Environmental Engineering Reference
In-Depth Information
synthetics on its outside. In order to avoid cold bridges, connecting flanges and
fixings have to be especially insulated against thermal losses. If the solar thermal
installations for domestic hot water supply are designed properly, the annual aver-
age loss including the remaining thermal bridges is between 10 and 15 % of the
heat released to the heat store from the collector.
An internal heat exchanger of the collector circuit has to be located at the bot-
tom of the storage due to the temperature layering within the boiler - the heavier
cold water is located at the bottom, whereas the specifically lighter warm water is
at the top. The temperature profile inside the heat store is also the reason why the
cold water inlet is at the bottom and the hot water outlet is at the top of the storage
in the case of direct charging and discharging. The heat exchanger for the auxil-
iary heating is located in the upper part of the storage. Thus, the lower volume can
be entirely used by the solar system.
One variation of this type of water storage is the thermo-siphon storage. In con-
trast to traditional heat stores, the bottom heat exchanger is positioned vertically in
an ascending pipe that is open at the bottom. Thus, during operation of the solar
system, water can flow past the heat exchanger and the heated water flows up-
wards. The ascending pipe has specially designed openings all the way through.
Depending on the temperature level, the heated water is released through these
openings into the storage volume, if rising further is no longer possible due to the
higher temperature in the layer above. With this stratification unit the heated water
is always released to the storage at a height where both temperatures of storage
and heated water are the same. This process is dependent on the collector effi-
ciency and thus dependent on the solar radiation supply. Such an ascending pipe
is called a stratification charging unit (see Fig. 4.15, right).
For larger collector areas, an external heat exchanger has to be applied as it is
no longer possible to transport heat at the required minimum temperature loss
using an internal heat exchanger. This requires an additional pump between the
heat exchanger and the storage. The storage is either charged with water heated up
by the collector at one or two fixed heights or via a stratification charging unit
/4-6/.
Solid storage.
Solid matter storages are mainly used in systems with air collectors
and are often directly integrated into the building. They are normally fills of peb-
bles or other rock or mass-intensive parts of the building (e.g. walls, floors, ceil-
ings). To give some examples, the fill can be underneath the basement floor or
vertically integrated into the wall of the building. Solid matter storages can also be
run with liquids as heat carriers.
In loose rock fills, the hot air from the collector is let in from the top; it releases
its heat into the rocks before leaving the storage again at its bottom. Heat release
works the other way round. If the parts of the building are used directly as storage,
they are called hypocausts. Warm air is transferred through channels to the indi-
vidual components and heats them up. The components then release the heat with
a time lag and at lower amplitude to the building. In contrast to rock storages,
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