Structural Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 166

insulations with different thicknesses and thermal conductivities, calculate the
R-value for each, per square meter with relation:

d
k ¼

R ¼ R w A ð 11 Þ

This is a measure of how much heat loss is reduced.
Equation ( 10 ) can be written in a new form:

mc d

# i
dt ¼

P a
A

A as G þ U ð # i # e Þ ð 12 Þ

where

Cm 2 ).

1
R w

k
d

is the ' overall heat loss coef cient or U -value ' (W/ °

U ¼

A ¼

U-value is a measure of how much heat is lost through a given thickness of any
speci c material, which includes conduction, convection, and radiation. The

U-value of a material (or several materials in series, e.g., brick and insulation in a
wall) is calculated by taking the reciprocal of the R-value (i.e., 1/R-value), and
adding convection and radiation heat losses. The lower U-value indicates the better
thermal performance.
The thermal capacity of the building s elements delays the heat transfer to the

interior of the building, by soaking up excessive heat for several hours. During the
night, when the external temperature is lower, the stored heat is slowly expelled to
the environment by radiation and by convection. The time delay due to the thermal
mass is known as time lag. The thicker the walls are and more resistive the material
is, the longer it will take for heat waves to pass through building walls. The
reduction in cyclical temperature on the inside surface compared to the outside
surface is knows as the decrement factor.
Time lag and decrement factor are very important characteristics to determine
the heat storage capabilities of any material. The time it takes for the heat wave to
propagate from the outer surface to the inner surface is named as

'

'' time lag '' and the

decreasing ratio of its amplitude is named as . The schematics of

time lag and decrement factor are shown in Fig. 14 . At the cross-section of the outer
wall of a building, there are different temperature pro

'' decrement factor ''

les during any instant of a

1-day period. These pro les are function of inside temperature, outside temperature,

and materials of the wall layers.
During this transient process, a heat wave

flows through the wall from outside to
inside and the amplitude of this wave shows the temperature magnitudes and
wavelength shows the time. The amplitude of the heat wave on the outer surface of
the wall is based on solar radiation and convection in between the outer surface of
the wall and ambient air. During the propagation of this heat wave through the wall,
its amplitude will decrease depending on the material and the thickness of the wall