Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 161

bined systems (SFH-I and SFH-II) and the multi-family house (MFH) are de-
signed as Low-Flow systems via storage with an external heat exchanger and
stratified charging. The solar system of the SFH-III operates according to the
High-Flow principle. The specifications in Table 1.2 (Chapter 1) are valid for the
district heating system DH-I.
The system efficiency includes all steps of energy transformation, from solar
radiation incident on the collector area up to the useful heat at the storage outlet
(for the district heating system the values are indicated without and with thermal
losses of the district heating system and the house transfer stations). The relatively
high system efficiency of the decentralised domestic water heating (SFH-III) and
the district heating network (DH-I) are due to the solar summer yields that can be
utilised almost entirely. The lower efficiency of the combined systems (SFH-I und
SFH-II) is due to the system being unable to use the excess heat during the sum-
mer for domestic water heating and space heating because of the large collector
area; it often stands still. The high system efficiency of the system MFH is due to
the low solar fractional and the fact that solar radiation can be used during the
whole summer, low return-flow temperatures from the heat distribution network.
When calculating the assumed degrees of utilisation of the conventional heat-
ing boiler that is needed in addition to the solar installation, the seasonal depend-
ency of the boiler efficiency has to be considered to the same extent as the varying
fractional saving of the solar plant during the summer and in winter. At an annual
fractional saving of around 60 % for solar heating of domestic water (SFH-III) the
fractional saving for example is between 80 and 100 % during the summer
months, and sometimes drops to even below 20 % in winter. Thus, the efficiency
of the fossil fuel boiler for domestic water heating is lower in summer than in
winter. Therefore, the mean efficiency of the boiler is lower than the annual aver-
age at times when the heat can also be supplied by a solar system. This is why an
efficiency of 80 % is assumed for the decentralised solar thermal domestic water
heating (SFH-III) and the combined systems (SFH-I und SFH-II), where oil is
substituted. For the centralised solar thermal system for multi-family houses
(MFH) and the solar-supported district heating network (DH-I) a condensing
boiler with a mean annual efficiency of 98 % is assumed.
In order to evaluate the costs generated by solar thermal heat utilisation, first of
all the investment and the operating and maintenance costs of solar thermal sys-
tems are described. Subsequently, the specific solar heat generation costs plus the
specific equivalent fuel costs will be determined on that basis. The latter are the
costs of useful solar energy at the storage outlet evaluated with the efficiency of a
conventional heating boiler providing heat in connection with the solar system
(i.e. the costs for the (fossil) fuel avoided by solar thermal heat generation).
Investments. Investments into systems for solar thermal low-temperature heat
generation can vary significantly. In the following only average costs can be indi-
cated. The actual costs can sometimes differ tremendously from these average
costs.