Structural Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 176

inertia. Other factors could render unreliable the values of the thermal map gen-
erated by the thermographic survey, for example, the presence of moisture in the
wall, radiant heat exchange, or the effect of solar radiation.
The effect due to the presence of moisture is controlled by identifying, in the
thermal mapping provided by the infrared camera, a homogeneous area of the wall
in which to evaluate the average value of the surface temperature, while the effect
due to solar radiation is controlled by choosing walls with an orientation that, in the
winter season, is not subject to the sunshine. The effect of the non-steady state heat

flow condition can be partially reduced by identifying periods in which to perform
the measurements with external climatic stability.
The main factors affecting the thermographic study relevance during a building
envelope thermal behavior inspection [ 10 ] are shown below:

Climatic conditions: insulation, wind, ambient temperature, relative humidity,
greenhouse gases concentration (water vapors, CO 2 ).

￿

Pattern characteristics: emissivity/re ectivity, roughness or unevenness, stains

and color of wall surface; construction of wall

￿

finish (e.g., extremely thick finish).

￿ Environmental conditions and de ciencies: angle of vision and survey distance,

orientation of building to the path of sunshine during the survey, existence of
any heat generating plants or machines inside the building; screening objects (e.
g., trees, shade of eaves, or adjacent building).

Climatic conditions. At the interface between air and wall heat ux by con-

duction is equal to the convective heat ux. From Eq. ( 30 ) it is possible to observe

the signi cient h we . Its value, in fact, has an

effect proportionally to the U w value of the U-value.
In the standard calculations of the external heat transfer coef

cant in uence of the convective coef

cient, pre-calcu-
lated values or simple equations are provided by the technical standards, such as the
ISO 6946 standard. The convective coef cient is used to calculate the heat loss

during the design phase of the building envelope and have ISO value, equal to 25.
In reality, the convective coef cient is function of the wind speed. For calculate the

convective coef cient, the Jurges equation [ 11 ] is given as follows:

h we ¼ 5 : 8 þ 3 : 805 m w with m w \ 5m/s ð 32 Þ

where:
h we is convective heat transfer coef cient;

v w is wind velocity near the building element

For three different values [ 12 ] of wind sped the h we coef cient have the fol-

lowing values: