 / m2.annum (€ 0.09 -€ 0.32)

mild temperate < $0.10/ m2.annum (< € 0.06)



In Australia

Mostly better to REFLECT Solar Radiation Than

ABSORB to save energy in a building.

(warm – very important; cool – not so important)

This is recognised in the Australian Building

Code through Insulation Concessions.


Saving Energy – Real Building Study.

FSEC Roof Study

Street scene

• 6 identical homes (roofing material varied)

• R3.4 ceiling insulation.

White tile Terracotta tile White metal

White Metal Roof Performed Best

• 23% less total energy than dark shingles

• better than white tiles because it cools quickly at night.

• Peak energy reduced by 28% relative to dark

shingles.


Broader Benefits of REFLECTING Heat.

Cool Roof

Long lasting

roof system

Less

Maintenance

Reduce AC Use

(Total / Peak)

Lower Outdoor

TemperatureLess Smog

Improved

Air Quality

Reduce

Energy Demand

Financial

Savings

Less GGE

Material

Sustainability

+ Comfort

+ Aesthetics


Tailoring Solar Reflectance - Solar Radiation.

0.2 0.4 0.7 2

UV VisibleSpectrum

NearInfrared

Wavelength(µm)

Relative Energy

More than half of the suns energy.

Reflectance is strongly influenced by pigment

choice

Sets the Appearance

-colour, brightness.

Least energy BUT

Most damaging. Low UV

transmittance to avoid

delamination.

Comments

SOLAR RADIATION

3% 42% 55%


Tailoring Solar Reflectance.

0.2 0.4 0.7 2

UV VisibleSpectrum

NearInfrared

Wavelength(µm)

Relative Energy

SOLAR RADIATION

3% 42% 55%

Controlled by colour.

Cool Colours

Pure White

Jet Black

Coolest

Hottest

Coolest where colour is set.


Tailoring Solar Reflectance – A natural progression.

MID RANGE COLOUR.

0

10

20

30

40

50

60

70

80

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Wavelength (nm)

Refl

ecti

vit

y (

%)

Lab Improved - SR = 40%

Current COLORBOND - SR = 34%

1998 - SR = 31%

1982 - SR = 22%

UV Visible NIR


Smart Roofs.

Generally Better to REFLECT Heat

UNLESS You can use the Heat Usefully.

Trap & Transfer Heat

-supplement air conditioner

(evaporate refrigerant)

Create Air Currents

-expel hot air / cool air in

-drive evaporative process

Hot Water

Heating

Cooling


Smart Roofs – Steel Roof Application Advantages.

High conductivity

– heat can be rapidly transferred to another medium.

Low thermal mass

– responsive to changes in thermal conditions, e.g. small periods of sunshine.

Can create water tight and sealed spaces.

Lightweight

– aiding installation and limiting need for additional support.


Smart Roofs – BlueScope Steel Involvement.

Utilising the heat between profiled ribs and insulation.

Solar air heater – experimental.

100 mm

Slate Grey COLORBOND® steel

double sided reflective foil laminate

BlueScope Steel Testing

On a warm winters day achieved

cavity air temperature of 50°C.

(air temp 19°C)

Performance affected by wind.




Solar air heater / cooler – commercialised (currently small volume).

Daytime Heating





Daytime Cooling





Nighttime cooling



Placing translucent material above sheeting.

Solar air heater / water heater - experimental

Sunny Autumn Day about 20 °C

– water 30 to 62°C (flow rate).

– air 50 to 65°C (flow rate).



Placing translucent material above sheeting.

Solar air heater

- installed on home.

- small scale commercialisation planned late 2007.

- Cheap PCM storage required to justify costs (to make stand alone system).

- amenable to hot water.

Good performance / costs prohibitive to large market growth / currently green minded only.

Glazing

Air Flow

Corrugated

Plate

Barrier

Solar

Radiation



Placing translucent sheeting above

Better performance and easily amenable to water heating

BUT

Complexity (system & installation), overheating, durability of components.

Versus Unglazed

Simple, easier to install, cheaper

BUT

Potentially at the cost of performance.

BEST – System cost / benefit.


Smart Roofs – PV in Australia.

Photovoltaic Cells

In Australia becoming a viable option with large government subsidy.

1 kW back to the grid system has a payback of about 16 years.

retail cost about $12,000 (€ 7,560).

less gov. rebate $8,000 (€ 5,040).

cost $4,000 (€ 2,520).

approximate saving $250 p.annum (€ 158).

simple payback 16 years.

Continued Excellent Growth Prospects:

� Costs of all components expected to continue to fall.

� New thin film lower cost technologies evolving.

� Conventional coal fired power station electricity generation set to rise.

(Current residential rate about $0.15 kWh or ( € 0.09 kWh)


Smart Roofs – PV BlueScope Steel Involvement.

BlueScope Steel looking at the natural advantage arising from steel roofs for thin films technology.

For new or existing roofs.

Custom made profile (non structural).


CONCLUSIONS

• Roofs can have a large thermal influence on buildings.

• In Australia it is Better to REFLECT heat - financial + broader environmental benefits.

Generally true for any climate that has a warm or hot summer.

HOWEVER

• Better to ABSORB heat if it can be utilised to supplement air heating or cooling or water heating.

• SMART Roofs (Multi-function) will have increasing demand.

Documents

4 Thermal Performance of Roofs - J. Adams