Quantitative application of Infrared Thermography
Some earlier posts referred to the importance of improving the thermal efficiency of existing buildings in order to reduce carbon emissions associated with heating.
Retrofit 2050 concentrated on methods of measuring the thermal performance
of buildings when the thermal properties of the materials used are
unknown.
Retrofit of buildings: citations and extracts referred to recent work on
improving methods of thermal measurement.
Energy Efficiency in Buildings provided links to energy performance
standards.
Many
papers on thermal measurement focus on assessing the U-values of
existing structures. Standard methods involve the use of both
temperature and heat-flow measurements and are typically
time-consuming and demanding. Effort has been directed to reducing
the difficulties and producing quicker results, in some cases by
eliminating the need for heat flow measurement. Reducing measurement
time without loss of accuracy remains a goal.
The
infrared thermography technique (ITT) has been widely used in thermal
surveying as a qualitative method, intended to quickly identify areas
of heat loss for further investigation. Brief notes follow on a 2018
Ph.D. thesis which investigates the use of ITT as a quantitative
method.
by
M.
O’Grady
It
is focussed on a particular aspect of thermal assessment, that of
thermal bridging, where certain parts of a building lose heat at a
greater rate than their surroundings due to the method of
construction in the local area. Window frames are a familiar example.
The
thermodynamic principle underlying the work is surface energy
balance; energy leaving a surface is evaluated from surface
temperature measurements, and equated to energy reaching the surface.
Thus temperature measurements on the exterior surface of a building
can allow computation of energy loss from the building. One of the
objectives of the work is to show that ITT used on the outside of a
building can produce a thermal assessment of useful accuracy in a variety of weather conditions.
Thermodynamic
literature is reviewed, with particular regard to heat transfer
coefficients. It is claimed that the exterior convection coefficient
hce, on which analysis depends, can be
calculated with sufficient accuracy from the Nusselt number, Nu,
the thermal conductivity of air k, and the appropriate
characteristic length lch in the
relation
hce
= (Nu k)/lch
Post
-processing of the infra red image allows the appropriate values of
hce to be calculated and assigned pixel by
pixel if required, and in some situations greater accuracy is
achieved as a result.
Comparisons
are made between ITT and laboratory measurements for a series of
test cases, and good agreement is reported.
The
results may be of interest to community energy workers involved in the retrofitting of buildings for fuel saving. The work described by O'Grady prompts consideration of whether practical thermal surveys can be carried out
using ITT only, thus saving time and reducing the amount of equipment needed.
However, the thesis described is specific in its focus on thermal
bridging, and it may not follow that the ITT method alone is suitable
for more general thermal assessment.
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