Last edited 13 Jul 2016

Heat transfer in buildings


[edit] Introduction

Heat transfer is the process of thermal exchange between different systems. Generally the net heat transfer between two systems will be from the hotter system to the cooler system.

Heat transfer is particularly important in buildings for determining the design of the building fabric, and for designing the passive and active systems necessary to deliver the required thermal conditions for the minimum consumption of resources.

Very broadly, the mechanisms of heat transfer can be described as:

The thermal behaviour of a system is a function of the dynamic relationship between these mechanisms.

[edit] Conduction

Conduction is the diffusion of internal heat within a body as a result of a temperature difference across it.

This is particularly important in buildings where there may be a temperature difference between the inside and outside of a building, such as in a heated building during winter. Conduction is one of the main potential heat transfer mechanisms by which the internal heating or cooling can be lost to the outside, resulting in high operating costs, high carbon emissions and occupant discomfort.

For building materials it is sometimes thought that conductivity is expressed by the U-Value, however, U-values are the reciprocal of the sum of the thermal resistances of a body plus its inside and outside surface thermal resistances. Conductivity is more accurately expressed by a material's R-Value, which is the reciprocal of its thermal resistance and does not include a surface component. See U-Value for more information.

Conduction can be inhibited by insulating materials which have a high thermal resistance and so help reduce heat transfer between the inside and outside. See insulation for more information.

An insulating effect can also be achieved by the thermal mass of building components. Thermal mass describes the ability of a material to absorb, store and release heat energy. Thermal mass can be used to even out variations in internal and external conditions, absorbing heat as temperatures rise and releasing it as they fall. In building design, this can useful for evening-out and delaying extremes in thermal conditions, stabilising the internal environment and so reducing the demand for building services systems.

[edit] Convection

Convection is the movement of a fluid, such as the air, by advection and diffusion. This is a very important mechanism in the design of buildings, where air movement is necessary to:

  • Moderate internal temperatures.
  • Reduce the accumulation of moisture, odours and other gases that can build up during occupied periods.
  • Improve the comfort of occupants.

Convection is also a heat transfer mechanism, resulting from the movement of air of different temperatures.

Air movement in buildings can be 'forced' (for example driven by fans), or 'natural' resulting from pressure differences from one part of a building to another. Natural air movement can be either wind driven, or buoyancy driven. For more information see: Natural ventilation.

Accurately predicting the movement of air within buildings is extremely complicated and can require the use of computational fluid dynamics (CFD) modelling software. See CFD for more information.

See convection for more information.

NB Fluids can also be used to transfer heat within a building by 'mass transfer', for example by the flow of a refrigerant, chilled water or hot water around a building to provide heating or cooling.

[edit] Radiation

All bodies which are hotter than 0°K emit thermal radiation. They also absorb thermal radiation emitted by their surroundings. The difference in the total amount of radiation emitted and absorbed by a body at any given moment may result in a net heat transfer which will produce a change in the temperature of that body.

The range of terrestrial temperatures experienced within the built environment is relatively small, and relative to the temperature of the sun this range is 'cold' and so radiating at a 'long' wavelength compared to the sun. This anomaly allows us to categorise thermal radiation as short-wave solar radiation and terrestrial or long wave infra-red radiation. Surfaces in the built environment will tend to absorb solar radiation and emit long wave infra-red radiation.

This difference also produces effects such as the greenhouse effect. The atmosphere is relatively transparent to solar radiation, this means it allows sunlight to enter the atmosphere and heat the Earth's surface. These surfaces then re-radiate that heat as long-wave infra-red radiation, which greenhouse gases tend to absorb rather than transmit. The result is that the long-wave infra-red radiation is 'trapped' and heat accumulates in the atmosphere causing a warming process. See greenhouse gases for more information.

The thermal optical properties of a material are a function of three basic parameters; transmittance, reflectance, and absorptance (or emissivity) , describing the ratio of the transmitted, radiated or absorbed radiation to the incident radiation. These properties vary depending on the wavelength and angle of the incident radiation. See thermal optical properties for more information.

[edit] Phase change

When substances change phase, for example changing from liquid to gas, they absorb or release heat energy. For example, when water evaporates, it absorbs heat, producing a cooling effect, and when it condenses it releases heat. So when water evaporates from the surface of a building, or when sweat evaporates from the skin, it has a cooling effect.

This is also important in refrigeration, where refrigerant gases absorb heat from the cooling medium (typically water) as they evaporate, and when they condense, they release heat which is rejected to the outside (or recovered). See refrigerants for more information.

Phase change materials can also be used in construction to reduce internal temperature changes by storing latent heat in the solid-liquid or liquid-gas phase change of a material. See phase change materials for more information.

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