Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 156

tank. Within this double tank, natural convection and vertical temperature layering
are utilised. In the top part of the hot water storage there is always enough hot
water to fill one bath tub. The higher costs for the double storage are a disadvan-
tage.
The system on the right is an integration of heat store for heating and domestic
hot water plus the auxiliary heating device (e.g. a gas-condensing boiler). Such
compact heat stores are also called combistores. One advantage is its compact
form and the little installation work to be performed on site in order to obtain a
solar combined system. The solar system feeds into the heat store via a stratifying
charging unit. The burner for the conventional fuel is directly integrated into the
storage with a flange. The domestic hot water is generated using a single pass
external heat exchanger with controllable mass flow on the store side. Thus, stor-
age of domestic hot water and the possibility of the emergence of legionella is
avoided (see /4-6/, /4-13/).
Solar-supported district heating systems. In contrast to systems supplying indi-
vidual houses with heat by individual solar systems, several heat users join to use
heat from a single solar plant. If that is the case, the system is called solar-
supported district heating system.
Usually, there are numerous measures of improved heat insulation that are less
costly than the supply with district solar heat. In order to technically and eco-
nomically optimise the overall system, a reduction in space heating in the build-
ings in question needs to be analysed before installing a heat supply with solar
district heating systems. Particularly, low flow and return-flow temperatures of
the heat distribution network have a very positive effect (e.g. 80/40 °C).
Solar-supported district heating systems are normally further differentiated as
solar-supported district heating systems without and with long-term heat storage
(Fig. 4.16). From the collectors installed near the heat store, the heat is transferred
to the central storage via pipes and a heat exchanger. In addition, a hot water dis-
tribution network in order to distribute the heat from the heating centre to the
private households is required. Two-pipe networks and four-pipe networks can be
distinguished in this respect.
− The two-pipe network uses decentralised domestic hot water heating - heating
is performed via the heating network either with domestic hot water stores or
domestic hot water heat exchangers in the individual houses (Fig. 4.16, left); in
smaller networks the heater is directly integrated, whereas it is coupled via a
heat exchanger in larger networks. In order to keep the thermal losses in low-
temperature networks low, heating up the domestic hot water store is done dur-
ing specific time windows, for example during the night and at the time of
highest solar radiation with increased flow and return-flow temperatures of the
network. In such two-pipe networks, the domestic hot water circulation pipe
that normally leads to thermal losses and destroys the layering, does not have to
be used. In addition, the danger of legionella due to small domestic hot water
volumes is low.