Environmental Engineering Reference
In-Depth Information
radiation; yet, this only corresponds to an approximate efficiency on an annual
average of between 13 and 14 %.
Losses incurred outside of the cell are mainly composed by Ohmic losses
within the direct current (DC) cabling, the inverter and the required alternating
current (AC) cabling. In relation to the radiated solar energy these losses are low
and in most cases in the order of magnitude of a few percent. Under the assumed
standard test conditions (STC) they result in system efficiencies between 11 and
14 %. The annual average overall system efficiencies of silicon solar cells are thus
between 10 and 12 %. It has to be stated that modern photovoltaic plants can
show significant better overall efficiencies even on an annual basis.
Characteristic power curve. Radiated solar energy is transformed into electric
energy by the described conversion chain. There is a defined correlation between
the solar energy radiated onto the cell material within a given period of time and
the electric energy effectively provided by the photovoltaic cell or the inverter.
But the specific output power or the cell efficiency is reduced by approximately
0.5 %/K due to an increasing module temperature as a result of increasing solar
radiation. Fig. 6.31 shows the corresponding characteristic power curves for two
different cell types and inverter designs. Such characteristic curves are created by
timely summation of solar radiation (kWh/m 2 ) and supplied alternating current
(AC) power generation (kWh/m 2 ). Fig. 6.31 illustrates the respective daily totals
of solar radiation along with the corresponding DC and AC power generation.
Direct current
generation
Mono-crystalline
cells
Alternating current
generation
Multi-crystalline
cells
Total global radiation
Fig. 6.31 Characteristic performance curve for photovoltaic power generation for different
cell types and inverter designs (see /6-11/)
Considering the typical correlation between high radiation and high ambient
temperature, the diagram reveals why efficiencies during time periods with high
solar radiation (primarily during the summer months) in average are clearly below
those achieved in periods with low radiation (especially during the winter
months). Fig. 6.31 makes this obvious by the slightly deviating characteristic per-
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