Introduction and Structure - Renewable Energy: Technology, Economics and Environment - page 20

duced during the non-heating summer period. Annual efficiencies between 91
and 93 % (in relation to the heating value of the fuel) can thus be achieved.
− Condensing boiler. Condensing boilers allow for the most extensive exploita-
tion of the energy contained in the fuel. By extensive cooling of hot flue gases
with the in-flow of the heating system, the perceptible heat of the exhaust gases
as well as the latent heat (evaporation heat) of steam contained in the flue gas
can almost entirely be utilised. Yet, this heat can only be used if the return (in-
flow) temperature of the heating system is below the dew point temperature of
the flue gas released by the boiler; only in this case part of the steam contained
in the flue gas can be condensed by releasing energy (heat). Such condensing
boilers are available for oil and gas. Natural gas-fired condensing boilers show
annual efficiencies of up to 104 %, referred to the heating value of the gas.
Domestic hot water generation is mainly performed by means of storage domestic
hot water heaters, located on top, below, or beside the heating boiler. The water is
either heated by a heat exchanger located inside the storage facility (i.e. directly
heated storage tank) or by an external heat exchanger (i.e. indirectly heated stor-
age tank). In addition, electrically heated domestic hot water storages are in use.
Heat provision for domestic hot water generation and/or space heating by oil-
fired or natural gas-fired boilers is characterised by corresponding costs, which
are briefly discussed in the following. However, first the assessed reference plants
are defined.
For the heat provision according to the defined supply tasks (Table 1.1) - de-
pending on the required thermal capacity - the application of natural gas-fired
boilers with condensing technology (5 kW for SFH-I; 8 kW for SFH-II; 18 kW
for SFH-III; 60 kW for MFH), of atmospheric low temperature natural gas-fired
boilers (9 kW for SFH-II), and of oil-fired low temperature boilers (20 kW for
SFH-III; 67 kW for MFH) have been assumed (Table 1.6). Domestic hot water
generation is provided via a storage system which is charged by an external heat
exchanger in case of the multi-family house (MFH) and by internal heat exchang-
ers for the single-family houses (SFH-I, SFH-II, SFH-III) respectively.
The applied amount of fossil fuel energy is determined by the amount of heat
provided to the domestic hot water storage and to the supply point of the heat dis-
tribution system within the supplied building, and by the overall system efficiency
of the heat generation system. Therefore the losses of the domestic hot water stor-
age as well as the lower boiler efficiency for domestic hot water generation during
the non-heating summer months have been considered. Especially for buildings
with specific low heat demand (e.g. SFH-I) the annual mean system efficiency
may be significantly lower than the efficiency of the boiler.
The defined district heating networks (Table 1.2) will not be analysed within
the scope of this topic for the use of fossil fuel energy. For economic as well as
for environmental reasons, to date preferably decentralised solutions have been
implemented.