Case Study – Admittance Method Windows Application
The challenge
All building materials have thermal mass, which is an ability to store heat.
Designers of buildings are increasingly using thermal mass as a way of preventing
overheating of a building on summer days and as a way of maintaining the
temperature of a building on winter nights. The thermal admittance is a
measure of a material's ability to absorb heat from or release heat to a space in
response to a change in temperature in the space. Materials with a high thermal
admittance have a high thermal mass and an ability to release or absorb heat
quickly in response to temperature changes. Materials such as concrete or
blockwork have a high thermal admittance, but they can only be effective in
reducing temperature variations in the space if they are deployed close to the
inner surface of the building structure with a minimum of other materials in
between. To make the best use of thermal mass, designers need a method which they
can use to determine the thermal admittance of any composite building structure.
The solution
Atkinson Science has created a Windows application, called the Admittance Method,
which enables designers to determine the thermal admittance of any planar building
structure, such as an interior or exterior wall, a floor, or a roof section. The
designer builds up the structure by selecting materials from a database,
specifying their thickness, and adding them to the structure. As the structure is
built up, it is displayed in a window on the left side of the application. The
designer can click on any material displayed in the window and change its thermal
properties (density, specific heat capacity, thermal conductivity) from the
default values held in the database or delete the material from the structure. The
designer also specifies a sinusoidal temperature wave with a period of 24 hours on
one side of the structure representing a diurnal temperature variation. When the
designer clicks the Calculate button, the application displays the temperature
wave along with the heat fluxes on the two sides of the structure. The heat flux
on the side to which the temperature wave is applied always leads the temperature
wave, and the heat flux on the opposite side always lags the temperature wave. The
differences in amplitude and phase of the two heat fluxes provide a visual
indication of the thermal mass of the structure. The application displays the
thermal admittance and the thermal admittance time lead, as well as other useful
dynamic properties, such as the decrement factor, the decrement factor time
lag, the surface factor and the surface factor time lag. In addition, it displays
some useful steady-state properties, such as the thermal transmittance
(U value), and the mean heat flux, given the difference in mean temperature
on the two sides of the structure.
The benefits
The Admittance Method Windows application provides the designer of buildings with a
rapid method of determining the thermal admittance of building components and
structures, so that he/she can make the best use of thermal mass to even out
temperature fluctuations in a building.