Give 3 reasons why two different walls with the same U-value might give rise to substantially different energy requirements?
- different thermal capacity;
- different arrangement of layers
- different layer conductivities.
Describe the principal factors affecting the following heat transfer processes within a building: natural convection, longwave radiation exchange, infiltration and inter-room air flow, intra-room air movement.
- Natural convection: surface-to-air temperature difference, surface roughness, direction of heat flow and characteristic dimensions.
- Longwave radiation: inter-surface temperature difference, surface emissivity, room geometry and content (dictating view factors), and surface reflection (diffuse, specular or mixed).
- Infiltration: pressure distribution, leakage distribution, buoyancy forces, and occupant interactions.
- Intra-room air movement: flow regimes (laminar, turbulent or mixed), boundary temperatures and velocities, distribution of heat flux and momentum inputs, and occupant interactions.
Explain the effect of applying a low-ε coating to 1) the inside of a double glazed window, and 2) the innermost cavity facing surface of the window?
- The low-ε coating reduces the longwave flux transfer between the room surfaces and the window and so reduces the overall heat loss. The window inside surface is cold.
- Now the longwave flux transfer can take place with the window inside surface and transfer, via conduction, to the innermost cavity-facing surface. At this point, the heat loss is arrested because the low-ε coating reduces the radiation exchange across the cavity. Thus, the window inside surface is warmer at the expense of a slightly higher heat loss.
Describe two circumstances in which it would be advisable to model wall conduction in three dimensions.
- At corners - where dissimilar materials may inter-penetrate or where the material comprising the corner is a significant proportion of the overall construction and therefore its omission may seriously impact on the result.
- Where thermal bridges are present -such as at the junction of the wall and a window frame.
State the advantages and disadvantages of locating insulation on the innermost position of the walls of a room.
Advantages:
- Hides the thermal capacity of the wall that may otherwise give rise to an increased peak plant demand due to the initial rush of energy to capacity at plant start-up in an intermittent scheme.
- The risk of surface condensation is lower because the inside surface temperature is likely to remain above the dew point temperature of the adjacent air.
Disadvantages:
- Hides the thermal capacity of the wall that may otherwise help to minimise the peaks and maximise the troughs of plant demand and so promote good load levelling.
- Solar radiation penetrating windows and striking the internal surface cannot be readily stored in the construction since the insulation will act as a barrier. A space experiencing high solar energy penetration is likely to overheat or need cooling.
- 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 in the absence of an effective vapour barrier.
Differentiate between short- and long-wave radiation.
Short-wave: visible light and ultraviolet radiation. solar radiation is usually considered shortwave radiation as the sun radiates energy mainly in the form visible light.
Long-Wave: Infrared radiation. The radiation emitted from bodies at terrestrial temperatures is longwave.
List 5 influencing factors that must be considered when modelling external surface longwave radiation exchange.
- cloud cover/type;
- effective sky temperature;
- temperature of surrounding buildings;
- ground temperature; and
- view factors between the surface and the sky, ground and surroundings.
What is the purpose of a passive solar element in the context of building design?
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.
