Environmental Engineering Reference
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does not exceed 27% throughout the year. Multiplying these values by the actual
panel surface installed, the panel efficiency and the overall system efficiency, we
will be able to estimate the electricity production of the solar facility.
Regarding the available photovoltaic solar module technologies, in general PV
cells can be classified as either crystalline (single crystal or multicrystalline) or
thin film. According to EPIA-Greenpeace (2008), the most commercialized tech-
nologies in 2007 were polycrystalline Silicon (pc-Si) with module efficiencies 12-
14%, monocrystalline Silicon (mc-Si) with module efficiencies 13-15%, amor-
phous silicon (a-Si) with module efficiencies 5-7%, and cadmium telluride (CdTe)
with module efficiencies about 10%. Less relevant technologies in terms of market
share but with promising technological aspects were copper indium diselenide
(CIS) and copper indium gallium diselenide (CIGS) with module efficiencies 7-
11%. For the configuration proposals of this case study the comparison between
the results using CIS, pc-Si, and a-Si was considered.
6.3.2 Sequence for the Dimensioning of the PV System
Once the electrical power needs and available solar irradiation have been deter-
mined, a standard procedure is followed to estimate the size, power and layout of
each intended photovoltaic arrangement First, the minimum (nominal) design
power to be installed to cover the annual electricity consumption is calculated.
The formula includes the average daily demand level, the average daily solar inci-
dent power on the plane of array chosen, and a factor for the overall system effi-
ciency called PR (Performance Rate), with values between 0 and 1. In this case a
mean value of 0.75 will be adopted, as no progressive loss of performance of the
solar modules throughout their lifetime will be considered. The value obtained
from the calculation is that 25.5 kWp of nominal power are needed. The area cov-
ered by the PV arrays to be installed can be calculated by dividing the nominal
power by the generation efficiency of the PV panels. An additional consideration
is the spacing between modules to avoid shading, which on occurance (even only
partially) causes significant losses of power generation. With a tilt angle of 18ยบ,
the distance to avoid shading will mean an additional 54% (of the module side
length) in the North to South direction.
In the next step it will be necessary to calculate the minimal cable section to be
used in the DC (continuous current) section, that is between the photovoltaic ar-
rays and the current inverters, to assure the losses by heat dissipation do not ex-
ceed a certain tolerated value. This can be an important cost factor for the installa-
tion. For that there is a formula relating all physical parameters of the installation.
Additionally, the minimum section established by the normative of the country
(for low voltage electrical installations of buildings) to avoid overheating due to
the amperage of the transmitted current will have to be considered.
The final choice of section will be the minimum values indicated by the norms
and the calculation. Afterwards the selection of the current inverter has to be done.
For that, one should try to keep the quantity of inverters as low as possible and op-
timize their use by choosing an operation range where their efficiency is high.
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