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Heat flow - PowerPoint PPT Presentation

Heat flow. Heat is energy: measured in Watts ( = J/s) Temperature is the amount of heat in a material. Measured in degrees Celsius (C o ) Measured in Kelvin (K) 0 K (-273 C o )= material with no heat. Vibration of molecules in a substance. Electromagnetic radiation. Heat flow.

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Heat flow

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Heat flow

Heat flow

  • Heat is energy:

    • measured in Watts ( = J/s)

  • Temperature is the amount of heat in a material.

    • Measured in degrees Celsius (Co )

    • Measured in Kelvin (K)

    • 0 K (-273 Co)= material with no heat


Heat flow1

Vibration of molecules in a substance

Electromagnetic radiation

Heat flow

  • Heat will flow from a hot substance to a cold substance.

  • Heat can flow in three ways:

    • Conduction

    • Convection

    • Radiation


Heat flow2

Heat flow

  • Vibration of molecules:

    • conduction

    • convection


Heat flow3

Heat flow

  • Electromagnetic radiation:


Heat flow4

Heat flow

  • Conduction

  • Conduction occurs through the transfer of vibrational energy from one molecule to the next.

  • Depends on:

    • The cross sectional area

    • The conductivity of the material

    • The temperature difference


Heat flow5

Heat flow

  • Conduction


Heat flow6

Heat flow

  • Convection

  • Convection occurs from a solid to a fluid or gas

    • As temperature increases, density in the fluid decreases

    • The lower density fluid rises

    • This results in convection currents


Heat flow7

Heat flow

  • Convection


Heat flow8

Heat flow

  • Radiation

  • All object emit some electromagnetic radiation.

    • The hotter the object, theshorter the wavelength

    • A hot fire will emit infrared radiation

    • The sun (much hotter) also emits short wave radiation


Heat flow9

Heat flow

  • Radiation


Heat flow through materials

Heat flow through materials

  • Heat flow through materials occurs mainly by conduction. The ability of a material to conduct heat varies:

    • Metals are good conductors

    • Gases are poor conductors


Heat flow through materials1

Heat flow through materials

  • Heat flow through materials depends on:

    • Cross-sectional area

    • Temperature difference

    • Resistivity or conductivity of the material


Heat flow through materials2

Heat flow through materials

  • Resistivity of a material:

    • Ability to resist heat flow

  • Conductivity of a material

    • Ability to allow heat flow

  • Resistivity is the opposite (inverse) of conductivity. If resistivity is low, conductivity is high.


Heat flow through materials3

Heat flow through materials

  • Measuring resistivity and conductivity:

    • Resistance is measures as a R-Value

    • m2 K/W

    • Conductance is measured as a U-Value

    • W/m2 K

    • R = 1/U and U = 1/R

    • If R = 2, then U = 0.5


Heat flow through materials4

1 m2

20 oC

21 oC

Heat flow through materials

  • Conductance : How much energy (in watts) passes through the material?

    • 1 m2 of a material

    • subjected to 1 oC of temperature difference


Heat flow through materials5

Heat flow through materials

  • A building structure usually consists of different materials. The overall R-Value or U-Value can be calculated:

  • Two possibilities:

    • Materials in series

    • Materials in parallel


Heat flow through materials6

Heat flow through materials

  • In series:

    • Resistances are added

    • R = R1 + R2 + R3 …

  • In parallel:

    • Conductances are added

    • U = U1 + U2 + U3 …


Heat flow through materials7

Heat flow through materials

  • Cavities:

    • Cavities increase resistance

  • Heat flow through the cavity

    • Conduction: depends on depth

    • Convection: depends on depth and height

    • Radiation: depends on reflectivity of internal surface


Heat flow through materials8

Heat flow through materials

  • Insulation:

    • Any material with low overallconductance

  • Ways to reduce overall conductance

    • Materials with air bubbles. E.g. glass fibre, mineral wool, polystyrene

    • Reflective materials that reduce the absorption of infrared radiation


Heat flow in buildings

Heat flow in buildings

  • A typical building structure

    • Many components

    • Many building systems (e.g. walls, roofs)

    • Many different thermal conductances

    • In series and in parallel


Heat flow in buildings1

Heat flow in buildings

  • A typical building structure

    • Many components

    • Many building systems (e.g. walls, roofs)

    • Many different thermal conductances

    • In series and in parallel

  • What is the overall result?

    • What is the overall heat loss for a building

    • What is the overall heat gain for a building?


Heat flow in buildings2

Heat flow in buildings

  • Cold climate


Heat flow in buildings3

Heat flow in buildings

  • Hot climate


Heat flow in buildings4

Heat flow in buildings

  • Heat loss = heat out:

    • Through the building fabric

    • Walls, floor, roof, etc

    • Ventilation through windows and doors

  • Heat gain = heat in:

    • from outside sources (e.g. sunlight)

    • from internal sources (e.g. people, lighting, equipment)


Heat flow in buildings5

Heat flow in buildings

  • If heat loss is less than heat gain

    • Building heats up

    • AC is required

  • If heat loss is greater than heat gain

    • Building cools down

    • Heating is required


Heat flow in buildings6

Heat flow in buildings

  • There are six main types of heat loss/gain:

  • Fabric gains

  • Indirect solar gains

  • Direct solar gains

  • Ventilation Gains

  • Internal Gains

  • Inter-zonal gains


Heat flow in buildings7

Heat flow in buildings

  • There are six main types of heat loss/gain:

  • Fabric gains: due to differentials in air temperature between inside and outside the space

  • Indirect solar gains

  • Direct solar gains

  • Ventilation Gains

  • Internal Gains

  • Inter-zonal gains


Heat flow in buildings8

Heat flow in buildings

  • There are six main types of heat loss/gain:

  • Fabric gains

  • Indirect solar gains: due to the effects of solar radiation on external opaque surfaces

  • Direct solar gains

  • Ventilation Gains

  • Internal Gains

  • Inter-zonal gains


Heat flow in buildings9

Heat flow in buildings

  • There are six main types of heat loss/gain:

  • Fabric gains

  • Indirect solar gains

  • Direct solar gains: due to solar radiation entering the space through a window or void

  • Ventilation Gains

  • Internal Gains

  • Inter-zonal gains


Heat flow in buildings10

Heat flow in buildings

  • There are six main types of heat loss/gain:

  • Fabric gains

  • Indirect solar gains

  • Direct solar gains

  • Ventilation Gains: due to the movement of air through through cracks and openings in the building

  • Internal Gains

  • Inter-zonal gains


Heat flow in buildings11

Heat flow in buildings

  • There are six main types of heat loss/gain:

  • Fabric gains

  • Indirect solar gains

  • Direct solar gains

  • Ventilation Gains

  • Internal Gains: due to people, equipment (such as computers) and lighting

  • Inter-zonal gains


Heat flow in buildings12

Heat flow in buildings

  • There are six main types of heat loss/gain:

  • Fabric gains

  • Indirect solar gains

  • Direct solar gains

  • Ventilation Gains

  • Internal Gains

  • Inter-zonal gains: due to differentials in air temperature between zones


Heat flow in buildings13

Heat flow in buildings

  • Designers job:

    • Reduce as much as possible the difference between

    • heat loss and

    • heat gain


Heat flow in buildings14

Heat flow in buildings

  • What is HVAC?

    • Heating, Ventilation and Air conditioning

    • It is expensive!

    • It uses a lot of energy

  • What is HVAC for?

    • To add or remove heat

    • To make up for the difference between heat loss and heat gain


Modelling with zones

Modelling with zones

  • With lighting, it was possible to simulate just one room. This is not the case with thermal simulations.

    • Spaces in the building interact with each other

    • Must model all spaces in the building

  • Therefore, thermal models must be highly simplified


Modelling with zones1

Modelling with zones

  • The basic unit of a thermal simulation is the zone:

    • A zone is a volume of air that is relatively homogeneous

    • Usually a room

    • No holes or gaps (windows are OK)

Zone 1

Zone 2


Modelling with zones2

Modelling with zones

  • The simulation software must calculate

  • heat flow between zones:

    • Adjacency is important

    • The construction may be different

adjacency


Modelling with zones3

Modelling with zones

  • The enclosing surfaces must have thermal properties.

  • For Ecotect these are:

    • U-values (U: W/m²K)

    • Specific admittance (Y: W/m²K)

    • Solar absorption (Abs: 0-1)

    • Thermal decrement (Decr: 0-1)

    • Thermal lag (Lag: hrs)


Modelling with zones4

Modelling with zones

  • U-values (U: W/m²K)

    • thermal conductance of materials and the convective and radiative effects of surfaces and cavities

  • Specific admittance (Y: W/m²K)

    • ability to absorb and release heat energy

  • Solar absorption (Abs: 0-1)

    • portion of solar radiation that is absorbed

  • Thermal decrement (Decr: 0-1) and lag (Lag: hrs)

    • dynamic thermal behaviour


Modelling with zones5

Modelling with zones

  • A zone may contain partitions

    • Partitions must be internal

    • The air on both side must be similar

    • Furniture can also be defined as a partition

partition


Modelling with zones6

Modelling with zones

  • A number of properties can be set for each zone:


Modelling with zones7

Modelling with zones

  • A number of properties can be set for each zone:

    • Operation: AC On/off times

    • Comfort Band: Max and min temperatures

    • Occupancy: Number of people and activity

    • Air Change Rates: Air infiltration, wind sensitivity

    • HVAC system: Type of HVAC system

    • Heat Gains: people, lighting, equipment

    • Schedules: daily and yearly behaviour


Modelling with zones8

Modelling with zones

  • HVAC system

  • None:

    • All the windows are doors remain shut

  • Natural Ventilation:

    • Occupants may open the windows

  • Mixed-Mode System:

    • A combination of heating, AC and natural ventilation

  • Full Air Conditioning, Heating Only or Cooling Only:

    • Heating and/or AC. Windows are never opened.


Modelling with zones9

Modelling with zones

  • Heat gains

  • There are two types of heat gains:

  • Sensible heat gains:

    • lighting, computers, etc

    • value given per square metre of floor area

    • Typical office lighting: 20 W/m2

    • Typical office equipment: 40 W/m2

  • Latent heat gains:

    • This value is not used in Ecotect, so ignore it


Modelling with zones10

Modelling with zones

  • Schedules:

    • operational schedules

    • occupancy schedules

  • Schedules allow you to assign different profiles to weekdays, holidays, weekends, etc:

    • a profile consists of a percentage for each hour in the day

    • profiles are assigned to different days

    • up to 12 different daily profiles can be created


Thermal analysis

Thermal analysis

  • Thermal analysis in Ecotect:


Thermal analysis1

Thermal analysis

  • Types of thermal calculation:


Thermal analysis2

Thermal analysis

  • Hourly Temperature Profile:

    • Outside temp:the temperature outside

    • Beam solar:direct solar radiation

    • Diffuse solar:indirect solar radiation

    • Wind speed:typical wind conditions

    • Zone temp:the temperature inside

    • Selected zone:the selected zone is shown in bold


Thermal analysis3

Thermal analysis

  • Hourly Heat Gains/Losses:

    • HVAC load: overall cooling and heating

    • Conduction: through building fabric

    • Sol Air: through solar gains on opaque surfaces

    • Direct Solar: through transparent windows

    • Ventilation: through crack and openings

    • Internal: lights, people and equipment

    • Inter-Zonal: heat flow between adjacent zones


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