Environmental Engineering Reference
In-Depth Information
(3.32)
with the resulting heat transfer fluid temperature:
(3.33)
However, this equation is only valid for small time intervals because the
continuously changing temperature of the heat transfer fluid must be
considered for the calculation of the heat flow. For longer time intervals, the
temperature of the heat transfer fluid
ϑ
HTF
(
t
2
) at time
t
2
can be calculated with
the time interval
t
2
-
t
1
subdivided into n small time intervals
∆
t
. With
(3.34)
the temperature of the heat transfer fluid becomes:
(3.35)
Finally,
∆
t
0,
t
1
= 0 and
t
2
=
t
gives:
(3.36)
A 20-m-long 15
1 copper pipe with an insulation thickness of 30 mm (
k'
=
0.15 W/(m K)) and with (
c
·
m
)
eff
= 3.6 Wh/K at an ambient temperature
ϑ
A
= 20°C cools down from
ϑ
HTF
(
t
2
) = 33°C in 1 hour. If
the collector cycle starts up again the pipes must be heated up again with the
above-described heat-up losses.
ϑ
HTF
(
t
1
) = 50°C to
T
HERMAL
S
TORAGE
Different storage systems are used for solar thermal systems depending on the
application. The objective of heat storage systems is to provide desired heat
even at locations with fluctuating solar irradiances. They can be divided into
two groups:
•
short-term storage systems (daily cycle)
•
long-term storage (inter-seasonal storage) systems.
Long-term storage systems should compensate for seasonal fluctuations,
whereas short-term storage systems generally are only required to store heat
