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# Energy - 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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## PowerPoint Slideshow about 'Energy' - jana

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Presentation Transcript

• 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

Vibration of molecules in a substance

Heat flow

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

• Heat can flow in three ways:

• Conduction

• Convection

• Vibration of molecules:

• conduction

• convection

• 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

• Conduction

• 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

• Convection

• 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 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 materials depends on:

• Cross-sectional area

• Temperature difference

• Resistivity or conductivity of the material

• 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.

• 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

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

• 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

• In series:

• R = R1 + R2 + R3 …

• In parallel:

• U = U1 + U2 + U3 …

• 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

• 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

• A typical building structure

• Many components

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

• Many different thermal conductances

• In series and in parallel

• 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?

• Cold climate

• Hot climate

• 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)

• 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

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

• Fabric gains

• Indirect solar gains

• Direct solar gains

• Ventilation Gains

• Internal Gains

• Inter-zonal gains

• 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

• 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

• 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

• 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

• 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

• 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

• Designers job:

• Reduce as much as possible the difference between

• heat loss and

• heat gain

• 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

• 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

• 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

• The simulation software must calculate

• heat flow between zones:

• The construction may be different

• The enclosing surfaces must have thermal properties.

• For Ecotect these are:

• U-values (U: W/m²K)

• Solar absorption (Abs: 0-1)

• Thermal decrement (Decr: 0-1)

• Thermal lag (Lag: hrs)

• U-values (U: W/m²K)

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

• 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

• 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

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

• 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

• 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.

• 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

• 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 in Ecotect:

• Types of thermal calculation:

• 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

• 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