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Heat flow - PowerPoint PPT Presentation


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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
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
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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 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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