Environmental Engineering Reference
In-Depth Information
adjusted to the total mass flow. High-Flow installations with collector areas below
15 m² are mostly combined by internal heat exchangers for heat generation,
whereas Low-Flow systems try to load the heat store using stratification units in
order to prevent cooling the heated water in the store by mixing.
Collectors are connected to distributing pipes both for charging and discharg-
ing. In order to equally distribute the heat carrier to the individual absorbers and
thus keep the pressure loss in the connecting pipe and with it, the electricity de-
mand of the circulation pump at a low level, the distributing pipe should be larger
in diameter than the absorber pipes. For the same reason, the flow channels in the
collectors connected in parallel should also be the same, if at all possible. Inlets
and outlets of the pipes should be connected at opposite ends /4-2/. However,
together with the number of collectors connected in parallel, the differences in
flow and thus the differences in the temperature rise in the collector (θ
out
- θ
in
)
increase. Therefore, for large collector arrays, the circuits connected in parallel
should be adjusted with control valves.
4.2.2
Further system elements
Heat store.
A heat store is not required for the physical basic principle of solar
thermal heat accumulation. However, heat stores are part of most solar thermal
installations. The reason is the general non-correlation between the solar radiation
supply and the demand for heat.
The heat store accumulates the heat generated in the collector by solar radia-
tion, and stores it for times when it is required. For that purpose, a heat store has
to consist of the heat accumulation medium, a solid cover with insulation material
plus heat inlet and outlet devices.
The heat capacity is an important parameter for the heat accumulation medium.
It is the amount of heat required to increase the temperature of a specific sub-
stance by 1 K. The specific heat capacities (in relation to mass and volume), plus
the densities of various heat accumulation media are shown in Table 4.4. Further
technical criteria determining a substance to be used as a heat accumulation me-
dium are the availability, the compatibility with other substances (e.g. corrosion
risk) and the environmental friendliness.
Several types of heat store designs are used. They can be differentiated accord-
ing to the type of heat accumulation (chemical, thermal) and the condition of the
accumulative substance. In the segment of low-temperature heat storage (up to
approximately 80 °C) mainly thermal heat accumulation is used. Liquid heat
stores (water heat stores), solid-matter storage and latent heat stores can be distin-
guished.
Liquid storage (Water storage)
. This is the form of storage used in most cases.
The simplest case is a multi-functional open-air swimming pool. In most cases,
separately installed, pressure-free tanks or tanks under pressure are used.
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