Environmental Engineering Reference
In-Depth Information
pump is switched off. For common solar thermal domestic hot water systems the
set value for the temperature difference when switched on is between 5 and 7 K.
A set value for the temperature difference when switched off is normally at ap-
proximately 3 K. The control should be exact up to 1 K. Additionally the use of
time-lag devices is useful as temperature swings might occur in longer pipes. A
recently introduced control strategy uses the pressure rise of the collector loop
when the collector is heated up. If a certain pressure rise is detected, the collector
loop pump switches on. Two temperature sensors on the hot and cold side of the
collector loop are used to switch the pump off again. Such systems can be prefab-
ricated units. No sensors have to be mounted on site /4-8/.
Apart from controlling the circulation pump within the collector circuit in
forced circulation systems, maintenance of temperature limits in the storage and
the collector circuit must be guaranteed. The storage temperature must not exceed
a certain maximum value. In standard tanks of solar thermal domestic hot water
supply systems, lime stone deposits can be caused by temperatures over 70 °C.
Furthermore, an evaporation of the heat carrier in the collector circuit must defi-
nitely be avoided or, the resulting steam has to be condensed by system parts de-
signed for that process (i.e. the heat exchanger in the heat store).
There are several ways /4-6/ of avoiding problems that may occur in the case of
a collector standstill. If the maximum allowed storage temperature is exceeded,
the circulation pump in the collector circuit can be switched off completely in
order to avoid charging the storage with further energy. In that case the collector
reaches its standstill temperature, which is significantly above 140 °C for selec-
tively coated collectors. The collector content is evaporated. Due to the increase in
volume during the evaporation process, in the best case, the entire liquid content
is pressed out of the absorber and captured by an expansion tank designed for that
purpose. In the worst case, the entire liquid content of the collector has to be
evaporated and condensed again within the system. This normally occurs in the
heat exchanger to the storage. In that case the expansion tank has to be able to
also absorb the volume of the pipes /4-9/. The evaporation strategy is often used
because no auxiliary energy is required. Recently, temperature-resistant heat ex-
changers are also offered so that there is no risk of premature ageing of the heat
exchanger with this type of operation. The circulation pump should only be
switched on after a collector standstill if the collector temperature is below
100 °C. Thus, it is ensured that the collector is free of evaporated media.
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The Drain-Back collector system solves the standstill problem by including a
gaseous volume (nitrogen or air) into the circuit from the collector to the stor-
age - either into the storage itself or into an intermediary integrated tank. When
operating the system, the heat transfer medium flows through the gaseous vol-
ume. At collector standstill, the gaseous volume moves into the collector and
the collector liquid fills the space filled by the gaseous volume beforehand.
This process requires no auxiliary energy. It however requires the ability of the
collector to empty itself (falling pipes, no "liquid sacks"). The gas in the collec-
tor can heat itself up to standstill temperature now without the heat carrier hav-
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