lecture vka -7 [Compatibility Mode]

8/7/2019 lecture vka -7 [Compatibility Mode]

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Thermal Quantities

BAR029

Ar. Jatish B.


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Thermal Mass

What is Thermal Mass?

Types of Thermal Mass

Historical Applications

Thermal Pro erties of Materials

Analyzing Heat/Cool Storage

Strategies

Other Factors


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Thermal Mass Thermal mass refers to materials have the

capacity to store thermal energy forextended periods.

Thermal mass can be

used effectively to

absorb daytime heat

gains (reducing coolingload) and release the

heat during the night

(reducing heat load).


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Types of Thermal Mass Traditional types of thermal mass include

water, rock, earth, brick, concrete, fibrous

cement and ceramic tile.

Phase change materials store energy while

ma n a n ng cons an empera ures, us ngchemical bonds to store & release latent heat.

PCMs include solid-liquid Glaubers salt, paraffin wax, and the newer

solid-solid linear crystalline alkyl hydrocarbons (K-18: 77oF phasetransformation temperature). PCMs can store five to fourteen times

more heat per unit volume than traditional materials. (source: US

Department of Energy).


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Historical Applications The use of thermal mass in shelter dates

back to the dawn of humans, and untilrecently has been the prevailing strategy for

buildin climate control in hot re ions.

Egyptian mud-brick storage rooms (3200 years old).


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Today, passive techniques such as thermal mass are ironically

considered alternative methods to mechanical heating and

cooling, yet the appropriate use of thermal mass offers an efficient

integration of structure and thermal services.

The lime-pozzolana (concrete) Roman Pantheon


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Thermal Terms for Architects

Conductance

Multilayer body

Surface conductance

Cavities

Convection

Radiation

Measurement of radiation

Sol-air temperature

Solar gain factor


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Specific heat of a substance is the amount of heat energy necessary

to cause unit temperature increase of a unit mass of the substance.

It is measured in: J/kg degC Latent heat of a substance is the amount of heat energy absorbed

by unit mass of the substance at change of state (from solid to

liquid or liquid to gaseous) without any change in temperature, It is

measured in: J/kg

Thermal capacity of a body is the product of its mass and the

specific heat of its material. It is measured as the amount of heat

required to cause unit temperature increase of the body, in units ofJ/degC

It must be noted that density is often taken as an indicator of

conductivity: higher density materials normally have a higher

conductivity or /r-value, but there is no direct or causalrelationship between the two quantities. The apparent relationship

is due to the fact that air has a very low conductivity value, and as

lightweight materials tend to be porous, thus containing more air,

their conductivity tends to be less


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Thermal Properties of MaterialsThe basic properties that indicate the thermal behavior of

materials are: density (p), specific heat (cm), and conductivity (k).

The specific heat for most masonry materials is similar (about0.2-0.25Wh/kgC).

Material Density(kg/m3)

Concrete 600-2200Stone 1900-2500

Bricks 1500-1900

Earth 1000-1500 (uncompressed)

Earth 1700-2200 (compressed)

,

of masonry materials, regardless of its type (concrete, brick,

stone, and earth).


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The more porous a material, the greater the increase in conductivity

with increased moisture content. Whilst conductivity and resistivity are properties of a material, the

corresponding properties of a body of a given thickness are

described as conductance (C). Conductance is the heat flow rate

through a unit area of the body (i.e. the density of heat flow rate)

when the temperature .difference between the two surfaces is 1

degC.

In addition to the resistance of a body to the flow of heat , aresistance will be offered by its surfaces, where a thin layer of air

film separates the body from the surrounding air. A measure of this

is the surface or film-resistance, denoted thus: 1/f(m2 degC/W)f

being the surface or film-conductance (W/m2 degC). Conductance

has been defined in these terms. If the heat flow from air on one

side, through the body, to air on the other side is considered, both

surface resistances must be taken into account. R =1/f + R + 1/f


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The reciprocal of this air-to-air resistance is the air-to-air

transmittance, or Uvalue. This is the quantity most often used in

building heat loss and heat gain problems, as its use greatly

simplifies the calculations. Values for everyday constructions are

given but if the U value of a particular construction is not found in

the table, it can be computed from its component factors . Its unit ofmeasurement is the same as for conductance W/m2 deg C , the

only difference being that here the air temperature difference (and

.

If an air space or cavity is enclosed within a body, through which

the heat transfer is considered, this will offer another barrier to the

passage of heat. It is measured as the cavity resistance, (Rc) which

can be added to the other resistances described above. Radiation received by a surface can be partly absorbed and partly

reflected: the proportion of these two is expressed by

Absorbance (a) + Reflectance (r) = 1for perfect black body : a = e (emittance)=1 & r = 0


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For building design purposes it is useful to combine the heating effect of

radiation incident on a building with the effect of warm air. This can bedone by using thesol-air temperature concept. A temperature value is

found, which would create the same thermal effect as the incident

radiation in question, and this value is added to the air temperature:

Ts = To + (I x a) / fo

T0 = outside air temperature, in C

I = radiation intensity, in W/m2

a = absorbance of the surface fo = surface conductance (outside), W/m2degC

The solar gain factor is defined as the heat flow rate through the

construction due to solar radiation expressed as a fraction of the incident

solar radiation. As this value can be related to the increase in the innersurface temperature, a performance requirement can be established on the

basis of experience, in terms of this solar gain factor.

q/I = (U x a) / fo


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Thermal Time ConstantOne of the more important mathematical constructs to imagine thebehavior of thermal mass is the Thermal Time Constant of an

building envelope, defined as the product of the heat capacity (Q)

and the resistance (R) to heat transmission. The TTC is

representative of the effective thermal capacity of a building.To calculate the TTC of an area, the heat capacity per unit area (QA) is multiplied by the resistance to

heat flow of that area ( where QA=thickness*density*specific heat, R=thickness/conductivity).

A , A ,

the outside and inside air layers, is calculated in sequence. The QAR for each layer is calculated fromthe external wall to the center of the section in question, thus:

QAiRi= (cm*l*p)i*(R0+R1++0.5Ri)

For a composite surface of n layers, TTCA=QA1R1+QA2R2+QAnRn .

The TTCs for each surface is the product of the TTCA multiplied by the area. Glazed areas areassumed to have a TTC of 0. The total TTC total of the buliding envelope equals the sum of all TTCsdivided by the total envelope area, including the glazing areas.

A high TTC indicates a high thermal inertia of the building and

results in a strong suppression of the interior temperature swing.


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Example TTC Calculations

Wall 1: exterior insulation

insideThermal

Wall 2: interior insulation

outside insideThermal

mass

mass

TTC = 43.8

TTC = 7.8 Source: Givoni


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TTC and DHCRelative values of TTC indicate the thermal capacity of the

building when a building is affected mostly by heat flow across

the opaque parts of the envelope (i.e., when it is unventilated, and

when solar gain is small relative to the total heat transfer throughthe building envelope).

Relative values of DHC, on the other hand, indicate the thermal

capacity for buildings where solar gain is considerable. The DHCalso is a measure of how much coolth the building can store

during the night in a night ventilated building.

Both measures indicate the amount of interior temperature swing

that can be expected based on outdoor temperatures (higher

values indicate less swing).Delta T(swing)= 0.61Qs/DHCtotal,

Qs is the daily total solar energy absorbed in the zone.


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TTC and DHC Examples

Building which is externally insulated with internal exposed mass.Here, both TTC and DHC are high. When the building is ventilated at night and closed

during the day, it can absorb the heat in the mass with relatively small indoor temperature

rise. Best for hot-dry regions.

Building with mass insulated internally.

Here, both the TTC is and DHC are low. The mass will store energy and release energy

mostly to the exterior, and the thermal response is similar to a low mass building.

u ng w t g mass nsu ate externa y an nterna y.

Here, the building has a high TTC, but a negligible DHC, as the interior insulation separates

the mass from the interior. When the building is closed and the solar gain is minimized, the

mass will dampen the temperature swing, but if the building is ventilated, the effect of the

mass will be negated. With solar gain, the inside temperature will rise quickly, as the

insulation prevents absorption of the energy by the mass.

Building with core insulation inside two layers of mass.

Here the TTC is a function of mostly the interior mass and the amount of insulation,

and the DHC is a function on the interior mass. The external mass influences heat loss

and gain by affecting the delta T across the insulation.


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StrategiesSlow rate of indoor heating in summer (minimize solar gain).

Fast rate of indoor cooling and ventilation in summer evenings.Higher indoor temperatures during the day in winter.

Slow release of stored heat during winter night.


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Other Factors to Consider Hygroscopic & vapor diffusion properties,

enthalpic response Ventilation, convective heat exchangers,

and eva orative coolin methods

Insulative additives to cast thermal mass

Fire resistance, earthquake behavior, and

building codes Acoustics

Life Cycle Analysis

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