Environmental Engineering Reference
In-Depth Information
Another interesting aspect of planning is how the land is used. So that the mirrors
do not throw shadows on one another, they can only be installed on about one-third
of the available land area. The land area for a concentrating solar power plant must
therefore be at least three times bigger than the surface area of the mirrors.
The mirror area should be coordinated optimally with the rest of the plant. If it is
too small, the plant will constantly be running in partial-load operation. As a result,
both effi ciency and yield will drop. If the size of the mirror area is too large, a higher
amount of solar radiation will fall onto the collectors than can ultimately be con-
verted into electricity. In this case some of the mirrors have to be turned away from
the sun.
The effi ciency depends on the technology used. Dish-Stirling power plants and solar
tower plants with pressure receivers can reach effi ciencies of 20% or more. Solar
tower or trough plants with steam turbines are currently at about 15% effi ciency.
An increase in DNI irradiance values also increases effi ciency as a power plant will
then be running for shorter periods in partial-load operation. In Germany effi ciency
in the order of 10% is all that can be expected from its solar thermal power plants
- and this is based on moderate DNI irradiance values of about 1000 kWh/m 2 and
by year. The annual yield from concentrating solar power plants in places like Spain
would be roughly three times higher than in Germany or Great Britain.
7.3.2 Solar Chimney Power Plants
With solar chimney power plants the annual yield is calculated on an analogous
basis. Instead of the DNI, the global radiation is used because these plants can also
use diffuse sunlight. The total effi ciency is only about 1% - and then only if the
tower is more than 1000 m high. Thus the effi ciency of a solar chimney power plant
is directly correlated to the height of the tower. At half the optimal tower height the
effi ciency is also halved. The collector area would have to be doubled in size to
achieve the same yield. In contrast to a concentrating power plant, a solar chimney
plant can use the entire land area. No unused gaps are necessary to prevent shading.
7.3.3 Concentrating Photovoltaic Power Plants
The annual yield from concentrating photovoltaic power plants is also calculated
analogously. Compared to non-concentrating photovoltaic plants, an effi ciency
of more than 20% is possible. As concentrating photovoltaic plants can only
use the direct-normal irradiance portion, the yield in sunny countries rises
overproportionally.
7.4 Economics
Solar thermal plants can currently generate electric power more cost-effectively than
photovoltaic plants - at least in regions with high solar irradiation. In Germany at
best the same electricity generation costs can be expected from both solar thermal
 
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