Environmental Engineering Reference
In-Depth Information
Note: Location: Berlin, Collector Inclination: 30°, Heat Demand: 10 kWh/day
Figure 3.19 Solar Fraction as a Function of the Collector Surface:
Simulation by the Software Getsolar.
Solar energy systems for domestic water heating in moderate climates are
usually designed for solar fractions of about 50-60 per cent. This is a
compromise between desired high solar fractions and economic considerations.
However, the solar fraction in regions with high annual irradiations can be
much higher. If there are only small differences between summer and winter,
the solar fraction can approach 100 per cent.
Figure 3.19 shows the solar fraction as a function of the collector surface
for two different volumes of storage tank calculated for Berlin. The heat
demand was constant for all calculations. The curve shows that the solar
fraction increases very quickly with the collector surface area for relatively
small collectors. However, in the given example, the collector surface must be
doubled to increase the solar fraction from 60 to 70 per cent. Large storage
volumes are counter-productive for small collector surfaces because the storage
losses are disproportionately high in such a case. If the solar collector surface
increases significantly, the storage volume should be increased as well.
Another important parameter for analysis is the solar collector cycle
efficiency
η CC . It describes the total efficiency of the solar thermal system. It is
defined as the ratio of the annual heat that the solar cycle feeds into the storage
system to the annual solar radiant energy on the collector surface. With annual
irradiation H Solar on the collector surface and collector area A C , the collector
cycle efficiency becomes:
(3.60)
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