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Heat flow PowerPoint Presentation

Heat flow

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Heat flow

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  1. 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

  2. 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

  3. Heat flow • Vibration of molecules: • conduction • convection

  4. Heat flow • Electromagnetic radiation:

  5. 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

  6. Heat flow • Conduction

  7. 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

  8. Heat flow • Convection

  9. 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

  10. Heat flow • Radiation

  11. 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

  12. Heat flow through materials • Heat flow through materials depends on: • Cross-sectional area • Temperature difference • Resistivity or conductivity of the material

  13. 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.

  14. 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

  15. 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

  16. 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

  17. Heat flow through materials • In series: • Resistances are added • R = R1 + R2 + R3 … • In parallel: • Conductances are added • U = U1 + U2 + U3 …

  18. 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

  19. 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

  20. 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

  21. 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?

  22. Heat flow in buildings • Cold climate

  23. Heat flow in buildings • Hot climate

  24. 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)

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. Heat flow in buildings • Designers job: • Reduce as much as possible the difference between • heat loss and • heat gain

  34. 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

  35. 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

  36. 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

  37. Modelling with zones • The simulation software must calculate • heat flow between zones: • Adjacency is important • The construction may be different adjacency

  38. 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)

  39. 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

  40. 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

  41. Modelling with zones • A number of properties can be set for each zone:

  42. 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

  43. 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.

  44. 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

  45. 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