Mod 5

Climate and Built Form I - Module 5: Heat Transfer of Buildings

1. Thermodynamic Properties of Materials

  • Specific Heat

    • Definition: Quantity of heat required to raise the temperature of 1 g of a substance by 1 degree Celsius (°C).

    • Values for specific heats (cal/g°C):

      • Water: 1

      • Ice: 0.480

      • Average for human body: 0.83

      • Soil: 0.2 to 0.8 (dependent on water content)

      • Aluminum: 0.214

      • Protein: 0.4

    • Gases:

      • Volumetric specific heat for air: Approximately 1,300 J/m³°C (varies with pressure and humidity).

  • Latent Heat

    • Definition: Heat energy absorbed by unit mass during a change of state (e.g., solid to liquid) without temperature change; measured in J/kg.

    • Values for water:

      • Latent heat of fusion (0°C ice to 0°C water): 335 kJ/kg

      • Latent heat of evaporation at 100°C: 2261 kJ/kg

      • Latent heat of evaporation around 20°C: 2400 kJ/kg

2. Thermal Capacity

  • Definition: Quantity of heat required to raise the temperature of a body/mass by one degree Celsius.

  • Formula:

    • For mass of m and specific heat s:

      • Thermal capacity = ms

    • For composite body:

      • Thermal capacity = m1s1 + m2s2 + m3s3 + ...

3. Heat Flow

  • Heat energy distributes evenly to achieve a uniform thermal field.

  • Heat flows from high temperature to low temperature zones by:

    • Conduction

    • Convection

    • Radiation

  • Motive Force: The temperature difference between zones is the driving factor; greater differences increase heat flow rate.

  • Power: Ability to perform work over time; measured in joules per second (J/s), termed 'Watt'.

4. Density of Heat Flow Rate

  • Measured in watts (W) or kilowatts (kW), essential for measuring heat loss in identifiable units (e.g., buildings) or processes (e.g., boilers).

  • Heat flow rate can also be expressed relative to unit area, termed intensity, measured in watts per square meter (W/m²).

5. Conductivity

  • Conduction is the molecular movement that constitutes heat flow.

  • Thermal Conductivity (k-value):

    • Describes how well materials conduct heat, measured in W/m°C.

    • Ranges from 0.03 W/m°C (insulating materials) to 400 W/m°C (metals).

    • Lower values indicate better insulation.

    • Resistivity is the reciprocal of conductivity (1/k), measured in m°C/W.

  • Density often relates to conductivity; denser materials generally conduct heat better, although the relationship isn't causative.

6. Thermal Properties of Multiple Layers

  • Thermal Conductance (G):

    • Measures how readily a material conducts heat, units: watts per kelvin (W/K).

  • Thermal Resistance (R):

    • Opposition to heat current, measured in kelvins per watt (K/W).

    • R-value reflects a material's ability to resist heat flow; higher R-values equate to better insulation.

7. Surface Resistance

  • Resistance offered by surfaces due to air film separating the body from surrounding air.

  • Surface or Film Resistance:

    • Denoted by 1/f (m²°C/W), f is the surface conductance (W/m²°C).

  • Conductance includes convective and radiant components of heat exchange.

  • Overall air-to-air resistance (R) combines material resistance and surface resistances.

8. Relevant Considerations for Multilayer Walls

  • Understanding heat transfer dynamics through multilayer wall structures aids in improving energy efficiency in built forms.