Environmental Engineering Reference
In-Depth Information
4.2.3 Energy conversion chain and losses
Energy conversion chain. A solar thermal system built with the described system
components converts solar radiation energy into utilisable heat. Fig. 4.9 shows the
entire energy conversion chain of such a solar thermal installation, together with
collector, heat carrier and heat store (optional). According to this presentation,
photons of solar radiation are absorbed by the absorber and cause the absorber
atoms to vibrate. Thus the temperature in the absorber increases and heat is gener-
ated. Part of this heat is transported by thermal conduction within the absorber to
the absorber pipes that the heat carrier flows through. This heat is released to the
heat carrier and transported further. In most cases the heat is then transferred via a
heat exchanger to a heat store before it is passed on to the consumer.
Heat-
Heat
exchanger
heat carrier/
heat store
media
Transformation of
conduction
photon energy
absorber/
heat
carrier
into heat in
absorber material
Solar
Heat
Heat
Heat
radiation energy
in absorber
in heat exchanger
in heat store
media
Fig. 4.9 Energy conversion chain of solar thermal heat utilisation (e.g. /4-6/)
Losses. Due to the various loss mechanisms only part of the solar radiation is
available as heat to the consumer. Fig. 4.10 shows the energy flow of a solar ther-
mal installation with a flat-plate collector, forced circulation and one to two day
storage to support the domestic hot water supply for a private household of 3 to 5
people according to the current state of technology. With a collector area of ap-
proximately 6 m 2 , the mean annual solar fractional saving is 50 to 60 %. During
the summer it is proportionally higher - over 90 % -, and during the winter it
sinks to below 15 %.
The relative losses described in Fig. 4.10 are mean annual values. They are
typical for Central European meteorological conditions and are related to the ra-
diation on the collector. Large losses of approximately 25 % thus occur due to a
collector standstill if the storage has already been heated up to its maximum tem-
perature, or the temperature required to charge the storage has not yet been
reached by the collector. The greatest losses, with a total of around 38 %, occur in
the collector when converting solar radiation into heat or before transporting it
further with the heat transfer medium.
Such solar systems have a total annual system efficiency of around 25 %, start-
ing from solar radiation to the actual utilisable heat of domestic hot water (here all
losses of the domestic hot water storage are allocated to the solar installation) or
of 32 % up to the release of the collector heat into the domestic hot water storage.
 
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