Solar energy use in buildings - Solar Energy Sciences and Engineering Applications - page 607

Figure 18.9.4 Heat flux into wood and concrete at an air temperature jump of 10 K.

Figure 18.9.10 Amount of heat stored in the materials on an air temperature jump of 10 K.

that conduct heat well. Deep in the component, on the other hand, a temperature
jump of air continues only very slowly with poorly conducting materials (see Figure
18.9.4). Since the temperature difference of the surface T ( x

=

0) and air T o (here zero)
is larger at the concrete surface than at the wood surface, the larger heat flows and
stored amounts of energy occur there.
To calculate the amount of heat Q (as shown in Figure 18.9.10) for each unit area
A that has flowed into the component until the point in time t 1 , the heat flux density
Q/A must be integrated over the time. A solution to this problem is probably very
complex. It is simpler to calculate the heat flow density in smaller time intervals and
total afterwards.
Apart from temperature jumps, periodic changes in temperature caused by the
external climate are of particular importance for practical applications. The outside
temperature T o and the irradiance converted to a fictitious sol-air temperature can
be approximated as periodic functions of time t with a period t 0 of 24 hours and an
amplitude T om .