Week 3 - Energy flow-paths

Question 1

Give 3 reasons why two different walls of the same U-value might give rise to substantially different energy requirements

Answer 1

1. different thermal capacity;
2. different arrangement of layers; and
3. different layer conductivities.

Question 2

Describe the principal factors affecting each of the following heat transfer processes within buildings: natural convection at internal surfaces; longwave radiation exchange; infiltration and interroom air flow; and intra-room air movement.

Answer 2

1. Natural convection: surface-to-air temperature difference, surface roughness, direction of heat flow and characteristic dimensions.
2. Longwave radiation: inter-surface temperature difference, surface emissivity, room geometry and content (dictating view factors), and surface reflection (diffuse, specular or mixed).
3. Infiltration: pressure distribution, leakage distribution, buoyancy forces, and occupant interactions.
4. Intra-room air movement: flow regimes (laminar, turbulent or mixed), boundary temperatures and velocities, distribution of heat flux and momentum inputs, and occupant interactions.

Question 3

Explain the effect on a room’s heating load in winter of applying a low emissivity coating to 1) the inside surface of a double glazed window; and 2) the innermost, cavity-facing surface of the same window.

Answer 3

1. The window inside surface is cold. The longwave flux transfers between the room surfaces and the window and so reduces the overall heat loss.
2. The window inside surface is warmer at the expense of a slightly higher heat loss. The longwave flux transfer takes place with the window inside surface and transfers, via conduction, to the innermost cavity-facing surface.

Question 4

Describe two circumstances in which it would be advisable to model wall conduction in three dimensions.

Answer 4

1. At corners, where dissimilar materials may inter-penetrate.
2. Where thermal bridges are present such as at the junction of the wall and a window frame.

Question 5

State the advantages and disadvantages of locating insulation on the innermost position of the walls of a room

Answer 5

Advantages:

1. Hides the thermal capacity of the wall and heating the room more quickly i.e. when the heating is turned on (turn the heating down, it cools down quickly).
2. Reduces the risk of surface condensation because the inside surface temperature is likely to remain above the dew point temperature of the adjacent air.

Disadvantages:

1. Hides the thermal capacity of the wall that may otherwise help to maintain the temperature in the room. (turn the heating down, it takes longer for the room to cool)
2. Solar radiation penetrating windows and striking the internal surface cannot be readily stored in the construction since the insulation will act as a barrier.
3. The risk of interstitial condensation is greater in the case of internally located insulation since a substantial portion of the construction may fall below the dew point temperature of moist air permeating through the construction.

Question 6

Differentiate between long- and short-wave radiation.

Answer 6

Shortwave radiation comprises visible light and ultraviolet radiation.

Longwave radiation is infrared radiation. Longwave is the radiation emitted from bodies at terrestrial temperatures

The Sun radiates energy mainly in the form of visible light, with small amounts of ultraviolet and infrared radiation; for this reason solar radiation is usually considered shortwave radiation.

Question 7

When modelling external surface longwave radiation exchange, list five influencing factors that must be taken into account.

Answer 7

1. cloud cover/type;
2. effective sky temperature;
3. temperature of buildings;
4. ground temperature; and
5. view factors between the surface and the sky, ground and surroundings.

Question 8

What is the purpose of a passive solar element in the context of building design? Identify some typical elements.

Answer 8

The purpose is to capture, store and/or transport energy without recourse to mechanical systems.

Examples include: solar control devices, sunspaces, phase change materials, desiccant materials, evaporative cooling, induced ventilation and mass walls.