Environmental Engineering Reference
In-Depth Information
Collector Effi ciency
The effi ciency and effi ciency curve of a solar collector can be determined using the
three parameters
0 describes which portion of
sunlight is converted into heat by the absorber. The absorber itself or the front panel
of glass actually refl ects a portion of the sunlight before it can even be absorbed.
Depending on the type of collector, optical effi ciency is between 70 and 90%. The
two loss coeffi cients k 1 and k 2 indicate how high the heat loss is in the collector.
The hotter the collector is, the higher the heat loss and the less useful heat the col-
lector can emit. High loss coeffi cients also mean high heat loss. The formula for
collector effi ciency is as follows:
η
0 , k 1 and k 2 . The optical effi ciency
η
=−
k
Δ
ϑ
+⋅
k
Δ
ϑ
2
1
2
ηη
.
0
E
Here
indicates the temperature difference between the collector and the environ-
ment and E the intensity of the solar radiation.
With an ambient temperature of 25 °C and a collector temperature of 55 °C, the
result is a temperature difference of
Δ
ϑ
= 30 ° C, or 30 K. On a pleasant summer 's
day with a solar radiation intensity of E = 800 W/m 2 , the calculation of a fl at - plate
collector with an optical effi ciency of
Δ
ϑ
0 = 0.8 and the loss coeffi cients k 1 = 3.97 W/
(m 2 K) and k 2 = 0.01 W/(m 2 K 2 ) gives a collector effi ciency of
η
2
08 3 97
.
30
K
+
0 01
.
(
30
K
)
W
mK
W
mK
2
2
2
η =−
.
=
064 64
. .
=
800
W
m
2
A collector 4.88 m 2 in size then supplies 2500 watts of power. This is suffi cient to
heat 100 litres of water from 33.5 °C to 55 °C in one hour.
The collector effi ciency curve describes the progression of effi ciency based on the
temperature difference between the collector and the environment (Figure 6.3). It
shows that effi ciency drops as the temperature rises until the collector reaches an
effi ciency of zero and fi nally cannot supply any more power.
Almost all thermal solar systems require storage in addition to a collector. There
are very few cases when the sun shines at exactly the same time as heat is needed.
Even a simple water tank can function as a thermal store and should be well insu-
lated to reduce heat loss. The storage size depends mainly on the heat requirement
and on how long the heat needs to be stored. Daytime storage cylinders for hot water
systems in single-family homes are designed to bridge a few days of demand and
usually only hold a few hundred litres. If, in addition to hot water, very large quanti-
ties of heat for heating are also to be stored, then seasonal heat storage cylinders
are required. These store heat in the summer and then release it again in the winter.
Large heat storage cylinders for small housing estates reach sizes of several hundred
or even thousand cubic metres. Large storage cylinders generally have less specifi c

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