Quiz 2 - Heat - Arch140

Black Body

Absorbs all SW and LW

High emissivity - Emits max LW (can emit SW)

Low solar reflectivity (albedo)

High heat gain

High heat loss

Examples:

• Dark asphalt

• Sand

• Roofing tar/gravel

• Black paint

White Body

Absorb zero SW and LW

Emit zero LW (Zero Emissivity)

Reflects SW and LW

Low heat loss

Low heat gain

Examples:

• Polish aluminum foil

• Low-e suspended coated polymer film

Perfect Dissipater

Absorbs zero SW

Emits max LW

High heat loss

High solar reflectivity

Examples:

• white plaster/paint

Perfect Collector

Absorbs all SW

Emits zero LW

High heat gain

Low Heat loss

Examples:

• Spectrally selective black coating

Solar reflectivity

Albedo

reflection of shortwave radiation

how much incoming solar energy, or light, is immediately reflected back to space

LW Emissivity vs LW Reflectivity

LW Emissivity:

• a material's ability to emit infrared radiation within the longwave spectrum

• how well it radiates heat

• high emissivity means the material readily releases heat

• only LW

LW Reflectivity:

• how much of that longwave infrared radiation a material reflects back

• high reflectivity means it bounces back most of the incoming longwave radiation

latent heat

energy transferred through a change in state

sensory heat

thermal mass

object property

the ability of a material to absorb, store, and release heat

depends on both the material's properties (like density and specific heat capacity) and the size and volume of the object or system.

relationship between absorption (α), transmission (T), and reflectivity (ρ)

100% incident radiation = radiation absorbed (a) + radiation transmitted (T) + radiation reflected (p)

radiation transmitted = the fraction of radiation that passes through the material (important for transparent materials).

• In opaque materials, T = 0, so:

100% incident radiation =absorbed (α)+reflected (ρ)

balance point temp

outdoor temp at which heat losses from the building exactly match internal gains

the temp below which the furnace turns on

in BPT, heat gain = heat loss (steady state)

If outdoor temp < BPT, heat loss (Temp inside drops)

A lower BPT is achieved by:

• high internal gains to match higher heat loss due to cold outdoor temp

• well-insulated thermal envelope to retain more indoor heat

“__ivity”

property of a material (independent of amount)

2 dimensional

“__ance”

property of an object (depends on amount)

1 dimensional

conductivity

symbol: k

describes how well a material conducts heat

It is a measure of the ability of a material to transfer heat from the hotter side to the cooler side when there is a temperature difference.

conductance

symbol - C

Thermal conductance refers to the ability of an object or material to conduct heat.

property of the object as a whole, not the material itself

depends on:

• thermal conductivity of the material

• the size (area) and thickness of the object

specific heat capacity

material property

the amount of heat energy required to raise the temperature of a substance by one degree Celsius (°C) per unit mass

If a material has low specific heat capacity, it means that the material requires less energy to raise its temperature by a certain amount. In other words, it heats up or cools down faster when energy is added or removed.

volumetric heat capacity

object property

dependent on volume of thing

the amount of heat energy needed to raise the temperature of a unit volume of a substance by one degree

thermal mass functions

• stores energy

• moderates indoor temp fluctuations

• reduces heat flow through envelope

• reduces and shifts peak heating loads

thermal mass applications

• passive solar heating

• passive cooling (night flush)

• commercial buildings

  • passive (structural mass)

  • active (ice storage, radiant systems)

Thermal mass needs

• outdoor temp swings on both sides of comfort zone (dry climates better)

• to be exposed to the indoor air (insulate from exterior)

• to be well-distributed

• cycle continuously

When thermal mass doesn’t work

• humid climates, small diurnal temp swing

• cold climates with overcast skies (little solar gain)

• too much mass not connected to interior

• intermittent occupancy or need for fast response time

direct-gain passive solar system

• south facing windows (building positioning)

• overhang prevents summer solar gain

• insulation on our outside of envelope (keeps heat in)

• thermal mass on inside stabilizes temp fluctuations

• nighttime insulation on glass

indirect-gain passive solar system

sunspace/attached greenhouse

• vents open in day maximizing heat gains, vents close at night to prevent heat loss

trombe wall

• absorbing wall collects/stores heat energy

• natural convection circulates heat

power

energy/time

rate of energy transferred

ACH

air changes per hour (room volumes/hour)

the number of times the air within a defined space is replaced with outdoor air in one hour

thermal bridging

a break in the thermal envelope through which heat can easily transfer (thin-cross section)

example:

• slab, beam, etc

cons:

• can make moisture control an issue “ghosting” “fogging”

sensible heat

temp change

thermometer

convection

transfer of heat between surface and fluid

• induced by temperature differential

forced convection:

• when fluid motion is caused by differences in temperature

• hot air rising

forced convection:

• mechanical force (like a fan, pump, or blower) is used to move the fluid

• mechanically increase rate of heat transfer

radiation

energy transferred by electromagnetic waves across a transparent medium

solar radiation:

• shortwave

• uv, visible light, near infrared

terrestrial radiation

• longwave

• energy radiated from essentially every surface

Series heat flows

sum the resistance of the different layers heat must move through

parallel heat flows

sum the different “conductive” paths of series heat flow

• more than one path

the perfect wall

1. rain control layer

2. air control layer

3. vapor control layer

4. thermal control layer

institutional wall

• long-lasting

• insulation adapted to climate

• hella layers (drainage, insulation, vapor barrier, concrete, etc)

clever wall

like institutional wall but has all-in-one layer with all control layers

bad for environment

commercial wall

uninsulated steel stud cavity

slab-on-grade

a shallow concrete slab that rests directly on the ground

common in warmer climates

interrupt the flow of heat

vapor barrier location

cold, dry air

• inside

warm and humid climates

• outside