Environmental Engineering Reference
In-Depth Information
In the four-pipe network, the space heating and domestic hot water are distrib-
uted separately. The advantage of this separate distribution of heating and do-
mestic hot water (four-pipe-network, Fig. 4.16, right) is the better utilisation of
the heat store and the solar system, as domestic hot water is still pre-heated,
even at low storage temperatures.
Without long-term storage, solar fractional savings of approximately 10 to 20 %
can be achieved for district heating systems with large collector arrays mainly
supplying private households. This fractional saving is related to the energy de-
mand for space heating and domestic hot water. Higher savings can be achieved
using seasonal long-term storage. If collectors are predominantly installed on the
roofs of buildings to save space, fractional savings of a maximum of 50 to 60 %
can nowadays be achieved in many Central and Northern European states with
their current heat control regulations, the available roof space and long-term heat
storage. If enhanced measures for heat insulation are applied, the fractional saving
iincreases /4-15/. Only if even higher degrees of fractional savings are aimed at,
and in cases where large sunny areas, close to the consumer not used otherwise
are available, the collectors are installed on such free areas.
Collector
collecting
loop
System
direct space
heat,
DHW once-
through HX
System
indirect
space heat,
DHW-storage
Space heat
distribution
loop
DHW
production
DHW
distrib.
line
District heating-
network
Boiler
Collectors
Fuel
Heating
station
Fuel
Cold
water
Boiler
Short-term
heat
storage
Seasonal
heat
storage
Fig. 4.16 Solar supported district heating systems: two-pipe system with decentralised
domestic hot water heating (left) (see /4-18/) and four-pipe system with long-term heat
store (right) (DHW Domestic Hot Water, HX heat exchanger, distrib. Distribution; see
/4-15/)
For solar supported district systems, the utilisation of highly efficient flat-space
collectors with optical efficiencies of almost 80 % and thermal loss factors of
approximately 3 W/(m 2 K) is effective. They can be connected in series and/or in
parallel as large collector modules (> 10 m 2 ) with low pressure losses to form
large collector arrays. Considering such collector types, for systems with partial
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